Toward CO2 Electroreduction under Controlled Mass Flow Conditions: A Combined Inverted RDE and Gas Chromatography Approach
- Pavel Moreno-García*Pavel Moreno-García*Email: [email protected]Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, SwitzerlandMore by Pavel Moreno-García
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- Noémi KovácsNoémi KovácsDepartment of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, SwitzerlandDepartment of Physical Chemistry, Eötvös Loránd University of Budapest, Pázmány Péter sétány 1/A, H−1117 Budapest, HungaryMore by Noémi Kovács
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- Vitali GrozovskiVitali GrozovskiDepartment of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, SwitzerlandMore by Vitali Grozovski
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- María de Jesús Gálvez-VázquezMaría de Jesús Gálvez-VázquezDepartment of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, SwitzerlandMore by María de Jesús Gálvez-Vázquez
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- Soma Vesztergom*Soma Vesztergom*Email: [email protected]Department of Physical Chemistry, Eötvös Loránd University of Budapest, Pázmány Péter sétány 1/A, H−1117 Budapest, HungaryMore by Soma Vesztergom
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- Peter BroekmannPeter BroekmannDepartment of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, SwitzerlandMore by Peter Broekmann
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Abstract
The use of rotating disk electrodes (RDEs) is probably the most convenient way of studying simple electrode reactions under well-defined transport conditions. Standard RDEs become, however, less expedient when the studied electrode process is a complex one, leading to the formation of various reaction products. In these cases, the accurate detection and quantification of the formed products are desirable. If the formed products are gaseous, then the usual way of quantifying them is the use of online gas chromatography (GC), a method that is not compatible with open RDE cells. In order to overcome these difficulties, we present here a sophisticated inverted RDE (iRDE) cell design. The design combines various advantages: it is amenable to the same mathematical treatment as standard (downward-facing) RDEs; it can be operated airtight and coupled to online GC; and due to its upward-facing design, the electrode surface is less prone to blockage by any formed gas bubbles. The iRDE&GC design is tested using simple model reactions and is demonstratively used for studying the electrochemical reduction of CO2, accompanied by parasitic hydrogen evolution, on a silver electrode.
Easy to construct with a variety of electrode materials and amenable to rigorous theoretical treatment, the rotating disk electrode (RDE) is the most widely employed hydrodynamic method used for the investigation of electrode processes. (1) In an RDE system, stationary concentration profiles are attained rather quickly and steady-state current/potential characteristics can be measured. The rate of mass transfer in an RDE configuration is typically higher than that of diffusion alone in quiescent systems, which makes RDEs useful for electrocatalysis research. By using RDEs, the supply of reactants to the electrode surface can be controlled, enabling the distinction between mass transport and kinetic limitations, e.g., by means of analysis based on the Koutecký–Levich equation. (1,2)
The interpretation of RDE measurements becomes less straightforward when the studied reaction has complex kinetics (leading to the formation of more than one reaction product) or if it is accompanied by other (parasitic) side reactions. Examples include the deposition of base metals, (3,4) almost inevitably accompanied by hydrogen evolution; the chloralkali process competing with oxygen evolution; (5) and the electrochemical reduction of either carbon dioxide (6−11) or nitrogen. (12,13) These latter processes, apart from being accompanied by hydrogen evolution, can themselves lead to the formation of various products. In these cases, the accurate (quantitative) determination of the formed products is a prerequisite of any valid RDE analysis. Since many of the products are gaseous, online gas chromatographic (GC) headspace analysis (14) seems to be an obvious choice; however, applying GC in a traditional (i.e., open-to-air) RDE cell is not straightforward.
To the best of our knowledge, there exist only a few designs in the literature for hermetically closed RDE cells that could be applicable to, although they were not applied to, online GC detection. (15,16) These designs utilized magnetic coupling in order to transfer the momentum necessary to rotate an electrode through the cell wall. The approach successfully circumvents the problem of sealing between the rotating shaft and the stagnant cell wall: it may not ensure, however, a full transfer of momentum under high-friction conditions.
A possible alternative to magnetic coupling (15,16) is offered by the use of direct momentum transfer through a gland seal. While airtight seals are difficult to design, liquid-tight seals are relatively easy to manufacture, with the only requirement that the RDE is brought into contact with the electrolyte solution by insertion through the cell bottom. In this case, the headspace itself can be tightly connected to a GC instrument, and the top of the cell also allows an insertion point for a (fixed) reference electrode. As in the case of other “H-type” cell designs, (14) the counter electrode compartment must be separated, by a membrane, from the inverted RDE (iRDE) compartment so that any counter electrode products are excluded from the GC analysis.
Apart from the direct transfer of momentum, a further advantage of using an iRDE design for GC analysis arises from the fact that in iRDEs the electrode is upward-facing. This has no adverse effect on the validity of the hydrodynamic calculations necessary for the mathematical description of transport (17) (i.e., simple expressions such as the Levich and Koutecký–Levich equations remain applicable). It aids, however, in the removal of bubbles that often cause electrode surface blockage in standard (downward-facing) RDE designs.
Some iRDE designs have already been published, (18−22) and some of these were operated in airtight cells. Hermetic cells were, however, used only in order to ensure oxygen-free conditions, and so far, no GC-based product detection was carried out in an iRDE configuration. Furthermore, the previous iRDE designs were validated only for simple model reactions (namely, the ferro-/ferricyanide redox system (18−20)), and either no validation for gas-evolving reactions was used or the interpretation of these measurements was not conclusive. (21,22)
This contribution aims to fill the gap of combining an iRDE configuration with GC-based product detection. The constructed iRDE&GC design is validated by means of simple test measurements. We show that the limiting currents measured in a ferro-/ferricyanide test system and those measured for the hydrogen evolution reaction (HER) in mildly acidic solutions are in perfect agreement with the Levich equation. In the case of the latter reaction, we are also able to detect 100% of the formed hydrogen by means of GC.
Subsequently, we employ the developed iRDE&GC system, fitted with a polycrystalline Ag electrode, for a demonstrative test measurement on the electrochemical CO2 reduction reaction (CO2RR). The iRDE&GC system is a powerful tool in this case for a combined product distribution–reaction kinetics study. We show that by using the iRDE&GC hyphenation, partial currents of CO2RR and HER can be distinguished and made subject, individually, to kinetic analysis.
Experimental Section
ARTICLE SECTIONS
Cell and iRDE Design
The hermetically tight iRDE cell (Figure 1) consists of two separable compartments made of round borosilicate glass flasks (nominal volume: 80 cm3). Necks for inserting the reference and counter electrodes and the gas inlets and outlets are mounted on the flasks and are equipped with custom-winding poly(tetrafluoroethylene) (PTFE) caps and O-ring fittings, allowing gastight operation. A Nafion ion-exchange membrane (Nafion 117, Sigma-Aldrich) connects the working and counter electrode compartments. The housing of the iRDE, machined from polyoxymethylene (POM), provides rigid connection to a standard rotator unit (AFMSRCE, Pine) and bears a cone joint to fit to the outer socket of the electrochemical cell (size 29/32, ground glass joint standard). Rotation is transferred by a modified RDE shaft that fits into the AFMSRCE rotator. It bears two ceramic sleeves, passing through radial shaft seals.
Figure 1
Figure 1. H-type cell equipped with an iRDE. Parts of the design: (a) glass cell body, (b) purging gas inlet (outlet not shown), (c) reference electrode inlet, (d) membrane and sealing junction, (e) counter electrode inlet, (f) PCTFE iRDE tip with an electrode embedded, (g) spring contact node, (h) PTFE tip groove for the O-ring fitting, (i) radial shaft seals (upper and lower), (j) ball bearing, (k) pressurized gland chamber, (
), ceramic fittings (upper and lower), (m) pressurizer gas inlet, (n) POM housing, and (o) rotating shaft (stainless steel).
As a safety component, the design also includes a pressure chamber: a single POM tube confined between the radial shaft seals, fed by N2 or Ar (99.999%, Carbagas) through a lateral pressurizing gas inlet. Through it, a constant overpressure of 0.2–0.4 bar is maintained in the gland seal, preventing the leakage of electrolyte solution into the seal. The exact coaxial position of the rotating shaft is maintained by a ball bearing, situated in the upper part of the pressure chamber. The radial shaft seals are made of high-quality Viton rubber and operate oil- and grease-free. The upper seal comes in contact with the solution in the working electrode compartment.
The RDE working electrodes are pressed into a laboratory-made poly(chlorotrifluoroethylene) (PCTFE) holder, fitted into the upper ceramic sleeve. Tightness is maintained by an O-ring (not shown in Figure 1), placed in the groove of the PTFE holder. A spring contact in the shaft provides electrical connection to the working electrode. The whole system is easy to maintain, and the radial shaft seals can be exchanged once they are worn out.
Gas Chromatography
Any gaseous products generated in the cell can be detected by connecting the purging gas outlet to a GC analyzer (SRI Instruments Multigas Analyzer no. 3). The continuous flow of a carrier gas (usually Ar or CO2, both 99.999% pure, Carbagas) through the electrolysis cell carries volatile reaction products from the headspace into the sampling loops of the gas chromatograph. The partial current Ii, corresponding to the formation of a gaseous product i, can be calculated as (14)
where xi denotes the mole fraction of the products, determined by GC using an independent calibration standard gas (Carbagas), ni is the number of electrons involved in the reduction reaction to form a particular product (n = 2 for both CO and H2 formation), F = 96 485.3 C mol–1 is Faraday’s constant, and vm is the molar gas flow rate measured with a universal flowmeter (7000 GC flowmeter, Ellutia) at the exit of the electrochemical cell.
(1)
The Faradaic efficiency (FE) of a given reaction product can be determined by dividing the respective partial current, determined from eq 1, by the total current, measured electrochemically. During the operation of the iRDE&GC cell, aliquots are analyzed in intervals of 7 to 20 min during steady-state electrolyses. For the measurements reported, we managed to detect, within the range of error, 100% of the products because no soluble products were formed. The latter was also checked by a postmortem analysis of the electrolyte using ion exchange chromatography (Metrohm).
Electrochemistry
Electrochemical measurements were performed at room temperature with a potentiostat/galvanostat system (Metrohm Autolab 128N) in a three-electrode configuration. A “leakless” Ag|AgCl|3 mol dm–3 KCl reference electrode (Pine) and a Pt foil counter electrode (0.8 cm × 2.0 cm, Goodfellow) were used. The glassy carbon, Pt, and Ag working electrodes were 5-mm-diameter disk electrodes purchased from Pine. When reporting current densities (j) instead of the current (I), we used the geometric surface area (0.196 cm2) for normalization.
All electrodes were polished to a mirror finish with 0.05 μm alumina particles (Micropolish, Buehler) on a polishing cloth (Buehler) and thoroughly rinsed with Milli-Q water (18.2 M cm, TOC ≤ 5 ppb, Millipore) prior to electrochemical measurements.
All of the reported potentiostatic or potentiodynamic measurement results were obtained by using automatic IR compensation, following an impedance-spectroscopy-based determination of the cell resistance.
Chemicals
All solutions were prepared with as-received chemicals and Milli-Q water. Potassium ferrocyanide (K4[Fe(CN)6], ≥99.5), potassium ferricyanide (K3[Fe(CN)6], ≥99%), and potassium chloride (KCl, ≥99.5%) were purchased from Fluka. Potassium sulfate (K2SO4, ≥99%), sulfuric acid (96% H2SO4, suprapure), sodium perchlorate (NaClO4, 99.99%), and perchloric acid (70% HClO4, suprapure) were purchased from Merck.
Results and Discussion
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Validation of the iRDE Design in the Absence of Gas Evolution
In order to check the hydrodynamic performance of the iRDE design, we measured linear sweep voltammograms (LSVs, sweep rate 20 mV s–1) on a glassy carbon iRDE in a 1 mol dm–3 KCl electrolyte solution containing the ferro-/ferricyanide redox couple in equimolar (5 mmol dm–3) concentrations. The voltammograms, shown in Figure 2, were recorded at seven different rotational rates, distributed equidistantly on a square-root scale between 100 and 1600 min–1. Well-defined limiting currents were reached for both the oxidation and reduction reactions, showing an excellent linear dependence (R2 = 0.9996, with a zero offset) on the square root of the rotational rate. This dependence was analyzed by using the Levich equation (1)
where n = 1 is the number of electrons transferred, D denotes the diffusion coefficient and c denotes the bulk concentration of the reacting species, ν = 0.008917 cm2 s–1 is the kinematic viscosity of water at 25 °C, and ω is the angular frequency of rotation.
