Grand Canonical Potential Kinetics of CO2 Reduction Reaction over Graphene-Supported Single-Atom Catalysts

Abstract

The Electrochemical reduction of CO2 to CO offers a promising strategy for managing the global carbon balance, but lack of selectivity towards traditional elctrocatlysts forces us to find an effective catalysts with high selectivity and stability. The single atom catalysts has emerged as potential candidate in the field of heterogeneous catalysis by offering single active site for different chemical reaction.1-3 Here, we studied the CO2 reduction mechanism over graphene supported Ni-single atom catalysts with the help of grand canonocal potential kinetics (GCP-K) formulation based on thermodynamics from quantum calculations.4 The GCP-K formulation obtained from the minimization of free enrgy using a legendare transformation relating the net charge of the system and applied potential. Using the transformation we are able to make a relation between GCP-K formulation with traditional Butler-Volmer kinetics, which allows us to describe the CO2 to CO reduction process elaborately at neutral medium and different applied voltages. We found that the geometry of transition states are continuously changes towards reaction coordinates with the applied potential as in Figure 1. The figure shows the transition states (TS) at lower potential looks similar to product geometry with high energy barrier while when we applied potential the TS moves towards reactant with lowering the reaction barrier. We also observed fraction of charges transferred from electrode to the reaction speices during the reaction at specific potential. Moreover, according to our GCP-K formulation, we describe the charge transfer from electrode to adsorbed species are continuous throughout the reaction path over traditional Butler-Volmer kinetics where a complete electron transfer from electrode to product shows discontinuty within reaction plane. The calcualtion results showed that the trans intermediate is more stable at higher potential than cis form of *COOH and the reaction barrier is lower along this pathways. We have shown that HER reaction is kinetically hindered on single Ni active sites, which provides almost 100% faradic CO efficiency. Finally, we found that the potential reaquied to produce 10 mA/cm2 current density is about -0.85 V, showing excellect aggrement with experimental literatures.5, 6 L. Yang, D. Cheng, H. Xu, X. Zeng, X. Wan, J. Shui, Z. Xiang and D. Cao, Proceedings of the National Academy of Sciences, 2018, 115, 6626-6631.M. D. Hossain, Z. Liu, M. Zhuang, X. Yan, G.-L. Xu, C. A. Gadre, A. Tyagi, I. H. Abidi, C.-J. Sun, H. Wong, A. Guda, Y. Hao, X. Pan, K. Amine and Z. Luo, Advanced Energy Materials, 2019, 9, 1803689.Y. Chen, S. Ji, C. Chen, Q. Peng, D. Wang and Y. Li, Joule, 2018, 2, 1242-1264.Y. Huang, R. J. Nielsen and W. A. Goddard, Journal of the American Chemical Society, 2018, 140, 16773-16782.H. B. Yang, S.-F. Hung, S. Liu, K. Yuan, S. Miao, L. Zhang, X. Huang, H.-Y. Wang, W. Cai, R. Chen, J. Gao, X. Yang, W. Chen, Y. Huang, H. M. Chen, C. M. Li, T. Zhang and B. Liu, Nature Energy, 2018, 3, 140-147.C. Zhao, X. Dai, T. Yao, W. Chen, X. Wang, J. Wang, J. Yang, S. Wei, Y. Wu and Y. Li, Journal of the American Chemical Society, 2017, 139, 8078-8081. Figure 1. Transition state (TS) changes with applied potential for trans-COOH to CO formation step. (a) TS moving towards reactant with decreasing reaction barrier as a function of potential, (b), (c) Reaction coordinates changes linearly with charges within the TS when potential is applied to begin the conversion process. Figure 1