Film Deposition of Electrochromic Metal Oxides through Spray Coating: A Descriptive Review

Abstract

Electrochromism induces reversible changes of coloration in specific organic and inorganic materials through electrical charge/discharge reactions. When processed into thin films, electrochromic metal oxides can be integrated into glazing applications such as displays, rearview mirrors, goggles and, most notably, smart windows in energy-efficient buildings. Over the years, the use of spray coating as a liquid-based approach has been acknowledged for its cost-efficient, high-throughput samples production with a low volume consumption. It represents an interesting alternative to vacuum processes and to other wet methods, suitably responding to the current limitations of electrochromic thin films production by offering improved control over deposition parameters and capacities of up-scaling, together with lowered energetic and economic costs. The present review summarizes the main theoretical and practical aspects of spray coating, notably distinguishing room-temperature methodologies from pyrolysis-based, under heating protocols. The main families of functional electrochromic metal oxides are then screened and discussed, establishing how spray processing can challengingly lead to higher levels of optical contrast, commutation kinetics, coloration efficiency and cycling durability, and how low-toxic and environment-friendly precursors can be favored while sustaining large deposition areas.

1. Electrochromism: Materials, Properties, and Fabrication Methods

Electrochromism consists in the capacity of a material to reversibly change its color aspect and, more generally, its optical characteristics (absorbance, transmittance, reflectance) from the application of an appropriate electric stimulus. Originally observed in the 19th century, it has been properly studied starting from the 1960–1970s, notably through the founding works of J. Platt [1] and S. Deb [2,3] on operating mechanisms and resulting optical behavior. Since then, it has developed into a booming technological field active in both fundamental research and practical applications, covering a.o. fenestration devices for energy-efficient buildings—so-called smart windows (conceptualized for the first time by C. Lampert [4] and C. Granqvist [5] in 1984–1985); displays, mirrors, and other glazing devices (goggles, helmets…); labels and stickers; energy storage devices including (super)capacitors and batteries; wearable electronics such as intelligent clothes, electronic skin, or paper; etc. [6,7,8,9,10,11,12,13,14,15,16].

Electrochromic (EC) materials are sorted into three constituting categories: organics, with viologens and conductive polymers based on polystyrenes (PEDOT:PSS), polyanilines (PANI), polythiophenes (P₃HT), or polypyrroles (PPy)…; inorganics, with metal oxides, nitrides and sulfides, polyoxymetalates, and carbon-based materials, such as carbon nanotubes (CNTs), graphene, carbon aerogel, and activated carbon…; and others, including organometallics (metal coordination complexes, metal organic frameworks (MOFs), synthetic pigments such as Prussian blue…) [17,18,19,20,21,22,23,24,25]. Organic EC materials provide a broader range of colors with faster response times, while inorganic counterparts—especially metal oxides—are more easily processed and show higher mechanical, chemical, and thermal stability together with improved reversibility and durability. EC metal oxides are further divided into anodic and cathodic materials, depending on how they respond to electrical stimuli: oxides of tungsten (WO₃), molybdenum (MoO₃), niobium (Nb₂O₅), titanium (TiO₂), and bismuth (Bi₂O₃) are examples of cathodic electrochromic compounds, coloring upon the reduction of the metallic element M and ionic X (proton H⁺ or alkali ion Li⁺, Na⁺, K⁺, Cs⁺…) insertion (Equation (1)), while oxides of nickel (NiO), iridium (IrO₂), and manganese (Mn₃O₄) constitute anodic electrochromic compounds, coloring upon oxidation and ionic extraction (Equation (1)); in addition, oxides of vanadium (V₂O₅) and cobalt (Co₃O₄) stand as multichromic compounds coloring differently upon reduction or oxidation.

$${\mathrm{M}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}+{\mathrm{ne}}^{-}+{\mathrm{nX}}^{+}\underset{\left(\mathrm{b}\right)}{\stackrel{\left(\mathrm{a}\right)}{\rightleftarrows }}{\mathrm{x}}_{\mathrm{n}}{\mathrm{M}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}$$

(1)

These oxides can be integrated as active EC materials at the working electrode of five-layer architectured devices (Figure 1), being combined with an ion storage or counter-electrode material (which can be of an electrochromic nature as well, complementarily to the working electrode compound—e.g., NiO with WO₃) through an electrolyte, ion conducting layer, disposed between two transparent electron-conducting layers covering the two transparent substrates. The latter typically consist of glass for a well-known applied use as smart windows and other glazing components, standing notably as energy-saving components of novel efficient buildings for lowering their consumption of heating, air conditioning, and lightening by up to 30%. Meanwhile, EC structures can also deal with flexible substrates such as polymers, plastics, or even papers for designing multiple sorts of light-weight smart electronic devices, which are increasingly developing nowadays so as to respond to the constant huger consumption of connected appliances. Specifically, as current electronic objects such as displays, labels, (bio)sensors, and even textiles are regularly lacking multifunctionality and tunability in their operation mode, flexible EC devices have recently emerged as conceptually simple and cost-efficient systems consuming low power, as the transition from one optical state to another demands minimal energy in a matter of seconds or minutes (depending on the substrate size). EC technology can therefore contribute to the reusability and recyclability of these electronic devices, reducing the probability and risk of them ending up in biological environment, either on purpose or accidentally, after too short of a lifecycle [11,13,15,26,27,28,29].

Two main approaches may be considered for fabricating EC devices, either through a layer-by-layer continuous deposition process, i.e., a multilayer EC stack (Figure 2A), or by laminating the two separate working and counter electrodes, i.e., permanently assembling them by the action of pressure, heat, or adhesive strengths, with the intercalated electrolyte layer typically being of a gel polymer nature (Figure 2B).

Most of the currently developed and commercialized EC glazing systems conveniently address the modulation of visible light transmittance. However, the capacity to simultaneously but independently filtrate the infrared radiation, accounting for about half of the incident solar energy, still constitutes an important and critical issue. An ideal EC technology should indeed provide the selective, dynamic control of solar heating (near-infrared NIR range) and daylighting (visible VIS range) so as to better correlate users’ needs and preferences depending on locations, climates, and seasons. Hence, there is the recent occurrence of dual-band EC materials and devices, allowing for selectively regulating the NIR and VIS regions of the solar spectrum through suitable, progressive electrical bias that will enable the switching of the EC devices between up to four filtration modes (Figure 3): bright (VIS and NIR transparent), cool (VIS transparent and NIR opaque), warm (VIS opaque and NIR transparent), and dark (VIS and NIR opaque) [9,30,31,32,33,34]. In specific configurations of EC materials and devices, dual-band properties can even be combined with polychromism, leading to multicolored glazing systems providing even more aesthetics and adaptability in daily life [35,36].

While the conventional EC mechanism of VIS light modulation consists in combined reduced polaron absorption and in-band transition, the tuning of the NIR region mostly relies on localized surface plasmon resonance LSPR effects, specifically occurring in selected nanocrystals (NCs) of metal oxides doped with hetero-elements and/or oxygen vacancies, including indium-tin oxide ITO, TiO_2−x, Nb₂O_5−x, and WO_3−x [37,38,39]. LSPR induces a strongly enhanced light absorption when the frequency of the incident light is resonating with the frequency of collective oscillations of the electrons contained in the (semi)conducting material. The relationship (Equation (2a,b)) between the free carrier concentration n of such doped metal oxide NCs (~10²¹ cm⁻³) and their LSPR frequency ω_LSPR is given by:

$${\omega }_{LSPR}=\sqrt{\frac{{{\omega }_P}^2}{1+2{\epsilon }_m}-{\gamma }^2}$$

(2a)

$${\omega }_P=\sqrt{\frac{n{e}^2}{{\epsilon }_0{m}_e^{*}}}$$

(2b)

With ω_P being the bulk plasma frequency of electrons, $m_e^{*}$ being the effective mass of an electron, ε₀ being the permittivity of free space, ε_m being the dielectric constant of the surrounding environment, and γ being the bulk collision frequency. The tuning of n can thus be achieved synthetically by changing the doping type and content, or post-synthetically by varying the applied potential. This consecutively induces the shift of the plasma frequency and of the corresponding LSPR wavelength, and therefore modulates the intensity and range of EC-driven optical absorption throughout NIR and VIS ranges.

The figures of merits of EC devices are commonly defined in terms of optical and electrochemical performance criteria [6,12,28,31,34], including:

• the optical contrast (i.e., the degree of transmittance modification ΔT% between clear and dark states, recorded at a single wavelength or within a specific wavelength range);
• the solar AM 1.5 coverage or integrated solar transmittance, defined as Equation (3):

  $$T_{sol}=\frac{{\displaystyle \int }T\left(\lambda \right)\psi \left(\lambda \right)d\lambda }{{\displaystyle \int }\psi \left(\lambda \right)d\lambda }$$

  (3)

  with $T\left(\lambda \right)$ being the transmittance at λ wavelength and $\psi \left(\lambda \right)$ being the incident solar intensity in the AM1.5G spectrum;
• the apparent color (regularly quantified in terms of coordinates from the International Commission on Illumination CIE color space with specific chromaticity plots) as well as the Haze factor (defined as the ratio between diffuse and total transmittances, which should be <2% to avoid an undesirable, milky, and troubling aspect in the smart window device);
• the switching speed or response time (the time required for a specific transmittance change, in terms of coloration t_col or bleaching t_ble, routinely equal to 90% of its full commutation run);
• the coloration efficiency CE, expressed in cm²/C, as the change in optical density OD at a defined wavelength induced by every unit of charge density Q inserted or extracted per unit area, i.e., Equation (4):

  $$CE=\frac{\mathsf{\Delta }OD}{\mathsf{\Delta }Q}$$

  (4)
• the memory effect (i.e., the capacity to maintain a transmittance state without applying an additional charge);
• the spectral selectivity (i.e., the ability to selectively control different wavelength ranges of transmittance, typically NIR independently of VIS);
• and the cycling stability and durability, characterized by the number of switching cycles during which the optical change can be preserved without significant degradation, being influenced by cycling testing conditions—the bias type and duration, electrolyte nature, temperature and pressure, illumination conditions, etc. To this end, the reader could refer to the recent report of J. Padilla et al. [40], presenting a detailed procedure that aims at adequately defining the testing conditions and the analytical description of the evolution of the performance of EC materials through continuous cycling. Their evaluation method relies on three steps: (i) define the reference switching conditions for each material, (ii) define the testing conditions used during the stability test, and (iii) use an analytical description of the contrast vs. the cycling run, like the number of cycles corresponding to an 80% performance retention. Various stability assessments were performed on common EC compounds, including conducting polymers, metal oxides, metallo-supramolecular polymers, and viologens, under a variety of test conditions.

