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Gas diffusion electrodes for the oxidation of sulphur dioxide
Int. J. Hydrogen Energy, Vol. 9,No. 11, pp. 901-906, 1984. Printed in Great Britain.
036(~-3199/84 $3.00 + 0.00 Pergamon Press Ltd. © 1984 International Association for Hydrogen Energy.
GAS DIFFUSION ELECTRODES FOR THE O X I D A T I O N OF S U L P H U R D I O X I D E K . PETROV,
Iv. NIKOLOVand T. VITANOV
Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria (Received for publication 14 February 1984)
Abstract--The catalytic activity of modified active carbon MC and organo-metallic compounds (CoTMPP and CoTAA) deposited and pyro|ized upon active carbon P-33 displayed toward the electrochemical oxidation of sulphur dioxide in H2SO4 media is investigated. The structure of the gas diffusion electrodes (GDE) prepared with these catalysts is optimized. The relationship between electrode performance and the method by which the binding material (PTFE) is introduced is studied. Electrodes which are catalyzed with modified active carbon show the best characteristics, reaching current densities 200 mAcm 2 at 600 mV (HE). Similar electrodes can operate 500 h continuously at a current density of 60 mAcm -2 without a noticeable increase of polarization.
NOMENCLATURE HAB AG-3 P-33 CoTMPP CoTAA GDE HE MC PTFE d
[4] which are Pt catalyzed have shown excellent characteristics: at current density 100mAcm -2, overvoltage is 265 mW, corresponding to a potential of +525 mV (HE) [4]. Similar electrodes, catalyzed with activated graphite PG-50 have been developed by Gorbachev et al. [3]. At current density 75-150 mAcm -2 the potential is 0.5-0.65 V [3, 5]. Tarasevich [6] have investigated the performance of gas diffusion electrodes catalyzed with dibenzo cobalt tetraazo anulen (CoTAA), cobalt tetra-methoxy-phenyl-porphirine (CoTMPP), and platinum, deposited upon active carbon AG-3. The best results were obtained with Pt-catalyzed electrodes-+625 mV at 100 mAcm -2 [6]. The use of gas diffusion electrodes for the oxidation of sulphur dioxide offers some advantages as compared with the immersed electrode version. In this case, a possibility is offered to design an electrolytic cell for the production of hydrogen and sulphuric acid, without using a membrane for the separation of the cathodic and anodic compartments, provided the corrosion problems connected with the decomposition of concentrated sulphuric acid can be avoided. This investigation is aimed at the determination of the relationship between the macro-structure of the G D E and the catalytic activity.
Acetylene black Active carbon Active carbon Cobalt tetra-methoxy-phenyl-porphirine Dibenzo cobalt tetraazo anulen Gas diffusion electrodes Hydrogen electrode Modified active carbon Polytetrafluoroethylene Thickness of the active layer INTRODUCTION
The electrolysis of water as an alternative method for the production of hydrogen, replacing the decomposition of hydrocarbons, is a relatively new area of investigation. The high voltage required by the conventional electrolysis cells, due to the kinetic limitations of the oxygen evolution process, renders the cost of electrolytic hydrogen uncompetitive as compared with the price of hydrogen obtained from oil. The use of sulphur dioxide as an anodic depolarizer decreases the thermodynamic voltage from 1.23 V to 0.29 V in a 50 wt% H2SO4 aqueous solution at 25°C [1]. Another advantage of this method is that it can use flue or exhaust gases from various industrial sources (power stations, ore dressing plants, etc.). Noble metals and their oxides are the usual catalysts for the oxidation of sulphur dioxide. Palladium and platinum are considered the most efficient catalysts among the noble metals. The expectations that iridium and rhutenium oxides, which show a superb catalytic activity and stability for the chlor-alkaline electrolysis process, will prove efficient also in this case did not materialize [1]. Better substitutes proved to be the metal porphirines [2] and activated graphite [3]. In the routine practice, the catalysts are deposited upon porous electrodes--immersed or gas diffusion type. The immersed electrodes, developed by Lu et al.
