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Design, simulation, fabrication and testing ofelectrochemical NO2 gas sensor

CONFERENCE PAPER MARCH 2015

DOI: 10.1109/ISPTS.2015.7220127

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4 AUTHORS, INCLUDING:

Umesh Yadav

Savirtibai Phule Pune University

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Ravindra Sarje


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Shashikala Gangal


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Available from: Ravindra Sarje

Retrieved on: 09 November 2015
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Umesh Yadav, Ravindra Sarje, A.D.Shaligram, S.A.GangalDept. of Electronic Science, SP Pune University, Pune-411007

[email protected],

Abstract- An electrochemical amperometric sensor measures theelectric current on the surface of the working electrodes and

estimates the concentration of chemical or biological species.

Electrochemical sensor is fast and selective but electrochemical

sensing is a complex phenomenon involving both physical and

chemical processes. With a view to understand the basic operation

of electrochemical sensor, present study demonstrates a steady-

state analysis of the current drawn in a cell of electrolyte solution

with two and three electrode configuration. Simulations are

carried out for 2- dimensional geometry using COMSOL

Multiphysics suite. Effect of variation in cell dimensions,

electrolyte concentrations and electrode dimensions on the

performance of the sensor is studied. For validation of simulationresult, a two electrode electrochemical cell is fabricated and tested

for NO2 gas sensing

KeywordsElectrochemical sensor, Gas sensor,

Simulation.

I. INTRODUCTION

Electrochemical gas sensors are gas detectors that measure the

concentration of a target gas by oxidizing or reducing the sameat an electrode and measuring the resulting current. The sensorshave two or three electrodes, occasionally four, in contact withan electrolyte. The electrodes are typically made-up by fixing ahigh surface area noble metal on to the porous hydrophobicmembrane. The working electrode is in contact with both theelectrolyte and the ambient air to be monitored usually via aporous membrane. The electrolyte most commonly used is anacid, but organic electrolytes are also used for some sensors.The electrodes and electrolyte are usually placed in a plastichousing which contains an exit hole for the gas and electricalcontacts.

The test gas diffuses into the sensor, through the back

of the porous membrane to the working electrode where it isoxidized or reduced. (See Fig.1). This electrochemical reactionresults in an electric current that passes through the externalcircuit. Voltage is applied across the sensor between theworking and counter electrodes for two electrode configurationor between the working and reference electrodes for threeelectrode configuration. At the counter electrode an equal andopposite reaction occurs, such that if the working electrode is

performing an oxidation, then at the counter electrode it isreduction.

Fig.1. Electrochemical gas sensor schematic

Present paper deals with the electrochemical gassensor for detection of the NO2 gas. The magnitude of thecurrent is controlled by how much of the NO2 gas is oxidized atthe working electrode. The oxidant, after its reaction with NO2,can be re-oxidized directly at the electrode [1]. Manyresearchers used H2SO4 as electrolyte for the sensing of NO2gas [2, 3, 4]. Some of them used Ionic liquids and otherelectrolytes like AgCl3, Ag/AgCl3 M NaCl and 10% PVC, 3%

tetrabutylammonium hexafluorophosphate [5, 6, 7].P. Hrncirova et.al [5] reported solid electrolyte sensor.

Platinum is used as electrode material, with the cell dimensionsof 25x25x2 mm3 and obtained the output current of the order of300nA for the NO2 gas sensing. Martina Nadherna et al [2] usedionic liquid as electrolyte, glassy carbon and platinum aselectrode for sensing NO2 gas. Using these materials theyobtained the resultant current of the order of 200nA for theconstant potential 900mV. Pratic Jacquinot et al [8] got theoutput current of the order of 1-5A for ppb levels of NO2 with0.5 M H2SO4 acid as electrolyte. Jing-Shan Do et al [3] usedpolyaniline/Nafion/Au electrode to get 30 - 240A current forthe 20100ppm NO2.

In electrochemical sensor physical size, geometry,selection of various components e.g. porosity of hydrophobicmembrane, electrolyte composition, sensing electrode materialsand the configuration such as two electrodes, three electrodeetc. usually depends on its intended use. Quite often the finaldesign results in a compromise between various performanceparameters of electrochemical sensors. They are sensitivity,selectivity, response time, and operating life. A lowconcentration gas sensor with very high sensitivity requires acoarse-porosity hydrophobic membrane with less restrictedcapillary. Larger pore size allows more gas molecules to pass

Proceedings of the 2015 2nd InternationalSymposium on Physics and Technology of Sensors,8-10th March, 2015, Pune, India

978-1-4673-8018-8/15/$31.00 2015 IEEE

Design, simulation, fabrication and testing

of Electrochemical NO2gas sensor

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through to produce enough signal (current or voltage) for bettersensitivity but at the same time it allows more of theelectrolytes water molecules to escape out to the environmentdue to evaporation of moisture through the porous membrane.This in turn reduces operating life of the sensor. Similarly, theelectrolyte composition and the sensing electrode material areselected on the basis of chemical reactivity of the target gas.The electrolyte and/or the sensing electrode can be selected to

achieve the selectivity towards the target gas, but at the sametime the sensitivity may reduce. Theoretical background on thesubject of electrochemistry may be referred from the referencenos. [3] and [7].

