Graphene-based single-atom catalysts for electrochemical CO reduction: unraveling the roles of metals and dopants in tuning activity.

Discovering electrocatalysts that can efficiently convert carbon dioxide (CO) to valuable fuels and feedstocks using excess renewable electricity is an emergent carbon-neutral technology. A single metal atom embedded in doped graphene, , single-atom catalyst (SAC), possesses high activity and selectivity for electrochemical CO reduction (COR) to CO, yet further reduction to hydrocarbons is challenging. Here, using density functional theory calculations, we investigate stability and reactivity of a broad SAC chemical space with various metal centers (3d transition metals) and dopants (2p dopants of B, N, O; 3p dopants of P, S) as electrocatalysts for COR to methane and methanol. We observe that the rigidities of these SACs depend on the type of dopants, with 3p-coordinating SACs exhibiting more severe out-of-plane distortion than 2p-coordinating SACs. Using CO adsorption energy as a descriptor for COR reactivity, we narrow down the candidates and identify seven SACs with near-optimal CO binding strength. We then elucidate full reaction mechanisms towards methane and methanol generation on these identified candidates and observe highly dopant-dependent activity and rate-limiting steps, divergent from conventional mechanistic understanding on metallic surfaces, calling into question whether previous design principles established on metals are directly transferrable to SACs. Consequently, we find that zinc embedded in boron-doped graphene (Zn-B-C) is a highly active catalyst for electrochemical COR to C hydrocarbons. Our work reveals the opportunities of tuning SAC reactivity engineering dopants and metals and highlights the importance of re-elucidating COR reaction mechanisms on SACs towards unearthing new design principles for SAC chemistry.