Electroanalysis, 4 (1992) 133-137

An Amperometric Solid-state Sensor for Nitrogen Dioxide Based on a Solid Polymer Electrolyte

Frantis‘ek Opekar’ LrNESCO Lahomtoiy of Environmental Electrochenaisty, J Hqroris@, Institute of Physical Chemisty and Electrockmktq: Czecboslouak Academy of Sciences, Dokjikoua 3, 182 2-3 Prugue 8, Czechosloiukia Receiued April 2, 19-9 I

ABSTRACT

A new sensor is described consisting of a Nafion membrane to which a very fine gold square grid indicator electrode is mechanically pressed. h platinum counter and a Pt/air reference electrode are prepared on the membrane using a chemical plating method. Nitrogen dioxide was determined in air by reduction at -0.317 vs. Pt/air electrode (0.75 \’ vs. SHE). The response dependence on the NO, concentration was linear, with a sensitivity of 59 nA.ppm-’ (42% rel. humidity). The time constant and the time required t o attain 95% of the steady-state response were 2.2 and 10 s, respectively. The re- sponse was independent of the air flow rate, but increased linearly with increasing relative humidity. The sensor sensitivity decreased with time-the longer the sensor was in contact with NOz. The de- crease amounted to ca. 5% per 24 hours during constant exposure to NO,. The indicator electrode can be reactivated by cyclic polarization. On the basis of the sensitivity attained, the indicator electrode can be considered as an array o f microband electrodes.

KEY WORDS: Nitrogen dioxide, solid polymer electrolyte, sensor.

INTROD TJCTION

The importance of electrocheniical sensors with solid electrolytes for detection of substances in the gaseous phase has been established. An important group of gas .sensors consists of those containing solid polymer electro- lpes (SPE); the most common SPE is a copolymer of poly(tetrafluoroethy1ene) with poly(sulfony1fluoride vi- nylether), inanufactured by DuPont under the iiame Nafion. Several designs ofgas sensors (mostly amperomei- ric) have been described with this electrolyte [ 1-71,

The electrochemical reactions in gas sensors take place at a three-boundary interface, where the test gas phase, the indicator electrode, and the electrolyte come in contact with one another. Therefore, the sensor properties depend not only upon the catalytic activity of the indicator electrode material, but also upon the dimensions of the gas-electrode-SPE interface, i.e., the design ofthe indicator electrode.

It has been demonstrated 17-91 that the signal-to- background current ratio increases with increasing P‘IA ratio, where P is the perimeter of the indicator electrode

‘Present address: Department ofhalytical Chemistry. Charles University, rllbertov 2030, 128 40 Prague 2, Czechoslovakia

(the length of the gas-electrode-SPE interface) and A its geometric area. Electrodes with rigorously defined P / A ratios (square grids of various densities) have been pro- ciuced by vacuum-plating gold onto a sheet of Nafion membrane through a suitable mask. Indicator electrodes of a required design can also be created on a solid support using the methods for preparation of planar mi- croelectrochemical sensors. The electrodes in these sen- sors are covered by a thin Nafion film and thus the indicator electrode perimeter and the three-phase bound- aryare not rigorously defined. Nevertheless, an increase in the sensitivity of measurement with an increase in the P2/A ratio has also been observed in this case [8, 91. It can be assumed that the signal of these electrodes is determined not only by the length of the three-boundary interface, but also by the edge effects and transport phenomena oper- ative with microelectrodes and their arrays.

Sensors with indicator electrodes vacuum-plated on the surface of a Nafion membrane, and microsensors are not particularly mechanically stable. With the former sen- sors, the electrode structure may be destroyed due to changes in the Nafion dimensions caused by humidity fluctuations 171; while with the latter, problems arise with corrosion of the metals forming the electrode system and their poor adhesion to the support 181. The small size of the electrodes in microsensors ma)’ lead to further difficulties

0 1992 VCH Publishers, Inc. 1040-0397/’32/$3.50 + .25 133

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caused by instability of the reference electrode potential

This article describes a sensor containing an elec- troformed gold minigrid (common in spectro- electrochemical measurements) pressed onto a Ndion membrane, as the indicator electrode. The gold minigrid is sufficiently strong to withstand fluctuations in the membrane dimensions with humidity variations and its fine square grid structure ensures a high P2/A ratio. A platinum counter and a reference electrode with a large active surface area are produced by chemical plating on the membrane. This solid-state sensor has indicator and reference electrodes with properties suitable for detection in the gaseous phase, and is mechanically stable.

