Anodes materials

Anode materials in MFCs should have a good biocompatibility, low resistance and large surface area, to facilitate bacterial growth and electron transfer. A large variety of carbonaceous and several metal materials with unique configurations and surface areas have been developed for this purpose. A summary of commonly used anodes and fheir characteristics are listed in Table 2.6. 

Comparison of the Characteristics of Commonly Used Anodic Base Materials in MFCs. 

Carbon paper Easy to coimect wiring Lack of durability, brittle [88] 

Carbon cloth Flexible, large porosity Thin, expensive [33] 

Carbon felt Large porosity Large resistance [89] 

Galvanic anodes of cast iron were already in use in 1824 for protecting the copper cladding on wooden ships (see Section 1.3). Even today iron anodes are still used for objects with a relatively positive protection potential, especially if only a small reduction in potential is desired, e.g., by the presence of limiting values U (see Section 2.4). In such cases, anodes of pure iron (Armco iron) are mostly used. The most important data are shown in Table 6-1. 

The Ni/YSZ anode has been widely used and studied for H2 SOFCs [53]. The Ni/YSZ cermet provides a satisfactory performance at 800-1000 °C, but suffers from low tolerance to the sulfur compounds and a substantial activation polarization loss at low temperature ( 600 °C). The H2 SOFCs require the use of a ZnO-based sorbent to remove sulfur compoimds in the H2 feed stream. An alternative to the Ni anode is the perovskites. Perovskites such as Sro.sLao.4Ti04 have exhibited sulfur resistance, but low performance [18,54]. 

We have demonstrated that LSCF exhibited high activity for the direct CH4 fuel cells for more than 72 h [59]. Transient response studies revealed that the electrochemical oxidation of CH4 on the anode produced electricity, CO2, and CO through a parallel pathway C + 20 C02 C + 0 C0, where the intrinsic rate constant for the formation of CO2 is greater than that of CO. This study and many subsequent studies with various perovskites revealed that perov-skite materials are promising for the direct CH4 SOFC [60-66]. Contaminants such as H2S have been foimd to decrease oxidation activity of perovskites and lead to the increase in carbon deposition [67]. One approach to enhance oxidation activity of perovskites is through the addition of metals such as Ag, Au, and Cu that exhibit good oxidation activities. 

2 Budiman, A Song, S.-H., Chang, T.-S. et at (2012) Dry reforming of methane over cobalt catalysts a literature review of catalyst development. Catal. Sum. Asia, 

3 Kapokova, L., Pavlova, S., Bunina, R. et aL (2011) Dry reforming of methane over LnFeo,7Nio.303 s perovskites influence of Ln nature. Catal. Today, 164 (1), 227-233. 

4 Rida, K Pena, MA., Sastre, E., and Martinez-Arias, A. (2012) Effect of calcination temperature on structural properties and catalytic activity in oxidation reactions of LaNiOa perovskite prepared by Pechini method. /. Rare Earths, 30 (3), 210-216. 

Graphite is the first kind of anode material for commercial lithium-ion batteries, and it is still widely used. However, the cointercalation of solvents such as propylene carbonate during lithium insertion may lead to the decomposition and intensive exfoliation of graphene sheets [20]. Besides, graphite is veiy sensitive to the electrotyte, and if not properly dealt with, it 

Despite the good performance with Ni-based anodes, their degradation with time and nickel volatilization are critical issues for stable, long-term cell operation [21]. 

Carbon paper, cloth, foams, and R VC. The use of carbon-based electrodes in paper, cloth, and foam forms for the MFC anode is very common. These materials have high conductivity and appear to be well suited for bacterial growth. Carbon paper is stiff and slightly brittle but it is easily connected to a wire (Fig. 5.1 A). It should be sealed to the wire using epoxy, with all exposed surfaces of the wire covered or sealed with epoxy as well. Copper wire can be used but it corrodes over time, either releasing copper into solution (which can be toxic to the bacteria) or causing the electrode to detach from the wire. Stainless steel or titanium wires work better in MFCs. Carbon paper is commonly 

Testing of the smaller brush electrode in a cube-type MFC produced the highest power density yet achieved for an air cathode MFC of 2400 mW/m at a current density of 0.82 mA/cm (R xi = 50 Q) Logan et al. 2007). The power was 73 W/m based on normalizing power to the reactor liquid volume. Ce ranged from 40% to 60% depending on current density. 

