- Email: [email protected]
Metal electrodes bonded on solid polymer electrolyte membranes (SPE)—II. The polarization resistance of Pt-Nafion electrode
METAL ELECTRODES BONDED ON SOLID POLYMER ELECTROLYTE MEMBRANES (SPE)-II. THE POLARIZATION RESISTANCE OF Pt-NAFION ELECTRODE H.
KITA,
IL
FUJIKAWA* and H. NAKAJIMA
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan and Institute
l
of Technology, Mworan 050, Japan
(Received 13 February
Muroran
1984; in revisedform 12 July 1984)
Abstracti-The hydrogen and oxygen electrodereactions were examinedon a Pt-SPE (Nafion)electrode with the condition that tbe platinum side was in contact with gaseous phase. The polarization curve shows an Ohmic relation determined mainly by the solution and membrane resistances.The removal of the solution resistanceimproves the above polarization characteristicsto a large extent.
INTRODUCTION
EXPERIMENTAL
Solid polymer electrolytes (SPE) are very attractive material in the field of electrolysis. Nation (Du Pont) is a typical cation exchange membrane and its electrochemical nature, especially its ionic conductance, has been extensively studiedC1-41. Furthermore, an attempt has been made to use the membrane as an electrode by bonding a metal layer on its surface[ 5-71. We have reported that the platinum bonded to Nafion shows a similar voltammogram to that of a platinum metal, independent of the presence or absence of electrolytic solution at the platinum side of the membrane[S]. The absence of electrolytic solution at the platinum side provides a great advantage that hydrogen or oxygen can adsorb directly from the gas phase and then be oxidized or reduced electrochemically without the transfer process in the solution. In fact, polarization of the electrode in this condition causes a large current exceeding the limiting value expected in the presence of electrolytic solution[S]. In addition, the polarization curve shows interestingly an Ohmic behaviour with a common slope for the hydrogen ionization, oxygen reduction and hydrogen evolution reactions. In the present paper, we examine the nature of the Ohmic component and found that the removal of the solution resistance improves the polarization characteristics of Pt-SPE electrode to a large extent.
Pt-SPE was prepared by the Takenaka-Torikai method[9]. A sheet of SPE (Nafion 315) was first immersed in boiling water for 0.5-2 h, and then mounted at a bottom of a cylinder so that the upper side faced 0.01-0.02 mol dm-’ H,PtCI, aqueous solution in the cylinder and the lower side faced 1.3 mol dm-” sodium borohydride alkaline aqueous solution (ca. 5Oml) in a beaker. Platinum ions were reduced at the upper side of the membrane. The apparent density of the platinum bonded onto the SPE was 4-12 mg crnm2 (called Pt-SPE I). The same technique was repeated by mounting the above Pt-SPE upside-down when both the sides of SPE are required to have a platinum layer (called Pt-SPE II). A schematic diagram of the cell is shown in Fig. 1. Compartments A and B of the cell in Fig. 1 were separated by Pt-SPE (an exposed area was 3.14 or 14.5 cm2). The platinum facing the gas phase (Hz or 0,) of compartment A served as a test electrode. In order to obtain air-tightness we used gaskets of Unisheets (NRK Co. Ltd., polymer made from silicon and Teflon) between Pt-SPE and glass flanges. Electrical contact to the Pt-SPE was made by a ring of gold foil inserted between Pt-SPE and the gasket. The cell in Fig. l(b) did not include the Luggin capillary. The solution in compartment B was aqueous HClO, solution of different concentrations. Electrochemical
(a)
(b)
Fig. 1. Schematicdiagram of the cell used (a) with and (b) without the Luggin capillary for referenceelectrode. C, T and R; counter, test and reference electrodes. 1721
1722
H. KITA, K. FUJIKAWAAND I-L NAKAJIMA 9
I
V
vs.
