solid polymer electrolyte-based hydrogen sulfide sensor
TRANSCRIPT
Solid polymer electrolyte-based hydrogen sulfide sensor
Yourong Wang, Heqing Yan*, E’feng WangDepartment of Chemistry, Wuhan University, Wuhan 430072, China
Received 17 September 2001; received in revised form 8 February 2002; accepted 29 May 2002
Abstract
The performance of the solid polymer electrolyte (SPE)-H2S sensor has been studied. It was found that the electrochemical oxidation of H2S was
controlled by the gas diffusion. That is the basis of the quantitative determination of H2S. The factors affecting the stability of the sensor have been
studied. The results indicated that elemental sulfur was the main factor. In contrast with the H2S sensors reported in the liquid electrolyte systems,
the stability of SPE-H2S sensor is better. The result was believed to be directly related to the porous and channeled structure of SPE-H2S electrode.
In addition, the SPE-H2S sensor has many advantages, including a fast response, a satisfactory linearity and good reproducibility.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Solid polymer electrolyte (SPE); Hydrogen sulfide; Sensor
1. Introduction
Hydrogen sulfide is a malodorous, corrosive and toxic
gas. In the world, about 100 million tonnes hydrogen sulfide
is given off every year [1]. It is an extremely toxic com-
pound. It produces severe effects on the nervous system at
low concentrations and causes fatalities at higher concen-
trations. So determination of hydrogen sulfide is of great
importance, especially in situ monitoring of hydrogen sul-
fide. A large number of methods have been reported for the
determination of hydrogen sulfide. Solid electrolyte sensors
have been reported [2–4], but it must work at higher
temperature. Another common sensors [5–8] for monitoring
hydrogen sulfide in the air contain a liquid electrolyte
generally and have leakage problems. The development
of solid polymer electrolyte (SPE) provides a possibility
to make electrochemical sensors without liquid solution,
room temperature, solid state electrochemical sensors. Our
group has reported the application of SPE hydrophobic gas
electrodes for the measurements of O2 and CO [9–13]. The
measurement is based on a quantitative relationship between
the current of O2 reducing or CO oxidation on a SPE-Pt
electrode and the amount of O2 or CO present in sample gas.
In the paper, a SPE-Pt hydrophobic gas diffusion electrode-
based electrochemical hydrogen sulfide sensor is reported.
The sensor does not contain any liquid electrolyte and avoids
the problems of drying, corrosion and pollution caused by
the leakage of liquid electrolyte. To the best of our knowl-
edge, there is no report yet about applying SPE-Pt hydro-
phobic gas electrode technology to quantify hydrogen
sulfide based on the current measurement. The SPE electro-
chemical sensor for hydrogen sulfide exhibited good repro-
ducibility, a fast response time (<10 s), a satisfactory
linearity (0–100 ppm).
2. Experimental
The scheme of the SPE hydrogen sulfide sensor used in
the experiment is shown in Fig. 1. A piece of acid-treated
(H2SO4, 4 mol/dm3) Nafion 117 membrane (perfluorinated
sulfonic cation exchange membrane, DuPont product) was
used as the SPE membrane. The working electrode (s ¼0:5 cm2) was Teflon-bonded Pt black membrane pressed on
one side of the membrane. On the other side of the mem-
brane were pressed two pieces of Teflon-bonded Pt black
membrane. The bigger one (s ¼ 0:5 cm2) served as the
counter electrode, the smaller one (s ¼ 0:1 cm2) faced
the surrounding air and served as the reference electrode.
In the sensor, there was a protective porous gas-permeable
Teflon sheet in front of the working electrode. On top of
the protective Teflon sheet was a perforated plastic plate to
help the electrode/SPE assembly to be kept in position. The
sensor cell was controlled using a laboratory potentiostat
(model SHD-1 potentiostat, Yanbian Electrochemical
Instruments Factory, China). The current generated by
the electrochemical oxidation of the hydrogen sulfide was
Sensors and Actuators B 87 (2002) 115–121
* Corresponding author. Fax: þ86-27-8764-7617.
E-mail address: [email protected] (H. Yan).