(2)
Figure 2
Figure 2. Validation of the hydrodynamic performance of the iRDE setup with a non-gas-evolving reaction. Limiting currents (both anodic and cathodic) measured by linear sweep voltammetry on a glassy carbon iRDE in a solution containing the K4[Fe(CN)6]/K3[Fe(CN)6] redox couple in equimolar concentrations scale linearly with the square root of the rotation rate. Green and red dashed lines show the cathodic and anodic limiting currents, respectively, predicted by eq 2 and the diffusion coefficient values mentioned in the text, at the given rotation rates. Sweep rate 20 mV s–1.
The analysis yielded the diffusion coefficients, (8.25 ± 0.25) × 10–6 and (9.42 ± 0.17) × 10–6 cm2 s–1, for the [Fe(CN)6]4– and [Fe(CN)6]3– ions, respectively. These are in good agreement with the literature data, (23) confirming that in this simple redox system (where no gas evolution is taking place) the hydrodynamic behavior of the iRDE is the same as that of normal RDEs.
Validation of the iRDE Design for a Gas-Evolving Reaction
We used the hydrogen evolution reaction (HER) as a model reaction in order to study the influence of gas formation on the hydrodynamic properties of the iRDE system.
For these measurements, a 0.1 mol dm–3 NaClO4 electrolyte solution was prepared, the pH of which was adjusted to the value of 2.56 (checked with a pH meter) by the addition of a small amount of perchloric acid. Linear sweep voltammograms (sweep rate 50 mV s–1) recorded on a Pt iRDE immersed in this solution exhibited a well-defined diffusion-limited plateau, as shown in Figure 3. Although in the case of this gas-evolving reaction the noise of the current signal was considerably higher compared to that of the previous case, the limiting currents did show a linear dependence on the square root of the rotational rate, and an analysis based on the Levich equation (eq 2) yielded a value of (8.79 ± 0.12) × 10–5 cm2 s–1 for the diffusion coefficient of H+ ions, again matching previous reports well.
Figure 3
Figure 3. Validation of the hydrodynamic performance of the iRDE setup with HER, a gas-evolving reaction. Limiting currents measured by linear sweep voltammetry (sweep rate 50 mV s–1) on a Pt iRDE in a pH 2.56 HClO4/NaClO4 solution scale linearly with the square root of the rotational rate. The dashed green lines are limiting current predictions of the Levich equation (eq 2), calculated using the diffusion coefficient value mentioned in the text at all of the given rotational rates.
At this point we note that the aforementioned diffusion coefficient value was obtained by assuming that cH+ = 10–pH mol dm–3 in eq 2. In other words, we considered a unity relative activity coefficient of H+ ions, and we assumed that the limiting current is determined solely by the concentration of free H+ in the solution.
While both of the above assumptions are fairly valid for a HClO4/NaClO4 electrolyte solution, they do not hold for more complex (i.e., buffered) systems. In K2SO4 solutions acidified with sulfuric acid, for example, we measured voltammograms that exhibited higher than expected limiting currents, confirming the previous experimental results of Nierhaus et al. (22) obtained with another iRDE design. In ref (22), this peculiar current enhancement was explained by an “extra stirring” of the electrolyte due to the produced H2 bubbles. We believe, however, that there exists an alternative, more straightforward explanation, namely, that the current increase is due to the buffered nature of the H2SO4/K2SO4 system that contains not only H+ but also HSO4– ions acting as a proton source. (10,24,25)
The above argument is supported by Figure 4, where three linear sweep voltammograms are compared. These LSVs were recorded in electrolyte (either NaClO4 or K2SO4) solutions that were acidified to a pH value of about 2.5 by the addition of small volumes of the native, concentrated acid (either HClO4 or H2SO4). Although the bulk pH and also other parameters (such as the sweep and rotational rates) of the recorded LSVs are essentially the same, Figure 4 reveals a pronounced difference in the limiting currents. We ascribe this 2- to 6-fold increase in the limiting current (depending on the sulfate concentration) to the buffering capacity of HSO4– ions.
Figure 4
Figure 4. Although the bulk pH is about the same, LSVs obtained in different electrolyte solutions exhibit varying limiting currents for H+ reduction. The sweep rate is 50 mV s–1, the rotational rate is 900 min–1, and the electrolyte compositions and the pH are shown in the graph. The given pH was set by adding a few drops of the respective concentrated acid to the electrolyte solution.
Validation of the iRDE&GC Hyphenation
The hydrogen evolution reaction, leading to the formation of a single reaction product (H2), is an ideal platform for validating the hyphenation of the iRDE design with GC detection. In order to achieve this, we applied a continuous Ar flow to the cell and led the gas in the headspace to the sampling loop of a gas chromatograph. After some 30 min of electrolysis, practically independent of the applied rotational rate, we were able to detect 100 ± 5% of the formed hydrogen gas. The latency can be explained by the gaseous product requiring a certain time to reach and fill up the sampling loop.
Figure 5a shows electrochemically measured and, based on eq 1, chromatographically determined currents of HER measured in a pH 3.75 HClO4/ NaClO4 solution at potentials in the limiting current region. Note that both currents, although matching each other relatively well, decay significantly and drop by about 28% of their initial values over the approximately 80 min time frame of the electrolysis.
Figure 5
Figure 5. Results of long-term electrolysis (hydrogen evolution from a HClO4/NaClO4 electrolyte solution) measured by iRDE&GC. Currents measured electrochemically (full black curve) and chromatographically (calculated using eq 1, dots) at E = −625 mV vs Ag|AgCl are shown in (a). A slow drift (decay) over time can be observed as a result of the electrolysis becoming exhaustive. Values of pH measured before and after the electrolysis, along with limiting currents estimated using the respective H+ concentrations and the diffusion coefficient of 8.79 × 10–5 cm2 s–1 are shown by the dashed horizontal lines. This pH change is in alignment with the shifting of the LSV plateaus shown in (b) and also corresponds to the estimated H+ concentration change calculated by taking into account the charge of the electrolysis, shown as the hatched area in (a), and a cell volume of 87 cm3. The applied rotational rate was 1600 min–1, and the linear sweep voltammograms were recorded at a sweep rate of 50 mV s–1.
This current drop, seen only in H-type iRDE cells equipped with a membrane, can be explained by a permanent pH change (also in the bulk of the solution), caused by the long-lasting electrolysis becoming partially exhaustive. Indeed, as shown by Figure 5b, the linear sweep voltammograms measured before and after electrolysis exhibit different limiting current values, and a corresponding pH change can also be measured directly with a meter. For the experiment shown in Figure 5b, the pH increased from a value of 3.76 to 3.93. This change scales well with the decrease in the limiting current (measured before and after the electrolysis) and is also in agreement with the H+ concentration change that the charge of the electrolysis is expected to cause in the total electrolyte volume of the working compartment (about 87 cm3 for this experiment).
It should further be noted with respect to the iRDE&GC hyphenation that for this system to deliver correct “chromatographic currents” it is crucial to make sure that all of the gas bubbles formed during the electrolysis reach the headspace and do not remain adhered to the tip surface. This can be assured by directing the purging gas inlet tube as close as possible to the tip (however, not directly to the electrode) surface. Although the approach results in an elevated noise level of the electrochemical measurement, it has only a small effect on the hydrodynamics (cf. Figure 3, where this configuration was already used; the measured limiting currents, however, did remain well-defined).
Using the iRDE&GC System to Investigate the CO2RR
In a further demonstrative experiment, we attempt to use the developed iRDE&GC system to study the electroreduction of carbon dioxide as it occurs on a silver iRDE in a 0.1 mol dm–3 K2SO4 solution saturated with CO2 (pH ∼4.17). We chose this specific electrolyte composition in order to get well-defined potential ranges where the predominant reduction product is either H2 or CO.
Linear sweep surveys, shown in Figure 6a, revealed that HER goes on and becomes diffusion-limited in the potential range between −1.2 and −1.4 V vs Ag|AgCl: this range is displayed in detail in Figure 6b. It can be assumed that in this potential range the sole electrode reaction taking place is that described by eq 3,
where the reactant H+ ions are present either in the form of free H+ or in the form of HSO4–.
(3)
Figure 6
Figure 6. (a) Survey voltammograms recorded at a sweep rate of 50 mV s–1 on a Ag iRDE in a 0.1 mol dm–3 K2SO4 solution saturated with CO2 (pH ∼4.17). Cathodic currents increase with increasing rotational rates (100, 225, 400, 625, 900, 1225, and 1600 min–1). (b) Hydrogen evolution attains a limiting current at lower overpotentials. At higher cathodic overpotentials, the electroreduction of CO2 competes with the reduction of water. In this potential range, CO is the primary product of electrolysis.
At potentials more cathodic than −1.6 V vs Ag|AgCl, we see the onset of another process: the electroreduction of CO2 that yields CO as the primary product
Although the voltammograms in Figure 6a exhibit no clear plateaus for this process, they do confirm the limiting role of transport because the currents measured below −1.6 V clearly depend, more or less linearly, on the square root of the applied rotational rate. This could be caused by a concentration limitation of either CO2 or H+ or both in the system under study: note that H+ also appears as a reactant in eq 4.
(4)
It is very probable that at extremely cathodic potentials (E < −1.8 V vs Ag|AgCl) a third process, namely, the electrolysis of water molecules, should also occur
again favoring the production of H2 over that of CO. With respect to this process, it is important to note that its exact onset potential can be heavily affected by effects such as the buffer (HSO4–) concentration of the solution (25) and even the choice of the polishing material. Polishing with alumina particles, for example, was recently shown to shift the onset of water reduction to less cathodic potentials. (26) (More about the effect of alumina polishing can be read in the Supporting Information.)
(5)
The above-described scenario can be confirmed and even quantified by applying iRDE&GC hyphenation. By carrying out galvanostatic electrolyses of CO2-saturated K2SO4 solutions at a fixed rotational rate (625 min–1), we applied GC detection in order to determine the product distribution of the cathode process. In agreement with literature data, (27,28) the only detectable products were H2 and CO, and within the first 40 min of the electrolyses, a 100% detection efficiency was practically achieved. After each electrolysis, the current was switched off and the electrolyte solution was replaced in order to avoid the accumulation of the exhaustion effects described in the previous section.
The recorded Faradaic efficiency vs current data were subjected to a numerical interpolation in order to determine the product distribution shown in Figure 7a. This figure allows a distinction among three current density regions with remarkably different product yields.
Figure 7
Figure 7. IRDE&GC system applied to the study of the electrolysis of a 0.1 mol dm–3 K2SO4 solution saturated with CO2 (pH ∼4.17). Applied rotational rate 625 min–1. (a) Faradaic efficiencies of H2 and CO formation are determined chromatographically (dots) and are interpolated using an arbitrary function (exponential decay superimposed on a straight line, black curve). (b) This interpolation allows the separation of the recorded LSVs: the total current and the partial currents of H2 and CO production are shown as black, red, and green curves, respectively. An LSV measured in a CO2-free (Ar-saturated) K2SO4 solution, the pH of which was set to 4.15 by direct H2SO4 addition, is shown as a reference (dashed gray curve).
First, at currents not exceeding the limiting current of HER from acidic media (i.e., the maximal current that can be supplied by the reaction in eq 3), it seems that H2 is the primary product of electrolysis, and there is very little, if any, detectable CO. At current densities exceeding this limiting current (about 0.1 mA for the system shown in Figure 7), there is a marked increase in CO productivity due to the onset of CO2RR, as described by eq 4. The growing CO productivity trend continues up to the point where the current becomes approximately an order of magnitude higher than the limiting current of (acidic) HER, at which point the Faradaic efficiency of H2 evolution will again increase. This is due to the onset of the direct reduction of water molecules according to eq 5.
The two aforementioned processes (CO2RR and the reduction of water molecules) occur concomitantly, and the LSV recorded in the system (black curve in Figure 7b) exhibits no clear limiting current plateau for CO2RR. Nevertheless, the interpolation shown in Figure 7a does create some means to separate the individual contributions of CO production (electroreduction) and H2 formation (H+ or water reduction) to the total current.
