Recent progress in Prussian blue electrode for electrochromic devices

Abstract

Great progress has been made in the electrochromic (EC) technology with potential applications in various fields. As one of the most promising EC materials, Prussian blue (PB) has attracted great attention due to its excellent EC performance, such as low cost, easy synthesis, rich color states, chemical stability, suitable redox potential, and fast color-switching kinetics. This review summarizes the recent progress in PB electrodes and devices, including several typical preparation techniques of PB electrodes, as well as the recent key strategies for enhancing EC performance of PB electrodes. Specifically, PB-based electrochromic devices (ECDs) have been widely used in various fields, such as smart windows, electrochromic energy storage devices (EESDs), wearable electronics, smart displays, military camouflage, and other fields. Several opportunities and obstacles are suggested for advancing the development of PB-based ECDs. This comprehensive review is expected to offer valuable insights for the design and fabrication of sophisticated PB-based ECDs, enabling their practical integration into real-world applications.

Graphical abstract

Keywords

Prussian blue / electrochromism / energy storage / smart windows

1 Introduction

The electrochromic (EC) phenomenon refers the reversible color change of EC materials when a certain voltage is applied [1–5]. In recent years, electrochromic devices (ECDs) have been used in displays, smart windows, military camouflage, anti-glare rearview mirrors, wearable electronics and energy storage devices [6–10]. EC materials are the basic materials to construct the ECDs toward real-world applications. Traditional EC materials are primarily classified into organic and inorganic categories. The organic EC materials can be mainly categorized into two major classes, which include p-conjugated organic molecules (such as viologens) and conductive polymers (such as polyaniline), which have the advantages of high coloration efficiency, fast response rate, and high contrast, but a poor chemical stability due to their low adhesion ability. Inorganic EC materials comprise cathodic EC materials (such as tungsten trioxide (WO₃)) and anodic EC materials (such as Prussian blue (PB)), with the former exhibiting colored state upon cation intercalation, while the latter colored state during cation extraction. WO₃ has become one of the most extensively studied inorganic EC materials due to its wide optical modulation range, excellent electrochemical stability, and excellent reversibility. However, the poor conductivity of WO₃ leads to a slow response rate and complicated synthesis processes. PB, one of the earliest investigated EC materials, was initially overlooked in related research due to its drawbacks such as a slow response rate, a narrow EC range, and a low color contrast. Consequently, its development progresses slowly. However, in recent years, its favorable durability and cycling stability, appropriate redox reaction region, simple fabrication process, and suitability for large-area coating have once again captured the attention.

PB is named as Iron (III) hexacyanoferrate (II) (Fe₄[Fe(CN)₆]₃), which belongs to coordination compound. It has been historically employed as a prevalent blue pigment [11,12] until Neff and the colleagues [13,14] discovered the EC properties of PB for the first time and Itaya et al. [15,16] first used PB as an EC material. As an inorganic anode EC material, PB has excellent EC performance and a good charge storage capacity due to its unique open-framework structure similar to the metal-organic framework (MOF) (Fig.1(a)–Fig.1(c)) and its property of mixed valence compounds [17]. The open-framework of PB octahedral units composed of alternating Fe^II and Fe^III can provide an abundance of well-organized and spacious channels that are conducive to efficient and swift cation accommodation. PB exhibits a cubic symmetry structure with 4 X ions (X could be H⁺, Li⁺, K⁺, Na⁺ and their corresponding hydrated ions) in a unit cell. During ion and electron co-insertion/co-extraction, the Fe^II/Fe^III couples within the lattice framework can coherently undergo valence changes to ensure the maintenance of overall electroneutrality [18]. As shown in Fig.1(a)–Fig.1(d), when applied with cathodic scan, PB (which could be expressed as XFe^IIIFe^II(CN)₆) can be reduced to a transparent Prussian white (PW) (which could be expressed as X₂Fe^IIFe^II(CN)₆). This process entails the co-insertion of X ions and electrons. During the reverse anodic scan, X ions and electrons co-extract from the crystal structure, and PW transforms to PB first. This process can be elucidated by Eq. (1).

(1)\begin{array}{rl}& \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}{(\mathrm{C}\mathrm{N}{)}_6}^{-}+{\mathrm{e}}^{-}\leftrightarrows \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}{(\mathrm{C}\mathrm{N}{)}_6}^{2-}\\ & (\mathrm{P}\mathrm{B})\leftrightarrows (\mathrm{P}\mathrm{W})\end{array}

With the anode scanning voltage increasing, X ions and electrons continue to be extract from the crystal structure. PB is electrochemically oxidized to Prussian green (PG) and Prussian yellow (PY) (which could be expressed as Fe^IIIFe^III(CN)₆) in turn [19]. This process entails the co-extraction of X ions and electrons, which result in the fact that there is no X ion in the crystal structure of PX. These can be described by Eqs. (2) and (3).

(2)\begin{array}{rl}& [\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}(\mathrm{C}\mathrm{N}{)}_6{]}^{-}\to \\ & \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}\{[\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}(\mathrm{C}\mathrm{N}{)}_6{]}_{2/3}[\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}(\mathrm{C}\mathrm{N}{)}_6{]}_{1/3}{\}}^{1/3-}+2/3{\mathrm{e}}^{-}\\ & (\mathrm{P}\mathrm{B})\to (\mathrm{P}\mathrm{G})\end{array}

(3)\begin{array}{rl}& [\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}(\mathrm{C}\mathrm{N}{)}_6{]}^{-}\to [\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}(\mathrm{C}\mathrm{N}{)}_6{]}^0+{\mathrm{e}}^{-}\\ & (\mathrm{P}\mathrm{B})\to (\mathrm{P}\mathrm{X})\end{array}

PB is an excellent anodically coloring EC material due to its excellent cycle stability, ease synthesis, and fast response [6,20]. In addition, PB has found extensive application in the realm of EC energy storage owing to its substantial charge storage capacity. A PB electrode can be fabricated by immobilizing PB material in the form of a thin film onto a conductive substrate. PB electrode coupled with a counter electrode and electrolyte collectively can form the PB-based ECD. Even though the pure PB electrode exhibits a relatively long EC response time due to its poor conductivity, researchers are committed to the development of PB electrodes after modification which shows a great promise as viable candidates for the large-scale development of ECDs, including compositing with functional groups, constructing multilayer composite structures, and constructing core-shell nanostructures. The physical structure of PB film, such as surface roughness, thickness of the film, and substrate adhesion play pivotal roles in dictating the attributes of electrochemical devices such as complementary EC windows, EC batteries, EC pseudocapacitors, and EC supercapacitors [21]. As an important EC material, PB has been widely studied and applied in many fields due to its excellent EC performance and low cost. A comprehensive understanding of the current development status of PB as an EC material is crucial for overcoming existing challenges and advancing the research on high-performance, low-cost, and large-scale ECDs. However, there is a lack of review on PB. Hence, it is imperative to undertake a comprehensive synthesis and examination of the recent progress in PB electrode and PB-based ECDs. This effort will furnish valuable insights for the formulation of high-performance PB-based ECDs, thereby propelling their practical utilization.

In this review, the structure of PB-based ECDs, its evaluation index, as well as several preparation techniques of PB electrodes are introduced. Moreover, an overview of recent key strategies is conducted aimed at augmenting the EC performance of PB electrodes, and the current applications of PB materials in smart windows, electrochromic energy storage devices (EESDs), wearable electronics, smart displays, and military camouflage. The advantages of PB including excellent energy storage capacity and stable electrochemical performance are very attractive in practical production applications. Furthermore, to further promote the practical application of PB materials within the realm of electrochromism, design and guidance on PB electrodes and devices are also provided. Anticipated outcomes of this review encompass novel perspectives for the advancement of PB-based ECDs.

