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The reduction of oxygen on platinised Sb doped SnO2 in 85% phosphoric acid
THE
REDUCTION OF OXYGEN ON PLATINISED SnO, IN SSy/;, PHOSPHORIC ACID
Sb DOPED
A. C. C. TSLL~G and S. C. DHAHA Department
of Chetmstry,
‘L‘he City University.
St. John
Street,
London,
EC1
Abstract-The
addition of small amounts of Pt to Sb doped SnO, significantly incrcascd the activity of these electrodes for rhe reduction of 0, in 85”;, H,PO,. inilicaring the existence of a synergistic effect Steady state and potentlodynamlc studies suggest a spill-over mechanism may be involved. The: chemixorhed O2 on the Pt migrates to the Sh doped Sn02 surface to Llndergo further electrochcmic;~l rcactinn
INTRODUCTlOh
Most of the work on supported Pt electrocatalysts for the reduction of oxygen has been performed m alkaline media. The commonly used support is carbon or graphite powder. The continuing interest in impure H2 fuel cellsLl], using 85’:;, HIPO, at 150 C has prompted us to investigate the use of acid resistant semiconducting oxides as electrocatalyst supports since carbon or graphite is not stable in hot phosphoric media. Very few semi-conducting oxides possess adequate corrosion resistance and electrical conductivity in acidic media and we decided to limit our choice to Sb doped SnOz[Z, 31.
mesh PI screens. The catalyst loading was nominally 10 mgjcm’ and the catalystjteflon ratio was 3: 1. The clcctrodc was then dried at IOO’C for I hr and thcrcafter cured at 300-C for 1 hr in air to form Teflon bonded electrodes[4]. Similarly, electrodes were prepared from non-platinised SnOz powder and Johnson Matthcy Fuel Cell grade Pt black. The electrodes were tested in a floating ce11[5], using a S x S cm Pt sheet as the counter electrode and a dhr[6] (Dynamic Hydrogen Electrode) as the reference electrode. This electrode has been calibrated against a rhe (Reversible Hydrogen
Electrode) and found to bc 30 mV more ncgativc than rhe.
A 3 A Chemical Electronics Potentiostat was used to control the potential of the working electrode. b’or potentiostatic pulse studies of transient performance. a Chemical Electronics Pulse Generator and a Solarton Oscilloscope was used. The details of the experimental setup has been reported clscwhcrc[7]. All the results were corrected for ir drop by the USC of the interruptor technique[8].
EXPERIMENTAL Prcpururinrl
und
charnctuisation
yj‘Sh
doped
SnOz
Unless stated otherwise, all the starting materials were of Analar Grade. A 10”~~~ HCI solution containing the appropriate amounts of SKI, and SbCl, (Sn:Sh = 97 atm/3 atm) was heated at X&90 C and continuously stirred with a magnetic stirrer until the solution bccamc clear. The solution was then cooled to room temperature ant1 1 S’;, KOH solution was added drop-wist: to the continuously stirred HCL solution until the pH of the solution reached 8. After filtering and washing, the precipitated hydroxides were calcined at SO0 c‘ for 5 hr-. X-ray and electl-ical reslstivity measurements confirmed that Sb had entered the SnOz lattice. The resistivity of the powder, compressed at I TSI in a 1 in. Teflon-lined steel die. was less than 1 ohm-cm. The BET surface area of the powder was 55 m’,‘g.
RESULTS
AND DISCL’SSIOK
The corrosion resistance of Sb doped SnO, at 150 C, 85’:;, H,PO, was determined in the presence of bubbling, de-oxygenated, white spot nitrogen. The results showed that the corrosion resistance of Sb doped SnOz is quite satisfactory over the range 0 1200 mV and that at the likely operating range for an oxygen electrocatalyst in acidic medium (6OG-1000 mV). the maximum corrosion current is less than 6.3 x IO-’ A/m’. The corresponding corrosion current for the Pt black electrode is 6.3 x 10e4 A/m’.
