Until the hydrogen economy is well established, it is more sensible to generate electricity directly from alcohols or hydrocarbons. SOFCs may become very attractive for portable, transportation, and stationary applications if alcohols and hydrocarbons can be utilized directly without applying any fuel pretreatments. The main advantage of liquid hydrocarbons is their relatively higher energy density compared to alcohols. However, most hydrocarbon fuels such as natural gas, bioderived gases, diesel, and gasoline contain impurities such as hydrogen sulfide and halogens, which may lead to poisoning of the SOFC electrode mate­rials. Particularly, sulfur content in such fuels should be reduced through pre­treatments to prevent the fuel cell electrodes from poisoning. Alternatively, progress is being made toward development of sulfur-resistant electrode materials for long-lasting operation of SOFCs using hydrocarbon fuels, which generally contain sulfide compounds in relatively high concentrations. For instance, a highly sulfur-tolerant anode composed of Cu, CeO2, and YSZ was developed to operate a SOFC using hydrogen with H2S levels up to 450 ppm at 1073 K [6]. Another study based on LaxSr1-xVO3- as anode material for SOFC showed a maximum power density of 135 mW/cm2 at 280 mA/cm2 when the fuel was a 5% H2S-95% H2 mixture at 1273K [7].

The advantage of liquid oxygenated hydrocarbons, such as alcohols, in com­parison to gasoline is that they are cleaner (low sulfur content) and can be derived from agricultural by-products and biomass as a renewable energy source. Alcohol is an ideal fuel for the fuel cells because of ease of transportation, storage, and handling, as well as their high energy density. Partially oxidized (hydrated) fuels may be easily reformed, such as alcohols, as they contain oxygen, in a liquid form. Since water is often used for internal reforming of the fuel, water solubility of alcohols (especially methanol, ethanol, and propanol) offers the advantage that additional fuel processing may not be necessary for operation of the fuel cell.

An anode-supported SOFC utilizing direct alcohol was reported by Jiang and Virkar [8]. A thin-film YSZ electrolyte was deposited on a Ni-YSZ anode with a composite of Sr-doped LaMnO3 and YSZ as a cathode. Pure methanol and an equivolume mixture of ethanol and water were used as fuels to operate the cells over a range of temperatures. Power densities achieved with ethanol and water mixtures were between 0.3 W/cm2 at 650°C and 0.8 W/cm2 at 800°C, and with methanol between 0.6 W/cm2 at 650°C and 1.3 W/cm2 at 800°C as shown in Figures 11.2 and 11.3. Carbon deposition on the electrodes was not observed when methanol was used as fuel. On the other hand, maximum power density using humidified H2 was 1.7 W/cm2 at 800°C. This indicates that a lack of H2 in the fuel may substantially increase concentration polarization thus limiting the performance of the cell.

Another study on direct-alcohol SOFCs reported a comparison of methanol, ethanol, propanol, and butanol as fuel sources [9]. With an increasing carbon number of the alcohol, a decrease in cell voltage was observed, which was attributed to slower decomposition and/or reforming kinetics of alcohols. Decreasing operational temperatures led to an increase of unreacted alcohols, aldehydes, and aromatic compounds. Thermochemical calculations were used to reveal the equilibrium amounts of reaction products of fuels during fuel cell operation [10,11]. Figure 11.4 shows the limit lines of carbon deposition as a function of temperature in the C-H-O diagram. No carbon deposition is expected if the carbon-to-oxygen ratio is less than unity. It is shown that the addition of

C

Further development of electrode materials that do not require introduction of water, will lead to better performance of the SOFCs, provided carbon deposi­tion can be suppressed. Recent studies showed that coking issues can be resolved through selection of appropriate catalysts and anode materials in fuel cell devel­opment [4,5]. Because nickel is an excellent catalyst for hydrocarbon cracking, Ni/ZrO2 cermets are used as anode materials for YSZ-based SOFCs. As mentioned earlier, these cermets can only be used in hydrocarbon or alcohol fuels if excess water is present to ensure complete fuel reforming. Mixing /Vo-octane with water, alcohol, and surfactant to produce an oil in water microemulsion was successful in reducing the carbon formation significantly, while retaining a high octane number [12]. It has been shown that the problem of carbon deposition may be avoided by using a copper-ceria anode [13] or applying an yttria-ceria interface between YSZ and Ni-YSZ cermet anode [3]. A nickel-free SOFC anode, La0.75Sr0 25Cr0.5Mn0 5O3 with comparable electrochemical performance to Ni/YSZ cermets was developed for methane oxidation without using excess steam [5]. A recent study showed that a Ru-CeO2 catalyst layer with a conventional anode allows internal reforming of /Vo-octane without cocking and yields stable power densities of 0.3 to 0.6 W/cm2 in a SOFC design operating at intermediate tem­peratures [14].