Excess heat generated from gasification can be captured and used for process heat in any application. Since there will be no edible residuals from gasification, fish feed will have to be produced in another manner. To this end, we will conduct production and use trials with feeds ranging from alfalfa leaf, small grains, soy, corn, legume lawn clippings, algae (spirulina), and duckweed. The feed ration does not need to be complex, only complete. Field crops will be irrigated with fish effluent during the summer in a controlled test. The control will be plain water and yields will be noted and recorded. All other parameters will be constant.

Compost-Based Aquaponic Greenhouses

As vegetables are grown and sold, there will be a certain amount of waste plant parts — roots, stems, and trimmings — which will be composted along with grass clippings, leaves, and other materials the project has available, including the ashes from the outdoor furnaces. Ashes are a rich source of boron — an element lacking in most northeastern soils. The project will use the compost in a separate growing system that will allow for the growth of root crops in pure compost, and irrigated with fish tank water.

Through gasification solid biomass is converted into synthesis gas. The funda­mentals have extensively been described by, among others, Katofsky (1993). Basically, biomass is converted to a mixture of CO, CO2, H2O, H2, and light hydrocarbons, the mutual ratios depending on the type of biomass, the gasifier type, temperature and pressure, and the use of air, oxygen, and steam.

Many gasification methods are available for synthesis gas production. Based on throughput, cost, complexity, and efficiency issues, only circulated fluidized bed gasifiers are suitable for large-scale synthesis gas production. Direct gasifi­cation with air results in nitrogen dilution, which in turn strongly increases downstream equipment size. This eliminates the TPS (Termiska Processer AB) and Enviropower gasifiers, which are both direct air blown. The MTCI (Manu­facturing and Technology Conversion International, affiliate of Thermochem, Inc.) gasifier is indirectly fired, but produces a very wet gas and the net carbon conversion is low. Two gasifiers are selected for the present analysis: the IGT (Institute of Gas Technology) pressurized direct oxygen fired gasifier and the BCL (Battelle Columbus) atmospheric indirectly fired gasifier. The IGT gasifier can also be operated in a maximum hydrogen mode by increasing the steam input. Both gasifiers produce medium calorific gas, undiluted by atmospheric nitrogen, and represent a very broad range for the H2:CO ratio of the raw synthesis gas.

The yeast Saccharomyces cerevisiae is specialized for fermentation, with approx­imately 45% of cellular proteins devoted to glycolysis and ethanol fermentation

[18]. Glucose and maltose are fermented to ethanol by S. cerevisiae via the same fermentation pathway (Figure 4.3) [19] used to make beverage alcohol. In glyc­olysis, glucose is converted through a series of reactions to pyruvate, and energy is extracted in the form of four ATP molecules. Then, pyruvate is converted to ethanol in a two-step reaction; pyruvate is decarboxylated to form the more reactive acetaldehyde, which is reduced to ethanol. The second part of the fer­mentation pathway reoxidizes NADH to NAD+ and thus serves to recover the reducing equivalents that were consumed in the conversion of glucose to pyruvate.

For each glucose fermented, two ethanol and two CO2 molecules are produced (Table 4.1, Figure 4.3). The theoretical mass yield is only 0.51 g of ethanol per g of fermented glucose. The actual yield is closer to 90-95% of 0.51 g because some glucose is converted to cell mass and side-products such as glycerol, citric acid cycle intermediates, and higher alcohols. Contaminating microorganisms can also lower the yield by converting glucose to other fermentation products such

as acetic, lactic, and succinic acids. Because ethanol is used as a fuel, it is also appropriate to consider ethanol yield on an energy basis. The thermodynamic yield can be calculated by comparing the heats of combustion for the products and reactants (Table 4.1). By this measure, converting glucose to ethanol has an amazing theoretical yield of 98-99%, which means that the yeast actually gains little energy benefit from fermenting glucose to ethanol. In other words, ethanol fermentation is an excellent process for generating fuel, because most of the energy from glucose is retained in the fermentation product.