Platinlsed Sb doped SnO, samples, containing different percentages of Pt were prepared by impregnatmg chloroplatinic acid solution onto the oxide powder, followed by subsequent reduction in H, at IOO’C for I hr.Thecatalys;l was then mixed with ICI PTFE (polytetrafluorcthylene) dispersion and painted onto 100
Figure 1 shows that there is a significant improvcment in the performance even for small additions of Pt to the Sb doped SnO, electrode. The mechanism of enhancement IS not clear at this stage. It is important to x45
A. c‘. C. Tsr:uw
ANI) 5. c. DEIAKA
I Currentdensity
state performance
Fig. 1. Steady
cm’;
2--(06mg
Pt -0.1
I
1
I50
I 00
50
200
mA/crn2
of P&black, platinized
SnO, and SnO, electrodes. l-10 mg Pt black/ black + 1Omg Sb/SnO,)/cm’; 3-403 mg Pt black + 1Omg Sb/SnO,)/cm’; mg Pt black + 10 mg Sb/SnO,)/cm’; 5-10 mg Sb/Sn0,/cm2.
determine whether the improvement is due to a simple mixture effect, or only synergistic effect, involving a In a study of oxygen reduc“spill-over” mechanism[9]. tion on platinised Na,WO, crystals, Bockris and McHardy[ IO] posiulated that the rate determining step in the reduction of oxygen in acidic media is O2 + H ‘CY (absorbed intermediate). It is possible that the adsorbed intermediates produced would diffuse from the platinum to the Sb doped SnO, and undergo further electro-chemical reduction to water. If a synergistic mechanism is involved it would be expected that the calculated pseudo-exchange current (obtained
from LLi curves, Fig. 1) of the platinised Sb doped SnOz would be very much higher than the theoretical value predicted by a simple mixture rule* (Fig. 2). The surface area of the Pt black in each electrode was determined by galvanostatic H2 stripping[7]. The surface area of Sn02 can be calculated from the weight of Sn02 per cm’ of electrode, since the BET area has been determined previously (55 m’/g). Figure 2 shows that the experimentally determined i” Pt/SnO, values are significantly higher than the “theoretical” i0 Pt/ SnOI values for a whole range of Pt loadings. Eflivc of H202 uddirion I?ILlIIL’P
* Theoretical: i’P,,sso where A represents
-
i”pr 4
+ insnO ,Lo,
the electrochemical
material/cm’
electrode.
iOE Kpt, Pt/Sn
O2
/ 6x1 O-6
a 5% I i” Theo.Pt/Sn
-----0.1
Amountof
Fig. 2. Relationship
0.2
0.3
perfor-
The reduction of 0, in acidic media may also involve the formation of H,O, as an intermediate. Thus it is possible that the addition of Pt to SnO, enhances the rate of HIOL decomposition, regenerating more 0,
Apt + Asnoi
tl~e area of the particular
on
0.4
impregnated
between experimental
05
Platinum
0.6
mg/cm*
07
otm
H3 PO,, I5OYI 0,
OF
i’SnO
2
0.6
electrode
iD Pt + SnO, and theoretica
io
Pt + SnO,
The reduction of oxygen on GO, Table 1. Oxygen
coverage on the various electrodes at open
Surface area
22.4
100~1
4.9
8.9
54
5.0
9.2
cm-‘.
Tar the oxygen reduction reaction[ll]. This may account for the higher observed i” Pt/SnQ. However. though this is a possible explanation at lower temperatures (-7S'C), it is unlikely to be a significant factor at higher temperatures, since H,02 is highly unstable at 15O’C. The addition of 1% H,O, to 857<, H,PO, at 150 C did not airect the open circuit voltage of the Pt/ Sn02 clcctrode: a profound effect was observed on electrodes operated at 7O’C, 5N HISO,.
85%
The galvanostatic “oxygen” stripping method was used to determine the oxygen coverage on the electrodc surface[S]. Basically, the process is very similar to the galvanostatic “H, stripping” method. Oxygen was bubbled through the test cell for I5 min, after which the O2 dissolved in the electrolyte was purged by a stream of de-oxygenated white spot N2. A galvanostatic pulse was then applied to reduce the oxygen chemisorbed on the electrode surface. The potentialtime relationship was displayed on the oscilloscope screen and photographed. A typical oscilloscope trace is shown in Fig. 3. The calculated O2 coverage on the different electrodes is reported in Table 1. There is close agreement between the Pt black results obtained by BET and the “oxygen stripping” method. indicating that the surface coverage of O2 on Pt is complete. Most probably the PtO is formed on the surface. However, the oxygen coverage on Sb doped SnO, is low. A low oxygen coverage need not necessarily result in low ekctro-chemical performance, provided the rate of oxygen chemisorption and the subsequent surface electrochemical reaction is fast.
