Porous electrolyte matrix

There are indications that these poorly sintered materials are unstable upon reduction of the copper oxides. Two separate studies, one with Cu—YZT and the other with Cu—GDC, have found that the Cu migrates out of the porous electrolyte matrix during reduction. 

Among new materials suggested for the porous electrolyte matrix in PAFCs, we mention a mixture of silicon carbide (SiC) and PTFE (Mori et ah, 1998). A suspension of the components is mixed in a ball mill for a long time, then spread onto the surfaces of the cathode and anode. This assures good contact between the electrodes and the electrolyte immobilized in the matrix. 

In MCFCs, which operate at relatively high temperature, no materials are known that wet-proof a porous structure against ingress by molten carbonates. Consequently, the technology used to obtain a stable three-phase interface in MCFC porous electrodes is different from that used in PAFCs. In the MCFC, the stable interface is achieved in the electrodes by carefully tailoring the pore structures of the electrodes and the electrolyte matrix (LiA102) so that the capillary forces establish a dynamic equilibrium in the different porous structures. Pigeaud et al. (4) provide a discussion of porous electrodes for MCFCs. 

Figure 25. Adler s ID macrohomogeneous model for the impedance response of a porous mixed conducting electrode. Oxygen reduction is viewed as a homogeneous conversion of electronic to ionic current within the porous electrode matrix, occurring primarily within a distance A from the electrode/electrolyte interface (utilization region). (Adapted with permission from ref 28. Copyright 1998 Elsevier.)...

We have studied the oxygenation of ferroheme bound to a polymer ligand in the solid state98. The profiles of oxygen uptake by powders of polymer-heme complexes were measured by volumetry, as shown in Fig. 22. The heme complex embedded in the porous polymer matrix or in the poly(electrolyte) aggregate takes up... 

O Regan and Gratzel, 1991). In these cells, a dye is incorporated in a porous inorganic matrix such as Ti02, and a liquid electrolyte is used for positive charge transport. This type of cell has a potential to be low-cost. However, the efficiencies at present are quite low, and the stability of the cell in sunlight is unacceptable. Research is needed to improve performance in both respects. 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

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 . 

Lim and Winnick [110] examined removal of H2S from a simulated hot coal-gas stream fed to the cathode while elemental sulfur gas was evolved at the anode. This process was performed in a cell that was similar in construction to a molten carbonate fuel cell (Fig. 23). The electrolyte was a mixture of Na2S and Li2S retained in a porous inert matrix material (MgO). The cathodic reaction involved the two-electron reduction of hydrogen sulfide to hydrogen (information on the equilibrium potential for H2S reduction can be obtained from [111] ... 

However, it is also possible to cycle CM made from pyrolyzed polyacrylonitrile in aqueous electrolytes, according to Beck and Zahedi [378]. Figure 30 shows relatively flat redox peaks around the quinone/hydroquinone center (f/s — 0 V, about 0.7 V vs. SHE). Protons are the counterions in this case. A polyquinonimine structure is concluded from (electro)chemical and FTIR data (cf. Fig. 34). These acceptor-type compounds have relatively high specific capacities of about 300 Ah/kg in the steady state. The initial capacities are even higher. It should be mentioned that graphite nanotubules were synthesized in the nanopores of a porous AI2O3 matrix at 250/ 600 °C [433]. 

The electrodes are flat. The anode is composed of porous sintered nickel along with additives, which inhibit the loss of surface area during operation. The anode is in direct contact with the electrolyte matrix. The cathode is a porous nickel oxide, which is initially fabricated in the form of a porous sintered nickel and is subsequently oxidized during the cell operation. 

Carbon dioxide transfers between the anode, where it is produced, and the cathode, where it is consumed. The anode electrode material is porous nickel or a Ni-Cr alloy, while the cathode is porous NiO. The electrolyte is a mixture of LiA102, K2CO3, and Li2C03, which is absorbed into a porous inert matrix. These cells operate at about 650°C, where a cell voltage of +0.9 V is obtained at a current density of about 150 mA/cm. Systems of 10 kW to 2 MW have been tested for electric utility applications. 

The main difference between the two types is in the electrolyte. The MCFC uses a molten carbonate immobilized in a porous LiA102 matrix. The SOFC uses a ceramic membrane of cubic stabilized zirconia. An illustration of the operation of a SOFC is shown in Figure 30.22. 

In contrast to other components of ohmic resistance, the electrolyte resistance depends on temperature. As a rule (though not always) this electrolyte resistance is higher than the electrodes ohmic resistance. When in the cell porous electrodes are used, the resistance of the liquid electrolyte inside pores (from reaction zone to interface with the electrolyte) must also be added to the electrolyte s ohmic resistance. The use of separators in the interelectrode gap or the use of a porous electrolyte-soaked matrix increases the electrolyte resistance. For different separators the corresponding conductance attenuation factors are quoted in Section 5.3. The electrolyte resistance is also increased when gas bubbles accumulate in the interelectrode gap. If the bubble volume comes to 30% of the total gap volume, the resistance increment will be 60-80%. 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

The electrolyte matrix is porous a- or y-lithium aluminum oxide with ca. 50 % porosity and 1 mm thickness. To immobilize the molten carbonate by capillary force, the pore diameter is less than 0.5 pm. To fabricate such fine pores, the matrix is made from an ultrafine powder. The stable condition of the a-lithium aluminum oxide is at a lower temperature than that of the y-lithium aluminum oxide, and the boundary is around 650 °C. Recently, the operating temperature is generally below 650 °C based on the durability therefore, the a phase is selected. The change in the phase leads to a pore stmcture change, so fine control of the operating conditions is required. To maintain the fine pores, a uniform particle size is important to decrease the Ostwald ripening, coarsening process [2, 4, 5]. 

The electrolyte management is essential for the performance and durability. The function of the electrolyte plate is ionic conduction and gas separation therefore, the pores of the matrix are fully filled by the electrolyte with a strong capillary force. The gas diffusion electrode requires gas diffusion and an ionic conduction path therefore, the pore of the porous electrode is partially filled by the electrolyte with a medium capillary force. The amount of electrolyte and relative pore diameter of the electrolyte matrix, anode, and cathode must be then maintained during whole of the lifetime [2, 4, 5, 7-9]. 

In principle, the electrophoretic separation can take place in free solution, and only longitudinal diffusion contributes to band broadening when the temperature is constant. For all practical purposes, however, constant temperature in the whole separation medium is difficult to obtain, and due to temperature differences, convection occurs and gives rise to significant band broadening. In order to reduce the convection, one solution is to use a totally porous soM matrix as a carrier for the electrolyte solution. This matrix, called a gel, is now used in most of the traditional electrophoretic techniques. In these porous matrices, eddy diffusion and adsorption effects can contribute to band broadening in addition to the longitudinal diffusion. 

Based on Figure 2.12, particle size can also affect the exposed areas of carbons particle to the electrolyte solution. In general, the more porous the matrix layer, the larger the exposed area. Therefore, the carbon particle size should be optimized to yield the best porosity. However, if the porosity of the electrode matrix layer is too high, the electric conductivity of the matrix layer will be reduced, leading to high resistance of the electrode layer and lower power density of the supercapacitor. Therefore, there is a trade-off between the porosities and conductivities of the electrode materials. 

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