RHE
Fig. 2. Cathodic i-4 curves (2H+ + 2e- * H,) on Pt-SPE I [Fig. 1 (a)] at various concentrations of HCIO,. Sweep rate, 10 mV s-l, N, flow (I atm) in compartment A.
with two sheets of platinized platinum. Results are shown in Table 1. The solution resistance, n,,,, decreases rapidly with the increase of the HCIOc concentration but the membrane resistance, a,,,,, , decreases only by a half. A N&on catalogue quotes a specific resistance of 175 Dcm at equilibration with 0.6 M KCL, 23°C for EW = 1300. Hence, the present membrane (thickness O.O2cm, area 3.i4cm2) is expected to have a resistance of I. 1 11. The value of Table 1 is reasonable taking into account the difference of cation used in the present work. The resistance of the platinum layer deposited on the SPE is estimated to be small for the following reason. Current, uniformly reaching the surface, passes through the Pt layer radially towards the rim where an annular Au current collector is attached. The potential drop, d#, formed across the two rings at r and I + dr in the Pt layer (loops shown in the Pt layer of Fig. 4) will be given as d@ = (irrr’)
measurements were carried out by the potential sweep or potentiostatic method at room temperature. Potentials are referred to the reversible hydrogen electrode.
x
dr
(>’ p-
2xrd
= p’rdr, 2d
where i is the current density, r the distance from the centre of the Pt layer of a thickness d, and p the specific resistance, respectively. Integration gives
REXJLTS We first examined the hydrogen evolution reaction at Pt-SPE I under N, flow in compartment A of Fig. l(a). Cathodic sweep of the electrode potential revealed i+ curves as shown in Fig. 2 (independent of a sweep rate) where the concentration of HClO., was varied from 0.063 to 4 M. A linear relation holds in each case except for the region near the reversible potential where the hydrogen evolution still takes place under N, atmosphere. Such a linear relation clearly excludes an electrode reaction from the factors responsible far the i-4 relation. The reciprocal of the slope represents the resistance and is plotted as a function of the HC104 concentration in Fig. 3. The resistance sharply drops with the concentration and becomes almost constant at concentrations beyond 2 M. The resistance may consist of three parts due to: (1) the solution present between the tip of the Luggin capillary and the membrane surface, (2) the membrane and (3) the platinum layer (see Fig. 4). These components are separately estimated as follows. The solution resistance was obtained from the measured conductivity (Toa Denpa Co. Ltd., CM-XET) and the geometrical factors of the surface area of the membrane exposed to the solution (3.1 cm’) and the distance between the tip of the Luggin capillary and the membrane surface (5 mm). The membrane resistance was directly measured by sandwiching SPE film, preequilibriated with the solution of interest overnight,
B
c
6
4E
k
-4
a
!
$p
2
-
t
I
Oa
. --:a.1.0
Rmcm.
I
_*__-_-_-__--_-__-____lc
2.0
3,O
4.0
5 ‘,
_
,
11.4
c IM
Fig. 5. Apparent resistance of Pt-SPE I as a function of [HCIO.,] (from Fig. 2): ---observed, ---sum of the solution and membrane resistances.
Fig. 4. Solution, membrane and Pt layer resistances.
Table 1. Solution and membrane resistance 0.063
0.125
C (mol dm-‘) 0.25 0.50
1.0
2.0
4.0
6.4 0.86
3.3 0.70
1.6 0.57
0.5 0.45
0.3 0.45
0.2 0.45
0.8 0.51
11.4 0.5 0.51
Metal electrodesbonded on SPE-II
where 1 is the radius of the Pt layer. Hence, the membrane resistance will be given by dividing A$ by the total current, I = ial’, as O_,,, = A@/[ = p/4xd. Calculating d from the amount of Pt deposited (5 mgcm-‘) and a lattice constant (3.92A) as d = 1W 4 cm, and assuming a bulk resistance of Pt as p = 10.6 x 10m6Rem, we have D2,,, c 0.01 Q in order of magnitude which is negligibly small. Thus, the sum of the solution and membrane resistances is compared with the observed resistance in Fig. 2. Agreement is satisfactory. Hence, we conclude that the factors which control the polarization behaviour are the solution and membrane resistances. Next we used Pt-SPE II (apparent surface area, 14.5cm*) where the platinum layer facing the electrolytic solution worked as a reference electrode [Fig. l(b)J In this case, the solution