Operating temperatures, PEFC system

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

PEFC The PEFC, like the SOFC, has a solid electrolyte. As a result, this cell exhibits excellent resistance to gas crossover. In contrast to the SOFC, the cell operates at a low 80°C. This results in a capability to bring the cell to its operating temperature quickly, but the rejected heat cannot be used for cogeneration or additional power. Test results have shown that the cell can operate at very high current densities compared to the other cells. However, heat and water management issues may limit the operating power density of a practical system. The PEFC tolerance for CO is in the low ppm level. 

Polymer electrolyte fuel cells (PEFC) deliver high power density, which offers low weight, cost, and volume. The immobilized electrolyte membrane simplifies sealing in the production process, reduces corrosion, and provides for longer cell and stack life. PEFCs operate at low temperature, allowing for faster startups and immediate response to changes in the demand for power. The PEFC system is seen as the system of choice for vehicular power applications, but is also being developed for smaller scale stationary power. For more detailed technical information, there are excellent overviews of the PEFC (1,2). 

One particular application for which supported Au catalysts may find a niche market is in fuel cells [4, 50] and in particular in polymer electrolyte fuel cells (PEFC), which are used in residential electric power and electric vehicles and operate at about 353-473 K. Polymer electrolyte fuel cells are usually operated by hydrogen produced from methane or methanol by steam reforming followed by water-gas shift reaction. Residual CO (about 1 vol.%) in the reformer output after the shift reaction poisons the Pt anode at a relatively low PEFC operating temperature. To solve this problem, the anode of the fuel cell should be improved to become more CO tolerant (Pt-Ru alloying) and secondly catalytic systems should be developed that can remove even trace amounts of CO from H2 in the presence of excess C02 and water. 

The vulnerability of Pt and Pt alloy catalysts to poisoning by trace contaminants at operation temperatures typical for a PEFC is well documented and is of clear concern in the design of a power system based on a PEFC stack. Sources of contaminants include both fuel and air feed streams as well as processes derived from chemical instability of cell component(s). As to the feed streams, polishing of anode feed streams generated by fuel processing upstream the cell should leave very low levels of CO to be dealt with effectively within the cell (see Sect. 8.3.7.1), whereas any traces of sulfur or ammonia have to be perfectly eliminated upstream the anode... 

Depending on operation temperature the material chosen for the bipolar plate is graphite (or graphite composites) or iron-based alloys (stainless steel) for low- and medium-temperature fuel cells (PEFC, PAFC, MCFC) and chromium or ceramic-based materials for the high-temperature systems (SOFC). In SOFC, for sealing reasons, tubular concepts have also been developed (see Section 8.1.4.6). 

If a fuel should be used which is available right now all over the world, LPG is still the best choice. For this reason, reforming systems for LPG have been developed for combination with a PEFC. The reaction temperature for the steam reforming of LPG is as high as 650-700 °C, thus a longer start-up period for heating the system to the required operation temperature is inevitable. Due to the temperature level, the CO concentration at the reformer exit is high and has to be further reduced by CO shift conversion and if needed also a CO fine purification like selective CO oxidation reaction. Thus, the required low CO concentration in the... 

The main difference between the AFC and PAFC is the gas-tight solid polymer electrolyte membrane, a sohd proton exchange membrane which has as its main function the transport of protons from anode to cathode. To investigate the physical and electrochemical origins of the performance loss in PEFC—operated at different conditions like high current densities, fuel composition (neat H2, H2 -1- lOOppm CO, H2O), flow rates, temperature, air or pure oxygen, etc.—electrochemical impedance studies on different PEFC systems with different electrodes and membranes were performed, as mentioned in Section 4.5.4.1. First impedance measurements and interpretation of FIS performed to characterize PEFC were reported by Srinivasan et al. [1988], Fletcher [1992], Wilson et al. [1993] and Poltarzewski et al. [1992], With increasing research and development effort to improve the PEFC performance and availability of suitable instrumentation the number of publications has increased. 

In order to design a basic flow sheet, aU operating conditions must be defined for chemical reactors and for the fuel cell. Different operating temperatures require a set of different heat exchangers. During the first phase of process analysis, the exchangers are not coimected to each other. An HT-PEFC system based on jet fuels as the energy carrier is analyzed here as an example. The thermodynamic conditions must be defined for each flow line ... 

Reformate from fuel processing can be directly used in aU fuel cells except PEFCs. In PEFC systems, an additional ZnO adsorbent is required at operating temperatures of 350-400 °C. The sorbent capacity for a threshold of 0.1 ppmv H2S is 9mg of S per gram of ZnO at Ippmv H2S in the reformate [31]. For European gasoHne with 10 ppmv S and a ISkWth fuel processor, an adsorbent mass of 1.3 gh operating time is necessary to decrease the sulfur concentration to 0.1 ppmv H2S. 

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