Article pubs.acs.org/JPCC
Studies on Ionic Conductivity of Sr2+-Doped CeP2O7 Electrolyte in Humid Atmosphere Bhupendra Singh,†,‡ Ha-Ni Im,† Jun-Young Park,§ and Sun-Ju Song*,†,‡ †
Ionics Lab, School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwang-Ju 500-757, Republic of Korea ‡ Research Institute for Catalysis, Chonnam National University and §Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea
ABSTRACT: Sr2+-doped cerium pyrophosphate (Ce1−xSrxP2O7, x = 0.05, 0.1, 0.15, and 0.2) electrolytes are synthesized by digesting cerium oxide and strontium nitrate with 85% H3PO4. The ionic conductivity behavior of Ce1−xSrxP2O7-sintered pellets is analyzed by electrochemical impedance spectroscopy in a dry and humid air atmosphere. The time-dependent variation of ionic conductivity during humidification process and the variation of conductivity with temperature are studied. In humid atmosphere in the 90−230 °C range, the variation of ionic conductivity is explained on the basis of the combined effect of ionic mobility and the availability of charge carriers in the samples. The ionic conductivity shows dependence on dopant concentration and water vapor pressure (pH2O). In dry air, conductivity of Ce1−xSrxP2O7 is very low, with Ce0.9Sr0.1P2O7 showing a maximum conductivity of 4.3 × 10−6 S/cm at 430 °C. Among various Ce1−xSrxP2O7 samples, Ce0.9Sr0.1P2O7 shows the highest conductivity in humid air (pH2O = 0.12 atm) with a maximum conductivity of 6.3 × 10−3 S/cm at 90 °C, and the conductivity of Ce0.9Sr0.1P2O7 was 3.5 × 10−3 S/cm at 190 °C and pH2O = 0.12 atm. On the basis of XRD results, the stability of Ce1−xSrxP2O7 in a humid atmosphere is analyzed.
1. INTRODUCTION
Cerium pyrophosphate (CeP2O7) has been reported to be a good proton conductor in the intermediate temperature range, although there has been a large difference in the variation of conductivity of cerium pyrophosphate with the variation in temperature in the dry and humid atmosphere.7,12,13 In previous works, many authors have reported high ionic conductivity in pyrophosphates in the dry atmosphere and assign this conductivity as proton conductivity, but because cerium pyrophosphate has no structural protons, their argument in the support of proton incorporation in pyrophosphates has been the incorporation of proton from the ambient atmosphere or due to the proton incorporation during the synthesis.2 However, in our recent work,13 by synthesizing a single-phase CeP2O7 and by measuring its electrical conductivity under dry air conditions, and which was very low
The development of electrolytes having high ionic conductivity in the temperature range of 100−400 °C has been a crucial requirement for the lowering of operational temperature of fuel cells.1 Several phosphate-based anhydrous proton conducting ceramics have shown to be potential electrolyte for the lowtemperature proton-conducting ceramic-electrolyte fuel cells (LT-PCFCs) in this temperature range, and a number of tetravalent metal pyrophosphates (MP2O7, where M = Sn, Ge, Zr, Si, Ce, Ti) have shown significant proton conductivity (∼10−2 S cm−1) in the 150−400 °C temperature range.2−7 Unlike other phosphate-based proton conductors, such as ammonium polyphosphate-based proton conductors, and CsH2PO4 and related proton conductors, metal pyrophosphates such as CeP2O7 do not have structural protons.1,8−11 The protons are incorporated into pyrophosphates as impurity during synthesis or as external elements from fuel or humid air.1 © 2013 American Chemical Society
Received: November 22, 2012 Revised: January 10, 2013 Published: January 21, 2013 2653
dx.doi.org/10.1021/jp311547g | J. Phys. Chem. C 2013, 117, 2653−2661
The Journal of Physical Chemistry C
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