PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00089/1496
Ultrathin Solid Oxide Fuel Cell Fabrication
Peter M. Martin, Dean W. Matson, Kerry Meinhardt
The development of ultrathin electrolyte layers for use in solid oxide fuel cells may allow both a significant reduction in
operating temperature and improvement in power output compared with cells produced using conventional ceramic
processing methods.
Project Description
The purpose of this project was to demonstrate the
production and application of a vapor-deposited ultrathin
electrolyte layer for solid oxide fuel cells. Ultrathin
electrolyte layers are predicted to allow operation of solid
oxide fuel cell units at lower temperatures than are
possible using the thicker elecrolyte layers produced
using conventional ceramic processed technology. Thin
(1 to10 micrometer) yttria-stabilized zirconia (YSZ)
coatings were deposited onto nickel-zirconia cermet
electrodes using reactive sputtering from metal alloy
targets. We have shown that, despite irregular surfaces on
the underlying cermet substrates, dense, defect-free yttriastabilized
zirconia electrolyte layers can be produced.
The microstructure and crystallographic phase of the
yttria-stabilized zirconia electrolyte were related to the
sputtering parameters used to produce the coatings.
Introduction
Solid oxide fuel cells provide a highly efficient means to
convert fossil fuels to electricity. This is produced by
migration of oxygen ions from a porous airside electrode
through a solid gas impermeable electrolyte to a porous
fuel-side electrode. The maximum power of these
devices is limited by the resistance of the electrolyte to
oxygen ion permeation and electrode/electrolyte
interfacial reaction kinetics. In most solid oxide fuel cell
designs, the electrolyte layer ranges in thickness from 40
to 200 microns. The maximum power is less than
400 milliwatts per square centimeter when operated at
1000°C. Recent work has shown that solid oxide fuel
cells made with thin electrolyte layers (~10 microns) that
were produced using colloidal processing methods may
allow both a reduction in operating temperature and an
increase in power to greater than 1 watt per square
centimeter. Further reduction in the thickness of the
electrolyte layers that were produced using colloidal
processes is considered impractical without introducing
pinholes and other defects detrimental to solid oxide fuel
cells operation. Vapor deposition methods available at
330 FY 2000 Laboratory Directed Research and Development Annual Report
PNNL allow the production of very thin (micrometer
scale) electrolyte layers that will permit solid oxide fuel
cells operating temperatures to be lowered by hundreds of
degrees while maintaining high power outputs.
Approach
This project focused on the production of thin yttriastabilized
zirconia electrolyte layers using reactive
magnetron sputtering. The process involves removal of
atomic species from an yttrium/zirconium metal alloy
target by bombarding it with ionized argon atoms in the
presence of a background oxygen concentration. The
oxygen atoms combine with the atomized metal to
produce oxide coatings on substrates placed in the path of
the sputtered material. To produce yttria-stabilized
zirconia that is stable at temperatures required to operate
fuel cells, the yttria-stabilized zirconia deposit was made
with an yttrium doping at an elevated temperature. The
yttria-stabilized zirconia thin film electrodes formed in
this study were produced from yttrium/zirconium targets
containing either 7 or 12 atom percent yttrium and with
the substrates heated to temperatures of up to 600°C
during the deposits.
The substrates used for the yttria-stabilized zirconia
electrolyte deposit were thin wafers of a porous
nickel/zirconia cermet that was produced using
conventional tape casting methods. The cermet layer
acted both as a support for the yttria-stabilized zirconia
electrolyte layer and as the fuel-side electrode for the fuel
cell. The anodes were pre-sintered in air at 1350°C for
1 hour. The porous air-side electrode of strontium-doped
lanthanum manganite was applied to the exposed face of
the yttria-stabilized zirconia electrolyte using a
conventional ceramic screen-printing process after the
electrolyte deposition was completed. The cathode was
sintered on at 1200°C. A platinum grid was screen
printed and sintered on each electrode as a current
collector. Platinum meshes with attached current and
voltage leads were sintered to the platinum current
collectors.