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Indicator and reference platinum | solid polymer electrolyte electrodes for a simple solid-state amperometric hydrogen sensor
ELSEVIER
Journal
of Electroanalytical
Chemistry
379 (1994) 301-306
Indicator and reference platinum 1solid polymer electrolyte electrodes for a simple solid-state amperometric hydrogen sensor Frantigek Opekar a, Jan Langmaier
b, ZderGk Samec b,*
aDepartment of Analytical Chemistry, Charles University, Albertov 2030, 128 40 Prague 2, Czech Republic b J. Heyrovsky’ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejSkova 3, 182 23 Prague 8, Czech Republic Received
14 March
1994; in revised form 13 May 1994
Abstract Factors that influence the open-circuit potential of a platinum 1solid polymer electrolyte (SPE) electrode in air containing low concentrations of hydrogen were investigated. The electrodes were prepared from platinum in the form of a wire or a grid mechanically pressed, or a Pt layer chemically deposited, onto a Nafion @ 117 membrane. The parameter underlying the behaviour of the Pt ISPE electrodes is the ratio of the rate of reduction of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. Depending on the value of the roughness factor, the Pt 1SPE electrode can be employed as either an indicator or a reference electrode. The design of a simple amperometric hydrogen sensor, which combines the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode, is outlined. Keywords:
Platinum electrode; Solid polymer electrolyte -
1. Introduction
In a previous study [l] we examined the possibility of detecting hydrogen in air using a two-electrode solid-state galvanic cell Pt(H,,
air) ]S(Pt’(air).
(1)
where S represents the Nafion@ membrane in the H-form, which is coated with platinum on both sides. The membrane serves as the solid polymer electrolyte (SPE) and the proton supply for the cell reaction. Under open-circuit conditions, the indicator electrode can be assumed [2] to reach a mixed potential, which is determined by the rate of the diffusion-controlled oxidation of hydrogen H,-2H++2e-
(2)
and the rate of the kinetically controlled oxygen or of Pt surface oxides +O, + 2H++ 2e- -
* Corresponding
H,O
reduction of (3)
author.
0022-0728/94/$07.00 0 1994 Elsevier SSDI 0022-0728(94)03563-I
Science
S.A. All rights reserved
As a result, the values of the open-circuit potential or the short-circuit current can depend on the hydrogen concentration at the indicator electrode [1,2]. Solid-state hydrogen sensors based on cell (l), but employing other proton conductors such as antimonic acid [2], hydrogenuranyl phosphate [3] or zirconium phosphate [4], have been described. In most applidations, however, the use of the Pt 1air reference elec trode is not very convenient from the practical point 01 view because it is necessary to introduce the reference gas (air) into the detector. Therefore it is desirable tc replace this electrode by a reference electrode whicl can be exposed to measured gas without inducing i change in its potential. Electrodes made from Ag [4] or Au [5] in contac with the proton conductor have- been reported as suit able pseudo-reference electrodes because of their in ertness to dilute hydrogen. In another reference elec trode design [5], Pt was sputtered onto the protor conductor and separated from the measured gas by : layer of the porous alumina, which presumably slow down the permeation of hydrogen to the electrode surface. Obviously, all these attempts stem from the same general principle, although this has’ not beei
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of Electroanalytical Chemistry 379 (I 994) 301-306
stated explicitly. Namely, the mixed potential arising from opposite charge transfer reactions, such as the reactions (2) and (3), should reflect the ratio of the catalytic and transport resistances. In particular, a suitable reference electrode should exhibit low catalytic and high transport resistance. The aim of this work has been to investigate the factors that influence the open-circuit potential of Pt ISPE electrode in air containing low concentrations of hydrogen in order to rationalize the design of indicator and reference Pt electrodes for the solid-state hydrogen sensor Pt(H,, air) IS ]Pt’(air, H2) (4) We shall show that the potential of the Pt ISPE electrodes depends mainly on the ratio of the true to the geometric area of the interface (i.e. the roughness factor). 2. Experimental An H-form Nafion@ 117 membrane (Aldrich, cat. no. 29,256-7) 0.17 mm thick was used as the SPE. A 15 mm X 15 mm piece of Nafion@ was first bathed in ethanol for 24 h and then boiled in water for 1 h. Three types of Pt I SPE electrode were examined, which incorporated platinum wire, platinum grid or chemically a deposited platinum disc. An S-shaped Pt wire, 0.1 mm in diameter and 20 mm long, was first cleaned by anodic polarization in 6 M HCl, then electrolytically covered with platinum black from a 2% aqueous solution of H,PtCl, and finally conditioned by a cathodic polarization in 0.1 M H,SO,. The electrode was then dipped into a 5% solution of Nafiona (Aldrich, cat. no. 27,240-4) and mechanically pressed onto the Nafion@ membrane. The 5.5 mm X 5.0 mm Pt grid consisted of 16 X 18 Pt wires, 0.06 mm in diameter. The grid was cleaned, platinized, conditioned and pressed onto the membrane in the same way as the Pt wire electrode. A Pt disc, 10 mm in diameter and with a density of about 20 mg Pt cm*, was deposited onto the Nafion@ membrane from 2 ml of the aqueous solution of 0.05 M H,PtCl, by reduction with 0.1 M N,H, in 1 M NaOH as described previously [l]. The Pt-coated membrane was boiled for 30 min in 1 M H,SO,, rinsed thoroughly with water and then bathed in water for 24 h. The geometric area of the Pt ISPE electrodes was calculated from the geometric dimensions given above for the Pt wire, grid and disc as 0.063 cm*, 0.33 cm* and 0.785 cm* respectively. Voltammetric measurements were carried out using a three-electrode system with a Pt disc counter-electrode prepared by the chemical deposition of platinum on the opposite side of the membrane. The construction of the cell is shown in Fig. 1. The Pt I SPE electrode was clamped between two parts of a
Q’
*
B
Fig. 1. Schemes of (A) the Pt ISPE electrode and (B) the experimental arrangement for measurements of its open-circuit potential against a SCE (1) Nafion” 117 membrane; (2) platinum; (3) Plexiglas body; (4) strip of Nafion@ membrane; (5) polyethylene grid; (6) assembling bolts; (7) platinum contact; (8) chamber with the controlled gas phase; (9) reference electrode; (10) auxiliary vessel; (11) measuring device.
