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Ti electrodes in neutral pH media
Pergamon 0013_46%6(94)E0147-R
EkctrochimicoActa. Vol. 39, No. II/K?,pp. 15974602, 1994 Copyrisht 0 1994ElsetirscienceLtd. PrintedinGreat Britain. Allri$iurcaervcd 0013~4686/w $7.00 + 0.00
INFLUENCE OF ANIONS ON OXYGEN/OZONE EVOLUTION ON PbOJspe AND Pb02/Ti ELECTRODES NEUTRAL pH MEDIA
IN
A. A. BABAK,* R. AMADELLI,~A. DE BATTISTI~and V. N. FATBBV* *Russian Scientific Center, “Kurchatov Institute”, Institute of Hydrogen Energy and Plasma Technology, Kurchatov Square, 123182 Moscow, Russia tCentro di Studio su Fotoreattivita e Catahsi (C.N.R.) and Dipartimento di Chimica, Universita di Ferrara, via Borsari 46,441OOFerrara, Italy (Received 8 November 1993; accepted 25 January 1994)
Abstract-The formation of 0,/O, on PbO, from the electrolysis of water in neutral solutions is shown here to present some analogies and some differences with respect to the same process in acid media. The electrolyte composition affects the current efliciency for 0, formation (n) and the cell potential (E) of a Membrel type electrochemical assembly. An improvement in r~and a decrease in E is observed upon addition of relatively low amounts of Na,SO, or NaClO, in pure water. We observe no effect of NaNO, on either parameters. In agreement with literature data in acid solutions, F- causes an increase in both u and E. The results of electrochemical kinetic investigations with electrodes of PbO, electrodeposited on Ti confirm the above data. Current-potential curves constructed from measurements in NaNO, show a region of Tafel linearity with slopes of 2RT/F and RT/F in the low and high currents range, respectively. Addition of Na,SO, and NaClO, to NaNO, has an effect on the process at more positive potentials only: the RT/F slope decreases toward a value of RT/2F as the concentration of the “foreign” salt is increased. As an explanation of the observed behaviour, the possibility is advanced that a step following the discharge of water is rate determining at high positive potentials with the adsorption of intermediates described by Temkin conditions. The composition of the electrolyte is expected to influence the yield of ozone formation by affecting the coverage and free energy of adsorbed oxygen intermediates. Key words: ozone, oxygen, electrocatalysis, PbO, , anions.
INTRODUCTION
Ozone is attractive in wastewater treatment since, due to its high oxidizing power, it is able to attack a large number of organic and inorganic compounds. Several studies have addressed the problem of ozone electrogeneration. Lead dioxide anodes are currently used for this purpose since the material is cheap and it is rather stable under the high positive potentials required. By far most of the literature reports are concerned with the evolution of ozone by electrolysis of concentrated aqueous solutions of mineral acids[l-31. Investigations which are more directly focused on practical applications use PbOJsolid polymer electrolyte (spe) electrodes[4,5].
In the present we report on the influence of anions on the production of ozone from the electrolysis of water at PbO,/spe in the neutral pH range. Although this problem has been addressed in some detail by previous research work[l-3, 63, conclusive results cannot be claimed and the interpretation is still sufficiently open to question to justify additional work. We carried out experiments using both PbO,/spe anodes in Membrel-type cells and PbOr/Ti anodes in conventional electrochemical cells. The work dis-
cussed herein is focused mainly on results obtained with the latter experimental set-up. However, consistent behaviour is observed from a comparison of the two sets of data on the effect of anions, independent of the presence of the membrane. We think that the conclusions drawn are preliminary and relevant to future research using spe cells under practical working conditions.
EXPERIMENTAL
Experiments with PbO,/spe electrodes have been conducted in a titanium housing which held together the electrodes separated by a Nafion@ membrane. In some cases, the Russian-made pertluorosulphonated membrane MF4SK was used instead of Nafion@ and only minor differences were noted. The cathode was a platinum film supported on a porous titanium current collector; the anode was prepared by spreading a paste of B-Pb02 with 1% liquid Nafion@ on porous titanium. The cell was fed with MilliQ water. A constant current of 1 Acme2 was supplied to the cell using a Good Will Instrument Co. power supply. All chemicals were obtained from Fluka and
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al.
Nafion@ membranes were purchased from Aldrich. Sodium salts were used in all experiments.
