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Electrochromism in Metal Oxide Thin Films: Towards long-term durability and materials rejuvenation
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electrochromic thin films can effectively regulate the visible and infrared light passing through a window, demonstrating great potential to save energy and offer a comfortable indoor environment in buildings. However, long-term durability is a big issue and the physics behind this is far from clear. This dissertation work concerns two important parts of an electrochromic window: the anodic and cathodic layers. In particular, work focusing on the anodic side develop a new Ni oxide based layers and uncover degradation dynamics in Ni oxide thin films; and work focusing on the cathodic side addresses materials rejuvenation with the aim to eliminate degradation.

In the first part of this dissertation work, iridium oxide is found to be compatible with acids, bases and Li+-containing electrolytes, and an anodic layer with very superior long-term durability was developed by incorporating of small amount (7.6 at. %) of Ir into Ni oxide. This film demonstrated sustained cycle-dependent growth of charge density and electrochromic modulation even after 10,000 CV cycles. The (111) and (100) crystal facets in Ni oxide are found to possess different abilities to absorb cation and/or anion, which yields different degrees of coloration and this is very significant for the electrochromic properties. The degradation of charge capacity in Ni oxide has an inevitable rapid decay in the first hundreds of cycles, subsequently combined with a more gradual decay, which is independent of applied potential and film composition. The consistent phenomenon can be very well modeled by power-law or stretched exponential decay; however the two models are indistinguishable in the current stage. Interestingly, in both models, the power-law exponent is 0.2 ≤ p ≤ 0.8, with most of the values around 0.5, in line with normal or anomalous diffusion models.

The second part of dissertation work deals with cathodic WO3 and TiO2. WO3 suffers from ion trapping induced degradation of charge capacity and optical modulation upon electrochemical cycling. This speculation is strongly supported by direct evidence from Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA). Most importantly, this ion trapping induced degradation can be eliminated by a galvanostatic de-trapping process. Significant ion-trapping takes place when x exceeds ~0.65 in LixWO3. The trapped ions are stable in the host structure, meaning that the ions cannot de-trap without external stimuli. The similar work done on TiO2 significantly complements and extends the work on the recuperation of WO3; the difference is that the trapped ions in host TiO2 seem to be less stable compared with the trapped ions in WO3.

    Overall, this dissertation presents a refined conceptual framework for developing superior electrochromic windows in energy efficient buildings.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1323
Keyword [en]
electrochromic, smart windows, long-term durability, degradation kinetics, ion trapping, de-trapping, materials rejuvenation
National Category
Nano Technology Condensed Matter Physics Materials Engineering Energy Systems Composite Science and Engineering
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
URN: urn:nbn:se:uu:diva-267111ISBN: 978-91-554-9421-6 (print)OAI: oai:DiVA.org:uu-267111DiVA: diva2:873012
Public defence
2016-01-14, Polhemalen, Ångströmlaboratoriet, Lägerhyddsv. 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Available from: 2015-12-14 Created: 2015-11-18 Last updated: 2016-01-28
List of papers
1. Electrochromic iridium oxide films: Compatibility with propionic acid, potassium hydroxide, and lithium perchlorate in propylene carbonate
Open this publication in new window or tab >>Electrochromic iridium oxide films: Compatibility with propionic acid, potassium hydroxide, and lithium perchlorate in propylene carbonate
2014 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 120, no Part A, 151-156 p.Article in journal (Refereed) Published
Abstract [en]

Porous thin films of It oxide were prepared by reactive dc magnetron sputtering onto unheated substrates. The crystallite size was similar to 5 nm, and a small amount of unoxidized Ir was present. The electrochromic performance was studied by optical transmittance measurements and cyclic voltammetry applied to films in aqueous and non-aqueous electrolytes, specifically being 1 M propionic acid, 1 M potassium hydroxide (KOH), and 1 M lithium perchlorate in propylene carbonate (Li-PC). Cyclic voltammetry measurements indicated that the films had a fractal surface structure. Good electrochromism, with mid-luminous transmittance modulation between similar to 55 and similar to 90% in similar to 100-nm-thick films, was documented in all of the electrolytes.

