Versatile electrochromic energy storage smart window utilizing surfactant-assisted niobium oxide thin films

Inorganic oxides have considerable potential in electrochromic energy storage applications, as their working performance can be dynamically monitored through structural deformations. One of the key approaches for achieving the desired surface structure is to integrate surface-directing agents during synthesis. This paper reports the successful development of hydrothermally grown niobium oxide (Nb2O5) thin films with the chemical assistance of surfactant derivatives (hexamethylene tetramine, poly(ethylene glycol), and poly(acrylic acid)) for electrochromic energy storage applications. The Nb2O5 layer was confirmed structurally, morphologically, and compositionally using physico-chemical characterization techniques. Electrochemical studies for Nb2O5 electrodes were accomplished, wherein they realized the function integration of adaptive solar radiation regulation and extended their role to energy storage applications. The results showed that the highly crystalline N–H (hexamethylene tetramine-assisted Nb2O5) electrode displayed nanospheres morphology with a porous network. Consequently, at a current density of 0.6 mA cm−2, the N–H electrode had a high areal capacitance and energy density of 26.52 mFcm−2 and 0.045 mWhcm−2, respectively. In-situ coloration monitoring showed that the Nb2O5 electrodes underwent substantial color variations upon redox cycling. Particularly, the N–H electrode displayed intense dark blue coloration, unveiling a large optical contrast of 83.1 % at 600 nm with a coloration efficiency of 126.97 cm2C−1. The electrode achieved long-term electrochromic energy storage cycling stability and excellent rate capability. Incorporating the N-H electrode into a device form boosted its overall functionality, delivering remarkable electrochemical performance. The device exhibited substantial optical modulation of 77.1 % while maintaining excellent supercapacitor performance. The built-in voltage of the device-powered red and green light-emitting diodes, highlighting the energy efficiency of the material. These results significantly broaden the potential applications of Nb2O5 material for electrochromic energy storage systems.