Yeast are ideally suited for use in the fuel ethanol industry. Fermentations run 360 days a year, in tanks containing thousands of gallons of beer, with no pH adjustments and only approximate temperature control (reactors are cooled with well water). As a consequence of the absence of pH control and the pro­duction of CO2, the pH drops steadily during the fermentation and ends up below 4.0. Furthermore, the yeast withstand extreme environmental stresses including high osmolality (beginning solids of 25-30% or higher) and high ethanol con­centrations (final concentrations of 12-18% vol.), as well as organic acids pro­duced by contaminating bacteria. The constant contamination of the fermentation is a consequence of the need to run the process in an “open system”— non — aseptically — because the fermentation volumes are quite large and the selling profit margin for ethanol is very low. Fortunately, most bacterial contaminants do not grow below pH 4, and the ability of yeast to do so provides a natural method of suppressing the growth of these contaminants.

Environmental stresses are additive and often synergistic in nature, which means that a combination of many minor stresses, from the perspective of the yeast, equals a single large stress. For example, yeast have reduced tolerance to ethanol at higher temperatures and reduced tolerance to organic acids at lower pH. Despite all of these challenges, S. cerevisiae produces ethanol at rates in excess of 3 g l-1 h-1 and at yields close to 95% of the theoretical maximum. Efforts in the yeast research field are directed at developing strains that produce less glycerol, grow at slightly elevated temperatures (38°C), and withstand even higher ethanol concentrations.

Corn steep water (CSW) is a by-product of the corn wet-milling industry and contains large amounts of substances derived from the fermentative conversion of carbohydrates, proteins, and lipids during corn steeping. Currently, CSW is evaporated to 50% solids and marketed primarily as an economical livestock feed supplement in the cattle industry. CSW is a rich complement of important nutri­ents such as nitrogen, amino acids, vitamins, and minerals and was proposed to be a good substitute for yeast extract (Hull et al., 1996). This finding is important as it impacts the economics of butanol production from corn.

An economic analysis performed by Qureshi and Blaschek (2000a), demon­strated that the fermentation substrate was one of the most important factors that influenced the price of butanol. Development of a cost-effective biomass-to- butanol process can only be commercially viable if cheaper commercial substrate such as liquefied corn starch and CSW can be used in combination with toxic product removal by gas stripping (Ezeji et al., 2005). It is interesting to note that C. beijerinckii BA101 when grown on liquefied corn starch-CSW medium pro­duced levels of acetone-butanol equal to or higher than the levels produced when grown on glucose-based yeast extract medium (Ezeji et al., 2005). The fermen­tation time for liquefied corn starch — and saccharified liquefied corn starch-CSW media were 120 and 78 h, respectively, while the fermentation time for glucose — based yeast extract medium was 68 h. The presence of sodium metabisulfite (Na2S2O5; a preservative) in the liquefied starch and CSW was found to result in inhibition of C. beijerinckii BA101 and also may have affected the secretion of amylolytic enzymes by the culture, which is necessary for efficient hydrolysis and utilization of starch and oligosaccharides. However, it appears that the use of CSW has a great potential for the bioconversion of corn to acetone-butanol. The presence of Na2S2O5 in the CSW may be a major problem in a long-term fermentation by C. beijerinckii BA101. During a long-term fermentation using CSW and C. beijerinckii BA101, removal of Na2S2O5 from CSW by oxidation is recommended (Ezeji et al., 2005).

The following is a brief treatment of the thermodynamics governing the methanol oxidation reaction of a DMFC. Also, the impact of surface kinetics on the practical efficiency of the cell are presented. Some intriguing reports suggesting a new general direction for CO-tolerant catalyst development are cited [14,15,16].

