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potential curves for platinum
Electrochimica Acta. 1967. Vol. 12. pp. 1457 to 1469. Pergamon Press Ltd. Printed in Northern Ireland
ON SOME DEBATED ASPECTS OF THE CAPACITANCE/ POTENTIAL CURVES FOR PLATINUM* Laboratory
L. FORMARO and S. TRASATTI of Electrochemistry and Metallurgy, University of Milan, Milan, Italy
Abstract-Two points are discussed here: (a) the actual value of the capacitance in the potential range where no adsorption of hydrogen or oxygen takes place; (b) the general shape of the capacitance curves in the oxygen-adsorption region. Resultsin the literature can be explained by taking into account the time elapsing between electrode cleaning and capacitance reading. The effect of impurities is studied and discussed by making use of two solutions at different purity level. Only within a narrow range of anodic potentials from 400 to 500 mV (he) are platinum electrodes free of adsorbed hydrogen and oxygen; at this potential the capacitance value is 34 pF/cma (real surface area). For E > 400 mV, capacitance falls with time; the possible reasons for this phenomenon are examined and discussed. The capacitance curves obtained by the technique adopted in the present work are compared with steady-state curves. R&um&-On discute deux points: (a) la valeur reelle de la capacitC dans le domaine de potentiel oti il n’y a ni hydrog&ne ni oxyg&ne adsorbes; (b) la forme generale des courbes de capacit6 dans le domaine de l’adsorption d’oxygkne. On montre que les r&.ultats trouvables en litterature peuvent dtre expliques en considtrant le temps qui s’&coule entre l’activation de 1’6lectrode et la mesure de la capacit6. On discute l’effet des impure& en employant dew solutions avec de diffkrents degrCs d’impuret6s. Seulement autour de 400-500 mV (he) l’blectrode de platine est libre d’hydrogkne ou d’oxygkne adsorb&; il ce potentiel-ill la capacitC a une valeur de 34 pF/crn* (r&f&&e il la surface r&lle). Pour des potentiels > 400 mV la capacit6 d&-o& dans le temps, et l’on discute les causes possibles de ce phenom&ne. Enfin l’on compare et analyse les courbes de capacite obtenues en condition stationnaire et non-stationnaire. Zusammenfaw-In der vorliegenden Arbeit werden zwei Punkte diskutiert: (a) der wirkliche Wert der Kapazitgt impotentialbereich, in dem keinewasserstoffoder Sauerstoff-Adsorption erfolgt ; (b) die allgemeine Form der Kurven im Sauerstoffadsorptionsbereich. Es wird gezeigt, dass die in der Literatur auffmdbaren Ergebnisse gedeutet werden kSnnen, wem die zwischen der Elektroden-Reinigung und der KapazitBtsmessung abgelaufene Zeit beriicksichtigt wird. Die Wirkung von Verunreinigungen wurde durch die Verwendung von zwei verschieden reinen LBsungen untersucht. Die Platinelektrode ist frei von adsorbiertem Wasserstoff und Sauerstoff erst in einem eingebei diesem Potential betrlgt die Kapazitlt schr&kten Bereich von 400-500mV (WE) aufwlrts; 34 pF/cma (unter Bezugnahme auf die reale Fllche). Bei Potentialen oberhalb 400 mV nimmt die Kapazitlt mit der Zeit ab; die Urachen von derartigen Phlnomenen werden diskutiert. Schliesslich werden die durch instationlre Messungen erlangten Ergebnisse mit denjenigen in stationken Zustand verglichen. INTRODUCTION
STUDIES on the differential capacitance of platinum electrodes were numerous in past times1 but in the most recent years they have been less frequent; furthermore, in the measurements are few recent papers2s3 where this subject is dealt with, capacitance used only to support conclusions derived by other experimental methods. A probable reason for this is that the field has appeared less fruitful than expected, for poor agreement is normally observed between the experimental data of different authors, which leads to a difficult interpretation of measurements in terms of double layer structure. Whilst in the case of mercury the knowledge of the double layer l Manuscript received 3 December 1966. 1457 6
1458
L. FORMARO and S. TRASA~
structure is in an advanced stage, in the case of platinum even the correct experimental procedure is not yet set up. Attempts to explain platinum capacitance by comparison with the behaviour of mercur>A led to conclusions not so evident as to be conclusive. The determination of the potential of zero charge on platinum, by exploiting the relationship found on mercury between the pzc and the potential of maximum adsorption for uncharged organic molecules2*6 must be considered in the light of Frumkin’s point,s that the platinum behaviour is strongly affected by the adsorption of hydrogen and oxygen. Several authors did not refer their capacitance values to electrode real surface areas, and this has also contributed to discrepant experimental data, because the roughness factor of a platinum electrode depends on several factors (material manufacture, electrode preparation, electrode treatment etc). Leaving out the theoretical interpretation of capacitance data, we shall deal in particular with the