Ultra-large optical modulation of electrochromic porous WO₃film and the local monitoring of redox activity

Biomimetic solid‐state nanofluidic diodes have attracted extensive research interest due to the possible applications in various fields, such as biosensing, energy conversion, and nanofluidic circuits. However, contributions of exterior surface to the transmembrane ionic transport are often ignored, which can be a crucial factor for ion rectification behavior. Herein, a rational design of robust sandwich‐structured nanofluidic diode is shown by creating opposite charges on the exterior surfaces of a nanoporous membrane using inorganic oxides with distinct isoelectric points. Potential‐induced changes in ion concentration within the nanopores lead to a current rectification; the results are subsequently supported by a theoretical simulation. Except for providing surface charges, functional inorganic oxides used in this work are complementary electrochromic materials. Hence, the sandwich‐structured nanofluidic diode is further developed into an electrochromic membrane exhibiting a visual color change in response to redox potentials. The results show that the surface‐charge‐governed ionic transport and the nanoporous structure facilitate the migration of Li+ ions, which in turn enhance the electrochromic performance. It is envisioned that this work will create new avenues to design and optimize nanofluidic diodes and electrochromic devices.

The studied EC materials usually include viologens (as polymeric films), conjugated conducting polymers, transition metal oxides (e.g., WO 3 and NiO), metal coordination complexes (e.g., polymeric, evaporated, sublimed films). [7,11] Bene fitted by the diversity in composition/structure and the superior electrochromic performance, the electrochromism based on the thin-film transition metal oxides has become a hot topic. Recently, there have been some important breakthroughs in the transition metal oxide-based electrochromism, ranging from the development of new materials, [12] the introduction of new nanostructures, [13][14][15] element doping, [16,17] and composites [18][19][20][21] to novel designs and concepts of visible light and near-infrared radiation (NIR) dual-modulated smart windows [8,14,20] and the self-powered electrochromic batteries. [22,23] Therefore, the exploration of electrochromic devices with multifunctionalities and unique features by integrating both energy storage and electrochromism has been attracting great attention.There are several elaborate reviews serving as good references for this field, [1,3,4,6,[8][9][10] however, more comprehensive integration and up-to-date summaries embracing all relevant materials and applications are still lacking. Under such a background, as outlined in Figure 1, we attempt to provide a systematic and recapitulative review on the material developments and application status of electrochromism with a focus on transition metal oxides. Additionally, tailored synthesis and material design toward enhanced optical modulation and fast switching are also illustrated. Finally, an outlook on the future research trends in electrochromism is presented. This review will be very instructive and of special importance for directing other researchers in the future. Electrochromism: Fundamentals, Principles, and Device Structure Fundamentals and PrinciplesElectrochromism refers to the reversible change of colors and transparency under charge insertion/extraction or chemical reduction/oxidation induced by the application of an electrical current or a potential difference. [4][5][6] In the mid-1980s, the acceptance of electrochromism in fenestration technology as an approach to achieve energy efficiency in buildings captured the interest of researchers and the public. [3,24] This concept was coined as "smart window," which was capable of achieving Electrochromism has received increasing attention due to its unique electrochromic feature and the induced applications. This review provides an overview on the recent developments in the synthesis and application of the electrochromic metal oxides in smart windows, display devices, e-skins, and multifunctional energy storage devices. The main cathodic and anodic electrochromic metal oxides are surveyed here, with the discussion of representative examples in details. The current state-of-the-art research activities of the derived multifunctional electrochromic devices are highlighted, i.e., material systems, components, structures, m...

and implementation of alternative energy technologies, e.g., renewable energy such as wind turbine and photovoltaics. However, such technologies by themselves are insufficient. Effective management of energy consumption is also crucial in order to properly address the issue of energy shortage. As countries become increasingly developed, the construction of many high-rise buildings quickly follows. Consequently, the smart utilization of solar radiation through glass windows becomes a primary concern. Therefore, smart window technology, an effective energy conservation method, is introduced to assist the reduction of energy consumption of indoor lighting and airconditioning. In this field, electrochromic (EC) materials, which are able to electrically modulate the transmittance of solar radiation, are one of the most widely investigated smart window materials. [1] Various transition metal oxides and conjugated polymers, such as tungsten oxides (WO 3 ), [2] vanadium oxide (V 2 O 5 ), [3] nickel oxide (NiO), [4] polyaniline, [5] and poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), [2c,6] have been studied for this application. Amongst transition metal oxides, WO 3 possesses the best EC effectivity and is thus the focus of many researches to date. The ability of WO 3 to induce color changes was first explored in the early 19th century, while subsequent electrochemical studies began around 1930. [7] It was reported that WO 3 exhibited reversible changes in optical property (colored/bleached) corresponding to the electrochemical insertion/extraction of small cations such as H + or Li + (Equation (1), M = H, Li,…) [8] WO M e MWO 3 bleached 3 colored