Sensors and Actuafors B, 17 (1994) 165-168 165

A solid polymer electrolyte-based electrochemical carbon monoxide sensor

Heqing Yan* and Chung-Chiun Liu Electmnics Design Center and Edison Sensor Technology Centeq Case Western Reserve University, Cleveland, OH 44106 (USA)

(Received September 1, 1992; in revised form June 17, 1993; accepted June 21, 1993)

Abstruct

A solid polymer electrolyte (SPE)-based electrochemical sensor for carbon monoxide detection has been designed, fabricated and evaluated. Nafion membrane is used as the solid electrolyte, and a hydrophobic Teflon-based platinum black thin-layer electrode is used as the working electrode. The sensor is tested over the CO concentration range O-1.04% and shows good reproducibility. The response time to a step change of carbon monoxide is about 3 s. Operation of the sensor over 90 days in an ambient environment shows good sensor stability.

In recent years attempts have been made to develop solid-state electrochemical sensors that can be operated at ambient temperature. Solid polymer electrolyte (SPE) electrochemical sensors have gained increasing interest because of their good stability and reproducibility ad- vantages. Nafion”, a perfluorated cation exchange mem- brane, is one of the SPEs commonly used in sensor development. Sensing electrode elements of gold or platinum films can be deposited onto the Naflon mem- brane by either evaporation or chemical deposition [l, 21. The SPE-metal electrodes formed by these methods are nonhydrophobic gas electrodes. Kita and Nakajima [3] reported the results of CO oxidation of SPE-Au electrodes produced by chemical deposition. They re- ported that the reaction zone of the electrode is always changing as the electrochemical reaction takes place [3]. Thus, an SPE-hydrophobic gas diffusion electrode with a stable reaction zone was suggested, and a new type of oxygen sensor was developed based on this approach [4]. This SPE-hydrophobic gas ditTusion elec- trode with a stable reaction zone can be used as the sensing element for a CO gas sensor.

The mechanism of electrochemical sensing of CO in the gas phase is suggested as [5, 61

CO + H,O - CO, + 2H+ + 2e- working electrode

*Present address: Department of Chemistry, Wuhan University Wuhan, China.

‘Author to whom correspondence should bc addressed.

and

2H+ + 4 0, + 2e- - Hz0 counter electrode

and the overall reaction is

co+ do*--+ co*

When a solid polymer electrolyte is used, the electrolyte film should be permeable to the gases CO, CO, and 02, and dilfusable for H’ ions and H,O. The oxidation current of the CO at the working electrode can then be used to quantify the CO present. Although H,O does not appear in the overall reaction, the presence of H,O is essential in this reaction mechanism, as demonstrated in the half-cell reactions. Yasuda et al. [5] also pointed out that the sensor sensitivity would be affected by the partial pressure of H,O in the sample as well as the water content in the SPE. These factors will have to be included in the design of the sensor structure. In this report, the construction of a SPE CO gas-sensor prototype and its evaluation will be given and discussed.

Structure of the CO sensor

This SPE CO sensor is basically an electrochemical cell with a three-electrode conhguration. Figure 1 shows the schematic of this sensor. This sensor structure is similar to that used for oxygen sensing [4]. Nation membrane 117 (Aldrich, Milwaukee, WI) with an EW value of 1100 and a thickness of 0.17 mm was used as the SPE. The proper-sized Nafion membrane was first boiled in distilled water for about an hour. The membrane was then removed from the water and the

09754005/94/$07.M) 0 1994 Elsevier Sequoia. All rights reserved

166

SAMPLE IN SAMPLE OUT

Fig. 1. Schematic structure of the SPE CO electrochemical sensor: 1, distilled water; 2, counter electrode; 3, Nation membrane; 4, working electrode; 5, 7, thin plastic plates with holes; 6, sample gas compartment; 8, reference electrode; 9, air cavity.

electrodes for the sensor were formed on the membrane surface.

On the surface of one side of the membrane, two hydrophobic gas diffusion electrodes composed of Te- flon-bonded platinum black were pressed onto the Nafion membrane. One of these two electrodes had a surface area of 1 cm* and served as the working electrode, whereas the other, with a surface area of 0.3 cm*, functioned as the reference electrode. Both electrodes had a platinum loading of 7 mg/cm’. The thickness of each electrode was approximately 0.25 mm and their exact porosity was not determined. The ref- erence electrode in this structure was in contact with the air, forming a platinum/air reference electrode similar to the concept suggested by Otagawa et al. [6]. On the other side of the Nafion membrane, a chemically deposited platinum layer [2] was used as the counter electrode.

