Development of Electrochemical Sensors for Hydrogen, Oxygen, and

Chapter 14

Development of Electrochemical Sensors for Hydrogen, Oxygen, and Water Using Perfluorosulfonic Acid Membranes

Downloaded by COLUMBIA UNIV on September 8, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0480.ch014

David R. Morris, Xiadong Sun, and Lietai Yang Department of Chemical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada The development of solid state potentiometric sensors for hydrogen and oxygen utilizing Nafion polymer electrolyte is described. The reference electrode comprises a mixture of Fe and Fe sulphate hydrates. For pure hydrogen at 1 atm. II

III

pressure, EOH = + 0 . 6 8 V ; for pure oxygen at 1 atm. pressure, EOO = - 0 . 3 6 V .

For hydrogen inert

gas

- 4

mixtures in the pressure range ~ 1 0 < p ≤ 1 atm, and for oxygen-inert gas mixtures in the pressure range ~ 1 0 < p ≤ 1 atm, Nernstian response is observed. For mixtures of hydrogen, oxygen and an inert gas, a mixed potential is observed which is a linear function of the ratio of the pressures of hydrogen and oxygen over the range ~0.25 < Ρ /P H

- 2

o

Η

O^)

reactions

occurring, and kinetic parameters are paramount. The results of typical measurements of sensor voltage E with various reactive gas mixtures are presented in Figure 4. The empirical regression line equation is, H

E

H

0

0

= 0.291 + 0.252 log (H/O), r = 0.994

(5)

where H/O represents the ratio of partial pressures of hydrogen and oxygen in the mixture. Relatively few tests have been conducted with reactive mixtures, and it is uncertain whether a sensor would operate satisfactorily over extended periods of time. Sensing of Hydrogen in Steel. It is well known that hydrogen in steel causes embrittlement of the metal, blistering and failure, particularly in a "sour" environment of sulphides. In a corrosive situation sulphides promote the entry of atomic hydrogen into the steel by suppressing the formation of hydrogen gas molecules. The sensor described appears to have application for monitoring hydrogen in steel. Responsiveness of the sensor to hydrogen in steel has been demonstrated by clamping the sensor to the side of a small steel vessel on which a flat surface had been milled; the thickness of the steel was -1.8 mm. A smear of silicone grease was applied between the surfaces to exclude air (1) . The sensor voltage, E , measured between the stainless steel ram, and the steel vessel was recorded as a function of time. The results of a typical experiment are presented in Figure 5. The following notes summarise the observations:

In Polyelectrolyte Gels; Harland, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Sensor

Voltage ,

Λ/3


14. MORRIS ET AL.

0.6

-

0.5

-

245

Development of Electrochemical Sensors

0.4 Legend: 0.3

χ

H-O-N Gas

ο

D-0-C0

2

Gas

0.2

01

1

-0.6

ι

-0.4

1

-0.2

0 log

1

•0.2 (H/0)

1

»0.4

1

1

«0.6

1

«0.8

• 1.0

Figure 4. Response of Sensor to Hydrogen-Oxygen Pressure Ratio. Humidified hydrogen-oxygen-inert gas mixtures at ambient temperature.


246

POLYELECTROLYTE GELS

1.

Sensor mounted on outer surface of steel vessel. Sensor voltage stabilised at -0.55V after approximately 30 h. The equivalent hydrogen pressure was - 4 X 10" atm, Equation 3. 2. Hydrochloric acid solution was introduced to the steel vessel at time 78 h. Sensor voltage rose rapidly to ~ 0.70V, equivalent to a hydrogen pressure of - 4 atm. 3. A small quantity of zinc sulphide was added to the acid solution at time 119 h. The sensor voltage rose rapidly to -0.72V. The equivalent hydrogen pressure was - 30 atm. 4. Thereafter, the sensor voltage declined as the acid was consumed and removed from the steel vessel at time 160 h. 5. In order to verify the interchangeability of sensors, the sensor was replaced at time 305 h with a second sensor. The voltage generated by the second sensor fluctuated initially as indicated by the dashed line. 6. Fresh hydrochloric acid was introduced to the steel vessel at time 353 h. The sensor voltage rose rapidly as previously observed to - 0.72V. 7. Thereafter, the sensor voltage declined following consumption of the acid and removal of the acid from the steel vessel at time 380 h. The above observations are attributed to the diffusion of hydrogen introduced into the steel by the cathodic reaction,


5

Η 0 3

+ ( & ς )