2 Device structure and performance criteria of PB-based ECDs

2.1 Device structure

The structure of PB-based ECDs usually includes transmissive-types and reflective-types. The transmissive-type PB-based ECD mainly consists of transparent substrates, transparent conductive layers, PB layer, ion storage layer, and electrolyte layer (Fig.2(a)). The reflective-type PB-based ECDs usually do not require the transparent substrate, and the conductive layer is not necessarily transparent. However, the conducting layer, PB layer, and electrolyte layer are necessarily layers. As depicted in Fig.2(b), the position of the conductive layer is different from the structure of transmissive-type ECDs, which is in direct contact with the electrolyte layer. Li et al. [22] prepared a symmetric reflective-type PB-based ECD with PB as an ion storage layer. The non-transparent gold (Au) was used as a conductive layer, and the porous nylon 66 was used to adsorb electrolytes and separate two conductive layers. The development of transmissive-type PB-based ECDs is more mature.

2.1.1 Transparent conductive layer

The transparent conductive layer needs a strong conductivity to generate appropriate voltages for driving the EC phenomenon. For transmission rigid PB-based ECDs, the conducting layer is usually indium tin oxide-coated glass (ITO glass) and fluorine-doped tin oxide-coated glass (FTO glass), while for flexible ECDs, the conducting layer is usually indium tin oxide-coated flexible polyethylene terephthalate (ITO-PET) substrate.

2.1.2 PB layer

PB layer is the core of the PB-based ECD since it performs the role of coloring/bleaching by switching the certain voltage. The oxidations to PG and PY are not fully reversible, which result in a loss of cycling stability. Besides, PG and PY have a less optical modulation for PW than PB. To make the device work stably and obtain a large optical modulation, PB is usually reversibly switched between blue and transparent. With the co-extraction of ions and electrons, the colored PB could be reduced to the bleached PW. Additionally, with the co-extraction of ions and electrons, the bleached PW could be oxidized to the colored PB. Hence, the EC performance of the PB-based ECD primarily relies on the characteristics of the PB layer.

2.1.3 Ion storage layer

Ion storage layer serves the purpose of storing or providing ions to maintain the charge balance in the EC system, which coordinates with the PB layer to facilitate the reversible exchange of ions and electrons between the electrodes (PB electrode and ion storage electrode) and the electrolyte layer. The ion storage layer effectively inhibits the accumulation of small ions on the electrode surface and prevents their introduction into the electrodes. The ion storage layer can be an ion storage material that imparts minimal optical alteration or a cathodically coloring material that complements the PB layer. WO₃, as a cathodic EC material exhibiting color transition between transparency and dark blue, is the most commonly used ion storage layer in PB-based transmissive-type ECDs. When the ions and electrons are co-inserted into the PB layer and bleached to transparency, the ions and electrons co-extracted from the WO₃ ion storage layer and bleached to transparency. Similarly, when the PB layer is oxidized to blue, the WO₃ ion storage layer is reduced to deep blue, resulting in the co-coloring and co-bleaching of the PB layer and the ion storage layer, which effectively improve the optical modulation and coloring efficiency.

2.1.4 Electrolyte layer

Electrolyte layer is also called the ion conductor layer, which assumes a crucial role in both providing and conducting ions, thereby exerting a substantial influence on the EC performance of PB-based ECDs. The electrolyte layer locates between the PB layer and the ion storage layer to avoid their direct electric contact and provide a transport channel for ions to move between two electrodes. In PB-based ECDs, monovalent ions like H⁺, Li⁺, and K⁺ are commonly employed as insertion ions. It is important to highlight that the electrolyte layer functions as an ion conductor rather than an electron conductor. Enhanced ion conductivity contributes to improved kinetic attributes in PB-based ECDs, thereby resulting in an enhanced cycling stability to a certain extent. Besides, reduced electron conductivity could potentially mitigate current leakage, which leads to a good optical memory and energy saving. Generally, the electrolytes employed in PB-based ECDs encompass aqueous electrolytes and organic electrolytes. In the aqueous electrolyte, the formation of Fe(OH)₃ can be inhibited by adding HCl acid [23]. Additionally, surplus H⁺ ions within the electrolyte can also function as insertion ions, thereby enhancing the performance of PB-based ECDs [23,24]. The efficacy of this acidulation approach is similarly applicable when utilizing organic electrolytes. The incorporation of acetic acid into LiClO₄/PC electrolyte can improve the cycling performance and lifetime of PB electrodes [23,25]. However, it is noted that the security issues caused by the acidified electrolyte leakage further limit the applications of the ECDs. To solve this problem, several acid-free organic electrolyte alternatives have been suggested, such as LiClO₄-acetonitrile [26,27], KTFSI-succinonitrile (SN) [28], and KCF₃SO₃-triethyl phosphate [29].

2.2 Performance index

To expedite and precisely assess the behavior of PB electrodes and PB-based ECDs, some key performance indicators including optical modulation, coloration efficiency, switching time, cycling performance, and optical memory are listed systematically in this section.

2.2.1 Optical modulation

Optical modulation refers to the difference of transmittance (ΔT), absorbance(ΔA) or reflectance (ΔR) of PB electrodes and PB-based ECDs before and after color switching at a specific wavelength. It serves as a crucial performance index for appraising the EC ability of PB electrodes and PB-based ECDs. It can be obtained from Eq. (4).

(4)\mathrm{\Delta }T=T_{\mathrm{c}}-T_{\mathrm{b}},\mathrm{\Delta }A=A_{\mathrm{c}}-A_{\mathrm{b}}\mathrm{o}\mathrm{r}\mathrm{\Delta }R=R_{\mathrm{c}}-R_{\mathrm{b}},

where T_c, T_b, A_c, A_b, R_c, and R_b correspond to the transmittance in the colored state, transmittance in the bleached state, absorbance in the colored state, absorbance in the bleached state, reflectance in the colored state, and reflectance in the bleached state, respectively. Generally, a higher optical modulation can be obtained by preparing a thicker film [30,31].

2.2.2 Coloration efficiency

Coloration efficiency (CE) is defined as the change in optical density change (ΔOD) per unit charge density (ΔQ), which can be used to evaluate the power requirements of the coloring process, and it can be expressed as Eq. (5).

(5)\mathrm{C}\mathrm{E}={\displaystyle \frac{\mathrm{\Delta }\mathrm{O}\mathrm{D}}{\mathrm{\Delta }Q}}={\displaystyle \frac{\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{C}\mathrm{R}}{{\displaystyle \frac{Q}{A}}}}={\displaystyle \frac{\mathrm{l}\mathrm{o}\mathrm{g}{\displaystyle \frac{T_{\mathrm{b}}}{T_{\mathrm{c}}}}}{{\displaystyle \frac{Q}{A}}}},

where ΔOD represents the alteration in optical density, CR represents contrast ratio, ΔQ denotes the charge density inserted into or extracted from the EC material (cm²/C), Q signifies the injected charge, A stands for the active area, T_b corresponds to the transmittance of the bleached state, and T_c represents the transmittance of the colored state. According to Eq. (5), PB electrodes or PB-based ECDs possessing a higher CE requires a reduced charge input to achieve equivalent optical modulation, which consumes less energy.

2.2.3 Switching time

Switching time can be defined as the duration needed for an electrode or ECD to switch from 0% to 90% of its full optical modulation. It is partitioned into coloring switching time t_c and bleaching switching time t_b. t_c and t_b respectively correspond to the time required for the coloring and bleaching processes, which correspond to the duration needed for the coloring and bleaching processes respectively. The PB electrodes and PB-based ECDs with a short switching time have a faster response rate, which can be utilized to gauge the capability for rapid optical alteration. As a kind of inorganic EC materials, PB has a lower response rate compared to the organic EC materials due to the combined impact of weak interaction forces and limited electrical conductivity. Toward this problem, the core-shell nanostructures show a shorter switching time due to the short diffusion path of ions and expansive surface area of PB (e.g., ITO@PB [21,32], TiO₂@PB [33,34], Au@PB [35,36], TiO₂@graphene (G)@PB [37], and TiO₂@carbon@PB [38]).