At each and every point on the LSV shown by the black curve in Figure 7b, we can calculate, using the interpolation of Figure 7a, the partial currents that correspond to CO and to H2 formation. These partial currents are plotted as green and red curves, respectively, in Figure 7b. The sum of the partial currents, by definition, equals the total current shown by the black curve.
Identifying the two partial currents, as shown in Figure 7b, allows for a better understanding of the kinetics of CO2RR: a reaction that is inevitably coupled to hydrogen evolution. The figure reveals that at potentials less cathodic than the onset potential of CO2RR, all measured currents can be attributed to hydrogen evolution, and this section of the LSV is identical to that measured in a CO2-free electrolyte solution of the same pH (dashed gray curve shown as a reference in Figure 7b).
The onset of CO2 reduction allows the cathodic current to increase beyond the limiting current of (acidic) hydrogen evolution at E < −1.5 V vs Ag|AgCl. This current increase, at least initially, can be fully ascribed to CO2 electroreduction: the reduction of water molecules only seems to commence at more negative potentials, E < −1.75 V. Note that the apparent onset potential of water reduction is about 150 mV more negative in the CO2-saturated solution than in the CO2-free reference system and that the partial current of H2 production does not rise monotonically with the applied potential. It seems more than plausible to assume that this peculiar feature, revealed by the iRDE&GC hyphenation, can be explained by the H+ consumption of CO2RR, as described by eq 4. Due to the autoprotolysis equilibrium of water, the H+ consumption of CO2RR leads to an increase in near-surface OH– concentrations, and as OH– ions appear as a product in eq 5, this shifts the onset of water reduction toward more cathodic values.
Conclusions
ARTICLE SECTIONS
We presented the design of a custom-made, hermetically sealed inverted rotating disk electrode coupled to a gas chromatographic detection system. The developed iRDE&GC system is suitable for electrochemical kinetic studies with the simultaneous analysis of the formed (gaseous) reaction products. The performance of the iRDE&GC hyphenation was evaluated using the ferro-/ferricyanide redox system and the hydrogen evolution reaction as test settings.
Apart from having conducted a successful validation of the iRDE&GC system, we pointed out two major caveats of the design. Probably the most important condition of using the iRDE&GC hyphenation is related to an inherent property of any GC-based headspace analyses in electrochemistry, namely, that the electrolyses must hold long enough that the reaction products can accumulate in the sampling loop in a sufficient amount in order to facilitate 100% detection. Even in the case of an upward-facing iRDE system it seems unavoidable to carefully orient the purging gas flow to remove any formed gas bubbles that could otherwise remain adhered to the electrode tip, resulting in detection deficiencies. Special care must be taken in this situation so that the purging gas flow does not interfere with the convective transport of the rotating disk.
Another important point that deserves emphasis is related to the transport conditions of the iRDE cell. In this hydrodynamic configuration, convection allows for a much higher rate of transport, compared to that in other quiescent systems. Although it is usually assumed that on rotating electrodes stationary current/potential characteristics can be attained, this condition may not hold for long-lasting experiments during which the electrolysis becomes at least partially exhaustive. This second limitation may, however, be overcome if the electrolyte solution is replaced from time to time or when a continuous flow of electrolyte guarantees that no permanent bulk concentration changes can be caused by the electrolysis.
We have further shown, by means of one demonstrative experiment, that the developed iRDE&GC hyphenation can have great potential in understanding the kinetics of technologically relevant electrochemical processes. We demonstrated that in the case of carbon dioxide reduction on silver electrodes the iRDE&GC setup allows for a resolution of the voltammetric response to individual contributions of actual reduction and hydrogen evolution.
Supporting Information
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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.analchem.9b04999.
Scanning electron micrographs and energy-dispersive X-ray spectra of alumina and diamond-polished silver RDE surfaces (PDF)
Terms & Conditions
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Author Information
ARTICLE SECTIONS
- Pavel Moreno-García - Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, Switzerland;
http://orcid.org/0000-0002-6827-787X;
- Soma Vesztergom - Department of Physical Chemistry, Eötvös Loránd University of Budapest, Pázmány Péter sétány 1/A, H−1117 Budapest, Hungary;http://orcid.org/0000-0001-7052-4553;
- Peter Broekmann - Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH−3012 Bern, Switzerland;http://orcid.org/0000-0002-6287-1042
Acknowledgments
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We kindly acknowledge the efforts dedicated to this project by Mr. Thomas Hübscher and Mr. René Schraner of the mechanical and electronics workshops of the University of Bern. Support by the CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage) is gratefully acknowledged. P.B. acknowledges financial support from the Swiss National Foundation (grant 200020-172507). S.V. acknowledges support from the National Research, Development and Innovation Office of Hungary (NKFIH grant PD124079). M.d.J.G.-V. and N.K. acknowledge the financial support of the Swiss Government Excellence Scholarships for Foreign Scholars (ESKAS).
References
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- 1Vesztergom, S. In Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry; Wandelt, K., Ed.; Elsevier: Amsterdam, 2018; pp 421– 444.Google ScholarThere is no corresponding record for this reference.
- 2Levich, B. The Theory of Concentration Polarisation. Discuss. Faraday Soc. 1947, 1, 37– 49, DOI: 10.1039/df9470100037Google ScholarThere is no corresponding record for this reference.
- 3Kuhn, A. T.; Chan, C. Y. pH Changes at Near-Electrode Surfaces. J. Appl. Electrochem. 1983, 13, 189– 207, DOI: 10.1007/BF00612481Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXhs12itLo%253D&md5=ecf246db110f1c9b0bb1b77078e7c663pH Changes at near-electrode surfacesKuhn, A. T.; Chan, C. Y.Journal of Applied Electrochemistry (1983), 13 (2), 189-207CODEN: JAELBJ; ISSN:0021-891X.Anodic or cathodic processes in which protons are released or consumed, resp., lead to local pH changes at the electrode surface. The phenomenon is important in several branches of metal finishing in corrosion, electroorg. synthesis and other branches of applied electrochem., as well as being a factor which can invalidate mechanistic investigations. The concept of pHs (near-surface pH) is examd., as are the exptl. techniques which were used to measure it. Many of these are inherently weak. Published results are shown to be largely discrepant, both in respect to the magnitude of the effect and even its incidence. Attempts to treat the phenomenon theor. are briefly summarized.
- 4Ying, R. Y. Electrodeposition of Copper-Nickel Alloys from Citrate Solutions on a Rotating Disk Electrode. J. Electrochem. Soc. 1988, 135, 2964– 2971, DOI: 10.1149/1.2095470Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1MXkslWktg%253D%253D&md5=c28ae3b4871624359bafde44ee58de6cElectrodeposition of copper-nickel alloys from citrate solutions on a rotating disk electrode. II. Mathematical modelingYing, Ramona Y.; Ng, Patrick K.; Mao, Z.; White, Ralph E.Journal of the Electrochemical Society (1988), 135 (12), 2964-71CODEN: JESOAN; ISSN:0013-4651.A math. model is developed to simulate the electrodeposition of Cu-Ni alloy from citrate solns. onto a rotating disk electrode under potentiostatic control. The model includes the influence of diffusion, ionic migration, forced convection, and homogeneous equil. The 3 major electrochem. reactions treated in the model are Cu deposition, Ni deposition, and H evolution. Using exptl. parameters when available, the model was fitted and tested against exptl.-obtained results for different bath compns. and operating conditions. The model agreed reasonably well with the exptl. results particularly in the codeposition region, -1.0 to -1.2 V vs. SCE, where H evolution is not the major reaction. Also, the model's predictive capabilities are evaluated.
- 5Vos, J. G.; Koper, M. T. M. Measurement of Competition between Oxygen Evolution and Chlorine Evolution Using Rotating Ring–Disk Electrode Voltammetry. J. Electroanal. Chem. 2018, 819, 260– 268, DOI: 10.1016/j.jelechem.2017.10.058Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhslOisb%252FL&md5=01573b2aec0aa64422dd552b109a5c76Measurement of competition between oxygen evolution and chlorine evolution using rotating ring-disk electrode voltammetryVos, J. G.; Koper, M. T. M.Journal of Electroanalytical Chemistry (2018), 819 (), 260-268CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)Selectivity between Cl evolution and O evolution in aq. media is a phenomenon of central importance in the chlor-alkali process, H2O treatment, and saline H2O splitting, which is an emerging technol. for sustainable energy conversion. An apparent scaling between O vs. Cl evolution was established, making it challenging to carry the two reactions out individually with 100% faradaic efficiency. To aid selectivity detn., the authors developed a new method to quickly measure Cl evolution rates using a conventional RRDE setup. A Pt ring fixed at 0.95 V vs. RHE in pH 0.88 can selectively reduce the Cl2 formed on the disk and this allows precise and flexible data acquisition. Using this method, O evolution and Cl evolution on a glassy C supported IrOx catalyst proceed independently, and the selectivity towards Cl evolution (εCER) rapidly approaches 100% as [Cl-] increases from 0 to 100 mM. The authors' results suggest that on IrOx, O evolution is not suppressed or influenced by the presence of Cl- or by the Cl evolution reaction taking place simultaneously on the surface.
- 6Hall, A. S.; Yoon, Y.; Wuttig, A.; Surendranath, Y. Mesostructure-Induced Selectivity in CO2 Reduction Catalysis. J. Am. Chem. Soc. 2015, 137, 14834– 14837, DOI: 10.1021/jacs.5b08259Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslKlt7rM&md5=f279b110a31a7cdf8d0dffe4e683cbdcMesostructure-Induced Selectivity in CO2 Reduction CatalysisHall, Anthony Shoji; Yoon, Youngmin; Wuttig, Anna; Surendranath, YogeshJournal of the American Chemical Society (2015), 137 (47), 14834-14837CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Gold inverse opal (Au-IO) thin films are active for CO2 redn. to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO2-to-fuels catalysis.
- 7Bandi, A. Electrochemical Reduction of Carbon Dioxide on Conductive Metallic Oxides. J. Electrochem. Soc. 1990, 137, 2157– 2160, DOI: 10.1149/1.2086903Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXls1Wisb4%253D&md5=7ab3a48be2b87c7c83256f43c85915acElectrochemical reduction of carbon dioxide on conductive metallic oxidesBandi, A.Journal of the Electrochemical Society (1990), 137 (7), 2157-60CODEN: JESOAN; ISSN:0013-4651.Faradaic efficiencies and current-voltage characteristics for the redn. of CO2 on various conductive oxide mixts. (RuO2, TiO2, MoO2, Co3O4, Rh2O3) were detd. Product anal. was performed by gas chromatog. Electrodes composed of RuO2 + TiO2 [35 + 65 mol percent (m/o)] and RuO2 + Co3O4 + SnO2 + TiO2 (20 + 10 + 8 + 62 m/o) show high current efficiencies for methanol prodn. when polarized near the equil. potential of H evolution in solns. of 0.2M Na2SO4 (pH 4) satd. with CO2. It can be concluded from the Tafel slopes that the first electron transfer is the rate-limiting step in the redn. of CO2 on the investigated oxides in acidic media.
- 8Aoki, A.; Nogami, G. Rotating Ring–Disk Electrode Study on the Fixation Mechanism of Carbon Dioxide. J. Electrochem. Soc. 1995, 142, 423– 427, DOI: 10.1149/1.2044046Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXjs1KjtL0%253D&md5=c6d040d8105b8be1a044e8ee4920cb9fRotating ring-disk electrode study on the fixation mechanism of carbon dioxideAoki, Akihiko; Nogami, GyoichiJournal of the Electrochemical Society (1995), 142 (2), 423-7CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)Formic acid formed on a SnO2 disk electrode as a redn. product of carbon dioxide was detected at a Pt ring electrode, and the fixation mechanism of carbon dioxide was studied. Org. products such as HCOOH could be detected electrochem. by a rotating ring-disk electrode technique which does not require a long-term electroredn. and offers a much more sensitive method than gas- and liq.-chromatog. do. CO2 was reduced to HCOOH on the SnO2 surface in competition with hydrogen formation: both HCOOH and H2 were formed quite slowly in a few seconds. The rotational speed dependence of the reaction current revealed that a considerably slow electron transfer gave some intermediates, from which HCOOH will be formed under the mass-transfer-limited condition of proton. The formation of HCOOH is most pronounced at pH 3.5.