Despite many advances and much progress over the last decades and years, EC systems still suffer from both conceptual and technical issues in the optimization of their formulation and structure, and of the conditions of materials preparation and device assembly towards proper transfer/integration into practical applications. Among key prospective actions, the improvement of device fabrication processes is of crucial importance and interest, as the suitable thin-film processing of their constitutive layers has a critical impact on obtaining EC systems with efficient functionality and performance. Film processing may be achieved by various vacuum- or wet-based methodologies [6,8,11,22,41,42]. Chemical vapor deposition (CVD) with atmospheric pressure (APCVD) and aerosol-assisted (AACVD) conditions, thermal evaporation, and sputtering (DC, RF, magnetron…) are among the vacuum processes commonly exploited to manufacture EC materials, while wet protocols include electrodeposition, sol-gel, inkjet printing, dip coating, spin coating, blade coating, and spray coating. Despite being less controllable, solution-based approaches are considered as simpler and more versatile in terms of deposition conditions and to be of a lower energetic and financial cost. Vacuum technologies present the advantages of a higher purity and precision (with a high degree of control provided on deposition parameters, lowering the contamination rates) but high constraints in terms of vacuum quality and temperature dependence, therefore leading to a higher complexity and increased fabrication costs, especially for large-area substrates.

2. Spray Deposition: Principles and Methods

The present contribution proposes to review how the spray deposition approach is precisely exploited for the manufacturing of various sorts of metal oxide-based EC coatings, to be ultimately integrated in functional devices for smart glazing and other (opto)electronic applications.

As an atmospheric pressure technique, spray coating presents a huge potential for producing highly homogeneous films of good adhesion on various substrates—glass, metals and alloys, plastics, polymers, papers, etc., with reduced energy consumption when compared to vacuum processes; it also provides higher upscaling capacities than other lab-level wet routes, therefore being easily transferrable to industrial pilot lines without important production costs. Spray deposition favors the use of low-toxic solvents with efficient precursor utilization, guaranteeing fast and continuous production speeds [8,43,44,45,46,47,48]. Beyond EC materials, spray coating is widely used in numerous fundamental and applied fields of research and development related to energy, the environment, and health, including semiconductors, solar cells, (bio)sensors, batteries, as well as optical and electronic devices [49,50,51,52,53,54,55,56].

A spray coating setup (Figure 4) generally involves a spray nozzle, being an atomizer component that converts the precursor fluid feed into a mist of droplets of a specific size and size distribution. These droplets are then conveyed, “sprayed” onto a target substrate, left to stand at room temperature (RT) or heated with a hot plate or in a furnace/oven enclosure, and positioned on a stage at a certain distance of the nozzle. Depending on the setups, either the nozzle or the substrate stage (or both of them) can be moved in the three spatial directions X/Y/Z during the spraying process, following (or not) a pattern of advanced movement control and automation that may be repeated and/or paused on demand. Thanks to its high level of sophistication combined with its practical simplicity, spray deposition may be used for the uniform coverage of uncommon and/or very irregular surfaces, especially flexible substrates such as polymers, plastics, and papers, and/or provide rapid coating conditions leading to stable and adherent functional layers.

Three types of spray atomization methods are regularly used [44,47,51,54,56]:

• Air or pneumatic spray (Figure 5a), in which compressed air is used to apply high pressure on the precursor fluid discharged from the nozzle, colliding at a high speed with the remaining air and then being split up and slowed down due to air resistance, before finally turning into a mist of droplets directed towards the target. Because they disperse a large quantity of precursor fluid, air spray setups can cause a significant excess of lost material, i.e., overspray.
• Electrostatic spray (Figure 5b), in which the precursor fluid is first charged with static electricity by applying a relatively high voltage within the spray nozzle (up to several thousands of V) and then changed into a droplet mist through electrostatic repulsion; the spray mist is then specifically attracted to the target surface standing on a grounded stage, therefore reducing the overspray quantity. Thanks to the strong electrical field in play, electrospray systems can be used with solutions and suspensions of quite high viscosities as well as with pastes and slurries.
• Ultrasonic spray (Figure 5c), in which the nozzle is equipped with a chip (atomization surface) that vibrates upon ultrasonic waves, causing the precursor fluid to spread over the chip and ruffle; when the ultrasonic output becomes higher than the surface tension, the fluid is transformed into a fine droplet mist of a highly uniform size and size distribution (in comparison with other spray modes), being ultimately conveyed onto the substrate surface through a carrier gas of a passive (inert) or active (oxidative, reducing…) nature. By controlling the liquid flow and vibration frequency, ultrasonic spray has been shown to produce high-quality coatings with important uniformity, regular thickness, and strong adherence, also involving very limited overspray by producing almost no splashing or avoidable liquid spreading.

Corresponding nozzles can have different dimensions and spray shaping conditions depending on the targeted function and application, particularly where high precision or uniform deposition is needed; shaping can also be adapted to the targeted size area, involving atomizers design favoring either fine or large cones of sprayed droplets as well as tandem/series mounting of spray nozzles.

When substantial heating conditions are applied during the spray protocol, pyrolysis processes (i.e., the thermal decomposition of a material at an elevated temperature, typically metal salts being converted into corresponding oxide(s) formulations) can occur depending on the nature and conditioning of the involved chemical precursors, leading to so-called spray pyrolysis deposition [43,44,54,58]. As a function of the substrate temperature or of the initial droplet size, the precursor fluid reaches the substrate surface as a liquid or a precipitate (at a too low temperature or with a too large initial droplet), a vapor (ideal temperature or initial droplet size), or a powder (at a too high temperature or at a too small initial droplet); this is defined according to the Viguié-Spitz [59] and Siefert [60] models (Figure 6) and shown to critically impact the resulting morphological, structural, and functional properties of the obtained films. Other issues must also be considered so to rationalize the basic aspects of the spray process, including (but not limited to): the size, nature, and form of the sprayed materials/particles; the film’s structural, kinetic, and topographical characteristics; the chemical bonding and associated structural considerations (including an amorphous or crystalline nature) of the as-deposited layers; and the effects of annealing on the bonding and thickness.

Spray coating can also be achieved at RT (or under relatively mild heating conditions involving no pyrolysis reaction)—for instance, for the deposition of “pre-formed” functional micro- or nanoparticles as liquid suspensions (Figure 7A) or as dry powders in a so-called nanoparticle deposition system (NPDS; Figure 7B) capable of depositing both nano- and micron-sized particles under low vacuum with low-cost compressed air. NPDS has already been reported for the coating of metals, ceramics, or polymer substrates with various metals and ceramics, including Cu, Ni, TiO₂, Al₂O₃, WO₃, NiO, and Sb-doped SnO₂ (ATO) [8,61,62,63,64,65].

The suitable selection of the spraying conditions, and the corresponding impact on the film’s functional properties, are very often reported to be critical and of high importance for all sorts of precursor compounds and formulations. Factors influencing spray deposition can be sorted along the material-based parameters, including the precursor’s nature, the solvent, the solution/suspension concentration, the effect of incorporated dopants and/or other additives, etc. Setup-related parameters are also shown to impact the deposited layers’ properties, including the nozzle-to-substrate distance, the carrier gas nature and pressure, the liquid feed flow rate, the nozzle and/or substrate movement/pattern, the spray duration (as a single or sequential run), the substrate temperature during deposition together with the heating conditions (hot plate, furnace, oven, microwaves…), and the (possible) consecutive post-treatment of a thermal/chemical/physical nature, etc. This is of course the case for all EC metal oxides being considered for the present review and examined on a case-by-case basis in the following sections.

3. Spray Deposition of Electrochromic Metal Oxides

3.1. Tungsten Oxide WO₃

The EC behavior of WO₃ was first evidenced by S. Deb in 1969 [2], making it one of the most popular EC materials since then. Coloring cathodically upon the reduction of W(+VI) and cationic insertion, its optical state turns from transparent to dark blue. Praised for their high optical modulation and coloration efficiency, strong cycling stability, and low processing cost, WO₃ films are well-established materials for the large-scale development of EC devices, despite a quite slow response time (typically of tens of seconds to minutes) and a mitigated response to the life cycle (aging effect). As for other EC metal oxides (see below), their optical and electrochemical performances rely on various morphological and structural factors, being strongly influenced by the selected fabrication technique, including the crystallinity, roughness, porosity, and conductivity; EC properties can also depend on further material design strategies such as nanostructuration, regulation of crystallinity (between crystalline, polycrystalline, amorphous, and hybrid), doping (with hetero-elements and/or oxygen vacancies), incorporation in hybrid and/or composite structures, as well as film’s density and electrolyte nature and concentration [14,66,67,68,69,70,71].

The spray deposition of WO₃ films for EC applications has been covered in about 45 publications since 1986, including 25 (14) over the last 10 (5) years (

Table 1. Review of spray-coated WO₃ layers in terms of spray conditions, resulting thickness, and EC metrics. N.p. means “not provided”; O.D. means “optical density”. T_ble and T_col data are represented as “highest–lowest” values obtained on all presented datasets in the corresponding references (which can be related to different samples); however, the reported ranges of ΔT are related to common datasets (and so are t_ble, t_col, CE, and the number of cycles data). e.g., a reference covers three samples: T_ble,1 = 95%, T_ble,2 = 90%, and T_ble,3 = 85%, and T_col,1 = 30%, T_col,2 = 5%, and T_col,3 = 15%; reported values in the table will be: T_ble = 85–95%, T_col = 5–30%, and ΔT = 65–85% (and not ΔT = 55–90%, as it would be calculated from the difference in the maximal T_ble and minimum T_col values). The same terminology is used in the following tables.