Plexiglas body together with a strip of the Nafion@ membrane, which was 5 mm wide and 50 mm long, and a rough polyethylene grid covering the round hole in the Plexiglas body (Fig. l(A)). The electrode was exposed to the gas phase in a sealed chamber (Fig. l(B)). A strip of the Nafion@ membrane, which serves as an electrolyte bridge 161,is led out of the chamber through a gas-tight seal and immersed in an aqueous solution of 0.1 M H,SO, or saturated KC1 in the auxiliary vessel, together with the saturated calomel electrode (SCE) or an Ag IAgCl I KC1 (sat) reference electrode. The gas phase, which was composed of pure air, 60-5000 ppm (v/v) hydrogen in air or pure hydrogen, was moisturized by passing it through a saturated solution of Zn(NO,), or NaCl, ensuring a relative humidity of 42% or 76%, respectively. Defined concentrations of hydrogen in air were generated by the electrolysis of water in a separate cell. Pure hydrogen was supplied directly from the gas cylinder. Air or air + hydrogen was pumped into the vessel, through a four-way valve, which permitted the introduction of pure air or air + hydrogen, using a membrane pump (Cole-Palmer). Air was purified by passing it through a filter containing active charcoal. Potentiometric and voltammetric measurements were carried out with an apparatus assembled from operational amplifiers. Impedance was measured using a 1250 Solartron frequency response analyser and a 1286 Solartron electrochemical interface (Solartron Instruments). A 20 mV peak-to-peak ac voltage (O.Ol65000 Hz) was applied across the cell at the equilibrium cell potential difference. The measurements were performed at ambient temperature (23 + 2°C).
F. Opekar et al. /Journal
of Electroanalytical
3. Results and discussion
0.80
The true surface area of the Pt (SPE electrodes was determined from the charge consumed in the oxidation of the adsorbed hydrogen during polarization of the electrodes in a three-electrode system by a triangular voltage in a nitrogen stream at a relative humidity of 76% [7]. A typical voltammogram is shown in Fig. 2. The calculation was based on a value of 0.21 mC cmp2 for 1: 1 adsorption of H on polycrystalline Pt [81. The values of the ratio of the true to geometric surface area, i.e. the roughness factor (RF), are summarized in Table 1. Fig. 3 illustrates the change in the potential of the Pt disc, grid and wire electrodes after switching from a flow of air through the cell to a flow of air containing a low concentration of hydrogen and vice versa. In the absence of hydrogen, all three Pt electrodes have a potential of about 0.78 V, i.e. 1.02 V/SHE, corresponding to the Pt I air electrode. This value agrees well with the steady-state potential of 1.06 V which the Pt electrode attains in oxygen-saturated 1 M H,SO, solution [9,10]. In the presence of hydrogen, the steady-state potentials of various Pt electrodes differ considerably (Fig. 3). Fig. 4 shows the steady-state potential as a function of the hydrogen concentration in air. Two slopes can be seen in each plot. On going from Pt wire through Pt grid to Pt disc, the change in the slope occurs at a
1
0
4
-1
-2
0.0
303
Chemistry 379 (1994) 301-306
0.2
0.4
0.75
0.70
> -
0.65
W
0.60
0
800
1200
1600
2000
t/s Fig. 3. Change in the potential E (vs. SCE) of (1) Pt disc, (2) grid and (3) wire electrodes with time t after switching from a flow of air through the cell to a flow of air containing 1270 ppm hydrogen and vice versa. Relative humidity, 42%.
higher concentration. At low hydrogen concentrations, the potential approaches the limit corresponding to the Pt lair electrode and the slope is close to zero. At higher concentrations, the plots tend to be linear with the logarithm of hydrogen concentration with a slope of - 184 mV (Pt wire), - 144 mV (Pt grid) or - 113 mV (Pt disc) per decade. The last value is less certain owing to the lack of data in the descending part of the plot. A simple theory of the effect displayed in Fig. 4 can be developed on the basis of the mixed-potential concept. In contrast with a gas-sensing mechanism that assumes the occurrence of only two opposite electron transfer reactions (2) and (3) [1,2], we shall consider an additional anodic reaction involving an oxygen atom, cf. the theory of the oxygen electrode [ill. In particular, we assume that in the presence of hydrogen in air. Table 1 Roughness factor (RF) and the parameters of the mixed-potentia theory evaluated from the potentiometric measurements for variou! Pt ISPE electrodes in air containing low concentrations of hydroger Electrode
E/‘f Fig. 2. Cyclic voltammogram of hydrogen adsorption and desorption at a Pt disc ISPE electrode in a nitrogen stream at a relative humidity of 76% and a polarization rate of 20 mV s-‘. The potential E is given versus SCE.
400
Pt wire Pt grid
Pt disc a Two-parameter
RF
a
E0 /mV
41
0.32
194 717
0.41 (0.41)
fit for a = 0.41.