2.2.4 Cycling performance

Cycling performance refers to the alterations in the optical and electrochemical performance of PB electrodes and PB-based ECDs after a number of coloring/bleaching cycles. The cycling behavior of PB is mainly determined by the morphology and structure, electrolyte, and adhesion of the PB film on the conducting substrate, as well as external perturbations. The porous structure mitigates volume expansion during the electrochemical cycle, enhancing the cycle stability of PB. The stronger the adhesion between the PB film and the conductive substrate, the better the cycling stability of PB electrodes. As for aqueous electrolyte, OH⁻ anions dissociated from coordinated water molecules can react with bound Fe^II, accelerating the creation of byproducts (e.g., Fe(OH)₃) and leading to the poor cycling performance of PB [29,39]. The cycling durability of PB electrodes and PB-based ECDs can be further enhanced by acidifying the liquid electrolyte with HCl. The reason for this is that the acidified medium suppresses the side reaction that leads to the formation of Fe(OH)₃, thereby improving the cycling durability [24]. In addition, the outstanding durability is accomplished owing to the activation of low-spin Fe within PB [39].

2.2.5 Optical memory

Optical memory, also called bistability, can be used to assess the capacity of the material to maintain its redox/colored state after the external bias is removed. PB electrodes and PB-based ECDs could be switched to a colored state or bleached state by applying a certain potential. It is preferable to sustain the state without requiring the application of supplementary charges. Consequently, EC materials exhibiting a robust bistability also entail a lower energy consumption during the optical state-switching procedures. This attribute holds a significant importance for widespread adoption in commercial applications.

3 Progress of PB electrochromic materials

3.1 Preparation methods for PB electrode

PB electrode can be prepared by immobilizing PB film onto a conductive substrate. There exist diverse methods to prepare PB electrodes, such as electrochemical deposition [25,35,39–57], hydrothermal [34,58–60], and sol-gel method [61–65]. Different preparation methods have different operating costs, difficulty of operation, and dependence on equipment. The different preparation methods result in the EC properties variation of PB electrodes. Here, the advanced preparation technology of PB electrode is systematically summarized.

3.1.1 Electrochemical deposition

Electrochemical deposition (EDS), also called electrodeposition, stands as a versatile approach for synthesizing a range of nanostructured materials due to its capacity to control the nanoporous structure (including pore size, surface area, etc.) [47–49,51]. The fabrication of PB film using this technique is executed within a three-electrode system (Fig.3(a)). Compared with other methods, such as the hydrothermal method, which require several hours of reaction, the EDS method has some advantages, such as short time, easy operation, and low cost. As an in situ growth method, it can be directly grown on the substrate to obtain self-supporting electrode without the transfer step of materials. The EDS method mainly include constant voltage deposition, constant current deposition, and cyclic voltammetry (CV) deposition. The PB film prepared by using the EDS technique is prone to wide cracks, which aid in the insertion and extraction of electrolyte ions into the film structure. However, these extensive cracks are detrimental to the cycling performance of PB electrodes. Furthermore, the adhesion between the PB film prepared using EDS and the conductive substrate is extremely poor, which is also detrimental to the cyclic stability of the PB electrode.

Fu et al. [48] fabricated PB thin films with different thicknesses by multi-step EDS. According Fig.3(b), an increase in the thickness of the PB film led to a concurrent rise in the enclosed area of the CV curve and the peak current density, which meant that the quantity of ions was being inserted or extracted with the increasing film thickness. However, when the thickness reached 490 nm, both the enclosed area of the CV curve and the redox current density exhibited a decline. As shown in Fig.3(c), an analogous trend was observed concerning charge density as the film thickness increased. Notably, a film with a thickness of 410 nm displayed the highest charge density. Nonetheless, a continuous increase in thickness was found to lead to a subsequent reduction in charge density, primarily due to the increased resistance inherent in thicker films. Isfahani et al. [47] prepared PB films with different EDS times by using the constant voltage method. A straightforward and precise electrochemical technique was employed to determine the optimal voltages required for coloration and bleaching of distinct PB films. By controlling the EDS time and applying an appropriate voltage, the EC performance of the PB electrode was enhanced. The outcomes indicated that an augmentation in deposition time led to a greater number of accessible electroactive units within the PB films. Besides, Sekhavat & Ghodsi [49] explored an inventive technique known as electrothermophoresis (ETP) for PB film fabrication, wherein a temperature gradient and pulsed potential are employed. As shown in Fig.3(d) and 3(e), all samples produced using various methods exhibited an amorphous nature. Scanning electron microscope (SEM) images revealed that employing reverse ETP resulted in smaller grain sizes. By using pulsed electric field, the coloration efficiency decreased first and then increased with the increase of temperature gradient.

3.1.2 Hydrothermal growth

The hydrothermal growth technique is a method for preparing PB electrode with a good crystal form, high crystallinity, and uniform shape [34,58–60]. It is necessary to transfer the configured precursor solution to the hydrothermal reactor. The substrate is positioned against the inner wall of a Teflon container, with its conductive surface facing downward. Fig.3(f) shows the process of hydrothermal growth. By employing this technique, the adhesion between the PB film and the conductive substrate is augmented, leading to an ultimate enhancement in the cyclic stability of PB-based ECDs.

Qian et al. [59] fabricated a nanostructured PB film by employing a facile and template-free hydrothermal growth method, which can be directly synthesized onto the FTO glass substrate. The as-grown film characterized by abundant well-defined nanochannels, and exhibited a strong adhesion to the substrate. The results showed that the PB-based ECD displayed a quick coloration/bleaching response of 2.4/1.0 s, along with a high CE of 87.4 cm²/C. Chu et al. [58] fabricated a PB electrode by employing the hydrothermal growth method on the FTO substrate. The EC properties demonstrated that the PB electrode synthesized over one hour exhibited excellent coloration/bleaching response times of 5.5 and 9.0 s respectively. Yang et al. [60] fabricated a PW film by utilizing the hydrothermal growth method onto the ITO glass. Fig.3(g) presents the surface image of the PW film, which comprises smooth and uniformly shaped microcubes with a size distribution around 1 μm. The film exhibited a substantial CE of 149.3 cm²/C, and a substantial optical transmittance contrast of over 70% across a broad wavelength spectrum (650–800 nm) (Fig.3(h)), and a short switching time (t_b = 5.5 s and t_c = 2.5 s). In addition, the PW electrode was able to cycle more than 10000 times without obvious decay (Fig.3(i)). Moreover, the hydrothermal method could control the morphology and grain size of PB precisely. Shen et al. [66] prepared the PB powder with different morphologies by employing the hydrothermal method. The morphological synthesis of PB nanocubes and nanospheres could be controlled by modifying the reaction temperature. Moreover, the size of PB nanocrystals could be tuned by altering the molar ratio of poly(vinylpyrrolidone) (PVP) to K₃[Fe(CN)₆]. Ming et al. [67] synthesized small, medium and large (about 20–200 nm) PB particles by using the hydrothermal method, and explained the growth mechanism of PB synthesized by using this method. However, limited by the capacity of the hydrothermal reactor, this method is exclusively suitable for laboratory-scale preparation, and difficult to prepare large-area PB electrodes.

3.1.3 Sol-gel method

In addition to the method of directly preparing the PB films onto conductive substrates, it is also feasible to prepare the PB powder first and then disperse it into the liquid (which could be called PB precursor solution) and transfer it to the conductive substrate. The commonly used method for preparing PB nanostructures is the simple room temperature chemical method [28,62,64,65, 68,69].

