The Art of Electrocatalysis

Charge transfer is not unique to electrocatalysis as even a cursory survey of the catalytic literature can show. Indeed, oxidation (18-21), desulfurization (21), and reduction (22) mechanisms have been proposed, involving electron transfer between catalyst and reactant, to explain activity and selectivity effects. Electronic interactions between adsorbate bonds and d-band electrons of the catalyst are also used commonly to explain strength of adsorption (21,23,24). This electron exchange or transfer in conventional catalysis and electrocatalysis, and steps such as adsorption, surface reaction, and desorption, point toward expected similarities between the two catalytic 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

From this brief overview, it becomes apparent that selection of a suitable electrocatalyst should be based upon the following criteria  

maximum specificity and control of the desirable reaction path  

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Many electrode reactions only occur at a measurable rate at a very high overpotential, i.e. the exchange current is low. The art of electrocatalysis is to provide alternative reaction pathways which avoid the slow step and permit the reaction to be carried out with a high current close to the reversible potential, i.e. to increase the exchange current density. 

KEY MILESTONES ON THE WAY TO THE PRESENT STATE OF THE ART OF FUEL CELL ELECTROCATALYSIS... 

Part I presents the state of the art of the theory of catalysis and electrocatalysis at clusters and nanoparticles. This section provides the current frame of modeling of the interaction of clusters with substrates as well as catalytic and electrocatalytic kinetics on clusters and nanoparticles, including ab initio quantum mechanical calculations, and epitomizes recent advances in understanding the relation between electronic structure and catalytic/electrocatalytic activity. 

We stress again here that in the actual state of the art, the EM of organic pollutants with simultaneous production of electrical energy (fuel cell regime) is not feasible due to the lack of active electrocatalytic anode material. Bio-electrocatalysis is a new active field and can overcome this problem as it has been demonstrated recently in the development of bio-fuel cells. 

II. CURRENT STATE OF THE ART IN SURFACE SCIENCE TAILORED FOR ELECTROCATALYSIS INVESTIGATIONS... 

The use of synchrotron based in situ x-ray absorption spectroscopy (XAS) for the study of catalysis, both heterogeneous and electrocatalysis has matured over the last decade with simultaneous efforts in the United Sates, European Union and Japan. Some recent exemplification of the state of the art can be obtained in the following references, " and an extensive database of literature on its application to catalysis can be accessed electronically (www.exafs.chem.msu.su/ papers). Detailed aspects on application of the technique and methodology used for data analysis has been recently published. " ... 

Carbon supported Pt and Pt-alloy electrocatalysts form the cornerstone of the current state-of-the-art electrocatalysts for medium and low temperature fuel cells such as phosphoric and proton exchange membrane fuel cells (PEMECs). Electrocatalysis on these nanophase clusters are very different from bulk materials due to unique short-range atomic order and the electronic environment of these cluster interfaces. Studies of these fundamental properties, especially in the context of alloy formation and particle size are, therefore, of great interest. This chapter provides an overview of the structure and electronic nature of these supported... 

The discussion of a number of topics in electrocatalysis, including adsorption phenomena, surface reaction mechanisms and investigation techniques, electrocatalytic activity and selectivity concepts, and reaction engineering factors, may seem at first too diverse. We believe, however, that fundamental principles cannot be divorced from their natural counterpart, praxis. Here, we attempt to establish ties between basic and applied electrocatalysis and with their conventional similes, catalysis, surface physics (and spectroscopy) and reaction engineering. By taking a vitae parallelae perspective, we hope that a synthetic analysis of the present state of the art emerges. 

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... 

Wendt H, Spinace EV, Oliveira Neto A, Linardi M. Electrocatalysis and electrocatalysts for low temperature fuel cells fundamentals, state of the art, research and development. Qurm Nova 2005 28 1066-75. 

The preparation, stmcture, and performance of SOFC used for power generation is described in Chapter 12 of this handbook. Both the electrocatalysis and the catalysis of the NiAi"SZ anode are reasonably well understood and the same applies for the electrocatalysis of the Lai xSrxMn03 5 and Lai xSrxCo03 g cathodes. State-of-the-art SOFC units with anodic and cathodic overpotentials of less than 150 mV each at current densities up to 0.5 AJcm at T = 950°C are currently available. ... 

Lei, H., Atanassova, P., Sun, Y., and Blizanac, B. (2009) State-of-the-art electrocatalysts for direct methanol fuel cells, in Electrocatalysis of Direct Methanol Fuel Cells From Fundamentals to Applications (eds. H. Liu and J. 

Refs. [i] Inzelt G (2005) / Solid State Electrochem 9 245 [ii] Horanyi G (1980) Electrochim Acta 25 45 [iii] Horanyi G (2004) In Horanyi G (ed) Radiotracer studies of interfaces. Elsevier, Amsterdam, chapters 1,2,4,6 [iv] Horanyi G (2002) State of art present knowledge and understanding. In Bard AJ, Stratmann M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, Weinheim, Chap. 3 [v] Horanyi G (1999) Radiotracer studies of adsorption/sorption phenomena at electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry. Marcel Dekker, New York, pp 477 [vi] Horanyi G, Inzelt G (1978) / Electroanal Chem 87 423 [vii] Horanyi G, Inzelt G, Szetey E (1977) / Electroanal Chem 81 395 [viii] Vertes G, Horanyi G (1974) / Electroanal Chem 52 47 [ix] Horanyi G (1994) Catal Today19 285 [x] Horanyi G (2003) Electrocatalysis - heterogeneous. In Horvath IT (ed) Encyclopedia of catalysis, vol. 3. Wiley Interscience, Hoboken, pp 115-155 [xi] Inzelt G, Horanyi G (2006) The nickel group (nickel, palladium, and platinum). In Bard AJ, Stratmann M, Scholz F, Pickett CJ (eds) Inorganic chemistry. Encyclopedia of electrochemistry, vol 7a. Wiley-VCH, Weinheim, chap. 18... 

This work was supported by the Serbian Ministry of Education and Science (Contraet III45014). S.V.M. acknowledges the support provided by the Serbian Academy of Science and Arts through the project Electrocatalysis in the eontemporary processes of energy conversion . 

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