4 Two quite different aspects of the field of electrochemistry
5 Reactions and electrodes The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe the real chemical changes at the two electrodes.Most of the time one is interested in only one of these reactions, and the electrode at which it occurs is called the working (or indicator) electrode, coupled with an electrode that approaches an ideal nonpolarizable electrode of known potential, called the reference electrode. In experiments, the current is passed between the working electrode and an auxiliary(or counter) electrode.Three electrodes are frequently placed in three compartments separated by a sintered-glass disk.
6 Three-electrode cell and notation for the different electrodes
7 Reference electrode● The potential of the working electrode is monitored relative to a separate reference electrode, positioned with its tip near the working electrode.● The internationally accepted primary reference is the standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE), which isPt/H2(a=1)/H+(a=1,aqueous)●By far the most common reference is the saturated calomel electrode (SCE), which isHg/Hg2Cl2/KCl(sat’d in water)Its potential is V vs. NHE.
8 Scope of electrochemistry IntroductionIntroductionInvestigation of chemical phenomena associated with a charge transfer reactionTo assure electroneutrality two (or more) half-reactions take place in opposite directions (oxidation/reduction)If the sum of free energy changes at both electrodes is negative electrical energy is released batteryIf it is positive, external electrical energy has to be supplied to oblige electrode reactions electrolysis
9 Factors affecting electrode reaction rate In general, the electrode reaction rate is governed by rates of processes such as:Mass transfer (e.g., from the bulk solution to the electrode surface).(2) Electron transfer at the electrode surface.(3)Chemical reactions preceding or following the electron transfer.(4)Other surface reactions.◆ The magnitude of this current is often limited by the inherent sluggishness of one or more reactions called rate-determining steps.
10 Conditions for electrochemical experiments Reproducible experimental conditions must be givenInterfering side effects must be avoided asMigration effectsHigh solution resistance-these effects can be minimised by adding an inert supporting electrolyte (around 1 mol/L)Undefined or large diffusion layerA complete study of the electrode process requires the measurement of kinetic as well as thermodynamic parameters.
11 Faradaic and nonfaradaic processes Charges (e.g., electrons) are transferred across the electrode-solution interface and causes oxidation or reduction to occur. Since these reactions are governed by Faraday’s law, they are called faradaic processes.Under some conditions, processes such as adsorption and desorption can occur, and the structure of the electrode-solution interface can change with changing potential or solution composition, these processes are called nonfaradaic processes.
12 Capacitance and charge of an electrode The behavior of the electrode-solution interface is analogous to that of a capacitor. When a potential is applied across a capacitor, charge will accumulate on its electrode plates.At a given potential there will exist a charge on the metal electrode, qM, and a charge in the solution, qs. At all times, qM=-qs.At a given potential the electrode-solution interface is characterized by a double-layer capacitance, Cd, typically in the range of 10 to 40μF/cm2.
14 The nature of electrode reactions Electrode reactions are heterogeneous and take place in the interfacial region between electrode and solution diffusion layerThe charge separation at each electrode is represented by a capacitancethe difficulty of charge transfer by a resistanceThe electrode can act as (1) a source of electrons (cathode) reduction ,(2) a sink of electrons transferred from species in solution (anode) oxidationThe amount of electrons transferred is related to the current flowing between the two electrodes
15 Thermodynamics and kinetics The potential at which a reduction or oxidation takes place (measured relative to the normal hydrogen electrode) is given by the Nernst equationi : stoichiometric numbers: positive for reduced species, negative for oxidised speciesE0 : standard electrode potentialci : concentration (ai has to be applied if activity coefficient is not 1)E = E0 – (RT/nF) i ln ci
16 Thermodynamics and kinetics ●The concentration of species at the electrode interface depends on its mass transport coefficient kd and●The rate of the electrode reaction is expressed by the standard rate constant k0 which is the rate when E = E0reversible reaction k0 >> kdirreversible reversible reaction k0 << kd,an overpotential has to be applied additionally to overcome this kinetic barrier●A behaviour in between these extremes is called quasireversible reaction