The methods of transferring PB precursor solution to conductive substrate by using the sol-gel method mainly include spin-coating and spraying. As depicted in Fig.4(a), the characteristics of the film prepared by using the spin-coating method are directly influenced by the concentration and viscosity of the precursor solution. Additionally, the spin coating rate also determines the quality of the film. Therefore, it is very important to prepare a certain concentration of the precursor solution and select the appropriate spin coating rate. Shiozaki et al. [62] dispersed PB nanoparticles (NPs) to water by surface modification and fabricated the PB thin films easily with the ink by using the spin-coating method on ITO glass. The thickness of the PB film was estimated to be in the range of 40 to 430 nm, and exhibited a stable EC property and reversible color alteration between blue and transparent. Liao et al. [70] synthesized the aqueous PB NPs ink with or without 10 vol.% poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) used as an additive for PB electrode by utilizing the spin-coating technique. A uniform PB film with a substantial visual contrast was applied onto the ITO glass through successive spin-coating of the PB NPs ink, as mentioned above. As shown in Fig.4(b), the PB NPs film show a color change of reversible transparent (PW), blue (PB), green (PG), and yellow (PY) at –0.3, 0.5, 1.3, and 1.7 V, respectively. The notable visual contrasts of the PB film for the PB/PW and PY/PB redox couples can be described by the transmittance variations (ΔT) at 690 nm (73%) and 440 nm (38%). Furthermore, the impact of the PEDOT: PSS additive was also explored. It was discovered that the PEDOT: PSS increased the diffusion coefficients for anodic reaction and cathodic reaction of the PB electrode. The adhesion of the PB thin films prepared by using the spin-coating method to the conductive substrate is adequate, and they are less prone to detachment. In addition, Fan et al. [28] fabricated the PB powder through a simple process, which could uniformly be dispersed in water to prepare the PB ink. From Fig.4(c), it is observed the mean PB particle size dispersed in water is approximately 10 nm. The PB film was fabricated by spraying PB ink on ITO glass at 90 °C. Compared with the PB film fabricated by electrodeposition (EDPB), the PB film fabricated by using the spraying method (wPB) has more uniform surface, less cracks, and smaller charge transfer resistances (Fig.4(d) and Fig.4(e)). As shown in Fig.4(f), the charge transfer resistances (R_ct) of the wPB film are approximately one order of magnitude lower than those of the EDPB film, resulting in significantly accelerated electrochemical kinetics at the interface between the wPB film and the electrolyte. Compared with the spin coating method, the spraying method offers a simpler approach to create a uniform film that exhibits robust adhesion to the substrate.

3.1.4 Others methods

Demiri et al. [72] prepared a PB electrode by using the chemical method. The film was readily fabricated through a sequential immersion process, involving the conductive substrate being dipped into an acidic aqueous solution containing Fe₂(SO₄)₃ and K₄[Fe(CN)₆]. The PB thin film based on the chemical method had an excellent stability reversibility, which made the film favorable for ECD. Elshorbagy et al. [73] fabricated a PB electrode by adopting the spray pyrolysis (SPS) technique. The SPS deposition process could be segmented into three primary stages, the atomization of the precursor solution, the conveyance of the resulting aerosol, and the decomposition of the precursor on the substrate at a high temperature. The CE of the SPS was measured to be 124.3 cm²/C, whereas for the EDS technology utilizing the same precursor materials and substrate, the CE was 47 cm²/C. The results showed that the PB electrode prepared by the SPS displayed a superior stability, remarkable reversibility, and consistent homogeneity throughout the coloration and bleaching processes than that prepared by EDS method. Similarly, Kim et al. [74] introduced an innovative micro-printing approach for generating PB patterns. This method involved the localized crystallization of FeFe(CN)₆ on a substrate, which was confined by the meniscus of an ink comprising acidic-ferric-ferricyanide. Subsequently, a thermal reduction process was conducted at 120 °C, leading to the formation of PB patterns. The PB pattern that was printed exhibited consistent and reversible transformation into PW over the course of 200 CV cycles in the 0.1 mol/L KCl (pH 4) electrolyte. Ding et al. [71] fabricated a PB electrode by galvanic-driven deposition, which was illustrated in Fig.4(g). The redox couple of Fe³⁺/Fe²⁺ exhibits a notably elevated standard electrode potential (φ°) of 0.771 V vs. standard hydrogen electrode (SHE), granting it a galvanic potential larger than that of reductive metals. Specifically, metals such as Cu (φ° = 0.337 V), Ni (φ° = − 0.250 V), and Zn (φ° = − 0.760 V) were utilized as the reductive metals. When the metal plate came into close electrical contact with the ITO substrate within the precursor solution, the substantial galvanic potential prompted the oxidation of the metal alongside a swift electron flow to the ITO substrate. Consequently, Fe³⁺ ions at the ITO interface accepted the electrons, leading to the formation of Fe²⁺ ions. These ions then participated in reactions with Fe(CN)₆³⁻ ions, facilitating the deposition of the PB film onto the ITO glass within a matter of minutes. In comparison to the previously employed methods like hydrothermal (requiring high temperature and pressure) and EDS (necessitating specialized equipment), this energy-efficient technique represents a straightforward solution-based treatment conducted at room temperature, making it conducive to large-scale production. The achievement of a high-quality PB electrode with a size of 13 cm×13 cm as depicted in Fig.4(h), which exhibited a robust mechanical flexibility, was attributed to the galvanic-driven deposition approach.

Presented in Tab.1 is a performance comparison of the abovementioned various preparation methods.

3.2 Performance optimization strategies of PB electrode

The single PB has inherent shortcomings such as poor conductivity because it is a semiconductor material with a band gap of approximately 1.73 eV [75], which leads to a slow charge transfer speed and a long response time. It is of great importance to design and fabricate PB electrodes with high performances.

3.2.1 Compositing with functional groups

(1) Conductive nanoparticles

The poor conductivity of PB leads to a slow charge transfer speed, which affects the response speed of the PB electrode and cannot meet its practical application requirements. Compounding with other conductive NPs could compensate for the shortcoming and combine the advantages of different materials. Many studies have shown that the film formed by compounding PB with conductive NPs (such as ITO-NPs [21,32] and Au-NPs [35,36]) could greatly improve the conductivity. Song et al. [21] developed a straightforward and efficient approach for fabricating PB electrodes (immersion), with the aid of ITO-NPs by using the spin-coating techniques. This process relies on the interfacial precipitation reaction between Fe(CN)₆⁴⁻ ions present in the electrolyte and Fe³⁺ ions derived from the acid-etched sacrifice layer. As shown in Fig.5(a), the ITO-NPs dispersion is spin-coated onto the FTO substrate, and annealed under 300 °C to improve the strength of the film. Then the film is immersed into a certain solution for one hour to accomplish the replacement reaction. As a transparent conductive component, the ITO-NPs is used to enhance the conductivity and adhesion strength of the PB electrode without having a serious effect on transmittance. The composite film shows excellent EC properties (ΔT = 59.9% at 633 nm, t_c = 8 s, t_b = 12 s, and a 77% ΔT retention after 1200 cycles), along with an outstanding charge storage/release capability.

