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Wu et al. Soft Sci 2024;4:29 https://dx.doi.org/10.20517/ss.2024.21 Page 5 of 22

In addition to the material selection and modification of the electrochromic active layer, the design
strategies for active layer structure should also be emphasized. With these structural design strategies, the
ECDs are expected to realize multicolor, self-powered abilities and further enhance long-term stability. The
first category is to fabricate the Fabry-Perot (F-P) type electrochromic electrodes [68-70] . They have the
structure of a WO layer (varied thickness)/partially reflective metal W layer/conducting layer on the
3
flexible substrate, which resulted in versatile structural colors of electrodes [Figure 2A]. The second category
is to introduce a complementary electrochromic counter electrode, which functions as an ion storage layer
for reversible electrochemical reactions and provides additional coloration ability. Examples of the working/
counter electrochromic pair electrodes of MnO /PB , WO /NiO , poly(3-methylthiophene) (P3MT)/PB
[58]
[20]
2
3
[41]
[71]
[Figure 2B] , WO /P3MT , PEDOT/PPy , PANI/V O 5 [59] have been extensively studied in high-
[61]
3
2
performance ECDs. The third category is to introduce other types of active counter layers, such as the active
metal electrode (Zn/Al) releasing ions and providing the build-in potential of the device [Figure 2C] [21,28,53,72] ,
and redox mediators [Ferrocene, dimethyl ferrocene (dmFc)] in the electrolyte as counter electrode
material [19,73] .
Electrolyte
In ECDs, the electrolytes provide ionic conduction and prevent direct electrical contact between
electrodes [74,75] . The polymer matrix is usually chosen as part of the electrolyte component for the flexible
and stretchable ECDs because of the superior mechanical flexibility. The selection of polymer materials is of
great importance to electrochromic devices as it aims to ensure the electrolyte possesses superior
mechanical strength, a wide potential window, and stable thermal, chemical, and electrochemical
properties [76,77] . To date, the extensively studied polymer materials are polyacrylamide (PAAM), polyethylene
oxide (PEO) , poly methyl methacrylate (PMMA) , poly(vinylidene fluoride-co-hexafluoropropylene)
[19]
[62]
(PVDF-HFP) , polyvinyl alcohol (PVA) , and other natural and biodegradable polymers
[60]
[53]
(agarose/gelatin) [78,79] . The basic properties of each polymer used in the electrolytes of electrochromic devices
have been illustrated in our previous published review in detail . Polymer electrolytes are usually
[80]
composed of the dissolved salt in the polymer host structure, and the choice of salt greatly influences the
ionic conductivity, potential window, and working temperature range of the electrolyte . Polymer-salt-
[74]
solvent-based electrolytes are commonly developed in electrochromic devices such as the PMMA-LiClO -
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PC organic gel and PAAM-LiCl hydrogel, which shows high ionic conductivity and good electrochemical
stability at ambient temperature. To further improve the high/ultralow temperature adaption and ionic
conductivity and widen the electrochemical window of the electrolyte, the emerging ionic liquid-based
polymer electrolytes can achieve these targets [81,82] . Santiago-Malagón et al. reported an iongel comprising
PVDF-HFP, 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM-Tf), and potassium
trifluoromethane sulfonate (KTf) applied in the PB-based-ECD which provides a stable chemical
[60]
environment for device operation . The deep eutectic solvents (DES), as the eutectic mixtures of Lewis,
maintain similar favorable properties (wide tolerant temperature, high ionic conductivity, and large
electrochemical window) as ionic liquids, which also have been successfully fabricated as DES-based gel in
ECD. The Litfsi in N-methylacetamide (NMA) as the DES integrating with the copolymer poly
(ethoxyethoxyethyl acrylate-co-isobornyl acrylate) [P(DEEA-co-IOBA)] exhibits high optical transmittance
over 90% and ionic conductivity of 0.63 mS/cm and stretching strain over 2,000% [Figure 2D], resulting in
the excellent electrochromic performances of the display . Furthermore, to increase the compatibility of
[20]
polymer matrices and ionic liquids, our group has proposed a novel electrolyte in flexible ECD in which the
poly[ionic liquid, 1-butyl-3vinylimidazolium bis(trifluoromethanesulfonyl)imide (BVIMTFSI)] as the
polymer backbone that anchors another ionic liquid (EMIMTFSI) with rapid photopolymerization method.
The prepared ionogels have favorable physicochemical (thermal, electrochemical, mechanical) stability
[Figure 2E], ensuring high performances of both WO and iron-centered coordination polymer (FeCP)-
3
[83]
based flexible ECD . In addition to the above strategies, a series of electrochromic ion gels were designed