Morphological and Electronic Optimization of Nanostructured FeCoNi-Based Electrocatalysts by Al Dopants for Neutral/Alkaline Water Splitting

A “hard–soft chemistry engineering” strategy is developed to assemble the Al–FeCoNi2/Ni/NF electrode with the optimized electronic/morphological structure through Al doping. The resulting 3D hierarchical Al–FeCoNi2/Ni/NF electrode affords the current density of 10 mA/cm2 at a cell voltage of 1.73 V, compared to that (1.81 V) of Pt/NF||IrO2/NF electrode-pair setting, in neutral PBS buffer at 25 °C. As an alkaline electrolyzer, the Al–FeCoNi2/Ni/NF electrode-pair device requires the voltage of 1.73 and 1.97 V to achieve the current density of 100 and 500 mA/cm2 in 1 M NaOH aqueous solution at 25 °C, respectively. At 80 °C, the anion exchange membrane (AEM) water electrolyzer employing bifunctional Al–FeCoNi2/Ni/NF electrodes is demonstrated to deliver the current densities of 0.5 A/cm2 (at a cell voltage of 2.00 V) in 1 M PBS and 2.0 A/cm2 (at a cell voltage of 1.82 V) in 1 M KOH electrolytes, respectively. Material and kinetic studies of Al–FeCoNi2 reveal that the keys for the observed activity/stability are ascribed to (a) the proposed charge-polarized [Alδ+/Feδ+Co0Niδ−] motif (evidenced by X-ray photoelectron spectroscopy (XPS)/X-ray absorption near-edge spectroscopy), facilitating interfacial electron transfer from metallic core to the catalytic shell, (b) the increased defects/disorders concentrating on the surface-adsorbed intermediates (characterized by high-resolution transmission electron microscopy and extended X-ray absorption fine structure), (c) the facile dissociation of the adsorbed water into H+ and OH– promoted by Al3+ dopant electrochemically on the catalyst surface (kinetic isotope effect result), (d) the upshift of d-band center and the increased work function induced by the hard nature of Al3+ dopant (evidenced by ultraviolet photoelectron spectroscopy/XPS/L-edge X-ray absorption spectroscopy), optimizing the binding energy of M–H*/M–O* intermediates (M = Fe, Co, and Ni), and (e) the good stability caused by redox-resistant nature of Al3+ dopant at a high current density (1 A/cm2) in the AEM electrolyzer. That is, Al0–FeCoNi metallic core and Al3+-doped spinel/(oxy)hydroxide shells work in concert to optimize the electrical conductivity and electronic structure of Al–FeCoNi2 nanoparticles for hydrogen evolution reaction/oxygen evolution reaction kinetics in neutral/alkaline electrolytes.