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Modeling Electrified Pt(111)-Had/Water Interfaces from Ab Initio Molecular Dynamics

  • Jia-Bo Le
    Jia-Bo Le
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
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  • , 
  • Ao Chen
    Ao Chen
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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  • , 
  • Lang Li
    Lang Li
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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  • , 
  • Jing-Fang Xiong
    Jing-Fang Xiong
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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  • , 
  • Jinggang Lan
    Jinggang Lan
    Department of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
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    Orcidhttp://orcid.org/0000-0001-6353-2539
  • , 
  • Yun-Pei Liu
    Yun-Pei Liu
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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  • , 
  • Marcella Iannuzzi
    Marcella Iannuzzi
    Department of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
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    Orcidhttp://orcid.org/0000-0001-9717-2527
  • , and 
  • Jun Cheng*
    Jun Cheng
    State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    *Email: [email protected]
    More by Jun Cheng
    Orcidhttp://orcid.org/0000-0001-6971-0797
Cite this: JACS Au 2021, 1, 5, 569–577
Publication Date (Web):April 6, 2021
https://doi.org/10.1021/jacsau.1c00108

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Unraveling the atomistic structures of electric double layers (EDL) at electrified interfaces is of paramount importance for understanding the mechanisms of electrocatalytic reactions and rationally designing electrode materials with better performance. Despite numerous efforts dedicated in the past, a molecular level understanding of the EDL is still lacking. Combining the state-of-the-art ab initio molecular dynamics (AIMD) and recently developed computational standard hydrogen electrode (cSHE) method, it is possible to realistically simulate the EDL under well-defined electrochemical conditions. In this work, we report extensive AIMD calculation of the electrified Pt(111)-Had/water interfaces at the saturation coverage of adsorbed hydrogen (Had) corresponding to the typical hydrogen evolution reaction conditions. We calculate the electrode potentials of a series of EDL models with various surface charge densities using the cSHE method and further obtain the Helmholtz capacitance that agrees with experiment. Furthermore, the AIMD simulations allow for detailed structural analyses of the electrified interfaces, such as the distribution of adsorbate Had and the structures of interface water and counterions, which can in turn explain the computed dielectric property of interface water. Our calculation provides valuable molecular insight into the electrified interfaces and a solid basis for understanding a variety of electrochemical processes occurring inside the EDL.

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1. Introduction

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Electrochemical devices enable energy conversion between chemicals and electricity, and one of their key components includes the interfaces immersed between the electrodes and the solution, where Faradaic processes occur. In order to understand the mechanisms of interfacial reactions and thereby rationally design electrode materials, the structures of electrochemical interfaces have been extensively studied both experimentally and theoretically. (1−8) Measuring differential double-layer capacitance from voltammetry is one of the traditional means in electrochemistry to investigate the interfaces, (9) which enables the early establishment of the well-known Gouy–Chapman–Stern (GCS) model for the electric double layers (EDL). To obtain a detailed microscopic picture of the electrochemical interfaces, many advanced experimental techniques have been developed in the past few decades, including surface enhanced infrared spectroscopy, (6,10,11) Raman spectroscopy, (5,12,13) sum frequency generation spectroscopy, (14,15) X-ray absorption spectroscopy, (2) and surface X-ray scattering. (16,17) They have been very helpful for revealing the hydrogen bonding network of interface water, (2,5,14) density distribution of ions and water, (4,16,17) coverages and adsorption sites of surface adsorbates, (18,19) and so on.
Theoretical computation, as a complementary tool to experimental techniques, also plays an important role in studying the structures of the electrochemical interfaces. (5,7,20−27) For example, with the aid of ab initio molecular dynamics (AIMD) simulations, it has been shown that there exists a layer of chemisorbed water on metal electrodes such as Pd and Pt, (23,28) which can induce electronic redistribution and significant potential shifts at the interfaces. More interestingly, it has been found recently that the coverage of the chemisorbed water is strongly dependent on the electrode potential at the Pt(111)/water interface. (29) This can result in a peculiar negative component of Helmholtz capacitance, providing a molecular insight into the bell-shaped profile of the differential double-layer capacitance measured by experiment. (30) It should be mentioned that the work (29) focused on the double-layer region at the Pt(111)/water interface (0.4–0.6 V vs RHE), (31) and hence, specifically adsorbed ions such as adsorbed H and OH were omitted in the models.
In the nondouble-layer region, e.g. < 0.4 V vs RHE, the Pt(111) surface is covered by certain amount of adsorbed H (Had). One would logically anticipate that the Had could compete for the surface chemisorption sites with water and hence would affect the interface structure and double-layer capacitance. Moreover, the Had on the electrode surface is the key intermediate of hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR), and its adsorption energy is widely used as a descriptor for the reaction activity. (32−36) In view of these interests, many efforts have been devoted to modeling the Pt(111)-Had/water interfaces in recent years. (37−40) For example, Kristoffersen et al. (39) have shown that the Had atoms are mostly distributed on the hollow and top sites on Pt(111) and effectively replace the chemisorbed water on the surface. It has been also reported that the Had on Pt(111) can ionize spontaneously at the high Had coverage condition during AIMD runs. (37,39) Kronberg and Laasonen (40) have found the presence of Had on Pt(111) surface can apparently accelerate the dynamics of the interface water. However, the EDL has not been realistically represented in their interface models, and as such, the electrochemical conditions are not well-defined, rendering it difficult to make direct comparison with experiment.
In this work, to unravel the microscopic structures of electrified Pt(111)-Had/water interfaces, the EDL is modeled at different potentials using AIMD simulations. The EDL is built by employing counterions at the vicinity of the surface to compensate the surface charge, representing a realistic Helmholtz layer. A detailed description of the interface model and the computational setup are given in the following Methods section. In the Results and Discussion section, we first calculate the Helmholtz capacitance of the Pt(111)-Had/water interface and compare with experimental data for validating the EDL model and the accuracy of the AIMD simulations. We then investigate the distribution of Had adsorption sites, interface water, and counterions on the Pt(111) surface at different potentials. In particular, we find that the saturation coverage of Had completely inhibits water chemisorption, thus removing the negative capacitive response owing to water adsorption/desorption processes at the interface, and giving rise to a ∼ 20 μF/cm2 Helmholtz capacitance consistent with the experimental value in the hydrogen underpotential deposition potential region. Finally, we end this paper with some conclusions.

2. Methods

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2.1. Electrified Interface Models

The Pt(111) surface was modeled with a p(6 × 6) periodic slab of 4 atomic layers, and separated from its periodic images with a vacuum region. The overall size of the cell of the Pt(111) surface was 16.869 × 16.869 × 30 Å3, and the calculated work function (5.8 eV) was very close to the experimental value. (41,42) Both sides of the clean Pt(111) surface model were then covered by 2/3 monolayer (ML) Had, which was suggested by Markovic (43) as the saturation coverage of the H deposited at underpotential (Hupd) on Pt(111). The electrified Pt(111)-Had/water interfaces were modeled by fully filling the vacuum region with water molecules (the water density of the bulk region was kept close to 1 g/cm3). To represent Helmholtz layers in the models, Na atoms were initially placed at the vicinity of both Pt(111)-Had surfaces, as shown in Figure 1. It should be mentioned the Na ions do not diffuse into the bulk water in the time scale of ∼10 ps due to electrostatic attraction by the charged surface. It has been indicated in Figure S2 that the Na atoms are indeed ionized, and the Mulliken population analysis shows the charge of Na at Pt(111)-Had/water interfaces is ∼+0.85 e0. Since the overall model is kept charge neutral, the Pt(111)-Had surfaces are accordingly negatively charged. Varying the number of Na+ ions in the model amounts to controlling the surface charge density (σ), and with this approach, four electrified Pt(111)-Had/water interface models were built up with σ = −13, −19.5, −26, and −32.5 μC/cm–2, respectively. A neutral Pt(111)-Had/water interface was also modeled, and it was found that a few Had atoms on the surface are spontaneously ionized, forming hydronium ions at the interface. This indicates that the Had atoms are not stable at the neutral Pt(111)-Had/water interface. In addition, it should be mentioned that the Gouy–Chapman layers are not included in the interface models, and thus, our EDL models correspond to the high concentration limit when the surface charges are effectively balanced within the Helmholtz layer. The ion pairs in the bulk electrolyte are also ignored for accelerating the equilibrium of the interface because the interfacial potential is determined by the charge excess in the double layer, not the charge balanced ion pairs, and the capacitive behavior of the Helmholtz layer should not be considerably affected by missing of bulk electrolyte ions. Separating from the longer timescale of ion exchange with bulk solution, we did not observe the counterions diffusing away from the interface in our AIMD simulations of tens of ps, which in some sense resembles the equilibrium situation that there is dynamic ion flux but with no net charge passing the EDL boundary. Similar EDL models have been successfully used to simulate the Au(111) and Pt(111)/water interfaces. (5,29)

Figure 1

Figure 1. AIMD models of the electrified Pt(111)-Had/water interfaces with surface charge density (a) σ = −13 μC/cm–2, (b) σ = −19.5 μC/cm–2, (c) σ = −26 μC/cm–2, and (d) σ = −32.5 μC/cm–2. The Pt, O, Hw (H of water), Na, and Had (adsorbed H) atoms are colored by gray, red, white, blue, and orange, respectively. The Pt(111) surface is saturated with a 2/3 monolayer of Had.

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2.2. Computational Details

All the calculations were performed with freely available CP2K/Quickstep package. (44) The DFT implemented in CP2K is based on a hybrid Gaussian plane wave (GPW) scheme. The orbitals were described by an atom centered Gaussian-type basis set, and an auxiliary plane wave basis set was used to re-expand the electron density in the reciprocal space. The 2s, 2p electrons of O, 2s, 2p, 3s electrons of Na, and 5d, 6s electrons of Pt were treated as valence, and the remaining core electrons were represented by Goedecker–Teter–Hutter (GTH) pseudopotentials. (45,46) The Gaussian basis sets were double-ζ with one set of polarization functions (DZVP), (47) and the energy cutoff was set to 400 Ry. We employed the Perdew–Burke–Ernzerhof (PBE) functional (48) to describe the exchange-correlation effects, and the dispersion correction was applied in all calculations with the Grimme D3 method. (49)
One of the AIMD families, the second generation Car–Parrinello molecular dynamics (SGCPMD) (27,50,51) was used to sample configurations for these interface models, and the target temperature was set to 330 K. The correction step was obtained by 5 iterations of the orbital transformation (OT) optimization, (52) and the integration time for each MD step was 0.5 fs. The Langevin friction coefficient (γL) was set to 0.001 fs–1, and the intrinsic friction coefficients (γD) were 5 × 105 fs–1 for Pt, 2.2 × 104 fs–1 for H2O and Na, and 2.2 × 103 fs–1 for Had, respectively. For each AIMD simulation, an initial ∼5 ps (∼10 000 steps) of MD trajectory was used to equilibrate the system, which was then followed by a production period of ∼10 ps (∼20 000 steps) for data analysis. Because of the large size of the cells, only the Γ point in the reciprocal space was used in all the simulations. The velocity density of states (VDOS) of Had was calculated by the Fourier transformation of the velocity-velocity autocorrelation function from the AIMD trajectories. Note that the inner two layers of the Pt(111) slab were fixed during AIMD simulations, and to prevent water dissociation, all the O–H bonds of water were constrained by harmonic springs. In order to check if the constraint on the O–H bonds has any noticeable effect on the Pt(111)-Had/water interfaces, an independent simulation was performed, in which the O–H bonds of the water were relaxed. As shown in Table S1 and Figure S3, the difference in the electrode potential, the structure of water and Had are indeed very small by comparing the results from the unconstrained trajectory with the constrained trajectories.
In order to investigate the effect of Had adsorption on the work function of the Pt(111) surface in vacuum, we built a series of models with Pt(111) surfaces covered by 1/3, 2/3, and 1 ML Had on the top or hollow site (see Figure S4 in the Supporting Information (SI)). To avoid the dipole interaction with periodic images, the periodicity in the surface normal direction (z-coordinate) of these models was removed. The BFGS minimizer was employed to optimize the geometries of these models. Moreover, to study the effect of Had on surface charge distribution, the excess charge on Pt(111) surface was compensated by a counter charge plane, which was modeled by an array of classical point charges with the same amount of charge.

3. Results and Discussion

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3.1. Helmholtz Capacitance

The electrode potential (U) corresponding to each interface model is first calculated with the recently developed computational standard hydrogen electrode (cSHE) method, which is briefly described in the SI. (23,53) The four models containing 2, 3, 4, and 5 Na+ ions for each interface in Figure 1 correspond to −0.03, −0.36, −0.51, and −0.84 V vs SHE, respectively (the convergence of the computed U can be found in Figure S1). It should be mentioned that we only model the Pt(111)-Had/water interfaces at negative potentials because only under that experimental condition (43,54) is the coverage of the Had saturated. As seen in Figure 2, it is clear that the data points σ-U can be well fitted by a straight line. It indicates the differential Helmholtz capacitance (CH = dσ/dU) of the Pt(111)-Had/water interface is almost constant in this negative potential region, and the obtained value of 24 μF/cm2 is in good agreement with the experimental double-layer capacitance (30) measured under the same condition (i.e., electrode potential and Had coverage).

Figure 2

Figure 2. Plot of surface charge density (σ) as a function of computed electrode potential (U). The solid dots with error bars represent the computed data from AIMD simulations, which can be linearly fitted. The slope of the fitted line indicates the Helmholtz capacitance CH = 24 μF/cm2.

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3.2. Had on Pt(111) Surface

We then study the properties of the adsorbed Had atoms on Pt(111) surface at different potentials. Figure 3a shows the density distribution profiles of Had () along the surface normal direction, and it is found that all the surface Had atoms are located within 2 Å from the Pt(111) surface. This indicates that surface Had does not spontaneously desorb from the negatively charged surface during our AIMD simulations, in contrast to the neutral surface where desorption of Had to form aqueous proton was observed in the previous work. (37,39) Also, we do not observe the production of H2 molecule via the coupling of two Had, which is in line with a previous calculation (55) showing that there is a significant barrier of ∼0.6 eV for the Tafel step (2Had → H2) to proceed on Pt(111). Furthermore, it is interesting to notice from Figure 3a that there are two peaks at ∼1 and ∼1.5 Å, indicating two types of Had at Pt(111)-Had/water interface. Similar to the previous work, (38) these two peaks at ∼1 Å and ∼1.5 Å can be assigned to the Had at the hollow and top site of Pt(111), respectively. At negative potentials below −0.5 V vs SHE, nearly half ML of Had sits at the hollow site, while the remaining ∼1/6 ML is at the top site, and the abundance slightly shifts to the top site when increasing the electrode potential. The adsorption sites of Had can also be characterized by computing the vibrational density of states (VDOS). As can be seen in Figure 3b, the sharp peak at ∼2100 cm–1 is attributable to the stretching vibrational mode between the top site Had and Pt surface, in agreement with the surface-enhanced infrared reflection absorption spectroscopy (SEIRAS) experiment. (18) We also note that there is a small red-shift (∼20 cm–1) in the vibrational frequency at negative potentials. The broad band around ∼1100 cm–1 corresponds to Had at the hollow site of Pt(111), as also indicated by the previous calculation in gas phase. (55)

Figure 3

Figure 3. (a) Density distribution profiles of Had () along the surface normal (z-coordinate) at −0.03, −0.36, −0.51, and −0.84 V vs SHE. The zero in z-coordinate indicates the position of the nuclei of the uppermost layer of Pt(111). (b) Calculated VDOS profiles of Had on Pt(111). (c) A representative snapshot of Pt(111)-Had surface from an AIMD trajectory. The Pt and Had atoms are represented by gray and orange balls, respectively. The black parallelogram in the model denotes the periodic boundary condition. (d) A ∼10 ps trajectory of one Had on Pt(111) surface.

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Figure 3c shows a snapshot of Pt(111)-Had surface from the AIMD trajectory, which indicates the lateral distribution of Had on Pt(111). We do not observe an obvious pattern formed by the surface Had, other than the Had mainly populating at the top and hollow sites of Pt(111), consistent with the density distribution and VDOS analysis above. There is a small fraction of Had at the bridge site, which is supposed to be the intermediate state for Had diffusion on Pt(111). We keep track of one Had atom along the AIMD trajectory, as illustrated in Figure 3d, and we find that the Had can readily diffuse on Pt(111) surface through the bridge sites. This finding is consistent with the previous calculation showing a small barrier of ∼0.1 eV for Had diffusion on Pt(111). (56)
It would be interesting to understand how the Had adsorption impacts the interface potential. Thus, we first investigate the effect of adsorption site and coverage on the work function of the Pt(111) surface () in vacuum. As shown in Figure 4a, we find that the is strongly dependent on the Had coverage, decreasing from ∼6 eV to ∼5 eV by increasing the Had coverage from 0 to 1 ML. A similar trend has been observed in an experiment for work function versus Had coverage. (57) The decrease in the work function indicates that upon adsorption, there is a charge redistribution from Had to Pt(111) surface, inducing a dipole pointing upward to the Had. This finding suggests that in the potential region where the Had coverage changes (e.g., H underpotential deposition (UPD) region), the change in due to charge redistribution induced by adsorption can lead to a significant capacitive response. Note that this response is a dipole effect and should be distinguished from the pseudocapacitive response for a Faradaic process like H UPD.

Figure 4

Figure 4. (a) Calculated work functions of Pt(111) surfaces with different coverages of Had at top (red circle) or hollow (blue triangle) site in vacuum. (b) Work functions of Pt(111)-Had surfaces, the configurations of which are taken from the AIMD trajectories of electrified interfaces at different potentials and with the solution removed.

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In contrast, the work function is less sensitive to the adsorption site of Had, differing by ∼0.3 eV between the top and hollow site at the 1 ML coverage. As for the interface structure, we compute the work functions of the Pt(111)-Had slabs taken from the AIMD trajectories at different potentials, as shown in Figure 4b. Since the Had coverage is fixed at 2/3 ML and there is only a very small variation of site distribution of Had (see Figure 3a), the work function keeps rather constant under different potential conditions. This implies that the small variation in Had adsorption has little impact on interface potential and capacitance, and we thus conclude that the CH of the Pt(111)-Had/water interface mainly stems from the response of the interface water to the electrode potential.

3.3. Interface Water

We then analyze the structure of interface water at the electrified Pt(111)-Had/water interfaces. Figure 5a shows the profiles of water density profiles () at different potentials, and there is a distinct peak at z = 3.3 Å in all the profiles. When we make comparisons to the Pt(111)/water interface, (23,28,29,58) we notice the peak at z = 2.3 Å disappears at the Pt(111)-Had/water interface, which indicates no chemisorbed water present at the Pt(111)-Had/water interface because the 1 ML Had blocks all the adsorption sites for water. Figure 5b plots the profiles of dipole orientation (, where φ is the angle between the bisector of water and the surface normal) and shows that the dipole of the interface water becomes more oriented toward the surface when the potential shifts to negative, as expected. It is interesting to note that the dipole orientation of the interface water has almost no further change at the potential more negative than −0.51 V vs SHE, suggesting that the dielectric saturation of the interface water is reached. Figure 5c shows the probability distribution profiles of the angle φ, and the prominent peak at 135° indicates most interface water molecules take the “one-H-down” configuration orienting one O–H bond of the water toward the surface and the other parallel to (see the inset in Figure 5b), which is beneficial to forming hydrogen bonding network with the neighboring water as reported in the recent work. (5) Consistently, the hydrogen bond analysis shown in Figure 5d reveals the number of the hydrogen bond donor (Ndonor) of the interface water is close to 1.

Figure 5

Figure 5. Profiles of water density (, (a)) and dipole orientation (, (b)) along the surface normal (z-coordinate) at −0.03, −0.36, −0.51, and −0.84 V vs SHE. The positions of water molecules are indicated by those of the oxygen atoms, and the zero in z-coordinate indicates the position of the nuclei of the uppermost layer of Pt(111). The inset shows a representative snapshot of the electrified Pt(111)-Had/water interface. The Pt, O, Hw, and Had are colored by gray, red, white and orange, respectively. (c) Probability distribution profiles of angle φ (between the bisector of water and the surface normal, P(φ)) of interface water. (d) Number of hydrogen-bond donors (Ndonor) of interface water as a function of potential. A hydrogen bond is defined when the O–O distance is shorter than 3.5 Å and the O–O–H angle is less than 35°.