H3P04.1 50°C
I aim 700
0, coverage (%)
22 55
Fig. 3. Galvanostatic oxygen stripping transient oscilloscope trace on Pt black clcctrodc. Electrode arca = 00625 cm* ; Pt loading = IO m&m2 ; Pt area = 260 cm2 ; current = 100mA: “x” axis 450mscm-’ ; “Y” axis 200mV
circuit potential
Surface area “oxygen stripping” method (m*/g)
BET method (m’ig) Pt black Sb doped SnOl 2”,, Pt on Sb doped SnO,
84.7
0,
-
600LL1 E 2
5OOI 400300
-
200
-
Current Fig. 4. Transient polarization
density
mA/ern’
curve for Pt. platinizcd SnO, and SnO, electrodes
1, 2, 3, 4, 5 a6 Fig. 1.
A. C. C.
x3x
Mramr-erwrlt of’trmsient
TSWNG
pri-forrnmce
The steady stale current-voltage curves shown in Fig. I cannot provide information on the rate of oxy&en chemisorption ur the ralt: of subsequent electrochemical reaction, since its performance could hc governed by the rate of mass transfer of the reactank or resultant products inside the pores of the Teflon bonded electrodes. By measuring the transient performance. the results will he relatively free from mass transfer el_fccts[7]. Basically, a potentiostatic pufsc is applied to the electrode and the faradaic current is monitored after double layer charging has been completed. Figure 4 shows that the transient performance is almost of Sb doped SnO: at 100 mV overpotential as good as Pt black. However, the steady state performance of 310, is very much poorer than the Pt black electrode under steady state conditions (Fig. I). When the electrodes are tested under potentiostatic pulse mode, only the chemisorbed O2 on the electrode surface is used. Thus even if the rate of replenishment of OL is slow, the performance could still be high provided the ssurface electrochemical reaction and the subsequent desorption is rapid. On the other hand, under steady state conditions, the performance is very dependent on the rate of oxygen chemisorption on the surface. This may be a possible cxplanatlon for the low performance of Sb doped Sn02 clectrodcs. Since the rate of O2 chemisorption on Pt is faster than on Sb doped Sn02. the addition of small amounts of Pt may cnablc some of the chemisorbed oxygen on the Pt sur-
AND
S. C.
DHARA
face to migrate to the SnO, surface and be reduced there -undergoing a “spill over” process. analogous to the Pt/H,WO,[12] and Pt/Na,.W0,[9] systems. Acknowledgeme,lt-This Ministry of Defence.
study
was
supported
by
the
REFERENCES P. Stonehart and P. A. Zucks. Elrcwo~lrirr~. AC-IU17, 2333 (1972). W. H. Hansen, R A. Osteryoung and T. Kurana. J. AIII. cheln. Sot. 88, IO62 (I 966). L. D. Lath, J. dectr~chr~~. SOC., 110, 1081 (1963). A. D. S. Tantram and A. C. C. Tseung, Nrrruw 221, 167 (1969). J. Giner, J. M. Parry, S. Smith and M. Turchan, J. elw rrochrm. Sot. 116. 1962 11969). J. Giner, ibid. 111. 376 (i964): B. S. Hobbs, P. R. Vassie and A. C. C. Tseung, Electrachemicd Enyineeriny Symposium (Edited by 1. D. Thornton), Institution of Chemical Engineers, London. I23 (1973). 8. K. R. Williams, An introducrion to fuel cells, Else&r, 5R {1966). 9. M. Boudart, M. A. Vannice and J. E. Bevan. Z. phys. Ckrrr. N.F. 64, I71 (1969). IO. J. O’M Bockris and J. McHardy. J. ckwochum. Sot. 120, 61 (1972). Il. J. R. Goldstein and A. C. C. Tseung. J. pkJ~.s.Chew. 76. 3646 (I 972). I?. B. S. Hobhs and A. C. C. Tseung. J. c,~~,~~~oL./x~III. Sot. 120, 766 (1973).
