Electrolyte matrix

Molten Carbonate -650 Some fuel flexibility High-grade waste heat Fragile electrolyte matrix Electrode sintering Distribute power Utilities... 

Cathode dissolution (NiO) and reduction of dissolved nickel to a metal particle precipitate in the electrolyte 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. 

One manufacturer reported cell power densities of 4.3 w/cm at a cell voltage of 0.8 V with an advanced space application cell configuration incorporating a 50 pm thick electrolyte matrix. 

The tape casting and electrophoretic deposition processes are amenable to scaleup, and thin electrolyte structures (0.25-0.5 mm) can be produced. The ohmic resistance of an electrolyte structure and the resulting ohmic polarization have a large influence on the operating voltage of MCFCs (14). FCE has stated that the electrolyte matrix encompasses 70% of the ohmic loss (15). At a current density of 160 mA/cm, the voltage drop (AVohm) of an 0.18 cm thick electrolyte structure, with a specific conductivity of -0.3 ohm cm at 650°C, was found to obey the relationship (13). 

The coupling of these improvements needs to be proven to meet endurance goals operation at pressure will definitely require design changes. The studies described in the recent literature provide updated information on promising development of the electrodes, the electrolyte matrix, and the capability of the cell to tolerate trace constituents in the fuel supply. The objectives of these works are to increase the life of the cells, improve cell performance, and lower cell component costs. Descriptions of some of this work follow. 

Life is shortened by a decrease in the electrolyte matrix thickness (38). Concurrently, an increase in matrix thickness brings about an increase in life. This is due to an increase in the Ni++ diffusion path, which lowers the transport rate and shifts the Ni desposition zone. Developers found that an increase in electrolyte thickness from 0.5 mm to 1.0 mm increased the time to shorting from 1000 hours to 10,000 hours. Along with this, data showed that if the Pco2 was reduced one-third, then the Ni dissolution decreased by a third. U.S. developers concluded that a two-fold improvement in the time-to-short can be achieved using a 60% increase in matrix thickness and an additive of CaCOs. However, this combined approach caused an approximately 20 mV reduction in performance at 160 mA/cm (23). 

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. 

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 . 

The sulfide ion migrates across the membrane-electrolyte matrix to the anode, where either elemental sulfur vapor is generated or, if hydrogen gas is present at the anode hydrogen sulfide is formed. 

For molten carbonate fuel cells (MCFCs), a full life-cycle analysis has been attempted (Lunghi and Bove, 2003). Both electrodes and the electrolyte matrix are manufactured by mixing powdered constituents with binders and... 

Life-cycle impact Negative electrode Positive electrode Electrolyte matrix Bipolar plate Total Unit... 

Recent technology for electrolytic aluminum production employing aluminum chloride has also been of interest because of the about 30% power savings possible [21]. Since aluminum chloride melts at much lower temperatures and forms a much more fluid melt than the standard Hall-Heroult electrolyte matrix, much higher voltage efficiencies are possible. However, sublimation and control problems limit the utility of direct, one-component electrolytic methods. The essence of this idea is employed in the process, developed by C. Toth of Alcoa, which has the additional advantage of enabling clay sources of alumina to be tapped [22] (Fig. 12.4). 

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. 

Alternatively, an additional layer constructed by using fine nickel powder, L1A102, and NiO is positioned between the anode and the electrolyte and filled with molten carbonate electrolyte. The purpose of this additional layer is to prevent gas crossover from one electrode to the other if cracks develop in the electrolyte structure. This bubble barrier layer serves as a reinforcement of the electrolyte matrix. This bubble pressure barrier (BPB) can be fabricated as an integral part of the anode structure. Typically, the pores of this barrier layer are smaller than the anode pores and provide ionic transport through the cell. ... 

Arendt, R.H. Curran, M.J. Alternate synthesis of electrolyte matrix for molten carbonate fuel cells. J. Electrochem. Soc. 1980, 127 (8), 1660-1663. 

Birk, S. Ibeh, C.C. Plastics in fuel cell applications an in-lab developed and fabricated molten carbonate fuel cell (MCFC) electrolyte matrix support with polyolefin-based binders. 57th Annual Technical Conference of the Society of Plastic Engineers, 1999 Vol. 2, 2629-2633. 

To prevent short circuiting between the separator plates the electrolyte matrix made of LiA102 extends to the outer edge of the separator plates. The molten carbonate electrolyte penetrates the matrix tile up to the edges, thus providing the necessary wet seal. This sealing is necessary to take care of small pressure differences (some tens of mbar) between gas chambers and the ambience and also between the fuel and oxidant gas chambers. From this short description of the geometry of the bipolar separator plates, it becomes clear that we have three different situations to take care of if we are to prevent corrosion of the separator plates ... 

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