Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
LIQUID ELECTROLYTE FUEL CELL
USING MIXTURE OF NABH4 AND KOH
JongPil Choi, ByeongHee Kim and Young Ho Seo
Mechatronics Division, Kangwon National University, Chuncheon, South Korea
Abstract: This paper presents a simple and low-cost liquid electrolyte fuel cell(direct borohydride fuel cell,
DBFC) with mixture of a fuel (sodium borohydride, NaBH4) and liquid electrolyte (potassium hydroxide, KOH) in
order to improve three-phase contact in the micro fuel cell. It consists of the anode, cathode, and a chamber for
fuel-electrolyte mixture. For the anode catalyst, gold was sputtered on a Pyrex® glass, and manganese dioxide
supported on carbon (MnO2/C) was coated on nickel-foam for the cathode catalysts. The NaBH4 and 1M KOH
were used as hydrogen source and electrolyte, respectively. It also has a simple configuration without a balance of
plant and is cost effective because platinum catalyst and ion exchange membranes were not used. Overall size of a
DBFC was 20mm×14mm×5mm, and the active area was 10mm×10mm. A DBFC has showed 1.18mW at 0.26V,
and serially connected three-cells have turned on 2 LEDs.
Key words: Direct borohydride fuel cell(DBFC), Sodium borohydride(NaBH4), manganese dioxide(MnO2),
Potassium hydroxide (KOH)
overall reaction of the DBFC is given in the following
equations.
1. INTRODUCTION
Recently, there is a growing demand on the
alternative power source for portable electronic
devices. Micro fuel cells, such as polymer electrolyte
fuel cell (PEMFC) and direct methanol fuel cell
(DMFC) are attractive candidates to replace the
batteries due to their high efficiency, high power
density, short charging time and zero emission of
environment pollutants. The main issues facing the
micro fuel cell are to produce a compact, lightweight
system [1-2]. However, these cells require not only
very expensive polymer electrolyte membranes and
noble metal catalysts, but also require strict operating
conditions such as purified fuels, strict humid, uniform
contact, and temperature managements. The hydrogen
storage technology also cannot meet the application
requirements of micro PEMFC [3-4]. These problems
greatly disturb a commercial development of micro
fuel cells.
Moreover, in DMFC, it has two problems which
are still not solved. The first is that its anode
performance is poor compared with that in the PEMFC
because of the lower activity of methanol than that of
hydrogen. The other problem is methanol crossover
from the anode side to the cathode side due to gradient
of methanol concentration. Because of these two
major problems, the DMFC has been lower power
density and open circuit voltage (OCV) than PEMFC
has [5-7].
Recently, there are increasing interests on liquid
electrolyte type fuel cells such as direct borohydride
fuel cells (DBFC) because DBFC doesn’t require
polymer electrolyte and platinum catalysts. The
Anode
BH4- +8OH-→ BO2- +6H2O + 8eE0anode = -1.24V
(1)
Cathode 2O2 +4H2O + 8e → 8OH
E0cathode = 0.40V
(2)
Total reaction BH4- +2O2→ BO2- +2H2O
E0total = 1.64V
(3)
-
-
E0a is standard anode potential, E0c is standard
cathode potential and E0total is the open circuit voltage
of the DBFC. Compared with PEMFC and DMFC,
DBFC have higher theoretical open circuit voltage
(OCV). The DBFC also benefits from the absence of
the CO poisoning for the anode catalyst. In addition,
the borohydride is a fuel that is easily stored,
distributed and is chemically stable in highly alkaline
media [8-9].
In this paper, the liquid electrolyte fuel cell with
mixture of a fuel and a liquid electrolyte. NaBH4 and
1M KOH were used as hydrogen source and
electrolyte, respectively. The deposited Au on a glass
and MnO2/C on nickel-foam were used, respectively,
as anode and cathode catalysts.
2. FABRICATION
Fig.1 shows the schematic of the DBFC which is
simple configuration without polymer membrane and
any balance of plants. It consists of the anode, cathode
69
Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
and a fuel chamber. The both electrode were used as
current collector to reduce the fuel cell volume,
simultaneously.
To prepare cathode catalyst, MnO2 powder (ETEK Co.) was mixed together Nafion® solution (5wt%,
Dupont Co.) to form a paste, and carbon powder
(Vulcan XC-72, E-TEK Co.) was added to increase the
surface areas of the electrochemical reaction region
and improve the three-phase contact. Then, the paste
was brushed on one side of a Ni-foam (thickness =
1mm, porosity > 95%) whose size is 10mm×10mm.
