LTCC-Based Micro-Scale PEM Fuel Cell Uwe Partsch, Adrian Goldberg, Michael Stelter Fraunhofer Institute Ceramic Technologies and Systems, 01277 Dresden, WinterbergstraBe 28
[email protected], 5R +49-351-2553-696
Abstract LTCC (=Low Temperature Cofired Ceramics) is well-known as a technology for the production of highly integrated ceramic microelectronic packages. Furthermore LTCC offers the possibility to integrate 3-Dstructures for e.g. mechanical sensors or the transport offluid or gaseous substances. With regard to the present needfor self substaining energy source systems LTCC-technology is suited very wellfor the assembling of micro-scaledfuel cells. LTCC technology offers the possibility to unite a system with all necessary components for a micro-scale fuel cell on one substrate carrier. Because of the variety in 3D structuring of the LTCC all kinds of the geometrical variants of the cell dimensions as well as different flowfields are possible. Furthermore electronic components which are needed in a self-substainingfuel cellsystem like electrical circuits ofDCIDC converter or charging circuits can be integrated. 1. Introduction The energy demand of electronic products such as mobile phones, notebooks and others will rapidly increase. Beside lithium batteries miniaturized fuel cells seems to be a way to satisfy the actual need for high density energy sources (Fig. 1). 800 -
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100
200
300
400
500
Specific Energy (Wh/kg)
Figure 1: Energy sources for mobile electronics [3]. In comparison with accumulator batteries fuel cells have many advantages. Fuel cell systems show a good cycle firmness of the stored hydrogen or methanol, a missing self discharge, a short reloading time and there are no memory effects as well as the expected longevity. Fuel cells are modular units, with a total electrical output which can be adapted accurately to the power need of the consumer. More flexible possibilities for the equipment adjustment
1-4244-0553-1/06/$20.00 (D2OO6 IEEE
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and equipment integration result from the separation from energy transformation (fuel cell) and energy storage (hydrogen or methanol). Furthermore fuel cells have a positive ecological balance compared to batteries because of the used and largely unproblematic materials as well as the arising reaction products (damp air). The cost level of the lithium batteries should be attainable for fuel cell systems for broad market penetration with a mass production. The LTCC-technology (LTCC=Low Temperature Cofired Ceramics) offers the possibility to unite a system with all components of a micro fuel cell system on one substrate carrier. LTCC-technology offers many 3D-structures (e.g. channels, cavities, diaphragms) which allows to integrate each kind of geometrical variants of the cell dimensions as well as different flow fields. A further advantage of LTCC is the possibility of the integration of passive and active electronic components (e.g. DC/DC converter or charging circuits). Particularly by the use of chemically aggressive or dangerous materials such as methanol or hydrogen LTCC shows a very good chemical stability. 2. PEMFC Basics 2.1 Working Principle Basically, during the generation of current in a PEM-fuel cell (PEM=Proton Exchange Membrane) the chemical reactions proceed contrary to the electrolysis of hydrogen (Fig. 2).
2006 Electronics Systemintegration Technology Conference Dresden, Germany
U=~~~~~~~~~~~I
-
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Figure 3: Series cell connection [3]. Oxidation
H2 -> 2H+ s2e
H+
Reduction
2H++2e-+50X1 -l> l7
02
2.2 Gas Distribution and Biopalar Plates Gas distributors (or rather flow distributors) lead the reaction gases as effectively as possible into the gas diffusion layer. One side open channel structures are integrated into the LTCC-substrate for channeling the fuel. In order to realize electrical contacts at the same time, electrically conducting thick-film materials are integrated. Apart from material and technical factors the characteristics of the flow field as well as the electrical contact areas play an important role. Table 1 shows possible flow fields.
+
Heat
Figure 2: Principle reactions [2].
At the anode, which is the negative pole of the PEMFC, hydrogen is supplied. There occurs the controlled oxidation of the hydrogen, supported by a noble-metalliferous catalyst. Hydrogen ions (protons) and electrons separate themselves (1). The hydrogen ions move by the electrolyte to the cathode side. The electrons are led over an outside electrical conductor to the cathode side and perform electrical work at an inserted load.
H2
-
2H+ + 2e-
Table 1: Possible flow fields for a PEMFC.
(1)
At the cathode the reduction of oxygen takes place. Subsequently electrons, hydrogen ions and reduced oxygen combine to water (2).