Because the electrochemical reaction of the CO involved water in the sensing electrode, the presence of water is critical to the sensor performance. In this prototype, a distilled water reservoir was incorporated in the back of the sensor. This reservoir had a volume of approximately 2.5 X 4 X 7 cm3. For testing purposes, the quantity of water was always maintained at the same level to avoid any water loss.

Two thin plastic plates were used to hold the mem- brane in place. On each plate, small holes were drilled to permit either the sampling gas or the water to diffise towards the membrane. The hole size is 1 mm in diameter and there are approximately 25 holes per cm’ of plate surface.

Experimental

The CO sensor prototype was evaluated using various CO and nitrogen gas mixtures (Matheson Gas Products, Twinsburg, OH). The different concentrations of CO

in the CO-N, mixtures were prepared using a gas proportioner (model 7472-33, Matheson Gas Products). Electrochemical measurements of the sensor were con- ducted using a laboratory potentiostat (model RDE- 3, Pine Instruments, Grove City, PA) A digital elec- trometer (model 616, Keithley Instruments, Solon, OH) and anX-Yrecorder (model 3023, Yokogawa, Cleveland, OH) were used for data display. All experiments were carried out at room temperature, i.e., approximately 21-22 “C.

Results and discussion

Cuyent-polential relationship The oxidation current of CO as a function of the

polarization potential of this CO sensor was examined. Figure 2 shows this current-potential relationship for the SPE CO sensor in the presence of 0.03% CO in a CO-N, mixture. This polarization curve was obtained at each fixed potential point by point. The corresponding oxidation current was measured at each potential. The current measurement was made at each potential when the potential was stabilized. As shown in Fig. 2, the oxidation current of CO increases significantly with the increase in potential up to +O.l V versus the reference electrode. At a higher anodic potential, i.e., +0.2 V versus the reference electrode, the oxidation current of CO in fact decreases slightly. This change of the oxidation current of CO as a function of the polarization potential is due to the change of the oxidation state of the platinum electrode surface, as suggested by Blurton and Sedlak [7]. The oxidation current of CO reaches a plateau in the polarization potential range +0.125-+0.20 V versus the reference electrode. The limiting current value in this potential region is con- trolled by the mass transfer of CO through the porous working electrode [8]. Therefore, the limiting current produced in this polarization region can then be used to quantify the CO present in the CO-N, mixture. Thus, a polarization potential of +0.2 V versus the

E, mV

Fig. 2. Relationship between current and applied potential (vs. reference) of the CO sensor in the presence of 0.03% CO.

reference was chosen to carry out our studies. This polarization potential was within the region used by others [6].

Effect of the flow rate of the gas mixture For the present sensor structure, it is possible that

the flow rate of the gas sample may have a direct effect on the oxidation current output of the CO sensor. Figure 3 shows the typical flow-rate effect on the sensor output. The CO concentration of this CO-N, gas mixture is 1.03%. The oxidation current reaches a difhtsion- limited value at a flow rate higher than 200 ml/min. This observation agrees well with the calculated results reported by others [8]. Therefore, the sensor output must take this effect into consideration.

Calibration of the CO sensor As mentioned above, the limiting current output of

the sensor can be used to quantify the CO present in the gas samples. Throughout the calibration, a potential of +0.2 V versus the reference electrode was applied to the working electrode. Under this condition, the working electrode appears to have good electrocatalytic activity and a relatively low residual current, approx- imately 1 4. Figure 4 shows the sensor current output in three different CO concentration regions. However, the sensitivity, namely the slope of each linear rela- tionship, decreases as the CO concentration increases. As shown in Fig. 4, in the 20-100 ppm CO concentration region, the sensitivity is 0.06 I.c4/ppm cm’, whereas in the 100-500 ppm region it is 0.05 @/ppm cm2 and in the O.l-1.04% region it is 0.042 Nppm cm2. This observation is in agreement with those reported by Xing and Liu [9]. The observed phenomenon is probably related to the nature of the adsorption isotherm of CO on the platinum surface. In this study, the variation in sensitivity over the three CO concentration regions is smaller than those reported by Xing and Liu [9]. This can be attributed to the difference of the platinum electrode structures used.

The reproducibility of the CO sensor prototype ap- pears to be very good. Repeated measurements show

600 CO Concsnlmtion=l.03* in Ne

1 OO

I 50 too 150 200

FLOW RATE, ml/min

Fig. 3. The effect of flow rate on the sensor current output.

167

4-

a =L _

!s w g

2-

u’

OO 1 I 40 80

(a) CO CONCENTRATION, ppm

30-

OO 200 400 600

@I CO CONCENTRATDN, ppm

400 -

a a F c g 200.