Reference	Spray Conditions (Tdep for Deposition Temperature, t.t. for Thermal Treatment)	Thickness (nm)	Tble (%)	Tcol (%)	ΔT (%)	tble (s)	tcol (s)	CE (cm2/C)	Cycling Runs
Colbow [72]	WCl6 in ethanol or DMF Tdep = 400 °C	N.p.	75	2	73	N.p.	N.p.	N.p.	N.p.
Colbow [73]	WCl6 in DMF Tdep = 200–400 °C	250–650	N.p.	N.p.	N.p.	N.p.	40–120	N.p.	N.p.
Colbow [74]	WCl6 in DMF Tdep = 200–400 °C	450	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Livage [75]	Tungstate/WOCl4 in H2O/alcohol Tdep = RT with drying at 80 °C	500	N.p.	N.p.	0.7 (O.D.)	N.p.	N.p.	N.p.	N.p.
Estrada [76]	Ammonium tungstate in H2O Tdep = 430 °C	1000	75	35	40	N.p.	N.p.	N.p.	100
Regragui [77]	WCl6 in H2O–ethanol Tdep = 200–400 °C with t.t. at 500 °C	150	60–72	10–50	22–60	N.p.	N.p.	N.p.	N.p.
Regragui [78]	WCl6 in H2O–ethanol Tdep = 300 °C with t.t. at 500 °C	150	>80	<5	75	<5	<10	N.p.	N.p.
Patil [79]	NH4 tungstate & molybdate in H2O Tdep = 300 °C with t.t. at 400 °C	600	N.p.	N.p.	0.6–1.3 (O.D.)	2–3	4–5	42–63	N.p.
Patil [80]	WO3 powder and C10H14O5Ti in NH4OH and methanol; Tdep = 525 °C	N.p.	45–85	25–70	20–25	N.p.	N.p.	7–13	N.p.
Patil [81]	WO3 powder and TiCl4 in NH4OH/H2O Tdep = 450 °C	N.p.	50–73	45–67	2–10	3–6	4–5	8–26	N.p.
Patil [82]	WO3 and Nb2O5 powders in NH4OH/H2O Tdep = 450 °C	520–530	73–85	55–67	6–30	1–3	3–5	17–26	N.p.
Patil [83]	WO3 and Nb2O5 powders in NH4OH/H2O Tdep = 450 °C	510–530	65–75	52–58	10–20	6–8	5–7	13–19	1000
Patil [84]	Ammonium tungstate in NH4OH/H2O Tdep = 450 °C	200	70	65	5	3–4	5–6	25	1200
Patil [85]	Ammonium tungstate and MWCNTs in H2O; Tdep = 450 °C	250	75–80	30–50	30–45	3–6	6–8	43–79	N.p.
Patil [86]	Ammonium tungstate in NH4OH/H2O Tdep = 450 °C	250–750	25–80	10–63	10–50	N.p.	N.p.	14–43	1000
Sivakumar [87]	Ammonium tungstate in NH4OH/H2O Tdep = 250 °C with t.t. at 500 °C	600	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Zelazowska [88]	WO(C5H7O2)4 in acetylacetone and dichloromethane; Tdep = 650 °C	120	58	6	52	N.p.	N.p.	N.p.	100
Sahay [89]	WCl6 or ammonium tungstate in DMF Tdep = up to 400 °C	450–500	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Acosta [90]	WCl6 and MoO2Cl2 in DMF Tdep = 500 °C	480–520	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	2000
Sahay [91]	Ammonium tungstate and MoCl5 in DMF Tdep = 390 °C	N.p.	18–48	1–18	13–30	25–50	1–25	14–43	100
Sahay [92]	Ammonium tungstate and SnCl4 in H2O Tdep = 400 °C	500–550	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Acosta [93]	WCl6 and TiCl3 in DMF Tdep = 500 °C	480–520	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	3000
Sahay [94]	Ammonium tungstate and SbCl3 in H2O Tdep = 400 °C	450–470	N.p.	N.p.	N.p.	3–6	3–10	N.p.	50
Mohagheghi [95]	WO3 powder, NiSO4, and CoCl2 in H2O Tdep = 400 °C with t.t. at 500 °C	250	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mohagheghi [96]	WO3 powder + CoCl2 in H2O Tdep = 400 °C with t.t. at 500 °C	250	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Wolden [97]	WCl6 + ethanol + P123 Tdep = RT with t.t. at 300–400 °C	400	70–92	14–38	35–75	6–120+	6–20	26–51	10
Wolden [98]	WCl6 + ethanol + P123 Tdep = RT with t.t. at 300–400 °C	400	70–80	45–50	40–75	5–24	5–9	43–50	2500
Wolden [99]	WCl6 + ethanol + P123 Tdep = RT with t.t. at 350 °C	1000	77–100	0–30	70–100	27–67	3	36–45	1000
Duta [100]	WCl6 + ethanol Tdep = 200–300 °C	200	65–80	15–35	21–64	N.p.	N.p.	28–35	N.p.
Duta [101]	WCl6, HTAB, and PEG in ethanol and acetylacetone Tdep = 250 °C with t.t. at 410 °C	350–375	65–80	45–50	13–35	47–58	13–19	10–43	50–100
Cloots [102]	AMT and PEG in H2O Tdep = 350 °C	550	92	9	83	42	72	28	500
Cloots [103]	AMT and PEG in H2O Tdep = 350 °C	100–120	87–92	7–9	78–85	46–51	77–91	26–31	100
Cloots [104]	APTA and Brij-56 in H2O Tdep = 190 °C or 100 °C with t.t. at 350 °C	200	55–85	20–55	10–40	35–200	60–300	6–40	20
Lin [105]	WCl6 and P123 in ethanol Tdep = RT with t.t. at 350 °C	1000	78–95	32–38	40–55	1–3	5–7	N.p.	3000
Gesheva [106]	W powders and H2O2 in ethanol Tdep = RT with t.t. at 100–275 °C	200	N.p.	N.p.	10–70	N.p.	N.p.	12–134	N.p.
P.S. Lee [107]	H2WO4 and MoO3 in H2O and ethylene glycol; Tdep = RT with drying	200	80	37	43	8	10	36	2000
Elezzabi [108]	Ti–W–Mo oxides NPs in H2O Tdep = RT with drying at 60 °C	N.p.	22–85	7–9	15–76	N.p.	N.p.	N.p.	100
Elezzabi [109]	W-Mo NPs + PEDOT:PSS Tdep = 60 °C, layer-by-layer process	900	75–85	10–30	55–65	11–14	18–22	22–53	300
C.S. Lee [62]	WO3 NPs Tdep = 80 °C, NPDS	400–700	60–70	5–20	44–64	N.p.	N.p.	N.p.	N.p.
C.S. Lee [63]	WO3 NPs Tdep = RT-200 °C, NPDS	400–700	55–65	14–35	10–42	N.p.	N.p.	N.p.	N.p.
C.S. Lee [64]	WO3 NPs Tdep = 80 °C, NPDS	400–700	2–15	55–62	43–55	>40	18–30	42–77	N.p.
C.S. Lee [110]	WO3 NPs Tdep = RT, NPDS	N.p.	37–65	8–47	2–50	Fixed time	Fixed time	N.p.	600+ and over 1M
Tsukagoshi [111]	WO3 NPs in H2O Tdep = 150 to 300 °C	450	79	19	60	2	4	37	80
Liu [112]	WO3 and MoO3 NPs in H2O and ethanol Tdep not provided	N.p.	70–75	12–36	45–60	6–11	4–9	43–80	2000
Choi [113]	WO3 NPs + Ti3C2Tx MXene Tdep = RT with t.t. at 100 °C	200	68–72	10–57	17–58	2–3	3–7	35–127	1000
Pugliese [114]	WO3 and TiO2 NCs Tdep = 100 °C with t.t. at 300 °C	500	VIS: 71 NIR: 70	VIS: 11 NIR: 5	VIS: 60 NIR: 65	VIS: 4 NIR: 22	VIS: 22 NIR: 5	VIS: 38 NIR: 29	500

). Notable initiatory works were conducted by K. Colbow et al. in the late 1980s and early 1990s [72,73,74,75], reporting the spray pyrolysis deposition at 400 °C of polycrystalline WO₃ layers from solutions of tungsten chloride WCl₆ dissolved in ethanol or dimethylformamide DMF, resulting in 250 to 650 nm thick films showing up to 73% (75–2) of optical contrast in VIS and coloration times of 40 to 60 s. New developments in pyrolytic spraying were achieved in the 2000s [76,77,78,79,80,81,82,83,84,85,86,87,88,89], notably by P. Patil and co-workers who addressed multiple compositions of precursor solutions—the combination of WO₃ with other oxides (MoO₃, TiO₂, Nb₂O₅), the nature of W salt (mostly tungstates, being less corrosive and toxic than chloride derivatives) and/or the solvent (mostly water), etc.—and of substrate temperatures during and/or after the deposition process (as annealing post-treatments), being between 300 and 525 °C (typically 450 °C). Films of thickness between 200 and 750 nm were consecutively obtained, presenting moderate VIS contrasts ΔT (up to 50%) but quite fast commutation kinetics (t_col and t_ble less than 10 s), and tested over 1000 to 2000 cycles; high coloration efficiency CE values were particularly noticed for mixed WO₃–MoO₃ formulations (42–63 cm²/C) [79] and for WO₃ layers included with multi-walled carbon nanotubes (43–79 cm²/C) [85]. Other groups further investigated the effect of WO₃ doping with various aliovalent compounds, notably Mo [90,91], Sn [92], Ti [93], Sb [94], Ni [95], or Co [95,96], with the most promising performances obtained by P. Sahay et al. with ammonium tungstate (NH₄)₂WO₄ precursor solutions mixed with up to 5% at. of molybdenum chloride MoCl₅ in DMF and sprayed at 390 °C [91], leading to ~30% VIS ΔT, 25–50 s switching speeds, and 14–43 cm²/C CEs.

