An oxide ion and proton co-ion conducting Sn_0.9In _0.1P₂ O₇ electrolyte for intermediate- temperature fuel cells

The ionic conductivity of Sn_0.9In_0.1P₂ O₇ ceramic was investigated under various atmospheres within the temperature range of 130-230°C. Similar to mixed-conductive perovskite oxides at high temperatures (such as SrCe_0.95Yb_0.05 O _3-α, La_0.9 Sr_0.1Ga_0.8 Mg _0.2 O_3-α at 600-1000°C), Sn_0.9In _0.1P₂ O₇ can conduct both protons and oxide ions at low temperatures (130-230°C). The conductivity of Sn _0.9In_0.1P₂ O₇ reaches 0.019 Scm at 200°C in wet nitrogen. Its transport numbers determined by steam concentration cells are around 0.76 for a proton and 0.12 for an oxide ion. The performance of direct methanol fuel cells at 170°C using mixed-ion conductive Sn_0.9In_0.1P₂ O₇ electrolyte is higher than that at 235°C using pure proton conductive CsH₂PO₄ electrolyte. This is attributed to direct oxidation of CO at the anode by the oxide ions generated at the cathode and moved through the Sn_0.9In_0.1P₂ O₇ electrolyte.