This paper describes the development of metallic bipolar plate fabrication
using micro-electroforming process for mini-DMFC (direct methanol fuel cell)
stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels
using a photomask and exposure process. Micro-fluidic channels mold with 300
micrometers thick and 500 micrometers wide were firstly fabricated in a
negative photoresist onto a stainless steel plate. Copper micro-electroforming
was used to replicate the micro-fluidic channels mold. Following by sputtering
silver (Ag) with 1.2 micrometers thick, the metallic bipolar plates were
completed. The silver layer is used for corrosive resistance. The completed
mini-DMFC stack is a 2x2 cm2 fuel cell stack including a 1.5x1.5 cm2 MEA
(membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks
using the completed metallic bipolar plates. All test results showed the
metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power
density is 9.3mW/cm2 and current density is 100 mA/cm2 when using 8 vol. %
methanol as fuel and operated at temperature 30 degrees C. The output power
result is similar to other reports by using conventional graphite bipolar
plates. However, conventional graphite bipolar plates have certain difficulty
to be machined to such micro-fluidic channels. The proposed
micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC
stacks for further portable 3C applications.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Lee, J. -H.

Shyu, R. F.

Yang, H.

English

arXiv

This paper describes the development of metallic bipolar plate fabrication using micro-electroforming process for mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels using a photomask and exposure process. Micro-fluidic channels mold with 300 μm thick and 500 μm wide were firstly fabricated in a negative photoresist onto a stainless steel plate. Copper micro-electroforming was used to replicate the micro-fluidic channels mold. Following by sputtering silver (Ag) with 1.2μm thick, the metallic bipolar plates were completed. The silver layer is used for corrosive resistance. The completed mini-DMFC stack is a 2x2 cm2 fuel cell stack including a 1.5x1.5 cm2 MEA (membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks using the completed metallic bipolar plates. All test results showed the metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power density is 9.3mW/cm2 and current density is 100 mA/cm2 when using 8 vol. % methanol as fuel and operated at temperature 30°C. The output power result is similar to other reports by using conventional graphite bipolar plates. However, conventional graphite bipolar plates have certain difficulty to be machined to such micro-fluidic channels. The proposed micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC stacks for further portable 3C applications

Shyu, R.F.

Lee, J.-H.

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
Micro-electroforming Metallic Bipolar Electrodes for 
Mini-DMFC Stacks 
 
R. F. Shyu1, H. Yang2, J.-H. Lee2 
1Department of Mechanical Manufacturing Engineering, National Formosa University, Yunlin, Taiwan 632  
2Institute of Precision Engineering, National Chung Hsing University, Taiwan 402 
 
 
Abstract- This paper describes the development of metallic 
bipolar plate fabrication using micro-electroforming process for 
mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) 
lithography was used to define micro-fluidic channels using a 
photomask and exposure process. Micro-fluidic channels mold 
with 300 μm thick and 500 μm wide were firstly fabricated in a 
negative photoresist onto a stainless steel plate. Copper 
micro-electroforming was used to replicate the micro-fluidic 
channels mold. Following by sputtering silver (Ag) with 1.2μm 
thick, the metallic bipolar plates were completed. The silver 
layer is used for corrosive resistance. The completed 
mini-DMFC stack is a 2x2 cm2 fuel cell stack including a 1.5x1.5 
cm2 MEA (membrane electrode assembly). Several MEAs were 
assembly into mini-DMFC stacks using the completed metallic 
bipolar plates. All test results showed the metallic bipolar plates 
suitable for mini-DMFC stacks. The maximum output power 
density is 9.3mW/cm2 and current density is 100 mA/cm2 when 
using 8 vol. % methanol as fuel and operated at temperature 
30°C. The output power result is similar to other reports by 
using conventional graphite bipolar plates. However, 
conventional graphite bipolar plates have certain difficulty to be 
machined to such micro-fluidic channels. The proposed 
micro-electroforming metallic bipolar plates are feasible to 
miniaturize DMFC stacks for further portable 3C applications. 
  
