Study of proton conductivity of composite metal-ceramic materials based on neodimium tugstates using a four-electrode technique with ionic probes

Abstract

Ceramic solid proton electrolytes La0.99Ca0.01NbO4, Nd5.5WO11.25- Nd5.5W0.5Mo0.5O11.25-δ were synthesized and their proton conductivity was measured in the temperature range of 300–650 °С in an atmosphere of dry and humid air. It is shown that solid electrolytes have a high proton conductivity ∼10−4 S/cm at 500 °С. Dense metal-ceramic composites containing phases of metal Ni0.5Cu0.5 and oxides Nd5.5WO11.25 or Nd5.5W0.5Mo0.5O11.25-δ with the total conductivity typical of metals were obtained using hot pressing technique an argon atmosphere. An experimental determination of the contribution of the ionic (in this case proton) conductivity to the overall conductivity of the material comprises is an actual problem in investigation of the properties of metal-ceramic materials. The values of proton conductivity can be estimated from the results of studying the hydrogen permeability of membranes and the diffusion of hydrogen, but 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 La0.99Ca0.01NbO4 in an atmosphere of moist hydrogen and at the temperature range of 300–650 °C. In the low-temperature region, the values of the partial proton conductivity 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. At high temperatures, 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.