Solid polymer electrolyte-based hydrogen sulfide sensor

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  • Solid polymer electrolyte-based hydrogen sulfide sensor

    Yourong Wang, Heqing Yan*, Efeng 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 [24], but it must work at higher

    temperature. Another common sensors [58] 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 [913]. 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 (

  • 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 H2SN2 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 and16 mA 12:5 kV was used in the experiment. The samplewas detected under 2 107 Pa. The reference energy wasversus C1s (284.6 eV). In the study of the humidity effects on

    the stability of the SPE sensor, Drierite, saturated

    MgCl26H2O, NaCl and K2SO4 solutions were used tomaintain 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 H2SN2 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 KCH2Swhere K is a constant. So at fixed potential (for example,

    j 0:3 V), the output of the sensor was proportional to theconcentration 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) 115121

  • 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 experiments results can further prove

    this conclusion.

    The rest potential of the SPE-Pt electrode decreases

    nearly linearly with increasing H2S concentrations in

    H2SN2 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 metalmembrane 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 4H2O ! SO42 10H 8e (A)cathodic : O2 4H 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 a bIBwhere 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 KCH2Swhere 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 a b0CH2SThis 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 H2SN2 gas mixtures were carried

    out at a controlled potential (E 0:3 V versus referenceelectrode). 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 Teflonmembrane; (*) 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) 115121 117

  • RC / rCL2, where r, C and L are the apparent specific ionicresistance, 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) 115121

  • 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 MgCl26H2O, NaCland 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 H2SN2 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 aft...

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