Proton Sponge Tailoring of Interfacial H‐Bond Networks Enables Seawater Electrosynthesis for Concomitant Alkenol and Mg(OH) ₂

Seawater electrolysis provides a sustainable hydrogen source for electrocatalytic semi-hydrogenation (ECSH) of alkynols, but it suffers from severe hydrogen evolution and a lack of effective interfacial management. Here, we construct a proton sponge (1,8-bis(dimethylamino)naphthalene) modified PdIn intermetallic metallene (Pd₁In₁ene@DMAN). In situ FTIR and ab initio molecular dynamics simulations reveal that this modification disrupts the hydrogen-bond network of interfacial water molecules, while DFT calculations indicate facilitated water dissociation and enhanced generation of active hydrogen species. The Pd₁In₁ene@DMAN achieves a Faradaic efficiency (FE) of 94.43% for 2-methyl-3-buten-2-ol (MBE) production at -100 mA cm^-2, a dramatic increase from the 44.93% attained by the Pd₁In₁ene, with an operational stability over 500 h. In a membrane-electrode assembly (MEA) electrolyzer, it enables the efficient conversion of alkynol to alkenol at a current of 2 A, delivering a FE of 84.16%, a selectivity of 98.54% toward MBE, and excellent stability for up to 200 h. Concurrently, the cathodically generated OH^- selectively precipitates Mg²⁺ as high-purity Mg(OH)₂ by precise pH control, enabling the co-production of value-added MBE and Mg(OH)₂. This work establishes a seawater-based electrochemical system, which regulates the hydrogen bond microenvironment through molecular interface engineering, providing a new strategy for efficient electrochemical hydrogenation in complex media.