Focused Plasma- and Pure Water-Enabled, Electrode-Emerged Nanointerfaced NiCo Hydroxide–Oxide for Robust Overall Water Splitting

Bimetallic, bifunctional electrocatalysts capable of driving both oxygen (OER) and hydrogen (HER) evolution half-reactions on both electrodes in commercial water electrolysis cells are among the most promising materials systems for clean hydrogen energy generation. However, insufficient hydrogen and oxygen production activity at industry-relevant current densities and long-term catalyst stability on the electrode surface prevent this approach from industrial translation. This work resolves these challenges by advancing the promising, yet far-from-successful attempts to sprout bimetallic electrocatalytic nanostructures directly from electrode frames. For the first time, we utilize magnetic-field-focused, atmospheric-pressure plasma jets in oxygen-argon gas mixtures to successfully induce the nanointerfaced bimetallic NiCo hydroxide and oxide catalyst phases. After a simple hydrothermal treatment in pure water, NiCo bimetallic hydroxide nanosheets are densely covered with strongly bonded bimetallic NiCo oxide nanoparticles which ensure high catalytic activity evidenced by the low overpotentials for both HER and OER for delivering a current density of 100 mA cm^-2 (j₁₀₀) of only 306 and 484 mV, respectively. The electrode-emerged nanointerfaced NiCo hydroxide-oxide bimetallic system (NiCo₂O₄-NiCo(OH)_x) shows an ultrastable electrocatalytic performance under a high current density of j₂₀₀, which only decays 5.8% and 6.3% for HER and OER processes within 100 h. The competitive H₂ and O₂ production rates are about 1.27 and 0.69 mmol h^-1 cm^-2 (near to 2:1, under j₁₀ conditions), meeting a nearly 100% Faradaic efficiency. Furthermore, the theory calculation indicates that the Ni and Co sites of NiCo₂O₄-NiCo(OH)_x are the catalytic centers for the HER process. Our new plasma-enabled approach for the controlled production of bimetallic hydroxide-oxide active nanointerfaced systems is generic and is potentially suitable for diverse materials systems and applications well beyond electrocatalysis.