752
761 770 =
122
4 0.42 ”
F. Opekar et al. /Journal
304
of Electroanalytical Chemistry 379 (1994) 301-306
n,ADK/Gn,A,k,a,. Moreover, we shall assume that in the first approximation (Ye= 1 -(Y* = (Y. Then, using Eq. (5), the following equation can be derived:
800
P-” = PP” + yc
z \ w
600
400
3
2
4
log (c/wm) Fig. 4. Plot of the potential E (vs. SCE) against the logarithm of the hydrogen concentration c for the Pt ISPE electrodes: (v) Pt disc; (01 Pt grid; (01 Pt wire. Full curves represent the non-linear least-square fits to eqn. (9).
the mixed potential is composed of the reduction of oxygen or the Pt oxide, the oxidation of water or the Pt surface and the oxidation of hydrogen, with the corresponding electrical currents I,, Z, and Z3 respectively. Under open-circuit conditions, the net current is zero, i.e. I, = z2 + z3
(5) Since the first two processes are probably controlled by charge transfer kinetics, the currents I, and Z2 should take the form of the Tafel equation:
I, = n,FA,k,a,
exp( -(Y,FE/RT)
(6)
z, = ~,FA,~,u,
exp[( 1 - (y2jzqzu]
(7)
where ni, ki, ai and (Y~denote the number of electrons, the rate constant, the activity of the electroactive species and the charge transfer coefficient respectively. A, is the true area of the interface and E is the potential. However, the oxidation of hydrogen should be controlled by the steady-state transport of hydrogen through a layer adjacent to the electrode surface, at the outer boundary of which the hydrogen concentration is kept constant. Then the current Z3 can be given by Z3= n3 FADKc/G
(8) where n3 = 2, A is the geometric area, D is the diffusion coefficient of hydrogen in the layer of a thickness 6, K is the distribution coefficient of hydrogen between the gas phase and the layer and c is the hydrogen concentration in the gas phase. The number of parameters in Eqs. (6)-(8) can be reduced by introducing new parameters, p =n,k,a,/n,k,a, and y =
(9) where P = exp(FE/RT), the potential E and the hydrogen concentration c represent the variables, and (Y, p and y represent the parameters. When c + 0, the potential E reaches a limiting value E, = -CRT/ 2~xF) In p. On the other hand, when the concentration term on the right-hand side of Eq. (9) prevails, the potential E should be linear with the logarithm of the concentration c, with the slope controlled by the value of the charge transfer coefficient (Y, i.e. E = -CRT/&F) ln(yc). The relative position of the E vs. log c plots for various electrodes should reflect the difference in the parameter y, for example, being the result of different roughness factors A/A. Table 1 summarizes the values of the parameters [Y, E, and y, which were obtained from the non-linear least-squares fit to potentiometric data in Fig. 4. The three-parameter fitting procedure was applied to data for the Pt wire and grid electrodes. Owing to the lack of data on the descending part of the E vs. log c plot, the charge transfer coefficient (Y for the Pt disc electrode was fixed and assumed to have the value found for the Pt grid electrode, i.e. a two-parameter fitting procedure was used. The value of the charge transfer coefficient (Y is close to that obtained in kinetic studies of the Pt oxygen electrode [ill. As expected, the potential E, approaches the value of the potential of the Pt ISPE electrode in air, which is given above. Table 2 compares the change in the parameter y with that in the roughness factor of the Pt I SPE electrode, both relative to the Pt wire electrode. A good correlation between these two parameters indicates that the differences in the potential of the Pt ISPE electrodes can be due to the roughness factor. The value of the parameter y can also depend on the linear transport rate of hydrogen D/6 through the layer adjacent to the Pt ISPE interface, although the boundary of this layer in the region of the solid phase of the electrode is difficult to define. In order to clarify
Table 2 Changes in the parameter Y relative to the Pt wire electrode evaluated from the potentiometric measurements (PI, or estimated from the roughness factor (RF) and impedance spectroscopy of hydrogen electrode reaction (IS) Electrode
Pt wire Pt grid Pt disc
Y/Ywire P
RF
IS
1.000 0.033 0.003
1.000 0.211 0.057
1.000 0.041 0.005
F. Opekar et al. /Journal
of Electroanalytical
the role of this factor, the transport properties of the hydrogen Pt I SPE electrode were examined by impedance spectroscopy in the two-electrode cell (Fig. l(B)) with an Ag I AgCl reference electrode (-0.197 V/SHE). Impedance measurements were carried out at the equilibrium potential of the Pt I SPE electrode in pure hydrogen, the values of which are given in Table 3. Fig. 5(A) displays the impedance data in the complex plane for all three Pt ISPE electrodes. In the frequency range 0.05-65000 Hz, the impedance plots exhibited a high frequency limit, a semicircle in the intermediate frequency range and a much less pronounced low frequency line with a slope of about 45” (cf. also Fig. 5(B)). The data were analysed by assuming that the system can be represented by a Randlestype equivalent circuit consisting of a resistance R, in series with the parallel combination of the capacitance C and the faradaic impedance. The assumption of the steady-state hydrogen transport through a layer adjacent to the electrode surface leading to Eq. (8) implies the presence of a finite-length Warburg element in the equivalent circuit [12]. Therefore the faradaic impedance was assumed to be the kinetic resistance R, in series with the finite-length Warburg impedance Zw of the H+/H, redox couple. Since the diffusion coefficient of hydrogen in Nafion@ (ca. 5 x lo-’ cm2 s-l [13]) is much lower than the diffusion coefficient of protons (3.5 X lo+ cm2 s- ’ [14]), and the solubility of hydrogen in Nafion@ (ca. 1.5 x 10-j mol cmw3 1131)is much lower than the proton concentration (1.2 X 10e3 mol cm-3), the contribution of the hydrogen transport to Z, should dominate. The Warburg impedance is then given by [12] Z, = Y;‘(iw)
200
150 B ’ k
100 q
u
0 0
50
0
0
100
150
Z’ / kR
(a)
‘Ooo r-----l _-
N
0.1 0.1
-1’2 tanh[ B(io)1’2].
305
Chemistry 379 (1994) 301-306
1
10
100
1000
10000
(10) f / Hz
where o is the angular frequency, i = (- 1)‘12, B = ~3D-l’~ and the admittance Y0 = (n:F2A/RT)D’/2 Kc. Impedance data were analysed using non-linear leastsquares fit software [15]. The results of the analysis are summarized in Table 3. The agreement between the non-linear least-squares fit and the experimental data was found to be very good (cf. Fig. 5(B)), and the estimated relative error of the parameters given in Table 3 was typically less than 10%.
