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We have used density functional theory calculations to study the stability of high density bulk defects in carbon with and without nitrogen. It was found that all the structures are energetically unstable, and the stability of the defects decreases with increasing number of nitrogen atoms independent of synthesis route and the carbon reference state. The results also highlight the significance of stabilizing factors for these defects, such as transition metals which are often used during synthesis of the carbon supports.

Electrochemical reduction of CO2 (CO2RR) to value-added liquid fuels or chemical feedstocks using off-peak electricity or intermittent renewable energy sources is a green approach for energy storage and carbon recycling. However, the difficult activation of CO2 to form the CO2 ∙− intermediate and the competitive hydrogen evolution (HER) in aqueous electrolytes make the Faradaic efficiency and selectivity for the CO2RR remain very low. From another perspective, CO2 reduction to CO, with co-production of H2 from water, is very appealing as the mixture of CO and H2 with controlled ratio can be used as syngas for methanol synthesis and Fischer-Tropsch reactions. In this approach, catalyst search is therefore no longer limited to metals with low HER activity such as Au and Ag. Materials with fair HER activity can be potentially considered for the CO2RR. Interestingly, Pd, a good HER catalyst, has been found to continuously produce CO in the CO2RR, which is however counterintuitive to that Pd is prone to CO poisoning.[1] In this work, we have re-examined Pd for the CO2RR using in situ XAS and in situ XRD, and have found that Pd transforms into β-PdH under the CO2RR condition. DFT calculations suggest that CO adsorption energy on the PdH surface is greatly reduced compared to that on the Pd surface, which is most likely the cause for high selectivity of CO2RR to CO when using Pd as the electrocatalyst.[2] [1] Y. Hori, Modern Aspects of Electrochemistry, Springer, New York, 2008, vol. 42, ch. 3, pp. 89–189. [2] Wenchao Sheng, Shyam Kattel, Siyu Yao, Binhang Yan, Zhixiu Liang, Christopher J. Hawxhurst, Qiyuan Wu and Jingguang G. Chen, Energy & Environmental Science, 10, 1180-1185 (2017)

Proton exchange membrane fuel cells (PEMFCs) have attracted enormous attention as a clean and sustainable source of power generation. However, the overall performance of a PEMFC is controlled by the sluggish oxygen reduction reaction (ORR) at the cathode which requires an efficient catalyst for effective reduction of O2 to H2O. Currently, Pt-based catalysts are the best performing ORR catalysts.1 The price of rare and expensive Pt-based catalysts contributes significantly towards the total cost of a PEMFC and hinders its scaled up application. Thus, it is desirable to explore cost-effective non-Pt based ORR catalysts as alternatives to Pt-based catalysts. Of particular interests are transition metals (Me=Fe, Co, Ni) and nitrogen (N) containing materials (Me-N-C catalysts) with embedded MeN4 clusters in carbon support.2-5 Previous experimental studies have illustrated that heat treated Fe-N-C catalysts promote a 4e- reduction of O2 to H2O with their catalytic activity for ORR comparable to that of the benchmark Pt catalyst.2,3 However the chemical nature of ORR active sites and the mechanism of ORR in Fe-N-C catalysts are still poorly understood. Here, we report density functional theory (DFT) study of the mechanism of ORR on Fe-N-C catalysts. Firstly, we studied the thermodynamic stability of various nitrogen Nx (x=0, 1, 2, 3, and 4) and combined transition metal and nitrogen FeNx (x=0, 1, 2, 3, and 4) clusters in a monolayer graphene. We found that the formation energies are very high for all Nx clusters. In contrast, the formation energies of FeNx clusters are significantly smaller than those of the corresponding Nx clusters. Our DFT results show that the formation energy of FeNx clusters decreases with increasing N coordination to Fe and FeN4 cluster has the lowest formation energy. Thus, we predict that FeN4 clusters are thermodynamically stable in a monolayer graphene. The possible mechanism of ORR on the FeN4 clusters in graphene is investigated through DFT transition state calculations. We calculated the activation energies (Ea) for all the possible elementary chemical reactions in ORR on the FeN4 clusters embedded in graphene. O2 dissociation on the FeN4 clusters (Fig. 1a) has high activation energy of 1.20 eV. In contrast, O2 hydrogenation chemical reaction (Fig. 1b) to form OOH is nearly barrierless (Ea=0.01 eV). Following the formation of OOH, the O-O bond scission in OOH can occur either by its direct dissociation to form O and OH (Fig. 1c) or by its hydrogenation chemical reaction to form 2OH (Fig. 1d). The activation energy for the former reaction is 0.64 eV and the latter reaction is 0.84 eV. The activation energies for subsequent reduction of O to OH and OH to H2O are 0.24 and 0.44 eV respectively. Based on our calculated activation energies of the elementary steps in ORR on the FeN4 clusters, we predict that O2 hydrogenation to form OOH is kinetically favorable over direct O2 dissociation. At U=0V, OOH formation would be followed by its dissociation to O and OH. Thus our results show that the ORR on the FeN4 clusters would favorably proceed via an OOH dissociation ORR mechanism. Since the activation energies for O-O bond scission in OOH by its direct dissociation to form O and OH or by its hydrogenation to form 2OH are not very different, a competing (H+OOH) dissociation ORR mechanism cannot be ruled at non-zero electrode potential. In either mechanisms, our calculated activation energy of the rate determining step is comparable to that (0.79 eV)6 on the Pt(111) surface. Hence we predict a comparable catalytic activity of FeN4 clusters embedded in graphene for ORR to that of Pt-based catalysts in agreement with the experimental findings. Our theoretical calculations provide an explanation for the experimentally observed 4e- ORR on Fe-N-C catalysts. Acknowledgements This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant no. DE-FG02-09ER16093). We also acknowledge the research grant from the EERE program of the U.S. Department of Energy (Grant no. DE-AC02-06CH11357). References (1) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247. (2) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443-447. (3) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Science 2009, 324, 71-74. (4) Kattel, S.; Atanassov, P.; Kiefer, B. Phys. Chem. Chem. Phys. 2013, 15, 148-153. (5) Kattel, S.; Wang. G. J. Mater. Chem. A 2013, 1, 10790–10797. (6) Duan, Z. Y.; Wang, G. F. Phys. Chem. Chem. Phys. 2011, 13, 20178-20187.