Keyword
Ir oxide, Porous thin films, Electrochromism, Transmittance, Durability
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-218594 (URN)10.1016/j.solmat.2013.08.035 (DOI)000329595400018 ()
Projects
EU GRINDOOR
Funder
EU, FP7, Seventh Framework Programme, 267234
Available from: 2014-02-13 Created: 2014-02-13 Last updated: 2017-12-06Bibliographically approved
2. Anodic Electrochromism for Energy-Efficient Windows: Cation/Anion-Based Surface Processes and Effects of Crystal Facets in Nickel Oxide Thin Films
Open this publication in new window or tab >>Anodic Electrochromism for Energy-Efficient Windows: Cation/Anion-Based Surface Processes and Effects of Crystal Facets in Nickel Oxide Thin Films
2015 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 22, 3359-3370 p.Article in journal (Refereed) Published
Abstract [en]

Anodic electrochromic (EC) oxides are of major interest as counter electrodes for smart window applications owing to their unique optical properties upon charge insertion and extraction. However, performance optimization of such oxides has been hampered by limited understanding of their EC mechanism, particularly in Li+-conducting electrolytes. This paper reports on NiOx films with 1.16 x 1.32, prepared by sputter deposition. These films are immersed in an electrolyte of lithium perchlorate in propylene carbonate, and EC properties are studied by cyclic voltammetry and in situ optical transmittance measurements. The electrochromism is significantly enhanced at large values of x. It has been found that charge exchange in Ni oxide is mainly due to surface processes and involves both cations and anions from the electrolyte, which is different from the case of cathodic EC materials such as WO3. Whereas previous studies of Ni oxide have focused on cation intercalation, the cation/anion-based mechanism presented here offers a new paradigm for designing and developing EC devices such as smart windows for energy efficient buildings.

Keyword
cation, anion adsorption, electrochromism, nickel oxides, surface process
National Category
Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-256995 (URN)10.1002/adfm.201500676 (DOI)000355992600012 ()
Funder
EU, European Research Council, 267234
Available from: 2015-06-30 Created: 2015-06-29 Last updated: 2017-12-04Bibliographically approved
3. Cyclic voltammetry on sputter-deposited films of electrochromic Ni oxide: Power-law decay of the charge density exchange
Open this publication in new window or tab >>Cyclic voltammetry on sputter-deposited films of electrochromic Ni oxide: Power-law decay of the charge density exchange
2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 16, 163502- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Engineering and Technology Physical Sciences
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-234931 (URN)10.1063/1.4899069 (DOI)000344363000084 ()
Projects
EU GRINDOOR
Funder
EU, FP7, Seventh Framework Programme, 267234
Available from: 2014-10-27 Created: 2014-10-27 Last updated: 2017-12-05Bibliographically approved
4. Anodic Electrochromism for Energy-Efficient Windows: Decay of Charge Density upon Extensive Electrochemical Cycling of Nickel Oxide Thin Films
Open this publication in new window or tab >>Anodic Electrochromism for Energy-Efficient Windows: Decay of Charge Density upon Extensive Electrochemical Cycling of Nickel Oxide Thin Films
(English)Article, review/survey (Refereed) Submitted
National Category
Energy Systems Manufacturing, Surface and Joining Technology Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-267132 (URN)
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2016-01-13
5. Strongly Improved Electrochemical Cycling Durability by Adding Iridium to Electrochromic Nickel Oxide Films
Open this publication in new window or tab >>Strongly Improved Electrochemical Cycling Durability by Adding Iridium to Electrochromic Nickel Oxide Films
2015 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 18, 9319-9322 p.Article in journal (Refereed) Published
Abstract [en]

Anodically colored nickel oxide (NiO) thin films :are of much interest as counter electrodes in tungsten oxide based electrochromic devices such as "smart windows" for energy-efficient buildings. However, NiO films are prone to Suffering severe charge density degradation upon prolonged electrochemical cycling, which can lead to insufficient device lifetime. Therefore, a means to improve the durability of NiO-based films is an important challenge at present. Here we report that the incorporation of a modest amount of iridium into NiO films [Ir/(Ir + Ni) = 7.6 atom %] leads to remarkable durability, exceeding 10000 cycles in a lithium-conducting, electrolyte, along with significantly improved optical modulation during extended cycling. Structure characterization showed that the face-centered-cubic-type NiO structure remained after iridium addition. Moreover, the crystallinity of these films was enhanced upon electrochemical cycling.