Table 3 Equilibrium potential E (vs. Ag IAgCl reference electrode) and the parameters of the Randles-type equivalent circuit for the Pt ISPE electrodes in pure hydrogen (0.1 MPa) at relative humidity of 76% Electrode Pt wire Pt grid Pt disc
E/
R,/
C/
R2/
mV
kR
pF
kR
104Ya/ 0-t SW2
B/ 9/r
-105 -113 -116
50 47 50
141 166 127
162 50 71
2.1 3.1 2.8
1.3 2.0 1.7
Fig. 5. (A) Impedance diagram of the Pt ISPE electrodes in pure hydrogen (0.1 MPa) at the equilibrium potential: (0) Pt wire; (v 1 Pt disc; and ( T ) Pt grid. (B) (1) Real and (2) imaginary components 2’ and Z” of the impedance of the Pt disc ISPE electrode vs. frequency f: experimental data (points) and the non-linear least-squares fit (solid line) yielding the parameters given in Table 3. Relative humidity, 76%.
The high frequency limit R, of the cell impedance can be ascribed to the ohmic resistance of the solid electrolyte connecting the Pt ISPE to the reference electrodes. As expected, its value is similar for all three Pt 1SPE electrodes. The values of parameter B in Table 3 correspond to a rather thin layer with a thickness 6 of lo-20 pm, assuming that D = 5 X lo-’ cm’ s-l. An important conclusion can be drawn from the ratio of the parameters Y0 and B in Table 3; Y,/B
= (ntF2A/8RT)DKc.
Negligible differences in its value (ca. 1.6 X lop4 0-l: indicate that the hydrogen transport rates for tht
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F. Opekar et al. /Journal
of Electroanalytical Chemistry
Pt ISPE electrodes tested are approximately equal. However, hydrogen transport in the layer adjacent to the electrode surface appears to be slower than that in Nafion@. Indeed, the values of the admittance Y0 themselves are lower than those estimated from the diffusion coefficient and the concentration of hydrogen in Nafion@: 6.7 X 10-3, 0-l sl/* 3.5 X lo-* 0-l sl/* and 8 x lo-* 0-l sl/* for the Pt wire, Pt grid and Pt disc electrodes respectively. The value of the exchange current density j, = 2.5 X 10d4 A cm-* for reaction (2) at a Pt electrode in an aqueous sulphuric acid solution [16] corresponds to kinetic resistances R, = RT/n,FAj, of 815 0, 156 0 or 65 R for Pt wire, grid and disc electrodes respectively. In contrast, the data analysis yields much higher values of 162 kR, 50 kR and 71 kR (Table 3). Thus the resistance R, does not seem to be associated with the kinetics of the reaction (2), but rather with the charge transport across the thin layer or film between the metal surface and Nafion@, which has a higher resistance than Nafion@ to the transport of both the ions and hydrogen. Since the capacitance C = 150 pF is also rather low and is unlikely to represent the capacitance of the Pt ISPE interface, R, and C appear to be the components of the geometric impedance of this layer [17]. Such a layer can be formed from, for example, water produced by the reduction of oxygen or surface oxides. Although the interpretation of impedance data may not be unambiguous, it is clear that the impedance spectra of the three Pt I SPE electrodes are very similar and the differences in the hydrogen transport rates appear to be rather small. Hence, the parameter y should reflect mainly the change in the true surface area A,. Indeed, the ratio y/ywire estimated on this basis is closer to that derived from the potentiometric measurements (Table 2).
4. Conclusions Changes in the open-circuit potential of a Pt ISPE electrode in air containing a low concentration of hydrogen can be explained quantitatively using a simple theory based on the mixed-potential concept. The parameter underlying the behaviour of the Pt I SPE electrodes is the ratio of the rate of the reduc-
379
(1994)
301-306
tion of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. A simple solid-state amperometric hydrogen sensor based on the cell (4) can be constructed by combining the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode.
Acknowledgements
The authors wish to acknowledge financial support from the Grant Agency of the Czech Republic (grant no. 203/93/0050) and the European Economic Community (grant no. CIPA-CT93-0097).
References [l] F. Opekar, J. Electroanal Chem., 260 (19891 451. [2] N. Miura, H. Kato, Y. Ozawa, N. Yamazoe and T. Seiyama, Chem. Lett., (1984) 1905. [3] R.V. Kumar and D.J. Fray, Sensors Actuators, 15 (19881 185. [4] N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Chem. Lett., (1983) 1573. [51 N. Miura, T. Harada, Y. Shimizu and N. Yamazoe, Sensors Actuators, Bl (19901 125. 161F. Opekar, Sensors Actuators, in press. [71 A. Katayama-Aramata, H. Nakajima, K. Fujikawa and H. Kita, Electrochim. Acta, 28 (1983) 777. 181 A.N. Frumkin, Adv. Electrochem. Electrochem. Eng., 3 (1967) 287. 191J.P. Hoare, J. Electrochem. Sot., 109 (1962) 858. [lOI S. Schuldiner and R.M. Roe, J. Electrochem. Sot., 110 (1963) 332. [ill J.P. Hoare, The Electrochemistry of Oxygen, Interscience, New York, 1968, p. 32. WI M. Sluyters-Rehbach and J.H. Sluyters in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 4, New York, 1970. (131 Z. Ogumi, T. Kuroe and Z. Takehara, J. Electrochem. Sot., 132 (1985) 2601. [141 M.W. Verbrugge and R.F. Hill, J. Electrochem. Sot., 137 (1990) 893. [I51 B.A. Boukamp, Equivalent Circuit Users’ Manual, University of Twente, The Netherlands, 1988-89. D61 S. Trassatti, J. Electroanal. Chem., 39 (19721 163. I171 R.P. Buck, Electrochim. Acta, 38 (1993) 1837.