0.502 + 2H+ + 2e-
-
H20
(2)
AGO = 24lkJ/mol
Characteristics
(+) - fast water evacuation - employment in prototypes -
unequal flow field
- pressure loss
The complete reaction (3)
H2+0.502 ->H20
Channel Geometry Meander
Parallel- Meander
(3)
shows that the free standard reaction enthalpy AGO of 241kJ/mol in the fuel cell is converted into electricity and warmth [4]. The conventional way of energy production by thermal and mechanical energy can be avoided, whereby the Camot limitation of the efficiency drops out. Due to the load-sensitive small output voltages of a single cell (0,4... 1V) for most applications it is necessary to connect several single cells electrically in series for a so called "stack". Thus of that a higher voltage can be produced, with which it is e.g. possible to operate an electrical consumer or rather integrated electronics for a fuel cell control. The presented system is a planar cells stack connected into series to reach an even gas flow distribution as well as a homogeneous temperature distribution. In addition building a self breathing system is now easier (Fig. 3).
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Finger- Topology
(+) - low pressure loss - constant flow field - bad water transport - isolated electrically bars
(+) -
separate gas- and water
Lg1&)- transport good gas flow through GDL Matrix- Structure
____I___l
- high pressure loss (+) - constant flow field - simple structuring
ilililiii (I III' _I
- bad water transport - pressure loss
2.3 Membrane Electrode Assembly (MEA) The MEA - consisting of polymer diaphragm (Nafion) and gas diffusion electrodes (GDE) - is the core element of the PEMFC. Nafion belongs to the 2006 Electronics Systemintegration Technology Conference Dresden, Germany
wide class of solid superacids catalysts and has a PTFE-relative chemical structure. In PEMFC applications Nafion works as a hydrogen-ion-conductor. At anode and cathode side there are different carbon-based microlayers (GDE=Gas Diffusion Electrode, GDL= Gas Diffusion Layer) consist of low degrees of catalyst materials (mostly platinum). On the basis of the definition of the goal system of one watt the MEA which was intended for this system by GORE PRIMEA SERIES 56 [5] was selected on its attainable current density per square centimeter, to determine the size and number of necessary cells. Fig. 4-6 show the structure of a PEMFC-MEA, a MEA detail as well as a SEM picture of a PEMFC-GDL.
On the basis of the measurements a system with 4 cells in series was drafted. 3 LTCC-Technology The term LTCC means a ceramic multilayer technology, which is used particularly in 3-Delectronic packaging as a robust substrate technology [6].
At LTCC technology unfired ceramic (green) tapes are separately processed (cutting, preconditioning, via-forming/ filling, screen printing of R,L,C-structures). After this proceedure the unfired tapes are collated and laminated (typ. 70°C, 200 bar). A firing process (875 °C) finishes the LTCC processing (Fig. 7). Thus a compact ceramic packages originates with integrated electrical and/or mechanically functional elements. LI
L2
Lx
Cutting 'I,F
I
Pre-Conditioning
Figure 4: PEMFC MEA-configuration.
I
Via-forming
Via-metallization
Screen printing
Collate/ Laminate
Figure 5: MEA-detail.
Hi
L
Bum-Out/ firing
Figure 7: LTCC process (Layer 1 ..x).
A very important point at the manufacturing of the LTCC-based PEMFC is the formation of the gas channel structures in the LTCC material. Table 2 shows different technologies for LTCC-structuring . Figure 6: PEMFC GDL (SEM-picture). 546
2006 Electronics Systemintegration Technology Conference
Dresden, Germany
2j(
Table 2. Technologies for LTCC-structuring. Technology Punching
Milling Laser
Embossing Sacrificial Volume Material (SVM)
Properties Many geometries possible, fast Inexpensive, treatment of laminates and single layers possible quick and variable structuring, universal geometry Very fast, variable depths No additional equipment necessary, still in development
.
0
Figure 8: CAD-model of a LTCC-based PEMFC.
1-Electronic circuits (DC/DC-converter, charging circuit) ; 2-Lithium-battery; 3- Fuel connector; 4- Anode; 5-MEA; 6-Current collector (Gold); 7-Cathode.
For the used variants of flow fields (meander, parallel and matrix flow fields) a suited structuring technology was selected and tested in each case. Fig. 8 shows an example.
The electronic components of the PEMFCsystem play an important role for the overall efficiency. The particular efficiency degrees are directly proportional to the overall efficiency of the system ( nGes )
7Ges
=PEMFC
(4)
'?DC IDC 7Ch arg e _Circiut
Therefore all electronic components ted for the highest possible efficiency.
were
selec-
4.1 DC/DC Converter Due to the relative low cell voltage of the single cells (0,4.. 1V) the serial interconnecting of the four cells reaches a voltage from 1,6 to 4 V. However for the charging of the lithium battery a higher voltage level is needed.