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Moreover, we can deduce from Figure 5 that the layer of interface water becomes more ordered at more negative potentials, particularly the distribution of angle φ becoming much narrower and the Ndonor is reduced from 1.08 to ∼1 at very negative potentials. Thus, one can speculate that it would become more difficult to reorient an interface water at very negative potentials, not only due to alignment with the enhanced electric field but also due to the higher energy penalty required to break more ordered hydrogen bonding network. It is supported by the dipole–dipole autocorrelation analysis shown in Figure S5 that the interface water at −0.03 V has faster relaxation time (∼5 ps) compared with those at more negative potentials (∼11 ps). Indeed, water reorganization at interfaces has been proposed by Koper and co-workers (59) as a pH-dependent descriptor for HER activity. This can also explain why modifying Pt(111) with a small amount of Ni(OH)2 can decrease the HER overpotential. (60) It is because Ni(OH)2 lowers the potential of zero charge of Pt(111), leading to a weaker electric field at the interface.
The main difference of interface water structure between the Pt(111)-Had/water and Pt(111)/water interface is that in the absence of surface Had, water can chemisorb on the Pt(111) surface, and moreover its coverage varies with the applied potential, leading to a significant capacitive response. Figure 6 compares the profiles of the differential capacitance CH at the Pt(111)/water interface and Pt(111)-Had/water interface (see CH as a function of σ in Figure S6). We can find that they converge to ∼20 μF/cm2 at a very negative potential, while they diverge when the potential shifts to positive. It has been shown in the recent work (29) that the CH of the interface water at the Pt(111)/water interface can be decomposed into two capacitive components connected in series; one is the commonly known dielectric response due to water reorientation (Csol), usually having a nearly constant capacitance value of ∼20 μF/cm2 at metal/water interfaces, (30,61−64) and the other is due to the variation of the coverage of the chemisorbed water in response to the electrode potential (CA). As sketched in the inset of Figure 6, the “one-H-down” water can reorient to chemisorb on the Pt(111) surface when shifting the potential positively. Electronic redistribution due to water chemisorption can induce a dipole (p) that compensates the electric field, giving rise to a peak in CH. In contrast, this capacitive component (CA) disappears at the Pt(111)-Had/water interface since the presence of Had completely inhibits water chemisorption (see Figure 5a), leaving only the Csol component. This discrepancy clearly demonstrates that the adsorption state of the Pt(111) surface has an important impact on the dielectric property of interface water.

Figure 6

Figure 6. Plots of Helmholtz capacitance (CH) as a function of electrode potential (U) at the Pt(111)/water interface (red) and Pt(111)-Had/water interface (blue). The red curve is taken from the recent work. (29) The insets show the change of the structure of interface water in response to the electrode potential on the Pt(111) and Pt(111)-Had surface. The red, white, and orange balls represent the O, Hw, and Had atom, respectively. The p denotes the induced dipole due to water chemisorption on Pt(111).

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3.4. Charges in Helmholtz Layer

Counter ions (i.e., Na+) are an important component of EDL, and they serve as more than just counter charges to the charges of electrode surfaces. In recent years, many experimental studies have reported that the choice of ions has a significant impact on the double-layer capacitance, (65) hydrogen adsorption, (66) selectivity of CO reduction, (67) and so on. However, the microscopic structures and behaviors of the ions at electrified interfaces are still lacking. We thus investigate the structures of counter Na+ ions at the Pt(111)-Had/water interface. Figure 7a shows the dependence of the distance between the Na+ ions and Pt(111) surface (dNa–Pt) on the electrode potential, that is, a monotonic decrease when shifting the potential from −0.03 V to −0.84 V vs SHE. Taking the averaged position of the Na+ ions in our simplified models as an approximate estimate to the width of Helmholtz layer, our calculation indicates that the width of the Helmholtz layer becomes smaller along with the negative shift in the electrode potential. It has been indeed suggested (68) that the Helmholtz layer can become narrower at charged surfaces due to enhanced electrostatic force. In the absence of chemisorbed water, the Helmholtz layer resembles a parallel-plate capacitor, and thus, its capacitance can be expressed as CH = εHε0/lH, in which lH denotes the Helmholtz layer width, and εH and ε0 are the relative permittivity of interface water in the Helmholtz layer and vacuum permittivity, respectively. A decrease of lH would suggest an increase of CH when the potential shifts more negative, which however is not what our calculation in Figure 2 shows. We suppose that εH would also become smaller with the negative sweep of the potential due to the saturation of the dipole orientation of the interface water at the negative potential, as illustrated in Figure 5b. Thus, the compensating effect between lH and εH may result in a nearly constant Helmholtz capacitance CH.

Figure 7

Figure 7. (a) Plot of the distance between the averaged positions of the Na+ ions and the uppermost Pt layer (dNa–Pt) at Pt(111)-Had/water interfaces as a function of potential. (b) The averaged number of water molecules coordinated to Na+ ions () at different potentials. (c,d) Snapshots of the Pt(111)-Had/water interfaces at −0.03 and −0.86 V vs SHE. Pt, O, Hw, Na, and Had are colored by gray, red, white, blue, and orange, respectively. The water molecules coordinated to Na+ are highlighted with the ball–stick model in comparison to other water with the line model.

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Furthermore, we analyze the solvation structure of Na+ ions as a function of the potential. As shown in Figure 7b, the coordination number of Na+ ions () shows a similar decreasing trend to dNa–Pt when shifting the potential to negative. At a slightly negative potential of −0.03 V vs SHE, the is 5.3, which is very similar to that of Na+ ions in aqueous solution. (69) This indicates that the Na+ ions have a full solvation shell at this potential, as illustrated in Figure 7c (see also the radial distribution function (RDF) of Na+ and O atoms in water in Figure S7 in the SI). The partial desolvation of Na+ starts at ∼ −0.4 V vs SHE, and the decreases to 4.5 at the potential of −0.84 V vs SHE, that is, almost one water in the solvation shell of Na+ is removed, which is in line with the surface enhanced Raman spectroscopic measurement. (12) It is worth mentioning that the ion desolvation at the interface is the consequence of the competition between the strength of the electrostatic attraction to the charged surface and the coordination of the ion solvation shell, and it should be less affected by ion packing at the interface. Note that we do not see any obvious decrease of the Mulliken charges on Na+ ions upon partial desolvation, which can also be confirmed by the projected density of states of Na in Figure S8 in the SI. Without partial charge transfer, (70) the inert Na+ ions do not specifically adsorb on the Pt surface, as commonly accepted in the field. In addition, we notice that as the Na+ ions approach closer to the surface with the negative shift of the potential, the surface Had atoms become in close proximity within ∼3 Å to the Na+ ions (see a representative snapshot in Figure 7d and the RDF between Na+ and Had in Figure S7b in the SI), probably driven in part by the attraction between the fractional negative charge on Had and the cations. We suspect that a similar attraction between surface intermediates and ions in the Helmholtz layer will have a direct impact on the catalytic activity of the interfaces, which would merit future investigations.
Finally, we calculate the location of net electronic charge on the Pt surface by taking the difference in charge density between the neutral Pt slab and the one with one excess electron. The solvent is ignored in this analysis because it has no charge transfer with the Pt–Had surface, as shown in Figure S2, and thus should not affect the distribution of the excess electron on the surface. As illustrated in Figure 8, we can find that the excess electron is located at ∼1.8 Å and ∼1.3 Å from the uppermost layer of the Pt(111) surface for Had adsorbed at the top and hollow site, respectively. For comparison, the excess electron is located at ∼1.3 Å from the uppermost layer on clean Pt(111). Thus, it is clear that Had at the hollow site hardly changes the position of the surface charge, while Had at the top site can push the net charge away from the Pt(111) surface by as much as 0.5 Å. Furthermore, Mulliken charge analysis indicates that the top site Had (+0.17 e0) donates more electrons to the neutral Pt(111) surface than the hollow site Had (+0.06 e0). The former decreases to +0.15 e0 at the negatively charged surface, confirming that the Had at the top site carries the surface excess electron (see the iso-surfaces of excess electron density in Figure 8a). This may weaken the Had binding and thus lead to the red-shift of the vibrational frequency of Had on Pt(111) at negative potentials observed by the SEIRAS experiment, (6) as well as in our calculated VDOS in Figure 3b. Considering Had at the top site is the key intermediate for HER and HOR on Pt(111), we suppose the change of its charge state and binding energy at varying potentials will have direct impact on the thermodynamics and kinetics of these reactions.

Figure 8

Figure 8. Net charge distribution on Pt(111) with 2/3 ML Had at the top site (a) and hollow site (b), and without Had (c). The profiles for the net negative charge distribution are averaged along the surface normal (z-coordinate), and the dashed lines in profiles indicate the positions of surface Had. The Pt and Had atoms are colored by gray and orange, respectively. The yellow isosurfaces in the models indicate the excess electron.

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3.5. pH Effects

In the electrochemical experiment, the pH, on a par with the potential, is a key parameter that determines the state of the electrode surface and thus the structure and dielectric properties of the interface, consequently having a significant impact on electrode activity for catalytic processes such as HER. Our simulated models at saturated 2/3 ML coverage of Had at different surface charge densities (i.e., potentials) can be related to electrochemical conditions at different pH, by applying the Nernstian relation. It has been shown that the saturated coverage of Had on Pt(111) can be reached at ∼0.05 V vs RHE for a wide range of pH conditions (e.g., ∼0 V vs SHE at pH = 1.2 and ∼−0.8 V vs SHE at pH = 13.1). (54) As for our simulated models at fixed coverage of Had, assuming the Nernstian relation is valid for the electrochemical condition range, varying the surface charge density and potential amounts to changing the pH condition with a rate of 59 meV per pH unit. In other words, the electrified interface models at −0.03 V and −0.84 V vs SHE can be estimated to correspond to solution pH = 1 and 15, respectively. Therefore, our calculation can actually offer information on electrified interfaces at different pH.
Informed by the differences of interface structures and capacitances caused by H adsorption, which in turn is affected by pH, we can envision how the pH may affect the hydrogen UPD. In acidic conditions, hydrogen starts to adsorb already at positive potential (e.g., 0.3 V vs SHE at pH = 2). (54) In these conditions, there are still some chemisorbed water molecules, which can lead to a high, total Helmholtz capacitance CH (red curve in Figure 6). Upon hydrogen adsorption, surface water desorbs at the same time, rendering the potential changing to a lesser extent. However, under alkaline conditions, hydrogen UPD occurs at rather negative potentials (e.g., −0.4 V vs SHE at pH = 13 (54)), when there is no chemisorbed water any more. The Helmholtz capacitance is expected to be low and constant (∼20 μF/cm2). This may explain the subtle difference observed in the hydrogen UPD regions in cyclic voltammetries at different pH.
As for HER which happens under the conditions at which the Pt(111) surface is saturated with adsorbed H atoms, our simulated models in this work can be regarded as the onset conditions at different pH. As mentioned above, the model at −0.84 V vs SHE, equivalent to a very basic solution pH = 15, has a rather rigid hydrogen bond network of interface water and a long relaxation time for dipole reorientation of interface water. This would indicate slower charge transfer kinetics than those under acidic conditions. (59)

4. Conclusions

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In order to understand the atomistic structure and Helmholtz capacitance of the Pt(111)-Had/water interface, a series of EDL models with various surface charge densities have been computed with extensive AIMD simulations. We have calculated the electrode potentials using the computational standard hydrogen electrode method as a function of surface charge density and obtained the capacitance of the Helmholtz layer. We have further analyzed the structures of water, Had, and counterions at the electrified interfaces and discussed the possible pH effects. The following main conclusions have been obtained.
(i)

The diffusion of surface Had atoms is fast on Pt(111), and neither spontaneous Had desorption nor H2 formation has been observed at negatively charged interfaces during the tens of ps AIMD simulations. We find that the majority of the Had adsorb at the hollow site and the rest sit at the top site, with the electrode potential hardly affecting the Had distribution. Interestingly, at the negatively charged interface, the Had atoms at the top site carry a fraction of surface charge and can push the negative charge away from the Pt surface compared with the clean Pt surface and the surface with the Had at the hollow sites.

(ii)

The saturation coverage of Had on Pt(111) eliminates the chemisorption of water and consequently the negative capacitive response induced by water adsorption/desorption processes at the interface. Thus, the calculated Helmholtz capacitance of the Pt(111)-Had/water interface, as expected from the usual dielectric property of interface water, is ∼20 μF/cm2 in the potential window of −1 to 0 V vs SHE, consistent with the experiment.

(iii)

Our simulated models can be connected to different pH conditions if applying the Nernstian relation. Understanding the surface state controlled by potential and pH, that is, with coadsorption of hydrogen and water, can help explain the differences in hydrogen UPD regions in cyclic voltammetries and HER kinetics at different pH.

(iv)

The width of the Helmholtz layer of the Pt(111)-Had/water interface becomes smaller when the potential shifts to the more negative direction, due to increased electrostatic attraction between the charged surface and counterions. Yet, the differential Helmholtz capacitance of the Pt(111)-Had/water interface at negative potentials remains almost constant, which must result from the compensating effect of dielectric (i.e., water orientation) saturation of interface water.

Finally, we believe that AIMD simulations of electrified interfaces can offer valuable atomic level understanding of the EDL structures and capacitive behaviors, which would be helpful to elucidating the molecular mechanisms of electrocatalytic reactions and searching for better performing catalysts.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00108.

  • Electrode potential calculation method; excess charge distributions at the Pt(111)-Had/water interface; models for Pt(111)-Had surface; time accumulative Fermi energies and electrostatic potentials; solvation structures of Na+; projected density of states; dipole–dipole autocorrelation of the interface water; plots of Helmholtz capacitance as a function of surface charge; verification of simulation time and the constraint of the O–H bonds in water (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Jia-Bo Le - State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China;  Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
    • Ao Chen - State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    • Lang Li - State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    • Jing-Fang Xiong - State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    • Jinggang Lan - Department of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland;  Orcidhttp://orcid.org/0000-0001-6353-2539
    • Yun-Pei Liu - State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
    • Marcella Iannuzzi - Department of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland;  Orcidhttp://orcid.org/0000-0001-9717-2527
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 21991151, 21991150, 21902136, 21861132015, and 21621091), the Principal’s fund of Xiamen University (Grant No. 20720190047) and the China Postdoctoral Science Foundation (Grant Nos. 2018M642563 and 2020T130361).

