In situ XRD and operando spectra-electrochemical investigation of tetragonal WO_3-x nanowire networks for electrochromic supercapacitors

Electrochromic supercapacitors (ESCs) are appealing for smart electronic device applications due to their advantages of dual-function integration. Unfortunately, the synchronous dual-function evaluation and the essential reaction mechanism are ambiguous. Herein, we constructed a 3D WO_3-x nanowire networks/fluorine-doped tin oxide (WO_3-x NWNs/FTO) bifunctional electrode for ESCs by a solvothermal self-crystal seeding method. The synchronous correspondence relationship between the optical and electrochemical performances of the WO_3-x NWNs/FTO electrode was explored using an operando spectra-electrochemical characterization method. It reveals an excellent areal capacity of 57.57 mF cm⁻² with a high corresponding optical modulation (ΔT) of 85.05% and high optical-electrochemical cycling stability. Furthermore, the synergistic reaction mechanism between the Al³⁺ ion intercalation behavior and the surface pseudocapacitance reaction during electrochemical cycling is revealed utilizing in situ X-ray diffraction. Based on these results, an ESC device was constructed by pairing WO_3-x/FTO as the cathode with V₂O₅ nanoflowers/FTO (V₂O₅ NFs/FTO) as the anode, which simultaneously deliver high capacity and large optical modulation. Moreover, the energy storage level of the ESC device could be visually monitored by rapid and reversible color transitions in real time. This work provides a promising pathway to developing multi-functional integrated smart supercapacitors.