Thermodynamic Optimum

When an organic fuel is used, essentially as a hydrogen source in a fuel cell, the expectation is that the fuel will be completely oxidized to carbon dioxide. For methanol, this is summarized thermodynamically [17] in terms of the reduction potentials as

CO2 + 6H++ 6e ^ CH3OHW+ H2O E0 = 0.016V (9.1)

While methanol is oxidized at the anode, oxygen is reduced at the cathode: O2 + 4H ++ 4e ^ 2H2O E0 = 1.229V (9.2)

The net cell reaction is

CH3OHW + 1.5O2 ^ 2H2O + CO2 (9.3)

where the standard cell potential (electromotive force, emf) is E0e„ = 1.229 — 0.016 = 1.213 V For a six-electron process (n = 6), the standard free energy is AG0 = — nFE0eU = -702.2 kJ/mol for methanol. With a molecular mass, M, of 0.03204 kg/mol, the theoretical specific energy for methanol is W = — AG0/(M x 3600s/hr) = 6.088kWh/kg; because the density of methanol is 0.7914 kg/l, this corresponds to an energy density of 4.818kWh/l. The standard enthalpy [17], AH0 = -726kJ/mol, is similar to AG0, consistent with a small entropy term.

Formally, the complete oxidation of methanol can be viewed thermodynam­ically as a series of two-electron/two-proton oxidation steps. The reduction poten­tials for this sequence in acid are given as [18]:

HCHO(aq) + 2H + + 2e ^ CH3OHw E0 = 0.232V (9.4)

HCOOH(aq) + 2H + + 2e ^ HCHO(aq) + H2O E0 = 0.034F (9.5)

H2CO3(aq) + 2H+ + 2e ^ HCOOH(aq) + H2O E0 = -0.166F (9.6)

where the oxidation of formic acid is reported to carbonic acid, consistent with the solubility of carbon dioxide and its equilibrium with carbonate [19].

CO2(g) ^ CO2W Kco2 = 0.034 (9.7)

CO2(aq) + H2O ^ HCO — + H+aq) pK = 6.36 (9.8)

The acidity constants for carbonic acid are pKa1 = 6.352 and pKa2 = 10.329; for formic acid, pKa = 3.745. Note that these reaction steps embed information about the complexities of the solution chemistry in the fuel cell as reaction products build and local pH changes. Note also, that reactions are reported in acid because practical DMFCs are usually run under acidic conditions. Under basic conditions, the formation of insoluble carbonates dramatically complicates the design of plant and limits applicability as electrolytes must be replaced as carbonate levels build.

For species in solution, the standard potentials (reactions 4 to 6) are such that thermodynamically, the oxidation of methanol proceeds cleanly and sequentially from alcohol to aldehyde to acid to CO2/carbonic acid with approximately 200 mV separating each successive two proton/two electron transfer. The specific energy and energy density of methanol are high. Thus, thermodynamically, the expectation is that methanol is an excellent fuel for a direct reformation fuel cell. However, the thermodynamics do not capture the complexity of the surface reactions that dictate the fate of methanol in a direct reformation fuel cell.

Realities of Surface Kinetics

The kinetic limitations of DMFCs have been well reviewed in detail from several different perspectives in recent years [17,20,21]; an early and thorough review is provided by Parsons and VanderNoot [22]. For effective utilization of methanol as a fuel, the catalyst must provide a good surface for adsorption of methanol and its sequential breakdown to carbon dioxide/carbonate through loss of paired protons and electrons. Under acidic conditions, this has largely restricted practical catalysts to platinum and its alloys and bimetallics. Methanol will adsorb to platinum and platinum serves as an excellent electron transfer catalyst. The difficulty is that platinum passivates as carbon monoxide by­product accumulates and adsorbs to the platinum surface. To oxidize carbon monoxide to carbon dioxide/carbonic acid, oxygenated species such as water must adsorb to the catalyst surface. Because platinum is not strongly

CO,

SCHEME 9.1 Reaction pathways for methanol oxidation.

hydrophilic, platinum bimetallics and alloys formed with more hydrophilic metals such as ruthenium are typically used to facilitate CO oxidation.

