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 micro-systems [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

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 . 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

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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 CNC-milled 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

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the device could be successfully realised. Furthermore no degradation of the device’s performance was detected during the tests.

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 set-up, optimisation and characterisation of a second generation micro power source. Therefore a distinct improvement of runtime and electrical performance of the device is expected.

Fuel cell accumulator

Transmitter-Unit

Sensor-actuator-Unit

µ-Controller

Energy HarvestingDevice

Figure 8 - Vision of a chip integrated energy self-sufficient microsystem powered by the chip integrated fuel cell accumulator as “key component”

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 set-up of this vision.

REFERENCES

[1] B. Warneke, in: M. Ilyas and I. Mahgoub (Eds.): Smart Dust: Sensor Network Applications, Architecture and Design, Miniaturizing sensor networks with MEMS.5-1 – 5-19 CRC Press (2006).

[2] M. J. Wolf, R. Schacht, H. Reichl, The “e-Grain” Concept, Building Blocks for Self-Sufficient Distributed Microsystems, Frequenz, 58 (2004).

[3] Erdler et al.: Chip integrated fuel cell, Sensors and Actuators A 132 (2006) 331-336.

[3] B. J. Neudecker, N. J. Dudney, and J. B. Bates, “Lithium-Free” Thin-Film Battery with In Situ Plated Li Anode, J.Electrochem. Soc. Volume 147 Issue 2 517-523 (February 2000).

[4] N. J. Dudney, Solid-state thin-film rechargeable batteries, Materials Science and Engineering B 116 (2005).

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