results as regards experimental technique. From published work one sees that two points are debated: (a) the actual value of the platinum capacitance in the potential range where no adsorption of hydrogen or oxygen takes place; (b) the general shape of the capacitance/potential curves in the oxygen-adsorption region. The results of the various authors are in agreement in the hydrogen adsorption region; here, the double layer capacitance is masked’ by a very high faradaic capacitance due to H+ + e z Hads. Data at present available on the two points under discussion are the following: (1) The actual value of platinum capacitance without hydrogen or oxygen adsorbed on the surface has hitherto been considered to be 18 pF/cm2,8*11and this value was even recommended4 and used2 for the determination of the real surface area of platinum. However, Gilman12 proved that a platinum electrode in a solution has the surface partly covered with organic impurities contained in the solution itself’, and concluded that the value of 18 pF/cm2 is too small, and due to the poisoning of the electrode surface. On the basis of these results, TrasattP carried out capacitance measurements, examining in particular the effect of the time elapsed after electrode surface cleaning; he found that the capacitance falls with time and that the decrease rate depends on stirring and temperature, the whole process being controlled by mass transfer in the solution. (2) Some authorss*Q.14stated that in the oxygen-adsorption region the platinum capacitance rises nearly proportionally to the amount of adsorbed oxygen. Gilman12 and BreiteP showed on the contrary that platinum capacitance rises steeply from 400 mV to 800 mV (he) and then drops when the surface commences to oxidize. In Gihnan’s and Breiter’s opinion the impurity effects account for the behaviour observed by others. However, TrasattP observed that at potentials ranging between 600 and 1000 mV (he) the decrease in capacitance, just after electrode cleaning, is much more marked than one could expect only from the impurity effect; he suggested that a slow rearrangement of the surface state takes place in this potential range. Topics (a) and (b), briefly considered by Trasatti in his previous paper,is are taken up and investigated in the present work, by using two solutions of different purity in respect of organic pollution. Thus, a clear separation of impurity effects from any other surface phenomenon is possible.
Capacitance/potential curves for platinum EXPERIMENTAL
1459
TECHNIQUE
Generalities All measurements were made in 1 M HClO, solutions prepared by dilution of analysed reagent-grade 60% perchloric acid. Experiments were carried out at 2X, temperature beingregulated to within fO*l”C by means of an air-thermostat. 99.999 % nitrogen (with traces of rare gases) was used when necessary. The gas line was made up with either copper or glass or Teflon, plastic and rubber tubes being excluded. No grease was used in any part of the apparatus employed for these experiments. The bridge for capacitance measurements has been described.13 Test electrode The electrode was prepared by melting the end of a 99.98 % pure platinum wire to a small sphere in a Hs-0, flame and then sealing the wire into a soft glass tubing. The electrode was then introduced into a vacuum furnace and kept at 500°C for 3 h to anneal the metal and eliminate all gases absorbed in it. Before each experiment the platinum bead and the glass tubing were cleaned in cont. H$O,, then washed and boiled in triply distilled water. All capacitance values are reported on the basis of the real surface area of the electrode (O-0493 cm2) as determined by cathodic charging curves.16 Electrode potentials were measured using a type 610-A Keithley electrometer and are referred to the hydrogen electrode (he) in the same 1 M HClO, solution, as described by Bianchi et a1.l’ Solutions Solutions were prepared using triply distilled water, the last distillation being made from alkaline permanganate in a nitrogen atmosphere. The resistivity of the water was 2 x lo6 s2 cm, measured in air. Since the main purpose of the present work was to separate impurity effects from any other surface phenomena, two kinds of solution were used, the content in organic impurities being controlled and defined by cathodic charging curvePr (see later). (i) Solutions “A”. The 1 M HClO, solutions were pre-electrolysed in a separate vessel under a nitrogen stream between platinized platinum electrodes for at least 24 h at a current density of about 0.1 mA/cm2 (referred to real surface). The solutions were then introduced into the test cell for experiments. The impurity contents of the solutions “A” were defined as about 2.3 per cent.ls (ii) Solutions “B”. Since the main cause of the presence of impurities in solutions in the contact between these and the atmosphere,ls solutions “A” were introduced into the cell and pre-electrolysed again for 20 h. The pre-electrolysis was carried out at controlled potential, not at controlled current. The two electrodes used for this purpose were kept at about 60 mV and 1500 mV, respectively, by means of a potentiometer directly connected to them. The voltage drop across the potentiometer divided between the two electrodes so as to fulfill the equality between the rates of anodic and cathodic processes. The cell voltage can thus be kept at such a value as water does not decompose; since in these conditions any detectable evolution of oxygen as well as hydrogen is quite absent, any nitrogen flowing through solutions, which may cause further contaminations, can be avoided. For higher effectiveness and rapidity of the pre-electrolysis, the polarites of the two electrodes were reversed every 30 s with