The sensor was tested for detection of nitrogen diox- ide in air. The NO, reduction at a gold cathode obeys the net equation, NO, + 2Hf + 2e- = NO + H,O [lo-121. The protons and electrons required for the reduction are formed at the counter electrode, by the reaction, H,O = 2H’ + 2e- + $0,; the protons readily pass through Nafion (the proton conductor), the electrons pass through the external circuit, and the corresponding current (sen- sor response) is proportional to the nitrogen dioxide concentration.

[a

EXpERMEhTi%L

Sensor Design and Fabrication The sensor was created on a 1.5 x 1.5 cm2 piece of Nafion 117 membrane (Aldrich, cat. no. 29,256-7). The counter and reference electrodes were obtained by chemically plating platinum from a 0.04 M water solution of H,PtCI, on action of the reducing solution, 0.1 M in NZH, and 1 M in NaOH [13], using the procedure described in [3]. The electrodes contain ca. 20 mg of platinum per cm’. The geometric area of the Ptiair reference electrode was 0.3 cm’ and its active surface area, determined from the amount of charge required to oxidize the adsorbed hydro- gen on polarization of the electrode in 1 M HClO,$ (0.21 mC for a true area of 1 cm2 j, was about 600 times greater. Thus, it can be assumed that its potential (about 1 V vs. SHE) is sufficiently stable. The surface area of the counter electrode is either the same as that of the reference electrode or twice as large, depending on the sensor configuration (see below).

The indicator electrode consisted of a 0.6 x 0.7 cm piece of a gold minigrid (500 wiredin., part no. MG-41, Buckbee Mears Co.). The geometric area of the electrode (one side of the grid), A, was 0.18 cm2 and the perimeter (the length of the gold-Nafion interface), P, was 250 cm. The electrode was pressed onto the Nafion membrane by a rigid polyethylene grid (ca. 20 mesh, 0.1 cm thick). It was found that mere mechanical pressing does not ensure good contact of the whole area of the gold minigrid with‘ the membrane, and thus the minigrid was “glued to the membrane with the 10 PI, of a Nafion solution (Aldrich, cat. no. 27,470-4) prior to the mechanical pressing. Assuming that this amount of Nafion solution covers an area of 0.5 cm2 (it could be seen that the solution spread over an area slightly larger than the area covered by the minigrid), the

thickness of the Nafion film after solvent evaporation was estimated at 5 x 10- cm (the densities of the solution and dry Nafion were 0.87 and 1.8 g - ~ m - ~ , respectively).

Two sensor types were tested, differing in configura- tion of the counter and reference electrodes, as shown in Figure 1A. The counter and reference electrodes in sensor a are placed on the opposite side of the membrane with respect to the indicator electrode, whereas only the coun- ter electrode is on the opposite side of the membrane in sensor b.

The overall arrangement of the sensor is depicted in Figure 1B. The electrical contacts for the electrodes were flattened platinum or gold wires pressed onto the elec- trodes with the polyethylene grid. Figure 1 also specifies the dimensions of the most important parts of the sensor.

Sensor Testing System A membrane pump (Cole Parmer, cat. no. 7056-41) pumped air into the apparatus. The air was purified in a filter containing active charcoal and was moisturized by passage through two 250 mL vessels connected in series which contained one of the following saturated solutions: LiCl (121, MgC1, (331, Zn(NO,), (421, Mg(NO,), (541, or NaCl (76); the numbers in the parentheses specify the corresponding relative humidity (RH) in %.

Nitrogen dioxide was produced by a permeation wafer (Chemoprojekt, Prague) at a rate of 4.4 ngs-’. Different Concentrations were obtained by varying the flow rate of air around the NO, source. Samples were taken froni this NO,-enriched air and fed to the sensor at a rate of 0.3 mks-’. The air flow rate was varied from 0.33 to 9.0 mks-’, thus obtaining the test concentrations from 0.26 to 7.09 ppm (v/v) NO,.

