Oxygen Evolution Reaction on Nitrogen-Doped Defective Carbon Nanotubes and Graphene

The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3–1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone–Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone–Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties.

: Structures of the model N-doped and/or defective GRA-based systems (top views are shown; in addition, the upper panels of subfigures (d-k) show the side views). Atoms in the vicinity of the studied sites are shown as spheres (carbon, gray; nitrogen, blue; oxygen, red; hydrogen, white) and labeled. The calculated overpotential η OER (in V) at a studied site is also shown. For each system, the optimal site for OER is circled. The corresponding information for the SWNT-based systems is shown in Figure 1 of the main text. With regard to panel (e), since graphene lacks an axis, this attempt to create a GRA-based system with a pyrrolic nitrogen results instead in a system with a pyridinic nitrogen atom. Thus, since the system in panel (e) differs from the one in panel (d) merely by a clockwise rotation of 120 • , it will not be discussed further. S6 The introduction of the more electrophilic nitrogen atoms into the carbon lattice causes charge transfer from nearby carbon atoms, thereby activating the region for adsorption of intermediates and reactivity. In the SWNT-based systems (GRA-based systems were similar unless a reconstruction occurred), the N atoms induce positive charges of about 0.10-0.14 (Hirshfeld population analysis, see Table S3) into the nearest neighbor C atoms. The partial charges on N atoms range from -0.24 (graphitic N) to -0.26 (pyrrolic N; protonated or true pyrrolic N has a similar charge of -0.29) to about -0.36 (pyridinic N). This trend is in agreement with experimental XPS N1s ionization energy seen for N atoms in these environments in carbon systems. For example, Ding et al. 36 have estimated ionization energies of 401.3 (socalled N3 peak, see, also, e.g., ref. 37 ), 400.7 (N2 peak, see, also, e.g., ref. 38 ), and 398.6 eV (N1 peak) for, respectively, the graphitic, pyrrolic, and pyridinic N atoms in graphene. Similar peaks were seen by Davodi et al. 17 in their OER study of the nitrogen-doped MWNT-based system.
As expected, the decrease in ionization energy is correlated with a more negative partial charge of the N atom (however, in addition to charge transfer, the nature of the chemical bond influences the XPS shift). Note that protonated pyrrolic (or true pyrrolic) N has a similar partial charge as (unprotonated) pyrrolic N, and thus is expected to contribute to the same peak. Likewise, it has recently been proposed that protonated pyridinic N is thought to contribute to the N2 peak (still usually labeled the 'pyrrolic peak'), 38 which is reasonable given that it also possesses a similar partial charge. Thus, the so-called pyrrolic peak, N2, likely has contributions from unprotonated and protonated pyrrolic N and also from protonated pyridinic N atoms, which may account for its broader nature compared to the relatively more sharply defined peaks usually labeled graphitic (N3) and pyridinic (N1).
Besides peaks due to nitrogen in graphitic (N3 peak, strong), pyridinic (N1 peak, strong), and perhaps pyrrolic and the other discussed functionalities (N2 peak, weaker, broader), the S9 XPS N1s spectrum of the N-doped MWNT-based system employed by Davodi et al. 17 also shows evidence of adsorbed N atoms or N 2 molecules (the so-called N4 peak, ≈ 405 eV, weak) and oxidized pyridinic nitrogen (N5 peak, 405-407 eV, weak). As shown in the table, our modeled system thought to contribute to the N5 peak, the oxidized pyridinic N system, has an N atom whose calculated partial charge is close to zero. Instead, the attached O atom with its greater electrophillicity acquires a negative charge.
With regard to protonation, in our simulations, we observed that only the protonated pyridinic N was strongly acidic and surrendered its H atom to form water when attacked by the OH moiety (as opposed to protonated pyrrolic N and also protonated apical C atoms, which retained their protonation). Thus, the protonated pyridinic N systems may be less relevant than the unprotonated pyridinic N systems in the context of OER under alkaline conditions. In summary, the N atom partial charges we have calculated in our models of N-doped systems show a trend consistent with the experimentally observed XPS N 1s spectra of such systems. This serves to further validate our use of these models to represent the nitrogen functionalities in actual SWNT-based and GRA-based systems. Moreover, we have fully explored the full range of relevant nitrogen functionalities suggested by experimental XPS N1s measurements of N-doped CNT and GRA systems. S10 Table S3: Ranges of (Hirshfeld) partial charges ∆q [au] on the SWNT-based (GRA-based in parentheses) bare systems of N atoms and C nearest neighbor atoms; charges on a protonating H atom or an oxidizing O atom attached to the N atom are also shown where appropriate. The data is ordered by increasing magnitude of partial charge on the N atom in the SWNT-based systems (the ordering is the same for the GRA-based systems with one exception, the oxidized system, see the footnotes). A tentative correlation is made to observed peaks in XPS N1s measurements of N-doped carbon systems (bold text for commonly accepted assignments, italic text for other peaks) System(s) Type of N atom

