The HER/OER mechanistic study of an FeCoNi-based electrocatalyst for alkaline water splitting

Abstract

Herein, the electrodeposited-film electrode CFeCoNiP was fabricated to serve as a bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Kinetic Tafel slope analysis suggests that the HER follows the Volmer–Tafel mechanism (29 mV dec⁻¹), indicating that the recombination of the two adsorbed hydrogen atoms is the rate-determining step. The FeCoNi-based thick film (thickness: 168.3 μm) shows a metallic state favorable for electron transfer; on the other hand, in the case of the FeCoNi-based thin film (thickness: 389.2 nm), the in operando XAS investigation reveals that Fe³⁺-assisted water dissociation promotes the formation of Co²⁺–μ-H–Ni³⁺ (catalyst-H_ad) species, which subsequently undergoes reductive elimination to furnish H₂ gas via the HER process. During the OER, the CoNi–oxide matrix acts as a chemical and electroconductive host to build/stabilize the key intermediate [Fe⁴⁺O/Fe³⁺–O˙] motifs; this subsequently triggers the catalytic O–O bond formation (30 mV dec⁻¹) through the radical–radical coupling of the adjacent [Fe⁴⁺O/Fe³⁺–O˙] motifs or/and OH⁻ attack on the Fe⁴⁺-induced electrophilic oxygen center, leading to the release of O₂. The mechanistic experiments provide advanced insights into the catalytic kinetics/intermediates and demonstrate that the electronically cooperative interplay among Fe/Co/Ni leads to enhanced alkaline water electrolysis. The CFeCoNiP catalyst exhibits an excellent HER activity (specific activity j_s = 0.227 mA cm⁻²) with a low charge transfer resistance (3.9 Ω) and an overpotential of 37 mV, achieving the current density of 10 mA cm²; moreover, it shows good OER activity (j_s = 1.798 mA cm⁻²) with low charge transfer resistance (2.1 Ω) and an overpotential of 250 mV, approaching a current density of 10 mA cm⁻² in a 1 M NaOH aqueous solution. The CFeCoNiP/NF (electrodeposited on Ni foam) electrode-pair device achieved the current densities of 100 and 500 mA cm⁻² at the voltages of 1.65 and 1.86 V, respectively, under alkaline conditions.