|For solid-oxide fuel cell applications, high operating temperatures in excess of 700°C are necessary in order to attain sufficient oxygen-ion conductivity and reduce series resistance in the membrane in order to achieve reasonable power density. Further reducing the membrane thickness would possibly enable the operating temperature to be reduced, thereby relaxing the requirements for thermal insulation and management. Tsuchiya, et. al., describe a scalable method to integrate an ultra-thin SOFC membrane structure onto a silicon wafer support.|
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
- Tsuchiya M, Lai BK, and Ramanathan S. 2011. Scalable Nanostructured Membranes for Solid-oxide Fuel Cells. Nature Nanotechnology Online publication 3 April 2011. doi:10.1038/NNANO.2011.43
Oxygen-ion conducting membranes find applications for a range of energy and environmental technologies including solid-oxide fuel cells (SOFCs), and electrolyzers for NOx reduction, oxygen separation, and exhaust gas sensors. Typical solid-oxide membrane synthesis routes utilize ceramic powder processing or vacuum coating techniques resulting in membrane thicknesses ranging from micron to millimeter scale. For SOFC applications, high operating temperatures in excess of 700°C are necessary in order to attain sufficient oxygen-ion conductivity and reduce series resistance in the membrane in order to achieve reasonable power density.
Further reducing the membrane thickness would possibly enable the operating temperature to be reduced, thereby relaxing the requirements for thermal insulation and management. Lower operating temperature would ultimately allow for a more compact stack configuration, as well as provide a SOFC based portable power source. In this capacity, a microscale SOFC (µSOFC) offers advantages over alternative power sources due to it’s high conversion efficiency, and superior specific power and energy properties. The challenge is in establishing a highly integrated approach to fuel cell system design.
Recently, Tsuchiya et. al., described a scalable method to integrate an ultra-thin SOFC membrane structure onto a silicon wafer support. Utilizing radio frequency (RF) sputter deposition techniques, the authors coated a 100 nm thick film of yttria-stabilized zirconia (YSZ) electrolyte at 550°C onto a silicon wafer passivated with silicon nitride. The YSZ film is then coated with a layer of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) for the fuel cell cathode. To form the free-standing SOFC membrane, the silicon substrate is patterned and etched from the backside using potassium hydroxide (KOH) exposing the silicon nitride layer on the top-side of the substrate. After removing the silicon nitride layer exposing the YSZ film from the backside, a porous film of platinum was deposited by D.C. sputtering to form the anode. In order to scale the thin-film SOFC membrane, the issue of mechanical instability during subsequent thermal cycling must be addressed. Since the typical relationship of stress to failure in thin films scales as film thickness to membrane radius, significant challenges are presented in scaling the SOFC membrane to active areas in the many tens of square millimeter range. To address this, the authors patterned a metal grid on the LCSF using a liftoff technique. The metal grid, formed from silver or platinum, was 1.5 µm thick, with 100 µm spacing’s exposing the SOFC cathode. The overall area utilization for the integrated fuel cell structure was >50%. The metal grid provided the necessary mechanical support to prevent failure of the thin-film SOFC structure during operation.
Testing of the integrated SOFC demonstrated 150 mW/cm2 at 510°C for a 5 mm x 5 mm membrane area. The free-standing SOFC membrane exhibited compressive stress at room temperature which relaxed after the first thermal cycle, and remained in tensile stress during subsequent thermal cycles. The ultra-thin membrane further exhibited no failure or cracking during multiple thermal cycles. Thus a scalable approach has been demonstrated for integration of thin-film membranes over many tens of square millimeters. The approach resolves critical performance issues for subsequent thermal cycling and stability at moderate to high temperature operation.
Images reproduced from Tsuchiya M,et.al., Scalable Nanostructured Membranes for Solid-oxide Fuel Cells. Nature Nanotechnology Online publication 3 April 2011. doi:10.1038/NNANO.2011.43. Permission pending.