| 1. |
- Arvizu, Miguel, et al.
(författare)
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Galvanostatic ion de-trapping rejuvenates oxide thin films
- 2015
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 7:48, s. 26387-26390
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Tidskriftsartikel (refereegranskat)abstract
- Ion trapping under charge insertion-extraction is well-known to degrade the electrochemical performance of oxides. Galvano-static treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li+ trapping associated with the degradation of the EC properties and, importantly, that Li+ detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li+ detrapping.
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| 2. |
- Baloukas, Bill, et al.
(författare)
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Galvanostatic Rejuvenation of Electrochromic WO3 Thin Films : Ion Trapping and Detrapping Observed by Optical Measurements and by Time-of-Flight Secondary Ion Mass Spectrometry
- 2017
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 9:20, s. 16996-17002
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Tidskriftsartikel (refereegranskat)abstract
- Electrochromic (EC) smart windows are able to decrease our energy footprint while enhancing indoor comfort and convenience. However, the limited durability of these windows, as well as their cost, result in hampered market introduction. Here, we investigate thin films of the most widely studied EC material, WO3. Specifically, we combine optical measurements (using spectrophotometry in conjunction with variable-angle spectroscopic ellipsometry) with time-of-flight secondary ion mass spectrometry and atomic force microscopy. Data were taken on films in their as-deposited state, after immersion in a Li-ion-conducting electrolyte, after severe degradation by harsh voltammetric cycling and after galvanostatic rejuvenation to regain the original EC performance. Unambiguous evidence was found for the trapping and detrapping of Li ions in the films, along with a thickness increase or decrease during degradation and rejuvenation, respectively. It was discovered that (i) the trapped ions exhibited a depth gradient; (ii) following the rejuvenation procedure, a small fraction of the Li ions remained trapped in the film and gave rise to a weak short-wavelength residual absorption; and (iii) the surface roughness of the film was larger in the degraded state than in its virgin and rejuvenated states. These data provide important insights into the degradation mechanisms of EC devices and into means of achieving improved durability.
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| 3. |
- Granqvist, Claes Göran, 1946-, et al.
(författare)
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Electrochromic materials and devices for energy efficiency and human comfort in buildings : A critical review
- 2018
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Ingår i: Electrochimica Acta. - : Elsevier. - 0013-4686 .- 1873-3859. ; 259, s. 1170-1182
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Forskningsöversikt (refereegranskat)abstract
- Electrochromic (EC) materials can be integrated in thin-film devices and used for modulating optical transmittance. The technology has recently been implemented in large-area glazing (windows and glass facades) in order to create buildings which combine energy efficiency with good indoor comfort. This critical review describes the basics of EC technology, provides a case study related to EC foils for glass lamination, and discusses a number of future aspects. Ample literature references are given with the object of providing an easy entrance to the burgeoning research field of electrochromics.
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| 4. |
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| 5. |
- Niklasson, Gunnar A., Professor, 1953-, et al.
(författare)
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Durability of electrochromic films : Aging kinetics and rejuvenation
- 2017
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Ingår i: ECS Transactions. - : Electrochemical Society. ; , s. 1659-1669
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Konferensbidrag (refereegranskat)abstract
- A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices over aservice life of more than 20 years. In this paper, we report recent results from a fundamental study of the aging kinetics of EC tungsten oxide and nickel oxide thin films and describe electrochemical rejuvenation mechanisms that are able to restore the films to their initial state. The aging kinetics displays an approximate power-law decrease of the charge capacity as a function of cycle number. This decay of charge capacity can be understood in terms of models built on so-called dispersive chemical kinetics. Tungsten oxide and nickel oxide EC films can be rejuvenated by applying a high electrochemical potential or a small constant current. Trapped ions in the bulk or at the surface of the films can be released by these procedures.
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| 6. |
- Niklasson, Gunnar A., 1953-, et al.
(författare)
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Electrochemical degradation and rejuvenation of electrochromic tungsten oxide thin films
- 2016
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Konferensbidrag (refereegranskat)abstract
- Tungsten oxide is the most widely used cathodic electrochromic material for smart window applications. One of the main challenges for smart window technology is to ensure the durability of the electrochromic devices over a service life of more than 20 years. Hence, in order to facilitate large-scale practical application of electrochromic materials, their degradation under operating conditions must be better understood and preferably prevented. In this paper we address these issues by three different approaches. First we show that the electrochemical ageing of electrochromic tungsten oxide, under stressed conditions, can be described by stretched exponential kinetics. The goal of such accelerated ageing studies is eventually to be able to predict service life using this empirical relationship. Secondly, we report on a recently discovered rejuvenation processes for restoring aged coatings to their initial state. During severe ageing of the coatings, Li ions are trapped in the film, and subsequently these ions can be released by application of a high electrochemical potential for a few hours. We estimate activation energies for the release process from chronoamperometric measurements during rejuvenation. Thirdly we address the issue of the growth of a solid-electrolyte interface. Impedance spectroscopy measurements on tungsten oxide films were used to obtain the interfacial charge transfer resistance. After the films had been subjected to low potentials known to induce degradation, the charge transfer resistance in the usual operating range showed a marked increase. This is interpreted as a signature of the development of a solid-electrolyte interface. A similar increase of the charge transfer resistance has been observed in electrochromic devices subjected to accelerated aging at an elevated temperature of 80oC for a thousand cycles.