An interesting approach of surfactant-assisted spray (pyrolysis) deposition was then explored and reported starting from the early 2010s [97,98,99,100,101,102,103,104,105]. Various sorts of surfactants—cationic and anionic, gemini, and non-ionic—could be used to produce WO₃ films with a high order and a controlled particle/grain size, porosity, and crystallinity. Different nucleation and growth processes were observed after the addition of the surfactant(s), impacted by the variation in the surface tension in the corresponding sprayed solution. Notably, C. Wolden and co-workers investigated how the incorporation of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer (P123) to an ethanol-based solution of WCl₆ could provide nanoscale porosity to consecutively sprayed WO₃ layers (of a 400 nm thickness). Electrochromic properties were then directly correlated to the annealing conditions following the RT (ultrasonic) spray deposition—namely, between 300 and 450 °C under “fast” (hot plate) or “slow” (oven with a 5 °C/min ramp) thermal post-treatments: ΔT in VIS was shown to evolve from 41 to 77%, t_col and t_ble were shown to evolve from 20/>120 to 6 s each, and CEs were shown to evolve from 26 to 51 cm²/C, considering a 1 h fast treatment at 350 °C for removing the polymer additive [97]. A. Duta et al. considered a mix of anionic and non-ionic surfactants, namely, hexadecyltrimethylammonium bromide (HTAB) and polythelyene glycol (PEG), being added to WCl₆-ethanol/acetylacetone solutions sprayed at 200 °C; the obtained 350 nm thick films of a high smoothness and homogeneity delivered up to 35% of VIS ΔT, 19 and 51 s of t_col and t_ble, and 43 cm²/C of CEs [101]. Finally, R. Cloots et al. proposed a single-step surfactant-assisted ultrasonic spray pyrolysis (USP) process at 350 °C, using aqueous solutions of ammonium metatungstate (AMT) mixed with PEG at a 1:10 mass ratio (Figure 8) [102]. The obtained layers, of a 550 nm thickness and a very high smoothness, showed a very high VIS ΔT of 83% (92–9), moderate kinetics (72 s for t_col and 42 s for t_ble), and a good CE (28 cm²/C), together with a charge reversibility maintained between 89 and 99% over a 500-cycles coloration/bleaching run. Such excellent EC performances were attributed to the beneficial consequence of the interactions between the PEG surfactant and the W precursor (via hydrogen bonding), specifically preventing droplet coalescence during spray deposition and particle aggregation during film formation. The same group kept exploring the surfactant-assisted USP processing of EC WO₃ layers in two additional studies. The first one addressed the USP deposition of the same solution in similar temperature conditions onto alternative transparent conducting surfaces, namely, glass covered with tin-doped (ITO) or tungsten-doped indium oxide (IWO) instead of fluorine-doped tin oxide (FTO); the levels of VIS ΔT (up to 85%), switching kinetics (between 46 and 91 s), and CEs (up to 31 cm²/C) were maintained at a high degree of efficiency [103]. The second one considered alternative precursors and surfactants (namely, acetylated peroxotungstic acid APTA and polyethylene glycol hexadecyl ether Brij-56) for adapting the spray conditions down to 190 °C in single-step or to 100 °C with 250 °C post-annealing in multi-step, ultimately leading to 40% VIS ΔT and 40 cm²/C CE (Figure 9) [104]. All in all, these studies paved the way for conducting the spray processing of (WO₃) EC layers at a much lower energetical cost.

The latest works kept investigating this “cooler” trend by considering even lower temperatures for the spray coating processes [62,63,64,106,107,108,109,110,111,112,113,114]. Several teams focused on the RT deposition of “pre-formed” WO₃ nanoparticles: notably, P.S. Lee et al. discussed the hydrothermal synthesis and consecutive RT spray coating of tungsten molybdenum oxide nanoparticles dispersed in water (Figure 10A) [107], while A. Elezabbi et al. considered colloidally synthesized titanium-substituted molybdenum–tungsten oxide (MTWO) nanoparticles being spray-deposited at RT and then dried in air at 60 °C for 24 h [108], or Mo–W oxide nanoparticles mixed with PEDOT:PSS and sprayed at 60 °C (Figure 10B) [109]. The optical contrasts in VIS reached 43 (Lee) to 65–76% values (Elezzabi), with 10–20 s commutation kinetics (Lee & Elezzabi) and 36 (Lee) to 53 cm²/C CEs (Elezzabi); the materials were cycled between 100–300 (Elezzabi) and 2000 runs (Lee). C.S. Lee and co-workers developed an original kinetic spray apparatus called a nanoparticle deposition system (NPDS; see Section 2), allowing for the low-temperature (from RT to 200 °C) deposition of commercial or lab-synthesized WO₃ NPs, achieving maximal EC performances of 64% of VIS ΔT and 77 cm²/C of CE [64,65,66,110]. More recent publications reported the RT deposition of hybrid formulations of WO₃ and titanium carbide Ti₃C₂T_x (“MXene”), showing up to 58% of VIS ΔT, <10 s of t_col and t_ble, and 127 cm²/C of CE (Figure 10C) [113], and of core-shell TiO₂@WO_3−x colloidal nanocrystals for dual-band EC, sprayed at 100 °C into 500 nm thick films showing 67% of NIR shielding while maintaining 60% of VIS transparency in the “cool mode” and 89% of the screening of both VIS and NIR radiations in the “dark mode” [114].

3.2. Nickel Oxide NiO

NiO thin films are of high importance for many energy and optoelectronic applications, including semiconductors, sensors and photodetectors, solar cells, and, of course, electrochromic devices. NiO colors anodically by switching from transparent to brownish upon the oxidation of Ni(+II) into Ni(+III) and cation extraction. The presence of free hydroxyl ions and adsorbed water, as well as Ni vacancies inducing additional Ni(+III) content, also play crucial roles in the EC behavior and in the resulting optical and electrochemical properties. Typically used as a complementary counter electrode to WO₃, NiO is also praised as an individual EC material of a large dynamic contrast range, high coloration efficiency, good cyclic reversibility, and low material cost; however, its relatively poor stability and low charge capacity vs. WO₃ still somehow limit its widespread application in EC technology, and its coloration mechanism remains not fully understood due to the evolution of its chemical nature during the coloration/bleaching processes [43,68,115,116,117,118].

More than 35 articles on sprayed EC NiO have been referenced, with 26 (12) of them being published during the last 10 (5) years (

Table 2. Review of spray-coated NiO layers in terms of spray conditions, resulting thickness, and EC metrics; N.p. means “not provided”, O.D. means “optical density”, and T_asdep means “transmittance of the as-deposited sample”.

Reference	Spray Conditions (Tdep for Deposition Temperature, t.t. for Thermal Treatment)	Thickness (nm)	Tble (%)	Tcol (%)	ΔT (%)	tble (s)	tcol (s)	CE (cm2/C)	Cycling Runs
Arakaki [120]	Ni(NO3)2 in H2O Tdep = 220–400 °C	N.p.	50+	N.p.	35	N.p.	N.p.	30	100+
Patil [121]	NiCl2 in H2O Tdep = 350 °C	300	80	60	20	40	20	37	500
Mahmoud [122]	NiCl2 in H2O Tdep = 350 °C	50–200	N.p.	N.p.	2–23	N.p.	N.p.	N.p.	150
Wang [123]	Ni(NO3)2 in H2O Tdep = 200–400 °C	4500	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Abdel-Hady [124]	NiCl2 in H2O Tdep = 225–425 °C	50–400	50–80	5–50	30–60	13	5	44	150
Lin [125]	Ni(NO3)2 in ethanol Tdep = 200 °C with t.t. at 300 °C	500	20–30	70–80	48–51	9–10	7–12	N.p.	3000
Leinen [126]	Ni acetate in H2O Tdep = 350–450 °C	230–840	20–70 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Ismail [127]	NiCl2 in H2O Tdep = 280–400 °C	200–900	65–70 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Sharma [128]	NiCl2 in H2O Tdep = 350 °C	70–110	60–90 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Sanjeeviraja [129]	NiCl2 in H2O Tdep = 400 °C	1100–2700	45–95 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Benhaoua [130]	Ni(NO3)2 in H2O Tdep = 500 °C	170–200	45–85 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Gomaa [131]	NiCl2/Ni(NO3)2/Ni(OCOCH3)2 in H2O Tdep = 450 °C	270–300	35–50 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Chtouki [132]	NiCl2 in H2O Tdep = 350 °C with t.t. at 500 °C	390	75 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Ravi Dhas [133]	Ni(NO3)2 in H2O Tdep = 300 °C with t.t. at 400–550 °C	N.p.	N.p.	N.p.	6–29	0.3–1.1	0.5–1.8	8–29	10
Obaida [134]	Ni acetylacetonate in H2O Tdep = 400–500 °C	190–400	20–60 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	500
Krunks [135]	Ni acetate in H2O and isopropanol Tdep = 300–420 °C	20–430	20–85 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Ortiz [136]	Ni and Li acetates in H2O and methanol Tdep = 360–440 °C	300–1000	10–50 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Yahia [137]	NiCl2 + AgNO3 in H2O Tdep = 450 °C	400–420	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Naik [138]	Ni(NO3)2 + Co(NO3)2 in H2O Tdep = 400 °C	700	5–20 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Garcia-Lobato [139]	Ni and Co acetates in H2O and EtOH Tdep = 225 °C with t.t. at 300 °C	310–380	50–60	4–6	46–54	13–18	18–25	27–32	65
Ganesan [140]	NiCl2 and CoCl2 in H2O Tdep = 300 °C	350	55–85 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Ganesan [141]	NiCl2 and CuCl2 in H2O Tdep = 350 °C	N.p.	35–50 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Manickam [142]	NiCl2 and CuCl2 in H2O Tdep = 450 °C	N.p.	20–45 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Anslin Ferby [143]	Ni(NO3)2 and Mn(NO3)2 in H2O Tdep = 300 °C	N.p.	60–75	30–45	29–37	0.2–1	0.4–1.1	29–105	N.p.
Jbilou [144]	NiCl2 and InCl3 in H2O Tdep = 450 °C	120	60–70	22–47	24–38	N.p.	N.p.	31–94	N.p.
Mrabet [145]	NiCl2 and LaCl3 in H2O Tdep = 450 °C	N.p.	30–55 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Dillon [146]	Ni(NO3)2 and LiNO3 in H2O Tdep = 330 °C	N.p.	83	33	50	57	29	33	50
Sahay [147]	Ni(NO3)2 and Li acetate in H2O Tdep = 400 °C with t.t. at 500 °C	N.p.	78–85	55–75	10–25	N.p.	N.p.	3–35	N.p.
Cloots [148]	Ni(NO3)2 + LiNO3 + PEG in H2O Tdep = 350 °C	300	89–97	45–58	39–43	8–17	29–115	31–41	100
Cloots [149]	Ni(NO3)2 + PEG in H2O Tdep = 350 °C with t.t. at 350–600 °C	200–300	70–80	65–70	5–10	N.p.	N.p.	N.p.	10
Torres-Torres [150]	Ni acetate in PEG Tdep = 250 °C	110	75	30	45	N.p.	N.p.	N.p.	N.p.
Garcia-Lobato [151]	Ni(NO3)2 + MWCNTs in ethanol Tdep = 110 °C with t.t. at 350 °C	110–200	80–90	10–30	40–80	2–7	4–10	26–31	5000
Garcia-Lobato [152]	Ni(NO3)2 + MWCNTs in ethanol Tdep = 110 °C	20–245	80–95	35–40	45–55	N.p.	N.p.	25–30	5000
C.S. Lee [63]	NiO powder Tdep = RT-200 °C, NPDS	400	75–80 (Tasdep)	N.p.	55–65 (device)	N.p.	N.p.	N.p.	N.p.
Cai [153]	NiO NPs in ethanol Tdep not provided	330–1600	90	10	83	N.p.	N.p.	76	N.p.
Dini [154]	NiO NPs in alcohol Tdep = RT with t.t. at 450 °C	200–4000	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Dini [155]	NiO NPs in methanol Tdep = RT with t.t. at 450 °C	200–3500	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	100

