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Effect of solution pH on the surface properties of NiCo2O4 electrodes
J. Electroanal. Chem., 143 (1983) 419--423
419
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note E F F E C T OF SOLUTION pH ON THE S U R F A C E P R O P E R T I E S OF NiCo204 ELECTRODES*
A. CARUGATI**, G. LODI*** and S. T R A S A T T I
Laboratory o f Electrochemistry o f the University, Via Venezian 21, 20133 Milan (Italy) (Received l l t h October 1982; in revised form 29th October 1982)
NiCo204 is of interest as a material for oxygen reduction [1--6], oxygen evolution [6--10] and for other technological applications [11,12]. The performance of an oxide electrode depends on the state of the surface in the potential range where the desired reaction takes place and on the modifications occurring upon prolonged use at high current densities. Although various preparation procedures have been suggested [ 5] the most practical route to prepare NiCo2 04-activated titanium electrodes consists in the thermal decomposition of the mixed nitrate solution. Previous studies on the physicochemical properties of NiCo204 layers are due to Trunov et al. [2], King and Tseung [5], Yeager and co-workers [4] and Bagotzky et al. [3]. However, investigations, of the surface properties under conditions of electrochemical work have been marginal and occasional. Trunov and Verenikina [13] have investigated the surface changes accompanying oxygen reduction. Yeager and co-workers [ 4] have published one voltammetric curve of NiCo2 04 prepared at 400°C in 1 M NaOH. One voltammogram can also be found in a paper of Jasem and Tseung [ 7]. No discussion of the state of the surface of NiCo204 electrodes prior to oxygen evolution is found in these papers. In the course of our work on the study of the properties of oxide electrodes for oxygen and chlorine evolution, different sets of NiCo2 O4 electrodes were prepared by brushing solutions of Co and Ni nitrates in isopropanol onto Ti plates and firing at 300--500°C. The nominal thickness was as a rule 2 t~m. The structural and morphological characterization of these layers has been reported elsewhere [14] and will be published subsequently. The purpose of this note is to report on the effect of the pH of the solution on the surface properties of these electrodes. One set of electrodes was used to investigate the pH dependence of the open circuit potential [ 14]. These samples worked for many days in different solutions of NaC1 + HC1 and NaC104 + HC104 in the pH range 2--14. At the end of these experiments, voltammetric curves were recorded in 1 M KOH solution to investigate the surface features. A typical ]--E curve of an "aged" electrode *From a paper presented at the 32nd I.S.E. Meeting, Dubrovnik/Cavtat, Yugoslavia, September 14--18, 1981. **Present address: Assoreni, 20097 San Donato Milanese, Italy. ***Permanent address: Chemical Institute of the University, 44100 Ferrara, Italy. 0022-0728/83/0000--0000/$03.00
©
1983 Elsevier Sequoia S.A.
420
BA
-o
15-3
]
lo-2
Bi/~.//~B "',"',,,,. ,! J ,
, - ' , " ~ 0.1 ........
-10 -2
-X.
,
,
, 02 0.3 ~/, /, 0.5
kkJ
Fig. 1. V o l t a m m e t r i c c u r v e s at 20 m V s -1 o f " f r e s h " ( e l e c t r o d e s p r e p a r e d a t 4 5 0 ° C. 1 M K O H ; 25 ° C.
) and "aged" (.....
