Modified electrode study techniques

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

A method for the preparation of thin films of Fe4[Ru(CN)6]3 ( ruthenium purple ) involving electrochemical reduction of K3[Ru(CN)6] in a solution of Fe2(S04)3 has been developed.28 This ruthenium purple modified electrode is claimed to be one of the best catalysts for evolution of oxygen and chlorine. Electrochemical studies on polyammonium macrocyclic complexes of [Ru(CN)6]4 indicate a 1 1 stoichiometry with a monoelectronic, reversible, oxidation for these complexes this illustrates the control of redox potential of anions by complexation with appropriate receptor molecules.29 The kinetics of oxidation of [Ru(CN)6]4 by [Mn04] in HC104 have been investigated by stopped-flow techniques. It is found that [Ru(CN)6]4" is quantitatively oxidized to [Ru(CN)6]3 in accordance with equation (1) and that two protonated intermediates [RuH(CN)6]3 and [RuH2(CN)6]3 are involved in the oxidation process.30... 

Numerous studies have now focussed on this technique of using diazonium salts for modifying electrode surfaces for a whole host of applications[9,57,58,63-65]. For example, Hong and Porter[66] have reported the electrochemical reduction of benzenediazonium tetrafluoroborate in acetonitrile containing tetrabutylammonium tetrafluoroborate to... 

J. G. Gaudiello, P. K. Gosh, and A. J. Bard, Polymer films on electrodes. 17. The application of simultaneous electrochemical and electron spin resonance techniques for the study of two viologen-based chemically modified electrodes, J. Am. Chem. Soc. 107, 3027-3032 (1985). 

We have applied this technique to the study of the proton flux that takes place when a modified electrode, the thionine-coated electrode, is either oxidised or reduced. We were particularly interested in the question as to whether the proton and electron fluxes were in time with one another or not. Typical results for proton and electron fluxes for reduction and oxidation at a number of different values of pH are displayed in Fig. 7. At first sight, we were bewildered by the variety of behaviour. However, we can explain the different transients as follows. In Table 2, we set out the scheme of squares [18, 19] for the thionine/leucothionine system with a number of vital pKk values. Starting at pH 4 in the oxidation direction (LH + - Th+ + 2e + 3H+), we see that the proton flux is indeed larger than the electron flux and that both fluxes are in time with each other. In the opposite reduction direction, the electron flux is similar but the proton flux is smaller and delayed. The reason for this is that, to start with, protons are used up and the pH crosses the pKa at 5.5 (Th+ + 3H+ + 2e - LH +). However, for pH > 5.5, the reaction can utilise the H+ stored in the coat (Th+ + 2 LH2+ + 2 e - 3 LH2+). This means that bulk H+ is not consumed, leading to a smaller H+ transient. When the electron flux dies away, the pH drifts back to the equilibrium value of 4. As it does so, there is an H+ flux from the relaxation LH2+ + H+ - LH +. The explanation of the transients at pH 5 is similar. In the reduction direction, the H+ flux has almost completely collapsed. In this case, the pH crosses the pKa boundaries at 8.5 where there will be no H+ flux (Th+ + 2e -> L ). The relaxation flux after the electron flux has died away will also be small since the bulk concentration of H+ (pH = 5) is so small. At pH 6, the reduction transients are similar to those at pH 5. In the oxidation direction, the pH rapidly crosses the pKa = 5.5 boundary. Now the coat mops up the H+, releasing no H+ to the solution (3LH2+ - ... 

Within the last two decades Electron Spin Resonance-(ESR) spectroscopy has become a standard experimental technique in electrochemical research. The main interest was in the field of electrochemical generation of radicals to characterize their structure by ESR spectroscopy or to prove their presence in electrode reactions. The studies have been extended to the kinetics of radical reactions and the set up of reaction mechanism, to the solvation phenomena in radical electron densities and to radical conformation and ion complex structure. The latest development is the study of the electrode materials and their surface layers in electrochemical systems by simultaneous ESR spectroscopic and electrochemical measurements, e.g., of polymer modified electrodes. 

Traditionally, UV-visible spectroscopy has been the main spectroscopic probe of electrochemical systems, covered in Chapter 3.4 by Crayston. This chapter describes the main techniques, theory and appKcations in the study of solution species (inorganic and organic), thin films and modified electrodes, along with other topics. A wide variety of cells have been described, to allow the use of UV-visible spectroscopy in electrochemistry, and consequently these are described in some detail. 

---