EXPERIMENTAL The following catalysts were used for the preparation of the GDE: C o T A A , CoTMPP (15 wt% deposited and pyrolized upon active carbon P-33 [7]) or modified active carbon (MC). The electrodes (S = 15 cm 2) were double-structured, with a porous gas-supply layer, similar to the air electrodes developed at this laboratory [8], and a porous catalytic layer. The binding agent was PTFE, introduced by two different methods: (a) PTFE powder and (b) PTFE previously precipitated upon acetylene black (HAB). The following parameters of the macro-structure were changed, keeping all others constant: (a) the
901
K. PETROV, IV. NIKOLOV AND T. VITANOV
902
amount of the catalyst, i.e. the thickness of the catalytic layer; (b) the quantity of PTFE or HAB. The activity of the electrodes was checked by tracing the steadystate galvanostatic volt-ampere characteristics in an electrolytic cell with cathode catalyzed with WC [9], and a 4.5 N H2504 solution. The potential of the electrodes was measured against a (Hg)2SO4 reference electrode, in turn checked against a hydrogen standard electrode immersed in the same solution. The sulphur dioxide depolarizer gas was fed to the chamber of the G D E with no residual overpressure. (1) Optimization of the GDE, catalyzed with organometallic compounds The structure of the active layer of the G D E catalyzed with CoTMPP and C o T A A was optimized. The optimization with respect to the catalyst content in the active layer was carried out when a constant amount of PTFE (4.5 wt%), introduced as HAB. Figure 1 shows the relationship between the potential of the electrode and the amount of catalyst, i.e. the thickness of the active layer (d) at different current densities. A clear-cut minimum is observed within the region d = 0 . 1 4 m m , corresponding to 13.3mg P-33 + 15% CoTMPP cm -2. The optimization of the structure with respect to the amount of PTFE was performed O
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®
with two-catalyst loadings: 13.3 and 20mg.cm -z. Figures 2 and 3 show the potential traced vs the amount of PTFE for the two catalyst loadings, at different current densities. It can be noted that thinner electrodes (13.3 mg P-33 + 15% CoTMPP.cm -2) show no substantial differences in the activity when PTFE content is increased up to 6%. Further augmentation evidently results in a partial blocking of the catalyst surface, and the corresponding decrease in the activity of the electrodes. At a catalyst loading of 20 mg.cm -2, the best characteristics are obtained with electrodes containing 3% PTFE in the active layer (Fig. 3). High current densities up to 200 mAcm -2 can be obtained only with similar electrodes. On the basis of the studies, two optimum compositions of the electrodes were determined: 13 mg.cm -2 catalyst loading, containing 3-6% PTFE, and 20 mg.cm -2 with 3% PTFE, respectively. This finding provides evidence that the amount of binding agent may vary depending on the catalyst loading, i.e. on the thickness of the catalytic layer. In a similar manner the structure of the G D E containing the catalyst C o T A A was optimized. At a constant content of the binding agent (4.5 wt% PTFE introduced as HAB), the optimum of the relationship potential-catalyst loading is reached at 20 mg P-33 + 15% C o T A A cm -2 (Fig. 4). The investigation of the effect exerted by the binding agent at two different catalyst loadings 13.3 and 20mg.cm -2 clearly shows, that electrodes containing 20 mg.cm -2 catalyst display a substantially better characteristic. Figure 5 shows the potential-PTFE content relationship in this case.
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Fig. 1. Potential vs amount of catalyst (catalyst loading) (P-33 + 15% CoTMPP) at constant content of P'ITE (4.5 wt% introduced as HAB) and different c.d.'s. (U) 40mAcm-2; (O) 60mAcro-2; (x) 80mAcm-Z; (O)o 100mAcro-2; (~) 120 macro-2; (ID) 150 mAcm-2; t = 20 C; 4.5 N H2SO4.
~nn ~ n n . ~ ~ n n
550 I
I
t
3
6
9
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Fig. 2. Potential of the GDE catalyzed with 13.3 mg.cm -2 P-33 + 15% CoTMPP vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
GAS DIFFUSION ELECTRODES FOR THE OXIDATION OF SULPHUR DIOXIDE ID
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O
700
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550
I1~11 I
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3
6
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%,PTFE Fig. 3. Potential of a GDE catalyzed with 20mg.cm -2 P-33 + 15% CoTMPP vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1. (~) 200 mAcm-2. •
%~PTFE Fig. 5. Potential of a GDE catalyzed with 20 mg P-33 + 15% CoTAA cm -2 vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
•
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o m-,
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I 30
catalyst loading,mg/cm2 Fig. 4. Potential of a GDE vs catalyst loading (P-33 + 15% CoTAA) at a constant content of PTFE (4.5% introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
I I ........_
0.2 I 2~0
0,3 I
d,mm
30 cafatys, toading,mglcm2
Fig. 6. Potential vs amount of catalyst (MC) at constant content of PTFE (4.5% introduced as HAB) at different c.d.'s. (B) 40mAcm 2; (©) 60mAcro-2; (Q) 100mAcm 2; (~) 150 mAcm-2; (~) 200 mAcm 2; t = 20°C; 4.5 N H2SO4.
