A 3D Dendritic Cu0.94Ni0.06 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 Cu0.94Ni0.06 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 *OCH2OH, accelerating *H and formate formation while lowering the H2 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.