Conventional b-PbO, electrodes were prepared by electrodeposition from Pb(NO& solutions onto Ti wires previously etched in concentrated hot oxalic acid and platinized. These are referred to as /IPbO,/Ti electrodes. Electrochemical experiments were carried out with an EG&G/PARC model 273A potentiostat interfaced with an Olivetti model M380 computer. For the cyclic voltammetry and chronopotentiometry experiments the EG&G M270 software was used. In some experiments the capacity of the electrodes was obtained from impedance data using an EG&G 5208 lock-in amplifier with EG&G M388 electrochemical impedance software. During steady-state measurements with PbO,/Ti electrodes, the solution pH was continuously monitored using a Radiometer PHM 93 pH meter. RESULTS
F-
AND DISCUSSION
fi-PbO,/spe In Fig. 1 are reported the data on the effect of anions on cell potential (E) and current efficiency of OJ production (q %). No effect is observed on addition of NaNO,, even for concentrations as high as 0.1 M, on either (E) or (q). Conversely, in the case of F- both the cell potential and the current efficiency are significantly affected. It is seen that as the concentration of NaF is increased, q increases at the expense of a rise in cell potential, in line with reported literature data obtained for acid electrolytes[l-31. In the case of phosphate, an increase in rl was observed already for concentrations below 10-4moldm-3 while, in the same range of concentrations, no appreciable variation of E was observed. Interestingly, with added SOi- the cell potential decreased and 1 increased, albeit to a more modest degree than in the case of F-. A large decrease in the current efficiency is, however, observed for SO:concentrations higher than 0.05 mol dme3 while the cell potential remains essentially constant. The behaviour observed upon addition of ClO; is qualitatively the same as that of SOi- although the effects are less pronounced: the effect on cell potential is illustrated in Fig. la. It is noteworthy that the effect of SOi- and ClO, on the cell potential is reversible. This can be varied between reproducible values when water in the absence or in the presence of a given concentration of SO:- (or ClOJ is alternatively fed to the cell (Fig. 2). In contrast, the cell potential is permanently altered by addition of F-. Preliminary results of an ESCA examination of the PbO, anode, after it worked in a NaF containing solution, gave an indication of the occurrence of a substantial surface modification. A thorough examination of the behaviour of F- is presently being carried out in our laboratories, and because a much more comprehensive effort is needed, the effect of F- is not discussed further in the following. We can reasonably rule out the possibility that diffusion of Na+ through the membrane is responsible for the observed phenomena since this is the common counter-cation of anions which have
ELECTROLYTE (MO1
CONCENTRATION DM-3)
Fig. 1. EfTect of the nature and concentration of electrolytes on: (a) the cell potential; and (b) current efficiency for 0, formation on PbO,, at neutral pH and room temperature, in a Membrel type electrochemical cell: applied current = 1 Acme*.
different and even opposite effects. In addition, impedance measurements showed that an appreciable increase in the membrane resistance is observed only when the concentration of added sodium salts is higher than 0.1 mol dm - 3. /I-PbO,film
on platinized titanium
We carried out measurements with b-PbOJTi electrodes to buttress the results described above for the /I-PbO,/spe and get the complementary information needed for an interpretation of the data. All the results reported below have been obtained using NaNO, as a supporting electrolyte. In Fig. 3 are reported the results of experiments showing the effect of SOi- on constant current charging curves for /I-PbOJTi. The curves have been recorded after anodic pretreatment at low constant currents[7]. Negative potentials of the opencircuit value were avoided to prevent the formation of PbSO,. The buildup of oxygen overvoltage at constant current has been described as a four-step processC7-J. Two such steps are identified in the curves of Fig. 3. In the region where the potential
Influence of anions on oxygen/ozone evolution
0
60
120
180
240
1599
300
360
TIME (min) Fig. 2. Influence of the concentration of Na,SO, on the cell potential during water electrolysis in a Membrel type cell with a PbO, anode and a Pt cathode separated by a Nafion membrane. Consecutive alternative cycles in which deionized water in the absence of (c) and (e) or in the presence of (a)
0.042moldm-3;
(b) 0.083moldm-3;
and (d) 0.025mol dm - 3 is fed to the cell: constant
current = 1Ac1n-~, T = 22°C.
increase with time is linear (clearly seen in Fig. 3a), the process involved is the accumulation of oxygen species on the surface. Other conditions being equal, the presence of sulphate causes a slower rate of increase of potential with time. This indicates a slower accumulation of oxygen species due to the presence of adsorbed SO:-. At higher currents the buildup of oxygen species is fast and the potential rises steeply with time approaching asymptotically the rate of gas evolution. As seen in Fig. 3, the potential at which the plateau region is observed is shifted to less positive values in the presence of sulphate. This confirms the data observed with the PbO,/spe electrode described above. The degree of potential decrease depends on
the amount of Na,SO, added and on the value of the constant current applied. Concomitant with the potential decrease one observes a rise in the production of ozone, in fine with the data reported for the PbOJspe system (uide supra). The data seemed to carry the message that the adsorbed anion might bring about a change in the reaction mechanism, and to verify it we carried out an examination of Tafel plots derived from measurements in NaNO, without and with added Na,SO, and NaCIO,. The effect of SO:- and ClO; was qualitatively the same, and data reported in Fig. 4 show the effect of the latter species. In agreement with the results on the 0,/O, evolution reported by Kdtz and Stucki[3] for PbO, in
2.3 20 mA
0.0
40
80
120
160
200
TIME (s) Fig. 3. Effect of sulphate addition on the charging curves for B-PbOJTi electrodes: dashed line: 0.1 moldm-” NaNO,; solid line: 0.1 moldme + 1.4 x 10-3moldm-3 Na,SO,.
