Recycling Cobalt\nin Spent Lithium-Ion Batteries to\nDesign Two-Dimensional Porous CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> Nanosheets for Full Water Splitting and\nNonenzymatic Glucose Detection

Enhancing catalytic efficiency, achieving controlled\nsynthesis,\nand lowering production costs are crucial for promoting the use of\ncobalt oxide (CoO<sub><i>x</i></sub>)/cobalt nitride (CoN<sub><i>x</i></sub>) composites in full water splitting and\nelectrochemical sensing. In this study, a three-dimensional (3D) hierarchically\nporous material composed of numerous two-dimensional (2D) porous CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> nanosheets\n(denoted as CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NSs) was synthesized by using cobalt oxalates (COs) recycled\nfrom spent lithium-ion batteries. In a 1.0 M KOH solution, the 2D\nporous CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NSs exhibited higher oxygen evolution reaction (OER) electrocatalytic\nactivity than commercial ruthenium oxide (RuO<sub>2</sub>). Meanwhile,\nthe CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NSs exhibited a slightly lower hydrogen evolution reaction (HER)\nelectrocatalytic activity than 20 wt % platinum/carbon (Pt/C). Moreover,\nthe CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NS-based full water-splitting electrolyzer achieved a current density\nof 10 mA cm<sup>–2</sup> at 1.562 V, which was 49 mV more positive\nthan the 1.489 V of the Pt/C||RuO<sub>2</sub> benchmark. Furthermore,\nthe 2D porous CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NSs featured significantly higher HER, OER, and full water-splitting\ncatalytic stability than Pt/C/RuO<sub>2</sub>. Moreover, the CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NS-based\nnonenzymatic electrochemical sensor exhibited advanced glucose detection\nperformance. Particularly, the sensor had a wide linear range of 1.0–10.5\nmM), a low detection limit of 5.0 μM), and a fast response time\nof 0.6 s. The 2D porous CoO<sub><i>x</i></sub>/CoN<sub><i>x</i></sub> NSs exhibited excellent catalytic activity owing\nto their 3D hierarchically porous architectures, large surface area,\ngood conductivity, uniformly dispersed active sites, and synergistic\neffects of CoN<sub><i>x</i></sub> and CoO. This study presents\nan advanced model for recycling Co metal from spent devices and designing\nadvanced Co-based electrocatalysts for water splitting and electrochemical\nsensing.