Greigite Fe₃S₄-Derived α-FeO(OH) Promotes Slow O-O Bond Formation in the Second-Order Oxygen Evolution Reaction Kinetics

The mechanistic pathway of oxygen evolution reaction (OER) invariably changes with the surface structure of the catalyst and difficult to envisage. Concerning the mechanistic interpretation of the rate-limiting step, the order of the reaction, and other intrinsic parameters, electrokinetic study is thereby of particular interest. Greigite Fe3S4 nanosheets prepared herein via a solvothermal route while deposited on the nickel foam (NF) electrode surface act as an acceptable OER catalyst with a recorded overpotential of 251 (±6) mV at a j value of 10 mA cm-2 comparable to some benchmark iron/nickel oxy-hydroxide catalysts. Ex situ post-OER analysis of a Fe3S4/NF anode has affirmed the formation of α-FeO(OH) as the active species for the alkaline OER. The OER performance however can be improved upon elevating the cell temperature from 303 to 338 K. Electrokinetic data obtained at variable temperatures provide several important intrinsic parameters. The experimentally determined transfer coefficient α (1.90), exchange current density js′ (3.06 × 10-6 mA cm-2), and a Tafel slope of 45.9 mV dec-1 implicate the O-O bond formation as the rate-limiting step following a Rossmeisl's peroxide path (RPP) where a nucleophilic attack of OH- to the Fe(IV)=O results in the formation of iron(III) hydroperoxo (Fe(III)-OOH) in the slowest step. From the variable temperature OER kinetics, the calculated activation energy (Ea) 53 (±2.1) kJ mol-1 is comparable to the noble metal oxide (IrO2) but lower than other active transition metal (Co, Ni) oxide/-oxyhydroxide. Calculated Ea and other kinetic data further evidence that the fair OER performance is due to a low activation barrier for the O-O bond formation step on the electro-generated FeO(OH) materials. The bimolecular RPP mechanism, nucleophilic attack of OH- to the Fe(IV)=O, has further been validated by the experimentally derived second-order rate of reaction with respect to [OH-]. Moreover, the observed linear drop of the Tafel slope with increasing size of the electrolyte's cation (Li+, Na+, K+), inferred a change in the mechanism, albeit it provides evidence on the weak non-covalent interaction of the surface adsorbed -OH with the cation of the electrolyte. The detailed electrokinetic study presented herein with the electrogenerated α-FeO(OH) can provide some useful guidelines to establish further the accurate OER mechanism on iron oxide/oxy-hydroxide materials.