resistance is completely excluded. Results on the hydrogen ionization reaction are shown in Fig. 5 in comparison with those at Pt-SPE I, where the compartment A in Fig. l(a) and (b) is fed a flow of atmospheric hydrogen gas. We have again an Ohmic relation for the hydrogen ionization reaction and with a much steeper slope on Pt-SPE II. It is seen that the current far exceeds the limiting value (ca. 1 mAcm- ‘[lo] ) for a usual gas electrode in solution. The reciprocal of the slope on Pt-SPE II gives a resistance of 0.127 R being much smaller than 1.68 R of Pt-SPE I. The decrease of 1.55 Q must be attributed to a solution resistance which is calculated as 1.31 R from a specific conductivity of 0.343 Q- 1 cm-’ (1 M HClOJ, a surface area of 14.5 cm2 and a distance of 6.5 cm between the counter electrode of a platinized platinum foil and the membrane surface. Hence, the value of 0.127 D of Pt-SPE 11 will be taken as the resistance of the membrane and platinum layer. The latter contribution will be small as discussed earlier. The value of 0.127 SLis apparently smailer than a,,,
1723
in Table 1 but becomes coincident when the difference in the surface area of the membrane is taken into account, i.e. 0.127 x 14.5/3.1= 0.59 R. On the same Pt-SPE I and II electrodes, we examined the reduction of oxygen by flowing O2 gas in compartment A. Results are shown in Fig. 6. In this case, a current at a given potential shows a gradual decay for the first 1&20min. We plotted a steady current at co. 30 min. Cathodic current increases in an accelerated manner with the less positive polarization and then becomes proportional to the potential on both the electrodes. The apparent resistance is estimated from the respective slopes as 2.10 and 0.29 R for Pt-SPE I and II. These values are somewhat larger than those of Fig. 5 but to be stressed is that the exclusion of the solution resistance again leads to a much larger current. CONCLUDING
REMARKS
The Pt-SPE electrode removes the serious difficulty present in a gas electrode that the current is controlled by a mass transfer in the solution and reveals an Ohmic relation over a wide range of current. Such a linear relation between the current and potential excludes the possibility of the electrode reaction being ratecontrolling. The slope gives a resistance which depends on the concentration of electrolyte. The removal of the solution resistance improves the polarization characteristics of the hydrogen ionization and oxygen reduction reactions. Thus, the Pt-SPE electrode has a promising feature in its use for the fuel cells of the third generation. In this respect, we are now examining the electrochemical behaviour of the SPE electrode which carries platinum layers on both sides of the membrane exposed directly to hydrogen and oxygen gases without an electrolyte solution. Results will be reported in the subsequent Paper. +lV
Pt-SPE
vs.RHE
0
I
t1
-04 PI-SPE
I
t 4 -
-OS-
-0.8
a
0
’ 0.2
0-L 91
V
0.6
s.RHE
0.6
i-4 curves (H, -+ 2H+ + b-) on Pt-SPE I and II. Potentiostatic steady polanzation, I M HCIO, in compartmentB, HZ Bow (1 atm) incompartment A. Apparent Fig. 5. Anodic
surface area, 14.5 cm’.
-
-1.0-
Fig. 6. Cathodic i-4 curves(0, +4H+ + 4e -+ 2H10) on Pt-
SPE I and II. Conditions are the same as in Fig. 5 except that compartment B is fed a flow of oxygen of l-07-1.3 atmospheric pressures.
1724
H. KITA,K.
FUJIKAWAAND
Acknowledgement-This work is partly supported by a grantin-aid for scientific research, Ministry of Education, Science and Culture. No. 57470001.
REFERENCES I. 2. Twardowski, H. L. Yeager and 8. O’Dell, J. elsctrothem. Sot. 129, 328 (1982). 2. R. S. Yea, J. .&ctrochem Sot. 130, 535 (1983). 3. W. Y. Hsu and T. D. Gierke, J. memb. Sci. 13,307 (1983).
H. NAKAJIMA
4. A. Steck and H. L. Yeager, J. rlectrockem Sor. 130,1297 (1983). 5. 2. Ogumi, K. Nishio and S. Yoshizawa, Electrockim. Acto 26, 1779 (1981). 6. I. Bergman, J. electroonal. Ckem. L57, 59 (1983). 7. A, KatayamaAramata and R. Ohnishi, J. Am. ckem. Sot. 105, 658 (1983). 8. A. Katayama-Aramata, H. Nakajima, K. Fujikawa and H. Kita, Eleccrrcchim. Acta 28, 777 (1983). 9. H. Takeuaka and E. Torikai, KokaiTokkyo Koko (Japan Patent) 55, 38934 (1980). IO. S. Schuldiner, J. electrockem. Sot. 106. 891 (1959).
