Charge‐Balancing Redox Mediators for High Color Contrast Electrochromism on Polyoxometalates

Abstract

POMs) recently emerged as promising inorganic materials due to their varying structures, high electrochemical activities, and EC performances. [11,12] The most common and widely investigated POM structure is the Keggin structure, which is applied in biochemical, electrochemical, and catalytic areas. [13] However, studies on the EC properties of Keggin-structured POMs are limited, and their reported EC performances were inferior to other POM types. [14] Several attempts were made to improve the EC performance, including modifying the molecular structure or atomic composition, and optimizing the nanostructures [15-19] Notably, for POM-based EC materials, these approaches were employed by simultaneously selecting and controlling their nanostructures. Porous TiO 2 nanoparticle films (TNFs) are successful POMs' hosts displaying satisfactory EC performances due to their large specific surface area and porous morphology. [20-23] Further increases in ΔT were obtained by assisting the EC reaction by charge-balancing species between the working and counter electrodes in a full cell structure. Although some full cell type ECDs were achieved with POMs, [20,23] charge-balancing reactions were rarely examined, possibly because of the complicated EC mechanism, which makes it difficult to select CBMs for POMs. To create a CBM, a redox active material or electron reservoir was coated on the counter electrode (film type) or dissolved in the electrolyte (solution type). To simplify the ECD structure and avoid increasing its thickness, the solution type could be promising for high-performance ECDs. For example, a WO 3-based ECD showed enhanced EC performance when it was assembled with an electrolyte containing a tetramethylthiourea redox couple. [24,25] Similarly, viologen-based ECDs displayed fast switching times and high cycling stability in the presence of an I − /I 3 − redox mediator in a gelatin-electrolyte. [26,27] However, POM-based ECDs with redox couples in the electrolyte are not reported in literature. Noticeably, POM-based ECDs suffer from limited electrolyte choices and low optical contrast (ΔT), and POM-based EC films are mainly operated in aqueous media. [28] Also, the limited electrochemical window (1.23 V) of the aqueous electrolyte restricted their broader application in ECDs. [29] In general, water can be electrolyzed, but generates bubbles at the voltage range for coloration and bleaching of EC materials. In a A nonaqueous charge-balancing electrolyte with a redox couple (I − /I 3 −) is investigated to improve the electrochromic (EC) performance of polyoxometalate (POM)-based electrochromic devices (ECDs). PW 12 O 40 3− (PW 12)-coated porous TiO 2 nanoparticle films (PWTNF) with different thicknesses are used as the working electrode. The optical contrast (ΔT) of the optimized PW 12-ECDs reached 59.4% with the electrolyte containing the I − /I 3 − redox couple. PW 12-ECDs also show a fast response time for bleaching (t b,90% = 1.9 s) and coloration (t c,90% = 3.1 s). On the contrary, without the I − /I 3 − redox ...

Reversible Zn2+/Al3+intercalation in niobium-substituted polyoxometalates and demonstration of energy storage smart windows

“…[14][15][16] It is worth noting that POM-based electrochromic materials suffer from limited electrolyte choices. 17 Acid aqueous electrolyte media facilitate the reversible and rapid multi-electron transfer process within POMs, and therefore they usually exhibit high electrochemical activity. 18 Unfortunately, acid-aqueous electrolyte media are not appropriate for device fabrication, since the H + tends to corrode the electrode surface, and extra H + in water media could be polarized easily to generate hydrogen bubbles.…”

Vanadium-Substituted Dawson-Type Polyoxometalate–TiO2 Nanowire Composite Film as Advanced Cathode Material for Bifunctional Electrochromic Energy-Storage Devices

“…This work provided an alternative method for improving the performance of POMs-based capacitor electrode materials. In addition, Wang et al [14] reported a high-performance PW 12 -based EC device, wherein the optical contrast of the optimised device containing an I − /I 3− redox couple in the electrolyte reached 59.4%. The PW 12 -EC device also showed a fast response time for bleaching and colouration.…”

Copper complex/polyoxometalate‐based tunable multi‐color film for energy storage

“…19 These properties make them an ideal material for EC and pseudocapacitor materials. For example, Wang et al 20 reported POMs-based electrochromic devices (ECDs) with high optical contrast (59.4%) and fast response time (t c = 3.1 s, t b = 1.9 s), in which Keggin type polyanion PW 12 O 40 3À coated porous TiO 2 nanoparticle films were used as the working electrode. A non-aqueous charge-balancing electrolyte with a redox couple I À /I 3À can improve the EC performance of ECDs.…”