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K. PETROV, IV. NIKOLOV AND T. VITANOV
(2) Optimization of the structure of the active layer of the GDE containing modified active carbon (MC) The optimization was carried out according to the same pattern. Figure 6 shows the electrode potentialcatalyst loading, i.e. thickness of the active layer relationship. A t low current densities this parameter does not exert a substantial effect. A t higher current densities (above 100 mAcm -2) the curves change abruptly their shape, passing through a minimum. In this case also optimum characterisics are displayed by electrodes with a thickness of the active layer d = 0 . 2 m m (20 mg.cm -2 catalyst loading). The optimization of the structure of the electrodes with respect to the amount of PTFE introduced as H A B was carried out for two different catalyst loadings--13.3 and 20 mg.cm -2. Figures 7 and 8 show the potentialPTFE content curves at different c.d. Electrodes containing 4.5% PTFE and 20 mg.cm-: catalyst show a minimum polarization. A t 13.3 mg.cm -2 the respective PTFE content is 6%. Similarly, as in the case of CoTMPP-catalyzed electrodes, the optimum content of H A B varies for the different catalyst loadings. The electrodes with optimum composition, containing 13.3 mg.cm -2 catalyst show a better performance than those with optimum composition at a catalyst loading 20 mg.cm -2 MC. The effect exerted by the method by which PTFE is introduced in the active layer was investigated using G D E , catalyzed with active carbon. Figure 9 shows the relationship: potential of the G D E catalyzed with 20 mg.cm -2 MC vs PTFE content (introduced in the active layer as powder) at different current densities. This figure must be juxtaposed with Fig. 7, where the same relationship is shown, when PTFE is introduced as hydrophobized acetylene black. In both cases the curves display a minimum. The comparison of both graphs shows, that up to current densities 100 mAcm -2 the introducing of PTFE as H A B brings about the decrease of the PTFE content only in the active layer. A t higher drains, however, it becomes clear that stable characteristics can be obtained only with electrodes which contain H A B in the active layer. Probably acetylene black increases the homogeneity of the pores, reducing transport limitations, hence improves the utilization of the catalyst. The V - A characteristics of the types of G D E were traced using a depolarizing gas mixture of SO2 + Ar, in an effort to provide evidence for this presumption. These measurements offer a possibility to determine the magnitude AE, which is defined as the difference between the potentials of the electrode when pure SO2 or a SO2 + A r gas mixture is used as the depolarizer. According to Iliev et al. [10, 11], this magnitude provides information for the transport limitations in the gaseous phase. Figure 10 shows that electrodes with PTFE introduced as a powder display more severe transport limitations. The following Fig. 11 shows the V - A characteristics of the best G D E for each catalyst. It can be noted, that
700
O
O
650 ,7, > E ,o600
O
550 w.._L--r/~ ~
%,PTFE Fig. 7. Potential of GDE catalyzed with 20 mg modified active carbon cm 2 vs the amount of P'ITE (in %, introduced as HAB) at different c.d.'s. (11) 30 mAcm-e; other symbols as for Fig. 6.
O
700
_650
O ~k O
600
~
O
f
O
O
•
550
:
I
I
I
3
6
9
%PTFE Fig. 8. Potential of a GDE catalyzed with 13,3 mg active carbon cm -2 vs the amount of PTFE (in % introduced as HAB) at different c.d.'s. All symbols as in Fig. 7.
905
GAS DIFFUSION ELECTRODES FOR THE OXIDATION OF SULPHUR DIOXIDE
g
650
./
650
m~l
E
600
•
550
0~
t~
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e J
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/7
//7-"
II-'"11 ~ 500
I
;o
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%,PTFE
1
Fig. 9. Potential of GDE catalyzed with 20 mg modified active carbon cm -z vs the amount of PTFE (powder) at different c.d.'s; (11) 30mAcm-2; (O) 60mAcm-2; (@) 100mAcro-2; (®) 120 mAcro-2; t = 20°C; 4.5 N H2SO4.