The sensor was placed in a flow-through, cylindrical chamber with a volume of ca. 5 mL; only when testing the

FIGURE 1. Geometric and mechanical arrangement of the sensor. A - Electrode configuration on the Nafion membrane: (1) membrane side with the indicator electrode; (1 ‘1 the opposite side of the membrane; (2) Au indicator electrode [s = 39.1 pm, w = 11.7 prn]; (3) PWair refer- ence electrode; (4) Pt counter electrode. The dimensions are given in mm. B - Mechanical arrangement of the sensor: (5) plexiglass body; (6) membrane with the electrode system; (7) poly- ethylene grid; (8) leads to the electrodes; (9) assembling bolts.

J El 1

Solid Polymer Sensor for Nitrogen Dioxide 135

dynamic behavior was the sample fed through a tube directly to the side of the membrane with the indicator electrode. A four-way valve at the chamber inlet permitted introduction of pure air or air with NO, to the sensor.

The potentiostat and the current follower were assem- bled from operational amplifiers. The indicator electrode potential was - 0.3 V vs. the Wair reference electrode (0.75 V vs. S€IE). The measurements were performed at laboratory temperature, 23 2 2°C.

RESULTS AND DISCUSSION Geometric Con figuration The sensors with electrode configuration a and b (Figure 1A) yielded almost identical results. With configuration a, the measuring circuit was unstable and the potentiostat oscillated; the instability was completely eliminated by connecting a 10 nF capacitor between the counter and the reference electrode. Sensor b can be used to detect NO, in air; for detection in gases that do not contain oxygen, configuration a must be used, as it is then possible to feed air separately to the reference and counter electrode. In principle, the sensor can also be used in the two-electrode arrangement (i.e., with the counter and reference elec- trode interconnected in configuration a or with the refer- ence electrode disconnected in configuration b). How- ever, the passage of current may alter the platinum electrode potential.

Cyc lic Voltammogram Cyclic polarization of the indicator electrode from 1.0 to - 0.8 V vs. Pt/air electrode yielded a polarization curve identical a7ith that obtained on compact gold in an acid solution. Comparing the potentials of the Au oxide reduc- tion in solution (1M H,SO,, vs. SHE) and in Nafion (vs. Ptfair), the potential of the Ptkair reference electrode was estimated at 1.05 V vs. SHE.

The actual surface area of the indicator electrode was determined from the charge consumed for the formation of the Au oxides (using the value, 0.42 mCcm-’), ob- taining 0.50 cm2. As the total geometric surface area of the gold minigrid is 0.36 cn? (2 X 0.18 cm,), the roughness factor equals 1.4 which can be expected for bright gold. Hence, it can be concluded that the gold minigrid is mostly covered with a Nafion film and the electrochemical oxida- tion occurs on both sides of the minigrid.

When the sensor sensitivity decreases, the indicator electrode can be reactivated by cyclic polarization within the above potential range.

Indicator Electrode Potential The sensor response dependence on the electrode poten- tial is given in Figure 2 . When the potential increases toward negative values within the range suitable for NO, detection, the background current, i.e., the oxygen reduc- tion current, also rapidly increases. Moreover, the re- sponse to NO, is distorted, as shown in Figure 3. Therefore, the working potential for determination of NO, was set at - 0.3 V, which corresponds to the beginning of the limit-

300

I /nA

200

100

0 200 400

- E / V 300

FIGURE 2. Dependence of the sensor response on the indicator electrode potential [42% RH, 4 ppm NOJ (0) NO, reduction current; (0) background current.

ing current while the effect of the background current is still acceptable. This value is close to the potentials recom- mended for the NO, reduction in the literature [lo-131.