1600
A. A.
I
I
I
I
1.9
2.0
2.1
2.2
POTENTIAL Fig.
(V
vs.
NHE
BABAK et al.
)
4.
Tafel plot for /I-PbO,/Ti at room temperature and neutral pH; electrode area: 0.2cm2. (a) 0.1moldm-3 NaNO,; (b) 0.1 moldm-” NaNO, + 4 x 10-3moldm-3 NaCIO,. Solid line: ir drop corrected by the interrupter method; dashed line: ir drop corrected by the feedback method. 3 M H,SO,, we observe two straight lines in the region of Tafel linearity, with slopes 2RT/F (130140mVdec-r) and RT/F (62-68mVdecr) at low and high currents, respectively. The effects of choice of the ir compensation method on the Tafel plots have been described in detail by the above authors. In accord with them, we found that the observation of the second linear region is subject to the correction of the ohmic drop by the dynamic compensation method. As seen in Fig. 4, on addition of ClO; the first Tafel line shifts slightly to more positive potentials with no noticeable change in the slope value, while for the line in the higher currents range the slope decreases from RT/F to _ RT/2F. Interestingly, the results are in accordance with other data of the present work, but they are in contrast with those reported by Katz and Stucki[3] on the effect of Fon the 0,/O, evolution on PbO, in concentrated sulphuric acid. In that case, the presence of the anion affects the slope of the first Tafel line only. We have not carried out measurements on the effects of F- on Tafel curves since, as mentioned earlier in this paper, we plan a more detailed investigation of this particular case. At present, it is dillicult to state whether the observed difference between our data and those reported in [3] are due to the peculiar behaviour of F- or to a difference in the nature and pH of the supporting electrolyte solution. There are indeed differences which seem to be attributable to a pH effect in the case of PF;. We observe no effect due to
NaPF, (pH 6.5-6.8) on either the cell potential or efficiency of 0, formation, whereas a large enhancement of the efficiency of 0, formation has been reported for HPF,[Z]. There are few attempts to analyse the mechanism that leads to 0, and 0, evolution[3,8]. In effect, the presence of parallel multistep processes introduces several parameters, and this makes it difficult to identify a unique possible pathway that explains the experimental data. It is generally recognized that the yield of 0, production depends on the nature of the anions of the electrolyte[l-3, 61. Attempts to correlate the effect of anions on the efficiency of 0, evolution to their electronegativity[2] have been questioned[3]. It is instead established that the largest effects are observed with adsorbable anions, and there exists a critical coverage by the anions for which the yield of 0, is maximum[l, 23. In this respect, the data reported in this work agree with literature reports that sulphate, phosphate and perchlorate are adsorbed on PbO, whereas nitrate is not[9]. The detection of oxygen from oxoanions in the evolved gases reported by Foller and Tobias[l, 21 was not confirmed by later work[lO]. The current explanation of the influence of adsorbed anions is that they compete with oxygen species for surface sites and reduce the adsorption strength of oxygen[ 1,
21. Despite the fact that the formulation of a reaction mechanism is objectively complicated, it is worth commenting briefly on some possibilities, in the light of the data available from this work and from literature. Thus the observed Tafel slope value of 2RT/F at the lower currents agrees with previous data on 0, evolution on /I-PbO,[3] and it is consistent with the first electron transfer, ie the oxidation of water, being the rate determining step: H,O + OH’ + H+ + e-. In a likely mechanistic quently followed by: 2(OH),,,
pathway,
reaction
(1) (1) is subse-
-, H,O + (O),,, 9
(2)
or (OH’)+(O),,,+H++e-,
(2’)
and 2(OH),,,
+ (O,),,,
+ (0,).
At some higher positive potential, forms ozone, probably as follows (O),,, + (Or),,, + 0,
a parallel
(3) route
(4)
We can rule out the mechanistic pathways that imply 0, formation through the direct involvement of the oxide since, based on the current information, oxidation of PbO, to higher valency states is not possible[9]. The fact that reaction (4) seems to be fast and diffusion controlledC3, 51 makes it an unlikely ratedetermining step. If we accept this, then the kinetics will be controlled by reactions (l)-(3), and the yield of 0, will be determined by a competition between reactions (3) and (4) which, in turn, depends on the
1601
Influence of anions on oxygen/ozone evolution
The above results point in favour of a significant coverage by strongly adsorbed oxygen species which can be displaced by SOi- or ClO;. In addition, we recall that the current interruption experiments of Riietschi et al.[7], which showed a recovery of overvoltage before the potential decayed to the opencircuit value, are also indicative of adsorbed (and absorbed) species. Thus, in the light of the available data, it does not seem possible to explain the data reported herein, for the process in NaNO,, on the basis of low coverage Langmuir conditions. A Tafel slope approaching RTf2F, observed when perchlorate or sulphate is added to NaNO, (Fig. 4), does not lend itself to a univocal interpretation. In the above line of reasoning, the observed behaviour may reflect, for example, a change in the adsorption isotherm, ie a decrease in surface coverage toward 0 + 0 with step (2’) rate determining. Alternatively, within the range of intermediate coverages, it would be also consistent with a transition from activated to non-activated oxygen desorption, with coadsorbed sulphate or perchlorate anions playing a key role.
