Atsuko Tomita

A Proton-Conducting In3 + -Doped SnP2O7 Electrolyte for Intermediate-Temperature Fuel Cells

We report proton conduction in In 3+ -doped SnP 2 O 7 in the temperature range from 100 to 300°C, and the performance of a H 2 -air fuel cell using this material as the electrolyte. The proton conductivity of In 3+ -doped SnP 2 O 7 was more than 10 -1 S cm -1 between 125 and 300°C, and a conductivity value of 1.95 × 10 -1 S cm -1 was achieved at 250°C. The resulting fuel cell exhibited a reasonable power density of 264 mW cm -2 at 250°C (electrolyte thickness = 0.35 mm), together with perfect tolerance toward 10% CO and good thermal stability in unhumidified conditions.

Proton Conduction in In3 + -Doped SnP2O7 at Intermediate Temperatures

SnP 2 O 7 -based proton conductors were characterized by Fourier transform infrared spectroscopy (FTIR), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and electrochemical techniques. Undoped SnP 2 O 7 showed overall conductivities greater than 10 -2 S cm -1 in the temperature range of 75-300°C. The proton transport numbers of this material at 250°C under various conditions were estimated, based on the ratio of the electromotive force of the galvanic cells to the theoretical values, to be 0.97-0.99 in humidified H 2 and 0.89-0.92 under fuel cell conditions. Partial substitution of In 3+ for Sn 4+ led to an increase in the proton conductivity (from 5.56 X 10 -2 to 1.95 X 10 -1 S cm -1 at 250°C, for example). FTIR and TPD measurements revealed that the effects of doping on the proton conductivity could be attributed to an increase in the proton concentration in the bulk Sn 1-x In x P 2 O 7 . The deficiency of P 2 O 2 ions in the Sn 1-x In x P 2 O 7 bulk decreased the proton conductivity by several orders of magnitude, which was explained as due to a decrease in the proton mobility rather than the proton concentration. The mechanism of proton incorporation and conduction is examined and discussed in detail.

Intermediate-Temperature Proton Conduction in Al3 + -Doped SnP2O7

Al 3+ -doped SnP 2 O 7 proton conductors were prepared by controlling the initial composition of the reactants [SnO 2 , Al(OH) 3 , and H 3 PO 4 ]. Sn 1-x Al x P y O z with y 2 exhibited conductivities at a maximum of 1.99 times higher. However, because the conductivity values of Sn 1-x Al x P y O z with y > 2 were not stable, the optimal value of y in Sn 1-x Al x P y O z was determined to be 2. Partial substitution of Al 3+ for Sn 4+ in Sn 1-x Al x P 2 O 7 led to an increase in the conductivity up until x = 0.05. As a result, the conductivity reached 0.045 S cm -1 at 100°C, 0.15 S cm -1 at 200°C, and 0.19 S cm -1 at 300°C when the x and y values were 0.05 and 2, respectively. A hydrogen concentration cell with this material demonstrated that the ionic transport number was ∼ 1, and a fuel cell using this material demonstrated that the dc conductivity was comparable to the ac conductivity.

Comparative Performance of Anode-Supported SOFCs Using a Thin Ce0.9Gd0.1O1.95 Electrolyte with an Incorporated BaCe0.8Y0.2O3 − α Layer in Hydrogen and Methane

Multilayered Ce 0.9 Gd 0.1 O 1.95 /BaCe 0.8 Y 0.2 O 3-α /Ce 0.9 Gd 0.1 O 1.95 (GDC/BCY/GDC) electrolytes were prepared by tape casting on a Ni-Ce 0.8 Sm 0.2 O 1 9 anode support. The overall electrolyte thickness ranged from 30 to 35 μm, including a 3 μm thick BCY layer. When the multilayered electrolyte cell was tested with hydrogen at the anode and air at the cathode in the temperature range of 500-700°C, it yielded open-circuit voltages (OCVs) of 846-1024 mV, which were higher than the OCVs of 753-933 mV obtained for a single-layered GDC electrolyte cell under the same conditions. The corresponding peak power densities reached 273, 731, and 1025 mW cm -2 at 500, 600, and 700°C, respectively. The multilayered electrolyte cell could also be applied to direct methane solid oxide fuel cell (SOFC) and single-chamber SOFC operating in a mixture of methane and air. These SOFCs yielded OCVs of 880-950 mV and reasonable power densities without coking.

