Upcycling Spent Cu‐Containing Catalysts to CuO Humidity Sensors for Physiological Detection: Calcination Temperature Optimization

To achieve high‐value utilization of spent catalysts, we recovered copper (Cu) from Cu‐bearing spent catalysts via acid‐base leaching to fabricate CuO and systematically studied how calcination temperature affects their microstructure and humidity‐sensing performance. Characterizations show that CuO‐500 (calcined at 500°C) has a large specific surface area (181.96 m 2 g −1 ) and abundant O V (56.83%). Residual Al and Fe from the spent catalyst induce in situ lattice self‐doping, conferring typical n‐type semiconducting properties. Humidity‐sensing tests show the CuO‐500 sensor achieves a high response of 16,300% across 11%–85% RH, with 71 s/3 s response/recovery times and 3.44% low hysteresis. It also exhibits excellent repeatability and 30‐day stability, showing great potential for noncontact detection and wearable respiratory monitoring. This superior performance stems from the synergistic effects of its structural and electronic advantages. This work provides a facile sustainable strategy for spent catalyst upcycling and low‐cost physiological humidity sensor fabrication.