etc.In power plants, gaseous hydrogen is used for removing friction heat in turbines. In nickel-hydrogen batteries used in satellites, hydrogen is stored at high ...
Ionics 10 (2004)
63
Nation Based Amperometric Hydrogen Sensor G. Velayutham, C. Ramesh*, N. Murugesan*,V. Manivannan K.S. Dhathathreyan, and G. Periaswami* Centre For Electrochemical & Energy Research, SPIC Science Foundation, Chennai 600 032 *Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 Tamil Nadu, India
Abstract. A Nation based amperometric hydrogen sensor that operates at room temperature has been developed. The electrolyte used in the sensor is Nation 117, which is a proton conducting solid polymer electrolyte. Palladium catalyst was used on the sensing side and platinum supported on carbon on the air side. The sensor functions as fuel cell, H2/Pd // Nation // Pt/O2 and the short circuit current is measured. The short circuit current is found to be linear with respect to concentration of hydrogen on the sensing side. The sensor is able to detect the concentration of hydrogen in argon down to ppb level. Details of assembly of the sensor, response behavior and applications are discussed.
1.
Introduction
The future economy is projected as hydrogen economy and fuel cells may become the energy source either replacing or augmenting the present oil based technology [1,2]. The pollution effect and the global warming due to environmentally unfriendly technology of today have adverse effect on the sustenance of life system on this planet [3]. There is a sustained effort today to develop commercially viable fuel cell technology. For automotive and portable power source applications, proton exchange membrane based fuel cells are the optimal choice because of low temperature operation and high power density. In fuel cells hydrogen rich gases are used either directly or via a reformer. For successful utility of any technology the same should be user friendly and safe. For safe usage of
etc.In power plants, gaseous hydrogen is used for removing friction heat in turbines. In nickel-hydrogen batteries used in satellites, hydrogen is stored at high pressure and has to be monitored for leaks [4]. In fast reactors, hydrogen monitoring in the cover gas is essential to detect steam generator leaks [5]. Research and development on hydrogen sensors is necessary for development of reliable sensors with good sensitivity, selectivity, response time and long-term stability. Detection and measurement of hydrogen at low concentration has always been a technological challenge. Types of hydrogen sensors available include
electro-
chemical, catalytic, semiconductor and resistivity sensors. Electrochemical sensors include both potentiometric [6,7] and amperometric [8,9] devices.
fuel cell technology hydrogen used as a fuel in fuel cells has
Even though the potentiometric sensors have a wide
to be monitored continuously. Hydrogen forms an explosive
dynamic range compared to amperometric sensor, they lack accuracy at higher concentrations because of their loga-
mixture with oxygen when concentration of hydrogen exceeds the lower explosion limit of 4% hydrogen in air. Hence the development of hydrogen sensors has gained importance these days for application in fuel cells. In addition to fuel cell application hydrogen sensors find
rithmic response. Amperometric sensors are linear in their response and are more accurate. Energy consumption is more for catalytic sensors which work at higher temperatures. Since the electrochemical sensor reported here works
utility in other areas also. Hydrogen is used extensively in
at room temperature on the fuel-cell principle,it does not
many industries as a chemical, as cryogenic-fuel in rockets
require power supply to operate it. Besides, this sensor can
64 also operate in the absence of oxygen on the sensing side,
Ionics 10 (2004) In the present
sensor,
nation
is
used
as
proton
whereas the availability of oxygen is essential for pellister
conducting electrolyte due to the better proton conductivity
sensors. In applications like cover gas monitoring in fast
of Nafion. Both palladium and platinum were attempted on the sensing side as electrode material. The electrodes were
reactors, where there is no oxygen on the sensing side, the present sensor based on a proton-conducting solid electrolyte becomes advantageous. An ideal sensor should be cost effective and relatively low in maintenance. Solid and liquid proton-conducting electrolytes may be used in electrochemical hydrogen sensors. Corrosion and leakage can occur when liquid is used as electrolytes. Solid electrolytes have advantage in this respect over liquid electrolytes. Most of the solid state proton conductors reported in literature (HUP, hydrated WO3 ) need humidification of the sample gas. Conducting solid polymer electrolytes
prepared by a chemical procedure called decal method. It was observed that palladium is more suitable as anode when hydrogen concentration is very low. The sensor is sensitive to less than 5 ppm of hydrogen in argon. At higher concentrations of hydrogen, platinum gave better response behaviour as anode material. The paper discusses the sensor design, and the sensor response to hydrogen.
