Electrochemical hydrogenation (ECH) represents a sustainable approach for organic transformations, utilizing H₂O as an environmentally benign hydrogen source under ambient conditions. However, its efficiency is often limited by kinetic constraints in proton activation and transfer processes. In this study, we develop ultrathin PdCu alloy metallenes as high-performance electrocatalysts to overcome these challenges. The as-synthesized PdCu metallenes (~ 3 nm thickness) demonstrate exceptional performance for benzaldehyde (BzH) reduction to benzyl alcohol, achieving a conversion of ~ 80.0% and a Faradaic efficiency of ~ 85.0% at a potential of −0.4 V versus reversible hydrogen electrode (RHE), which significantly surpass pristine Pd counterparts. Combined experimental and theoretical analyses reveal that the Cu alloying strategically modulates the electronic structure of Pd, which enhances the BzH chemisorption and makes the energy barrier appropriate for the proton-coupled electron transfer followed by electrochemical hydrogen-atom transfer. This work establishes a fundamental structure–activity relationship for Pd-based electrocatalysts through surface engineering and demonstrates broad substrate compatibility, highlighting the potential of alloy metallenes as promising catalysts for sustainable electrochemical synthesis.