Electrolyte design for reversible metal electrodeposition-based electrochromic energy-saving devices

Electrochromic devices have promising energy-saving applications in smart windows, energy-efficient displays, and military camouflage.^1–7 This benefits from the basic characteristic of electrochromic materials that can reversibly change their optical properties in the ultraviolet–visible–infrared (UV–Vis–IR) region caused by electrochemical redox reactions under an external electric field.^8–10 Over the past decades, the majority of electrochromic research has focused on the exploitation of new functional electrochromic materials such as transition metal oxides (NiO, WO₃, V₂O₅, etc.), conducting polymers, and organic frameworks.^11–18 Meanwhile, various approaches are being carried out to engineer these traditional materials into thin films that can achieve vivid color change and tunable light regulation in the device. The corresponding benchmarking standards have also been properly established to evaluate the optical properties of electrochromic materials and devices.

Nonetheless, their energy-saving applications in commercial products are still in an immature stage due to their relatively high cost, complex manufacture, and limited service life. Therefore, reversible metal electrodeposition (RME) offers an alternative strategy of electrochromism that has been rapidly developed in recent years.^19–22 The working principle of this RME strategy is based on the deposition and dissolution of metal cations under an external electric field (Fig. 1).^23–27 Electrochemical reduction of metal cations in the electrolyte causes the formation of a thin metal layer on the transparent conducting substrate, which can induce the color change from a transparent state to an opaque state. The reverse color transformation can be completed by the metal dissolution in the electrolyte. Therefore, one of the essential features of RME devices is their simple configuration consisting of the substrate and electrolyte, i.e., low-cost fabrication without traditional electrochromic film construction. Owing to its high intrinsic extinction coefficient, this metal layer with only tens of nanometers of thickness would achieve an ideally opaque state,²⁸ endowing electrochromic devices with excellent optical properties in transmittance and reflectance modes of the UV–Vis–IR region. From this perspective, RME-based devices are regarded as promising candidates for the applications of smart windows, energy-efficient displays, rearview mirrors, and camouflage.

Although RME-based devices do not require the film construction of traditional devices, their electrochromic behavior is realized depending on the deposition and dissolution of metals, in which the conversion of the metal and metal cations in the electrolyte needs to be highly reversible under the stimulation of an electric field. This indeed puts forward more critical conditions for their electrolyte system because uniform deposition and dissolution of metals at various scales are highly related to the electrolyte conditions. Historically, silver (Ag) and bismuth (Bi) are the two main metals utilized as reversible electrodepositions for electrochromic display and optical modulation. Dating back to the early 1990s, Howard and Ziegler found the first primary Bi-based electrochromism in the aqueous electrolyte with a mixture of BiCl₃, CuCl₂, and LiBr.²⁹ Afterward, a great deal of effort has been devoted to studying the optimization of the Bi³⁺ aqueous electrolyte system to achieve comparable optical properties (durability, optical modulation, etc.) of RME devices with traditional electrochromic devices. This Bi³⁺ aqueous electrolyte system was mainly utilized for the application of smart windows that could realize the color change with a transparent-to-neutral state and dynamic regulation in the visible–near-infrared (Vis–NIR) regions, greatly promoting the indoor comfort of buildings and vehicles. Another considerable amount of attention has been focused on exploring RME systems in an organic dimethyl sulfoxide (DMSO)-based electrolyte.³⁰ Ag-based RME systems in DMSO electrolytes, for instance, have shown excellent reflection of mirror state, multicolor phenomenon, and large infrared emissivity tunability, which has promising applications in energy-efficient displays and camouflages. Unfortunately, up until now, it has been challenging for RME electrochromic devices to reach widespread practical applications, and some critical issues should be urgently solved such as poor reversibility, low memory stability at open circuits, and slow processing. Considering the significance and merits of electrolytes in the RME system (Fig. 1), this review first presents the discussion of recent advances in the current RME-based electrochromic devices in electrolytes, including aqueous (acid and mild) electrolytes, organic electrolytes, ionic liquids, and eutectic electrolytes. The issues and potential applications of RME-based devices are also summarized. Furthermore, we propose some opportunities and future directions for the electrolyte development of RME-based devices.

Aqueous electrolyte systems feature low cost, environmental friendliness, and high ionic conductivity (two orders of magnitude higher than organic electrolytes),^31,32 despite electrochemical reactions in such systems being generally limited by water-splitting reactions. Moreover, water, as an ideal solvent, can dissolve various metal salts and dissociate metal cations from their counter-anions in acidic, mild, or alkaline environments.³³ In recent years, considering the merit of aqueous electrolytes, Bi/copper (Cu), zinc (Zn), and Ag-based RME electrochromic devices have emphatically attracted attention not only in the field of electrochromism but also in energy storage applications.^26,34–36 The RME electrolytes for reversible electrodeposition should be selected according to the corresponding intrinsic nature and reversibility of the deposited metals.

