1、 Highlights of this article


1) A mixed aqueous zn2+/al3+ electrochromic battery system was introduced;


2) The mixed aqueous electrolyte endows the electrochromic battery with excellent electrochemical performance;


3) The mixed water zn2+/al3+ electrochromic battery system can be applied to the electrochromic smart window with recyclable energy consumption.


2、 Research background


Electrochromic technology can be applied to a variety of optical devices because of its reversible color change characteristics. The most promising application is electrochromic smart window. The electrochromic smart window can achieve the effect of building energy saving by reversibly adjusting the solar transmittance. However, the traditional electrochromic smart window always needs to be driven by external voltage, which is far from achieving the optimal energy-saving effect. In recent years, scientists in the field of electrochromism have always hoped to achieve the recovery of energy consumed by smart windows. Therefore, an electrochromic battery structure is proposed, which provides a feasible scheme for recovering the energy consumed by the electrochromic smart window.


Tungsten oxide (WO3) is the core material of electrochromic smart window, but it is often limited by driving ions. Recently, zhaozhigang research group of Suzhou Institute of nanotechnology, Chinese Academy of Sciences and Jim Yang Lee research group of National University of Singapore found that the use of al3+ electrolyte can greatly improve the electrochromic performance of WO3 electrochromic electrode (ref: adv. funct. mater. 2015, 25, 5833-5839; energy environ. sci., 2018, 11, 2884-2892). Therefore, the author proposed a mixed electrolyte containing zn2+ and al3+, which not only solved the irreversible problem of Anodic Deposition dissociation in electrochromic batteries, but also greatly improved the electrochemical performance of WO3 electrochromic electrodes. It is worth noting that this electrolyte system is suitable for all WO3 electrochromic electrodes currently in existence.

Figure 1 Schematic diagram of the structure of hybrid zn2+/al3+ electrochromic battery with rechargeable aqueous solution, in which Zn is used as the anode of the electrochromic battery, the traditional WO3 electrochromic film is used as the cathode, and 1m znso4-alcl3 solution is used as the electrolyte.


Figure 1 shows the working mechanism of this new electrochromic smart window with recyclable energy consumption. During charging, the electrochromic battery consumes energy, so that the reduced WO3 is oxidized, and zn2+ is deposited on the Zn anode. Finally, the electrochromic battery achieves the fading effect. When discharging, WO3 is restored to achieve the coloring effect, and the energy consumed in the fading process can be partially recovered. This coloring process does not need to be driven by external voltage like the traditional electrochromic smart window. It is a spontaneous thermodynamic downhill process.


3、 Experimental verification


In order to verify the feasibility of this novel rechargeable aqueous hybrid zn2+/al3+ electrochromic battery, amorphous tungsten oxide thin films with high performance were prepared by common three electrode electrodeposition method. Figure 2A shows the most common three electrode electrodeposition method. Figure 2B shows that WO3 film is composed of uniform nanoparticles. Figure 3C confirms its amorphous state. In addition, the WO3 thin film prepared by the electrodeposition method is in the oxidation state (W is 6valent tungsten, FIG. 2D), which proves that the WO3 thin film is in the charged state. Therefore, WO3 film can directly generate self coloring effect. Figure 2E shows the self coloring effect of WO3 film under different electrolytes. It can be clearly seen from the figure that WO3 film has faster self coloring time (0.5s versus 1.9s) under 1m znso4-alcl3 electrolyte. Figure 2F shows the valence state of W in the self colored WO3 film. It is found that w6+ can be oxidized to w4+ due to the ultra-high activity of al3+

Figure 2 Characterization of Electrodeposited WO3 films and their self coloring properties.

Figure 3 Al3+ and zn2+ intercalation proof.

In order to prove that the excellent performance of WO3 film in 1m znso4-alcl3 electrolyte is mainly due to the insertion of al3+ rather than a small amount of h+ in the electrolyte. The authors also used STEM-EDS mapping and TOF-SIMS to characterize the colored tungsten oxide films. Figure 3 fully demonstrates the embedding of al3+ and zn2+ in WO3 thin films.

Figure 4 Comparison of electrochemical properties of WO3 thin films in different electrolytes.


In addition, the electrochemical properties of WO3 thin films in different electrolytes were compared. It is obvious from figure 4A that WO3 film has higher electrochemical activity in 1m znso4-alcl3 electrolyte. Figure 4B verifies the high area specific capacitance resulting from this high electrochemical activity. Figure 4C shows the optical modulation range of WO3 film in 1m znso4-alcl3 electrolyte, reaching 88% of the optical modulation range. Figure 4D shows that WO3 film has faster response time in 1m znso4-alcl3 electrolyte (coloring time: 3.9s versus 6.9s; fading time: 5.1s versus 6.6s). In addition, WO3 thin films also showed better cycle life in 1mznso4-alcl3 electrolyte.


In order to verify the application of this water-based hybrid electrochromic battery system in the field of electrochromic smart window, a new electrochromic device was assembled. Figure 5A reveals the structure of this new electrochromic smart window with recyclable energy consumption. As can be seen from figure 5a, the author uses two WO3 electrochromic films to clamp Zn sheets in the middle to assemble an electrochromic smart window with recyclable energy consumption. This structure can greatly reduce the transmittance of the smart window in the colored state. Figure 5B shows the transmittance curve of the electrochromic smart window with recyclable energy consumption under different states. It can be seen from the figure that the transmittance of the smart window can reach 79% under fading state, and the smart window under fading state has an open circuit voltage of 1.15V, which is enough to light a 0.5V led and produce coloring effect (Figure 5C). Therefore, the optical modulation range of the smart window can reach 77%. Figure 5D shows the rapid charging / fading process of this smart window. It can be seen from the figure that this smart window can completely fade within 10s. Figure 5E confirms the fast response characteristics of this smart window, and its coloring time and fading time are 5.7s and 10.3s respectively

Figure 5 Display of electrochromic smart window with recyclable energy consumption.


4、 Conclusion


The author first proposed a water mixed zn2+/al3+ electrochromic battery system. This new electrochromic battery system can greatly improve the electrochemical performance of the traditional WO3 thin film electrode, and thus construct a fast response electrochromic smart window. In addition, this system eliminates the need of the traditional electrochromic smart window for the counter electrode and solves the problem of the cycle stability of the counter electrode. Most importantly, this new electrochromic battery smart window does not generate energy consumption in the coloring process, and can recover part of the energy consumed in the fading process. This kind of water mixed zn2+/al3+ electrochromic battery smart window will continue to promote the development of more efficient and energy-saving electrochromic devices.


Haizeng Li, Curtis J. Firby, Abdulhakem Y. Elezzabi, Rechargeable Aqueous Hybrid Zn2+/Al3+Electrochromic Batteries, Joule, 2019, DOI:10.1016/j.joule. 2019.06.021


About the author


The first author and chief corresponding author of this article is Dr. lihaizeng. Dr. lihaizeng graduated from Donghua University in 2016, studied under Professor wanghongzhi, and has been engaged in the research of electrochromic smart windows since 2012. In February, 2017, he joined the University of Alberta, and the cooperative tutor is Professor A. y. elezzabi of research chair in Canada. Dr. lihaizeng's recent research in the University of Alberta is mainly based on his previous accumulation in Professor wanghongzhi's research group. Since joining the University of Alberta, Dr. lihaizeng has published the following representative articles:

Adv. Mater, 2019, 31, 1807065；

Nano Energy, 2018, 47, 130-139;

ACS Appl. Mater. Interfaces 2018, 10, 1210520-10527;

ACS Appl. Mater. Interfaces 2019, 11, 2220378-20385