1460
L. FORMARO and S. TRASAW
a relay; in this way, alternately, the electrodes oxidized anodically the impurities adsorbed during the cathodic working. The aim of such a pre-electrolysis was exclusively to oxidize organic impurities; also, since no detectable evolution of oxygen took place at the anode, residual oxygen in solution was completely reduced at the cathode. The impurity content of solutions “B” was found to be go.7 % (see later). During the course of the pre-electrolysis, solutions were stirred by means of a magnetic stirrer. Since our experiments were carried out in differently impure solutions and the impurity contents were scarcely reproducible and could not be corrected at all, it was necessary to perform a single run of measurements in the solution “B”, whilst a certain amount of the solution “A” was stored under nitrogen atmosphere. Cells The cell previously13 described was used to perform experiments in solutions “A”. These were degassed in the cell for 0.5 h before each experiment, using a nitrogen stream. A closely similar cell was used for experiments in solutions “B”, but in this case the auxiliary electrode for alternating current consisted of two semi-cylindricalshaped platinum sheets which were employed as pre-electrolysis electrodes. At the end of the pre-electrolysis, the two platinum electrodes were connected to the same point of the impedance bridge to form the auxiliary single electrode. The solutions were carefully degassed by nitrogen before the pre-electrolysis in this case as well. During the pre-electrolysis and successive measurements the cell was kept hermetically sealed by means of Teflon stopcocks. Electrode pre-treatment
A platinum electrode, kept either in the air or in a solution for a long time, is partly covered with impurities. 12*13Therefore, it is necessary to remove them to get a clean surface and reproducible as well as reliable capacitance values. Before each measurement the electrode surface was cleaned by the potentiostatic sequence shown in Fig. 1. Such a kind of pre-treatment was first introduced by Gilman.ls The rationale behind the various potential steps is as follows. Anodization at 1500 mV (he) removes impurities largely and forms a layer of adsorbed or combined oxygen. Solutions were kept stirred so that traces of molecular oxygen anodically evolved and residuals of the impurity oxidation could be swept away. At 1200 mV the oxygen layer is retained13 and hinders re-adsorption of impurities. Solutions were kept stirred for 20 s so as to restore normal conditions of concentration at the interface. Solutions “A” were stirred by gas bubbling and solutions “B” by a magnetic stirrer. Final results were found to be not affected by using either stirring. Gas or stirrer was then turned off to allow solutions to become quiescent, so as to get reproducible conditions in the solution side as well. At 60 mV the oxygen layer is reduced within the first few ms. The duration of this step was chosen as 2 s, a sufficiently long time for any oxygen adsorbedlD to be reduced, and a sufficiently short one for successive measurements to be unaffected by any hydrogen adsorption.20 At the potential E each capacitance measurement was made after the potentiostat (type-557 Amel) had been disconnected by switching to a potentiometric circuit, which allowed ac to be superimposed. The duration of this step is afterwards denoted as t and the main
Capacitance/potential
*With
curves for platinum
1461
stirring
FIG. 1. Potential/time
sequence used to clean the electrode.
purpose of this work was to investigate the influence of the time of measurement on the capacitance value. RESULTS
The capacitance/potential curves obtained by the above technique appear in Fig. 2. Curves b and c were obtained with solutions “B” with t = 30 s and t = 3 min, respectively. The electrode is still impurity-free within this time period (see later). Curve a was obtained by Gilman12 by the poten ti a 1-ste p method with t = 1 s using a similar pre-treatment. Curve d was obtained in solutions “A” under steady conditions, ie measuring the capacitance only after its variation with time became negligible. x I =30 This
work
120 o Gilman” 2 %
A I ‘3
{.
s (S0l.B) min
(Sol.B)
Steady-stale /=
(Sol.
