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Oxygen and ozone evolution at fluoride modified lead dioxide electrodes
Electrochimica Acta 45 (1999) 713 – 720 www.elsevier.nl/locate/electacta
Oxygen and ozone evolution at fluoride modified lead dioxide electrodes R. Amadelli a,*, L. Armelao b, A.B. Velichenko c, N.V. Nikolenko c, D.V. Girenko c, S.V. Kovalyov c, F.I. Danilov c Centro di Studio su Fotoreatti6ita` e Catalisi (CNR), Dipartimento di Chimica, 6ia Borsari 46, Ferrara, Italy Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Uni6ersita` di Pado6a, 6ia P. Loredan 4, 35131 Pado6a, Italy c Department of Physical Chemistry, Ukrainian State Chemical Technology Uni6ersity, Gagarin a6e. 8, Dniepropetro6sk 320005, Ukraine a
b
Received 21 October 1998; received in revised form 7 April 1999
Abstract This work examines the behaviour of fluorine modified b-PbO2 electrodes in the processes of O2 and O3 evolution in sulphuric acid. The electrochemical kinetic analyses of these processes are based on quasi-steady-state polarisation and impedance data. The good agreement between the two sets of measurements allows some basic conclusions to be drawn. In particular, the O2 evolution process is always inhibited at F-doped PbO2 electrodes, and impedance results suggest possible changes in the mechanism, with electrodesorption of intermediates becoming more important as the concentration of the doping element increases. The interpretation of the data for the less positive potentials region invokes the specific adsorption of SO24 − as a factor influencing the kinetics of O2 evolution. The current efficiency for O3 formation as a function of the amount of NaF added to the PbO2 growth solution reaches a maximum for a concentration of 0.01 mol dm − 3. A plausible cause for the decrease on the higher concentration side is the discharge of adsorbed SO24 − (or HSO− 4 ) eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluoride in the electrolyte. An analysis of the results of modified neglect of diatomic differential overlap (MNDO) calculations on Pb cluster models and of X-ray photoelectron spectroscopy (XPS) data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Electrocatalysis; Ozone; Oxygen evolution; Lead dioxide; Fluoride
1. Introduction The behaviour of anodes at high potentials is interesting because of the electrochemical synthesis of important oxidants used in the chemical industry such as perchlorate, peroxysulphate and ozone, and in studies on the abatement of recalcitrant pollutants by electrochemical methods. Lead dioxide electrodes are still
* Corresponding author.
widely used for these purposes as the material is cheap and relatively stable under the high positive potentials required. Extensive literature data have shown that the electrochemical activity of these electrodes depends considerably on the composition of the electrolyte. In particular, the nature of anions is known to have a marked influence on ozone and persulphate production. Thus, for example, an enhancement of these processes by F− added to the electrolyte has long been known [1 – 12].
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R. Amadelli et al. / Electrochimica Acta 45 (1999) 713–720
The electrocatalytic activity of PbO2 electrodes, as well as their stability, can often be considerably enhanced by the incorporation of some foreign ions added to the electrodeposition solution. Among these, Bi3 + , Fe3 + and F− gave very good PbO2 electrodes for oxygen transfer reactions, including O3 formation [13– 19]. In a recent work, we studied the influence of F− on the electrodeposition of PbO2 on platinum [20]. We observed a rise in the Pb(II) oxidation rate in the presence of fluoride, which we explained on the basis of a mechanism where the effect of fluoride is that of increasing the surface concentration of oxygen species that are more strongly bound to the electrode surface. In that paper we also anticipated some results on the electrocatalytic activity of FPbO2 (and FePbO2) doped electrodes in the processes of O2 and O3 evolution. Herein we report on the results of a deeper investigation into the processes of O2 and O3 electrogeneration at PbO2 and FPbO2 anodes, using steady-state and impedance techniques. We also discuss the results of modified neglect of diatomic differential overlap (MNDO) calculations using cluster models containing or not containing fluorine.
2. Experimental Electrochemical kinetic and impedance experiments were performed on an EG&G model 273A potentiostat/galvanostat, EG&G model 5210 lock-in amplifier using EG&G software. Simulation calculations of impedance data were done using B.A. Boukamp’s EQUIVALENT CIRCUIT simulation program. Measurements were carried out in 1 M H2SO4 prepared from ultrapure sulphuric acid (Merck) and Millipore water, using a conventional three compartment cell. The counter electrode was a large Pt flag, or a cylindrical Pt gauze surrounding the working electrode in the case of impedance measurements. A saturated calomel electrode was used as reference. This was in contact with the working electrode compartment through a Luggin tip. Ohmic drop correction was carried out using the dynamic compensation method [9,12]. The cell used for ozone evolution experiments was either a Nafion® membrane cell described in an earlier paper [12] or a conventional cell. In this case, the current efficiency of ozone formation was determined with an AF-2 UV absorption ozone photometer, by sweeping the anodic gases with argon at a flow of 120 ml min − 1 to satisfy the flow requirements of our analytical instrument. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Perkin–Elmer f 5600ci instrument.
PbO2 samples were electrodeposited onto a Pt wire (S= 1.5 cm2) in 1 M HClO4 containing 0.02 M Pb(II) at 1.75 V over 30 min. Lead and fluoride were added to the solution as Pb(NO3)2 and NaF. Doped PbO2 electrodes obtained from solutions containing 0.01 M and 0.04 M NaF in the growth solution are referred to as F(001)PbO2 and F(004)PbO2, respectively. For the particular case of O3 measurements with a Nafion® membrane cell, PbO2 was electrodeposited onto platinised porous titanium disks having a diameter of 3 cm. All electrodes were pretreated as described by Ho et al. [21] and impedance measurements were taken only after a reproducible Tafel plot was obtained.
