Incorporation of Organic Counter‐Cations Into Poly(Heptazine Imide) Networks for Promoting Proton‐Coupled Electron Transfer During Photocatalytic H ₂ O ₂ Evolution

Nitrogen based onium ions such as methylammonium (MA + ), formamidinium (FA + ), and tetramethylammonium (TMA + ) were used as counterions in poly(heptazine imide) (PHI) networks via a protonation‐acid base reaction strategy. This creates all organic PHIs. Chemical composition analysis confirmed the successful accommodation of target species, resulting in a maximum incorporation of 1 cation per ideal PHI unit cell. Structural characterization confirmed crystal phase consistency to that of parent Na‐PHI. Photocatalytic H 2 O 2 evolution was chosen as a model reaction. Under ambient conditions, MA + ‐PHI showed the best performance, producing 7.6 mmol g − 1 H 2 O 2 after 1 h of 427 nm LED irradiation, corresponding to an AQY of 13.9% and a 42% activity enhancement over the Na‐PHI analogue. When an additional oxygen pressure of 3 bars is applied, the H 2 O 2 yield increased to 36.5 mmol g − 1 , while AQY increased proportionally to ∼66.8%, being one of highest outputs reported to date. Photochemical and photo‐charging experiments suggest that methylammonium moieties stabilize photoelectrons though coulombic interaction in an effective manner, while proton conductivity and photocatalytic experiments point out toward a PCET driven mechanism responsible for the recorded H 2 O 2 rate enhancements. These results showcase that counterion replacement, as applied in organic perovskites, also improves carbon nitride photosynthesis.