- 9Bandi, A.; Kühne, H.-M. Electrochemical Reduction of Carbon Dioxide in Water: Analysis of Reaction Mechanism on Ruthenium-Titanium-Oxide. J. Electrochem. Soc. 1992, 139, 1605– 1610, DOI: 10.1149/1.2069464Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XktlGmsro%253D&md5=bfd89719ae54867716c4e19ec5f1e25cElectrochemical reduction of carbon dioxide in water: analysis of reaction mechanism on ruthenium-titanium-oxideBandi, A.; Kuehne, H. M.Journal of the Electrochemical Society (1992), 139 (6), 1605-10CODEN: JESOAN; ISSN:0013-4651.Mixed ruthenium/titanium oxide electrodes were prepd. by thermal decompn. of the halides on titanium sheets. The electrocatalytic activity of the metallic conductive electrodes towards hydrogen evolution and carbon dioxide redn. was investigated in aq. solns. with rotating-disk electrodes. Current-voltage curves at pH 4 to 7 show a current-limiting behavior at small overpotentials for hydrogen evolution. In the presence of carbon dioxide, the current efficiency for CO2 redn. reaches a max. under current-limiting conditions for hydrogen ion redn., before a steep onset of hydrogen evolution takes place due to the redn. of water. An anal. of the electrode kinetics for the formation of reaction intermediates is given which suggests that the surface recombination of adsorbed hydrogen with CO2 is the rate-limiting step in the electrochem. redn. of CO2.
- 10Ooka, H.; Figueiredo, M. C.; Koper, M. T. M. Competition betweenHydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic Media. Langmuir 2017, 33, 9307– 9313, DOI: 10.1021/acs.langmuir.7b00696Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmvVGntL4%253D&md5=ad2663ea95e8f1c53a702dacbbae4060Competition between Hydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic MediaOoka, Hideshi; Figueiredo, Marta C.; Koper, Marc T. M.Langmuir (2017), 33 (37), 9307-9313CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Understanding the competition between H evolution and CO2 redn. is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO2 redn. in aq. electrolytes. Here, by using a Cu rotating disk electrode, the major H evolution pathway competing with CO2 redn. is H2O redn., even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited redn. of protons takes place at potentials for which there is no significant competition with CO2 redn. This selective inhibitory effect of CO2 on H2O redn., as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between H2O redn. and proton redn. pathways for H evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO2 redn. is the primary intermediate responsible for inhibiting the H2O redn. process, which may be one of the main mechanisms by which Cu maintains a high faradaic efficiency for CO2 redn. in neutral media.
- 11Dutta, A.; Kuzume, A.; Kaliginedi, V.; Rahaman, M.; Sinev, I.; Ahmadi, M.; Roldán Cuenya, B.; Vesztergom, S.; Broekmann, P. Probing the Chemical State of Tin Oxide NP Catalysts during CO2 Electrore- duction: A Complementary Operando Approach. Nano Energy 2018, 53, 828– 840, DOI: 10.1016/j.nanoen.2018.09.033Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvVeru7rP&md5=67fb0d9291ae9dd4c90ccea0cc9ea2dbProbing the chemical state of tin oxide NP catalysts during CO2 electroreduction: A complementary operando approachDutta, Abhijit; Kuzume, Akiyoshi; Kaliginedi, Veerabhadrarao; Rahaman, Motiar; Sinev, Ilya; Ahmadi, Mahdi; Roldan Cuenya, Beatriz; Vesztergom, Soma; Broekmann, PeterNano Energy (2018), 53 (), 828-840CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)In this paper we combine two operando methods, Raman spectroscopy and X-ray absorption spectroscopy (XAS), in order to probe reduced graphene-oxide supported tinIV oxide nanoparticles (SnO2NPs@rGO) as they are being used to catalyze CO2 electroredn. To achieve high reaction rates it is necessary to apply sufficiently cathodic electrode potentials. Under such conditions, however, not only CO2 is reduced electrochem., but also the catalyst particles may be transformed from the initial SnIV state to SnII or, in an extreme case, to metallic Sn. While SnII species still favor CO2 electroredn., yielding formate as a primary product, on metallic Sn CO2 redn.is disfavored with respect to the competing hydrogen evolution reaction (HER). We show that operando XAS, a robust technique yielding information averaged over a large surface area and a relatively large thickness of the catalyst layer, is a very expedient method able to detect the redn.of SnO2NPs@rGO to metallic Sn. XAS can thus be used to establish an optimum potential for the electroredn.in practical electrolyzing cells. It takes, however, a complementary method offered by operando Raman spectroscopy, having greater sensitivity at the catalyst/electrolyte soln.interface, to probe redn.intermediates such as the SnII state, which remain undetectable for ex situ methods. As it is shown in the paper, Raman spectroscopy may also find further use when investigating the recovery of catalyst particles following exposure to extreme reducing conditions.
- 12Murakami, T.; Nishikiori, T.; Nohira, T.; Ito, Y. Electrolytic Synthesis of Ammonia in Molten Salts under Atmospheric Pressure. J. Am. Chem. Soc. 2003, 125, 334– 335, DOI: 10.1021/ja028891tGoogle Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XpsVens7s%253D&md5=c29a3860689d64b932bc1d39ac563e04Electrolytic Synthesis of Ammonia in Molten Salts under Atmospheric PressureMurakami, Tsuyoshi; Nishikiori, Tokujiro; Nohira, Toshiyuki; Ito, YasuhikoJournal of the American Chemical Society (2003), 125 (2), 334-335CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)NH3 was successfully synthesized by using a new electrochem. reaction with high current efficiency at atm. pressure and at lower temps. than the Haber-Bosch process. In this method, nitride ion (N3-), which is produced by the redn. from nitrogen gas at the cathode, is anodically oxidized and reacts with hydrogen to produce NH3 at the anode.
- 13Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell- Worth, C.; Sun, C.; Zhang, X.; MacFarlane, D. R. Electro-Synthesis of Ammonia from Nitrogen at Ambient Temperature and Pressure in Ionic Liquids. Energy Environ. Sci. 2017, 10, 2516– 2520, DOI: 10.1039/C7EE02716HGoogle Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvVSktbrM&md5=f11b8bab2e692594865c60405bc71e4aElectro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquidsZhou, Fengling; Azofra, Luis Miguel; Ali, Muataz; Kar, Mega; Simonov, Alexandr N.; McDonnell-Worth, Ciaran; Sun, Chenghua; Zhang, Xinyi; MacFarlane, Douglas R.Energy & Environmental Science (2017), 10 (12), 2516-2520CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)Ammonia as the source of most fertilizers has become one of the most important chems. globally. It also is being increasingly considered as an easily transported carrier of hydrogen energy that can be generated from "stranded" renewable-energy resources. However, the traditional Haber-Bosch process for the prodn. of ammonia from atm. nitrogen and fossil fuels is a high temp. and pressure process that is energy intensive, currently producing more than 1.6% of global CO2 emissions. An ambient temp., electrochem. synthesis of ammonia is an attractive alternative approach, but has, to date, not been achieved at high efficiency. We report in this work the use of ionic liqs. that have high N2 soly. as electrolytes to achieve high conversion efficiency of 60% for N2 electro-redn. to ammonia on a nanostructured iron catalyst under ambient conditions.
- 14Rudnev, A. V. In Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry; Wandelt, K., Ed.; Elsevier: Amsterdam, 2018; pp 321– 325.Google ScholarThere is no corresponding record for this reference.
- 15Fleige, M. J.; Wiberg, G. K. H.; Arenz, M. Rotating Disk Electrode System for Elevated Pressures and Temperatures. Rev. Sci. Instrum. 2015, 86, 064101 DOI: 10.1063/1.4922382Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVarsb7I&md5=6312fdbe77dd094955503a2b2aa2f2f8Rotating disk electrode system for elevated pressures and temperaturesFleige, M. J.; Wiberg, G. K. H.; Arenz, M.Review of Scientific Instruments (2015), 86 (6), 064101/1-064101/6CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)We describe the development and test of an elevated pressure and temp. rotating disk electrode (RDE) system that allows measurements under well-defined mass transport conditions. As demonstrated for the oxygen redn. reaction on polycryst. platinum (Pt) in 0.5M H2SO4, the setup can easily be operated in a pressure range of 1-101 bar oxygen, and temp. of 140 °C. Under such conditions, diffusion limited current densities increase by almost two orders of magnitude as compared to conventional RDE setups allowing, for example, fuel cell catalyst studies under more realistic conditions. Levich plots demonstrate that the mass transport is indeed well-defined, i.e., at low electrode potentials, the measured current densities are fully diffusion controlled, while at higher potentials, a mixed kinetic-diffusion controlled regime is obsd. Therefore, the setup opens up a new field for RDE investigations under temp. and c.d. conditions relevant for low and high temp. proton exchange membrane fuel cells. (c) 2015 American Institute of Physics.
- 16Jung, S.; Kortlever, R.; Jones, R. J. R.; Lichterman, M. F.; Agapie, T.; McCrory, C. C. L.; Peters, J. C. Gastight Hydrodynamic Electrochem- istry: Design for a Hermetically Sealed Rotating Disk Electrode Cell. Anal. Chem. 2017, 89, 581– 585, DOI: 10.1021/acs.analchem.6b04228Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitVWnu7nF&md5=6716bccf7c54006bbe6f57794ff446c8Gastight Hydrodynamic Electrochemistry: Design for a Hermetically Sealed Rotating Disk Electrode CellJung, Suho; Kortlever, Ruud; Jones, Ryan J. R.; Lichterman, Michael F.; Agapie, Theodor; McCrory, Charles C. L.; Peters, Jonas C.Analytical Chemistry (Washington, DC, United States) (2017), 89 (1), 581-585CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)Rotating disk electrodes (RDEs) are widely used in electrochem. characterization to analyze the mechanisms of various electrocatalytic reactions. RDE expts. often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace anal. challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript the authors describe a new, hermetically sealed electrochem. cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the working electrode to a rotating magnetic driver. Calibration of the RDE using a tachometer shows that the rotation speed of the electrode is the same as that of the magnetic driver. To validate the performance of this cell for hydrodynamic measurements, limiting currents from the redn. of a K ferrocyanide (K4[Fe(CN)6]·3H2O) were measured and shown to compare favorably with calcd. values from the Levich equation and with data obtained using more typical, nongastight RDE cells. Faradaic efficiencies of ∼95% were measured in the gas phase for O evolution in alk. media at an Inconel 625 alloy electrocatalyst during rotation at 1600 rpm. These data verify that a gastight environment is maintained even during rotation.
- 17Riddiford, A. C. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Interscience: New York, 1966; pp 47– 116.Google ScholarThere is no corresponding record for this reference.
- 18Zdunek, A. D. A Novel Rotating Disk Electrode Cell Design: The Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1992, 139, 2549– 2551, DOI: 10.1149/1.2221261Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38Xmt1Wqt7k%253D&md5=c6c6f159d1fdf5a808126c33242a9461A novel rotating disk electrode cell design: the inverted rotating disk electrodeZdunek, A. D.; Selman, J. R.Journal of the Electrochemical Society (1992), 139 (9), 2549-51CODEN: JESOAN; ISSN:0013-4651.A new cell configuration for use with the rotating disk electrode (RDE), the inverted rotating disk electrode (IRDE), is outlined. The IRDE faces upwards in soln. and rotates without electrolyte leakage, even in highly corrosive solns. This facilitates gas evolution and free convection studies. Mass transfer characteristics obtained by limiting current measurements agree well with literature for the RDE.
- 19Bressers, P. M. M. C.; Kelly, J. J. A New Cell Design for the Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1995, 142, L114– L115, DOI: 10.1149/1.2044331Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXntVCit7s%253D&md5=f86b89758600e9ccd41a99c2215b2b73A new cell design for the inverted rotating disk electrodeBressers, P. M. M. C.; Kelly, J. J.Journal of the Electrochemical Society (1995), 142 (7), L114-L115CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)To avoid the problem of gas bubbles blocking the surfaces of rotating disk electrodes, the authors propose a new cell configuration for an inverted rotating disk electrode. This simple construction gives the same hydrodynamic conditions as a conventional rotating disk electrode.
