Atmospheric-Pressure Plasma Jet-Induced Ultrafast Construction of an Ultrathin Nonstoichiometric Nickel Oxide Layer with Mixed Ni³⁺/Ni²⁺ Ions and Rich Oxygen Defects as an Efficient Electrocatalyst for Oxygen Evolution Reaction

An approach, i.e., atmospheric-pressure nonequilibrium oxygen plasma jet technology, has been reported for the first time to in situ prepare ultrathin nonstoichiometric nickel oxide layers with mixed Ni3+/Ni2+ ions and rich oxygen defects on the surface of three-dimensional (3D) nickel foams (APPJ-NixOy/NFs). It depends on the bombardment of the highly reactive energetic species (including O, OH•, O2+, and e) with the NF to form the oxide layer in an ultrafast way. The specific feature of the plasma technology gives rise to the simultaneous formation of mixed Ni3+/Ni2+ ions and rich oxygen defects in the APPJ-NixOy/NFs. Specifically, for the APPJ-NixOy/NFs formed in a short time of 5 min, they show remarkably high catalytic activities and excellent stability for the oxygen evolution reaction (OER). They only need an onset potential of 1.490 V and an overpotential of 355 mV to drive a current density of 10 mA cm–2. They also exhibit faster reaction kinetics and a lower Tafel slope (88 mV dec–1). The presence of mixed Ni3+/Ni2+ ions and rich oxygen defects plays an important role in the high catalytic activities of the APPJ-NixOy/NFs. Density functional theory (DFT) calculations indicate that the heterojunction between the NixOy layer and the NF greatly reduces the UL(OER) value, which also contributes to the high catalytic activities of the APPJ-NixOy/NFs. The work reported here provides a potential approach to fabricating transition metal oxide layers with enhanced catalytic activities for overall water splitting.