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Carbon dioxide methanation is well known to offer some advantages and be catalyzed by Ru, Rh, Pd, and Ni. In this study, Ni catalysts supported on various metal oxides were fabricated and their catalytic activity for CO2 methanation was evaluated. The CO2 conversion for most of catalysts drastically increased at 225-250 ºC and reached a maximal value at 300-350 ºC. The order of CH4 yield at 250 ºC was as follows; Ni/Y2O3 > Ni/Sm2O3 > Ni/ZrO2 > Ni/CeO2 > Ni/Al2O3 > Ni/La2O3. The catalytic activity could be partly explained by the basic property of the catalysts. Moreover, the chemical species formed on the catalyst surface during CO2 methanation were examined by in situ infrared spectroscopy. From the obtained results, the difference in the activity depending on the support material of Ni catalysts was discussed.

Nickel–yttria-stabilized zirconia (Ni–YSZ) cermet is a conventional anode for use in solid oxide fuel cells, and its composition and microstructure are carefully controlled to achieve high performance and long-term stability. In this study, the performance stability of the electrolyte-supported cell (Ni–YSZ|YSZ|LSM) was examined at 1000°C by feeding humidified fuel, x%normalH2O–(100−x)%normalH2 . The influence of the cermet composition on degradation was also studied. The degradation behavior was significantly dependent on the fuel humidity and cermet composition. In the case of Ni–YSZ with a volume ratio of 50 to 50, peculiar phenomena were observed. When the fuel of 30%normalH2O–70%normalH2 was supplied to the anode at the terminal voltage of 0.7 V, the current density decreased gradually soon after the discharge of up to 69 h, followed by a sudden drop in the current density. After the subsequent open-circuit holding, the performance was partially recovered in the discharge operation. This behavior of the degradation-recovery was reversibly repeated upon the discharge-open-circuit holding operation. Under the 40%normalH2O–60%normalH2 atmosphere, an irreversible performance deterioration was observed accompanied with a drastic decrease in the volume-specific triple-phase boundary (TPB) length. The TPB length of the degraded anode was evaluated to be 1.66–1.68μmμnormalm−3 by a focused ion beam scanning electron microscopy technique, whereas that of the as-prepared anode was 2.49μmμnormalm−3 .

No abstract

In recent years, solid oxide fuel cells fueled with ammonia have been attracting intensive attention. In this work, ammonia fuel was supplied to the Ni/yttria-stabilized zirconia (YSZ) cermet anode at 600 and 700 °C, and the change of electrochemical performance and microstructure under the open-circuit state was studied in detail. The influence of ammonia exposure on the microstructure of Ni was also investigated by using Ni/YSZ powder and Ni film deposited on a YSZ disk. The obtained results demonstrated that Ni in the cermet anode was partially nitrided under an ammonia atmosphere, which considerably roughened the Ni surface. Moreover, the destruction of the anode support layer was confirmed for the anode-supported cell upon the temperature cycling test between 600 and 700 °C because of the nitriding phenomenon of Ni, resulting in severe performance degradation.

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.

No abstract

The individual electrode processes from anode and cathode have been clearly identified from practical impedance spectra with high-frequency inductive impedances for the Mitsubishi segmented-in-series tubular solid oxide fuel cell by combining the distribution of relaxation time analysis and the complex nonlinear least square fitting. Anodic gas diffusion process and charge transfer reaction near anodic triple phase boundary appear at ∼0.3 and 1000Hz , respectively. Cathodic oxygen reduction processes appear at ∼10Hz at 700°C , whereas the impedance arc over 10000Hz is dominated by the cathodic oxygen ion transfer through the (normalLa,normalSr)normalMnO3 (LSM)/ normalY2normalO3 -stabilized normalZrO2 (YSZ) interface and YSZ of the composite. The results also exhibit an excellent agreement with those from the analysis of difference in impedance spectra.