Ion trapping in electrodes upon long-term cycling is found to be one of the main reasons for performance degradation in electrochromic devices. Galvanostatic and potentiostatic post-treatments can rejuvenate degraded electrochromic layers. However, these procedures require high oxidation potentials, which are neither safe for the electrode-electrolyte system nor compatible with the operation of a full device. In the present paper, we report that degraded electrochromic oxides can be rejuvenated by a photo-electrochemical synergistically induced ion detrapping procedure. The UV light-induced photocurrent assists ion detrapping and limits the applied potential to the safe range used for electrochromic switching. This approach has been demonstrated to be effective for several cathodic electrochromic oxides and can be directly implemented in a full device. Our findings provide new vistas for efforts to expand the lifespan of electrochromic devices and other ion intercalation-based devices.

Electrochromic oxides have tremendous potential applications in smart windows, displays, and camouflage due to their capability for selective modulation of visible and near-infrared optical spectra. Although these applications are dependent on the optical performance, the origin of the optical absorption in electrochromic oxides is not clear. Here, we demonstrate that the electrochromism of all amorphous cathodic electrochromic oxides can be described by a combination of polaron and bipolaron hopping. Based on the valences of the metallic constituents, we model experimental optical absorption spectra by polaron theory and assign two prominent absorption peaks to polaronic and bipolaronic charge transfer excitations. However, in the special case of V2O5, three peaks were necessary to fit the optical spectra. The activation energies of polaronic and bipolaronic hopping were remarkably similar for all the cathodic oxides studied. Within the framework of polaron absorption, V2O5 would be categorized as a cathodic oxide, rather than as a mixed anodic/cathodic material as in the conventional picture. We emphasize that our findings here not only offer a profound understanding of all amorphous cathodic electrochromic oxides but also pave the way for exploring electrochromic oxides with dual-band modulations.

We explore a novel method to obtain scattering and absorption (S&A) coefficients of inhomogeneous materials from measurements of regular transmittance and specular reflectance. We use a Spectral Projected Gradient Method (SPGM) to invert experimental spectra. The SPGM method requires an initial approximation to the S&A coefficients, which is needed to ensure convergence to an optimized solution. We discuss problems associated with multiple solutions and conditions for the obtained optimal solution to be a good approximation to the physical one. We present results for TiO2/polyvinylpyrrolidone (PVP)-water, plasmonic Au/PVP-water and ferromagnetic Fe3O4/PVP-water materials. The obtained S&A coefficients indicate that these materials are dilute suspensions of dense particle clusters in the dependent scattering regime. We argue that the SPGM gives physically realistic solutions for this class of materials. The SPGM has the advantages that it is much less expensive in computational terms and easier to implement than previously used simulated annealing methods.

Ni-oxide-based thin films, prepared by sputtering, are investigated by electrochemical impedance spectroscopy in a KOH electrolyte. The films are electrochromic, and an increase of the applied potential leads to a variation from a bleached to a colored state as a result of proton (H+) extraction together with extraction of electrons from the valence band. The complex frequency-dependent impedance displays different features in different potential ranges. At low potentials, where coloration is weak, the spectra give evidence for a constant-phase element in parallel with a leak resistance. At intermediate and high potentials, the impedance spectra indicate the presence of a diffusion process, and a model for anomalous diffusion gives excellent fits to the spectra except in a crossover region. The applicability of this model in a significant part of the studied potential range suggests the presence of a multiple-trapping process for the ions. The potential dependence of the chemical capacitance, as well as the diffusion coefficient of un-trapped ions, are analyzed. The electrochemical density-of-states of the charge-compensating electrons gives indications of the top of the valence band and of a band tail extending into the band gap. Diffusion coefficients are found to increase steeply in the crossover potential region to very high values at high potentials. These features are discussed and related to the electrochromic behavior of Ni-oxide-based thin films.

Tungsten oxide and titanium doped tungsten oxide thin films, deposited by sputtering, were immersed in a viscous electrolyte comprised of LiClO4 in propylene carbonate and 2.0 wt% of polyethylene oxide (PEO). Electrochromic properties of the films were investigated by electrochemical techniques and in situ transmittance measurements. Cyclic voltammetry data were taken in the voltage ranges 2.0-4.0 and 1.5-4.0 V vs Li/Li+ for up to 500 cycles. A potentiostatic rejuvenation treatment was then performed on the degraded electrochromic films, at 6.0 V for 20 h, which was subsequently followed by another cyclic voltammetry measurement. Titanium incorporation into tungsten oxide resulted in a small cyclic stability improvement in the 2.0-4.0-V range, whereas less pronounced effects were observed for cycling in the 1.5-4.0-V range. Combining the results of the present study with our previous work, we are able to assess the relative merits of titanium incorporation and PEO addition to the electrolyte for the durability of electrochromic tungsten oxide thin films. Titanium addition was found advantageous for electrochemical durability in the 2.0-4.0-V range, but no clear benefits of PEO in the electrolyte were seen. On the other hand, in the wider 1.5-4.0-V range, tungsten oxide exhibited better durability than titanium-containing films, and this was especially so after rejuvenation in the PEO-containing electrolyte.

