Electrocatalysis of Oxygen Evolution Reaction on Iron-Loaded Nickel Phosphide from Microkinetic Theory

Understanding the effect of electrochemical conditions, such as pH and applied potential, on complex electrochemical systems is key to designing an efficient catalyst. In the current study, we have investigated the oxygen evolution reaction catalyzed by experimentally reported iron-tuned nickel phosphide supported on the graphene sheet using five different atomic ratios, 1:1, 1:2, 1:3, 1:4, and 1:5, of Ni and Fe. We analyzed the structural, electronic, and energetic properties to find fitting parameters of a recently theorized microkinetic model following the Marcus electron transfer theory. The DFT-calculated free energy barriers of the OER elementary steps were used as the primary input of the microkinetic model. Irrespective of the Ni:Fe atomic ratio, OOH* formation is the potential-determining step. The barrier of the O* formation and its binding energy were used as descriptors for several physical phenomena. For the Ni1.50Fe0.50P, we found a maximum d-band center shift, away from the EFermi, compared to other NixFe2–xP. As a result, the highest free energy barrier of the O* formation was observed due to inefficient overlapping among Ni d-orbitals and O p-orbitals. Inefficient oxo species binding leads to a feasible formation of an OOH* with the least energy barrier. Hence, the obtained reaction-free energy among the elementary steps was used as the input for the rate function of the microkinetic model. Four-parameter fitting, namely, two reorganization free energies and two net work values in transferring ions and water, was performed on the experimental current density vs external potential data points. The magnitude of the reorganization free energy to transfer OH– (λOH–) and the work expended to transfer ions (ωion) and water (ωwater) to the Ni1.50Fe0.50P is minimum. This implies that the structural reorganization required to regain equilibration after a vertical electron transfer is a minimum for the Ni1.50Fe0.50P surface, hence explaining the efficient OER activity compared to other Ni:Fe ratios. We calculated the separate contribution of OH– equilibrated and proton-coupled electron transfer pathways. The net work done suggests that hydroxide-mediated steps are the leading pathway for the OER. The pH-dependent polarization curve indicates an increase in the current density for the same URHE. The correlation among the physical parameters obtained from microkinetic fitting of experimental current density and DFT-calculated electronic properties of the model catalyst surface provides a better understanding of the catalytic mechanism. This microkinetic model, explaining OER, is transferable to complex electrochemical systems.