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| 7. |
- Niklasson, Gunnar A., 1953-, et al.
(författare)
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Electrochromic oxides: Electrochemical density of states, degradation kinetics and rejuvenation of degraded films
- 2015
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Konferensbidrag (refereegranskat)abstract
- Electrochromic (EC) materials change their optical properties upon the application of a voltage signal. EC devices are of interest for energy efficient smart windows, as well as for a number of niche applications. An EC device can be viewed as a thin-film electrical battery whose charging state is manifested by optical absorption. The central part of the device comprises EC thin films based on amorphous tungsten oxide (WO3) and nano-crystalline nickel oxide (NiOx) joined by a layer of polymer electrolyte. This three-layer arrangement is positioned between transparent and electrically conducting thin films of indium tin oxide (In2O3:Sn; ITO) backed by polyester foils. Applying a voltage between the ITO films induces transport of ions between the WO3 and NiO films, together with charge compensating electron transport through the outer circuit. Insertion of ions and electrons in WO3 and extraction of them from NiO lead to enhanced optical absorption. In this paper we report novel developments regarding the electronic density of states (DOS), the ageing kinetics and the rejuvenation of aged EC materials. The optical absorption in EC oxides is due to electronic transitions between localized states close to the band edges. In order to model the optical absorption it is necessary to have a good understanding of the electronic DOS. We have found that it is possible to derive an effective DOS from electrochemical measurements, in particular from measurements of quasi-steady-state potential curves, during the ion/electron insertion process. The obtained “electrochemical DOS” exhibits good agreement with state-of-the-art density functional calculations of the DOS for a number of amorphous and nano-crystalline oxides. EC devices are prone to slowly degrade under repeated electrochemical cycling and a thorough understanding of the kinetics of this degradation is necessary in order to estimate the lifetime of smart window products. The ageing is often ascribed to irreversible incorporation of ions at the film surfaces or inside the film. We present results which show that the degradation displays dispersive kinetics and the chemical reaction behind the process can be modelled by using a rate constant with power-law time dependence. Results obtained for WO3 and NiO thin films show both similarities and differences. We have very recently found that rejuvenation of previously degraded WO3 films is possible. Films with heavily degraded charge capacity and optical switching were subjected to a galvanostatic treatment for a certain time. After subjecting the film to a small current for a number of hours, it regained its initial charge capacity and optical properties. The rejuvenation is ascribed to de-trapping of ions from deep traps, induced by a high electrochemical potential over the film. The optical absorption and switching as well as the durability of EC material are key issues for the successful commercial development of smart windows. The results presented here pave the way for an increased understanding of the optical switching as well as of the degradation kinetics. The possibility of rejuvenating degraded films is highly exciting, and further studies are necessary in order to develop this idea into a practical process.
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| 8. |
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| 9. |
- Wen, Rui-Tao, et al.
(författare)
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Anodic Electrochromic Nickel Oxide Thin Films : Decay of Charge Density upon Extensive Electrochemical Cycling
- 2016
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Ingår i: ChemElectroChem. - : Wiley-VCH Verlagsgesellschaft. - 2196-0216. ; 3:2, s. 266-275
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Tidskriftsartikel (refereegranskat)abstract
- Electrochromic (EC) Ni oxide thin films are a critical component in the smart windows. However, long-term decay of the EC performance in aprotic electrolytes is persistent and poorly understood, and it is difficult to assess lifetimes of EC devices. Here we report on charge density decline upon electrochemical cycling. The charge density decay was modeled with a power law or, alternatively, a stretched exponential; both models describe a rapid drop of charge density during the first hundreds of cycles and a subsequent slower decline. The decay is independent of film composition and applied potential range as long as the upper limit of the potential is 4.4V vs. Li/Li+. Our decay models are interpreted in terms of dispersive chemical reaction kinetics and point at ion diffusion as the rate-limiting step. Power-law exponents are consistent with diffusion. The results provide a framework for evaluating EC durability of Ni-oxide-based thin films and may be important for assessing the durability of EC devices.
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| 10. |
- Wen, Rui-Tao, et al.
(författare)
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Cyclic voltammetry on sputter-deposited films of electrochromic Ni oxide : Power-law decay of the charge density exchange
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:16, s. 163502-
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Tidskriftsartikel (refereegranskat)abstract
- Ni-oxide-based thin films were produced by reactive direct-current magnetron sputtering and were characterized by X-ray diffraction and Rutherford backscattering spectroscopy. Intercalation of Li+ ions was accomplished by cyclic voltammetry (CV) in an electrolyte of LiClO4 in propylene carbonate, and electrochromism was documented by spectrophotometry. The charge density exchange, and hence the optical modulation span, decayed gradually upon repeated cycling. This phenomenon was accurately described by an empirical power law, which was valid for at least 10(4) cycles when the applied voltage was limited to 4.1V vs Li/Li+. Our results allow lifetime assessments for one of the essential components in an electrochromic device such as a "smart window" for energy-efficient buildings.
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