Metal-Molecule Interactions Govern CO₂ Reduction with Potential-Dependent Charge Transfer Effects

Molecular catalysts offer atomically defined active sites with tunable catalytic performance, which is strongly substrate-dependent. Nevertheless, a mechanistic understanding of how substrate effects govern activity remains lacking, particularly under electrochemical conditions where the applied potential can dynamically reshape interfacial electronic structure. Here, using density functional theory and constant-potential calculations, we systematically investigate how metal substrates regulate the CO₂ reduction activity of transition-metal phthalocyanines (TMPcs). Across FePc, CoPc, and NiPc supported on Au, Ag, and Pt surfaces with different facets, we revealed that metal substrates regulate the activity through two coupled mechanisms: (i) tuning the static charge state of the metal center, which controls *CO binding strength under vacuum, and (ii) enabling potential-driven dynamic charge transfer that reshapes reaction energetics under working conditions. Accordingly, electron transfer at the transition-metal center emerges as an effective electronic descriptor for *CO adsorption and the support-dependent activity trend. More importantly, metallic substrates exhibit a pronounced electronic response to applied potential, acting as charge reservoirs that dynamically modulate the active site. As a result, CoPc/Pt(111) becomes thermodynamically favorable for CO₂ reduction at -0.6 V versus RHE, whereas graphene-supported CoPc remains limited under the same conditions. These findings show that both substrate nature and applied potential are critical for determining the activity of molecular electrocatalysts and provide design principles for supported molecular catalysts for CO₂ reduction.