Solid Anodes

Data on impressed current anodes are assembled in Tables 7-1 to 7-3. 7.2.1 Solid Anodes 

Scrap iron is seldom used as anode material today. When plied, it includes old steel girders, pipes, tram lines or railway lines (30 to 50 kg m ) which are welded 

Anode material Iron High-silicon iron Graphite Magnetite titanium  

Danger of fracture None Moderate High Moderate None 1 

Recommended Extended anode installation Mostly used for Aggressive soils and Soils Deep o 

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1), 

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Draw a single vertical line to represent the phase boundary between the solid anode and the solution Zn(s) ... 

Here subscripts a and c denote anode and cathode respectively, iref is the reference exchange current density, y is the concentration dependence exponent, [ ] and [ ]ref represent the local species concentration and its reference concentration, respectively. Anode transfer current, Ra, is the source in the electric potential equations at the anode/electrolyte interface with positive sign on membrane (electrolyte) side and negative sign on solid (anode) side. Similarly, near the cathode interface, the source on membrane (electrolyte) side is negative of the cathode transfer current, Rc and that on solid (cathode) side is positive of Rc. The activation over-potentials, in Equations (5.35) and (5.36) are given by... 

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

In contrast to stationary applications, portable applications require frequent start and stop procedures. Therefore for SOFC, a robust cell design and adapted electrode-electrolyte assemblies are an important issue. Frequent thermal cycles between room temperature and an operation temperature of about 600-800 °C pose challenges to the layered system consisting of solid anode, ceranfic electrolyte and solid cathode with respect to thermal and mechanical stability. For several years, different approaches to developing tubular nficro SOFC have been undertaken but did not lead to a commercial product yet. As SOFC can be operated with pure hydrogen, reformate and hydrocarbons as fuel as well - the latter option means direct internal reforming at the anode catalyst — various investigations focused on reduced operation temperature and a parallel conversion of fuels [21]. 

Carbon materials, such as carbon cloth and carbon paper, are also good substrates for the deposition of photocatalysts, due to their low resistivity and cost. They are also widely used in the proton exchange membrane fuel cell (PEMFC), and are also commercially available at www.fuelcellstore.com. Carbon cloth/paper is supplied with an untreated surface or reinforced with PTFE. Since the photocatal5ftic reactions at the anode side involve three phases liquid electrol)4e, solid anode photocatalyst, and the produced gaseous CO2, carbon materials with hydrophilic surfaces are preferred in the fabrication of photoanodes. Compared to the previous two substrates, carbon material can be used directly without any pretreatment. 

LiAl and Li(Si) alloys are processed into powders, which are cold-pressed into anode wafers or pellets that range in thickness from 0.75 to 2.0 mm. In the cell, the alloy pellet is backed with an iron, stainless steel, or nickel current collector. Lithium alloy anodes function in activated cells as solid anodes, and must be maintained below melt or partial melt temperatures. Forty-four weight percent Li(Si) alloy will partially melt at 709°C, while a, 13-LiAl will exhibit partial melting at 600°C. If these melting temperatures are exceeded, the melted anode may come in contact with cathode material, allowing a direct, highly exothermic chemical reaction and cell short-circuiting. 

The majority of easily detected compounds at solid anodes under constant applied potentials are self-stabUized via tt-resonance. Therefore, a desirable characteristic of electrodes in dc amperometry is inert. The electrode serves as a sink to provide and remove electrons with no direct involvement in the reaction mechanism. Since TT-resonance does not exist in polar ahphatic compounds (e.g., carbohydrates), stabilization of reaction intermediates is actively achieved via adsorption at clean noble metal electrodes. Faradaic processes that benefit from electrode surface interactions are described as electrocatalytic. Unfortunately, an undesirable consequence of this apiproach is the accumulation of adsorbed carbonaceous materials, which eventually foul the electrode surface. 

Metal-air batteries are different from conventional batteries because metal-air batteries are connected to the atmosphere, and need this access to operate. Metal-air batteries are also different from fuel cells because metal-air batteries have a self-contained anode within the battery case itself. Metal-air batteries are part conventional" battery and part fuel cell. Sometimes metal-air batteries are called semi-fuel cells. "Conventional" batteries have the active components of both the anode and cathode within the battery case (see Figure 1.1). Fuel cells have both of the "active" components (or fuels) of the anode and cathode supplied from outside the fuel cell case. A metal-air battery, or semi-fuel cell, has the solid anode within the case (like a battery), while the cathode fuel is brought into the cell (like a fuel cell). Oxygen gas... 

The lithium-iodine battery has a solid anode of lithium and a polyphase cathode of poly-2-vitiyl-pyridine (P2VP), which is largely iodine (at 90% by weight). The solid electrolyte is constituted by a thin film of Lil. The discharge reaction is given by... 

Silver may also be determined by coulometric dissolution of silver deposits (210) or by spontaneous electrolysis against a bismuth amalgam anode (215). The oxidation of silver (I) to silver (II) at solid anodes is well known but is not suitable for direct coulometric purposes owing to the rapid reaction of silver (II) with... 

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