Catalysts for the oxygen evolution reaction (OER) play an important role in the conversion of solar energy to fuel of earth-abundant water into H2 and O2 through splitting/electrolysis. Heterogeneous electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) exhibit catalytic activity that depends on the electronic properties, oxidation states, and local surface structure. Spinel ferrites (MFe2O4; M = Ni and Co) based materials have been attractive for the catalytic water oxidation due to their well-known stability in alkaline medium, easy synthesis, existence of metal cations with various oxidation states, low cost, and tunable properties by the desired metal substitution. To understand the better catalytic activity of MFe2O4 in detail the role of Ni and Co was studied through MxNi1-xFe2O4 (M = Co; 0 < x < 1), which was prepared by the sol-gel method. The results showed that bare NiFe2O4 has better catalytic activity (η = 381 mV at 10 mA cm-2 and Tafel slope of 46.4 mV dec-1) compared to Co-containing MxNi1-xFe2O4 (η = 450-470 mV at 10 mA cm-2 and Tafel slope of 50-73 mV dec-1) in alkaline medium, and the substitution of Co is found to suppress the catalytic activity of NiFe2O4. The degradation of catalytic activity with an increase in Co content was accounted for in further detailed investigations.
Keywords: alkaline medium; effect of metal substitution; electrocatalysts; oxygen evolution reaction (OER); spinel-based metal oxides; water electrolysis.