MICRO SOLID OXIDE FUEL CELL IN SILICON TECHNOLOGY WITH
NICKEL GRID FOR ELECTROLYTE REINFORCEMENT
S. Rey-Mermet, Y.Yan, G. Deng, and P. Muralt
Ceramics Laboratory, Swiss Federal Institute of Technology EPFL, Lausanne, Switzerland
Abstract: We report on a study to realize solid oxide fuel cells as micro power generators burning hydrogen or
eventually butane. The sputter deposited electrolyte layer of yttria stabilized zirconia (YSZ) was 200 to 400 nm
thick. The membrane was supported by a nickel grid that was fabricated by electrochemical deposition. Open
circuit voltages (OCV) were observed above 380 °C. Cells with a diameter of 0.5 mm showed an OCV of 850 mV
at 550 °C in an H2/Ar fuel mixture.
Keywords: micro solid oxide fuel cell, yttria, thin films, nickel
INTRODUCTION
temperature. Formally, these two requirements go
well together, if the electrolyte layer is realized with a
thin film in the 100 to 1’000 nm range. In practice,
there are many open questions related to material and
microstructure of the electrolyte film, the mechanical
stability, and the choice of electrodes for optimal gas
exchange, ionic and electronic transport. The
packaging and control of the cells is a further
challenge in order to achieve a safe operation and a
low housing temperature [3]. Various works on single
cells or cell arrays have been published since 1999 [46]. One of the issues we worked on since 2005 was
the stabilization of the brittle ceramic membrane made
of yttria stabilized zirconia (YSZ) and (Ce,Gd)O2 by
means of a nickel grid . The goal of the present paper
is to report on the status of this work [7, 8].
Fuel cells have a number of advantages over other
transportable power generators. They are inherently
more efficient than combustion engines such as Otto
motors, diesel motors, or gas turbines (see, e.g. [1, 2]).
The reason is that chemical energy is directly
transformed to electricity, and not through an
intermediate transformation step from thermal to
mechanical energy. The high energy content of
organic fuels, combined with the fact that fuel cells do
not contain substantial portions of heavy metals, leads
to the conclusion that fuel cells are very attractive
power sources for portable applications. Especially
the solid oxide fuel cell transforming hydrocarbons
such as propane and butane look very advantageous in
terms of the energy density (Fig. 1).
Fig. 2: The nickel grid concept: Schematic structure
of the cell with the positive-electrolyte-negative (PEN)
membrane structure.
Fig. 1: Energy densities of portable power sources
including batteries and micro fuel cells in
comparison. The µ-SOFC concept assumes burning of
hydrocarbons such as butane.
MATERIALS AND FABRICATION
Most of the published works concentrate on yttria
stabilized zirconia (8 mol. % Y2O3, i.e, 8YSZ) as
electrolyte material, which indeed shows a good ionic
conduction at high temperatures [9] and therefore is
The challenge for portable SOFC’s is the
downscaling of dimensions, and the reduction of
0-9743611-5-1/PMEMS2009/$20©2009TRF
490
PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
the standard choice for conventional solid oxide fuel
cells [10, 11]. In conventional SOFCs operating at
around 1000°C, the electrolyte is about 100 µm thick
[12] and exhibits a homogenous morphology of
randomly oriented, large grain. Micro-SOFCs should
work at the lowest possible temperatures and thus
require much thinner electrolyte membranes [13],
because the ionic conductivity (σ) is thermally
activated
(activation
energy
Ea),
i.e.
.
For
operation
at
500°C,
σ = ( A / kT ) ⋅ exp(−Ea / kT )
the µSOFCs electrolyte should be thinner than about 1
µm [14]. In our work we focus on sputter deposited
YSZ. In the past work, the best open circuit voltage
(OCV) we achieved was 280 mV with a single layer
YSZ thin film [7], thus much less than the expected 1
V. Sputtered films exhibit one obvious difference to a
classical ceramics: the grains are oriented (YSZ
nucleates with the (111) plane parallel to the substrate
plane), and they reach from the bottom to the top of
the film (columnar growth). It is thus possible that
such a microstructure is unfavorable. It could be that
the grain boundaries allow for some electronic
conduction, leading to an internal leakage of the cell.
