Matrix electrode structures

Although the matrix may have a well-defined planar surface, there is a complex reaction surface extending throughout the volume of the porous electrode, and the effective active surface may be many times the geometric surface area. Ideally, when a battery produces current, the sites of current production extend uniformly throughout the electrode structure. A nonuniform current distribution introduces an inefficiency and lowers the expected performance from a battery system. In some cases the negative electrode is a metallic element, such as zinc or lithium metal, of sufficient conductivity to require only minimal supporting conductive structures. 

Figure 9. Schematic porous electrode structure (A) Electrons from the external circuit flow in the current collector which has contact to the conductive matrix in the electrode structure. The redox reaction at the electrode produces electrons that enter the external circuit and flow through the load to the cathode, where the reduction reaction at the cathode accepts the electron from the external circuit and the reduction reaction. The ions in the electrolyte carry the current through the device. (B) The reaction distribution in the porous electrode is shown for the case where the conductivity of the electrode matrix is higher than the conductivity of the electrolyte.

Most battery electrodes are porous structures in which an interconnected matrix of solid particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte. When the active mass is nonconducting, conductive materials, usually carbon or metallic powders, are added to provide electronic contact to the active mass. The solids occupy 50% to 70% of the volume of a typical poious battery electiode. Most battery electrode structures do not have a well-defined planar surface but have a complex surface extending throughout the volume of the porous electrode Macroscopically, the porous electrode behaves as a homogeneous unit. 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

However, it is useful first to illustrate the complexity of such systems by showing a hierarchy of equivalent RC circuits that can be developed, from which an ultimate model of a porous-electrode CR device can be constructed. Note that the behavior of an electrochemical capacitor device is, electrically, far from that of a pure capacitor in its ac response spectrum to AV modulation this is primarily due to the complexity of the distributed, internal connections within the matrix, associated with resistivity of electrolyte channels and the intrinsic resistance of the C microparticles or fibrils and their interparticle contact resistances which usually depend on the pressure applied during fabrication of electrode structures. 

The parameters 3 and e control the current distribution in the electrode structure. 3 represents the competition between the reaction current and the ohmic resistance, with low values (small L, high conductivities, and low current densities) tending to produce a more uniform current distribution (Fig. 5.32)." The second parameter s, viewed in conjunction with 3 as the ratio of electrolyte to electrode conductivities, has a significant effect on current distribution. In practical electrodes the conductivity of the metallic matrix will usually be very close to that for the pure electronic conductor hence, tr /c. This corresponds to the situation when s = 3, Most of the activity is near the counterelectrode (Fig. 5.32). 

Four different strategies for immobilizing zeoUtes on the surface of an electrode can be identified (106). The desired zeoUte can be (1) dispersed within a solid matrix (2) compressed onto a conductive substrate (3) embedded in a polymerie film or (4) covalently anchored. Figure 8.14 outlines these broad approaches and the many different possible electrode structures. Further elaboration on these immobilization schemes can be found in Tables I and II of references (102) and (106), respectively. These references also contain specific examples of preparation procedures for zeolite modified electrodes. 

Electrolyte decomposition is a major concern when intercalating Li ions on a carbonaceous matrix. When graphite is used as the anode, exfoliation of the electrode structure occurs when LiC10.,/PC is used as the electrolyte, but the same electrolyte system can be used for disordered carbons such as those derived from petroleum coke. The most common nonaqueous electrolyte is LiPF6 in EC/DEC. A number of products from electrolyte decomposition have been identified by Aurbach et Also, inorganic compounds such as LiCO, LijO, CO, and Hj have been reported as being produced by reactions with the organic products or trace water. ... 

High performance displays, such as color displays on mobile phones and computer screens, require high image refresh rate in combination with high display brightness. The patterned electrode structures in these active matrix devices contain electronic switches in the display area. High performance active matrix devices are silicon-based. 

The generic demonstrator shown in Fig. 7.15B consists of a glass fiber-reinforced polyamide 6 (GF-PA 6) component. Additionally, especially developed matrix identical thermoplastic compatible piezoceramic modules (TPM) were embedded in the demonstrator structure during its manufacturing process. In Fig. 7.14 the built up of different TPM configurations are shown, consisting of thermoplastic carrier films, metallized with electrode structures and piezoelectric functional layers (eg, piezoceramic plates, fiber composites, or printing pastes) in the middle. 

The PAFC and MCFC are similar types of fuel cell in that they both use a liquid electrolyte that is immobilised in a porous matrix. We have seen that in the PAFC, PTFE serves as a binder and wet-proofing agent to maintain the integrity of the electrode structure and to establish a stable electrolyte/gas interface in the porous electrode. The phosphoric acid is retained in a matrix of PTFF and SiC sandwiched between the anode and cathode. There are no materials available that are stable enough for use at MCFC temperatures that are comparable to PTFF. Thus, a different approach is needed to establish a stable electrolyte/gas interface in MCFC porous electrodes. The MCFC relies on a balance in capillary pressnres to establish the electrolyte interfacial boundaries in the porous electrodes (Maru and Marianowski, 1976 and Mitteldorf and Wilemski, 1984). This is illnstrated in Fignre 7.12. 

Zinc electrodes can also be manufactured by the plastic-bonded method, similar to that of the nickel electrode described above. The zinc oxide dry powder, PTFE binder and other additives are blended with an organic solvent and then the mixture is passed through a calendaring process similar to that in Fig. 31.1. The PTFE fibriUates into a nano-structured three-dimensional fiber matrix. This electrode structure for the calcium zincate electrode is shown in Fig. 31.3 for a freshly prepared electrode. The active materials are locked into the electrode structural matrix which helps to reduce the tendency towards shape change and dendritic growth. 

Corrosion of matrix and the cell components in the presence of impurities also cuts short the cell life. Prevention of dust is an absolute necessity, as apart from choking the fine electrode structure it may react with the acid as well (most of the dust particles contain silica). 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

Multi-walled CNTs (MWCNTs) are produced by arc discharge between graphite electrodes but other carbonaceous materials are always formed simultaneously. The main by-product, nanoparticles, can be removed utilizing the difference in oxidation reaction rates between CNTs and nanoparticles [9]. Then, it was reported that CNTs can be aligned by dispersion in a polymer resin matrix [10]. However, the parameters of CNTs are uncontrollable, such as the diameter, length, chirality and so on, at present. Furthermore, although the CNTs are observed like cylinders by transmission electron microscopy (TEM), some reports have pointed out the possibility of non-cylindrical structures and the existence of defects [11-14]. 

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