) Ti/NiCo204
is shown in Fig. 1 (dashed line). Only one anodic peak is observed (although a shoulder is probably present at more negative potentials) over a potential region of 0.7 V prior to oxygen evolution. The peak appears at about 0.35 V (SCE) -- corresponding to about 1.43 V(RHE) -- for all five electrodes prepared at temperatures between 300 and 500 ° C. Although some ohmic drop is probably associated with a TiO2 interlayer between Ti and NiCo2 04 (as was also observed by Yeager and co-workers [4] ), this appears to distort the Tafel line for oxygen evolution at current densities higher than about 10 mA cm -2 [ 15]. Therefore, no substantial distortion of the voltammogram is expected to take place. The cathodic peak is less sharp and probably consists of a doublet. The prevalence of one or the other of the two c o m p o n e n t peaks appears to depend on the temperature of preparation. The more cathodic peak prevails as the firing temperature decreases. Nevertheless, the charge integrated under the curves is fairly independent of the direction of the potential sweep. Figure 2 shows that, as a rule for oxide electrodes, the voltammetric charge decreases monotonically as the firing temperature is increased. As long as nothing is known about the nature of the charging process, it can be assumed to involve a change in the valency state of surface active sites. Therefore, the charge q* can be taken as a measure of the surface concentration of the metal ions participating in the electrochemical process. These are also responsible for the catalysis of oxygen evolution since a fairly good linear relationship of unit slope can be found between the reaction rate at a given potential in the Tafel line region and q* [15]. The latter set of NiCo204 electrodes was used as prepared directly to record voltammetric curves in 1 M KOH. Figure 1 shows that the shape of the curve for a " f r e s h " electrode (curye B) is remarkably different from that for " a g e d " ones. In particular, the two peaks are sharper and more symmetrical even
421 100 E 0
o E
50
o
o" 8 I
1
I
300
400
500 T/°C
Fig. 2. Anodic voltammetric charge of (o) " f r e s h " and (o) " a g e d " Ti/NiCo= 0 4 electrodes as a function of the preparation temperature. Arrows indicate the direction of change of the voltammetric charge of " f r e s h " electrodes upon acid treatment.
though the hysteresis is more appreciable. The position of the anodic peak appears to depend on the firing temperature and increases from about 0.3 V (SCE) at 350°C up to about 0.4 V at 500°C. The opposite is observed with the cathodic peak, so that the hysteresis apparently increases as the firing temperature is increased. The most peculiar result is t h a t the voltammetric charge, while fairly independent of the direction of the potential sweep, changes the other way round as a function of the firing temperature, as illustrated in Fig. 2. No similar observation has ever been reported with other oxide electrodes, which suggests that the freshly prepared NiCo204 surfaces may be in a very particular state. However, this does not appear to be a metastable state in alkaline solution. After prolonged oxygen evolution the voltammogram exhibited the same features and the charge differed by less than 2% with respect to the unused surface. The state of the NiCo204 surface has been found to change with the pH of the solution. The freshly prepared electrodes were kept for 1--2 h in different solutions at decreasing pH and the voltammetric curve was recorded after each treatment. The voltammogram was observed to start modifying as the pH became slightly acidic. A drastic change was observed as the pH decreased below about 3. Ultimately, a curve very close to curve A in Fig. 1 was obtained, which indicates that the acid treatment reproduces the state of " a g e d " surfaces. The most striking observation was however that the voltammetric charge decreased dramatically after immersion in low pH solutions. Figure 2 illustrates very clearly this phenomenon. The charge of (so-called) " a g e d " electrodes is fairly equal to that of the other set of electrodes. Such a change in charge corresponds to a marked deactivation of the electrodes which cannot be restored by alkaline treatment. For the sample prepared at 500°C a tenfold decrease in charge was accompanied by a parallel decrease in the current associated with oxygen evolution. The first explanation which comes to mind is in terms of some recrystallization and sintering of the surface operated by the acidic solution. However,
422
the fact that the modification does n o t involve simply the extension of the surface area is manifested by the occurrence of a shift in the peak position and n o t only of a variation in the peak height. Any assignment of the current peaks to a specific surface reaction is hard because there are no definite thermodynamic data for the spinel helping the identification of the redox couples involved. While the Co304 electrode exhibits fairly symmetric peaks [6,16], the Ni(OH)2 electrode shows [17] a hysteresis of about 0.1 V. The peak potential of "fresh" NiCo, 04 electrodes is closer to that of Ni oxide than to that of Co304. Since the charge at 350°C is little affected by the solution pH, a possible interpretation of the experimental picture may invoke an increasing surface segregation of Ni oxide as the firing temperature is increased, which could be leached out upon contact with acidic solutions. Formation of a mixture of NiCo204 + NiO has been in fact observed by X-ray analysis of powders fired at higher temperatures than 450°C [14]. This view implies that Ni ions play the role of surface active sites. While Ni oxides are certainly sensitive to acid solutions, also Co304 is known to dissolve or modify at pH < 3 [18,19]. Therefore, it is difficult to discriminate among the various possibilities on the basis of the present results alone. On the other hand, the mechanism of oxygen evolution was n o t observed [15] to change detectably from "fresh" to "aged" NiCo204 electrodes. While only a surface chemical analysis is likely to be able to yield some definite information about the origins of the observations reported here, work is in progress to elucidate all of the aspects of the electrochemical behaviour of NiCo2 O4 electrodes. ACKNOWLEDGEMENTS
Work carried out with the financial support of the National Research Council (C.N.R., Rome). A.C. is grateful to O. De Nora S.p.A., Milan, for a fellowship during which this work was carried out.
