Hydrogen electrode reactions, formulated

Great promise exists in the use of graphitic carbons in the electrochemical synthesis of hydrogen peroxide [reaction (15.21)] and in the electrochemical reduction of carbon dioxide to various organic products. Considering the diversity in structures and surface forms of carbonaceous materials, it is difficult to formulate generalizations as to the influence of their chemical and electron structure on the kinetics and mechanism of electrochemical reactions occurring at carbon electrodes. 

However, this formulation yields very little information on the actual course of the reaction, i.e. of which partial processes it consists. This set of partial processes is termed the mechanism of the electrode reaction. In the first case, the electrode reaction at some electrodes involves the formation of an adsorbed hydrogen atom, followed by the recombination reaction ... 

This is demonstrated by combining an oxygen electrode and a hydrogen electrode as the two half cells of an electrochemical cell. The half cell reactions are commonly formulated as reductions ... 

A special case of heterogeneous catalytic kinetics, that we will briefly discuss, is electrocatalysis, which is defined as the acceleration of an electrodic reaction by a substance that which is not consumed in the overall reaction. For a catalytic reaction with the following step Sads+Hads=>SHads this is only one of the possible mechanisms for the involvement of hydrogen in the catalytic reduction. Another mechanism is the electrochemical or ionic mechanism where the adsorbed hydrogen serves only as an electron source for the reduction process. This type of reaction is formulated by the following reaction steps ... 

The concept of stoichiometric number was introduced in electrochemistry by Horiuti and Ikusima [42, 43] for the hydrogen electroreduction reaction. We need to introduce the stoichiometric number V in complex multielectron electrode kinetics in order to distinguish different possible mechanisms. The International Union of Pure and Applied Chemistry (lUPAC) defines the stoichiometric number in electrochemistry as a positive integer that indicates the number of identical activated complexes formed and destroyed in the completion of the overall reaction as formulated with the charge number, n [44, 45]... 

Since the solution is alkaline, the electrode reactions are formulated with hydroxyl ions OH rather than with hydrogen ions, and the electrode potentials are referred to the alkaline variant of the SHE. This explains the difference between equations (4.19) and (4.2), which from a thermodynamic point of view are completely equivalent. 

Hydrogen evolution, the other reaction studied, is a classical reaction for electrochemical kinetic studies. It was this reaction that led Tafel (24) to formulate his semi-logarithmic relation between potential and current which is named for him and that later resulted in the derivation of the equation that today is called "Butler-Volmer-equation" (25,26). The influence of the electrode potential is considered to modify the activation barrier for the charge transfer step of the reaction at the interface. This results in an exponential dependence of the reaction rate on the electrode potential, the extent of which is given by the transfer coefficient, a. 

If the reversible element is a metal or hydrogen, which is in equilibrium with positive ions (cations), the reaction occurring at the electrode when it forms part of a cell may be formulated as... 

Generation of photocurrent at the semiconductor/electrolyte interface upon its illumination makes it possible to carry out photoelectrochemical reactions which can be used either for chemical fuel production, or purification of waters. Principles of operation of electrochemical cells with semiconductor electrodes for solar energy conversion to electrical and chemical energy are formulated. Most efficient cells for electricity and hydrogen production are surveyed. Certain processes for photo-destruction of pollutants, recovery of metals, etc. with making use of semiconductor dispersions are briefly discussed. 

In hydrogen-oxygen fuel cells with an alkaline electrolyte, the reactions at the electrodes and the overall current-generating reaction can be formulated as follows ... 

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