Gabriel A A Diab, Vitor G S Pastana, Sara Stolfi, Rayse M Ferreira, Izadora F Reis, Marcos A R da Silva, Caio V C Ribeiro, Kathrin Küster, Piero Torelli, Paolo Ghigna, Viola Duppel, Maurizio Fagnoni, Sebastian Bette, Bruna R Serino, Andre F de Moura, Davide Ravelli, Bettina V Lotsch, Ivo F Teixeira
Optimizing charge transfer at the solid-liquid interface remains a central challenge for photocatalytic hydrogen evolution, particularly in polymeric semiconductors with limited electron-donating ability. Here, we demonstrate that Mg2+ incorporated into poly(heptazine imide) (PHI) acts as a catalytic promoter by synergistically coupling semiconductor charge transport with Pt co-catalysts, thereby overcoming kinetically limited interfacial electron transfer. Using H2 evolution as a model reaction, Mg incorporation into Na-PHI enhanced the hydrogen evolution rate by more than 2.5-fold and introduced a linear dependence on Pt loading, an effect absent in pristine Na-PHI. In situ XANES reveals reversible electronic modulation of Mg2+ exclusively in the presence of Pt and water, confirming its direct participation in interfacial charge mediation. Complementary photophysical, (photo)electrochemical, and molecular dynamics simulations indicate that Mg2+ polarizes water molecules through a dynamic Mg2+-OHδ-Hδ+-Pt interfacial motif, facilitating proton transfer toward Pt while stabilizing hydroxide species. Beyond the activity enhancement, Mg-PHI exhibits improved structural stability by resisting framework protonation during recycling. Collectively, these findings establish Mg2+ as a catalytic promoter and demonstrate that interfacial charge-bridge effects can be rationally heterogenized, providing a general strategy for enhancing photocatalytic hydrogen evolution through targeted interface engineering.