Improving the Chemical Stability of BaCe0.9Y0.1O3-d (BCY10) Towards Humidification and Carbonation By Microstructural Design
Authors: Graca Vanessa Cristina Diniz , Loureiro Francisco José Almeida , Mikhalev Sergey Mikhaylovich , Fagg Duncan Paul 
Published: July 2021

Protonic ceramic fuel cells are rapidly gaining interest as a promising technology for energy conversion. These devices use proton-conducting electrolytes allowing them to operate at considerably lower temperatures (e.g., 400 – 600 °C) than those of the oxide-ion-conducting counterparts (e.g., > 600 °C). This decrease in temperature can bring significant benefits, including shorter start-up times, lower energy inputs to heat the cell up to the operating temperature and greater longevity [1].

Owing to its very high equilibrium constant for hydration, yttrium-doped barium cerate, BaCe1−xYxO3−δ (BCY), stands out as being an interesting electrolyte material to use at lower temperatures, due to its high proton conduction under these conditions (e.g., ∼ 10−3 S cm−1 at 400 °C under humidified atmospheres, p
H2O ∼ 10−2 atm). Nonetheless, this composition has been, typically, discarded for real world applications due to its very poor chemical stability towards carbonate or hydroxide formation at low temperatures. In this respect, the presence of amorphous intergranular phases in BaCeO3 polycrystalline materials can be highly reactive to acidic atmospheres and have been suggested to be a likely cause of the structural disintegration of this composition [1,2].

To try to overcome this limitation, we report a potential solution based on the modification of BaCe0.9Y0.1O3−δ (BCY10) composition by the introduction of small amounts of NiO (1 vol%) during the sintering process. While the addition of transition metals has been widely adopted for enhancing the sinterability of these materials [1], such behaviour has not been previously tested as a method to improve the chemical stability and lifetime of this material.

To accomplish this goal, four different samples were pelletised and sintered: the undoped samples, BCY10 sintered at 1450 °C (BCY1450) and 1600 °C (BCY1600); and the NiO-modified samples, sintered at 1450 °C (Ni-BCY1450) and 1350 °C (Ni-BCY1350). All materials were characterised for phase composition by X-ray diffraction (XRD), and for microstructure by scanning electron microscopy, coupled with chemical analysis by Energy Energy-dispersive X-ray spectroscopy (SEM/EDS).

The XRD results reveal all samples to be single phase, indexed as the orthorhombic perovskite BaCe0.9Y0.1O3-
d. Nevertheless, slight changes of the cell volume were noted upon Rietveld refinement of the XRD data, suggesting that nickel is incorporated in the perovskite structure of BCY10. From the SEM results, it was possible to observe that all samples showed different average grain sizes. Particularly, the NiO-modified samples showed considerably higher grain sizes and levels of densification than that observed for the unmodified BCY10 samples.

Electrochemical Impedance Spectroscopy (EIS) was utilised to evaluate the electrical properties of bulk samples in wet O2 (p
H2O = 0.033 atm) in the temperature range 100 – 600 °C. It was found that the bulk response is similar for all samples under study, with no notable dependence from any compositional differences. Conversely, the specific grain boundary conductivity, a factor where the grain boundary response is normalised as a function of microstructural differences, was found to be higher in the case of the BCY1450 and Ni-BCY1350 samples, followed by Ni-BCY1450 and BCY1600.

To evaluate the chemical stability, fresh samples were exposed to a humidified CO2 atmosphere (p
H2O = 0.033 atm) at 400 °C for 30 days and EIS was used to record impedance data. The results revealed that the unmodified BCY10 samples suffered a substantial drop in total conductivity in this timeframe, while the NiO-modified samples (Ni-BCY1350 and Ni-BCY1450) remained stable through the tests.

This result is important as it reveals that the simple addition of 1vol% of NiO not only offers its typical role of decreasing the sintering temperature from 1450 to 1350 °C, but also is sufficient for dramatically improving the stability of BCY10 against humidification and carbonation. The work offers a significant advance to the knowledge of this family of materials at low temperatures, potentially opening a way to use a previously discarded proton-conducting perovskite material in real world applications.

Acknowledgements The authors acknowledge to FCT for the PhD grant SFRH/BD/130218/2017 and the projects, PTDC/CTM-CTM/2156/2020, PTDC/QUI-ELT/3681/2020, POCI-01-0247-FEDER-039926, POCI-01-0145-FEDER-032241, UIDB/00481/2020 and UIDP/00481/2020 and CENTRO-01-0145-FEDER-022083 - Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).

References

[1] F.J.A. Loureiro, N. Nasani, G.S. Reddy, N.R. Munirathnam, D.P. Fagg, A review on sintering technology of proton conducting BaCeO3-BaZrO3 perovskite oxide materials for Protonic Ceramic Fuel Cells, J. Power Sources. 438 (2019) 226991.

[2] F.J.A. Loureiro, D. Pérez-Coll, V.C.D. Graça, S.M. Mikhalev, A.F.G. Ribeiro, A. Mendes, D.P. Fagg, Proton conductivity in yttrium-doped barium cerate in nominally dry reducing conditions for application in chemical synthesis, J. Mater. Chem. A. 7 (2019) 18135–18142.