Electrochromic reactions

Ru 2,2 -bipyridine complexes can form a large number of colored compounds upon successive reduction, with the formal Ru oxidation state from +2 to -4. In the case of highly reduced complexes, proper representation of the electrochromic reaction is actually the reduction of the hgand, not that of the metal center. 

Since PB and WO32 are respectively anodically and cathodically coloring electrochromic materials, they can be used together in a single device121-125 so that their electrochromic reactions are complementary (Equation (15)) ... 

Electrochromism can be defined as a colour change induced in a material by an applied electric field or current. Some ions in solid compounds can be reduced or oxidised (redox) electrochromically with a consequent change in colour. WO3 and M0O3 solid films have been extensively used for this purpose. The electrochromic reaction is expressed by... 

Improvements in Photoelectrochemical and Electrochromic Reactions at Chemically Modified Electrodes... 

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. 

A further electro-optical effect which is studied for applications in display is electrochromism. Electrochromism is characterized as a reversible colour change induced in some organic and inorganic materials by an applied electric field or current. According to Chang [263], the physical mechanisms responsible for these phenomena are electronic and electrochemical in nature. A schematic electrochromic reaction can be formulated as follows ... 

Electrochromic applications of the PAn and substituted PAn films [307a-h], composite films of PAn or its derivatives with other electrochromic materials [307i-m], silanized PAn films [306o] and photo-induced electrochromic reactions on semiconductor particles [307p]. Again, most other polymers can be used for this purpose as long as their oxidized and reduced states are stable and have different colors from that of neutral polymers. 

Mixed conductivity, ionic and electronic, is a feature of electrochromics. Both transport rates affect the kinetics of the electrochromic reaction The coloration kinetics and the stability of the electrode are strongly influenced both by the electroljde solution and the preparation of the electrode material. 

The basic device configuration typically consists of (i) an electrochromic layer, or mixed conductor, deposited onto a transparent electronic conductor this layer can be coloured or bleached by double injection or ejection of electrons and ions (ii) an electrolyte, or purely ionic conductor, in contact with the above described layer (hi) a counter electrode for storing the ions involved in the electrochromic reaction. Depending on the application, the electrolyte, the counter electrode and the whole cell may possess certain specific characteristics. 

For display devices, a pigment has to be mixed into the electrolyte to provide a diffuse backgrounds (Fig. 38.1a). Moreover, very fast switching properties and a lifetime of 10 cycles are needed. The counter electrode needs only to be a material which is electrochemically reversible to the ion involved in the electrochromic reaction without having any particular optical properties. 

Proposed mechanisms for the electrochromic reaction of nickel hydroxide or nickel oxide in aqueous electrolyte are given by the following reactions (Svaisson, 1986 Yu, 1986 Agrawal, 1992 Nagai, 1993 Azens, 1998). Without doubt, the electrochromic coloration is related to the redox process Ni + (darkj/Ni (bleached). 

In addition to the possible provision of a non-electrochromic counter electrode reaction, several studies have incorporated an electrochromic counter electrode to supplement the electrochromism of the CP. These have a complementary color change (they darken when the CP lightens, etc.). Examples of this include prussian blue and WO3 glasses the latter use the cation of the dopant/electrolyte, e.g. Li of LiC104, in the electrochromic reaction. 

---