REFERENCES 1 M.R. Tarasevich, V.S. Vilinskaya, A.M. Khutornoi, R.Kh. Burshtein, F.V. Makordei and Yu.A. Tkach, Elektrokhimiya, 12 (1976) 504. 2 A.M. Trunov, V.A. Presnov, M.V. Uminskii, O.F. Rakityanskaya, T.S. Bakutina and A.I. Kotseruba, Elektrokhimiya, 11 (1975) 552. 3 V.S. Bagotzky, N.A. Shumilova and E.I. Khrushcheva, Electrochim. Acta, 21 (1976) 919. 4 H. Takahashi, W. Aldred, P. Bindra and E. Yeager, Extended Abstracts, 31st I.S.E. Meeting, Venice, 1980, p. 783. 5 W.J. King and A.C.C. Tseung, Electrochim. Acta, 19 (1974) 4 8 5 , 4 9 3 . 6 M.R. Tarasevich and B.N. Efremov in S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part A, Elsevier, Amsterdam, 1980, p. 221. 7 S.M. Jasem and A.C.C. Tseung, J. Electrochem. Soc., 126 (1979) 1353. 8 A.C.C. Tseung, S. Jasem and M.N. Mahmood in T.N. Verizoglu and W. Seifritz (Eds.), Hydrogen Energy System, Vol. 1, Pergamon, Oxford, 1978, p. 215. 9 C.R. Davidson, G. Kissel and S. Srinivasan, J. Electroanal. Chem., 132 (1982) 129. 10 D.B. Hibbert, J. Chem. Soc. Chem. Commun., (1980) 202. 11 V.I. Lyubushkin, V.A. Smir~ov and E.T. Lyubushkina, Elektrokhimiya, 17 (1981) 828.
423 12 13 14 15 16 17 18 19
S. Trasatti and G. Lodi in S. Trasatti (Ed.), Electrodes o f Conductive Metallic Oxides, Part B, Elsevier, Amsterdam, 1981, p. 521. A.M. Trunov and N.N. Verenikina, Elektrokhimiya, 17 (1981) 135. C.M. Mari, G. Gilardoni, A. Carugati and S. Trasatti, Extended Abstracts, 32nd I.S.E. Meeting, Dubrovnik/Cavtat, 1981, p. 96. A. Carugati and S. Trasatti, Extended Abstracts, 33rd I.S.E. Meeting, Lyon, 1982, p. 118. L.D. Burke, M.E. Lyons and O.J. Murphy, J. Electroanal. Chem., 132 (1982) 247. R.S. Schrebler Guzm~n, J.R. Vilche and A.J. Arvfa, J. Appl. Electrochem., 9 (1979) 183. D.M. Shub, A.N. Chemodanov and V.V. Shalaginov, Elektrokhimiya, 14 (1978) 595. M.B. Konovalov, V.I. Bystrov and V.L. Kubasov, Elektrokhimiya, 12 (1976) 1266.