I.  INTRODUCTION 
 Global warming effect results in serious environmental 
problems. Fossil energy consumption generated lots of carbon 
dioxide and related compound is the major reason.  Clean 
energy with high conversion efficiency and low pollution 
effect is human’s common objective. Fuel cell is an electrical 
power generator, it converts chemical energy to electrical 
power. There are two major fuel cells including PEMFC 
(proton exchange membrane fuel cell) and DMFC. Both 
systems need a FC stack as the reaction site. The key 
components include proton exchange membrane, gas diffusion 
layer (electrode), catalyst, and bipolar plate. The combination 
of proton exchange membrane, catalyst and gas diffusion 
layers is called MEA (membrane electrode assembly). 
Hydrogen fuel with catalyst help reacting with oxygen 
generates electricity and water. Bipolar plates are responsible 
for hydrogen and water flow in the FC stack.  
Many conductive materials are suitable for mini-DMFC 
bipolar plates, silicon and stainless steel ate two major items 
[1]. Jiang et al used (100) silicon as the substrate and etched 
microchannels as bipolar plates [2]. A reactive area of 
1.3×1.1cm2 MEA was applied and generated the maximum 
output power density 2.31 mW/cm2. Methanol solution (1 M 
volume concentration) was operated at room temperature. 
Though silicon bipolar plates can be fabricated for 
micro-DMFC stacks, the brittle problem is easy to make 
cracks in assembly and results in failure [1]. Stainless steel 
provides low cost and high strength advantages compare to 
silicon. The skillful metal work can realize stainless steel for 
commercial applications [3]. Yang et al used stainless steel 
(SS316) as bipolar plate material for micro-DMFC stacks. A 
MEA with 5 cm2 was assembly to a single DMFC. The 
maximum output power was 13mW/cm2 when using 6 vol. % 
methanol solution and operated at temperature 31°C [4]. 
Different microchannel flow patterns have direct influence 
on fuel cell stack performance. Serpentine flow pattern has the 
best result based on practical test compared to other patterns. 
Bipolar plates with serpentine flow pattern contributed a better 
CO2 exhaust on anode side. The further study also indicated 
the generated output efficiency using serpentine flow pattern 
is suitable for micro-DMFC stacks [5]. Microchannel profile 
related to liquid flow pattern is the other concerned issue. 
Microchannel with 580μm wide for mini-DMFC has a better 
performance than other sizes [6]. Different depths were 
studied and concluded mcirochannel with 300 μm deep 
suitable for the optimum efficiency [7].  Based on previous 
studies, many researchers suggested that fluidic channel with 
300 μm in depth and 500 μm in width provides an efficient 
flow.         
The experiment was to design such pattern in the mask. 
UV-LIGA process was applied to fabricate such pattern for 
bipolar plates with low electrical resistance. Stainless steel 
(SS316) with 1.5 mm thick was used as the substrate. The 
negative photoresist (JSR THB-120N) with 300 μm thick was 
triple-spun coating on the substrate. UV exposure and 
development then generated the fluidic channel mold in 
photoresist. Copper electroforming then filled the mold onto 
the substrate. The dimensional measurement is required to 
confirm the geometry accuracy by using an optical 3D profile. 
Electrical performance measurement of mini-DMFC to verify 
the experimental bipolar plates is measured by a commercial 
MEA and test system.  
 