). EC in NiO has been studied starting from the mid-1980s [119] and, specifically, by spray coating since 1995 and the pioneering study of J. Arakaki et al. [120], in which NiO layers were pyrolytically deposited at 400 °C and provided a 35% optical contrast in VIS and a 30 cm²/C CE. The impact of spray pyrolysis conditions has been screened in multiple following studies, notably in terms of the precursor nature (chlorides, nitrates, acetates), solvent (water, methanol, ethanol, isopropanol), and deposition and/or annealing temperature dependence (from 200 to 500 °C) [121,122,123,124,125,126,127,128,129,130,131,132,133,134,135]. Selected contributions from P. Patil [121], K. Abdel-Hady [122], S.-H. Lin [125], and C. Ravi Dhas [132] highlighted the maximal VIS ΔT between 20 and 60%, switching kinetics between 1 and 40 s, CE values of 8 to 44 cm²/C, and cyclability tests performed up to 3000 runs.

Doping strategies were attempted in several studies with Li [136], Ag [137], Co [138,139,140], Cu [141,142], Mn [143], In [144], and La [145] elements. In particular, Co doping led to interestingly high contrasts of 46%–54% [139], while increased CE values were reported with Mn (105 cm²/C) [143] and In dopants (94 cm²/C) [144]. Li doping was also shown to be successful for improving EC performances, notably by the teams of A. Dillon [146] and P. Sahay [147]. R. Cloots et al. also considered the Li doping of NiO layers [148], reporting a VIS ΔT enhanced from 39 to 44% and CE values from 31 to 41 cm²/C, while the switching kinetics were slightly slowed down (from 2 to 12 s of t_col, and 8 to 17 s of t_ble); films of 300 nm thickness were ultrasonically sprayed at 350 °C without further post-treatment, using precursor solutions of Ni(NO₃)₂ and 5% LiNO₃ mixed with a PEG surfactant in a 3:1 mass ratio with Ni salt (Figure 11). The same PEG-assisted USP protocol was used in another study to generate undoped NiO layers whose Ni/O stoichiometry could be straightforwardly predicted from optical and electrochemical (impedance analysis) measurements [149].

Other teams further explored the impact of incorporating additives in the sprayed liquid fluid, including D. Torres-Torres et al., who used Ni acetate as a precursor and PEG as a solvent and surfactant, obtaining 110 nm thick films at a deposition temperature of 250 °C, showing a VIS ΔT of 45% (75–30) [150]. Also, M. Garcia-Lobato et al. showed the very promising effect of adding CNTs into a precursor Ni(NO₃)₂/ethanol solution being electrostatically sprayed at 110 °C into 20 to 250 nm thick films, obtaining high contrasts (up to 80%) and good CE values (up to 30 cm²/C) after a quick thermal annealing of 1 min at 350 °C (Figure 12) [151,152].

Similar to WO₃ formulations, the RT deposition of “pre-formed” NiO NPs was also considered quite recently for the “low energetical cost” spray fabrication of corresponding EC layers [63,153,154,155]. C.S. Lee et al. used their acknowledged kinetic spray approach (previously described) to design 400 nm thick films from different-sized NiO powders, showing diameters from 1–2 µm down to 20 nm; EC devices made by assembling these NiO counter-electrode materials with WO₃ counterparts presented 56%–65% VIS ΔT [63]. G. Cai et al. (Figure 13) considered an electrostatic spray deposition process with solvothermally synthesized NiO NPs dispersed in ethanol, leading to 330 to 1600 nm thick films, allowing for an extremely high optical contrast (more than 83%) and CE values (up to 76 cm²/C) [153].

3.3. Vanadium Pentoxide V₂O₅

The vanadium element exists in several oxidation states, namely, V(+III), V(+IV), and V(+V), involving multiple stoichiometries and phases that are often difficult to control. The VO₂ formulation is an acknowledged thermochromic material of great interest, as it exhibits a transition temperature at 68 °C (not too far from RT) combined with a conversion from a monoclinic to rutile crystal structure, and the V₂O₃ phase presents a phase transition coupled with a magnetic disordering varying with the temperature. The V₂O₅ stoichiometry, the most stable one, is well known as a performant battery material thanks to its layered structure, and also as a multicolor material exhibiting both anodic (coloring from green to orange–yellow) and cathodic EC (coloring into blue) as a result of the occurrence of various oxidation states associated with different colors; however, it generally suffers from low conductivity and instability during cycling [156,157,158,159,160].

Sprayed films of EC V₂O₅ have been reported in about 30 articles, among which 20 (13) were published over the last 10 (5) years (

Table 3. Review of spray-coated V₂O₅ layers in terms of the spray conditions, resulting thickness, and EC metrics; N.p. means “not provided”, O.D. means “optical density”, and T_asdep means “transmittance of the as-deposited sample”.

Reference	Spray Conditions (Tdep for Deposition Temperature, t.t. for Thermal Treatment)	Thickness (nm)	Tble (%)	Tcol (%)	ΔT (%)	tble (s)	tcol (s)	CE (cm2/C)	Cycling Runs
Bouzidi [161]	VCl3 in H2O Tdep = 250 °C	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mathieu [162]	VCl3 in H2O Tdep = 150 °C	N.p.	40 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Patil [163]	VCl3 in methanol Tdep = 300 °C with t.t. at 400 °C	120	49	32	17	N.p.	N.p.	13	N.p.
Patil [164]	VCl3 and MoO3 in methanol Tdep = 400 °C + t.t. at 400 °C	130–370	45–61	23–44	13–24	9–11	9–11	15–35	N.p.
Patil [165]	VCl3 in methanol Tdep = 300–500 °C	N.p.	36–61	23–44	8–17	N.p.	N.p.	10–15	N.p.
Patil [166]	VCl3 and ammonium tungstate Tdep = 400 °C	300–370	48–61	27–44	7–22	10–20	9–23	15–49	1000
Kim [167]	V tri-isopropoxide in ethanol Tdep = 200 °C with t.t. at 200 °C	N.p.	75 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Wei [168]	V2O5 NPs in H2O Tdep = RT with drying at 110 °C	30–120	70–85 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mohammad [169]	VCl3 in H2O Tdep = 350 °C	N.p.	75–80 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Ramamurthi [170]	NH4VO3 in H2O Tdep = 300–400 °C	N.p.	62–78 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Eshghi [171]	VCl3 in H2O Tdep not provided with t.t. at 200 °C	600–2000	35–50 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mouratis [172]	NH4VO3 in H2O Tdep = 250 °C with t.t. at 400 °C	1000	18–26	6–8	10–20	16–34	10–35	N.p.	N.p.
Mouratis [173]	NH4VO3 in H2O Tdep = 250 °C with t.t. at 400 °C	N.p.	40–43	31–32	8–12	15	10	4–6	N.p.
Chen [174]	V2O5 powders Tdep = RT with drying at 70 °C	690–800	80–36	33–39	4–42	4–9	1–7	8–83	1000
Mahmoud [175]	V(NO3)5 in H2O Tdep = 350 °C with t.t. at 400–550 °C	120–140	45–70 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Unalan [176]	VO(acac)2 and V2O5 in methanol Tdep = 100 °C with t.t. at 450–550 °C	60–80	75–94	64–94	0–18	N.p.	N.p.	N.p.	25 (1000 in devices)
Suchea [177]	NH4VO3 in H2O Tdep = 250 °C with t.t. at 400 °C	100–800	12–65	1–60	5–11	2–34	2–35	N.p.	N.p.
Mousavi [178]	VCl3 and (NH2)2CS in H2O Tdep = 400–450 °C	110–270	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Abyazisani [179]	VCl3 and NH4F in H2O Tdep = 450 °C	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Kovendhan [180]	VCl3 and LiCl in H2O Tdep = 450 °C	50–1400	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Tabatabai [181]	VCl3 and InCl3 in H2O Tdep = 430 °C	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mrigal [182]	VCl3 and MoCl6 in H2O Tdep = 500 °C with t.t. at 400 °C	N.p.	50–77	10–43	29–67	N.p.	N.p.	42–80	N.p.
Sreelatha [183]	NH4VO3 and Triton-X 100 surfactant in H2O; Tdep = 300 °C with t.t. at 350–450 °C	250	15–30 (Tasdep)	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Hsiao [184]	VCl3 and PEDOT:PSS Tdep = RT with t.t. at 120 °C	N.p.	68	14–42	20–46	3–7	4–8	39–89	10000
Wang [185]	VCl3 and 1,4-benzendicarboxylic acid and HTAB in H2O/ethanol Tdep = 50 °C with t.t. at 50 °C	N.p.	29–73	22–49	20–55	1–2	1–3	20–109	2000
Kim [186]	V2CTx in H2O Tdep = 70 °C with t.t. at 350 °C	600–1500	49–59	5–44	5–54	5–25	7–21	5–91	200

