Modeling, Fabrication and Characterization of a Silicon-based μDMFC
Xiaohong Wang, Kewen Xie, Yingqi Jiang, Lingyan Zhong, Yan’an Zhou, and Litian Liu
Institute of Microelectronics, Tsinghua University
Beijing, 100084, China
Tel +86-10-62789151, Fax +86-10-62771130, E-mail wxh-ime@mail.tsinghua.edu.cn
Abstract
This paper reports a 3-D anode model of a direct methanol fuel cell (DMFC) and some simulation results. The design,
fabrication using MEMS technology, and characterization of a silicon-based μDMFC are also presented. The cell with an
active area of 8.6mm×8.6mm was assembled by sandwiching the MEA between two silicon bipolar plates using PDMS. The
latest μDMFC has an open-circuited potential of 0.73V, and the maximum power density of 7.04mW/cm2 when 1M-methanol
is fed at room temperature. The output has supplied the power for a small motor.
Keywords: Micro Fuel Cell, DMFC, Modeling, PDMS, MEMS
1 INTRODUCTION
+
−
Anode: CH 3OH + H 2O → CO2 + 6 H + 6e
The demand for Micro Power Sources (MPS) is expected to
increase significantly with the widespread and growing use
of compact information and communication devices with
many advanced features. Conventional battery solutions are
hardly able to keep up with the power requirements of
rapidly evolving existing applications, resulting in frequent
recharging or disposal of battery cells. Emerging
applications such as wireless sensing, wearable computing,
autonomous robots, and other systems that are yet to hit the
marketplace, will require even more efficient power sources
to deliver revolutionary new functionality and convenience.
Due to the potential advantages such as high
energy-conversion efficiency, low operating temperature,
easy storage of liquid fuel, and environment-friendly
emissions, micro direct methanol fuel cells (μDMFC) have
been emerging as one of the favorable candidates for future
micro-systems and portable electronic devices [1-3].
Cathode:
3
2
(1)
O2 + 6H + + 6e − → 3H 2O
(2)
3
Net reaction: CH 3OH + 2 O2 + H 2O → CO2 + 3H 2O
(3)
To optimize the structure and improve the performance of
DMFCs, several models that focus on one-dimensional flow
with transport of reactant and products in the flow-field
channels and across the membrane has been developed [4-5].
This work presents a 3-dimensional anode model of DMFC,
in which the vertical transmissions of methanol through
diffusion layer, catalyst layer and PEM as well as the
distributions of parameters from the inlet to outlet have all
been considered. The half-cell cross-section of the model is
shown in Fig. 1. There are three different zones consisting of
flow-field channels, diffusion layer, and catalyst layer in the
DMFC anode to be considered. The model is computed with
the commercial fluid dynamics (CFD) software — Fluent®.
This work reports the efforts made in our group on
developing the μDMFC, including a 3-D anode model
simulation, fabrication using MEMS technology and original
PDMS assemble technology, and the experimental results.
z axis
z3
z2
z1
2 MODELING
CH 3OH
H2 O
H+
CH 3OH
H2 O
CO2
Catalyst layer
Diffusion layer
Flow field
channels
2.1 Modeling
z0
The μDMFC designed herein consists of two silicon plates
with micro channels, one for the anode and the other for the
cathode, and a MEA sandwiched in between. Methanol is
supplied at the anode where is dissociates into charged
protons and electrons. The protons diffuse through the
membrane onto the cathode side while the electrons flow
through the outer circuit generating electricity. And these
protons and electrons react with oxygen to produce water.
The electrochemical reactions are as follows:
Fig.1. Half-cell cross-section of the DMFC model
In this model, the Navier-Stokes equations used as the basic
transport equations were written for each zone of
computation domain. For the flow in the fuel cell involving
species mixing and electrochemical reactions, a species
conservation equation was solved. The equation of
conservation for the chemical species i.e. methanol can be
124
written as follows:
(
∂ ρYCH 3OH vx
∂x
) + ∂(ρY
CH 3 OH v y
∂y
∂YCH 3OH
∂ ⎛
w
+ ⎜⎜ ρDCH
3OH
∂y ⎝
∂y
) + ∂(ρY
CH 3 OH v z
∂z
)=
(
∂Y
∂⎛ w
⎜ ρDCH 3OH CH 3OH
∂x ⎜⎝
∂x
∂YCH 3OH
⎞ ∂⎛
w
⎟⎟ + ⎜⎜ ρDCH
3OH
∂z
⎠ ∂z ⎝
∂ ρv x
∂x
⎞
⎟⎟
⎠
+
⎞
⎟⎟ + RCH 3OH
⎠
(4)
w
DCH
3OH
is the diffusion coefficient for the methanol in the
mixture, and νx, νy, νz are the components of the velocity in
γ
⎡ ⎛ 6αF
⎞
⎛ 6(1 − α )F
⎞⎤
η anode ⎟⎥
⎢exp⎜ RT η anode ⎟ − exp⎜ −
RT
⎠
⎝
⎠⎦
⎣ ⎝
(6)
Avi0 ⎛⎜ cCH 3OH
ref
6 F ⎜⎝ cCH
3 OH
⎛ 6(1 − α )F
⎞⎤
− exp⎜ −
η anode ⎟⎥
RT
⎝
⎠⎦ ,
⎞
⎟
⎟
⎠
γ
⎡ ⎛ 6αF
⎞
⎢exp⎜ RT η anode ⎟
⎠
⎣ ⎝
∂v ⎞
∂p ∂ ⎛
+ ⎜ 2μ x ⎟
∂x ∂x ⎝
∂x ⎠
(9)
z 0 ≤ z ≤ z1
vx
β x , z1 ≤ z ≤ z 3
(10)
(11)
The results of simulations include not only definite numbers
such as the output voltage, the average output operating
current density, methanol crossover, but also the
distributions of operating current density, methanol
concentration, the amount of methanol that has crossed over
the PEM, and other important parameters of the fuel cell.