Figure 8: LTCC-flowfield cathode side (matrix structure, Au-metallized).
4 LTCC-based PEMFC System Figure 8 shows the general 3-D-structure of the PEMFC-system consisting of the anode side with the hydrogen channels and connections as well as the electronic circuits (DC/DC conversion, charging circuit) and the cathode side with the oval openings for the self breathing of the cells. Besides LTCCbased anode and cathode the 5-Layer-MEA (GDL/GDE/membrane/GDE/GDL) can be seen.
From these requirements a DC/DC Integrated Circuit of Maxim Dallas Semiconductor was selected (MAX 1674) which supplies an exact voltage with high efficiency with only a few external components (Fig. 9).
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The test-equipment allows to characterize the fuel cell system under different conditions (hydrogen flow rate, electric load). Current densities up to OOmA/cm2 were reached.
Figure 9: DC/DC converter block diagram.
Figure 11: PEMFC test-equipment.
4.2 Charging Circuit The charging circuit was selected for the present most common accumulators, the lithium-ionaccumulator. In principle each kind of accumulator could be used, Super Caps are possible too. The developed charging control cosists of an integrated circuit from Maxim Dallas Semiconductor (MAX 1508). This IC has also a high efficiency and gets along with only a few construction units. Figure 10 shows the block diagram of the PEMFC-charging control.
[mAlcm2] w2 r, 0.0
0
30
45
60
75
90
105
120
3.0 2.5 2.0 1.5 -
0.0
7
15
0.1
0.2
[A]
0.3
0.4
Figure 12: LTCC-based PEMFC - characteristic curves (hydrogen flow 60 ml/min, p=0.6 bar, 25°C, 4 cells serial connection, without electronics). -16
i
Bati
aPc1BAT1
6~~~~~~~r G =rD
6 Conclusions LTCC-technology was used as an integration platform for a PEMFC-system. All electrical (current collectors, internal conductors, DC/DCconverter, charging circuit) and non-electrical (channels, cavities) 3-D-structures can be united on one board in a very compact way. All electrical components were selected for a maximum efficiency. The built up PEMFC-systems work stable and show high current densities (up to 100 mA/ cm2). LTCC-based micro-scale PEMFC promise to be an alternative to standard lithium batteries. A main difficulty for the common usage of PEMFC is the
Figure 10: PEMFC-charging control block diagramm. 5 LTCC-PEMFC Characterization For the characterization of the PEMFC-system a computer controlled test-equipment consists of a hydrogen supply, a Keithley 2400 electrometer as electric load and a Keithley 2000 multimeter was built-on (Fig 1 1). 548
2006 Electronics Systemintegration Technology Conference Dresden, Germany
hydrogen storage. Different solutions points out that there are by all means possibilities for miniaturized storage reservoirs (metal hydride storage, pressure storage). However the regular recharging with hydrogen or the changing of cartouches from time to time could be a problem. The change to a direct methanol fuel cell would be a possibility to make the hydrogen storage easier. Unfortunately there would be a substantial additional complexity of the overall system (pumps, sensors, water management, C02-separator). In addition the power density of the DMFC cell at ambient temperature stays far behind the power density of a hydrogen system. To the most important questions during the introduction of PEMFC's in the low power range belongs the establishment of a surface covering hydrogen infrastructure. Overall as long as there are no commercial fuel cells, there is also no hydrogen infrastructure and vice versa [3].
[3] Christopher Hebling, Fraunhofer ISE, "Brennstoffzellen im kleinen Leistungsbereich", f-cell 2001. [4] M.H. Eikerling "Theoretische Modellierung der elektrophysikalischen Eigenschaften, der Struktur und Funktion von NiedertemperaturIonenaustauschmembranen", Dissertation TUMulnchen 1999. [5] Gore Technologies Worldwide, www.gore.com/fuelcell, " Series 56" , 2006. [6] Uwe Partsch, "LTCC-kompatible Sensorschichten und deren Applikationen in LTCC-Drucksensoren" Dissertation TU Dresden, Verlag Dr. Detert 2002.
Figure 13: LTCC-PEMFC (cathode side).
7 References [1] Christopher Hebling , Fraunhofer ISE, "Fuel Cell in Micro Energy Engineering", FVS 2004. [2] Torsten Schirgott, " Entwurf, Aufbau und Charakterisierung einer PEM-Mikrobrennstoffzelle in LTCC-Technik", Diplomarbeit TU Dresden 2004.
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