Analyzing the Surface Chemistry of Ni-Fe Hydroxide Alloy Nanoparticles as Catalysts for the Oxygen Evolution Reaction by X-ray Photoelectron Spectroscopy Analysis

Water electrolysis has been proposed as a renewable source of hydrogen, a possible replacement to harmful fossil fuels. This process can be broken down into two half reactions, the hydrogen evolution reaction and the oxygen evolution reaction. Because the oxygen evolution reaction has slow kinetics and a high overpotential to overcome, a catalyst is needed to speed up the kinetics and ensure faster and more efficient production of molecular hydrogen. The use of Ni-Fe hydroxide alloy nanoparticles as a catalyst has been shown to significantly improve the efficiency of the reaction by decreasing the overpotential, making the process more energy efficient [1]. Although most catalysts for electrochemical reactions degrade over time, past research conducted in the Greenlee Lab at the University of Arkansas has shown that the Ni-Fe catalysts tend to have low degradation and, in some cases, even improvement in the overall activity [1]. This change in overall activity, however, indicates there may be a significant change in the surface chemistry of the nanoparticle during electrocatalysis. In this research, the atomic level changes on the surface of the Ni-Fe catalyst were analyzed by using x-ray photoelectron spectroscopy (XPS) after the potential was cycled. Ultra-high vacuum (UHV)-XPS and near ambient pressure (NAP)-XPS revealed that metallic forms of nickel and iron are present at the surface of the synthesized nanocatalyst but are quickly gone after electrocatalysis, with only more oxidized forms present on the surface after use in the oxygen evolution reaction (OER). The XPS characterization also revealed that the ratio of nickel to iron significantly increases after catalytic use, showing an increase from 1:1 nickel to iron to 7:2 after just one cyclic voltammetry cycle. Overall, the results of this research enable a better understanding of the origins of the durability and increased performance of the catalyst after use in OER. [1] Acharya, Prashant, et al. Chemical Structure of Fe–Ni Nanoparticles for Efficient Oxygen Evolution Reaction Electrocatalysis. ACS Omega (2019)