;:

-:,

&I 040 080

(c) CO CONCENTRATION, %

0

Fig. 4. Calibration curves of the sensor as a function of CO concentration: (a) 2Q-100 ppm CO; (b) 100-500 ppm CO; (c) O.l-1.04% co.

a reproducibility within f3% over the whole CO con- centration range, i.e., O-1.04%.

Figure 5 shows the sensor prototype response to a step change of CO concentration from 1.04 to 0.45%. A 90% response time of approximately 3 s is observed, which is relatively fast for CO sensing. The high degree of electrocatalytic activity of the working platinum black electrode may have led to the sensor’s good response time.

Continued testing of the long-term stability of the sensor was carried out in the presence of 1.04% CO in a CO-N, mixture over a period of 90 days. Throughout this testing period, the working electrode was polarized at +0.2 V versus the reference electrode. Figure 6 shows that the sensor current output decreases to approximately 85% of its initial value after a period of 40 days and remains at this level for the next 50

168

500 Co=l.O4% I

4 x

5 300

-L

E

a co= 0.45%

I

I

100 ’ 8 I I

0 4 0

TIME, set

Fig. 5. Time response of the Sensor to a step change in CO concentration.

Q 4400 . !s . k! I------ 2

2ooO-

TIME, day

Fig. 6. Long-term sensor performance characteristics.

days. One possibility for this decay in sensor output might be the absorbed state of CO on the electrode surface. However, it is rather unlikely that any CO in the absorbed state would be present at the electrode surface, which is biased at + 0.2 V versus the reference electrode [6]. It is feasible that the slow formation of oxide film on the catalytic surface [g] leads to diminished electrode activity, and consequently to an overall decay in sensor output.

Conclusions

A solid polymer electrolyte CO sensor employing a hydrophobic gas diffusional platinum black electrode was constructed and evaluated at 21-22 “C. This CO sensor prototype shows fast response and satisfactory linearity between the sensor current output over a reasonable CO concentration range. However, the sen- sitivity of the sensor decreases as the CO concentration increases. The long-term stability of the sensor is fair and the sensor shows a 15% decrease of sensor output over a period of 40 days and then remains at this level for another 50 days. This sensor shows promise for long-term minimal-maintenance operation.

Acknowledgements

This work was partially supported by the Edison Sensor Technology Center, State of Ohio, and by Na- tional Institutes of Health grant number RR02024.

References

G.J. Maclay, W.J. Buttner and J.R. Stetter, Microfabricated amperometric gas sensors, IEEE Trans. Elmron Devices, ED- 35 (1988) 793-799. H. Kita, K. Fujikawa and H. Nakajima, Metal electrodes bonded on solid polymer electrolyte membranes (SPE)-II The polarization resistance of Pt-Nafion electrode, Elecrro- chim Am, 29 (1984) 1721-1724. H. Kita and H. Nakajima, Metal electrodes bonded on solid polymer electrolyte membranes (SPE)-III CO oxidation at Au-SPE electrodes, Electichim. Acta, 31 (1986) 193-200. H. Yan and J. Lu, Solid polymer electrolyte-based electro- chemical oxygen sensor, Sensors and Achufors, 19 (1989) 3340. A. Yasuda, N. xamaga, K Doi, T. Fujioka and S. Kusanagi, A planar electrochemical carbon monoxide sensor, J. Elec- no&em. Sot., 139 (1992) 1091-1095. T. Otagawa, M. Madou, S. Wing and J. Rich-Alexander, Planar microclectrochemical carbon monoxide sensors, Sensors and Actuators, Bl (1990) 319-325. K.F. Blurton and J.M. Sedlak, The electro-oxidation of carbon monoxide on platinum, J. Electmchem. Sot., 121 (1974) 1315-1317. H.W. Bay, K.F. Blurton, J.M. Sedlak and A.M. Valentine, Electrochemical technique for the measurement of carbon monoxide, Anal. Chem, 46 (1974) 1837-1839. X.K. Xing and CC. Liu, Electrochemical oxidation and the quantitative determination of carbon monoxide on a metal-solid polymer electrolyte (SPE) system, Electroanalysis, 3 (1991) 111-117.

Biographies

Heqing Yun is an assistant professor in the Department of Chemistry, Wuhan University, Wuhan, China. He was a visiting scholar at the Electronics Design Center, Case Western Reserve University in 1990-1992. His research interest is the development of electrochemical sensors.

Chung-Chiun Liu is the Wallace R. Persons professor of Sensor Technology and Control as well as the director of the Electronics Design Center at Case Western Reserve University. His research interests include elec- trochemical sensors, microelectronic fabrication pro- cesses and electrochemical sciences.