Electrochromic (EC) technology allows control of the transmission of visible light and solar radiation through thin-film devices. When applied to "smart" windows, EC technology can significantly diminish energy use for cooling and air conditioning of buildings and simultaneously provide good indoor comfort for the buildings' occupants through reduced glare. EC "smart" windows are available on the market, but it is nevertheless important that their degradation under operating conditions be better understood and, ideally, prevented. In the present work, we investigated EC properties, voltammetric cycling durability, and potentiostatic rejuvenation of sputter-deposited WO3 thin films immersed in LiClO4-propylene carbonate electrolytes containing up to 3.0 wt% of-7-nm-diameter SiO2 nanoparticles. Adding about 1 wt% SiO2 led to a significant improvement in cycling durability in the commonly used potential range of 2.0-4.0 V vs. Li/Li+. Furthermore, X-ray photoemission spectroscopy indicated that O-Si bonds were associated with enhanced durability in the presence of SiO2 nanoparticles.

Sunscreen lotions are used to protect the skin from damage due to solar ultraviolet (UV) radiation. The active UV blocking components can be organic molecules or inorganic particles, for example TiO2. While both in vivo and in vitro methods exist for assessing the protective capacity of sunscreens, their basic optical properties have received little attention. In this paper we take the first steps to address this issue by presenting a method to obtain spectra of absorption and scattering coefficients of a sunscreen by analysis of experimental transmittance and reflectance spectra. We model the optical properties of the sunscreen by two flux theory, develop approximate expressions for interface reflection coefficients and obtain the remaining optical parameters by fitting the two-flux theory to measurements. We analyze spectrophotometric reflectance and transmittance spectra of experimental narrow band UV blocking materials based on mixtures of mesoporous magnesium carbonate and TiO2, as well as some commercial sunscreens. Our method is demonstrated in vitro using quartz substrates and is further used to model in vivo measurements on the upside of the hand of one of the authors. The obtained absorption and scattering coefficient spectra give information on the relative contributions of these processes to the UV blocking effect as well as the scattering of visible light responsible for the whitening of the skin upon application of a sunscreen lotion. Our analysis method may be useful for computational optimization of novel sunscreen formulations as well as for the development of in vivo test methods.

The rapidly expanding field of intelligent ion-based devices has increased interest in the use of anodically-coloring electrochromic nickel oxide thin films. The degradation and coloration mechanisms of nickel oxide, especially in Li+-based electrolytes, are yet to be well understood. Herein we demonstrate that high potentials have a positive effect on the electrochromic performance of nickel oxide thin films. Our studies show that Cl- ions involved in the electrochromic process have been accumulated on the surface of the films upon extended electrochemical cycling, as confirmed by the X-ray Photoelectron Spectroscopy. X-ray Absorption Spectroscopy results indicate that the formation of Ni-Cl bonds influence the structural distortion and that the hybridization between Ni 3d and O 2p orbitals has been enhanced. Density functional theory calculations provide further insights for the band structures and how they change when Li+ and Cl- are adsorbed. Our results have revealed the underlying physical and chemical origins associated with the coloration mechanism and the degradation of nickel oxide thin films and highlighted the key role of Cl-. These new understandings will advance the development of superior electrochromic materials and the designing of efficient and durable electrochromic devices, both experimentally and theoretically.

The coloration mechanisms in electrochromic (EC) systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques$\text{---}$that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy (CIS). This was done at different bias potentials, which can be associated with different intercalation levels. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components participating in the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron-ion couple in the film. A quantity denoted complex optical capacitance is compared to the complex electrical capacitance$\text{---}$in particular, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. For perfectly synchronized electrical and optical responses, their complex spectra are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented in this work, can help in the understanding of the dynamics of the coloration mechanisms in EC materials at various conditions$\text{---}$for example, at different intercalation levels and optical wavelengths.

