Electrocatalysis of the oxygen reduction reaction

The kinetics and mechanism of electrochemical oxygen reduction at different catalysts has been widely studied from the late 1950s onward, as can be seen in the monographs of Hoare (1968), Breiter (1969), and Kinoshita (1992). 

One important question that up to now has not been answered conclusively is that of the nature of the open-circuit potential exhibited by an oxygen electrode (Section 12.6.1). 

An important requirement that must be met by a catalyst for the oxygen electrode is that of accelerating the reduction reaction that follows the four-electron mechanism  

Recent work related to oxygen elecffodes in fuel cells has mainly been as follows  

The main reaction occurring at the oxygen electrode in fuel cells, 

---

The effect of the distance between the active center and the electrode on the reaction rate has been studied using as an example the electrocatalysis of the oxygen reduction reaction by laccase adsorbed on soot. Variation in the distance between the active center and the electroconductive substrate was achieved by inserting an intermediate monolayer of lipid molecules flatly and vertically oriented cholesterol molecules and vertically oriented lecithin molecules (scheme in Figure 36). In this case, the conditions of obtaining compact lipid monolayers were fulfilled. The subsequent setting of laccase did not lead to their desorption. 

Specific processes involved in the ORR will be dealt with in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum in Chapter 3. Here, the focus is on basic phenomenological concepts that are required to incorporate electro-catalytic reactions in device-level modeling of PEFCs. For a more detailed treatment of fundamental concepts in electrocatalysis, see the recent edition of the textbook by Schmickler and Santos (2010), as well as the classical textbook by Bard and Faulkner (2000). 

Pathways, mechanisms, and corresponding kinetic parameters of the ORR have been discussed in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum. In a highly simplified picture, derived originally on the basis of a series of experimental studies by Damjanovic and coworkers (Damjanovic, 1992 Gatrell and MacDougall, 2003 Sepa et al., 1981,1987), it was proposed that the rate-determining reaction step is the initial adsorption. 

The electrocatalysis of the oxygen reduction reaction is of theoretical and practical interest because of its paramount importance for electrochemical energy conversion and industrial electrolysis. Au nanoparticles deposited on a BDD electrode show excellent activity towards catalytic reduction of oxygen [23], which also depends on the particle size, the nature of the support and the preparation method [24]. 

The effect of Pt crystallite size on the kinetics of the oxygen reduction reaction is a long-standing problem in electrocatalysis. An excellent review of experimental work with carbon-supported Pt was presented by Kinoshita [33] for work prior to about 1990. Kinoshita concluded that the change in fraction of surface atoms on the... 

For a given electrochemical system, the increase of the voltage efficiency is directly related to the decrease of the overpotentials of the oxygen reduction reaction, t]c, and alcohol oxidation reaction, T]a, which needs to enhance the activity of the catalysts at low potentials and low temperature, whereas the increase of the faradic efficiency is related to the ability of the catalyst to oxidize completely or not the fuel into carbon dioxide, i.e. it is related to the selectivity of the catalyst. Indeed, in the case of ethanol, taken as an example, acetic acid and acetaldehyde are formed at the anode [10], which corresponds to a number of electrons involved of 4 and 2, respectively, against 12 for the complete oxidation of ethanol to carbon dioxide. The enhancement of both these efficiencies is a challenge in electrocatalysis. 

Wang, J. and Swain, G. M. (2003) Fabrication and evaluation of platinum/diamond composite electrodes for electrocatalysis - Preliminary studies of the oxygen-reduction reaction, J. Electrochem. Soc. 150, E24-E32. 

Recent intensive research efforts have led to the development of less expensive and more abundant electrocatalysts for fuel cells. This book aims to summarize recent advances of electrocatalysis in oxygen reduction and alcohol oxidation, with a particular focus on low- and non-Pt electrocatalysts. The book is divided into two parts containing 24 chapters total. All the chapters were written by leading experts in their fields from Asia, Europe, North America, South America, and Africa. The first part contains six chapters and focuses on the electro-oxidation reactions of small organic fuels. The subsequent eighteen chapters cover the oxygen reduction reactions on low- and non- Pt catalysts. 

Until now, the methodology available to study charge transfer reactions at soft interfaces has been rather mature, and studies in the field have shifted to the study of catalyzed reactions such as the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), or even oxygen evolution reaction (OER). Eor this, two classes of catalysts have been used (i) molecular catalysts and (ii) nanoparticle solid catalysts. These two approaches draw their inspiration from classical molecular catalysis and from electrocatalysis, respectively. 

Dodelet JP (2013) The controversial role of the metal in Fe- or Co-based electrocatalysts for the oxygen reduction reaction in acid medium. In Shao M (ed) Electrocatalysis in fuel cells a non- and low-platinum approach. Springer London, United Kingdom, pp. 271-338, ISBN 978-1 71 910-1... 

This chapter concentrates on the first principles studies of fiiel cell air-cathode electrocatalysts, i.e., oxygen reduction reaction (ORR) electrocatalysis. Recent progress in theoretical methodologies will be introduced and the current status of first principles studies of the ORR will be reviewed. The chapter is organized as follow first, the background theory and the oxygen reduction reaction will be introduced, and then theoretical applications in the study of the ORR will be... 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

Pt is, of course, not a good electrocatalyst for the O2 evolution reaction, although it is the best for the O2 reduction reaction. However, also with especially active oxides of extended surface area, the theoretical value of E° has never been observed. For this reason, the search for new or optimized materials is a scientific challenge but also an industrial need. A theoretical approach to O2 electrocatalysis can only be more empirical than in the case of hydrogen in view of the complexity of the mechanisms. However, a chemical concept that can be derived from scrutiny of the mechanisms mentioned above is that oxygen evolution on an oxide can be schematized as follows [59] ... 

---