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

Fonseca Reis I, Diab GA, Nenaniya S, Da Silva MAR, Ferreira R, Shah H, Da Silva GMGD, Hutzler A, Isaacs MA, Lopes CW, Teixeira IF, Jiménez Calvo P (2026)

Publication Type: Journal article

Publication year: 2026

Journal

Energy & Environmental Materials Wiley

Original Authors: Izadora F. Reis, Gabriel A. A. Diab, Sachin Nenaniya, Marcos A. R. da Silva, Rayse M. Ferreira, Heer Shah, Geraldo M. G. da Silva, Andreas Hutzler, Mark Isaacs, Christian W. Lopes, Ivo F. Teixeira, Pablo Jiménez‐Calvo

Article Number: e70475

DOI: 10.1002/eem2.70475

Abstract

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 Co2+ centers and facilitate interfacial charge transfer due to their favorable CoN4 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.

Authors with CRIS profile

Izadora Fonseca Reis Lehrstuhl für Chemistry of thin film materials Pablo Jiménez Calvo Lehrstuhl für Chemistry of thin film materials

Involved external institutions

Universidade Federal de São Carlos (UFSCar) / Federal University of São Carlos BR Brazil (BR) Forschungszentrum Jülich / Research Centre Jülich (FZJ) DE Germany (DE) University College London (UCL) GB United Kingdom (GB) Universidade Federal do Paraná (UFPR) BR Brazil (BR)

How to cite

APA:

Fonseca Reis, I., Diab, G.A., Nenaniya, S., Da Silva, M.A.R., Ferreira, R., Shah, H.,... Jiménez Calvo, P. (2026). Cobalt Single‐Atom Sites Engineered in Poly(Heptazine Imide) Photocathodes as Benchmark for Alkaline Hydrogen Evolution: A Comparative Transition Metal Study. Energy & Environmental Materials. https://doi.org/10.1002/eem2.70475

MLA:

Fonseca Reis, Izadora, et al. "Cobalt Single‐Atom Sites Engineered in Poly(Heptazine Imide) Photocathodes as Benchmark for Alkaline Hydrogen Evolution: A Comparative Transition Metal Study." Energy & Environmental Materials (2026).

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