Fast proton-conducting solid-state electrolytes for highly efficient energy conversion

Highly densified BaCeO3 polycrystals free from a grain-boundary amorphous phase, through a combination of atomic-scale chemical analysis and physical imaging, were shown to be easily fabricated by a conventional ceramic process and to possess sufficiently high proton conductivity along with significantly improved chemical stability.

Article  |  Fall 2018

Since the first reports on the high-temperature proton conduction behavior discovered in SrCeO₃ in the early 1980s, a substantial amount of research on the perovskite-type proton-conducting oxides has been carried out to elucidate the incorporation process of protons during hydration, the conduction mechanism and path in the lattice, and the correlation between vacancies and dopants from a defect chemical viewpoint.

Although initial studies largely dealt with SrCeO₃, a notable report in 1988 on the significantly high proton conductivity in BaCeO₃ led to many relevant follow-up studies on the Ba-based perovskites. As a result, a variety of electrolyte membrane applications at elevated temperatures have been proposed in solid-oxide fuel cells, in steam electrolyzers for hydrogen production, and even in electrochemical cell reactors for methane conversion to versatile hydrocarbon materials.

In many ion-conducting polycrystalline oxides, grain boundaries are generally accepted as rate-limiting obstacles to rapid ionic diffusion, often resulting in overall sluggish transport. Therefore, highly resistive grain boundaries, compared with bulk grains, are one of the significant shortcomings that should be overcome in the Ba-based perovskites to achieve sufficient conduction performance. Several notable experimental methods, including solid solutions with multiple cations, the development of micron-scale thin films, the fabrication of textured and epitaxially oriented films by pulsed laser deposition, and even the suggestion of completely new perovskite composition, have been proposed over the past decade to overcome the major barriers encountered with perovskite proton conductors. Although significant performance leaps have been achieved in thin-film technologies and crucial insights into alternative ways of fabrication have been presented, control of polycrystalline microstructure on the basis of the conventional ceramic technique is still a key process to manufacture oxide electrolyte membranes on a large scale.

Prof. Sung-Yoon Chung’s group in the Graduate School of EEWS (Energy, Environment, Water, and Sustainability) and the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST) has successfully demonstrated that dense BaCeO₃ polycrystals presenting both significantly reduced grain-boundary impedance and notably enhanced tolerance to decomposition can be fabricated via a traditional sintering process. Focusing on BaCeO₃ doped with trivalent acceptor cations (BaCe_1-xM_xO_3-d, where M = Dy, Gd, Sm, Y), they first clarified that nanoscale Ba-excess intergranular amorphous phases were present at most grain boundaries in sintered polycrystals. By using two straightforward experimental approaches, it was also successfully shown that polycrystalline samples consisting of intergranular-phase-free grain boundaries reveal remarkable improvements of both proton conduction and chemical stability under CO₂- and H₂O-rich environments.

 

The findings in this work have several noteworthy implications regarding the fabrication and stability of BaCeO₃ polycrystals. First, this study demonstrates that highly dense polycrystalline BaCeO₃ (even >99% in relative density) free of grain-boundary phases can be easily fabricated by conventional sintering at 1350-1400°C with no sintering additives. Regardless of the composition of the dopant used in this study, the Ba-deficient Ba_0.95Ce_0.9M_0.1O_3-d samples free of intergranular phases are noted to consistently show nearly the same values of proton conductivity (1.6×10^-2 S/cm at 600°C and 0.9×10^-2 S/cm at 500°C on average). Revealing the presence of an intergranular amorphous phase at grain boundaries, the present study demonstrated that the nanoscale grain-boundary secondary phase was not only a major obstacle to rapid proton conduction but also a serious penetration path for H₂O and CO₂ gas molecules facilitating chemical decomposition. Therefore, the fabrication of highly densified polycrystals without the formation of intergranular amorphous phases is of major significance to achieve better ionic transport and microstructural durability in proton-conducting BaCeO₃.

Figure 2. Grain boundaries are clearly confirmed to be free of thin amorphous layers, showing direct graingrain contacts, as denoted by dotted yellow lines.

 

This work was published in Nano Letters (2018, 18, 1110-1117).

Additional links for more information:
https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b04655
https://sites.google.com/site/atomicscaledefects/home

Figure 1.