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Power Supply System Using On-Chip Polymer

Electrolyte Membrane (PEM) Fuel CellsMirko Frank, Matthias Kuhl, Gilbert Erdler, Ingo Freund, Yiannos Manoli, Claas Mller, and Holger

Reinecke

Abstract

A stabilized power supply realized by chip-integrated micro fuel cells

within an extended CMOS process is presented in this paper. The fuel cell system

delivers a maximum power output of 450 W/cm. The electronic control

circuitry consists of an LDO, an on-chip oscillator and a programmable timing

network. The core system consumes an average power of 620 nW. The system

reaches a current efficiency of up to 92% and provides a constant output voltage of

3.3 V.

Introduction

Macro-scale energy harvesting technologies in the form of windmills,

watermillsand passive solar power systems have been around for centuries. Now,as designersseek to cut the cords, they turn to microenergy harvesting systems thatcan scavenge milliwatts from solar, vibrational, thermal and biological sources.

However, understanding ultra-low power from the sourcing side brings challenges

as harvested power derived from ambient sources tends to be unregulated,

intermittent and small.

New technologies call for different forms of battery Electronics and electrics

are becoming ubiquitous, the devices appearing on and in higher and higher

volume products including e-labels and e-packaging. This calls for different forms

of battery, capacitor and other energy storage because priorities such as

environmental credentials, thinness and compatibility with energy harvesting

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(eg solar cells) come to the fore alongside life and cost. This unique new report is

directed towards those developing, marketing and using the new small electronic

and electrical devices, particularly those that are self-sufficient. It will also interest

those investing in new battery, capacitor and allied companies providing products

for these markets and those regulating and supporting these burgeoning industries.

Synopsis

Recent advances in miniaturization of both electronics and MEMS devices

have resulted in a considerable power reduction. On the other hand, the size of

power supplies for such miniaturized devices has scaled down only marginally.

Downsizing conventional batteries to wafer level causes various problems. For

example electrode materials or liquid electrolytes of the chip-integrated batteries

just as those of conventional systems have to be hermetically sealed in order to

prevent defects caused by interaction with oxygen or water in ambient atmosphere.

The system presented in this paper consists of fuel cells (FCs) connected in series,

so called fuel cell cascades (FCCs) (Fig. 1), a core system to control the output

voltage by a low dropout voltage regulator (LDO) and circuitry to check and

bypass empty or defective FCs to keep the FCCs functional.

Fig. 1. Prototype of the fuel cell cascades, 7 cascades consisting of 8 single

chip-integrated fuel cells in a PLCC68 ceramic package.

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Working Principle

Polymer Electrolyte Membrane (PEM) fuel cells also called Proton

Exchange Membrane fuel cells are the type typically used in automobiles. A PEM

fuel cell uses hydrogen fuel and oxygen from the air to produce electricity.

Fig 2: Diagram of a PEM fuel cell

A proton exchange membrane fuel cell transforms the chemical

energy liberated during the electrochemical reaction of hydrogen and oxygen to

electrical energy, as opposed to the direct combustion of hydrogen and oxygen

gases to produce thermal energy. A stream of hydrogen is delivered to

the anode side of the membrane electrode assembly (MEA). At the anode side it

is catalytically split into protons and electrons. This oxidation half-cell reaction is

represented by:

Eo = 0VSHE

The newly formed protons permeate through the polymer electrolyte

membrane to the cathode side. The electrons travel along an external load circuit to

the cathode side of the MEA, thus creating the current output of the fuel cell.

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Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the

cathode side oxygen molecules react with the protons permeating through the

polymer electrolyte membrane and the electrons arriving through the external

circuit to form water molecules. This reduction half-cell reaction is represented by:

Eo

= 1.229VSHE

Fig. 3. Layout of a chip-integrated fuel cell

Conventional PEM fuel cells consist of a polymer electrolyte membrane (PEM),

two gas diffusion electrodes, two diffusion layers and two flow fields. The

reactants e.g., hydrogen and oxygen are supplied to the gas diffusion electrodes

over feed pipes out of external tanks. The amount of supplied fuel is often

controlled by active system periphery like pressure reducers and valves. For the

chip integration a new setup principle of PEM fuel cells was developed, the new

kind of fuel cell is made up of a palladium based hydrogen storage and an air

breathing cathode both separated by a PEM. The layout of an integrated fuel cell is

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depicted in Fig. 3. Advantages of the new approach are the omission of active

devices for fuel supply and the reduction of system components like flow fields

and diffusion layers. Due to the simple assembly process, the fuel cells can be

produced by thin film technologies and can be fabricated within an extended

CMOS process.

Conclusion

The hybrid integration of a chip-integrated micro energy system based on

fuel cells and a CMOS control circuitry that stabilizes the output voltage of the

system to a constant level In future work the monolithic integration of fuel cell

cascades and the electronic control circuitry will be realized. A detailedcharacterization of the fully integrated system with fuel cell cascades and the

CMOS circuitry on a single chip will be carried out. Further integration of a sensor

and a signal processing unit will allow the realization of autonomous sensor

devices.