+ e- -> H

( a d s )

+H 0 2

(6)

where H ( j ) represents a hydrogen atom adsorbed on the surface of the steel. The adsorbed hydrogen may then form molecular hydrogen gas, a(

2H

s

H

7

(ads) * 2(g)

( )

or enter the steel as interstitial atoms, H

(ads) "> H

( F e

)

(8)

and diffuse through the steel. The diffusing H atoms may escape to the atmosphere at the external surface or at faults in the metal structure to form hydrogen gas. The latter phenomenon can result in the formation of hydrogen blisters as noted earlier (4) . The response of the sensor following the


14. MORRIS ET AL.


247

addition of zinc sulphide to the solution is also in accord with previous observations. (4) . A portion of a response chart similar to that presented in Figure 5 is shown in Fig. 6. The observed break-through time permitted an estimate of the diffusion coefficient (5) for hydrogen in steel as - 2 X 10' cm /s at ~ 2 5 ° C in agreement with published data (6) . Some recent experiments with zirconium into which hydrogen had been introduced by corrosion reactions with water suggest that the sensor may have application to this system also. Downloaded by COLUMBIA UNIV on September 8, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0480.ch014

6

2

Water Sorption and Transport Properties of Nafion. Water sorption isotherms for Nafion H 1100 were determined at 2 5 ° , 50° and 100°C with a Cahn microbalance and at 2 5 ° C by equilibration with the vapor phase of a solution of lithium chloride of known concentration (7) . A small piece of Nafion (~ 0.03 g) was boiled in hydrochloric acid solution and water, suspended in the microbalance and heated to ~ 1 5 0 ° C under vacuum ( ~ 1 0 mm Hg) to a constant mass. Variation of the temperature(±10°C ) caused a small (~ 0.7%) variation in the final mass; some darkening of the Nafion was observed. Water vapor was admitted to the system after cooling to 25°C and the mass of the Nafion recorded as a function of time. Upon attainment of a constant mass, additional water vapor was admitted and measurements continued. Sorption measurements were followed by desorption measurements at the temperatures noted. Sample preparation for the isopiestic measurements was identical; the specimen was periodically weighed until a constant mass was achieved. The equivalent weight of the Nafion was found to be 1097.7 ± 5.0 by elution with sodium chloride solution followed by titration (7) . The results of the water sorption experiments are presented in Figure 7 as a plot of the mol ratio Ν (s mois Η2Ο/Π10Ι H ) as a function of the water activity in the gas phase, H 2 0 (= partial pressure water / saturated vapor pressure). It is evident that all data form a single curve; at a n 0 = 1» Ν -14.0 mois Η2Ο/Π10Ι H corresponding to a mass fraction ~0.23 water in the Nafion. These data are in close agreement with those of Pushpa et al. (7) . Water sorption was accompanied by swelling -5

+

A

2

+



248


0.52

> UJ 0.50 -

ο

0.48

^ ο

0.46

ο

ο ο -

V) c

ο

ο

ο

_

ο

ο

ο

ο ο

_

ο ο _

-

) oo oo oo ο 20

40

ο

ο

/Ρ ι 60

ο

ι 80

ι

ι 100

ι

1 120

1

1 140

Time / min.

Figure 6. Breakthrough Time for Response of Sensor to Hydrogen in Steel. 1, Acid solution introduced to steel vessel; and 2, sensor voltage increasing.




14. MORRIS ET AL.

Figure 7. Sorption of Water by Nafion. Measurements with microbalance and equilibration with L i C l - H 0 solutions. 2


249

250


of the Nafion; the specific volume of the material increased from 0.49 cm /g for material dried at 130°C under vacuum, to 0.55 c m / g for material equilibrated with pure water. Water sorption data obtained from the microbalance permitted evaluation of the average diffusion coefficient for water in Nafion over the range of the concentration increment (or decrement). The diffusion coefficient D was determined from the data using the expression (8) , 3