3. Results and discussion
3.1. Electrocatalytic acti6ity of F-doped PbO2 Incorporation of fluoride into lead dioxide leads to a change in its electrocatalytic activity which is clearly seen in both the processes of oxygen evolution and O3 production. The current efficiency for O3 formation at F-doped PbO2 anodes was found to be a function of the NaF concentration used in the electrodeposition bath (Fig. 1). These data were obtained using a solid polymer electrolyte (spe) membrane cell (Section 2) at 25°C but the trend was the same as that observed using a conventional gas-tight three compartment cell. It is seen that O3 formation reaches a maximum for FPbO2 electrodes prepared from growing solutions containing 0.01 mol dm − 3 NaF and decreases on further increasing the fluoride amount. This decrease in the current efficiency is accompanied by a rise in the cell potential. Tafel plots of E versus log i, obtained from quasisteady-state polarisation measurements are given in Fig. 2. They agree with our previous data [12] and are also in accord with the observation of Kotz and Stucki [9] that the linear region in the more positive potentials region is subject to correction of the Ohmic drop by the dynamic compensation method. The current was read at each potential after it reached a constant value, which required at least 5 min. The Tafel slopes showed a time dependence, ranging from values close to 120 mV decade − 1 for a waiting time B 2 min to 150 – 160 mV decade − 1 for more than 5 min. This time dependence of Tafel slopes is generally attributed to blocking of active surface sites [22]. For the particular case examined here, the cause may be the adsorption of SO24 − , as pointed by Ruetschi et al. [23]. According to these authors, SO24 − is adsorbed on bPbO2 and contribute to the charge of the double layer. The increase of the double layer charge due to SO24 − ions increases the electrode potential for a given current, or a give number of reacting oxygen species on the surface. In effect, our data shows [24] that, in the Tafel
R. Amadelli et al. / Electrochimica Acta 45 (1999) 713–720
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Fig. 1. Ozone formation at F-doped PbO2 anodes in a solid polymer electrolyte (spe) electrochemical cell: effect of the concentration of fluoride on the O3 current efficiency of and cell potential.
potential region, double layer capacities are higher in sulphuric than in perchloric acid by 40%. It is interesting to point out that these phenomena were considered in a theory of electrode charge transfer processes that Ruetschi formulated in a series of papers some time ago [25–27]. In Fig. 2 we also report plots of the potential versus log(R − 1). In fact, the resistance is [28] R =(E/(i=exp(−bh)/(bi°)
(1)
and log(R − 1) =log(bi°) +bh
(2)
The reaction resistance was obtained from the simulation of the experimental data using the equivalent circuit previously employed by other authors in analogous studies with PbO2 electrodes [21,29] where the adsorption – desorption of intermediates is characterised by the resistance Rp and by the pseudo-capacitance Cp, and Rct is the charge transfer resistance.
Fig. 2. Tafel plots for PbO2 and FPbO2 anodes in 1 M H2SO4 at room temperature. Electrode area: 1.5 cm2. (A) E vs. log i from quasi-steady-state data; (B) E vs. log 1/Rp from impedance data.
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An examination of Fig. 2 shows that there is remarkable agreement between the set of data obtained by the point-by-point quasi-steady-state method and that obtained from impedance measurements using the Rp data. The general trend observed is that Rp is larger than Rct in the whole potential range examined and for all PbO2 electrodes. This is in keeping with the statement of Ho et al. [21] that the mechanism of oxygen evolution on PbO2 is strongly controlled by electrodesorption. The parameters derived from the simulation calculations of impedance data are given in Table 1(a–c) for PbO2, F(001)PbO2 and F(004)PbO2 electrodes. A closer look into these data provides some interesting information that is not evident from an analysis of the Tafel plots of Fig. 2 alone. They reveal, for example, a different effect on Rp and Rct of the concentration of fluoride used to prepare the F-doped oxides. From the comparison of F(001)PbO2 with the undoped oxide, it is clear, despite some scattering of the data, that both Rp and Rct increase; one observes, in addition, that the increase of the former is
more pronounced than that of the latter. On the other hand, with F(004)PbO2 a conspicuous increase in Rp is seen at EB1.8 V, above this value the tendency is reversed (Fig. 2), and Rct now decreases pronouncedly up to 2.0 V (Table 1a,c). The behaviour of F(001)PbO2 is similar to that observed before with PbO2 electrodes in F− containing electrolytes [9,12]. Evidently, it reflects a loss in activity of the surface toward O2 evolution, with both the processes of water discharge and electrodesorption of intermediates being inhibited. On the other hand, the trend observed with F(004)PbO2 is somehow more surprising since, in this case, the Tafel slope, in the more positive potential range, returns to the lower values observed with the unmodified electrodes. While Rp still increases (at least up to 1.8 V) compared to the undoped oxide, Rcr decreases consistently. This behaviour might be due to a change in the reaction mechanism, for example, a different rate determining step, as the opposite effect on Rct and Rp seems to suggest. These changes in the mechanism can possibly be