Design of a Reduction-Resistant Ce0.8Sm0.2 O 1.9 Electrolyte Through Growth of a Thin BaCe1−xSmxO3−α Layer over Electrolyte Surface

A method that can block off electronic current through a samaria-doped ceria (SDC, Ce 0 . 8 Sm 0 . 2 O 1 . 9 ) electrolyte is proposed. A thin BaCeO 3 -based layer 12 μm thick was grown by a solid-state reaction of the electrolyte substrate and a BaO film deposited previously over the substrate surface at 1500°C. A homogeneous junction between the layer and the electrolyte was formed, thus allowing no delamination and cracking of the layer. Tolerance of this layer to CO 2 was high enough to suppress decomposition into BaCO 3 and CeO 2 . Open-circuit voltages of a hydrogen-air fuel cell with the coated SDC electrolyte were near 1 V or more in the range of 600-950°C. The resulting peak power density was higher than that of a fuel cell with an uncoated SDC electrolyte.

Sn0.9In0.1P2O7-Based Organic/Inorganic Composite Membranes Application to Intermediate-Temperature Fuel Cells

2by reducing the electrolyte thickness to 60 m. The peak power densities achieved with unhumidified H2 and air were 109 mW cm �2

Direct Oxidation of Methane to Methanol at Low Temperature and Pressure in an Electrochemical Fuel Cell

Methane is an abundantly available fuel whose use is mainly limited to that of a primary energy source due to its low reactivity. Methanol, on the other hand, is a useful intermediate material in many chemical manufacturing processes as well as a safe-to-handle liquid fuel for transportation and storage. There is therefore a long-standing industrial interest in producing methanol from methane effectively. The conventional synthesis of methanol from methane involves multi-step processes, including the steam reforming of methane and subsequent catalytic reaction. These processes, however, require high temperatures (< 700 8C) and pressures (200–300 atm) operations, respectively, which lead to high running costs. The direct oxidation of methane to methanol has received much attention as the next step in methanol production since it avoids the above multi-step processes. However, this oxidation is regarded as a very difficult reaction, especially in the gas phase at low pressure, because of the need to operate at high temperatures (> 400 8C), where methanol is quickly oxidized to formaldehyde and COx. [2–7] One approach for oxidizing methane at lower temperatures is to apply an electrochemical cell to the reaction system. Otsuka and Yamanaka et al., for example, have reported the selective oxidation of light alkanes to oxygenates by the electrochemically activated oxygen species that are generated at the cathode in polymer electrolyte (PEFCs) and phosphoric acid fuel cells (PAFCs) [Eqs. (1) and (2)].

Single-Chamber SOFCs with a Ce0.9Gd0.1 O 1.95 Electrolyte Film for Low-Temperature Operation

Single-chamber solid oxide fuel cells (SOFCs) with an anode-supported Ce 0 . 9 Gd 0 . 1 O 1 . 9 5 electrolyte were operated in a mixture of butane and air at furnace temperatures of 200-300°C. The electromotive force (emf) of the cell and the voltage drop were strongly influenced by the catalytic activity of the anode for the partial oxidation of butane. The promotion of hydrogen formation by the addition of Ru to the anode caused an increase in the emf and a reduction in the voltage drop. As a result, stable power densities of 44 and 176 mW cm - 2 were obtained at 200 and 300°C, respectively.

A Single-Chamber SOFC Stack Operating in Engine Exhaust

Heat engines emit huge amounts of exhaust which contain considerable quantities of heat and chemical energies. An increasing public awareness of global warming and oil depletion has required the development of innovative methods of recovering energy from such exhaust. A promising approach is to produce electricity from unburned fuel by using a fuel cell system. The exhaust includes cracked-light hydrocarbons, offering the possibility of applying this system to a power generator. Solid oxide fuel cells (SOFCs) can work at elevated temperatures but have poor thermal and mechanical shock resistance. In this study, we address this problem by operating the fuel cell system in a single-chamber mode, wherein no separation between fuel and oxidant gases is required. This operation provides high tolerance toward thermal cycling and breakage of the electrolyte. As a result, a twelve-cell stack exhibits stable and high performance in the exhaust from a motorcycle.

Room-Temperature Hydrogen Sensors Based on an In3 + -Doped SnP2O7 Proton Conductor

A potentiometric solid-state gas sensor was fabricated using a proton-conducting Sn 0.9 In 0.1 P 2 O 7 electrolyte with an active Pt/C working electrode in order to study its sensing properties for small quantities (100 ppm 3%) of H 2 in air at room temperature. The sensor showed electromotive force (emf) response in the negative direction to changes in the H 2 concentration. Furthermore, the emf value varied linearly with the logarithm of the H 2 concentration, while it was minimally affected by the water-vapor concentration. The sensing mechanism was shown to be based on the mixed potential at the working electrode through measurements of the polarization curves of H 2 and air. The Sn 0.9 ln 0.1 P 2 O 7 electrolyte was also applied in two single-chamber H 2 sensors, wherein Pt/C and carbon were used as active and inactive electrodes, respectively; these electrodes were attached on the opposite surfaces or on the same surface of the electrolyte. Both single-chamber sensors could exhibit comparable H 2 sensitivities, compared to the dual-chamber sensor.