Electrocatalytic performances of high-entropy spinel oxide (Cr1/5Mn1/5Ni1/5Fe1/5Co1/5)3O4 for oxygen reduction/evolution reactions in alkaline electrolyte

High-entropy spinel oxide (Cr1/5Mn1/5Ni1/5Fe1/5Co1/5)3O4 (CMNFCO) and its low- and medium-entropy counterparts including FeCo2O4 (FCO), (Ni1/3Fe1/3Co1/3)3O4 (NFCO), (Mn1/4Ni1/4Fe1/4Co1/4)3O4 (MNFCO) were prepared via a simple hydrothermal method, and their electrocatalytic performances towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in 0.1 M KOH were investigated. Among all the synthesized spinel oxides, the high-entropy CMNFCO showed the best ORR activity with an onset potential Eonset = 0.88 V, a half-wave potential E1/2 = 0.76 V and excellent ORR durability with 91.5 % current retained after 33 h-chronoamperometry test at 0.287 V vs. RHE. On the other hand, CMNFCO showed mediocre OER activity with an overpotential of 411 mV at 10 mA·cm−2, which is more than 20 mV higher than the medium-entropy composition MNCFO. An Al-air battery using CMNFCO as the cathode catalyst was assembled and exhibited a maximum power density of 148.8 mW·cm−2. Surface reconstruction of the high-entropy spinel in the electrolyte was revealed by in situ Raman spectroscopy. Based on the present study, we conclude that increasing the number of constituent elements does not necessarily improve the OER activity, but it may lead to surface reconstruction to benefit ORR.