Table 1 Impedance data for PbO2 and FPbO2 in 1 M H2SO4a Rct
Rp
Cdl×103
(a)
Table 1a b PbO2 1.650 0.76 1.700 0.76 1.725 0.73 1.800 0.703 1.850 0.76 1.900 0.76 2.000 0.84 2.025 0.95
24.30 16.50 18.27 16.10 13.47 12.86 1.43 1.61
891.2 435.6 295 134.8 74.13 56.2 26.5 20.04
1.23 1.042 1.20 1.12 0.85 0.89 0.045 0.041
(0.78) (0.80) (0.77) (0.77) (0.80) (0.79) (1.0) (1.0)
7.22 7.17 5.75 3.57 3.20 2.20 8.95 6.36
(1.0) (0.95) (0.94) (1.0) (1.0) (1.0) (0.84) (0.76)
Table 1b cF(001) PbO2 1.65 1.06 1.70 1.06 1.80 1.07 1.90 1.1 2.00 0.51 2.05 0.54
24.86 24.1 23.12 8.6 6.8 2.3
998 602.5 239.8 89 33 14.12
0.98 0.97 0.95 0.526 0.574 0.074
(0.85) (0.85) (0.85) (0.91) (0.83) (1.0)
6.20 4.93 1.60 1.62 0.506 0.70
(0.87) (0.89) (1.0) (1.0) (1.0) (1.0)
Table 1c d F(004) PbO2 1.650 1.14 1.700 1.12 1.800 1.4 1.900 1.43 1.975 0.68 2.000 0.68 2.025 0.65
12.63 9.94 8.25 4.41 0.92 0.9 1.0
2,165 700 167.34 58.8 28.2 21 15.8
1.80 1.38 1.38 1.40 0.104 0.076 0.081
(0.78) (0.79) (0.77) (0.80) (1.0) (1.0) (1.0)
E (V)
Rs
Cp×104
12.5 13.0 7.75 6.0 11.3 9.95 8.85
(a)
(0.87) (0.89) (0.94) (0.89) (0.77) (0.80) (0.80)
a Electrode areas: 1.5 cm2. Room temperature. Potentials vs. SCE; resistances in ohms and capacities in Faradays. a represents a constant phase element introduced in the simulations. b PbO2 electrodeposited in the absence of F−. c PbO2 electrodeposited in the presence of 0.01 M F−. d PbO2 electrodeposited in the presence of 0.04 M F−.
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caused by consistent changes in the surface and interface structure involving not only surface fluorine and oxygen species but also variations in the above mentioned adsorption–desorption of the sulphate anions of the electrolyte. The pseudo-capacitances Cp, given in Table 1(a–c), are plotted for convenience in Fig. 3. Probably, the most interesting thing to note is the behaviour of the capacitance in the high potential range where the processes leading to ozone formation become important. A completely different behaviour is seen for PbO2 and F(004)PbO2 on the one side and F(001)PbO2 on the other. In the first case, Cp increases at E \1.9 V and reaches a peak around 2.0 V; in the second case it decreases continuously and no peak is seen. In the latter case, one can also notice that the double layer capacity Cdl has correspondingly higher values. The observed Cp peak is a characteristic feature of the sulphuric acid medium [24] and must then originate from processes involving SO24 − . We have already mentioned above the possibility of the involvement of adsorbed SO24 − leading to changes in the double layer. At more anodic potentials, however, SO24 − can participate in Faradaic reactions yielding S2O28 − according to: 2SO24 − S2O28 − +2e −
(3)
or 2HSO4− S2O28 − +2H+ +2e −
(4)
whose standard potentials are 1.77 and 1.88 V (SCE), respectively [30]. These reactions have been extensively
Fig. 3. Plots of the pseudo-capacitance Cp vs. potential for: (a) PbO2; (b) PbO2 electrodeposited in the presence of 0.01 M NaF; (c) PbO2 electrodeposited in the presence of 0.04 M NaF. Electrode area: 1.5 cm2. Room temperature.
717
Fig. 4. Cluster models used for MNDO calculations on the effects of oxygen substitution with fluorine.
studied at PbO2 [10], DSA-type [3 – 5] and Pt [2,31 – 33] electrodes, and are known to be favoured in the presence of relatively high amounts of F− in the electrolyte. One possible explanation of the experimental data can then be that low doping fluorine amounts reduce the rate of O2 evolution and persulphate formation, providing the best conditions for O3 production. As the PbO2 doping is carried out in the presence of higher F− concentrations, the formation of persulphate is favoured and the O3 yield decreases again. We did not check on the presence of S2O28 − , also because the concentration of sulphuric acid we used was rather low for these kind of studies. However, in a study on persulphate production on b-PbO2 electrodes in 1.5 M H2SO4 + 2.3 M (NH4)2SO4, Scheler and Wabner [10] mention analogous effects of the increase of F− in the solution as we discussed above, i.e. (i) a decrease in the O3 yield versus an increase in S2O28 − and (ii) a decrease and subsequent increase in the double layer capacitance. Thus we have reason to believe that the promoting effect of high fluoride concentration with respect to the discharge of SO24 − , can account for the trend in O3 formation reported in Fig. 1 of this work.
3.2. X-ray photoelectron spectroscopy and modified neglect of diatomic differential o6erlap calculations for the interaction of fluorine with a PbO2 surface The Tafel plots for oxygen evolution shift to more positive potentials in the case of F-doped PbO2 (Fig. 2),
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Table 2 Results of MNDO calculations using the cluster models given in Fig. 4 and showing the effect of fluorine substitution on the electronic property of the system Charge
Pb3(OH)12(H2O)2 Pb3(OH)10F2(H2O)2 -(Cluster I) -(Cluster II)
QPba (e) QO(OH) (e) QO(H2O) (e)
+0.72 −0.20}−0.52 −0.17
a
+0.71 −0.20}−0.54 −0.17
Charge of the central Pb.
