From Surface Buffering to Bulk Ion Exchange: Mapping the Acid Response of Potassium Poly(Heptazine Imide) (K-PHI)

Potassium poly(heptazine imide) (K-PHI) is a promising material for photocatalysis and energy storage, yet its application in aqueous media is limited by protonation. Cation exchange to protonated poly(heptazine imide) (H-PHI) causes structural and optoelectronic changes. Herein, a systematic study of its acid stability regime and protonation threshold is presented through simultaneous consideration of PHI's bulk, interface, and solution properties. Down to pH 3.0, solutions are buffered by proton adsorption while the particle bulk retains structural integrity, pointing to coexistence of H+ at the surface and K+ in the bulk. Within this regime down to pH 4.0, buffering coincides with a stable zeta potential, consistent with proton adsorption balanced by counterions. At yet lower pH, the buffer limit (pH 3.0) is characterized by surface destabilization, suggesting proton excess. At pH 2.0, protonation proceeds via bulk ion exchange and H-PHI formation.