The following mechanism is given in [ lo] for the reduction of NO, on a gold cathode: Adsorption of NOz on the clean electrode surface is followed by chemical oxida- tion ofgold, NO, - Au(ads) = NO + AuO. The next step is electrochemical reduction of AuO to gold, AuO + 2H’ + 2e ~ = Au + H,O. This mechanism can explain the distor- tion of the sensor response at potentials at which oxygen reduction occurs simultaneously with the reduction of NO, (Figure 3). When the electrode is covered with the oxide produced by the effect of NO,, the oxygen reduc- tion is suppressed and the corresponding cathodic cur- rent i.e. the background current, decreases. When the supply of NO, is interrupted, the surface oxide is electro- cheniically reduced, the reduction of oxygen ceases to be hindered, and the background current returns to its origi- nal value.

Dynamic Behavior The response rate was determined from the time depen- dence of the response to a step change in the NO, concen- tration from zero to 0.75 or 2.50 ppm and back to zero, at various gas flow rates. It has been found that the response

136 Opekar

-- 100 /

2 3

\

FIGURE 3. Response character at various reduction poten- tials (V vs. Wair reference electrode). The background current (nA) is given at the curves [42% RH, 4 pprn NO,]. (1) -0.30; (2) -0.35; (3) -0.40.

follows a first-order equation [ 121 up to ca. 85% of the final value and is symmetrical for the increase and decrease in the concentration. The time constant of the sensor, 7 = 2 . 2 k 0.1 s, was obtained from the linear portions of the time dependence of the terms, In[ 1 - 1411 and In[I)1], for the step increase and decrease in the NO, concentration, respectively, where I , is the current at time t and 1 is the steady-state current. Values of 0.95 I (on an increase) and (0.05 I) (on a decrease), in concentration of NO2 were attained within 10 s and steady-state or zero current within ca. 80 s. The time constant was independent of the gas flow rate.

The experimental value of 7 is in a very good agree- ment with the value 2.5 s, calculated from the expression, 7 = J / D , with an estimated value of the Nafion film thickness of x = 5 X cm and an approximate diffusion coefficient of gas in Nafion of U = 1 X lo-’ cm2.s-l. Hence the NO, transport toward the indicator electrode is controlled by diffusion through the Nafion film and the electrochemical reaction primarily occurs at the “front” side of the gold minigrid, which is exposed to the test gaseous phase through a Nafion film. Only a small fraction of the NO2 diffuses through the Nafion membrane to the “rear” side of the minigrid and causes an increase in the time required for the attainment of a current steady- state value.

Concentration Dependence The dependence of the sensor response on the NO, concentration was linear within the studied range. The calibration plot parameters obtained from 12 points, for 42% RH, and the sensor of highest activity (see below) were: an intercept of 1.3 nA, a slope of 59.0 nA-ppm - I , a standard deviation of 3.8 nA, and a coefficient of corre- lation of 0.998.

The standard deviation is rather high, as it involves the uncertainty in the preparation of calibration mixtures by dynamic dilution. This is also reflected in a relatively high limit of detection, 0.2 ppm, obtaincd from the ratio of the triple standard deviation and the slope of the calibration plot. It can be seen from the recording of the sensor response in Figure 4 that the real limit of detection might be almost one order of magnitude lower. The relative standard devialion for 10 repeated measurements at a constant concentration equalled 0.4%, for NO, concentra- tions of 0.75 and 4.0 ppm,

The steady-state current I at a planar electrode with transport o f the electroactive gas controlled by the dif- fusion (or more precisely, the permeation) through a membrane, is given by I = nFApp/x, where P is the per- meation coefficient and p the partial pressure of the test gas: the other symbols have their common signifi- cance. This expression permits an estimation of the theo- retical sensitivity of a sensor with the indicator elec- trode of geometric surface area A, unaffected by the electrode shape. The permeation coefficient of NO? in poly(tetrafluoroethy1ene) is P = G x 10-l- mol.( cm-s-Pa) ~ ’ [ 141; the value for the structurally analo- gous Nafion should be similar. The calculated sensitivity is then Up = 4.2 n4-Pa-l. The experimental value, 59 nA-ppm-’, which corresponds to 590 nA-Pa:l is ca. 140 times higher.

The sensor indicating electrode can be considered as an array of microband electrodes, or an array of hemicylinders [15] with a radius of Y = udn-, where w = 11.7 pm is the bandwidth (Figure 1A). Hence the differ- ence between the experimental and calculated sensi- tivities can be interpreted as a product of the transport effects characteristic of microelectrode arrays.