- 20Bradley, P. E.; Landolt, D. New Cell Design for Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1997, 144, L145– L148, DOI: 10.1149/1.1837705Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXksVegtbc%253D&md5=b2e9e06e5b7d75931bd70c07dcc54ac2New cell design for inverted rotating disk electrodeBradley, P. E.; Landolt, D.Journal of the Electrochemical Society (1997), 144 (6), L145-L148CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)A new inverted rotating disk electrode (IRDE) design, based on a gas sealing arrangement, is presented. An original procedure for changing the working electrode without the need for emptying the cell is described. Mass transport characteristics of the IRDE are tested by measuring limiting currents for ferricyanide redn. The cell is particularly useful for carrying out electrodeposition on flat substrates with concurrent gas evolution.
- 21Parys, H. V.; Tourwé, E.; Breugelmans, T.; Depauw, M.; Deconinck, J.; Hubin, A. Modeling of mass and charge transfer in an inverted rotating disk electrode IRDE reactor. J. Electroanal. Chem. 2008, 622, 44– 50, DOI: 10.1016/j.jelechem.2008.05.004Google ScholarThere is no corresponding record for this reference.
- 22Nierhaus, T.; Parys, H. V.; Dehaeck, S.; van Beeck, J.; Deconinck, H.; Deconinck, J.; Hubin, A. Simulation of the Two-Phase Flow Hy- drodynamics in an IRDE Reactor. J. Electrochem. Soc. 2009, 156, P139– P148, DOI: 10.1149/1.3155423Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXptV2lurk%253D&md5=e2360c055bb69c6958e6e252a852eac9Simulation of the Two-Phase Flow Hydrodynamics in an IRDE ReactorNierhaus, Thomas; Van Parys, Heidi; Dehaeck, Sam; van Beeck, Jeroen; Deconinck, Herman; Deconinck, Johan; Hubin, AnnickJournal of the Electrochemical Society (2009), 156 (9), P139-P148CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)Many industrial processes deal with gas bubbles, e.g., the chlor-alkali processes or a side reaction in metal deposition reactions. It is therefore very important to describe the influence of gas bubbles on the fluid flow in a quant. way. In the present paper, the two-phase flow is both exptl. characterized and numerically modeled in a reactor with a rotating flow field such as the inverted rotating disk electrode (IRDE). Polarization curves of the hydrogen evolution in 0.1 M Na2SO4 at pH 2.5 are recorded at different rotation speeds. The bubble dispersion and size distribution of the hydrogen bubbles are detd. by laser marked shadowgraphy and interferometric laser imaging for droplet sizing. Concerning the numerical investigations, in the first step the single-phase flow soln. in the vicinity of the IRDE is compared to the anal. soln. of the flow field, as proposed by Cochran [Proc. Cambridge Philos. Soc., 30, 365 (1934)]. In the following step, an Eulerian-Lagrangian two-phase flow model is used to track the bubbles. Two-way momentum coupling effects between bubbles and electrolyte flow are taken into account. The calcd. two-phase flow field compares well against the exptl. data of the two-phase flow field obtained from the optical imaging techniques.
- 23Konopka, S. J.; McDuffie, B. Diffusion Coefficients of Ferri- and Ferrocyanide Ions in Aqueous Media, Using Twin-Electrode Thin- Layer Electrochemistry. Anal. Chem. 1970, 42, 1741– 1746, DOI: 10.1021/ac50160a042Google Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE3MXhvVOmuw%253D%253D&md5=9b595e5ed88c8afc7b190544df278750Diffusion coefficients of ferri- and ferrocyanide ions in aqueous media, using twin-electrode thin-layer electrochemistryKonopka, S. J.; McDuffie, BruceAnalytical Chemistry (1970), 42 (14), 1741-6CODEN: ANCHAM; ISSN:0003-2700.The diffusion coeffs., Do and DR, of the species [Fe(CN)6]3- and [Fe(CN)6]4-, resp., in aq. KCl media were detd. by twin-electrode thin-layer electrochemistry by using a micrometer-type thin-layer cell. The values obtained for Do and DR at 25° in 1.00M KCl were 0.726 (±0.011) × 10-5 and 0.667 (±0.014) × 10-5 cm2/sec, resp., covering a concn. range of 0.61-6.36mM in total electroactive species. Identical values within exptl. uncertainty were found in 0.10M KCl medium. Criteria for the reliability of this abs. method for measuring the D values of a sol., stable redox pair are presented, and D values are compared with those obtained by other workers.
- 24Grozovski, V.; Vesztergom, S.; Láng, G. G.; Broekmann, P. Electro- chemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode Study. J. Electrochem. Soc. 2017, 164, E3171– E3178, DOI: 10.1149/2.0191711jesGoogle Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsFCltrvE&md5=1912434cff941bde06c3827aa433ef4aElectrochemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode StudyGrozovski, Vitali; Vesztergom, Soma; Lang, Gyozo G.; Broekmann, PeterJournal of the Electrochemical Society (2017), 164 (11), E3171-E3178CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)The authors study the effect of H+ and OH- diffusion on the H evolution reaction in unbuffered aq. electrolyte solns. of mildly acidic pH values. The cathodic polarization curves measured on a Ni rotating disk electrode in these solns. can be modeled by assuming two irreversible reactions, the redn. of H+ and of H2O mols., both following Erdey-Gra´a´u´z-Volmer-Butler kinetics. The redn. of H+ yields a transport-limited and thus, rotation rate-dependent current at not very neg. potentials. At more cathodic potentials the polarization curves are dominated by the redn. of H2O and no mass transfer limitation seems to apply for this reaction. Although prima facie the two processes may seem to proceed independently, by the means of finite-element digital simulations a strong coupling (due to the recombination of H+ and OH- to H2O mols.) exists between them. The authors also develop an anal. model that can well describe polarization curves at various values of pH and rotation rates. The key indication of both models is that hydroxide ions can have an infinite diffusion rate in the proximity of the electrode surface, a feature that can be explained by assuming a directed Grotthuss-like shuttling mechanism of transport.
- 25Auinger, M.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Biedermann, P. U.; Topalov, A. A.; Rohwerder, M.; Mayrhofer, K. J. J. Near- Surface Ion Distribution and Buffer Effects during Electrochemical Reactions. Phys. Chem. Chem. Phys. 2011, 13, 16384– 16394, DOI: 10.1039/c1cp21717hGoogle Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFSjsbjP&md5=1a08177eb2327ac6cf49f9b710e0f599Near-surface ion distribution and buffer effects during electrochemical reactionsAuinger, Michael; Katsounaros, Ioannis; Meier, Josef C.; Klemm, Sebastian O.; Biedermann, P. Ulrich; Topalov, Angel A.; Rohwerder, Michael; Mayrhofer, Karl J. J.Physical Chemistry Chemical Physics (2011), 13 (36), 16384-16394CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)The near-surface ion distribution at the solid-liq. interface during the Hydrogen Oxidn. Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disk electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theor. predict cyclic voltammetry behavior using only thermodn. and diffusion data obtained from the literature, which were confirmed by exptl. measurements. The effect of buffer addn. on the current signal, the surface pH and the ion distribution is quant. described by anal. solns. and the fragility of the surface pH during reactions that form or consume H+ in near-neutral unbuffered solns. or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.
- 26Monteiro, M. C.; Koper, M. T. Alumina Contamination through Polishing and Its Effect on Hydrogen Evolution on Gold Electrodes. Electrochim. Acta 2019, 325, 134915, DOI: 10.1016/j.electacta.2019.134915Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvVOksbjF&md5=6f6d3a884dc8c0ecff19f3c986025865Alumina contamination through polishing and its effect on hydrogen evolution on gold electrodesMonteiro, Mariana C. O.; Koper, Marc T. M.Electrochimica Acta (2019), 325 (), 134915CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)The contamination of Au electrodes with alumina particles by electrode polishing leads to an enhancement in activity for H evolution (HER). Polishing is one of the most used electrode treatments, however, particles from the polishing media cannot be easily removed from the electrode surface by std. cleaning procedures. Comparing the HER activity of Au disk electrodes polished with either diamond suspension or alumina paste, the latter leads to higher current densities, specifically for the H2O redn. to H. A similar enhancement in HER activity was obsd. by the addn. of Al3+ cations to the electrolyte, demonstrating that the particles are not catalytically active, but that the Al3+ species released in soln. due to corrosion promote the H2O redn. reaction. Due to an increase in the local OH- concn. during HER, the contaminating Al3+ cations ppt. and may deposit at the electrode surface as Al(OH)3. In the presence of a high enough Al3+ concn., layered Al(OH)3 plates cover the whole electrode surface. The plates are composed of Al(OH)3 sheets intercalated by sulfate anions. Surprisingly, the Al(OH)3 sheets do not affect the Au blank voltammetry, and therefore remain undetected by simple electrochem. characterization methods.
- 27Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochim. Acta 1994, 39, 1833– 1839, DOI: 10.1016/0013-4686(94)85172-7Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXls1GrtLs%253D&md5=f9795260a8e604a14f966ca1c8cc7998Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous mediaHori, Yoshi; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, OsamuElectrochimica Acta (1994), 39 (11-12), 1833-9CODEN: ELCAAV; ISSN:0013-4686.Electrochem. redn. of CO2 at metal electrodes yields CO, HCOO-, CH4, C2H4, and alcs. in aq. media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO- ones (Pb, Hg, In, Sn, Tl). Foreign atom modified electrode (the coverage is virtually unity) showed variable product selectivity between CO and HCOO-, which depends upon the combination of modifier atom and substrate electrode. Crit. investigation of foreign atom modified electrodes derived a series of CO selectively, as Au > Ag > Cu > Zn » Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 redn. are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 redn., and is rationalized in terms of stabilization of intermediate species CO2·- at the electrode surface. CO is produced from stably adsorbed CO2·-, and HCOO- is formed from free or weakly adsorbed CO2·-.
- 28Dutta, A.; Morstein, C. E.; Rahaman, M.; Cedeño López, A.; Broekmann, P. Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam Catalysts. ACS Catal. 2018, 8, 8357– 8368, DOI: 10.1021/acscatal.8b01738Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVSrsL3F&md5=1087d23218a418340cba0e9b02c73d65Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam CatalystsDutta, Abhijit; Morstein, Carina Elisabeth; Rahaman, Motiar; Cedeno Lopez, Alena; Broekmann, PeterACS Catalysis (2018), 8 (9), 8357-8368CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)Ag-foam catalysts were developed for the electrochem. CO2 redn. reaction (ec-CO2RR) based on a concerted additive- and template-assisted metal-deposition process. In aq. media (CO2-satd. 0.5M KHCO3 electrolyte), these Ag foams show high activity and selectivity toward CO prodn. at low and moderate over-potentials. Faradaic efficiencies for CO (FECO) never fell <90% within an extremely broad potential window of ∼900 mV, starting at -0.3 V and reaching up to -1.2 V vs. a reversible H electrode (RHE). An increased adsorption energy of CO on the Ag foam is discussed as the origin of the efficient suppression of the competing H-evolution reaction (HER) in this potential range. At potentials of <-1.1 V vs. RHE, the FEH2 values significantly increase at the expense of FECO. Superimposed on this anti-correlated change in the CO and H2 efficiencies is the rise in the CH4 efficiency to the max. of FECH4 = 51% at -1.5 V vs. RHE. As a minor byproduct, even C-C-coupled ethylene could be detected reaching a max. faradaic efficiency of FEC2H4 = 8.6% at -1.5 V vs. RHE. Extended ec-CO2RR reveals the extremely high long-term stability of the Ag foam catalysts, with CO efficiencies never falling <90% for >70 h of electrolysis at -0.8 V vs. RHE (potential regime of predominant CO prodn.). However, a more-rapid degrdn. is obsd. for extended ec-CO2RR at -1.5 V vs. RHE (potential regime of predominant CH4 prodn.), in which the FECH4 values drop to 32% within 5 h of electrolysis. The degrdn. behavior of the Ag-foam catalyst is correlated to time-resolved identical-location SEM studies that show severe morphol. changes, particularly at higher applied over-potentials (current densities) at -1.5 V vs. RHE. This study reports on the 1st ec-CO2RR catalyst beyond Cu that demonstrates a remarkably high selectivity toward hydrocarbon formation, reaching a max. of ∼60% at -1.5 V vs. RHE. The exptl. observations presented herein strongly suggest that this newly designed Ag-foam catalyst shares, in part, mechanistic features with common Cu catalysts in terms of ec-CO2RR product selectivity and catalyst degrdn. behavior.