?
i
/ / iiI
E
/
/
/
tt
I
iI
x
/ / ' .,' / / / ,,," ,'" x ;
20
,x" ,,s/
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/
,,/ 50
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1
150 i, mA/cm2
Fig. 11. V-A curves of a GDE with optimum composition catalyzed with (@) modified active carbon, P-33 + 15% CoTMPP (n) P-33 + 15% CoTAA. t = 20°C; 4.5 N H2SO4.
CONCLUSION
/
//~,~ ~
1
lOO
the characteristics of G D E catalyzed with modified active carbon are the best. Their polarization is only 50 mV higher than that of Pt-catalyzed electrodes [4]. The long-term investigations have shown, that the active layer of G D E catalyzed with CoTMPP and C o T A A is cracked after 30-50 h of operation at current density 100 mAcm -2, resulting in a drastic increase of polarization. G D E catalyzed with modified active carbon retain their good initial characteristics during 500 h continuous operation at current density 60mAcm -2, with no visible alterations of the surface (Fig. 12). The leakage of SO2h -1 into the electrolyte through the optimized G D E is 0.03 g.cm -2.
80
60
50
/
•
100
150
i mA/cm 2
Fig. 10. A E - i curves of a GDE, catalyzed with 20 mg modified active carbon cm -2 at the optimum PTFE content. (. . . . ) PTFE (powder); ( ) PTFE (as HAB), at different ratio of the components of gas mixture (SO2 and Ar) (×) 3/6; (O) 4/6; (11) 5/6; t = 20°C; 4.5 N H2SO4.
The investigations have provided information about the activity of G D E catalyzed with CoTMPP, C o T A A , and modified active carbon for the sulphur dioxide oxidation in H2SO4 medium. After optimization of the structure, their characteristics can be compared with those of immersed electrodes with platinum catalysts. G D E electrodes which are catalyzed with modified active carbon, retain their good initial characteristics during long-term tests, and therefore could be applied in the design of electrochemical cells for the production of hydrogen and sulphuric acid.
906
K. PETROV, IV. NIKOLOV AND T. VITANOV
7O0
600 E
500
I
100
I
200
350
,60
Fig. 12. Long-term tests of GDE catalyzed with 20 mg modified active carbon cm 2. i = 60 macro-2; t = 20°C; 4.5 N H2504; (0) first sample; (×) second sample.
Acknowledgements--The authors are greatly indebted to Professor Tarasevich, Institute of Electrochemistry, Soviet Academy of Sciences, Professor Wiesener, Technische Universit/it Dresden, and Dr. I. Iliev, Central Laboratory of Electrochemical Power Sources, Sofia, who kindly provided catalysts.
REFERENCES 1. P. W. Lu and R. L. Ammon, J. Electrochem. Soc. 127, 2610 (1980). 2. K. A. Radushkina, M. R. Tarasevich and E. A. Akhundov, Elektrokhimiya 15, 1884 (1979). 3. A. K. Gorbachev, F. K. Andrushtenko, V. K. Nikiforov, V. P. Bochin, L. I. Ishtenko, L. V. Oparin and V. I. Kupina, Vorp. atom. nayki i tehn., Atom. Vodorod. energ. i technol. 2/9, 18 (1981).
4. P. W. Lu, E. R. Garcia and R. L. Ammon, J. appl. Electrochem. 11,347 (1981). 5. A. K. Gorbachev, F. K. Andrushtenko, V. P. Bochin, V. K. Nikiforov and L. I. Ischenko, Tez. dokl. 6 Vses. konf. Elektrokhimii 2, 305 (1982). 6. M. R. Tarasevich, personal communication. 7. V. S. Bagotsky, M. R. Tarasevich, K. A. Radushkina, O. A. Levina and S. I. Andruseva, J. Power Sources 2, 233 (1977). 8. E. Budevski, I. Iliev, S. Gamburtsev, A. Kaisheva, E. Vakanova and I. Mukhovski, Dept Chem. Bulg. Acad. Sci. 7, 223 (1974). 9. I. Nikolov, K. Petrov and T. Vitanov, Int. J. Hydrogen Energy (in press). 10. I. lliev, A. Kaisheva and S. Gamburtsev, Dept Chem. Bulg. Acad. Sci. 8, 367 (1975). 11. I. Iliev, S. Gamburtsev, A. Kaisheva and J. Mrha, J. appl. Electrochem. 5,291 (1979).
036(~-3199/84 $3.00 + 0.00 Pergamon Press Ltd. © 1984 International Association for Hydrogen Energy.
