Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni₂O₃. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

Although WO3 thin films are promising as electrodes for electrochromic devices, they suffer from degradation of optical modulation. Ion-trapping-induced degradation can now be successfully eliminated by constant-current-driven de-trapping. There is keen interest in the use of amorphous WO3 thin films as cathodic electrodes in transmittance-modulating electrochromic devices1,2,3,4. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility on extended Li+-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping; that is, WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion trapping occurs when x exceeds ∼0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion-trapping site (intermediate and deep) in WO3, where the intermediate ones are most prevalent. Li+ ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.

Earth-abundant transition metal-based compounds are of high interest as catalysts for sustainable hydrogen fuel generation. The realization of effective electrolysis of water, however, is still limited by the requirement of a high sustainable driving potential above thermodynamic requirements. Here, we report dynamically self-optimized (DSO) NiFe layered double hydroxide (LDH) nanosheets with promising bi-functional performance. Compared with pristine NiFe LDH, DSO NiFe LDH exhibits much lower overpotential for the hydrogen evolution reaction (HER), even outperforming platinum. Under 1 M KOH aqueous electrolyte, the bi-functional DSO catalysts show an overpotential of 184 and −59 mV without iR compensation for oxygen evolution reaction (OER) and HER at 10 mA cm⁻². The material system operates at 1.48 V and 1.29 V to reach 10 and 1 mA cm⁻² in two-electrode measurements, corresponding to 83% and 95% electricity-to-fuel conversion efficiency with respect to the lower heating value of hydrogen. The material is seen to dynamically reform the active phase of the surface layer during HER and OER, where the pristine and activated catalysts are analyzed with ex situ XPS, SAED and EELS as well as with in situ Raman spectro-electrochemistry. The results show transformation into different active interfacial species during OER and HER, revealing a synergistic interplay between iron and nickel in facilitating water electrolysis.

The Maxwell Garnett and Bruggeman effective medium theories are derived for the average dielectric permeability of heterogeneous materials from a unified theoretical approach. It starts by specifying two random unit cells which represent different microstructures. Requiring that these cells should not be detectable by electromagnetic radiation when embedded in an effective medium, we show from an extended optical theorem that the forward scattering amplitude must vanish. Setting the leading term in the expansion series of this quantity equal to zero yields the effective medium theories pertaining to the two microstructures. The remaining terms provide estimates of the accuracy of the approximations. This approach is then used in numerical computations for Co–Al₂O₃ cermets.

Co‐Al₂O₃ composite films were produced by electron‐beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separated‐grain structure with regular hcp Co particles embedded in Al₂O₃ was found for Co contents (f_Cos’) up to ∼30 vol %. The complex dielectric permeability ε̄ was evaluated in the 0.3≲λ≲40‐μm wavelength range for samples with 0.11≲ f_Co ≲0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for ε̄ were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Garnett, Bruggeman, and others were thus rederived in a unified way. Large‐size limits of validity and extensions to nonspherical particles were treated. It was found that the Maxwell Garnett theory could reproduce the experimental ε̄’s in detail at low f_Co provided that some Co was taken to be dispersed in the insulating matrix. At larger f_Co’s we found discrepancies which are likely to be due mainly to dipole‐dipole coupling among adjacent particles. A comparison of the experimental ε̄’s and the rigorous Bergman–Milton bounds, which hold irrespective of detailed microgeometry, gave several interesting results: the ε̄’s varied monotonically along one of the bounds for isotropic materials as f_Co was increased, at large f_Co we found evidence for anisotropy, and at λ≊12.5 μm we noted certain cases of disagreement with even the most generous bounds. The empirical ε̄’s were used to construct surfaces which combine a high solar absorptance a_s with a low hemispherical thermal emittance e^H. From a computer optimization study we found that a_s =0.95 and e^H =0.07 could be obtained with 0.07 μm of Co‐Al₂O₃ ( f_Co≊0.6) antireflected with 0.07 μm of Al₂O₃ and laid on Ni. These results were verified by measurements on samples which approximate the ideal design.