Other Fuel Cells

The question is often asked, what is the best fuel cell system and when will we see it The answer is simple Like any commodity, the value and marketability must be compared to the alternatives available. The best fuel cell for a particular purpose depends on the application and the competing technologies. We will see the fuel cell introduced to the public when it becomes a better alternative to the competing technologies. As long as there are less expensive and more convenient alternatives, fuel cells will not be introduced in large quantities. 

The purpose of this chapter is to describe the development and operation of the various fuel cell systems besides the PEFC. With background information from Chapters 1-5, the reader should be able to fully appreciate and analyze the different designs, operational advantages, and technical challenges of the other fuel cell varieties discussed. 

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The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

The interconnect material is in contact with both electrodes at elevated temperatures, so chemical compatibility with other fuel cell components is important. Although, direct reaction of lanthanum chromite based materials with other components is typically not a major problem [2], reaction between calcium-doped lanthanum chromite and YSZ has been observed [20-24], but can be minimized by application of an interlayer to prevent calcium migration [25], Strontium doping, rather than calcium doping, tends to improve the resistance to reaction [26], but reaction can occur with strontium doping, especially if SrCr04 forms on the interconnect [27],... 

Interconnects are formed into the desired shape using ceramic processing techniques. For example, bipolar plates with gas channels can be formed by tape casting a mixture of the ceramic powder with a solvent, such as trichloroethylene (TCE)-ethanol [90], Coating techniques, such as plasma spray [91] or laser ablation [92] can also be used to apply interconnect materials to the other fuel cell components. 

In the EPD process, a DC electric field is used to deposit charged particles from a colloidal suspension onto an oppositely charged substrate, as illustrated in Figure 6.8. The graphite rod used for the deposition substrate is later burned out prior to cell operation, leaving a hollow tube. The other fuel cell layers can be deposited by a similar process onto the anode support tube. 

The Sequel is almost the size of a Cadillac SRX. It has a 300-mile range on a refueling of hydrogen and accelerates to 60 mph in less then 10 seconds. Other fuel cell cars have a driving range of 170-250 miles and cover 0-60 mph in 12-16 seconds depending on whether they use a battery. 

Research the advances made in the development of fuel cells since this book was written. Describe how any new types of fuel cells operate. Evaluate their advantages and disadvantages, as compared to the internal combustion engine and other fuel cells. 

The above discussion implies that MCFCs should be operated at low reactant gas utilizations to maintain voltage levels, but doing this means inefficient fuel use. As with other fuel cell types, a compromise must be made to optimize overall performance. Typical utilizations are 75 to 85% of the fuel. 

B.L. Halpern, J.W. Golz, Y. Di, "Jet Vapor Deposition of Thin Films for Solid Oxide and Other Fuel Cell Applications," in Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U S. DOE/METC, July, 1992. 

The advantages of PEMFCs over the other fuel cell systems are as follows (Chau-rasia et al., 2003) ... 

Direct methanol fuel cell technology is relatively new compared to that of fuel cells powered by pure hydrogen, and research and development are roughly 34 years behind that of other fuel cell types. Nonetheless, the DMFC appears to be the most promising as a battery replacement for portable applications such cellular phones and laptop computers, and a number of manufacturers are already introducing commercial versions of these applications. 

Although they are more resistant to impurities than other fuel cell types, scientists are looking for ways to make MCFCs resistant enough to impurities from coal, such as sulphur and particulates. 

Solid oxide fuel cells (SOFC) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity, however, calculations show that over 70 percent may be achievable. In applications designed to capture and utilize the system s waste heat (co-generation), overall fiiel use efficiencies could top 80-85 percent. 

The PEM fuel cells offers an order of magnitude higher power density than any other fuel cell system, with the exception of the advanced aerospace AFC. Recent advances in performance and design offer the possibility of lower cost than any other fuel cell system. 

Other fuel-cell developments have been underway by Baitelle Northwest und Brookhnvcn National Laboratory,... 

The SOFC, like other fuel cells, is an electrochemical device for the conversion of chemical energy of a fuel into electricity and heat. The fuel, for example, hydrogen is not combusted but electro-oxidized at the anode (fuel electrode) by oxygen ions conducted across the electrolyte according to the following overall reaction... 

