Fuel cell casing

At the positive electrode of a fuel cell, a more complex set of reactions must be at work in order for the hydrogen ions to join with oxygen and form water (as illustrated by the sequences in Fig. 3.3, read from top to bottom). Figure 3.3 does not indicate ionic charges, because part of the investigation is to find out at which stage molecules are ionised or electrons are captured to form neutral molecules. The electrons are taken from or donated to the electrode metal, which then becomes positively charged in the fuel cell case (or already is due to the external potential in the electrolyser case). It follows that the electrons likely react with ions quite near to the electrode surface. 

Case 1 Conv. H2 plant Case la Conv. H2 plant w/carbon membrane Case 2 Reformer W/AI2O3 membrane Case 2 Reformer W/AI2O3 and carbon membrane Case 3 Conv. H2 plant w/Fuel cell Case 3 Conv. H2 plant w/Fuel cell and carbon membrane Case 4 Auto-reformer design... 

This was probably a consequence of the hydrogen oxidation electrochemistry occurring at a significantly more positive potential than in the fuel cell case. The supported metal sulfide packing in the oxidation compartment was necessary for rapidly shifting equilibrium (7.42) upon hydrogen depletion. Typical H2S conversion rates were 95%. 

Elter, J., J.S. Cooper (2007) Sustainability tools Applying life cycle assessment A fuel cell case study, American Society of Mechanical Engineers (ASME) and American Institute of Chemical Engineers (AIChE) Sustainable Engineering Series, 5th Session. 

Metal-air batteries are different from conventional batteries because metal-air batteries are connected to the atmosphere, and need this access to operate. Metal-air batteries are also different from fuel cells because metal-air batteries have a self-contained anode within the battery case itself. Metal-air batteries are part conventional" battery and part fuel cell. Sometimes metal-air batteries are called semi-fuel cells. "Conventional" batteries have the active components of both the anode and cathode within the battery case (see Figure 1.1). Fuel cells have both of the "active" components (or fuels) of the anode and cathode supplied from outside the fuel cell case. A metal-air battery, or semi-fuel cell, has the solid anode within the case (like a battery), while the cathode fuel is brought into the cell (like a fuel cell). Oxygen gas... 

Electrical management, or power conditioning, of fuel cell output is often essential because the fuel cell voltage is always dc and may not be at a suitable level. For stationai y applications, an inverter is needed for conversion to ac, while in cases where dc voltage is acceptable, a dc-dc converter maybe needed to adjust to the load voltage. In electric vehicles, for example, a combination of dc-dc conversion followed by inversion may be necessary to interface the fuel cell stack to a, 100 V ac motor. 

Finally, the energy available from the above reaction might be used to operate a fuel cell such as those involved in the space program. In that case, as much as 818 kj/mol of useful electrical work could be obtained relatively litde heat is evolved. Summarizing this discussion in terms of an energy balance (per mole of methane reacting) ... 

The concept of a promoter can also be extended to the case of substances which enhance the performance of an electrocatalyst by accelerating the rate of an electrocatalytic reaction. This can be quite important for the performance, e.g., of low temperature (polymer electrolyte membrane, PEM) fuel cells where poisoning of the anodic Pt electrocatalyst (reaction 1.7) by trace amounts of strongly adsorbed CO poses a serious problem. Such a promoter which when added to the Pt electrocatalyst would accelerate the desired reaction (1.5 or 1.7) could be termed an electrocatalytic promoter, or electropromoter, but this concept will not be dealt with in the present book, where the term promoter will always be used for substances which enhance the performance of a catalyst. 

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. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The... 

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