Fundamentals of fuel cells including polymer electrolyte fuel cell (PEFC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC), are presented. Three types of overpotential, activation overpotential, ohmic overpotential and concentration overpotential, are described. Polarization curves in PEFC and SOFC are also presented for better understanding of dominant factors that determine overall cell performance.
Bio-fuel cell, or microbial fuel cells (MFCs) can convert chemical energy of organic compounds to electric power, with their aerobic degradation to CO2 and water. MFCs are analogous to polymer electrolyte fuel cell (PEFC) in terms of the reaction sequences: fuel oxidation, proton transfer and oxygen reduction, whereas MFCs can be also considered as a waste treatment reactor. In this review, in comparison with PEFC, the items explained are the mechanism of MFCs, application of MFCs to wastewater/organic waste (rice straw and excess sludge) disposal, and application of “exoelectrogens” to mediator-less MFCs. Feasibility of MFCs is discussed for designing reactor structures of MFCs as semi-continuous, mediator-less, and single-chambered form.
During low humidity operation of polymer electrolyte fuel cells (PEFCs), water management is a critical issue to enhancement of cell performance because low proton conductivity of electrolyte membrane is caused by membrane dehydration. In an operating PEFC at high current densities, water migrates significantly through the membrane from the anode to cathode owing to electro-osmotic drag, and the membrane dehydration occurs mainly at the anode. In order to alleviate this problem known as “dryout”, it is essential to understand the fundamental phenomena of the water transport within the fuel cell. In this study, the water distribution in the anode flow field of a PEFC with low humidity conditions was experimentally visualized by using water sensitive paper (WSP). Furthermore, the dew-point temperature at the anode outlet and the membrane resistance were simultaneously measured, and the effects of inlet gas humidification and current density on the water transport in the anode and the cell performance were discussed. It was found that the water vapor concentration in the anode increases gradually after the startup because of the back diffusion of the product water. Especially at high current densities, the humidification of the anode inlet gas is effective to achieve the proper water management on the anode side and decrease the membrane resistance.
Recently, portable terminal equipment such as notebook personal computers and mobile phones etc. have spread dramatically, and users are desirous of using portable terminal equipment and the like for office use. In particular, portable terminal equipment cannot even be charged for lack of commercial power sources on the sea, in mountains and at disaster sites, etc. Hydrogen cylinders comprising Proton Electrolyte Fuel Cells (PEFC) fuel are not obtainable anywhere. Although PEFC has been developed to satisfy such demands, because reliability and durability are low, these are not useful as the power supply source for portable devices. On the other hand, Molten Carbonate Fuel Cells (MCFC) is known for its long-term operation and stability. Because MCFC can use readily obtainable butane gas cylinders as fuel, MCFC is the best for portable power supply. However, because MCFC has been developed for large-scale use, the cell components and the stack technology should be optimized for downsizing. Therefore, it is intended to solve problems associated with developing a portable MCFC. This paper examines the following three points: (1) the optimization of the electrolyte impregnation method, (2) the optimization of the electrolyte sheet method and (3) the optimization of the manifold sealing method. As a result, stack twisting can be controlled by a twist prevention jig at the time of electrolyte pre-impregnation. The manifold sealing method can solve this by using an electrolyte sheet and a matrix sheet as the sealing component. Oxidation of the catalyst can be prevented by improving the electrolyte impregnation procedure, with the same Direct Internal Reforming Molten Carbonate Fuel Cells (DIR-MCFC) performance as with a conventional MCFC. Although the highest filler content of the electrolyte to the anode electrode is 97%, in a cathode electrode it is 129% if the cathode electrode was pre-oxidized at 500°C. Further improvement is necessary for the electrolyte pre-impregnation method, which cannot be applied to the Li/Na electrolyte in that the electrolyte cannot be sufficiently filled into both electrodes.
Nowadays, bio-fuel cells which utilize the enzymes of microbes are widely noticed. In this study, the authors made a bio-fuel cell which utilized dry yeast along with sucrose as fuel, and conducted experiments under various conditions in order to improve its power generation performance. The kinds and concentrations of the electron mediator and oxidizer solutions and the number of current collectors in the anode and cathode were optimized. As a result, the single bio-fuel cell with six sheets of current collectors (each area was 20 cm2) in each of the anode and cathode was able to generate the maximum power of about 9 mW. Moreover, the authors measured the performance of the fuel cell using wild yeast and analyzed the variations in the concentrations of reaction substances, and further conducted experiments using reagents affecting the metabolism of yeast in order to examine the electricity generation mechanism in the bio-fuel cell.