0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 2 2 7 - 7
displayed on a simple potentiostat. The response curve was
recorded on a X-Y recorder (model 3086 X-Y recorder, the
forth Instruments Factory, Sichuan, China). A mixture of
H2S and N2 as well as pure N2 was obtained from Beijing
Beifen Gas Products. The different concentrations of H2S in
the H2S–N2 mixtures were prepared using a gas proportioner
(model 7472-37, Matheson Gas Products, USA). XPS ana-
lysis was conducted using a model XSAM800 instrument
(KRATOS product). An Mg Ka target at 1253.6 eV and
16 mA � 12:5 kV was used in the experiment. The sample
was detected under 2 � 10�7 Pa. The reference energy was
versus C1s (284.6 eV). In the study of the humidity effects on
the stability of the SPE sensor, Drierite, saturated
MgCl2�6H2O, NaCl and K2SO4 solutions were used to
maintain the relative humidity in a chamber at 0, 35, 75
and 96%, respectively, at room temperature. All chemicals
used were chemical pure. All potentials reported were versus
air reference electrode. All experiments were conducted at
room temperature.
3. Results and discussion
3.1. Principle of quantitative measurement and linearity
A steady-state polarization curve of the oxidation of 1%
H2S in a H2S–N2 mixture on SPE-Pt electrode is shown in
Fig. 2. As is shown in this figure, the electrochemical oxida-
tion current of H2S increased significantly with the increase in
potential up to 0.1 V versus the reference electrode. The
oxidation current of H2S reaches a plateau when the polar-
ization potential is higher than 0.1 V. The limiting current
value in this potential region is controlled by the mass transfer
of H2S through the porous working electrode. According to
Fick law, the limited current of the sensor can be represented
by following equation under limiting diffusion conditions:
Id ¼ KCH2S
where K is a constant. So at fixed potential (for example,
j ¼ 0:3 V), the output of the sensor was proportional to the
concentration of H2S in sample gas. It is the reason that the
sensor can be used to determinate the concentration of H2S.
Fig. 3 shows the relationship between the oxidation current
of 1% H2S and the flow rate of the gas sample when the SPE-
Pt electrode was controlled at 0.3 V. It was found in experi-
ment that the sensor response was influenced by the rate when
the flow rate is less than 100 ml/min. But the output current of
the sensor reached a diffusion-limited value at flow rate higher
than 100 ml/min. Therefore, in practical application, the flow
rate was held at 100 ml/min. Besides, the different numbers of
Teflon porous membrane are put in front of the working
electrode had no evident effect on the limiting diffusion
current. As we know, there are three factors that affect the
output current of the sensor in the electrode process; they are
the flow rate of the sample gas, the rate of the gas diffusion
and the rate of H2S reduction. Because the working electrode
Fig. 1. Scheme of the SPE H2S sensor. 1, plate shell; 2, perforated Teflon
plate; 3, porous gas-permeable Teflon sheet; 4, working electrode; 5, Nafion
membrane; 6, counter electrode; 7, reference electrode; 8, opening to gas.
Fig. 2. The steady-state polarization curve of hydrogen sulphide.
116 Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121
is controlled at higher potential, the rate of H2S reduction
is fast. When the flow rate of the sample gas is less than
100 ml/min, the flow rate determines the current value. When
the flow rate is more than 100 ml/min, the rate of the gas
diffusion determines the current value. In other words, the
electrode process is controlled by the diffusion of H2S
through catalyst membrane of the working electrode at high
flow rate. Following experiment’s results can further prove
this conclusion.
The rest potential of the SPE-Pt electrode decreases
nearly linearly with increasing H2S concentrations in
H2S–N2 gas mixtures. Fig. 4 shows this linear relationship
with a slope of about �3.3 mV/100 ppm H2S. Miura et al.
