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Investigations on the storage life of oxygen electrodes by means of electrolytic catalysts based on CoTAA
639
Chem.;l80 (1984) 639-643 Elsevier Sequoia S_&. Lausanne- Printedin The Netherlands
.J_ EIe&vanal. .-
..
..
IiWE~TiGATIONS ON THiSTORAGELIFE OF OXYGEN B~MEAN~~FELE~~R~L~~CATALY~T~BASED.ON~OTAA*
ELECTRODES
: :. ,_ ..
k. WIESENER
and G. GRUNIG
Department qf Chemistsy, Dresden Technical Universi&
DDR : 802 7 Dresden (G. D. R.)
(Received 30~5 April 1984)
ABSTRACT When electrolytic catalysts preferred by pyrolysis of cobalt tetraazaannu?enes on active carbon are stored in air, a decrease of the electrolytic activity is observed. This deterioration is the result of irreversible absorption of oxygen from kir. This oxidation of electrode.materials which are very active in the cathodic reduction of oxygen can only be prevented by storage unde; a protective atmosphere or by immediate application.
INTRODUCTION
Since the seventies, the N,-chelates have been &knownas good electrolytic catalysts in cathodic oxygen reduction [l]. Pyrolysis of the organic catalyst on an active carbon support improves the electrochemical activity, i.e. .at constant potential the oxygen reduction flow is increased. On the basis of experimental investigations, a scheme of reactions @king place during the pyrtilysis was put forward [2]. The physico-chemical characteristics of these pyrolysates differ considerably from those of the starting materials. We suppose that the nitrogenous functionai groups on the carbon are connected with the electrocatalytic actikty of the materials. During storage of oxygen electrodes-and the electrocatalysts, it w&s found that the electrochemical activity deteriorates with increasing .duration of storage. In the present paper we try to explain the cause of this deterioration and, in addition, to characterize the change of the electrode material. EXPERIMENTAL
Gas-diffusion electrodes, +s described in ref_ 3, consisted of an active and a gas layer. The catalyst percentage on-the active carbon support (P 33)in rhe active layer was 15%.
After. this active material -had been pyrolysed ai 630°C in argon the pyrolysate yas mixed with Teflon and mcklded to an electrode equipcGd with a gas layer. After * Dedicated :o the memory of &f&or 0022-0728/84/.$03.00
Dr. Dr. h. c. Kurt-Schwabe.
Q 19S4 Elsevie; Sequoia SA.
-.
640 . ..
_. /,
.:a ,
.--
y
elec&de.tias. tested under storage in 2.25 k H,SO_,, the two-layer gas-diffusion galvanostatic, conditions_ A mercur$jmercur&--sulphate electrode. in --the. same ,. solution was used as the reference .electrode. Thi, electrodes yere stored ‘in iiir at25°C. in a desiccator at 25 “C and in a d.rymg .cupboard at. SOOC; I: .. T.. T -The investigation of the absorptionof oxygen was ckried out:by mea&of.the.g& volumetric apparatus illustrated.in Fig. 1 therm&tatting in air at 40 o C. The-sample was placed in a ground-glass bottle fitted with an extension-limb. The bottle was evacuated and then filled with oxygen until. normal pressure w& reached. Oxygen consumption was measured daily. Two days before the investigations were carried out, the samples were dried at 200 o C for 5 h and afterwards exposed to air. RESULTS
Electrocrlemical investigations In Fig. 2 the investigations of the galvanostatic properties indicate an unequivocal decrease in electrochemical activities of the oxygen electrodes with CoTAA (cobalt tetraazaannulenes) pyrolysates depending on the duration of storage in air. To investigate the reason for this deterioration more precisely, storage tests were carried out in a drying cupboard and a desiccator_ The r~zsults of the subsequent electrochemical investigations are shown in Table 1.
_Fig. 1. Gas volumetric apparatus used.to determine the oxygen consumption on electrode materials. Sample; (2) ground-glass bottle with extension limb;.(3) levelling bottle with measuiing liquid.-
(1)
.-
641
-.Theoxygen electro@k w&e stored in. a desiccator, in air and in a drying cupboard for .lOO days .at an, increased..temperature ,and consequently, the elecirochemical activity decreakd by. 35, 46 atid 74%~~resp&iveIy, whereas an electrode in the
Current density a! ERHE = 700 mV (iloo) and &sting potential (ERRfl~) of oxygen electrodes (P 33 + 15% CoTAA: 630 o C. 5 h. Ar) stored under different conditions, measured in 2.25 M HZSO, at 25 o C in 0, Temperature
Type of storage
/“C Without
storage
Duration of srorage r/day-
&&/mV
i,,,/mA
-
930
43.2
28 100
884
40.6
875
28.5
cm’
Storage in a desiccator
25
Storage in air
25
S:orage
28
-865
100
848
27.5 15.7
28 100
834
16.2
830
11.2
30
939
43.5
28 100.
