Varieties of SOFCs

In direct-flame SOFCs, part of the fuel gas is combusted in an ordinary burner with an open flame. When the combustible fuel-oxygen mixture is fuel-rich, then a partial fuel reforming wiU occur in the flame yielding hydrogen and carbon monoxide CO. The SOFC anode is set up in the immediate vicinity of the flame, within a few millimeters. There these reforming products are oxidized elec-trochemically. The surrounding air has immediate access to the cathode surface, located on the opposite side of the cell. The fuel cell itself is heated to its working temperature by the flame. 

Defects of this fuel cell variant are the relatively low fuel utilization efficiency and the high heat losses, just as in the case described above. Also, considerable mechanical stresses may arise in the fuel cell itself because of its considerable temperature gradients and because of the rapid changes in temperature. 

Fuel cell performance was studied by Kronemayer et al. (2007) as a function of a number of factors the fuel/air ratio in the gas mixture fed to the burner, the distance of the anode from the tip of the flame, and the temperature of the fuel cell itself. Methane, propane, and butane were used as fuels in these experiments. Values of the specific energy density of up to 120 mW/cm were attained under optimum conditions. 

In recent years, ammonia NH3 was suggested as a fuel for solid-oxide fuel cells by a number of workers (Maffei et al., 2005 Fournier et al., 2006). Ammonia is a large-scale product of the chemical industry. At temperatures above 500°C, ammonia is readily decomposed to nitrogen and hydrogen at nickel catalysts. 

Therefore, when ammonia is introduced into such a cell, it is completely converted to nitrogen and hydrogen at the nickel-containing anode, the hydrogen then undergoing electrochemical oxidation. This direct ammonia fuel cell is actually a direct internal ammonia-reforming fuel ceU. 

---

The combination of various SOFC component performance, microstructural, and property requirements has led to a variety of structures, such as the composite, graded, and multilayered electrodes and electrolytes described above. The need... 

Arthur D. Little has carried out cost structure studies for a variety of fuel cell technologies for a wide range of applications, including SOFC tubular, planar and PEM technologies. Because phenomena at many levels of abstraction have a significant impact on performance and cost, they have developed a multi-level system performance and cost modeling approach (see Figure 1-15). At the most elementary level, it includes fundamental chemical reachon/reactor models for the fuel processor and fuel cell as one-dimensional systems. 

High temperature solid oxide fuel cells (SOFCs) have become of great interest as a potentially economical, clean and efficient means of producing electricity in a variety of commercial and industrial applications (Singhal, 1991). A SOFC is based upon the ability of oxide ions to be conducted through a solid at elevated temperatures. Oxide ion conductivity was observed in Zr02 9 mol% YjOj by Nernst as early as 1899. In 1937, Bauer... 

Key material properties for SOFC, such as the ionic conductivity as a function of temperature, are available in refs 36—39. In addition, Todd and Young ° compiled extensive data and presented estimation methods for the calculation of diffusion coefficients, thermal conductivities, and viscosities for both pure components and mixtures of a wide variety of gases commonly encountered in SOFCs. Another excellent source of transport properties for gases and mixtures involved in a SOFC is the CHEMKIN thermodynamic database. ... 

The performance of SOFCs with Cu—ceria—YSZ anodes has been tested with a wide variety of hydrocarbon fuels, and this has been documented elsewhere.With the exception of methane, which is known to be relatively unreactive in normal heterogeneous reactions as well, all of the hydrocarbons we examined appear to give similar performance characteristics. The fuels that were tested include /2-butane, /2-decane, toluene, and a synthetic diesel. The main difference observed between the various fuels is that some fuels tend to form tars more readily via gas-phase free-radical chemistry. Otherwise, with the exception of CH4, all hydrocarbons that were investigated showed similar power densities. This is shown in Figure 20, which displays the voltage and current densities for /2-decane, toluene, and synthetic diesel as a function of time. In this case, the hydrocarbon fuels were diluted in dry N2 to a concentration of 40 wt % hydrocarbon to prevent condensation of unreacted fuels that leave the cell. (In our studies, the active area for the fuel cell is typically 0.5 cm, and a current density of 1 A/cm would require a flow... 

Solid oxide fuel cellsoperateatvery high temperatures, around 1,000°C. High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulphur-resistant fuel cell type they can tolerate several orders of magnitude more sulphur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal. 

Unlike molten carbonates, solid oxides use a hard ceramic electrolyte instead of a liquid. That means the fuel cell can be cast into a variety of useful shapes, such as tubes. With higher temperatures, sofcs may be able to cogenerate steam at temperatures as high as i,ooo°f. The Siemens Westinghouse Power Corporation has built the first advanced hybrid system, which combines a gas turbine with a tubular sofc. As of 2003, the 220 kW hybrid system has operated in California for more than 2,000 hours with a respectable 53 percent efficiency, comparable to current combined cycle gas turbines. The ultimate goal is an efficiency of 70 percent or more. 

Figure 2.56 shows a variety of stacked cell designs employed by SOFCs. Since an individual fuel cell produces a low voltage (typically < 1V), a number of cells are connected in series forming a fuel cell stack. An interconnect comprising a high-density material is used between the repeating anode-electrolyte-cathode units of... 

Solid oxide fuel cell (SOFC) uses solid ceramic material, such as Y2O3 stabilized Zr02 (YSZ), as an electrolyte. As SOFC operates at high temperature (600-1000° C), a variety of fuels, e.g., hydrogen, methane, and carbon monoxide, can directly be utilized. The high temperature places severe constraints on material selection and results in difficult fabrication process. Co-ZrO (or Ni-ZrO) and SrO doped LaMn03 have often been used for anode and cathode materials, respectively. 

One remarkable aspect is that, frequently, a variety of products result from electrochemical reactions in the fuel cell, so that selectivity for a given product is another factor to be considered in cell design. For instance, SOFC cells using bismuth... 

PEMFC) high-temperature models include molten carbonate fuel cell (MCFC) and SOFC. The wide range of power outputs available make fuel cells suitable for a variety of applications. 

SOFC Oxygen-ion conductor 700-1000 Sr-doped LaMnOs Ni- or Co-doped YSZ cermet Impure H2, variety of hydrocarbon fuels 45-55 1-5... 

---