Electrochromic reactions surfaces

As shown in this symposium, interest in chemical modification of electrode surfaces has been extended in many directions, including the study of light-assisted redox reactions, and the use of modified electrodes in electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (A,5). 

Further studies of the n-heptyl viologen reaction on modified surfaces are reported elsewhere (25). It is clear that the interaction of ion-beams or plasmas with the electrode surface can be a powerful modification tool, complementary to chemical modification procedures for application to either photoelectrochemical or electrochromic reactions. 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

The deliberate modification of electrode surfaces by coating with one or more layers of electroactive material has been used for a variety of purposes. Solar energy conversion, electrochromism, corrosion protection, and electrocatalysis are but a few of the applications which are currently of interest. The use of in situ Raman spectroscopic studies can help to determine the structural characteristics of electrode coatings at the molecular level and can provide information on the mechanisms of electrochemical reactions occurring at modified electrode surfaces. 

Electrochromism is the reversible change in optical properties of a material caused by redox reactions. The redox reactions can be initiated when the material is placed on the surface of an electrode. When the electro-chromic material is capable of showing several colors, it is addressed as polyelectrochromic. 

The field of chemistry concerned with the interrelation of electrical and chemical effects, especially the study of chemical changes caused by an electric current and the electrical energy production by chemical reactions, is termed electrochemistry [5]. While electrochemistry encompasses a huge array of different phenomena applied in a variety of technologies, applications, and characterization techniques, such as the surface area measurement by hydrogen adsorption discussed in Sect. 4.3.8, the main emphasis here will be focused on electrodeposition and devices based on electrochemistry, such as electrochemical supercapacitors and electrochromic displays. 

Electrochromism. Electrochromic materials have the property of a reversible and visible change in transmittance and/or reflectance associated with an electrochemically induced redox process involving electroactive species typically deposited onto an electrode surface as a thin film. The redox state of the material may be switched by an electron transfer reaction at an electrode and the observed colour change results from the generation of different electronic absorption bands according to the redox state. Such a colour change is commonly reported between a transparent ( bleached ) state and a coloured state, or between two coloured states. or even between multiple coloured states (multichromic). ... 

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