KITA,
IL
FUJIKAWA* and H. NAKAJIMA
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan and Institute
l
of Technology, Mworan 050, Japan
(Received 13 February
Muroran
1984; in revisedform 12 July 1984)
Abstracti-The hydrogen and oxygen electrodereactions were examinedon a Pt-SPE (Nafion)electrode with the condition that tbe platinum side was in contact with gaseous phase. The polarization curve shows an Ohmic relation determined mainly by the solution and membrane resistances.The removal of the solution resistanceimproves the above polarization characteristicsto a large extent.
INTRODUCTION
EXPERIMENTAL
Solid polymer electrolytes (SPE) are very attractive material in the field of electrolysis. Nation (Du Pont) is a typical cation exchange membrane and its electrochemical nature, especially its ionic conductance, has been extensively studiedC1-41. Furthermore, an attempt has been made to use the membrane as an electrode by bonding a metal layer on its surface[ 5-71. We have reported that the platinum bonded to Nafion shows a similar voltammogram to that of a platinum metal, independent of the presence or absence of electrolytic solution at the platinum side of the membrane[S]. The absence of electrolytic solution at the platinum side provides a great advantage that hydrogen or oxygen can adsorb directly from the gas phase and then be oxidized or reduced electrochemically without the transfer process in the solution. In fact, polarization of the electrode in this condition causes a large current exceeding the limiting value expected in the presence of electrolytic solution[S]. In addition, the polarization curve shows interestingly an Ohmic behaviour with a common slope for the hydrogen ionization, oxygen reduction and hydrogen evolution reactions. In the present paper, we examine the nature of the Ohmic component and found that the removal of the solution resistance improves the polarization characteristics of Pt-SPE electrode to a large extent.
Pt-SPE was prepared by the Takenaka-Torikai method[9]. A sheet of SPE (Nafion 315) was first immersed in boiling water for 0.5-2 h, and then mounted at a bottom of a cylinder so that the upper side faced 0.01-0.02 mol dm-’ H,PtCI, aqueous solution in the cylinder and the lower side faced 1.3 mol dm-” sodium borohydride alkaline aqueous solution (ca. 5Oml) in a beaker. Platinum ions were reduced at the upper side of the membrane. The apparent density of the platinum bonded onto the SPE was 4-12 mg crnm2 (called Pt-SPE I). The same technique was repeated by mounting the above Pt-SPE upside-down when both the sides of SPE are required to have a platinum layer (called Pt-SPE II). A schematic diagram of the cell is shown in Fig. 1. Compartments A and B of the cell in Fig. 1 were separated by Pt-SPE (an exposed area was 3.14 or 14.5 cm2). The platinum facing the gas phase (Hz or 0,) of compartment A served as a test electrode. In order to obtain air-tightness we used gaskets of Unisheets (NRK Co. Ltd., polymer made from silicon and Teflon) between Pt-SPE and glass flanges. Electrical contact to the Pt-SPE was made by a ring of gold foil inserted between Pt-SPE and the gasket. The cell in Fig. l(b) did not include the Luggin capillary. The solution in compartment B was aqueous HClO, solution of different concentrations. Electrochemical
(a)
(b)
Fig. 1. Schematicdiagram of the cell used (a) with and (b) without the Luggin capillary for referenceelectrode. C, T and R; counter, test and reference electrodes. 1721
1722
H. KITA, K. FUJIKAWAAND I-L NAKAJIMA 9
I
V
vs.
