Toward Efficient Hydrogen Production: Impact of Solid Solution of Tungsten on Nickel-Iron Hydroxide OER Catalysts

Designing catalysts for the oxygen evolution reaction (OER) that are platinum group metal-free (PGM-free) is vital for making the production of hydrogen via water splitting more cost-effective. A trimetallic catalyst, NiFeW-(OH)₂, was synthesized and studied using electrochemical methods, exhibiting higher catalytic performance than bare nickel-iron, manifested by faster reaction kinetics, evidenced by a lower Tafel slope and reduced effective resistance. This catalyst served as a parent compound for heat-treated catalysts in various conditions, such as air and inert atmosphere, to study the effect of the mixed oxide/hydroxide phase on electrochemical performance. X-ray Diffraction (XRD) revealed that tungsten addition expanded the crystal lattice by ∼30% in the c direction, which had a significant impact on the electronic environment, resulting in lowered binding energies, as revealed by X-ray photoemission spectroscopy (XPS). The most active composition was later studied in an anion exchange membrane water electrolyzer (AEM-WE) and showed high performance, reaching current densities of 2.12 A cm^-2 at ∼2.0 V. Density functional theory (DFT) calculations assisted in identifying iron as the active site. Electrochemical impedance spectroscopy (EIS), analyzed by distribution function of relaxation times (DFRT, a.k.a. DRT), revealed the contribution of tungsten toward reduced charge transfer resistance. The best performances were found with compositions close to the solubility limit of tungsten in the system.