419
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note E F F E C T OF SOLUTION pH ON THE S U R F A C E P R O P E R T I E S OF NiCo204 ELECTRODES*
A. CARUGATI**, G. LODI*** and S. T R A S A T T I
Laboratory o f Electrochemistry o f the University, Via Venezian 21, 20133 Milan (Italy) (Received l l t h October 1982; in revised form 29th October 1982)
NiCo204 is of interest as a material for oxygen reduction [1--6], oxygen evolution [6--10] and for other technological applications [11,12]. The performance of an oxide electrode depends on the state of the surface in the potential range where the desired reaction takes place and on the modifications occurring upon prolonged use at high current densities. Although various preparation procedures have been suggested [ 5] the most practical route to prepare NiCo2 04-activated titanium electrodes consists in the thermal decomposition of the mixed nitrate solution. Previous studies on the physicochemical properties of NiCo204 layers are due to Trunov et al. [2], King and Tseung [5], Yeager and co-workers [4] and Bagotzky et al. [3]. However, investigations, of the surface properties under conditions of electrochemical work have been marginal and occasional. Trunov and Verenikina [13] have investigated the surface changes accompanying oxygen reduction. Yeager and co-workers [ 4] have published one voltammetric curve of NiCo2 04 prepared at 400°C in 1 M NaOH. One voltammogram can also be found in a paper of Jasem and Tseung [ 7]. No discussion of the state of the surface of NiCo204 electrodes prior to oxygen evolution is found in these papers. In the course of our work on the study of the properties of oxide electrodes for oxygen and chlorine evolution, different sets of NiCo2 O4 electrodes were prepared by brushing solutions of Co and Ni nitrates in isopropanol onto Ti plates and firing at 300--500°C. The nominal thickness was as a rule 2 t~m. The structural and morphological characterization of these layers has been reported elsewhere [14] and will be published subsequently. The purpose of this note is to report on the effect of the pH of the solution on the surface properties of these electrodes. One set of electrodes was used to investigate the pH dependence of the open circuit potential [ 14]. These samples worked for many days in different solutions of NaC1 + HC1 and NaC104 + HC104 in the pH range 2--14. At the end of these experiments, voltammetric curves were recorded in 1 M KOH solution to investigate the surface features. A typical ]--E curve of an "aged" electrode *From a paper presented at the 32nd I.S.E. Meeting, Dubrovnik/Cavtat, Yugoslavia, September 14--18, 1981. **Present address: Assoreni, 20097 San Donato Milanese, Italy. ***Permanent address: Chemical Institute of the University, 44100 Ferrara, Italy. 0022-0728/83/0000--0000/$03.00
©
1983 Elsevier Sequoia S.A.
420
BA
-o
15-3
]
lo-2
Bi/~.//~B "',"',,,,. ,! J ,
, - ' , " ~ 0.1 ........
-10 -2
-X.
,
,
, 02 0.3 ~/, /, 0.5
kkJ
Fig. 1. V o l t a m m e t r i c c u r v e s at 20 m V s -1 o f " f r e s h " ( e l e c t r o d e s p r e p a r e d a t 4 5 0 ° C. 1 M K O H ; 25 ° C.
) and "aged" (.....