coverage and reactivity of adsorbed oxygen intermediates. The observed change in Tafel slope from 2RT/F to RTJF at higher potentials bears out the conclusion that a change in the mechanism is involved with a different reactivity of the adsorbed intermediates. We think it is unrealistic to explain this change with the influence of the adsorption of anions of the electrolyte, since the data which have been obtained in NaNO, and NO; do not appear to be adsorbed on PbO,[9]. In terms of a traditional mechanistic analysis, the RT/T Tafel slope indicates that a step following reaction (1) determines the rate at the more positive potentials, with the condition that the adsorption of intermediates is described by a Temkin isotherm[ 111. In particular, Langmuir conditions with 0 + 1 do not give Tafel slope values < ZRT/F. In the following we report on an attempt to point out the validity of the assumption that the coverage by intermediates is appreciable. The curves displayed in Fig. 5 show that after 0,/O, evolution in NaNO, the capacity at different potentials increases significantly (B) compared to the case when no gas evolution occurred (A). The value of the capacities observed in the presence of SO:- (or ClO;) added either after (C and D) or before gas evolution (E), were considerably lower than in its absence. The capacity curves were not modified permanently by SOi- : the original behaviour (A) was observed when the electrode was re-immersed in NaNO, free of SO;-. On this ground, roughening of the surface can be ruled out. In addition, the formation of a layer of PbSO, is excluded since potentials negative of the open-circuit value were avoided.
0.5
0.8
o-9
CONCLUSIONS The process of 0, formation on PbOz anodes in aqueous neutral solutions is strongly influenced by the electrolyte composition, in particular by the nature of the anions. Some of the data reported herein agree with published results on related experiments. So, for example, F- is found to increase both the current efficiency of 0, formation and the potential of an electrochemical cell working at a constant current.
kiiz
1.0
1.1 POTENTIAL
1.2 (V
VS.
1.3
1.4
1.5
1.6
NHE)
Fig. 5. Capacity vs. potential curves for /?-PbOJTi. (A) The capacity is measured in 0.1 moldm-3 NaNO,, from 1.6 V in the direction of less positive potentials. (B) Electrode scanned from 1.6 to 2.1 V at 0.5 mV s- ’ ; scan reversed, stopped at 1.6 V for 1 min and capacity measured as in (A). (C) Same as (B) but 1.4 x 10m3moldm-3 Na,SO, added as the potential was held at 1.6V before recording the capacity. (D) Same as (C) with 2.5 x 10-“moldm-3 Na,SO,. (E) Experiment carried out as in (B) but with 1.5 x 10-3moldm-3 Na,SO, added from the beginning: electrode area = 0.2cm2, T = 22°C.
1602
A. A.
BABAK et al.
New data discussed in this work show that addition of Na,SO, and NaClO, to NaNO, solutions improves the yield of 0, evolution while simultaneously lowering the cell potential; in this regard, sodium nitrate was found to behave as an inert electrolyte. The experimental approach followed in this work is based on the comparison of data obtained using both PbO,/spe electrodes in Membrel-type cells and /?-PbO,/Ti in conventional electrochemical cells. Since the results are qualitatively the same, we infer that the observed effect of anions does not reside in phenomena involving the spe membrane. We propose that the influence of SO:- and ClO; reflects their coadsorption with oxygen species at the electrode surface: from earlier literature both anions are known to adsorb significantly on PbOl whereas NO; does not[9]. Tafel plots derived from steady-state measurements on /?-PbO,/Ti in NaNO, show two regions of linearity as found previously in acid electrolyte[3]. However, at variance with those data, under our experimental conditions the anions are found to affect mainly the slope of the linear region at the higher currents. Due to the intrinsic difficulties of the system, there have been few attempts to formulate a mechanism based on electrochemical kinetic data. In the present work we advance a possible mechanism for the complex process of 02/0, evolution at PbO, anodes. We think that the results reported herein should stimulate further discussion and experimental
work directed at gaining more insight particularly
into the processes occurring in the high anodic potential where significant formation of ozone is observed.
Research
in this direction
is continuing
in
our laboratories. Acknowledgement-Support for this research by the Italian National Council of Research (C.N.R.) (Progetto Finalizzato Chirnica Fine) is gratefully acknowledged.
REFERENCES 1. P. C. Foller and C. W. Tobias, J. phys. Chem. 85, 3228 (1962). 2. P. C. Foller and C. W. Tobias, J. electrochem. Sot. 129, 506 (1982). 3. P. Kiitz and S. Stucki, J. electroanal. Chem. 228, 407 (1987). 4. S. Stucki, G. Theis, R. Kiitz, H. Devantay and H. J. Christen, J. electrochem. Sot. 132,367 (1985). 5. S. Stucki. H. Baumann. H. J. Christen and R. Kiitz, J. appl. Ele&ochem. 17,7?3 (1987). 6. J. C. G. Thanos, H. P. Fritz and D. W. Wabner, J. appl. Electrochem. 14,389 (1984). 7. P. Riietschi, R. T. Angstat and B. D. Cahan, J. electrothem. Sot. 106,547 (1959). 8. D. Wabner and C. Grambow, J. electroad. Chem. 1%. 108 (1985). Chem. Rev. 72. 679 9. J. P. Carr and N. A. Hamnson. . (1972). 10. J. C. T’hanos and D. W. Wabner, Electrochim. Acta 30, 753 (1985).