References

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    Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single mol. level, comparable to single-mol. fluorescence spectroscopy. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realize a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A no. of approaches have extended the technique to non-traditional substrates, most notably tip-enhanced Raman spectroscopy (TERS) where the probed substance (mol. or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to mols. with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as 'smart dust' over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various mols. adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.
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  14. 14
    Tong, Y.; Lapointe, F.; Thamer, M.; Wolf, M.; Campen, R. K. Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface. Angew. Chem., Int. Ed. 2017, 56, 4211– 4214,  DOI: 10.1002/anie.201612183
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    Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface
    Tong, Yujin; Lapointe, Francois; Thaemer, Martin; Wolf, Martin; Campen, R. Kramer
    Angewandte Chemie, International Edition (2017), 56 (15), 4211-4214CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Quant. description of reaction mechanisms in aq. phase electrochem. requires exptl. characterization of local H2O structure at the electrode/aq. interface and its evolution with changing potential. Gaining such insight exptl. under electrochem. conditions is a formidable task. The potential-dependent structure of a subpopulation of interfacial H2O with one OH group pointing towards a Au working electrode was characterized using interface specific vibrational spectroscopy in a thin film electrochem. cell. Such free-OH groups are the mol. level observable of an extended hydrophobic interface. This free-OH interacts only weakly with the Au surface at all potentials, has an orientational distribution that narrows approaching the potential of zero charge, and disappears on oxidn. of the Au electrode.
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  15. 15
    Noguchi, H.; Okada, T.; Uosaki, K. Molecular structure at electrode/electrolyte solution interfaces related to electrocatalysis. Faraday Discuss. 2009, 140, 125– 137,  DOI: 10.1039/B803640C
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    Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Voltage-dependent ordering of water molecules at an electrode-electrolyte interface. Nature 1994, 368, 444,  DOI: 10.1038/368444a0
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    Voltage-dependent ordering of water molecules at an electrode-electrolyte interface
    Toney, Michael F.; Howard, Jason N.; Richer, Joselyn; Borges, Gary L.; Gordon, Joseph G.; Melroy, Owen R.; Wiesler, David G.; Yee, Dennis; Sorensen, Larry B.
    Nature (London, United Kingdom) (1994), 368 (6470), 444-6CODEN: NATUAS; ISSN:0028-0836.
    The arrangement of water mols. at charged, aq. interfaces is an important question in electrochem., geochem. and biol. Theor. studies suggest that the mols. become arranged in several layers adjacent to a solid interface, with densities similar to that in the bulk, and that the mols. in the 1st layer are reoriented from oxygen-up to oxygen-down as the electrode charge changes from neg. to pos. Few of these predictions were verified exptl., however. Using x-ray scattering, the authors have measured the water d. profile perpendicular to a silver (111) surface at 2 applied voltages. The water mols. are ordered in layers extending ∼3 mol diams. from the electrode, and the spacing between the electrode and 1st water layer indicates an oxygen-up (oxygen-down) av. orientation for neg. (pos.) charge. Contrary to current models, however, the 1st layer has a far greater d. than that in bulk water. This implies that the hydrogen-bonding network is disrupted in this layer, and that the properties of the water in the layer probably are very different from those in the bulk.
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  17. 17
    Lucas, C. A.; Thompson, P.; Gründer, Y.; Markovic, N. M. The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte. Electrochem. Commun. 2011, 13, 1205,  DOI: 10.1016/j.elecom.2011.08.043
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    The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte
    Lucas, Christopher A.; Thompson, Paul; Gruender, Yvonne; Markovic, Nenad M.
    Electrochemistry Communications (2011), 13 (11), 1205-1208CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    The structure of the electrochem. double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte was probed using in-situ surface x-ray scattering (SXS). Detailed modeling of the SXS data at neg. potential (E = - 1.0 V vs. SCE) is consistent with the presence of an hydrated K+ cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at pos. potential (E = - 0.2 V), indicates that the presence of OHad stabilizes the hydrated K+ cations through a noncovalent interaction forming a compact double layer structure in which the Ag-K+ distance is reduced to 3.6 ± 0.2 Å.
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  18. 18
    Kunimatsu, K.; Senzaki, T.; Tsushima, M.; Osawa, M. A combined surface-enhanced infrared and electrochemical kinetics study of hydrogen adsorption and evolution on a Pt electrode. Chem. Phys. Lett. 2005, 401, 451– 454,  DOI: 10.1016/j.cplett.2004.11.100
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    A combined surface-enhanced infrared and electrochemical kinetics study of hydrogen adsorption and evolution on a Pt electrode
    Kunimatsu, Keiji; Senzaki, Takahiro; Tsushima, Minoru; Osawa, Masatoshi
    Chemical Physics Letters (2005), 401 (4-6), 451-454CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
    The adsorption of hydrogen atoms on a polycryst. Pt electrode was examd. in acid by surface-enhanced IR spectroscopy, and the IR data was quant. compared with the kinetics of hydrogen evolution reaction (HER). The Pt-H stretching vibration was obsd. around 2100 cm-1 in the H2 evolution potential range (E < 0.1 V vs. RHE). The current of HER was proportional to the square of the band intensity at 0.02 < E < 0.1 V, from which the adsorbed hydrogen atom is the reaction intermediate of HER and the combination of 2 adsorbed H atoms is rate-detg. At E < 0.02 V, however, an inconsistency was found between the IR and electrochem. measurements due to the supersatn. of H2 at the interface.
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    Carrasco, E.; Aumer, A.; Brown, M. A.; Dowler, R.; Palacio, I.; Song, S.; Sterrer, M. Infrared spectra of high coverage CO adsorption structures on Pt(111). Surf. Sci. 2010, 604, 1320– 1325,  DOI: 10.1016/j.susc.2010.04.021
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    Infrared spectra of high coverage CO adsorption structures on Pt(111)
    Carrasco, Esther; Aumer, Andreas; Brown, Matthew A.; Dowler, Rhys; Palacio, Irene; Song, Sundal; Sterrer, Martin
    Surface Science (2010), 604 (15-16), 1320-1325CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The adsorption of CO on Pt(111) was studied using polarization modulation IR reflection absorption spectroscopy (PM-IRAS) and sum frequency generation (SFG) spectroscopy. Two CO on-top signals at 2110 cm- 1 and 2097 cm- 1 were detected under continuous CO exposure in a pressure range from 10- 7 to 100 mbar and at 200-300 K The formation of the higher wavenumber signal is kinetically limited <200 K and by the presence of a stable c(4 × 2) adlayer in UHV. From the results presented in this study and previous exptl. findings the 2 on-top signals are related to different CO compression layers on Pt(111) with θ > 0.5, hexagonal Moire lattices and rectangular coincident site lattices.
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  20. 20
    Cheng, J.; Sprik, M. The electric double layer at a rutile TiO(2) water interface modelled using density functional theory based molecular dynamics simulation. J. Phys.: Condens. Matter 2014, 26, 244108,  DOI: 10.1088/0953-8984/26/24/244108
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    20
    The electric double layer at a rutile TiO2 water interface modeled using density functional theory based molecular dynamics simulation
    Cheng, J.; Sprik, M.
    Journal of Physics: Condensed Matter (2014), 26 (24), 244108/1-244108/11, 11 pp.CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    A fully atomistic model of a compact elec. double layer at the rutile TiO2(110)-water interface is constructed by adding protons to bridging oxygens or removing them from H2O mols. adsorbed on terminal metal cation sites. The surface charge is compensated by F- or Na+ counterions in outer as well as inner sphere coordination. For each of the protonation states the energy of the TiO2 conduction band min. is detd. relative to the std. hydrogen electrode by computing the free energy for the combined insertion of an electron in the solid and a proton in soln. away from the double layer using d. functional theory based mol. dynamics methods. Interpreted as electrode potentials, this gives an est. of the capacitance which is compared to the capacitance obtained from the difference in the av. electrostatic potentials in the solid and aq. phase. When aligned at the point of zero charge these two methods lead to almost identical potential-charge profiles. Inner sphere complexes have a slightly larger capacitance (0.4 F m-2) compared to outer sphere complexes (0.3 F m-2).
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  21. 21
    Otani, M.; Hamada, I.; Sugino, O.; Morikawa, Y.; Okamoto, Y.; Ikeshoji, T. Electrode dynamics from first principles. J. Phys. Soc. Jpn. 2008, 77, 024802,  DOI: 10.1143/JPSJ.77.024802
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    Electrode dynamics from first principles
    Otani, Minoru; Hamada, Ikutaro; Sugino, Osamu; Morikawa, Yoshitada; Okamoto, Yasuharu; Ikeshoji, Tamio
    Journal of the Physical Society of Japan (2008), 77 (2), 024802/1-024802/6CODEN: JUPSAU; ISSN:0031-9015. (Physical Society of Japan)
    The study of electrode dynamics was a major topic in the field of electrochem. for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte soln., transport of electrons and ions. The 1st-principles mol. dynamics simulation of the electrochem. system was hampered by the difficulty to describe the bias voltage and the complex soln.-electrode interface structure. Here the authors use a new algorithm called the effective screening medium to characterize the biased interface between Pt and liq. H2O, revealing the microscopic details of the 1st, Volmer, step of the Pt-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the H2O and the bias, the authors show why this reaction occurs so efficiently at the interface. The simulations make a significant step towards a deeper understanding of electrochem. reactions.
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  22. 22
    Surendralal, S.; Todorova, M.; Finnis, M. W.; Neugebauer, J. First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion. Phys. Rev. Lett. 2018, 120, 246801,  DOI: 10.1103/PhysRevLett.120.246801
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    First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion
    Surendralal, Sudarsan; Todorova, Mira; Finnis, Michael W.; Neugebauer, Joerg
    Physical Review Letters (2018), 120 (24), 246801CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calcns.under controlled potentiostat conditions for electrochem.systems. The proposed approach can be straightforwardly applied in std.d.functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chem. reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution.
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    Le, J.; Iannuzzi, M.; Cuesta, A.; Cheng, J. Determining potentials of zero charge of metal electrodes versus the standard hydrogen electrode from based on density-functional-theory-based molecular dynamics. Phys. Rev. Lett. 2017, 119, 16801,  DOI: 10.1103/PhysRevLett.119.016801
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    Determining potentials of zero charge of metal electrodes versus the standard hydrogen electrode from density-functional-theory-based molecular dynamics
    Le, Jiabo; Iannuzzi, Marcella; Cuesta, Angel; Cheng, Jun
    Physical Review Letters (2017), 119 (1), 016801/1-016801/6CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    We develop a computationally efficient scheme to det. the potentials of zero charge (PZC) of metal-water interfaces with respect to the std. hydrogen electrode. We calc. the PZC of Pt(111), Au(111), Pd(111) and Ag(111) at a good accuracy using this scheme. Moreover, we find that the interface dipole potentials are almost entirely caused by charge transfer from water to the surfaces, the magnitude of which depends on the bonding strength between water and the metals, while water orientation hardly contributes at the PZC conditions.
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    Cheng, J.; Sprik, M. Alignment of electronic energy levels at electrochemical interfaces. Phys. Chem. Chem. Phys. 2012, 14, 11245,  DOI: 10.1039/c2cp41652b
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    Alignment of electronic energy levels at electrochemical interfaces
    Cheng, Jun; Sprik, Michiel
    Physical Chemistry Chemical Physics (2012), 14 (32), 11245-11267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calcg. such shifts for electrochem. interfaces requires a combination of methods from computational surface science and phys. chem. The problem is closely related to the computation of potentials of electrochem. inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective the authors review two d. functional theory based methods that were developed for this purpose, the work function method and the H insertion method. The key expressions of the two methods are derived from the formal theory of abs. electrode potentials. As an illustration of the work function method the authors review the computation of the potential of zero charge of the Pt(111)-H2O interface as recently published by a no. of groups. The example of the H insertion method is from the authors' own work on the rutile TiO2(110)-H2O interface at the point of zero proton charge. The calcns. are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band max. and conduction band min. of the semiconductor) to the band edges of liq. H2O and the std. potential for the redn. of the hydroxyl radical. All potentials are calcd. at the same level of d. functional theory using the std. H electrode as common energy ref. Comparison to expt. identifies the treatment of the valence band of H2O as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.
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    Magnussen, O. M.; Groß, A. Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics. J. Am. Chem. Soc. 2019, 141, 4777– 4790,  DOI: 10.1021/jacs.8b13188
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    Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics
    Magnussen, Olaf M.; Gross, Axel
    Journal of the American Chemical Society (2019), 141 (12), 4777-4790CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. For the knowledge-based development of electrochem. processes, a better fundamental understanding of the interfaces between electrodes and electrolytes is necessary. This requires insight into the interface structure and dynamics on the at.-scale, including that of the liq. electrolyte in the near-surface region, i.e., in the inner and outer part of the electrochem. double layer. This perspective describes current studies of simple and well-defined electrochem. interfaces by 1st-principles electronic structure calcns. and in situ structure-sensitive methods. These exptl. and theor. studies are now approaching a level, where they can operate on the same footing, making direct comparison of the obtained results feasible. Using selected examples, progress in clarifying the structure and dynamics of the double layer, of adsorbed species on electrode surfaces, and of initial steps in electrochem. phase formation processes is discussed.
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    Huang, P.; Pham, T. A.; Galli, G.; Schwegler, E. Alumina(0001)/Water Interface: Structural Properties and Infrared Spectra from First-Principles Molecular Dynamics Simulations. J. Phys. Chem. C 2014, 118, 8944– 8951,  DOI: 10.1021/jp4123002
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    Alumina(0001)/Water Interface: Structural Properties and Infrared Spectra from First-Principles Molecular Dynamics Simulations
    Huang, Patrick; Pham, Tuan Anh; Galli, Giulia; Schwegler, Eric
    Journal of Physical Chemistry C (2014), 118 (17), 8944-8951CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We investigated the at. structure and IR spectra of the alumina(0001)/water interface, using first-principles mol. dynamics simulations based on d. functional theory within the generalized gradient approxn. The computed structural properties of the interface are in good agreement with the results of synchrotron X-ray expts. Detailed analyses of the computed IR spectra revealed two types of water mols. at the hydrophilic oxide/water interface: mols. participating in strong "ice-like" hydrogen bonding with the oxide surface and mols. involved in weaker "liq.-like" hydrogen bonding. Our results provide a mol. interpretation of the "ice-like" and "liq.-like" bands obsd. in sum-frequency vibrational spectroscopy expts. and underscore the significance of strong hydrogen-bonding interactions in detg. the orientation of interfacial water mols.
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    Lan, J.; Rybkin, V. V.; Iannuzzi, M. Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces. J. Phys. Chem. Lett. 2020, 11, 3724– 3730,  DOI: 10.1021/acs.jpclett.0c01025
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    27
    Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces
    Lan, Jinggang; Rybkin, Vladimir V.; Iannuzzi, Marcella
    Journal of Physical Chemistry Letters (2020), 11 (9), 3724-3730CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The enhanced probability of water dissocn. at the aq. electrode interfaces is predicted by path-integral ab initio mol. dynamics. The ionization process is obsd. at the aq. platinum interface when nuclear quantum effects are introduced in the statistical sampling, while minor effects have been obsd. at the gold interface. We characterize the dissocn. mechanism of the formed water ions. In spite of the fact that the concn. and lifetime of the ions might be challenging to exptl. detect, they may serve as a guide to future expts. Our observation might have a significant impact on the understanding of electrochem. processes occurring at the metal electrode surface.
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    Le, J.; Fan, Q.; Perez-Martinez, L.; Cuesta, A.; Cheng, J. Theoretical insight into the vibrational spectra of metal-water interfaces from density functional theory based molecular dynamics. Phys. Chem. Chem. Phys. 2018, 20, 11554,  DOI: 10.1039/C8CP00615F
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    Theoretical insight into the vibrational spectra of metal-water interfaces from density functional theory based molecular dynamics
    Le, Jiabo; Fan, Qiyuan; Perez-Martinez, Laura; Cuesta, Angel; Cheng, Jun
    Physical Chemistry Chemical Physics (2018), 20 (17), 11554-11558CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Understanding electrochem. interface structures at the at. level is key to developing efficient electrochem. cells for energy storage/conversion. Spectroscopic techniques have been widely used to examine interfacial structures and vibrational properties. Interpretation of these spectra is not straight-forward. This work used d. functional theory-based mol. dynamics simulations to examine the vibrational properties of Pt(111)/ and Au(111)/water interfaces. The specific adsorption of some surface water on Pt(111) led to a partial charge transfer to the metal and strong H bonding with neighboring water mols., which resolved the interpretation of the elusive O-H stretching peak at approx. 3000 cm-1 obsd. in some expts.
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    Le, J.-B.; Fan, Q.-Y.; Li, J.-Q.; Cheng, J. Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface. Sci. Adv. 2020, 6, eabb1219  DOI: 10.1126/sciadv.abb1219
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    Pajkossy, T.; Kolb, D. M. Double layer capacitance of Pt(111) single crystal electrodes. Electrochim. Acta 2001, 46, 3063,  DOI: 10.1016/S0013-4686(01)00597-7
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    Double layer capacitance of Pt(111) single crystal electrodes
    Pajkossy, T.; Kolb, D. M.
    Electrochimica Acta (2001), 46 (20-21), 3063-3071CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)
    To det. the double layer capacitance of the Pt(111) electrode, impedance and capacitance measurements were carried out in neutral and acidic aq. perchlorate solns. Sepn. of the double layer and adsorption contributions of the interfacial capacitance were based on the adsorption impedance theory. The double layer capacitance vs. potential plot exhibits a peak at ≈0.12 V vs. SCE in the 1.5-7 pH range; from here towards cathodic potentials the capacitance attains a value of ≈-20 μF/cm2. The peak may be related to the potential of zero free charge of the Pt(111) electrode.
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    Ojha, K.; Arulmozhi, N.; Aranzales, D.; Koper, M. T. M. Double Layer of Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening. Angew. Chem., Int. Ed. 2020, 59, 711,  DOI: 10.1002/anie.201911929
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    Double Layer of Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening
    Ojha, Kasinath; Arulmozhi, Nakkiran; Aranzales, Diana; Koper, Marc T. M.
    Angewandte Chemie, International Edition (2020), 59 (2), 711-715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We report, for the first time, the observation of a Gouy-Chapman capacitance min. at the potential of zero charge of the Pt(111)-aq. perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concns. ( < 10-3 m). By comparison to gold and mercury double-layer data, we conclude that the diffuse double layer structure at the Pt(111)-electrolyte interface deviates significantly from the Gouy-Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean-field Poisson-Boltzmann theory.
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    Cheng, T.; Wang, L.; Merinov, B. V.; Goddard, W. A. Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH. J. Am. Chem. Soc. 2018, 140, 7787,  DOI: 10.1021/jacs.8b04006
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    Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH
    Cheng, Tao; Wang, Lu; Merinov, Boris V.; Goddard, William A.
    Journal of the American Chemical Society (2018), 140 (25), 7787-7790CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    H oxidn. reaction (HOR) and H evolution reaction (HER) are both 2 orders slower in alk. electrolyte than in acidic electrolyte, but no explanation was provided. The 1st step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent H binding to the electrode, a perplexing behavior obsd. exptl. The authors carried out Quantum Mechanics Mol. Dynamics (QMMD) with explicit considerations of solvent and applied voltage (U) to in situ simulate H2O/Pt(100) interface in the condition of under-potential adsorption of H (HUPD). As U is made more neg., the electrode tends to repel H2O, which in turn increases the H binding. The authors predicted a 0.13 eV increase in H binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the exptl. observation of 8 to 12 meV/pH. Thus, the changes in H2O adsorption are the major causes of pH-dependent H binding on a noble metal. The new insight of crit. role of surface H2O in modifying electrochem. reactions provides a guideline in designing HER/HOR catalyst targeting for the alk. electrolyte.
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    Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy. Nat. Commun. 2015, 6, 5848,  DOI: 10.1038/ncomms6848
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    Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy
    Sheng, Wenchao; Zhuang, Zhongbin; Gao, Minrui; Zheng, Jie; Chen, Jingguang G.; Yan, Yushan
    Nature Communications (2015), 6 (), 5848CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    The hydrogen oxidn./evolution reactions are two of the most fundamental reactions in distributed renewable electrochem. energy conversion and storage systems. The identification of the reaction descriptor is therefore of crit. importance for the rational catalyst design and development. Here we report the correlation between hydrogen oxidn./evolution activity and exptl. measured hydrogen binding energy for polycryst. platinum examd. in several buffer solns. in a wide range of electrolyte pH from 0 to 13. The hydrogen oxidn./evolution activity obtained using the rotating disk electrode method is found to decrease with the pH, while the hydrogen binding energy, obtained from cyclic voltammograms, linearly increases with the pH. Correlating the hydrogen oxidn./evolution activity to the hydrogen binding energy renders a monotonic decreasing hydrogen oxidn./evolution activity with the hydrogen binding energy, strongly supporting the hypothesis that hydrogen binding energy is the sole reaction descriptor for the hydrogen oxidn./evolution activity on monometallic platinum.
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    Zheng, J.; Sheng, W.; Zhuang, Z.; Xu, B.; Yan, Y. Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy. Sci. Adv. 2016, 2, e1501602  DOI: 10.1126/sciadv.1501602
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    Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy
    Zheng, Jie; Sheng, Wenchao; Zhuang, Zhongbin; Xu, Bingjun; Yan, Yushan
    Science Advances (2016), 2 (3), e1501602/1-e1501602/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
    Understanding how pH affects the activity of hydrogen oxidn. reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is obsd. for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange c.d. and HBE is also obsd. on the four metals, indicating that they may share the same elementary steps and rate-detg. steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.
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    Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23,  DOI: 10.1149/1.1856988
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    Trends in the exchange current for hydrogen evolution
    Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U.
    Journal of the Electrochemical Society (2005), 152 (3), J23-J26CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    A d. functional theory database of hydrogen chemisorption energies on close packed surfaces of a no. of transition and noble metals is presented. The bond energies are used to understand the trends in the exchange current for hydrogen evolution. A volcano curve is obtained when measured exchange currents are plotted as a function of the calcd. hydrogen adsorption energies and a simple kinetic model is developed to understand the origin of the volcano. The volcano curve is also consistent with Pt being the most efficient electrocatalyst for hydrogen evolution.
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    Briega-Martos, V.; Ferre-Vilaplana, A.; Herrero, E.; Feliu, J. M. Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases. Electrochim. Acta 2020, 354, 136620,  DOI: 10.1016/j.electacta.2020.136620
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    Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases
    Briega-Martos, Valentin; Ferre-Vilaplana, Adolfo; Herrero, Enrique; Feliu, Juan M.
    Electrochimica Acta (2020), 354 (), 136620CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Platinum is a very effective electrode for the hydrogen evolution and oxidn. reactions (HER/HOR) in acidic media. However, the activity for the HOR on platinum falls two orders of magnitude from acidic to alk. media, which has not been completely understood yet. Here, we provide an explanation for that. Both the HER and the HOR were investigated on the three basal planes of platinum in a pH range near neutral pH conditions in buffered solns. in the absence of anion specific adsorption for guaranteeing the protons availability. Whereas changes in the pH from acid to neutral values produced negligible effects on the HER, the HOR was found to be pH sensitive, even under near neutral pH conditions. From these results, it can be consistently reasoned that the drastic fall in the activity of the HOR on platinum from acidic to alk. media is an effect of the charge on the electrode, which is more neg. as the pH increases. With the aid of d. functional theory calcns., kinetic arguments explaining the unfavorable effect that neg. charge on the electrode has on the HOR are provided.
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    Sakong, S.; Gross, A. The electric double layer at metal-water interfaces revisited based on a charge polarization scheme. J. Chem. Phys. 2018, 149, 084705,  DOI: 10.1063/1.5040056
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    The electric double layer at metal-water interfaces revisited based on a charge polarization scheme
    Sakong, Sung; Gross, Axel
    Journal of Chemical Physics (2018), 149 (8), 084705/1-084705/11CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The description of electrode-electrolyte interfaces is based on the concept of the formation of an elec. double layer. This concept was derived from continuum theories extended by introducing point charge distributions. Based on ab initio mol. dynamics simulations, we analyze the elec. double layer in an approach beyond the point charge scheme by instead assessing charge polarizations at electrochem. metal-water interfaces from first principles. We show that the at. structure of water layers at room temp. leads to an oscillatory behavior of the averaged electrostatic potential. We address the relation between the polarization distribution at the interface and the extent of the elec. double layer and subsequently derive the electrode potential from the charge polarization. (c) 2018 American Institute of Physics.
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    Hansen, M. H.; Nilsson, A.; Rossmeisl, J. Modelling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale. Phys. Chem. Chem. Phys. 2017, 19, 23505– 23514,  DOI: 10.1039/C7CP03576D
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    Modeling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale
    Hansen, Martin Hangaard; Nilsson, Anders; Rossmeisl, Jan
    Physical Chemistry Chemical Physics (2017), 19 (34), 23505-23514CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    At.-scale structures of a Pt(111)/water interface were prepd. by calcg. at. distance distributions as a function of pH. The Pt(111)/water interface structure is a particularly interesting model electrocatalysis system for proton exchange reactions, particularly the O redn. reaction in polymer electrolyte membrane fuel cells. Addnl. insight into such reactions requires accurate simulations of the interface electrolyte structure. This work displays many interesting details regarding electrolyte structure behavior, e.g., the electrolyte structure av. response to the presence of protons by a H down-water orientation; and hexagonally adsorbed water layers are present only when they are anchored at the surface by OH-. New adsorbate configurations were also obsd. at 5/12 ML OH- coverage, suggesting an explanation for reported cyclic voltammetry expts. This work is a step toward a more complete understanding of the electrochem. interface structure on an at. scale.
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    Kristoffersen, H. H.; Vegge, T.; Hansen, H. A. OH formation and H2adsorption at the liquid water-Pt(111) interface. Chem. Sci. 2018, 9, 6912,  DOI: 10.1039/C8SC02495B
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    OH formation and H2 adsorption at the liquid water-Pt(111) interface
    Kristoffersen, Henrik H.; Vegge, Tejs; Hansen, Heine Anton
    Chemical Science (2018), 9 (34), 6912-6921CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The liq.water-Pt(111) interface is studied with const.temp.ab initio mol.dynamics to explore the importance of liq.water dynamics of catalytic reactions such as the oxygen redn.reaction in PEM fuel cells. The structure and energetics of hydroxyls formed at the liq.water-Pt(111) interface are found to be significantly different from those of the hydroxyl formed on a bare Pt(111) surface and the hydroxyl formed on a Pt(111) surface with a static water layer. We identify 1/12 ML *OH, 5/12 ML *OH and 2/3 ML *OH as particularly stable hydroxyl coverages in highly dynamic liq.water environments, which - contrary to static water-hydroxyl models - contain adjacent uncovered Pt sites. Atomic surface oxygen is found to be unstable in the presence of liq.water, in contrast to static at.level simulations. These results give an improved understanding of hydroxide and surface oxide formation from Pt(111) cyclic voltammetry and allow us to draw detailed connections between the electrostatic potential and the interface structure. The study of hydrogen adsorption at the liq.water-Pt(111) interface finds competitive adsorption between the adsorbed hydrogen atoms and water mols. This does not adhere with exptl.observations, and this indicates that the Pt(111) surface has to be neg.charged for a correct description of the liq.water-Pt(111) interface at potentials where hydrogen adsorption occurs.
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    Kronberg, R.; Laasonen, K. Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer–Water Structure of Hydrogenated Single-Crystal Platinum Electrodes. J. Phys. Chem. C 2020, 124, 13706– 13714,  DOI: 10.1021/acs.jpcc.0c02323
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    Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer-Water Structure of Hydrogenated Single-Crystal Platinum Electrodes
    Kronberg, Rasmus; Laasonen, Kari