Formation energies of the bare systems
To better judge the likelihood of occurrence of a particular defective and/or nitrogen-doped and/or H-passivated system N i V j H k , or, in a more general case, where oxygen is added, reaction with the chemical potentials being defined as energies of atoms referenced to stable reservoir species: where N C,Pristine = 336 or = 240 for SWNT-or GRA-based systems.
Before preceding further in the discussion, it must be noted that since the nitrogen-doping and defect formation, whether simple or complex (i.e., a single isolated defect versus two or more merged adjacent defects), is a nonequilibrium process which often occurs at elevated temperatures (≈ 700 K) and/or under irradiation and is also highly process-dependent, the formation energies presented here should serve as merely a guide to the possible types of systems. More relevant than the actual formation energy is the possibility of quenching S12 or trapping the system due to the presence of barriers acting to hinder reversal of the defect formation. That such trapping occurs and results in stable yet defective carbon-based systems is borne out by XPS studies, where defective systems remain stable even under the relatively harsh conditions of thousands of OER cycles. For an example of a recent study on graphene of Stone-Wales defect formation (and of other defects) via a nonequilibrium process as well as the barriers stabilizing the defective systems, the reader is referred to the review article of Banhart et al. 40 In general, the trends are similar to those observed by Kaukonen et al. 39 As seen in Table S4, for the N-doped systems required for OER activity, the formation energies ranged from about 1 eV to 6 eV for the SWNT-based systems (about 1 eV to 5 eV for the GRA- On the other extreme, the nearly highest formation energy was that of a single vacancy, system V 1 (6 eV and about 8 eV for SWNT and GRA, respectively). The formation energy of a double vacancy, system V 2 , was less. Introducing N to the V 1 -SWNT and creating the pyrrolic N 1 V 1 system required a slightly larger energy, 6.3 eV, while the pyridinic N 1 V 1 and multiple N atoms were present at the vertices (see, e.g., the system Pyridinic N 3 V 1 compared to the system Pyridinic N 1 V 1 ). Passivation of vertex C or N atoms by H atoms also lowered S14 the energy, however, as mentioned previously, H atoms on pyridinic nitrogens were found to be highly acidic in the alkaline environment (in our simulations, they were quickly removed by interaction with approaching OH moieties, resulting in H 2 O formation). S15 Table S4: Bold or italic text corresponds to those systems found to be OER highly active (defined here as a system/site with η OER < 0.50 V) or active (η OER = 0 .50 -0 .59 V), while normal text corresponds to inactive systems (η OER ≥ 0.60 V). See Table 1. ' * ' marks those systems for which OER calculations were not performed. The labels (a-k) are shown for easy reference to the SWNT-and GRA-based structures shown in in Figure 1 and Figure S1, respectively. Structures for the 'Additional systems', (l-r), are not shown. Note that for SWNT-based systems, the presence of a single defect (V 1 ) leads to the standard reconstruction of the hexagonal structure resulting in the typical 5-9 rings structure. (Then, substitution of an N atom into the (stable) 5-ring allows the N atom to be labeled 'pyrrolic'.) This reconstruction does not occur and the 5-ring does not form if at least two of the three C vertex atoms are substituted by N atoms and/or if the 5-ring C vertex atoms are protonated. The behavior of the GRA-based systems was similar, but the reconstruction was weaker and the resulting 5-9 rings structure was much less pronounced. Another approach to create a 5-ring in both SWNT and GRA systems was through the Stone-Wales rotation, resulting in the typical 7-5-5-7 rings structure. (Then, substitution of an N atom into a (stable) 5-ring allowed the N atom to be labeled 'pyrrolic'.) We also considered a 'true pyrrolic' system, i.e., the one labeled 'protonated pyrrolic'. Pyridinic systems do not have the nitrogen protonated unless so labeled. b Since graphene lacks an axis, this attempt to create a GRA-based system with a pyrrolic nitrogen results instead in a system with a pyridinic nitrogen atom. Thus, since the system in panel (e) differs from the one in panel (d) merely by a clockwise rotation of 120 • , it will not be discussed further. It is seen in Figure S12 that the scaling relation between ∆E * OH and ∆E * O was only very roughly linear, even if the data were divided into two sets, i.e., the set where the * O intermediate attached on top ('t') of the C atom from the set where it attached via a bridge ('b'; further divided in the case of the SWNT-based systems into axial bridge, 'ba'; and 'quasicircumferential bridge, 'bqc') between two C atoms. The two sets can be roughly fitted by two separate linear scaling relations. Only 'best' and 'far' sites are plotted to avoid clutter.
It is seen that that the best-performing systems/sites have the * O intermediate attached on top rather than as a bridge, top attachment is stronger than bridge attachment, and thus the best-performing sites generally have stronger attachment of * O. They also have stronger attachment of * OH (and thus also * OOH, due to the scaling relation).

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(a) SWNT-based systems ('best' and 'far' sites in the eleven main systems seen in Figure 1(a-k)  (b) GRA-based systems ('best' and 'far' sites in the ten main systems seen in Figure S1  166t (3) 220bqc (2) 223t (1) 330ba(far) Stone−Wales Pyrrolic N 1b 166t (2) 172t (3) 223t (1) 330ba(far) (b) GRA-based systems ('best' and 'far' sites in the ten main systems seen in Figure S1 (3) 108t (2) 129t (1) 239b ( Figure S14: Volcano plot for OER on SWNT-based systems (eleven main systems, plus five additional systems). The inset shows the region of best performance. Only the best site or site(s) are shown for each system (multiple non-equivalent sites existed because in a few cases more than one site per system exhibited good OER performance in the form of a low overpotential, i.e., η OER 0.5 V). The most highly active systems/sites are highlighted. Note that the additional five SWNT-based systems not discussed in Figure 6(a) of main text shown here are V 1 , V 2 , and Stone-Wales (these three differ in not having an N atom); and Pyridinic N 1 V 1 H 1 and Pyridinic N 1 V 1 H 3 (these two differ in having a protonated pyridinic N site and/or protonated C vertex atoms). None of these five additional systems showed good OER activity. S34