11. J. O’M. Bockris and T. Otagawa, J. phys. Chem. 87, 2960 (1983).
EkctrochimicoActa. Vol. 39, No. II/K?,pp. 15974602, 1994 Copyrisht 0 1994ElsetirscienceLtd. PrintedinGreat Britain. Allri$iurcaervcd 0013~4686/w $7.00 + 0.00
INFLUENCE OF ANIONS ON OXYGEN/OZONE EVOLUTION ON PbOJspe AND Pb02/Ti ELECTRODES NEUTRAL pH MEDIA
IN
A. A. BABAK,* R. AMADELLI,~A. DE BATTISTI~and V. N. FATBBV* *Russian Scientific Center, “Kurchatov Institute”, Institute of Hydrogen Energy and Plasma Technology, Kurchatov Square, 123182 Moscow, Russia tCentro di Studio su Fotoreattivita e Catahsi (C.N.R.) and Dipartimento di Chimica, Universita di Ferrara, via Borsari 46,441OOFerrara, Italy (Received 8 November 1993; accepted 25 January 1994)
Abstract-The formation of 0,/O, on PbO, from the electrolysis of water in neutral solutions is shown here to present some analogies and some differences with respect to the same process in acid media. The electrolyte composition affects the current efliciency for 0, formation (n) and the cell potential (E) of a Membrel type electrochemical assembly. An improvement in r~and a decrease in E is observed upon addition of relatively low amounts of Na,SO, or NaClO, in pure water. We observe no effect of NaNO, on either parameters. In agreement with literature data in acid solutions, F- causes an increase in both u and E. The results of electrochemical kinetic investigations with electrodes of PbO, electrodeposited on Ti confirm the above data. Current-potential curves constructed from measurements in NaNO, show a region of Tafel linearity with slopes of 2RT/F and RT/F in the low and high currents range, respectively. Addition of Na,SO, and NaClO, to NaNO, has an effect on the process at more positive potentials only: the RT/F slope decreases toward a value of RT/2F as the concentration of the “foreign” salt is increased. As an explanation of the observed behaviour, the possibility is advanced that a step following the discharge of water is rate determining at high positive potentials with the adsorption of intermediates described by Temkin conditions. The composition of the electrolyte is expected to influence the yield of ozone formation by affecting the coverage and free energy of adsorbed oxygen intermediates. Key words: ozone, oxygen, electrocatalysis, PbO, , anions.
INTRODUCTION
Ozone is attractive in wastewater treatment since, due to its high oxidizing power, it is able to attack a large number of organic and inorganic compounds. Several studies have addressed the problem of ozone electrogeneration. Lead dioxide anodes are currently used for this purpose since the material is cheap and it is rather stable under the high positive potentials required. By far most of the literature reports are concerned with the evolution of ozone by electrolysis of concentrated aqueous solutions of mineral acids[l-31. Investigations which are more directly focused on practical applications use PbOJsolid polymer electrolyte (spe) electrodes[4,5].
In the present we report on the influence of anions on the production of ozone from the electrolysis of water at PbO,/spe in the neutral pH range. Although this problem has been addressed in some detail by previous research work[l-3, 63, conclusive results cannot be claimed and the interpretation is still sufficiently open to question to justify additional work. We carried out experiments using both PbO,/spe anodes in Membrel-type cells and PbOr/Ti anodes in conventional electrochemical cells. The work dis-
cussed herein is focused mainly on results obtained with the latter experimental set-up. However, consistent behaviour is observed from a comparison of the two sets of data on the effect of anions, independent of the presence of the membrane. We think that the conclusions drawn are preliminary and relevant to future research using spe cells under practical working conditions.
EXPERIMENTAL
Experiments with PbO,/spe electrodes have been conducted in a titanium housing which held together the electrodes separated by a Nafion@ membrane. In some cases, the Russian-made pertluorosulphonated membrane MF4SK was used instead of Nafion@ and only minor differences were noted. The cathode was a platinum film supported on a porous titanium current collector; the anode was prepared by spreading a paste of B-Pb02 with 1% liquid Nafion@ on porous titanium. The cell was fed with MilliQ water. A constant current of 1 Acme2 was supplied to the cell using a Good Will Instrument Co. power supply. All chemicals were obtained from Fluka and
1597
A. A.
1598
BABAK et
al.
Nafion@ membranes were purchased from Aldrich. Sodium salts were used in all experiments.
