n‐Type Conductivity in Gadolinia‐Doped Ceria

Abstract

The ionic and n-type conductivities of ceria-based electrolytes doped with gadolinium (10 and 20 cation %) or gadolinium and praseodymium (2% Pr and 18% Gd) were studied by impedance spectroscopy in air, between 573 and 1273 K, and by constant frequency (10 kllz) conductivity measurements as a function of the oxygen partial pressure (p02) between about 1020 atm and air in the temperature range 1073 to 1273 K. Ion blocking measurements were also performed between 1073 and 1273 K with air at the reversible electrode. From data obtained with different experimental techniques it was concluded that estimates for the electronic conductivity of all compositions were consistent and that all materials behaved in a rather similar manner. Estimated n-type conductivities suggest that Pr-doped materials have a slightly lower electronic conductivity and a larger electrolytic (and ionic) domain, but overall differences are quite small. The model behavior used in analyzing experimental results is discussed based on the existing knowledge of the defect chemistry of ceria-based electrolytes, and a range of working conditions is identified where the ionic conductivity can be assumed constant and the electronic conductivity proportional to p'4.

Propiedades de transporte de cerámicas Bicuvox dopadas

“…Experimental procedures for the characterisation, including determination of the oxygen-ion transference numbers by the e.m.f. method, have been reported elsewhere (6,(9)(10)(11)(12)(13)(14). The spectroscopic analysis showed no deviations in the cation composition of the ceramics with respect to their chemical formulas within the experimental error limits; the total concentration of impurities was found to be less than 0.01 at%.…”

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

“…the last relation assumes that electrons resulting from oxygen exchange are localized as trivalent cerium ions Ce 3+ , (Panhans & Blumenthal, 1993;Steele, 2000;Mogensen, 2000) producing small polarons (Tuller & Nowick, 1977;Panhans & Blumenthal, 1993;Navarro et al, 1997). If we assume that there is no interaction between defects, the mass action law of Eq.…”

“…As noted by Navarro et al (Navarro et al, 1997) the hopping conductivity should be proportional to [Ce Ce '][Ce Ce x ], which describes the probability of the small polaron to have a Ce 4+ as neighbour, and thus to provide charge transport. However, the mole fraction [Ce Ce x ] is usually included in the mobility term.…”

“…The HebbWagner procedure is a suitable technique based on blocking the transport of oxygen ions through the membrane submitted to a gradient of oxygen partial pressure (Hebb, 1952;Wagner, 1957;Navarro et al 1997;Lübke & Wiemhoefer, 1999). For this purpose, symmetrical Pt-electrodes are placed on both surfaces of a gas tight ceria sample, and an alumina impervious disk is then sealed on one of the electrodes, avoiding oxygen leakage to the internal chamber (Fig.…”

“…However, the decrease in the oxygen partial pressure exerted by the presence of the fuel increases the non-stoichiometry accompanied by the reduction of Ce 4+ to Ce 3+ . 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.…”