). Initiatory studies go back to the early 2000s with a.o. A. Bouzidi et al., who pyrolytically sprayed V₂O₅ layers at 250 °C from vanadium trichloride VCl₃ aqueous solutions [161], and C. Mathieu et al., who could lower the deposition temperature to 150 °C from similar precursor fluids [162]. P. Patil and co-workers followed with new protocols [163,164,165,166] mainly involving VCl₃ methanolic solutions sprayed between 300 and 450 °C (potentially with further annealing at 400 °C for 2 h), obtaining ~120 nm thick layers with up to 20% (50–30) of VIS ΔT between clear (oxidized) and dark (reduced) states and a CE of 10 to 15 cm²/C [163]. The same group then developed ~300 nm thick mixed layers of V₂O₅ and MoO₃ [164] or WO₃ [166] being (pulsed) spray pyrolyzed at 400 °C from VCl₃ solutions, leading to VIS contrasts of 7 to 25%, switching times of ~10 s, and CE values of 15 to 35 cm²/C (V₂O₅-MoO₃) or 49 cm²/C (V₂O₅-WO₃), being stable up to 1000 cycles (Figure 14). Other teams considered similar strategies [167,168,169,170,171,172,173,174,175,176,177] as well as doping approaches with aliovalent elements such as S [178], F [179], Li [180], In [181], and Mo [182]; in the latter case, the incorporation of 4% at. MoCl₆ powder into the VCl₃ aqueous precursor solution led to noticeably high values of VIS ΔT (67%) and CE (80 cm²/C) in comparison with undoped and other doped (2, 6% at.) formulations [182]. More recently, V₂O₅ formulations hybridized with carbon-based materials led to interesting EC properties [183,184,185,186], in some cases being significantly enhanced with respect to the state-of-the-art: these notably involved metal-organic-frameworks (MOFs) containing vanadium (V) sprayed at RT to 50 °C, resulting in highly functional layers with a VIS ΔT of 46 to 55%, a mean response time of 1 to 3 s, a CE of 89 to 109 cm²/C, and 81 to 96% of optical retention after several thousands of cycles [184,185]. Mixes of V₂O₅ with MXenes, especially V₂C₂T_x formulations, led to even more increased figures of merits, with the VIS ΔT improved from 5 (pristine V₂O₅) to 54% (V₂O₅-MXene), the switching times reduced from 21–25 to 5–7 s, and the CE values raised from 5 to 91 cm²/C (Figure 15) [186].

3.4. Other Oxides

Other EC oxides were also spray-coated into active EC films over the years, with about 30 related studies reported below (Table 4).

As cathodic EC oxides, one can cite:

• MoO₃, with works [187,188,189,190] reporting deposition protocols at high temperatures (250 to 400 °C), starting from precursor solutions constituting MoCl₅ salts or MoO₃ pre-formed powders and resulting in 100 to 550 nm thick layers with up to 23% of VIS optical contrast and 37 cm²/C of CE; further doping with W led to CE values improved up to 168 cm²/C [189].
• TiO₂, obtained as 100 to 800 nm thick layers from titanium alkoxide precursors at spray temperatures of 200 to 680 °C and showing VIS ΔT values as high as 61%, a switching time below 10 s, and CEs of 18 to 101 cm²/C [191,192,193,194,195,196]; selected articles investigated the further doping of TiO₂ with W [194], Nb [195], and Ni [196], the latter resulting in extremely high CE values of 474 cm²/C for 46% VIS ΔT. TiO₂ layers can also be used as passive counter electrodes in assembled EC devices [191].
• Nb₂O₅, starting from NbCl₅ dissolved in water–ethanol mixes or from Nb₂O₅ pre-formed powders combined with tartaric acid, sprayed at 300–500 °C into 400–500 nm thick layers of 15%–40% VIS ΔT, t_col and t_ble values below 10 s, and 4–26 cm²/C CEs [197,198,199].
• Bi₂O₃, spray-processed at 500 °C from bismuth carbonate aqueous solutions, into 125–450 nm thick films presenting 71% optical contrast and 12–18 cm²/C [200].
• SnO₂, for which P. Patil et al. explored various substrate temperatures between 450 and 500 °C for the pyrolytic transformation of tri-n-butyl tin acetate solutions into layers of 400–500 nm thickness [201]; a modest EC performance of 13% VIS optical contrast and 14 cm²/C of CE could be highlighted. In a later study, C.S. Lee and co-workers used pre-formed NPs of Sb-doped SnO₂ (ATO) for NPDS-based spray coating at RT, leading to a ~35% VIS transmittance change and a CE of 16 cm²/C [61].

More recently, D. Milliron, A. Maho and co-workers considered indium-tin oxide (ITO) nanocrystalline formulations for the spray deposition of VIS-transparent but NIR-modulating EC thin films. Owing to their plasmonic behavior (LSPR) and low carrier concentration (~10²⁰–10²¹ cm⁻³), ITO NCs strongly absorb NIR light between 1500 and 2000 nm; from there, their reversible electrochemical doping, when submitted to an external bias, achieves strong blue shifts in absorption intensity and frequency. With such electrochemical charging being capacitive and not faradic/intercalative, films of ITO NCs therefore exhibit unique NIR EC performances, including extremely fast switching kinetics, high CE, and extended cycling stability. Two articles accordingly reported the spray coating of such inks of ITO NCs synthesized either from solvothermal processing or from ligand-tuned Schlenk line colloidal methods. In the first case [202], ITO dispersions were straightforwardly deposited from low-toxic isopropanol media according to a RT procedure implying no harsh chemical or thermal post-treatment—only a mild drying step at 100 °C for 10 min; in situ spectroelectrochemical measurements completed on the films (of 70–415 nm thickness) showed appreciable NIR contrasts (up to 40%) upon electrochemical charge/discharge, together with a >80% VIS transmittance (Figure 16). The other study [203] introduced an original method for producing aqueous ITO inks by refunctionalizing the NCs’ surface, previously stripped of their native hydrophobic ligands, with a hydrophilic poly(acrylic acid) polymer featuring a low density of methoxy-terminated poly(ethylene oxide) grafts (PAA-mPEO₄). The polymer-wrapped ITO NC thin films did show the fast, stable, and reversible EC modulation of NIR (up to 39%), without the need to remove the polymer after deposition, as long as a wrapping density of ~20% by mass was not overpassed (Figure 17).

In terms of anodic EC oxides, IrO₂ [204,205,206] and Mn₃O₄ [207] were also sporadically considered for the spray coating of EC materials, using aqueous solutions of Ir (IrCl₃) and Mn (MnCl₂) chlorides as precursors and pyrolytic deposition conditions (250 to 520 °C for IrO₂, 350 °C for Mn₃O₄). Moderate optical contrasts—20%–30%—and CE values—13–26 cm²/C—were obtained with films of various thicknesses—from 100 nm to several µm. Other authors also highlighted the interest in Co₃O₄ formulations [208,209,210,211,212], responding anodically but also cathodically as multicolor EC, reporting the spray pyrolysis deposition of corresponding layers of various thicknesses (500–1000 nm) at 300–400 °C from Co chloride or nitrate solutions and obtaining VIS ΔT values as high as 38%, switching times below 5 s, and CE values up to 47 cm²/C.

EC devices can also integrate passive counter electrode materials instead of actively coloring compounds—such as NiO or V₂O₅ in complementarity with WO₃, as seen above. To this end, cerium oxide CeO₂ has been reported as an attractive material allowing for high optical transparency in the VIS region together with an excellent ability to insert/extract large charge densities and to remain fully transparent regardless of its cationic content (H⁺, Li⁺…); however, its low chemical stability in lithium-based electrolytes remains a critical issue for its further large-scale development in EC systems [213,214,215,216]. The spray deposition of CeO₂ layers was considered in a few publications over the last 20 years, all focusing on the pyrolytic decomposition at 300 to 500 °C of chloride CeCl₃ or nitrate Ce(NO₃)₂ precursor salts being dissolved in water or in methanol. The first works by B. El Idrissi [217] and P. Patil [218] reported films of 300 to 800 nm thicknesses leading to elevated VIS transparency (generally over 80%) being maintained over several hundreds to thousands of charging/discharging cycles, with ion storage capacities of 20 C/cm². Consecutive studies further explored the effect of incorporating surfactants or doping with V [219,220], Zr [221], and Ti [222] elements; notably, the use of the last two dopants showed noticeable improvements in the EC performance when counter-electrodes of CeO₂-ZrO₂ or CeO₂-TiO₂ (respectively) were combined with WO₃ working electrodes, enhancing the CE values of corresponding devices from 42–47 (CeO₂ only) to 107 (CeO₂-ZrO₂) and even 231 cm²/C (CeO₂-TiO₂).