Conventional b-PbO, electrodes were prepared by electrodeposition from Pb(NO& solutions onto Ti wires previously etched in concentrated hot oxalic acid and platinized. These are referred to as /IPbO,/Ti electrodes. Electrochemical experiments were carried out with an EG&G/PARC model 273A potentiostat interfaced with an Olivetti model M380 computer. For the cyclic voltammetry and chronopotentiometry experiments the EG&G M270 software was used. In some experiments the capacity of the electrodes was obtained from impedance data using an EG&G 5208 lock-in amplifier with EG&G M388 electrochemical impedance software. During steady-state measurements with PbO,/Ti electrodes, the solution pH was continuously monitored using a Radiometer PHM 93 pH meter. RESULTS
F-
AND DISCUSSION
fi-PbO,/spe In Fig. 1 are reported the data on the effect of anions on cell potential (E) and current efficiency of OJ production (q %). No effect is observed on addition of NaNO,, even for concentrations as high as 0.1 M, on either (E) or (q). Conversely, in the case of F- both the cell potential and the current efficiency are significantly affected. It is seen that as the concentration of NaF is increased, q increases at the expense of a rise in cell potential, in line with reported literature data obtained for acid electrolytes[l-31. In the case of phosphate, an increase in rl was observed already for concentrations below 10-4moldm-3 while, in the same range of concentrations, no appreciable variation of E was observed. Interestingly, with added SOi- the cell potential decreased and 1 increased, albeit to a more modest degree than in the case of F-. A large decrease in the current efficiency is, however, observed for SO:concentrations higher than 0.05 mol dme3 while the cell potential remains essentially constant. The behaviour observed upon addition of ClO; is qualitatively the same as that of SOi- although the effects are less pronounced: the effect on cell potential is illustrated in Fig. la. It is noteworthy that the effect of SOi- and ClO, on the cell potential is reversible. This can be varied between reproducible values when water in the absence or in the presence of a given concentration of SO:- (or ClOJ is alternatively fed to the cell (Fig. 2). In contrast, the cell potential is permanently altered by addition of F-. Preliminary results of an ESCA examination of the PbO, anode, after it worked in a NaF containing solution, gave an indication of the occurrence of a substantial surface modification. A thorough examination of the behaviour of F- is presently being carried out in our laboratories, and because a much more comprehensive effort is needed, the effect of F- is not discussed further in the following. We can reasonably rule out the possibility that diffusion of Na+ through the membrane is responsible for the observed phenomena since this is the common counter-cation of anions which have
ELECTROLYTE (MO1
CONCENTRATION DM-3)
Fig. 1. EfTect of the nature and concentration of electrolytes on: (a) the cell potential; and (b) current efficiency for 0, formation on PbO,, at neutral pH and room temperature, in a Membrel type electrochemical cell: applied current = 1 Acme*.
different and even opposite effects. In addition, impedance measurements showed that an appreciable increase in the membrane resistance is observed only when the concentration of added sodium salts is higher than 0.1 mol dm - 3. /I-PbO,film
on platinized titanium
We carried out measurements with b-PbOJTi electrodes to buttress the results described above for the /I-PbO,/spe and get the complementary information needed for an interpretation of the data. All the results reported below have been obtained using NaNO, as a supporting electrolyte. In Fig. 3 are reported the results of experiments showing the effect of SOi- on constant current charging curves for /I-PbOJTi. The curves have been recorded after anodic pretreatment at low constant currents[7]. Negative potentials of the opencircuit value were avoided to prevent the formation of PbSO,. The buildup of oxygen overvoltage at constant current has been described as a four-step processC7-J. Two such steps are identified in the curves of Fig. 3. In the region where the potential
Influence of anions on oxygen/ozone evolution
0
60
120
180
240
1599
300
360
TIME (min) Fig. 2. Influence of the concentration of Na,SO, on the cell potential during water electrolysis in a Membrel type cell with a PbO, anode and a Pt cathode separated by a Nafion membrane. Consecutive alternative cycles in which deionized water in the absence of (c) and (e) or in the presence of (a)
0.042moldm-3;
(b) 0.083moldm-3;
and (d) 0.025mol dm - 3 is fed to the cell: constant
current = 1Ac1n-~, T = 22°C.
increase with time is linear (clearly seen in Fig. 3a), the process involved is the accumulation of oxygen species on the surface. Other conditions being equal, the presence of sulphate causes a slower rate of increase of potential with time. This indicates a slower accumulation of oxygen species due to the presence of adsorbed SO:-. At higher currents the buildup of oxygen species is fast and the potential rises steeply with time approaching asymptotically the rate of gas evolution. As seen in Fig. 3, the potential at which the plateau region is observed is shifted to less positive values in the presence of sulphate. This confirms the data observed with the PbO,/spe electrode described above. The degree of potential decrease depends on
the amount of Na,SO, added and on the value of the constant current applied. Concomitant with the potential decrease one observes a rise in the production of ozone, in fine with the data reported for the PbOJspe system (uide supra). The data seemed to carry the message that the adsorbed anion might bring about a change in the reaction mechanism, and to verify it we carried out an examination of Tafel plots derived from measurements in NaNO, without and with added Na,SO, and NaCIO,. The effect of SO:- and ClO; was qualitatively the same, and data reported in Fig. 4 show the effect of the latter species. In agreement with the results on the 0,/O, evolution reported by Kdtz and Stucki[3] for PbO, in
2.3 20 mA
0.0
40
80
120
160
200
TIME (s) Fig. 3. Effect of sulphate addition on the charging curves for B-PbOJTi electrodes: dashed line: 0.1 moldm-” NaNO,; solid line: 0.1 moldme + 1.4 x 10-3moldm-3 Na,SO,.
