Cooperative Copper Single‐Atom Catalyst in 2D Carbon Nitride for Enhanced CO₂ Electrolysis to Methane

Abstract Renewable‐electricity‐powered carbon dioxide (CO₂) reduction (eCO₂R) to high‐value fuels like methane (CH₄) holds the potential to close the carbon cycle at meaningful scales. However, this kinetically staggered 8‐electron multistep reduction suffers from inadequate catalytic efficiency and current density. Atomic Cu‐structures can boost eCO₂R‐to‐CH₄ selectivity due to enhanced intermediate binding energies (BEs) resulting from favorably shifted d‐band centers. In this work, 2D carbon nitride (CN) matrices, viz. Na‐polyheptazine (PHI) and Li‐polytriazine imides (PTI), are exploited to host Cu–N₂ type single‐atom sites with high density (≈1.5 at%), via a facile metal‐ion exchange process. Optimized Cu loading in nanocrystalline Cu‐PTI maximizes eCO₂R‐to‐CH₄ performance with Faradaic efficiency (FE_CH4) of ≈68% and a high partial current density of 348 mA cm⁻² at −0.84 V vs reversible hydrogen electrode (RHE), surpassing the state‐of‐the‐art catalysts. Multi‐Cu substituted N‐appended nanopores in the CN frameworks yield thermodynamically stable quasi‐dual/triple sites with large interatomic distances dictated by the pore dimensions. First‐principles calculations elucidate the relative Cu–CN cooperative effects between the matrices and how the Cu local environment dictates the adsorbate BEs, density of states, and CO₂‐to‐CH₄ energy profile landscape. The 9N pores in Cu‐PTI yield cooperative Cu–Cu sites that synergistically enhance the kinetics of the rate‐limiting steps in the eCO₂R‐to‐CH₄ pathway.