RHE
Fig. 2. Cathodic i-4 curves (2H+ + 2e- * H,) on Pt-SPE I [Fig. 1 (a)] at various concentrations of HCIO,. Sweep rate, 10 mV s-l, N, flow (I atm) in compartment A.
with two sheets of platinized platinum. Results are shown in Table 1. The solution resistance, n,,,, decreases rapidly with the increase of the HCIOc concentration but the membrane resistance, a,,,,, , decreases only by a half. A N&on catalogue quotes a specific resistance of 175 Dcm at equilibration with 0.6 M KCL, 23°C for EW = 1300. Hence, the present membrane (thickness O.O2cm, area 3.i4cm2) is expected to have a resistance of I. 1 11. The value of Table 1 is reasonable taking into account the difference of cation used in the present work. The resistance of the platinum layer deposited on the SPE is estimated to be small for the following reason. Current, uniformly reaching the surface, passes through the Pt layer radially towards the rim where an annular Au current collector is attached. The potential drop, d#, formed across the two rings at r and I + dr in the Pt layer (loops shown in the Pt layer of Fig. 4) will be given as d@ = (irrr’)
measurements were carried out by the potential sweep or potentiostatic method at room temperature. Potentials are referred to the reversible hydrogen electrode.
x
dr
(>’ p-
2xrd
= p’rdr, 2d
where i is the current density, r the distance from the centre of the Pt layer of a thickness d, and p the specific resistance, respectively. Integration gives
REXJLTS We first examined the hydrogen evolution reaction at Pt-SPE I under N, flow in compartment A of Fig. l(a). Cathodic sweep of the electrode potential revealed i+ curves as shown in Fig. 2 (independent of a sweep rate) where the concentration of HClO., was varied from 0.063 to 4 M. A linear relation holds in each case except for the region near the reversible potential where the hydrogen evolution still takes place under N, atmosphere. Such a linear relation clearly excludes an electrode reaction from the factors responsible far the i-4 relation. The reciprocal of the slope represents the resistance and is plotted as a function of the HC104 concentration in Fig. 3. The resistance sharply drops with the concentration and becomes almost constant at concentrations beyond 2 M. The resistance may consist of three parts due to: (1) the solution present between the tip of the Luggin capillary and the membrane surface, (2) the membrane and (3) the platinum layer (see Fig. 4). These components are separately estimated as follows. The solution resistance was obtained from the measured conductivity (Toa Denpa Co. Ltd., CM-XET) and the geometrical factors of the surface area of the membrane exposed to the solution (3.1 cm’) and the distance between the tip of the Luggin capillary and the membrane surface (5 mm). The membrane resistance was directly measured by sandwiching SPE film, preequilibriated with the solution of interest overnight,
B
c
6
4E
k
-4
a
!
$p
2
-
t
I
Oa
. --:a.1.0
Rmcm.
I
_*__-_-_-__--_-__-____lc
2.0
3,O
4.0
5 ‘,
_
,
11.4
c IM
Fig. 5. Apparent resistance of Pt-SPE I as a function of [HCIO.,] (from Fig. 2): ---observed, ---sum of the solution and membrane resistances.
Fig. 4. Solution, membrane and Pt layer resistances.
Table 1. Solution and membrane resistance 0.063
0.125
C (mol dm-‘) 0.25 0.50
1.0
2.0
4.0
6.4 0.86
3.3 0.70
1.6 0.57
0.5 0.45
0.3 0.45
0.2 0.45
0.8 0.51
11.4 0.5 0.51
Metal electrodesbonded on SPE-II
where 1 is the radius of the Pt layer. Hence, the membrane resistance will be given by dividing A$ by the total current, I = ial’, as O_,,, = A@/[ = p/4xd. Calculating d from the amount of Pt deposited (5 mgcm-‘) and a lattice constant (3.92A) as d = 1W 4 cm, and assuming a bulk resistance of Pt as p = 10.6 x 10m6Rem, we have D2,,, c 0.01 Q in order of magnitude which is negligibly small. Thus, the sum of the solution and membrane resistances is compared with the observed resistance in Fig. 2. Agreement is satisfactory. Hence, we conclude that the factors which control the polarization behaviour are the solution and membrane resistances. Next we used Pt-SPE II (apparent surface area, 14.5cm*) where the platinum layer facing the electrolytic solution worked as a reference electrode [Fig. l(b)J In this case, the solution