In the early research stage, Bi-based RME electrochromic systems were found to exhibit competitive optical properties in terms of optical modulation due to the black neutral coloration and low reduction potential of Bi metal.^29,37–39 The electrolyte employed in this RME system typically consists of BiCl₃, CuCl₂, LiBr, and HCl, in which a strongly acidic environment has to maintain the Bi³⁺ solubility by preventing the precipitation of insoluble Bi(OH)₃. On the other hand, the Cu element in the electrolyte also plays an important role in adjusting the electrooxidation of Bi metal in its dissolution process, in which the generated Cu⁺ in the deposition process chemically oxidizes dendritic Bi atoms according to the galvanic displacement of Bi by Cu⁺ [Fig. 2(a)]. Therefore, the poor dissolution of Bi film can be greatly relieved, and a smooth spherical particle was formed, thereby promoting the reversibility of the Bi deposition–dissolution process. It was reported that a low Bi/Cu ratio in the electrolyte can achieve a fast switching speed for the coloration (60 s) and the bleaching (10 s) [Fig. 2(b)].⁴⁰ 

For the application of large-scale smart windows, Bi-based RME systems generally require a high potential to support sufficient current for optical modulation, according to Ohm’s law. However, high potential generally triggers unfavorable hydrogen evolution in aqueous electrolytes, which can cause water exhaustion and performance attenuation.⁴¹ Additionally, the heterogeneous surface chemistry of indium tin oxide (ITO) electrodes determines that the metal layer electrodeposited by the reduction reaction is not uniform.³³ Large-scale RME devices have to be tolerant to the voltage drop across the electrodes in the electrochromic process [Fig. 2(c)]. Strand et al. demonstrated that platinum nanoparticles (Pt NPs) fabricated on ITO surfaces can facilitate the nucleation of Bi/Cu at a large scale, resulting in improved optical modulation for 100 cm² devices at a fixed voltage of −600 mV [Figs. 2(d) and 2(e)].⁴² The further removal of surfactant molecules that are used to bridge the ITO surface and Pt NPs can reduce the nucleation potential by 70 mV. The inhomogeneous electrochromism of large-sized Bi-based RME smart windows is also caused by unstable or unreasonable counter electrodes. For instance, metal frame-type counter electrodes generally lead to the issue that metal ions in electrolytes are initially reduced to the metal layer at the edge of smart windows before at the center, originating from the difference in ion diffusion length at different sites between two electrodes. The redox reaction of Br⁻ and Br³⁻ on a transparent counter electrode can provide uniform ion transportation for the RME electrochromism.⁴³ However, this reaction system in an aqueous electrolyte is unstable because of the accumulation of colored Br³⁻ ions over cycling. One strategy conceived by Islam et al. was to incorporate a traditional LiNiO_x counter electrode, in which a high concentration of LiBr in electrolytes ensures Li⁺ ion insertion/extraction.⁴⁴ A triazole inhibitor coated on the LiNiO_x layer can significantly increase the overpotential and protect the counter electrode from a side reaction of metal electrodeposition to some extent. It was demonstrated that 100 cm² dynamic windows fabricated by this approach can uniformly switch between clear and black states with ∼65% optical modulation in one minute and cycle at least 4000 times without degradation.

