Electrocatalytic activity and stability of Co and Mn-based oxides for the oxygen reduction reaction in alkaline electrolyte

Abstract

The oxygen reduction reaction (ORR) was investigated on electrocatalysts composed of Co and Ni-modified Mn oxides, on Mn-modified Co oxides (spinel structure) and on these pure metal oxides, all supported on carbon powder. The transition metal oxides were synthesized via thermal decomposition of the respective nitrates under oxidizing atmosphere, and characterized by X-ray diffraction, before and after electrochemical potential cycling experiments. The ORR electrocatalytic activity was measured using steady state polarization curves using a rotating disk electrode. The comparison of the ORR polarization curves obtained for the pure metal oxides showed higher electrocatalytic activity for the Mn oxide electrocatalyst. However, durability tests showed an activity loss of this material due to the formation of the less active Mn3O4 phase. Co and Ni oxide showed similar ORR onset potential, but Co oxide presented much higher limiting current, evidencing higher number of electrons involved in the ORR. This was ascribed to its higher ability for the disproportionation reaction and for the O–O bond breaking, conducting the ORR via the 4-electron pathway. The Ni-modified Mn oxide material resulted in an increased stability during the ORR electrocatalysis. This was attributed to the introduction of Ni atoms into the Mn oxide structure, which leads to an electronic interaction between the Ni and Mn atoms, avoiding the formation of the less active Mn3O4. In the case of the Mn-modified Co oxide electrocatalyst, the addition of Mn into the Co oxide structure facilitated the electroreduction of the Co3+ to Co2+ during the ORR polarization curves. The faster electroreduction of Co(III) to Co(II) increased the mediated step, increasing the overall ORR rate.