3D conifer-like WO3 branched nanowire arrays electrode for boosting electrochromic-supercapacitor performance

Abstract Electrochromic-supercapacitors (ESCs) have shown great perspective in multifunctional smart devices due to their special function of monitoring the energy storage level by the visible color changes. However, the poor areal capacitance at high optical modulation(ΔT) and the inferior bifunctional lifespan hinder the practical application of ESCs. It is thus important to develop feasible ways to boost the optical-electrochemical properties and comprehend the corresponding inner mechanism. Inspired by nature’s conifer structure, herein, 3D hexagonal WO3 branched nanowire arrays/F-doped SnO2 (h-WO3 BNW/FTO) electrode have been successfully constructed via a single-step hydrothermal reaction followed by annealing process. Such biomimetic conifer-like structure is shaped by the separating crystal growth effect coupled with the specific topological structure of h-WO3. Benefiting from bionic structure, h-WO3 BNW/FTO electrode delivers a high areal specific capacity (0.0223 mAh cm-2) and large corresponding ΔT respond (80% ΔT at 650 nm) at 5 mV s-1, as well as outstanding optical-electrochemical cycle stability. Moreover, the aluminum storage mechanism during the optical-electrochemical process is affirmed using cyclic voltammetry and in-situ X-ray method. For practical application, the assembled ESC device based on the h-WO3 BNW/FTO electrode delivers a high 22.75 mF cm-2 areal specific capacitance, corresponding 45.81% ΔT (at 650 nm) and long-term bifunctional stability (over 90% bifunctional retention after 5000 cycles). Meanwhile, the visually monitoring capacity of the device is realized through the color changes. This study provides some insights into the biomimetic design for advanced bifunctional electrodes and the underlying mechanism affecting the electrochromic-charge storage performance.