The development of electrocatalysts for the oxygen evolution reaction (OER) is one of the principal challenges in the area of renewable energy research. Within this context, mixed-metal oxides have recently emerged as the highest performing OER catalysts. Their structural and compositional modification to further boost their activity is crucial to the wide-spread use of electrolysis technologies. In this work, we investigated a series of mixed-metal F-containing materials as OER catalysts to probe possible benefits of the high electronegativity of fluoride ions. We found that crystalline hydrated fluorides, CoFe₂F₈(H₂O)₂ and NiFe₂F₈(H₂O)₂, and amorphous oxyfluorides, NiFe₂F_4.4O_1.8 and CoFe₂F_6.6O_0.7, feature excellent activity (overpotential for 10 mA cm⁻² as low as 270 mV) and stability (extended performance for >250 hours with ∼40 mV activity loss) for the OER in alkaline electrolyte. Subsequent electroanalytical and spectroscopic characterization hinted that the electronic structure modulation conferred by the fluoride ions aided their reactivity. Finally, the best catalyst of the set, NiFe₂F_4.4O_1.8, was applied as anode in an electrolyzer comprised solely of earth-abundant materials, which carried out overall water splitting at 1.65 V at 10 mA cm⁻².

The synthesis and the electrochemical properties of iron hydroxyfluorides with open-structure, prepared from the thermal decomposition under ambient air of Fe2+Fe3+F5(H2O)2 and Fe2+Fe3+2F8(H2O)2, are reported. The combination of thermogravimetric analyses, X-ray thermodiffraction (XRD) and Infra-Red spectroscopy has evidenced two instable lacunar oxyfluorides that transform rapidly into hydroxyfluorides under ambient air. Their formulations and structural features have been determined from powder XRD, Mössbauer spectrometry and pair distribution function leading to Fe3+F2.5(OH)0.5 with the pyrochlore network and to Fe3+F2.66(OH)0.34 with the HTB structure. The electrochemistry of these hydroxyfluorides indicates excellent and similar capacities with a significant decrease after the first cycle to reach sustained values of ~110mAh/g upon cycling. In situ XRD measurements reveal that the pyrochlore network undergoes amorphisation (xLi+ > 0.5) while the HTB phase transforms into the ReO3-type phase (xLi+ > 0.33).