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production efficiencies, high-temperature thermo-chemical [9] or electrolytic processes [10] can be used. The required high temperature process heat can be based on concentrated solar energy [11] or on nuclear energy from advanced high- temperature reactors [12] From 2003 – 2009, development and demonstration of advanced nuclear hydrogen technologies were supported by the US Department of Energy under the Nuclear Hydrogen Initiative (NHI) [13]. High temperature steam electrolysis (HTE) was demonstrated as a feasible technology under this program, which included a 15 kW HTE technology demonstration, achieving a hydrogen production rate in excess of 5000 NL/hr [14] During 2009, the NHI program sponsored a technology down-selection activity by which an independent review team recommended HTE as the most appropriate advanced nuclear hydrogen production technology for near-term deployment [15]. HTE research is currently supported by the DOE Office of Nuclear Energy under the Next Generation Nuclear Plant (NGNP) Program. The ultimate cost of hydrogen production by any technology is dependent on both capital and operating costs. In order to achieve competitive capital costs, HTE cells and stacks must exhibit both high performance and low degradation rates. Although HTE has been successfully demonstrated, our experience to date has indicated that SOEC cell and stack degradation must be improved prior to deployment of HTE as a viable cost-competitive technology. Consequently, the current focus of our research is to identify the mechanisms responsible for accelerated degradation in the electrolysis mode. Once these mechanisms are fully understood and ranked in terms of importance, effective mitigation strategies can be developed. Possible degradation mechanisms include transport of impurities leading to electrode poisoning and deactivation [16], coarsening of electrodes [17], loss of electrolyte ionic conductivity [18], depletion of oxygen vacancies in mixed conducting electrodes [19, 20], and electrode delamination [21]. Under contract to NASA, Boeing Phantom Works conducted a system study to compare flight performance capabilities of a solid oxide fuel cell (SOFC), a proton exchange membrane fuel cell (PEM), and an internal combustion engine (ICE) system for a 14-day HALE (High Altitude Long Endurance) aircraft [22]. The study results indicated SOFC provided significantly longer flight endurance due to its higher efficiency, but that SOFC also required the greatest technology development. The study identified unique technical challenges for the HALE application showing especial importance to reduced stack weight (1 kW/kg target). Commercial fuel cells for land-based applications were ~0.2 kW/kg at the time of this study (2005). To meet NASA’s demands for high power to weight SOFC, the ceramics branch at NASA Glenn Research Center is working to prototype an alternative light weight, low volume planar SOFC for aerospace applications. Based on their SOFC technology, NASA has been developing single cells and stacks that are suited to operation in the electrolysis mode of operation for high temperature steam electrolysis. The focus of the NASA research is to develop material sets that lead to cells that exhibit low degradation rates and good efficiencies (low Area Specific Resistances). Under the U.S. Department of Energy (DOE) Next Generation Nuclear Plant (NGNP) program, Idaho National Laboratory (INL) provided funding for the development of the first generation 3-cell stacks and component 5 cm by 5 cm single cells for the evaluation of their hydrogen production efficiencies and degradation rates. The NASA cells are fabricated by casting and laminating green ceramic tapes. The electrolyte is made by doctor blade casting a solvent-based 8 molar percent yttria stabilized zirconia (8YSZ) ceramic slip to produce electrolyte tapes. The electrode skeletons or scaffolds are aqueous-based and produced by doctor blade casting on a chilled bed (<-20 °C). Micron-size ice crystals from on the Mylar tape that is in contact with the chilled bed and coarsen as they grow outward. Conditions are controlled to maximize in-plane texture. The frozen ceramic tape is placed in a freeze-dryer and the water is sublimated which results in a gradient porous structure of interconnected micro-channels. Macro-channels are subsequently cut into the green tapes using a laser in order to create gas flow channels. To produce cells, tapes are cut to size and laminated. Two freeze-case electrode skeletons are laminated with an electrolyte tape, placing the small pore sides against the electrolyte tape (see Figure 1). The laminated structures are sintered at a temperature of approximately 1450°C. After sintering, the laminated structures are infiltrated with active electrode materials that are typically composed of metal salts. For the cells tested in this paper, the anodes were infiltrated with nickel nitrate and the cathodes were infiltrated with a combination of nitrates to produce a cobalt-doped lanthanum strontium ferrite (LSCF). A final infiltration of the anodes and cathodes with samarium-doped ceria was also performed for stability enhancement. Figure 1: Layup for lamination of the freeze cast electrode skeleton and tape cast electrolyte materials prior to sintering of the SOEC cell structure. During testing of single cells in electrolysis and in regenerative (reversible) modes of operation at NASA Glenn, the single cells of this design exhibited verify high electrochemical voltage efficiencies (EVE) and high H2O conversion percentages. An overview of preliminary single cell testing activities and results obtained at INL is presented within this paper. 2PDF Image | Electrolysis Cells Operated Fuel Cell Steam Electrolysis
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