) Ti/NiCo204
is shown in Fig. 1 (dashed line). Only one anodic peak is observed (although a shoulder is probably present at more negative potentials) over a potential region of 0.7 V prior to oxygen evolution. The peak appears at about 0.35 V (SCE) -- corresponding to about 1.43 V(RHE) -- for all five electrodes prepared at temperatures between 300 and 500 ° C. Although some ohmic drop is probably associated with a TiO2 interlayer between Ti and NiCo2 04 (as was also observed by Yeager and co-workers [4] ), this appears to distort the Tafel line for oxygen evolution at current densities higher than about 10 mA cm -2 [ 15]. Therefore, no substantial distortion of the voltammogram is expected to take place. The cathodic peak is less sharp and probably consists of a doublet. The prevalence of one or the other of the two c o m p o n e n t peaks appears to depend on the temperature of preparation. The more cathodic peak prevails as the firing temperature decreases. Nevertheless, the charge integrated under the curves is fairly independent of the direction of the potential sweep. Figure 2 shows that, as a rule for oxide electrodes, the voltammetric charge decreases monotonically as the firing temperature is increased. As long as nothing is known about the nature of the charging process, it can be assumed to involve a change in the valency state of surface active sites. Therefore, the charge q* can be taken as a measure of the surface concentration of the metal ions participating in the electrochemical process. These are also responsible for the catalysis of oxygen evolution since a fairly good linear relationship of unit slope can be found between the reaction rate at a given potential in the Tafel line region and q* [15]. The latter set of NiCo204 electrodes was used as prepared directly to record voltammetric curves in 1 M KOH. Figure 1 shows that the shape of the curve for a " f r e s h " electrode (curye B) is remarkably different from that for " a g e d " ones. In particular, the two peaks are sharper and more symmetrical even
421 100 E 0
o E
50
o
o" 8 I
1
I
300
400
500 T/°C
Fig. 2. Anodic voltammetric charge of (o) " f r e s h " and (o) " a g e d " Ti/NiCo= 0 4 electrodes as a function of the preparation temperature. Arrows indicate the direction of change of the voltammetric charge of " f r e s h " electrodes upon acid treatment.
though the hysteresis is more appreciable. The position of the anodic peak appears to depend on the firing temperature and increases from about 0.3 V (SCE) at 350°C up to about 0.4 V at 500°C. The opposite is observed with the cathodic peak, so that the hysteresis apparently increases as the firing temperature is increased. The most peculiar result is t h a t the voltammetric charge, while fairly independent of the direction of the potential sweep, changes the other way round as a function of the firing temperature, as illustrated in Fig. 2. No similar observation has ever been reported with other oxide electrodes, which suggests that the freshly prepared NiCo204 surfaces may be in a very particular state. However, this does not appear to be a metastable state in alkaline solution. After prolonged oxygen evolution the voltammogram exhibited the same features and the charge differed by less than 2% with respect to the unused surface. The state of the NiCo204 surface has been found to change with the pH of the solution. The freshly prepared electrodes were kept for 1--2 h in different solutions at decreasing pH and the voltammetric curve was recorded after each treatment. The voltammogram was observed to start modifying as the pH became slightly acidic. A drastic change was observed as the pH decreased below about 3. Ultimately, a curve very close to curve A in Fig. 1 was obtained, which indicates that the acid treatment reproduces the state of " a g e d " surfaces. The most striking observation was however that the voltammetric charge decreased dramatically after immersion in low pH solutions. Figure 2 illustrates very clearly this phenomenon. The charge of (so-called) " a g e d " electrodes is fairly equal to that of the other set of electrodes. Such a change in charge corresponds to a marked deactivation of the electrodes which cannot be restored by alkaline treatment. For the sample prepared at 500°C a tenfold decrease in charge was accompanied by a parallel decrease in the current associated with oxygen evolution. The first explanation which comes to mind is in terms of some recrystallization and sintering of the surface operated by the acidic solution. However,
422
the fact that the modification does n o t involve simply the extension of the surface area is manifested by the occurrence of a shift in the peak position and n o t only of a variation in the peak height. Any assignment of the current peaks to a specific surface reaction is hard because there are no definite thermodynamic data for the spinel helping the identification of the redox couples involved. While the Co304 electrode exhibits fairly symmetric peaks [6,16], the Ni(OH)2 electrode shows [17] a hysteresis of about 0.1 V. The peak potential of "fresh" NiCo, 04 electrodes is closer to that of Ni oxide than to that of Co304. Since the charge at 350°C is little affected by the solution pH, a possible interpretation of the experimental picture may invoke an increasing surface segregation of Ni oxide as the firing temperature is increased, which could be leached out upon contact with acidic solutions. Formation of a mixture of NiCo204 + NiO has been in fact observed by X-ray analysis of powders fired at higher temperatures than 450°C [14]. This view implies that Ni ions play the role of surface active sites. While Ni oxides are certainly sensitive to acid solutions, also Co304 is known to dissolve or modify at pH < 3 [18,19]. Therefore, it is difficult to discriminate among the various possibilities on the basis of the present results alone. On the other hand, the mechanism of oxygen evolution was n o t observed [15] to change detectably from "fresh" to "aged" NiCo204 electrodes. While only a surface chemical analysis is likely to be able to yield some definite information about the origins of the observations reported here, work is in progress to elucidate all of the aspects of the electrochemical behaviour of NiCo2 O4 electrodes. ACKNOWLEDGEMENTS
Work carried out with the financial support of the National Research Council (C.N.R., Rome). A.C. is grateful to O. De Nora S.p.A., Milan, for a fellowship during which this work was carried out.