Nevertheless, the anions and additives in acidic conditions used in the Bi-based RME system are not optimized for the electrochromic smart windows, in which the resulting corrosion issues of transparent conducting electrodes lead to unsatisfactory durability of the smart windows. For instance, the anion in aqueous electrolytes can affect the electrochromic performance (e.g., durability) of the Cu-based RME system. Hernandez et al. compared Cu-based RME dynamic windows with different anions (NO₃⁻, SO₄²⁻, ClO₄⁻, Cl⁻, and Br⁻) and pH in an aqueous electrolyte.⁴⁵ Without acidic support, the side reactions are more pronounced such as the Cu₂O formation in the electrolyte with SO₄²⁻ or ClO₄⁻ anions. Relatively, the acidic electrolyte can lower the pH and prevent the formation of irreversible metal complexes in acid-free electrolytes, but the anions like Cl⁻ ions in acidic electrolytes can etch the ITO surface by breaking the In–O valence bond [Fig. 3(a)]. As a result, it was suggested that acidic ClO₄⁻ ions with a weak nucleophilic nature were identified as the most suitable anions [Fig. 3(b)], which can stabilize the ITO surface and also improve the electrochromic durability of the 225 cm² Bi-based RME system (Cu²⁺ ions as mediator) over 10 000 cycles. Recently, Sui et al. proposed an aqueous electrically switchable electrochromic energy-efficient building envelope for building thermoregulation based on reversible copper electrodeposition,⁴⁶ which can adjust the radiation temperature and have good long-term durability. This is also helped by the fact that perchlorate is a good anion for aqueous reversible Cu electrodeposition, as it avoids Cu⁺-related side effects compared with chlorides, which limits the durability of electrodeposition. To further optimize the optical modulation of large-sized Bi-based RME systems, Strand et al. found that polyvinyl alcohol (PVA) additives as an inhibitor in the electrolyte can effectively regulate the distribution of electric fields on Pt NPs-modified ITO surfaces, thus promoting the formation of smooth and compact metal films. It was shown that the windows employed with this polymer inhibitor can readily tint to below 0.001% visible transmittance in less than 3 min and exhibit high infrared reflectance (>70%). Benefiting from the enhanced coloration efficiency and reduction in sheet resistance of the electrode in PVA electrolyte, uniform deposition was achieved in a >900 cm² smart window at low current density, which has played a huge role in driving the commercial application of the RME system. Apart from Bi-based RME electrolytes, Ag and Pd metals can also be reversibly electrodeposited in acidic electrolytes. With the mediation of Cu²⁺ ions, uniform Ag and Pd electrodeposition on the Pt NPs-modified ITO surface has been achieved, in which the morphology evolution of metals involves the sphere-like particles during the first cycle and the increasingly smaller particles over 1000 cycles.³³ It was said that 25 cm² smart windows could be switched uniformly between transparent (∼80% transmission) and opaque states (<5% transmission) in less than 3 min, and the cycling stability could be maintained 5500 times without degradation in optical modulation and film uniformity. Side reactions can be effectively inhibited in the oxygen-free electrolyte in this system with the achievement of high Coulombic efficiency (99.9%).

The above-mentioned RME systems have a promising application in the energy-saving smart window of buildings, which can achieve eye-comfort neutral coloration with high optical modulation. Meanwhile, it is also necessary to strictly consider the application of smart windows in a low-temperature environment. Even though aqueous electrolytes have a temperature stability range of ∼0 to ∼100 °C, the ice crystal below 0 °C can affect the window transparency and cause its electrochromic failure. In this case, Alcaraz et al. constructed a Bi-based RME smart window with thermal stability from −40 to 60 °C by introducing various alcohol-based antifreezes (methanol, ethylene glycol, and glycerol) in aqueous electrolytes.⁴⁷ This research indicates that RME windows with the antifreeze-type additive in electrolytes indeed have stable reversibility of metal deposition/dissolution, but the performance evaluation and optimization of RME systems that adapt to various extreme environments should be further in-depth studied in the future.

An acidic electrolyte-based RME system can also assist nanoparticles for continuous color tunability and thermal barrier capacity in the full-visible range according to the localized surface plasmon resonance (LSPR) effect,^48–50 making it promising for use in light modulating devices such as multicolor smart windows and low-power displays. The LSPR originates from the collective oscillation of the free electrons on the surface of nanoparticles when the frequency of incident light matches the oscillation frequency of the surface plasmons. The accompanying resonance absorption or scattering peak in the spectrum is related to the morphology of nanoparticles. In the previous study, the random self-nucleation of Ag nanoparticles (Ag NPs) often caused uncontrollable morphologies and large agglomerates of particles over cycling use. To solve this issue, Li et al. proposed a plasmonic Au–Ag system through Ag deposition/dissolution on the Au shell structure, which can also avoid the use of complex templates such as anodized aluminum oxide (AAO) and SiO₂.⁴⁹ It was shown that such an Au–Ag hollow structure exhibits the capability of restraining the random self-nucleation of Ag NPs, leading to improved reversibility and excellent transmittance. Although the transparent film can be colored to light blue, purple, and eventually reddish in 30 s, the color change in the visible range should be further explored such as the color variety and chroma. Furthermore, He et al. recently explored the color tunability of Bi-based RME systems by combining the “constant voltage method” and the previously reported “voltage-step method,” as shown in Fig. 4(a).⁵⁰ The first voltage V₁ was applied for a short time to initiate the metal nucleation, and a subsequent second voltage V₂ (|V₂| < |V₁|) was used to promote the uniform growth of films. Different from the above-mentioned Bi-based RME system with a black coloration state, a high potential of 0.8 V in this strategy leads to the coloration of the purple transparent and purple mirror by controlling deposition time [Fig. 4(b)]. With the voltage-step method, two different colors of yellow transparent and yellow mirror were achieved. Those multiple optical states were attributed to the co-nucleation and growth of Bi and Cu metals and the LSPR effect of nanoparticles. It was also found that the colored-mirror states of devices had a higher proportion of Cu contents than those of colored-transparent states, owing to the higher oxidation of Cu⁺ than Bi metals.⁴⁰ Additionally, after 6.7 h of memory test without a power supply, the transmittance of the purple mirror state increased from 5.4% to 29.3%, and there was barely any change for the yellow mirror state. Su et al. further developed a five-state flexible dynamic window based on Bi and Cu metals by combining the constant voltage method and the voltage step method.⁵¹ Through precise voltage and time control, the device can display a transparent state, gray state, black state, and two mirror states, demonstrating a transmittance modulation range of more than 63% in the transmittance-to-black state and more than 60% in the reflectance modulation range in the transmittance-to-mirror state. Recently, Yeang et al. further demonstrated that the voltage-step pulse strategy can inhibit dendrite growth of Bi-metal and endow metal films with a smoother morphology relative to direct current electrodeposition.⁵² The result shows that the film based on this strategy can realize a deep coloration state with a limit transmittance value of 0.1%, fast switching speed, and robust cycling life.