Metals and metal coatings. The use of various metals and metal coatings on carbon materials has not been well-examined for MFCs. In one study, it was shown that addition of vapor-deposited iron oxide to a carbon paper cathode decreased acclimation time of a reactor but did not affect maximum power Kim et al. 2005). A two-chamber reactor was used, and thus power production was limited by high internal resistance so it is not known if the iron oxide would have resulted in an increase in power production in a reactor with less internal resistance. Over time the iron oxide coating dissolved into solution, leaving only the carbon paper electrode. However, the use of iron to enrich iron-reducing bacteria on the electrode may be a beneficial approach for acclimation of an electrode with exoelectrogenic bacteria. 

Metals added to electrodes have been shown to increase power in several studies, but whether this additional power resulted from galvanic increases due to the potentials of the 

FC304 and Fe304+ Ni were added to graphite paste, and Mn + Ni were added to a graphite-ceramic mixture to make anodes used in sediment fuel cells using the same procedure employed by Park and Zeikus (2002). These anodes produced 1.7- to 2.2-foId greater current than plain graphite electrodes under similar conditions Lowy et al. 2006). The Mn -based electrode, which performed best in these laboratory tests, was also field-tested in sediment MFCs and produced 105 mW/m, versus 20 mW/m obtained in previous tests. 

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For practical applicability, several aspects have to be considered such as tire anode material (sacrificial (e.g. zinc) or inert (e.g. Pt/Ti or graphite)), tlie conductivity of tlie medium and tlie current distribution. Catliodic protection is typically used for buried constmctions (e.g. pipelines), off-shore stmctures or ship hulls. 

The oxygen contribution from these reactions is dependent on the nature of the anode material and the pH of the medium. The current efficiency for oxygen is generally 1—3% using commercial metal anodes. If graphite anodes are used, another overall reaction leading to inefficiency is the oxidation of... 

Cells operating at low (2,80,81) and high (79,82) temperatures were developed first, but discontinued because of corrosion and other problems. The first medium temperature cell had an electrolyte composition corresponding to KF 3HF, and operated at 65—75°C using a copper cathode and nickel anodes. A later cell operated at 75°C and used KF 2.2HF or KF 2HF as electrolyte (83,84), and nickel and graphite as anode materials. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

Two observations relevant to ECM can be made. (/) Because the anode metal dissolves electrochemicaHy, the rate of dissolution (or machining) depends, by Faraday s laws of electrolysis, only on the atomic weight M and valency of the anode material, the current I which is passed, and the time t for which the current passes. The dissolution rate is not infiuenced by hardness (qv) or any other characteristics of the metal. (2) Because only hydrogen gas is evolved at the cathode, the shape of that electrode remains unaltered during the electrolysis. This feature is perhaps the most relevant in the use of ECM as a metal-shaping process (4). 

In electrolytic processes, the anode is the positive terminal through which electrons pass from the electrolyte. Anode design and selection of anode materials of constmction have traditionally been the result of an optimisation of anode cost and operating economics, in addition to being dependent on the requirements of the process. Most materials used in metal anode fabrication are characteristically expensive use has, however, been justified by enhanced performance and reduced operating cost. An additional consideration that has had increasing influence on selection of the appropriate anode is concern for the environment (see Electrochemical processing). 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

If the cations in solution are condensable as a soHd, such as copper, they can plate out on the cathode of the cell. As the same time, perhaps some hydrogen is also produced at the cathode. The SO can react with a copper anode material by taking it into solution to replace the lost copper ions. Thus the anode is a consumable electrode in the process. 

