Recycling Cobalt in Spent Lithium-Ion Batteries to Design Two-Dimensional Porous CoO_<i>x</i>/CoN_<i>x</i> 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 (CoOx)/cobalt nitride (CoNx) 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 CoOx/CoNx nanosheets (denoted as CoOx/CoNx NSs) was synthesized by using cobalt oxalates (COs) recycled from spent lithium-ion batteries. In a 1.0 M KOH solution, the 2D porous CoOx/CoNx NSs exhibited higher oxygen evolution reaction (OER) electrocatalytic activity than commercial ruthenium oxide (RuO2). Meanwhile, the CoOx/CoNx NSs exhibited a slightly lower hydrogen evolution reaction (HER) electrocatalytic activity than 20 wt % platinum/carbon (Pt/C). Moreover, the CoOx/CoNx 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||RuO2 benchmark. Furthermore, the 2D porous CoOx/CoNx NSs featured significantly higher HER, OER, and full water-splitting catalytic stability than Pt/C/RuO2. Moreover, the CoOx/CoNx 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 CoOx/CoNx 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 CoNx 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.