REDUCTION OF OXYGEN ON PLATINISED SnO, IN SSy/;, PHOSPHORIC ACID
Sb DOPED
A. C. C. TSLL~G and S. C. DHAHA Department
of Chetmstry,
‘L‘he City University.
St. John
Street,
London,
EC1
Abstract-The
addition of small amounts of Pt to Sb doped SnO, significantly incrcascd the activity of these electrodes for rhe reduction of 0, in 85”;, H,PO,. inilicaring the existence of a synergistic effect Steady state and potentlodynamlc studies suggest a spill-over mechanism may be involved. The: chemixorhed O2 on the Pt migrates to the Sh doped Sn02 surface to Llndergo further electrochcmic;~l rcactinn
INTRODUCTlOh
Most of the work on supported Pt electrocatalysts for the reduction of oxygen has been performed m alkaline media. The commonly used support is carbon or graphite powder. The continuing interest in impure H2 fuel cellsLl], using 85’:;, HIPO, at 150 C has prompted us to investigate the use of acid resistant semiconducting oxides as electrocatalyst supports since carbon or graphite is not stable in hot phosphoric media. Very few semi-conducting oxides possess adequate corrosion resistance and electrical conductivity in acidic media and we decided to limit our choice to Sb doped SnOz[Z, 31.
mesh PI screens. The catalyst loading was nominally 10 mgjcm’ and the catalystjteflon ratio was 3: 1. The clcctrodc was then dried at IOO’C for I hr and thcrcafter cured at 300-C for 1 hr in air to form Teflon bonded electrodes[4]. Similarly, electrodes were prepared from non-platinised SnOz powder and Johnson Matthcy Fuel Cell grade Pt black. The electrodes were tested in a floating ce11[5], using a S x S cm Pt sheet as the counter electrode and a dhr[6] (Dynamic Hydrogen Electrode) as the reference electrode. This electrode has been calibrated against a rhe (Reversible Hydrogen
Electrode) and found to bc 30 mV more ncgativc than rhe.
A 3 A Chemical Electronics Potentiostat was used to control the potential of the working electrode. b’or potentiostatic pulse studies of transient performance. a Chemical Electronics Pulse Generator and a Solarton Oscilloscope was used. The details of the experimental setup has been reported clscwhcrc[7]. All the results were corrected for ir drop by the USC of the interruptor technique[8].
EXPERIMENTAL Prcpururinrl
und
charnctuisation
yj‘Sh
doped
SnOz
Unless stated otherwise, all the starting materials were of Analar Grade. A 10”~~~ HCI solution containing the appropriate amounts of SKI, and SbCl, (Sn:Sh = 97 atm/3 atm) was heated at X&90 C and continuously stirred with a magnetic stirrer until the solution bccamc clear. The solution was then cooled to room temperature ant1 1 S’;, KOH solution was added drop-wist: to the continuously stirred HCL solution until the pH of the solution reached 8. After filtering and washing, the precipitated hydroxides were calcined at SO0 c‘ for 5 hr-. X-ray and electl-ical reslstivity measurements confirmed that Sb had entered the SnOz lattice. The resistivity of the powder, compressed at I TSI in a 1 in. Teflon-lined steel die. was less than 1 ohm-cm. The BET surface area of the powder was 55 m’,‘g.
RESULTS
AND DISCL’SSIOK
The corrosion resistance of Sb doped SnO, at 150 C, 85’:;, H,PO, was determined in the presence of bubbling, de-oxygenated, white spot nitrogen. The results showed that the corrosion resistance of Sb doped SnOz is quite satisfactory over the range 0 1200 mV and that at the likely operating range for an oxygen electrocatalyst in acidic medium (6OG-1000 mV). the maximum corrosion current is less than 6.3 x IO-’ A/m’. The corresponding corrosion current for the Pt black electrode is 6.3 x 10e4 A/m’.