Anode Corrosion Reaction. Ziac might at first appear to be an unusual choice for battery anode material, because the metal is thermodynamically unstable ia contact with water... 

Cadmium hydroxide is the anode material of Ag—Cd and Ni—Cd rechargeable storage batteries (see Batteries, secondary cells). Cadmium sulfide, selenide, and especially teUuride find utiUty in solar cells (see Solarenergy). Cadmium sulfide, Hthopone, and sulfoselenide are used as colorants (orange, yellow, red) for plastics, glass, glazes, mbber, and fireworks (see Colorants for ceramics Colorants forplastics Pigments). 

Calcium metal is also used in strip form as the anode material in thermal batteries (see Batteries), which ate used as the power source in artillery fuses (39). 

Fluorine. This appHcation uses carbon plates as the anode in a fluorine salt solution. Since the ordered crystal stmcture of graphite results in short life, carbon is the preferred anode material (see Fluorine). 

Ma.nga.neseDioxide. Graphite plates used as anodes in this process are coated with MnO during electrolysis. The anodes are removed from the solution periodically and the MnO is removed by mechanical methods. Graphite can also be used as the cathode material. Titanium is used as anode materials where high quaHty MnO is desired. 

Anode material used for anode technologies is coated t... 

Electrode materials and shapes may have a profound effect on cell designs. Anode materials encountered ia electrochemical processes are... 

Product Reactants Company Cell type Anode material Cathode material... 

Soluble anode materials are not always a pure metal. In acid, low chloride nickel solutions, pure nickel does not corrode well, and small amounts of specific impurities are added to make the nickel more active, allowing more efficient dissolution. For example, since the early 1960s, nickel anode material containing a small amount of nickel sulfide [16812-54-7] NiS, has been commercially available and important in nickel sulfamate [13770-85-3] Ni(H2N02S)2, plating baths. These anodes corrode at a lower potential then pure nickel or other nickel anode materials (see Nickel and nickel alloys). 

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

The use of dissimilar metals in contact with each other should generally be minimized, particularly if they are widely separated in their nominal positions in the galvanic series (see Table 28-1 ). If they are to be used together, consideration should be given to insulating them from each other or making the anodic material area as large as possible. 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

For water, organic and water-organic metal salts mixtures the dependence of integral and spectral intensities of coherent and non-coherent scattered radiation on the atomic number (Z), density, oscillator layer thickness, chemical composition, and the conditions of the registering of analytical signals (voltage and tube current, tube anode material, crystal-analyzer) was investigated. The dependence obtained was compared to that for the solid probes (metals, alloys, pressed powder probes). 

It is little known that Thomas Alva Edison tried to achieve cathodic protection of ships with impressed current in 1890 however, the sources of current and anodic materials available to him were inadequate. In 1902, K. Cohen achieved practical cathodic protection using impressed direct current. The manager of urban works at... 

Table 6-1 Properties of pure metals as anode materials...

Zinc was also already in use for protection in seawater in 1824 (see Section 1.3). In the beginning zinc material that was available from the hot-dip galvanizing industry was used but was less suitable because it became passive. Passivation does not occur with high-purity zinc. Super high grade zinc is the anode material with the least problems [5] and consists of 99.995% Zn and less than 0.0014% Fe without further additions. It is specified in Ref. 6 and permitted by the German Navy [7]. The most important properties of pure zinc are listed in Table 6-1. 

Fig. 6-4 J U) curves for zinc anodes (material No. 2.2301) in 3.5 wt. % NaCl solution, aerated and stirred — at the start of the experiment -----after 90 h.

In spite of a low driving voltage of about 0.2 V, about 90% of all galvanic anodes for the external protection of seagoing ships are zinc anodes (see Section 17.3.2). Zinc alloys are the only anode materials permitted without restrictions for the internal protection of exchange tanks on tankers [16] (see Section 17.4). 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

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