    Journal of Physical Chemistry C (2020), 124 (25), 13706-13714CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Atomically flat, single-crystal solid-liq. interfaces attract considerable interest through their electrochem. relevance and well-defined structure facilitating controlled atomistic characterization. Yet, crucial details esp. regarding the nanoscale adlayer-water dynamics remain uncertain. Here, the influence of adsorbate coverage on the interfacial structure and solvent relaxation on hydrogenated Pt(111) is examd. by extensive d. functional mol. dynamics simulations. Pronounced water dynamics is obsd. with increasing hydrogen coverage, for which an interpretation based on displacement of specifically co-adsorbed water and strong screening of the electrostatic interaction across the interface is proposed. However, the magnitude of the solvent fluctuations is argued to be partly overestimated by the employed RPBE-D3 exchange-correlation functional, which impedes water chemisorption and charge transfer to sparsely hydrogenated platinum. This manifests as overestimated equil. electrode potentials compared to exptl. adsorption isotherms, which are conversely well reproduced by static calcns. invoking the computational hydrogen electrode formalism. By coupling the interfacial structure with electrostatic properties, our work underscores the profound importance of functional choice as well as the persisting value and comparable precision of carefully employed static approxns. in electrochem. simulations.
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    The work function of the elements and its periodicity
    Michaelson, Herbert B.
    Journal of Applied Physics (1977), 48 (11), 4729-33CODEN: JAPIAU; ISSN:0021-8979.
    A review with 63 refs. is compiled based on a literature search for the period 1969-1976.
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    Cuesta, A. Measurement of the surface charge density of CO-saturated Pt(111) electrodes as a function of potential: The potential of zero charge of Pt(111). Surf. Sci. 2004, 572, 11,  DOI: 10.1016/j.susc.2004.08.014
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    Measurement of the surface charge density of CO-saturated Pt(111) electrodes as a function of potential: the potential of zero charge of Pt(111)
    Cuesta, Angel
    Surface Science (2004), 572 (1), 11-22CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    From measurements of the charge flowing upon immersion, at controlled potential, of a CO-covered Pt(111) electrode in a 0.1M HClO4 soln., the corresponding surface charge d. vs. potential curve was obtained, and from this the potential of zero charge (pzc) of the CO-covered Pt(111) electrode. From these data it was estd. that the error incurred when the potential of zero total charge (pztc) of Pt(111) electrodes is detd. by the CO-charge displacement method is of ∼50 mV at pH 1 and of ∼90 mV at pH 3. Also, the exptl. detd. pzc of the CO-covered Pt(111) electrode has allowed one to make an estn. of the potential of zero free charge (pzfc) of Pt(111) electrodes.
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    Markovic, N. Surface science studies of model fuel cell electrocatalysts. Surf. Sci. Rep. 2002, 45, 117– 229,  DOI: 10.1016/S0167-5729(01)00022-X
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    Surface science studies of model fuel cell electrocatalysts
    Markovic, N. M.; Ross, P. N.
    Surface Science Reports (2002), 45 (4-6), 117-229CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
    A review with refs. The purpose of this review is to discuss progress in the understanding of electrocatalytic reactions through the study of model systems with surface spectroscopies. Pure metal single crystals and well-characterized bulk alloys have been used quite successfully as models for real (com.) electrocatalysts. Given the sheer vol. of all work in electrocatalysis that is on fuel cell reactions, we will focus on electrocatalysts for fuel cells. Since Pt is the model fuel cell electrocatalyst, we will focus entirely on studies of pure Pt and Pt bimetallic alloys. The electrode reactions discussed include hydrogen oxidn./evolution, oxygen redn., and the electrooxidn. of carbon monoxide, formic acid, and methanol. Surface spectroscopies emphasized are FTIR, STM/AFM and surface X-ray scattering (SXS). The discussion focuses on the relation between the energetics of adsorption of intermediates and the reaction pathway and kinetics, and how the energetics and kinetics relate to the extrinsic properties of the model system, e.g. surface structure and/or compn. Finally, we conclude by discussing the limitations that are reached by using pure metal single crystals and well-characterized bulk alloys as models for real catalysts, and suggest some directions for developing more realistic systems.
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    VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Quickstep: Fast and accurate density functional calculations using a mixed Gaussian and plane waves approach. Comput. Phys. Commun. 2005, 167, 103,  DOI: 10.1016/j.cpc.2004.12.014
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    QUICKSTEP: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach
    VandeVondele, Joost; Krack, Matthias; Mohamed, Fawzi; Parrinello, Michele; Chassaing, Thomas; Hutter, Juerg
    Computer Physics Communications (2005), 167 (2), 103-128CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
    We present the Gaussian and plane waves (GPW) method and its implementation in which is part of the freely available program package CP2K. The GPW method allows for accurate d. functional calcns. in gas and condensed phases and can be effectively used for mol. dynamics simulations. We show how derivs. of the GPW energy functional, namely ionic forces and the Kohn-Sham matrix, can be computed in a consistent way. The computational cost of computing the total energy and the Kohn-Sham matrix is scaling linearly with the system size, even for condensed phase systems of just a few tens of atoms. The efficiency of the method allows for the use of large Gaussian basis sets for systems up to 3000 atoms, and we illustrate the accuracy of the method for various basis sets in gas and condensed phases. Agreement with basis set free calcns. for single mols. and plane wave based calcns. in the condensed phase is excellent. Wave function optimization with the orbital transformation technique leads to good parallel performance, and outperforms traditional diagonalisation methods. Energy conserving Born-Oppenheimer dynamics can be performed, and a highly efficient scheme is obtained using an extrapolation of the d. matrix. We illustrate these findings with calcns. using commodity PCs as well as supercomputers.
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    Goedecker, S.; Teter, M.; Hutter, J. Separable dual-space Gaussian pseudopotentials. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 1703,  DOI: 10.1103/PhysRevB.54.1703
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    Separable dual-space Gaussian pseudopotentials
    Goedecker, S.; Teter, M.; Hutter, J.
    Physical Review B: Condensed Matter (1996), 54 (3), 1703-1710CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    We present pseudopotential coeffs. for the first two rows of the Periodic Table. The pseudopotential is of an analytic form that gives optimal efficiency in numerical calculations using plane waves as a basis set. At most, even coeffs. are necessary to specify its analytic form. It is separable and has optimal decay properties in both real and Fourier space. Because of this property, the application of the nonlocal part of the pseudopotential to a wave function can be done efficiently on a grid in real space. Real space integration is much faster for large systems than ordinary multiplication in Fourier space, since it shows only quadratic scaling with respect to the size of the system. We systematically verify the high accuracy of these pseudopotentials by extensive at. and mol. test calcns.
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    Hartwigsen, C.; Goedecker, S.; Hutter, J. Relativistic separable dual-space Gaussian pseudopotentials from H to Rn. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 3641,  DOI: 10.1103/PhysRevB.58.3641
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    Relativistic separable dual-space Gaussian pseudopotentials from H to Rn
    Hartwigsen, C.; Goedecker, S.; Hutter, J.
    Physical Review B: Condensed Matter and Materials Physics (1998), 58 (7), 3641-3662CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    We generalize the concept of separable dual-space Gaussian pseudopotentials to the relativistic case. This allows us to construct this type of pseudopotential for the whole Periodic Table, and we present a complete table of pseudopotential parameters for all the elements from H to Rn. The relativistic version of this pseudopotential retains all the advantages of its nonrelativistic version. It is separable by construction, it is optimal for integration on a real-space grid, it is highly accurate, and, due to its analytic form, it can be specified by a very small no. of parameters. The accuracy of the pseudopotential is illustrated by an extensive series of mol. calcns.
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    VandeVondele, J.; Hutter, J. Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases. J. Chem. Phys. 2007, 127, 114105,  DOI: 10.1063/1.2770708
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    VandeVondele, Joost; Hutter, Jurg
    Journal of Chemical Physics (2007), 127 (11), 114105/1-114105/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    We present a library of Gaussian basis sets that has been specifically optimized to perform accurate mol. calcns. based on d. functional theory. It targets a wide range of chem. environments, including the gas phase, interfaces, and the condensed phase. These generally contracted basis sets, which include diffuse primitives, are obtained minimizing a linear combination of the total energy and the condition no. of the overlap matrix for a set of mols. with respect to the exponents and contraction coeffs. of the full basis. Typically, for a given accuracy in the total energy, significantly fewer basis functions are needed in this scheme than in the usual split valence scheme, leading to a speedup for systems where the computational cost is dominated by diagonalization. More importantly, binding energies of hydrogen bonded complexes are of similar quality as the ones obtained with augmented basis sets, i.e., have a small (down to 0.2 kcal/mol) basis set superposition error, and the monomers have dipoles within 0.1 D of the basis set limit. However, contrary to typical augmented basis sets, there are no near linear dependencies in the basis, so that the overlap matrix is always well conditioned, also, in the condensed phase. The basis can therefore be used in first principles mol. dynamics simulations and is well suited for linear scaling calcns.
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    Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865,  DOI: 10.1103/PhysRevLett.77.3865
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    Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344
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    A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
    Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
    Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    Kühne, T. D.; Krack, M.; Mohamed, F. R.; Parrinello, M. Efficient and accurate car-parrinello-like approach to born-oppenheimer molecular dynamics. Phys. Rev. Lett. 2007, 98, 66401,  DOI: 10.1103/PhysRevLett.98.066401
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    Lan, J.; Hutter, J.; Iannuzzi, M. First-Principles Simulations of an Aqueous CO/Pt(111) Interface. J. Phys. Chem. C 2018, 122, 24068,  DOI: 10.1021/acs.jpcc.8b05933
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    First-Principles Simulations of an Aqueous CO/Pt(111) Interface
    Lan, Jinggang; Hutter, Jurg; Iannuzzi, Marcella
    Journal of Physical Chemistry C (2018), 122 (42), 24068-24076CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The structural, electronic, and vibrational properties of an adsorbed CO monolayer in the presence of liq. H2O at room temp., as obtained by ab initio mol. dynamics simulations are discussed. The H2O mols. at closest distance from the substrate form a bilayer, where they are coadsorbed at empty surface sites and with relatively rigid orientation, at the difference with the bulk liq. above. Coadsorbed H2O strengthens the back-bonding to CO, which leads to the red shift of ∼40 cm-1. The synergy between the 1st bilayer and the bulk liq. induces further polarization of the CO bond, which changes the transition dipole and results in the CO intensity enhancement. It is also verified that no hydrogen bond is effectively present between CO and bulk H2O. These findings provide new insight into the physics of the surface/adsorbate/solvent interface, thus clarifying the exptl. observation to be exploited for the design of improved catalysts.
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    VandeVondele, J.; Hutter, J. An efficient orbital transformation method for electronic structure calculations. J. Chem. Phys. 2003, 118, 4365– 4369,  DOI: 10.1063/1.1543154
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    An efficient orbital transformation method for electronic structure calculations
    VandeVondele, Joost; Hutter, Jurg
    Journal of Chemical Physics (2003), 118 (10), 4365-4369CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    An efficient method for optimizing single-determinant wave functions of medium and large systems is presented. It is based on a minimization of the energy functional using a new set of variables to perform orbital transformations. With this method convergence of the wave function is guaranteed. Preconditioners with different computational cost and efficiency have been constructed. Depending on the preconditioner, the method needs a no. of iterations that is very similar to the established diagonalization-DIIS approach, in cases where the latter converges well. Diagonalization of the Kohn-Sham matrix can be avoided and the sparsity of the overlap and Kohn-Sham matrix can be exploited. If sparsity is taken into account, the method scales as O(MN2), where M is the total no. of basis functions and N is the no. of occupied orbitals. The relative performance of the method is optimal for large systems that are described with high quality basis sets, and for which the d. matrixes are not yet sparse. We present a benchmark calcn. on a DNA crystal contg. 2×12 base pairs, solvent and counter ions (2388 atoms), using a TZV(2d,2p) basis (38688 basis functions) and conclude that the electronic structure of systems of this size can now be studied routinely.
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    Cheng, J.; Liu, X.; VandeVondele, J.; Sulpizi, M.; Sprik, M. Redox potentials and acidity constants from density functional theory based molecular dynamics. Acc. Chem. Res. 2014, 47, 3522,  DOI: 10.1021/ar500268y
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    Redox Potentials and Acidity Constants from Density Functional Theory Based Molecular Dynamics
    Cheng, Jun; Liu, Xiandong; Vande Vondele, Joost; Sulpizi, Marialore; Sprik, Michiel
    Accounts of Chemical Research (2014), 47 (12), 3522-3529CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. All-atom methods treat solute and solvent at the same level of electronic structure theory and statistical mechanics. All-atom computation of acidity consts. (pKa) and redox potentials is still a challenge. In this Account, we review such a method combining d. functional theory based mol. dynamics (DFTMD) and free energy perturbation (FEP) methods. The key computational tool is a FEP based method for reversible insertion of a proton or electron in a periodic DFTMD model system. The free energy of insertion (work function) is computed by thermodn. integration of vertical energy gaps obtained from total energy differences. The problem of the loss of a phys. ref. for ionization energies under periodic boundary conditions is solved by comparing with the proton work function computed for the same supercell. The scheme acts as a computational hydrogen electrode, and the DFTMD redox energies can be directly compared with exptl. redox potentials. Consistent with the closed shell nature of acid dissocn., pKa ests. computed using the proton insertion/removal scheme are found to be significantly more accurate than the redox potential calcns. This enables us to sep. the DFT error from other sources of uncertainty such as finite system size and sampling errors. Drawing an analogy with charged defects in solids, we trace the error in redox potentials back to underestimation of the energy gap of the extended states of the solvent. Accordingly the improvement in the redox potential as calcd. by hybrid functionals is explained as a consequence of the opening up of the bandgap by the Hartree-Fock exchange component in hybrids. Test calcns. for a no. of small inorg. and org. mols. show that the hybrid functional implementation of our method can reproduce acidity consts. with an uncertainty of 1-2 pKa units (0.1 eV). The error for redox potentials is in the order of 0.2 V.
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    Rizo, R.; Sitta, E.; Herrero, E.; Climent, V.; Feliu, J. M. Towards the understanding of the interfacial pH scale at Pt(111) electrodes. Electrochim. Acta 2015, 162, 138,  DOI: 10.1016/j.electacta.2015.01.069
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    Towards the understanding of the interfacial pH scale at Pt(1 1 1) electrodes
    Rizo, Ruben; Sitta, Elton; Herrero, Enrique; Climent, Victor; Feliu, Juan M.
    Electrochimica Acta (2015), 162 (), 138-145CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The detn. of the potentials of zero total and free charge, pztc and pzfc resp., were made in a wide pH range by using the CO displacement method and the same calcn. assumptions used previously for Pt(1 1 1) electrodes in contact with non-specifically adsorbing anions. Calcn. of the pzfc involves, in occasions, long extrapolations that lead us to the introduction of the concept of potential of zero extrapolated charge (pzec). It was obsd. that the pztc changes with pH but the pzec is independent of this parameter. It was obsd. that the pztc > pzec at pH > 3.4 but the opposite is true for pH > 3.4. At the latter pH both pzec and pztc coincide. This defines two different pH regions and means that adsorbed hydrogen has to be cor. in the "acidic" solns. at the pztc while adsorbed OH is the species to be cor. in the "alk." range. The comparison of the overall picture suggests that neutral conditions at the interface are attained at significantly acidic solns. than those at the bulk.
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    Skúlason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jónsson, H.; Nørskov, J. K. Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode. Phys. Chem. Chem. Phys. 2007, 9, 3241,  DOI: 10.1039/B700099E
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    Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode
    Skulason, Egill; Karlberg, Gustav S.; Rossmeisl, Jan; Bligaard, Thomas; Greeley, Jeff; Jonsson, Hannes; Norskov, Jens K.
    Physical Chemistry Chemical Physics (2007), 9 (25), 3241-3250CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The authors present results of d. functional theory calcns. on a Pt(111) slab with a bilayer of H2O, solvated protons in the H2O layer, and excess electrons in the metal surface. In this way the authors model the electrochem. double layer at a Pt electrode. By varying the no. of protons/electrons in the double layer the authors study the system as a function of the electrode potential. The authors study the elementary processes involved in the hydrogen evolution reaction, 2(H+ + e-) → H2, and det. the activation energy and predominant reaction mechanism as a function of electrode potential. The authors confirm by explicit calcns. the notion that the variation of the activation barrier with potential can be viewed as a manifestation of the Broensted-Evans-Polanyi-type relation between activation energy and reaction energy found throughout surface chem.
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    Watson, G. W.; Wells, R. P. K.; Willock, D. J.; Hutchings, G. J. A comparison of the adsorption and diffusion of hydrogen on the {111} surfaces of Ni, Pd, and Pt from density functional theory calculations. J. Phys. Chem. B 2001, 105, 4889– 4894,  DOI: 10.1021/jp002864c
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    A Comparison of the Adsorption and Diffusion of Hydrogen on the {111} Surfaces of Ni, Pd, and Pt from Density Functional Theory Calculations
    Watson, Graeme W.; Wells, Richard P. K.; Willock, David J.; Hutchings, Graham J.
    Journal of Physical Chemistry B (2001), 105 (21), 4889-4894CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Gradient cor. d. function theory calcns. on the adsorption and thermally activated diffusion pathways of hydrogen on the {111} surfaces of Ni, Pd, and Pt are presented. It was found that the variation of the adsorption energy as a function of adsorption site shows considerable differences between the three metals. For Ni and Pd, the adsorption energies vary as a function of hydrogen coordination with the 3-fold hollow sites the most stable and the 1-fold atop site considerably less stable. On Pt the adsorption energies for all the sites are similar indicating that diffusion across the surface will be faster on Pt than on Ni or Pd. The activation energies for diffusion have been calcd. with that for Pt (2-3 kJ mol-1) considerably smaller than for Ni or Pd (13 kJ mol-1). The calcd. adsorption and activation energies are in good agreement with exptl. investigations of the structure, energetics, and diffusion properties.
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    Le, J.; Cuesta, A.; Cheng, J. The structure of metal-water interface at the potential of zero charge from density functional theory-based molecular dynamics. J. Electroanal. Chem. 2018, 819, 87,  DOI: 10.1016/j.jelechem.2017.09.002
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    The structure of metal-water interface at the potential of zero charge from density functional theory-based molecular dynamics
    Le, Jiabo; Cuesta, Angel; Cheng, Jun
    Journal of Electroanalytical Chemistry (2018), 819 (), 87-94CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
    The authors simulated transition metal-H2O interfaces, Pt(111), Au(111), Pd(111) and Ag(111), by d. functional theory based mol. dynamics, and found some common structural features for the surface H2O on different transition metal surfaces. Firstly, there exists a pronounced adsorption layer within ∼5 Å distance from metal surfaces, in which three main H2O species with different orientations (watA, watB-down and watB-up) could be identified. WatA and watB-down show a lower degree of H bonding, due to their interaction with the metal surface via one of the lone pairs of the O atoms and via one of their H atoms, resp. While, watB-up has an almost full coordination shell, indicating it not only forms H bonds in the adsorption layer, but also with the nonsurface H2O. As expected, the honeycomb-like bilayer model used as the starting point of the simulation was destructed into irregular patterns after ∼10 ps of mol. dynamics simulations, and the surface H2O coverage concomitantly increases from 0.66 ML to ∼0.8 ML.
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    Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes. Nat. Energy 2017, 2, 17031,  DOI: 10.1038/nenergy.2017.31
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    Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes
    Ledezma-Yanez, Isis; Wallace, W. David Z.; Sebastian-Pascual, Paula; Climent, Victor; Feliu, Juan M.; Koper, Marc T. M.
    Nature Energy (2017), 2 (4), 17031CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
    Hydrogen evolution on platinum is a key reaction for electrocatalysis and sustainable energy storage, yet its pH-dependent kinetics are not fully understood. Here we present a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) in a wide pH range. Electrochem. measurements show that hydrogen adsorption and hydrogen evolution are both slow in alk. media, consistent with the observation of a shift in the rate-detg. step for hydrogen evolution. Adding nickel to the Pt(111) surface lowers the barrier for hydrogen adsorption in alk. solns. and thereby enhances the hydrogen evolution rate. We explain these observations with a model that highlights the role of the reorganization of interfacial water to accommodate charge transfer through the elec. double layer, the energetics of which are controlled by how strongly water interacts with the interfacial field. The model is supported by laser-induced temp.-jump measurements. Our model sheds light on the origin of the slow kinetics for the hydrogen evolution reaction in alk. media.
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    Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 1256– 1260,  DOI: 10.1126/science.1211934
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    Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces
    Subbaraman, Ram; Tripkovic, Dusan; Strmcnik, Dusan; Chang, Kee-Chul; Uchimura, Masanobu; Paulikas, Arvydas P.; Stamenkovic, Vojislav; Markovic, Nenad M.
    Science (Washington, DC, United States) (2011), 334 (6060), 1256-1260CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Improving the sluggish kinetics for the electrochem. redn. of H2O to H2 in alk. environments is one key to reducing the high overpotentials and assocd. energy losses in H2O-alkali and chlor-alkali electrolyzers. A controlled arrangement of nanometer-scale Ni(OH)2 clusters on Pt electrode surfaces manifests a factor of 8 activity increase in catalyzing the H evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissocn. of H2O and the prodn. of H intermediates that then adsorbed on the nearby Pt surfaces and recombined into H2. The generation of these H intermediates could be further enhanced via Li+-induced destabilization of the HO-H bond, resulting in a factor of 10 total increase in activity.
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    El-Aziz, A. M.; Kibler, L. A.; Kolb, D. M. The potentials of zero charge of Pd(111) and thin Pd overlayers on Au(111). Electrochem. Commun. 2002, 4, 535,  DOI: 10.1016/S1388-2481(02)00362-4
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    The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)
    El-Aziz, A. M.; Kibler, L. A.; Kolb, D. M.
    Electrochemistry Communications (2002), 4 (7), 535-539CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
    The potential of zero charge (pzc) of Pd(1 1 1) was detd. in dil. NaF solns. by measuring the Gouy-Chapman min. of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of -0.12 V vs. SCE was found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) also were detd. While the pzc of the 1st, pseudomorphic Pd layer on Au(1 1 1) is -0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/H2O interface is estd.
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    Pajkossy, T.; Kolb, D. M. On the origin of the double layer capacitance maximum of Pt(111) single crystal electrodes. Electrochem. Commun. 2003, 5, 283,  DOI: 10.1016/S1388-2481(03)00046-8
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    On the origin of the double layer capacitance maximum of Pt(111) single crystal electrodes
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    Electrochemistry Communications (2003), 5 (4), 283-285CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
    The double layer capacitance vs. potential, Cdl(E), curve of Pt(111) electrodes in aq. KClO4 and NaF solns. exhibits a max. at about 0.1 V vs SCE. Since with lowered solute concns. no Gouy-Chapman min. can be found in Cdl(E), the peak is not related to the potential of zero free charge.
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    Kolb, D. M.; Schneider, J. Surface reconstruction in electrochemistry: Au(100-(5*20), Au(111)-(1*23) and Au(110)-(1*2). Electrochim. Acta 1986, 31, 929,  DOI: 10.1016/0013-4686(86)80005-6
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    Surface reconstruction in electrochemistry: gold(100)-(5 x 20), gold(111)-(1 x 23) and gold(110)-(1 x 2)
    Kolb, D. M.; Schneider, J.
    Electrochimica Acta (1986), 31 (8), 929-36CODEN: ELCAAV; ISSN:0013-4686.
    Au (100), Au (111) and Au (110) electrodes with reconstructed surfaces of the type (5 × 20), (1 × 23) and (1 × 2), resp., were prepd. by flame treatment and their properties investigated in various electrolytes by electrochem. and optical methods. The reconstructed (100) and (111) surfaces are found to be stable only in a potential range where no specific adsorption occurs. The Au (100)-(5 × 20) surface has optical properties which are distinctly different from those of the unreconstructed Au (100)-(1 × 1). This difference was used to monitor by in situ spectroscopy the adsorbate-induced (5 × 20) → (1 × 1) transition, in order to obtain information on the transition kinetics. For a certain fraction of the surface, electrochem. induced reconstruction, (1 × 1) → (5 × 20) and (1 × 1) → (1 × 23), was obsd. for Au (100) and Au (111). The 0-charge potentials of the reconstructed surfaces were detd. and compared with those of the unreconstructed ones.
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    Double layer on silver single crystal electrodes in contact with electrolytes having anions which are slightly specifically adsorbed. Part III. The (111) face
    Valette, Georges
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    Differential capacitance-potential curves for a (111) face of Ag in contact with aq. solns. of the electrolytes NaF, NaClO4 and KPF6 were analyzed. From the dependence of the potential of the capacity min. on electrolyte concn., it is found that the ionic specific adsorption decreases as: ClO4->F->PF6-≥O. The contributions of the interaction terms involved in the specific adsorption are discussed, taking into account results obtained previously for the (110) and (100) faces, and those for Hg. In particular, it is proposed that the stronger water-metal interactions for Ag than those for Hg could be responsible for the absence of specific adsorption at the potential of zero charge (pzc) for the PF6- ion on the three low-index faces. The potential of the capacity min. can therefore be identified with the pzc. The pzc for the Ag (111)/aq. electrolyte interface without specific adsorption is: -0.695±0.005 V (SCE). The elec. double-layer model in the absence of specific adsorption accounts for the capacitance-potential curves at different concns. for the Ag (111)/KPFl system in most of the electrode charge range, taking into account only the superficial roughness. In contrast, application of this model round the pzc requires a correction of the measured capacity for the cryst. heterogeneity due to surface defects. Ests. for the water-Ag interactions are proposed for the (110), (100) and (111) faces. Ag (111) would be almost as hydrophilic as Ga.
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    Hussain, G.; Pérez-Martínez, L.; Le, J.-B.; Papasizza, M.; Cabello, G.; Cheng, J.; Cuesta, A. How cations determine the interfacial potential profile: Relevance for the CO2 reduction reaction. Electrochim. Acta 2019, 327, 135055,  DOI: 10.1016/j.electacta.2019.135055
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    How cations determine the interfacial potential profile: Relevance for the CO2 reduction reaction
    Hussain, Ghulam; Perez-Martinez, Laura; Le, Jia-Bo; Papasizza, Marco; Cabello, Gema; Cheng, Jun; Cuesta, Angel
    Electrochimica Acta (2019), 327 (), 135055CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The strong effect of the electrolyte cation on the activity and selectivity of the CO2 redn. reaction (CO2RR) can only be understood and controlled if the cation's effect on the interfacial potential distribution is known. Using CO (the key intermediate in the CO2RR) adsorbed on Pt as a probe mol., and combining IR spectroscopy, capacitance measurements and ab initio mol. dynamics, the cation size dets. the location of the outer Helmholtz plane, whereby smaller cations increase not just the polarization but, most importantly, the polarizability of adsorbed CO (COad) and the accumulation of electronic d. on the O atom of COad. This strongly affects its adsorption energy, the degree of H bonding of interfacial H2O to COad and the degree of polarization of H2O mols. in the cation's solvation shell, all of which can deeply affect the subsequent steps of the CO2RR.
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    Chen, X.; McCrum, I. T.; Schwarz, K. A.; Janik, M. J.; Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chem., Int. Ed. 2017, 56, 15025– 15029,  DOI: 10.1002/anie.201709455
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    Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode
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    Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction
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    The partial charge transfer
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    Electrochimica Acta (2014), 127 (), 489-505CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    In this review, the problems are examd. related to the chemisorption of mols. on electrodes, where the adsorbate may possibly share its electrons with the metal. The "partial charge transfer" between adsorbate and electrode cannot be measured, since the division of the bonding electrons into parts pertaining to the adsorbate and to the electrode is arbitrary; at most, it can be tentatively estd. on the basis of model considerations. This point will be clearly demonstrated on the basis of simple thermodn. considerations. A thermodynamically significant and exptl. measurable quantity that has often been conceptually related to partial charge transfer is the "electrosorption valency". Discussed are methods by which it can be measured, both when only a single species is adsorbed and when its adsorption is accompanied by coadsorption or competitive adsorption with other species. A simple unified model picture will be adopted to illustrate the extra-thermodn. partial charge transfer coeff. for low surface coverages of ionic and neutral adsorbates and for compact chemisorbed monolayers both in soln. and in vacuum. The relation between partial charge transfer coeff. and the dipole moment of the chemisorption bond will be clarified and the est. of this bond in vacuum by work function or outer potential measurements will be described.
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  • Abstract