GAS DIFFUSION ELECTRODES FOR THE O X I D A T I O N OF S U L P H U R D I O X I D E K . PETROV,
Iv. NIKOLOVand T. VITANOV
Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria (Received for publication 14 February 1984)
Abstract--The catalytic activity of modified active carbon MC and organo-metallic compounds (CoTMPP and CoTAA) deposited and pyro|ized upon active carbon P-33 displayed toward the electrochemical oxidation of sulphur dioxide in H2SO4 media is investigated. The structure of the gas diffusion electrodes (GDE) prepared with these catalysts is optimized. The relationship between electrode performance and the method by which the binding material (PTFE) is introduced is studied. Electrodes which are catalyzed with modified active carbon show the best characteristics, reaching current densities 200 mAcm 2 at 600 mV (HE). Similar electrodes can operate 500 h continuously at a current density of 60 mAcm -2 without a noticeable increase of polarization.
NOMENCLATURE HAB AG-3 P-33 CoTMPP CoTAA GDE HE MC PTFE d
[4] which are Pt catalyzed have shown excellent characteristics: at current density 100mAcm -2, overvoltage is 265 mW, corresponding to a potential of +525 mV (HE) [4]. Similar electrodes, catalyzed with activated graphite PG-50 have been developed by Gorbachev et al. [3]. At current density 75-150 mAcm -2 the potential is 0.5-0.65 V [3, 5]. Tarasevich [6] have investigated the performance of gas diffusion electrodes catalyzed with dibenzo cobalt tetraazo anulen (CoTAA), cobalt tetra-methoxy-phenyl-porphirine (CoTMPP), and platinum, deposited upon active carbon AG-3. The best results were obtained with Pt-catalyzed electrodes-+625 mV at 100 mAcm -2 [6]. The use of gas diffusion electrodes for the oxidation of sulphur dioxide offers some advantages as compared with the immersed electrode version. In this case, a possibility is offered to design an electrolytic cell for the production of hydrogen and sulphuric acid, without using a membrane for the separation of the cathodic and anodic compartments, provided the corrosion problems connected with the decomposition of concentrated sulphuric acid can be avoided. This investigation is aimed at the determination of the relationship between the macro-structure of the G D E and the catalytic activity.
Acetylene black Active carbon Active carbon Cobalt tetra-methoxy-phenyl-porphirine Dibenzo cobalt tetraazo anulen Gas diffusion electrodes Hydrogen electrode Modified active carbon Polytetrafluoroethylene Thickness of the active layer INTRODUCTION
The electrolysis of water as an alternative method for the production of hydrogen, replacing the decomposition of hydrocarbons, is a relatively new area of investigation. The high voltage required by the conventional electrolysis cells, due to the kinetic limitations of the oxygen evolution process, renders the cost of electrolytic hydrogen uncompetitive as compared with the price of hydrogen obtained from oil. The use of sulphur dioxide as an anodic depolarizer decreases the thermodynamic voltage from 1.23 V to 0.29 V in a 50 wt% H2SO4 aqueous solution at 25°C [1]. Another advantage of this method is that it can use flue or exhaust gases from various industrial sources (power stations, ore dressing plants, etc.). Noble metals and their oxides are the usual catalysts for the oxidation of sulphur dioxide. Palladium and platinum are considered the most efficient catalysts among the noble metals. The expectations that iridium and rhutenium oxides, which show a superb catalytic activity and stability for the chlor-alkaline electrolysis process, will prove efficient also in this case did not materialize [1]. Better substitutes proved to be the metal porphirines [2] and activated graphite [3]. In the routine practice, the catalysts are deposited upon porous electrodes--immersed or gas diffusion type. The immersed electrodes, developed by Lu et al.
