Cation Modulation of Layered Self-Assembled Polyoxometalates Enables Efficient and Robust Hydrogen Evolution

Superstructures assembled from polyoxometalate (POM) with alkyl trimethyl ammonium bromide (TAB) surfactants have generated rising attention owing to their compositional tunability and structural diversity. However, the role of alkyl chain length in dictating architectures and performance remains poorly understood. Herein, we report the controllable self-assembly of four-layered superstructures based on the giant POM K₂₈Li₅H₇P₈W₄₈O₁₈₄·92H₂O (P₈W₄₈) and systematically elucidate the influence of chain lengths in alkyl TABs on their electrochemical hydrogen evolution reaction (HER) activity. Short chains limit the exposure of catalytic sites, whereas excessively long chains impede charge transport. The superstructures assembled with an optimal chain of cetyl TAB (denoted as P₈W₄₈-CTAB) exhibit the highest HER activity, demanding an overpotential of only 55 mV to afford 10 mA cm^-2. In situ electrochemical spectroscopy and theoretical calculations reveal that the pronounced interfacial electronic coupling between P₈W₄₈ and CTAB promotes electron redistribution at the active centers, increases the density of electrocatalytically active sites, and lowers the reaction energy barrier, thereby improving the adsorption free energy of *H. This work establishes a clear correlation between organic cation chain length and electrocatalytic performance, providing general guidance for designing multifunctional POM-based superstructures through molecular-level self-assembly.