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    Figure 1

    Figure 1. AIMD models of the electrified Pt(111)-Had/water interfaces with surface charge density (a) σ = −13 μC/cm–2, (b) σ = −19.5 μC/cm–2, (c) σ = −26 μC/cm–2, and (d) σ = −32.5 μC/cm–2. The Pt, O, Hw (H of water), Na, and Had (adsorbed H) atoms are colored by gray, red, white, blue, and orange, respectively. The Pt(111) surface is saturated with a 2/3 monolayer of Had.

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    Figure 2

    Figure 2. Plot of surface charge density (σ) as a function of computed electrode potential (U). The solid dots with error bars represent the computed data from AIMD simulations, which can be linearly fitted. The slope of the fitted line indicates the Helmholtz capacitance CH = 24 μF/cm2.

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    Figure 3

    Figure 3. (a) Density distribution profiles of Had () along the surface normal (z-coordinate) at −0.03, −0.36, −0.51, and −0.84 V vs SHE. The zero in z-coordinate indicates the position of the nuclei of the uppermost layer of Pt(111). (b) Calculated VDOS profiles of Had on Pt(111). (c) A representative snapshot of Pt(111)-Had surface from an AIMD trajectory. The Pt and Had atoms are represented by gray and orange balls, respectively. The black parallelogram in the model denotes the periodic boundary condition. (d) A ∼10 ps trajectory of one Had on Pt(111) surface.

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    Figure 4

    Figure 4. (a) Calculated work functions of Pt(111) surfaces with different coverages of Had at top (red circle) or hollow (blue triangle) site in vacuum. (b) Work functions of Pt(111)-Had surfaces, the configurations of which are taken from the AIMD trajectories of electrified interfaces at different potentials and with the solution removed.

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    Figure 5

    Figure 5. Profiles of water density (, (a)) and dipole orientation (, (b)) along the surface normal (z-coordinate) at −0.03, −0.36, −0.51, and −0.84 V vs SHE. The positions of water molecules are indicated by those of the oxygen atoms, and the zero in z-coordinate indicates the position of the nuclei of the uppermost layer of Pt(111). The inset shows a representative snapshot of the electrified Pt(111)-Had/water interface. The Pt, O, Hw, and Had are colored by gray, red, white and orange, respectively. (c) Probability distribution profiles of angle φ (between the bisector of water and the surface normal, P(φ)) of interface water. (d) Number of hydrogen-bond donors (Ndonor) of interface water as a function of potential. A hydrogen bond is defined when the O–O distance is shorter than 3.5 Å and the O–O–H angle is less than 35°.

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    Figure 6

    Figure 6. Plots of Helmholtz capacitance (CH) as a function of electrode potential (U) at the Pt(111)/water interface (red) and Pt(111)-Had/water interface (blue). The red curve is taken from the recent work. (29) The insets show the change of the structure of interface water in response to the electrode potential on the Pt(111) and Pt(111)-Had surface. The red, white, and orange balls represent the O, Hw, and Had atom, respectively. The p denotes the induced dipole due to water chemisorption on Pt(111).

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    Figure 7

    Figure 7. (a) Plot of the distance between the averaged positions of the Na+ ions and the uppermost Pt layer (dNa–Pt) at Pt(111)-Had/water interfaces as a function of potential. (b) The averaged number of water molecules coordinated to Na+ ions () at different potentials. (c,d) Snapshots of the Pt(111)-Had/water interfaces at −0.03 and −0.86 V vs SHE. Pt, O, Hw, Na, and Had are colored by gray, red, white, blue, and orange, respectively. The water molecules coordinated to Na+ are highlighted with the ball–stick model in comparison to other water with the line model.

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    Figure 8

    Figure 8. Net charge distribution on Pt(111) with 2/3 ML Had at the top site (a) and hollow site (b), and without Had (c). The profiles for the net negative charge distribution are averaged along the surface normal (z-coordinate), and the dashed lines in profiles indicate the positions of surface Had. The Pt and Had atoms are colored by gray and orange, respectively. The yellow isosurfaces in the models indicate the excess electron.

