A Biomimetic Twisting Strategy Enables Efficient Electrocatalytic Oxidation of Energy-Dense Hydrazine Hydrate on FeN2+2C4+4 Sites

Electrocatalytic hydrazine oxidation holds great promise for enabling fuel cell-powered transportation since hydrazine hydrate (N2H4·H2O) has the highest energy density of all liquid, CO2-free fuels (3.45 kWh/L), and the highest fuel cell voltage (1.56 V vs O2). Inspired by the ruffling of catalytic centers in oxidative enzymes, we designed a twisted single-atom nanozyme comprising twisted FeN2+2C4+4 sites, enabling high accessibility of N2H4 and OH– reactants. Experimental evidence shows that this nanozyme catalyst achieves the lowest oxidation overpotential of all Fe–N–C materials, both in the lab and in direct hydrazine fuel cells, with an open-circuit voltage of 0.95 V, unprecedented for an Fe-based anode. The structure of the catalytic site is elucidated through a combination of electrochemistry, 57Fe Mössbauer spectroscopy, high-resolution transmission electron microscopy, and X-ray absorption spectroscopy with crystal field multiplet simulations and fits of the pre-edge features, as well as density functional theory calculations and theoretical simulations of X-ray absorption near edge structure. The experimental and theoretical methods reveal that twisting the active site also shifts its oxidation potential positively and improves N2 bubble removal while limiting ammonia production to less than 10 ppm. This work demonstrates the potential of active site twisting to enhance the oxidation of energy-rich and liquid substrates, representing a crucial step toward building a sustainable society.