2. Experimental Procedure 2.1. Materials Preparation and
Sensor
Assembly.
include a PVA/H3PO4 blend, Nation, acid-doped polyben-
Schematic of the sensor assembly is given in Fig.
zimidazole, etc. where humidification is not essential.
Commercial
Nafion has already been used as a electrolyte for sensor applications [10]. We have previously developed an am-
(Nation117) was used as proton conducting solid polymer electrolyte. The membrane was pretreated using three weight
perometric sensor that uses a PVA/H3PO4 blend as electrolyte and Pd thin films as electrodes and tested its
percent hydrogen peroxide at 100 ~ for one hour to remove organic impurities.After washing,the membrane was put in
performance [11]. The combination of high lattice hydrogen solubility and high values of the hydrogen diffusion
one molar sulphutic acid at 100 ~ for thirty minutes to eliminate metallic impurities.The membrane was thourughly
coefficient and retention of metallic elasticity makes palladium the best choice for the sensor anode at low hydrogen
washed in deionised water.The membrane is ready for
partial pressures. Platinum is used as cathode owing to its lower activation energy for reduction of oxygen than palladium. The cell functions in fuel cell mode with hydrogen in argon on the sensing side and air on the counter electrode side. The sensor is sensitive at the parts per million (ppm) level of hydrogen in argon. The sensor was developed for monitoring hydrogen in argon cover gas in the fast reactor at IGCAR, Kalpakkam, India and also to extend its utility for other applications requiting hydrogen monitoring.
Nation
membrane
from
Dupont,
1.
USA
the
catalyst coating. Either palladium black or Platinum black was used as anode. A slurry of anode catalyst was prepared by mixing with water followed by ultrasonication for homogenization. A small amount of ethylene glycol was added and a known amount of Nation was added to the slurry. The slurry was sonicated for ten minutes .The slurry is now ready for coating on the substrate.The cathode on the counter electrode side of the sensor was platinum supported on carbon and the active area was 20% of the surface area. A slurry of the catalyst was prepared by adding water to platinum on carbon, mixing well and sonicating for a few minutes. To this mixture, isopropyl alcohol was added and sonicated until a homogeneous ink-like paste was obtained. The slurry was coated on the polyimide film(Kapton) sintered at 100 ~ in the atmosphere for one hour.
and
The pretreated Nafion membrane was kept over the catalyst coated polyamide film and was hot pressed under pressure. The catalyst gets adhered to the Nation membrane. Similar procedure was adapted for both anode and cathode for coating the catalyst. The film on the sensing side was exposed to the gas containing hydrogen. The electrode on the other side was exposed to air. The two electrodes wel~e Fig .1 Schematic of the sensor assembly.
short-circuited and the short-circuit current was measured using a sensitive current measuring device interfaced to a
Ionics 10 (2004)
65 6O
Ar Cylinder
:t
,00opP
4s -~
Mass Flow controller-1
'~1 2O
Mass Flow controller-2
15 "1 10-1 -I
150PPM
|
l I-I~/ArCylinder I
200PPM / ~
.
.
0
.
300PPM ~ I { 1 f
250PPM ('~ t ]
t
I
.
2000
I ]
I
I
=
4000
6000
.
!
8000
10000
Time (s)
Fig 2. A block diagram of the experimental set-up for hydrogen measurement.
Fig. 4. The response curve of the sensor(SEN2).
concentration was let into the sensing side at the same flow computer and related to the concentration of hydrogen on the
rate as argon and retained until a steady-state current was
sensing side.
reached. Once the measurement was over, the H J A r flow was stopped and A r flow resumed until the original baseline
3.