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      The mol. structure of the elec. double layer dets. the chem. in all electrochem. processes. Using x-ray absorption spectroscopy (XAS), we probed the structure of water near gold electrodes and its bias dependence. Electron yield XAS detected at the gold electrode revealed that the interfacial water mols. have a different structure from those in the bulk. First principles calcns. revealed that ~50% of the mols. lie flat on the surface with satd. hydrogen bonds and another substantial fraction with broken hydrogen bonds that do not contribute to the XAS spectrum because their core-excited states are delocalized by coupling with the gold substrate. At neg. bias, the population of flat-lying mols. with broken hydrogen bonds increases, producing a spectrum similar to that of bulk water.
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    3. 3
      Zhang, C.; Hutter, J.; Sprik, M. Coupling of Surface Chemistry and Electric Double Layer at TiO 2 Electrochemical Interfaces. J. Phys. Chem. Lett. 2019, 10, 3871– 3876,  DOI: 10.1021/acs.jpclett.9b01355
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      3
      Coupling of Surface Chemistry and Electric Double Layer at TiO2 Electrochemical Interfaces
      Zhang, Chao; Hutter, Jurg; Sprik, Michiel
      Journal of Physical Chemistry Letters (2019), 10 (14), 3871-3876CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Surfaces of metal oxides at working conditions are usually electrified because of the acid-base chem. The charged interface compensated with counterions forms the so-called elec. double layer. The coupling of surface chem. and the elec. double layer is considered to be crucial but is poorly understood because of the lack of information at the atomistic scale. Here, we used the latest development in d. functional theory-based finite-field mol. dynamics simulation to investigate the pH dependence of the Helmholtz capacitance at electrified rutile TiO2(110)-NaCl electrolyte interfaces. It is found that, because of competing forces from surface adsorption and from the elec. double layer, water mols. have a stronger structural fluctuation at high pH, and this leads to a much larger capacitance. It is also seen that interfacial proton transfers at low pH increase significantly the capacitance value. These findings elucidate the microscopic origin of the same trend obsd. in titrn. expts.
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    4. 4
      Liu, Y.; Kawaguchi, T.; Pierce, M. S.; Komanicky, V.; You, H. Layering and Ordering in Electrochemical Double Layers. J. Phys. Chem. Lett. 2018, 9, 1265– 1271,  DOI: 10.1021/acs.jpclett.8b00123
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      4
      Layering and Ordering in Electrochemical Double Layers
      Liu, Yihua; Kawaguchi, Tomoya; Pierce, Michael S.; Komanicky, Vladimir; You, Hoydoo
      Journal of Physical Chemistry Letters (2018), 9 (6), 1265-1271CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Electrochem. double layers (EDL) form at electrified interfaces. Whereas the Gouy-Chapman model describes moderately charged EDL, the formation of Stern layers was predicted for highly charged EDL. The authors' results provide structural evidence for a Stern layer of cations at potentials close to H evolution in alkali fluoride and chloride electrolytes. Layering was obsd. by x-ray crystal truncation rods and at.-scale recoil responses of Pt(111) surface layers. Ordering in the layer was confirmed by glancing-incidence in-plane diffraction measurements.
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    5. 5
      Li, C.-Y.; Le, J.-B.; Wang, Y.-H.; Chen, S.; Yang, Z.-L.; Li, J.-F.; Cheng, J.; Tian, Z.-Q. In situ probing electrified interfacial water structures at atomically flat surfaces. Nat. Mater. 2019, 18, 697,  DOI: 10.1038/s41563-019-0356-x
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      5
      In situ probing electrified interfacial water structures at atomically flat surfaces
      Li, Chao-Yu; Le, Jia-Bo; Wang, Yao-Hui; Chen, Shu; Yang, Zhi-Lin; Li, Jian-Feng; Cheng, Jun; Tian, Zhong-Qun
      Nature Materials (2019), 18 (7), 697-701CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Solid/liq. interfaces are ubiquitous and knowledge of their at.-level structure is essential in elucidating many phenomena in chem., physics, materials science and Earth science1. In electrochem., in particular, the detailed structure of interfacial H2O, such as the orientation and H-bonding network in elec. double layers under bias potentials, has a significant impact on the electrochem. performances of electrode materials2-4. To elucidate the structures of elec. double layers at electrochem. interfaces, the authors combine in situ Raman spectroscopy and ab initio mol. dynamics and distinguish 2 structural transitions of interfacial H2O at electrified Au single-crystal electrode surfaces. Towards neg. potentials, the interfacial H2O mols. evolve from structurally parallel to 1-H-down and then to 2-H-down. Concurrently, the no. of H bonds in the interfacial H2O also undergoes 2 transitions. The findings shed light on the fundamental understanding of elec. double layers and electrochem. processes at the interfaces.
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    6. 6
      Zhu, S.; Qin, X.; Yao, Y.; Shao, M. pH-Dependent Hydrogen and Water Binding Energies on Platinum Surfaces as Directly Probed through Surface-Enhanced Infrared Absorption Spectroscopy. J. Am. Chem. Soc. 2020, 142, 8748– 8754,  DOI: 10.1021/jacs.0c01104
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      6
      pH-Dependent Hydrogen and Water Binding Energies on Platinum Surfaces as Directly Probed through Surface-Enhanced Infrared Absorption Spectroscopy
      Zhu Shangqian; Qin Xueping; Yao Yao; Shao Minhua; Shao Minhua
      Journal of the American Chemical Society (2020), 142 (19), 8748-8754 ISSN:.
      The origins of the pH-dependent kinetics of hydrogen evolution and oxidation reactions on Pt surfaces are unsolved dilemmas that have lasted for over half a century. In this study, surface-enhanced infrared absorption spectroscopy is applied to directly monitor the vibrational behaviors of adsorbed hydrogen atoms and interfacial water molecules on Pt surfaces in a wide pH window from 1.1 to 12.9. For the first time, we successfully measure the pH-dependent changes of hydrogen and water binding strength according to their vibrational wavenumbers, which are both monotonously weakened as the solution pH increases. Their changes are the net results of altered electrochemical interface environments and are important contributions to the pH-dependent hydrogen reaction kinetics. Our results add significant new insights into the role of interfacial environments on electrocatalysis.
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    7. 7
      Bouzid, A.; Pasquarello, A. Atomic-Scale Simulation of Electrochemical Processes at Electrode/Water Interfaces under Referenced Bias Potential. J. Phys. Chem. Lett. 2018, 9, 1880,  DOI: 10.1021/acs.jpclett.8b00573
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      7
      Atomic-Scale Simulation of Electrochemical Processes at Electrode/Water Interfaces under Referenced Bias Potential
      Bouzid, Assil; Pasquarello, Alfredo
      Journal of Physical Chemistry Letters (2018), 9 (8), 1880-1884CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Based on const. Fermi-level mol. dynamics and a proper alignment scheme, the authors perform simulations of the Pt(111)/H2O interface under variable bias potential referenced to the std. H electrode (SHE). The authors' scheme yields a potential of zero charge μpzc of ∼0.22 eV relative to the SHE and a double layer capacitance Cdl of ≃19 μF cm-2, in excellent agreement with exptl. measurements. The authors study the structural reorganization of the elec. double layer for bias potentials ranging from -0.92 eV to +0.44 eV and find that Odown configurations, which are dominant at potentials above the pzc, reorient to favor Hdown configurations as the measured potential becomes neg. The authors' modeling scheme allows one to not only access at.-scale processes at metal/H2O interfaces, but also to quant. est. macroscopic electrochem. quantities.
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    8. 8
      Zhang, C.; Sayer, T.; Hutter, J.; Sprik, M. Modelling electrochemical systems with finite field molecular dynamics. J. Phys. Energy 2020, 2, 032005,  DOI: 10.1088/2515-7655/ab9d8c
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      There is no corresponding record for this reference.
    9. 9
      Gileadi, E. Physical Electrochemistry. Fundamentals, Techniques and Applications; Wiley-VCH, 2011; pp 1– 374.
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      There is no corresponding record for this reference.
    10. 10
      Ataka, K.-i.; Yotsuyanagi, T.; Osawa, M. Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy. J. Phys. Chem. 1996, 100, 10664,  DOI: 10.1021/jp953636z
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      10
      Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy
      Ataka, Ken-ichi; Yotsuyanagi, Takao; Osawa, Masatoshi
      Journal of Physical Chemistry (1996), 100 (25), 10664-10672CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      The structure and orientation of water mols. at a highly ordered Au(111) electrode surface in perchloric acid were studied in-situ as a function of applied potential by surface-enhanced IR absorption spectroscopy. This newly developed IR spectroscopy technique enables the observation of the electrode/electrolyte interface at a very high sensitivity without interference from the bulk soln. The spectrum of the interfacial water significantly differs from that of bulk water and drastically changes in peak frequencies and band widths around the potential of zero charge (pzc) of the electrode and at ∼0.3 V pos. from the pzc. The interfacial water mols. are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded ice-like structure at potentials slightly above the pzc. The ice-like structure is broken at more pos. potentials due to the specific adsorption of perchlorate ion, where one OH moiety of water is nonhydrogen-bonded and the other OH moiety is hydrogen-bonded to another water mol. The intensities of the fundamental modes of water are also a strong function of applied potential. They are very weak around the pzc and increase as the potential changes in both pos. and neg. directions. These results are explained in terms of the potential-dependent reorientation of water mols. from oxygen-up to oxygen-down as the surface charge changes from neg. to pos. The adsorption of hydronium and perchlorate ions on gold is also discussed.
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    11. 11
      Osawa, M.; Tsushima, M.; Mogami, H.; Samjeské, G.; Yamakata, A. Structure of Water at the Electrified Platinum-Water Interface: A Study by Surface-Enhanced Infrared Absorption Spectroscopy. J. Phys. Chem. C 2008, 112, 4248,  DOI: 10.1021/jp710386g
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      11
      Structure of Water at the Electrified Platinum-Water Interface: A Study by Surface-Enhanced Infrared Absorption Spectroscopy
      Osawa, Masatoshi; Tsushima, Minoru; Mogami, Hirokazu; Samjeske, Gabor; Yamakata, Akira
      Journal of Physical Chemistry C (2008), 112 (11), 4248-4256CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Surface-enhanced IR absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of H2O on a polycryst. Pt electrode in H2SO4 and HClO4 as a function of applied potential. The electrode surface covered with CO is used as the ref. in recording spectra, which enables one to obtain the abs. IR spectrum of the interfacial H2O layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk H2O. The spectrum of the interfacial H2O is largely different from that of bulk H2O and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of H2O mols. from a weakly hydrogen-bonded oxygen-up orientation at the neg. charged surface to a strongly hydrogen-bonded nearly flat orientation at the pos. charged surface in agreement with theor. simulations reported in the literature. Clear exptl. evidence of the formation of a stable ice-like structured H2O on the pos. charged surface is reported.
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    12. 12
      Tian, Z.-Q.; Ren, B.; Chen, Y.-X.; Zou, S.-Z.; Mao, B.-W. Probing electrode/electrolyte interfacial structure in the potential region of hydrogen evolution by Raman spectroscopy. J. Chem. Soc., Faraday Trans. 1996, 92, 3829,  DOI: 10.1039/ft9969203829
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      12
      Probing electrode/electrolyte interfacial structure in the potential region of hydrogen evolution by Raman spectroscopy
      Tian, Zhong-Qun; Ren, Bin; Chen, Yan-Xia; Zou, Shou-Zhong; Mao, Bing-Wei
      Journal of the Chemical Society, Faraday Transactions (1996), 92 (20), 3829-3838CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
      The detailed interfacial structure in the potential region of severe hydrogen evolution, to date, is far from clear due to lack of both exptl. data and correlated theoretic models. It is possible to surmount, to some extent, the disturbance of the spectroelectrochem. measurement by strong hydrogen bubbling in the potential region of severe hydrogen evolution by using a surface enhancement effect and a thin-layer cell configuration. Using this approach, the authors have obtained surface enhanced Raman scattering (SERS) spectra of water at an Ag electrode at very neg. potentials at various concns. of NaClO4. To explain the abnormal reversal of the peak intensity ratio of the bending to the stretching vibration, a preliminary model of the electrode/electrolyte interface is presented. The water mol. is oriented with one hydrogen attached to the surface and the oxygen towards an adsorbed cation which is partially dehydrated owing to the very strong electrostatic force. Raman spectra of hydrogen bound at a Pt electrode in solns. of varying pH from 0 to 14 at potentials of mild hydrogen evolution also were presented for the 1st time. The spectra reveal that the Pt-hydrogen interaction is influenced by both the potential and the interfacial structure. These primary studies may initiate more mol.-level research of electrochem. interfaces in the potential region of hydrogen evolution.
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    13. 13
      Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 2010, 464, 392,  DOI: 10.1038/nature08907
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      13
      Shell-isolated nanoparticle-enhanced Raman spectroscopy
      Li, Jian Feng; Huang, Yi Fan; Ding, Yong; Yang, Zhi Lin; Li, Song Bo; Zhou, Xiao Shun; Fan, Feng Ru; Zhang, Wei; Zhou, Zhi You; Wu, De Yin; Ren, Bin; Wang, Zhong Lin; Tian, Zhong Qun
      Nature (London, United Kingdom) (2010), 464 (7287), 392-395CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single mol. level, comparable to single-mol. fluorescence spectroscopy. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realize a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A no. of approaches have extended the technique to non-traditional substrates, most notably tip-enhanced Raman spectroscopy (TERS) where the probed substance (mol. or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to mols. with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as 'smart dust' over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various mols. adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.
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    14. 14
      Tong, Y.; Lapointe, F.; Thamer, M.; Wolf, M.; Campen, R. K. Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface. Angew. Chem., Int. Ed. 2017, 56, 4211– 4214,  DOI: 10.1002/anie.201612183
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      14
      Hydrophobic Water Probed Experimentally at the Gold Electrode/Aqueous Interface
      Tong, Yujin; Lapointe, Francois; Thaemer, Martin; Wolf, Martin; Campen, R. Kramer
      Angewandte Chemie, International Edition (2017), 56 (15), 4211-4214CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Quant. description of reaction mechanisms in aq. phase electrochem. requires exptl. characterization of local H2O structure at the electrode/aq. interface and its evolution with changing potential. Gaining such insight exptl. under electrochem. conditions is a formidable task. The potential-dependent structure of a subpopulation of interfacial H2O with one OH group pointing towards a Au working electrode was characterized using interface specific vibrational spectroscopy in a thin film electrochem. cell. Such free-OH groups are the mol. level observable of an extended hydrophobic interface. This free-OH interacts only weakly with the Au surface at all potentials, has an orientational distribution that narrows approaching the potential of zero charge, and disappears on oxidn. of the Au electrode.
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    15. 15
      Noguchi, H.; Okada, T.; Uosaki, K. Molecular structure at electrode/electrolyte solution interfaces related to electrocatalysis. Faraday Discuss. 2009, 140, 125– 137,  DOI: 10.1039/B803640C
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      Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Voltage-dependent ordering of water molecules at an electrode-electrolyte interface. Nature 1994, 368, 444,  DOI: 10.1038/368444a0
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      16
      Voltage-dependent ordering of water molecules at an electrode-electrolyte interface
      Toney, Michael F.; Howard, Jason N.; Richer, Joselyn; Borges, Gary L.; Gordon, Joseph G.; Melroy, Owen R.; Wiesler, David G.; Yee, Dennis; Sorensen, Larry B.
      Nature (London, United Kingdom) (1994), 368 (6470), 444-6CODEN: NATUAS; ISSN:0028-0836.
      The arrangement of water mols. at charged, aq. interfaces is an important question in electrochem., geochem. and biol. Theor. studies suggest that the mols. become arranged in several layers adjacent to a solid interface, with densities similar to that in the bulk, and that the mols. in the 1st layer are reoriented from oxygen-up to oxygen-down as the electrode charge changes from neg. to pos. Few of these predictions were verified exptl., however. Using x-ray scattering, the authors have measured the water d. profile perpendicular to a silver (111) surface at 2 applied voltages. The water mols. are ordered in layers extending ∼3 mol diams. from the electrode, and the spacing between the electrode and 1st water layer indicates an oxygen-up (oxygen-down) av. orientation for neg. (pos.) charge. Contrary to current models, however, the 1st layer has a far greater d. than that in bulk water. This implies that the hydrogen-bonding network is disrupted in this layer, and that the properties of the water in the layer probably are very different from those in the bulk.
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    17. 17
      Lucas, C. A.; Thompson, P.; Gründer, Y.; Markovic, N. M. The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte. Electrochem. Commun. 2011, 13, 1205,  DOI: 10.1016/j.elecom.2011.08.043
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      17
      The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte
      Lucas, Christopher A.; Thompson, Paul; Gruender, Yvonne; Markovic, Nenad M.
      Electrochemistry Communications (2011), 13 (11), 1205-1208CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      The structure of the electrochem. double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte was probed using in-situ surface x-ray scattering (SXS). Detailed modeling of the SXS data at neg. potential (E = - 1.0 V vs. SCE) is consistent with the presence of an hydrated K+ cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at pos. potential (E = - 0.2 V), indicates that the presence of OHad stabilizes the hydrated K+ cations through a noncovalent interaction forming a compact double layer structure in which the Ag-K+ distance is reduced to 3.6 ± 0.2 Å.
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    18. 18
      Kunimatsu, K.; Senzaki, T.; Tsushima, M.; Osawa, M. A combined surface-enhanced infrared and electrochemical kinetics study of hydrogen adsorption and evolution on a Pt electrode. Chem. Phys. Lett. 2005, 401, 451– 454,  DOI: 10.1016/j.cplett.2004.11.100
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      18
      A combined surface-enhanced infrared and electrochemical kinetics study of hydrogen adsorption and evolution on a Pt electrode
      Kunimatsu, Keiji; Senzaki, Takahiro; Tsushima, Minoru; Osawa, Masatoshi
      Chemical Physics Letters (2005), 401 (4-6), 451-454CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
      The adsorption of hydrogen atoms on a polycryst. Pt electrode was examd. in acid by surface-enhanced IR spectroscopy, and the IR data was quant. compared with the kinetics of hydrogen evolution reaction (HER). The Pt-H stretching vibration was obsd. around 2100 cm-1 in the H2 evolution potential range (E < 0.1 V vs. RHE). The current of HER was proportional to the square of the band intensity at 0.02 < E < 0.1 V, from which the adsorbed hydrogen atom is the reaction intermediate of HER and the combination of 2 adsorbed H atoms is rate-detg. At E < 0.02 V, however, an inconsistency was found between the IR and electrochem. measurements due to the supersatn. of H2 at the interface.
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    19. 19
      Carrasco, E.; Aumer, A.; Brown, M. A.; Dowler, R.; Palacio, I.; Song, S.; Sterrer, M. Infrared spectra of high coverage CO adsorption structures on Pt(111). Surf. Sci. 2010, 604, 1320– 1325,  DOI: 10.1016/j.susc.2010.04.021
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      Infrared spectra of high coverage CO adsorption structures on Pt(111)
      Carrasco, Esther; Aumer, Andreas; Brown, Matthew A.; Dowler, Rhys; Palacio, Irene; Song, Sundal; Sterrer, Martin
      Surface Science (2010), 604 (15-16), 1320-1325CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The adsorption of CO on Pt(111) was studied using polarization modulation IR reflection absorption spectroscopy (PM-IRAS) and sum frequency generation (SFG) spectroscopy. Two CO on-top signals at 2110 cm- 1 and 2097 cm- 1 were detected under continuous CO exposure in a pressure range from 10- 7 to 100 mbar and at 200-300 K The formation of the higher wavenumber signal is kinetically limited <200 K and by the presence of a stable c(4 × 2) adlayer in UHV. From the results presented in this study and previous exptl. findings the 2 on-top signals are related to different CO compression layers on Pt(111) with θ > 0.5, hexagonal Moire lattices and rectangular coincident site lattices.
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    20. 20
      Cheng, J.; Sprik, M. The electric double layer at a rutile TiO(2) water interface modelled using density functional theory based molecular dynamics simulation. J. Phys.: Condens. Matter 2014, 26, 244108,  DOI: 10.1088/0953-8984/26/24/244108
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      20
      The electric double layer at a rutile TiO2 water interface modeled using density functional theory based molecular dynamics simulation
      Cheng, J.; Sprik, M.
      Journal of Physics: Condensed Matter (2014), 26 (24), 244108/1-244108/11, 11 pp.CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      A fully atomistic model of a compact elec. double layer at the rutile TiO2(110)-water interface is constructed by adding protons to bridging oxygens or removing them from H2O mols. adsorbed on terminal metal cation sites. The surface charge is compensated by F- or Na+ counterions in outer as well as inner sphere coordination. For each of the protonation states the energy of the TiO2 conduction band min. is detd. relative to the std. hydrogen electrode by computing the free energy for the combined insertion of an electron in the solid and a proton in soln. away from the double layer using d. functional theory based mol. dynamics methods. Interpreted as electrode potentials, this gives an est. of the capacitance which is compared to the capacitance obtained from the difference in the av. electrostatic potentials in the solid and aq. phase. When aligned at the point of zero charge these two methods lead to almost identical potential-charge profiles. Inner sphere complexes have a slightly larger capacitance (0.4 F m-2) compared to outer sphere complexes (0.3 F m-2).
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      Otani, M.; Hamada, I.; Sugino, O.; Morikawa, Y.; Okamoto, Y.; Ikeshoji, T. Electrode dynamics from first principles. J. Phys. Soc. Jpn. 2008, 77, 024802,  DOI: 10.1143/JPSJ.77.024802
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      Electrode dynamics from first principles
      Otani, Minoru; Hamada, Ikutaro; Sugino, Osamu; Morikawa, Yoshitada; Okamoto, Yasuharu; Ikeshoji, Tamio
      Journal of the Physical Society of Japan (2008), 77 (2), 024802/1-024802/6CODEN: JUPSAU; ISSN:0031-9015. (Physical Society of Japan)
      The study of electrode dynamics was a major topic in the field of electrochem. for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte soln., transport of electrons and ions. The 1st-principles mol. dynamics simulation of the electrochem. system was hampered by the difficulty to describe the bias voltage and the complex soln.-electrode interface structure. Here the authors use a new algorithm called the effective screening medium to characterize the biased interface between Pt and liq. H2O, revealing the microscopic details of the 1st, Volmer, step of the Pt-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the H2O and the bias, the authors show why this reaction occurs so efficiently at the interface. The simulations make a significant step towards a deeper understanding of electrochem. reactions.
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    22. 22
      Surendralal, S.; Todorova, M.; Finnis, M. W.; Neugebauer, J. First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion. Phys. Rev. Lett. 2018, 120, 246801,  DOI: 10.1103/PhysRevLett.120.246801
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      22
      First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion
      Surendralal, Sudarsan; Todorova, Mira; Finnis, Michael W.; Neugebauer, Joerg
      Physical Review Letters (2018), 120 (24), 246801CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calcns.under controlled potentiostat conditions for electrochem.systems. The proposed approach can be straightforwardly applied in std.d.functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chem. reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution.
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      Le, J.; Iannuzzi, M.; Cuesta, A.; Cheng, J. Determining potentials of zero charge of metal electrodes versus the standard hydrogen electrode from based on density-functional-theory-based molecular dynamics. Phys. Rev. Lett. 2017, 119, 16801,  DOI: 10.1103/PhysRevLett.119.016801
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      Determining potentials of zero charge of metal electrodes versus the standard hydrogen electrode from density-functional-theory-based molecular dynamics
      Le, Jiabo; Iannuzzi, Marcella; Cuesta, Angel; Cheng, Jun
      Physical Review Letters (2017), 119 (1), 016801/1-016801/6CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      We develop a computationally efficient scheme to det. the potentials of zero charge (PZC) of metal-water interfaces with respect to the std. hydrogen electrode. We calc. the PZC of Pt(111), Au(111), Pd(111) and Ag(111) at a good accuracy using this scheme. Moreover, we find that the interface dipole potentials are almost entirely caused by charge transfer from water to the surfaces, the magnitude of which depends on the bonding strength between water and the metals, while water orientation hardly contributes at the PZC conditions.
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      Cheng, J.; Sprik, M. Alignment of electronic energy levels at electrochemical interfaces. Phys. Chem. Chem. Phys. 2012, 14, 11245,  DOI: 10.1039/c2cp41652b
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      Alignment of electronic energy levels at electrochemical interfaces
      Cheng, Jun; Sprik, Michiel
      Physical Chemistry Chemical Physics (2012), 14 (32), 11245-11267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calcg. such shifts for electrochem. interfaces requires a combination of methods from computational surface science and phys. chem. The problem is closely related to the computation of potentials of electrochem. inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective the authors review two d. functional theory based methods that were developed for this purpose, the work function method and the H insertion method. The key expressions of the two methods are derived from the formal theory of abs. electrode potentials. As an illustration of the work function method the authors review the computation of the potential of zero charge of the Pt(111)-H2O interface as recently published by a no. of groups. The example of the H insertion method is from the authors' own work on the rutile TiO2(110)-H2O interface at the point of zero proton charge. The calcns. are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band max. and conduction band min. of the semiconductor) to the band edges of liq. H2O and the std. potential for the redn. of the hydroxyl radical. All potentials are calcd. at the same level of d. functional theory using the std. H electrode as common energy ref. Comparison to expt. identifies the treatment of the valence band of H2O as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.
