Electrocatalysis, working electrodes

Fig. 27 Electrocatalysis of NOx reduction in the presence of 2 10 M CU4P4W30 in a pH 5 medium. The scan rate was 2 mV s, the working electrode was glassy carbon and the reference electrode was SCE. (a) Nitrate (b) nitrite. For more detailed information, see text (taken from Ref 115).

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

The selective facilitation of the charge transfer of the species of interest is called electrocatalysis. In such a case, the species of interest are transformed at energies substantially lower than those of the interferants. The higher selectivity therefore implies a lower applied potential at the modified working electrode, which exhibits such selective electrocatalytic properties. In such a situation, the choice of the... 

Electrocatalysis is a type of electrosynthesis that uses surface modified electrodes, or mediators/electrocatalysts to facilitate the redox reaction. Meyer reported the design and synthesis of a chemically modified electrode that consists of a thin polymer film with covalently attached redox sites,designed to facilitate rapid electron transport for electrocatalysis. Complexes of Fe, Ru, Os, Re, and Co were synthesized in such a way that when electrochemically reduced, they reacted to form smooth electroactive polymer films that adhered well to the working electrode to form a chemically modified electrode designed for electrocatalysis. 

An important aspect of the study of electrocatalysis is the accurate measurement of the overpotential, rj, of the electrocatalyst. The overpotential is the deviation of the electrocatalyst potential from its open-circuit (I = 0) value. The basic experimental setup for the study of electrocatalysis is the three-electrode system comprising the electrocatalyst under study, termed working electrode (W), a counterelectrode (C), and a reference electrode (R). 

Figure 8-13. Left Electrocatalysis of A. xylosoxidans Cu-nitrite reductase on cysteamine-modified Au(lll)-electrode 5 mM sodium acetate buffer, pH 6.0. Scan rate 10 mV s . Dashed line no KNO2 present. Solid lines Potassium nitrite concentrations (pM) a 70, b 110, c 250, d 800. Right In situ STM of A xylosoxidans Cu-nitrite reductase. Same conditions. Potassium nitrite concentration 200 pM. Working electrode potential +0.38 V. Bias voltage -1.10 V. Tunneling current 0.1 uA. From ref. 127 with permission.

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

Work on the modification of electrode surfaces is of more recent origin, but the research has already gathered considerable momentum. Coordination compounds have not played a unique role in this development, but they have on occasion made important advances possible. Some authors believe that this work will influence the direction of electrochemistry for many years to come. It certainly has implications for electrocatalysis, electronic devices, visual display units and photoelectricity to mention but a few topical objectives which currently drive the research. 

Joaquin Gonzalez is a Lecturer at the University of Murcia, Spain. He follows studies of Chemistry at this University and got his Ph.D. in 1997. He has been part of the Theoretical and Applied Electrochemistry group directed by Professor Molina since 1994. He is author of more than 80 research papers. Between 1997 and 1999, he also collaborated with Prof. Ms Luisa Abrantes of the University of Lisboa. He is the coauthor of four chapters, including Ultramicroelectrodes in Characterization of Materials second Ed (Kaufmann, Ed). He has taught in undergraduate and specialist postgraduate courses and has supervised three Ph.D. theses. His working areas are physical electrochemistry, the development of new electrochemical techniques, and the modelization, analytical treatment, and study of electrode processes at the solution and at the electrode surface (especially those related to electrocatalysis). 

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