smart windows commercialization must overcome the challenges posed by slow switching speed, nonuniform switching processes, optical contrast, and energy consumption as the device is scaled up. Consequently, having an inexpensive new energy-efficient and high optical contrast electrochromic platform with a rapid and uniform switching process is a welcomed opportunity that will accelerate the development and commercialization of large-scale electrochromic smart windows. Simultaneously, the development of such an large-scale electrochromic platform would further advance the abovementioned applications using electrochromic technologies. In a conventional electrochromic smart window, the requirement of an external energy source brings about a few drawbacks, including complicated installation, increased cost, and offsetting energy savings. [9,10] To overcome these issues, several efforts have been devoted to the implantation of photovoltaic (PV)-electrochromic window systems. [11] As schematically shown in Figure 1a, the PV device provides the needed electrical energy, via the photoelectron conversion of light photons, to operate the electrochromic smart window. [12] Although this configuration eliminates the need for an external power source, the electrical wiring is quite elaborate as it requires control circuitry to manage the current and voltage that power the electrochromic window. [10] The solar-generated power consumed by the electrochromic window renders this system far from being an energy-efficient platform. Furthermore, in a normal operation requiring active sunlight regulation, the electrochromic window needs to be colored during the day and bleached at night or during sunlight intermittency. However, unless the electrical energy is stored in an external energy storage unit (e.g., battery), the lack of solar electrical power source to bleach the electrochromic window at night or during sunlight intermittency at daytime introduces another significant challenge to this PV-electrochromic window platform. [9] Recently, we reported on a new and fundamentally different class of ECDs which facilitates the ability to efficiently retrieve the consumed energy while preserving the natural electrochromic characteristics. [2,13,14] In these devices (defined as zincanode-based electrochromic device, ZECD), an aqueous electrolyte compatible zinc anode is used as the counter electrode. Most importantly, due to the redox potential difference between Newly born zinc-anode-based electrochromic devices (ZECDs), incorporating electrochromic and energy storage functions in a single transparent platform, represent the most promising technology for next-generation transparent electronics. As the existing ZECDs are limited by opaque zinc anodes, the key focus should be on the development of transparent zinc anodes. Here, the first demonstration of a flexible transparent zincmesh electrode is reported for a ZECD window that yields a remarkable electrochromic performance in an 80 cm 2 device, including rapid switching times (3.6 and ...

Electrochromic displays have been the subject of extensive research as a promising colour display technology. The current state-of-the-art inorganic multicolour electrochromic displays utilize nanocavity structures that sacrifice transparency and thus limit their diverse applications. Herein, we demonstrate a transparent inorganic multicolour display platform based on Zn-based electrochromic devices. These devices enable independent operation of top and bottom electrochromic electrodes, thus providing additional configuration flexibility of the devices through the utilization of dual electrochromic layers under the same or different colour states. Zn-sodium vanadium oxide (Zn-SVO) electrochromic displays were assembled by sandwiching Zn between two SVO electrodes, and they could be reversibly switched between multiple colours (orange, amber, yellow, brown, chartreuse and green) while preserving a high optical transparency. These Zn-SVO electrochromic displays represent the most colourful transparent inorganic-based electrochromic displays to date. In addition, the Zn-SVO electrochromic displays possess an open-circuit potential (OCP) of 1.56 V, which enables a self-colouration behaviour and compelling energy retrieval functionality. This study presents a new concept integrating high transparency and high energy efficiency for inorganic multicolour displays.

Electrochromism, an emerging energy conversion technology, has attracted immense interest due to its various applications including bistable displays, optical filters, variable optical attenuators, optical switches, and energy-efficient smart windows. Currently, the major drawback for the development of electrochromism is the slow switching speed, especially in inorganic electrochromic materials. The slow switching speed is mainly attributed to slow reaction kinetics of the dense inorganic electrochromic films. As such, an efficient design of nanostructured electrochromic materials is a key strategy to attain a rapid switching speed for their real-world applications. In this review article, we summarize the classifications of electrochromic materials, including inorganic materials (e.g., transition metal oxides, Prussian blue, and polyoxometalates), organic materials (e.g., polymers, covalent organic frameworks, and viologens), inorganic-organic hybrids, and plasmonic materials. We also discuss the electrochromic properties and synthesis methods for various nanostructured inorganic electrochromic materials depending on structure/morphology engineering, doping techniques, and crystal phase design. Finally, we outline the major challenges to be solved and discuss the outlooks and our perspectives for the development of high-performance nanostructured electrochromic materials.