Xu et al. [35] prepared an Au/PB composite film by using the EDS method to enhance the EC performance of the PB electrode. The incorporation of Au-NPs yielded multifaceted benefits, encompassing a heightened conductivity and mitigation of polarization effects. Furthermore, it increased the interfacial contact area, thereby mitigating the stress and strain induced by the volumetric fluctuations intrinsic to the ion insertion and extraction processes. As shown in Fig.5(b) and Fig.5(c), with the Au-NPs composited with the PB film, the switching time shortened from (t_c = 1.92 s and t_b = 4.08 s) to (t_c = 1.36 s and t_b = 2.32 s). Fig.5(d) and Fig.5(e) displayed the variation of the ΔOD of the PB and the Au/PB electrode, which is contingent upon the intercalation charge density. Calculations revealed a CE value of 131.3 cm²/C for the Au/PB film, surpassing that of the PB film (92.6 cm²/C). This elevated CE value signified the capacity to achieve amplified optical modulation while minimizing energy expenditure, which was expected to achieve long-term cycle stability [60, 77]. The prepared Au/PB film had an excellent cycling performance (96.4% after 2000 cycles). In addition, PB can also compound with nanostructured carbon materials including fullerene, carbon nanotube (CNT), and graphene, etc. Ko et al. [78] prepared a PB film composited with transparent graphene to improve its response rate. Nossol & Zarbin [79] fabricated a PB film by combining with CNTs to obtain the high conductivity. Compared with the single ITO/PB film, the ITO/CNTS/PB composite film exhibited a shorter switching time and a higher coloration efficiency.

(2) Organic conductive polymer

Organic conductive polymers have the advantages of a fast response rate, a rich color, an easy solution processing, and a low cost. The composite film of PB and organic conductive polymer has a unique microstructure and apparent synergistic effect. The complementary advantages of PB and organic conductive polymers (such as polypyrrole (PPy), PEDOT: PSS) effectively enhance the overall EC performance of the composite film. Talagaeva et al. [80] prepared a composite film of PB and PPy via the chemical redox process. The response rate of the PB film was greatly improved due to the composition with PPy. In addition, the stability of a single PB was reduced to about 55% after 300 cycles, while a composite film could only be reduced to about 78% after 3000 cycles. Hong & Chen [61] prepared a composite film of PB and PEDOT: PSS. PEDOT: The PSS ink was used as a dispersive medium, which has a high transparency and a high EC activity. Besides, PEDOT: PSS could also be used as a pseudocapacitor to accommodate charges, which made electron transfer between nanoparticles easier. DeLongchamp & Hammond [81] fabricated a composite film of PB and polyaniline (PANI) to enhance the conductivity of the film and improve the switching rate (t_c = 1.52 s and t_b = 1.62 s).

3.2.2 Constructing multilayer composite structures

To ensure a large optical modulation, it is necessary to fabricate the PB film with a certain thickness. Nonetheless, the propensity for PB NPs to aggregate is inevitable owing to their pronounced surface energy. This phenomenon culminates in a low utilization of the active sites inherent to PB-NPs, consequently elongating the switching time [47]. To solve this issue, creating an effective multilayer architecture with well-dispersed PB-NPs would be a feasible strategy to improve the utilization of PB-NPs active sites. DeLongchamp & Hammond [81] fabricated a composite film of PB and PANI by LBL assembly. The composite film was generated through leveraging the inherent electrostatic attraction between the polycationic material PANI and a dispersion of negatively charged PB-NPs. As a typical conductive polymer material, PANI has a good electrical conductivity, which made the PB@PANI composite film achieve an extremely fast response rate and excellent performance (ΔT = 61%, t_c = 1.52 s, and t_b = 1.62 s). Liu et al. [76] prepared a PB electrode through exfoliated LDH nanosheets and PB-NPs by using the LBL method, which named as (LDH/PB)_n. As shown in Fig.5(f), the multilayered electrode denoted as (LDH/PB)_n was fabricated through a sequential deposition process involving LDH nanosheets and colloidal suspension of PB-NPs for multiple (n) cycles. Structural and morphological analyses unveiled a long-range stacking order in the (LDH/PB)_n film, wherein the PB-NPs exhibited notable dispersion and were securely immobilized in a monolayer configuration within the interlayers of LDH. Moreover, the film showed a superior EC performance (ΔT = 56%, t_c = 0.71 s, and t_b = 0.94 s) (Fig.5(g)) due to the elevated dispersion of PB-NPs within an ordered film structure, which ensured the smooth flow of cation channels for diffusion and the availability of active sites.

3.2.3 Constructing core-shell nanostructures

The core-shell nanostructures can be constructed in order to shorten the ions diffusion distance, and enlarge the contact area of films with electrolyte. As a result, the switching time of the PB electrode will be shortened obviously and the coloring efficiency can also be improved greatly [33,34,37,38]. Chen et al. [33] demonstrated an effective approach (the hydrothermal method combined with the EDS method) to enhance the EC properties of PB electrode through constructing TiO₂@PB core-shell nanostructures. The TiO₂ nanorods grew perpendicular to the conductive substrate and formed the TiO₂ nanostructure array (TNRA), which served as supporting templates of PB. In addition, the PB uniformly formed a thin layer on the surface of TNRA, which led to a reduced penetration depth of ions during diffusion and an increased contact area with active materials on the surface. As a consequence, the ion storage capacity of the film surpassed that of a compact PB film. Xu et al. [34] successfully fabricated a PB/TNRA film on FTO conductive glass by using the two-step hydrothermal technique to enhance the cycling performance and the optical modulation of the PB electrode. The PB/TNRA film presented a more superior cycling performance (83.8% ΔT retention after 1000 cycles) than the PB film fabricated by using the same method (83.8% ΔT retention after 400 cycles). The remarkable cycling stability observed in the PB/TNRA film can be attributed to the factors that the nanostructured TNRA augmented the surface roughness of the substrate, thereby enhancing the adhesion between the PB film and the conductive substrate, and that the presence of gaps and numerous cracks in the TNRA nanostructure mitigated the adverse effects of volume expansion occurring during ion intercalation and deintercalation processes [34]. The transmittance of PB/TNRA at colored state was almost the same as that of PB film, while the bleached state was 9.7% higher than that of PB film. The results may attribute to the fact that the nanostructure of films was changed by TNRA, which led to an excellent antireflection performance.

The combination of PB with conductive NPs can compensate for the poor conductivity of PB. The construction of core-shell nanostructures has the potential to augment the contact interface between PB and the electrolyte, as well as reducing the ion transport distance. In addition, TNRA is able to increase the transmittance of faded PB films by changing the film structure. These approaches can effectively improve the EC properties of PB electrodes. Based on these, some studies combined the conductive NPs and core-shell nanostructure to fabricate the PB electrode with a better EC performance. Xu et al. [38] fabricated a TNRA on the FTO glass by using the hydrothermal technique as depicted in Fig.6(a). Additionally, the carbon layer was modified on the TNRA by using the hydrothermal method. PB was prepared using the EDS technique to fabricate the TiO₂@carbon@PB film. As depicted in Fig.6(b)–6(d), TiO₂ nanorods were uniformly organized on the FTO substrate, which displayed a high density and featured numerous pores. After carbon coating, the morphology of TiO₂@carbon film showed no significant changes except for the rough surface. After the EDS process, the PB achieved uniform surface modification of the TiO₂@carbon nanorods. The vertical orientation and porous configuration were retained from the initial TNRAs. As shown in Fig.6(e), the enlarged transmission electron microscope (TEM) image confirmed a well-defined core-shell nanostructure with a seamless interface achieved, in contrast to a mere blended structure. This observation provided evidence of the effective connection between the carbon layer and the PB shell, which facilitates efficient interfacial electron transport. The dense pure PB film in Fig.6(f) had a long ion diffusion distance and a relatively small contact area with the electrolyte, which led to a poor EC performance of the film. As depicted in Fig.6(g), the PB uniformly grew on the TNRA, which increased the contact area of the film and the electrolyte. Meanwhile, the core-shell nanoarchitecture offered an increased number of reactive sites conducive to ion storage during the EC process. Moreover, the thinner PB film on TNRA shortened the ion diffusion distance and accelerated the electrochemical reaction. Finally, the carbon layer functioned as a bridge between the FTO glass and the PB layer, as depicted in Fig.6(h). This intermediary role expedited electron transfer and contributed to the mitigation of charge transfer resistance.

