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Thin Film Manufacturing Techniques Produce Nanoscale Components for Solid Oxide Fuel Cell With Higher Power, Lower Operating Temperatures and No Carbon Deposition


A solid oxide fuel cell for direct electrochemical oxidation of hydrocarbons that generates greater power densities at lower temperatures without carbon deposition earned U.S. Patent 7,709,124, for Northwestern University (Evanston, IL). Nanostructured fuel cell components were created by Professor Scott A. Barnett (Robert  R. McCormick School of Engineering and Applied Science) with researchers Tammy Lai and Jiang Liu.  The components are manufactured using thin film techniques similar to those used to produce micro and nanoscale devices for the semiconductor industry.
The novel low-temperature solid oxide fuel cells demonstrate the feasibility of direct hydrocarbon electrochemical oxidation. For instance, power densities up to 0.37 W/cm2 were measured for single cells that were operated at 650 degrees C  with atmospheric-pressure air as the oxidant and pure methane as the fuel. The measured power densities are competitive with fuel cells operated on hydrogen.  The fuel cell eliminates the need and cost for a reformer to separate hydrogen from hydrocarbon fuels prior to the electrochemical conversion process. 
Solid oxide fuel cells can generate temperatures in excess of 1200 degrees C which limit the materials that can be used in their production. High temperatures create problems associated with coefficient of thermal expansion mismatches which can lead to failure of fuel cell components. Lower temperature solid oxide fuel cells is a goal of many company and public laboratories which will allow the use of more common materials in their production.  
The cathode comprises lanthanum strontium manganite (LSM), the anode comprises nickel- yttria stabilized zirconia (Ni—YSZ), the electrolyte comprises yttria stabilized zirconia (YSZ). and the substrate support comprises a partially stabilized zirconia (PSZ).  The operating temperature is between about 600 degree  C. and about 800 degree  C.
The Northwestern fuel cell increases the rate of hydrocarbon oxidation so as to increase and provide useful power densities. Such densities can be increased by utilizing various catalytic metals in the fabrication of fuel cell anodes, such anodes as can be used in conjunction with a ceria material.

The Northwestern invention provides various anodes and related cellular components having nanoparticle size obtainable by sputter deposition processes and related fabrication techniques.  The process uses nano-scale YSZ for the electrolyte to reduce sintering temperatures. It  can produce several micron thick YSZ films in single step. Multiple deposition steps were used to make thicker layers.

Another aspect of this process is co-sintering. Fuel cells require a thin dense YSZ layer with porous cathodes, anodes, and support. Difficulties can include matching shrinkages (to avoid cracking of the layers and/or sample curvature) and maintaining electrode porosity at the high temperatures required for YSZ sintering.
Fabrication procedures can be viewed as analogous to various thin-film/layer techniques used in the fabrication of micro- and nano-dimension integrated devices. The Northwestern invention introduces a novel stacking geometry devised to enhance the benefits available from this technology. The geometry involved has all active SOFC components and the interconnect deposited as thin layers on an electrically insulating support. This configuration allows the choice of a support material that provides optimal mechanical toughness and thermal shock resistance. The supports can be in the form of flattened tubes, providing relatively high strength, high packing densities, and minimizing the number of seals required.
The integration of SOFCs and interconnects on the same support provides several other advantages including the reduction of electrical resistances associated with pressure contacts between the cells and interconnects, relaxation of fabrication tolerances required for pressure contacts, reduction of ohmic losses, and reduction of interconnect conductivity requirements. The materials used in the integrated stacks can be similar to or the same used with conventional SOFCs, and long-term stable operation will be achievable. Use of thin layer cell-active components helps to lower overall material costs.
FIG. 26 shows fracture cross-sectional SEM micrographs from a typical four-cell array, along with a portion of the support, taken after cell testing. The low magnification image shows that the PSZ support had porosity striations, the result of the starch pore-former added during processing.

Fuel cells are promising electrical power generation technologies, with key advantages including high efficiency and low pollution. Most potential near-term applications of fuel cells require the use of hydrocarbon fuels such as methane, for which a supply infrastructure is currently available. However, fuel cells typically operate only with hydrogen as the fuel. Thus, current demonstration power plants and planned fuel-cell electric vehicles must include a hydrocarbon fuel reformer to convert the hydrocarbon fuel to hydrogen. Fuel cells that could operate directly on hydrocarbon fuels would eliminate the need for a fuel reformer, providing considerable system and economic advantages and presumably improving the viability of the technology.