The local distribution of current density of the half-cell is
shown in Fig. 2 for the condition of the average current
density of 511mA/cm2, output voltage of 0.53V, and the
2M-methanol feed concentration. On the whole, the current
density decreases along the streamwise direction, especially
at the beginning of the channel.
In the equation above MCH3OH is the mole weight of
methanol. Combining Eqs. (5) and (6), and considering that
no reaction occurs in the flow distributor and diffusion layer,
so the net reaction rate in different zones can be given as
follows:
RCH 3OH = − M CH 3OH
=−
Numerical simulations were performed with CFD software
FLUENT 6.1. The electrochemical reaction rate of methanol
oxidation (i.e. Butler-Volmer equation) in the anode was
incorporated into solver through the User Defined Functions
(UDFs) code written in C programming language. It was
assumed that the cathode polarization is a constant. With
designated conditions, the fuel cell unit electrical
performance and local distributions could be obtained.
From Eq. (1), the relationship between the rate of methanol
consumption and current density distribution can be
obtained as following:
z0 ≤ z ≤ z2
)
2.2 Simulations and Results
(5)
3
,
z
In the equations above, βx denote the permeability of the
porous material.
ref
RCH 3OH = 0
,
S x = −μ
CCH3OH is the local methanol concentration, CCH OH is the
reference methanol concentration, which is associated with
i0, α is the anodic transfer coefficient, ηanode is the anode
overpotential, and the term di/dz presents the local proton
current density distribution in the catalyst region.
M CH 3OH di
6 F dz
x
∂z
∂ ⎡ ⎛ ∂v y ∂vx ⎞⎤ ∂ ⎡ ⎛ ∂vz ∂vx ⎞⎤
⎟⎥ +
μ⎜
+
+
⎟ + ρf x + S x
⎢μ ⎜
∂y ⎣ ⎜⎝ ∂x ∂y ⎟⎠⎦ ∂z ⎢⎣ ⎝ ∂x ∂z ⎠⎥⎦
Sx = 0
where Aν is the specific area of reaction surface, i0 is the
reference exchange current density, γ is the order of reaction,
RCH 3OH = −
y
3
For the electrochemical reaction occurs in the catalyst layer,
methanol and water are consumed there. Butler-Volmer
equation was used to describe the electrochemical reaction.
It is usually given as follows:
⎞
⎟
⎟
⎠
x
∂y
When the fuel flows through porous media, the diffusion and
catalyst layers, the pressure drop is given by Darcy’s law:
CH OH
is the net
x-, y-, z-direction, respectively.
electrochemical reaction rate of methanol consumption.
⎛ CCH OH
di
= Av i0 ⎜ ref 3
⎜ CCH OH
dz
3
⎝
) + ∂(ρv v ) + ∂(ρv v
where Sx is the source term in the above equations for the x
directions momentum. μ denotes the viscosity of the fluid in
the medium. p is the intensity of pressure. fx is denotes the
viscous force.
The equation above is a convection-diffusion equation,
where ρ is the density of the mixture solution, YCH3OH is the
mass fraction of the methanol in the mixture solution fuel,
R
2
Methanol crossover has great influence on the performance
of DMFC, and it results from both the electro-osmotic drag
and diffusion. Fig. 3 shows the variation of the methanol
crossover with current density for various feed
concentrations of methanol. It can be seen that, as the
methanol feed concentration increases, the methanol
crossover increases accordingly.
(7)
(8)
z2 ≤ z ≤ z3
Because the steady state is presumed, the conservation
equations of momentum are given as follows (only take xdirection as an example):
The influence of the flow-field channels on the performance
was studied. Fig. 4 shows the results of the fuel cell
performances with two kinds of flow-field, serpentine and
125
parallel patterns. The comparison shows that the limiting
current density of the serpentine flow-field channels is
higher than that of the parallel. So the maximum power
output of the serpentine flow-field channels is higher as well.
