Basic Electrocatalytic Concepts

For the benefit of the would-be electrochemist or electrochemical engineer, we shall outline briefly some principles that would facilitate understanding of the origin of electrochemical reactions and the rate at which they proceed. 

The potential distribution of Fig. 1 is typical of the compact-diffuse layer models (42-44). The potential varies almost linearly within the compact double layer, decaying exponentially within the diffuse layer. The thickness of the latter depends on the electrolyte ionic strength and becomes negligible in strong electrolytic solutions (42). This feature becomes important in electrocatalytic studies, since it is the potential difference (0 — e) that can be measured or fixed experimentally versus a reference electrode, while reacting ions and molecules experience a potential difference ((j) — 

For an electrochemical reaction at equilibrium, the standard electrode potential °( = / m - / mp) can be calculated from the standard free energy, AGr of the reaction at 1 atm, 1 N solution and 25 C  

Reaction (kcal/mol) (kcal/mol) / (V) (estimated) Er (V) (observed) References 

For conditions other than standard state, the reversible electrode potential can be calculated from the chemical potentials of reactants and products, Hj  

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In view of this need, we discuss here a variety of electrocatalytic topics, ranging from basic and microscopic concepts to phenomenological principles. Thus, the origin of electrodic reactions, electrosorption, and electrode kinetics are introduced briefly for the benefit of the nonelectrochemist. Since electrocatalytic reactions take place at the electrode surface, attention is given to recent efforts to link catalyst activity with microscopic surface properties. These include surface crystallographic orientation, crystallite size and distribution, adsorbate-adsorbent-support synergism, multiple adsorption states, identification of surface intermediates, and electrocatalytic surface reaction mechanisms. 

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. 

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