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Literature review on high temperature proton conducting materials
Gepubliceerd door: Publicatie datum:
ECN Waterstof en Schoon Fossiel 2-2-2009
ECN publicatienummer: Publicatie type:
ECN-E--08-091 ECN rapport
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This report contains then main results of a literature survey on proton-conducting ceramic oxides to be applied at high temperatures, i.e. in the range from 600 to 800 ºC. These materials are broadly divided in two classes: 1. materials that require an external circuit for electronic conduction, and 2. materials that show both protonic and electronic conduction. They are denoted in this report as class I and class II conductors, respectively. Class I conductors Most research has been performed on the perovskites, notably doped barium and strontium ce-rates and their mixtures. These show good protonic conductivity (in the order of 10 mS/cm) in the temperature range 500 - 900 ºC, though their chemical stability in a H2O and CO2 containing atmosphere is not so good. Among the alternatives with comparable protonic conductivity are fluorite-related structures, e.g. the tungstates La5.8WO11.7 and La5.7Ca0.3WO11.85 of which the chemical stability is not well investigated as yet, and pyrochlores like La1.95Ca0.05Zr2O6.975, which have good stability though protonic conductivity only up to 600 ºC. Fergusonites are currently investigated by the group of Norby at the University of Oslo. Though their stability has not been well investigated as yet, it is expected to be good. The conductivity is an order of mag-nitude less than that of the perovskites. The main drive for research on class I conductors is their application in SOFCs (solid oxide fuel cells) and electrolysis cells, allowing lower operation temperature and thus cheaper stack construction materials than cells with oyxgen ion conducting electrolytes. Other applications in-clude steam-methane reforming reactors, water-gas shift reactors, dehydrogenation reactor, ammonia synthesis reactors, H2 pumps and H2 sensors. Only perovskites have been tested for these applications. The power densities obtained in lab-scale SOFCs with proton conducting electrolyte is approx. half of that of lab-scale SOFCs with oxygen ion conducting electrolyte. In all of these applications, except for the last three, oxygen ion conducting membranes can also be applied. In contrast with solid oxide fuel cells with oxygen ion conducting electrolyte, all H2O is produced at the cathode in proton conducting electrolyte fuel cells. This brings new system designs into view, like the temperature swing reformer. Class II conductors These can be divided in two groups: those derived from a class I conductor by doping it with a metal that enhances the electronic or hole conduction, and the cermets, i.e. composites derived from a class I conductor and an electronic conductor. Their main application is H2 separation in the temperature range 600 - 1000 ºC. The H2 permeability of the former group is in general much lower than that of the latter group. The most successful application so far is a composite developed by Eltron Research being an equimolar mixture of BaCe0.8Eu0.2O2.9 and Ce0.8Y0.2O2.9, of which the H2 permeability is comparable with that of Pd. Many materials classes are currently investigated for H2 separation membranes as alternative of pure Pd, though all of these operate at temperatures below 600 ºC: Pd-alloys, non-Pd crystalline materials, porous ceramics, bulk metallic glasses, ceramic-metal composites (e.g. Pd in Y-stabilized zirconia), and ceramic-salt composites (e.g. RbNO3 - La1-xSrxCoO3). On the other hand, for applications at operation temperatures above 600 ºC, only solid oxides can be used as membrane material. Thus, class II conductors and mixed oxygen ion-electronic conducting membranes are competing in this temperature range. The only system investigated so far using a class II conducting membrane is the HMR (hydrogen membrane reformer) concept, which is developed in CCP (Carbon Capture Project). Systems studies as well as experimental data concerning the long-term kinetic and mechanical stability of proton conductors, both class I and class II, are most wanted. Other class I materials than the cerates, zirconates and their mixtures should be tested in a fuel cell. Given the great number of promising materials and the uncertainties related to all problems that must be overcome, class II conductors should not be considered as alternative of pure Pd for H2 separation.

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