Platinlsed Sb doped SnO, samples, containing different percentages of Pt were prepared by impregnatmg chloroplatinic acid solution onto the oxide powder, followed by subsequent reduction in H, at IOO’C for I hr.Thecatalys;l was then mixed with ICI PTFE (polytetrafluorcthylene) dispersion and painted onto 100
Figure 1 shows that there is a significant improvcment in the performance even for small additions of Pt to the Sb doped SnO, electrode. The mechanism of enhancement IS not clear at this stage. It is important to x45
A. c‘. C. Tsr:uw
ANI) 5. c. DEIAKA
I Currentdensity
state performance
Fig. 1. Steady
cm’;
2--(06mg
Pt -0.1
I
1
I50
I 00
50
200
mA/crn2
of P&black, platinized
SnO, and SnO, electrodes. l-10 mg Pt black/ black + 1Omg Sb/SnO,)/cm’; 3-403 mg Pt black + 1Omg Sb/SnO,)/cm’; mg Pt black + 10 mg Sb/SnO,)/cm’; 5-10 mg Sb/Sn0,/cm2.
determine whether the improvement is due to a simple mixture effect, or only synergistic effect, involving a In a study of oxygen reduc“spill-over” mechanism[9]. tion on platinised Na,WO, crystals, Bockris and McHardy[ IO] posiulated that the rate determining step in the reduction of oxygen in acidic media is O2 + H ‘CY (absorbed intermediate). It is possible that the adsorbed intermediates produced would diffuse from the platinum to the Sb doped SnO, and undergo further electro-chemical reduction to water. If a synergistic mechanism is involved it would be expected that the calculated pseudo-exchange current (obtained
from LLi curves, Fig. 1) of the platinised Sb doped SnOz would be very much higher than the theoretical value predicted by a simple mixture rule* (Fig. 2). The surface area of the Pt black in each electrode was determined by galvanostatic H2 stripping[7]. The surface area of Sn02 can be calculated from the weight of Sn02 per cm’ of electrode, since the BET area has been determined previously (55 m’/g). Figure 2 shows that the experimentally determined i” Pt/SnO, values are significantly higher than the “theoretical” i0 Pt/ SnOI values for a whole range of Pt loadings. Eflivc of H202 uddirion I?ILlIIL’P
* Theoretical: i’P,,sso where A represents
-
i”pr 4
+ insnO ,Lo,
the electrochemical
material/cm’
electrode.
iOE Kpt, Pt/Sn
O2
/ 6x1 O-6
a 5% I i” Theo.Pt/Sn
-----0.1
Amountof
Fig. 2. Relationship
0.2
0.3
perfor-
The reduction of 0, in acidic media may also involve the formation of H,O, as an intermediate. Thus it is possible that the addition of Pt to SnO, enhances the rate of HIOL decomposition, regenerating more 0,
Apt + Asnoi
tl~e area of the particular
on
0.4
impregnated
between experimental
05
Platinum
0.6
mg/cm*
07
otm
H3 PO,, I5OYI 0,
OF
i’SnO
2
0.6
electrode
iD Pt + SnO, and theoretica
io
Pt + SnO,
The reduction of oxygen on GO, Table 1. Oxygen
coverage on the various electrodes at open
Surface area
22.4
100~1
4.9
8.9
54
5.0
9.2
cm-‘.
Tar the oxygen reduction reaction[ll]. This may account for the higher observed i” Pt/SnQ. However. though this is a possible explanation at lower temperatures (-7S'C), it is unlikely to be a significant factor at higher temperatures, since H,02 is highly unstable at 15O’C. The addition of 1% H,O, to 857<, H,PO, at 150 C did not airect the open circuit voltage of the Pt/ Sn02 clcctrode: a profound effect was observed on electrodes operated at 7O’C, 5N HISO,.
85%
The galvanostatic “oxygen” stripping method was used to determine the oxygen coverage on the electrodc surface[S]. Basically, the process is very similar to the galvanostatic “H, stripping” method. Oxygen was bubbled through the test cell for I5 min, after which the O2 dissolved in the electrolyte was purged by a stream of de-oxygenated white spot N2. A galvanostatic pulse was then applied to reduce the oxygen chemisorbed on the electrode surface. The potentialtime relationship was displayed on the oscilloscope screen and photographed. A typical oscilloscope trace is shown in Fig. 3. The calculated O2 coverage on the different electrodes is reported in Table 1. There is close agreement between the Pt black results obtained by BET and the “oxygen stripping” method. indicating that the surface coverage of O2 on Pt is complete. Most probably the PtO is formed on the surface. However, the oxygen coverage on Sb doped SnO, is low. A low oxygen coverage need not necessarily result in low ekctro-chemical performance, provided the rate of oxygen chemisorption and the subsequent surface electrochemical reaction is fast.