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Abstract
Figure 1
Figure 1. H-type cell equipped with an iRDE. Parts of the design: (a) glass cell body, (b) purging gas inlet (outlet not shown), (c) reference electrode inlet, (d) membrane and sealing junction, (e) counter electrode inlet, (f) PCTFE iRDE tip with an electrode embedded, (g) spring contact node, (h) PTFE tip groove for the O-ring fitting, (i) radial shaft seals (upper and lower), (j) ball bearing, (k) pressurized gland chamber, (
), ceramic fittings (upper and lower), (m) pressurizer gas inlet, (n) POM housing, and (o) rotating shaft (stainless steel).Figure 2
Figure 2. Validation of the hydrodynamic performance of the iRDE setup with a non-gas-evolving reaction. Limiting currents (both anodic and cathodic) measured by linear sweep voltammetry on a glassy carbon iRDE in a solution containing the K4[Fe(CN)6]/K3[Fe(CN)6] redox couple in equimolar concentrations scale linearly with the square root of the rotation rate. Green and red dashed lines show the cathodic and anodic limiting currents, respectively, predicted by eq 2 and the diffusion coefficient values mentioned in the text, at the given rotation rates. Sweep rate 20 mV s–1.
Figure 3
Figure 3. Validation of the hydrodynamic performance of the iRDE setup with HER, a gas-evolving reaction. Limiting currents measured by linear sweep voltammetry (sweep rate 50 mV s–1) on a Pt iRDE in a pH 2.56 HClO4/NaClO4 solution scale linearly with the square root of the rotational rate. The dashed green lines are limiting current predictions of the Levich equation (eq 2), calculated using the diffusion coefficient value mentioned in the text at all of the given rotational rates.
Figure 4
Figure 4. Although the bulk pH is about the same, LSVs obtained in different electrolyte solutions exhibit varying limiting currents for H+ reduction. The sweep rate is 50 mV s–1, the rotational rate is 900 min–1, and the electrolyte compositions and the pH are shown in the graph. The given pH was set by adding a few drops of the respective concentrated acid to the electrolyte solution.
Figure 5
Figure 5. Results of long-term electrolysis (hydrogen evolution from a HClO4/NaClO4 electrolyte solution) measured by iRDE&GC. Currents measured electrochemically (full black curve) and chromatographically (calculated using eq 1, dots) at E = −625 mV vs Ag|AgCl are shown in (a). A slow drift (decay) over time can be observed as a result of the electrolysis becoming exhaustive. Values of pH measured before and after the electrolysis, along with limiting currents estimated using the respective H+ concentrations and the diffusion coefficient of 8.79 × 10–5 cm2 s–1 are shown by the dashed horizontal lines. This pH change is in alignment with the shifting of the LSV plateaus shown in (b) and also corresponds to the estimated H+ concentration change calculated by taking into account the charge of the electrolysis, shown as the hatched area in (a), and a cell volume of 87 cm3. The applied rotational rate was 1600 min–1, and the linear sweep voltammograms were recorded at a sweep rate of 50 mV s–1.
Figure 6
Figure 6. (a) Survey voltammograms recorded at a sweep rate of 50 mV s–1 on a Ag iRDE in a 0.1 mol dm–3 K2SO4 solution saturated with CO2 (pH ∼4.17). Cathodic currents increase with increasing rotational rates (100, 225, 400, 625, 900, 1225, and 1600 min–1). (b) Hydrogen evolution attains a limiting current at lower overpotentials. At higher cathodic overpotentials, the electroreduction of CO2 competes with the reduction of water. In this potential range, CO is the primary product of electrolysis.
Figure 7
Figure 7. IRDE&GC system applied to the study of the electrolysis of a 0.1 mol dm–3 K2SO4 solution saturated with CO2 (pH ∼4.17). Applied rotational rate 625 min–1. (a) Faradaic efficiencies of H2 and CO formation are determined chromatographically (dots) and are interpolated using an arbitrary function (exponential decay superimposed on a straight line, black curve). (b) This interpolation allows the separation of the recorded LSVs: the total current and the partial currents of H2 and CO production are shown as black, red, and green curves, respectively. An LSV measured in a CO2-free (Ar-saturated) K2SO4 solution, the pH of which was set to 4.15 by direct H2SO4 addition, is shown as a reference (dashed gray curve).
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- 3Kuhn, A. T.; Chan, C. Y. pH Changes at Near-Electrode Surfaces. J. Appl. Electrochem. 1983, 13, 189– 207, DOI: 10.1007/BF00612481Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3sXhs12itLo%253D&md5=ecf246db110f1c9b0bb1b77078e7c663pH Changes at near-electrode surfacesKuhn, A. T.; Chan, C. Y.Journal of Applied Electrochemistry (1983), 13 (2), 189-207CODEN: JAELBJ; ISSN:0021-891X.Anodic or cathodic processes in which protons are released or consumed, resp., lead to local pH changes at the electrode surface. The phenomenon is important in several branches of metal finishing in corrosion, electroorg. synthesis and other branches of applied electrochem., as well as being a factor which can invalidate mechanistic investigations. The concept of pHs (near-surface pH) is examd., as are the exptl. techniques which were used to measure it. Many of these are inherently weak. Published results are shown to be largely discrepant, both in respect to the magnitude of the effect and even its incidence. Attempts to treat the phenomenon theor. are briefly summarized.
- 4Ying, R. Y. Electrodeposition of Copper-Nickel Alloys from Citrate Solutions on a Rotating Disk Electrode. J. Electrochem. Soc. 1988, 135, 2964– 2971, DOI: 10.1149/1.2095470Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1MXkslWktg%253D%253D&md5=c28ae3b4871624359bafde44ee58de6cElectrodeposition of copper-nickel alloys from citrate solutions on a rotating disk electrode. II. Mathematical modelingYing, Ramona Y.; Ng, Patrick K.; Mao, Z.; White, Ralph E.Journal of the Electrochemical Society (1988), 135 (12), 2964-71CODEN: JESOAN; ISSN:0013-4651.A math. model is developed to simulate the electrodeposition of Cu-Ni alloy from citrate solns. onto a rotating disk electrode under potentiostatic control. The model includes the influence of diffusion, ionic migration, forced convection, and homogeneous equil. The 3 major electrochem. reactions treated in the model are Cu deposition, Ni deposition, and H evolution. Using exptl. parameters when available, the model was fitted and tested against exptl.-obtained results for different bath compns. and operating conditions. The model agreed reasonably well with the exptl. results particularly in the codeposition region, -1.0 to -1.2 V vs. SCE, where H evolution is not the major reaction. Also, the model's predictive capabilities are evaluated.
- 5Vos, J. G.; Koper, M. T. M. Measurement of Competition between Oxygen Evolution and Chlorine Evolution Using Rotating Ring–Disk Electrode Voltammetry. J. Electroanal. Chem. 2018, 819, 260– 268, DOI: 10.1016/j.jelechem.2017.10.058Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhslOisb%252FL&md5=01573b2aec0aa64422dd552b109a5c76Measurement of competition between oxygen evolution and chlorine evolution using rotating ring-disk electrode voltammetryVos, J. G.; Koper, M. T. M.Journal of Electroanalytical Chemistry (2018), 819 (), 260-268CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)Selectivity between Cl evolution and O evolution in aq. media is a phenomenon of central importance in the chlor-alkali process, H2O treatment, and saline H2O splitting, which is an emerging technol. for sustainable energy conversion. An apparent scaling between O vs. Cl evolution was established, making it challenging to carry the two reactions out individually with 100% faradaic efficiency. To aid selectivity detn., the authors developed a new method to quickly measure Cl evolution rates using a conventional RRDE setup. A Pt ring fixed at 0.95 V vs. RHE in pH 0.88 can selectively reduce the Cl2 formed on the disk and this allows precise and flexible data acquisition. Using this method, O evolution and Cl evolution on a glassy C supported IrOx catalyst proceed independently, and the selectivity towards Cl evolution (εCER) rapidly approaches 100% as [Cl-] increases from 0 to 100 mM. The authors' results suggest that on IrOx, O evolution is not suppressed or influenced by the presence of Cl- or by the Cl evolution reaction taking place simultaneously on the surface.
- 6Hall, A. S.; Yoon, Y.; Wuttig, A.; Surendranath, Y. Mesostructure-Induced Selectivity in CO2 Reduction Catalysis. J. Am. Chem. Soc. 2015, 137, 14834– 14837, DOI: 10.1021/jacs.5b08259Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslKlt7rM&md5=f279b110a31a7cdf8d0dffe4e683cbdcMesostructure-Induced Selectivity in CO2 Reduction CatalysisHall, Anthony Shoji; Yoon, Youngmin; Wuttig, Anna; Surendranath, YogeshJournal of the American Chemical Society (2015), 137 (47), 14834-14837CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Gold inverse opal (Au-IO) thin films are active for CO2 redn. to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO2-to-fuels catalysis.
- 7Bandi, A. Electrochemical Reduction of Carbon Dioxide on Conductive Metallic Oxides. J. Electrochem. Soc. 1990, 137, 2157– 2160, DOI: 10.1149/1.2086903Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXls1Wisb4%253D&md5=7ab3a48be2b87c7c83256f43c85915acElectrochemical reduction of carbon dioxide on conductive metallic oxidesBandi, A.Journal of the Electrochemical Society (1990), 137 (7), 2157-60CODEN: JESOAN; ISSN:0013-4651.Faradaic efficiencies and current-voltage characteristics for the redn. of CO2 on various conductive oxide mixts. (RuO2, TiO2, MoO2, Co3O4, Rh2O3) were detd. Product anal. was performed by gas chromatog. Electrodes composed of RuO2 + TiO2 [35 + 65 mol percent (m/o)] and RuO2 + Co3O4 + SnO2 + TiO2 (20 + 10 + 8 + 62 m/o) show high current efficiencies for methanol prodn. when polarized near the equil. potential of H evolution in solns. of 0.2M Na2SO4 (pH 4) satd. with CO2. It can be concluded from the Tafel slopes that the first electron transfer is the rate-limiting step in the redn. of CO2 on the investigated oxides in acidic media.
- 8Aoki, A.; Nogami, G. Rotating Ring–Disk Electrode Study on the Fixation Mechanism of Carbon Dioxide. J. Electrochem. Soc. 1995, 142, 423– 427, DOI: 10.1149/1.2044046Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXjs1KjtL0%253D&md5=c6d040d8105b8be1a044e8ee4920cb9fRotating ring-disk electrode study on the fixation mechanism of carbon dioxideAoki, Akihiko; Nogami, GyoichiJournal of the Electrochemical Society (1995), 142 (2), 423-7CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)Formic acid formed on a SnO2 disk electrode as a redn. product of carbon dioxide was detected at a Pt ring electrode, and the fixation mechanism of carbon dioxide was studied. Org. products such as HCOOH could be detected electrochem. by a rotating ring-disk electrode technique which does not require a long-term electroredn. and offers a much more sensitive method than gas- and liq.-chromatog. do. CO2 was reduced to HCOOH on the SnO2 surface in competition with hydrogen formation: both HCOOH and H2 were formed quite slowly in a few seconds. The rotational speed dependence of the reaction current revealed that a considerably slow electron transfer gave some intermediates, from which HCOOH will be formed under the mass-transfer-limited condition of proton. The formation of HCOOH is most pronounced at pH 3.5.