912 881
41.4 37.6
in a drying
cupbtiard
80
Storage in argon
-. 25
Without siorage (loading of the electrode i 7 5 mA/cm’)
-
I I
T I 0
-
‘20
’
Ijo
-.
60
-
‘0
-
100 r/d
Fig.. 2: Depkndencc of ihe ctin-tint density of ari oxygen’i!ec!rode (P 33 t- 15% COTA& Ar, 630 OC; 5 h) at E,,, =‘700 mY in 2.25 M HiSO, a& 25OC with.air on the duration’of storage at 25 OC in air.
_ -
642 TABLE
.’
2
_‘..: _-,-
0, consumption and specific surface of the active layer material agd of the in~etiediatd stag&.at 40 o 9;
.:.
total value of the first 20 days Material
O2 consumption/ml g-* BET surface/m2 g-’
.-
Active layer
Active material pyrolysed
Active material
Active carboq
I?33 -i-15% CoTAA 630°C,5h,Ar + 15% Teflon
P33 +15% CoTAA 630°C,5h,Ar
P33 .. -i-15% CoTAA
P33
20.4 850
15.8 912
1.0
1.6 l&O
’
340
:
‘7
’
:.
7
receiving oxygen constantly during 100 days and being permanently loaded by 5 mA/cm2 only lost 18% of its initial activity.
electr&yte
Gas vclumetric invesiigatiok During a continuous test, the oxygen consumption of the active layer material (P 33 +- 15% CoTAA; 630 OC, 5 h, Ar) + 15% Teflon and the starting materials were determined by the apparatus shown in Fig. 1. In Table 2 the results for the different stages of the active material are summarized. After about 20 days, the 0, consumption real:hed a limiting value; after 50 days no measurable change in volume was observed. It follows from Table 2 that the reactivity of the active material is considerably increased by the pyrolysis. On the one hand, this reactivity is due to a three-fold increase of the BET surface of the sample. On the other hand, there must be another important effect as active carbon with a specific surface of 1000 m’/g absorbs less oxygen than pyrolysed active material with a surface of 912 m”/g. The even slightly increased 0, absor$on of the teflonized material indicates that the hydrophobicity increasing agent does not positively influence the “ageing” of the electrode material. DISCUSSION
The measurements of specific electrical resistance indicates that the resistance increases with increasing duration of storage. Compared with a “fresh” electrode, it amounts to l-3-fold for an electrode stored for 100 days [23. Consequently, the decrease in activity due to storage-can be caused by the oxygen influence on the active material resulting in a irreversible oxygen absorption. this ,tiould explain why the activity of the electrodes stored in the desiccator is higher while in the course of time the amount of oxygen is decreased, a& is the case with electrodes stored in air. At increased temperature the rate of oxygen absorption is higher, therefore the eiectrode activity is even more decreased after storage in the drying cupboard.
643
For comparison, an electrode was stored in argon in a desiccator for 30 days. In Table 1 it can be seen that there are hardly any differences in the parameters compared with those of a “fresh” electrode. The following consideration could explain the slight decrease in activity of an electrode “stored in the electrolyte with a current load in contrast to the electrodes stored in air. The preservation of -the cathodic activity of the electrode loaded in the electrolyte can only be interpreted in such a way that on reducing the oxygen in the working electrode, metastable surface compounds of oxygen on the catalytic electrode arise, react with the electrolyte and thus disappear from the surface. In addition, the oxidation of the carbon surface is impeded by the cathodic c:urrent. However, the potential of an electrode stored in air is higher, promoting the irreversible absorption of oxygen and the formation of oxidation products at the carbcn surface. If we suppose that similar metastable sirface products are formed also during the reaction between oxygen and the catalyst in the absence of an electrolyte, it can be concluded that these products are convertr-d into a stable state impairing the catalyst by a possible consecutive reaction_ Tltis could result in the oxidation of double bonds between the pyrolysates of the catalyst and the carbon and thus in a conductivity decrease of the catalyst by the consumption of rr-electrons as well as in the deactivation of the electrode surfaces. REFERENCES 1 H. Jaknke, M. Schiinbom and G. Zimmermann, Topics Cm-r. Chem.. 61 (1976) 133. 2 G. Gtinig, Doctoral Thesis, Dresden Teeb&cal University, 1983. 3 E. Budevski, I. Iliev, S. Gamburzev and 1%. Kaisheva, GB Patent 139 235 3 (1975).
