High Temperature Membrane Reactor System for Hydrogen Permeation Measurements and Validation with Pd Based Membranes

Abstract: Hydrogen separation membranes are under development for integration with a coal gasifier or natural gas reformer for pre-combustion separation of hydrogen and carbon dioxide. Because of the high operating temperatures and pressures, a robust reactor and associated control systems are required for fast screening of membrane materials with a strong emphasis on operator and plant safety. In this paper, the design, construction and commissioning of a reliable membrane reactor and a versatile test station for evaluation of hydrogen permeation membrane materials (metals, ceramics or cermets) at high temperatures and high differential pressures has been described. The membrane reactor system has been designed to operate at temperatures up to 800 o C and pressure differentials across the membrane to 1.0MPa. The system has multiple levels of safety redundancy built-in which include a range of controls and monitors for both operator and system safety. A number of Pd and Pd-Ag alloys of nominal thicknesses in the 20 and 140 m range were sourced and alumina based porous ceramic support structure were fabricated for evaluation of metal membranes. The test station has been validated with Pd and Pd-Ag alloys of different thicknesses. The data obtained from the reactor for various membrane types and thicknesses are in agreement with those reported in the literature

Investigation of the stability of ceria-gadolinia electrolytes in solid oxide fuel cell environments

Doped ceria-based materials are potential electrolytes for use in lower operating temperature (500-700 degrees C) solid oxide fuel cells because of their high ionic conductivity. In this study, impedance behaviour and microstructure of the (Ce0.8Gd0.2)O-1.9 exposed to mild fuel environments (H-2-N-2 mixtures) have been investigated. The exposure of specimens to H-2-N-2 mixtures at 1000 degrees C resulted in a substantial expansion of the lattice as a consequence of the reduction of Ce4+ to Ce3+, which in turn led to the development of microcracks and loss of continuity at the grain boundary region and increase in both the grain boundary (major effect) and the lattice (minor effect) resistivity. The behaviour for the grain boundary resistivity after the 800 degrees C exposure was somewhat similar although expansion of the lattice at 800 degrees C (or lower temperatures) was considerably less conspicuous. After exposure to H-2-N-2 atmosphere at lower temperatures (650 and 500 degrees C), although no significant increase in the grain boundary resistivity for exposures up to 1000 h was observed, the shape of the grain boundary are was clearly affected. The large increase in the grain boundary resistivity in reduced specimens has been attributed to the observed microcracking, loss of continuity between grains and possibly the formation of new phase regions with extremely poor oxygen-ion conductivity along grain boundaries during the reduction. The disruption to the microstructure is not recovered on subsequent oxidation in air. (C) 1999 Elsevier Science B.V. All rights reserved

An investigation of conductivity, microstructure and stability of electrolyte compositions in the system 9 mol% (Sc2O3-Y2O3)-ZrO2(Al 2O3)

In search for better ionically conducting ceramics for oxygen separators, fuel cells and sensors, the electrical conductivity and micro structure of the 9 mol% (ScO-YO)-ZrO system with varying ScO/YO ratios has been investigated in detail with XRD, SEM, TEM and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 850 and 1000°C. Impedance spectroscopy was employed for determining the contribution of the grain boundary resistivity. The role of alumina additions to selected ScO-YO-ZrO compositions was studied as alumina is known to reduce the grain boundary resistivity by scavenging silica impurities and enhance mechanical properties in zirconia-based systems. AlO-containing compositions show much higher conductivity degradation compared with alumina-free materials. This behaviour has been investigated in more detail with XRD and TEM analysis

Structural and microstructural stability of ceria - gadolinia electrolyte exposed to reducing environments of high temperature fuel cells

Doped CeO is widely used in intermediate temperature solid oxide fuel cells (500-650 °C) due to its high ionic conductivity, low reactivity to other cell components and ability to facilitate charge transfer reactions at the electrode/electrolyte interface. However, on exposure to hydrogen above 650 °C doped cerates can be reduced leading to catastrophic microstructure failure and loss of mechanical integrity. The effect of other fuels such as C and CO on the stability of ceria based electrolytes remains largely unexplored despite the increased significance in developing fuel cells that operate on these fuels. In this paper a systematic investigation has been carried out on the effect of carbon monoxide on the electrical conductivity, ionic transport, crystal structure and microstructure of CeGd O, with particular emphasis on the mechanisms of reduction and the long term stability of the material for use in a direct carbon fuel cell (DCFC) where this material will be exposed to a reducing environment containing little or no hydrogen. These investigations have been carried out at temperatures typically found during the operation of a DCFC (800 °C) and the results have been compared with similar investigation carried out in dry hydrogen atmosphere. A wide range of techniques including synchrotron X-ray powder diffraction, high resolution transmission and scanning electron microscopy, four-probe DC conductivity measurements and electrochemical impedance analysis have been used to investigate the structural, microstructural and electrical properties of CeGdO exposed to the operating environments of a DCFC

Characterisation of conductivity of the (Ce xY0.2&#x2D;x)Sc0.6 Zr3.2O8&#x2D;&#948; (0 < x < 0.2) system and composition Ce0.04Y0.02Sc0.67Zr3.27O 7.66 as function of time Caracterização da condutividade do sistema (Ce xY0,2&#x2D;x)Sc0,6 Zr3,2O8&#x2D;&#948; (0 < x < ) e da composição Ce0,04Y0,02Sc0,67Zr3,27O 7,66 em função do tempo

The conductivity behaviour of the (Ce xY0.2-x)Sc0.6 Zr3.2O8-&#948; (0 < x < 0.2) system and composition Ce0.04Y0.02Sc0.67Zr3.27O 7.66 have been investigated as function of time using impedance spectroscopy. All samples were prepared by sol-gel and combustion process, sintered at 1500 °C for 12 h and the densities obtained were between 92 and 97%. The electrical measurements were performed at 600 °C. The conductivity values were fairly stable during the first 1800 h of the experiment but after this time the conductivity decreases. For some compositions of the system a semi-circle is detected through time, with capacitance values of an order of magnitude lower, 10-8 F/cm. This semi-circle becomes well defined with time. After the experiment, SEM pictures show that grain boundary is well defined and an increase of pore inside grains, in some cases the surface is damaged showing cracks and fissures indicating microstructure deterioration.<br>O comportamento da condutividade do sistema (Ce xY0,2-x)Sc0,6Zr3,2O 8-&#948; (0 < x < 0,2) e da composição Ce0,04Y0,02Sc0,67Zr3,27O 7,66, foi investigado em função do tempo usando espectroscopia de impedância. Todas as amostras foram preparadas pelo processo de sol gel e combustão combinados, sinterizadas a 1500 ºC durante 12 h e as densidades obtidas estão entre 92 e 97%. Esta experiência foi executada a 600 °C. Os valores da condutividade apresentaram-se relativamente estáveis durante as primeiras 1800 h da experiência mas depois deste tempo de funcionamento a condutividade decresceu. Para algumas composições do sistema um semi-circulo apareceu ao longo do tempo, com valores de capacidade com ordem de grandeza 10-8 F/cm. Estes semi-círculos tornam-se melhor definidos com o tempo. Depois desta experiência as micrografias de microscopia eletrônica de varredura destas amostras mostram que a fronteira de grão é bem definida e que ocorreu um aumento de porosidade, e em algumas amostras a superfície ficou danificada mostrando fissuras e trincas na superfície indicando deterioração na microestrutura