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J. Phys. Chem. C 2009, 113, 6730–6734
Nitrogen-Treated Graphite and Oxygen Electroreduction on Pyridinic Edge Sites Kiera A. Kurak and Alfred. B. Anderson* Chemistry Department, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland Ohio 44106-7078 ReceiVed: December 31, 2008
On the basis of literature reports, some nitrogen-treated graphite electrodes have been found to catalyze the four-electron electroreduction of O2 to water in acid. In this study the linear Gibbs energy relationship is used to predict the reversible potentials for forming intermediates during O2 reduction in acid over graphene doped with two N atoms substituting for adjacent edge CH groups. This procedure, generally accurate within ∼0.2 V, is useful for estimating overpotentials for electrode surface catalyzed reactions. Using bond strengths from VASP slab-band density functional calculations, it is predicted that one of the edge N has H bonded to it at potentials of ∼1.70 V and below. In the first reduction step, the OOH that forms then dissociates on the edge into O that bonds strongly to N with OH weakly associated with it. The calculated reversible potential is ∼0.89 V. The OH is proposed to abstract H from an edge NH, forming H2O. The H is then replaced in a reduction reaction. The reversible potential for reducing the O(ads) to OH(ads) on the edge with is ∼-0.60 V, well negative of the potential range of interest for oxygen reduction, which means this edge structure will be stable at the potentials of interest. The edge has an unpaired electron and OOH bonds to the C atom bridging ON · · · NH with a strength corresponding to a reversible potential of ∼0.73 V. This means that the two-electron reduction product, H2O2, can form at a potential close to the ∼0.695 V standard reversible potential. The absence of any apparent pathway for the direct four-electron reduction suggests that (i) some other catalytic site structure involving substituent N is involved or (ii) the peroxide pathway is being followed with O2 and H2O generation when peroxy intermediates disproportionate or (iii) impurity transition metals are contributing to direct four-electron reduction. Introduction The high overpotentials and high cost of oxygen cathodes using platinum continue to be bottlenecks to the wide scale implementation of polymer electrolyte membrane (PEM) fuel cells in transportation applications,1 though hydrogen powered fuel cells are finding applications where high power density is important and cost is a lesser issue, such as in laptop computers and cellular phones.2 To make fuel cells more generally useful, additional advancements with electrocatalysts are needed. Carbon supports are generally used in PEM fuel cells due to their stability and electrical conductivity, among other properties.3 The carbon supports by themselves are poor catalysts for the fourelectron reduction of oxygen to water, though they do provide activity for its two-electron reduction to hydrogen peroxide.4 Heat-treated carbon electrodes containing certain transition metals and nitrogen have long been known to be active toward four-electron oxygen reduction to water; an introduction to this literature may be found in ref 1. Ostensibly metal-free carbon materials containing nitrogen can be active toward oxygen reduction, as noted by Franke et al.5 Even after purification, the P33 carbon support material used in ref 5 contained 700 ppm iron. Others, including Yeager in his excellent survey of electrocatalysts for oxygen reduction, have warned that iron impurity may be involved in oxygen reduction over carbon nitride.6 Wang et al.7 showed that carbon support material Vulcan XC-72R, when heat-treated with HNO3 and NH3, possessed higher activities for hydrogen peroxide generation than the untreated carbon. However, the overpotential relative to the 1.229 V standard potential for the desired four-electron reduction to water was high, with an onset for reduction current * To whom correspondence should be addressed. E-mail
[email protected], phone 216-368-5044, fax 216-368-3006.