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      Magnussen, O. M.; Groß, A. Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics. J. Am. Chem. Soc. 2019, 141, 4777– 4790,  DOI: 10.1021/jacs.8b13188
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      Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics
      Magnussen, Olaf M.; Gross, Axel
      Journal of the American Chemical Society (2019), 141 (12), 4777-4790CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. For the knowledge-based development of electrochem. processes, a better fundamental understanding of the interfaces between electrodes and electrolytes is necessary. This requires insight into the interface structure and dynamics on the at.-scale, including that of the liq. electrolyte in the near-surface region, i.e., in the inner and outer part of the electrochem. double layer. This perspective describes current studies of simple and well-defined electrochem. interfaces by 1st-principles electronic structure calcns. and in situ structure-sensitive methods. These exptl. and theor. studies are now approaching a level, where they can operate on the same footing, making direct comparison of the obtained results feasible. Using selected examples, progress in clarifying the structure and dynamics of the double layer, of adsorbed species on electrode surfaces, and of initial steps in electrochem. phase formation processes is discussed.
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    26. 26
      Huang, P.; Pham, T. A.; Galli, G.; Schwegler, E. Alumina(0001)/Water Interface: Structural Properties and Infrared Spectra from First-Principles Molecular Dynamics Simulations. J. Phys. Chem. C 2014, 118, 8944– 8951,  DOI: 10.1021/jp4123002
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      26
      Alumina(0001)/Water Interface: Structural Properties and Infrared Spectra from First-Principles Molecular Dynamics Simulations
      Huang, Patrick; Pham, Tuan Anh; Galli, Giulia; Schwegler, Eric
      Journal of Physical Chemistry C (2014), 118 (17), 8944-8951CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We investigated the at. structure and IR spectra of the alumina(0001)/water interface, using first-principles mol. dynamics simulations based on d. functional theory within the generalized gradient approxn. The computed structural properties of the interface are in good agreement with the results of synchrotron X-ray expts. Detailed analyses of the computed IR spectra revealed two types of water mols. at the hydrophilic oxide/water interface: mols. participating in strong "ice-like" hydrogen bonding with the oxide surface and mols. involved in weaker "liq.-like" hydrogen bonding. Our results provide a mol. interpretation of the "ice-like" and "liq.-like" bands obsd. in sum-frequency vibrational spectroscopy expts. and underscore the significance of strong hydrogen-bonding interactions in detg. the orientation of interfacial water mols.
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      Lan, J.; Rybkin, V. V.; Iannuzzi, M. Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces. J. Phys. Chem. Lett. 2020, 11, 3724– 3730,  DOI: 10.1021/acs.jpclett.0c01025
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      27
      Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces
      Lan, Jinggang; Rybkin, Vladimir V.; Iannuzzi, Marcella
      Journal of Physical Chemistry Letters (2020), 11 (9), 3724-3730CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The enhanced probability of water dissocn. at the aq. electrode interfaces is predicted by path-integral ab initio mol. dynamics. The ionization process is obsd. at the aq. platinum interface when nuclear quantum effects are introduced in the statistical sampling, while minor effects have been obsd. at the gold interface. We characterize the dissocn. mechanism of the formed water ions. In spite of the fact that the concn. and lifetime of the ions might be challenging to exptl. detect, they may serve as a guide to future expts. Our observation might have a significant impact on the understanding of electrochem. processes occurring at the metal electrode surface.
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      Le, J.; Fan, Q.; Perez-Martinez, L.; Cuesta, A.; Cheng, J. Theoretical insight into the vibrational spectra of metal-water interfaces from density functional theory based molecular dynamics. Phys. Chem. Chem. Phys. 2018, 20, 11554,  DOI: 10.1039/C8CP00615F
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      28
      Theoretical insight into the vibrational spectra of metal-water interfaces from density functional theory based molecular dynamics
      Le, Jiabo; Fan, Qiyuan; Perez-Martinez, Laura; Cuesta, Angel; Cheng, Jun
      Physical Chemistry Chemical Physics (2018), 20 (17), 11554-11558CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Understanding electrochem. interface structures at the at. level is key to developing efficient electrochem. cells for energy storage/conversion. Spectroscopic techniques have been widely used to examine interfacial structures and vibrational properties. Interpretation of these spectra is not straight-forward. This work used d. functional theory-based mol. dynamics simulations to examine the vibrational properties of Pt(111)/ and Au(111)/water interfaces. The specific adsorption of some surface water on Pt(111) led to a partial charge transfer to the metal and strong H bonding with neighboring water mols., which resolved the interpretation of the elusive O-H stretching peak at approx. 3000 cm-1 obsd. in some expts.
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      Le, J.-B.; Fan, Q.-Y.; Li, J.-Q.; Cheng, J. Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface. Sci. Adv. 2020, 6, eabb1219  DOI: 10.1126/sciadv.abb1219
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      Pajkossy, T.; Kolb, D. M. Double layer capacitance of Pt(111) single crystal electrodes. Electrochim. Acta 2001, 46, 3063,  DOI: 10.1016/S0013-4686(01)00597-7
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      Double layer capacitance of Pt(111) single crystal electrodes
      Pajkossy, T.; Kolb, D. M.
      Electrochimica Acta (2001), 46 (20-21), 3063-3071CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Science Ltd.)
      To det. the double layer capacitance of the Pt(111) electrode, impedance and capacitance measurements were carried out in neutral and acidic aq. perchlorate solns. Sepn. of the double layer and adsorption contributions of the interfacial capacitance were based on the adsorption impedance theory. The double layer capacitance vs. potential plot exhibits a peak at ≈0.12 V vs. SCE in the 1.5-7 pH range; from here towards cathodic potentials the capacitance attains a value of ≈-20 μF/cm2. The peak may be related to the potential of zero free charge of the Pt(111) electrode.
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    31. 31
      Ojha, K.; Arulmozhi, N.; Aranzales, D.; Koper, M. T. M. Double Layer of Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening. Angew. Chem., Int. Ed. 2020, 59, 711,  DOI: 10.1002/anie.201911929
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      Double Layer of Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening
      Ojha, Kasinath; Arulmozhi, Nakkiran; Aranzales, Diana; Koper, Marc T. M.
      Angewandte Chemie, International Edition (2020), 59 (2), 711-715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We report, for the first time, the observation of a Gouy-Chapman capacitance min. at the potential of zero charge of the Pt(111)-aq. perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concns. ( < 10-3 m). By comparison to gold and mercury double-layer data, we conclude that the diffuse double layer structure at the Pt(111)-electrolyte interface deviates significantly from the Gouy-Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean-field Poisson-Boltzmann theory.
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    32. 32
      Cheng, T.; Wang, L.; Merinov, B. V.; Goddard, W. A. Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH. J. Am. Chem. Soc. 2018, 140, 7787,  DOI: 10.1021/jacs.8b04006
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      32
      Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH
      Cheng, Tao; Wang, Lu; Merinov, Boris V.; Goddard, William A.
      Journal of the American Chemical Society (2018), 140 (25), 7787-7790CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      H oxidn. reaction (HOR) and H evolution reaction (HER) are both 2 orders slower in alk. electrolyte than in acidic electrolyte, but no explanation was provided. The 1st step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent H binding to the electrode, a perplexing behavior obsd. exptl. The authors carried out Quantum Mechanics Mol. Dynamics (QMMD) with explicit considerations of solvent and applied voltage (U) to in situ simulate H2O/Pt(100) interface in the condition of under-potential adsorption of H (HUPD). As U is made more neg., the electrode tends to repel H2O, which in turn increases the H binding. The authors predicted a 0.13 eV increase in H binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the exptl. observation of 8 to 12 meV/pH. Thus, the changes in H2O adsorption are the major causes of pH-dependent H binding on a noble metal. The new insight of crit. role of surface H2O in modifying electrochem. reactions provides a guideline in designing HER/HOR catalyst targeting for the alk. electrolyte.
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      Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy. Nat. Commun. 2015, 6, 5848,  DOI: 10.1038/ncomms6848
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      Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy
      Sheng, Wenchao; Zhuang, Zhongbin; Gao, Minrui; Zheng, Jie; Chen, Jingguang G.; Yan, Yushan
      Nature Communications (2015), 6 (), 5848CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      The hydrogen oxidn./evolution reactions are two of the most fundamental reactions in distributed renewable electrochem. energy conversion and storage systems. The identification of the reaction descriptor is therefore of crit. importance for the rational catalyst design and development. Here we report the correlation between hydrogen oxidn./evolution activity and exptl. measured hydrogen binding energy for polycryst. platinum examd. in several buffer solns. in a wide range of electrolyte pH from 0 to 13. The hydrogen oxidn./evolution activity obtained using the rotating disk electrode method is found to decrease with the pH, while the hydrogen binding energy, obtained from cyclic voltammograms, linearly increases with the pH. Correlating the hydrogen oxidn./evolution activity to the hydrogen binding energy renders a monotonic decreasing hydrogen oxidn./evolution activity with the hydrogen binding energy, strongly supporting the hypothesis that hydrogen binding energy is the sole reaction descriptor for the hydrogen oxidn./evolution activity on monometallic platinum.
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      Zheng, J.; Sheng, W.; Zhuang, Z.; Xu, B.; Yan, Y. Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy. Sci. Adv. 2016, 2, e1501602  DOI: 10.1126/sciadv.1501602
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      Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy
      Zheng, Jie; Sheng, Wenchao; Zhuang, Zhongbin; Xu, Bingjun; Yan, Yushan
      Science Advances (2016), 2 (3), e1501602/1-e1501602/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      Understanding how pH affects the activity of hydrogen oxidn. reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is obsd. for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange c.d. and HBE is also obsd. on the four metals, indicating that they may share the same elementary steps and rate-detg. steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.
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      Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23,  DOI: 10.1149/1.1856988
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      Trends in the exchange current for hydrogen evolution
      Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U.
      Journal of the Electrochemical Society (2005), 152 (3), J23-J26CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A d. functional theory database of hydrogen chemisorption energies on close packed surfaces of a no. of transition and noble metals is presented. The bond energies are used to understand the trends in the exchange current for hydrogen evolution. A volcano curve is obtained when measured exchange currents are plotted as a function of the calcd. hydrogen adsorption energies and a simple kinetic model is developed to understand the origin of the volcano. The volcano curve is also consistent with Pt being the most efficient electrocatalyst for hydrogen evolution.
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      Briega-Martos, V.; Ferre-Vilaplana, A.; Herrero, E.; Feliu, J. M. Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases. Electrochim. Acta 2020, 354, 136620,  DOI: 10.1016/j.electacta.2020.136620
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      Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases
      Briega-Martos, Valentin; Ferre-Vilaplana, Adolfo; Herrero, Enrique; Feliu, Juan M.
      Electrochimica Acta (2020), 354 (), 136620CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Platinum is a very effective electrode for the hydrogen evolution and oxidn. reactions (HER/HOR) in acidic media. However, the activity for the HOR on platinum falls two orders of magnitude from acidic to alk. media, which has not been completely understood yet. Here, we provide an explanation for that. Both the HER and the HOR were investigated on the three basal planes of platinum in a pH range near neutral pH conditions in buffered solns. in the absence of anion specific adsorption for guaranteeing the protons availability. Whereas changes in the pH from acid to neutral values produced negligible effects on the HER, the HOR was found to be pH sensitive, even under near neutral pH conditions. From these results, it can be consistently reasoned that the drastic fall in the activity of the HOR on platinum from acidic to alk. media is an effect of the charge on the electrode, which is more neg. as the pH increases. With the aid of d. functional theory calcns., kinetic arguments explaining the unfavorable effect that neg. charge on the electrode has on the HOR are provided.
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      Sakong, S.; Gross, A. The electric double layer at metal-water interfaces revisited based on a charge polarization scheme. J. Chem. Phys. 2018, 149, 084705,  DOI: 10.1063/1.5040056
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      The electric double layer at metal-water interfaces revisited based on a charge polarization scheme
      Sakong, Sung; Gross, Axel
      Journal of Chemical Physics (2018), 149 (8), 084705/1-084705/11CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The description of electrode-electrolyte interfaces is based on the concept of the formation of an elec. double layer. This concept was derived from continuum theories extended by introducing point charge distributions. Based on ab initio mol. dynamics simulations, we analyze the elec. double layer in an approach beyond the point charge scheme by instead assessing charge polarizations at electrochem. metal-water interfaces from first principles. We show that the at. structure of water layers at room temp. leads to an oscillatory behavior of the averaged electrostatic potential. We address the relation between the polarization distribution at the interface and the extent of the elec. double layer and subsequently derive the electrode potential from the charge polarization. (c) 2018 American Institute of Physics.
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      Hansen, M. H.; Nilsson, A.; Rossmeisl, J. Modelling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale. Phys. Chem. Chem. Phys. 2017, 19, 23505– 23514,  DOI: 10.1039/C7CP03576D
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      Modeling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale
      Hansen, Martin Hangaard; Nilsson, Anders; Rossmeisl, Jan
      Physical Chemistry Chemical Physics (2017), 19 (34), 23505-23514CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      At.-scale structures of a Pt(111)/water interface were prepd. by calcg. at. distance distributions as a function of pH. The Pt(111)/water interface structure is a particularly interesting model electrocatalysis system for proton exchange reactions, particularly the O redn. reaction in polymer electrolyte membrane fuel cells. Addnl. insight into such reactions requires accurate simulations of the interface electrolyte structure. This work displays many interesting details regarding electrolyte structure behavior, e.g., the electrolyte structure av. response to the presence of protons by a H down-water orientation; and hexagonally adsorbed water layers are present only when they are anchored at the surface by OH-. New adsorbate configurations were also obsd. at 5/12 ML OH- coverage, suggesting an explanation for reported cyclic voltammetry expts. This work is a step toward a more complete understanding of the electrochem. interface structure on an at. scale.
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      Kristoffersen, H. H.; Vegge, T.; Hansen, H. A. OH formation and H2adsorption at the liquid water-Pt(111) interface. Chem. Sci. 2018, 9, 6912,  DOI: 10.1039/C8SC02495B
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      OH formation and H2 adsorption at the liquid water-Pt(111) interface
      Kristoffersen, Henrik H.; Vegge, Tejs; Hansen, Heine Anton
      Chemical Science (2018), 9 (34), 6912-6921CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The liq.water-Pt(111) interface is studied with const.temp.ab initio mol.dynamics to explore the importance of liq.water dynamics of catalytic reactions such as the oxygen redn.reaction in PEM fuel cells. The structure and energetics of hydroxyls formed at the liq.water-Pt(111) interface are found to be significantly different from those of the hydroxyl formed on a bare Pt(111) surface and the hydroxyl formed on a Pt(111) surface with a static water layer. We identify 1/12 ML *OH, 5/12 ML *OH and 2/3 ML *OH as particularly stable hydroxyl coverages in highly dynamic liq.water environments, which - contrary to static water-hydroxyl models - contain adjacent uncovered Pt sites. Atomic surface oxygen is found to be unstable in the presence of liq.water, in contrast to static at.level simulations. These results give an improved understanding of hydroxide and surface oxide formation from Pt(111) cyclic voltammetry and allow us to draw detailed connections between the electrostatic potential and the interface structure. The study of hydrogen adsorption at the liq.water-Pt(111) interface finds competitive adsorption between the adsorbed hydrogen atoms and water mols. This does not adhere with exptl.observations, and this indicates that the Pt(111) surface has to be neg.charged for a correct description of the liq.water-Pt(111) interface at potentials where hydrogen adsorption occurs.
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      Kronberg, R.; Laasonen, K. Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer–Water Structure of Hydrogenated Single-Crystal Platinum Electrodes. J. Phys. Chem. C 2020, 124, 13706– 13714,  DOI: 10.1021/acs.jpcc.0c02323
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      Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer-Water Structure of Hydrogenated Single-Crystal Platinum Electrodes
      Kronberg, Rasmus; Laasonen, Kari
      Journal of Physical Chemistry C (2020), 124 (25), 13706-13714CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Atomically flat, single-crystal solid-liq. interfaces attract considerable interest through their electrochem. relevance and well-defined structure facilitating controlled atomistic characterization. Yet, crucial details esp. regarding the nanoscale adlayer-water dynamics remain uncertain. Here, the influence of adsorbate coverage on the interfacial structure and solvent relaxation on hydrogenated Pt(111) is examd. by extensive d. functional mol. dynamics simulations. Pronounced water dynamics is obsd. with increasing hydrogen coverage, for which an interpretation based on displacement of specifically co-adsorbed water and strong screening of the electrostatic interaction across the interface is proposed. However, the magnitude of the solvent fluctuations is argued to be partly overestimated by the employed RPBE-D3 exchange-correlation functional, which impedes water chemisorption and charge transfer to sparsely hydrogenated platinum. This manifests as overestimated equil. electrode potentials compared to exptl. adsorption isotherms, which are conversely well reproduced by static calcns. invoking the computational hydrogen electrode formalism. By coupling the interfacial structure with electrostatic properties, our work underscores the profound importance of functional choice as well as the persisting value and comparable precision of carefully employed static approxns. in electrochem. simulations.
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      The work function of the elements and its periodicity
      Michaelson, Herbert B.
      Journal of Applied Physics (1977), 48 (11), 4729-33CODEN: JAPIAU; ISSN:0021-8979.
      A review with 63 refs. is compiled based on a literature search for the period 1969-1976.
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      Cuesta, A. Measurement of the surface charge density of CO-saturated Pt(111) electrodes as a function of potential: The potential of zero charge of Pt(111). Surf. Sci. 2004, 572, 11,  DOI: 10.1016/j.susc.2004.08.014
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      Measurement of the surface charge density of CO-saturated Pt(111) electrodes as a function of potential: the potential of zero charge of Pt(111)
      Cuesta, Angel
      Surface Science (2004), 572 (1), 11-22CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      From measurements of the charge flowing upon immersion, at controlled potential, of a CO-covered Pt(111) electrode in a 0.1M HClO4 soln., the corresponding surface charge d. vs. potential curve was obtained, and from this the potential of zero charge (pzc) of the CO-covered Pt(111) electrode. From these data it was estd. that the error incurred when the potential of zero total charge (pztc) of Pt(111) electrodes is detd. by the CO-charge displacement method is of ∼50 mV at pH 1 and of ∼90 mV at pH 3. Also, the exptl. detd. pzc of the CO-covered Pt(111) electrode has allowed one to make an estn. of the potential of zero free charge (pzfc) of Pt(111) electrodes.
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      Markovic, N. Surface science studies of model fuel cell electrocatalysts. Surf. Sci. Rep. 2002, 45, 117– 229,  DOI: 10.1016/S0167-5729(01)00022-X
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      Surface science studies of model fuel cell electrocatalysts
      Markovic, N. M.; Ross, P. N.
      Surface Science Reports (2002), 45 (4-6), 117-229CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
      A review with refs. The purpose of this review is to discuss progress in the understanding of electrocatalytic reactions through the study of model systems with surface spectroscopies. Pure metal single crystals and well-characterized bulk alloys have been used quite successfully as models for real (com.) electrocatalysts. Given the sheer vol. of all work in electrocatalysis that is on fuel cell reactions, we will focus on electrocatalysts for fuel cells. Since Pt is the model fuel cell electrocatalyst, we will focus entirely on studies of pure Pt and Pt bimetallic alloys. The electrode reactions discussed include hydrogen oxidn./evolution, oxygen redn., and the electrooxidn. of carbon monoxide, formic acid, and methanol. Surface spectroscopies emphasized are FTIR, STM/AFM and surface X-ray scattering (SXS). The discussion focuses on the relation between the energetics of adsorption of intermediates and the reaction pathway and kinetics, and how the energetics and kinetics relate to the extrinsic properties of the model system, e.g. surface structure and/or compn. Finally, we conclude by discussing the limitations that are reached by using pure metal single crystals and well-characterized bulk alloys as models for real catalysts, and suggest some directions for developing more realistic systems.
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      VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Quickstep: Fast and accurate density functional calculations using a mixed Gaussian and plane waves approach. Comput. Phys. Commun. 2005, 167, 103,  DOI: 10.1016/j.cpc.2004.12.014
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      QUICKSTEP: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach
      VandeVondele, Joost; Krack, Matthias; Mohamed, Fawzi; Parrinello, Michele; Chassaing, Thomas; Hutter, Juerg
      Computer Physics Communications (2005), 167 (2), 103-128CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
      We present the Gaussian and plane waves (GPW) method and its implementation in which is part of the freely available program package CP2K. The GPW method allows for accurate d. functional calcns. in gas and condensed phases and can be effectively used for mol. dynamics simulations. We show how derivs. of the GPW energy functional, namely ionic forces and the Kohn-Sham matrix, can be computed in a consistent way. The computational cost of computing the total energy and the Kohn-Sham matrix is scaling linearly with the system size, even for condensed phase systems of just a few tens of atoms. The efficiency of the method allows for the use of large Gaussian basis sets for systems up to 3000 atoms, and we illustrate the accuracy of the method for various basis sets in gas and condensed phases. Agreement with basis set free calcns. for single mols. and plane wave based calcns. in the condensed phase is excellent. Wave function optimization with the orbital transformation technique leads to good parallel performance, and outperforms traditional diagonalisation methods. Energy conserving Born-Oppenheimer dynamics can be performed, and a highly efficient scheme is obtained using an extrapolation of the d. matrix. We illustrate these findings with calcns. using commodity PCs as well as supercomputers.
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      Goedecker, S.; Teter, M.; Hutter, J. Separable dual-space Gaussian pseudopotentials. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 1703,  DOI: 10.1103/PhysRevB.54.1703
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      Separable dual-space Gaussian pseudopotentials
      Goedecker, S.; Teter, M.; Hutter, J.
      Physical Review B: Condensed Matter (1996), 54 (3), 1703-1710CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      We present pseudopotential coeffs. for the first two rows of the Periodic Table. The pseudopotential is of an analytic form that gives optimal efficiency in numerical calculations using plane waves as a basis set. At most, even coeffs. are necessary to specify its analytic form. It is separable and has optimal decay properties in both real and Fourier space. Because of this property, the application of the nonlocal part of the pseudopotential to a wave function can be done efficiently on a grid in real space. Real space integration is much faster for large systems than ordinary multiplication in Fourier space, since it shows only quadratic scaling with respect to the size of the system. We systematically verify the high accuracy of these pseudopotentials by extensive at. and mol. test calcns.
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      Hartwigsen, C.; Goedecker, S.; Hutter, J. Relativistic separable dual-space Gaussian pseudopotentials from H to Rn. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 3641,  DOI: 10.1103/PhysRevB.58.3641
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      Relativistic separable dual-space Gaussian pseudopotentials from H to Rn
      Hartwigsen, C.; Goedecker, S.; Hutter, J.
      Physical Review B: Condensed Matter and Materials Physics (1998), 58 (7), 3641-3662CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      We generalize the concept of separable dual-space Gaussian pseudopotentials to the relativistic case. This allows us to construct this type of pseudopotential for the whole Periodic Table, and we present a complete table of pseudopotential parameters for all the elements from H to Rn. The relativistic version of this pseudopotential retains all the advantages of its nonrelativistic version. It is separable by construction, it is optimal for integration on a real-space grid, it is highly accurate, and, due to its analytic form, it can be specified by a very small no. of parameters. The accuracy of the pseudopotential is illustrated by an extensive series of mol. calcns.
