Single-Atom Engineered Poly(Heptazine Imide) Photocathodes for Efficient Alkaline Hydrogen Evolution

The development of sustainable photoelectrocatalysts for the hydrogen evolution reaction (HER) is key for advancing solar-to-X energy conversion technologies. Ionic crystalline poly(heptazine imides) (PHI) emerges as a versatile platform owing to their 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 2 activity. CoPHI displays a current densities at voltages vs RHE of ≈ −60 mA cm-2 at −0.76 V (or −5 A mg-1 at ≈ −0.72 V), with a favorable onset potential (≈ 0.55 V), a low Tafel slope (53 mV dec-1), reduced charge-transfer resistance (≈ 120 Ω), and an enhanced photocurrent response (≈ 9 μA cm-2). Advanced spectroscopic and microscopic analyses reveal that atomically dispersed Co sites operate as Co 2+ centers, facilitating interfacial charge transfer, due to their lower electronegativity and favorable electronic coupling with the PHI framework. These findings suggest CoPHI as an efficient carbon-based proof-of concept platform for single-atom-engineered photoelectrodes in alkaline HER.