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.To complete the circuit ions must cross the cell(cations towards the cathode competing with anions towards the anode).Clearly, the energy necessary to pass electrons right across the cell involvesthe inherent energetics of electron transfers at both electrodes ( electrode poten-tials of the processes involved) and a component relating to mass transport,which involves the diffusion of species to and from the electrode as well as themobility of ions crossing the centre of the cell.Highly oxidized and reducedspecies are simultaneously formed, however we may only be interested in one ofthese, and because they could react together unfavourably if allowed to meet, wemay keep them apart by a semi-permeable membrane between the cell compart-ments.This will increase the overall cell resistance to some extent and will affectthe cell voltage required to maintain electrolysis.Now if there were two highly reactive species (for the oxidation and reduc-tion process), it would require only a low voltage to drive the two electrodereactions.* In fact a battery is a device that contains species that are suffi-ciently reactive so that instead of needing any externally supplied voltage tomove electrons, the electrons are driven externally under the energy of reac-tion.Alternatively, two unreactive species (or even just one, if sufficientlyunreactive) will require a higher voltage for electrolysis.The upshot is that the measured voltage necessary to drive a cell with theminimum two electrodes is a complex mixture of potentials at both elec-trodes together with various voltage losses in the system, and we do notknow, if the required voltage should suddenly be seen to rise after a period ofelectrolysis, whether this is due to some time-dependent effect at the anode,the cathode or elsewhere in the system.Since the measured voltage in the above arrangement is ambiguous, it iscommon to run a two-electrode cell at constant current, so that the number ofelectrons being transferred in unit time is known, and the charge passed duringelectrolysis, which is an important figure of merit to be monitored, is simplythe product of the constant current and the time of passing.This is procedu-rally simple, requiring only a straightforward power source as apparatus,but there may be complexities in interpretation of results.The electrolysis mix-ture contains a solvent, an electrolyte salt, and the electroactive species ofinterest; and although initially this electroactive species will necessarily bechosen to be the most reactive, as time goes by and the species becomes* The voltage between the two electrodes in a simple cell of this type is highly dependent upon a num-ber of factors and more usually a three-electrode cell containing a chemical standard is used (see text).160 6: Conducting polymers from heterocyclesdepleted, in order to maintain constant current the power source will increaseits applied voltage so that another species will start to react.This could be thesolvent, for instance, forming disadvantageous by-products.One way to spotthis happening is to note the rise in cell voltage if the power source has a read-out to display the voltage it is delivering to maintain current (or else the opera-tor should make sure there is a voltmeter in parallel across the cell).However,a voltage rise could indicate depletion at the counter-electrode, partial foulingof a cell membrane or some other effect that does not compromise the reactionof interest at the working electrode.Furthermore, it may be that the operatorhas selected too high a current density (current per unit area of electrode) per-haps even from the start of electrolysis, such that the concentration and diffu-sion properties of the desired electroactive species cannot maintain thiscurrent value.In this case other processes will interfere.Now, this is not usually a problem for producing polypyrrole and related poly-mers, since a typical film on the electrode weighs only some tens of milligrams,and an electrolyte containing, for example, 0.01 M pyrrole can support its forma-tion without appreciable depletion of the monomer.Constant-current electroly-sis is therefore often used for these polymers; although it should be noted thatthe exact properties of the film can vary with preparation conditions, and withthis methodology the exact electrode potential is not known
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