 
 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
II. EXPERIMENTAL PROCEDURE 
Metallic bipolar plates with different patterns for 
microchannel flow were studied. Previous studies showed that 
the optimum microchannel with width 500μm and depth 
300μm provides the best efficiency. Pattern design on the 
mask is shown in Fig. 1. Serpentine (left diagram) and 
interdigital (right diagram) flow patterns were designed as 
shown. Desired patterns are transferred from the designed 
mask in the lithographic process. In this experiment, a plastic 
mask was fabricated using laser writing onto a PET 
(Polyethylene terephthalate) used for PCBs (print circuit 
boards). The stainless steel (SS316) substrate (Fig. 2) was then 
spun with a layer of negative photoresist (JSR THB-120N) 
100μm thick. The spin condition was 150rpm for 15 seconds. 
Prebaking in a convection oven at 100°C for 3 minutes is a 
required procedure. This removes the excess solvent from the 
photoresist and produces a slightly hardened photoresist 
surface. Repeat the coating and prebaking process in three 
times resulted in multi-layer thickness 300 μm. The mask was 
not stuck onto the substrate. 
The sample was exposed through the plastic mask using a 
UV mask aligner (EVG620). This aligner had soft, hard 
contact or proximity exposure modes with NUV (near 
ultra-violet) wavelength 350-450nm and lamp power range 
from 200-500 W.  The exposure time was 180 seconds and 
developing time for 40 minutes. The hardbake temperature 
was 120°C for 3 minutes by the hot plate. Commercial cooper 
sulfite electrolyte was used to electroform metallic plates. The 
specimen with microchannel flow pattern on the stainless steel 
substrate was immersed into the electrolyte. Cooper atoms 
were deposited into resist mold from the substrate. The input 
current density was 5ASD and the growth rate was 1 μm/min 
in theory. The plating elapsed for 5 hours to obtain 
microchannel thickness 300μm in cooper. Sodium hydroxide 
solution 150 ml containing 3.8 g NaOH was heated to 60°C. 
Photoresist mold was detached from the substrate after 15 
minutes immersion. After cleaning with DI water and spray 
nitrogen air, metallic cooper microchannel bipolar plates were 
completed. After stripping the photoresist and sputtering silver 
with 1.2 μm, the metallic bipolar plate with surface resistance 
284 mΩ was completed. The completed metallic bipolar plates 
for mini-DMFC stacks are shown in Fig. 3.  
 
Fig. 1. Schematic diagrams of microchannel flow pattern on the mask. 
 
 
Fig. 2. Stainless steel (SS316) substrates as the plating base material, each piece 
is 2x2 cm2. 
 
 
 
Fig. 3. Completed metallic bipolar plates for mini-DMFC stack.  
 
III. EXPERIMENTAL RESULTS 
A. Dimensional measurement 
Serpentine microchannel profiles of bipolar plates were 
measured using a 3D surface profiler to measure their widths 
and depths. As shown in Fig. 4(a), the measured width is 
498μm and the depth is 350μm. The three dimensional profile 
is also shown in Fig. 4(b). Rough surface came from large 
grains of copper deposition. That can be further modified in 
electroforming conditions such decreasing growth rate and 
adding some additives for grain refining. Interdigital 
microchannel pattern of bipolar plates was measured as shown 
in Fig. 5. The dimensional measurement results are close to the 
design size. There is about 10% dimensional error existed.    
 
 
(a) 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
(b) 
Fig. 4.  Measurement results of serpentine microchannel pattern bipolar plate; (a) 
cross-sectional view and (b) 3D morphology. 
 
 
(a) 
 
(b) 
Fig. 5.  Measurement results of interdigital microchannel pattern bipolar plate; (a) 
cross-sectional view and (b) 3D morphology. 
 
B. Mini-DMFC stack assembly 
The mini-DMFC stack includes a 1.5x1.5cm2 MEA from 
Micropower Tech, metallic bipolar plates from this 
experiment, and other fixtures as shown in Fig. 6. To 
understand the commercial DMFC performance using 
Micropower Tech’s MEA, Fig. 8 shows its performance 
characteristics. Test equipment was the Antig’s DMFC tes 
platform. The maximum output power density 12 mW/cm2 
with the current density 50 mA/cm2 was measured. That’s a 
reference to verify the experimental metallic bipolar plates 
since the commercial bipolar plate is made of graphite. 
Serpentine and interdigital microchannel patterns of metallic 
bipolar plates in mini-DMFC were assembly and measured 
their performance characteristics. Fig. 9 shows the 
performance characteristics of serpentine microchannel 
bipolar plates, its output performance is similar to the 
commercial product. Interdigital microchannel bipolar plates 
have the poor performance as shown in Fig. 10. Once again 
showing serpentine flow pattern is more suitable for DMFC 
stacks. Fig. 11 shows the functional test of mini-DMFC by 
using a mini-fan as the load. The output voltage 0.37 V is 
shown to drive the mini-fan running.    
 
