Cobalt Single‐Atom Sites Engineered in Poly(Heptazine Imide) Photocathodes as Benchmark for Alkaline Hydrogen Evolution: A Comparative Transition Metal Study

The development of sustainable photoelectrocatalysts for the hydrogen evolution reaction (HER) is key for advancing solar‐to‐fuel conversion technologies. Ionic crystalline poly(heptazine imide) (PHI) emerges as a versatile platform owing to its facile synthesis, high chemical stability, and tunable electronic structure under visible‐light irradiation. Herein, Fe, Cu, and Co transition metals are incorporated into PHI via cation‐exchange strategy, acting as co‐catalytic centers that enhance charge separation photoelectrochemically. Photoelectrochemical tests reveal that metal‐modified PHIs outperform pristine NaPHI for alkaline HER upon blue‐light activation, with CoPHI showing the highest activity. CoPHI displays a current density of ≈ −60 mA cm −2 at −0.76 V vs RHE (or −5 A mg −1 at ≈ −0.72 V), together with an onset potential (≈0.55 V vs RHE under illumination), a low Tafel slope (53 mV dec −1 ), reduced charge‐transfer resistance (≈120 Ω), and an enhanced photocurrent response (≈40 mA cm −2 ). Advanced spectroscopic and microscopic analyses reveal that predominantly atomically dispersed Co sites operate as Co 2+ centers and facilitate interfacial charge transfer due to their favorable CoN 4 coordination geometry and strong electronic coupling with the PHI framework. These findings establish CoPHI as an efficient carbon‐based proof‐of‐concept photoelectrocatalyst featuring atomically dispersed metal sites for alkaline HER.