In-situ activation of three-dimensional porous NiFe alloy: Synergistic NiFe hydroxide/alloy as composite active sites for efficiently catalyzing alkaline water splitting

The strategic engineering of bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media is critical for water-splitting technologies. This study presented a catalyst consisting of in situ grown nickel iron (NiFe) hydroxide/porous alloy composite catalytic sites on a nickel foam (NF) substrate. Initially, a porous NiFe alloy was electrodeposited onto NF, employing hydrogen (H2) bubbles as dynamic templates to induce a three-dimensional (3D) architecture. Subsequently, ultrathin layers of NiFe hydroxide nanosheets were synthesized in situ via electrochemical activation. The distinctive porous structure, coupled with the in-situ activated nanosheets, augmented the exposure of active sites, optimized electrolyte interaction, and expedited the expulsion of gaseous byproducts, collectively enhancing the kinetics of water electrolysis. The resultant a-Ni2Fe/NF demonstrated superior OER/HER activities, achieving overpotentials of merely 240 mV for OER and 183 mV for HER at a current density of 100 mA cm-2 in alkaline solution. Moreover, a voltage of just 1.53 V was requisite to attain a current density of 10 mA cm-2 for overall water splitting. Operando Raman spectroscopy revealed the formation of NiOOH and FeOOH active phases during HER, indicating the hydroxide's role in adsorbing hydroxyl groups and shielding NiFe alloy's H* adsorption sites from oxidation. The presence of the more reactive β-NiOOH phase was discerned during OER process, acting as an intermediate. This straightforward yet efficacious strategy heralded a new paradigm in the design of NiFe-based bifunctional catalysts, significantly bolstering the efficiency of water electrolysis catalysis.