observed to be ∼0.5 V in cyclic voltammograms. Such a value is a relatively low, 0.2 V, overpotential for the two-electron reduction to hydrogen peroxide, for which the standard reversible potential is 0.695 V. In the absence of nitrogen treatment, the onset potential for reduction was ∼0.2 V, which is at 0.5 V overpotential to the two-electron oxygen reduction potential. Later work examined similarly prepared nitrogen-treated carbon electrode based on Ketjen Black EC 300J and the same onset potentials were found by Sidik et al.8 Reference 8 included a theory component whereby the reversible potentials for OOH(ads) and H2O2(aq) formation were predicted using a linear Gibbs energy relationship. The predicted reversible potentials were in good agreement with observed onset potentials for the untreated carbon and carbon nitride in ref 7. In the theory study, N substituting for C, referred to as graphitic nitrogen, was placed in the center of a 14-ring sheet of graphene with edge carbon atom valence satisfied by C-H bonds. The graphitic N atom provided a delocalized unpaired electron and its spin density was mainly on the neighboring C atoms. The first reduction step intermediate, OOH, was able to bond to one of these C atoms with a bond strength that corresponded, in the linear Gibbs energy relationship, to a 0.54 V reversible potential for H2O2(aq) formation. In the absence of graphitic nitrogen, the OOH was able to bond to C atoms of the edge C-H with a strength corresponding to 0.31 V reversible potential for H2O2(aq) formation. Both of the estimated potentials were in good agreement with the measurements. As shown in ref 8, the dominant product of reduction was hydrogen peroxide over graphite and nitrogen-treated graphite. For maximum energy extraction from the fuel in a fuel cell, four-electron reduction to water is the goal. Matter et al. have also explored the idea that nitrogen-treated graphite by itself, without metal present, can electrocatalyze the
10.1021/jp811518e CCC: $40.75 2009 American Chemical Society Published on Web 03/27/2009
Electroreduction on Pyridinic Edge Sites four-electron reduction of oxygen.9-11 Matter et al. performed X-ray photoelectron spectroscopic (XPS) measurements on catalysts prepared by heat treatment and found a correlation of increasing activity for four-electron reduction of O2 with increased amount of pyridinic nitrogen atoms, which are twocoordinate N occurring on edges of graphene with filled lonepair orbital completing the trigonal sp2 structure.9 In that study no effort was made to remove the iron and nickel used in catalyst synthesis. They subsequently learned from Mo¨ssbauer measurements on catalysts synthesized using iron particles that iron particle and iron carbide and other oxidized iron phases were in the catalyst.10 Acid washing removed measurable oxidized iron species but the iron particles remained, encapsulated within the graphite. Activity for oxygen reduction increased after acid washing, implying oxidized forms of iron are not participating in the electrocatalysis, and the activity was an apparent function of the amount of edge nitrogen atoms.11 Faubert et al. found that catalysts prepared by pyrolyzing iron macrocycles contained iron coordinated to pyridinic nitrogen atoms even after acid washing.12 Also, Wang et al. showed in ref 7 that when iron was added to the heat treated nitrided Vulcan XC-72R, the most active catalysts had one iron atom per 100 N atoms on the surface of the carbon support. This corresponded to an iron concentration of ∼1000 ppm, which is at the detection limit for XPS. These catalysts generated current at significantly higher potentials than the nitrided supports without added iron. Since iron below the detection limit by XPS is active, uncertainty remains about whether nitrogen-treated graphite can have high activity toward oxygen reduction in the absence of transition metal atoms at the active sites. This laboratory used the linear Gibbs energy relationship to estimate reversible potentials for oxygen reduction on a Co site coordinated to two edge pyridinic N atoms.13 It was found that CoII in the form of Co(OH)2 would support four-electron reduction to water. The site was, however, predicted to be unstable against anodic dissolution. Recently Nallathambi et al. prepared supported catalysts without the use of transition metals in their synthesis,14 as was the case in the work of Matter et al., and without adding transition metal as was the case in the work of Wang et al. Measurements of oxygen reduction in 0.5 M H2SO4 using ringdisk electrodes and several types of preparation of nitrogendoped carbon catalysts showed the best activity was for pyrolyzed selenourea-formaldehyde. The reduction onset was about 0.7 V and little H2O2 was formed. It was suggested that a peroxy intermediate might be chemically decomposing, regenerating O2. The net reduction was a nearly four-electron one but at a high overpotential relative to 1.229 V for the direct four-electron to water and a low overpotential relative to 0.695 V for the two-electron reduction to hydrogen peroxide. A twostep mechanism has also been proposed by Maldonado and Stevenson for O2 reduction in neutral and basic solutions.15 The conclusion can be drawn from the studies discussed above that nitrided graphite has activity toward oxygen reduction. When a small amount of iron is known to be present, the overpotential relative to the 1.229 V reversible potential for the direct fourelectron reduction to water is the least. When transition metals may not be involved in activating the reduction, the reduction onset potential seems to be limited by the 0.695 V hydrogen peroxide formation potential. More hydrogen peroxide is formed in some preparations than in others. The best one, prepared with selenium in the reactants prior to heat treatment, forms little hydrogen peroxide. In this case it is not definitely known whether the reaction is direct four-electron reduction to water or a two-electron reduction
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6731
Figure 1. C16H4N2 translational cell used in the VASP calculations. Gray spheres are carbon, white circles are hydrogen, and the blue are nitrogen.
