High-temperature polaronic transport in PrBaCoTa(Nb)O₆ perovskite-like phases

Highlights

• Electrical conductivity in Ta and Nb-doped cobaltite PrBaCo₂O_6–δ was measured.

• Theoretical model for electron transport was developed and validated.

• Co ions spin distribution was shown to influence the electrical properties.

Abstract

The electrical transport properties of dense PrBaCoTaO₆ and PrBaCoNbO₆ polycrystalline phases are thoroughly studied. Both measured conductivity and thermopower temperature dependencies are consistent with p-type adiabatic small polaron hopping mechanism. Detailed analysis based on DFT calculations reveals Co t_2g orbitals play crucial role in charge transfer process. The proposed theoretical transport model predicts most trivalent Co ions tend to acquire an intermediate spin state acting as self-trapped electron holes. Estimated carrier mobility and polaron jump frequency values for both studied compounds are found to be in a reasonable coincidence with previously obtained results.

Introduction

Despite numerous efforts undertaken the development of stable and efficient intermediate temperature solid oxide fuel cells (IT-SOFCs) still remains a desirable goal to achieve [[1], [2], [3], [4]]. In particular, optimal chemical composition for cathode materials wasn't found yet. Layered cobaltites with general formula AA’Co₂O₆ where A and A′ denote rare- and alkaline-earth elements, respectively, are often considered as perspective candidates for practical implementation in IT-SOFCs [[5], [6], [7]]. These materials are distinguished by very high mixed ionic/electronic conductivity values [[8], [9], [10], [11]] and fast oxygen exchange kinetics even at 900 K [12,13]. However, several drawbacks restrict their application potential, in particular, relatively fast degradation in CO₂ containing atmospheres [14] and unacceptably high values of thermal expansion coefficient (TEC) leading to mechanical instabilities in high-temperature devices [11]. In this regard various modifying strategies were implemented in order to improve the overall cathode performance of cobaltites [[15], [16], [17], [18], [19]]

Recently, cobalt based electrodes with additions of niobium ant tantalum were developed [16,17,[20], [21], [22]]. The injections of 4d/5d elements in cobalt-containing matrix occurred to be an attractive doping strategy because such problems as TEC mismatch and cathodes’ degradation under CO₂ ambient can be solved simultaneously without sufficient suppression of cathode functional properties. One should notice the nature of the resulting Nb/Ta-doped materials differs significantly depending on the total dopant concentration. The single phase samples from the solid solution PrBa_1-xCo_2-yTa_yO_6-δ with 0.01≤y ≤ 0.05 were successfully synthesized [17]. On the other hand, electrochemical properties of the composite (1–x) PrBaCo₂O_6–δ – (x) PrBaCoNbO₆ electrodes [20] where Nb-containing phase maintains ordered double perovskite structure were studied. The results obtained evidence these chemical compositions are promising cathode materials. However, numerous questions regarding the nature of PrBaCoNbO₆ additive remain unclear, for example a mechanism of electrical conductivity, oxygen ion transfer and thermal expansion features.

In previous work [23] we've tried to answer these questions using a combination of theoretical and experimental approaches. It was shown PrBaCoNbO₆ (PBCN) and PrBaCoTaO₆ (PBCT) possess very small oxygen stoichiometry changes even at 1223 K thus excluding the possibility of their appreciable contribution to ionic transport in composite electrodes studied in Ref. [20]. Moreover, the respective TEC values appeared to be commensurate with those for common electrolytes. Calculated electron structure for both PBCN and PBCT revealed these materials are wide-gap semiconductors that is qualitatively confirmed by the results obtained in Ref. [20,24].

In this paper we continue to develop theoretical concepts underlying the functional properties observed for PBCN/PBCT phases, in particular, electron transport model is proposed and verified using experimental data.