In recent years, reversible Zn electrodeposition in aqueous electrolytes has shown attractive charm due to its low redox potential [−0.76 V vs standard hydrogen electrode (SHE)] and high overpotential for hydrogen evolution reaction (HER), especially for the application of aqueous batteries.^31,36,41 In comparison with acidic electrolytes, aqueous, mild electrolytes can decrease the chemical reaction between Zn metal and protons, alleviating Zn metal corrosion. In this case, the rapid development of Zn anodes with deposition/dissolution chemistry has enlightened researchers studying Zn-based RME, which could be a potential alternative strategy for smart windows in weak acid or neutral electrolytes.^53–57 

A bright mirror state in Zn-based RME could be realized by depositing Zn in a ZnSO₄/gelatin electrolyte while viologen constitutes the cathode, as early reported by Wang et al.⁵³ This mirror state benefits from the small and uniform phase of the ITO conducting layer, which can result in nanosized Zn electrodeposition. Nevertheless, pure ZnSO₄ electrolyte without additives greatly affects the reversibility and long-term cycling of the RME process. Madu et al. systematically studied the Zn reversibility properties by comparing the values of Coulomb efficiency (CE) in different electrolytes.⁵⁴ In the electrolyte recipes of ZnCl₂ and different halogen/fluorine-substituted acetates, the highest CE (98%) of Zn-based RME is shown in the ZnCl₂-acetate electrolyte. The format with a shorter chain length than that of the acetate possesses faster dissolution kinetics with a resulted CE of 99%. By comparing halogen-based Zn salts, the fast switching speed of Zn-based RME was found in ZnBr₂ electrolyte. It is due to the morphology deposited in ZnBr₂ electrolyte exhibiting a lower density of larger particles than that in ZnCl₂ electrolyte. Even so, the cycling performance of Zn-based smart windows could maintain 250 cycles to the maximum extent. This recession in cycling originates from the by-product formation of Zn(OH)₂ and ZnO, which are kinetically sluggish.⁵⁸ To overcome this issue, the addition of metal ion electrolytes is an effective strategy to slow down their formation during Zn deposition. The Cu(CH₃COO)₂ additive was recently reported to improve the Zn nucleation and the cycling performance (2500 cycles) due to the more positive reduction potential of Cu/Cu²⁺ than that of Zn/Zn²⁺. Slow scan rates in electrochromism and deoxygenation of the electrolyte can also decrease the amount of Zn(OH)₂ and ZnO [Figs. 5(a)–5(c)]. However, it is still difficult to use the current approach to thoroughly eliminate their negative effects. Be that as it may, the current RME systems (Bi, Zn) still have promising applications in large-sized smart windows due to their large optical modulation, transparent bleaching state, and neutral coloration state.

