STUDY OF TRANSPORT PROPERTIES OF COMPOSITE METAL-CERAMIC MEMBRANE MATERIALS FOR SELECTIVE SEPARATION OF HYDROGEN

Ceramic solid proton electrolytes La_0,99Ca_0,01NbO₄, Nd_5,5WO_11,25-δ и Nd_5,5W_0,5Mo_0.5O_11,25-δ were synthesized and their proton conductivity was measured in the temperature range of 300-650 оС in an atmosphere of dry and humid air. Solid electrolytes are shown to have a high proton conductivity ~ 10^-4 S/cm at 500 оС. Dense metal-ceramic composites containing phases of metal Ni_0,5Cu_0,5 and oxides Nd_5.5WO_11.25 or Nd_5.5W_0.5Mo_0.5O_11.25-δ with the typical total conductivity of metals were obtained using hot pressing technique an argon atmosphere. The problem arises with an experimental determination of the ionic (in this case proton) conductivity contribution to the overall conductivity of the material comprises when studying the metal-ceramic materials properties. The proton conductivity values can be estimated from the results of studying the hydrogen permeability of membranes and the diffusion of hydrogen; however, these methods are rather complicated in instrumentation. Therefore the use of relatively simple and accessible electrical measurement methods to solve this problem is very relevant. In this paper, the partial proton conductivity of the composite materials mentioned above was first measured using a 4-electrode cell with ion probes made of a ceramic proton conductor La_0,99Ca_0,01NbO₄in an atmosphere of moist hydrogen and at the temperature range of 300-650 оC. In the low temperature region, the partial proton conductivity values measured in the 4-electrode cell are in good agreement with those obtained by standard complex impedance analysis for the pure ceramics not containing metal. In high temperature region, the values obtained by two independent techniques differ. This can be explained by the contribution of the electrochemical reaction proceeding at the interface between the ion probe and the metal phases and accompanied by the dissolution of atomic hydrogen in the metal. In general, the measured value of the ionic conductivity can be either underestimated or overestimated in comparison with the real one, depending on the rate of chemical reactions occurring at the electrodes. Nevertheless, in a limited temperature range, the use of four-electrode measurements with ionic (proton) probes allows one to obtain correct results.

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