Prussian blue analogue-derived Mn–Fe oxide nanocubes with controllable crystal structure and crystallinity as highly efficient OER electrocatalysts

A series of ternary manganese iron oxides with different crystal structures, oxidation states and crystallinities were successfully fabricated by modulating the calcination conditions of Mn–Fe Prussian blue analogue (PBA) precursor (Mn3 [Fe(CN)6]2·nH2O). The obtained Mn–Fe oxides retained the nanocubic morphology of the PBA precursor, and a mesoporous structure was acquired as a result of gas molecule release during the pyrolysis process. Electrochemical oxygen evolution reaction (OER) activity of the as-prepared catalysts was tested, and among the bimetallic oxides, the catalyst that had a crystal structure similar to cubic bixbyite Mn1.2Fe0.8O3 (space group: Ia-3) with low crystallinity exhibited the most advanced OER activity. An overpotential of only 245 mV was required to achieve a current density of 10 mA cm−2, and the Tafel slope value was only 38 mV dec−1. The excellent OER activity is likely due to the hollow porous morphology of the samples, the synergistic effect of Mn and Fe, the defect-rich low crystallinity of the catalyst, and the cubic Mn1.2Fe0.8O3 structure (space group: Ia-3), which has an intrinsic activity superior to that of spinel Mn1.8Fe1.2O4.