The metal layer formed in the RME process is also able to act as a flat-plate solar collector, which can be used to heat the flowed RME electrolyte in smart windows. For instance, Wang et al. proposed a Zn-based RME system with a flowed ZnSO₄/CuCl₂ solution as the electrolyte and Prussian blue (PB) as the counter electrode [Fig. 5(d)].⁵⁶ The smart window can show four different states (a transparent state, a blue state, a black state, and a mirror state) upon the RME process and the metal-ion insertion/extraction in the PB film [Fig. 5(e)]. With such an electrolyte-flow strategy, the indoor temperature of the window can be effectively modulated, in which up to 42% of the solar energy is converted/stored as thermal energy for indoor heating, bringing new insights into the design of next-generation green buildings. Moreover, our group has recently proposed a new concept to integrate electrochromic and reversible metal deposition/dissolution in a comprehensive electrochromic device.⁵⁹ In the 0.3M Zn (ClO₄)₂/polycarbonate (PC)/acetonitrile (ACN)/polymethyl methacrylate (PMMA) electrolyte, with a decreasing negative potential toward −1.75 V (vs Ag wire), Zn²⁺ reversible electrodeposition was carried out. The reversible electrodeposition of Zn²⁺ on the films significantly broadens the transmittance of the films in the visible, near-infrared, and ultraviolet regions. In the present report, optical modulation in the Vis-NIR region is more significant than that in the same films deposited by Zn²⁺ and other reported conventional electrochromic materials without metal deposition.

Compared with aqueous electrolytes, organic electrolytes provide a wide potential window for stable reversible metal electrodeposition.^60–62 For instance, dimethyl sulfoxide (DMSO)-based electrolyte with a formula of (CH₃)₂SO has received considerable attention due to its good thermal stability, high conductivity, and low viscosity.^63,64 Moreover, DMSO, as a versatile polar solvent, can dissolve most types of inorganic salts. To date, the majority of DMSO-based electrolytes have been employed in the research of reversible Ag electrodeposition, mainly for the applications of smart windows and radiative heat management.^{21,30,43,65–84}

One advantage of the RME approach is the stable management of the metal thickness by controlling the current/voltage, thus realizing the dynamic optical modulation of electrochromic devices between a transparent state and a coloration state. Ag NPs, as a prominent representative, can yield various plasmonic color states by modulating their size, composition, and configuration. Therefore, progress in the Ag-based RME system with the LSPR effect has been further made and provides an alternative route for multicolor electrochromic devices. For instance, Wang et al. explored an Au–Ag hybrid core-shell system fabricated by reversible Ag electrodeposition on top of Au nanostructure arrays based on the AAO template.⁸³ By controlling the tunable thicknesses of the Ag shell, the device LSPR color throughout the full visible region can be adjusted continuously from 650 to 430 nm. This is attributed to the red-shifting of the peak wavelength due to the intrinsic plasmonic spectral properties of Au. On this basis, a mechanical chameleon integrated with color sensors and a control system was realized by changing its colors to match the background. Han et al. use a specially designed array of ordered SiO₂ nanoholes as a deposition template for the direct electrodeposition of Ag NPs.⁸⁵ By changing the deposition time and adjusting the thickness of the deposition Ag nanocylinder structure in this template, the property of the tunable LSPR excited by the deposited Ag nanocylinders can be realized dynamically. In general, the Mie resonance can also make the nanostructures appear structure colored. Dong et al. designed a DMSO gel-electrolyte-based reversible switching that relies on the electrocoagulation of Ag NPs between silicon nanostructures that support Mie resonances.⁸⁶ Electrodeposited Ag NPs can excite hybrid plasma-Mie resonance supported by Ag–silicon nanostructures, resulting in large spectral conversion. Araki et al. demonstrated that three optical states (transparent, mirror, and black) of Ag-based RME could be achieved on rough/flat ITO surfaces.⁴³ In DMSO-based gel with AgNO₃, CuCl₂, and tetrabutylammonium bromide (TBABr), it was explained that the stable complex anion of AgBr_n⁽¹⁻ⁿ⁾ would slow down the Ag electrodeposition and, therefore, lead to the formation of a specular film on a flat ITO surface.⁸⁷ The transmittance and reflectance modulation of the Ag-based RME device are all over 80% and almost maintained after the 2500th cycle, revealing the high stability of Ag RME in the DMSO electrolyte. The color change can be accomplished in more than 10 s.