A)
Is
100 -
I 0.3
I 0.5
Electrode
I 0.7
I 0.9
potential,
I I.1
I I.3
I I.5
V (rhe)
FIG. 2. Capacitance/potential
curves obtained as indicated. 5 KHz.
At 400 mV (he) curves a, b and c merge and the capacitance has a value of about 34 ,uF/cm2. At potentials ranging between 500 and 1000 mV (he) the capacitance drops with time, so that b and c are below a. However, we observed that curve b was slightly displaced upward in this potential range when measurements at t = 30 s were taken in rapid succession without keeping the electrode at the potential E for a too long time between two successive measurements. The above drop in capacitance appears in any case. On the contrary, at 400 mV (he) the duration of previous measurements had no effect on the capacitance value. For E > 1000 mV (he) as well,
L. FORMAROand S. T~~sarrr
1462
the capacitance was relatively little affected in this sense; curves a, b and c are considerably closer to one another. As regards curve d, it is remarkably lower than curve c throughout the potential range investigated and tends to merge with it only at E = 1500 mV (he). The capacitance variation with time in solutions “A” is shown in Fig. 3. At 400 mV (he) the capacitance behaviour just after pre-treatment is markedly different from that at any other potential. At 500 mV an initial decrease in capacity can
1
I 1 I 12345
I
I
I
I
I
7
9
I2
Time,
16
min
FIG. 3. Time-dependence of capacitance at the various potentials. Solutions “A”. 5 KHz.
indeed be seen. At 700 and 1000 mV the decrease is still marked, yet the more positive the potential, the less marked is the decrease. A comparison between solutions “A” and “B” in respect of the phenomenon of decrease in capacitance with time is shown in Figs. 4-8. At 400 mV (he), curves merge at t = 30 s, but for longer times they are very divergent ; capacitance decreases much more in solutions “A” (less pure) and tends to a value as low as 17 pF/cm* (curve d, Fig. 2). The capacitance at 400 mV appears to be constant after 2 min in solutions “B”; it decreases less than one per cent in the third minute. At 500 mV (he) (Fig. 5) the behaviour is very similar to that above for long times, but just after pre-treatment a marked fall in capacitance can be seen, similar in both kinds of solution. At 800 mV (he) (Fig. 6), the curves are even more similar. There is an initial large fall in capacitance in both solutions; for intermediate times the curve obtained in solutions “B” remains slightIy above the other one, but they merge for very long
1463
Capacitance/potential curves for platinum
II
I
2
I
3
1 5I
4
I
I
7
9
Time,
I
12
I
16
min
FIG. 4. Time-dependence of capacitance at 400 mV (he).
I I I I I 12345
I
I
7
9
Time, FIG.
I
12
I
I6
min
5. Time-dependence of capacitance at 500 mV (he).
times. This last fact may be related to a lower starting value of capacitance for the curve in solutions “B”. At potentials where a large fall in capacitance occurs, capacitance values were in fact found to be reproducible within 5 per cent; at 400 mV, on the contrary, the reproducibility was better than one per cent. At 1000 mV (he) (Fig. 7) the difference in shape between curves “A” and 73” is even less marked. At 1300 mV (Fig. 8) the two curves are practically parallel; an equal starting value of capacitance would cause the two curves to merge.
L. FORMAROand S. TRASA~-~I
1464
min
Time,
FIG. 6. Time-dependence of capacitance at 800 mV (he).
I I I 12345
I
I
I
7
Time,
I
9
I 12
I
16
min
FIG. 7. Time-dependence of capacitance at 1000 mV (he). DISCUSSION
The purpose of this work was to examine (a) the actual value of platinum capacitance in the potential range where no adsorption of hydrogen or oxygen takes place (b) the general shape of the curves in the oxygen-adsorption region. The two points will be separately discussed, though for the discussion of each reference to the other must be made.
1465
Capacitance/potential curves for platinum
In the following discussion the time t is chiefly taken into consideration, and we attempt to show that a different duration of the electrode at the potential of measurement can account for the observed disagreement between the previous results. Actid
value of capacitance
The general shape of the curves in Fig. 2 suggests that the electrode is free of either hydrogen or oxygen species only in a very narrow range of potential from
I I I 12345
I
I
I
I
I
I
7
9
12
I6
Time,
min
FIG. 8. Time-dependence of capacitance at 1300 mV (he).
