Electrochromic Displays Based on NiO Electrodes

As naturally abundant and low-cost semiconductor, NiO is widely used in electrochromic windows [20], batteries [21], supercapacitors [22], and sensors [23], While all these applications benefit from an interconnected, three-dimensional NiO nanostructure that combines a high specific surface area with a good electric conductivity, the performance enhancement becomes vividly evident as an increased coloration contrast and improved switching behavior when applied in electrochromic devices. NiO nanomaterials recently employed in electrochromic studies include nanocomposites [24], inverse opals [25], macroporous [26] and mesoporous films [27-29], 

To investigate the effect of the nanostructure on the electrochromic performance of NiO, transparent devices were assembled from nontemplated and DG-structured films with a FTO counter electrode and a 1M KOH(aq) electrolyte, see Fig. 6.9a. The active electrode material used was limited in area to 0.95mm. During the nickel electroplating process limiting the deposition area improve the control and quality of the deposit. 

In alkaline solution the reversible optical absorption change caused by an electron-transfer reaction accompanied by a compensating transfer of mobile H cations between the hydrated NiO and electrolyte can be summarized by the following reaction scheme [30, 31] 

A second important aspect of electrochromism is the temporal response under alternating potentials ( 0.55 V). The DG showed sharp and distinct transitions between the colored/oxidized and bleached/reduced state across the entire visible spectrum (Fig. 6.10b). This time-resolved switching behavior was analyzed in more detail at A = 630 nm (Fig. 6.10c). The DG device showed short characteristic response times of 53 ms for the bleaching step and 63 ms for the reverse process, determined by fitting exponential functions to the switching curves. These short response times which are close to video rate (24 frames per second) are enabled by the short ion diffusion distance through the only = 5 nm thick NiO nanotube wall. 

Cycle stability and stability of the structural integrity of the nanostructured material are further important requirements of electrochromic materials. The CV curves presented in Fig. 6. lOd, e indicate that the nanotubular NiO structures exhibits good long-term cycle stability, giving rise to a pair of well pronounced redox peaks. Fur- 

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