Measurement
current was reached. The experiment was repeated for
A block diagram of the experimental set-up for hydrogen
various concentrations of H2 in Ar and the calibration curve
measurement is shown in Fig. 2. Commercially available
was obtained by plotting the steady-state current against the
calibration gas mixtures (SGL, Mumbai) were used for
concentration of hydrogen in argon.
calibration of the sensor. The required concentration of
Since the current measuring device was interfaced to a
hydrogen in argon was prepared by blending a commercially
computer, it was possible to observe online the response of
available calibrated Ar/H2 mixture with argon, using a gas
the sensor to varying concentrations of hydrogen in argon.
blending system consisting of two mass flow controllers
The response of the sensor was also observed over a period
(MKS, USA) connected to a mixing chamber. Initially,
of time so as to assess its long-term stability. The sensor
commercial argon was passed (flow rate 300 standard cm 3
was tested at ambient temperature.
min q ) on the sensing side of the sensor until the baseline current was steady. Airflow was maintained at the counter
4.
Results and Discussion
electrode side continuously. A H:/Ar mixture of known
The Nation based sensors were calibrated for its response to 70-
80-
60
5-25 PPM
f
EO-
50
50-
30 20
10
lOo"
0
-10
!
0
510
1:;00
1530
2r
;5~
319
m
350O
[
2
-
i
4
9
I
6
,
i
8
9
i
10
,
i
12
,
i
14
9
i
9
16
C o n c . o f II=(PPM)
Tram (~
Fig. 3. The response curve of the sensor (SEN1).
i
18
Fig. 5. The calibration plots for SEN1.
9
i
20
9
i
22
9
u
24
9
i
26
,
i
28
66
Ionics 10 (2004)
240
/
220
Jr/'/"
\.
"G" 200
,I(
\
\ \
,= ?80
j-.
\,
j,
c"
\
4['"
3
J
~6o-
/
\
140 -
.mr
Y -200
i 0
m
.......
W
120 i"' 200
,'
i 400
Conc. o f
600
800
i I000
i 1200
Conc.of H2(PPM)
H2(PPN)
Fig. 6. The calibration plots for SEN2.
hydrogen in argon in the concentration range 5 ppm - 1000 ppm. Figure 3 is the response curve of the sensor in the concentration range 5 ppm - 25 ppm.This sensor operable at low concentration of hydrogen (SEN1) has palladium on the sensing side and platinum on the counter electrode side. Figure 4 is the response curve of the sensor(SEN2) in the concentration range 50 ppm - 1 0 0 0 ppm. This sensor operable at higher concentration (SEN2) has platinum on
Fig. 7a. Plot of response time vs. concentration of Hydrogen for SEN1.
240220,,.,200' IN
,2G 180~ .E t6o. ~140-
both the sensing side and counter electrode side.Figures 5 and 6 are the calibration plots for SEN1 and SEN2 obtained by plotting the limiting current against the concentration of hydrogen in argon. It is clear from the response curves for hydrogen in argon for SEN1 that the sensor is able to respond fast to change in hydrogen concentration. The 90% response time
80-' 60-
4
:60
460
660
soo
000
1200
Conc. of H2 (ppm) Fig 7b. Plot of response time vs. concentration of Hydrogen for SEN2.
for SEN 1 is less than two minutes at concentration 25 ppm and this is evident from Fig. 7a. However, at higher concentration the 90% response time for SEN2 is less than
anodic process no more a diffusion controlled process. The
a minute at concentration of hydrogen more than 100 ppm
oxidation is now no longer dependant on the supply of hydrogen at the Palladium electrode-electrolyte interface
as seen Fig. 7b.
since dissolved hydrogen available at the electrode elec-
Details of the mechanism of the sensor behavior are described in [12,13]. The oxidation of hydrogen at the
trolyte interface is able to cater to the removal of hydrogen
sensing electrode is a diffusion-controlled process and is the
due to electrochemical process at the interface. Since Pla-
rate determining step. Hence the limiting current in short circuit mode is linearly dependant on the concentration of
tinum has lesser solubility for hydrogen, plateauing of the
hydrogen 9 Since palladium has high sticking coefficient
concentration gradient occur at much higher concentration and hence response is linear with concentration up to much
compared to platinum, palladium is more suitable as sen-
higher concentration of hydrogen in argon. In our study,
sing electrode at low concentration of hydrogen .At higher
SEN1 was linear up to 25 ppm and SEN2 up to 1000 ppm.