Journal
of Electroanalytical
Chemistry
379 (1994) 301-306
Indicator and reference platinum 1solid polymer electrolyte electrodes for a simple solid-state amperometric hydrogen sensor Frantigek Opekar a, Jan Langmaier
b, ZderGk Samec b,*
aDepartment of Analytical Chemistry, Charles University, Albertov 2030, 128 40 Prague 2, Czech Republic b J. Heyrovsky’ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejSkova 3, 182 23 Prague 8, Czech Republic Received
14 March
1994; in revised form 13 May 1994
Abstract Factors that influence the open-circuit potential of a platinum 1solid polymer electrolyte (SPE) electrode in air containing low concentrations of hydrogen were investigated. The electrodes were prepared from platinum in the form of a wire or a grid mechanically pressed, or a Pt layer chemically deposited, onto a Nafion @ 117 membrane. The parameter underlying the behaviour of the Pt ISPE electrodes is the ratio of the rate of reduction of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. Depending on the value of the roughness factor, the Pt 1SPE electrode can be employed as either an indicator or a reference electrode. The design of a simple amperometric hydrogen sensor, which combines the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode, is outlined. Keywords:
Platinum electrode; Solid polymer electrolyte -
1. Introduction
In a previous study [l] we examined the possibility of detecting hydrogen in air using a two-electrode solid-state galvanic cell Pt(H,,
air) ]S(Pt’(air).
(1)
where S represents the Nafion@ membrane in the H-form, which is coated with platinum on both sides. The membrane serves as the solid polymer electrolyte (SPE) and the proton supply for the cell reaction. Under open-circuit conditions, the indicator electrode can be assumed [2] to reach a mixed potential, which is determined by the rate of the diffusion-controlled oxidation of hydrogen H,-2H++2e-
(2)
and the rate of the kinetically controlled oxygen or of Pt surface oxides +O, + 2H++ 2e- -
* Corresponding
H,O
reduction of (3)
author.
0022-0728/94/$07.00 0 1994 Elsevier SSDI 0022-0728(94)03563-I
Science
S.A. All rights reserved
As a result, the values of the open-circuit potential or the short-circuit current can depend on the hydrogen concentration at the indicator electrode [1,2]. Solid-state hydrogen sensors based on cell (l), but employing other proton conductors such as antimonic acid [2], hydrogenuranyl phosphate [3] or zirconium phosphate [4], have been described. In most applidations, however, the use of the Pt 1air reference elec trode is not very convenient from the practical point 01 view because it is necessary to introduce the reference gas (air) into the detector. Therefore it is desirable tc replace this electrode by a reference electrode whicl can be exposed to measured gas without inducing i change in its potential. Electrodes made from Ag [4] or Au [5] in contac with the proton conductor have- been reported as suit able pseudo-reference electrodes because of their in ertness to dilute hydrogen. In another reference elec trode design [5], Pt was sputtered onto the protor conductor and separated from the measured gas by : layer of the porous alumina, which presumably slow down the permeation of hydrogen to the electrode surface. Obviously, all these attempts stem from the same general principle, although this has’ not beei
302
F. Opekar et al. /Journal
of Electroanalytical Chemistry 379 (I 994) 301-306
stated explicitly. Namely, the mixed potential arising from opposite charge transfer reactions, such as the reactions (2) and (3), should reflect the ratio of the catalytic and transport resistances. In particular, a suitable reference electrode should exhibit low catalytic and high transport resistance. The aim of this work has been to investigate the factors that influence the open-circuit potential of Pt ISPE electrode in air containing low concentrations of hydrogen in order to rationalize the design of indicator and reference Pt electrodes for the solid-state hydrogen sensor Pt(H,, air) IS ]Pt’(air, H2) (4) We shall show that the potential of the Pt ISPE electrodes depends mainly on the ratio of the true to the geometric area of the interface (i.e. the roughness factor). 2. Experimental An H-form Nafion@ 117 membrane (Aldrich, cat. no. 29,256-7) 0.17 mm thick was used as the SPE. A 15 mm X 15 mm piece of Nafion@ was first bathed in ethanol for 24 h and then boiled in water for 1 h. Three types of Pt I SPE electrode were examined, which incorporated platinum wire, platinum grid or chemically a deposited platinum disc. An S-shaped Pt wire, 0.1 mm in diameter and 20 mm long, was first cleaned by anodic polarization in 6 M HCl, then electrolytically covered with platinum black from a 2% aqueous solution of H,PtCl, and finally conditioned by a cathodic polarization in 0.1 M H,SO,. The electrode was then dipped into a 5% solution of Nafiona (Aldrich, cat. no. 27,240-4) and mechanically pressed onto the Nafion@ membrane. The 5.5 mm X 5.0 mm Pt grid consisted of 16 X 18 Pt wires, 0.06 mm in diameter. The grid was cleaned, platinized, conditioned and pressed onto the membrane in the same way as the Pt wire electrode. A Pt disc, 10 mm in diameter and with a density of about 20 mg Pt cm*, was deposited onto the Nafion@ membrane from 2 ml of the aqueous solution of 0.05 M H,PtCl, by reduction with 0.1 M N,H, in 1 M NaOH as described previously [l]. The Pt-coated membrane was boiled for 30 min in 1 M H,SO,, rinsed thoroughly with water and then bathed in water for 24 h. The geometric area of the Pt ISPE electrodes was calculated from the geometric dimensions given above for the Pt wire, grid and disc as 0.063 cm*, 0.33 cm* and 0.785 cm* respectively. Voltammetric measurements were carried out using a three-electrode system with a Pt disc counter-electrode prepared by the chemical deposition of platinum on the opposite side of the membrane. The construction of the cell is shown in Fig. 1. The Pt I SPE electrode was clamped between two parts of a
Q’
*
B
Fig. 1. Schemes of (A) the Pt ISPE electrode and (B) the experimental arrangement for measurements of its open-circuit potential against a SCE (1) Nafion” 117 membrane; (2) platinum; (3) Plexiglas body; (4) strip of Nafion@ membrane; (5) polyethylene grid; (6) assembling bolts; (7) platinum contact; (8) chamber with the controlled gas phase; (9) reference electrode; (10) auxiliary vessel; (11) measuring device.
