Energy-efficient Electrochromic Smart Windows

Abstract

Smart windows, having electrically-controlled transmission and zero-energy consumption when maintaining a colored or colorless state, increase the energy efficiency of buildings because they offer the potential to greatly reduce the energy cost of lighting, heating and cooling. Currently, the operation of traditional smart windows requires external voltages to trigger the coloration/bleaching processes, which makes traditional smart windows far from being a net-zero energy-consumption technology. The purpose of this work was to develop energy-efficient electrochromic smart windows, which are addressed by electrochromic supercapacitors and electrochromic batteries. Herein, the first study involved electrodeposition of MoO2+x thin films with oxygen deficiencies. The electrodeposited MoO2+x electrode exhibits a super-capacitive performance of 89 mF cm−2 at 1 mA cm−2. This enhanced super-capacitive performance makes these electrodeposited MoO2+x films highly promising candidates as counter electrodes in a complementary electrochromic device (i.e. electrochromic supercapacitor). As such, in comparison to the single-active-layer electrochromic device, the introduction of MoO2+x electrode accelerates redox reactions at the working electrode (WO3). We show that the coloration potential of the complementary electrochromic device decreases to −0.5 V and the bleaching potential reaches as low as 0.5 V. This research provides a new and facile strategy to fabricate sub-stoichiometric molybdenum oxide nanofilms and reveals the functions of super-capacitive materials in a complementary electrochromic device. The second study involved the synthesis of aqueous V3O7 nanoparticle inks, which offers the potential for fabricating large-scale thin films via low-cost solution-processed techniques. The fabricated V3O7 electrode can be utilized in a Zn-V3O7 electrochromic battery display system that exhibits an optical transmittance contrast of 21% at 632.8 nm and rapid switching times of 10.4/28.6 s (coloration at 0.2 V/bleaching at 1.6 V). Moreover, the Zn-V3O7 electrochromic battery display system eliminates the external voltage requirement for the coloration process and retrieves 15.2 mWh g-1 (32.6 mWh m-2) energy consumed for the bleaching process. For a proof of concept, a prototype aqueous Zn-V3O7 electrochromic battery display is constructed by sandwiching a Zn anode between two V3O7 cathodes. The demonstrated electrochromic battery display possesses an open circuit potential (OCP) of 1.38 V, which enables the self-coloration behavior and energy retrieval functionality. We also show that the prototype display reversibly switches between the multi-colors (fully yellow, fully grayish-blue and half yellow-half grayish-blue maple leaves). This research presents a facile strategy to synthesize aqueous V3O7 inks, as well as a novel electrochromic battery display having energy retrieval functions, thus facilitating the development of energy-efficient electrochromic displays.