Title : 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 the development of commercially viable hydrogen energy systems. Natural hydrogenases catalyze this process with exceptional efficiency under mild conditions; however, their structural complexity and limited operational stability outside biological environments motivate the development of robust synthetic molecular catalysts. In particular, molecular systems capable of efficiently catalyzing both the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR) remain comparatively rare. Inspired by the catalytic activity and metal-ligand cooperativity of hydrogenases, we have developed a water-soluble, non-heme mononuclear iron complex, Fe (pap)3(ClO4)2 (pap = 2-(phenylazo)pyridine), as a bifunctional molecular catalyst for hydrogen conversion in aqueous media.
The redox-active azo-pyridine ligands provide an electronically flexible coordination environment around the iron center and facilitate proton-coupled electron transfer during catalysis. C1 exhibits efficient HOR activity with a turnover number (TON) of 800 at pH 6, while HER proceeds with a TON of 800 and > 90% Faradaic efficiency at pH 2 with a low overpotential of 142 mV. The catalyst retains its activity across a broad aqueous pH range (pH2-8) and demonstrates appreciable oxygen tolerance under mild operating conditions. Spectroscopic, electrochemical, and computational investigations provide mechanistic insights into how the redox-active ligand framework and metal-ligand cooperativity contribute to proton and electron transfer during both HER and HOR.
Beyond molecular-level electrocatalysis, the practical applicability of C1 was investigated by integrating it into hydrogen-conversion devices. When incorporated into a gas-diffusion-layer electrode and evaluated as an anode catalyst in a proton exchange membrane fuel cell, C1 delivers an open-circuit voltage of 0.98 V at 60ºC and a peak power density of 0.87 mW·cm-2 in HOR mode. In a separate PEM electrolyzer configuration, C1 also enables sustained hydrogen evolution under acidic conditions, demonstrating the compatibility of the molecular catalyst with device-relevant architectures.
Overall, this study establishes a structurally defined, non-heme iron molecular platform for bifunctional hydrogen electrocatalysis and highlights the role of redox-active ligand frameworks in expanding the reactivity of earth-abundant iron centers. The successful integration of C1 into both fuel-cell and electrolyzer configurations provides a proof of concept for translating mechanistically well-defined molecular electrocatalysts toward practical hydrogen energy technologies.