Similarly, Wang et al. [37] fabricated a TNRA@G/PB composite film coating on TNRA with the core-shell nanostructure by utilizing the hydrothermal technique and spin coating methods. The PB film and the TNRA@PB film were prepared by identical method to compare the properties. As depicted in Fig.7, the TNRA@G/PB film showed a shorter switching time, higher coloring efficiency, and more excellent cycling performance than PB film and TNRA@PB film.

Functioning as a supportive template, the TNRA facilitates an enhanced electrolyte infiltration and swift ion replenishment to active sites. Simultaneously, the porous framework assists in mitigating volume expansion during cycling, thereby enhancing the cyclic stability of ECDs. Additionally, the incorporated conductive material serves as a connecting link between the conductive and active layers, minimizing electron losses at the interfacial regions during transport. This material establishes a rapid electron transfer pathway, fostering prompt electrochromic response.

4 Applications of PB-based ECDs

4.1 Smart windows

The windows of various vehicles and buildings assist in clear viewing. The transmission of the intense light and the lack of heat insulation have promoted researchers to develop smart windows, capable of regulating the extent of natural light ingress into buildings and diverse vehicles. In particular, the next generation smart windows demand elevated optical modulation capabilities, coupled with flexibility to accommodate diverse intelligent designs. To obtain a stable reaction and high optical modulation in multiple cycles, the ion storage layer of PB-based ECDs can also use EC materials to construct complementary ECDs [25,28,51,53,63–65,69,82–91]. The term “complementary” refers to the utilization of a pair of EC materials for electrodes, wherein PB undergoes coloration through oxidation, while the other EC material manifests coloration through reduction, such as WO₃. The complementary electrode and PB electrode are simultaneously colored and bleached, which lead to a high coloration efficiency, an obvious color change, an excellent cycling performance, and a low energy consumption owing to factors like charge equilibrium and redox balance [85,92–95]. Cai et al. [83] prepared a complementary ECD, where PB and WO₃ were used as EC electrodes. As shown in Fig.8(a), the ECD demonstrated the ability to be colored at 1.1 V and subsequently be bleached at −2.4 V. As depicted in Fig.8(b)–Fig.8(d), the ECD exhibited an excellent EC performance at a wavelength of 678 nm (ΔT = 74.8%, t_c = 2.1 s, t_b = 1.7 s, and CE = 154.5 cm²/C). This high CE value signified the potential for achieving substantial optical modulation with a minimal energy input, thus enhancing the overall energy efficiency. Fig.8(e) depicted the alteration in optical transmittance throughout the cyclic operation of the ECD. ΔT gradually decreased slightly during the 13500 cycles, and maintained 90.6% of the initial ΔT.

Significant advancements have been made by researchers in the investigation of PB-based complementary ECDs, as summarized in Tab.2.

Besides, the traditional EC smart window only involves visible light adjustment and is not able to selectively regulate visible (VIS) and near-infrared (NIR) light, which leads to a poor shielding effect on solar radiant heat [98–103]. NIR light constitutes of approximately 50% of the overall solar radiation and significantly increases the temperature of the room through the window. In this regard, exploring dual-band EC smart windows to achieve selective and independent manipulation of VIS and NIR light has received increasing attention. Previous investigations did not showcase the capacity of PB materials to selectively modulate the NIR region. Tang et al. [39] developed a robust single-component PB film, which demonstrated the unique capability to independently modulate VIS and NIR light for the first time. This film exhibited an extensive array of reversible color changes, encompassing blue, green, yellow, and transparent states. This diversity enabled the activation of three distinct operational modes (bright, cool, and dark) during the EC process. At the same time, significant optical modulation for both VIS and NIR light, coupled with elevated coloration efficiency and rapid response rate were effectively achieved. As depicted in Fig.9(a), the PB electrode yielded a substantial dual-band ΔT at the potential of −0.4 and 0.7 V, recording 76.6% at 650 nm, 79.8% at 1400 nm, and 79.6% at 2000 nm. Illustrative images at various potentials are shown in Fig.9(b). At −0.4 V, the PB film maintained the transmittance across both VIS and NIR spectral ranges, with values of 77.0% in the VIS region and 75.8% in the NIR region, corresponding to the bright and warm mode. Shifted to 0.7 V, the PB film obstructed 77.9% of the total solar energy and 75.6% of VIS light, signifying the dark and cool mode. In the colored mode (1.1 and 1.4 V), the PB film permitted a substantial portion of VIS light to pass through (54.5% at 1.1 V and 63.2% at 1.4 V), while simultaneously impeded NIR light (50.1% at 1.1 V and 31.8% at 1.4 V). As shown in Fig.9(b), the adaptability of the film to transition various modes based on individual preferences effectively curtails energy consumption in buildings, encompassing lighting and air conditioning needs. Liu et al. [82] fabricated a PB-based complementary ECD based on WO₃ and PB, which were employed as EC active layers. The smart window made by the ECD demonstrated a continuously adjustability in optical modulation and temperature regulation in correspondence to transitions among various operational modes. As illustrated in Fig.9(c), when the device was just prepared, the PB film exhibited a colored state, while the WO₃ film appeared bleached. At this time, the ECD enabled the transmittance of nearly 40% of both VIS and NIR light, which tended to keep the room warm. When the ECD was under short-circuit conditions and illuminated, ions were inserted into the PB film, which caused the conversion of PB to PW, according to the bright mode. In addition, when the ECD was switched to open-circuit configuration and illuminated, the WO₃ film would correspond to the cold mode. Dark mode was achievable at 1.5 V. As depicted in Fig.9(d), the smart window demonstrated a dynamic responsiveness in terms of VIS light transmittance and NIR inhibition rate during the four-mode switching process. Notably, the smart window showed an exceptional thermal insulation performance in the dark mode, and the NIR inhibition rate was closed to 100%. PB-based EC smart windows possess the capability to dynamically modulate spectral attributes, enabling them to effectively control the transmission of both light and heat between the interior of the building and its surroundings. This feature contributes to reducing supplementary energy consumption and upholding indoor temperature levels.

4.2 Electrochromic energy storage devices (EESDs)

The ECDs are similar to the energy storage devices (such as supercapacitors and lithium-ion batteries) in term of the device construction and working principle. In recent years, the applications of PB-based ECDs have expanded to the field of energy storage. Interestingly, the energy storage state of EESDs can be usually visualized by changes in their optical properties due to the one-to-one correspondence between electrochemical and optical processes. Ding et al. [104] fabricated an inorganic flexible lithium-based EESD, which combined the PW@MnO₂ composite electrode and WO₃ electrode. The collaborative impact of PW and MnO₂ exhibited a constructive influence on both energy storage and EC properties. The energy level could be quantified through transmission spectrum and chrominance difference assessments, while the charge–discharge process can be real-time monitored through the optical modulation of specific wavelengths. As shown in Fig.10(a), the EESD displayed a noticeable color differentiation in both colored and bleached conditions. Leveraging voltage-driven transmittance visualization, the energy storage status of the EESD can be accurately assessed through the alterations in transmittance and visual appearance. Fig.10(b) illustrated that in a fully charged state, the curved EESD could power a digital watch with a driving voltage of 1.5 V for a duration of 47 min. Furthermore, the EESD exhibited a remarkable stability, enduring 10000 cycles of CV without noticeable degradation across broad voltage ranges (from −2 to 2.5 V) (Fig.10(c)). During the cycle progresses, the change trend of transmittance was consistent with the charge density. When the cycle reached 2000th cycle, it could obtain approximately 25% ΔT at 510 nm. The energy status of the EESD could be actively monitored in real time through observed color alterations (Fig.10(a)).