1600
A. A.
I
I
I
I
1.9
2.0
2.1
2.2
POTENTIAL Fig.
(V
vs.
NHE
BABAK et al.
)
4.
Tafel plot for /I-PbO,/Ti at room temperature and neutral pH; electrode area: 0.2cm2. (a) 0.1moldm-3 NaNO,; (b) 0.1 moldm-” NaNO, + 4 x 10-3moldm-3 NaCIO,. Solid line: ir drop corrected by the interrupter method; dashed line: ir drop corrected by the feedback method. 3 M H,SO,, we observe two straight lines in the region of Tafel linearity, with slopes 2RT/F (130140mVdec-r) and RT/F (62-68mVdecr) at low and high currents, respectively. The effects of choice of the ir compensation method on the Tafel plots have been described in detail by the above authors. In accord with them, we found that the observation of the second linear region is subject to the correction of the ohmic drop by the dynamic compensation method. As seen in Fig. 4, on addition of ClO; the first Tafel line shifts slightly to more positive potentials with no noticeable change in the slope value, while for the line in the higher currents range the slope decreases from RT/F to _ RT/2F. Interestingly, the results are in accordance with other data of the present work, but they are in contrast with those reported by Katz and Stucki[3] on the effect of Fon the 0,/O, evolution on PbO, in concentrated sulphuric acid. In that case, the presence of the anion affects the slope of the first Tafel line only. We have not carried out measurements on the effects of F- on Tafel curves since, as mentioned earlier in this paper, we plan a more detailed investigation of this particular case. At present, it is dillicult to state whether the observed difference between our data and those reported in [3] are due to the peculiar behaviour of F- or to a difference in the nature and pH of the supporting electrolyte solution. There are indeed differences which seem to be attributable to a pH effect in the case of PF;. We observe no effect due to
NaPF, (pH 6.5-6.8) on either the cell potential or efficiency of 0, formation, whereas a large enhancement of the efficiency of 0, formation has been reported for HPF,[Z]. There are few attempts to analyse the mechanism that leads to 0, and 0, evolution[3,8]. In effect, the presence of parallel multistep processes introduces several parameters, and this makes it difficult to identify a unique possible pathway that explains the experimental data. It is generally recognized that the yield of 0, production depends on the nature of the anions of the electrolyte[l-3, 61. Attempts to correlate the effect of anions on the efficiency of 0, evolution to their electronegativity[2] have been questioned[3]. It is instead established that the largest effects are observed with adsorbable anions, and there exists a critical coverage by the anions for which the yield of 0, is maximum[l, 23. In this respect, the data reported in this work agree with literature reports that sulphate, phosphate and perchlorate are adsorbed on PbO, whereas nitrate is not[9]. The detection of oxygen from oxoanions in the evolved gases reported by Foller and Tobias[l, 21 was not confirmed by later work[lO]. The current explanation of the influence of adsorbed anions is that they compete with oxygen species for surface sites and reduce the adsorption strength of oxygen[ 1,
21. Despite the fact that the formulation of a reaction mechanism is objectively complicated, it is worth commenting briefly on some possibilities, in the light of the data available from this work and from literature. Thus the observed Tafel slope value of 2RT/F at the lower currents agrees with previous data on 0, evolution on /I-PbO,[3] and it is consistent with the first electron transfer, ie the oxidation of water, being the rate determining step: H,O + OH’ + H+ + e-. In a likely mechanistic quently followed by: 2(OH),,,
pathway,
reaction
(1) (1) is subse-
-, H,O + (O),,, 9
(2)
or (OH’)+(O),,,+H++e-,
(2’)
and 2(OH),,,
+ (O,),,,
+ (0,).
At some higher positive potential, forms ozone, probably as follows (O),,, + (Or),,, + 0,
a parallel
(3) route
(4)
We can rule out the mechanistic pathways that imply 0, formation through the direct involvement of the oxide since, based on the current information, oxidation of PbO, to higher valency states is not possible[9]. The fact that reaction (4) seems to be fast and diffusion controlledC3, 51 makes it an unlikely ratedetermining step. If we accept this, then the kinetics will be controlled by reactions (l)-(3), and the yield of 0, will be determined by a competition between reactions (3) and (4) which, in turn, depends on the
1601
Influence of anions on oxygen/ozone evolution
The above results point in favour of a significant coverage by strongly adsorbed oxygen species which can be displaced by SOi- or ClO;. In addition, we recall that the current interruption experiments of Riietschi et al.[7], which showed a recovery of overvoltage before the potential decayed to the opencircuit value, are also indicative of adsorbed (and absorbed) species. Thus, in the light of the available data, it does not seem possible to explain the data reported herein, for the process in NaNO,, on the basis of low coverage Langmuir conditions. A Tafel slope approaching RTf2F, observed when perchlorate or sulphate is added to NaNO, (Fig. 4), does not lend itself to a univocal interpretation. In the above line of reasoning, the observed behaviour may reflect, for example, a change in the adsorption isotherm, ie a decrease in surface coverage toward 0 + 0 with step (2’) rate determining. Alternatively, within the range of intermediate coverages, it would be also consistent with a transition from activated to non-activated oxygen desorption, with coadsorbed sulphate or perchlorate anions playing a key role.