400 mV (he). At E > 400 mV the initial fall in capacitance will be later shown to be probably due to a surface rearrangement with respect to adsorbed oxygenated species. At 400 mV (Fig. 4) the capacitance remains constant as long as the electrode surface remains clean; the purer the solution, the longer the time during which capacitance does not vary. This fact shows that after pre-treatment the electrode surface is thoroughly clean and the successive variations, at this potential, are not due to rearrangements connected with the previous pre-treatments. Our results show that just after pre-treatment, the electrode reaches a steady capacitance at 400 mV. The subsequent decrease in capacitance is due to impurities reaching the electrode surface at a rate depending on their concentrations; the purer the solution, the less marked the decrease in capacitance, so that the steady value is a function of the purity of the solution. This suggests also that a quasi-equilibrium exists between adsorbed impurities and those in solution, as in the case of most common organic substances.2*5*21A value of 17 ,uF/cm2 for capacitance was found in solutions “A” in agreement with those obtained by other authors4*8-11J4 working under steady conditions; this is the capacitance of a platinum electrode the surface of which is partly covered with impurities. The agreement on this value of the results of various authors
around
1466
L. FORMARO and S. T~~s~rrr
indicates that the solution purity was approximately the same in all cases. A higher purity degree may lead to values a little higher,s but steady-state measurements are entirely reliable only when made in absolutely pure solutions in which the platinum capacitance must remain indefinitely constant at 400 mV (he) after pre-treatment. The constancy of the capacitance at 400 mV (he) proves the double layer structure to be not affected by pre-treatment or, more probably, the pre-treatment produces so labile a modification that it disappears within a few ms.12 Capacitance measurements performed by the above technique are fully reliable for real surface area determinations,16 since the whole electrode surface is in the same condition. Capacitance measurements on mercury show why the true capacitance of platinum can be determined only just after electrode pre-treatment. On mercury the capacitance is measured with a dropping electrode precisely to avoid impurity effects, as shown by Proskurnin and Frumkin. 22 With platinum it is obviously not possible to work in the same manner, but it is reasonable to choose a pre-treatment which, in a sense, can create a new surface for each measurement. TABLE 1. CAPACITANCEAND HYDROOEN ADSORPTIONAS A FUNCTION OF ELECTRODEPOLARIZATION TIME IAT 4OOmV(he)
t S
Solutions “A”
Solutions “B”
c
,uF/cma
-
223 223 223 223 223 222 219 218 212
-
223 223 223 223 223 223 223 223 221.5 212
-
33.2 32.5 31.6 29.8
0.3 1.0 1.9 3.7
5 10 20 30 45 60 90 120 180 600
33.5 33.5 33.5 33.5 33-l 30.0
0 0 0 0.4 3.5
* qa,* is 223 pC/cnP,
AH
&/cm2
5 10 20 30 45 60 90 120 180
33.5 -
4a
AC
pc Pcm= 0 0 0 0 1 4 5 11 0 0 0 0 0 0 0 1.5 11
AC/C
Aqa/qa,s*
-
-
0009 0.030 0.057 0.110
0 0 0 0 0.005 0018 0.023 0.050
-
-
0 0 0 0.012 @104
0 0 0 0 0 0 0 0.007 0.050
the value for monolayer hydrogen adsorption.
When measuring capacitance for qualitative purposes, even steady-state measurements may be performed, as shown elsewhere,21 but the results must always be examined in the light of possible poisoning of the electrode surface. Our results and views are in full agreement with Gilman’s.12 At 400 mV (he) our capacitance and Gilman’s have practically the same value, ca 34 pF/cm2. This shows the effectiveness of pre-treatment and supports the validity of the charging-curve method16 for the determination of the real surface area of the electrode. To display fully the results obtained, a comparison is presented in Table 1 between the rate of decrease in capacitance at 400 mV (he) and the rate of decrease in qH, qH denoting the charge consumed during a cathodic charging curve to cover every available site with one hydrogen atom. Experimental details for determining qH are
Capacitance/potential curves for platinum
1467
described elsewhere.13 The drop in qH (AqH) is directly proportional to the fraction of surface covered with adsorbed impurities. Table 1 shows that the electrode remains clean after pre-treatment for about 45 s in solutions “A” and for 25-3 min in solutions “B”. Also, AC is, in per cent, about twice Aqn, in agreement with Gilman’s findings, l2 for not very high coverages, namely at not too long times. The Table cotirms that cathodic charging curves can be used to define conventionally the purity of a solution, as regards organic contamination, through the percentage of electrode surface covered by impurities after a chosen time.ls Oxygen adrorption region For E > 400 mV (he) our results are also in agreement with Gilman’s,12 but only qualitatively, which suggests that in this potential range the time t plays an important role. We think Gilman found such high capacitance values (Fig. 2) mainly because he performed his measurements at very short times. Figure 6 clearly shows that the capacitance decreases with time at 800 mV (he), independent of impurity effects, since such a decrease can also be observed in solutions “B” where the electrode remains clean for about 3 min after pre-treatment (Table 1). Gilman also observed a marked decrease in capacitance within the first few ms, but this decrease was no longer detected between 100 ms and 1 s. In our opinion, the decrease rate may be too low at these times to be detected over such a short time interval. On the other hand, the above discrepancies do not seem to be due to different frequency effects when measuring capacitance, since we worked with a bridge at 5 KHz and Gihnan employed the potential-step method with double layer charging times corresponding to an ac frequency of 5-50 KHz. Nevertheless, the possibility of an additional faradaic capacity may be taken into consideration. In the very potential range where a maximum in capacitance is observed, 750-950 mV (he), platinum shows, according to Breiter,ls a faradaic capacitance due to Pt + H,O e Pt-OH
+ H+.