Table 3 Results of MNDO calculations using the cluster models given in Fig. 4 and showing the effect of fluorine substitution on the energy parameters of for O2 evolution assuming the ‘oxide path’ mechanism Step
D(DH), (kcal mol−1) Pb3(OH)12(H2O)2 (Cluster I)
163.8 S(OH−)2 S(OH)2+2e S(OH)2 170.3 S(O)2+2H++2e
Pb3(OH)10F2(H2O)2 (Cluster II) 167.8 178.0
intermediates. According to XPS data for PbO2, which was deposited from perchloric or nitric acid solutions in the presence of 0.02 M F−, the fluoride content in the oxide bulk was about 4 at.%. For the conditions that give a uniform distribution of fluoride within the PbO2 layer with UOH + UF = 1, the average surface coverage by fluoride was calculated to be about 6%. Using the MNDO method, we tried to estimate the fluoride influence on the geometric and electronic factors of the modified PbO2 surface and on the metal – oxygen intermediate bond strength on the process of O2 evolution. As a model of the PbO2 surface we adopted the structure indicated as Cluster I (Fig. 4). We further assume that the oxygen evolution kinetics are described by the so-called electrochemical oxide path mechanism. Two adjoining oxygen species take part in the oxygen evolution process (Fig. 4, boldface lines in the Cluster I). In our simulation of fluoride modified PbO2, two OH− ions are replaced by two fluoride ions (Fig. 4, Cluster II). In this case we expect the maximum influence of fluoride on the system parameters. Actually, the calculation showed that the influence of fluoride on electronic system properties is essentially negligible (Table 2). The O2 formation, for the Cluster I model, may be presented as a two-steps process: Pb3(OH)12(H2O)2 Pb3(OH)12(OH − )2 + 2H+ Pb3(OH)12(OH)2 + 2H+ + 2e
indicating an inhibition of this reaction. In principle, this might be due to surface blocking by fluoride (decrease of the active site concentration) with a consequent change in the bond energy of the oxygen
Pb3(OH)12(O)2 + 2H+ + 2e Pb3(OH)12O2 Pb3(OH)12(. . .)+O2
Fig. 5. X-ray photoelectron spectra of O 1s for (a) PbO2; (b) PbO2 electrodeposited in the presence of 0.01 M NaF in an unstirred solution; (c) PbO2 electrodeposited in the presence of 0.01 M NaF with stirring. Binding energies are given vs. C1s taken as 285 eV.
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Calculations for Cluster I and II demonstrated that fluoride shows no influence on the energy parameters for evolution of O2 from H2O (Table 3). Thus, within the limitations of the model, the relatively high surface concentration of fluoride versus its negligible influence on the electronic and the energy properties of the studied system, seems to suggest that the effect of the fluoride modification is mainly that of changing the surface coverage of adsorbed oxygen species. This is clearly a simplified model which does not take into account, for example, the above mentioned role that specifically adsorbed sulphate anions seem to play in determining the reactivity of intermediate oxygen species. However, despite the limitations, the above conclusion on the importance of changes in the surface coverage of oxygen species is supported by the XPS data. In Fig. 5 the XPS spectra in the O 1s region shows that F-doping causes a marked decrease of the broad signal at the higher binding energies which attributed to adsorbed OH and H2O [34]. Conversely, neither the position of the peak at lower binding energies, assigned to oxide ions within the structure, nor that of the Pb4f undergo major changes, and the difference, D(O 1s − Pb 4f7/2) [34] is 392 9 0.25 eV for both PbO2 and F-doped PbO2 samples. While more work is needed, the conclusion that the present data seems to bring out is that fluorine affects the weakly bound oxygen species, i.e. those ultimately involved in the O2 evolution process.
4. Conclusions In this research work we examined the electrocatalytic behaviour of fluorine-doped PbO2 electrodes in the processes of O2 and O3 evolution. By comparison with unmodified PbO2, F-doping shifts the O2 evolution process to higher potentials for a given current, for the higher F-doping levels, however, this increase is seen mainly at lower currents. On the other hand, as the concentration of F− in the PbO2 growth solution is increased, the current efficiency of O3 formation first rises and then declines to lower values. Investigations were focused on electrodes prepared from 0.01 M NaF (F(001)PbO2) and from 0.04 M NaF (F(004)PbO2). By comparison with the undoped oxide, the activity for O3 formation of the first was higher and that of the latter lower. We show that there is agreement between the results of quasi-steady-state and impedance measurements. In addition, from the results of impedance data analysis, based on an equivalent circuit with components that describe a charge transfer process producing adsorbed species, we can distinguish an effect of doping on charge transfer (resistance Rct) from that on the ad-
719
sorbed intermediates (Rp). With respect to PbO2, both parameters increase for F(001)PbO2 while for F(004)PbO2 Rp increases and Rct decreases, reflecting a change to an O2 evolution mechanism controlled by the electrodesorption of intermediates. Sulphate anions have a relevant role in the anodic processes examined. In the less positive potential region their specific adsorption would affect O2 evolution through screening charge effects on oxygen intermediates, at higher potentials their discharge is not only thermodynamically possible but, in addition, it is known from literature to be enhanced by increasing amounts of fluoride. An analysis of the results of MNDO calculations on Pb cluster models and of XPS data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping.