coverage and reactivity of adsorbed oxygen intermediates. The observed change in Tafel slope from 2RT/F to RTJF at higher potentials bears out the conclusion that a change in the mechanism is involved with a different reactivity of the adsorbed intermediates. We think it is unrealistic to explain this change with the influence of the adsorption of anions of the electrolyte, since the data which have been obtained in NaNO, and NO; do not appear to be adsorbed on PbO,[9]. In terms of a traditional mechanistic analysis, the RT/T Tafel slope indicates that a step following reaction (1) determines the rate at the more positive potentials, with the condition that the adsorption of intermediates is described by a Temkin isotherm[ 111. In particular, Langmuir conditions with 0 + 1 do not give Tafel slope values < ZRT/F. In the following we report on an attempt to point out the validity of the assumption that the coverage by intermediates is appreciable. The curves displayed in Fig. 5 show that after 0,/O, evolution in NaNO, the capacity at different potentials increases significantly (B) compared to the case when no gas evolution occurred (A). The value of the capacities observed in the presence of SO:- (or ClO;) added either after (C and D) or before gas evolution (E), were considerably lower than in its absence. The capacity curves were not modified permanently by SOi- : the original behaviour (A) was observed when the electrode was re-immersed in NaNO, free of SO;-. On this ground, roughening of the surface can be ruled out. In addition, the formation of a layer of PbSO, is excluded since potentials negative of the open-circuit value were avoided.
0.5
0.8
o-9
CONCLUSIONS The process of 0, formation on PbOz anodes in aqueous neutral solutions is strongly influenced by the electrolyte composition, in particular by the nature of the anions. Some of the data reported herein agree with published results on related experiments. So, for example, F- is found to increase both the current efficiency of 0, formation and the potential of an electrochemical cell working at a constant current.
kiiz
1.0
1.1 POTENTIAL
1.2 (V
VS.
1.3
1.4
1.5
1.6
NHE)
Fig. 5. Capacity vs. potential curves for /?-PbOJTi. (A) The capacity is measured in 0.1 moldm-3 NaNO,, from 1.6 V in the direction of less positive potentials. (B) Electrode scanned from 1.6 to 2.1 V at 0.5 mV s- ’ ; scan reversed, stopped at 1.6 V for 1 min and capacity measured as in (A). (C) Same as (B) but 1.4 x 10m3moldm-3 Na,SO, added as the potential was held at 1.6V before recording the capacity. (D) Same as (C) with 2.5 x 10-“moldm-3 Na,SO,. (E) Experiment carried out as in (B) but with 1.5 x 10-3moldm-3 Na,SO, added from the beginning: electrode area = 0.2cm2, T = 22°C.
1602
A. A.
BABAK et al.
New data discussed in this work show that addition of Na,SO, and NaClO, to NaNO, solutions improves the yield of 0, evolution while simultaneously lowering the cell potential; in this regard, sodium nitrate was found to behave as an inert electrolyte. The experimental approach followed in this work is based on the comparison of data obtained using both PbO,/spe electrodes in Membrel-type cells and /?-PbO,/Ti in conventional electrochemical cells. Since the results are qualitatively the same, we infer that the observed effect of anions does not reside in phenomena involving the spe membrane. We propose that the influence of SO:- and ClO; reflects their coadsorption with oxygen species at the electrode surface: from earlier literature both anions are known to adsorb significantly on PbOl whereas NO; does not[9]. Tafel plots derived from steady-state measurements on /?-PbO,/Ti in NaNO, show two regions of linearity as found previously in acid electrolyte[3]. However, at variance with those data, under our experimental conditions the anions are found to affect mainly the slope of the linear region at the higher currents. Due to the intrinsic difficulties of the system, there have been few attempts to formulate a mechanism based on electrochemical kinetic data. In the present work we advance a possible mechanism for the complex process of 02/0, evolution at PbO, anodes. We think that the results reported herein should stimulate further discussion and experimental
work directed at gaining more insight particularly
into the processes occurring in the high anodic potential where significant formation of ozone is observed.
Research
in this direction
is continuing
in
our laboratories. Acknowledgement-Support for this research by the Italian National Council of Research (C.N.R.) (Progetto Finalizzato Chirnica Fine) is gratefully acknowledged.
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11. J. O’M. Bockris and T. Otagawa, J. phys. Chem. 87, 2960 (1983).