- 9Bandi, A.; Kühne, H.-M. Electrochemical Reduction of Carbon Dioxide in Water: Analysis of Reaction Mechanism on Ruthenium-Titanium-Oxide. J. Electrochem. Soc. 1992, 139, 1605– 1610, DOI: 10.1149/1.2069464Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XktlGmsro%253D&md5=bfd89719ae54867716c4e19ec5f1e25cElectrochemical reduction of carbon dioxide in water: analysis of reaction mechanism on ruthenium-titanium-oxideBandi, A.; Kuehne, H. M.Journal of the Electrochemical Society (1992), 139 (6), 1605-10CODEN: JESOAN; ISSN:0013-4651.Mixed ruthenium/titanium oxide electrodes were prepd. by thermal decompn. of the halides on titanium sheets. The electrocatalytic activity of the metallic conductive electrodes towards hydrogen evolution and carbon dioxide redn. was investigated in aq. solns. with rotating-disk electrodes. Current-voltage curves at pH 4 to 7 show a current-limiting behavior at small overpotentials for hydrogen evolution. In the presence of carbon dioxide, the current efficiency for CO2 redn. reaches a max. under current-limiting conditions for hydrogen ion redn., before a steep onset of hydrogen evolution takes place due to the redn. of water. An anal. of the electrode kinetics for the formation of reaction intermediates is given which suggests that the surface recombination of adsorbed hydrogen with CO2 is the rate-limiting step in the electrochem. redn. of CO2.
- 10Ooka, H.; Figueiredo, M. C.; Koper, M. T. M. Competition betweenHydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic Media. Langmuir 2017, 33, 9307– 9313, DOI: 10.1021/acs.langmuir.7b00696Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmvVGntL4%253D&md5=ad2663ea95e8f1c53a702dacbbae4060Competition between Hydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic MediaOoka, Hideshi; Figueiredo, Marta C.; Koper, Marc T. M.Langmuir (2017), 33 (37), 9307-9313CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)Understanding the competition between H evolution and CO2 redn. is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO2 redn. in aq. electrolytes. Here, by using a Cu rotating disk electrode, the major H evolution pathway competing with CO2 redn. is H2O redn., even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited redn. of protons takes place at potentials for which there is no significant competition with CO2 redn. This selective inhibitory effect of CO2 on H2O redn., as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between H2O redn. and proton redn. pathways for H evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO2 redn. is the primary intermediate responsible for inhibiting the H2O redn. process, which may be one of the main mechanisms by which Cu maintains a high faradaic efficiency for CO2 redn. in neutral media.
- 11Dutta, A.; Kuzume, A.; Kaliginedi, V.; Rahaman, M.; Sinev, I.; Ahmadi, M.; Roldán Cuenya, B.; Vesztergom, S.; Broekmann, P. Probing the Chemical State of Tin Oxide NP Catalysts during CO2 Electrore- duction: A Complementary Operando Approach. Nano Energy 2018, 53, 828– 840, DOI: 10.1016/j.nanoen.2018.09.033Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvVeru7rP&md5=67fb0d9291ae9dd4c90ccea0cc9ea2dbProbing the chemical state of tin oxide NP catalysts during CO2 electroreduction: A complementary operando approachDutta, Abhijit; Kuzume, Akiyoshi; Kaliginedi, Veerabhadrarao; Rahaman, Motiar; Sinev, Ilya; Ahmadi, Mahdi; Roldan Cuenya, Beatriz; Vesztergom, Soma; Broekmann, PeterNano Energy (2018), 53 (), 828-840CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)In this paper we combine two operando methods, Raman spectroscopy and X-ray absorption spectroscopy (XAS), in order to probe reduced graphene-oxide supported tinIV oxide nanoparticles (SnO2NPs@rGO) as they are being used to catalyze CO2 electroredn. To achieve high reaction rates it is necessary to apply sufficiently cathodic electrode potentials. Under such conditions, however, not only CO2 is reduced electrochem., but also the catalyst particles may be transformed from the initial SnIV state to SnII or, in an extreme case, to metallic Sn. While SnII species still favor CO2 electroredn., yielding formate as a primary product, on metallic Sn CO2 redn.is disfavored with respect to the competing hydrogen evolution reaction (HER). We show that operando XAS, a robust technique yielding information averaged over a large surface area and a relatively large thickness of the catalyst layer, is a very expedient method able to detect the redn.of SnO2NPs@rGO to metallic Sn. XAS can thus be used to establish an optimum potential for the electroredn.in practical electrolyzing cells. It takes, however, a complementary method offered by operando Raman spectroscopy, having greater sensitivity at the catalyst/electrolyte soln.interface, to probe redn.intermediates such as the SnII state, which remain undetectable for ex situ methods. As it is shown in the paper, Raman spectroscopy may also find further use when investigating the recovery of catalyst particles following exposure to extreme reducing conditions.
- 12Murakami, T.; Nishikiori, T.; Nohira, T.; Ito, Y. Electrolytic Synthesis of Ammonia in Molten Salts under Atmospheric Pressure. J. Am. Chem. Soc. 2003, 125, 334– 335, DOI: 10.1021/ja028891tGoogle Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XpsVens7s%253D&md5=c29a3860689d64b932bc1d39ac563e04Electrolytic Synthesis of Ammonia in Molten Salts under Atmospheric PressureMurakami, Tsuyoshi; Nishikiori, Tokujiro; Nohira, Toshiyuki; Ito, YasuhikoJournal of the American Chemical Society (2003), 125 (2), 334-335CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)NH3 was successfully synthesized by using a new electrochem. reaction with high current efficiency at atm. pressure and at lower temps. than the Haber-Bosch process. In this method, nitride ion (N3-), which is produced by the redn. from nitrogen gas at the cathode, is anodically oxidized and reacts with hydrogen to produce NH3 at the anode.
- 13Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell- Worth, C.; Sun, C.; Zhang, X.; MacFarlane, D. R. Electro-Synthesis of Ammonia from Nitrogen at Ambient Temperature and Pressure in Ionic Liquids. Energy Environ. Sci. 2017, 10, 2516– 2520, DOI: 10.1039/C7EE02716HGoogle Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvVSktbrM&md5=f11b8bab2e692594865c60405bc71e4aElectro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquidsZhou, Fengling; Azofra, Luis Miguel; Ali, Muataz; Kar, Mega; Simonov, Alexandr N.; McDonnell-Worth, Ciaran; Sun, Chenghua; Zhang, Xinyi; MacFarlane, Douglas R.Energy & Environmental Science (2017), 10 (12), 2516-2520CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)Ammonia as the source of most fertilizers has become one of the most important chems. globally. It also is being increasingly considered as an easily transported carrier of hydrogen energy that can be generated from "stranded" renewable-energy resources. However, the traditional Haber-Bosch process for the prodn. of ammonia from atm. nitrogen and fossil fuels is a high temp. and pressure process that is energy intensive, currently producing more than 1.6% of global CO2 emissions. An ambient temp., electrochem. synthesis of ammonia is an attractive alternative approach, but has, to date, not been achieved at high efficiency. We report in this work the use of ionic liqs. that have high N2 soly. as electrolytes to achieve high conversion efficiency of 60% for N2 electro-redn. to ammonia on a nanostructured iron catalyst under ambient conditions.
- 14Rudnev, A. V. In Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry; Wandelt, K., Ed.; Elsevier: Amsterdam, 2018; pp 321– 325.Google ScholarThere is no corresponding record for this reference.
- 15Fleige, M. J.; Wiberg, G. K. H.; Arenz, M. Rotating Disk Electrode System for Elevated Pressures and Temperatures. Rev. Sci. Instrum. 2015, 86, 064101 DOI: 10.1063/1.4922382Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVarsb7I&md5=6312fdbe77dd094955503a2b2aa2f2f8Rotating disk electrode system for elevated pressures and temperaturesFleige, M. J.; Wiberg, G. K. H.; Arenz, M.Review of Scientific Instruments (2015), 86 (6), 064101/1-064101/6CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)We describe the development and test of an elevated pressure and temp. rotating disk electrode (RDE) system that allows measurements under well-defined mass transport conditions. As demonstrated for the oxygen redn. reaction on polycryst. platinum (Pt) in 0.5M H2SO4, the setup can easily be operated in a pressure range of 1-101 bar oxygen, and temp. of 140 °C. Under such conditions, diffusion limited current densities increase by almost two orders of magnitude as compared to conventional RDE setups allowing, for example, fuel cell catalyst studies under more realistic conditions. Levich plots demonstrate that the mass transport is indeed well-defined, i.e., at low electrode potentials, the measured current densities are fully diffusion controlled, while at higher potentials, a mixed kinetic-diffusion controlled regime is obsd. Therefore, the setup opens up a new field for RDE investigations under temp. and c.d. conditions relevant for low and high temp. proton exchange membrane fuel cells. (c) 2015 American Institute of Physics.
- 16Jung, S.; Kortlever, R.; Jones, R. J. R.; Lichterman, M. F.; Agapie, T.; McCrory, C. C. L.; Peters, J. C. Gastight Hydrodynamic Electrochem- istry: Design for a Hermetically Sealed Rotating Disk Electrode Cell. Anal. Chem. 2017, 89, 581– 585, DOI: 10.1021/acs.analchem.6b04228Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitVWnu7nF&md5=6716bccf7c54006bbe6f57794ff446c8Gastight Hydrodynamic Electrochemistry: Design for a Hermetically Sealed Rotating Disk Electrode CellJung, Suho; Kortlever, Ruud; Jones, Ryan J. R.; Lichterman, Michael F.; Agapie, Theodor; McCrory, Charles C. L.; Peters, Jonas C.Analytical Chemistry (Washington, DC, United States) (2017), 89 (1), 581-585CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)Rotating disk electrodes (RDEs) are widely used in electrochem. characterization to analyze the mechanisms of various electrocatalytic reactions. RDE expts. often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace anal. challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript the authors describe a new, hermetically sealed electrochem. cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the working electrode to a rotating magnetic driver. Calibration of the RDE using a tachometer shows that the rotation speed of the electrode is the same as that of the magnetic driver. To validate the performance of this cell for hydrodynamic measurements, limiting currents from the redn. of a K ferrocyanide (K4[Fe(CN)6]·3H2O) were measured and shown to compare favorably with calcd. values from the Levich equation and with data obtained using more typical, nongastight RDE cells. Faradaic efficiencies of ∼95% were measured in the gas phase for O evolution in alk. media at an Inconel 625 alloy electrocatalyst during rotation at 1600 rpm. These data verify that a gastight environment is maintained even during rotation.
- 17Riddiford, A. C. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Interscience: New York, 1966; pp 47– 116.Google ScholarThere is no corresponding record for this reference.
- 18Zdunek, A. D. A Novel Rotating Disk Electrode Cell Design: The Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1992, 139, 2549– 2551, DOI: 10.1149/1.2221261Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38Xmt1Wqt7k%253D&md5=c6c6f159d1fdf5a808126c33242a9461A novel rotating disk electrode cell design: the inverted rotating disk electrodeZdunek, A. D.; Selman, J. R.Journal of the Electrochemical Society (1992), 139 (9), 2549-51CODEN: JESOAN; ISSN:0013-4651.A new cell configuration for use with the rotating disk electrode (RDE), the inverted rotating disk electrode (IRDE), is outlined. The IRDE faces upwards in soln. and rotates without electrolyte leakage, even in highly corrosive solns. This facilitates gas evolution and free convection studies. Mass transfer characteristics obtained by limiting current measurements agree well with literature for the RDE.
- 19Bressers, P. M. M. C.; Kelly, J. J. A New Cell Design for the Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1995, 142, L114– L115, DOI: 10.1149/1.2044331Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXntVCit7s%253D&md5=f86b89758600e9ccd41a99c2215b2b73A new cell design for the inverted rotating disk electrodeBressers, P. M. M. C.; Kelly, J. J.Journal of the Electrochemical Society (1995), 142 (7), L114-L115CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)To avoid the problem of gas bubbles blocking the surfaces of rotating disk electrodes, the authors propose a new cell configuration for an inverted rotating disk electrode. This simple construction gives the same hydrodynamic conditions as a conventional rotating disk electrode.
- 20Bradley, P. E.; Landolt, D. New Cell Design for Inverted Rotating Disk Electrode. J. Electrochem. Soc. 1997, 144, L145– L148, DOI: 10.1149/1.1837705Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXksVegtbc%253D&md5=b2e9e06e5b7d75931bd70c07dcc54ac2New cell design for inverted rotating disk electrodeBradley, P. E.; Landolt, D.Journal of the Electrochemical Society (1997), 144 (6), L145-L148CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)A new inverted rotating disk electrode (IRDE) design, based on a gas sealing arrangement, is presented. An original procedure for changing the working electrode without the need for emptying the cell is described. Mass transport characteristics of the IRDE are tested by measuring limiting currents for ferricyanide redn. The cell is particularly useful for carrying out electrodeposition on flat substrates with concurrent gas evolution.
