Electrochemical Engineering

Abstract

As discussed in Chap. 2 any chemical reaction involving charge exchange be-tween two different redox reactants, see Eqs. (3.1 a,b) can be performed virtual-ly – and very often also practically – by performing the redox reactions of the two redox-couples separately at two different electrodes but jointly in a divided electrochemical cell (Fig. 3.1): total reaction (3.1) redox couple A (3.1 a) redox couple B (3.1 b) thus the oxidation of ferrous ions by ceric ions (3.1 c) may be performed in two half cells, one containing the ferric/ferrous system , the other containing the ceric/cerous system . Fig. 3.1. Schematic of a cell reaction composed of two separate redox reactions which jointly would establish a homogeneous redox reaction between an oxidant and a reductant. () + − + + → 1 Fe Ce Fe Ce 2 4 3 3 + + + + + → + () Fe Fe e 2 3 + + − → + () Ce e Ce 4 3 + − + + → 18 3 Electrochemical Thermodynamics Both being connected by a salt bridge 1) , they deliver an electrical potential dif-ference U 0 between two Pt-electrodes inserted into the two separated electro-lytes (Fig. 3.2) Likewise the formation of hydrochloric acid, dissolved at a given concentra-tion in water, from gaseous chlorine and gaseous hydrogen, realized in two half cells connected by an electrolyte bridge, generates a cell potential. If the reaction is performed electrochemically, and , by dipping a chlorine electrode (made of platinized platinum and being supplied and sparged with elemental chlorine) and a hydrogen elec-trode (platinized, H 2 -sparged platinum electrode) into an aqueous solution of hydrochloric acid of the respective concentration, the equilibrium cell potential can be measured between the two platinum electrodes at vanishing current with a voltmeter of high internal resistance. This cell potential is sometimes called " electromotive force " . Under condi-tions of vanishing cell current and established reversibility of the two different electrode reactions, (which is accomplished by effective electrocatalysis of the two electrode reactions by applying for instance platinum black), the so called open cell potential becomes the equilibrium cell voltage U 0 . U 0 equals the free energy ∆ G of the cell reaction per mol of product divided by the number ν e of Faradays (1F=96,500 As) necessary for the electrochemical generation of one mol of product. (3.2 a)