EXPERIMENTAL The following catalysts were used for the preparation of the GDE: C o T A A , CoTMPP (15 wt% deposited and pyrolized upon active carbon P-33 [7]) or modified active carbon (MC). The electrodes (S = 15 cm 2) were double-structured, with a porous gas-supply layer, similar to the air electrodes developed at this laboratory [8], and a porous catalytic layer. The binding agent was PTFE, introduced by two different methods: (a) PTFE powder and (b) PTFE previously precipitated upon acetylene black (HAB). The following parameters of the macro-structure were changed, keeping all others constant: (a) the
901
K. PETROV, IV. NIKOLOV AND T. VITANOV
902
amount of the catalyst, i.e. the thickness of the catalytic layer; (b) the quantity of PTFE or HAB. The activity of the electrodes was checked by tracing the steadystate galvanostatic volt-ampere characteristics in an electrolytic cell with cathode catalyzed with WC [9], and a 4.5 N H2504 solution. The potential of the electrodes was measured against a (Hg)2SO4 reference electrode, in turn checked against a hydrogen standard electrode immersed in the same solution. The sulphur dioxide depolarizer gas was fed to the chamber of the G D E with no residual overpressure. (1) Optimization of the GDE, catalyzed with organometallic compounds The structure of the active layer of the G D E catalyzed with CoTMPP and C o T A A was optimized. The optimization with respect to the catalyst content in the active layer was carried out when a constant amount of PTFE (4.5 wt%), introduced as HAB. Figure 1 shows the relationship between the potential of the electrode and the amount of catalyst, i.e. the thickness of the active layer (d) at different current densities. A clear-cut minimum is observed within the region d = 0 . 1 4 m m , corresponding to 13.3mg P-33 + 15% CoTMPP cm -2. The optimization of the structure with respect to the amount of PTFE was performed O
•
®
with two-catalyst loadings: 13.3 and 20mg.cm -z. Figures 2 and 3 show the potential traced vs the amount of PTFE for the two catalyst loadings, at different current densities. It can be noted that thinner electrodes (13.3 mg P-33 + 15% CoTMPP.cm -2) show no substantial differences in the activity when PTFE content is increased up to 6%. Further augmentation evidently results in a partial blocking of the catalyst surface, and the corresponding decrease in the activity of the electrodes. At a catalyst loading of 20 mg.cm -2, the best characteristics are obtained with electrodes containing 3% PTFE in the active layer (Fig. 3). High current densities up to 200 mAcm -2 can be obtained only with similar electrodes. On the basis of the studies, two optimum compositions of the electrodes were determined: 13 mg.cm -2 catalyst loading, containing 3-6% PTFE, and 20 mg.cm -2 with 3% PTFE, respectively. This finding provides evidence that the amount of binding agent may vary depending on the catalyst loading, i.e. on the thickness of the catalytic layer. In a similar manner the structure of the G D E containing the catalyst C o T A A was optimized. At a constant content of the binding agent (4.5 wt% PTFE introduced as HAB), the optimum of the relationship potential-catalyst loading is reached at 20 mg P-33 + 15% C o T A A cm -2 (Fig. 4). The investigation of the effect exerted by the binding agent at two different catalyst loadings 13.3 and 20mg.cm -2 clearly shows, that electrodes containing 20 mg.cm -2 catalyst display a substantially better characteristic. Figure 5 shows the potential-PTFE content relationship in this case.
700 700
65O ID
650
•
6OO •
x
0
6O0
- - - - -
,;
2'0
3'o
catalyst Loading,mglcm 2
Fig. 1. Potential vs amount of catalyst (catalyst loading) (P-33 + 15% CoTMPP) at constant content of P'ITE (4.5 wt% introduced as HAB) and different c.d.'s. (U) 40mAcm-2; (O) 60mAcro-2; (x) 80mAcm-Z; (O)o 100mAcro-2; (~) 120 macro-2; (ID) 150 mAcm-2; t = 20 C; 4.5 N H2SO4.
~nn ~ n n . ~ ~ n n
550 I
I
t
3
6
9
°/oPTF E
Fig. 2. Potential of the GDE catalyzed with 13.3 mg.cm -2 P-33 + 15% CoTMPP vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
GAS DIFFUSION ELECTRODES FOR THE OXIDATION OF SULPHUR DIOXIDE ID
0
903
O
700
700
650
650 -rE
¢
600
w
x~l(
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x ~ l ( J :.._....._ ~)J
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550
550
I1~11 I
f
I
3
6
9
%,PTFE Fig. 3. Potential of a GDE catalyzed with 20mg.cm -2 P-33 + 15% CoTMPP vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1. (~) 200 mAcm-2. •
%~PTFE Fig. 5. Potential of a GDE catalyzed with 20 mg P-33 + 15% CoTAA cm -2 vs the amount of PTFE (in %, introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
•
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650 :z:
,6
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°~°~o--------~o
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o m-,
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0il
012
013 d, m m
I 10
210
I 30
catalyst loading,mg/cm2 Fig. 4. Potential of a GDE vs catalyst loading (P-33 + 15% CoTAA) at a constant content of PTFE (4.5% introduced as HAB) at different c.d.'s. Symbols as in Fig. 1.