Plexiglas body together with a strip of the Nafion@ membrane, which was 5 mm wide and 50 mm long, and a rough polyethylene grid covering the round hole in the Plexiglas body (Fig. l(A)). The electrode was exposed to the gas phase in a sealed chamber (Fig. l(B)). A strip of the Nafion@ membrane, which serves as an electrolyte bridge 161,is led out of the chamber through a gas-tight seal and immersed in an aqueous solution of 0.1 M H,SO, or saturated KC1 in the auxiliary vessel, together with the saturated calomel electrode (SCE) or an Ag IAgCl I KC1 (sat) reference electrode. The gas phase, which was composed of pure air, 60-5000 ppm (v/v) hydrogen in air or pure hydrogen, was moisturized by passing it through a saturated solution of Zn(NO,), or NaCl, ensuring a relative humidity of 42% or 76%, respectively. Defined concentrations of hydrogen in air were generated by the electrolysis of water in a separate cell. Pure hydrogen was supplied directly from the gas cylinder. Air or air + hydrogen was pumped into the vessel, through a four-way valve, which permitted the introduction of pure air or air + hydrogen, using a membrane pump (Cole-Palmer). Air was purified by passing it through a filter containing active charcoal. Potentiometric and voltammetric measurements were carried out with an apparatus assembled from operational amplifiers. Impedance was measured using a 1250 Solartron frequency response analyser and a 1286 Solartron electrochemical interface (Solartron Instruments). A 20 mV peak-to-peak ac voltage (O.Ol65000 Hz) was applied across the cell at the equilibrium cell potential difference. The measurements were performed at ambient temperature (23 + 2°C).
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3. Results and discussion
0.80
The true surface area of the Pt (SPE electrodes was determined from the charge consumed in the oxidation of the adsorbed hydrogen during polarization of the electrodes in a three-electrode system by a triangular voltage in a nitrogen stream at a relative humidity of 76% [7]. A typical voltammogram is shown in Fig. 2. The calculation was based on a value of 0.21 mC cmp2 for 1: 1 adsorption of H on polycrystalline Pt [81. The values of the ratio of the true to geometric surface area, i.e. the roughness factor (RF), are summarized in Table 1. Fig. 3 illustrates the change in the potential of the Pt disc, grid and wire electrodes after switching from a flow of air through the cell to a flow of air containing a low concentration of hydrogen and vice versa. In the absence of hydrogen, all three Pt electrodes have a potential of about 0.78 V, i.e. 1.02 V/SHE, corresponding to the Pt I air electrode. This value agrees well with the steady-state potential of 1.06 V which the Pt electrode attains in oxygen-saturated 1 M H,SO, solution [9,10]. In the presence of hydrogen, the steady-state potentials of various Pt electrodes differ considerably (Fig. 3). Fig. 4 shows the steady-state potential as a function of the hydrogen concentration in air. Two slopes can be seen in each plot. On going from Pt wire through Pt grid to Pt disc, the change in the slope occurs at a
1
0
4
-1
-2
0.0
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Chemistry 379 (1994) 301-306
0.2
0.4
0.75
0.70
> -
0.65
W
0.60
0
800
1200
1600
2000
t/s Fig. 3. Change in the potential E (vs. SCE) of (1) Pt disc, (2) grid and (3) wire electrodes with time t after switching from a flow of air through the cell to a flow of air containing 1270 ppm hydrogen and vice versa. Relative humidity, 42%.
higher concentration. At low hydrogen concentrations, the potential approaches the limit corresponding to the Pt lair electrode and the slope is close to zero. At higher concentrations, the plots tend to be linear with the logarithm of hydrogen concentration with a slope of - 184 mV (Pt wire), - 144 mV (Pt grid) or - 113 mV (Pt disc) per decade. The last value is less certain owing to the lack of data in the descending part of the plot. A simple theory of the effect displayed in Fig. 4 can be developed on the basis of the mixed-potential concept. In contrast with a gas-sensing mechanism that assumes the occurrence of only two opposite electron transfer reactions (2) and (3) [1,2], we shall consider an additional anodic reaction involving an oxygen atom, cf. the theory of the oxygen electrode [ill. In particular, we assume that in the presence of hydrogen in air. Table 1 Roughness factor (RF) and the parameters of the mixed-potentia theory evaluated from the potentiometric measurements for variou! Pt ISPE electrodes in air containing low concentrations of hydroger Electrode
E/‘f Fig. 2. Cyclic voltammogram of hydrogen adsorption and desorption at a Pt disc ISPE electrode in a nitrogen stream at a relative humidity of 76% and a polarization rate of 20 mV s-‘. The potential E is given versus SCE.
400
Pt wire Pt grid
Pt disc a Two-parameter
RF
a
E0 /mV
41
0.32
194 717
0.41 (0.41)
fit for a = 0.41.
