Fuel cell temperature considerations

Thus, the heat release is directly related to the amount of product water. The next consideration is the amount of heat needed to raise fuel cell temperature from, for example, -30 to 0°C (AT = 30 K). The thermal mass of the fuel cell components comes in large part from the bipolar plates (BPPs), neglecting the end plates. With graphite bipolar plates of 1 mm thickness each, and assuming an adiabatic system, the required heat is... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Second, sensors are often intended for a single use, or for usage over periods of one week or less, and enzymes are capable of excellent performance over these time scales, provided that they are maintained in a nfild environment at moderate temperature and with minimal physical stress. Stabilization of enzymes on conducting surfaces over longer periods of time presents a considerable challenge, since enzymes may be subject to denaturation or inactivation. In addition, the need to feed reactants to the biofuel cell means that convection and therefore viscous shear are often present in working fuel cells. Application of shear to a soft material such as a protein-based film can lead to accelerated degradation due to shear stress [Binyamin and Heller, 1999]. However, enzymes on surfaces have been demonstrated to be stable for several months (see below). 

These effects can all be enhanced if the point defects interact to form defect clusters or similar structures, as in Fej xO above or U02, (Section 4.4). Such clusters can suppress phase changes at low temperatures. Under circumstances in which the clusters dissociate, such as those found in solid oxide fuel cells, the volume change can be considerable, leading to failure of the component. 

POX reactor exit temperatures vary widely. Noncatalytic processes for gasoline reforming require temperatures in excess of 1,000°C. These temperatures require the use of special materials and significant preheating and integration of process streams. The use of a catalyst can substantially reduce the operating temperature allowing the use of more common materials such as steel. Lower temperature conversion leads to less carbon monoxide (an important consideration for low temperature fuel cells), so that the shift reactor can be smaller. Lower temperature conversion will also increase system efficiency. 

Over the last decade, novel carbonaceous and graphitic support materials for low-temperature fuel cell catalysts have been extensively explored. Recently, fibrous nanocarbon materials such as carbon nanotubes (CNTs) and CNFs have been examined as support materials for anodes and cathodes of fuel cells [18-31], Mesoporous carbons have also attracted considerable attention for enhancing the activity of metal catalysts in low-temperature DMFC and PEMFC anodes [32-44], Notwithstanding the many studies, carbon blacks are still the most common supports in industrial practice. 

To obtain a useful fuel cell, polarization must therefore be kept as slight as possible. This can be done by choosing an electrolyte of good conductivity and above all by accelerating the electrode reactions. In low-temperature cells, that operate with aqueous electrolytes the reactions at both cathode and anode can be considerably accelerated by the addition of very active catalysts. These materials are incorporated in the appropriate electrode, so that the electrode not only conducts the current but in addition catalyzes the reaction. The rest of this paper is devoted exclusively to cells of this type. 

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