Electrochromism coloured electrode type

In the case of the coloured electrolyte type, the two complementary electrochromes are dissolved in the electrolyte between the transparent electrodes. One becomes coloured by oxidation and the other by reduction and consequently the electrolyte becomes coloured (see 1.5.2.1). The electrolyte remains coloured only whilst a current is being passed, becoming colourless once the charge is removed. 

Dnring the process of colouration in electrochromic cells by passing a charge in one direction, a colour can form in one or both of the electrodes or in the electrolyte adjacent to the electrodes. When the colour is formed by reduction at a negative electrode it is called cathodic coloration and, conversely, at the anode it is anodic coloration. Two different types of cells are shown schematically in Figure 1.31. ... 

This type of electrochrome, in its pale or colourless state, is soluble in the electrolyte. However, on electron transfer, the coloured form of the electrochrome that is produced is insoluble and is deposited onto the surface of an electrode. Viologens with hydrophobic chains, such as heptyl (1.90 R = form deeply coloured radical... 

Vanadium oxide is basically a cathodic ion insertion ECM (see section 8.3) and this should in principle rule it out from the class of counter-electrodes suitable for W03-based EWs. However, the colour changes upon lithiation are not too marked, i.e. from yellowish to pale blue-grey (see equation [8.5]). Therefore, the cathodic coloration of lithiated vanadium oxide, Li,V2 05, can be considered to be sufficiently weak to suggest its application as counter-electrode with WO3. Effectively, EWs of the type LiV205/W03, where the electrochromic process can be indicated as ... 

---