Chem.;l80 (1984) 639-643 Elsevier Sequoia S_&. Lausanne- Printedin The Netherlands
.J_ EIe&vanal. .-
..
..
IiWE~TiGATIONS ON THiSTORAGELIFE OF OXYGEN B~MEAN~~FELE~~R~L~~CATALY~T~BASED.ON~OTAA*
ELECTRODES
: :. ,_ ..
k. WIESENER
and G. GRUNIG
Department qf Chemistsy, Dresden Technical Universi&
DDR : 802 7 Dresden (G. D. R.)
(Received 30~5 April 1984)
ABSTRACT When electrolytic catalysts preferred by pyrolysis of cobalt tetraazaannu?enes on active carbon are stored in air, a decrease of the electrolytic activity is observed. This deterioration is the result of irreversible absorption of oxygen from kir. This oxidation of electrode.materials which are very active in the cathodic reduction of oxygen can only be prevented by storage unde; a protective atmosphere or by immediate application.
INTRODUCTION
Since the seventies, the N,-chelates have been &knownas good electrolytic catalysts in cathodic oxygen reduction [l]. Pyrolysis of the organic catalyst on an active carbon support improves the electrochemical activity, i.e. .at constant potential the oxygen reduction flow is increased. On the basis of experimental investigations, a scheme of reactions @king place during the pyrtilysis was put forward [2]. The physico-chemical characteristics of these pyrolysates differ considerably from those of the starting materials. We suppose that the nitrogenous functionai groups on the carbon are connected with the electrocatalytic actikty of the materials. During storage of oxygen electrodes-and the electrocatalysts, it w&s found that the electrochemical activity deteriorates with increasing .duration of storage. In the present paper we try to explain the cause of this deterioration and, in addition, to characterize the change of the electrode material. EXPERIMENTAL
Gas-diffusion electrodes, +s described in ref_ 3, consisted of an active and a gas layer. The catalyst percentage on-the active carbon support (P 33)in rhe active layer was 15%.
After. this active material -had been pyrolysed ai 630°C in argon the pyrolysate yas mixed with Teflon and mcklded to an electrode equipcGd with a gas layer. After * Dedicated :o the memory of &f&or 0022-0728/84/.$03.00
Dr. Dr. h. c. Kurt-Schwabe.
Q 19S4 Elsevie; Sequoia SA.
-.
640 . ..
_. /,
.:a ,
.--
y
elec&de.tias. tested under storage in 2.25 k H,SO_,, the two-layer gas-diffusion galvanostatic, conditions_ A mercur$jmercur&--sulphate electrode. in --the. same ,. solution was used as the reference .electrode. Thi, electrodes yere stored ‘in iiir at25°C. in a desiccator at 25 “C and in a d.rymg .cupboard at. SOOC; I: .. T.. T -The investigation of the absorptionof oxygen was ckried out:by mea&of.the.g& volumetric apparatus illustrated.in Fig. 1 therm&tatting in air at 40 o C. The-sample was placed in a ground-glass bottle fitted with an extension-limb. The bottle was evacuated and then filled with oxygen until. normal pressure w& reached. Oxygen consumption was measured daily. Two days before the investigations were carried out, the samples were dried at 200 o C for 5 h and afterwards exposed to air. RESULTS
Electrocrlemical investigations In Fig. 2 the investigations of the galvanostatic properties indicate an unequivocal decrease in electrochemical activities of the oxygen electrodes with CoTAA (cobalt tetraazaannulenes) pyrolysates depending on the duration of storage in air. To investigate the reason for this deterioration more precisely, storage tests were carried out in a drying cupboard and a desiccator_ The r~zsults of the subsequent electrochemical investigations are shown in Table 1.
_Fig. 1. Gas volumetric apparatus used.to determine the oxygen consumption on electrode materials. Sample; (2) ground-glass bottle with extension limb;.(3) levelling bottle with measuiing liquid.-
(1)
.-
641
-.Theoxygen electro@k w&e stored in. a desiccator, in air and in a drying cupboard for .lOO days .at an, increased..temperature ,and consequently, the elecirochemical activity decreakd by. 35, 46 atid 74%~~resp&iveIy, whereas an electrode in the
Current density a! ERHE = 700 mV (iloo) and &sting potential (ERRfl~) of oxygen electrodes (P 33 + 15% CoTAA: 630 o C. 5 h. Ar) stored under different conditions, measured in 2.25 M HZSO, at 25 o C in 0, Temperature
Type of storage
/“C Without
storage
Duration of srorage r/day-
&&/mV
i,,,/mA
-
930
43.2
28 100
884
40.6
875
28.5
cm’
Storage in a desiccator
25
Storage in air
25
S:orage
28
-865
100
848
27.5 15.7
28 100
834
16.2
830
11.2
30
939
43.5
28 100.