for developing bistable displays due to their zero-energy consumption while maintaining a colored or colorless state. [3] However, operation of traditional electrochromic displays requires external voltages to trigger the coloration/bleaching processes, which makes the traditional electrochromic displays far from a netzero energy-consumption technology. The majority of the current electrochromic display research has focused on developing nanostructure electrochromic materials for fast switchings [4,5] without paying attention to how to reduce the consumed energy of the electrochromic displays.Recently, we developed a promising Zn-based electrochromic battery technology for smart windows. [6,7] The electrochromic battery exhibits self-coloration behaviors and eliminates the external voltage requirement for triggering the coloration process. The aqueous compatible Zn anode implements a much lower charged/bleached voltage for the electrochromic battery compared to the Li-and Al-based electrochromic batteries. [8][9][10] The lower charged/bleached potential indicates a lower energy consumption during the bleaching process. As such, the Zn-based aqueous electrochromic battery platform is an energy-efficient technology for reducing the energy consumption of electrochromic devices. To date, no reports exist on the utilization of electrochromic battery systems for developing energy-efficient electrochromic displays.Electrochromic materials play an important role in the development of electrochromic battery displays. Transition metal oxides (TMOs) have shown excellent electrochromic properties due to their remarkable multivalence states and the corresponding color evolutions. [11][12][13][14][15] Among the TMOs, vanadium oxide is considered the most promising material for electrochromic displays because of its multicolor behaviors. [16][17][18] However, the low electrical conductivity, significant volume expansion during cycling, and the slow reaction kinetics of bulk vanadium oxide prevent its widespread use. [19,20] In recent years, nanosized vanadium oxides have been studied to mediate these drawbacks because nanostructures provide abundant active sites on the surface and shorten the diffusion paths of ions. [20] Techniques, such as electrodeposition, [19] Electrochromic displays have attracted increased attention owing to their reversible switch of multicolors. However, the external voltage requirement for triggering the color switching makes them far from an optimum energyefficient technology. The newly developed electrochromic batteries eliminate the energy consumption for coloration while they can retrieve the consumed energy for bleaching. Such features make the electrochromic battery technology the most promising technology for energy-efficient electrochromic displays. Here, a scalable method to synthesize colloidal V 3 O 7 nanoparticles is presented, which is compatible with solution-process techniques for aqueous Zn-V 3 O 7 electrochromic battery displays. The Zn-V 3 O 7 electrochromic battery display shows an ...

Molybdenum oxides have been widely studied in recent years, owing to their electrochromic properties, electrocatalytic activities for hydrogen evolution reactions (HERs) and excellent energy storage performance. These characteristics strongly depend on the valence states of Mo in the oxides such as IV, V, and VI, which can be efficiently altered through oxygen deficiencies within the oxides. Here, we present a colloidal electrodeposition method to introduce oxygen vacancies in such Mo oxide films. We prepared uniform MoO x films and investigated their electrochemical characteristics under different valence states IV, V, and VI. In this paper, we demonstrate that MoO 2+x films, where Mo in valence states IV and V, can be used for high-performance supercapacitor electrodes. Due to their high conductivity, they exhibit an areal capacitance of 89 mF cm −2 at 1 mA cm −2 and negligible capacitance loss within 600 cycles. Additionally, we demonstrate that, in a complementary electrochromic device configuration, the introduction of an MoO 2+x counter electrode remarkably lowers the activation potential of WO 3 from −2 to −0.5 V and achieves a fully bleached state at 0.5 V. These properties make the MoO 2+x film an ideal counter electrode to store ions for an electrochromic device. Furthermore, MoO 3−y films, where Mo in the valence states V and VI, are obtained by annealing the electrodeposited MoO 2+x film under 200 °C for 24 h. Such films exhibit an excellent catalytic for the HER with an overpotential of 201 mV. Furthermore, we show that MoO 3 films, where Mo at its highest oxidation state (VI), can be obtained via annealing the MoO 2+x film at 300 °C for 6 h, and the resulting films exhibit battery characteristics. Our research provides a new and facile strategy to fabricate substoichiometric molybdenum oxide nanofilms and reveals the effect of different valences on the electrochemical performance of molybdenum oxide films, which opens new doorways for future research in the electrochemical applications of transition metal oxides.

Electrochromic devices with a wide color gamut distribution have long been sought after for non-emissive display technologies. The current state-ofthe-art multicolor electrochromic displays utilize a single electrochromic layer, which restricts their color tunability within a linear or curved segment scope in International Commission on Illumination (CIE) color space and thus leads to limited color hues. Herein, it is demonstrated vivid electrochromic displays with broadened color hues via fabricating Zn-based multicolor electrochromic displays having 2D CIE color space tunability. In addition, it is revealed that a Fabry-Perot nanocavity structure can further tune the color hues via altering the coordinate of the 2D CIE color space. It is known that this is the first demonstration of 2D CIE color space tunability realization from a single transparent or reflective electrochromic device. These findings represent a novel strategy for fabricating multicolor electrochromic displays and are expected to advance the development of electrochromic displays.

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The development of electrochromic materials has opened the door to the development of numerous devices including smart windows, color displays, optical filters, wearable camouflages, among others. Although the current electrochromic devices do not consume energy while maintaining their colored or colorless states, their bistable operation requires external electrical energy to be consumed during switching. To reduce the energy consumption of an electrochromic device, an emerging Zn anode‐based electrochromic device concept was recently introduced to partially retrieve the consumed electrical energy. In this Review, key technological developments and scientific challenges are presented for a broad range of Zn anode‐based electrochromic device configurations with emphasis on the inherent distinctions between the Zn anode‐based and conventional electrochromic devices. Specifically, a comprehensive comparison of the two classes of electrochromic devices in the high‐performance device design is provided. For the electrochromic layer, the methods for obtaining high‐quality electrochromic materials and thin films are reviewed. For the electrolytes, the effect of the dual ion system on the electrochromic performance is discussed. Also, some critical but unresolved issues in the device design and fabrication are discussed. The perspectives and outlook at the end of this Review provide recommendations to improve performance for future electrochromic studies.