I I ........_
0.2 I 2~0
0,3 I
d,mm
30 cafatys, toading,mglcm2
Fig. 6. Potential vs amount of catalyst (MC) at constant content of PTFE (4.5% introduced as HAB) at different c.d.'s. (B) 40mAcm 2; (©) 60mAcro-2; (Q) 100mAcm 2; (~) 150 mAcm-2; (~) 200 mAcm 2; t = 20°C; 4.5 N H2SO4.
904
K. PETROV, IV. NIKOLOV AND T. VITANOV
(2) Optimization of the structure of the active layer of the GDE containing modified active carbon (MC) The optimization was carried out according to the same pattern. Figure 6 shows the electrode potentialcatalyst loading, i.e. thickness of the active layer relationship. A t low current densities this parameter does not exert a substantial effect. A t higher current densities (above 100 mAcm -2) the curves change abruptly their shape, passing through a minimum. In this case also optimum characterisics are displayed by electrodes with a thickness of the active layer d = 0 . 2 m m (20 mg.cm -2 catalyst loading). The optimization of the structure of the electrodes with respect to the amount of PTFE introduced as H A B was carried out for two different catalyst loadings--13.3 and 20 mg.cm -2. Figures 7 and 8 show the potentialPTFE content curves at different c.d. Electrodes containing 4.5% PTFE and 20 mg.cm-: catalyst show a minimum polarization. A t 13.3 mg.cm -2 the respective PTFE content is 6%. Similarly, as in the case of CoTMPP-catalyzed electrodes, the optimum content of H A B varies for the different catalyst loadings. The electrodes with optimum composition, containing 13.3 mg.cm -2 catalyst show a better performance than those with optimum composition at a catalyst loading 20 mg.cm -2 MC. The effect exerted by the method by which PTFE is introduced in the active layer was investigated using G D E , catalyzed with active carbon. Figure 9 shows the relationship: potential of the G D E catalyzed with 20 mg.cm -2 MC vs PTFE content (introduced in the active layer as powder) at different current densities. This figure must be juxtaposed with Fig. 7, where the same relationship is shown, when PTFE is introduced as hydrophobized acetylene black. In both cases the curves display a minimum. The comparison of both graphs shows, that up to current densities 100 mAcm -2 the introducing of PTFE as H A B brings about the decrease of the PTFE content only in the active layer. A t higher drains, however, it becomes clear that stable characteristics can be obtained only with electrodes which contain H A B in the active layer. Probably acetylene black increases the homogeneity of the pores, reducing transport limitations, hence improves the utilization of the catalyst. The V - A characteristics of the types of G D E were traced using a depolarizing gas mixture of SO2 + Ar, in an effort to provide evidence for this presumption. These measurements offer a possibility to determine the magnitude AE, which is defined as the difference between the potentials of the electrode when pure SO2 or a SO2 + A r gas mixture is used as the depolarizer. According to Iliev et al. [10, 11], this magnitude provides information for the transport limitations in the gaseous phase. Figure 10 shows that electrodes with PTFE introduced as a powder display more severe transport limitations. The following Fig. 11 shows the V - A characteristics of the best G D E for each catalyst. It can be noted, that
700
O
O
650 ,7, > E ,o600
O
550 w.._L--r/~ ~
%,PTFE Fig. 7. Potential of GDE catalyzed with 20 mg modified active carbon cm 2 vs the amount of P'ITE (in %, introduced as HAB) at different c.d.'s. (11) 30 mAcm-e; other symbols as for Fig. 6.
O
700
_650
O ~k O
600
~
O
f
O
O
•
550
:
I
I
I
3
6
9
%PTFE Fig. 8. Potential of a GDE catalyzed with 13,3 mg active carbon cm -2 vs the amount of PTFE (in % introduced as HAB) at different c.d.'s. All symbols as in Fig. 7.
905
GAS DIFFUSION ELECTRODES FOR THE OXIDATION OF SULPHUR DIOXIDE
g
650
./
650
m~l
E
600
•
550
0~
t~
O"-"'-4L""-~ ®
OB~ ~
e J
°
/7
//7-"
II-'"11 ~ 500
I
;o
3'0
%,PTFE
1
Fig. 9. Potential of GDE catalyzed with 20 mg modified active carbon cm -z vs the amount of PTFE (powder) at different c.d.'s; (11) 30mAcm-2; (O) 60mAcm-2; (@) 100mAcro-2; (®) 120 mAcro-2; t = 20°C; 4.5 N H2SO4.