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Oxygen and ozone evolution at fluoride modified lead dioxide electrodes R. Amadelli a,*, L. Armelao b, A.B. Velichenko c, N.V. Nikolenko c, D.V. Girenko c, S.V. Kovalyov c, F.I. Danilov c Centro di Studio su Fotoreatti6ita` e Catalisi (CNR), Dipartimento di Chimica, 6ia Borsari 46, Ferrara, Italy Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Uni6ersita` di Pado6a, 6ia P. Loredan 4, 35131 Pado6a, Italy c Department of Physical Chemistry, Ukrainian State Chemical Technology Uni6ersity, Gagarin a6e. 8, Dniepropetro6sk 320005, Ukraine a
b
Received 21 October 1998; received in revised form 7 April 1999
Abstract This work examines the behaviour of fluorine modified b-PbO2 electrodes in the processes of O2 and O3 evolution in sulphuric acid. The electrochemical kinetic analyses of these processes are based on quasi-steady-state polarisation and impedance data. The good agreement between the two sets of measurements allows some basic conclusions to be drawn. In particular, the O2 evolution process is always inhibited at F-doped PbO2 electrodes, and impedance results suggest possible changes in the mechanism, with electrodesorption of intermediates becoming more important as the concentration of the doping element increases. The interpretation of the data for the less positive potentials region invokes the specific adsorption of SO24 − as a factor influencing the kinetics of O2 evolution. The current efficiency for O3 formation as a function of the amount of NaF added to the PbO2 growth solution reaches a maximum for a concentration of 0.01 mol dm − 3. A plausible cause for the decrease on the higher concentration side is the discharge of adsorbed SO24 − (or HSO− 4 ) eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluoride in the electrolyte. An analysis of the results of modified neglect of diatomic differential overlap (MNDO) calculations on Pb cluster models and of X-ray photoelectron spectroscopy (XPS) data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Electrocatalysis; Ozone; Oxygen evolution; Lead dioxide; Fluoride
1. Introduction The behaviour of anodes at high potentials is interesting because of the electrochemical synthesis of important oxidants used in the chemical industry such as perchlorate, peroxysulphate and ozone, and in studies on the abatement of recalcitrant pollutants by electrochemical methods. Lead dioxide electrodes are still
* Corresponding author.
widely used for these purposes as the material is cheap and relatively stable under the high positive potentials required. Extensive literature data have shown that the electrochemical activity of these electrodes depends considerably on the composition of the electrolyte. In particular, the nature of anions is known to have a marked influence on ozone and persulphate production. Thus, for example, an enhancement of these processes by F− added to the electrolyte has long been known [1 – 12].
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The electrocatalytic activity of PbO2 electrodes, as well as their stability, can often be considerably enhanced by the incorporation of some foreign ions added to the electrodeposition solution. Among these, Bi3 + , Fe3 + and F− gave very good PbO2 electrodes for oxygen transfer reactions, including O3 formation [13– 19]. In a recent work, we studied the influence of F− on the electrodeposition of PbO2 on platinum [20]. We observed a rise in the Pb(II) oxidation rate in the presence of fluoride, which we explained on the basis of a mechanism where the effect of fluoride is that of increasing the surface concentration of oxygen species that are more strongly bound to the electrode surface. In that paper we also anticipated some results on the electrocatalytic activity of FPbO2 (and FePbO2) doped electrodes in the processes of O2 and O3 evolution. Herein we report on the results of a deeper investigation into the processes of O2 and O3 electrogeneration at PbO2 and FPbO2 anodes, using steady-state and impedance techniques. We also discuss the results of modified neglect of diatomic differential overlap (MNDO) calculations using cluster models containing or not containing fluorine.
2. Experimental Electrochemical kinetic and impedance experiments were performed on an EG&G model 273A potentiostat/galvanostat, EG&G model 5210 lock-in amplifier using EG&G software. Simulation calculations of impedance data were done using B.A. Boukamp’s EQUIVALENT CIRCUIT simulation program. Measurements were carried out in 1 M H2SO4 prepared from ultrapure sulphuric acid (Merck) and Millipore water, using a conventional three compartment cell. The counter electrode was a large Pt flag, or a cylindrical Pt gauze surrounding the working electrode in the case of impedance measurements. A saturated calomel electrode was used as reference. This was in contact with the working electrode compartment through a Luggin tip. Ohmic drop correction was carried out using the dynamic compensation method [9,12]. The cell used for ozone evolution experiments was either a Nafion® membrane cell described in an earlier paper [12] or a conventional cell. In this case, the current efficiency of ozone formation was determined with an AF-2 UV absorption ozone photometer, by sweeping the anodic gases with argon at a flow of 120 ml min − 1 to satisfy the flow requirements of our analytical instrument. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Perkin–Elmer f 5600ci instrument.
PbO2 samples were electrodeposited onto a Pt wire (S= 1.5 cm2) in 1 M HClO4 containing 0.02 M Pb(II) at 1.75 V over 30 min. Lead and fluoride were added to the solution as Pb(NO3)2 and NaF. Doped PbO2 electrodes obtained from solutions containing 0.01 M and 0.04 M NaF in the growth solution are referred to as F(001)PbO2 and F(004)PbO2, respectively. For the particular case of O3 measurements with a Nafion® membrane cell, PbO2 was electrodeposited onto platinised porous titanium disks having a diameter of 3 cm. All electrodes were pretreated as described by Ho et al. [21] and impedance measurements were taken only after a reproducible Tafel plot was obtained.
