Introduction
Conclusions
Stationary Fuel Cells
Overview
In the MCFC, hydrogen and carbonate ions from the electrolyte react at the anode to produce water and carbon dioxide while releasing electrons to the anode. The carbon dioxide (collected from the anode exhaust) and oxygen from the oxidant stream combine with electrons to produce carbonate ions that enter the electrolyte. The carbon dioxide from the spent anode gas has to be collected and any residual hydrogen burned before it can be combined with the incoming air stream at the cathode. Future systems may use membranes to remove the hydrogen for recirculation back to the fuel stream.
The reaction for the cell is as follows:
Design
The molten carbonate fuel cell uses a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a porous, insulating, and chemically inert ceramic matrix. The anode is typically made of Ni-Cr/Ni-Al/Ni-Al-Cr and is 0.20 – 0.50 mm thick. The pores are 3-6 micrometers in diameter and it typically has 45-70% porosity. Cathode is typically composed of lithiated NiO-MgO with 70-80% initial porosity. After litigation and oxidation, the porosity decreases to 60-65%. The cathode pores are larger than the anode pores, measuring 7-15 micrometers, and the cathode is also significantly thicker than the anode, at 0.5 – 1.0 mm. The major considerations with Ni-based anodes is structural stability and with NiO cathodes is NiO dissolution. Ni ions diffuse in the electrolyte towards the anode and metallic Ni can precipitate in a hydrogen reducing environment. At atmospheric pressure, Ni dissolution and precipitation should not be a problem for getting the desired 5yr life span, but problems arise with increased pressure.
The electrolyte support is lithium aluminum oxide and is 0.5 – 1.0 mm thick, and the electrolyte is lithium potassium or a lithium sodium salt. Because it operates at about 650˚C, the salt is liquid and a good ionic conductor. The high operating temperature also means that it could, theoretically, operate directly on hydrocarbon fuels. A highly successful method to fabricate the solid electrolyte has been to assemble it using tape casting, where the electrolyte is assembled onto the fuel cell using an organic binder that is subsequently removed by thermal decomposition. Lithium potassium carbonate is normally used for atmospheric pressure operation and lithium sodium carbonate for pressurized operation and life extension. The electrolyte composition affects the fuel cells in many ways. Thinner electrolytes have improved cell performance but shorter life spans and lithium rich electrolytes have increased ionic conductivity and lower ohmic polarization compared to sodium and potassium carbonates because gas solubility and diffusability are lower. Corrosion is also more rapid but can be largely overcome by coating the vulnerable locations in the cell.
The cell performance is very sensitive to operating temperature, and a drop in the temperature leads to increased ionic and electrical resistance and decreased electrode kinetics. As the operating temperature increases the maximum fuel efficiency and theoretical operating voltage decreases but the rate of the electrochemical reaction and the current that can be obtained increases. Consequently, the real operating voltage is higher than that of the phosphoric acid fuel cell at the same current density. More power is available at a higher fuel efficiency from a MCFC than a PAFC of the same electrode area. This means that a MCFC should be smaller and cheaper than a comparable PAFC. Like the PAFC, the MCFC produces enough excess heat to produce high pressure steam and run a turbine to produce additional electricity. With the two cycles combined, electrical efficiencies over 60% have been suggested. The MCFC could also have its own internal processor for reforming fuel where the heat from the fuel cell reaction could drive the reaction of methanol and water to form carbon monoxide and hydrogen, giving the MCFC another advantage over the PAFC.
Future
Many improvements and optimizations still need to be made for the MCFC, however. For instance, increased operating pressures enhance cell voltages, however, it also causes undesirable side reactions, such as carbon deposition, methane formation, and methane decomposition to carbon and hydrogen. Halogen compounds can severely corrode cathode hardware and increase electrolyte loss and NOx, As, Pb, Cd, Hg, and Sn could also damage cell performance. There is also a need for better sulfur tolerance in MCFC’s, and decreased cathode dissolution. Sulfur compounds in low parts per million in fuel are detrimental to MCFC’s. Sulfur compounds block active electrochemical sites by chemisorption on Ni surfaces, poison catalytic reaction sites, and react with carbonate ions in the electrolyte. The tolerance is highly dependent on temperature, pressure, gas composition, cell components, and system operation; the limits increase with increased temperature but decrease with increased pressure. Cathode dissolution is the main life-limiting constraint of MCFC’s, and some proposed solutions are increasing the basicity for the electrolyte, increasing the Li fraction, or lowering carbon dioxide (acid) partial pressure to produce a milder cell environment. Another challenge is the electrolyte structure material; the particles grow at higher temperatures, low carbon dioxide gas atmospheres, and in base, leading to detrimental changes in the particle structure. Consequently, a more uniform particle size distribution is necessary. There is significant voltage loss throughout the lifespan of the fuel cell and the majority is lost in the electrolyte and cathode components. Increasing the porosity of the electrolyte and changing the melt from Li/Na to Li/K or preventing gas crossover from one electrode to the other may alleviate some of the voltage losses and studies are being done to optimize the electrolyte. There is also a tendency for the electrolyte to migrate from the positive end of the stack to the negative end, causing the end cells to lose performance and new designs are trying to address this problem. Another source of power loss is low oxygen partial pressure on the anode side of the plate, leading to heat-resistant alloy formation, which increases resistance on the cathode side.
