This study describes and presents the results of a new electrochemical approach to co-production of hydrogen and electric power using a steam-carbon fuel cell, within which carbon-containing species are kept physically separate from the hydrogen stream by a solid oxide electrolyte membrane. The fuel cell used for this purpose consists of H 2 , H 2 O (g) /Pt/YSZ/Pt/C (s) ,CO,CO 2 and measurements are taken between 600 and 900 C. Peak electrical power generated at 900 C is 8 mW/cm 2 at a current density of 40.5 mA/ cm 2 corresponding to simultaneous production of carbon-free hydrogen at a rate of 354 g H 2 /m 2 day. Electrochemical behavior and cell loss mechanisms are studied using impedance spectroscopy in different cell arrangements operating in steam-carbon and air-carbon modes. Exchange current densities extracted from these measurements indicated activation energies of 80.3 6 7.9 kJ/ mol for oxygen reduction, 132 6 12 kJ/mol for CO oxidation, and 189 6 35 kJ/mol for steam reduction. These results indicate that steam reduction is the dominant loss mechanism with significant contribution from CO oxidation kinetics. Modeling results for the carbon bed indicate that a bed height of 7 mm is capable of supporting cell current densities of 700 mA/cm 2 at 85% effective char utilization, allowing for high performing steam-carbon fuel cells for the simultaneous production of hydrogen and electrical work. Hydrogen is a desirable fuel because it is both an effective energy carrier as well as a clean burning fuel with minimal impact on the environment. Unfortunately, hydrogen does not occur naturally, and must be generated from other sources. If widespread use of hydrogen is to be achieved, an efficient method for distributed hydrogen production is desired due to the cost and technical challenges posed by hydrogen transportation and storage.Currently, the majority of hydrogen production is performed centrally using steam reforming of methane, and to a lesser extent, coal. The fuel is reformed with steam and then undergoes the water-gas shift reaction to produce hydrogen. Unfortunately, this process produces a hydrogen stream that is contaminated by various carbonaceous species, and therefore requires expensive separation techniques to clean the gas. Carbon monoxide remnants in the hydrogen gas are of particular concern, as even trace amounts of CO in the hydrogen stream render the hydrogen product undesirable for catalytic processes due to CO poisoning. This is a critical problem especially for low temperature fuel cells. In addition, the reforming process is only economically feasible at large scales, requiring expensive storage and distribution of the hydrogen product to its point of use.A variety of alternative hydrogen production schemes that attempt to solve or avoid the limitations of reforming have been proposed. Thermal decomposition of steam into hydrogen and oxygen is one such option, but this reaction is thermodynamically uphill and energetically expensive. Another approach is the electrolysis of water in an e...

Pulverized samples of charred biomass from rice straw, wood, almond shell, and corn stover were converted within a solid carbon fuel cell (SCFC) using a yttria stabilized zirconia oxide ion conducting electrolyte. Open circuit cell potentials against air for all solid fuels tested at 900 • C were in the range 1.00 to 1.07 V, in good agreement with expected values. Measurements of cell performance indicated peak power densities of 34 to 39 mW/cm 2 for the biomass fuels, which compared favorably with a peak power density of 38 mW/cm 2 for an activated carbon fuel used for benchmarking purposes. Comparison with previous measurements for activated carbon in our laboratory, where peak power densities in excess of 220 mW/cm 2 were demonstrated in optimized cells, suggests great promise for biomass utilization in a SCFC. Further modeling of corn stover and activated carbon revealed that the lower specific surface area and bulk density of the biomass chars is offset by a higher fuel reactivity in the dry gasification environment inside the SCFC anode compartment. The modeling also defined the limits of the SCFC operating window for high effective char utilization and demonstrated that high current densities can be supported when biomass chars are employed.

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A novel electrochemical conversion scheme for the spontaneous and simultaneous production of carbon-free hydrogen and electric power is proposed and modeled. The design combines a carbon fuel cell and a steam-carbon fuel cell into a steam-carbon-air fuel cell that shares a single solid carbonaceous fuel bed in the anode chamber. A model of the combined cell is developed and a new definition for overall cell efficiency, which accounts for the hydrogen product generated, is produced. Results indicate that the proposed cell can reach efficiencies above 78%, and can also realize a six-fold increase in hydrogen production rate compared to a steam-carbon fuel cell with only a doubling of the cell active area.

This study describes a novel electrochemical approach for the simultaneous and spontaneous production of carbon-free hydrogen and electricity. Hydrogen was generated from steam at the cathode of a yttria stabilized zirconia electrolyte by utilizing a solid carbonaceous fuel at the cell anode to maintain low oxygen activity levels within the anode chamber. The results verified that a steam-carbon cell is capable of producing carbon-free hydrogen in a fuel cell mode while generating electric power at the same time. The open circuit voltages of the cells agreed with expected values. The current-voltage behavior indicated that cell performance was governed mostly by resistive losses. A gas transport model of the solid carbon bed based on known Boudouard kinetics was developed and analyzed. The model demonstrates that the fuel bed is capable of supporting a cell current density in excess of 700 mA/cm2 while maintaining a fuel utilization rate of over 85%.