Analyzing Ceramics in Fuel Cells
At first, fuel cells appear to be simple devices, containing no moving parts and only four functional components: electrolyte, cathode, anode and interconnect. They are vibration free; therefore, the noise pollution associated with power generation is also eliminated. Fuel cells are also are clean, reliable and almost completely nonpolluting.
Much development has focused on solid oxide fuel cells (SOFCs) because they can convert a variety of fuels into electricity at higher efficiency (40-60% alone; up to 70% in a pressurized hybrid system) than engines and modern thermal power plants (30-40%).
Multiple RequirementsFor SOFCs to be commercially viable, cost-effective materials and processing must be developed. The first successful SOFC used platinum as both the cathode and anode, but less expensive alternatives are available today.
Temperature and atmosphere requirements drive material selection for all the components. Materials must limit reactivity and interdiffusion between the components. The thermal expansion coefficients of the components must match as closely as possible to minimize thermal stresses that could cause cracking and mechanical failure. The air side of the cell must operate in an oxidizing atmosphere, and the fuel side must operate in a reducing atmosphere. Finally, to produce sufficiently high current densities and power output, fuel cells must run at extremely high temperatures.
The cathode must meet all the requirements noted above while also being porous to allow oxygen molecules to reach the electrode/electrolyte interface. In some SOFCs, the cathode accounts for over 90% of cell weight and provides structural support for the cell. The most commonly used cathode material is lanthanum manganite, which is typically doped with rare earth elements to improve conductivity. Most often, it is doped with strontium and called lanthanum strontium manganite (LSM).
The electrolyte allows oxygen ions to migrate to the fuel side of the cell. This material must have high ionic conductivity and no electrical conductivity. It also must be dense enough to prevent gases from short-circuiting through it. Electrolyte materials include yttria-stabilized zirconia (YSZ), doped cerium oxide and doped bismuth oxide. Of these, YSZ has emerged as the most suitable because it allows operation at up to 1000°C, stabilizes the zirconia in the structure at high temperatures and provides oxygen vacancies at one vacancy per mole of dopant.
Cerium oxide can operate at lower temperatures (under 700°C), allowing the use of less expensive materials for the other components. However, cerium oxide is susceptible to reduction on the fuel side.
The anode must meet the same requirements for electrical conductivity, thermal expansion and porosity as the cathode. It also must survive in a reducing atmosphere. These requirements make metals such as nickel the most widely used materials. However, nickel’s thermal expansion is too high to pair it with YSZ, and it tends to lose its porosity at operating temperatures. These problems have been solved by making the anode from a nickel-YSZ composite.
Finally, the interconnect must survive the high operating temperature of the cells and the severe environments. It also must meet the same requirements as the other components: 100% electrical conductivity, no porosity, compatible thermal expansion, and inertness with respect to the other fuel cell components. For YSZ SOFCs operating at about 1000°C, the preferred material is lanthanum chromium oxide (LaCrO3) doped with a rare earth element to improve conductivity.
Analysis ChallengesA SOFC must sustain on/off thermal cycling without cracking, which would degrade its functional performance. Therefore, high fracture toughness is essential to prevent premature failure and sustain structural integrity during operation. This high fracture toughness, however, also makes preparing samples for analysis difficult, and special proprietary techniques must be used to ensure accurate analysis.
In addition, three other factors can affect the accuracy of the analysis. Each factor affects the analysis in different ways, and, unless samples are prepared carefully, the analysis can produce inaccurate readings of part composition and the impurities present. These factors include the types of ceramics in the part, the individual elements and the concentration ranges.
The first two factors can cause inter-element interferences that can lead to false readings of the actual element concentrations. These interferences can be compensated for by applying well-known correction factors to the measurements.
The third factor controls the type of analysis that can be used. A number of analytical tests can be used to verify the composition and purity of ceramic materials. Each has benefits and limitations that must be considered before selecting the appropriate method.
For bulk analysis to determine the concentration of the major chemical constituents of the ceramic material, X-ray fluorescence (XRF) and inductively coupled plasma optical emission spectroscopy (ICPOES) are used. To determine the presence of elements in trace amounts, particularly impurities, DC arc and inductively coupled plasma/mass spectrometry (ICP/MS) are used.
XRF determines element concentration by analyzing the emission of secondary (or fluorescent) X-rays from a material that has been excited by high-energy X-rays or gamma rays. The radiation can be analyzed either by sorting the photon energies emitted (energy-dispersive analysis) or by separating the radiation wavelengths (wavelength-dispersive analysis). Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material.
Samples for XRF are made into pressed powders or fused glass beads. Pressed powders consist of the sample mixed with a small amount of binder and pressed into a pellet. If the sample has uniform particle size, elements can be detected at the parts per million (ppm) level.
One challenge of using pressed powders is acquiring matching material with known concentrations to calculate the final concentration. These may not be available for the element of interest and the ceramic material matrix. Fused beads overcome this problem because measurement standards can be created synthetically for comparison.
Beads are created by mixing the sample with a flux and heating to form a glass bead. The main drawback of fusion beads is that the sample material is diluted by the flux, limiting detection accuracy to about 0.1-0.5%, depending on the fusion technique.
ICP instrumentation introduces an aqueous solution into an extremely hot plasma gas. The intensity of the light emitted is compared to previously measured standards of known element concentration, and a concentration is computed.
One advantage of ICP analysis is the ability to match the sample solution to a synthetically created standard once the sample has been dissolved by various digestion techniques. Also, concentrations from trace to major levels can be quantified using the same sample preparation technique.
Using DC arc analysis to determine trace elements involves vaporizing a 20- to 50-mg solid sample in a high-intensity DC arc that creates a 5000°C corona. As in ICP analysis, the intensity of the light emitted is compared to standards of known concentration, and a concentration is computed. Measurement accuracy ranges from parts per billion (ppb) to 0.2%.
The main advantage of DC arc analysis is that it does not dilute the sample, which minimizes the potential for contamination (provided no additional preparation is required). However, because the sample size is so small, material homogeneity is critical for accurate results. In addition, fine particle size is required to ensure accuracy, so the material may have to be crushed and blended prior to preparing a sample. This process can potentially introduce contaminants.
ICP/MS is a highly sensitive type of mass spectrometry that can determine element concentrations below one part per trillion. Coupling mass spectrometry with ICP allows for low-level detection in ceramics, from parts per billion to 0.1%. Unlike ICPOES, the ICP/MS instrument detects and quantifies the specific atomic mass of an element vs. the measuring of photons at specific wavelengths. As in ICP analysis, light intensity is measured to determine the presence or absence of specific elements.
Advantages of ICP/MS include the ability to identify and quantify a large group of elements, and measurement standards are readily available. The drawback to ICP/MS is that the ratio of sample to diluent can cause analysis errors and uncertainty. Also, because the sample must undergo digestion to break down the material, contamination is a potential problem.
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