Revisiting the oxygen reduction reaction activity of two-dimensional TM-C2N electrocatalysts via constant-potential density functional theory: crucial impact of the spin state and coordination†

Abstract

Single-atom catalysts (SACs) have shown great potential in catalyzing the oxygen reduction reaction (ORR) in fuel cell batteries. In carbon-based SACs, besides the most representative TM-N₄-C, other 2D carbon nitrides are used as substrates to fabricate SACs, such as C₂N, which receives less attention than TM-N₄-C. In addition, the significant effects of spin multiplicity and spin evolution in TM-N₄-C have been proposed, underlining the necessity to include spin evolution in mechanistic studies. To understand the influence of spin and coordination of TM-C₂N SACs in ORR catalysis, we employed first-principles density functional theory (DFT) calculations with a constant-potential model (CPM) to systematically investigate the ORR mechanism with various TM centers (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu). A spin-dependent ORR pathway was found to dominate the reaction rate, depending on electrode potential. The *OH adsorption energy on the TM site is the key factor to determine the valence, spin state and coordination number of active sites. By fully exploring the constant-potential free energy diagram of all ORR pathways, the potential-dependent switchable ORR path was found in Fe-, Co-, and Ni-based TM–C₂N, accompanied with spin-state transition in active centers. However, the most excellent ORR activity was found in Cu–C₂N with a predicted onset potential of 0.9 V vs. SHE and subtle spin variation on the Cu center during the ORR process. Decomposed polarization current indicates that overall ORR kinetics is jointly determined by the partition and activity of active moieties, which are both correlated with G_*OH and magnetic moment on the TM center. Our work reveals the voltage-driven evolution of the spin state and coordination on TM–C₂N in the ORR process, which could provide significant insights into the development of spin-related catalytic mechanism and SACs.