Efforts done to understand the dependence of some ofthese parameters on sensor performance parameters especiallysensitivity, are reported in the present paper.

II. METHODOLOGY

1) Simulations of Electrochemical Cell:

In this work, simulations are carried out on the

electrochemical sensor model for detection of lowconcentration of NO2 gas. As stated the variables of theelectrochemical sensor are physical size, geometry, porosity ofhydrophobic membrane, electrolyte composition, sensingelectrode material and configuration of electrochemical cellsuch as two electrode or three electrode.In this work we have used two as well as three electrode system.2- Dimensional geometry is used to simulate the electrochemicalsensor. Fig.2 shows the geometry of the electrochemical cell.Simulations are carried out for three physical sizes of the sensorviz 4x4, 4x3 and 2x2 cm2. These are decided on the basis oftypical sizes of the commercial sensors. In this simulation thetarget gas is inserted near the working electrode and thats whythe porosity of the hydrophobic membrane is not taken in to

consideration. Many researchers have reported H2SO4as theelectrolyte for NO2 gas sensing. The same is used here.Concentration of the electrolyte is varied from 0.5M to2.0M.The sensitivity of the sensor depends on surface area of theworking electrode. The electrode dimensions are varied from0.4x 0.02cm2to 0.4x 0.01cm2.Larger is the surface area higheris the sensitivity .Output of the sensor is in terms of currentflowing through the external circuit. Simulations of the sensorare carried out using the electrochemical module of COMSOLMultiphysics suite. It uses basic electrochemical equations fromelectrochemical theory [2].The current density for this reactionis given by the electro analytical ButlerVolmer equation for anoxidation.

(1)Where, k0 is the conversion rate of the reaction, c is thecathodic transfer coefficient, and is the supplied potential at

the working electrode.The diffusivity (conversion rate) of theNO2gas in the electrolyte is taken as 5 X 10-4 cm2 /s [11]. Thecathodic transfer coefficient is taken by the software itself whichis equal to 1X10-9.. The viscosity of the electrolyte is taken as26.7 cP. [7].This is standard viscosity of the H2SO4acid. Thesimulation results are in the form of current density. From the

current density we can calculate the current by using thefollowing formula. Eqn. (2)

(2)The ion mobility is not considered in the simulation. The rate ofthis reaction (mol/m3) is given by a MichaelisMenten rate law

as

(3)

Where, Vmaxis the maximum rate of the reaction, CNO2is the gasconcentration and the parameter Km is a characteristicMichaelis-Menten coefficient. The reaction rate is calculatedfrom the data obtained. Results on the simulations are reportedin this paper and discussed.

III.

EXPERIMENTAL

Two electrode, electrochemical cell was fabricated forvalidation of simulation results. Dimensions were decided on

the basis of the simulation results. Experimental set up fortesting the performance of the electrochemical cell is shown inFig.3.Fig.3 (a) shows the block diagram of the set up and Fig.3(b) shows the photograph of the set up. Experiments werecarried out for finding out the sensitivity of the cell. NO2 gas ofknown concentration was introduced in the gas chamber inwhich the electrochemical cell is kept (refer Fig.3 (b).The finalgas concentration was calculated from the known values of gaschamber volume and the volume of the gas introduced. The cellconsists of electrolyte and electrodes. The results obtained arereported and discussed in this paper.

Fig.2.The geometry of the sensor cell.

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IV.

RESULTS AND DISCUSSION

Results on simulations carried out for 2-dimensional geometry

by varying the cell dimensions, electrolyte concentrations andelectrode dimensions and configuration (2 or 3 electrodes) arediscussed in this section. Fig.4 and fig.5 show the response ofthe electrochemical cells of three different dimensions 2x2, 4x3and 4x4 cm2 with 3- electrode system and 2 electrode systemrespectively for sensing the NO2 gas in the gas concentrationrange of 0 to 10 ppm. The electrolyte concentration is keptconstant at 0.5M and electrode dimensions are taken as 0.4 x0.02 cm2. All the electrodes are taken of gold. The outputcurrent is found to increase linearly with the gas concentrationand the slope is different for different dimensions of the cell.

