Abstract:
Searching a non-noble metal catalyst for electrochemical energy conversion, such as oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), has been a holy grail for bringing electrocatalyst systems into more widespread applications. Among them, the transition metal complexes, characterized by a redox active single-metal-atom with biomimetic ligands, have been attractive for both ORR and HER. For instance, acid treated py-B12 (an MN4 structure) supported on reduced graphene oxide shows an enhanced HER activity with extremely high turnover frequencies per cobalt site in acid. Recently, we have developed a couple of facile methods to co-dope the MN4 structure with heteroatoms, either in coplanar or axial configuration. Two representative cases are (i) atomically dispersed cobalt on nitrogen and sulfur co-doped graphene to form Co-N3S electrocatalyst for HER, and (ii) axial chlorine integrated atomically dispersed single Fe-N4 electrocatalyst for ORR. The electronic and geometric properties of the MN4 catalyst can be effectively tuned by co-doping, leading to enhanced HER or ORR activities.
To study the binding and electronic structures of these nano-architectures, synchrotron radiation-based techniques, such as X‐ray absorption spectroscopy (XAS), X‐ray absorption near edge structure (XANES), and extended X‐ray absorption fine structure (EXAFS), have been employed. Moreover, the ORR mechanism of py-B12 is studied through operando XAS coupled with electrochemical impedance spectroscopy. Our results revealed preferential adsorption of oxygen at the Co2+ center with end-on coordination forming an oxo-like species. These experimental findings, corroborated with first-principles calculations, provide new insight into metal active-site geometry and structure evolution during ORR and can aid in developing material design strategies for high-performance electrocatalysts for fuel cell application.
Keywords: X-ray absorption spectroscopy, Electrocatalyst, Single-atom catalyst, MN4, in situ/operando.
References:
[1] P. Sabhapathy, P. Raghunath, A. Sabbah, I. Shown, K. S. Bayikadi, R.-K. Xie, M.-C. Lin, K. H. Chen and L. C. Chen, ‘Mechanistic Paradigm and Axial Chlorine Integrated Atomically Dispersed Single Fe-N4 Electrocatalyst for Oxygen Reduction Reaction’, submitted (2022).
[2] P. Sabhapathy, I. Shown, A. Sabbah, P. Raghunath, Jeng-Lung Chen, W. F. Chen, M. C. Lin, K. H. Chen, and L. C. Chen, ‘Electronic Structure Modulation of Isolated Co-N4 Electrocatalyst by Sulfur: Improved pH-universal Hydrogen Evolution Reaction’, Nano Energy 80, 105544 (2021).
[3] H.-T. Lien, S.-T. Chang, P.-T. Chen, D. P. Wong, Y.-C. Chang, Y.-R. Lu, C.-L. Dong, K. H. Chen, and L. C. Chen, ‘Probing the Active Site in Single-atom Oxygen Reduction Catalysts via Operando X-Ray and Electrochemical Spectroscopy’, Nature Comm. 11, 4233 (2020).
[4] P. Sabhapathy, C.-C. Liao, W.-F. Chen, T.-C. Chou, I. Shown, A. Sabbah, Y.-G. Lin, J.-F. Lee, M.-K. Tsai, K. H. Chen and L. C. Chen, ‘Highly Efficient Nitrogen and Carbon Coordinated N-Co-C Electrocatalyst on Reduced Graphene Oxide Derived from Vitamin-B12 for Hydrogen Evolution Reaction’, J. Mater. Chem. A 7, 7179 (2019).