Interfacial Phenomena of Alkalimetal Carbonate on Sm-Doped Ceria for Composite Electrolytes

It is well-known that high temperature fuel cells as MCFCs and SOFCs operated at ca. 923-1273 K,generate electricity with high power density and fuel efficiency. Since component materials and cell configuration that sustainable at high operating temperature under oxidation/ reduction atmosphere are undefined, comprehensive improvement for included electrodes and electrolyte is needed. Composite electrolyte materials including solid oxide and carbonate indicate high conductivity at relative low temperature and are expected that various materials can be utilized for the operation of low temperature SOFCs etc. Wang et al. consider that there is an interaction at the interface of solid oxide particles and sodium carbonate phase, leading to the superior proton conduction [1].

In case of heterophase system such as carbonate/ceria based oxide composite system, it is important to consider the interfacial properties and behavior of ionic species at interface. In this study, to clarify the properties of solid oxide/carbonate composite system, behavior of carbonate ion at ca. 300—800 K was investigated by spectroscopic method.

CeO₂ and Sm-doped CeO₂ samples(SDCs) were synthesized by co-precipitation method[2]. Obtained precursor was separated, dried, milled and calcined at 1073 K for 5 hours under air atmosphere. Samples are indicated as SDC10(10 mol%), SDC20(20 mol%), SDC30(30 mol%), and SDC50(50mol%), according to amount of dopant Sm.

For eutectic mixture defined as NK. Na₂CO₃ and K₂CO₃ was dried at 473 K for 48 hours under CO₂ atmosphere, and mixed at Na:K = 59:41 and milled throughly. The carbonate mixture is added to CeO₂ or SDCs powder indicated as CeO₂-NK or SDCs-NK, respectively. Raman spectroscopy and TG-DTA were used for characterization of the samples as described elsewhere[3].

According to XRD patterns of solid oxide samples, CeO₂, SDC10, 20 and 30 have fluorite structure specific to pure CeO₂. On the other hand, SDC50 has both patterns of CeO₂ fluorite structure and pure Sm₂O₃. This result indicates there are two coexisting phases of CeO₂-Sm₂O₃ solid solution and segregated pure Sm₂O₃phase in SDC50.

Surface state and defects were observed by Raman spectroscopy. The band at about 460 cm^-1is called as F_2g band, which is assigned to Ce-O8 breathing mode [3]. F_2g band in samples CeO₂, SDC20 and SDC30 shifted to lower wavenumber in associate with dope amount increasing, but in SDC50, F_2g band shifted to higher wavenumber. Quantitative analysis by XPS and XRF, deviation of Sm amount was observed. Change of surface composition and structure associated with solid solution formation and segregation of Sm₂O₃makes a profound difference on surface state. From these results, it is projected that properties of composite materials do not follow Sm content change linearly.

In Raman spectra with temperature of carbonate breathing mode of NK in initial heating process, whereas separated n₁(CO₃^2-) bands assigned to Na₂CO₃ and K₂CO₃are observed clearly at lower temperature, unifying of these bands was observed due to solid state migration of carbonates at temperature lower than eutectic point (983 K).  Degree of migration is affected by apparent average thickness (AAT) and Sm amount in oxide (Fig. 1).

[1] X. Wang et al., J. Power Sources, 196, 2754 (2011).

[2] Z. Ning, Int. J. Hydrog. Energy, 37, 797(2012).

[2] M. Mizuhata et al. Int. J. Hydrogen Energy, 37, 24, 19407(2012).