The largest current is found to be 18 A in 3 electrode systemand 15 A with two electrode system in case of cell dimensionof 4x4cm2, increase in current with increase in gasconcentration is attributed to the increased reactivity. It is alsoclear from the figure that sensitivity (A/ppm slope of thecurve) of the sensor increases with increase in cell dimensions.This increase is attributed to the increase in number of ions inincreased volume of electrolyte solution [9].

The output current in two electrode system is littlehigher than that of the three electrode system. This happensbecause, in two electrode system the current flowing in theelectrode randomly changes the potential applied between them[10]. To avoid these fluctuations in the potential the thirdelectrode is (reference electrode) used in the three electrodesystem. Reference electrode is given constant voltage equal toredox reaction potential.

In order to see the effect of electrolyte concentrationon the performance of the cell, simulations are done at threedifferent electrolyte concentrations viz. 0.5, 1.0 and 2.0 M.Fig.6 and Fig.7 show the output response of the sensor usingthree electrode system and two electrode systems respectivelywith varying electrolyte concentration. Here the cell dimensionis taken a 2 x2 cm2, electrode dimensions are taken as 0.4 x 0.02cm2.

Fig.3b. Photograph of Experimental test setup

Fig. 4. Simulation result for varying cell dimensions (3-Electrode s stem)

Fig.5. Simulation result for varying cell dimensions(2- Electrode system)

Fig.3a.Block diagram of the experimental test set up

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All the electrodes are of gold. It is found that the sensorresponse is same for all the concentrations of the electrolyte.The output current is not changing with the change inelectrolyte concentration because of constant redox potentialapplied to the working electrode.Fig.8 and Fig.9 show the sensor response using three electrodesystem and two electrode system with at different electrodedimensions 0.4 x 0.02 cm2 and 0.4 x 0.01 cm2. Here the cell

dimension is taken a 2x2 cm2 and electrolyte concentration iskept constant at 0.5M. The output current of the sensorincreases with increase in electrode dimension. This is becausethe higher surface area interacts with more number of NO2atoms and the reaction rate is increases. Maximum current of 7A in three electrode system and 6.2 A in two electrodesystem with NO2gas of 10 ppm is observed.

The current in two electrode system is lower than the threeelectrode system. This happens because of the change inpotential of working electrode.

For validation of simulation results an electrochemical cell isfabricated with dimensions of 2 X 4 X 2 cm3(length x height xwidth). Electrode dimensions are taken to be 4 X 0.002cm2=0.0008 cm2. The area of electrodes is same as that taken forsimulation (0.4 X 0.02 cm2). Electrolyte concentration was keptat 0.5M. In the validation experiment gas concentration wasvaried from 2 ppm to 10 ppm. Fig.10 show response (outputcurrent versus gas concentration) of the fabricated sensor. Thecurrent varies from 7.5 to 8.5 A when the concentration of NO2gas is varied between 2 ppm to 10ppm. The simulated andpractical results are compared. Since simulations are done in twodimensions the simulated curve for 2 x4 (width x height) withthe symbol ( ) in Fig.4 is compared with the practical results.Simulated sensitivity (0.8 A / ppm) of the sensor is found to bemuch larger than the one practically obtained (o.12 A/ppm).The difference seen may be attributed to the fact that simulationcurrent is zero for zero ppm whereas practically the current isaround 7.5 A. The difference may also be due to the thirddimension in practical electrochemical cell. As can be seen fromthe fig. 10 slope of the current versus concentration curve isdifferent above gas concentration of 10 ppm. This may beattributed to the change in reactivity of the sensor. The order of

Fig.6. Simulation results for varying Electrolyte concentration(3- Electrode s stem)

Fig.7. Simulation results for varying Electrolyte concentration

(2- Electrode system)

Fig.8. Simulation results for varying Electrode dimensions (3-Electrode system)

Fig. 9. Simulation results for varying Electrode dimensions (2-Electrode s stem

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currents obtained in the present experiments are similar to theones reported in the literature [2, 8]

V.

CONCLUSION

The electrochemical NO2gas sensor was designed, simulated,fabricated and tested successfully. The simulation of the sensorwas done by the electrochemical module from the COMSOLMultiphysics suite. The sensor shows the good sensitivity forgold electrode with 0.5 molar concentration H2SO4 aselectrolyte.

VI. ACKNOWLEDGEMENT

Authors acknowledge with thanks the financial supportprovided by ISRO-UoP STC for this work.

VII.

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Fig.10. Result of fabricated sensor

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