Engineering Porous Hollow Metal‐Poly(Heptazine Imide) Spheres: An Optimized Synthetic Strategy for Controlling Surface, Morphology, and Properties

The utilization of carbon nitride materials is significantly constrained by their inherently low specific surface area (SA). Specifically, introducing porosity into their ionic derivatives, such as poly(heptazine imide) (PHI), proves challenging as the high‐temperature ionothermal synthesis produces more regular and stacked structures. This study introduces a novel synthetic strategy for porous hollow PHI spheres (HS_MPHIs) via a hard‐templating method. HS_MPHIs exhibit significantly higher SA (145 m 2 g −1 vs 44 m 2 g −1 of bulk KPHI), precise metal content control, improved visible light absorption, adjustable band positions, and tunable hydrophilicity. The increased SA facilitates the intercalation of a higher content of metal cations, with PHI channel system accommodating various cations, including potassium, iron, nickel, and cobalt. Transition metal containing PHIs show notable morphological, structural and optical changes, such as flower‐like shapes, varied interlayer distances and SA, extended light absorption to 700 nm, and more negative valence band positions. The optimized HS_NiPHI_3_0.4 achieves a 1.4‐fold higher H 2 O 2 production than the bulk material via a 2e ‒ O 2 reduction pathway under scavenger‐free conditions, with relatively stable performance over four cycles.