Mechanistic Insights into CO₂ Electroreduction on Ni₂P: Understanding Its Selectivity toward Multicarbon Products

Recently, nickel phosphides (Ni_xP_y) have been reported to enable selective electrochemical formation of multicarbon products (C₃ and C₄) via the CO₂ reduction reaction (CO₂RR); nevertheless, their activities remain low. In order to understand the roots of their high selectivity and low activity and to direct the design of more active Ni_xP_y-based CO₂RR catalysts, we investigate the CO₂RR mechanism on Ni₂P using density functional theory (DFT) calculations. We reveal that the reaction proceeds through the formate pathway, followed by formaldehyde (H₂CO*) formation and self-condensation. Moreover, we demonstrate that surface hydride transfer steps, along with surface-mediated C–C coupling, are essential in order to avoid C₁ product formation and boost selectivity toward multicarbon products. In addition, we find that the thermal surface hydride transfer from the surface to the physisorbed CO₂ is one of the key rate-limiting steps, and since it is not electroactive, it cannot be accelerated by applying an overpotential. Finally, our results also show that the hydrogen affinity of the surface and the dynamic surface reconstruction via H adsorption facilitate selective CO₂ reduction and C–C coupling on Ni₂P. These findings provide an impetus for exploring materials design space to identify the physical principles that govern the thermodynamics of rate-limiting thermal steps in electrocatalytic processes.