Table 1 shows the detailed composition of the cathode
catalyst for several types of catalysts. High porous Nifoam has 3-dimensional network structure, so it can
load more amount of the MnO2/C catalyst paste and
strongly combine with catalyst. The scanning electron
microscopy (SEM) image of the catalyst-loaded Nifoam is shown in Fig.2. The anode used in the cell
was the Au-deposited Pyrex® glass. Thickness of the
sputtered Au was 200nm.
The fuel and electrolyte were prepared by
dissolving NaBH4 in alkaline solutions of KOH.
Table 1. Several compositions of the cathode catalyst
of 50mg/cm2.
®
Reference
MnO2/C
Ratio of
Nafion
Number
(wt%)
MnO2:C
(wt%)
1
30:70
2
50:50
30
70
3
70:30
4
100:0
The self hydrolysis rate on NaBH4 solutions
depends on the pH and temperature of the liquid
electrolyte. Its rate is empirically represented by:
t1/2 = pH - (0.034 T - 1.92)
(4)
The t1/2 is the time that it takes for one-half of a
NaBH4 solution to decompose in minutes [10]. We
used 1M KOH (pH14) and 4wt% NaBH4 solution.
The tests in all the experiment were carried out at
room temperature around 293K and ambient
conditions.
3. EXPERIMENTAL RESULTS
Fig. 3 shows the fabricated components and DBFC.
The fuel chamber was made from acrylic and its
volume was 0.3ml. The UV adhesive and water
proof adhesive(467MP, 3M Co.) were used to bond
electrode to the fuel chamber. The dimension of
DBFC was 20mm×14mm×5mm and the active area
of both electrodes was 10mm×10mm. After the
assembling, a fuel-electrolyte mixture of 4wt%
NaBH4 in 1M KOH was filled in the fuel chamber.
The DBFCs were operated in fully passive condition
without any pumps and other auxiliary devices.
Fig.1: Schematic diagram of the proposed DBFC.
Fig.2: SEM image of MnO2/C catalyst-loaded Ni-foam
for the cathode.
Fig.3: Photographs of component and assembly of the
fabricated DBFC.
70
Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
We characterize the performance of the DBFC by
polarization curve. Also, a long-term measurement
was performed. The polarization curves were
measured by scanning the various resistance steps
ranging from 100kΩ to 1Ω and simultaneously
measuring the voltage of the DBFC. The current was
kept to stabilize for 20sec at each measurement point
before the next current step.
Fig.4 illustrates the cell performance with several
mixture ratios of MnO2 and carbon. Total amount of
the loaded MnO2/C was 50mg/cm2.
With increasing the amount of MnO2, OCV was
increased from 1.1 to 1.6V and the cell performance
was improved due to a high oxidation reaction rate of
the MnO2 by high ionic conductivity in the cathode.
However, the cell with the MnO2 of 100wt% in
cathode, it shows that the cell voltage in the low
current density region was dramatically decreased
because of the low electronic conductivity.
Maximum power density of 1.18mW/ cm2 at 0.26V
was obtained for the DBFC with mixture ratio of ref.
no.3 (MnO2 of 70wt% and C of 30wt%).
In order to optimize the weight of cathode catalyst
of MnO2/C, three different cathodes were prepared
including catalyst weight of 30mg, 50mg, and 70mg,
and then their polarization curves were measured.
For the three cases, the composition of MnO2/C was
fixed at ref. no.3 of 70:30.
Fig.5(a) and 5(b) show photographs of prepared
three different cathodes, and their polarization curves,
respectively.
The DBFC with a MnO2/C of
50mg/cm2 exhibited the best performance. This
result is attributed to the higher catalyst loading that
could facilitate the electrochemical reaction by
increasing the active area, but interfere with mass
transport of the air because MnO2/C blocks the pore
of nickel-form.
From the Fig.4 and Fig.5, we found that best
composition of the cathode catalyst was 70wt% of
MnO2 and 30wt% of C, and best amount of MnO2/C
was 50mg/cm2, which is the specimen of ref. no.3 in
Table 1. Long-term stability of the DBFC (ref. no.3)
was tested by monitoring cell voltage change during
the working time. Fig.6 shows the behavior of cell
voltage under a constant load of 1mA/cm2. The cell
power gradually decreased to 0.22mW after 1hour.
The reason of degradation is that the fuel is consumed
and the carbon dioxide in the air reacts with the
hydroxide ion, in which they formed carbonate.
Therefore, the electrolyte conductivity is reduced,
increasing the ohmic losses.
1.4
1.8
Ref. No.1
Ref. No.2
Ref. No.3
Ref. No.4
1.6
1.4
(MnO2-30wt% / C-70wt%)
(MnO2-50wt% / C-50wt%)
(MnO2-70wt% / C-30wt%)
(MnO2-100wt%)
1.2
1.0
1.0
0.8
0.8
0.6
0.6
0.4
Power [mW]
Voltage [V]
1.2
0.4
0.2
0.2
0.0
0
1
2
3
4
5
6
7
0.0
Current [mA]
Fig.4: Polarization curve of DBFCs with respect to
mixture ratios of MnO2 and Carbon in the cathode.