752
761 770 =
122
4 0.42 ”
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n,ADK/Gn,A,k,a,. Moreover, we shall assume that in the first approximation (Ye= 1 -(Y* = (Y. Then, using Eq. (5), the following equation can be derived:
800
P-” = PP” + yc
z \ w
600
400
3
2
4
log (c/wm) Fig. 4. Plot of the potential E (vs. SCE) against the logarithm of the hydrogen concentration c for the Pt ISPE electrodes: (v) Pt disc; (01 Pt grid; (01 Pt wire. Full curves represent the non-linear least-square fits to eqn. (9).
the mixed potential is composed of the reduction of oxygen or the Pt oxide, the oxidation of water or the Pt surface and the oxidation of hydrogen, with the corresponding electrical currents I,, Z, and Z3 respectively. Under open-circuit conditions, the net current is zero, i.e. I, = z2 + z3
(5) Since the first two processes are probably controlled by charge transfer kinetics, the currents I, and Z2 should take the form of the Tafel equation:
I, = n,FA,k,a,
exp( -(Y,FE/RT)
(6)
z, = ~,FA,~,u,
exp[( 1 - (y2jzqzu]
(7)
where ni, ki, ai and (Y~denote the number of electrons, the rate constant, the activity of the electroactive species and the charge transfer coefficient respectively. A, is the true area of the interface and E is the potential. However, the oxidation of hydrogen should be controlled by the steady-state transport of hydrogen through a layer adjacent to the electrode surface, at the outer boundary of which the hydrogen concentration is kept constant. Then the current Z3 can be given by Z3= n3 FADKc/G
(8) where n3 = 2, A is the geometric area, D is the diffusion coefficient of hydrogen in the layer of a thickness 6, K is the distribution coefficient of hydrogen between the gas phase and the layer and c is the hydrogen concentration in the gas phase. The number of parameters in Eqs. (6)-(8) can be reduced by introducing new parameters, p =n,k,a,/n,k,a, and y =
(9) where P = exp(FE/RT), the potential E and the hydrogen concentration c represent the variables, and (Y, p and y represent the parameters. When c + 0, the potential E reaches a limiting value E, = -CRT/ 2~xF) In p. On the other hand, when the concentration term on the right-hand side of Eq. (9) prevails, the potential E should be linear with the logarithm of the concentration c, with the slope controlled by the value of the charge transfer coefficient (Y, i.e. E = -CRT/&F) ln(yc). The relative position of the E vs. log c plots for various electrodes should reflect the difference in the parameter y, for example, being the result of different roughness factors A/A. Table 1 summarizes the values of the parameters [Y, E, and y, which were obtained from the non-linear least-squares fit to potentiometric data in Fig. 4. The three-parameter fitting procedure was applied to data for the Pt wire and grid electrodes. Owing to the lack of data on the descending part of the E vs. log c plot, the charge transfer coefficient (Y for the Pt disc electrode was fixed and assumed to have the value found for the Pt grid electrode, i.e. a two-parameter fitting procedure was used. The value of the charge transfer coefficient (Y is close to that obtained in kinetic studies of the Pt oxygen electrode [ill. As expected, the potential E, approaches the value of the potential of the Pt ISPE electrode in air, which is given above. Table 2 compares the change in the parameter y with that in the roughness factor of the Pt I SPE electrode, both relative to the Pt wire electrode. A good correlation between these two parameters indicates that the differences in the potential of the Pt ISPE electrodes can be due to the roughness factor. The value of the parameter y can also depend on the linear transport rate of hydrogen D/6 through the layer adjacent to the Pt ISPE interface, although the boundary of this layer in the region of the solid phase of the electrode is difficult to define. In order to clarify
Table 2 Changes in the parameter Y relative to the Pt wire electrode evaluated from the potentiometric measurements (PI, or estimated from the roughness factor (RF) and impedance spectroscopy of hydrogen electrode reaction (IS) Electrode
Pt wire Pt grid Pt disc
Y/Ywire P
RF
IS
1.000 0.033 0.003
1.000 0.211 0.057
1.000 0.041 0.005
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the role of this factor, the transport properties of the hydrogen Pt I SPE electrode were examined by impedance spectroscopy in the two-electrode cell (Fig. l(B)) with an Ag I AgCl reference electrode (-0.197 V/SHE). Impedance measurements were carried out at the equilibrium potential of the Pt I SPE electrode in pure hydrogen, the values of which are given in Table 3. Fig. 5(A) displays the impedance data in the complex plane for all three Pt ISPE electrodes. In the frequency range 0.05-65000 Hz, the impedance plots exhibited a high frequency limit, a semicircle in the intermediate frequency range and a much less pronounced low frequency line with a slope of about 45” (cf. also Fig. 5(B)). The data were analysed by assuming that the system can be represented by a Randlestype equivalent circuit consisting of a resistance R, in series with the parallel combination of the capacitance C and the faradaic impedance. The assumption of the steady-state hydrogen transport through a layer adjacent to the electrode surface leading to Eq. (8) implies the presence of a finite-length Warburg element in the equivalent circuit [12]. Therefore the faradaic impedance was assumed to be the kinetic resistance R, in series with the finite-length Warburg impedance Zw of the H+/H, redox couple. Since the diffusion coefficient of hydrogen in Nafion@ (ca. 5 x lo-’ cm2 s-l [13]) is much lower than the diffusion coefficient of protons (3.5 X lo+ cm2 s- ’ [14]), and the solubility of hydrogen in Nafion@ (ca. 1.5 x 10-j mol cmw3 1131)is much lower than the proton concentration (1.2 X 10e3 mol cm-3), the contribution of the hydrogen transport to Z, should dominate. The Warburg impedance is then given by [12] Z, = Y;‘(iw)
200
150 B ’ k
100 q
u
0 0
50
0
0
100
150
Z’ / kR
(a)
‘Ooo r-----l _-
N
0.1 0.1
-1’2 tanh[ B(io)1’2].
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Chemistry 379 (1994) 301-306
1
10
100
1000
10000
(10) f / Hz
where o is the angular frequency, i = (- 1)‘12, B = ~3D-l’~ and the admittance Y0 = (n:F2A/RT)D’/2 Kc. Impedance data were analysed using non-linear leastsquares fit software [15]. The results of the analysis are summarized in Table 3. The agreement between the non-linear least-squares fit and the experimental data was found to be very good (cf. Fig. 5(B)), and the estimated relative error of the parameters given in Table 3 was typically less than 10%.