Power generation is one of the key challenges in the current century due to decreasing natural energy resources, population growth and the goal to reduction of carbon emissions. Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention for potential power generation technology especially for mobile application. However, PEMFC currently faces several challenges: high cost of Platinum group metals catalysts, the slow kinetics of oxygen reduction reaction (ORR) at the cathode and the durability of cathode catalysts in harsh acidic environment. Alloying Pt modifies the surface electronic structures and at the same time reduces the use of expensive Pt from the catalyst design. Recently, 3d-transition metals (TMs) alloyed Pt catalysts have been reported to show superior ORR activity as compared to pure Pt catalyst.1-5 For example, the ORR activity enhancement for bimetallic Pt3Ni(111) extended single crystal surface is reported to be 10 times (10´) higher than pure Pt(111) surface and 90´ higher than the commercial Pt/C nanoparticle catalysts.1 With these encouraging studies, catalysts design by controlling composition, sizes and shapes of Pt-based alloy nanoparticle are now under extensive investigations. Within this contribution, we employed an atomistic Monte Carlo (MC) simulation method to predict the equilibrium surface structures of Pt alloy surfaces. Based on the calculated surface composition profile, we built Pt surface-segregated surface slab models for Pt and Pt alloy catalysts. The possible ORR mechanism on the (111) and (100) surfaces of Pt and Pt alloys are explained by first-principles density functional theory (DFT) calculations. Our DFT computations predict that alloying Pt with transition metal alters the adsorption energetics of the ORR intermediates and changes the mechanism of ORR compared to pure Pt catalyst. The ORR on Pt and Pt alloy catalysts can proceed via one of the following ORR mechanisms: O2 dissociation, OOH dissociation or H2O2 dissociation ORR mechanism. Our calculations show that the ORR occurs via an OOH dissociation mechanism on the pure Pt(111) surface. In contrast, we predict that the ORR on Pt(100) surface proceeds via an O2 dissociation ORR mechanism. Our calculated activation energy for the rate determining step (RDS) on the Pt(111) surface (0.79 e V)3 is very similar to that (0.80 eV)4 on the Pt(100) surface. This suggests that the (111) and (100) facets of Pt nanocrystals show similar catalytic activity for ORR. We further studied the mechanism of ORR on Pt surface segregated modified Pt/TM(111) (TM =Fe, Co, Ni) and Pt3TM(111) (TM=Ti, V, Fe, Ni) surfaces. We found that the activation energy of the RDS on modified Pt/TM(111) and Pt3TM(111) surfaces is always smaller than that on the pure Pt(111) surface. Thus modified Pt/TM(111) and Pt3TM(111) surfaces have superior catalytic activity for ORR compared to the ORR activity of pure Pt(111) surface. The DFT calculations further predict that the most favorable ORR mechanism on the modified Pt(111) surfaces is H2O2 dissociation ORR mechanism. We found that Ni modified Pt/TM(111) and Pt3TM(111) surfaces show highest catalytic activity for ORR. Contrarily, our DFT calculations demonstrates that the catalytic activity of modified Pt/Ni(100) surface is very similar to that of pure Pt(100) surface. All these predictions are in excellent agreement with previous experimental measurements. Acknowledgements This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant no. DE-FG02-09ER16093). We also acknowledge the research grant from the EERE program of the U.S. Department of Energy (Grant no. DE-AC02-06CH11357). References (1) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493-497. (2) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247. (3) Duan, Z. Y.; Wang, G. F. Phys. Chem. Chem. Phys. 2011, 13, 20178-20187. (4) Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 6284−6292. (5) Kattel, S.; Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 7107−7113.