concentration of hydrogen at the palladium anode, the high
The high sticking coefficient of hydrogen for palladium is
solubility of hydrogen leads to non-linear behavior and
responsible for its high sensitivity at low concentration
saturation in limiting current. The high solubility leads to decreased concentration gradient which in turn makes the
while lesser sticking coefficient of platinum limits the sensitivity at low concentration.
Ionics 10 (2004)
67
A polymer electrolyte based amperometric hydrogen sensor based on Nation was designed, assembled and tested for its response. The sensor uses Nation membrane as proton conducting solid polymer electrolyte. A calibration run of the sensor was carried out for hydrogen in argon and the sensor is found to respond linearly with increase in hydrogen concentration. Further work is in progress to check the long-term behavior of the sensor.
[4]
5. Conclusion A polymer electrolyte fuel cell based amperometric hydrogen sensor using a Nation as electrolyte and Pd and Pt as anode and Pt as cathode has been developed and its response to hydrogen studied in 5 ppm to 1000 ppm. The sensor is operable at room temperature and can monitor hydrogen in argon atmosphere. The variation of response time with various anode has been investigated. When Pt is used on the sensing side the response behaviour is linear with respect to concentration of hydrogen ranging from 50 to 1000 ppm. However, at lower concentration Palladium anode is the preferred catalyst compared to Platinum on the sensing side. Further studies are required to widen the concentration range of hydrogen and also for the longterm stability of the sensor.
[7]
[5]
[6]
[8] [9] [10] [11] [12]
[13] 6. Reference [1] Karl Kordesch, Gunter Simader (1996) Fuel Cells and their applications: VCH publishers, Inc., New York, U.S.A., Chapter 4. [2] K.S. Dhathathreyan, P. Sridhar,, G. Sasikumar, K.K. Ghosh, G.V. Velayutham, N. Rajalakshmi, C.K. Subramaniam, M. Raja, and K. Ramya, Development of Polymer Electrolyte Membrane Fuel Cell Stack, Int. J. Hydrogen Energy 24, 1107 (1999). [3] B.K. Datta and G Velayutham "Fuel cell power source for cold region" paper presented in Seventh Grove Fuel Cell Symposium, London, UK, Sep. 2001.
David Linden, Handbook of Fuel Cell and Batteries, MacGraw-Hill, Chapter 22. J. Jung, H. Runge, Monitoring systems for the surveillance of non metallic impurities in sodium and cover gas, Proc. 4th Int. Conf. on Liquid Metal Engineering and Technology Vol. 3, Avignon, France, 1988, 603-10. N. Miura, H. Kato, Y. Ozawa, N. Yamazoe and T. Seiyama, Chem. Letters 1573-1576 (1983). N. Miura and N. Yamazoe, Development of a solidstate gas sensor using proton conductor operative at room temperature, Chemical Sensor Technology, Vol. 1, Kodansha, Tokyo/Elsevier, Amsterdam, 1988, 123-139. N. Miura, H. Kato, Y. Ozawa, N. Yamazoe and T. Seiyama, Chem. Letters 1905-1908 (1984). N. Miura, T. Harada, N. Yamazoe, J. Electrochem. Soc. 136, 1215 (1989). Marc Madou and Takaaki Otagawa, Solid State Ionics 28-30, 1653 (1988). C. Ramesh. et al., Indian patent No. 186660. N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Fundamentals and Applications of Chemical Sensors, 1986, 203-14. C. Ramesh, N. Murugesan, V. Ganesan and G. Periaswami, Journal of Solid State Electrochemistry 7, 510 (2003).
Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 2830, 2003. Manscript rec. Dec. 1, 2003; rev. Jan. 30, 2004; acc. Feb. 13, 2004.
