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Study of the Oxygen Evolution Reaction Catalytic Behavior of Co<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>Fe<sub>2</sub>O<sub>4</sub> 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
H<sub>2</sub> and O<sub>2</sub> 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 (MFe<sub>2</sub>O<sub>4</sub>; 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 MFe<sub>2</sub>O<sub>4</sub> in detail the
role of Ni and Co was studied through M<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>Fe<sub>2</sub>O<sub>4</sub> (M = Co; 0 < <i>x</i> < 1), which was prepared by
the sol–gel method. The results showed that bare NiFe<sub>2</sub>O<sub>4</sub> has better catalytic activity (η = 381 mV at
10 mA cm<sup>–2</sup> and Tafel slope of 46.4 mV dec<sup>–1</sup>) compared to Co-containing M<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>Fe<sub>2</sub>O<sub>4</sub> (η
= 450–470 mV at 10 mA cm<sup>–2</sup> and Tafel slope
of 50–73 mV dec<sup>–1</sup>) in alkaline medium, and
the substitution of Co is found to suppress the catalytic activity
of NiFe<sub>2</sub>O<sub>4</sub>. The degradation of catalytic activity
with an increase in Co content was accounted for in further detailed
investigations