REFERENCES 1 M.R. Tarasevich, V.S. Vilinskaya, A.M. Khutornoi, R.Kh. Burshtein, F.V. Makordei and Yu.A. Tkach, Elektrokhimiya, 12 (1976) 504. 2 A.M. Trunov, V.A. Presnov, M.V. Uminskii, O.F. Rakityanskaya, T.S. Bakutina and A.I. Kotseruba, Elektrokhimiya, 11 (1975) 552. 3 V.S. Bagotzky, N.A. Shumilova and E.I. Khrushcheva, Electrochim. Acta, 21 (1976) 919. 4 H. Takahashi, W. Aldred, P. Bindra and E. Yeager, Extended Abstracts, 31st I.S.E. Meeting, Venice, 1980, p. 783. 5 W.J. King and A.C.C. Tseung, Electrochim. Acta, 19 (1974) 4 8 5 , 4 9 3 . 6 M.R. Tarasevich and B.N. Efremov in S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part A, Elsevier, Amsterdam, 1980, p. 221. 7 S.M. Jasem and A.C.C. Tseung, J. Electrochem. Soc., 126 (1979) 1353. 8 A.C.C. Tseung, S. Jasem and M.N. Mahmood in T.N. Verizoglu and W. Seifritz (Eds.), Hydrogen Energy System, Vol. 1, Pergamon, Oxford, 1978, p. 215. 9 C.R. Davidson, G. Kissel and S. Srinivasan, J. Electroanal. Chem., 132 (1982) 129. 10 D.B. Hibbert, J. Chem. Soc. Chem. Commun., (1980) 202. 11 V.I. Lyubushkin, V.A. Smir~ov and E.T. Lyubushkina, Elektrokhimiya, 17 (1981) 828.
423 12 13 14 15 16 17 18 19
S. Trasatti and G. Lodi in S. Trasatti (Ed.), Electrodes o f Conductive Metallic Oxides, Part B, Elsevier, Amsterdam, 1981, p. 521. A.M. Trunov and N.N. Verenikina, Elektrokhimiya, 17 (1981) 135. C.M. Mari, G. Gilardoni, A. Carugati and S. Trasatti, Extended Abstracts, 32nd I.S.E. Meeting, Dubrovnik/Cavtat, 1981, p. 96. A. Carugati and S. Trasatti, Extended Abstracts, 33rd I.S.E. Meeting, Lyon, 1982, p. 118. L.D. Burke, M.E. Lyons and O.J. Murphy, J. Electroanal. Chem., 132 (1982) 247. R.S. Schrebler Guzm~n, J.R. Vilche and A.J. Arvfa, J. Appl. Electrochem., 9 (1979) 183. D.M. Shub, A.N. Chemodanov and V.V. Shalaginov, Elektrokhimiya, 14 (1978) 595. M.B. Konovalov, V.I. Bystrov and V.L. Kubasov, Elektrokhimiya, 12 (1976) 1266.