Galvanostatic Rejuvenation of Electrochromic WO₃ Thin Films: Ion Trapping and Detrapping Observed by Optical Measurements and by Time-of-Flight Secondary Ion Mass Spectrometry

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, WO. 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.

Electrochromic materials and devices for energy efficiency and human comfort in buildings: A critical review

“…The above results suggest strongly that Li ion de-trapping is possible, and conclusive evidence on this matter was obtained by analyzing the Li content-including its depth profile-by time-of-flight elastic recoil detection analysis (ToF-ERDA) [165] and time-offlight secondary ion mass spectroscopy (ToF-SIMS) [166]. Fig.…”

Electrochemical Rejuvenation of Anodically Coloring Electrochromic Nickel Oxide Thin Films

“…12,13 Nevertheless, there has remained an urgent need for other means to extend device longevity, and it was recently shown that electrochemical treatments can rejuvenate degraded EC films and restore their original properties. This work was initiated with WO 3 films [14][15][16][17][18] and was later extended to films of MoO 3 19 and TiO 2 . 20 These oxides are cathodic and similar in many ways, and it is commonly accepted that their electrochromism is associated with the intercalation/deintercalation of small cations (H + or Li + ) and concurrent insertion/extraction of electrons.…”

“…Rejuvenation of WO 3 has been firmly related to electrically induced extraction of trapped Li + ions from degraded films. 14,15,18 However, the situation is more complex for the case of anodically coloring nickel oxide in the Li + -based electrolytes. [21][22][23] Its coloration mechanism remains contentious and even the ionic species involved in the EC process are controversial, furthermore, the physical and chemical origins of the films' irreversibility remain obscure, experimentally as well as theoretically.…”

Life-cycling and uncovering cation-trapping evidence of a monolithic inorganic electrochromic device: glass/ITO/WO3/LiTaO3/NiO/ITO

“…Sung et al 13 also reported an improvement in the durability with an inorganic protective layer for the laminated device V 2 O 5 /WO 3 , but organic electrolytes rather than inorganic ones were used for device lamination. Baloukas et al 14 have demonstrated ion trapping and detrapping limited in single EC layer WO 3 films.…”