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.