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      VandeVondele, J.; Hutter, J. Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases. J. Chem. Phys. 2007, 127, 114105,  DOI: 10.1063/1.2770708
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      Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases
      VandeVondele, Joost; Hutter, Jurg
      Journal of Chemical Physics (2007), 127 (11), 114105/1-114105/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      We present a library of Gaussian basis sets that has been specifically optimized to perform accurate mol. calcns. based on d. functional theory. It targets a wide range of chem. environments, including the gas phase, interfaces, and the condensed phase. These generally contracted basis sets, which include diffuse primitives, are obtained minimizing a linear combination of the total energy and the condition no. of the overlap matrix for a set of mols. with respect to the exponents and contraction coeffs. of the full basis. Typically, for a given accuracy in the total energy, significantly fewer basis functions are needed in this scheme than in the usual split valence scheme, leading to a speedup for systems where the computational cost is dominated by diagonalization. More importantly, binding energies of hydrogen bonded complexes are of similar quality as the ones obtained with augmented basis sets, i.e., have a small (down to 0.2 kcal/mol) basis set superposition error, and the monomers have dipoles within 0.1 D of the basis set limit. However, contrary to typical augmented basis sets, there are no near linear dependencies in the basis, so that the overlap matrix is always well conditioned, also, in the condensed phase. The basis can therefore be used in first principles mol. dynamics simulations and is well suited for linear scaling calcns.
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      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865,  DOI: 10.1103/PhysRevLett.77.3865
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      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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      Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344
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      A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
      Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
      Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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      Kühne, T. D.; Krack, M.; Mohamed, F. R.; Parrinello, M. Efficient and accurate car-parrinello-like approach to born-oppenheimer molecular dynamics. Phys. Rev. Lett. 2007, 98, 66401,  DOI: 10.1103/PhysRevLett.98.066401
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      Lan, J.; Hutter, J.; Iannuzzi, M. First-Principles Simulations of an Aqueous CO/Pt(111) Interface. J. Phys. Chem. C 2018, 122, 24068,  DOI: 10.1021/acs.jpcc.8b05933
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      First-Principles Simulations of an Aqueous CO/Pt(111) Interface
      Lan, Jinggang; Hutter, Jurg; Iannuzzi, Marcella
      Journal of Physical Chemistry C (2018), 122 (42), 24068-24076CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The structural, electronic, and vibrational properties of an adsorbed CO monolayer in the presence of liq. H2O at room temp., as obtained by ab initio mol. dynamics simulations are discussed. The H2O mols. at closest distance from the substrate form a bilayer, where they are coadsorbed at empty surface sites and with relatively rigid orientation, at the difference with the bulk liq. above. Coadsorbed H2O strengthens the back-bonding to CO, which leads to the red shift of ∼40 cm-1. The synergy between the 1st bilayer and the bulk liq. induces further polarization of the CO bond, which changes the transition dipole and results in the CO intensity enhancement. It is also verified that no hydrogen bond is effectively present between CO and bulk H2O. These findings provide new insight into the physics of the surface/adsorbate/solvent interface, thus clarifying the exptl. observation to be exploited for the design of improved catalysts.
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      VandeVondele, J.; Hutter, J. An efficient orbital transformation method for electronic structure calculations. J. Chem. Phys. 2003, 118, 4365– 4369,  DOI: 10.1063/1.1543154
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      An efficient orbital transformation method for electronic structure calculations
      VandeVondele, Joost; Hutter, Jurg
      Journal of Chemical Physics (2003), 118 (10), 4365-4369CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      An efficient method for optimizing single-determinant wave functions of medium and large systems is presented. It is based on a minimization of the energy functional using a new set of variables to perform orbital transformations. With this method convergence of the wave function is guaranteed. Preconditioners with different computational cost and efficiency have been constructed. Depending on the preconditioner, the method needs a no. of iterations that is very similar to the established diagonalization-DIIS approach, in cases where the latter converges well. Diagonalization of the Kohn-Sham matrix can be avoided and the sparsity of the overlap and Kohn-Sham matrix can be exploited. If sparsity is taken into account, the method scales as O(MN2), where M is the total no. of basis functions and N is the no. of occupied orbitals. The relative performance of the method is optimal for large systems that are described with high quality basis sets, and for which the d. matrixes are not yet sparse. We present a benchmark calcn. on a DNA crystal contg. 2×12 base pairs, solvent and counter ions (2388 atoms), using a TZV(2d,2p) basis (38688 basis functions) and conclude that the electronic structure of systems of this size can now be studied routinely.
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      Cheng, J.; Liu, X.; VandeVondele, J.; Sulpizi, M.; Sprik, M. Redox potentials and acidity constants from density functional theory based molecular dynamics. Acc. Chem. Res. 2014, 47, 3522,  DOI: 10.1021/ar500268y
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      Redox Potentials and Acidity Constants from Density Functional Theory Based Molecular Dynamics
      Cheng, Jun; Liu, Xiandong; Vande Vondele, Joost; Sulpizi, Marialore; Sprik, Michiel
      Accounts of Chemical Research (2014), 47 (12), 3522-3529CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. All-atom methods treat solute and solvent at the same level of electronic structure theory and statistical mechanics. All-atom computation of acidity consts. (pKa) and redox potentials is still a challenge. In this Account, we review such a method combining d. functional theory based mol. dynamics (DFTMD) and free energy perturbation (FEP) methods. The key computational tool is a FEP based method for reversible insertion of a proton or electron in a periodic DFTMD model system. The free energy of insertion (work function) is computed by thermodn. integration of vertical energy gaps obtained from total energy differences. The problem of the loss of a phys. ref. for ionization energies under periodic boundary conditions is solved by comparing with the proton work function computed for the same supercell. The scheme acts as a computational hydrogen electrode, and the DFTMD redox energies can be directly compared with exptl. redox potentials. Consistent with the closed shell nature of acid dissocn., pKa ests. computed using the proton insertion/removal scheme are found to be significantly more accurate than the redox potential calcns. This enables us to sep. the DFT error from other sources of uncertainty such as finite system size and sampling errors. Drawing an analogy with charged defects in solids, we trace the error in redox potentials back to underestimation of the energy gap of the extended states of the solvent. Accordingly the improvement in the redox potential as calcd. by hybrid functionals is explained as a consequence of the opening up of the bandgap by the Hartree-Fock exchange component in hybrids. Test calcns. for a no. of small inorg. and org. mols. show that the hybrid functional implementation of our method can reproduce acidity consts. with an uncertainty of 1-2 pKa units (0.1 eV). The error for redox potentials is in the order of 0.2 V.
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      Rizo, R.; Sitta, E.; Herrero, E.; Climent, V.; Feliu, J. M. Towards the understanding of the interfacial pH scale at Pt(111) electrodes. Electrochim. Acta 2015, 162, 138,  DOI: 10.1016/j.electacta.2015.01.069
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      Towards the understanding of the interfacial pH scale at Pt(1 1 1) electrodes
      Rizo, Ruben; Sitta, Elton; Herrero, Enrique; Climent, Victor; Feliu, Juan M.
      Electrochimica Acta (2015), 162 (), 138-145CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The detn. of the potentials of zero total and free charge, pztc and pzfc resp., were made in a wide pH range by using the CO displacement method and the same calcn. assumptions used previously for Pt(1 1 1) electrodes in contact with non-specifically adsorbing anions. Calcn. of the pzfc involves, in occasions, long extrapolations that lead us to the introduction of the concept of potential of zero extrapolated charge (pzec). It was obsd. that the pztc changes with pH but the pzec is independent of this parameter. It was obsd. that the pztc > pzec at pH > 3.4 but the opposite is true for pH > 3.4. At the latter pH both pzec and pztc coincide. This defines two different pH regions and means that adsorbed hydrogen has to be cor. in the "acidic" solns. at the pztc while adsorbed OH is the species to be cor. in the "alk." range. The comparison of the overall picture suggests that neutral conditions at the interface are attained at significantly acidic solns. than those at the bulk.
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      Skúlason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jónsson, H.; Nørskov, J. K. Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode. Phys. Chem. Chem. Phys. 2007, 9, 3241,  DOI: 10.1039/B700099E
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      Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode
      Skulason, Egill; Karlberg, Gustav S.; Rossmeisl, Jan; Bligaard, Thomas; Greeley, Jeff; Jonsson, Hannes; Norskov, Jens K.
      Physical Chemistry Chemical Physics (2007), 9 (25), 3241-3250CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The authors present results of d. functional theory calcns. on a Pt(111) slab with a bilayer of H2O, solvated protons in the H2O layer, and excess electrons in the metal surface. In this way the authors model the electrochem. double layer at a Pt electrode. By varying the no. of protons/electrons in the double layer the authors study the system as a function of the electrode potential. The authors study the elementary processes involved in the hydrogen evolution reaction, 2(H+ + e-) → H2, and det. the activation energy and predominant reaction mechanism as a function of electrode potential. The authors confirm by explicit calcns. the notion that the variation of the activation barrier with potential can be viewed as a manifestation of the Broensted-Evans-Polanyi-type relation between activation energy and reaction energy found throughout surface chem.
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      Watson, G. W.; Wells, R. P. K.; Willock, D. J.; Hutchings, G. J. A comparison of the adsorption and diffusion of hydrogen on the {111} surfaces of Ni, Pd, and Pt from density functional theory calculations. J. Phys. Chem. B 2001, 105, 4889– 4894,  DOI: 10.1021/jp002864c
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      A Comparison of the Adsorption and Diffusion of Hydrogen on the {111} Surfaces of Ni, Pd, and Pt from Density Functional Theory Calculations
      Watson, Graeme W.; Wells, Richard P. K.; Willock, David J.; Hutchings, Graham J.
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      Gradient cor. d. function theory calcns. on the adsorption and thermally activated diffusion pathways of hydrogen on the {111} surfaces of Ni, Pd, and Pt are presented. It was found that the variation of the adsorption energy as a function of adsorption site shows considerable differences between the three metals. For Ni and Pd, the adsorption energies vary as a function of hydrogen coordination with the 3-fold hollow sites the most stable and the 1-fold atop site considerably less stable. On Pt the adsorption energies for all the sites are similar indicating that diffusion across the surface will be faster on Pt than on Ni or Pd. The activation energies for diffusion have been calcd. with that for Pt (2-3 kJ mol-1) considerably smaller than for Ni or Pd (13 kJ mol-1). The calcd. adsorption and activation energies are in good agreement with exptl. investigations of the structure, energetics, and diffusion properties.
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      The structure of metal-water interface at the potential of zero charge from density functional theory-based molecular dynamics
      Le, Jiabo; Cuesta, Angel; Cheng, Jun
      Journal of Electroanalytical Chemistry (2018), 819 (), 87-94CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
      The authors simulated transition metal-H2O interfaces, Pt(111), Au(111), Pd(111) and Ag(111), by d. functional theory based mol. dynamics, and found some common structural features for the surface H2O on different transition metal surfaces. Firstly, there exists a pronounced adsorption layer within ∼5 Å distance from metal surfaces, in which three main H2O species with different orientations (watA, watB-down and watB-up) could be identified. WatA and watB-down show a lower degree of H bonding, due to their interaction with the metal surface via one of the lone pairs of the O atoms and via one of their H atoms, resp. While, watB-up has an almost full coordination shell, indicating it not only forms H bonds in the adsorption layer, but also with the nonsurface H2O. As expected, the honeycomb-like bilayer model used as the starting point of the simulation was destructed into irregular patterns after ∼10 ps of mol. dynamics simulations, and the surface H2O coverage concomitantly increases from 0.66 ML to ∼0.8 ML.
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      Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes. Nat. Energy 2017, 2, 17031,  DOI: 10.1038/nenergy.2017.31
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      Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes
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      Nature Energy (2017), 2 (4), 17031CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
      Hydrogen evolution on platinum is a key reaction for electrocatalysis and sustainable energy storage, yet its pH-dependent kinetics are not fully understood. Here we present a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) in a wide pH range. Electrochem. measurements show that hydrogen adsorption and hydrogen evolution are both slow in alk. media, consistent with the observation of a shift in the rate-detg. step for hydrogen evolution. Adding nickel to the Pt(111) surface lowers the barrier for hydrogen adsorption in alk. solns. and thereby enhances the hydrogen evolution rate. We explain these observations with a model that highlights the role of the reorganization of interfacial water to accommodate charge transfer through the elec. double layer, the energetics of which are controlled by how strongly water interacts with the interfacial field. The model is supported by laser-induced temp.-jump measurements. Our model sheds light on the origin of the slow kinetics for the hydrogen evolution reaction in alk. media.
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      Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 1256– 1260,  DOI: 10.1126/science.1211934
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      Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces
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      Science (Washington, DC, United States) (2011), 334 (6060), 1256-1260CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Improving the sluggish kinetics for the electrochem. redn. of H2O to H2 in alk. environments is one key to reducing the high overpotentials and assocd. energy losses in H2O-alkali and chlor-alkali electrolyzers. A controlled arrangement of nanometer-scale Ni(OH)2 clusters on Pt electrode surfaces manifests a factor of 8 activity increase in catalyzing the H evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissocn. of H2O and the prodn. of H intermediates that then adsorbed on the nearby Pt surfaces and recombined into H2. The generation of these H intermediates could be further enhanced via Li+-induced destabilization of the HO-H bond, resulting in a factor of 10 total increase in activity.
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      El-Aziz, A. M.; Kibler, L. A.; Kolb, D. M. The potentials of zero charge of Pd(111) and thin Pd overlayers on Au(111). Electrochem. Commun. 2002, 4, 535,  DOI: 10.1016/S1388-2481(02)00362-4
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      The potential of zero charge (pzc) of Pd(1 1 1) was detd. in dil. NaF solns. by measuring the Gouy-Chapman min. of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of -0.12 V vs. SCE was found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) also were detd. While the pzc of the 1st, pseudomorphic Pd layer on Au(1 1 1) is -0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/H2O interface is estd.
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      The double layer capacitance vs. potential, Cdl(E), curve of Pt(111) electrodes in aq. KClO4 and NaF solns. exhibits a max. at about 0.1 V vs SCE. Since with lowered solute concns. no Gouy-Chapman min. can be found in Cdl(E), the peak is not related to the potential of zero free charge.
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      Surface reconstruction in electrochemistry: gold(100)-(5 x 20), gold(111)-(1 x 23) and gold(110)-(1 x 2)
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      Au (100), Au (111) and Au (110) electrodes with reconstructed surfaces of the type (5 × 20), (1 × 23) and (1 × 2), resp., were prepd. by flame treatment and their properties investigated in various electrolytes by electrochem. and optical methods. The reconstructed (100) and (111) surfaces are found to be stable only in a potential range where no specific adsorption occurs. The Au (100)-(5 × 20) surface has optical properties which are distinctly different from those of the unreconstructed Au (100)-(1 × 1). This difference was used to monitor by in situ spectroscopy the adsorbate-induced (5 × 20) → (1 × 1) transition, in order to obtain information on the transition kinetics. For a certain fraction of the surface, electrochem. induced reconstruction, (1 × 1) → (5 × 20) and (1 × 1) → (1 × 23), was obsd. for Au (100) and Au (111). The 0-charge potentials of the reconstructed surfaces were detd. and compared with those of the unreconstructed ones.
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      Differential capacitance-potential curves for a (111) face of Ag in contact with aq. solns. of the electrolytes NaF, NaClO4 and KPF6 were analyzed. From the dependence of the potential of the capacity min. on electrolyte concn., it is found that the ionic specific adsorption decreases as: ClO4->F->PF6-≥O. The contributions of the interaction terms involved in the specific adsorption are discussed, taking into account results obtained previously for the (110) and (100) faces, and those for Hg. In particular, it is proposed that the stronger water-metal interactions for Ag than those for Hg could be responsible for the absence of specific adsorption at the potential of zero charge (pzc) for the PF6- ion on the three low-index faces. The potential of the capacity min. can therefore be identified with the pzc. The pzc for the Ag (111)/aq. electrolyte interface without specific adsorption is: -0.695±0.005 V (SCE). The elec. double-layer model in the absence of specific adsorption accounts for the capacitance-potential curves at different concns. for the Ag (111)/KPFl system in most of the electrode charge range, taking into account only the superficial roughness. In contrast, application of this model round the pzc requires a correction of the measured capacity for the cryst. heterogeneity due to surface defects. Ests. for the water-Ag interactions are proposed for the (110), (100) and (111) faces. Ag (111) would be almost as hydrophilic as Ga.
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      Hussain, G.; Pérez-Martínez, L.; Le, J.-B.; Papasizza, M.; Cabello, G.; Cheng, J.; Cuesta, A. How cations determine the interfacial potential profile: Relevance for the CO2 reduction reaction. Electrochim. Acta 2019, 327, 135055,  DOI: 10.1016/j.electacta.2019.135055
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      How cations determine the interfacial potential profile: Relevance for the CO2 reduction reaction
      Hussain, Ghulam; Perez-Martinez, Laura; Le, Jia-Bo; Papasizza, Marco; Cabello, Gema; Cheng, Jun; Cuesta, Angel
      Electrochimica Acta (2019), 327 (), 135055CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The strong effect of the electrolyte cation on the activity and selectivity of the CO2 redn. reaction (CO2RR) can only be understood and controlled if the cation's effect on the interfacial potential distribution is known. Using CO (the key intermediate in the CO2RR) adsorbed on Pt as a probe mol., and combining IR spectroscopy, capacitance measurements and ab initio mol. dynamics, the cation size dets. the location of the outer Helmholtz plane, whereby smaller cations increase not just the polarization but, most importantly, the polarizability of adsorbed CO (COad) and the accumulation of electronic d. on the O atom of COad. This strongly affects its adsorption energy, the degree of H bonding of interfacial H2O to COad and the degree of polarization of H2O mols. in the cation's solvation shell, all of which can deeply affect the subsequent steps of the CO2RR.
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      Chen, X.; McCrum, I. T.; Schwarz, K. A.; Janik, M. J.; Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chem., Int. Ed. 2017, 56, 15025– 15029,  DOI: 10.1002/anie.201709455
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      Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode
      Chen, Xiaoting; McCrum, Ian T.; Schwarz, Kathleen A.; Janik, Michael J.; Koper, Marc T. M.
      Angewandte Chemie, International Edition (2017), 56 (47), 15025-15029CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The successful deployment of advanced energy-conversion systems depends critically on the authors' understanding of the fundamental interactions of the key adsorbed intermediates (H *H and hydroxyl *OH) at electrified metal-aq. electrolyte interfaces. The effect of alkali metal cations (Li+, Na+, K+, Cs+) on the non-Nernstian pH shift of the step-related voltammetric peak of the Pt(553) electrode was studied over a wide pH window (1 to 13) by exptl. and computational methods. The co-adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a pos. shift of the potential of the step-related peak on Pt(553). D. functional calcns. explain the observations on the identity and concn. of alkali cations on the non-Nernstian pH shift, and demonstrate that cation-hydroxyl co-adsorption causes the apparent pH dependence of H adsorption in the step sites of Pt electrodes.
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      Li, J.; Li, X.; Gunathunge, C. M.; Waegele, M. M. Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 9220– 9229,  DOI: 10.1073/pnas.1900761116
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      Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction
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      Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (19), 9220-9229CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liq. side of the electrocatalytic interface. The electrocatalytic redn. of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liq. reaction environment poses a formidable exptl. challenge. As a result, the mol. origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examd. the chem. and electrostatic interactions of surface-adsorbed CO with its liq. reaction environment. Using a series of quaternary alkyl ammonium cations (methyl4N + , ethyl4N + , propyl4N + , and butyl4N + ), we systematically tuned the properties of this environment. With differential electrochem. mass spectrometry (DEMS), we show that ethylene is produced in the presence of methyl4N + and ethyl4N + cations, whereas this product is not synthesized in propyl4N + - and butyl4N + -contg. electrolytes. Surface-enhanced IR absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial elec. field is too small to account for the obsd. changes in selectivity. However, SEIRAS shows that an intermol. interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a crit. mol.-level insight into how interactions of surface species with the liq. reaction environment control the selectivity of this complex electrocatalytic process.
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      A method is proposed for the detn. of the distance of closest approach of a model solvent mol. to the surface of a jellium-like metal. The mol.-metal potential consists of a Van der Waals attraction and a repulsive part obtained from the metal electron d. and a model for the electron mol. pseudopotential by a perturbative approach. The statistical mechanics for the soln. side is solved by replacing the external potential due to the metal by an equiv. hard wall, whose position is detd. self-consistently with the metal electron d. The method is illustrated in the case of a Ag electrode. The contribution to the capacitance directly related to the distance of closest approach and the metal electrostatic potential drop were investigated for 2 different values of the strength of the pseudopotential. Their dependence upon the crystallog. orientation for the 3 main faces of Ag is discussed.
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      Gaiduk, A. P.; Zhang, C.; Gygi, F.; Galli, G. Structural and electronic properties of aqueous NaCl solutions from ab initio molecular dynamics simulations with hybrid density functionals. Chem. Phys. Lett. 2014, 604, 89,  DOI: 10.1016/j.cplett.2014.04.037
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      We present a study of a dil. soln. (1 M) of NaCl in water, carried out using ab initio mol. dynamics with semilocal and hybrid functionals. We showed that the structural and electronic properties of the solute and the solvent are the same as those obtained in the infinite diln. limit, i.e. for aq. ions in the presence of a uniform compensating background. Compared to semilocal functionals, simulations with hybrid functionals yield a less structured soln. with a smaller no. of hydrogen bonds and a larger coordination no. for the Cl- anion. In addn., hybrid functionals predict qual. correct positions of the energy levels of the ions with respect to the valence band of water.
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      The partial charge transfer
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      In this review, the problems are examd. related to the chemisorption of mols. on electrodes, where the adsorbate may possibly share its electrons with the metal. The "partial charge transfer" between adsorbate and electrode cannot be measured, since the division of the bonding electrons into parts pertaining to the adsorbate and to the electrode is arbitrary; at most, it can be tentatively estd. on the basis of model considerations. This point will be clearly demonstrated on the basis of simple thermodn. considerations. A thermodynamically significant and exptl. measurable quantity that has often been conceptually related to partial charge transfer is the "electrosorption valency". Discussed are methods by which it can be measured, both when only a single species is adsorbed and when its adsorption is accompanied by coadsorption or competitive adsorption with other species. A simple unified model picture will be adopted to illustrate the extra-thermodn. partial charge transfer coeff. for low surface coverages of ionic and neutral adsorbates and for compact chemisorbed monolayers both in soln. and in vacuum. The relation between partial charge transfer coeff. and the dipole moment of the chemisorption bond will be clarified and the est. of this bond in vacuum by work function or outer potential measurements will be described.
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    • Electrode potential calculation method; excess charge distributions at the Pt(111)-Had/water interface; models for Pt(111)-Had surface; time accumulative Fermi energies and electrostatic potentials; solvation structures of Na+; projected density of states; dipole–dipole autocorrelation of the interface water; plots of Helmholtz capacitance as a function of surface charge; verification of simulation time and the constraint of the O–H bonds in water (PDF)


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