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RESEARCH PRODUCT

New synthetic strategies for the enhancement of the ionic conductivity in Ce0.8Sm0.2O2-x

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Introduction Samarium-doped ceria has been widely investigat-ed for electrochemistry applications as full density elec-trolyte for intermediate temperature solid oxide fuel cells (IT-SOFC), between 500°C and 800°C [1]. One of the main challenges for the ceria-based electrolytes is to decrease the resistivity at the grain boundary zone, especially in this low temperature range. Microstructure and morphology of the powder play a fundamental role on the densification behaviour and grain boundary properties, thus affecting the overall conductivity of electrolytes [2]. In this light, solution combustion syn-thesis (SCS) represents a flexible method to produce ultra fine powders with suitable microstructural proper-ties [3]. In this work, we investigated the effect of the synthesis fuels on microstructural features and on ionic transport properties of Ce0.8Sm0.2O2-x. Experiments Ce0.8Sm0.2O2-x was prepared by SCS using four dif-ferent combustion fuel mixtures: i) citric acid and am-monia (sample 1), ii) cellulose and citric acid (sample 2), iii) sucrose (sample 3), iv) sucrose and PEG20000 (sam-ple 4). Sintering of the pellets was performed both in air and in H2/N2 flow until 1400°C for 12h. The electro-chemical properties of the electrolytes were studied in air by AC impedance spectroscopy. Results and Discussion Samples prepared with different fuel mixtures showed different surface areas, relative densities, linear shrinkages and porosities. A different electrochemical behaviour was also evidenced by the impedance spec-tra of the samples sintered both in air and in H2/N2. Fig. 1. Impedance plots collected in air at 300°C of samples prepared with fuel mixture 3 (black circles) and 2 (blue circles). As a matter of the fact, the fuel mixtures have a di-rect influence on ionic conductivity of Ce0.8Sm0.2O2-x, even after high-temperature sintering. As an example, in Fig. 1 the impedance spectra, measured at 300°C in air, are shown for the air-sintered sample 2 and sample 3. As it is shown, a gain in grain boundary conductivity is evident for sample 3 (sucrose fuel) compared to sample 2 (cellulose-citric acid mixture). This result cannot be solely ascribed to a higher relative density (92% vs 90%), but also to a different distribution of dopants and defects between bulk and grain boundary, where the cerium oxidation state plays a fundamental role [4]. In facts, the Ce3+/Ce4+ ratio is probably influenced by the reducing power of the combustion fuel mixture, as re-cently demonstrated for other IT-SOFC materials pre-pared by SCS [5]. This thesis is enforced by the differ-ent bulk conductivities showed by sample 2 and sample 3, and by the fact that Rietveld analysis of the XRD data does not reveal any difference in the cell parame-ters of SDC in the pre and post-sintered powders. More-over, the ionic conductivity was influenced by the re-duction of Ce4+ to Ce3+ during the processing proce-dures, highlighting that, probably, there is an optimum Ce3+/Ce4+ ratio which enhances the Ce0.8Sm0.2O2-x per-formance. Conclusions We pointed out that it is possible to improve the to-tal ionic conductivity of Ce0.8Sm0.2O2-x electrolyte by a careful choice of the SCS conditions. In particular, we demonstrated that the combustion fuel directly shapes the microstructure and the grain boundary of the sin-tered powders. The effect of the fuel mixtures was still preserved after high temperature sintering. We found that the grain boundary conductivity was considerably improved by sucrose-based fuels with high reducing power. References [1] Z. Liu, D. Ding, M. Liu, X. Ding, D. Chen, X. Li, C.Xia, M. Liu, J. Power Sources 241 (2013) 454. [2] V.Esposito, E. Traversa, J. Am. Ceram. Soc. 91 (2008) 1037. [3] F. Deganello, G. Marcì, G. Deganello, J. Eur. Ceram. Soc. 29 (2009) 439. [4] E.C.C. Souza, W.C. Chueh, W. Jung, E.N.S. Muccil-lo, S.M. Haile, J. Electrochem. Soc. 159 (2012) K127. [5] F. Deganello, L.F. Liotta, G. Marcì, E. Fabbri, E. Traversa, Mater. Renew. Sust. Energ. 2 (2013) 2:8.