A Study of Transition Metal Doping on the Electrical Properties of Perovskite Type Proton Conductor
Authors: Lee Young-Sung , Leonard Kwati , Okuyama Yuji , Matsumoto Hiroshige 
Published: February 2020

Proton-conducting oxides have been studied as intermediate-temperature electrolyte materials for fuel cells and steam electrolysis. These materials can operate at intermediate temperatures because the activation energy for proton conductivity is lower than that of oxide ion conductivity. When proton conducting oxides are used as the electrolyte for fuel cells and steam electrolysis, nickel or its composite with electrolyte materials, so-called Ni cermet, are used as the fuel-side electrode, and transition-metal-containing perovskite such as (La,Sr)(Co,Fe)O3 (LSCF) and (La,Sr)MnO3 (LSM), are used for the air-side electrodes. The electrode materials contain Ni, Fe, Co, Mn, and so forth, which possibly react with the electrolyte oxides during processing and operation of the fuel cells and electrolysers. A possible concern is that the diffusion of these transition metals will degrade the electrical conductivity of the electrolyte. Shimura et al. reported a significant decrease in the electrical conductivity of BaCe0.9Y0.1O3-δ-based (BCY) proton conductors on partially substituting Fe, Mn and Co for Ce [1]. In this study, transition metal doping of perovskite type oxide have been performed, i.e., BaCe0.85Y0.1
M
0.05O3-δ, BaZr0.85Y0.1
M
0.05O3-δ, SrCe0.85Y0.1
M
0.05O3-δ and SrZr0.85Y0.1
M
0.05O3-δ which are referred to hereafter as BCYM, BZYM, SCYM and BZYM, respectively (M = Co, Fe, Mn and Ni), and their electrical conduction properties investigated to understand the effect of introducing transition metals to the proton conductor oxides.

(Ba,Sr)(Ce,Zr)0.9-x
Y0.1
Mx
O3-δ were prepared by a solid-state reaction method. BaCO3, SrCO3, CeO2, ZrO2, Y2O3 and transition metal oxides were appropriately weighed, mixed, and calcined at 1200-1300 °C for 10 h in air, ball-milled, pressed into pellets, and sintered at 1400-1700°C for 10 h in air. Phase identification was carried out by X-ray diffraction. The conductivity was measured by a four-terminal AC impedance method. The electromotive force of gas concentration cells was measured in the temperature range from 873 to 1073 K.

The electrical conductivity change was observed by introducing transition metals. These results suggest that the introduction of transition metals causes the change in the conductivity of the proton conducting electrolytes in most cases, and that the change in the conductivity depends on the sort of transition metals. This means that the impact of introducing transition metals to the electrical conduction properties depends on the oxide. The above two cases suggests that either A-site (Sr or Ba) or B-site (Ce or Zr) possibly governs for the impact of conductivity is sensitive to the transition metals.

Acknowledgement

This work was supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP), International Institute for Carbon Neutral Research (I2CNER) and World Premium International Research Center Initiative (WPI), Japan.

[1] T. Shimura, H. Tanaka, H. Matsumoto, T. Yogo, Solid State Ionics, 176 (2005) 2945–2950