The grain boundaries are possibly reduced by
hydrogen penetrating from the anode side. Prior
anneal in oxygen did not help. We investigated the
possibility to change the microstructure of the film,
and especially to interrupt grain boundaries going
through the whole membrane. The target is to keep at
least in part a dense and strong membrane layer, and
to add a film with different or no texture of smaller
grains, leading to an elongation of grain boundary
paths. In addition, the mechanical stress of the
membrane must be small enough in order to avoid
cracking during temperature cycling [15].
The
employed thin film deposition technique was reactive
RF sputtering from an alloyed metal target, a
technique allowing for a later scale up to industrial
fabrication.
Using different substrate bias voltages, it was
possible to turn the orientation of the YSZ film: A
dense film of (111) texture was obtained at low RF
bias voltage and higher deposition temperature, and a
rather porous (200) orientation was obtained at lower
temperature and higher substrate RF bias power (see
Transmission Electron Microscopy (TEM) image in
Fig. 3). Such an effect of the RF bias was earlier
reported by Rudell et al. [16].
Our µSOFC fabrication was based on double side
polished (100) silicon wafers. In a first step, the
wafers were wet oxidized to obtain a 1.5 µm thick
layer of silicon dioxide. It was patterned by dry
etching on the back side of the wafer to serve later as
a mask for the liberation of the membranes by deep
silicon etching. On the front side, the oxide layer was
thinned down to 200 nm in a buffered HF bath. This
layer served as electrical insulation from the Si wafer,
to avoid conduction through the latter. A platinum
layer of 50 nm was deposited by sputtering on the
front side serving as current collector for the cathode.
This layer was dry etched through a photoresist mask
in a chlorine atmosphere. Then the YSZ electrolyte
was deposited through a metallic hard mask covering
the contact area to the cathode. A platinum layer of
100 nm is deposited by sputtering on the YSZ. This
layer was patterned through a photoresist mask, and
served subsequently as seed layer for the nickel
electroplating. The nickel grid was deposited in a
photoresist mould from a commercial nickel speed
bath with the following composition: nickel sulfamate
600 g/l, nickel chloride 10 g/l, boric acid 40 g/l and
additives. The grid height was comprised between 4
and 6 µm with a linewidth of 10 µm. Finally the
silicon wafer was dry etched through the backside
oxide mask. The insulating oxide layer remaining at
the bottom of the holes was dry etched afterwards.
Finally porous Pt electrodes were deposited on both
sides to serve as anode and cathode in between the
grid lines.
Fig: 3. TEM cross-section of double layer YSZ
electrolyte membrane. Lower layer is 200 nm thick.
Fig. 4: Top view of realized 0.5 mm cell with nickel grid.
491
kB T ⎛ pc (O2 ) ⎞
ln⎜
(1)
⎟
ne ⎝ pa (O2 ) ⎠
Oxygen ions carry the charge 2e (n=2). A partial
oxygen pressure 10-4 atm is derived for the anode side.
Without fuel no current can be delivered. The
observation of an OCV in pure Ar means that eventual
leakage currents are small.
When supplying hydrogen in addition to argon,
the OCV went up to 850 mV. Figure 6 shows OCV,
cell and gas temperature as function of time during
various manipulations of hydrogen supply and
resistive load. The heating by hydrogen oxidation
makes operation unsteady. Temperature fluctuations
and inhomogeneities couple back to the ionic
conductivity of the membrane, and the heat dissipated
locally. Nevertheless, a more or less stable operation
was achieved once the “burn-in” phenomena (like Pt
dewetting) were passed (around 2500s in the Fig. 6).
At events A and B, the resistive load was lowered to
achieve higher currents. Unfortunately, the collected
current was too small. The connections to the cathode
had a too large resistance.
