Introduction

Like all other types of display device, mechanical or electronic, no electrochromic device will continue to function indefinitely. For this reason, cycle lives are reported. The definition of cycle life has not been conclusively settled. Even by the definition in Section 1.4, reported lives vary enormously: some workers suggest their devices will degrade and thereby preclude realistic use after a few cycles while others claim a device surviving several million cycles. Table 16.1 contains a few examples; in each case the cycle life cited represents ‘deep’ cycles, as defined on p. 12. Some of these longer cycle lives were obtained via methods of accelerated testing, as outlined below.

It is important to appreciate that results obtained with a typical three-electrode cell in conjunction with a potentiostat can yield profoundly different results from the same components assembled as a device: most devices operate with only two electrodes.

The results of Biswas et al., who potentiostatically cycled a thin film of WO3 immersed in electrolyte, are typical insofar as the electrochemical reversibility of the cycle remained quite good with little deterioration. Their films retained their physical integrity, but the intensity of the coloration decreased with the number of cycles.

Some devices are intended for once-only use, such as the freezer indicator of Owen and co-workers; other applications envisage at most a few cycles, like the Eveready battery-charge indicator. Clearly, degradation can be allowed to occur after no more than a few cycles with applications like the latter.