Nir Ben Oved

Commonly, hydrogen is used as a fuel in fuel cells. However, it has several disadvantages. The hydrogen produced in the world today is produced mainly by reforming hydrocarbons, a process which requires a large input of energy from another source to extract the hydrogen. In this case, the hydrogen is not a carbon free fuel, since carbon dioxide is produced in its production. In principle, hydrogen can be produced in a clean way by electrolysis of water,utilizing sustainable power generation technologies such as wind power or solar energy.However, currently less than 2% of hydrogen is produced in this manner. The process also requires large amounts of electrical power, which may be more efficiently used directly as electricity rather than by conversion to hydrogen and then back to electricity in a fuel cell. Moreover, the current situation is that hydrogen is not readily available as a fuel, since there is no widespread infrastructure to produce, compress, store, and supply hydrogen to large numbers of end users. This limits the use of hydrogen as a fuel for large scale power generation.

Fueling fuel cells with existing carbon-containing fuels, such as natural gas, may overcome the problem of the lack of infrastructure for hydrogen, as well as the large amounts of energy required to produce it from those fuels by reforming reactions. Since fuel cells, especially SOFCs, have very high efficiency and produce no air pollution emissions, they can be more suitable for electricity production than conventional power generation technologies. Due to their high efficiency, they can contribute to reducing green house gas (GHG) emissions even when utilizing hydrocarbon fuels.

My research involves producing SOFCs that are able to directly oxidize natural gas or other carbon containing fuels. Commonly, SOFCs utilize carbon-containing fuels by internal reforming. In this method, the carbon containing fuel, such as methane, is mixed with steam to produce carbon monoxide and hydrogen. Both carbon monoxide and hydrogen are then utilized by the fuel cell to produce electricity. Using internal reforming has the advantage of facilitating thermal management of the cell, since this reaction is endothermic for methane (it absorbs heat). However, internal reforming creates thermal gradients because it occurs in an unevenly distributed manner in the anode of the cell. This may lead to degradation of the cell or to cell failure due to thermal stresses. In addition, the extent of conversion of fuel by internal reforming is strongly dependent on the temperature, with lower equilibrium conversions possible at low and intermediate operating temperature ranges (500°C-750°C) than at higher temperatures.Operating SOFCs in the lower temperature range is desirable because it allows the use of cheaper materials, specifically metals for the interconnect. The reforming reaction is also less efficient with fuels other than methane. One potential future application of SOFCs is as auxiliary power units to generate electricity in vehicles, for example, for trucks. In this case, it is desirable that the SOFC be able to utilize a larger variety of carbon containing fuels, for example, low-sulfur diesel. This encourages attempts to develop SOFCs that directly oxidize the fuel without the need for reforming, known as direct oxidation.

Commonly, SOFC anodes utilize nickel as the catalyst for the reforming reaction. Nickel, however, is also an excellent catalyst for carbon formation, and attempts to directly utilize carbon containing fuel on a nickel based anode without steam have resulted in carbon deposition on the anode and fast degradation in cell performance until the anode becomes inactive. In order to allow direct oxidation of hydrocarbon fuels in SOFCs, materials other than nickel may be required in the anode. Several studies have suggested copper as a replacement for the nickel as the electronic conductor in the anode. Copper is catalytically inert for carbon formation, and therefore it is suitable as an electronic conductor in the anode. However, since copper is catalytically inert to hydrogen and CO oxidation, a catalytic material should be added to the anode. SDC (samaria doped ceria, Ce0.8Sm0.2O1.9) has been utilized instead of YSZ for direct oxidation anodes. SDC has some catalytic activity for oxidation of carbon based fuels, but it does not catalyze carbon formation on the anode surface. Recent studies also suggest using cobalt or cobalt-copper alloys as a catalyst for direct oxidation reactions. Adding cobalt in small quantities to the anode increases the catalytic activity of the anode, and therefore, the cell performance, without causing carbon deposition, as nickel does in similar quantities.

Currently, SOFCS are produced with multi-step wet ceramic processing techniques, for example tape casting, screen printing, and sintering. The complex multi-step processing procedures are time consuming and require large capital costs. In addition, they make the introduction of controlled gradients in the electrode microstructure and material composition and the use of metallic structural supports for the cells difficult. Improved control over the microstructure and material composition across the electrode may lead to better performance and reduced thermal stresses, thereby increasing the reliability and durability of the cell, while use of metal supports can lower the materials costs and improve durability.

Plasma spray processing has also been studied as a processing technique for the manufacturing of SOFCs. Plasma spraying has the advantage of short processing time, material composition flexibility, and a wide range of controllable spraying parameters. Controlling the spraying and feedstock parameters during spraying allows control of the coating characteristics during the spraying process, which may lead to better electrical performance and higher cell reliability. It also allows manufacturing of an entire cell in one process. The process can also be scaled up easily for rapid, automated mass-production, as well as the use of metallic structural supports for the cells, which may allow the reduction of manufacturing and material costs.

In my research I produce SOFCs for direct oxidation of carbon containing fuels by utilizing plasma spray processing. Our challenge is to control a large number of spraying and feedstock parameters during the plasma spray process to produce anodes with acceptable performance for direct oxidation of carbon containing fuels. Because of the large melting temperature difference between the CuO and the SDC used as spraying feedstock, the primary challenge is to obtain well-mixed, porous microstructures with high surface areas that lead to good anode performance.