Volcano‐Shaped Relationship Between Interfacial K⁺‐H₂O Ratio and CO₂ Reduction Activity in Tandem Electrocatalysts

Modulating surface-active hydrogen (*H) supply represents a critical strategy to boost the electrocatalytic CO₂ reduction reaction (ECRR), yet the mechanistic interplay between *H dynamics and catalytic behavior remains ambiguous. Herein, we construct tandem catalysts (M₄/Ni₁NC, M = Fe, Co, Cu, or Mn) by coupling tetranuclear metal clusters (M₄) with single-atom Ni sites on N-doped carbon (Ni₁NC) to regulate *H supply. Experimental and theoretical results reveal that the *H supply is governed by both thermodynamics and kinetic factors. The M₄ clusters provide the thermodynamic feasibility for *H supply for CO₂ activation. The *H supply rate in kinetic perspective is tuned by the K⁺-H₂O ratio of interfacial water, determined by work function of the decorated M₄ clusters. The increased K⁺-H₂O ratio can promote water dissociation to maintain optimal *H coverage for intermediate hydrogenation, whereas excessive *H accumulation triggers competitive hydrogen evolution. Therefore, a volcanic relationship was observed between the K⁺-H₂O ratio and ECRR performance. Among these samples, Cu₄/Ni₁NC with moderate *H supply rate in kinetic exhibits exceptional ECRR performance, achieving >95% Faradaic efficiency for CO across a 0.8 V potential range (-0.2 to -1.0 V versus RHE) and industrial-relevant current densities (∼385 mA cm^-2 at -1.0 V) in a flow cell.