By using the “voltage-step method,” reversible Ag NPs electrodeposition in the DMSO gel electrolyte (Ag⁺, Cu²⁺, and Br⁻ ions) can also be achieved.⁵⁰ The LSPR-shifted extinction band was displayed at a longer length, resulting in clear color changes from transparent to red (particle size <40 nm) and then to blue (particle size ≈50 nm). It was said that the color depth and the number of nucleations can be well controlled by the first voltage, V₁, while the short time of the first nucleation process and the small V₂ are beneficial to the formation of vivid colors. Obviously, the small nucleation of Ag NPs is a prerequisite to achieving the plasmonic multicolor electrochromic device. Uji et al. recently applied dark-field microscopy (DFM) to an in situ study of the LSPR changes of Ag NPs fabricated by the “voltage-step method” [Fig. 6(a)].⁶⁵ By applying various nucleation voltages, V₁, the scattered light of the Ag NPs during voltage V₂ application was observed: With the voltage V₁ of −2.4 V, the light scattering peak located at around 470 nm appeared. With a higher voltage V₁ of −2.5 V or −2.7 V, light scattering peaks appear at 540 nm and 610 nm, in addition to the peaks at 470–480 nm, due to the LSPR bands of the coalescent Ag NPs [Figs. 6(b) and 6(c)]. Nevertheless, Cu²⁺ ions in the electrolyte easily lead to the loss of the mirror state and deteriorate the bistability of the device. To solve this issue, introducing an ion storage-type counter electrode (e.g., Prussian blue, WO₃) in place of Cu²⁺ ions would be a feasible strategy due to its capacity for charge compensation in the redox reaction, in which improved stability of the mirror state can be maintained within 2 h.^67–69 Tao et al. recently found that absent Cu²⁺ ions in electrolytes also cause the dissolution of the Ag metal layer due to the corrosion of halide ions.⁸² In this regard, they designed an additional Ag metal layer on the counter electrode as a mediator layer [Fig. 6(d)]. By controlling the Ag:Br atom ratio to 1:2, there are no excessive Br⁻ ions in the DMSO electrolyte, avoiding Ag dissolution by Br⁻ corrosion. The result also shows that this Ag-based RME system can maintain bistability for 24 h without a power supply [Fig. 6(e)].

Even though an Ag-based RME system can achieve a black state on rough ITO surfaces, the properties of bleaching time and reversibility are unsatisfactory. In this regard, Guo et al. recently reported a Ni-based RME system in the DMSO electrolyte (NiCl₂, CuCl₂ as mediator, and LiClO₄ as supporting salt) due to the identical crystal structure and similar lattice parameters of Ni and Cu metals.⁸⁴ It was studied that excessive Ni²⁺ ions in the electrolyte result in limited reversibility, while an insufficient amount of Ni²⁺ ions leads to poor optical modulation. The optimized RME device can realize excellent cycling stability over 1500 cycles and a maximum transmittance modulation of 55.2% in short coloration/bleaching times (6.2 s/13.2 s). 35.0% of IR radiation can be effectively blocked in 400 s due to the low colored transmittance of the Ni metal layer. Afterward, to overcome the shortcomings of leakage and poor chemical stability induced by the liquid electrolyte, Guo et al. developed a type of solid PVA hydrogel electrolyte with a mixture of DMSO/water solvent based on the freezing-thawing method.⁸⁸ The water in the electrolyte can break the weak hydrogen bond between DMSO and PVA, promoting the cross-linking and gelation of solid electrolytes. Such a solid electrolyte endows the Ni/Cu RME-based device with a transmittance modulation of up to 53.1% and an excellent cycling life of 2000 cycles at 550 nm. In addition to the mediation effect, Cu²⁺ ions can also participate in the reversible electrodeposition in DMSO/PVA electrolytes and offer a blue coloration state (Cu⁺) and a mirror state (Cu⁰).⁷³ KI in the electrolyte mediated the reduction of Cu²⁺ to Cu⁺ via its oxidation reaction. The mirror state of deposited Cu was attributed to the solution resistance of PVA gel to the movement of the ions, leading to slower electrodeposition and controllable formation of nanoparticles. Nevertheless, the cycling reversibility of the Cu-based RME system was only limited to 200 cycles due to the side reaction of Cu oxidation. To solve this issue, Eh et al. designed a type of CuSn-based DMSO electrolyte containing CuCl₂, SnCl₂, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and KI [Fig. 6(f)].⁷⁷ Stannum (Sn) metal has a higher reduction potential and a lower oxidation potential than Cu²⁺ ions, thus mediating the dissolution of Cu during the bleaching process. The combination of Cu and Sn elements in DMSO-based electrolytes can support the three coloration states of transparent, greyish blue, and mirror. The cycling stability can thus be enhanced to 1000 cycles without significant degradation. Additionally, the CuSn alloy layer possesses good dynamic NIR-blocking properties at 80 and 180 °C, offering promising applications in energy-saving fields.

