Solid Acid Reforming Methanol Fuel Cells and Hydrogen Generators
Solid acid fuel cells (SAFCs) operate at intermediate temperatures (~250°C), are inherently impermeable to gases, and transport "bare" protons through a solid electrolyte. Solid acids are compounds whose chemistry and properties are intermediate between those of a normal acid, such as H2SO4 or H3PO4, and a normal salt, such as Cs2SO4. The proton conductivity of these materials can reach values higher than 10-2 Ω-1cm-1 when heated to moderately elevated temperatures (150-250°C). In the case of the standard SAFC electrolyte, cesium dihydrogen phosphate (CsH2PO4), the conductivity is 2.5 x10-2 Ω-1cm-1 at 250 °C. To date, solid acid fuel cells (SAFCs) utilizing this electrolyte as thin (10-25 μm) gas tight electrolyte layers have demonstrated peak power densities of over 330 mW/cm2on hydrogen/air with lifetimes of thousands of hours. Recent development efforts at SAFCell have demonstrated SAFC stacks with robustness to thermal cycling, power outputs of over 1 kW, and lifetimes in the thousands of hours.
SAFC stacks have also demonstrated very high tolerances to typical anode catalyst poisons such as carbon monoxide (CO), ammonia (NH3), and hydrogen sulfide (H2S): measured tolerances are 20%, 100 ppm, and 200 ppm, respectively, without significant performance decreases. These high impurity tolerances and the intermediate operating temperatures allow SAFC power systems to operate on reformed fuels with simplified, and therefore, less costly reforming and gas clean-up sub-systems.
Internally Reforming Methanol Solid Acid Fuel Cells:
The application of SAFCs as reforming methanol fuel cells (RMFCs) is particularly compelling due to the nearly ideal operational temperature match between the fuel cell stack (235-275ºC) and standard methanol steam reforming catalysts (250-300ºC). The operational temperature match allows for methanol steam reforming inside the stack to create a solid acid-based RMFC with higher performance and fuel efficiencies than current polymer-based RMFCs and direct methanol fuel cells (DMFCs). In fact, SAFCell has demonstrated single cell performance similar to diluted hydrogen on vaporized methanol/water fuel streams using a solid acid RMFC (255 mW/cm2 versus 237 mW/cm2, respectively). Further results at the stack level show the effectiveness of the solid acid RMFC configuration to run directly on methanol for at least thousands of hours. This system design is so simple that a solid acid RMFC unit is projected to cost nearly half of an equivalent DMFC system, exceeding most of the DOE’s 2015 target values for such a system.
Talk will describe the development of a solid acid RMFC stacks and systems for O&NG remote power applications. Stack and system is designed for extended use without maintenance and under harsh environmental conditions expected for remote power applications (e.g, temperatures from -40°C to 45°C), and to run on “field methanol” which can be easily delivered to remote locations in large quantities.
Internally Reforming Methanol Solid Acid Electrochemical Hydrogen Generators:
The high impurity tolerances of SAFC stacks also enable them be used as electrochemical hydrogen separation (EHS) systems with superior properties to those currently in existence. Preliminary testing has been done in EHS mode to show the tolerance of SAFC stacks to carbon monoxide (CO) in EHS mode, and cyclic voltammetry experiments show that CO can be directly oxidized on SAFC anodes. Moreover, EHS testing on synthetic reformate with a CO water gas shift catalyst incorporated into the anode suggests significant chemical conversion of CO to H2 can be achieved. As fuel cell results described above show little affect on performance of H2S, NH3, and CH4, SAFC stacks should be equally tolerant to these impurities in EHS mode. As such, a solid acid EHS unit should separate almost all hydrogen from “dirty” hydrogen containing gas streams without severe performance penalties.
The key difference of a solid acid EHS system is the higher operating temperature (~250°C / 482°F) versus the 90°C to 160°C of alternate EHS technologies. As described above, this higher temperature makes the system highly tolerant impurities and contaminants, and provides better integration with cooling and shifting of the supply gas, increasing hydrogen production. It also provides waste heat of a higher value than lower temperature technologies.
Talk will describe the development of solid acid EHS stacks using internally reformed methanol, and other hydrocarbon reformates (e.g., natural gas and propane).