A 3D Dendritic Cu 0.94 Ni 0.06 Electrocatalyst Enables 39% Solar‐to‐Hydrogen Efficiency via Coupled Seawater Electrolysis and Formaldehyde Oxidation

Hydrogen energy emerges as a pivotal carbon‐neutral alternative to fossil fuels due to its exceptional energy density and sustainability. While seawater electrolysis presents a promising avenue for scalable hydrogen generation, persistent challenges resulting from chlorine evolution reactions and chloride‐induced corrosion significantly impair system durability. Here, we introduce a breakthrough strategy coupling formaldehyde oxidation (FOR) with hydrogen evolution (HER) in seawater electrolysis, which drives the overall process at an ultralow voltage while simultaneously producing value‐added formate and achieving dual hydrogen generation at both electrodes. A rationally designed 3D dendritic Cu 0.94 Ni 0.06 electrocatalyst exhibits unprecedented FOR activity, delivering a remarkable current density of 629.9 mA cm −2 at 0.2 V vs. RHE. Density functional theory (DFT) calculations elucidate that Ni doping facilitates C–H bond cleavage in *OCH 2 OH, accelerating *H and formate formation while lowering the H 2 evolution barrier. An electrochemical system integrating HER and FOR achieves dual hydrogen output with a Faradaic efficiency of ∼200% alongside ∼100% formate selectivity. When paired with photovoltaic cells, the hybrid configuration attains a record solar‐to‐hydrogen efficiency of 39%. This work establishes an economically viable paradigm for marine hydrogen production, offering critical insights into the engineering of reaction mechanisms and the development of scalable clean energy infrastructure.