[14] actually observed a similar phenomenon but with a
linearity occurring between the rest potential and logarithm
of the CO concentration for metal–membrane systems. This
kind of dependence of the rest potential on the concentration
of electroactive species is usually due to the coupled anodic
and cathodic reactions, which take place simultaneously on
the electrode. As a result, the apparent potential observed
(i.e. the rest potential) is actually a mixed potential, directly
related to the coupled reactions and thus to the concentra-
tions of species. For the given system, previous work [15]
showed that reaction product of electrode oxidation of
hydrogen sulfide was sulfate ion. These coupled reactions
are as follows:
anodic : H2S þ 4H2O ! SO42� þ 10Hþ þ 8e� (A)
cathodic : O2 þ 4Hþ þ 4e� ! 2H2O (B)
These coupled reactions decided the potential of the sensing
electrode. This reduction of oxygen may take place on SPE-
Pt electrodes in a low overvoltage or in the so-called linear
polarization region under the given conditions. That is,
EB ¼ a þ bIB
where EB is the cathodic potential, IB the cathodic current,
and a and b the constants. The anodic reaction could take
place under limiting diffusion conditions, thus:
IA ¼ KCH2S
where IA is the anodic current and K a constant. The mixed
potential or OCP of the electrode, at which the anodic
current IA is equal to the cathodic one IB, can be expressed
as follows:
EM ¼ a þ b0CH2S
This equation indicates a linear relationship between OCP and
the concentration of H2S. The experimental results support
above theoretical prediction. The quantitative measurements
of H2S concentrations in H2S–N2 gas mixtures were carried
out at a controlled potential (E ¼ 0:3 V versus reference
electrode). The oxidation current of H2S is shown in Fig. 5.
A linear relation between the oxidation currents and the H2S
concentration was found under the experimental condition.
3.2. The response time
Hydrogen sulfide was oxidized electrolytically after intro-
duction of a sample; the current generated rapidly reached a
steady-state value. The response curve of 100 ppm H2S was
showed in Fig. 6. According to Fig. 6, it was concluded that
90% response time was 6.3 s. The rapid response of the
sensor to hydrogen sulfide is attributed to the high electro-
catalytic activity of the working electrode and the small RC
time constant of the catalyst layer of the working electrode
and the small RC time constant of the catalyst layer of the
working electrode and the small RC time constant of the
catalyst layer of the working electrode. (RC describes
the process of potential redistribution of the gas sample,
Fig. 3. The relationship between the oxidation current and the flow rate:
(~) no gas-permeable Teflon membrane; (&) single gas-permeable Teflon
membrane; (*) double gas-permeable Teflon membranes.
Fig. 4. the relationship between the mixed and the H2S concentration.
Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121 117
RC / rCL2, where r, C and L are the apparent specific ionic
resistance, apparent specific capacitance and thickness of the
porous catalyst layer of the working electrode, respectively.)
A detailed analysis of the response time has been published
elsewhere [16].
3.3. The stability of the sensor
The stability of the SPE-H2S sensor was tested consecu-
tively over 7 months in ambient temperature. In the experi-
ment, 100 ppm H2S was introduced into the sensor for 4 h
per day. The potential of the working electrode was kept at
0.3 V. The output current of the SPE-H2S sensor in first 2
months decayed significantly as in Fig. 7. Subsequently the
output current had no evident change. In the literature [15],
we compared the decay on the SPE-Pt electrode to one on
the smooth Pt electrode in the liquid electrolyte system,
which showed the decay on the SPE-Pt electrode is slowerFig. 5. A linear relation between the oxidation currents and the H2S
concentration.
Fig. 6. Response curve of SPE-Pt electrode.
Fig. 7. The curve of the stability in natural humidity environment.
118 Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121
than one on the smooth Pt electrode. It means the stability of
SPE-H2S sensor is better. The decay of the output current of
the sensor may attribute to the deterioration of the electro-
catalytic activity of the working electrode. This deterioration
can be sustained by measuring the half-wave potential of the
H2S sensor. The half-wave potential is closely related to the
standard potential of a specific reaction and is independent of
the reactant concentration. Thus, the measurement of the half-
wave potential under the conditions of our studies would
provide insight into the specific reaction, such as sensor
deterioration. Fig. 8 shows the characteristic curve of the
H2S sensor. Curve ‘a’ was the sensor characteristic on day 1,
whereas curve ‘b’ was the sensor characteristic when H2S
(1% H2S, 40 ml/min, 1 h per day) was introduced into the
sensor for 20 days. As was shown, the half-wave potential
positively shifts positively to approximately 150 mV. It sug-
gested that the electrocatalytic activity of the sensor deterio-
rated. To investigate the reasons that cause the deterioration of
the electrocatalytic activity of the H2S sensor, the effects of
humidity and deposited sulfur have been studied.
3.4. The study of humidity level effects on the stability
of the sensor
The SPE-Pt sensor was placed inside a sealed chamber.
