Four-electron reduction electrocatalysis

The first accounts that seemed to give direct enzyme electrochemistry were the reports concerning a soluble blue Cu oxidase, laccase, which catalyzed the rapid four-electron reduction of dioxygen to water. An efficient electrocatalysis of O2 reduction by adsorbed fungal laccase on pyrolytic graphite, glassy carbon, and C02-treated carbon black electrodes was first described by Tarasevich and co-workers (48). Several control experiments were carried out to verify direct electron transfer from the electrode to the Cu sites of the enzyme. 

Collman, J.P, P. Denisevich, Y. Konai, M. Marrocco, C. Koval, and F.C. Anson (1980). Electrocatalysis of the four-electron reduction of oxygen to water by dicobalt face-to-face porphyrins. J. Am. Chem. Soc. 102, 6027-6036. 

When the four electron reduction is seeked to achieve the maximum energy release by the electrodiemical reduction of O2, as in fuel cells, different strategies have been proposed to get a rtytid multiple electron transfer. An efficient four electron electrocatalysis was claimed using dicobalt dimers (foce-to face cobalt poi yrins) [7] or complexes containing multiple centers vliich can serve as electron donors are used [9]. 

In recent years, electrocatalysis has been widely employed to reductively activate dioxygen [22b, 146,147]. The reduction of proceeds through two pathways, which are mainly determined by the electrocatalyst and electrode potential two-electron reduction into (Eq. 14.34) and four-electron reduction into H O (Eq. 14.35). 

Figures 5.4 and 5.5 summarize results of a recent study of P. versicolor laccase electrochemistry based on cyclic and rotating disk voltammetry [60]. Figure 5.4 shows unequivocally that this laccase is voltammetrically active and gives a kinetically controlled, unpromoted four-electron peak at edge-plane pyrolytic graphite. Electrochemical reduction of 02 catalyzed by an immobilized laccase monolayer is close to reversible, and unrestricted by mass transport. The electrocatalysis follows, moreover, a Michaelis-Menten pattern (Fig. 5.5). Finally, there is a characteristic bell-shaped functional pH-profile with a pronounced maximum at pH 3.1.

The ORR [Eq. (15.2)] is considered one of the major challenges in electrocatalysis, from both fundamental and apphed points of view. Some recent reviews have summarized the state-of-the-art and actual developments in this field [6,7). Despite extensive research, the ORR is not well understood, and practical oxygen reduction catalysts and electrodes for fuel cells experience a large overjxitential for this reaction, that is, a kinetic barrier, which contributes a voltage loss of 25% (see above). One of the reasons for the sluggishness of this reaction is that four electrons have to be transferred and also four protons added for the complete reduction of oxygen to water in an acidic electrolyte. Hence the ORR exhibits... 

---