A Redox-Active Iron Azo-Pyridine Complex for Aqueous H2 Evolution and H2 Oxidation: From Mechanism to Devices

Abstract The efficient and reversible interconversion of protons and molecular hydrogen is essential for commercially viable hydrogen energy systems. Natural hydrogenases catalyze this process with unmatched efficiency but lack operational stability outside biological conditions. Inspired by their catalytic activity, a water-soluble nonheme iron complex, Fe(pap)3(ClO4)2 (pap = 2-(phenylazo)pyridine), has been developed as a molecular catalyst exhibiting bifunctional homogeneous hydrogen evolution (HER) and hydrogen oxidation (HOR) activity in aqueous media. The complex achieves a turnover number of ∼800 for HOR and delivers HER with ∼90% Faradaic efficiency at overpotentials near 150 mV, maintaining activity under mild conditions. Ligand redox activity facilitates electron and proton transfer, enabling efficient HER and HOR. Beyond mechanistic relevance, the catalyst is integrated into a prototype fuel cell and electrolyzer. In HOR fuel cell mode, an open-circuit voltage of 0.98 V and a peak power density of 0.87 mW·cm–2 is observed, while in a separate PEM electrolyzer operated in HER mode, sustained hydrogen evolution is observed under acidic conditions. This study highlights the potential of nonheme molecular iron catalysts to enable translation of molecular electrocatalysis toward device-relevant hydrogen conversion.