 
Fig. 6.  Illustration of mini-DMFC stack parts. 
 
 
 
Fig. 7. Comparison photos of conventional DMFC stack and mini-DMFC 
stack. 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
0
100
200
300
400
500
600
0 20 40 60 80 100
Current density(mA/cm2 )
V
ol
ta
ge
(m
V
 ) 
  
 
(a) 
0
2
4
6
8
10
12
0 20 40 60 80 100
Current density(mA/cm2 )
Po
w
er
 d
en
si
ty
(m
W
/c
m
   2
 )
 
(b) 
Fig. 8. Conventional DMFC performance characteristics by using MEA 
(3.5×3.5cm2) from Micropower Tech.; (a) I-V curve and (b) output power.  
 
 
0
100
200
300
400
500
600
0 20 40 60 80 100 120
Current density(mA/cm2 )
V
ol
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(m
V
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 .
 
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0
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8
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0 20 40 60 80 100 120
Current density(mA/cm2 )
Po
w
er
 d
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ty
(m
W
/c
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   2
 )
 
Fig. 9. Mini DMFC performance characteristics using serpentine 
microchannel pattern bipolar plates; (a) I-V curve and (b) output power.  
 
 
0
100
200
300
400
0 20 40 60 80
Current density(mA/cm2 )
V
ol
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V
) .
 
(a) 
 
0
0.5
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1.5
2
2.5
3
0 20 40 60 80
Current density(mA/cm2 )
Po
w
er
 d
en
si
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(m
W
/c
m
   
2 
  )
 
(b) 
Fig. 10. Mini DMFC performance characteristics using interdigital 
microchannel pattern bipolar plates; (a) I-V curve and (b) output power.  
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
Fig. 11. Functional test of mini DMFC stack with output voltage 0.37 V. 
 
IV.  CONCLUSION 
Commercial MEAs with the fabricated bipolar plates were 
assembly into a 2x2 cm2 stack. As found, the output power 
mainly dominated by MEA. MEA is the most important factor 
effect on the output power, though fluidic channel types on 
bipolar plates have a little effect on the stack performance. The 
completed mini-DMFC stack has the maximum power 
9.33mW/cm2 was measured. The power density is similar to 
macro-FC stacks. It proves the proposed micro-electroforming 
bipolar plates feasible to mini-DMFC stacks. The further 
applications trend to apply the mini-DMFC stacks for portable 
electronic products. 
 
ACKNOWLEDGMENT 
This research is supported by the National Science Council 
of Taiwan under the grand number 
NSC96-2221-E-005-069-MY3. 
 
REFERENCES 
[1]     N. T. Nguyen and S. H. Chan, J. Micromech. Microeng., Vol. 16, 
pp.1-12, 2006. 
[2]     Y. Jiang, X. Wang, L. Zhong and L. Liu, J. Micromech. Microeng., Vol. 
16, pp. 233-239, 2006. 
[3]    B. R. Padhy and R. G. Reddy, J. Power Sources, Vol. 153, pp. 125–129, 
2006. 
[4]   W. M. Yang, S.K. Chou and C. Shu, J. Power Sources, Vol. 164, pp. 
549–554, 2007. 
[5]     K. Tuber, A. Oedegaard, M. Hermann and C. Hebling, J. Power Sources, 
Vol. 131, pp. 175-181, 2004. 
[6]   C. W. Wong, T. S. Zhao, Q. Ye and J. G.. Liu, J.  The Electrochemical 
Society, Vol.152, no. 8, pp. 1600-1605, 2005. 
[7]     C. W. Wong, T. S. Zhao, Q. Ye and J. G. Liu, J. Power Sources, Vol. 155, 
pp.291-296, 2006. 
 
 


Micro-electroforming metallic bipolar electrodes for mini-DMFC stacks

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