Supercapattery Electrode Materials by Design: Plasma-Induced Defect Engineering of Bimetallic Oxyphosphides for Energy Storage

Although transition metal hydroxides are promising candidates as advanced supercapattery materials, they suffer from poor electrical conductivity. In this regard, previous studies have typically analyzed separately the impacts of defect engineering at the atomic level and the conversion of hydroxides to phosphides on conductivity and the overall electrochemical performance. Meanwhile, this paper uniquely studies the aforementioned methodologies simultaneously inside an all-in-one simple plasma treatment for nickel cobalt carbonate hydroxide, examines the effect of altering the nickel-to-cobalt ratio in the binder-free defect-engineered bimetallic Ni-Co system, and estimates the respective quantum capacitance. Results show that the concurrent defect-engineering and phosphidation of nickel cobalt carbonate hydroxide boost the amount of effective redox and adsorption sites and increase the conductivity and the operating potential window. The electrodes exhibit an ultra-high capacity of 1462 C g⁻¹, which is among the highest reported for a nickel–cobalt phosphide/phosphate system. Besides, a hybrid supercapacitor device was fabricated that can deliver an energy density of 48 Wh kg⁻¹ at a power density of 800 W kg⁻¹, along with an outstanding cycling performance, using the best performing electrode as the positive electrode and graphene hydrogel as the negative electrode. These results outperform most Ni-Co-based materials, demonstrating that plasma-assisted defect-engineered Ni-Co-P/PO_x is a promising material for use to assemble efficient energy storage devices.