3. Results and discussion
3.1. Electrocatalytic acti6ity of F-doped PbO2 Incorporation of fluoride into lead dioxide leads to a change in its electrocatalytic activity which is clearly seen in both the processes of oxygen evolution and O3 production. The current efficiency for O3 formation at F-doped PbO2 anodes was found to be a function of the NaF concentration used in the electrodeposition bath (Fig. 1). These data were obtained using a solid polymer electrolyte (spe) membrane cell (Section 2) at 25°C but the trend was the same as that observed using a conventional gas-tight three compartment cell. It is seen that O3 formation reaches a maximum for FPbO2 electrodes prepared from growing solutions containing 0.01 mol dm − 3 NaF and decreases on further increasing the fluoride amount. This decrease in the current efficiency is accompanied by a rise in the cell potential. Tafel plots of E versus log i, obtained from quasisteady-state polarisation measurements are given in Fig. 2. They agree with our previous data [12] and are also in accord with the observation of Kotz and Stucki [9] that the linear region in the more positive potentials region is subject to correction of the Ohmic drop by the dynamic compensation method. The current was read at each potential after it reached a constant value, which required at least 5 min. The Tafel slopes showed a time dependence, ranging from values close to 120 mV decade − 1 for a waiting time B 2 min to 150 – 160 mV decade − 1 for more than 5 min. This time dependence of Tafel slopes is generally attributed to blocking of active surface sites [22]. For the particular case examined here, the cause may be the adsorption of SO24 − , as pointed by Ruetschi et al. [23]. According to these authors, SO24 − is adsorbed on bPbO2 and contribute to the charge of the double layer. The increase of the double layer charge due to SO24 − ions increases the electrode potential for a given current, or a give number of reacting oxygen species on the surface. In effect, our data shows [24] that, in the Tafel
R. Amadelli et al. / Electrochimica Acta 45 (1999) 713–720
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Fig. 1. Ozone formation at F-doped PbO2 anodes in a solid polymer electrolyte (spe) electrochemical cell: effect of the concentration of fluoride on the O3 current efficiency of and cell potential.
potential region, double layer capacities are higher in sulphuric than in perchloric acid by 40%. It is interesting to point out that these phenomena were considered in a theory of electrode charge transfer processes that Ruetschi formulated in a series of papers some time ago [25–27]. In Fig. 2 we also report plots of the potential versus log(R − 1). In fact, the resistance is [28] R =(E/(i=exp(−bh)/(bi°)
(1)
and log(R − 1) =log(bi°) +bh
(2)
The reaction resistance was obtained from the simulation of the experimental data using the equivalent circuit previously employed by other authors in analogous studies with PbO2 electrodes [21,29] where the adsorption – desorption of intermediates is characterised by the resistance Rp and by the pseudo-capacitance Cp, and Rct is the charge transfer resistance.
Fig. 2. Tafel plots for PbO2 and FPbO2 anodes in 1 M H2SO4 at room temperature. Electrode area: 1.5 cm2. (A) E vs. log i from quasi-steady-state data; (B) E vs. log 1/Rp from impedance data.
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An examination of Fig. 2 shows that there is remarkable agreement between the set of data obtained by the point-by-point quasi-steady-state method and that obtained from impedance measurements using the Rp data. The general trend observed is that Rp is larger than Rct in the whole potential range examined and for all PbO2 electrodes. This is in keeping with the statement of Ho et al. [21] that the mechanism of oxygen evolution on PbO2 is strongly controlled by electrodesorption. The parameters derived from the simulation calculations of impedance data are given in Table 1(a–c) for PbO2, F(001)PbO2 and F(004)PbO2 electrodes. A closer look into these data provides some interesting information that is not evident from an analysis of the Tafel plots of Fig. 2 alone. They reveal, for example, a different effect on Rp and Rct of the concentration of fluoride used to prepare the F-doped oxides. From the comparison of F(001)PbO2 with the undoped oxide, it is clear, despite some scattering of the data, that both Rp and Rct increase; one observes, in addition, that the increase of the former is
more pronounced than that of the latter. On the other hand, with F(004)PbO2 a conspicuous increase in Rp is seen at EB1.8 V, above this value the tendency is reversed (Fig. 2), and Rct now decreases pronouncedly up to 2.0 V (Table 1a,c). The behaviour of F(001)PbO2 is similar to that observed before with PbO2 electrodes in F− containing electrolytes [9,12]. Evidently, it reflects a loss in activity of the surface toward O2 evolution, with both the processes of water discharge and electrodesorption of intermediates being inhibited. On the other hand, the trend observed with F(004)PbO2 is somehow more surprising since, in this case, the Tafel slope, in the more positive potential range, returns to the lower values observed with the unmodified electrodes. While Rp still increases (at least up to 1.8 V) compared to the undoped oxide, Rcr decreases consistently. This behaviour might be due to a change in the reaction mechanism, for example, a different rate determining step, as the opposite effect on Rct and Rp seems to suggest. These changes in the mechanism can possibly be