While at low current density, the effect of different channels
shape is insignificant. Such results agree with the
experimental results in [6].
3 DESINGN AND FABRICATION
With the simulation results, we chose plates with serpentine
flow-field channels in our practical experiments. The bipolar
plates with micro channels were fabricated by
micro-machining silicon wafers with photolithography,
bulk-silicon etching, sputtering and other standard silicon
MEMS techniques. After a series of experiments,
considering physical stiffness of silicon wafer and fuel
transmission, we finally determined the following
dimensions of the micro channels: 8600μm in length, 200μm
in width, and 120μm in depth, respectively. Toray carbon
paper was chosen as the diffusion layer and Nafion® 117
attached with catalyst layers was used as the membrane
electrode assembly (MEA).
Fig. 5 shows the fabrication process of the μDMFC, roughly
involving three steps: fabrication of plates, fabrication of a
half-cell, and final assembly of the whole cell using PDMS.
First, silicon plates were fabricated by using MEMS
technologies, including LPCVD, KOH etching and
sputtering (Fig. 5 (a) to (e)). The plate has an area of 18mm
×18mm with the active area of 8.6mm×8.6mm. Second, to
make the half-cell, a piece of cured PDMS was glued to the
back of the silicon plate with liquid-state PDMS. Two plastic
pipes were fixed onto the PDMS block, and then Al foils and
carbon papers were glued onto the front side of the plate
using silver paste (Fig. 5 (f)). Finally, a MEA was
sandwiched between two half-cells with PDMS to create a
μDMFC (Fig. 5 (g)). This fabrication process combines the
advantages of MEMS technology and the outstand
properties of PDMS.
Fig. 2. Local distribution of current density (mA/cm2) of the
half-cell for the condition of the average current density of
511mA/cm2, output voltage of 0.53V, and 2M-methanol.
Fig. 6 shows the photograph of an assembled μDMFC with a
total size of 22mm×24mm×8mm.
Fig. 3. Methanol crossover vs. current density for
various feed concentrations of methanol
SiO2/Si3N4
（a）
PDMS
（b）
Aluminum foil
（e）
（f）
（c）
（d）
（g）
Fig. 5. Fabrication process of the μDMFC
Fig. 4. Comparison of performances of parallel and
serpentine flow-field channels
126
Pt/Ti
Carbon
paper
0.1M
1M
2M
4M
8
2
Power Density(mW/cm )
7
6
5
4
3
2
1
0
-5
0
5
10 15 20 25 30 35 40 45 50 55
2
Current Density (mA/cm )
Fig. 8. Power density curves of the µDMFC tested for different
methanol concentration; methanol: 0.40ml/min and oxygen:
15ml/min; room temperature(21℃) and ambient pressure
Fig. 6. Assembled μDMFC
4 CHARACTERIZATION
5 CONCLUSIONS
The performance of the μDMFC was measured at room
temperature under ambient pressure. 1M-methanol solution
with a feeding rate of 0.6ml/min was fed into the anode and
oxygen was supplied to the cathode with a flow rate of
15ml/min. An electrochemical interface, Solartron 1287, was
used to test the performance by the constant current method.
This work reports the recent achievements made in our
group on developing the μDMFC, including a 3-D anode
model simulation, design, fabrication and assembly of a
μDMFC with MEMS technology and original PDMS
assemble technology, and its testing performance. The latest
µDMFC has an open-circuited potential of 0.73V, and the
maximum power density of 7.04mW/cm2 when
1M-methanol was fed at room temperature. The output has
supplied the power for a small motor. It demonstrates that
the power density of the μDMFC fabricated in our
laboratory satisfies the requirements of power consumption
of many electronic devices and has the potential of being
used for practical applications.
Fig. 7 presents the typical performance curves of the
µDMFCs developed in our group at different periods. The
latest µDMFC has an open-circuited potential of 0.73V, and
the maximum power density of 7.04mW/cm2. The output
has successfully supplied the power for a small motor. The
characterization of the μDMFC has also been investigated.
Fig. 8 presents the effect of methanol concentration on the
performance. The best performance was achieved at
2M-methanol solution. Lower and higher concentrations
both cause the performance degradation. It verifies the
simulation results as Fig. 3 suggests that methanol crossover
can cause the performance degradation at higher
concentrations.
ACKNOWLEDGEMENTS
The authors are grateful for the support of this work by
National Natural Science Foundation of China, Major
Research Plan, and Grant No. 90207023.
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8
0.6
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2
Voltage(1G)
Power Denstiy(1G)
Voltage(2G)
Power Density(2G)
0.7
Power Density (mW/cm )
0.8
0.1
0.0
0
20
40
60
0
80
2
Current Density(mA/cm )
Fig. 7. Progress in performance of a single cell. The
cell operated fed with 1M-methanol solution and
pure oxygen at room temperature.
1G(generation) ⎯ fabricated and tested in 2004
2G(generation) ⎯ fabricated and tested in 2005
127