Incorporation of Organic Counter-Cations Into Poly(Heptazine Imide) Networks for Promoting Proton-Coupled Electron...

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 H₂O₂ evolution was chosen as a model reaction. Under ambient conditions, MA⁺-PHI showed the best performance, producing 7.6 mmol g^{- 1} H₂O₂ 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 H₂O₂ 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 H₂O₂ rate enhancements. These results showcase that counterion replacement, as applied in organic perovskites, also improves carbon nitride photosynthesis.