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

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.