CHIP INTEGRATED MICRO POWER SOURCE BASED ON A FUEL CELL
ACCUMULATOR
M. Frank1, G. Erdler2, H.-P. Frerichs2, C. Müller1, H. Reinecke1
1
University of Freiburg, Laboratory for Process Technology,
IMTEK-Department of Microsystems Engineering, Germany
2
Micronas GmbH, Freiburg, Germany
Abstract: The paper presents a new approach to realise a chip integrated rechargeable micro power
source. The system combines a self breathing fuel cell with an integrated hydrogen storage and an
electrolyser. Connecting the electrolyser to an external energy source, hydrogen will be generated and
directly absorbed within the hydrogen storage. Thus the metal hydride hydrogen storage is refilled. The
fuel cell of the chip integrated device converts the hydrogen incorporated within the storage to electric
energy. The realisation of this chip integrated accumulator opens up new possibilities to build
autonomous, energy self-sufficient microsystems. Recharging the system, e.g. by a chip integrated
Energy Harvesting Device, the accumulator stores electrical energy and provides microsystems with
electricity by the fuel cell.
Key Words: CMOS-compatible chip integrated fuel cell accumulator, rechargeable chip integrated micro
power source, self-breathing fuel cell, chip integrated electrolyser cell, metal hydride.
1. INTRODUCTION
Autonomous, energy self-sufficient microsystems [1, 2] require micro power sources with
high energy density. The chip integrated fuel cell
(Figure 1) complies with the demands of such
energy systems [3]. The completely passive, self
breathing device is integrated in a silicon
substrate, its typical lateral dimensions are less
than 10 mm; the thickness of the palladium
hydrogen storage is in the range of 100 µm and
the whole system’s thickness is less than 200 µm.
More than 50 % of the system’s volume consists
of hydrogen storage material. Therefore the
theoretical
volumetric
energy
density
(1.5 mWh/mm³) is significantly higher compared
to other wafer level batteries [4, 5] or
conventional battery systems. Operating the fuel
cell, hydrogen atoms incorporated within the
palladium hydrogen storage are split up into
protons and electrons at the interface of the metal
hydrogen storage and the membrane electrode
assembly (MEA). Electrons pass through the
electric circuit driving an external load. Protons
are being conducted through the MEA and react at
the fuel cell’s cathode with electrons and ambient
oxygen producing water. The working principle of
the chip integrated fuel cell is similar to that of
primary battery systems. Thus the chip integrated
fuel cell can not be recharged and the entire
microsystem that is powered by it has to be
replaced after the hydrogen’s emptying.
Figure 1 – Principle set-up of the chip integrated fuel
cell [3]
An electrolyser cell is an electrochemical
element that inverts the chemical reaction of a fuel
cell. The fuel cell requires hydrogen and oxygen
to generate electrical energy and water whereas
the electrolyser cell utilises water and electrical
energy to generate oxygen and hydrogen. By
attaching an electrolyser cell to the fuel cell with
integrated hydrogen storage, a rechargeable fuel
cell a so called fuel cell accumulator is realised.
The micro power source’s principle set-up is
shown in Figure 2.
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Figure 2 - Principle set-up of the fuel cell accumulator
2. EXPERIMENTAL
2.1 The macroscopic fuel cell accumulator
In order to verify the function of the device and
to analyse the specific components a macroscopic
set-up was fabricated. If possible, for the
macroscopic and the later on realised chip
integrated set-up, identical materials were used to
assure the transferability of the gained experiences
and results. These components of the macroscopic
fuel cell accumulator, anode of the electrolyser
cell with integrated electrolyte reservoir (1),
aluminium oxide isolator (2), palladium hydrogen
storage (3) as well as polymer electrolyte
membrane (4) and carbon flowfield with platinum
particles (5) are illustrated (Figure 3). The real
components used for the assembly of the
macroscopic fuel cell accumulator are also
displayed (Figure 4).
The anode of the electrolyser cell was
manufactured by CNC milling out of a 2 mm
thick graphite plate. Furthermore a cavity to store
electrolyte was milled into the anode. The
palladium hydrogen storage was fabricated by
wire-cut EDM. For the electrical isolation of
graphite anode and palladium cathode of the
electrolyser cell a porous aluminium oxide plate
was inserted between them. The electrolyser cell’s
assembly was carried out using a polymer blend
solution; properties of the cured polymer are good
adhesion to the palladium surface and high proton
conductivity. The polymer solution was dispensed
onto the components and then they were stuck
together by thermally curing the polymer solution.
Afterwards the fabrication of the fuel cell was
carried out. The identical polymer solution
already utilised for the set-up of the electrolyser
cell was dispensed onto the fuel cell side of the
palladium hydrogen storage. By thermal curing of
the dispensed solution the fuel cell’s polymer
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Figure 3 - Schematic exploded view of the macroscopic
fuel cell accumulator
electrolyte membrane was formed. On top of the
membrane the carbon flow field, also fabricated
by CNC milling and catalytically activated by
spray coated platinum particles, was mounted.
Thus the assembly of fuel cell was accomplished.
