Hiroki Muroyama

Comparison Between Internal Steam and CO2 Reforming of Methane for Ni-YSZ and Ni-ScSZ SOFC Anodes

The performance and durability for Ni-YSZ and Ni-ScSZ anodes of solid oxide fuel cells were compared between internal steam and CO 2 reforming of methane in the carbon deposition conditions. With a supply of H 2 -CO-CO 2 mixtures to the anode, the polarization resistance increased with a rise in CO concentrations at 1023 K because of the difficulty of electrochemical oxidation of carbon monoxide. Carbon was produced by the disproportionation of carbon monoxide in CO―CO 2 mixture (CO:CO 2 = 95:5) at 1023 K, which led to low performance and much degradation of the cells. In comparison between steam and CO 2 reforming of methane, the performance and durability in a gaseous mixture at H 2 O/CH 4 = 0.5 were better than that at CO 2 /CH 4 = 0.5. The durability of the Ni-ScSZ anode was superior to that of the Ni-YSZ anode at 1273 K. Amorphous carbon covered the Ni-YSZ anode surface after power generation, which deactivated the nickel catalyst and inhibited gas diffusion. However, crystalline graphite with a rod morphology was grown on the Ni-ScSZ anode, which affected the catalytic activity less than amorphous carbon. The crystallinity and morphology of deposited carbon are important in determining the performance and durability of the cells at low H 2 0/CH 4 and CO 2 /CH 4 ratios.

Development of Novel Proton Conductors Consisting of Solid Acid/pyrophosphate Composite for Intermediate-temperature Fuel Cells

Fuel cells are attractive energy conversion devices with high efficiencies and low emissions, and many studies have been conducted so far. Among them, fuel cells operating at 200-600°C are promising technologies which combine the many advantages of high- and low-temperature fuel cells. However, they have not been developed due to the lack of good ionic-conductors with high thermal stability at intermediate temperatures. Recently, we have developed new proton-conductive electrolytes consisting of solid acid and pyrophosphate, and evaluated their electrochemical, structural and thermal properties at intermediate temperatures. For the composite based on CsH2PO4/SiP2O7, the interfacial chemical reaction between CsH2PO4 and SiP2O7 during heat-treatment gave rise to the formation of a new phase of CsH5(PO4)2. The temperature dependence of conductivity for this composite was different from that for pure CsH2PO4, and the maximum conductivity achieved was 44 mS·cm−1 at 266°C. Using potassium and rubidium salts, MH2PO4 (M = K, Rb), as the solid acids for the composite electrolytes, analogous phenomena were confirmed despite the alkaline metal. Operation of a fuel cell employing CsH2PO4/SiP2O7-based composite electrolyte (thickness: ca. 1.3 mm) was demonstrated at 200°C and generated electricity up to 220 mA·cm−2 at 0.2 V. CsH5(PO4)2 composites with SiP2O7 and SiO2 were fabricated, and the composite effects were investigated at intermediate temperatures based on conductivity measurement, thermal analysis, and wettability evaluation. The melting and dehydration processes of CsH5(PO4)2 in composites were different depending on the matrix species. The composite with SiP2O7 matrix showed the highest conductivity of all composites. The conductivity of the composites appears to correlate with the wettability between the components as examined by contact angle measurement. These findings should be attributed to the differences in the interfacial interactions between CsH5(PO4)2 and the matrix.

In Situ Attenuated Total Reflection Infrared Spectroscopy on Electrochemical Ammonia Oxidation over Pt Electrode in Alkaline Aqueous Solutions

The electrochemical oxidation of ammonia over Pt electrode in alkaline aqueous solutions was studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. In 0.1 M NH3-1 M KOH, the band ascribable to the HNH bending mode of adsorbed NH3 was confirmed at 1662-1674 cm(-1) in the potential range of 0.1-1.1 V. The intensity of this band decreased continuously with a rise in potential, indicating the oxidative consumption of adsorbed ammonia. In response to this behavior, the band at 1269 cm(-1) appeared alternatively above 0.2 V, and its intensity reached the local maximal value at ca. 0.4 V. Note that this potential of ca. 0.4 V agreed well with the onset potential of ammonia oxidation, ca. 0.45 V, in the linear sweep voltammogram. This 1269 cm(-1) band was assigned to the NH2 wagging mode of N2H4, which was one of the active intermediates, N2H(x+y,ad) (x = 1 or 2, y = 1 or 2), according to the mechanism proposed by Gerischer and Mauere. To the best of our knowledge, this is the first report for the detection of N2H4 as a reaction intermediate over Pt electrode. Furthermore, the formation of bridged NO was also observed above the onset potential of ammonia oxidation, ca. 0.5 V. Such adsorbed NO species probably inhibit the electrochemical reaction due to the occupation of reaction sites at higher potential.