It is generally believed that PB-based complementary ECDs in Section 4.1 are thin film devices characterized by a relatively modest capacity. Throughout the charging process, the externally applied voltage served as the impetus that induced cation to insert into the working electrode, consequently prompting a color shift. On the contrary, throughout the discharging process, cations were liberated from the working electrode and integrated into the counter electrode. However, the low redox potential difference between working electrode and counter electrode made it difficult for stored cations to self-retrieve during the discharging process [107]. In other words, the operation of the PB-based complementary ECDs requires the external bias to furnish the voltage or current, and the inherent energy consumption will weaken its energy-efficient characteristics. In this context, self-powered EESDs [39,54–57,104–106,108–115] were proposed, which could independently achieve color switching and partially recover the energy consumed by the device during charging [116]. In most PB-based self-powered EESDs, the PB film acts as a cathode, and the active metal (such as Mg [57,109], Zn [29,39,54,105,106,108,112,114], Al [110,111,115], Ni [113]) as anode, which displays a rapid response rate throughout the reduction process. Wang et al. [111] first proposed a PB-based self-powered EC battery utilizing Al as the anode and PB as the cathode. The bleaching process of the PB electrode could be easily achieved by connecting the two electrodes using a conductor. The substantial redox potential disparity between the Zn and PB electrodes (approximately 1.12 V) was anticipated to function as the driving potential for initiating the autogenic discoloring/discharging process of the PB electrode. In addition, Li and the colleges [54,105,108] developed a new EESD with the Zn anode functioning as a counter electrode, while the PB cathode serving as the EC layer with the dual-ion aqueous electrolytes of Zn²⁺ and K⁺. The autochromism through the redox potential gradient disparity between the Zn anode and the PB EC cathode could be achieved. In 2020, Li & Elezzabi [108] further fabricated the PB-based EESD by interleaving a slender Zn foil strip amid dual PB electrodes. The as-fabricated EESD supplied an approximate voltage of 1.46 V, enabling the illumination of an LED (regulated at 0.5 V) for a duration exceeding 60 min. The rapid charging/coloring of the self-bleaching device mandates the application of an external voltage of 1.8 V. To achieve self-powered energy-efficient EESD, Li et al. [105] further integrated the device with the PV cell to charge the bleached EESD. As shown in Fig.10(d), the PV cell furnished the requisite electrical power to drive the PB-Zn based EESD during the daytime. Throughout the periods of nighttime or intermittent sunlight, the colored EESD could undergo spontaneous bleaching without energy consumption and recycle the power provided by the PV battery to power the electronic equipment. Fig.10(e) described the solar charging by PV panels and the EESD self-powered bleaching process. The solid blue curve showed the optical modulation (ΔT ≈ 63%) of the EESD at 632.8 nm. After being colored by solar cell throughout daylight hours, the EESD stored enough power to illuminate the LED (regulated at 0.5 V) and bleached its colored state. Then, in 2023, Li & Elezzabi [108] coupled the Zn anode-based EC platform into a rocking-chair type ECD, and realized a dual-mode ECD with self-coloring and self-bleaching functions. It is demonstrated that the EESD induced a self-powered behavior, and enabled the independent operation of an individual EC electrode through the comprehensive exploitation of redox potential disparities among three electrodes, as shown in Fig.10(f). Besides, Li et al. [106] developed a PB-Zn-based dual-function battery for skin-interface wearable electronics with a thickness of less than 50 μm, which could be put on the finger and light up the LED bulb (Fig.10(g)). Fig.10(h) showed the galvanostatic charging and discharging curves of the PB-Zn battery, in which a wide voltage plateau was observed at approximately 1.25 V. It also showed that the self-powered PB-Zn EESD had a relatively high specific capacity. In fact, all of the aforementioned self-powered EESDs are categorized as semi-self-powered configurations. While certain self-powered electrochromic systems can indeed be recharged using solar cells, these setups impede their practical utility in portable devices. In this regard, Ma et al. [57] introduced a novel integrated ECD named “Mg~PB~MnO₂,” as illustrated in Fig.10(i). Driven by the potential disparity between the Mg electrode and the PB electrode, the PB electrode could be bleached from blue to colorless, and harnessing the released energy to illuminate a 1.5 V LED. By using the potential difference between the PW electrode and the MnO₂ electrode, the bleached PW film could be recolored to the original state and store energy undergoing the coloration procedure. The PW electrode and the MnO₂ electrode could be serially connected to supply power to a 1.5 V LED. The multifunctional self-powered EC battery (Mg~PB~MnO₂) accomplished swift self-bleaching/coloring and self-discharging/charging sequences devoid of external input.

4.3 Wearable electronics

Flexible ECDs have garnered significant interest in recent times owing to their substantial potential in wearable intelligent electronics. As shown in Fig.10(b), Ding et al. [104] fabricated the PB@MnO₂ on flexible ITO-PET and constructed the EESD with a WO₃ electrode. The device could be worn on the wrist in a curved state and provide power to a digital watch (operating at a 1.5 V) for a duration of 47 min. The EESD offered a visual representation of its capacity by means of real-time alterations in color, all without the need for an extra power source. However, the flexible PET substrate has a limited degree of bending, and the emergence of on-skin electronics for human–machine interfaces and on-body sensing necessitates high-performance development of smart flexible batteries. Li et al. [106] prepared an ultra-thin, dual-functional, PB-based wearable electronic battery that can be directly applied to the skin. They used a simple and scalable transfer printing technique to fabricate this wearable battery for direct skin application. This method resulted in a thickness of under 50 μm for electronic components (Fig.10(g)). The LED illuminated effectively when the “switch” was turned on. Furthermore, the LED remained undamaged and did not flicker while the batteries were bent, demonstrating a consistent voltage output from the batteries even during periods of dynamic mechanical deformation.

4.4 Smart displays

Inorganic EC materials still face great challenges in achieving multicolor conversion due to their monochromatic tone changes. PB is a material that exhibits multiple colors, encompassing blue, green, yellow, and even transparency. However, most researchers focused on the color change between transparent and blue due to the excellent stability of PB. Since the ECD has two active electrodes, a multicolor PB-based ECD can be created by superimposing the colors of the two electrodes. Ding et al. [55] designed a multicolor ECD, in which the PB and MnO₂ were utilized as the asymmetrical electrodes, with exhibition of blue, green and yellow, as shown in Fig.11(a). During the EC process, MnO₂ could be switched between brown and pale yellow, and PB could transition between blue and transparent, resulting in the rich color of the mixed electrode. Fig.11(b) showed the color transformation of PB-based multicolor ECD observed through visual analysis of the photographs alongside corresponding simulation images. The ECD could display varying degrees of yellow, green, and blue colors at different voltages. The chromaticity coordinates in the 1976 color space (CIE XYZ) were represented in Fig.11(c). It was evident that the primary wavelength of the color points underwent a gradual shift from approximately 570 to about 485 nm, encompassing the three primary color zones from yellow to green and blue. This observation confirmed the capability for multicolor transformation of the EESD. As shown in Fig.11(d), the ECD was continuously colored/bleached 5000 cycles at 1.6 and −1 V. The ECD exhibited notable EC performance even after undergoing 5000 rapid cycles, with only a minor reduction in its electrochemical and optical characteristics.