to hydrogen peroxide followed by a two-electron reduction of the hydrogen peroxide to water. Prior theory shows that the unpaired electron associated with a graphitic N atom can activate adjacent C atoms for H2O2 generation. Theory has not yet been applied to exploring the oxygen reduction capability of pyridinic N atoms at the edges of graphite sheets when no metal is present. What is the state of these N atoms? Will they activate the direct four-electron reduction? These are the questions addressed in this paper. Theory An approximate linear relationship between reaction energies and reaction Gibbs free energies has been found to apply to surface redox reactions.7,13,16-23 These references demonstrate capability for estimating reversible potentials for forming adsorbed surface intermediates as functions of adsorption bond strengths and standard reversible reduction potentials, U° for the reactions in bulk solutions:
Urev(surface) ) U° + ∆E(adsorption)/nF
(1)
Here ∆E(adsorption) is the adsorption bond strength of the product minus the adsorption bond strength of the reactant. This approximate equation makes useful predictions because (or when) the P∆V - T∆S contribution to the reaction Gibbs energy is approximately the same when the reactant and product are adsorbed on the catalyst surface and when they are in bulk solution. In this limit, for reactant and product, the Gibbs energy of part of the ion stabilization due to its solvation shell in bulk solution is replaced by the energy of adsorption to the electrode. Assuming that activation energies at the reversible potentials for the reduction steps are low, it is the value of the reversible potential itself that can be compared to the overpotentials for the steps. Comparisons between predicted and measured values in the above references indicate that predictions of reversible potentials made for adsorbed reaction intermediates in this way have errors of ∼0.2 V. To calculate molecular structures and adsorption energies on extended graphene edges, the Vienna ab initio Software Package (VASP)24-26 was used. The plane wave band calculations employed the 1991 Perdew-Wang27 method and the generalized gradient approximation with ultrasoft pseudo potentials for the C, N, O, and H atoms.28 Integrations were over a 5 × 5 × 1 k-point mesh. The C16H4N2 graphene translational cell is shown in Figure 1: the front edge has two nitrogen atoms substituting for C-H and the back edge is terminated with C-H. When translated, the cell produces a one-dimensional ribbon. Each
6732 J. Phys. Chem. C, Vol. 113, No. 16, 2009
Kurak and Anderson
TABLE 1: Experimental Values29 for the Standard Reversible Potentials, U°, for Reactions in This Study +
reaction
U° (V/SHE)
-
-0.046 -2.757 -0.664 1.436 2.813 2.047 -2.110
O2(g) + H (aq) + e a OOH(aq) O2(g) + H+(aq) + e- a O(aq) + OH(aq) OOH(aq) + H+(aq) + e- a 2OH(aq) OOH(aq) + H+(aq) + e- a H2O2(aq) OH(g) + H+(aq) + e- a H2O(aq) O(aq) + H+(aq) + e- a OH(aq) H+(aq) + e- a H(aq)
ribbon is separated by about 7 Å from its neighbors in the ribbon plane and the planes are separated by 21.2 Å. In the adsorption studies, the atoms in the zigzag edge containing the two N atoms were giving full freedom to relax, the C atoms in the central zigzag row were allowed to relax only in the plane, and, finally, atoms in the other zigzag edge were not allowed to relax. The unrelaxed atoms were in positions determined by fully relaxing a ribbon of twice the width. Some calculations were done with a model for the edge of undoped graphene. The C18H6 translational cell was fully optimized for them and the adsorption calculations followed the procedure used for C16H4N2. Results and Discussion Calculations using the C16H4N2 translational cell produced no band gap, which is consistent with the electrical conduction property of graphene. The question of edge pyridinic N being blocked by under-potential-deposited hydrogen was investigated first. According to the VASP band calculations, the first H forms a bond with strength 3.804 eV to an edge. Using experimental U° (Table 1) the resulting reversible potential is 1.69 V for the reaction
The strength of bonding of another H to the adjacent edge N is 2.007 eV which corresponds to a reversible potential of -0.10 V for the reaction:
Thus, one N is predicted to be blocked by hydrogen, and the other N remains unblocked at potentials of -0.10 V up to 1.69 V. Could the H on the N, which introduces an unpaired electron, cause ORR reactivity by strengthening the absorption of OOH to the catalyst, as in ref 8? Bonds strengths of possible oxygen reduction intermediates to the edge N were calculated. The structures and energies are shown in Figure 2: O2 did not bond, H2O formed a weak hydrogen bond of 0.229 eV strength to the N, and OOH bonded to N with the O-O bond elongated to 2.62Å. The OOH is formed by the reaction
Figure 2. Strengths of adsorption bonds (eV) on the nitrogenated graphite edge with one N blocked by H.