Gas Transport Impedance in Segmented-in-Series Tubular Solid Oxide Fuel Cell

Gas phase transport is a very important electrode process in practical solid oxide fuel cells. In this study, we have identified gas conversion impedance and gas diffusion impedance in the Mitsubishi segmented-in-series tubular solid oxide fuel cell. Gas conversion impedance is caused by the weak convection transport in the gas flow channel. It is observed that both the insufficient anode and cathode gas flow rates can result in the gas conversion impedance. Gas conversion impedance appears at less than 0.1 Hz, and its magnitude strongly depends on the gas flow rates. It disappears when the gas flow rates of both the anode and cathode are improved sufficiently. Anode gas diffusion through the porous substrate appears at ~0.5 Hz and dominates the overall diffusion impedance. Cathode gas diffusion through the porous current collecting layer appears at ~3 Hz, which significantly contributes to the overall gas diffusion impedance under low cathode oxygen partial pressures.

Activation of LSM Electrode Related to the Potential Oscillation under Cathodic Polarization

Solid oxide fuel cells with the conventional configuration of Ni―ytttria-stabilized zirconia (Ni-YSZ)|YSZ|(La,Sr)MnO 3±δ (LSM) have been reported to exhibit performance improvement in the initial discharge operation, which is attributed to the activation of LSM. In this study, the time course of potential for LSM was monitored under cathodic polarization to precisely analyze this enhancement. A characteristic phenomenon of potential oscillation was found during the constant current loading. Regardless of electrolyte materials and LSM composition, the potential was oscillated with a frequency of ca. 0.67 Hz soon after current passing. The potential oscillation appeared under the following conditions: (i) at low partial pressure of oxygen, (ii) at high cathodic current loading, and (iii) for an LSM electrode with low porosity. The amplitude of oscillation reduced with the elapsed time of cathodic polarization and then disappeared. The microscopic observation revealed that the structural change in the LSM electrode after cathodic polarization is responsible for the disappearance of potential oscillation. These findings suggest that the potential oscillation originates in the mass-transfer limitation, oxygen diffusion, and densified electrode.

COx-free hydrogen production via ammonia decomposition over molybdenum nitride-based catalysts

COx-free hydrogen generation via ammonia decomposition has received much attention as an important process for fuel cell applications. In the present study, non-precious Mo nitride-based catalysts with Co, Ni, and Fe additives were synthesized by temperature-programmed reaction of the corresponding oxide precursors with NH3. N2 adsorption, X-ray diffraction (XRD), NH3-temperature programmed surface reaction (NH3-TPSR), NH3 pulse reaction, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were carried out to gain better insight into the chemical and textural properties of the catalysts. The XRD analysis confirmed the formation of the Mo2N, Co3Mo3N, Ni3Mo3N, and Fe3Mo3N phases, which act as active species for ammonia decomposition reaction. The Co3Mo3N, Ni3Mo3N, and Fe3Mo3N catalysts were more active for ammonia decomposition than the Mo2N catalysts, indicating that the Co, Ni, and Fe species promoted the reaction. The increase in particle size and the decrease in surface area by the Co, Ni, and Fe addition did not negatively affect the ammonia decomposition activity. Interestingly, the Co, Ni, and Fe addition facilitated the recombinative desorption of N atoms from the active components, resulting in the enhancement in the activity. In addition, kinetic analysis was also conducted in detail to investigate the effects of the NH3 and H2 partial pressures. The Co, Ni, and Fe addition alleviated the negative effect of the H2 poisoning on the active sites.

Ammonia Decomposition over Nickel Catalysts Supported on Rare-Earth Oxides for the On-Site Generation of Hydrogen

Ammonia decomposition has attracted the interest of many researchers as a promising process for the on‐site generation of H2. In this study, Ni catalysts supported on various rare‐earth oxides were prepared by the impregnation method and employed for ammonia decomposition. The Ni/Y2O3 catalyst exhibited the best performance among the samples investigated. The reaction kinetics study indicated that most of rare‐earth oxide supports were effective for the alleviation of the hydrogen inhibition phenomenon in the decomposition reaction. The desorption behavior of hydrogen has revealed that the amount of hydrogen atoms adsorbed strongly on the Ni surface up to high temperatures was relatively small in the case of Ni/Y2O3. Furthermore, for Ni/Y2O3 the optimal Ni loading was 40 wt % in terms of the catalytic activity because of the appropriate Ni dispersion.