912 881
41.4 37.6
in a drying
cupbtiard
80
Storage in argon
-. 25
Without siorage (loading of the electrode i 7 5 mA/cm’)
-
I I
T I 0
-
‘20
’
Ijo
-.
60
-
‘0
-
100 r/d
Fig.. 2: Depkndencc of ihe ctin-tint density of ari oxygen’i!ec!rode (P 33 t- 15% COTA& Ar, 630 OC; 5 h) at E,,, =‘700 mY in 2.25 M HiSO, a& 25OC with.air on the duration’of storage at 25 OC in air.
_ -
642 TABLE
.’
2
_‘..: _-,-
0, consumption and specific surface of the active layer material agd of the in~etiediatd stag&.at 40 o 9;
.:.
total value of the first 20 days Material
O2 consumption/ml g-* BET surface/m2 g-’
.-
Active layer
Active material pyrolysed
Active material
Active carboq
I?33 -i-15% CoTAA 630°C,5h,Ar + 15% Teflon
P33 +15% CoTAA 630°C,5h,Ar
P33 .. -i-15% CoTAA
P33
20.4 850
15.8 912
1.0
1.6 l&O
’
340
:
‘7
’
:.
7
receiving oxygen constantly during 100 days and being permanently loaded by 5 mA/cm2 only lost 18% of its initial activity.
electr&yte
Gas vclumetric invesiigatiok During a continuous test, the oxygen consumption of the active layer material (P 33 +- 15% CoTAA; 630 OC, 5 h, Ar) + 15% Teflon and the starting materials were determined by the apparatus shown in Fig. 1. In Table 2 the results for the different stages of the active material are summarized. After about 20 days, the 0, consumption real:hed a limiting value; after 50 days no measurable change in volume was observed. It follows from Table 2 that the reactivity of the active material is considerably increased by the pyrolysis. On the one hand, this reactivity is due to a three-fold increase of the BET surface of the sample. On the other hand, there must be another important effect as active carbon with a specific surface of 1000 m’/g absorbs less oxygen than pyrolysed active material with a surface of 912 m”/g. The even slightly increased 0, absor$on of the teflonized material indicates that the hydrophobicity increasing agent does not positively influence the “ageing” of the electrode material. DISCUSSION
The measurements of specific electrical resistance indicates that the resistance increases with increasing duration of storage. Compared with a “fresh” electrode, it amounts to l-3-fold for an electrode stored for 100 days [23. Consequently, the decrease in activity due to storage-can be caused by the oxygen influence on the active material resulting in a irreversible oxygen absorption. this ,tiould explain why the activity of the electrodes stored in the desiccator is higher while in the course of time the amount of oxygen is decreased, a& is the case with electrodes stored in air. At increased temperature the rate of oxygen absorption is higher, therefore the eiectrode activity is even more decreased after storage in the drying cupboard.
643
For comparison, an electrode was stored in argon in a desiccator for 30 days. In Table 1 it can be seen that there are hardly any differences in the parameters compared with those of a “fresh” electrode. The following consideration could explain the slight decrease in activity of an electrode “stored in the electrolyte with a current load in contrast to the electrodes stored in air. The preservation of -the cathodic activity of the electrode loaded in the electrolyte can only be interpreted in such a way that on reducing the oxygen in the working electrode, metastable surface compounds of oxygen on the catalytic electrode arise, react with the electrolyte and thus disappear from the surface. In addition, the oxidation of the carbon surface is impeded by the cathodic c:urrent. However, the potential of an electrode stored in air is higher, promoting the irreversible absorption of oxygen and the formation of oxidation products at the carbcn surface. If we suppose that similar metastable sirface products are formed also during the reaction between oxygen and the catalyst in the absence of an electrolyte, it can be concluded that these products are convertr-d into a stable state impairing the catalyst by a possible consecutive reaction_ Tltis could result in the oxidation of double bonds between the pyrolysates of the catalyst and the carbon and thus in a conductivity decrease of the catalyst by the consumption of rr-electrons as well as in the deactivation of the electrode surfaces. REFERENCES 1 H. Jaknke, M. Schiinbom and G. Zimmermann, Topics Cm-r. Chem.. 61 (1976) 133. 2 G. Gtinig, Doctoral Thesis, Dresden Teeb&cal University, 1983. 3 E. Budevski, I. Iliev, S. Gamburzev and 1%. Kaisheva, GB Patent 139 235 3 (1975).