Amorphous Iron–Manganese Oxyfluorides, Promising Catalysts for Oxygen Evolution Reaction under Acidic Media

The development of earth-abundant catalysts for the oxygen evolution reaction (OER) in acidic media represents a significant challenge in the context of polymer electrolyte membrane (PEM)-based electrolysis. In this scope, transition metal oxides constitute an emerging alternative to Ir and Ru oxides. Notably, Mn oxides are among the few that have sufficient stability in acidic electrolytes, but their performance still lacks behind Ir/Ru oxides. To this end, the modification of Mn oxides’ structure, crystallinity (or amorphous structure), and/or composition may work to enhance their catalytic activity. In this report, we focused our attention on the development of mixed-metal Mn–Fe-based catalysts containing highly electronegative fluorine ions as acid-stable OER catalysts. Inspired by our recent work on the preparation Mn-Fe-based fluorinated materials MnFe2F8(H2O)2 and MnFe2F5.8O1.1, a new hydrated fluoride, MnFeF5(H2O)2, was prepared by microwave-assisted solvothermal synthesis and its subsequent thermal treatment under air yielded the corresponding oxyfluoride MnFeF4.6O0.2. The resultant composition and structure of the materials were determined from powder X-ray diffraction (XRD), Mössbauer spectrometry, thermal analyses, and electronic microscopies. The crystalline-hydrated fluorides (MnFeF5(H2O)2 and MnFe2F8(H2O)2) and the corresponding amorphous oxyfluorides (MnFeF4.6O0.2 and MnFe2F5.8O1.1) were subsequently evaluated for the first time as OER electrocatalysts in highly acidic (0.5 M H2SO4) media. Notably, the oxyfluorides featured sustained OER activity at 10 mA/cm2 for more than 10 h and, thus, present an important addition to the growing library of earth-abundant alternatives to Ir and Ru oxides.