Mg²⁺ Ions as Interfacial Charge Bridges for Enhanced Photocatalytic Hydrogen Evolution in Poly(Heptazine Imide)

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⁺-OH^δ-H^δ+-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.