Microstructure and Microfabrication Considerations for Self‐Supported On‐Chip Ultra‐Thin Micro‐Solid Oxide Fuel Cell Membranes

Abstract

La0.6Sr0.4Co0.8Fe0.2O3 – δ (LSCF) has been sputtered on bare Si and Si3N4 and yttria‐stabilised zirconia (YSZ) thin films to investigate annealing temperature‐ and thickness‐dependent microstructure and functional properties, as well as their implications for designing failure‐resistant micro‐solid oxide fuel cell (μSOFC) membranes. The LSCF thin films crystallise in the 400–450 °C range; however, after annealing in the 600–700 °C range, cracks are observed. The formation of cracks is also thickness‐dependent. High electrical conductivity, ∼520 Scm–1 at 600 °C, and low activation energy, ∼0.13 eV, in the 400–600 °C range, are still maintained for LSCF films as thin as 27 nm. Based on these studies, a strong correlation between microstructure and electrical conductivity has been observed and an annealing temperature‐thickness design space that is complementary to temperature‐stress design space has been proposed for designing reliable membranes using sputtered LSCF thin films. Microfabrication approaches that maintain the highest possible surface and interface quality of heterostructured membranes have been carefully examined. By taking advantage of the microstructure, microfabrication and geometrical structural considerations, we were able to successfully fabricate large‐area, self‐supported membranes. These results are also relevant to conventional or grid‐supported SOFC membranes using ultrathin nanocrystalline cathodes and μSOFCs using cathode thin films other than LSCF.

Nanoindentation of porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3−δ

“…Our recent study on crack formation in fuel cell electrode films [39] has revealed that rather than shrinkage during sintering, or differential contraction during cooling as reported in many studies [40,41], the critical factor for obtaining crack-free and smooth films in this study was the ability of the ink to be self-leveling in the earlier wet state. Cracking was initiated at the drying stage if the particles were prevented from packing more effectively as the liquid was removed.…”

Low temperature thin film solid oxide fuel cells with nanoporous ruthenium anodes for direct methane operation

“…Detailed fabrication design and procedures are discussed elsewhere. 37,38 For fuel cell measurements on methane, room temperature humidified pure methane gas was fed at a flow rate of 200 ml min À1 to the anodes of mSOFCs from the bottom of the chips. For hydrogen fuel cell measurements, room temperature humidified 5% hydrogen/Ar balanced gas was supplied at a flow rate of 100 ml min À1 .…”

Surface quality improvement of porous thin films suitable for nanoindentation

“…As can be seen in Table 2, the surface roughness for the films made from 1:2 ink was reduced to merely 10% that of films made from the as-receivedink.This is much smaller than the indentation depth, resulting in a steep reductionin the relative standard deviationofmeasured elastic modulus from 63.2% to 6.2%. The experiments above revealed that rather than shrinkage during sintering, or differential contraction during cooling as proposed in many studies [11,17], the more critical factor for obtaining crack-free and flat films in the current study was the ability of the ink to be self-leveling in the early wet state. Cracking is most likely to initiate at the drying stage if the particles are prevented from packing more effectively as the liquid content was removed.…”