Long Term Stability The sensor attained the highest sensitivity (59 nA-ppni - I )

after several potential cycles applied to the indicator elec- trode between 1.0 and - 0.9 V; the electrode was generally activated by ten cycles at scan rate of 50 mVs-’.

Pure air was fed to the activated sensor for 800 s, followed by feeding of air with 4 ppm of NO, for 200 s. These cycles wvre continued for 96 hours. The sensitivity decreased by 13% after the first 24 hours, after which followed a decrease of ca. 5% every 24 hours. The sensor was disconnected from the measuring circuit and main- tained in pure air of a constant humidity (42%). It was found that after about 20 hours the sensitivity was restored almost to the original value (56 nA-ppm-’). When the sensor was used daily for detection of NO,, but for rela-

Solid Polymer Sensor for Nitrogen Dioxide 137

flGURE 4. Sensor response to NO, concentrations close to the limit of detection. The concentrations (ppm) are given at the curves [42% RH, -0.3 V, gas flow rate of 0.3 mL-s '1

tively short periods of time (1 to 2 hours), its sensitivity remained virtually unchanged.

The deterioration in the indicator electrode activity can be explained from the above mechanism of the NO, reduction [lo]. The reduction only occurs on a clean (oxide free) electrode surface. A residual fraction of the AuO, produced on the electrode surface by the action of NO,, requires a rather long time to be reduced (cf. the increase in sensitivity when the sensor is not used). When NO, is fed to the electrode continuously or in frequent intervals, the electrode surface covered by AuO gradually increases and the sensitivity decreases. This residual frac-

tion may consist of different gold oxides which are pro- duced in minor amounts by the reaction of NO, with the electrode surface. A monolayer of gold oxide, strongly adherent to the electrode surface, could play a role as well. Cyclic polarization frees the electrode surface from all the oxides and the sensor attains the maximum sensitivity.

The sensor response was virtually independent of the gas flow rate within the range studied, i.e., 0.33 to 1.25 mL.s-'.

Dependence on the Relative Humidity The effect of relative humidity was determined after a 12 hour exposure of the sensor to an environment with a certain KH value. The sensor sensitivity increases linearly with increasing RH within the range studied, i.e., 12 to 76% RH, with a slope of 1.4 nA.ppm-' per 1% RH. The depen- dence passes through the origin.

The problems with the dependence of the sensitivity of SPE sensors on the test gas humidity are chiefly dealt with by maintaining the humidity at a constant value, most easily at 100% RH [8, 91, or by continuous wetting with water o f the membrane side opposite that of the indicator electrode [ 1,101. The influence of the water content on the transport properties of Nafion and its structure is complex and is being intensely studied [ 161. The assumption that the sensor sensitivity increases with increasing humidity due to a mere increase in the Nafion conductivity [S, 91 is unrealistic. It is important from the analytical point ofview, however, that the sensor response is a linear function of RH and thus can readily be corrected for RI-I fluctuations.

CONCLUSIONS The described arrangement of the functional elements has produced a solid-state sensor with a stable geometric configuration suitable for detection of substances in the gaseous phase, with a long life expectancy, and small maintenance demands. The Nafion film covering the indi- cator electrode was originally considered an undesirable consequence of the measures taken to improve the contact between the electrode and the Nafion membrane. How- ever, it is probable that this film contributes to the protec- tion of the electrode against external effects and maintains the electrode surface activity, similar to the situation with platinum electrodes [ 171.

The response dependence on the humidity is a prob- lem from a practical point of view, since it is difficult to moisturize the test gas in a defined way without a loss of analyte. Simultaneous measurement of the RH using an independent detector and an electronic correction of the sensor signal seems to be a better approach.

The use of the sensor in laboratory detection should not be affected since the experimental conditions can readily be kept constant. The sensor diffusion geometry is stable and thus the sensor should be a useful detection element in analytical methods that employ membrane electrodes, especially those with porous membranes, in the detection of gaseous substances [18].

Some other general properties of the sensor, such as

138 Opekar

the temperature dependence ofthe signal or interferences have nor been studied here, as they have been described in the literature [I , 121 dealing with NO2 sensors based on analogous principles.

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