ORIGINAL PAPER Duncan P. Fagg Æ Vladislav V. Kharton Jorge R. Frade Transport in ceria electrolytes modified with sintering aids: effects on oxygen reduction kinetics Received: 11 April 2003 / Accepted: 29 September 2003 / Published online: 23 March 2004 Ó Springer-Verlag 2004 Abstract Small (2 mol%) cobalt oxide additions to ceria-gadolinia (CGO) materials considerably improve sinterability, making it possible to obtain ceramics with 95–99% density and sub-micrometre grain sizes at 1,170–1,370 K. The addition of Co causes a significant shift of the electrolytic domain to lower pO 2 . This modification to the minor electronic conductivity of the electrolyte material has influence on the cathodic oxygen reduction reaction. The impedance technique is shown to provide information not only about polarisation resistance, but also about the active electrode area from analysis of the current constriction resistance. It is demonstrated that this current constriction resistance can be related to the minor electronic contributions to total conductivity in these materials. A simple imbedded grid approach gives control of the contact area allowing the properties of the electrolyte materials to be studied. A much lower polarisation resistance for the Co-con- taining CGO electrolyte is observed, which can be clearly attributed to an increased three-phase reaction area in the Co-containing material, as a consequence of elevated p-type conductivity. Keywords Gadolinia-doped ceria Æ Electrolyte Æ Sintering aids Æ Oxygen reduction kinetics Æ Impedance spectroscopy Introduction Solid electrolytes based on doped cerium dioxide, Ce(M)O 2-d (M: rare earth cations), are considered one of the most promising alternatives to yttria-stabilised zir- conia (YSZ) ceramics for applications in electrochemical devices operating at intermediate temperatures (770– 970 K), due to the four–five times higher ionic conduc- tivity of ceria-based materials in this temperature range [1, 2, 3, 4, 5, 6, 7, 8]. At temperatures below 1,000 K, electronic leakage due to reduction of Ce 4+ to Ce 3+ has been shown to be minimal and successful incorporation in devices such as solid oxide fuel cells (SOFCs) has been demonstrated. Recently, Kleinlogel and Gauckler [6, 7] have sintered dense Gd-substituted CeO 2 (CGO) with nano-sized grains at temperatures as low as 1,170 K by the addition of small quantities (<5 mol%) of binary transition metal oxides such as cobalt or copper oxide. The nano-sized grain structure is likely to have greater mechanical stability than conventionally processed CGO sintered at higher temperatures. Previous results from our group [9, 10] have shown that, whilst these minor dopant additions have no essential effect on the total and ionic conductivity, the p- type conduction in the transition metal-containing materials at 900–1,200 K is 8–30 times higher than that in undoped CGO. The oxygen ion transference numbers of the Co-, Fe- and Cu-doped ceramics, determined by the modified electromotive force (EMF) technique under oxygen/air gradient, are in the range 0.89–0.99. The contribution of the electron-hole conductivity to the total conductivity increases with temperature, as the activation energy for ionic conduction, 72 to 83 kJ/mol, is significantly lower than that for the p-type electronic transport (118–176 kJ/mol). Ion-blocking results have shown the inverse result that the n-type conductivity is depleted in Co-doped CGO materials when compared to that of undoped CGO [10]. Consequently, doping with trace amounts of transi- tion metal oxides could beneficially enhance the oxygen exchange rate in conditions where a higher electronic conductivity at the electrolyte surface would be created [11, 12]. For example, incorporation of transition metal cations into the surface layers of YSZ ceramics was shown to considerably decrease electrode polarisation [13, 14]. A similar effect was observed for 2% Pr-doped CGO, where the variable-valence Pr cations simulta- D. P. Fagg (&) Æ V. V. Kharton Æ J. R. Frade Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal E-mail: duncan@cv.ua.pt Tel.: +351-234-370263 Fax: +351-234-425300 J Solid State Electrochem (2004) 8: 618–625 DOI 10.1007/s10008-004-0509-x