Electrochromic device secondary electrode

The future prospects for polymer electrolytes look promising because it has been appreciated that they form an ideal medium for a wide range of electrochemical processes. Other than primary and secondary batteries, and high and low temperature fuel cells, practical applications for polymer electrolytes that are under consideration include electrochromic devices, modified electrode/sensors, solid-state reference electrode systems, supercapacitors, thermoelectric generators, high-vacuum electrochemistry and electrochemical switching. Device applications that have been the main driving force behind the development of polymer electrolytes are treated hereafter. 

An electrochromic device is basically a two-electrode cell as mentioned in the introduction (Figure 1). In the construction of a two-electrode cell with only a single electrochromic material, there may be a problem with degradation reactions of the electrolyte diat occur because of the absence of an effective counter-electrode reaction. This limits operating lifetimes through destruction of the electrolyte and a build-iqi of degra tion products in the cell. The use of a reversible electrochromic material as a counter electrode or secondary electrode helps to avoid this problem. The secondary electrode, in electrochromic terms. 

By combining two electrochromic materials whose neutral states are transparent in file visible, with one electrode anodically colored while the other is cathodically colored, allows fabrication of a device that can switch between highly transmissive and absorptive states. These kinds of ECDs are used as smart windows. In the case of electrochromic materials where the neutral state shows absorption in the visible, it is possible to choose as the secondary electrode a material whose neutral state absmption is conqilementaiy to fiiat of the primary electrode, and its reduced form shows file same color as t of file oxidized form of file primary electrode. In other words, file combination of the spectra of two con lementary colors leads to absorption over the whole visible spectrum turning file device black. Upon potential switching between file two redox states, the electrochromic cell will turn from black to the characteristic color of the original forms. 

As an environmentally stable conducting polymer, polyaniline has been recognized as a potential candidate for application in secondary battery electrodes, sensors, electrochromic display devices, microelectronics, ion-exchange resins etc. [151-161]. 

Electrically conducting polymers have been a subject of extensive studies in view of both academic interest and potential technological applications. This chapter describes the synthesis and properties of electrically conducting polymers and their applications as functional materials, such as electrode materials for secondary batteries, photoactive materials for photovoltaic devices, electrochromic materials, and materials for use in organic electroluminescent devices. 

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