?
i
/ / iiI
E
/
/
/
tt
I
iI
x
/ / ' .,' / / / ,,," ,'" x ;
20
,x" ,,s/
/"
/
,,/ 50
•
1
150 i, mA/cm2
Fig. 11. V-A curves of a GDE with optimum composition catalyzed with (@) modified active carbon, P-33 + 15% CoTMPP (n) P-33 + 15% CoTAA. t = 20°C; 4.5 N H2SO4.
CONCLUSION
/
//~,~ ~
1
lOO
the characteristics of G D E catalyzed with modified active carbon are the best. Their polarization is only 50 mV higher than that of Pt-catalyzed electrodes [4]. The long-term investigations have shown, that the active layer of G D E catalyzed with CoTMPP and C o T A A is cracked after 30-50 h of operation at current density 100 mAcm -2, resulting in a drastic increase of polarization. G D E catalyzed with modified active carbon retain their good initial characteristics during 500 h continuous operation at current density 60mAcm -2, with no visible alterations of the surface (Fig. 12). The leakage of SO2h -1 into the electrolyte through the optimized G D E is 0.03 g.cm -2.
80
60
50
/
•
100
150
i mA/cm 2
Fig. 10. A E - i curves of a GDE, catalyzed with 20 mg modified active carbon cm -2 at the optimum PTFE content. (. . . . ) PTFE (powder); ( ) PTFE (as HAB), at different ratio of the components of gas mixture (SO2 and Ar) (×) 3/6; (O) 4/6; (11) 5/6; t = 20°C; 4.5 N H2SO4.
The investigations have provided information about the activity of G D E catalyzed with CoTMPP, C o T A A , and modified active carbon for the sulphur dioxide oxidation in H2SO4 medium. After optimization of the structure, their characteristics can be compared with those of immersed electrodes with platinum catalysts. G D E electrodes which are catalyzed with modified active carbon, retain their good initial characteristics during long-term tests, and therefore could be applied in the design of electrochemical cells for the production of hydrogen and sulphuric acid.
906
K. PETROV, IV. NIKOLOV AND T. VITANOV
7O0
600 E
500
I
100
I
200
350
,60
Fig. 12. Long-term tests of GDE catalyzed with 20 mg modified active carbon cm 2. i = 60 macro-2; t = 20°C; 4.5 N H2504; (0) first sample; (×) second sample.
Acknowledgements--The authors are greatly indebted to Professor Tarasevich, Institute of Electrochemistry, Soviet Academy of Sciences, Professor Wiesener, Technische Universit/it Dresden, and Dr. I. Iliev, Central Laboratory of Electrochemical Power Sources, Sofia, who kindly provided catalysts.
REFERENCES 1. P. W. Lu and R. L. Ammon, J. Electrochem. Soc. 127, 2610 (1980). 2. K. A. Radushkina, M. R. Tarasevich and E. A. Akhundov, Elektrokhimiya 15, 1884 (1979). 3. A. K. Gorbachev, F. K. Andrushtenko, V. K. Nikiforov, V. P. Bochin, L. I. Ishtenko, L. V. Oparin and V. I. Kupina, Vorp. atom. nayki i tehn., Atom. Vodorod. energ. i technol. 2/9, 18 (1981).
4. P. W. Lu, E. R. Garcia and R. L. Ammon, J. appl. Electrochem. 11,347 (1981). 5. A. K. Gorbachev, F. K. Andrushtenko, V. P. Bochin, V. K. Nikiforov and L. I. Ischenko, Tez. dokl. 6 Vses. konf. Elektrokhimii 2, 305 (1982). 6. M. R. Tarasevich, personal communication. 7. V. S. Bagotsky, M. R. Tarasevich, K. A. Radushkina, O. A. Levina and S. I. Andruseva, J. Power Sources 2, 233 (1977). 8. E. Budevski, I. Iliev, S. Gamburtsev, A. Kaisheva, E. Vakanova and I. Mukhovski, Dept Chem. Bulg. Acad. Sci. 7, 223 (1974). 9. I. Nikolov, K. Petrov and T. Vitanov, Int. J. Hydrogen Energy (in press). 10. I. lliev, A. Kaisheva and S. Gamburtsev, Dept Chem. Bulg. Acad. Sci. 8, 367 (1975). 11. I. Iliev, S. Gamburtsev, A. Kaisheva and J. Mrha, J. appl. Electrochem. 5,291 (1979).