Despite the highly modulated reflectance and transmittance of Ag-based RME systems, research on IR modulation (>3 mm) is lagging due to the limitations of IR opaque electrodes, substrates, or surrounding media. In this respect, the DMSO electrolyte shows a unique advantage in making IR light highly transmittable, except for its good thermal stability and high conductivity. Li et al. reported an Ag-based RME system on nanoscopic Pt films supported by an IR-transparent BaF₂ substrate in DMSO electrolyte.⁷⁰ With tiny amounts of Cu ions, Ag⁺ was electrochemically reduced to a uniform Ag layer on the Pt film surface, resulting in a large, uniform, and consistent IR modulation of devices in both mid-wave IR (3–5 mm) with the modulation of 0.77 and long-wave IR (7.5–13 mm) with the modulation of 0.71. Owing to the high electron migration rates and low carrier concentrations of graphene, the Ag-based RME device achieves precise and quick modulation of thermal radiation on IR-transparent conducting substrates by using graphene/polypropylene (PP) and graphene/BaF₂.⁷² Nevertheless, the cycling stability for both systems can only maintain five cycles. Rao et al. further designed a gold microgrid on the monolayer graphene/polyethylene (PE) membrane to achieve reversible Ag electrodeposition.²¹ As a result, this Ag-based RME device not only has emissivity modulation between 0.12 and 0.94 but can perform synergistic dual-band heat management of solar and IR (2.5–18 µm). Good cycling performance over 360 cycles for thermal radiation modulation can be maintained.

Recently, Moon et al. attempted to study the Zn RME-based device with the support of a DMSO-based electrolyte containing Zn (CH₃COO)₂, ZnBr₂, and NaCH₃COO. The finite-difference time-domain method was utilized to conduct an in-depth analysis of the optical phenomena in Zn dendrite and dendrite-free nanostructures, in which the found LSPR effect effectively promotes the blocking of visible light and supports the construction of large-area dynamic windows.⁸⁹ 

Ionic liquids (ILs) have been widely studied for the applications of protein stabilization and energy storage owing to their selective cation/anion pairing, low volatility, wide potential window, and high ionic conductivity.^90–92 Even possessing a relatively high cost, imidazolium-based ILs have proven to be a suitable medium for the generation and stabilization of soluble metal NPs.^93–98 He et al., in the early stages, proposed an Ag NPs deposition/dissolution strategy in which the electrolyte consists of silver acetate, acetonitrile, and IL, followed by the removal of acetonitrile.⁹⁵ This imidazolium-based IL acts as the electrolyte and stabilizer, providing both steric and electronic protection against Ag aggregation. The multicolor change was achieved from yellow, pink, and purple–violet to dark blue within 18 s due to the LSPR shift associated with the changed size of Ag NPs. Additionally, this device can maintain multicolor stability for more than 60 days in the voltage-off state without any obvious change. The stabilizing effect of ILs was also demonstrated in other works. For instance, Park et al. reported that 1-Methyl-4-hexylimidazolium bromide (MHImBr), as an anion-blocking layer, can protect the Ag mirror state for 2 h with high reflectance.⁹⁴ Poly(ionic liquids) (PILs), as a type of polyelectrolyte, comprise a polymeric backbone and IL monomer repeating units. The polymerized characteristic endows PILs with not only the ionic nature of ILs but also the mechanical stability of polymers.^99,100 Hou et al. synthesized a series of mono- and bis-imidazolium ILs and PILs with various substituents. The size and density of formed Ag NPs are correlated with the imidazolium cation charge density and also the length of hydrophobic alkyl chains. Owing to the high viscosity, PILs show smaller and denser Ag NPs than corresponding ILs, resulting in fast switching speed and high cycling stability over 1000 cycles.

In addition to the ILs, deep eutectic solvents (DESs) have emerged as environmentally nonaqueous solvents with ease of preparation, low cost, thermal stability, and chemical stability.¹⁰¹ The feature of DESs is that their component has a higher melting point than that of DES itself, and the formation mechanism lies in the strong intermolecular interactions between different components. One type of DES with hydrogen-bond interactions is composed of a hydrogen bond acceptor [e.g., choline chloride (ChCl)], a hydrogen bond donor (e.g., amide, alcohol), and metal halides. In this regard, Eh et al. proposed a hybrid electrolyte containing CuCl₂, ChCl, glycerol, and water for a Cu-based RME system.¹⁰² This hybridization strategy inherits the merits of a wide potential window of eutectics and a high current density of reversible deposition in water, offering trioptical-state switching between transparent, red, and mirror. Moreover, the hybrid electrolyte allows fast coloration, excellent memory retention, and impressive cycling stability (5000 cycles).