Consider the mechanistic constraints for oxidation of methanol. As in equa­tion 1, the complete oxidation of methanol to carbon dioxide proceeds by a six — proton, six-electron process. The mechanism presented in Scheme 9.1 outlines the basic route by which methanol is fully oxidized. The loss of paired protons and electrons is noted for each step. To account for all six electrons, recognize that the adsorption of water to the catalyst surface also generates an electron and proton. For a catalyst metal site, M,

M + H2O ^ M — OH + H + + e

Following the notation from Ref. [21], methanol first adsorbs to liberate one electron and one proton.

CH3OH + Pt ^ Pt — CH2OHads + H+ + e (9.10)

This is followed by two steps to form the formyl intermediate, — CHO.

Pt — CH2OH„* + Pt ^ Pt2CHOH + H+ + e

Pt2CHOH ^ Pt + Pt — CHO + H+ + e

On clean platinum surfaces, these oxidations proceed smoothly to provide two electrons and two protons. Consider Scheme 9.1. The weakly adsorbed -CHO is a point at which the oxidation mechanism breaks into two paths. One path yields adsorbed CO and the other adsorbed COOH. Adsorbed COOH is generated by reaction of — CHO and an adjacent M — OH to yield one proton and one electron and form weakly adsorbed — COOH. Adsorbed CO is generated by the direct oxidation of — CHO by one proton and one electron to form strongly adsorbed CO. Basic kinetic arguments would favor the strongly adsorbed CO over the weakly adsorbed — COOH because first, the oxidation of — CHO to — CO is direct and does not require an adjacent second species, M — OH, and second, because — CO is strongly bound and — COOH is weakly bound.

It should be pointed out that there is an alternative branch point in the oxidation process in which adsorbed — CHOH undergoes a one-electron and a one-proton oxidation to form adsorbed — COH.

Pt2CHOH + Pt ^ Pt3COH + H+ + e

The adsorbed — COH can then either undergo one-proton/one-electron oxida­tion to adsorbed — CO or react with an adjacent M — OH to form HCOOH in solution. Neither process leads to the efficient oxidation to carbon dioxide/car — bonic acid.

To the extent the platinum surface is passivated by CO, the reaction is terminated. Thus, the design of a system for the efficient and complete oxidation of methanol can be approached in two ways.

The first approach is to circumvent the formation of adsorbed CO by favoring the formation of — COOH. Experimentally, this is done by enhancing the proba­bility that — CHO is adjacent to an oxygen source, M — OH, by using bimetallics and alloys of platinum where M is more hydrophilic than platinum. There are questions of stability and cost associated with these catalysts although they have been shown to enhance conversion efficiency. But, based on the relative strengths of the adsorbates — CO and — COOH and the need for an additional catalyst site (M — OH), this approach poses some challenges.

The second approach is to consider why — CO is so difficult to oxidize; that is, why does CO adsorb so strongly. Thermodynamically, the oxidation of CO to CO2 in solution occurs at low potential [18].

CO2 + 2H+ + 2e ^ CO + H2O (1) in E0 = -0.106F (9.14)

But, the oxidation of CO on platinum in acidic solution occurs 600 to 700 mV positive of this value; Pt-Ru alloys are shown to oxidize CO at 200 to 300 mV lower overpotential than Pt [23]. The oxidation of adsorbed CO is strongly disfavored. There are two ways to think about overcoming this large overpotential. One is to design better catalysts. One common approach has been through the bifunctional mechanism where the bimetallic catalyst is designed to place Pt — CO adjacent to an oxygen source through M — OH. The other approach would rely on a paradigm shift in how the oxidation of -CO is viewed at a more fundamental level; better understanding could lead to better catalysts [14,15,16].

The above discussion is provided in a very general manner. Many factors significantly impact the catalytic efficiency of the conversion of methanol to carbon dioxide/carbonic acid. This includes surface structure, catalyst size, and catalyst crystal face as well as the history of the cell, the current coverage of CO, the pH, and the time since the start of the cell.

As we have mentioned above, one difficulty to be overcome for the practical and extensive use of biomass-derived ethanol as a hydrogen source to fuel-cell systems is supplying the energy needed to distill and/or vaporize the H2O/ethanol mix­tures, and that related to the endothermicity of the steam reforming reaction.

