Novel Material Design Concept Helps Develop New Ionic Conductors

A team led by Prof. Masatomo Yashima of Tokyo Institute of Technology (Japan) and Prof. John A. Kilner of Imperial College London (UK) has successfully visualized the oxygen diffusion pathway in an excellent oxide-ion and electronic mixed conductor, layered perovskite-type cobaltite PrBaCo₂O_5+δ.

The present work demonstrates a new concept in the material design for high oxide-ion conduction and this can lead to the development of new ionic conductors for better solid oxide fuel cells and oxygen concentrators.

Ionic conductors with high oxide-ion conductivity and mixed conductors with both high oxide-ion conductivity and high electron conductivity are indispensable in improving the efficiency of fuel cells, solid electrolyzers and oxygen concentrators. Therefore, much effort has been devoted to the development of ionic and mixed conductors. AA'₂O_5+δ-based oxides with a layered (A and A' cation-ordered) perovskite-type structure have extensively been studied as new mixed ionic-electronic conductors where the A = rare earth, A' = Ba, and B = Co, and 5+δ is the oxygen concentration, because they exhibit high oxide-ion conductivity. PrBaCo₂O_5+δ has highest oxide-ion self-diffusion coefficient among AA'B₂O_5+δ-based oxides (A = La, Pr, Nd, Sm, Gd and Y; A' = Ba and B = Co). However, the structural origin of the high oxide-ion conductivity and oxide-ion diffusional pathway of PrBaCo₂O_5+δ had been suggested by molecular dynamics calculations at Imperial College but were not known to be correct.

Dr. Juan Peña-Martínez (Present position: Assistant Professor of Complutense University of Madrid, Spain) and Prof. Dr. John A. Kilner of Imperial College, prepared and characterized the PrBaCo₂O_5+δ sample as single phase, including electrical properties. Prof. Dr. Masatomo Yashima and Ms. YiChing Chen of Tokyo Institute of Technology analyzed the crystal structure and nuclear density of PrBaCo₂O_5+δ at 596 and 1000 °C by Rietveld and maximum-entropy methods using in situ neutron diffraction data obtained by the neutron diffractometer HERMES at the Institute for Materials Research, Tohoku University, located at the research reactor JRR-3M of the Japan Atomic Energy Agency. The sample was characterized also by the high-resolution neutron diffraction (iMateria, J-Parc) and synchrotron X-ray diffraction (BL-4B2 beamline of the Photon Factory, KEK and BL02B2 and BL19B2 beam lines of SPring-8). It was found that the oxide ions of PrBaCo₂O_5+δ two-dimensionally and anisotropically diffuse through equatorial and deficient apical oxygen sites along the <101> directions in the Pr-Co–O ion-conducting slab (Fig. 1). The A–A' (= Pr–Ba) cation ordering makes (i) higher oxygen vacancy concentration at the O2 site and (ii) shorter O2–O3 interionic distance due to lower electrostatic energy (Fig. 2), which leads to the –O2–O3–O2– highway of mobile oxide ions in the conducting Pr–Co–O slab. The present work demonstrates a new concept in the material design for high oxide-ion conduction and this can lead to the development of new ionic conductors for better solid oxide fuel cells and oxygen concentrators.

Source: http://www.titech.ac.jp