Constant humidity inside the chamber was maintained
according to the method described by Spencer [17]. A
saturated salt aqueous solution was used to maintain con-
stant humidity. Based on the chemicals chosen, these would
maintain the humidity inside the chamber at room tempera-
ture. In our studies, Drierite, saturated MgCl2�6H2O, NaCl
and K2SO4 solution were used to maintain the relative
humidity level inside the chamber at 0, 32, 75 and 96%,
respectively, at room temperature. The potential of the
working electrode is controlled at 0.3 V. Fig. 9 shows the
results of the SPE-H2S sensor output over 2 months in
the conditions of different humidity at a fixed concentra-
tion of 100 ppm H2S in H2S–N2 mixture. As was shown, the
output of the SPE-H2S sensor decayed hardly at 0% relative
humidity. In the case of 32 and 96% relative humidity, the
sensor current outputs decreased to approximately 55% of
their initial values after a period of 30 days and remained at
this level for next 30 days. In the case of 75% relative
humidity, however, the current decreased to 77% in the same
conditions. The influence of humidity on SPE-H2S is similar
with that on SPE-O2 sensor [18]. In a high-humidity envir-
onment, for example, 96% relative humidity or more, the
SPE membrane absorbed more water from the atmosphere.
This leads to flooding of the working electrode; conse-
quently affecting the sensor performance adversely. In a
low-humidity environment, for example, 32%, the water in
the SPE membrane vaporized to the environment. This leads
to increasing of the SPE membrane resistance especially at
0% relative humidity, the sensor decayed hardly by fast
drying of the SPE membrane. In middling-humidity envir-
onment, for example, 75%, because of the less change of the
water quantity in the SPE membrane, this leads to less
deterioration of the output current. But according to litera-
ture [18], in the same conditions, the influence of humidity
on the SPE-O2 sensor is less than on the SPE-H2S sensor.
This indicated that the reason of the deterioration of SPE-
H2S sensor was not only the relative humidity level.
3.5. The study of deposited sulfur effects on the stability
of the sensor
Previous work [15] indicated that there was elemental
sulfur, which deposited on the SPE-Pt electrode when H2S
was introduced into the H2S sensor. Although the potential of
the working electrode of H2S sensor was higher, elemental
sulfur was still unavoidable. As we know, elemental sulfur
can make the SPE-Pt electrode poison. To investigate the
effect of elemental sulfur on the stability of the sensor, we
studied the percent of elemental sulfur in SPE-Pt electrode by
Fig. 8. I-E characteristics of hydrogen sulphide sensors after (a) 1 day; (b)
20 days.
Fig. 9. The relationship of the oxidation current with time at different
humidity: (&) 0%; (~) 32%; (*) 75%; (!) 96%.
Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121 119
XPS when 1% H2S was introduced continuously. The tested
result is shown in Fig. 10. The percent of elemental sulfur on
SPE-Pt electrode increased quickly from 0 to 50% in first
60 min, but when the time of introduced H2S increased, the
percent of elemental sulfur increased slower and slower
(from 50 to 55% in the next 120 min). This is analogous
with the decay of the stability of the sensor in Fig. 7. From
above experiment, a conclusion that deposited elemental
sulfur on the working electrode should be the main factor
influenced the stability of sensor could be drawn.
4. Answer the questions
1. The humidity influences the output of the SPE-H2S sensor
indeed. The reason is the change in water-retention capacity
of the membrane in the different humidity environment.
This has been discussed in our revised paper. But it is not
only one factor. According to literature [18], in the same
conditions, the influence of humidity on the SPE-O2 sensor
is less than on the SPE-H2S sensor. This indicated that the
reason of the deterioration of SPE-H2S sensor was not only
the relative humidity level. In other words, besides the
humidity, there are other factors that affect the deterioration
of SPE-H2S, for example, the deposited elemental sulfur.
2. Concerning elemental sulfur absorption, according to
the result of XPS, the percent of elemental sulfur on
SPE-Pt electrode increased quickly from 0 to 50% in
first 60 min, but when the time of introduced H2S
increased, the percent of elemental sulfur increased
slower and slower (from 50 to 55% in the next 120 min).