H3P04.1 50°C
I aim 700
0, coverage (%)
22 55
Fig. 3. Galvanostatic oxygen stripping transient oscilloscope trace on Pt black clcctrodc. Electrode arca = 00625 cm* ; Pt loading = IO m&m2 ; Pt area = 260 cm2 ; current = 100mA: “x” axis 450mscm-’ ; “Y” axis 200mV
circuit potential
Surface area “oxygen stripping” method (m*/g)
BET method (m’ig) Pt black Sb doped SnOl 2”,, Pt on Sb doped SnO,
84.7
0,
-
600LL1 E 2
5OOI 400300
-
200
-
Current Fig. 4. Transient polarization
density
mA/ern’
curve for Pt. platinizcd SnO, and SnO, electrodes
1, 2, 3, 4, 5 a6 Fig. 1.
A. C. C.
x3x
Mramr-erwrlt of’trmsient
TSWNG
pri-forrnmce
The steady stale current-voltage curves shown in Fig. I cannot provide information on the rate of oxy&en chemisorption ur the ralt: of subsequent electrochemical reaction, since its performance could hc governed by the rate of mass transfer of the reactank or resultant products inside the pores of the Teflon bonded electrodes. By measuring the transient performance. the results will he relatively free from mass transfer el_fccts[7]. Basically, a potentiostatic pufsc is applied to the electrode and the faradaic current is monitored after double layer charging has been completed. Figure 4 shows that the transient performance is almost of Sb doped SnO: at 100 mV overpotential as good as Pt black. However, the steady state performance of 310, is very much poorer than the Pt black electrode under steady state conditions (Fig. I). When the electrodes are tested under potentiostatic pulse mode, only the chemisorbed O2 on the electrode surface is used. Thus even if the rate of replenishment of OL is slow, the performance could still be high provided the ssurface electrochemical reaction and the subsequent desorption is rapid. On the other hand, under steady state conditions, the performance is very dependent on the rate of oxygen chemisorption on the surface. This may be a possible cxplanatlon for the low performance of Sb doped Sn02 clectrodcs. Since the rate of O2 chemisorption on Pt is faster than on Sb doped Sn02. the addition of small amounts of Pt may cnablc some of the chemisorbed oxygen on the Pt sur-
AND
S. C.
DHARA
face to migrate to the SnO, surface and be reduced there -undergoing a “spill over” process. analogous to the Pt/H,WO,[12] and Pt/Na,.W0,[9] systems. Acknowledgeme,lt-This Ministry of Defence.
study
was
supported
by
the
REFERENCES P. Stonehart and P. A. Zucks. Elrcwo~lrirr~. AC-IU17, 2333 (1972). W. H. Hansen, R A. Osteryoung and T. Kurana. J. AIII. cheln. Sot. 88, IO62 (I 966). L. D. Lath, J. dectr~chr~~. SOC., 110, 1081 (1963). A. D. S. Tantram and A. C. C. Tseung, Nrrruw 221, 167 (1969). J. Giner, J. M. Parry, S. Smith and M. Turchan, J. elw rrochrm. Sot. 116. 1962 11969). J. Giner, ibid. 111. 376 (i964): B. S. Hobbs, P. R. Vassie and A. C. C. Tseung, Electrachemicd Enyineeriny Symposium (Edited by 1. D. Thornton), Institution of Chemical Engineers, London. I23 (1973). 8. K. R. Williams, An introducrion to fuel cells, Else&r, 5R {1966). 9. M. Boudart, M. A. Vannice and J. E. Bevan. Z. phys. Ckrrr. N.F. 64, I71 (1969). IO. J. O’M Bockris and J. McHardy. J. ckwochum. Sot. 120, 61 (1972). Il. J. R. Goldstein and A. C. C. Tseung. J. pkJ~.s.Chew. 76. 3646 (I 972). I?. B. S. Hobhs and A. C. C. Tseung. J. c,~~,~~~oL./x~III. Sot. 120, 766 (1973).