Table 4. Review of spray-coated “other” metal oxide layers in terms of the deposition conditions, resulting thickness, and EC metrics; N.p. means “not provided”.

Reference	Spray Conditions (Tdep for Deposition Temperature, t.t. for Thermal Treatment)	Thickness (nm)	Tble (%)	Tcol (%)	ΔT (%)	tble (s)	tcol (s)	CE (cm2/C)	Cycling Runs
MoO3
Patil [187]	MoO3 powder in NH4OH Tdep = 350 °C	1100	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Patil [188]	MoO3 powder in NH4OH and TiAcAc; Tdep = 350 °C	130–540	47	31	16	N.p.	N.p.	19–37	1000
Sahay [189]	MoCl5 + ammonium tungstate in methanol; Tdep = 390 °C	N.p.	5–25	2–17	3–8	1–5	1–25+	35–168	100
Unalan [190]	MoCl5 in H2O Tdep = 300 °C	100–200	42–49	21–34	15–23	N.p.	N.p.	16–32	20
TiO2
Patil [191]	TiAcAc in methanol Tdep = 350–450 °C	200–400	65–85	50–80	5–10	3–4	4–5	3–4	N.p.
Sivakumar [192]	Titanium isopropoxide in acetylacetone Tdep = 200–350 °C with t.t. at 450 °C	800–1300	59–88	27–37	32–61	2	3	22–101	100
Eshghi [193]	Titanium isopropoxide in ethanol and acetylacetone Tdep = 200 °C with t.t. at 450 °C	300	90–95	45–75	17–53	< 1	1–2	18–92	N.p.
Zelazowska [194]	TiAcAc (and WAcAc) in dichloromethane and acetylacetone; Tdep = 680 °C	110	70	15–30	40–55	N.p.	N.p.	N.p.	N.p.
Zhao [195]	Ti(C4H9O)4 + NbCl5 in acetylacetone and ethanol; Tdep = 320–480 °C	100–400	60–76	30–69	4–26	8–21	35–59	N.p.	1000
Sivakumar [196]	Titanium isopropoxide and Ni acetate in ethanol and glycerol Tdep = 200 °C with t.t. at 450 °C	350–600	68–89	22–61	28–46	2	2–3	123–474	1000
Nb2O5
Patil [197]	Nb2O5 powder in tartaric acid Tdep = 300–400 °C	480	78	62	16	5	4	13	1000
Patil [198]	Nb2O5 powder in tartaric acid Tdep = 400 °C with t.t. at 500 °C	450–480	77–87	62–78	8–15	4–5	3–4	10–13	N.p.
Romero [199]	NbCl5 in H2O and ethanol and acetic acid Tdep = 350–500 °C with t.t. at 500–900 °C	100–450	65	25–60	5–40	6	10	4–26	20
Bi2O3
Bao [200]	Bi carbonate in H2O Tdep = 500 °C with t.t. at 550 °C	125–450	81	10	71	N.p.	N.p.	12–18	4
SnO2
Patil [201]	(C4H9)3Sn(OOCCH3) in methanol Tdep = 450–500 °C	400–500	78–85	68–83	2–13	<1	<1	8–14	N.p.
C.S. Lee [61]	Sb-doped SnO2 NPs Tdep = RT (NPDS)	800	72–78	37–54	24–43	~1	~5	15	10
ITO
Maho [202]	ITO NPs in isopropanol Tdep = RT with drying at 100 °C	70–415	53–100 (in NIR)	42–90 (in NIR)	10–40 (in NIR)	N.p.	N.p.	N.p.	10
Maho [203]	ITO NPs in H2O Tdep = RT	130	59–75 (in NIR)	36–42 (in NIR)	17–39 (in NIR)	N.p.	N.p.	535–802	N.p.
IrO2
Patil [204]	IrCl3 in H2O Tdep = 250–400 °C	100–1200	25–60	20–52	2–30	2	5–6	10–26	N.p.
Patil [205]	IrCl3 in H2O Tdep = 250 °C	1200–3900	37–47	27–38	3–20	2–3	2–5	10–59	N.p.
Hassanien [206]	IrCl3 in H2O Tdep = 250–520 °C with t.t. at 600 °C	100–350	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mn3O4
Sivakumar [207]	MnCl2 in H2O Tdep = 350 °C	940–1700	N.p.	N.p.	16–23	N.p.	N.p.	4–18	100
Co3O4
Patil [208]	CoCl2 in H2O Tdep = 300 °C	400	25	10	15	2	4	N.p.	1000
Sivakumar [209]	Co(NO3)2 in H2O Tdep = 300–400 °C	700–1000	14–95	10–66	4–29	1–2	1–3	1–12	N.p.
Sivakumar [210]	Co(NO3)2·6H2O and Mn(CH3COO)2 in H2O Tdep = 400 °C	685–850	19–61	9–26	6–35	1–4	2–3	10–29	N.p.
Sivakumar [211]	Co(NO3)2 and Cr(NO3)2 in H2O Tdep = 400 °C	680–785	29–45	7–32	8–38	2–4	2–3	3–47	N.p.
El Bachiri [212]	CoCl2 in H2O Tdep = 350 °C	500	40	10	31	2	4	19	N.p.
CeO2
El Idrissi [217]	CeCl3 in H2O Tdep = 300–500 °C + t.t. at 300 °C	300–800	80 (Tasdep)	N/A	N/A	N/A	N/A	N/A	100
Patil [218]	Ce(NO3)2 in methanol Tdep = 300–450 °C	500–790	85 (Tasdep)	N/A	N/A	N/A	N/A	N/A	600
Patil [219]	Ce(NO3)2 + VCl3 in methanol Tdep not provided	N.p.	68	62	6	N.p.	N.p.	N.p.	N.p.
El Habib [220]	CeCl3 + VCl3 in H2O Tdep = 450 °C	N.p.	50–65 (Tasdep)	N/A	<2	N/A	N/A	N/A	10
Patil [221]	Ce(NO3)2·6H2O and Zr(NO3)2 in methanol Tdep = 400 °C	540–600	86 (Tasdep)	N/A	N/A	<1	N/A	N/A	10000
Patil [222]	Ce(NO3)3 and titanium isopropoxide in methanol; Tdep = 400 °C	N.p.	75 (Tasdep)	72	3	N/A	N/A	N/A	30000

Table 4. Review of spray-coated “other” metal oxide layers in terms of the deposition conditions, resulting thickness, and EC metrics; N.p. means “not provided”.

Reference	Spray Conditions (Tdep for Deposition Temperature, t.t. for Thermal Treatment)	Thickness (nm)	Tble (%)	Tcol (%)	ΔT (%)	tble (s)	tcol (s)	CE (cm2/C)	Cycling Runs
MoO3
Patil [187]	MoO3 powder in NH4OH Tdep = 350 °C	1100	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Patil [188]	MoO3 powder in NH4OH and TiAcAc; Tdep = 350 °C	130–540	47	31	16	N.p.	N.p.	19–37	1000
Sahay [189]	MoCl5 + ammonium tungstate in methanol; Tdep = 390 °C	N.p.	5–25	2–17	3–8	1–5	1–25+	35–168	100
Unalan [190]	MoCl5 in H2O Tdep = 300 °C	100–200	42–49	21–34	15–23	N.p.	N.p.	16–32	20
TiO2
Patil [191]	TiAcAc in methanol Tdep = 350–450 °C	200–400	65–85	50–80	5–10	3–4	4–5	3–4	N.p.
Sivakumar [192]	Titanium isopropoxide in acetylacetone Tdep = 200–350 °C with t.t. at 450 °C	800–1300	59–88	27–37	32–61	2	3	22–101	100
Eshghi [193]	Titanium isopropoxide in ethanol and acetylacetone Tdep = 200 °C with t.t. at 450 °C	300	90–95	45–75	17–53	< 1	1–2	18–92	N.p.
Zelazowska [194]	TiAcAc (and WAcAc) in dichloromethane and acetylacetone; Tdep = 680 °C	110	70	15–30	40–55	N.p.	N.p.	N.p.	N.p.
Zhao [195]	Ti(C4H9O)4 + NbCl5 in acetylacetone and ethanol; Tdep = 320–480 °C	100–400	60–76	30–69	4–26	8–21	35–59	N.p.	1000
Sivakumar [196]	Titanium isopropoxide and Ni acetate in ethanol and glycerol Tdep = 200 °C with t.t. at 450 °C	350–600	68–89	22–61	28–46	2	2–3	123–474	1000
Nb2O5
Patil [197]	Nb2O5 powder in tartaric acid Tdep = 300–400 °C	480	78	62	16	5	4	13	1000
Patil [198]	Nb2O5 powder in tartaric acid Tdep = 400 °C with t.t. at 500 °C	450–480	77–87	62–78	8–15	4–5	3–4	10–13	N.p.
Romero [199]	NbCl5 in H2O and ethanol and acetic acid Tdep = 350–500 °C with t.t. at 500–900 °C	100–450	65	25–60	5–40	6	10	4–26	20
Bi2O3
Bao [200]	Bi carbonate in H2O Tdep = 500 °C with t.t. at 550 °C	125–450	81	10	71	N.p.	N.p.	12–18	4
SnO2
Patil [201]	(C4H9)3Sn(OOCCH3) in methanol Tdep = 450–500 °C	400–500	78–85	68–83	2–13	<1	<1	8–14	N.p.
C.S. Lee [61]	Sb-doped SnO2 NPs Tdep = RT (NPDS)	800	72–78	37–54	24–43	~1	~5	15	10
ITO
Maho [202]	ITO NPs in isopropanol Tdep = RT with drying at 100 °C	70–415	53–100 (in NIR)	42–90 (in NIR)	10–40 (in NIR)	N.p.	N.p.	N.p.	10
Maho [203]	ITO NPs in H2O Tdep = RT	130	59–75 (in NIR)	36–42 (in NIR)	17–39 (in NIR)	N.p.	N.p.	535–802	N.p.
IrO2
Patil [204]	IrCl3 in H2O Tdep = 250–400 °C	100–1200	25–60	20–52	2–30	2	5–6	10–26	N.p.
Patil [205]	IrCl3 in H2O Tdep = 250 °C	1200–3900	37–47	27–38	3–20	2–3	2–5	10–59	N.p.
Hassanien [206]	IrCl3 in H2O Tdep = 250–520 °C with t.t. at 600 °C	100–350	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.	N.p.
Mn3O4
Sivakumar [207]	MnCl2 in H2O Tdep = 350 °C	940–1700	N.p.	N.p.	16–23	N.p.	N.p.	4–18	100
Co3O4
Patil [208]	CoCl2 in H2O Tdep = 300 °C	400	25	10	15	2	4	N.p.	1000
Sivakumar [209]	Co(NO3)2 in H2O Tdep = 300–400 °C	700–1000	14–95	10–66	4–29	1–2	1–3	1–12	N.p.
Sivakumar [210]	Co(NO3)2·6H2O and Mn(CH3COO)2 in H2O Tdep = 400 °C	685–850	19–61	9–26	6–35	1–4	2–3	10–29	N.p.
Sivakumar [211]	Co(NO3)2 and Cr(NO3)2 in H2O Tdep = 400 °C	680–785	29–45	7–32	8–38	2–4	2–3	3–47	N.p.
El Bachiri [212]	CoCl2 in H2O Tdep = 350 °C	500	40	10	31	2	4	19	N.p.
CeO2
El Idrissi [217]	CeCl3 in H2O Tdep = 300–500 °C + t.t. at 300 °C	300–800	80 (Tasdep)	N/A	N/A	N/A	N/A	N/A	100
Patil [218]	Ce(NO3)2 in methanol Tdep = 300–450 °C	500–790	85 (Tasdep)	N/A	N/A	N/A	N/A	N/A	600
Patil [219]	Ce(NO3)2 + VCl3 in methanol Tdep not provided	N.p.	68	62	6	N.p.	N.p.	N.p.	N.p.
El Habib [220]	CeCl3 + VCl3 in H2O Tdep = 450 °C	N.p.	50–65 (Tasdep)	N/A	<2	N/A	N/A	N/A	10
Patil [221]	Ce(NO3)2·6H2O and Zr(NO3)2 in methanol Tdep = 400 °C	540–600	86 (Tasdep)	N/A	N/A	<1	N/A	N/A	10000
Patil [222]	Ce(NO3)3 and titanium isopropoxide in methanol; Tdep = 400 °C	N.p.	75 (Tasdep)	72	3	N/A	N/A	N/A	30000