Mixed nickel-iron (Ni-Fe) compounds have recently emerged as promising non-precious electrocatalysts for alkaline water splitting. The understanding of the charge-transfer mechanism involved in the multi-step Faradic reaction, however, is still limited for the overall electrochemical process. In this paper, electrochemical impedance spectroscopy (EIS) measurements of Fe incorporated Ni oxide nanosheets were used to study the reaction kinetics for both hydrogen (HER) and oxygen (OER) evolution reactions in alkaline media. Our results showed that Fe incorporation improves the catalytic property of NiO nanosheets because of the lower reaction resistance and faster intermediate transformations. Detailed EIS modeling enables a separation of the surface coverage relaxation from the charge transfer resistance, with an inductive behavior observed in the low-frequency range for HER, holding important information on the dominating reaction mechanism. For OER, the good agreement between the EIS experimental results and a model with an inductance loop indicated that similar inductive behavior would be determining the EIS response at very low frequencies. The physical significance of the elementary steps gives insight into the governing reaction mechanisms involved in the electron and hole charge transfer, as well as the inherent properties of catalysts and their surface coverage relaxation.

Solutions of the differential equations for the diffuse components of the four-flux model are obtained, including explicit expressions for the collimated-diffuse and diffuse-diffuse reflectance and transmittance for an optical three-layer system. We establish a method to invert collimated-diffuse reflectance and transmittance in order to obtain the spectral variation of all average pathlength parameters and forward scattering ratios, by applying a spectral simulated annealing method. The inversion procedure was tested with synthetic collimated-diffuse reflectance and transmittance spectra and showed very good convergence. Subsequently, the method was applied to measured spectra of a light scattering and absorbing medium containing TiO2 particles in a low-absorbing matrix, which was enclosed between two glass slides. Hence the present paper, together with our previous one, wherein we inverted collimated-collimated reflectance and transmittance spectra to obtain scattering and absorption coefficients, establishes a reliable inversion method for obtaining all parameters in the most general version of the four-flux theory from experimental data.

Solar-driven water splitting is an emerging technology with high potential to generate fuel cleanly and sustainably. In this work, we show that WO3 can be used as a cathodic electrocatalyst in combination with (Ag,Cu) InGaSe2 solar cell modules to produce hydrogen and provide electrochromic functionality to water splitting devices. This electrochromic effect can be used to monitor the charge state or performance of the catalyst for process control or for controlling the temperature and absorbed heat due to tunable optical modulation of the electrocatalyst. WO3 films coated on Ni foam, using a wide range of different sputtering conditions, were investigated as cathodic electrocatalysts for the water splitting reaction. The solar-to-hydrogen (STH) efficiency of solar-driven water electrolysis was extracted using (Ag,Cu)InGaSe2 solar cell modules with a cell band gap varied in between 1.15 and 1.25 eV with WO3 on Ni foam-based electrolyzers and yielded up to 13% STH efficiency. Electrochromic properties during water electrolysis were characterized for the WO3 films on transparent substrate (indium tin oxide). Transmittance varied between 10% and 78% and the coloration efficiency at a wavelength of 528 nm and the overpotential of 400 mV was 40 cm(2) C-1. Hydrogen ion consumption in ion intercalation for electrochromic and hydrogen gas production for water electrolysis processes was discussed.

Inorganic electrochromic materials are promising for applications in color-based chromogenic technologies. Limited color control in these materials has, however, hitherto hampered their applications. Here, we show that multicolored nickel oxide (NiO) films can be obtained due to the combined effect of the intrinsic color of NiO and the structural color of the inverse opal structures by tailoring anodic oxide NiO films, exhibiting an absorption tail in the visible region into three-dimensional ordered macroporous inverse opal photonic bandgap structures. Various colors were achieved by the synergistic mechanism of structural and electrochromic coloration, thus realizing a wide spectrum of blue, green, yellow, orange, and brown colors depending on pore size, wall thickness, and viewing angle. Importantly, it is shown that the depth of color can be varied by applying an external potential. The electrochromic coloring of the inverse opal NiO films is found to be very different from the typical optical switching of non-structural NiO films. Thus, our work brings insights into the development of inorganic colored electrochromic materials.

Tungsten oxide is a widely used electrochromic material with important applications in variable-transmittance smart windows as well as in other optoelectronic devices. Here we report on electrochemical impedance spectroscopy applied to hydrous electrochromic tungsten oxide films in a wide range of applied potentials. The films were able to reversibly bleach and color upon electrochemical cycling. Interestingly, the bleaching potential was found to be significantly higher than in conventional non-hydrous tungsten oxide films. Impedance spectra at low potentials showed good agreement with anomalous diffusion models for ion transport in the films. At high potentials, where little ion intercalation takes place, it seems that parasitic side reactions influence the spectra. The potential dependence of the chemical capacitance, as well as the ion diffusion coefficient, were analyzed. The chemical capacitance is discussed in terms of the electron density of states in the films and evidence was found for a band tail extending below the conduction band edge.