Recently, Dumesic and coworkers have shown that methanol, ethylene glycol, glycerol, and sorbitol can be reformed in the aqueous phase to H2 and CO2 at temperatures near 500 K and at pressures between 15-50 bar [57-59]. On the basis of these studies, the reforming of ethanol in the aqueous phase appears as a new approach to be considered for the production of H2 from ethanol reforma­tion. This process would have several advantages over steam reforming: i) it does not need energy to vaporize alcohol and water before the reaction; ii) the operating temperatures and pressures are suitable for the water-gas shift reaction, so it may be possible to generate hydrogen with low amounts of CO in a single step; and iii) the step of H2 purification or CO2 separation is simplified because of the pressure range of the effluent.

Another possibility that merits greater study is the operation under autother­mal conditions with ethanol/water/air mixtures. Here, the goal is to maximize the hydrogen yield, while minimizing the total combustion and the formation of by-products and carbon deposits on the catalysts. For both steam reforming and oxidative steam reforming, future research is needed to develop more stable, active, selective, and inexpensive catalytic systems that operate under the required final experimental conditions.

Finally, the integration of the ethanol reformation in an energetically favored total process is also an area, which, still in our day, remains to be completed from a technological point of view.

Efforts in the above-mentioned areas could lead to the practical use of ethanol as H2 supplier to generate clean electrical power in the not-so-distant future.

Methanol is produced by the hydrogenation of carbon oxides over a suitable (copper oxide, zinc oxide, or chromium oxide-based) catalyst:

CO + 2H2 о CH3OH (2.5)

CO2 + 3H2 о CH3OH + H2O (2.6)

The first reaction is the primary methanol synthesis reaction, a small amount of CO2 in the feed (2-10%) acts as a promoter of this primary reaction and helps maintain catalyst activity. The stoichiometry of both reactions is satisfied when R in the following relation is 2.03 minimally (Katofsky 1993). H2 builds up in the recycle loop; this leads to an actual R value of the combined synthesis feed (makeup plus recycle feed) of 3 to 4 typically.

H2 — CO2 CO + CO2

The reactions are exothermic and give a net decrease in molar volume. Therefore, the equilibrium is favored by high pressure and low temperature. During production, heat is released and has to be removed to keep optimum catalyst life and reaction rate. 0.3% of the produced methanol reacts further to form side products such as dimethyl ether, formaldehyde, or higher alcohols (van Dijk et al. 1995).

The catalyst deactivates primarily because of loss of active copper due to physical blockage of the active sites by large by-product molecules; poisoning by halogens or sulfur in the synthesis gas, which irreversibly form inactive copper salts; and sintering of the copper crystallites into larger crystals, which then have a lower surface area-to-volume ratio.

Conventionally, methanol is produced in two-phase systems, the reactants and products forming the gas phase and the catalyst forming the solid phase. The
production of methanol from synthesis gas was first developed at BASF in Germany in 1922. This process used a zinc oxide/chromium oxide catalyst with poor selectivity, and required extremely vigorous conditions—pressures ranging from 300-1000 bar and temperatures of about 400°C. In the 1960s and 1970s the more active Cu/Zn/Al catalyst was developed allowing more energy-efficient and cost-effective plants, and larger scales. Processes under development at present focus on shifting the equilibrium to the product side to achieve higher conversion per pass. Examples are the gas/solid/solid trickle flow reactor, with a fine adsorbent powder flowing down a catalyst bed and picking up the produced methanol, and liquid phase methanol processes where reactants, product, and catalyst are suspended in a liquid. Fundamentally different could be the direct conversion of methane to methanol, but despite a century of research this method has not yet proved advantageous.