Table 3 Equilibrium potential E (vs. Ag IAgCl reference electrode) and the parameters of the Randles-type equivalent circuit for the Pt ISPE electrodes in pure hydrogen (0.1 MPa) at relative humidity of 76% Electrode Pt wire Pt grid Pt disc
E/
R,/
C/
R2/
mV
kR
pF
kR
104Ya/ 0-t SW2
B/ 9/r
-105 -113 -116
50 47 50
141 166 127
162 50 71
2.1 3.1 2.8
1.3 2.0 1.7
Fig. 5. (A) Impedance diagram of the Pt ISPE electrodes in pure hydrogen (0.1 MPa) at the equilibrium potential: (0) Pt wire; (v 1 Pt disc; and ( T ) Pt grid. (B) (1) Real and (2) imaginary components 2’ and Z” of the impedance of the Pt disc ISPE electrode vs. frequency f: experimental data (points) and the non-linear least-squares fit (solid line) yielding the parameters given in Table 3. Relative humidity, 76%.
The high frequency limit R, of the cell impedance can be ascribed to the ohmic resistance of the solid electrolyte connecting the Pt ISPE to the reference electrodes. As expected, its value is similar for all three Pt 1SPE electrodes. The values of parameter B in Table 3 correspond to a rather thin layer with a thickness 6 of lo-20 pm, assuming that D = 5 X lo-’ cm’ s-l. An important conclusion can be drawn from the ratio of the parameters Y0 and B in Table 3; Y,/B
= (ntF2A/8RT)DKc.
Negligible differences in its value (ca. 1.6 X lop4 0-l: indicate that the hydrogen transport rates for tht
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Pt ISPE electrodes tested are approximately equal. However, hydrogen transport in the layer adjacent to the electrode surface appears to be slower than that in Nafion@. Indeed, the values of the admittance Y0 themselves are lower than those estimated from the diffusion coefficient and the concentration of hydrogen in Nafion@: 6.7 X 10-3, 0-l sl/* 3.5 X lo-* 0-l sl/* and 8 x lo-* 0-l sl/* for the Pt wire, Pt grid and Pt disc electrodes respectively. The value of the exchange current density j, = 2.5 X 10d4 A cm-* for reaction (2) at a Pt electrode in an aqueous sulphuric acid solution [16] corresponds to kinetic resistances R, = RT/n,FAj, of 815 0, 156 0 or 65 R for Pt wire, grid and disc electrodes respectively. In contrast, the data analysis yields much higher values of 162 kR, 50 kR and 71 kR (Table 3). Thus the resistance R, does not seem to be associated with the kinetics of the reaction (2), but rather with the charge transport across the thin layer or film between the metal surface and Nafion@, which has a higher resistance than Nafion@ to the transport of both the ions and hydrogen. Since the capacitance C = 150 pF is also rather low and is unlikely to represent the capacitance of the Pt ISPE interface, R, and C appear to be the components of the geometric impedance of this layer [17]. Such a layer can be formed from, for example, water produced by the reduction of oxygen or surface oxides. Although the interpretation of impedance data may not be unambiguous, it is clear that the impedance spectra of the three Pt I SPE electrodes are very similar and the differences in the hydrogen transport rates appear to be rather small. Hence, the parameter y should reflect mainly the change in the true surface area A,. Indeed, the ratio y/ywire estimated on this basis is closer to that derived from the potentiometric measurements (Table 2).
4. Conclusions Changes in the open-circuit potential of a Pt ISPE electrode in air containing a low concentration of hydrogen can be explained quantitatively using a simple theory based on the mixed-potential concept. The parameter underlying the behaviour of the Pt I SPE electrodes is the ratio of the rate of the reduc-
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tion of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. A simple solid-state amperometric hydrogen sensor based on the cell (4) can be constructed by combining the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode.
Acknowledgements
The authors wish to acknowledge financial support from the Grant Agency of the Czech Republic (grant no. 203/93/0050) and the European Economic Community (grant no. CIPA-CT93-0097).
References [l] F. Opekar, J. Electroanal Chem., 260 (19891 451. [2] N. Miura, H. Kato, Y. Ozawa, N. Yamazoe and T. Seiyama, Chem. Lett., (1984) 1905. [3] R.V. Kumar and D.J. Fray, Sensors Actuators, 15 (19881 185. [4] N. Miura, H. Kato, N. Yamazoe and T. Seiyama, Chem. Lett., (1983) 1573. [51 N. Miura, T. Harada, Y. Shimizu and N. Yamazoe, Sensors Actuators, Bl (19901 125. 161F. Opekar, Sensors Actuators, in press. [71 A. Katayama-Aramata, H. Nakajima, K. Fujikawa and H. Kita, Electrochim. Acta, 28 (1983) 777. 181 A.N. Frumkin, Adv. Electrochem. Electrochem. Eng., 3 (1967) 287. 191J.P. Hoare, J. Electrochem. Sot., 109 (1962) 858. [lOI S. Schuldiner and R.M. Roe, J. Electrochem. Sot., 110 (1963) 332. [ill J.P. Hoare, The Electrochemistry of Oxygen, Interscience, New York, 1968, p. 32. WI M. Sluyters-Rehbach and J.H. Sluyters in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 4, New York, 1970. (131 Z. Ogumi, T. Kuroe and Z. Takehara, J. Electrochem. Sot., 132 (1985) 2601. [141 M.W. Verbrugge and R.F. Hill, J. Electrochem. Sot., 137 (1990) 893. [I51 B.A. Boukamp, Equivalent Circuit Users’ Manual, University of Twente, The Netherlands, 1988-89. D61 S. Trassatti, J. Electroanal. Chem., 39 (19721 163. I171 R.P. Buck, Electrochim. Acta, 38 (1993) 1837.