Electrochemical degradation and rejuvenation of electrochromic tungsten oxide thin films

• 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

Nyckelord

Teknisk fysik med inriktning mot fasta tillståndets fysik

Engineering Science with specialization in Solid State Physics

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