Corn wet mills have a long history of converting starch to a wide variety of products in addition to or instead of ethanol. As described above in the discussion of wet milling, starch products from wet milling also have many applications beyond the food and beverage industries in pharmaceutical, cosmetic, paper, and packaging industries. Recently, corn wet millers have begun to adopt processes for converting starch into biodegradable polymers. Such products represent a fundamental shift from other nonethanol products, because they directly compete with petroleum-based products and have the potential for virtually unlimited growth. A number of corn processors in the United States have a biopolymer either on the market or under development. Cargill Dow produces PLA (polylac — tide) polymer fiber under the trade name Nature Works, for use in packaging, films, and resins. ADM and Metabolix have formed an alliance to scale-up and commercialize PHA (polyhydroxyalkanoate) polymers, for marketing as renew­able alternatives to traditional petrochemical plastics used in making molded goods, films, and coated papers. A joint venture between DuPont and Tate and Lyle was formed to produce 1,3-propanediol (Bio-PDO™) from corn starch as an alternative to petrochemically-derived PDO. Sorona®, a family of polymers made from PDO, is used in fibers and fabrics, films, and resins.

The use of ethanol in an internal combustion engine was first investigated in 1897 (1). Henry Ford originally designed the Model T in 1908 to run on ethanol, but increasing taxes limited its use (2). The concept of employing ethanol as a fuel was reintroduced during the fuel shortages during both World Wars, but the U. S. federal ethanol program was not started until the oil crisis of the 1970s (2). In 1973, OPEC quadrupled the cost of purchasing crude oil (3), which started the resurgence of promoting ethanol as an alternative fuel for combustion engines. However, ethanol as an alternative fuel has not infiltrated the fuel market in the way blended ethanol/gasoline fuels have for automobiles.

Although research in the United States from the Society of Automotive Engineers showed extensive engine testing of E10 (10% ethanol/90% gasoline) in 1933, it was not until 1978 that the U. S. government established a National Alcohol Fuel Commission (4). In 1980, President Carter signed into the law the Energy Security Act containing Title 11, which is commonly called the Biomass Energy and Alcohol Fuels Act of 1980 (4). The Clean Air Act of 1970 allowed the Environmental Protection Agency (EPA) to set standards for vehicle emissions of carbon monoxide, nitrogen oxides, and ozone (4). In 1992, the EPA started requiring cities that were considered to have serious or moderate carbon monoxide pollution problems to establish oxygenated fuel programs. The oxygen content of 2.7% by wt is a required minimum for gasoline sold in these cities. This corresponds to approximately 7.5% by volume ethanol and approximately 15.0% by volume methyl tertiary butyl ether (MTBE) in gasoline.

In 1994, the EPA proposed a policy that at least 30% of the oxygenate be derived from renewable resources (4). However, this proposed policy was not passed by Congress. Ninety-five percent of the oxygenate used in Chicago is ethanol (4). Ethanol has been marketed in every state except California (MTBE has been the mandated oxygenate) (4), but currently MTBE is being phased out of California and ethanol is being phased in due to environmental issues.

In view of the recent Kyoto Conference at which the United States committed to decreasing greenhouse gas emissions by 2012 to below the 1990 level (5), ethanol/gasoline blends from E10 to E85 are an excellent way to achieve these greenhouse gas reductions. Argonne National Laboratory has shown that green­house gas emissions is 2.4 to 2.9% less for E10 than 100% gasoline overall (5). Most of this decrease is due to a decrease in greenhouse gas emissions from vehicle combustion because there is actually a small increase in greenhouse gas emission from the fuel due to volatility.

Fatih Dogan

Department of Materials Science and Engineering University of Missouri-Rolla

CONTENTS

Introduction………………………………………………………………………………………………….. 204

Fuels for Solid-Oxide Fuel Cells…………………………………………………………………. 205

Single-Chamber Solid-Oxide Fuel Cells and Hydrocarbon Fuels…………………… 209

Summary………………………………………………………………………………………………………. 211

References…………………………………………………………………………………………………….. 212

Abstract This chapter addresses utilization of alcohol and other hydrocarbon — based fuels to generate electricity in solid-oxide fuel cells (SOFCs). One of the key advantages of SOFC is that both external as well internal fuel reforming is possible to operate the fuel cell under stable conditions. While alcohol fuels can be obtained sulfur-free and in high purity, hydrocarbon fuels have higher energy density and existing infrastructure of production and distribution. Development of more energy-efficient and chemically stable electrode materials is necessary for SOFC operating at high (800-1000°C) and intermediate (500-800°C) tem­peratures. Significant progress has been made in recent years in the development of carbon monoxide-tolerant fuel electrodes (anodes) to prevent carbon deposition on the catalyst that results in a reduced performance of the fuel cell. Development of fuel electrodes compatible with alcohol and hydrocarbon fuels will lead to more efficient and widespread applications of SOFCs in double-chamber and single-chamber modes.

