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

摘要

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 (MnxFe3-xO4) were formed on electroplated Fe-Mn films. The MnxFe3-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 cm(2), which is nine times that of the nanoporous structure on Fe (3.67 cm(2)). The onset potential of the NP/MC on Fe-15 at% Mn and Fe-36 at% Mn was 0.88 V vs. RHE (overpotential, similar to 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 (similar to 560 mV) and 1.74 V on Fe-36 at% Mn (similar to 510 mV). The OER and ORR activities of the MnxFe3-xO4 porous oxides are better than those of spinel iron oxide (similar to 510 and similar to 640 mV for the ORR and OER, respectively) because of the good intrinsic activity of MnxFe3-xO4 and greater surface area of the NP/MC. The ORR activities of the MnxFe3-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 MnxFe3-xO4 porous oxides retained a high level of activity during OER durability testing for 8 h.