Figure 3. Sites on the ON · · · NH edge where OH and OOH adsorptions were studied. Bond strengths on the sites of greatest stability are given.
On the basis of the calculated N-OOH bond strength of 0.864 eV, the reversible potential is 0.89 V. The strength of the stretched O-O bond in O · · · OH is only 1.197 eV and, in the presence of NO, the N-H bond strength is 3.540 eV, so, given the 5.947 eV calculated bond strength for the OH bond in H2O, the OH in O · · · OH is proposed to abstract a hydrogen atom from N-H, forming water. Given the strength of the N-H bond, the abstracted H will be replaced at potentials of 1.43 V and less. The N-O bond is strong, 3.649 eV, and the N-OH bond is much weaker at 1.002 eV due to the promotion of an electron to the conduction band when the H atom is added to the O. The resulting reduction potential is negative, -0.60 V, for the reaction
This means O will remain bonded to the edge N in the potential range of interest for O2 reduction. The net result of
Electroreduction on Pyridinic Edge Sites
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6733
Figure 4. Predicted catalytic cycle for the two-electron reduction on the catalytic ON · · · NH edge site.
the two one-electron transfer reactions is the formation of a stable ON · · · NH edge termination at potentials above -0.60 V. With H bonded to one N and the other N blocked by O, various OH and OOH adsorption sites were examined. An unpaired electron is present and it has the potential to stabilize adsorbed OH or OOH. Adsorption bond strengths were calculated for OOH over several sites, including the edge NH and surrounding C. The highest bond strength was 0.776 eV on the site consisting of C bridging the N atoms as shown in Figure 3. This corresponds to 0.73 V reduction potential, which is close to 0.695 V, the reversible potential for the two-electron reduction to H2O2. OH was calculated to adsorb on many of the sites and also bonded most strongly, by 2.251 eV, to the C bridging the N atoms. Taking into account the 0.214 eV strength of the H2O hydrogen bond to the O of the NO site, the reduction potential for the OH is calculated to be 0.78 V. This means the edge containing NO and NH is predicted to maintain an unpaired electron up to 0.78 V, which allows OOH(ads) formation at potentials up to the 0.695 V reversible potential for the twoelectron reduction to hydrogen peroxide. Using the 0.776 eV OOH adsorption bond strength, and assuming the hydrogen peroxide product is not adsorbed at the edge, the reversible potential for the second reduction step is 0.66 V. Thus, both electron transfer steps are calculated to take place close to the ideal potential. Figure 4 shows the predicted catalytic cycle for the two-electron reduction. Oxygen reduction to OOH adsorbed to a carbon atom of an edge CH in a cluster model of graphene was previously calculated to have 0.31 V reversible potential.8 This was recalculated with VASP using a ribbon model like the one used for the nitride edge. The predicted reversible potential for hydrogen peroxide generation on the pure graphite edge is 0.27 V, which is in satisfactory agreement with the earlier result from the cluster model. This potential is 0.39 V less than that for the 2N-modified edge. The predicted reduction in the overpotential suggests that two adjacent pyridinic N on graphite edges will, after becoming ON · · · NH, activate oxygen reduction by the twoelectron pathway at a possibly smaller overpotential than graphitic N. However, the O bonded to N cannot be reduced at sufficiently positive potential to participate in a useful direct four-electron reduction to water. Conclusions Theory indicates that, unlike on platinum electrodes,17 H2O is not oxidized to OH(ads), on graphene or nitrided graphene due to the weakness of the adsorption bond strengths and the consequent high reversible potentials. Adsorption of possible oxygen intermediates on edge pyridinic nitrogen atoms is blocked after initial reductions leading to O bonded to one N and H bonded to the other. Adsorbed oxygen will only be reductively eliminated from the edge N at the potential of -0.60 V and less. However, the ON · · · NH edge, having an unpaired electron, bonds OOH to the C bridging the N with the proper
