Electrolytes molten carbonates

Electrolyte management, that is, the control over the optimum distribution of molten carbonate electrolyte in the different cell components, is critical for achieving high performance and endurance with MCFCs. Various processes (i.e., consumption by corrosion reactions, potential driven migration, creepage of salt and salt vaporization) occur, all of which contribute to the redistribution of molten carbonate in MCFCs these aspects are discussed by Maru et al. (4) and Kunz (5). 

The major problems with Ni-based anodes and NiO cathodes are structural stability and NiO dissolution, respectively (9). Sintering and mechanical deformation of the porous Ni-based anode under compressive load lead to severe performance decay by redistribution of electrolyte in a MCFC stack. The dissolution of NiO in molten carbonate electrolyte became evident when thin electrolyte structures were used. Despite the low solubility of NiO in carbonate electrolytes ( 10 ppm), Ni ions diffuse in the electrolyte towards the anode, and metallic Ni can precipitate in regions where a H2 reducing environment is encountered. The precipitation of Ni provides a sink for Ni ions, and thus promotes the diffusion of dissolved Ni from the cathode. This phenomenon... 

A distinct minimum in NiO solubility is observed in plots of log (NiO solubility) versus basicity (-log aM2o), which can be demarcated into two branches corresponding to acidic and basic dissolution. Acidic dissolution is represented by a straight line with a slope of+1, and a NiO solubility that decreases with an increase in aM20- Basic dissolution is represented by a straight line with a slope of to either -1 or -V4, corresponding to Equations (6-9) and (6-10), respectively. The CO2 partial pressure is an important parameter in the dissolution of NiO in carbonate melts because the basicity is directly proportional to log Pcc>2 An MCFC usually operates with a molten carbonate electrolyte that is acidic. 

As a first example of the use of reaction mechanism graphs, consider the electrochemistry of molten carbonate fuel cell (MCFC) cathodes. These cathodes are typically nickel-oxide porous electrodes with pores partially filled with a molten carbonate electrolyte. Oxygen and carbon dioxide are fed into the cathode through the vacant portions of the pores. The overall cathodic reaction is 02 + 2C02 + 4e / 2C03=. This overall reaction can be achieved through a number of reaction mechanisms two such mechanisms are the peroxide mechanism and the superoxide-peroxide mechanism, and these are considered next. 

Figure 8.6 shows a schematic illustration of a molten carbonate electrolyte fuel cell (MCFC). 

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. ... 

The MCFC is a promising power generating source because of its unique characteristics such as high fuel efficiency and ability to use various carbonaceous fuels. Although Ni-10wt% Cr is used in the state-of-the-art MCFC as anode, it needs to be improved in terms of better creep and sintering resistance. In spite of the development in the alternate cathode material research, lithiated nickel oxide has been the choice of cathode material in the kilowatt-level MCFC stacks developed by many companies. Continuous research in the development of stable electrolyte retention matrix, identification of suitable molten carbonate electrolyte composition, and additives to the electrolyte will be a significant milestone. Also, research in the area of current collector/bipolar plate to overcome... 

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 ... 

High-temperature - with molten carbonate electrolyte (up to 650...750 C) Fuel Fuel 10 ppm 100 ppm... 

CO2 was used for the initial heating of the fuel cell with molten carbonate electrolyte. The rate of heating was 5°C/min for the uniform fuel cell heating. After heating of the fuel cell to the necessary temperatures, synthesis-gas and air were added into the streams of carbon dioxide and then CO2 consumption was reduced. 

Samples of fuel cells with molten carbonate electrolyte were made in CETl. Molten carbonate electrolyte on the basis of Na2C03, K2CO3, and Li2C03 was used. For the decrease in melting temperature of electrolyte, the part of Li2C03 was 50 mass %. Operating temperature of electrolyte was from 600 up to 650°C. The fixing of electrolyte was executed by the matrix. The matrix made of ceramics (MgO) with a 40% porosity. The anodes were made of nickel. The cathodes were made of NiO and NiO with Li addition. Ni and NiO were put on the matrix by plasma deposition. 

Composite solid oxide/molten carbonate electrolyte membrane... 

Only molten carbonate electrolytes figure in molten salt fuel cell designs.Other melts are not compatible with the use of hydrocarbon fuels, or, indeed, hydrogen and air feeds which have not been scrubbed free from carbon dioxide. Viable molten carbonate devices are based on eutectic mixtures of alkali-metal salts and are operated in the range 923-973 K. The overall electrode reactions are novel ... 

The schematic of CO2 capture in a conventional MCFC is shown in Figure 15.31. Thus, C02-laden flue gas from a coal-fired power plant is used directly as the oxidant on the cathode of an MCFC. The CO2 acts as a vehicle molecule (Figure 15.2) for the elementary anion 0 , forming the carbonate ion, which diffuses across the supported molten-carbonate electrolyte layer. It unloads its payload at the anode, where the fuel is oxidized into H2O and more CO2. Instead of recovering and recycling the CO2 from the anode back to the cathode to ferry more ions as in conventional MCFC, in the proposed schematic shown in Figure 15.31, the CO2 from the anode is recovered and sequestered. 

Another point to be mentioned for the dissemination of MCFCs is extending their lifetime. Electrolyte management is strongly related to the problem. The molten carbonate electrolyte is depleted mostly by corrosion with metals and weakening of electrolyte holding in the matrices. Cell design and surface treatment of metal should be considered. It is also necessary to search for appropriate material for the matrix. 

Attempts were made to realize the ancient dream of all electrochemists that is, building fuel cells working directly with natural fuels. It was with this aim that work on two high-temperature fuel cells was began (actually, as a resumption of very early work) with molten carbonate electrolytes (Chapter 7) and with solid-oxide electrolytes (Chapter 8). 

Molten electrolyte fuel cells-. In these devices, molten carbonate electrolytes are used at very high temperatures, ranging from 500° to 750°C. These fuel cells operate on impure hydrogen and do not require an expensive catalyst agent, thereby yielding the relatively cheapest device. These cells are best suited for industrial and commercial applications. 

The molten carbonate electrolyte fuel cell (MCFC) has a history that can be traced back at least as far as the 1920s. It operates at temperatures around 650°C. The main problems with this type of cell relate to the degradation of the cell components over long periods. The MCFC does, however, show great promise for use in CHP systems, and this is discussed in detail in Section 7.4. 

The high operating temperature of MCFCs provides the opportunity for achieving higher overall system efficiencies and greater flexibility in the use of available fuels compared with the low temperature types. Unfortunately, the higher temperatures also place severe demands on the corrosion stability and life of cell components, particularly in the aggressive environment of the molten carbonate electrolyte. 

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