Fuel Cell Anodes

The desulfurized natural gas is mixed with the recycled depleted fuel stream containing steam formed in the fuel cell. About 75 percent of the methane is converted to hvdrogen and carbon monoxide in the prereformer. The hvdrogen-rich fuel is then passed over the fuel cell anode, where 85 percent is converted to electricity. The balance is burned with depleted air in the combustion zone. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

The Pt/Ru catalyst is the material of choice for the direct methanol fuel cell (DMFC) (and hydrogen reformate) fuel cell anodes, and its catalytic function needs to be completely understood. In the hrst approximation, as is now widely acknowledged, methanol decomposes on Pt sites of the Pt/Ru surface, producing chemisorbed CO that is transferred via surface motions to the active Pt/Ru sites to become oxidized to CO2... 

Finally, we have discussed the effect of incomplete Cj oxidation product formation for fuel cell applications and the implications of these processes for reaction modeling. While for standard DMFC applications, formaldehyde and formic acid formation will be negligible, they may become important for low temperature applications and for microstructured cells with high space velocities. For reaction modeling, we have particularly stressed the need for an improved kinetic data base, including kinetic data under defined reaction and transport conditions and kinetic measurements on the oxidation of Ci mixtures with defined amounts of formaldehyde and formic acid, for a better understanding of cross effects between the different reactants at an operating fuel cell anode. 

Minteer and co-workers have also exploited the broad substrate specificity of PQQ-dependent alcohol dehydrogenase and aldehyde dehydrogenase from Gluconobacter species trapped within Nahon to oxidize either ethanol or glycerol at a fuel cell anode [Arechederra et al., 2007]. Although the alcohol dehydrogenase incorporates a series of heme electron transfer centers, it is unlikely that many enzyme molecules trapped within the mediator-free Nahon polymer are electronically engaged at the electrode. 

Song, H Qiu, X., Guo, D., and Li, F. (2008) Role of structural H2O in Ti02 nanotubes in enhancing Pt/C direct ethanol fuel cell anode electro-catalysts. Journal of Power Sources, 178 (1), 97-102. 

FC anode loop fuel cell anode loop (control)... 

Gasifiers typically produce contaminants that need to be removed before entering the fuel cell anode. These contaminants include H2S, COS, NH3, HCN, particulate, and tars, oils, and phenols. The contaminant levels are dependent upon both the fuel composition and the gasifier employed. There are two families of cleanup that can be utilized to remove the sulfur impurities hot and cold gas cleanup systems. The cold gas cleanup technology is commercial, has been proven over many years, and provides the system designer with several choices. The hot gas cleanup technology is still developmental and would likely need to be joined with low-temperature cleanup systems to remove the non-sulfur impurities in a fuel cell system. For example, tars, oils, phenols, and ammonia could all be removed in a low-temperature water quench followed by gas reheat. 

Sarantaridis D and Atkinson A. Redox cycling of Ni-based solid oxide fuel cell anodes A review. Fuel Cells 2007 7 246-258. 

Itoh H, Yamamoto T, and Mori M. Configurational and electrical behavior of Ni-YSZ cermet with novel microstructure for solid oxide fuel cell anodes. J Electrochem 5ocl997 144 641-646. 

Corbin SF and Qiao X. Development of solid oxide fuel cell anodes using metal-coated pore-forming agents. J Am Ceram Soc 2003 86 401-406. 

This is the most common test method employed to qualify the leak characteristics of a new seal material. The test method involves applying the seal between two ceramic discs or between a ceramic and a metal disc, pressurizing the cavity formed by the seal and monitoring the pressure decay as a function of time.22 Alternatively, a metal tube and a ceramic disc can also be used [34], Typically, the cavity is pressurized to about 2 psi and the leak rate is determined by the pressure decay as a function of time. These tests can be done at room temperature or elevated temperatures. Similar test arrangement has also been used to test a plastically deformable brazed metal seal between fuel cell anode material and Haynes 214 washer [35], The cavity is pressurized to measure the rupture strength of the seal material. 

Ruiz-Morales JC, Canales-Vazquez J, Savaniu C, Marrero-Lopez C, Zhou W, and Irvine JTS. Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation. Nature 2006 439 568-571. 