resistance is completely excluded. Results on the hydrogen ionization reaction are shown in Fig. 5 in comparison with those at Pt-SPE I, where the compartment A in Fig. l(a) and (b) is fed a flow of atmospheric hydrogen gas. We have again an Ohmic relation for the hydrogen ionization reaction and with a much steeper slope on Pt-SPE II. It is seen that the current far exceeds the limiting value (ca. 1 mAcm- ‘[lo] ) for a usual gas electrode in solution. The reciprocal of the slope on Pt-SPE II gives a resistance of 0.127 R being much smaller than 1.68 R of Pt-SPE I. The decrease of 1.55 Q must be attributed to a solution resistance which is calculated as 1.31 R from a specific conductivity of 0.343 Q- 1 cm-’ (1 M HClOJ, a surface area of 14.5 cm2 and a distance of 6.5 cm between the counter electrode of a platinized platinum foil and the membrane surface. Hence, the value of 0.127 D of Pt-SPE 11 will be taken as the resistance of the membrane and platinum layer. The latter contribution will be small as discussed earlier. The value of 0.127 SLis apparently smailer than a,,,
1723
in Table 1 but becomes coincident when the difference in the surface area of the membrane is taken into account, i.e. 0.127 x 14.5/3.1= 0.59 R. On the same Pt-SPE I and II electrodes, we examined the reduction of oxygen by flowing O2 gas in compartment A. Results are shown in Fig. 6. In this case, a current at a given potential shows a gradual decay for the first 1&20min. We plotted a steady current at co. 30 min. Cathodic current increases in an accelerated manner with the less positive polarization and then becomes proportional to the potential on both the electrodes. The apparent resistance is estimated from the respective slopes as 2.10 and 0.29 R for Pt-SPE I and II. These values are somewhat larger than those of Fig. 5 but to be stressed is that the exclusion of the solution resistance again leads to a much larger current. CONCLUDING
REMARKS
The Pt-SPE electrode removes the serious difficulty present in a gas electrode that the current is controlled by a mass transfer in the solution and reveals an Ohmic relation over a wide range of current. Such a linear relation between the current and potential excludes the possibility of the electrode reaction being ratecontrolling. The slope gives a resistance which depends on the concentration of electrolyte. The removal of the solution resistance improves the polarization characteristics of the hydrogen ionization and oxygen reduction reactions. Thus, the Pt-SPE electrode has a promising feature in its use for the fuel cells of the third generation. In this respect, we are now examining the electrochemical behaviour of the SPE electrode which carries platinum layers on both sides of the membrane exposed directly to hydrogen and oxygen gases without an electrolyte solution. Results will be reported in the subsequent Paper. +lV
Pt-SPE
vs.RHE
0
I
t1
-04 PI-SPE
I
t 4 -
-OS-
-0.8
a
0
’ 0.2
0-L 91
V
0.6
s.RHE
0.6
i-4 curves (H, -+ 2H+ + b-) on Pt-SPE I and II. Potentiostatic steady polanzation, I M HCIO, in compartmentB, HZ Bow (1 atm) incompartment A. Apparent Fig. 5. Anodic
surface area, 14.5 cm’.
-
-1.0-
Fig. 6. Cathodic i-4 curves(0, +4H+ + 4e -+ 2H10) on Pt-
SPE I and II. Conditions are the same as in Fig. 5 except that compartment B is fed a flow of oxygen of l-07-1.3 atmospheric pressures.
1724
H. KITA,K.
FUJIKAWAAND
Acknowledgement-This work is partly supported by a grantin-aid for scientific research, Ministry of Education, Science and Culture. No. 57470001.
REFERENCES I. 2. Twardowski, H. L. Yeager and 8. O’Dell, J. elsctrothem. Sot. 129, 328 (1982). 2. R. S. Yea, J. .&ctrochem Sot. 130, 535 (1983). 3. W. Y. Hsu and T. D. Gierke, J. memb. Sci. 13,307 (1983).
H. NAKAJIMA
4. A. Steck and H. L. Yeager, J. rlectrockem Sor. 130,1297 (1983). 5. 2. Ogumi, K. Nishio and S. Yoshizawa, Electrockim. Acto 26, 1779 (1981). 6. I. Bergman, J. electroonal. Ckem. L57, 59 (1983). 7. A, KatayamaAramata and R. Ohnishi, J. Am. ckem. Sot. 105, 658 (1983). 8. A. Katayama-Aramata, H. Nakajima, K. Fujikawa and H. Kita, Eleccrrcchim. Acta 28, 777 (1983). 9. H. Takeuaka and E. Torikai, KokaiTokkyo Koko (Japan Patent) 55, 38934 (1980). IO. S. Schuldiner, J. electrockem. Sot. 106. 891 (1959).