RESULTS
OCV =
800
A
B
600
500
400
400
300
200
OCV
TFurnace
TGas
200
100
0
A,B-Measure I-V Curve
0
1000
2000
3000
4000
0
5000
Time (s)
Fig: 6: OCV (lower curve) and cell temperature
(upper curves) as a function of time with H2/Ar fuel
during manipulation of fuel flow and external
impedance.
CONCLUSIONS
Fig. 5: Open circuit voltage (OCV) measured with
argon at anode side, as a function of temperature. The
value corresponds to an oxygen pressure of about 10-4
atm.
8YSZ thin films have been deposited as double
layer with different morphologies to prevent grain
boundary and pinhole continuity through the film. The
different morphologies have been obtained by
changing the temperature and the substrate bias during
sputtering. The (111) layer deposited at 450°C and
small bias shows a columnar, dense microstructure.
The (200) layer deposited with a large RF bias at
room temperature exhibits nano scale grains with
substantial porosity. The porous layer remains
The cell was first characterized supplying pure Ar
onto the anode side. A rising open circuit voltage was
observed above 360 °C (Fig. 5). The OCV can be
derived from the Nernst equation that is valid for no
net ionic flux:
492
o
OCV (mV)
600
Temperature C)
(
The electrical impedance of the multilayered YSZ
films was measured as a function of temperature. The
conductivity increases with the temperature as
expected for ionic conductors. The activation energy
for ionic conduction was derived as 1.04 eV. The
value of conductivity at 500° was obtained as 0.02
S/m. These values are somewhat lower that in bulk
and thick YSZ films for which respective values of
1.18 eV and 0.1 S/m, and 1.16 eV and 0.05 S/m are
reported at 500 °C [17]. With the assumption that the
area-specific resistance (ASR) of the electrolyte is
simply the ratio of its thickness h divided by its
conductivity [18] (i.e. with the dimensions of the
electrodes grains smaller than h), the thickness must
be smaller than 300 nm to have an ASR smaller than
the critical value of 0.15Wcm2 proposed as upper
limit in ref. [14].
The obtained double layer was integrated into a
micromachined cell of 0.5 mm diameter as shown in
Fig. 4, and following the process flow described
above. The transformation of this film was assessed
after operation of the cell at 500 °C in diluted
hydrogen at the anode side. The porous Pt film broke
up into small islands and meandering lines. Pt on YSZ
is obviously dewetting. YSZ (200) kept the porous
microstructure, meaning that no recrystallization of
YSZ is going on at this temperature.
Power Source 173 325-345
[14] Brandon N P, Skinner S, and Steele B C H 2003
Recent advances in materials for fuel cells Annual
Review of Material Research 33 183-213
[15] Tang Y, Stanley K, Wu J, et al. 2005 Design
consideration of micro thin film solid-oxide fuel cells
Journal of Micromechanics and Microengineering 15
S185-S192
[16] Ruddell D E, Stoner B R, and Thomspon J Y 2002
Effect of deposition interruption and substrate bias on
the structure of sputter-deposited yttria-stabilized
zirconia thin films Journal of Vacuum science and
technology A 20 1744-1748
[17] Kwon O J, Hwang S-M, Ahn J-G, and Kim J J 2006
Silicon-based miniaturized-reformer for portable fuel
cell applications Journal of Power Sources 156 253259
[18] Fleig J, Tuller H L, and Maier J 2004 Electrodes and
electrolytes in micro-SOFCs: a discussion of
geometrical constraints Solid State Ionics 174 261-270
structurally stable at operation temperature, and was
tested as interlayer to a porous Pt anode film.
Together with the dense (111) layer the membrane
withstood 500 °C when hydrogen was supplied to the
anode. Further works are needed to improve the
cathode contacts, which are in our design on the same
side as the anode contact. Moreover, more studies are
needed to find optimal electrodes.
ACKNOWLEDGEMENTS
This work was supported by the Swiss
Commission for Innovation and Technology, the
Swiss Center of Competence for Energy and Mobility,
and Swisselectric.
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