Conductivity dependence on oxygen partial pressure and transport number measurements of La 2 Mo 2 O 9
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S6.5.1
Conductivity dependence on oxygen partial pressure and transport number measurements of La2Mo2O9 A. Tarancón1, G. Dezanneau1, A. Morata1, F. Peiró1, J.R. Morante1, T. Norby2 EME (CEMIC&CeRMAE), Departament d'Electrònica, Universitat de Barcelona, Martí i Franquès, 1, 08028 Barcelona, SPAIN, e-mail: [email protected] 1
2
Center for Materials Science, Department of Chemistry, University of Oslo Gaustadalleen, 21, N-0349 Oslo, NORWAY ABSTRACT
Transport number measurements and impedance spectroscopy in controlled temperature and atmosphere were used to investigate the electrical properties of La2Mo2O9 sintered samples. These samples were prepared from nanocrystalline La2Mo2O9 powders as obtained from a new soft chemistry route involving the polymerisation of acrylamide. By means of the electromotive force method measurements, the sintered compound was found mainly oxygen conductor in the range of 400 ºC-800 ºC with oxide-ion transport number greater than 0.99. The effect of the oxygen partial pressure on the electrical conductivity of La2Mo2O9 was investigated by impedance spectroscopy from 1 to 10-22 atm, showing a highly stable conduction properties up to 10-17 atm at 800 ºC. INTRODUCTION Solid oxide fuel cells (SOFCs) are solid-state ionic electrochemical devices that directly transform chemical energy of a fuel gas into electrical energy. A SOFC (Fig.1) consists of a gas tight solid electrolyte separating two electrodes, cathode and anode, exposed to two independent atmospheres of different oxygen partial pressure. Oxygen is reduced at the cathode/electrolyte interface and incorporated to the electrolyte following the reaction: O2 cathode + 4e- = 2O2-electrolyte
(1)
At the anode the oxygen ions are oxidized at the interface electrolyte/anode to form oxygen molecules, reverse process in Eq. (1), or a different compound in the presence of a fuel gas, e.g. H2O with H2 as a fuel gas (Fig.1). The SOFC is therefore considered to be an oxygen concentration cell which generates an electromotive force (emf) given by the Nernst equation: Eemf
high RT pO2 ln = 4 F pOlow 2
This expression describes the system only in equilibrium conditions, i.e. without current flowing through the device. In practice, irreversible resistive and polarization losses appear causing the subsequent drop in fuel cell efficiency.
(2)
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Fuel: H2
Excess fuel H2+O2- = H2O+2e -
Anode
I external load
O2-
Electrolyte Cathode
eO2+4e - = 2O2Cathode exhaust
Oxidant: O2
Figure 1. Scheme and performance principle of a SOFC operating with H2 as a fuel gas. This work is focused on the electrical characterization of one of the most promising ionic conductor for SOFC electrolyte applications, i.e. the recently discovered1 La2Mo2O9, parent compound of the so-called LAMOX family. In order to establish the suitability of a material as a good solid electrolyte is essential to determine the fraction of the total conductivity due to ionic conduction and the set of experimental conditions over which it maintains a negligible el
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