Over the last decade, the RME strategy for electrochromism has attracted increasing attention owing to its ease of manufacture and fantastic optical properties. The rational selection of metal ions and associated mediums is highly significant for the reversibility of metal electrodeposition and thereafter exploring their applications, including smart windows, multicolor displays, heat management, and camouflage. On the basis of the solvent property, we introduced the recent progress of several types of electrolytes for the RME system in this critical review: aqueous electrolytes (acidic, mild), organic electrolytes, and other electrolytes (ILs, DESs). Representative electrolytes for the RME system with featured characteristics and applications are carefully emphasized.

Aqueous electrolytes are a good medium with high ionic conductivity for RME systems. Previous research indicates that Cu²⁺ ions always act as an indispensable mediator to enhance the reversibility of metal electrodeposition/dissolution (e.g., Bi, Pd, Ag, and Zn). In this case, significant advances in acidic electrolytes toward achieving a Bi-based RME system for the application of smart windows with neutral coloration include the research on component ratio, polymer additives, counter anions, and functional molecules. Furthermore, expansion in recent two years has been focused on mild electrolytes, greatly promoting the development of Zn-based RME systems with a mirror state. Different from aqueous electrolytes, DMSO, as the representative in organic electrolytes, can allow RME systems to achieve wideband modulation in the Vis–IR region, easily generated mirror states, and multicolor displays by nanoparticle LSPR effect. A prominent issue lies in the low memory property of mirror states caused by the corroding of Cu²⁺ and halide ions, which has been greatly relieved by the addition of ion-storage counter electrodes and the optimization of the component ratio. Hereto, IL electrolytes offer another approach to solving this issue via their anion-blocking effect. Recently, the development of a hybridization strategy of DESs and water has provided an effective way for RME electrolytes with high ionic conductivity and stable potential windows.

To further drive research on RME systems with high performance and additional functions for commercial application, some challenges for various electrolytes need to be addressed as follows:

1. One important issue is related to the optical properties of RME-based electrochromic devices. Due to the high extinction coefficient, a few tens of nanometers of the metal layer in the RME process can theoretically achieve an excellent coloration state in reflection and transmittance modes. Nevertheless, achieving high coloration in RME devices generally sacrifices other optical properties (e.g., response time and cycling stability). It should be noted that the optical memory and cycling stability of mirror-state RME devices in organic electrolytes are still limited, and more efforts should be made to find appropriate electrolytes and optimize interface compatibility between electrolytes and electrodes.

2. Importantly, the achievement of RME systems is based on the redox mechanism of the metal deposition/dissolution chemistry. It has been demonstrated that metal deposition/dissolution can deliver high energy storage capacity for electrochromic devices thanks to the rich redox process and the existence of multiple valence states. Therefore, the operation of RME systems requires a high current density under an external power supply, which is a non-negligible energy consumption for the application of large-scale smart windows compared to traditional electrochromic materials. In this regard, it is necessary to explore the additional function of energy storage associated with the RME-based electrochromic process. For instance, introducing counter electrodes with high-efficient ion intercalation/de-intercalation or other types of redox mechanisms to ensure charge-matching between electrodes.

3. Despite numerous studies on RME systems in acidic electrolytes and DMSO-based organic electrolytes, the in-depth understanding and exploration of the deposition/dissolution process are still not sufficient, especially for emerging metal species in a new type of electrolyte. For instance, the cycling process of Zn-based RME systems causes the formation of by-products and passivation in an aqueous electrolyte. Zn ion electrolytes with functional component designs should be developed and considered.

4. The alkaline electrolyte generally displays many merits, including high solubility and high ionic conductivity in electrochemical and electrochromic processes. Moreover, it offers fast kinetics similar to the acidic electrolyte that has been applied to conventional electrochromic cathode materials. However, the investigation of alkaline electrolyte-based RME devices is still in its infancy.^54,103,104 More efforts are suggested to achieve detailed recognition between the alkaline electrolyte and electrochromic performance in RME systems. In this regard, the alkaline electrolyte-based RME system not only provides a new route for fabricating electrochromic anodes for devices but would also be utilized as a complementary electrode for electrochromic cathodes such as nickel-based oxides.

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 52302361, 62222402, and U2004175), the China Postdoctoral Science Foundation (2022M711036), and the Henan Natural Science Foundation (Grant No. 232300421387).

The authors have no conflicts to disclose.

Jinhui Wang: Conceptualization (lead); Writing – original draft (lead). Ying Lv: Writing – original draft (supporting). Yiping Zhou: Writing – original draft (supporting). Sensen Jia: Writing – original draft (equal). Feng Zhu: Conceptualization (supporting); Writing – review & editing (equal). Oliver G. Schmidt: Writing – review & editing (supporting). Guofa Cai: Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.