High purity CH4 production from CO2via cascade electro-thermocatalysis using metal nanoclusters with high CO2 binding affinity†

Electrochemical CO₂ reduction reaction (CO₂RR) has emerged as a promising strategy to convert CO₂ into value-added chemicals and fuels. While methane is especially desirable owing to its extensive use as a fuel, existing infrastructure, and large global market, the direct electroreduction of CO₂ to CH₄ is hindered by challenges such as low product purity and high overpotentials. In this study, an efficient cascade electrolysis and thermocatalysis system for the high-purity production of CH₄ from CO₂ has been demonstrated. Electrochemical syngas production was carried out using CO₂RR-active electrocatalysts, including Au₂₅ and Ag₁₄ nanoclusters (NCs). While both NCs exhibited high CO₂-to-CO activity in alkaline media, Ag₁₄ NCs enabled syngas production with a varying ratio (H₂/CO) by adjusting the CO₂ flow rate, achieving near-theoretical single-pass conversion efficiency (SPCE) of over 45% (theoretical limit = 50%). Electrokinetic analysis revealed that the strong CO₂ binding affinity and exceptional CO selectivity of Ag₁₄ NCs contribute to superior syngas tunability and carbon conversion efficiency. Electrochemically generated syngas (H₂/CO = 3) at 800 mA cm⁻² was directly fed into a thermocatalysis reactor, producing CH₄ with a purity exceeding 85%.