Study of the Oxygen Evolution Reaction Catalytic Behavior of Co_xNi_1-xFe₂O₄ in Alkaline Medium

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 M_xNi_1-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 M_xNi_1-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.