Keyword
electrochromic, iridium-nickel oxide, improved durability, charge density, optical modulation
National Category
Nano Technology Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-256240 (URN)10.1021/acsami.5b01715 (DOI)000354906500002 ()25919917 (PubMedID)
Funder
EU, European Research Council, 267234
Available from: 2015-06-24 Created: 2015-06-22 Last updated: 2017-12-04Bibliographically approved
6. Electrochromic Iridium-Containing Nickel Oxide Films with Excellent Electrochemical Cycling Performance
Open this publication in new window or tab >>Electrochromic Iridium-Containing Nickel Oxide Films with Excellent Electrochemical Cycling Performance
2016 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 163, no 2, E7-E13 p.Article in journal (Refereed) Published
Abstract [en]

Electrochromic Ni oxide thin films attract much interest because of their large potential for applications as optically active layers in energy-saving and comfort enhancing smart windows in buildings. However Ni oxide, typically being the anodic counter electrode in a W-oxide-based device, may suffer severe charge capacity degradation upon extended electrochemical cycling. It is therefore important to identify improved Ni-oxide-based thin films for electrochromics. Here we describe a new class of such films wherein an addition of a small amount of Ir to Ni oxide is found to provide strongly improved electrochemical cycling durability. Best properties were achieved with Ir/(Ir + Ni) = 7.6%, and such films displayed charge capacity and optical modulation that, remarkably, were still increasing after 10,000 cycles.

Keyword
Electrochromic, Ir -Ni oxide, thin films, durability
National Category
Chemical Sciences Physical Sciences Engineering and Technology Materials Chemistry
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-267706 (URN)10.1149/2.0591602jes (DOI)000367324400057 ()
Funder
EU, European Research Council, 267234
Available from: 2015-11-25 Created: 2015-11-25 Last updated: 2017-12-01Bibliographically approved
7. Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films
Open this publication in new window or tab >>Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films
2015 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 14, 996-1001 p.Article in journal (Refereed) Published
Abstract [en]

There is keen interest in the use of amorphous WO3 thin films as cathodic electrodes in transmittance-modulating electrochromic devices1–4. However, these films suer from ion-trapping-induced degradation of optical modulation and reversibility on extended LiC-ion exchange. Here,we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminatedby 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.

Keyword
WO3 films, electrochromic, variable-transmittance windows, ion-trapping
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-264725 (URN)10.1038/nmat4368 (DOI)000361771800018 ()
Funder
EU, European Research Council, 267234
Available from: 2015-10-15 Created: 2015-10-15 Last updated: 2017-12-01Bibliographically approved
8. Sustainable rejuvenation of electrochromic WO3 films
Open this publication in new window or tab >>Sustainable rejuvenation of electrochromic WO3 films
2015 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 51, 28100-28104 p.Article, review/survey (Refereed) Published
Abstract [en]

Devices relying on ion transport normally suffer from a decline of their long-term performance due to irreversible ion accumulation in the host material, and this effect may severely curtail the operational lifetime of the device. In this work, we demonstrate that degraded electrochromic WO3 films can sustainably regain their initial performance through galvanostatic de-trapping of Li+ ions. The rejuvenated films displayed degradation features similar to those of the as-prepared films, thus indicating that the de-trapping process is effectively reversible so that long-term performance degradation can be successfully avoided. De-trapping did not occur in the absence of an electric current.

Keyword
electrochromic, WO3, optical, sustainable rejuvenation, de-trapping
National Category
Manufacturing, Surface and Joining Technology Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-266808 (URN)10.1021/acsami.5b09035 (DOI)000369448200013 ()26562589 (PubMedID)
Funder
EU, European Research Council, 267234
Available from: 2015-11-18 Created: 2015-11-11 Last updated: 2017-12-01Bibliographically approved
9. Galvanostatic ion de-trapping rejuvenates oxide thin films
Open this publication in new window or tab >>Galvanostatic ion de-trapping rejuvenates oxide thin films
Show others...
2015 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 48, 26387-26390 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Other Materials Engineering Atom and Molecular Physics and Optics Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-267134 (URN)10.1021/acsami.5b09430 (DOI)000366339100006 ()26599729 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 267234
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2017-12-01Bibliographically approved
10. Electrochromism for energy conservation: Eliminating degradation in amorphous TiO2 thin films
Open this publication in new window or tab >>Electrochromism for energy conservation: Eliminating degradation in amorphous TiO2 thin films
(English)Article, review/survey (Refereed) Submitted
Keyword
Smart windows, TiO2, electrochromism, ion-trapping, rejuvenating
National Category
Nano Technology Energy Systems Materials Engineering
Identifiers
urn:nbn:se:uu:diva-267127 (URN)
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2016-01-13

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