Thin films of W oxide were prepared by reactive DC magnetron sputtering (5 cm-diameter W target), and their electrochromic (EC) properties were investigated in an electrolyte of LiClO4 in propylene carbonate. The purpose of the study was to elucidate the role of critical deposition parameters-oxygen/argon gas flow ratio for the sputter plasma Gamma, total pressure in the sputter plasma p(tot) , and sputtering power P-s - on the EC performance with foci on electrochemical cycling durability and optical modulation range Delta T. Specifically, we used 0.15 <= Gamma <= 0.90, 5 <= p(tot) <= 30 mTorr, and 200 <= P-s <= 400 W and studied cycling durability for up to 500 voltammetric cycles in the range 2.0-4.0 V vs. Li/Li+ together with optical properties at a wavelength of 528 nm. Most significantly, we discovered that a judicious choice of deposition parameters could yield films with superior cycling durability. Thus a similar to 300 nm-thick film prepared at Gamma = 0.90, p(tot) = 10 mTorr, and P-s = 200 W showed Delta T approximate to 65% after an initial "training" during similar to 100 voltammetric cycles; higher values of p(tot), on the other hand, yielded films whose Delta Ts degraded by similar to 10% during the cycling, and a lower value of p(tot) led to dark films with only marginal electrochromism. Hence our work delineates a pathway towards W oxide films with excellent durability of the EC properties. (C) 2020 The Author(s). Published by Elsevier Ltd.

Four-flux radiative transfer models have been extensively used to describe reflectance and transmittance (R&T) spectra of light scattering and absorbing (S&A) media. Solutions to the differential equations corresponding to the collimated fluxes are obtained by subsequent application of boundary conditions. Explicit expressions for the collimated R&T of light are reported, when considering a light S&A medium contained between two glass slides, an experimental arrangement which is appropriate for liquid suspensions and viscous matrices containing solid particles. A spectral simulated annealing method is applied to retrieve, from measured R&T spectra of collimated light under normal incident radiation, the scattering and absorption coefficients of the composite medium. First, the accuracy of the method is established by applying it to synthetic collimated R&T data. Secondly, we apply the method to experimental data and use it to determine the S&A coefficients of a layer of TiO2 particles dispersed in a PVP/water matrix.

Interaction of light with magnetic nanoparticles, dispersed in solution or embedded in other materials, is of major interest in a range of applications, one example being optomagnetic sensors. In applied research, light absorption and scattering of nanoparticle composites are often quantified by the Kubelka-Munk two-flux radiative transfer model. In this paper, we synthesized magnetite (Fe3O4) nanospheres with different diameters and encapsulated them into a polymer matrix. Their spectral transmittance and reflectance were investigated by spectrophotometry, together with measurements of angle-resolved scattering in the forward and backward hemispheres. The measured angular distribution was applied to approximate the scattering-phase function inside the film, which could be well described by the revised Reynolds-McCormick model. The backscattering and absorption coefficients were derived by inversion of the Kubelka-Munk relations, using the interface reflectances obtained from angle-dependent measurements. We present detailed optical properties for samples with various particle concentrations and scattering layer thicknesses, consisting of the magnetite and polymer composites. The absorption and backscattering coefficients for particles of diameter 458 nm showed qualitative agreement with single-scattering Mie calculations. The optical properties of composites with smaller particles might be influenced by an oxidized Fe3O4-like surface layer. The present approach can be used to study different kinds of magnetic nanoparticle clusters, dispersed in a supporting medium, and thus provide optical parameters of relevance for interpreting results of optomagnetic sensing experiments.

To facilitate optical design of energy-efficient materials and devices, a detailed knowledge of their basic optical parameters is necessary. In this paper we present a novel method for determining scattering (S) and absorption (K) coefficients from total transmittance and reflectance measurements by inversion of the Kubelka-Munk theory. The reflectance parameters appearing in this theory depend on the angular distribution of scattered light inside the material. The versatility of our method is demonstrated by a reanalysis of experimental data for several materials of interest in energy-related applications. Specifically, we report spectra of S and K for: (a) pigmented polymer foils for radiative cooling applications; (b) suspended particle devices for smart windows; (c) solar reflecting TiO2-pigmented paints and (d) selective solar absorbing paints for solar collectors.