Takeou Okanishi

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.

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.

A Stability Study of Ni/Yttria-Stabilized Zirconia Anode for Direct Ammonia Solid Oxide Fuel Cells

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.

Polymer Electrolyte Fuel Cells Employing Heteropolyacids as Redox Mediators for Oxygen Reduction Reactions: Pt-Free Cathode Systems

In this study, the heteropolyacids of H3+xPVxMO12-xO40 (x = 0, 2, and 3) were applied as redox mediators for the oxygen reduction reaction in polymer electrolyte fuel cells, of which the cathode is free from the usage of noble metals such as Pt/C. In this system, the electrochemical reduction of heteropolyacid over the carbon cathode and the subsequent reoxidation of the partially reduced heteropolyacid by exposure to the dissolved oxygen in the regenerator are important processes for continuous power generation. Thus, the redox properties of catholytes containing these heteropolyacids were investigated in detail. The substitution quantity of V in the heteropolyacid affected the onset reduction potential as well as the reduction current density, resulting in a difference in cell performance. The chemical composition of heteropolyacid also had a significant impact on the reoxidation property. Among the three compounds, H6PV3Mo9O40 was the most suitable redox mediator. Furthermore, the pH of the catholyte was found to be the crucial factor in determining the reoxidation rate of partially reduced heteropolyacid as well as cell performance.

Anion-Exchange Membrane Fuel Cells with Improved CO2 Tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption

Over the last few decades, because of the significant development of anion exchange membranes, increasing efforts have been devoted the realization of anion exchange membrane fuel cells (AEMFCs) that operate with the supply of hydrogen generated on-site. In this paper, ammonia was selected as a hydrogen source, following which the effect of conceivable impurities, unreacted NH3 and atmospheric CO2, on the performance of AEMFCs was established. As expected, we show that these impurities worsen the performance of AEMFCs significantly. Furthermore, with the help of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, it was revealed that the degradation of the cell performance was primarily due to the inhibition of the hydrogen oxidation reaction (HOR). This is attributed to the active site occupation by CO-related adspecies derived from (bi)carbonate adspecies. Interestingly, this degradation in the HOR activity is suppressed in the presence of both NH3 and HCO3- because of the bicarbonate ion consumption reaction induced by the existence of NH3. Further analysis using in situ ATR-IR and electrochemical methods revealed that the poisonous CO-related adspecies were completely removed under NH3-HCO3- conditions, accompanied by the improvement in HOR activity. Finally, a fuel cell test was conducted by using the practical AEMFC with the supply of NH3-contained H2 gas to the anode and ambient air to the cathode. The result confirmed the validity of this positive effect of NH3-HCO3- coexistence on CO2-tolerence of AEMFCs. The cell performance achieved nearly 95% of that without any impurity in the fuels. These results clearly show the impact of the chemically induced bicarbonate ion consumption reaction on the realization of highly CO2-tolerent AEMFCs.