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

Abstract

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 F2g band, which is assigned to Ce-O8 breathing mode [3]. F2g band in samples CeO2, SDC20 and SDC30 shifted to lower wavenumber in associate with dope amount increasing, but in SDC50, F2g 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).