Nanoporous manganese ferrite films by anodising electroplated Fe–Mn alloys for bifunctional oxygen electrodes

Abstract

An electroplating–anodising method based on a facile and scalable electrochemical process was used to fabricate manganese ferrite porous oxide films for use as precious-metal-free oxygen reduction/evolution reaction (ORR/OER) electrodes. Porous oxide films of spinel manganese ferrites (Mn_xFe_3−xO₄) were formed on electroplated Fe–Mn films. The Mn_xFe_3−xO₄ porous oxide formed on microcracks in the Fe–Mn films constituted a nanoporous/microcrack hierarchical structure (NP/MC), which provided a large electrode surface area for ORR/OER. The electrochemically active surface area of the NP/MC on Fe–36 at% Mn was 33.3 cm², which is nine times that of the nanoporous structure on Fe (3.67 cm²). The onset potential of the NP/MC on Fe–15 at% Mn and Fe–36 at% Mn was 0.88 V vs. RHE (overpotential, ∼350 mV) for the ORR at −0.1 mA cm⁻². The OER onset potentials at 10 mA cm⁻² were 1.79 V on Fe–15 at% Mn (∼560 mV) and 1.74 V on Fe–36 at% Mn (∼510 mV). The OER and ORR activities of the Mn_xFe_3−xO₄ porous oxides are better than those of spinel iron oxide (∼510 and ∼640 mV for the ORR and OER, respectively) because of the good intrinsic activity of Mn_xFe_3−xO₄ and greater surface area of the NP/MC. The ORR activities of the Mn_xFe_3−xO₄ porous oxides decreased to about 30% during ORR durability testing for 7.5 h, and the same level of activity was retained after 24 h of use. The Mn_xFe_3−xO₄ porous oxides retained a high level of activity during OER durability testing for 8 h.