INTRODUCTION

Fuel cells are viewed as environmentally compatible and efficient energy conver­sion systems. A fuel cell works much like a battery with external fuel supplies. Chemical fuels are electrochemically converted into electricity at high efficiencies without producing significant amount of pollutants such as nitrogen oxides as compared to combustion engines. Hydrogen is the ideal fuel since it reacts with oxygen in the air to produce water and an electric current, but hydrogen is expensive and difficult to store. Until the hydrogen economy is well established, other fuels can be used indirectly with an external reformer or directly to operate fuel cells. Hydrogen is stored naturally in alcohols (e. g., ethanol and methanol) or hydrocarbons such as propane and methane, which are available to produce cleaner power if the electrochemical processes of hydrocarbon oxidation reactions are well understood.

Among various fuel cells, solid oxide fuel cells (SOFCs) and molten car­bonate fuel cells can be operated using hydrogen as well as carbon monoxide. Particularly, SOFC is viewed as the most flexible fuel cell system that can operate using various fuel gases directly supplied to the fuel electrodes [1-3]. Removal of CO from H2 fuel is essential for polymer electrolyte membrane fuel cells, which are generally considered to be the most viable approach for mobile applications.

The application of high and intermediate temperature SOFCs range from small-scale domestic heat and power to large-scale distributed power generation. SOFCs offer high efficiencies up to 60-70% in individual systems and up to 80% in hybrid systems by extracting the energy present in the high-temperature exhaust gases, e. g., by using gas or steam turbines [4]. High-temperature SOFC applica­tions include multimegawatt-scale centralized power generation, distributed power generation up to 1 MW and combined heat/power (CHP) plants in the 100-kW to 1-MW range. Potential areas of application for intermediate SOFCs are in the transport sector (up to 50 kW), military and aerospace (5 to 50 kW), domestic CHP (up to 10 kW) and miniaturized fuel cells “palm-power” in the 10-W range.

In SOFC, the electrolyte is typically a dense yttria-stabilized zirconia (YSZ), which is an ionic conductor blocking electron transport as shown in Figure 11.1. The electrolyte allows the transport of oxygen ions via the oxygen vacancies from the interface at the air electrode (cathode) to the interface with the fuel electrode (anode). The cathode is typically composed of a porous lanthanum strontium manganese oxide with YSZ and facilitates the reaction for the reduction of oxygen gas to oxygen ions at the electrode/electrolyte interface. The anode material is typically a porous Ni-YSZ composite allowing the oxi­dation of the fuel and transport of the electrons from the electrolyte/electrode interface to the interconnect of the fuel cell stack. The interconnect material is typically lanthanum strontium chromite for high-temperature operation while corrosion-resistant metallic alloys are employed in the development of SOFCs operating at intermediate temperatures. The role of the interconnect is to transfer

the electrons between the individual cells in the stack and to prevent mixing of fuel and oxidant gases [5].

A diverse range of fuels can be used in SOFCs since the internal temperature is high enough to initiate fuel conversion reactions. Hence, SOFCs have an efficiency advantage over polymer electrolyte membrane fuel cells when alcohol or hydrocarbon fuels are to be used, even though direct-methanol fuel cells with polymer electrolyte membranes are widely studied. The use of these fuels in SOFCs without preprocessing, however, requires further advances in development of appropriate electrode materials toward preventing unwanted reactions such as carbon formation on the anode, which significantly affects the performance of the fuel cell.