The Effect of Transition Metal Dopants on the Sintering and Electrical Properties of Cerium Gadolinium Oxide

摘要

Cerium gadolinium oxide (Ce_(0.9)_(0.9)Gd_(0.1)O_(1.95), CGO) is a promising candidate for use as an electrolyte material in intermediate temperature solid oxide fuel cells. Within this operating temperature range, CGO has shown some of the highest reported ionic conductivity values. One disadvantage of using CGO relates to its relatively poor densification behavior at lower sintering temperatures. The introduction of certain transition metal oxide (TMO) sintering aids has previously been reported to improve the densification behavior of CGO without having a deleterious effect on the conductivity. In particular, low concentrations of cobalt oxide (1-2 cat%) have been shown to be effective. The recent impetus to reduce the operating temperature to 500-700°C for small scale power generation has enabled the use of cheaper stainless steel interconnects, which share a similar thermal expansion coefficient to CGO and metal-supported electrolyte cells. It is however likely that the use of stainless steel supports and interconnects will lead to elements from the steel (in particular Cr) entering the electrolyte during manufacture, which will effectively lead to multiple doping of the electrolyte. In the current work the effects of low level TMO doping (Co and Cr) on the densification and electrical properties of CGO were analyzed singularly and in combination using dilatometry and AC impedance spectroscopy. The experiments show that Co promotes densification whilst Cr has a strong retarding effect. When both Co and Cr are present the Co nullifies the inhibiting effect of Cr. Neither of the TMOs has a detectable influence on the lattice ionic conductivity; although Co was shown to increase the grain boundary conductivity at low temperatures whilst Cr was shown to reduce it. In the case of Cr, the reduction is particularly severe and is apparent even at low concentrations.