Table 1 Impedance data for PbO2 and FPbO2 in 1 M H2SO4a Rct
Rp
Cdl×103
(a)
Table 1a b PbO2 1.650 0.76 1.700 0.76 1.725 0.73 1.800 0.703 1.850 0.76 1.900 0.76 2.000 0.84 2.025 0.95
24.30 16.50 18.27 16.10 13.47 12.86 1.43 1.61
891.2 435.6 295 134.8 74.13 56.2 26.5 20.04
1.23 1.042 1.20 1.12 0.85 0.89 0.045 0.041
(0.78) (0.80) (0.77) (0.77) (0.80) (0.79) (1.0) (1.0)
7.22 7.17 5.75 3.57 3.20 2.20 8.95 6.36
(1.0) (0.95) (0.94) (1.0) (1.0) (1.0) (0.84) (0.76)
Table 1b cF(001) PbO2 1.65 1.06 1.70 1.06 1.80 1.07 1.90 1.1 2.00 0.51 2.05 0.54
24.86 24.1 23.12 8.6 6.8 2.3
998 602.5 239.8 89 33 14.12
0.98 0.97 0.95 0.526 0.574 0.074
(0.85) (0.85) (0.85) (0.91) (0.83) (1.0)
6.20 4.93 1.60 1.62 0.506 0.70
(0.87) (0.89) (1.0) (1.0) (1.0) (1.0)
Table 1c d F(004) PbO2 1.650 1.14 1.700 1.12 1.800 1.4 1.900 1.43 1.975 0.68 2.000 0.68 2.025 0.65
12.63 9.94 8.25 4.41 0.92 0.9 1.0
2,165 700 167.34 58.8 28.2 21 15.8
1.80 1.38 1.38 1.40 0.104 0.076 0.081
(0.78) (0.79) (0.77) (0.80) (1.0) (1.0) (1.0)
E (V)
Rs
Cp×104
12.5 13.0 7.75 6.0 11.3 9.95 8.85
(a)
(0.87) (0.89) (0.94) (0.89) (0.77) (0.80) (0.80)
a Electrode areas: 1.5 cm2. Room temperature. Potentials vs. SCE; resistances in ohms and capacities in Faradays. a represents a constant phase element introduced in the simulations. b PbO2 electrodeposited in the absence of F−. c PbO2 electrodeposited in the presence of 0.01 M F−. d PbO2 electrodeposited in the presence of 0.04 M F−.
R. Amadelli et al. / Electrochimica Acta 45 (1999) 713–720
caused by consistent changes in the surface and interface structure involving not only surface fluorine and oxygen species but also variations in the above mentioned adsorption–desorption of the sulphate anions of the electrolyte. The pseudo-capacitances Cp, given in Table 1(a–c), are plotted for convenience in Fig. 3. Probably, the most interesting thing to note is the behaviour of the capacitance in the high potential range where the processes leading to ozone formation become important. A completely different behaviour is seen for PbO2 and F(004)PbO2 on the one side and F(001)PbO2 on the other. In the first case, Cp increases at E \1.9 V and reaches a peak around 2.0 V; in the second case it decreases continuously and no peak is seen. In the latter case, one can also notice that the double layer capacity Cdl has correspondingly higher values. The observed Cp peak is a characteristic feature of the sulphuric acid medium [24] and must then originate from processes involving SO24 − . We have already mentioned above the possibility of the involvement of adsorbed SO24 − leading to changes in the double layer. At more anodic potentials, however, SO24 − can participate in Faradaic reactions yielding S2O28 − according to: 2SO24 − S2O28 − +2e −
(3)
or 2HSO4− S2O28 − +2H+ +2e −
(4)
whose standard potentials are 1.77 and 1.88 V (SCE), respectively [30]. These reactions have been extensively
Fig. 3. Plots of the pseudo-capacitance Cp vs. potential for: (a) PbO2; (b) PbO2 electrodeposited in the presence of 0.01 M NaF; (c) PbO2 electrodeposited in the presence of 0.04 M NaF. Electrode area: 1.5 cm2. Room temperature.
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Fig. 4. Cluster models used for MNDO calculations on the effects of oxygen substitution with fluorine.
studied at PbO2 [10], DSA-type [3 – 5] and Pt [2,31 – 33] electrodes, and are known to be favoured in the presence of relatively high amounts of F− in the electrolyte. One possible explanation of the experimental data can then be that low doping fluorine amounts reduce the rate of O2 evolution and persulphate formation, providing the best conditions for O3 production. As the PbO2 doping is carried out in the presence of higher F− concentrations, the formation of persulphate is favoured and the O3 yield decreases again. We did not check on the presence of S2O28 − , also because the concentration of sulphuric acid we used was rather low for these kind of studies. However, in a study on persulphate production on b-PbO2 electrodes in 1.5 M H2SO4 + 2.3 M (NH4)2SO4, Scheler and Wabner [10] mention analogous effects of the increase of F− in the solution as we discussed above, i.e. (i) a decrease in the O3 yield versus an increase in S2O28 − and (ii) a decrease and subsequent increase in the double layer capacitance. Thus we have reason to believe that the promoting effect of high fluoride concentration with respect to the discharge of SO24 − , can account for the trend in O3 formation reported in Fig. 1 of this work.
3.2. X-ray photoelectron spectroscopy and modified neglect of diatomic differential o6erlap calculations for the interaction of fluorine with a PbO2 surface The Tafel plots for oxygen evolution shift to more positive potentials in the case of F-doped PbO2 (Fig. 2),
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Table 2 Results of MNDO calculations using the cluster models given in Fig. 4 and showing the effect of fluorine substitution on the electronic property of the system Charge
Pb3(OH)12(H2O)2 Pb3(OH)10F2(H2O)2 -(Cluster I) -(Cluster II)
QPba (e) QO(OH) (e) QO(H2O) (e)
+0.72 −0.20}−0.52 −0.17
a
+0.71 −0.20}−0.54 −0.17
Charge of the central Pb.
