Abstract
To be suitable as electrolytes in intermediate temperature solid oxide fuel cell (IT-SOFC), ceramic precursors have to be characterized by high sintering aptitude for producing fully densified products which are needed for this kind of application. Therefore, synthesis processes able to prepare highly reactive powders with low costs are noteworthy to be highlighted. It has been shown that amorphous coprecipitates based on cerium doped (and codoped) hydrated hydroxycarbonates can lead to synthesized ceramics with such desired characteristics. These materials can be prepared by adopting a simple coprecipitation technique using ammonium carbonate as precipitating agent. As a function of both the molar ratio between carbonate anions and total metallic cations, and the adopted mixing speed, the coprecipitate can be either amorphous, owning a very good morphology, or crystalline, owning worse morphology, packing aptitude, and sinterability. The amorphous powders, upon a mild calcination step, gave rise to the formation of stable solid solutions of fluorite-structured ceria maintaining the same morphology of the starting powders. Such calcined powders are excellent precursors for sintering ceramic electrolytes at low temperatures and with very high electrical conductivity in the intermediate temperature range (i.e., 500–700 °C). Therefore, irrespective of the actual composition of ceria-based systems, by providing an accurate control of both chemical conditions and physical parameters, the coprecipitation in the presence of ammonium carbonate can be considered as one of the most promising synthesis route in terms of cost/effectiveness to prepare excellent ceramic precursors for the next generation of IT-SOFC solid electrolytes.
Highlights
In the last few decades, ceria (CeO2 )-based ceramics have attracted much interest as functional materials in many applications such as chemical-mechanical polishing media, automobile exhaust catalysts and supports, dense ceramic membranes for oxygen separation, low temperature water-gas shift (WGS) reaction catalysts, oxygen sensors, etc. [1,2]
The readiest application for a real market stage exploitation of ceria-based ceramics is related to the production of solid electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFCs), due to their very high ionic conductivity
The reduction of SOFCs operating temperature is strongly required for efficient long-term operations and this reduction can be fulfilled by using a new class of ceramic electrolytes as substitute of the commonly used Y2 O3 -doped ZrO2 (YSZ) [5]
Summary
In the last few decades, ceria (CeO2 )-based ceramics have attracted much interest as functional materials in many applications such as chemical-mechanical polishing media, automobile exhaust catalysts and supports, dense ceramic membranes for oxygen separation, low temperature water-gas shift (WGS) reaction catalysts, oxygen sensors, etc. [1,2]. Ceria-based ceramics prepared by conventional solid-state synthesis are characterized by a poor sintering aptitude, requiring high temperatures, long dwell times, and low heating rates to produce sufficiently densified ceramic bodies [15]. Many synthesis routes fulfill this requirement and, among them, the most common ones are the hydrothermal treatment [18,19], the sol–gel route [20] and the coprecipitation method from aqueous solutions [4,21] Among those processes, the coprecipitation method is the simplest and cheapest one, even if powders prepared via coprecipitation from aqueous solutions are often characterized by high degrees of agglomeration causing the formation of hard aggregates during the calcination and sintering steps, impairing severely the final density of the electrolyte [22,23]. The either amorphous or crystalline nature of the precipitated powders can dramatically influence their morphology and, in turn, their forming aptitude and sinterability
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