5. Conclusions
The performance of the SPE-H2S sensor was studied. It
was found that the electrochemical oxidation of H2S was
controlled by the gas diffusion through the catalyst mem-
brane of working electrode. That is the basis of the quanti-
tative determination of H2S. As literature [15] reported, the
SPE working electrode has better stability than the smooth
Pt electrode in liquid electrolyte. Therefore, the develop-
ment of SPE sensor proves a possibility to make practical
electrochemical sensors without liquid solutions, i.e. room
temperature solid state electrochemical sensors. The better
stability of SPE-H2S sensor was believed to be directly
related to the porous and channeled structure of SPE-H2S
electrode. In our experiments, the factors affecting the
stability of the sensor had been studied. The results indicated
that elemental sulfur was the main factor. As we reported
[15], elemental sulfur could make the sensing electrode
poison. The sensing electrode used in our experiment was
a type of SPE hydrophobic gas diffusion electrode, which
consisted of the small Pt particles. These Pt particles were
distributed with a typical porous structure. The porous and
channeled structure increased the real surface area and thus
enhanced the electrocatalytic activity and capacity of the
electrode. The working electrode of sensor was controlled in
higher potential to decrease the forming of elemental sulfur.
All factories were useful for increasing the lifetime of the
SPE-H2S sensor. In addition, the SPE-H2S sensor has many
advantages, including a fast response, a satisfactory linearity
and good reproducibility.
Acknowledgements
This work was supported by the Science Funds of the
Academy of Science of Wuhan in China.
References
[1] C. Jiang, Zhong guo da bei ke quan shu, Huan Jing Ke Xue (1983) 253.
[2] Y. Yan, N. Miura, N. Yamazoe, Chem. Lett. 9 (1994) 1753.
[3] N. Miura, Y. Yan, G. Lu, N. Yamazoe, Sens. Actuators B 34 (1996) 367.
[4] N. Yamazo, N. Miura, M. Ando, C. Nakayama, Japanese Patent 04
62466 (1992).
[5] J.M. Sedlak, K.F. Blurton, Talanta 23 (1976) 445.
[6] X. Qi, Z. Wang, W. Tan, X. Li, Yun Nan Daxue Xuebao, Vol. 19, No.
1, 1997 (Chapter 14).
[7] A.V. Kroll, V.I. Smorchkov, A.Y. Nazarenko, Sens. Actuators B 21
(1994) 97.
[8] P. Jeroschewski, K. Haase, A. Trommer, P. Grundler, Electroanalysis
6 (1994) 769.
[9] H. Yan, J. Lu, Field Anal. Chem. Technol. 1 (3) (1997) 175.
[10] H. Yan, J. Lu, Sens. Actuators 19 (1989) 33.
[11] H. Yan, J. Lu, Sens. Actuators B 17 (1994) 165.
[12] J. Yang, H. Yan, J. Lu, Wu Han Da Xue Xue Bao (Wuhan Univ. Nat.
Sci.) 43 (6) (1997) 735.
[13] H. Yan, J. Lu, E. Wang, Gao Deng Xue Xiao Hua Xue Xue Bao 9
(1991) 1216.
[14] N. Miura, H. Kato, N. Yamazoe, T. Seiyama, in: Proceedings of the
International Meeting on Chemistry and the Senses, 1983, 233 pp;
X.-K. Xing, C.C. Liu, Electroanalysis 3 (1991) 115.
[15] Y. Wang, H. Yan, E. Wang, J. Electroanal. Chem. 497 (2001) 163.
[16] J. Yang, H. Yan, J. Lu, Wuhan Univ. J. Nat. Sci. 5 (3) (2000) 342.
Fig. 10. The curve of the percent of elemental sulfur, flow rate: 40 ml/min,
1% H2S.
120 Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121
[17] H.M. Spencer, Laboratory methods for maintaining constant humidity,
in: International Critical Tables, Vol. 1, McGraw-Hill, New York, 67 pp.
[18] H. Yan, C.C. Liu, Sens. Actuators B 10 (1993) 133.
Biographies
Yourong Wang is a graduate student at the Department of Chemistry,
Wuhan University, Wuhan, China. She is engaged in researching gas
electrochemical sensors.
Heqing Yan is a professor at the Department of Chemistry, Wuhan
University, Wuhan, China. He visited the Electronics Design Center at
Case Western Reserve University, USA, during 1990–1992 as a visiting
scholar. His research interests include electrochemical sensors and the
research of electrocatalysis.
E’feng Wang is an engineer at Wuhan University, Wuhan, China. She
graduated from the University in 1975. Her current field of interest is the
development of electrochemical sensors.
Y. Wang et al. / Sensors and Actuators B 87 (2002) 115–121 121