Discussion

As mentioned in section 7.2.1, the positions of the peaks observed on the cyclic voltammograms were in the region expected for the Eu^+/Eu^+ couple measured with respect to the Li+ /Li reference electrode. The most striking feature of the cyclic voltammograms however, was the reduced magnitude of the anodic peak (oxidation of Eu2+) as compared with the cathodic peak (reduction of Eu^+). This suggested that

after electrochemical formation of Eu^+, a following chemical step was removing it from the system. Further support of this hypothesis came from the fact that the ratio o f the peak currents, (1^ 1 ), was less than unity and increased with increasing sweep

rate. i.e. at faster sweep rates, the anodic peak was larger due to the fact that more of the Eu^+ was oxidised before it had a chance to react. The reaction scheme for the system can be described as an ec mechanism in which an electron transfer step (e) is followed by a chemical step (c).

Eu^+ + e V Eu 2+

7 - ii)

Eu^* --- ^ C

Although it was not possible to postulate with any degree of certainty the nature of the chemical step, it was clear that the products, C, other than Eu^+ were not electrocative in the potential range of the experiment. The strongly reducing nature o f the Eu^+ ion may enable it to scavenge any impurities present in the polymer or it may even have attacked the polymer itself.

In the light o f the proposed ec mechanism for the reduction o f the Eu^+ ions, the validity of determining the transfer coefficients from the Tafel plot figures 7-6 to 7-8 was called into question. A t any given potential, the anodic current would be less than ‘expected’ due to the removal of Eu^+ from the system by the chemical step following electron transfer. For this reason, the slope of the Tafel plot, particularly in the region of high positive potentials would be reduced and the value of the transfer coefficient, a ^, determined in section 7.2.1 lower than the true value. For simple electron transfer reactions, the sum o f the transfer coeficients , + a ^, should equal unity. The departure from this behaviour in the Eu^+ / Eu^+ system was therefore a reflection of the complexity of the redox process.

The small exchange current density, 0.12 pAcm-2, determined fo r the system (section 7.2.1), was indicative of slow kinetics for the electrode reaction. Optical microscopy experiments by Smith et al.^^z have suggested that Eu(CF^SOg)3 forms a

crystalline complex w ith PEO. In the temperature and composition range o f the experiment however, it is unlikely that much of the Eu^+ present in the system was in

the form o f a crystalline complex. It was probable that a PEOiLiCFgSOg complex was present in significant proportions. This would have the affect o f reducing the concentration of the supporting electrolyte in the amorphous regions of the polymer. The presence of these crystalline regions is likely to severely lim it the segmental motion of the polymer chains in these amorphous regions, since some chains pass through both crystalline and amorphous regions.

In classical liquid electrolytes, the reorganisation of the primary solvation sphere upon electron transfer is often a relatively facile process for small solvent molecules. For the polymer electrolyte case, reorganisation of the polymer coordination sphere is like ly to involve a significant amount of energy. This would give rise to a large activation energy for electron transfer and would thus severely lim it the kinetics.

In section 7.2.3, it was assumed that the Warburg impedance dominating the ac impedance plot for the cell (N i / PEO ^0Œu(O^SO3)2 ^ P^OgoLiCFgSOg /L i /

PE0 2oLiCF^S0 3 / L i) could be ascribed to the diffusion o f Eu^+ ions. It is likely

however that several diffusional processes occur in the system

i) diffusion of Eu^+ ions towards and away from the electrode, ii) diffusion of Eu^+ ions towards and away from the electrode, iii) diffusion o f the reactants involved in the chemical step towards the

Eii2+ ions prior to the reaction and

iv) diffusion of the products of the chemical reaction away.

The behaviour of the system was clearly dominated by an extremely slow diffusional process. O f the first two steps, step i) is likely to be the slowest since the stronger interaction between the polymer and the hard Eu^+ ions is likely to render them more immobile than the less highly charged Eu^+ ions. If step iii) was the rate lim iting step, then clearly the cyclic voltammograms would have exhibited more reversible behaviour than was illustrated in figure 7-3. Despite the unknown nature o f the chemical step, it is likely that Eu3+ w ill be one of the products. This step w ill therefore

involve diffusion of Eu^+ ions as in step i). In conclusion, it can be seen that it is likely that the diffusional feature of the impedance plot can indeed be ascribed to the motion of Eu^+ ions and that the diffusion coefficient determined was indeed that for Eu3+.

The diffusion coefficient determined appeared to indicate that mass transport of Eu^+ through the polymer was extremely slow. This is not unexpected due to the hard acid- hard base interaction between the Eu^+ ions and the ether oxygen atoms on the polymer. Strong coordination of the ion by the polymer would be expected to severely restrict the m obility of the ion through the polymer.

The presence of crystalline regions in the polymer would also inhibit the motion of ions by restricting the segmental motion of the polymer chains in the amorphous regions. Furthermore, the mean free path of the ions to the electrode would be greatly increased due to the tortuous routes taken to avoid these crystalline regions. These factors would all serve to reduce the determined value of the diffusion coefficient compared to the ‘true’ value in a completely amorphous system.

De Kreuk et aF®^ determined Dg^g^ = 8.5 x 10'^ cm^s’ l for a lOmM solution in IM KCl. Despite the vastly different experimental conditions, the im m obility of the Eu^+ ions in the PEO system, where Dg^g^ « 3.66 x 10"!^ cm^s“^ has been clearly demonstrated.