strength for the two-electron reduction to H2O2 to take place. The OH would block the C adsorption site only at potentials greater than 0.78 V. The OOH formation potential is predicted to be 0.73 V, which is near the ideal reversible potential of 0.695 V. The second reduction, forming H2O2 will also be near 0.695 V. Neither O2 nor OOH will dissociate without some further catalytic interaction. Such interactions might be provided by intentional addition of transition metals or by residual traces of metal atoms if present after acid washing. No evidence for the direct four-electron oxygen reduction to water has been found in the metal-free nitrided graphite according to structures employed so far, but there is strong evidence for two-electron reduction to hydrogen peroxide at low overpotential. This leaves a decomposition mechanism involving a peroxy intermediate to form O2 and H2O as worth considering in future analyses that are aimed at explaining the four-electron reduction in the absence of transition metal catalysts. In the case of catalysts prepared by pyrolysis of selenourea-formaldehyde, which show very little hydrogen peroxide formation, the role of selenium in aiding the disproportionation pathway or the direct fourelectron pathway presents an interesting problem. Acknowledgment. This work was supported by MultiUniversity Research Initiative (MURI) Grant DAAD 19-03-10169 from the Army Research Office to Case Western Reserve University and by the National Science Foundation, Grant No. CHE-0809209. References and Notes (1) Dodelet, J. P. Oxygen Reduction in PEM Fuel Cell Conditions: Heat-Treated Non-Precious Metal-N4 Macrocycles and Beyond. In N4 -Macrocyclic Metal Complexes; Zagal, J. H., Bedioui, F., Dodelet, J. P., Eds.; Springer Science + Business Media, LLC: New York, 2006; pp 83148. (2) Tu¨ber, K.; Zobel, M.; Schmidt, H.; Hebling, C. J. Power Sources 2003, 122, 1–8. (3) Inagaki, M. Applications of Polycrystalline Graphite. In Graphite and Precursors; Delhae´s, P., Ed.; World of Carbon; Gordon and Breach Science Publishers: Canada, 2001; Vol. 1, pp 179-198. (4) Boehm, H. P. Carbon Surface Chemistry. In Graphite and Precursors; Delhae´s, P., Ed.; World of Carbon; Gordon and Breach Science Publishers: Canada, 2001; Vol. 1, pp 141-178. (5) Franke, R.; Ohms, D.; Wiesener, K. J. Electroanal. Chem. 1989, 260, 63–73. (6) Yeager, E. Electrochim. Acta 1984, 29, 1527–1537. (7) Wang, H.; Cote, R.; Faubert, G.; Guay, D.; Dodelet, J. P. J. Phys. Chem. B 1999, 103. (8) Sidik, R. A.; Anderson, A. B.; Subramanian, N. P.; Kumaraguru, S. P.; Popov, B. N. J. Phys. Chem. B 2006, 110, 1787–1793. (9) Matter, P. H.; Zhang, L.; Ozkan, U. S. J. Catal. 2006, 239, 83–96. (10) Matter, P. H.; Wang, E.; Millet, J. M.; Ozkan, U. S. J. Phys. Chem. C 2007, 111, 1444–1450. (11) Matter, P. H.; Wang, E.; Arias, M.; Biddinger, E. J.; Ozkan, U. S. J. Mol. Catal. A: Chem. 2007, 264, 73–81. (12) Faubert, G.; Cote, R.; Dodelet, J. P.; Leferre, M.; Bertrand, P. Electrochim. Acta 1999, 44, 2589–2603. (13) Vayner, E.; Anderson, A. B. J. Phys. Chem. C 2007, 111, 9330– 9336. (14) Nallathambi, V.; Lee, J.-W.; Kumaraguru, S. P.; Wu, G.; Popov, B. N. J. Power Sources 2008, 183, 34–42.
6734 J. Phys. Chem. C, Vol. 113, No. 16, 2009 (15) Maldonado, S.; Stevenson, K. J. J. Phys. Chem. B 2005, 109, 4707– 4716. (16) Anderson, A. B.; Albu, T. V. J. Am. Chem. Soc. 1999, 121, 11855– 11863. (17) Roques, J.; Anderson, A. B. J. Electrochem. Soc. 2004, 151, E85E91. (18) Anderson, A. B.; Sidik, R. A. J. Phys. Chem. B 2004, 108, 5031– 5035. (19) Roques, J.; Anderson, A. B. J. Fuel Cell Sci. Technol. 2005, 2, 86–93. (20) Schweiger, H.; Vayner, E.; Anderson, A. B. Electrochem. SolidState Lett. 2005, 8, A585-A587. (21) Vayner, E.; Schweiger, H.; Anderson, A. B. J. Electroanal. Chem. 2007, 607, 90–100.
Kurak and Anderson (22) Sidik, R. A.; Anderson, A. B. J. Phys. Chem. B 2006, 110, 936– 941. (23) Sidik, R. A.; Anderson, A. B. J. Phys. Chem. B 2006, 110, 1787– 1793. (24) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (25) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (26) Kresse, G.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169. (27) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; et al. Phys. ReV. B 1992, 46, 6671. (28) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (29) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Marcel Dekker, Inc.: New York, 1985.
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