The processing of suitable electrolyte layers to be incorporated in EC device architectures also constitutes a critical and important challenge, with different types of relevant formulations of organic, inorganic, and organic–inorganic hybrid natures being conceivable. Most of them are (gel) polymer-based and are typically wet-deposited onto one of the two active electrodes (working or counter), before being compressed with the other and laminated together. On the other hand, inorganic electrolyte layers such as transparent lithium phosphorous oxynitride (LiPON) and lithium silicate formulations (aluminosilicates, borosilicates, phosphosilicates) can also be used, typically being intercalated between the working and counter electrode layers in layer-by-layer deposition processes [26,223,224,225]. The use of metal oxide-based compounds as electrolyte species and of spray coating as a processing methodology in this context, both for (gel) lamination and consecutive film deposition approaches, is only but scarcely reported, with, notably, the patent of R. Cloots et al. [226] related to the 350 °C USP of lithium aluminosilicate LAS Li₂O–Al₂O₃–SiO₂ layers from an aqueous precursor solution mixed with a PEG surfactant. Nevertheless, other metal oxide formulations such as Li₇La₃Zr₂O₁₂ [227], BaCe_0.72Zr_0.2Y_0.2O₃ [228], or Mg_0.5Ce_0.2Zr_1.8(PO₄)₃ [229] have been successfully spray-processed as electrolyte layers in batteries and fuel cells devices and could certainly be exploited further in EC counterparts considering appropriate processing adaptations (to reach a suitable optical transparency).

Finally, the latest literature clearly shows that using spray-coating approaches for the design of advanced, highly-functional EC materials and devices keeps developing, also expanding to emerging formulations beyond the sole category of metal-oxide-based compounds, with micro- and mesoporous metal–organic frameworks (MOFs) and other organometallic materials, organic–halide hybrid perovskites, composite materials, and 2D MXenes. A few very recent studies from 2023 are worth a special mention here—notably, the work of A. Mazel et al. [230] on Zn-based highly oriented surface-anchored metal–organic frameworks (SurMOFs) as spray-coated EC thin films being reversibly switched in a few seconds from orange to dark blue by electrochemical reduction, leading to remarkable 105 cm²/C CE values; in addition, interestingly, the color of the reduced state was shown to be easily changed from dark blue to cyan by modifying the electrolyte composition from LiTFSI-EMIMTFSI to pure EMIMTFSI. Also recently, P.S. Lee and co-workers did consider the spray processing of Fe-based metallo-supramolecular polymers (MSP-Fe) into uniform films exhibiting an excellent EC performance and energy storage property simultaneously, with reported ultrahigh values of CE (1104 cm²/C), fast switching speeds (<2 s), and a large optical contrast (over 72%); large-scale semi-solid state EC systems of a 225 cm² area could be successfully manufactured by including MSP-Fe films, achieving uniform, fast, and reversible color variations all across the device [231]. Finally, spray coating was originally used by M. Gusatti et al. to prepare novel all-solid-state EC devices free of an ITO transparent conductive electrode, a WO₃ active EC layer, and Li⁺ or any other liquid/gel electrolytes, obtained by spray-depositing a PEDOT:PSS solution on a tris-(8-hydroxyquinoline) aluminum(III) (Alq₃) film previously sprayed on a pre-formed Al-coated glass substrate [232]. The fabricated devices demonstrated a reversible and homogeneous color change between light-blue (off) and dark-blue (on) states, with CE values of 175 cm²/C and stability demonstrated over a minimum of 150 cycles.

4. Conclusions and Outcomes

From this review of the state of the art, it appears clear that the obtention of EC layers and devices with excellent optical, charging, and transport properties relies on the robust and efficient processing of suitable metal oxide layers, among which WO₃, NiO, and V₂O₅ are the three main formulations of interest. As screened in this paper, they all show high levels of functionality and efficiency, both optically and electrochemically, leading to qualitative and quantitative figures of merit such as contrasts, kinetics, coloration efficiency, and cycling stability. Nevertheless, important shortcomings remain to be dealt with, notably in terms of precursors and production costs and of more favorable transition schemes from the lab to the industrial scale.

To this end, the proper selection of the fabrication process is one of the most critical issues: clearly, the spray-coating approach can be acknowledged as a versatile and suitable methodology for obtaining extremely homogeneous, adherent, and robust EC metal oxide films of high functionality and very good performances, offering important advantages such as low operational costs, simple equipment facilities, atmospheric operation, reproducibility, rapid deposition speeds (over 100 nm/s), and a large surface coverage area as well as a high-throughput capacity. As a wet-based technique, it surely allows for an importantly lowered energy consumption compared to the vacuum processes that are mostly used in the current industrial production of EC materials, notably for glazing applications (smart windows). In comparison with other lab-scale wet approaches such as spin or dip coating, it is also one of the most promising film deposition techniques, if not the only one, that can be used first at the laboratory scale for quick and simple trials on small-area substrates but that can in the meantime be considered, and used, for larger-scale developments, with protocols being easily transferable towards industrial production levels and generation of meter-long devices.

However, the production of efficient (metal oxides) EC layers does not have to be based only on strategies of materials design (here, by spray coating) or on characterizations that focus on passing the performance evaluation through standard laboratory spectroscopic and electrochemical tools. One must also consider how the processing conditions are directly and indirectly related to the materials’ uniformity, stability under device operating and aging conditions, scalability, as well as recyclability. In this context, spray deposition allows almost uniquely for the progressive adaptation and optimization of all coating parameters, with multiple different micro/nanostructures, crystallinities, surface states, and, ultimately, optical and electronic/electrochemical properties being accessible, directly depending on the fine tuning of spray protocols. As could be established from the present literature screening, these include the properties of the liquid solution feed, especially the nature of precursors (especially those of a corrosive and/or toxic nature, i.e., transitioning from chlorides or nitrate salts to alkoxy derivates such as acetates) and solvents (from hydrophobic alkanes, toluene and chloroform to esters, alcohols, and water), the viscosity, the load, etc. Also the thermal and atmospheric (gas nature and concentration, use of low-vacuum conditions) conditions of layer growth and post-treatment are of high importance, as well as the spray nozzle design, the positioning and movement with respect to the surface, which can lead to roll-to-roll fabrication capacities, the adaptation to various natures and dimensions of substrates, etc.

Still, the specific correlation between how the fundamental properties of the EC metal oxides (and other) materials are being influenced by the spray conditions/parameters and how they will specifically be addressed towards scale-up transfer remains quite rarely considered in most of the reviewed articles, with only a few articles briefly discussing it either as a rapid proof-of-concept final demonstration result or as a perspective work [45,111,113,186,231,233]. In our opinion, this is one of the most important challenges to be addressed in upcoming developments, as such aspects illustrate the critical importance and significance of understanding EC systems from the nano- and microscale in the constitutive individual materials and interfaces, to the process(ability) of assembled devices at a (very) large scale. The latter will be strongly dependent of the synthetic conditions, the processing methods and parameters, the compatibility between components, the functional and economic viability and durability, and the strength towards environmental conditions. These are worth being clearly established and developed in future R&D works, gathering the EC community of chemists, physicists, material scientists, and engineers upon building new foundations for a more robust and efficient large-scale fabrication of EC materials and devices at both academic and industrial levels.