3

M / M . = (4/7T )(Dt/i )l/2 1/2

2

(9)


t

where M and ML are the mass of water at time t and mass at equilibrium and i is the thickness of the material. Plots of M / M - vs t showed good linearity to M / M - ~ 0.5, for both sorption and desorption. Diffusion coefficients thus determined are presented in Figure 8 as a function of the average mol ratio Ν for the particular increment (or decrement). It is seen that the diffusion coefficient is a strong function of the water content, and that data obtained from sorption experiments differ from those obtained from desorption at a given value of N . These differences are attributed to dimensional changes of the Nafion accompanying sorption and desorption. Furthermore, it is seen that D increases with increase of water content to N ~ 4 . 0 followed by decreasing D for Ν > 4.0. Data obtained at 100°C are least reliable due to the small mass change of the Nafion and the restricted experimental range. In particular at the lowest water activity levels, oscillation of the microbalance obscured the measured mass change of the specimen. At the water activity level a n o ~ 0.02, the activation energy for water diffusion has been estimated as ~ 3.46 kcal/mol. t

1/2

t

t

2

Electrical Conductivity conductivity of Nafion was schematic arrangement,

of Nafion. The measured with the

electrical following

Air, H 0 , Pt I Nafion I Pt, H 0 , Air 2

2

A Nafion disc, to each side of which Pt black had been applied, was held between platinum gauze and clamped with plexiglass flanges, provided with gas access ports. The unit was mounted in a small vessel provided with connections to an air


14. MORRIS ET AL.


251

supply of known humidity, and located in a constant temperature oven. The electrical resistance of the Nafion was measured with an A C conductivity bridge or by use of an A C power supply in conjunction with an ammeter. The electrical conductivity of the Nafion is primarily a function of the mol ratio Ν as shown in Fig. 9. The regression equation is,


log σ = 6.48 log Ν - 5.08; r = 0.996

(10)

It is evident that the electrical conductivity is a strong function of the water content of the Nafion; the Grotthuss mechanism of conduction is suggested to account for this dependency. Amperometric Sensor for water. The strong dependency of the electrical conductivity of Nafion on the water concentration indicates possible application as a water sensor. The responsiveness of the sensor, constructed in a similar manner to that described in the previous section, to changes in the humidity of the air stream at ambient temperature is shown in Figure 10. It is evident that the device shows good responsiveness and reproducibility. Conclusions 1.

2.

3. 4. 5. 6. 7.

Electrochemical sensors of the potentiometric type for hydrogen and oxygen in inert gas mixtures utilizing Nafion polymer electrolyte show a Nernstian response with good long-term voltage reproducibility and stability. With a reactive gas mixture, e.g., hydrogen and oxygen with an inert gas, the sensor voltage is interpreted as a mixed potential. The sensors have application for monitoring of hydrogen in metals, e.g., Η in steel and zirconium. The equilibrium water concentration in Nafion is determined by the activity of water in the gas phase. The diffusion coefficient of water in Nafion is a strong function of the water content. The electrical conductivity of Nafion is primarily a function of the water content. An amperometric sensor for water utilizing Nafion has been developed. This unit showed good reproducibility and responsiveness to changes in gas humidity.



252


Mol

Ratio

N : n^ /n . 0

H

Figure 8. Diffusion Coefficient of Water in Nafion as Function of Temperature and Water Content

ο ο

ο

χ*

χ

χ ο

1.0 Mol

Figure 9. Content.

2.0 ratio,

3.0 Ν

Legend : t= 85 to 95*C t=25'C

4.0

=η ^ /η · Η

0

Η

Electrical Conductivity of Nafion as Function of Water


5.0


14. MORRIS ET AL.


253

Literature Cited 1. Morris, D. R., Kumar, R. V.; Fray, D. J. Ironmaking and Steelmaking, 1989, vol. 16, pp. 429-434. 2. Fray, D. J.; Morris, D. R., U.S. Patent No. 4879005 Nov. 7, 1989. 3. Miura, N.; Kato, H.; Yamazoe, N.; Sieyama, T. in Chemical Sensors; Editors, T. Seiyama et al; Analytical Chemistry, Symposium Series vol. 17 , pp. 233-238, Elsevier 1983. 4. Fontana, M. G.; Greene, N.D. Corrosion Engineering 2nd ed. McGraw-Hill, New York, 1978. 5. Crank, J.; The Mathematics of Diffusion , Oxford University Press, Oxford, 1964, pp. 47-49. 6. Ellerbrock, H.G.; Vibrans, G.; Stuwe, H.P. Acta Metallurgica , 1972, vol. 20, pp. 53-60. 7. Pushpa, K.K.; Nandan, D.; Iyer, R.M., J. Chem. Soc. , Faraday Trans. I, 1988, vol. 84 , pp. 2047-2056. 8. see ref. 5, pp. 247-248. RECEIVED August 19, 1991


Development of Electrochemical Sensors for Hydrogen, Oxygen, and

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