Carbon dioxide reduction electrode potential

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

Fig. 5 Comparison of carbon dioxide reduction potentials observed on different electrodes together with thermodynamic potentials, E°.

A little later, Russell et al.19 tried to obtain methanol from carbon dioxide by electrolysis. Reduction of carbon dioxide to formate ion took place in a neutral electrolyte at a mercury electrode. On the other hand, formic acid was reduced to methanol either in a perchloric acid solution at a lead electrode or in a buffered formic acid solution at a tin electrode. The largest faradaic efficiency for methanol formation from formic acid was ca. 12%, with poor reproducibility, after passing 1900 C in the perchloric acid solution at Pb in a very narrow potential region (-0.9 to -1.0 V versus SCE). In the buffered formic acid solution (0.25 M HCOOH + 0.1 M... 

Reduction of carbon dioxide takes place at various metal electrodes. The main products are formic acid in aqueous solutions and oxalate, CO, and formic acid in nonaqueous solutions. An indium electrode is the most potential saving for C02 reduction. Due to the difference in optimum conditions between those for C02 reduction to formic acid and those for formic acid reduction to further reduced products, direct reduction of C02 in aqueous solutions without a catalyst to highly reduced products seems to be difficult at metal electrodes. However, catalytic effects of metal electrodes themselves have recently become more clear for example, on Cu, methane was detected, while on Ag and Au, CO was produced effectively in aqueous solutions. Furthermore, at a Mo electrode, methanol was obtained. The power efficiency is, however, still low at any electrode. 

In contrast to a variety of oxidizable compounds, only a few examples for the detection of strong oxidants with transition metal hexacyanoferrates were shown. Among them, hydrogen peroxide is discussed in the following section. Except for H202, the reduction of carbon dioxide [91] and persulfate [92] by Prussian blue-modified electrode was shown. The detection of the latter is important in cosmetics. It should be noted that the reduction of Prussian blue to Prussian white occurs at the lowest redox potential as can be found in transition metal hexacyanoferrates. 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

CO2 reduction at metallic electrodes is generally poorly selective [151]. Monoelec -tronic reduction of carbon dioxide may occur at a platinum cathode in non-aqueous solvents, but at very negative potentials. Catalytic activation of CO2 has been described (e.g. at a cathode modified by a rhenium complex in a hydroorganic solvent) the observed conversions did correspond to the formation of CO and formic acid. In organic synthesis, CO2 was mainly used as an electrophile (toward electrogenerated anions from jt -acceptors or electrogenerated nucleophiles when adequate transition metals ions were present in situ) for the purpose of carboxylation. 

The onset potential for the freshly etched p-CdTe for the photo-electrochemical reduction of carbon dioxide is -0.76V NHE. When the electrode was cycled between -0.56 to -2.24V under illumination, the onset potential for the photocurrent shifts to less cathodic potentials and remains constant at -0.66V after about c. 20 minutes. When the electrode is potentiostated at -2.0V the photocurrent remained constant for about 24 hours. These results are reproducible and consistent with the published data. Surface analysis of the etched surface of p-CdTe using XPS and SEM showed only trace amounts of carbon and oxygen. 

Determine kinetics and mechanisms of electrode reactions shown to be potentially useful (i.e., carbon monoxide and dioxide reduction, alkali metal deposition, solution redox reactions, and oxygen reduction) to permit the design of highly efficient electrolytic cells (55)... 

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