PNNL-13501 - Pacific Northwest National Laboratory

Component Fabrication and Assembly for High Power Density Solid Oxide Fuel Cells
Study Control Number: PN00023/1430
Kerry Meinhardt, Jeff Stevenson, Steve Simner
This project demonstrated our capability to design and fabricate an entire planar solid oxide fuel cell sub-stack using
zirconia-based technology.
Project Description
The objective of this project was to develop the
capabilities to fabricate key ceramic components for a
high power density solid oxide fuel cell. The main goals
were to fabricate thin, anode-supported electrolytes and
both ceramic and metal interconnect plates and to
assemble these into operable solid oxide fuel cell stacks.
Components were designed to accommodate high-volume
manufacturing techniques.
Introduction
Solid oxide fuel cells have been of interest for over
30 years due to their potential for high efficiencies and
low environmental impact. Currently, solid oxide fuel
cell technology is expensive, fragile, and from the
perspective of manufacturing, impractical. There are
numerous challenges with respect to making this
technology cost-effective and attractive for wide
application. Some of these challenges include lower
temperature operation to incorporate metal interconnects
(at the expense of power density), optimized designs that
minimize the mechanical stress within the various
ceramic components of the fuel cell stack, the
development of simple and practical gasoline/diesel
reformers, and the incorporation of ceramic processing
methods, which could lower the cost and complexity of
fabrication.
The conventional cell designs, with yttria-stabilized
zirconia electrolyte thickness in the range 100 to 200 µm,
require an operating temperature of near 1000°C to keep
the electrolyte resistance in a practical range. Such hightemperature
operation precludes use of metallic
components, requiring fabrication of (heretofore)
expensive ceramic interconnect plates. Acceptable power
densities can be achieved at lower temperatures (< 800°C)
by either replacing yttria-stabilized zirconia with a more
conductive material, or by reducing the thickness of the
yttria-stabilized zirconia electrolyte to below 50 µm.
306 FY 2000 Laboratory Directed Research and Development Annual Report
Conductivity of strontium-substituted lanthanum gallate is
at least 3 times higher than yttria-stabilized zirconia at
800°C. This may allow use of thicker, self-supporting
strontium-substituted lanthanum gallate structures at
800°C. However, mechanical strength is inferior to yttriastabilized
zirconia and cost is higher. On the other hand,
as yttria-stabilized zirconia electrolyte thickness is
decreased, the electrolyte ceases to be self-supporting and
must be fabricated with a porous support layer, typically
the anode electrode. This substrate is the mechanical
support for the electrolyte allowing it to be handled and
processed without breaking. However, fabrication of the
anode-supported components is challenging; the
electrolyte and anode must be co-sintered, requiring close
matching of shrinkage curves. An additional
complication is that the porous support layer will leak fuel
gas if it extends to the edge of the stack. It must therefore
be constrained to the active cell area, by surrounding it
with a picture frame, which adds considerable fabrication
and sealing complexity. Worldwide, the number of
groups currently pursuing development of conventional,
high-temperature, yttria-stabilized zirconia/ceramic
interconnect technology is roughly equal to the number
developing less conventional designs.
Results and Accomplishments
One of the primary goals of this project was to build
capability in the area of solid oxide fuel cell fabrication.
Our primary goal was to identify and characterize
candidate cathode materials for lanthanum gallate-based
cells. Electrolyte supported cells were fabricated by
sintering screen printed electrodes onto sintered
lanthanum gallate membranes which had been fabricated
by tape casting. After sintering of the electrode materials,
the cells were tested using air on the cathode side and
97% H2 and 3% H2O on the anode side. Representative
results (at 700°C) are shown in Figure 1. Electrode
studies (both cathode and anode) will continue in the
future to determine the optimum compositions and
morphologies for stable, high performance at
temperatures in the 600 to 700°C range.