Electrochemical and catalytic properties of Ni/BaCe0.75Y0.25O3-δ anode for direct ammonia-fueled solid oxide fuel cells.

In this study, Ni/BaCe0.75Y0.25O3-δ (Ni/BCY25) was investigated as an anode for direct ammonia-fueled solid oxide fuel cells. The catalytic activity of Ni/BCY25 for ammonia decomposition was found to be remarkably higher than Ni/8 mol % Y2O3-ZrO2 and Ni/Ce0.90Gd0.10O1.95. The poisoning effect of water and hydrogen on ammonia decomposition reaction over Ni/BCY25 was evaluated. In addition, an electrolyte-supported SOFC employing BaCe0.90Y0.10O3-δ (BCY10) electrolyte and Ni/BCY25 anode was fabricated, and its electrochemical performance was investigated at 550-650 °C with supply of ammonia and hydrogen fuel gases. The effect of water content in anode gas on the cell performance was also studied. Based on these results, it was concluded that Ni/BCY25 was a promising anode for direct ammonia-fueled SOFCs. An anode-supported single cell denoted as Ni/BCY25|BCY10|Sm0.5Sr0.5CoO3-δ was also fabricated, and maximum powder density of 216 and 165 mW cm(-2) was achieved at 650 and 600 °C, for ammonia fuel, respectively.

Additive effect of alkaline earth metals on ammonia decomposition reaction over Ni/Y2O3 catalysts

Recently, ammonia has attracted much attention as a promising hydrogen carrier due to various advantages for on-site hydrogen generation. In this study, Ni/Y2O3 catalysts modified by alkaline earth metals were prepared and their catalytic activity for ammonia decomposition was examined. The addition of a small amount of Sr or Ba species remarkably improved the performance of Ni/Y2O3, while the Mg- and Ca-modification were not effective. The XRD analysis elucidated that the composite oxides consisting of alkaline earth metals and nickel were formed in the as-calcined Sr- and Ba-modified Ni/Y2O3 catalysts, and were decomposed by the reduction treatment. This suggested that the Sr and Ba components were located in the vicinity of Ni particles, resulting in the strong interaction with the Ni species. The NH3-temperature programmed surface reaction measurement revealed that the desorption of nitrogen atoms strongly-adsorbed on the Ni surface has terminated at lower temperatures over the Sr- and Ba-modified catalysts than over the others. This characteristic desorption behavior would be mainly attributed to the enhancement of the electron density in the Ni metal by the strong basic property of Sr and Ba components and the strong Ni–Sr and Ni–Ba interaction. Considering the nitrogen desorption step was kinetically significant, this change in the electronic state of the Ni surface should be responsible for the promotion effect of the Sr- and Ba-modification.

Fabrication of Redox Tolerant Anode with an Electronic Conductive Oxide of Y-Doped SrTiO3

Composite anodes consisting of Ni, Y-doped SrTiO 3 (YST), and Y 2 O 3 -stabilized ZrO 2 (YSZ) were fabricated; the Ni species was impregnated to YST-YSZ powder and then the resulting powder was fired onto the electrolyte at 1200°C for 5 h in air. The conductive oxide of YST sintered in air showed a conductivity of 7.2 S cm ―1 after reduction in humidified hydrogen (1.2% H 2 O―H 2 ) for 100 h and behaved as a metallic conductor with respect to the temperature dependence of conductivity. The influence of the nickel content in the anode of Ni/YST―YSZ on the electrochemical property was studied under 0.6% H 2 O―H 2 atmosphere. The cermet of 10 wt % Ni/YST―YSZ showed the minimal polarization resistance among the catalysts with the nickel content in the range of 5-50 wt %. An electrolyte-supported cell employing this anode exhibited a power density of 0.34 W cm ―2 at 0.5 V at 1000°C despite the extremely low content of Ni. Stable power generation was attained at a constant terminal voltage of 0.7 V in humidified hydrogen (0.6% H 2 O―H 2 ) for 21 h. Even after five consecutive redox cycles, the cell performance remained unchanged mainly due to the stable porous framework of YST-YSZ with a low Ni content.