(a)
1.4
MnO2/C-30mg
MnO2/C-50mg
MnO2/C-70mg
1.6
1.4
1.0
1.2
0.8
1.0
0.8
0.6
0.6
0.4
Power [mW]
Voltage [V]
1.2
0.4
0.2
0.2
0.0
0.0
0
2
4
6
8
Current [mA]
(b)
Fig.5: Polarization curve of DBFC (a) Polarization
curve (b) Ni-foam images according to amount of
MnO2/C loading.
Fig. 6: Stability test of the DBFC(ref. no.3) for 1hour.
71
Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
We carried on the three-cell test after single cell
test. Fig.6 shows that the three cells in series
successfully turn on 2 LEDs at 2.4V. The operation
time was about 1hour due to degradation of cell
performance and the limited fuel.
ACKNOWLEDGMENT
The work was financially supported by Ministry of
Knowledge Economy, Republic of Korea. The authors
would like to give special thank to SAMSUNG
Electro-Mechanics Co., Ltd. for their financial
supports in this study. This work, also, was partly
supported by the 2nd stage of BK21 project funded by
the Ministry of Education & Human Resources
Development, Republic of Korea.
REFERENCES
[1]
(a)
[2]
[3]
[4]
(b)
[5]
Fig. 7: Picture of DBFC three-cell module lighting 2
LED lights (a) OFF state (b) ON state.
[6]
4. CONCLUSION
The performance of a direct borohydride fuel cell
was studied. The DBFC has been designed and
fabricated very simple configuration which consists of
an anode, cathode, and a fuel chamber. The deposited
Au on a glass and MnO2/C on Ni-foam were used,
respectively, as anode and cathode catalysts. The
NaBH4 and KOH were also used as hydrogen source
and electrolyte. We characterize the performance of
the fabricated DBFC by point-by-point and continuous
polarization scans. The MnO2/C cathode catalyst of
50mg/cm2 in the composition of 70wt%-MnO2 and
30wt%-C shows the best performance, because of
increase of the electrochemical reaction by increasing
the active area and the smooth mass transport of the air.
Maximum power density of 1.18mW/ cm2 at 0.3V was
obtained for the DBFC. Stability test was conducted
under constant current at 1mA/cm2. The cell power
gradually decreased to 0.22mW after 1hour and we
could turn on 2 LEDs for about 1 hour by using the
fabricated DBFC three-cell module.
[7]
[8]
[9]
[10]
72
K. Shah, R.S. Besser, Novel microfabrication
approaches for directly patterning PEM fuel cell
membranes, J. Power sources, 123 172-181
S. Aravamudhan, S. Bhansali, Porous silicon
based orientation independent, self-priming
micro direct ethanol fuel cell, Sensors and
Actuators A, 123-124 497-504
M.A. Priestnall, V.P. Kotzeva, D.J. Fish, E.M
Nilsson, Compact mixed-reactant fuel cells, J.
Power Sources, 106 21-30
Rong Zeng, Pei Kang Shen, Selective membrane
electrode assemblies for bipolar plate-free mixedreactant fuel cells, J. of Power Sources 170 286–
290
Bin Hong Liu, Zhou Peng Li, S. Suda,
Development of high-performance planar
borohydride fuel cell modules for portable
applications, J. Power Source, 175 226-231
Steven C. Amendola, Stefanie L. Sharp-Goldman,
M. Saleem Janjua, Nicole C. Spencer, Michael T.
Kelly, Phillip J. Petillo, Michael Binder, A safe,
portable, hydrogen gas generator using aqueous
borohydride solution and Ru catalyst, Int. J.
Hydrogen Energy, 25 967-975
R.X. Feng, H. Dong, Y.D. Wang, X.P. Ai, Y.L.
Cao, H.X. Yang, A simple and high efficient
direct borohydride fuel cell with MnO2-catalyzed
cathode, Electrochemistry Comm., 7 449-452
S.U. Jeong, R.K. Kim, E.A. Cho, H. J. Kim, S.
W. Nam, I. H. Oh, S. A. Hong, S.H. Kim, A
study on hydrogen generation from NaBH4
solution using the high-performance Co-B
catalyst, J. Power Sources, 144 129-134
Go Young Moon, Sang Seo Lee, Kwan Young
Lee, Sung Hyun Kim, Kwang Ho Song, Behavior
of hydrogen evolution of aqueous sodium
borohydride solutions, J. of Ind. and Eng.
Chemistry, 14 94-99
A. Verma, S. Basu, Development of the direct
borohydride fuel cell, J. of Alloys and
Compounds 404–406 648-652