Mesoporous Nickel−Yttria−Zirconia Fuel Cell Materials

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A novel synthesis method for mesoporous nickel−yttria−zirconia (meso-Ni−YZ) has been developed using a surfactant-templated co-assembly strategy, involving yttrium(III)−zirconium(IV) glycometalate and nickel(II) precursors. Nickel loading has been investigated over the range 0−45 Ni at. % and found to control the average pore size of the oxidized meso-NiO1+x−YZ and reduced meso-Ni−YZ products. Under the synthesis conditions utilized, the cationic surfactant glycometalate co-assembly is proposed to involve nickel in the form of Ni4(OH)44+, which subsequently transforms to a co-assembly containing nanoscale Ni(OH)2. Upon thermal posttreatment in air, the nickel hydroxide species is converted to the nanocrystalline p-type nickel oxide, NiO1+x, located in the pores of meso-NiO1+x−YZ. The channel walls of meso-NiO1+x−YZ are composed of nanocrystalline yttria−zirconia, and the gas adsorption isotherm appears as type I. Thermal reduction of the NiO1+x component of the mesostructure in hydrogen creates nickel nanocrystals. A concomitant dramatic increase in average pore diameter is observed with concurrent evolution of an adsorption isotherm of type IV. This behavior is attributed to a structural transformation of meso-NiO1+x−YZ to meso-Ni−YZ involving a reduction in NiO1+x nanocrystal size on conversion to Ni with accompanying migration and aggregation of Ni nanocrystals. The outcome of these combined effects widens the pores within meso-Ni−YZ relative to meso-NiO1+x−YZ. Preliminary ac impedance spectroscopy of meso-Ni−YZ at a loading of less than 40 at. % Ni depicts an electrical response dominated by oxygen ion conductive nanocrystalline YZ channel walls of the meso-Ni−YZ mesostructure. By contrast, higher nickel loaded meso-Ni−YZ samples give a metallic response most likely due to a better connectivity of Ni nanocrystals within the mesostructure.