Cathode-limited systems

Electrical power can also be applied to the BES to increase the electrical potential between the anode and cathode electrodes and enable cathodic reactions (reviewed in [1]) not attainable in cathode-limited systems such as MECs. Energy is added to the system with an external power source or by poising the anode potential with a potentiostat. This type of BES is often referred to as a MEC because it enables the electrochemical reduction of protons to hydrogen at the cathode (reviewed in [8]). These BESs are especially suitable for microbial studies and have been used extensively for Geohflcter bacteria. For this reason, we describe the theory and practice of MEC operation using Geobacter cultures in detail in the following sections. 

The cathode material need not be the same as the anode material. In fact, its selection is especially important in cathode-limited systems. In an MFC, the anode... 

Cathodic protection can be useful, although its ability to protect tube interiors is generally limited to the first 4 to 6 in. of tube length. Such systems, however, must be properly designed and maintained to be effective. Corrosion can be intensified if the polarity of the cathodic protection system is inadvertently reversed. 

Ideally, one would prefer to compare anodic and cathodic potential limits instead of the overall ionic liquid electrochemical window, because difference sets of anodic and cathodic limits can give rise to the same value of electrochemical window (see Figure 3.6-1). However, the lack of a standard reference electrode system within and between ionic liquid systems precludes this possibility. Gonsequently, significant care must be taken when evaluating the impact of changes in the cation or anion on the overall ionic liquid electrochemical window. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

A typical soil resistivity survey is shown in Fig. 10.22. Soil resistivities will normally indicate whether a cathodic-protection system is advisable in principle and whether impressed current or sacrificial anode schemes in particular are preferable. It may, as a result of the survey, be considered desirable to apply protection to the whole line or to limit protection to certain areas of low soil resistivity or hot spots . 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

In order to study the identity and nature of the intermediate, Aylmer-K.elly et al. (1973) employed modulated specular reflectance spectroscopy. They studied the reduction reaction at a lead cathode in both aqueous and non-aqueous electrolytes. A phase-sensitive detection system was employed by the authors, locked-in to the frequency of the potential modulation. The potential was modulated at 30 Hz between the reference potential of — 1.0 V vs. Ag/AgCl and a more cathodic limit. 

In fused-salt systems, reduction of traces of water (if present) or of the metal cations ordinarily will be the cathodic limiting process. 

Table 11 shows some representative results from the cathodic reduction of some aromatic hydrocarbons. These include cases with Ei j2 near the cathodic limit or in the discharge region of the SSE (benzene, toluene) and cases with Ex j2 at considerably more positive potential (naphthalene, anthracene again we must anticipate the discussion of reactivity and refer to Table 21). Reactions nos. 1, 2, 6, and 7 immediately demonstrate one difficulty with such studies in that the catholyte of a divided cell becomes strongly basic as electrolysis progresses. In sufficiently basic medium, the initial product, a 1,4-dihydro derivative (cf. the Birch reduction Birch and Subba Rao, 1972), will rearrange to a conjugated system which, in contrast to the 1,4-dihydro derivative, is further reducible to the tetrahydro product (nos. 1 and 6). In a non-divided cell the acid production at the anode balances the base production and thus only a little rearrangement occurs. It is therefore not a trivial problem to find out if the tetrahydro product is formed from the conjugated dihydro product, formed directly or by rearrangement [eqn (78)].

Figure 4.6 shows the EWs of chloroaluminate systems with the correction of the potential differences derived from the melt composition and the kind of cation using the redox potential of ferrocene. This figure clearly indicates the effect of using the ferrocene internal reference. The cathodic limiting or anodic limiting potential is almost the same when the same cation is contained in RTILs, and is independent of the measurement conditions such as the difference of melt composition, not considered in Figure 4.5. The potential variation seen in Figure 4.5 even for the same RTILs is almost eliminated.

The electrolyte systems El I, II, and III resemble one another and show relatively low cathodic limiting current densities. The cathodic limiting current densities of the various El II electrolytes clearly ascend with increasing value of the 1 2/1 1 complex ratio, as related to electrolyte prehandling. The cathodic limiting current den-... 

The useful potential range that can be obtained in a given system depends on the electrode material, the supporting electrolyte, the temperature [269], and the solvent only the solvent is discussed here. Approximate values for the anodic and cathodic limits of some... 

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