Finally the set-up of the macroscopic fuel cell
accumulator was completed by filling up the
cavity of the electrolyser cell with 0.1 Molar
sulphuric acid.
20 mm
Figure 4 - The macroscopic fuel cell accumulator’s
components
The macroscopic fuel cell accumulator was
charged by a constant current of 2.5 mA for eight
hours. The generated charge was 20 As, equal to a
capacity of 20 mAh. Afterwards the fuel cell was
operated with a load of 330 ȍ. A runtime of 20
hours could be achieved. The reclaimed charge
was calculated by integrating the current flow
over time. A charge of 46.8 As could be
reclaimed, correlating to a capacity of 13 mAh.
Therefore a macroscopic laboratory prototype fuel
cell accumulator’s over-all charge efficiency of
65 % could be reproducibly achieved. The proof
of principle could be verified by the successful
set-up of a rechargeable micro power source
based on a fuel cell accumulator.
2.2 The laboratory prototype chip integrated
fuel cell accumulator
The promising results obtained by the
characterisation of the macroscopic set-up
encouraged the realisation of a chip integrated
fuel cell accumulator. The schematic set-up is
displayed in Figure 5. The device’s design was
based on the set-up of the chip integrated fuel cell.
Simplifying and accelerating the fabrication
process solely clean room compatible physical
vapour deposition (PVD) processes were applied
for the fabrication of the metallic layers.
Additionally the integration of hydrogen diffusion
barriers was omitted. Drawback of the simplified
set-up is decreased hydrogen storage capacity by
two orders of magnitude since the thickness of the
vapour deposited palladium layer is limited to
1 µm. The palladium hydrogen storage, the
conducting gold paths to contact the palladium
and the electrolyser cell anode were fabricated by
physical vapour deposition processes and
structured by lift-off technique. Following the
fluidic structures were processed by SU-8
lithography. These structures conduct the
electrolyte from the reservoir to the electrodes oft
the electrolyser cell by capillary channels. The
clean room processing steps were finalised by
dispensing the polymer electrolyte membrane
onto the surface of the palladium hydrogen
storage. The prototype is displayed in Figure 6
(left), the dimensions of the chip are
19×19×0.5 mm³.
The fabrication of the accumulator was
finalised by laboratory assembly steps. The CNCmilled electrolyte reservoir chip was attached on
top of the fluidic SU-8 structures. Afterwards the
fuel cell was set up by mounting the CNC-milled
flowfield with spray coated catalyst particles on
top of the polymer electrolyte membrane.
Completing the laboratory prototype’s set-up
0.1 Molar sulphuric acid was poured in the
electrolyte reservoir. The completely assembled
device is shown in Figure 6 (right).
10 mm
Figure 6 – left: clean room processed accumulator
chip; right: completed set-up of the laboratory
prototype chip integrated fuel cell accumulator
The laboratory prototype fuel cell accumulator
was charged by a constant current of 30 µA.
Varying the charge times (between 30 minutes
and 4 hours) the open circuit voltage graph
showed a reproducible behaviour, during the
charge process the fuel cell’s open circuit voltage
raised to a level above 800 mV. Finishing the
charge process the voltage declined to a level
above 500 mV. A runtime up to 8 hours could be
achieved even though the storage’s volume is only
16×10-3 mm³ and the omission of hydrogen
diffusion barriers. The chip integrated fuel cell
accumulator’s voltage graph is displayed in
Figure 7. Multiple charge and discharge cycles of
Figure 5 - Principle set-up of the chip integrated fuel cell accumulator
175
the device could be successfully realised.
Furthermore no degradation of the device’s
performance was detected during the tests.
Energy Harvesting
Device
Sensor-actuatorUnit
Fuel cell accumulator
µ-Controller
TransmitterUnit
Figure 8 - Vision of a chip integrated energy selfsufficient microsystem powered by the chip integrated
fuel cell accumulator as “key component”
Figure 7 - Charge- and discharge open circuit voltage
graph of the chip integrated fuel cell accumulator
laboratory prototype
3. DISCUSSION
The evaluation of the macroscopic set-up and
the chip integrated fuel cell accumulator
laboratory prototype showed up promising results.
The hydrogen storage capacity of the laboratory
prototype accumulator is only 1 % of the chip
integrated fuel cell’s storage capacity. Therefore
the achieved runtime of 8 hours is a promising
result, the implementation of a thick film
palladium hydrogen storage and of hydrogen
diffusion barriers has the potential to increase the
device’s runtime by orders of magnitude.
4. CONCLUSION & OUTLOOK
The proof of concept of a novel kind of micro
power source was provided. The carried out
electrical characterisation of the chip integrated
fuel cell’s laboratory prototype showed up
promising results. The next steps will be the setup, optimisation and characterisation of a second
generation micro power source. Therefore a
distinct improvement of runtime and electrical
performance of the device is expected.
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The coupling of the chip integrated fuel cell
accumulator with an Energy Harvesting Device
creates a self-recharging micro power source with
high energy density and long lifetime. In the
future the realisation of such a micro power
source will be opening up the possibility of
realising autonomous, energy self-sufficient
microsystems. Figure 8 displays the principle setup of this vision.
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