Earlier Electrodic Suggestions

Historically, Lund in 1928 correlated the process of ionic transport, measured as an electrical current, to metabolic reactions, specifically to redox reactions, such as those occurring in mitochondria. Lund measured the amount of O2 consumed by a frog skin, as a function of the current flowing through the skin. Lund s idea of the possible relation of O2 consumption to some bioelectric phenomena was not well accepted at that time. 

In 1945, Lundegardh put forward an explanation of ion transport in terms of redox reactions. The redox reactions occurring in respiration were considered as the source of bioelectric phenomena. Describing the oxidation of Fe ion to Fe in enzymes, Lundegardh proposed that since Fe ion attracts one more anion than the Fe ion, the process of Fe /Fe redox reaction causes the movement of anions in the opposite direction to that of the electrons. Since the principal postulate of this theory was regarded as charge separation in connection with ionic trans- 

What Cope suggested was not literally possible because of the reversibility which he assumed, but it suggests the application of the Wagner-Traud hypothesis in biology, i.e., the idea of an overall reaction with no net electron transfer, but electron transfer controlled. What it needs is the application of the theory of interfacial charge transfer, in which the current density is related to the deviation of the potential drop at the interface, from the reversible value. 

The difference of this view, in explaining the potential difference across biological interfaces, compared with that of the earlier workers, is profound. Thus, ion transport through the membrane is a consequence of the electrode potential differences referred to above. Electron transfer at the solid/liquid interface sites is rate determining (one of the two kinds of sites will dominate). The concentration of alkali metal ions on the two sides of the membranes may be the result, rather than the cause, of the potential 

FIGURE 1. Enzyme particle showing electron transfer between x site and y site, giving rise to oxidation and reduction reactions, the rates of which are governed by the overvoltage phenomena at the sites and the electrical resistance across the enzyme particles.  

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The identity of the cathode materials is essential for the outcome of CO2 electroreduction. While an earlier electrode classification was based on whether the cathode metal belonged to the sp- or the d-metal group [50], Hori considered that the performance of various metals is loosely related to the periodic table. For aqueous electrolytes, Hori [82,83] suggested regrouping the electrode metals into two categories (1) CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni and Ft) and (2) metals that yield formate (Hg, Pb, Zn, In, Sn, Cd and Tl). As discussed in the previous section, copper represents a very special electrode material, enabling the formation of various hydrocarbons, such as methane and ethylene [82]. 

There are almost 100 papers that discuss benzene dithiol s conductance. As the point about geometric distributions became well understood, it was realized that statistical analysis was extremely useful. Accordingly, electrochemical break junction techniques, both in their original form of crashed electrodes being separated to form the gap or in the newer electrochemistry form, in which a gap is created and then electrochemically modified, have proliferated. The important thing is that statistical measurements can be made [24, 90], with hundreds or thousands of data points. Not surprisingly, distributions are observed (as the earlier computations had suggested). 

Some additional experiments relevant to the data in Figure k are suggested by the preceding discussion. In particular, if the corrosion sites also act as the recombination centers that control current onset in the absence of sulfide ions (as discussed earlier) then there are no oxidized recombination centers before exposure to light. In that case a CdS electrode biased at a voltage below the saturated portion of curve 1 in Figure k would show a higher initial current than indicated in curve 1 and then decay in time to the curve 1 value. This situation can be analyzed by ... 

In a much earlier study of the reduction of 4-NH2C5H4ASO at a Pb electrode another intermediate (ArAs=AsAr, Ar = 4-NH2CgH4) was isolated after preparative electrolysis in basic solution, and reactions 76 and 80 were suggested to account for this product . 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

All the results discussed above strongly suggest that the exposure of the Ti02 (anat-ase) electrode immersed in an alkaline solution to the band-gap illumination, under anodic bias or at open circuit, leads in all cases to the formation of very similar (if not identical) species bound to the oxide surface. Such conclusion is supported by the similarity of the potential, shape and decay characteristics of the corresponding cathodic reduction peaks shown in Figs. 4,5,7 and 8. These species have been identified, in the course of earlier experiments performed with aqueous alkaline dispersions of Ti02 (anatase) particles irradiated with near-UV light, as surface-bonded, peroxo-titanium com-... 

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