Table 3 Results of MNDO calculations using the cluster models given in Fig. 4 and showing the effect of fluorine substitution on the energy parameters of for O2 evolution assuming the ‘oxide path’ mechanism Step
D(DH), (kcal mol−1) Pb3(OH)12(H2O)2 (Cluster I)
163.8 S(OH−)2 S(OH)2+2e S(OH)2 170.3 S(O)2+2H++2e
Pb3(OH)10F2(H2O)2 (Cluster II) 167.8 178.0
intermediates. According to XPS data for PbO2, which was deposited from perchloric or nitric acid solutions in the presence of 0.02 M F−, the fluoride content in the oxide bulk was about 4 at.%. For the conditions that give a uniform distribution of fluoride within the PbO2 layer with UOH + UF = 1, the average surface coverage by fluoride was calculated to be about 6%. Using the MNDO method, we tried to estimate the fluoride influence on the geometric and electronic factors of the modified PbO2 surface and on the metal – oxygen intermediate bond strength on the process of O2 evolution. As a model of the PbO2 surface we adopted the structure indicated as Cluster I (Fig. 4). We further assume that the oxygen evolution kinetics are described by the so-called electrochemical oxide path mechanism. Two adjoining oxygen species take part in the oxygen evolution process (Fig. 4, boldface lines in the Cluster I). In our simulation of fluoride modified PbO2, two OH− ions are replaced by two fluoride ions (Fig. 4, Cluster II). In this case we expect the maximum influence of fluoride on the system parameters. Actually, the calculation showed that the influence of fluoride on electronic system properties is essentially negligible (Table 2). The O2 formation, for the Cluster I model, may be presented as a two-steps process: Pb3(OH)12(H2O)2 Pb3(OH)12(OH − )2 + 2H+ Pb3(OH)12(OH)2 + 2H+ + 2e
indicating an inhibition of this reaction. In principle, this might be due to surface blocking by fluoride (decrease of the active site concentration) with a consequent change in the bond energy of the oxygen
Pb3(OH)12(O)2 + 2H+ + 2e Pb3(OH)12O2 Pb3(OH)12(. . .)+O2
Fig. 5. X-ray photoelectron spectra of O 1s for (a) PbO2; (b) PbO2 electrodeposited in the presence of 0.01 M NaF in an unstirred solution; (c) PbO2 electrodeposited in the presence of 0.01 M NaF with stirring. Binding energies are given vs. C1s taken as 285 eV.
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Calculations for Cluster I and II demonstrated that fluoride shows no influence on the energy parameters for evolution of O2 from H2O (Table 3). Thus, within the limitations of the model, the relatively high surface concentration of fluoride versus its negligible influence on the electronic and the energy properties of the studied system, seems to suggest that the effect of the fluoride modification is mainly that of changing the surface coverage of adsorbed oxygen species. This is clearly a simplified model which does not take into account, for example, the above mentioned role that specifically adsorbed sulphate anions seem to play in determining the reactivity of intermediate oxygen species. However, despite the limitations, the above conclusion on the importance of changes in the surface coverage of oxygen species is supported by the XPS data. In Fig. 5 the XPS spectra in the O 1s region shows that F-doping causes a marked decrease of the broad signal at the higher binding energies which attributed to adsorbed OH and H2O [34]. Conversely, neither the position of the peak at lower binding energies, assigned to oxide ions within the structure, nor that of the Pb4f undergo major changes, and the difference, D(O 1s − Pb 4f7/2) [34] is 392 9 0.25 eV for both PbO2 and F-doped PbO2 samples. While more work is needed, the conclusion that the present data seems to bring out is that fluorine affects the weakly bound oxygen species, i.e. those ultimately involved in the O2 evolution process.
4. Conclusions In this research work we examined the electrocatalytic behaviour of fluorine-doped PbO2 electrodes in the processes of O2 and O3 evolution. By comparison with unmodified PbO2, F-doping shifts the O2 evolution process to higher potentials for a given current, for the higher F-doping levels, however, this increase is seen mainly at lower currents. On the other hand, as the concentration of F− in the PbO2 growth solution is increased, the current efficiency of O3 formation first rises and then declines to lower values. Investigations were focused on electrodes prepared from 0.01 M NaF (F(001)PbO2) and from 0.04 M NaF (F(004)PbO2). By comparison with the undoped oxide, the activity for O3 formation of the first was higher and that of the latter lower. We show that there is agreement between the results of quasi-steady-state and impedance measurements. In addition, from the results of impedance data analysis, based on an equivalent circuit with components that describe a charge transfer process producing adsorbed species, we can distinguish an effect of doping on charge transfer (resistance Rct) from that on the ad-
719
sorbed intermediates (Rp). With respect to PbO2, both parameters increase for F(001)PbO2 while for F(004)PbO2 Rp increases and Rct decreases, reflecting a change to an O2 evolution mechanism controlled by the electrodesorption of intermediates. Sulphate anions have a relevant role in the anodic processes examined. In the less positive potential region their specific adsorption would affect O2 evolution through screening charge effects on oxygen intermediates, at higher potentials their discharge is not only thermodynamically possible but, in addition, it is known from literature to be enhanced by increasing amounts of fluoride. An analysis of the results of MNDO calculations on Pb cluster models and of XPS data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping.
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