Reducibility of Ceria-Based Materials Exposed to Fuels and under Fuel/Air Gradients

Abstract

Ceria-based materials have been extensively studied in recent years for their potential use as solid electrolytes for alternative solid oxide fuel cells (SOFC) concepts, with emphasis on intermediate temperature SOFCs (Steele, 2000), single chamber SOFC (Yano et al., 2007), etc. However, the decrease in the oxygen partial pressure exerted by the presence of the fuel increases the non-stoichiometry accompanied by the reduction of Ce4+ to Ce3+. In these cases, reducibility of ceria-based solid electrolytes is a critical limitation, mainly because this implies onset of electronic conductivity (Blumenthal & Hofmaier, 1974; Blumenthal & Sharma, 1975; Tuller & Nowick, 1977; Navarro et al., 1997) and corresponding risks of internal short circuiting and decrease in cell voltage. The reducibility of cerias is affected by trivalent additives and their contents (Wang et al. 1997; Wang et al. 1998; Kobayashi et al., 1999), altering the mixed transport properties under low values of oxygen pressure according to the defect chemistry models. However some discrepancies are found in literature regarding the level of oxygen losses as well as the electronic conductivity (Mogensen et al., 2000; Zachau-Christiansen et al., 1996). On the other hand, ceria and related materials are also promising catalysts, including use as SOFC anode components for hydrocarbon conversion (McIntosh & Gorte, 2004; Tsipis et al., 2004). Performance in these prospective applications is likely to be promoted by the redox behaviour of cerias. Thus, one has studied the onset of electronic conductivity of the most promising ceria-based materials Ce1-xGdxO2-0.5x-Δδ (CGO) and Ce1-xSmxO2-0.5x-Δδ (CSO), including its dependence on temperature and composition (Perez-Coll et al., 2004; Abrantes et al., 2003). The impact of low temperature sintering with suitable additives on the onset of n-type contribution under reducing conditions has also been studied (Fagg et al., 2003). In this chapter the dependence on oxygen partial is revised in detail, and corresponding effects imposed by fuels such as hydrogen or methane are examined by taking into account thermodynamic correlations between oxygen partial pressure and fuel conditions. This is extended for fuel/electrolyte/air gradients, as expected for SOFC operation.