With the continuous progress of modern display technology, EC display has been widely concerned due to its typical non-emission (passive) display. Kim et al. [74] presented an innovative micro-printing technique to create patterns of PB. They used this method to manufacture a PB-based EC display, which was then integrated with a navigation function into a smart contact lens. The device provided real-time directional guidance to the user by receiving and displaying GPS (global positioning system) coordinates corresponding to the destination. As shown in Fig.12(a), the uniformity and line width of the printed PB pattern were effectively managed. The PB-based EC display demonstrated a consistent blinking stability even under a 0.1 s duration condition. This capability enabled rapid information transmission to users. Fig.12(b) showed the correlation between the extracted/inserted charge capacity and the number of CV cycles of the printed lines. The charging capacity changed slightly, suggesting that the property of the PB pattern remained consistent throughout the course of 200 cycles (Fig.12(c)). Fig.12(d) illustrated the integration of an EC navigation system into a smart contact lens. This system comprises components such as a GPS receiver module, an Arduino UNO serving as a processor, and a PB display. The direction arrows (straight, left, right) and marks (GO, STOP) were five independent working electrodes. According to the latitude and longitude information from the GPS, the voltage applied between the counter electrode and the working electrode was controlled by a programming logic (Fig.12(e)). In fact, this simple method for preparing PB micropatterns can find applications in advanced electronic displays and a variety of functional devices.

4.5 Military camouflage

Electrochromism is able to realize the multicolor change and has a wide application potential in military camouflage. Li et al. [22] fabricated a PB film on Au/nylon 66 flexible substrate and assembled a flexible ECD. PB is capable of reversible switching between blue and colorless, and the color of Au is stable golden yellow. Drawing upon the subtractive color-mixing theory [117,118], the superposition of Au and PB could transition in color between sandy yellow and leafy green, reminiscent of the hues found in deserts and oases, respectively (Fig.13(a)). Applying these devices for effective camouflage in such environments is indeed feasible (Fig.13(b) and Fig.13(c)).

5 Conclusions and prospects

In summary, this paper provided a comprehensive overview of the recent advancements in PB electrodes and devices within the realm of electrochromism. In addition, it has briefly described the device structure of PB-based ECDs and performance criteria of PB electrode and ECDs. Moreover, it introduced several typical methods for preparing PB electrodes, and summarized the strategies of improving the properties of PB electrodes, like compositing with functional groups, constructing multilayer composite structures and constructing core-shell nanostructures. The appealing features of PB materials, including the stable EC performance and strong energy storage capabilities, make PB highly attractive for various practical production applications. Thereby, PB-based ECDs have found applications in diverse fields, including but not limited to smart windows, electrochromic energy storage devices, wearable electronics, smart displays, and military camouflage (Fig.14). Furthermore, to further enhance the practical application of PB materials in the field of electrochromism, it proposed several key considerations for future scientific endeavors.

5.1 Performance improvement of PB-based ECDs

Researchers have introduced several approaches in their studies to fabricate PB electrodes and enhancing their EC performance. To compensate for the shortcomings of the slow response caused by the poor conductivity of PB materials, compositing PB with functional groups and constructing core-shell nanostructures are good ways to improve the EC properties of PB electrodes. Nevertheless, despite extensive research and notable progress, there remain challenges that need to be addressed within the EC field.

1) The addition of an appropriate amount of acid in the electrolyte can improve the cycle stability of PB effectively by inhibiting the production of by-product Fe(OH)₃. However, conventional liquid electrolyte has great disadvantages in device packaging, and the leakage of acidic electrolyte has a great potential safety hazard. To address this issue, it is crucial for researchers to develop gel electrolytes or solid electrolytes suitable for PB-based ECDs to prevent electrolyte leakage.

2) To obtain PB electrodes with better EC properties, it is imperative to urgently explore novel strategies for designing and constructing tailored compositions of PB materials using the nano-technology.

3) An appropriate device structure is equally essential for expanding the range of applications and enhancing the practicality of PB-based ECDs because of the interaction between various layers in the multilayer structure. The EC property and applications of PB-based ECDs could be significantly enhanced by properly designing the device structure, such as the double-layer ECDs structure integrating an electrolyte layer with an EC layer or ion storage layer [106], and the integrated ECDs that combines all layers together [22].

5.2 Large-scale preparation of high-performance PB-based ECDs

With the efforts of a large number of researchers, PB-based complementary ECDs with an excellent performance have been relatively mature. However, PB-based ECDs for smart windows in the buildings require an ultra-large area. There are still a series of problems in preparing large-scale PB electrodes with an excellent performance and applying them to ECDs. At present, there are several methods to prepare large-scale PB electrodes and devices, such as slit coating [120] and the sheet-to-sheet (S2S) lamination process [121]. These prepared large-scale PB-based ECDs are not able to meet the requirements of practical applications due to the poor cycle stability, poor adhesion between electrodes and films, long response time, and low optical contrast. In the process of rapid film formation, there is an interface effect between PB films and conductive substrates. The film has multi-dimensional electrochemical weaknesses such as crack defects and shedding, which cannot guarantee the uniformity of the film. Therefore, controlling the deposition rate of PB film is essential, as it allows for the enhancement of thin film crystallinity while strengthening the adhesion between the film and the substrate. Currently, the parameter settings of various methods for the fabrication of PB thin films often rely on inefficient empirical approaches, with the microscale growth kinetics remaining largely unexplored. Researchers can delve deeper into elucidating the connection between microscale growth kinetics and experimental parameters, thereby enabling precise control of thin film quality. Ultimately, the challenges of large-scale fabrication difficulties and poor EC performance can be more efficiently tackled, thus establishing a theoretical foundation for the extensive application of PB. Researchers still need to exert great effort in the preparation of PB-based large-scale ECDs with excellent EC properties.

5.3 Developing flexible PB-based ECDs

In recent years, researchers have shown considerable interest in flexible ECDs due to their potential applications in wearables and embeddable technologies. Flexible ECDs are required to be flexible and portable enough. Researchers [65,71,104] prepared the PB film on ITO-PET, and PB-based flexible ECDs with a sandwich structure. PET substrates are required to have low flexibility due to the brittleness of ITO, resulting in a limited bending angle for PB-based flexible ECDs. Therefore, there is a need for novel flexible conductive layers to enhance the flexibility of ECD. For instance, a conductive network constructed using silver nanowires exhibits an excellent conductivity and can accommodate the microstructural deformations of the film caused by a large number of ion insertions/extractions and bending stress on the electrode. Li et al. [106] prepared an ultra-thin and ultra-flexible PB-based ECD using silver nanowires as the conductive layer.

Besides, the complex multilayer structure of traditional ECDs and the requirement to withstand frequent bending and stretching also pose challenges. To avoid the displacement between the layers of the device during mechanical bending, researchers were committed to decrease the number of stacked layers of PB-based ECDs while ensuring the normal operation of the ECD. Simultaneously, it is equally important to enhance the adhesion between the various layers of the device. Ensuring that the device does not delaminate during repeated bending, twisting, and stretching processes is crucial. Heat treatment is a common method to improve the adhesion between the PB film and the flexible electrode. However, most flexible substrates (organic polymers) cannot undergo such high-temperature heat treatments. Therefore, there is an urgent need to develop methods that can enhance adhesion without requiring heat annealing. One approach to address this challenge is to leverage chemical bonds and electrostatic interactions between the different layers of the electronic components. Li et al. [106] fabricated a PB-based EC battery with a double-layer structure and the Zn foil was silanized to enhance its adhesion to the electrolyte layer. Li et al. [22] prepared the PB-based all-in-one flexible ECD, which concentrated the five-layer structure of traditional ECD in one layer to achieve an ultra-thin EC flexible device with any bending angle.

In summary, it is necessary to enhance the interlayer bonding force to improve the mechanical strength of the device, and the integration of the structure will be the general trend of PB-based flexible ECDs. With the advancement of the smart wearable technology, which holds promising prospects for applications in artificial skin, portable electronics, and sensor modules within the Internet of Things, flexible PB-based ECD will become increasingly common.

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