Thionine-coated electrodes

We have applied this technique to the study of the proton flux that takes place when a modified electrode, the thionine-coated electrode, is either oxidised or reduced. We were particularly interested in the question as to whether the proton and electron fluxes were in time with one another or not. Typical results for proton and electron fluxes for reduction and oxidation at a number of different values of pH are displayed in Fig. 7. At first sight, we were bewildered by the variety of behaviour. However, we can explain the different transients as follows. In Table 2, we set out the scheme of squares [18, 19] for the thionine/leucothionine system with a number of vital pKk values. Starting at pH 4 in the oxidation direction (LH + - Th+ + 2e + 3H+), we see that the proton flux is indeed larger than the electron flux and that both fluxes are in time with each other. In the opposite reduction direction, the electron flux is similar but the proton flux is smaller and delayed. The reason for this is that, to start with, protons are used up and the pH crosses the pKa at 5.5 (Th+ + 3H+ + 2e - LH +). However, for pH > 5.5, the reaction can utilise the H+ stored in the coat (Th+ + 2 LH2+ + 2 e - 3 LH2+). This means that bulk H+ is not consumed, leading to a smaller H+ transient. When the electron flux dies away, the pH drifts back to the equilibrium value of 4. As it does so, there is an H+ flux from the relaxation LH2+ + H+ - LH +. The explanation of the transients at pH 5 is similar. In the reduction direction, the H+ flux has almost completely collapsed. In this case, the pH crosses the pKa boundaries at 8.5 where there will be no H+ flux (Th+ + 2e -> L ). The relaxation flux after the electron flux has died away will also be small since the bulk concentration of H+ (pH = 5) is so small. At pH 6, the reduction transients are similar to those at pH 5. In the oxidation direction, the pH rapidly crosses the pKa = 5.5 boundary. Now the coat mops up the H+, releasing no H+ to the solution (3LH2+ - ... 

The beastly electrode is merely an efficient catalyst for the back reaction. In my third lecture we shall deal with the problems of electrode selectivity and the thionine coated electrode in particular. My fourth lecture vrill be concerned with the efficiency of photogalvanic cells and the progress (or lack of it) made to date. In this lecture I propose to outline the theoretical analysis which leads to the recipe for the ideal photogalvanic cell. 

Fig. 7. Typical cyclic voltammogram of the thionine coated electrode in background electrolyte.

The selective electrode kinetics of the thionine coated electrode extends to other systems. For instance, the coat hardly affects the kinetics of organic systems such as quinone / hydroquinone, but other inorganic systems such as the reduction of Ru (bpy) 3 " and Ce (IV) are blocked.26 por inorganic systems it appears that the ions are reduced by direct reaction with the leucothionine present in the coat rather than with the metal itself. This explains why the current voltage curves are shifted close to for thionine / leucothionine in the coat. Because of its more rapid kinetics Ru (bpy)3 requires much less L than Fe(lII) hence the former is reduced efficiently a hundred mV or so more positive than E oat hile Fe (III) is reduced inefficiently at a potential that is several hundred millivolts more negative than E oat- presume that... 

In the previous lecture we saw how the use of thionine coated electrode will give us the necessary electrode selectivity to separate L and Fe (III) and prevent their recombination. We consider that this problem can therefore be solved. We shall now explore how the efficiency depends on the behaviour of the homogeneous part of the system. We remember that it is always advantageous to reduce Xg. and absorb the light as close as possible to the electrode. We shall therefore present our results as a function of Xe. Other variables that can be changed in the cell are... 

However we found that it was possible to make a thionine coated modified electrode that had the right kinetic... 

Turning to the electrode kinetics current voltage curves for the reduction of thionine and of Fe (III) on clean and on coated electrodes are shown in figure 8. 

Various modifications have been attempted in order to increase the solar energy conversion efficiency of the iron-thionine PG cell, including adding sulfonate groups onto the thionine dye in order to increase solubihty [13] and irreversibly coating a thionine electrode with up to twenty thionine monolayers to increase absorbance of solar radiation and thus decrease E [26], However, research has broadened since early innovations by Rabinowitch, Albeiy, Archer, and Foulds, and the common approach is now motivated towards improvisations of the dye and reductant to induce wholesale changes in the efficiencies of PG cells. 

The electrode can then be removed, washed, dried, waved about in the air and put into background electrolyte. One can then observe cyclic voltammograms such as those in figure 7, which result from the reduction and oxidation of the coated thionine. 

Spectra taken by XPES confirm that thionine is coated on the electrode the coat is so thick that the Pt signal cannot be seen. 

It can be seen that the coating process hardly affects the electrode kinetics of the thionine reduction, while the current voltage curve for the Fe (III) reduction is shifted and much reduced. This is exactly the type of electrode that we need for an efficient photogalvanic cell. 

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