Electrochromic devices have wide application prospects in energy-saving buildings, low-power displays, anti-glare mirrors and other fields, and have received widespread attention. The inorganic color-changing material tungsten oxide (WO3-x) is considered one of the most promising electrochromic candidates due to its high optical transmission modulation rate and excellent cycle stability. However, its single color variation is not conducive to its application in areas such as multicolor displays. Therefore, it is important to enrich the color variation of WO3-x based electrochromic devices.
At present, the monotony of color has been reported by doping elements by WO3-x, synthesizing WO3-x composites, and designing WO3-x materials into plasma resonance or Fabry-Perot interferometer structures, but there are still shortcomings of high cost or sacrificing high transparency of the device.
Recently, the team of academician Yu Shuhong of the University of Science and Technology of China designed a new type of multi-color display electrochromic device based on nanowire assembly, which co-assembles W18O49 and V2O5 nanowires by using interface assembly technology, so that the device has specific optical, electrical and multicolor display characteristics (Figure 1a).
Compared with general electrochromic devices, the advantages of the device are reflected in the following two aspects: (1) Because the W18O49 and V2O5 nanowires display different colors under the same applied voltage, the nanowire co-assemblies can present dynamic color changes from orange, green to gray under the application of 2 V, 0 V and -0.5 V bias.
By controlling the proportion of the two nanowires, the electrochromic properties of the co-assembled film can be adjusted, including color display, transparency, response time, and coloring rate (Figure 1b) ;(2) Various patterns and components can be constructed with the assistance of LB technology in the mask plate to achieve more complex color display and information transmission (Figures 1c-d).
Figure 1. Based on the co-assembly of W18O49 and V2O5 nanowires, a multicolor display device and its performance regulation are constructed
At the same time, most of the electrochromic devices are prepared based on brittle ITO substrates, and require an external power supply system to drive the work, which affects the flexibility, independence and portability of the devices. In recent years, there have been reports of integrating solar cells or nano-generators with electrochromic systems for the manufacture of self-powered electrochromic devices. However, such devices require complex integrated systems that are often detrimental to practical use, so developing self-powered electrochromic devices with flexible and mechanical stability remains challenging.
Figure 2. Self-powered flexible electrochromic intelligent window constructed based on Ag and W18O49 nanowires
In order to solve this problem, the team's special associate researcher Wang Jinlong and others were inspired by the concept of primary batteries, and successfully built a self-powered flexible electrochromic device by integrating the primary battery system into the highly stable and flexible Ag nanowire (NWs) transparent conductive electrode (TCE) and the W18O49 nanowire electrochromic film (Figure 2a-b).
In self-powered flexible electrochromic devices, Ag nanowires play two roles, first as electrodes instead of ITO substrates, and then coupled with Al sheets to form two working electrodes for primary batteries. The open-circuit voltage of Ag nanowires and Al sheets is ~0.83 V, which is sufficient to drive the coloring of W18O49 nanowires. Unlike external power supplies, self-powered electrochromic devices avoid internal resistance, and their average coloring efficiency (CE) is approximately 20% higher than that of external power supplies, from ~50.7 c㎡/C to ~62.2 c㎡/C (Figure 2c).
The flexible self-powered electrochromic device consumes only ~6.8 mg/c㎡ of aluminum sheet after 450 electrochromic cycle cycles, and the cost is only 0.19 US dollars/c㎡. In addition, the dimensions of the portable flexible self-powered electrochromic device can be expanded to 20 × 20 c㎡ (Figure 2d). In addition, the device exhibits good sunlight irradiation regulation as a smart window, reducing the average temperature of simulated sunlight-illuminated objects from 33 oC to 25.6 oC when the window is shaded compared to the faded state (Figure 2e-f).
The results of the above research were published in Nano Letters under the titles "Manipulating Nanowire Assemblies toward Multicolor Transparent Electrochromic Device" and "Self-Powered Flexible Electrochromic Smart Window", respectively. It provides a new way for the development of high-performance multi-color display and flexible self-powered electrochromic devices.
The research has been funded by the National Natural Science Foundation of China Innovative Research Group Project, the National Natural Science Foundation of China Key Project, the Frontier Key Project of the Chinese Academy of Sciences, the Special Fund for Basic Scientific Research Business Expenses of Central Universities, the Collaborative Innovation Project of Universities in Anhui Province, and the Synchrotron Radiation Joint Fund of the University of Science and Technology of China. (Source: School of Chemistry and Materials Science, University of Science and Technology of China)
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