Nanogranular advancements in molybdenum-doped tungsten oxide for superior electrochromic energy storage

Abstract

Significant advancement has been achieved in the domain of compact and smart electronic devices by combining electrochromic (EC) materials and energy storage technologies into an integrated electrochemical system. This groundbreaking advancement enables the simultaneous functioning and reciprocal improvement of both electrochromic and energy storage applications. Electrochromic materials play a crucial role in visually displaying the real-time energy levels in EC energy storage devices by changing their optical features in response to voltage. In this scenario, amorphous molybdenum-doped tungsten oxide (W–Mo) thin films were fabricated using a one-step electrodeposition process, and the influence of Mo-doping on allied materials characteristics was investigated. X-ray diffraction analysis detected subtle changes in the diffraction patterns, whereas scanning electron microscopy unveiled variations in the film's structure. These variations encompassed the presence of porosity, clustering, densification, and the occurrence of cracks in the nanogranules within the film. The WMo-2 thin film, doped at a 2 wt%, displayed outstanding electrochromic energy storage capabilities. It efficiently accommodated lithium ions and showcased desirable bifunctional characteristics. The optimized electrode, denoted as WMo-2, delivered impressive electrochromic performance, boasting the highest optical modulation at 81.40 %, exceptional reversibility at 98 %, and a notably high coloring efficiency of 100.81 cm2/C. Moreover, the investigation into supercapacitive properties unveiled noteworthy findings, including an impressive areal capacitance of 63.27 mF/cm2 at 0.1 mA/cm2, a substantial energy density of 9.89 μWh/cm2 at a power density of 75 μWh/cm2, and an outstanding capacitive retention rate of 70.1 % observed over the course of 20,000 consecutive charge-discharge cycles. The multifunctional device is constructed employing optimized W–Mo and glass coated fluorine-doped tin oxide (FTO) as the positive and negative electrodes, respectively. An evaluation was carried out to assess both the electrochromic and supercapacitive performance of the device. The fabricated WMo-2 device was illuminated red and green light-emitting diodes, highlighting its energy efficiency. These discoveries broaden the scope of potential applications for WO3 materials in electrochromic energy storage devices, thus opening doors to the development of more compact and versatile intelligent electronic systems. © 2024 Elsevier Ltd