The working principles behind a solid oxide fuel cell (SOFC) are schematically illustrated in Figure 8.7, where, similar to the other fuel cell types, the three key parts of an SOFC, a cathode, an anode, and an electrolyte, are shown. The electrolyte is, in a majority of cases, an oxygen-anion ceramic conductor, which is, as well, an electronic insulator [5]. In the SOFC the fuel can be methane (CH4). Subsequently, in this case the oxidation reaction in the anode is given by... 

With respect to the marketplace, SOFC and SOFC/gas turbine hybrids are potentially an attractive basis for an efficient, clean, cost-competitive DG system, but they do not depend on having 1I2 fuel. However, they could facilitate a transition to a H2 economy by making use of H2 for distributed electricity and CHP, while other fuel cells for vehicles are becoming cost-effective, reliable, and efficient. It is important for the DOE to monitor the milestones and goals of the SECA Program and to fully fund it. 

The one and only fuel cell being manufactured, which makes use of circulators, as in Figure A.l, is the power storage system ESS-RGN, formerly Regensys, which features liquid reactants which are incompressible in contrast to the gaseous reactants of all other fuel cell systems. 

Gardiner F J, 1996, Thermodynamic Processes in Solid Oxide and Other Fuel Cells. Symposium S 455, Paper 3, Fuel Cells for Power and Propulsion, IMechE, London. 

Especially to be noted is the high potential of the cell relative to the value obtained when the IL is replaced by 98% H3PO4 with, (0.9 vs. 0.75 V), which is maintained under load when using ethylammonium nitrate as electrolyte. It is evident that with the protic ionic liquid, a much higher exchange current density at the oxygen electrode can be obtained than in other fuel cells. 

SOFCs share with other fuel cell types a very low tolerance to fuel impurities of H2S (< 1 ppm), NH3 (< WYo), HCl and other halogens (< 1 ppm), but in contrast to the low-temperature cells accept fuel impiuities of CO in addition to the methane and hydrocarbons reformed internally (Dayton et ah, 2001). 

The individual components used in an AFC are not necessarily expensive compared to those of other fuel cell t3q>es under development. Use of Ft catalysts can be avoided, while the bipolar plates collecting the electron flows typically have to be made of fairly expensive black carbon to avoid corrosion. The peripherals needed for water management and electrolyte draining add to the cost, but do not necessarily lead to drawbacks such as long start-up... 

In both PEMFCs and SOFCs, there is a strong need to improve the electrolyte materials and to find substitutes for the traditional electrolytes, which are Nafion for the PEMFC and yttria-stabilized zirconia (YSZ) for the SOFC. In this paper, we will focus on the discussion of new developments in the field of SOFC electrolytes. For recent developments concerning other fuel cell types, the reader is referred to review articles and books. " ... 

The MCFC because of its high operating temperature has higher efficiency (>50%o) and faster electrode kinetics than any other fuel cell system.At 650° C, almost a theoretical reversible potential is established at the interface with low electrode overpotentials, which does not require any noble metal catalysts. The CO does not poison the anode, because in the MCFC it is oxidized at the anode interface. 

Alkaline fuel cell (AFC) was used for Apollo and Space Shuttle program. Alkaline fuel cell employs liquid alkaline, e.g., KOH, as an electrolyte so that fuel, as well as air or oxygen, should be free of CO2 because the strong alkaline electrolyte reacts with CO2 to form carbonates, which reduces the ionic conductivity. Electrodes, e.g., Ni, Ag, and metal oxides, are relatively inexpensive compared to that of other fuel cells. 

Fuel cells, due to their higher efficiency in the conversion of chemical into electrical energy vhth respect to thermo-mechanical cycles, are another major area of R D that has emerged in the last decade. Their effective use, ho vever, still requires an intense effort to develop ne v materials and catalysts. Many relevant contributions from catalysis (increase in efficiency of the chemical to electrical energy conversion and the stability of operations, reduce costs of electrocatalysts) are necessary to make a step for vard in the application of fuel cells out of niche areas. This objective also requires the development of efficient fuel cells fuelled directly vith non-toxic liquid chemicals (ethanol, in particular, but also other chemicals such as ethylene glycol are possible). Together vith improvement in other fuel cell components (membranes, in particular), ethanol direct fuel cells require the development of ne v more active and stable electrocatalysts. 

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