Steigerwalt, S.E. et al., A Pt-Ru/graphitic carbon nanofiber nanocomposite exhibiting high relative performance as a direct-methanol fuel cell anode catalyst, J. Phys. Chem. B., 105, 8097, 2001. 

N. Kiratzis, P Holtappels, C. E. Hatchwell, M. Mogensen, and J. T. S. Irvine, Preparation and characterization of copper/yttria titania zirconia cermets for use as possible solid oxide fuel cell anodes, Fuel Cells 1,211-218 (2001). 

R. X. Liu, and E. S. Smotkin, Array membrane electrode assemblies for high throughput screening of direct methanol fuel cell anode catalysts, J. Electroanal. Chem. 535, 49-55 (2002). 

In fuel cells, the combustion energy of hydrocarbons can be converted directly into electrical energy. At the fuel cell anode, the hydrocarbon is in most cases converted to carbon dioxide because the intermediates are more easily oxidized than the starting hydrocarbon (Eq. 9a) at the fuel cell cathode oxygen is reduced to water (Eq. 9b). Most fuel cell research has involved the use of hydrogen as fuel. However, solid oxide fuels cells (SOCFs) can operate at higher temperature and can... 

PAFC systems achieve about 37 to 42% electrical efficiency (based on the LHV of natural gas). This is at the low end of the efficiency goal for fuel cell power plants. PAFCs use high cost precious metal catalysts such as platinum. The fuel has to be reformed external to the cell, and CO has to be shifted by a water gas reaction to below 3 to 5 vol% at the inlet to the fuel cell anode or it will affect the catalyst. These limitations have prompted development of the alternate, higher temperature cells, MCFC, and SOFC. 

Fuel Cell Anode Reaction Cathode Reaction... 

Fuel processing is defined in this Handbook as the conversion of a commercially available gas, liquid, or solid fuel (raw fuel) to a fuel gas reformate suitable for the fuel cell anode reaction. Fuel processing encompasses the cleaning and removal of harmful species in the raw fuel, the conversion of the raw fuel to the fuel gas reformate, and downstream processing to alter the fuel gas reformate according to specific fuel cell requirements. Examples of these processes are ... 

In general, reforming of the CH4 fuel with excess H2O outside the cell has been practiced both in molten carbonate and solid oxide fuel cell systems in order to produce H2, more reactive on a fuel cell anode, and to avoid the possible deposition of C. This reforming reaction... 

For every molecule of hydrogen (H2) that reacts within a fuel cell, two electrons are liberated at the fuel cell anode. This is most easily seen in the PAFC and PEFC because of the simplicity of the anode (fuel) reaction, although the rule of two electrons per diatomic hydrogen molecule (H2) holds true for all fuel cell types. The solution also requires knowledge of the definition of an ampere (A) and an equivalence of electrons. 

Under normal operation of an H2/O2 fuel cell, anodic oxidation of IT2 (or other hydrocarbons or alcoholic fuels)—that is, H2 —> 2H+ -1- 2e —produces protons that move through the polymer electrolyte membrane (PEM) to the cathode, where reduction of O2 (i.e., O2 -1- 2H+ -1- 2e —> H2O) produces water. The overall redox process is H2 -1-O2 —> H2O. The electronically insulating PEM forces electrons produced at the anode through an external electric circuit to the cathode to perform work in stationary power units, drive trains... 

As with the other reactor configurations, vaporizers, heat exchangers, and a heat source are also needed for microreactors.Unless the hydrogen is 99.999% pure, the PEM fuel cell typically will utilize 70—80% of the diluted hydrogen fed to it. The unreacted hydrogen from the fuel cell anode, augmented with additional fuel as needed, can be used as fuel for the combustor. The use of anode off-gas requires special controls for transient operating conditions for example, a mechanism is needed to... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

An approach similar to this avoids the use of a comparative nohle metal electrode and neglects overpotential losses at the electrodes. In this method, the potential applied at the hydrogen (or oxygen) electrode (in a three electrode configuration) is compared with the potential generated at an ideal fuel cell anode (or cathode). In the case of a n-type semiconductor photoanode ... 

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