Recycling Cobalt in Spent Lithium-Ion Batteries to Design Two-Dimensional Porous CoO_x/CoN_x Nanosheets for Full Water Splitting and Nonenzymatic Glucose Detection

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