Characterization of composite cermet with 68 wt.% NiO and BaCe0.2Zr0.6Y0.2O3−δ

Protonic ceramic fuel cells, steam electrolyzers, membrane reactors, and related intermediate temperature electrochemical devices require thin, dense membranes of protonic ceramic electrolytes that are supported by a porous substrate. A cofired cermet consisting of reduced 68 wt.% NiO and the proton conductor, BaCe0.2Zr0.6Y0.2O3−δ (68NiBCZY26) is described. It is shown that a composite of two distinct interpenetrating and percolating phases – Ni and BCZY26 – with about 25% open porosity, is produced from a fully dense sintered ceramic body. This is possible because the protonic ceramic phase can transport hydrogen to the NiO grains and remove the water vapor reaction product of reduction by solid-state diffusion, which makes it possible to begin reducing NiO grains even before gas percolation channels are fully formed. By eliminating the need for pore-formers in the cermet fabrication step, thin electrolyte membranes may be fabricated without the need to bridge open pores at the surface or develop pin-holes due to burn-out of pore-forming organic binders. Furthermore, the processing window for sintering is expanded, permitting the microstructure of the dense membrane to fully develop, since there is no requirement to limit the time at peak sintering temperature, that would otherwise cause the pores to collapse. The cermet described in this monograph was prepared by the process of solid-state reactive sintering, whereby the BCZY26 phase was formed during sintering from precursor oxides and carbonates. Since no preliminary calcination step is required, ceramic fabrication is very cost-effective. The subsequent in situ reduction of NiO by a combination of gas phase and solid-state diffusion results in an unusual nickel microstructure, and makes it possible to tailor electrical properties and pore size distribution for a wide range of specialized applications by adjusting the amount and grain size of NiO in the sintered body. The reduced cermet exhibits a bimodal pore diameter distribution, with one group centered at 0.5 μm and a second group centered at 7 nm.

► A cermet electrode is described that comprises a composite of NiO and the protonic ceramic, BaCe0.2Zr0.6Y0.2O3−δ (BCZY26). The ceramic is fired completely dense, facilitating application of thin, dense ceramic membranes.
► The unreduced composite ceramic consists of two interpenetrating phases NiO and BCZY26 – that each retain phase purity during sintering.
► The BCZY26 protonic ceramic phase is produced in situ by solid-state reactive sintering from precursor oxides and carbonates.
► The electrode becomes porous upon exposure to hydrogen atmosphere at moderate temperature (700 °C), which is made possible by solid-state diffusion of hydrogen to the encapsulated NiO grains and removal of water vapor by ambipolar diffusion in the protonic ceramic phase.
► The resulting porous structure has three percolating phases – BCZY26, Ni metal and porous channels. The structure is robust and the conductivity of the nickel phase approaches that of the pure metal.