Janus carbon shells with inner–outer functional asymmetry enable local proton enrichment for promoting CO2 methanation

• CoPc supported on hollow carbon nanospheres (HCNs) achieves a maximum CO2-to-CH4 selectivity of 15.1%, breaking the conventional CO-selective behavior of CoPc. • The Janus carbon shell architecture creates inner–outer functional asymmetry, where the inner surface promotes HER to enrich protons near the outer CoPc active sites. • Local proton enrichment is identified as the key mechanism enabling deep CO2 reduction to methane on a non-copper molecular catalyst. • This work demonstrates that catalyst structure can overcome intrinsic selectivity limits of molecular catalysts, offering a new design strategy for non-copper CO2 methanation.