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

Abstract Modulating surface‐active hydrogen (*H) supply represents a critical strategy to boost the electrocatalytic CO 2 reduction reaction (ECRR), yet the mechanistic interplay between *H dynamics and catalytic behavior remains ambiguous. Herein, we construct tandem catalysts (M 4 /Ni 1 NC, M = Fe, Co, Cu, or Mn) by coupling tetranuclear metal clusters (M 4 ) with single‐atom Ni sites on N‐doped carbon (Ni 1 NC) to regulate *H supply. Experimental and theoretical results reveal that the *H supply is governed by both thermodynamics and kinetic factors. The M 4 clusters provide the thermodynamic feasibility for *H supply for CO 2 activation. The *H supply rate in kinetic perspective is tuned by the K + ‐H 2 O ratio of interfacial water, determined by work function of the decorated M 4 clusters. The increased K + ‐H 2 O 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 + ‐H 2 O ratio and ECRR performance. Among these samples, Cu 4 /Ni 1 NC 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.