- 21Parys, H. V.; Tourwé, E.; Breugelmans, T.; Depauw, M.; Deconinck, J.; Hubin, A. Modeling of mass and charge transfer in an inverted rotating disk electrode IRDE reactor. J. Electroanal. Chem. 2008, 622, 44– 50, DOI: 10.1016/j.jelechem.2008.05.004Google ScholarThere is no corresponding record for this reference.
- 22Nierhaus, T.; Parys, H. V.; Dehaeck, S.; van Beeck, J.; Deconinck, H.; Deconinck, J.; Hubin, A. Simulation of the Two-Phase Flow Hy- drodynamics in an IRDE Reactor. J. Electrochem. Soc. 2009, 156, P139– P148, DOI: 10.1149/1.3155423Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXptV2lurk%253D&md5=e2360c055bb69c6958e6e252a852eac9Simulation of the Two-Phase Flow Hydrodynamics in an IRDE ReactorNierhaus, Thomas; Van Parys, Heidi; Dehaeck, Sam; van Beeck, Jeroen; Deconinck, Herman; Deconinck, Johan; Hubin, AnnickJournal of the Electrochemical Society (2009), 156 (9), P139-P148CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)Many industrial processes deal with gas bubbles, e.g., the chlor-alkali processes or a side reaction in metal deposition reactions. It is therefore very important to describe the influence of gas bubbles on the fluid flow in a quant. way. In the present paper, the two-phase flow is both exptl. characterized and numerically modeled in a reactor with a rotating flow field such as the inverted rotating disk electrode (IRDE). Polarization curves of the hydrogen evolution in 0.1 M Na2SO4 at pH 2.5 are recorded at different rotation speeds. The bubble dispersion and size distribution of the hydrogen bubbles are detd. by laser marked shadowgraphy and interferometric laser imaging for droplet sizing. Concerning the numerical investigations, in the first step the single-phase flow soln. in the vicinity of the IRDE is compared to the anal. soln. of the flow field, as proposed by Cochran [Proc. Cambridge Philos. Soc., 30, 365 (1934)]. In the following step, an Eulerian-Lagrangian two-phase flow model is used to track the bubbles. Two-way momentum coupling effects between bubbles and electrolyte flow are taken into account. The calcd. two-phase flow field compares well against the exptl. data of the two-phase flow field obtained from the optical imaging techniques.
- 23Konopka, S. J.; McDuffie, B. Diffusion Coefficients of Ferri- and Ferrocyanide Ions in Aqueous Media, Using Twin-Electrode Thin- Layer Electrochemistry. Anal. Chem. 1970, 42, 1741– 1746, DOI: 10.1021/ac50160a042Google Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE3MXhvVOmuw%253D%253D&md5=9b595e5ed88c8afc7b190544df278750Diffusion coefficients of ferri- and ferrocyanide ions in aqueous media, using twin-electrode thin-layer electrochemistryKonopka, S. J.; McDuffie, BruceAnalytical Chemistry (1970), 42 (14), 1741-6CODEN: ANCHAM; ISSN:0003-2700.The diffusion coeffs., Do and DR, of the species [Fe(CN)6]3- and [Fe(CN)6]4-, resp., in aq. KCl media were detd. by twin-electrode thin-layer electrochemistry by using a micrometer-type thin-layer cell. The values obtained for Do and DR at 25° in 1.00M KCl were 0.726 (±0.011) × 10-5 and 0.667 (±0.014) × 10-5 cm2/sec, resp., covering a concn. range of 0.61-6.36mM in total electroactive species. Identical values within exptl. uncertainty were found in 0.10M KCl medium. Criteria for the reliability of this abs. method for measuring the D values of a sol., stable redox pair are presented, and D values are compared with those obtained by other workers.
- 24Grozovski, V.; Vesztergom, S.; Láng, G. G.; Broekmann, P. Electro- chemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode Study. J. Electrochem. Soc. 2017, 164, E3171– E3178, DOI: 10.1149/2.0191711jesGoogle Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsFCltrvE&md5=1912434cff941bde06c3827aa433ef4aElectrochemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode StudyGrozovski, Vitali; Vesztergom, Soma; Lang, Gyozo G.; Broekmann, PeterJournal of the Electrochemical Society (2017), 164 (11), E3171-E3178CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)The authors study the effect of H+ and OH- diffusion on the H evolution reaction in unbuffered aq. electrolyte solns. of mildly acidic pH values. The cathodic polarization curves measured on a Ni rotating disk electrode in these solns. can be modeled by assuming two irreversible reactions, the redn. of H+ and of H2O mols., both following Erdey-Gra´a´u´z-Volmer-Butler kinetics. The redn. of H+ yields a transport-limited and thus, rotation rate-dependent current at not very neg. potentials. At more cathodic potentials the polarization curves are dominated by the redn. of H2O and no mass transfer limitation seems to apply for this reaction. Although prima facie the two processes may seem to proceed independently, by the means of finite-element digital simulations a strong coupling (due to the recombination of H+ and OH- to H2O mols.) exists between them. The authors also develop an anal. model that can well describe polarization curves at various values of pH and rotation rates. The key indication of both models is that hydroxide ions can have an infinite diffusion rate in the proximity of the electrode surface, a feature that can be explained by assuming a directed Grotthuss-like shuttling mechanism of transport.
- 25Auinger, M.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Biedermann, P. U.; Topalov, A. A.; Rohwerder, M.; Mayrhofer, K. J. J. Near- Surface Ion Distribution and Buffer Effects during Electrochemical Reactions. Phys. Chem. Chem. Phys. 2011, 13, 16384– 16394, DOI: 10.1039/c1cp21717hGoogle Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFSjsbjP&md5=1a08177eb2327ac6cf49f9b710e0f599Near-surface ion distribution and buffer effects during electrochemical reactionsAuinger, Michael; Katsounaros, Ioannis; Meier, Josef C.; Klemm, Sebastian O.; Biedermann, P. Ulrich; Topalov, Angel A.; Rohwerder, Michael; Mayrhofer, Karl J. J.Physical Chemistry Chemical Physics (2011), 13 (36), 16384-16394CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)The near-surface ion distribution at the solid-liq. interface during the Hydrogen Oxidn. Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disk electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theor. predict cyclic voltammetry behavior using only thermodn. and diffusion data obtained from the literature, which were confirmed by exptl. measurements. The effect of buffer addn. on the current signal, the surface pH and the ion distribution is quant. described by anal. solns. and the fragility of the surface pH during reactions that form or consume H+ in near-neutral unbuffered solns. or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.
- 26Monteiro, M. C.; Koper, M. T. Alumina Contamination through Polishing and Its Effect on Hydrogen Evolution on Gold Electrodes. Electrochim. Acta 2019, 325, 134915, DOI: 10.1016/j.electacta.2019.134915Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvVOksbjF&md5=6f6d3a884dc8c0ecff19f3c986025865Alumina contamination through polishing and its effect on hydrogen evolution on gold electrodesMonteiro, Mariana C. O.; Koper, Marc T. M.Electrochimica Acta (2019), 325 (), 134915CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)The contamination of Au electrodes with alumina particles by electrode polishing leads to an enhancement in activity for H evolution (HER). Polishing is one of the most used electrode treatments, however, particles from the polishing media cannot be easily removed from the electrode surface by std. cleaning procedures. Comparing the HER activity of Au disk electrodes polished with either diamond suspension or alumina paste, the latter leads to higher current densities, specifically for the H2O redn. to H. A similar enhancement in HER activity was obsd. by the addn. of Al3+ cations to the electrolyte, demonstrating that the particles are not catalytically active, but that the Al3+ species released in soln. due to corrosion promote the H2O redn. reaction. Due to an increase in the local OH- concn. during HER, the contaminating Al3+ cations ppt. and may deposit at the electrode surface as Al(OH)3. In the presence of a high enough Al3+ concn., layered Al(OH)3 plates cover the whole electrode surface. The plates are composed of Al(OH)3 sheets intercalated by sulfate anions. Surprisingly, the Al(OH)3 sheets do not affect the Au blank voltammetry, and therefore remain undetected by simple electrochem. characterization methods.
- 27Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochim. Acta 1994, 39, 1833– 1839, DOI: 10.1016/0013-4686(94)85172-7Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXls1GrtLs%253D&md5=f9795260a8e604a14f966ca1c8cc7998Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous mediaHori, Yoshi; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, OsamuElectrochimica Acta (1994), 39 (11-12), 1833-9CODEN: ELCAAV; ISSN:0013-4686.Electrochem. redn. of CO2 at metal electrodes yields CO, HCOO-, CH4, C2H4, and alcs. in aq. media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO- ones (Pb, Hg, In, Sn, Tl). Foreign atom modified electrode (the coverage is virtually unity) showed variable product selectivity between CO and HCOO-, which depends upon the combination of modifier atom and substrate electrode. Crit. investigation of foreign atom modified electrodes derived a series of CO selectively, as Au > Ag > Cu > Zn » Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 redn. are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 redn., and is rationalized in terms of stabilization of intermediate species CO2·- at the electrode surface. CO is produced from stably adsorbed CO2·-, and HCOO- is formed from free or weakly adsorbed CO2·-.
- 28Dutta, A.; Morstein, C. E.; Rahaman, M.; Cedeño López, A.; Broekmann, P. Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam Catalysts. ACS Catal. 2018, 8, 8357– 8368, DOI: 10.1021/acscatal.8b01738Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVSrsL3F&md5=1087d23218a418340cba0e9b02c73d65Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver-Nanofoam CatalystsDutta, Abhijit; Morstein, Carina Elisabeth; Rahaman, Motiar; Cedeno Lopez, Alena; Broekmann, PeterACS Catalysis (2018), 8 (9), 8357-8368CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)Ag-foam catalysts were developed for the electrochem. CO2 redn. reaction (ec-CO2RR) based on a concerted additive- and template-assisted metal-deposition process. In aq. media (CO2-satd. 0.5M KHCO3 electrolyte), these Ag foams show high activity and selectivity toward CO prodn. at low and moderate over-potentials. Faradaic efficiencies for CO (FECO) never fell <90% within an extremely broad potential window of ∼900 mV, starting at -0.3 V and reaching up to -1.2 V vs. a reversible H electrode (RHE). An increased adsorption energy of CO on the Ag foam is discussed as the origin of the efficient suppression of the competing H-evolution reaction (HER) in this potential range. At potentials of <-1.1 V vs. RHE, the FEH2 values significantly increase at the expense of FECO. Superimposed on this anti-correlated change in the CO and H2 efficiencies is the rise in the CH4 efficiency to the max. of FECH4 = 51% at -1.5 V vs. RHE. As a minor byproduct, even C-C-coupled ethylene could be detected reaching a max. faradaic efficiency of FEC2H4 = 8.6% at -1.5 V vs. RHE. Extended ec-CO2RR reveals the extremely high long-term stability of the Ag foam catalysts, with CO efficiencies never falling <90% for >70 h of electrolysis at -0.8 V vs. RHE (potential regime of predominant CO prodn.). However, a more-rapid degrdn. is obsd. for extended ec-CO2RR at -1.5 V vs. RHE (potential regime of predominant CH4 prodn.), in which the FECH4 values drop to 32% within 5 h of electrolysis. The degrdn. behavior of the Ag-foam catalyst is correlated to time-resolved identical-location SEM studies that show severe morphol. changes, particularly at higher applied over-potentials (current densities) at -1.5 V vs. RHE. This study reports on the 1st ec-CO2RR catalyst beyond Cu that demonstrates a remarkably high selectivity toward hydrocarbon formation, reaching a max. of ∼60% at -1.5 V vs. RHE. The exptl. observations presented herein strongly suggest that this newly designed Ag-foam catalyst shares, in part, mechanistic features with common Cu catalysts in terms of ec-CO2RR product selectivity and catalyst degrdn. behavior.
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Scanning electron micrographs and energy-dispersive X-ray spectra of alumina and diamond-polished silver RDE surfaces (PDF)
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