Ionic Metal Poly(heptazine Imides) and Single-Atoms Interplay: Engineered Stability and Performance for Photocatalysis, PhotoElectroCatalysis and Organic Synthesis

Poly(heptazine imides) (PHIs), a crystalline subclass of carbon nitrides, have emerged as transformative materials for energy conversion, chemical synthesis, and electrochemical applications. Distinguished by a nitrogen-rich ionic framework, structural stability, and a pseudohexagonal lattice, PHIs serve as ideal platform for single-atom catalysts (SACs), delivering exceptional atomic efficiency, catalytic activity, and selectivity. Their unique structural and electronical properties, including negatively charged sites, promote homogeneous SAC dispersion, enhanced light absorption, and efficient charge transport. Advances in ionothermal synthesis, structural engineering, and heterojunction formation have further elevated their functionality. This review highlights PHIs' versatile applications in photocatalysis, photoelectrocatalysis, and organic photoredox catalysis, specifically water splitting, hydrogen production, oxygen evolution, carbon dioxide reduction, methane oxidation, and ammonia synthesis. By systematically exploring their synthesis, structural properties, and catalytic synergy with SACs, this study underscores PHIs' potential to tackle critical challenges in green technologies, achieving efficiencies comparable to their metal oxide counterparts