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-Ni_xO_y/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-Ni_xO_y/NFs. Specifically, for the APPJ-Ni_xO_y/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-Ni_xO_y/NFs. Density functional theory (DFT) calculations indicate that the heterojunction between the Ni_xO_y layer and the NF greatly reduces the U_L(OER) value, which also contributes to the high catalytic activities of the APPJ-Ni_xO_y/NFs. The work reported here provides a potential approach to fabricating transition metal oxide layers with enhanced catalytic activities for overall water splitting.