The amount of the faradaic capacitance may be dependent on the amplitude of potential step. Gihnan employed a negative pulse of 6 mV; we worked with an alternating voltage across the cell never exceeding 5 mV (peak to peak). This may also partly account for the high capacitance values found by Gilman. Finally, we must bear in mind the above-mentioned slight ageing-effect of the electrode in this very potential range, consisting in a lowering in capacitance values when the pre-treatment is preceded by a lengthened polarization at not very positive potentials. This effect is not due to impurities, because it is absent at 400 mV. It must be, therefore, a surface phenomenon of platinum, and it affects the reaction of formation of oxygenated species. Thus our results agree qualitatively with Gilman’s, but they show that platinum capacitance, over a wide potential range where surface oxidation occurs, must be examined in the light of both the type of measurement and the time elapsed between pre-treatment and measurement. The fact that the decrease in capacitance appears, more or less strikingly, over the whole potential range from 500 to 1500 mV (he), leads us to believe it to be connected with the presence of oxygenated species on the electrode surface; moreover, it suggests that formation of such species, most probably adsorbed OH radicals, takes
1468
L. FORMAROand S. TRASAI-H
place even at 500 mV. Hence, the platinum electrode is certainly free of hydrogen or oxygen on the surface only in a really narrow potential range between 400 and 500 mV (he). The present results do not lead to a full explanation of what occurs on the platinum surface just after pre-treatment ; however, we may suggest probable reasons for the observed phenomena. The steep rise in capacitance with increase of potential in the range 400-800 mV is attributed by Gilman12 to a change in the structure of the double layer. The fall in capacitance with time cannot, however, be explained in this way because ionic equilibrium in the double layer is reached rapidly. We think that the formation, by the pre-treatment, of an unstable surface layer of atoms and their subsequent recrystallization23 is also to be excluded, because such a phenomen should affect the capacitance at 400 mV as well. Popat and Hackerman observed a similar decrease in capacitance, which they explained on the basis of slowness of ion adsorption-desorption processes. Ion adsorption can actually lead to a decrease in capacitance in the potential range referred to,15 but in our case other considerations seem to exclude that this may be the main reason. In fact, sulphate ions were found to adsorb on platinum beginning from O-17V;24 even considering that ClO,- ions adsorb on platinum less strongly than SO%,15 we conclude that a decrease in capacitance should be observed at 400 mV as well, if ion adsorption is the real cause. We prefer to explain the decrease in capacitance with time on the basis of a rearrangement of the electrode surface state, which might be due either to a variation in surface concentration of oxygenated species or, more likely, to a modification in the nature of the platinum-oxygen bond (a strengthening).25 Finally, the phenomenon of the decrease in capacitance enables us to re-consider to a some extent the steady-state measurements as regards the oxygen adsorption region. In Fig. 2, curve d, obtained with steady-state measurements and resembling the data reported by Laitinen and Enke,9 is the lowest. This curve was re-interpreted by Gilman12 exclusively on the basis of impurity effects. However, our results show that this effect is determinin g for E < 800 mV, but no longer for E > 800 mV, even though it may still slightly affect the results. In fact, Figs. 5-8 show the decrease in capacitance with time to be practically independent of the impurity level in the solution even at 800 mV. At 1300 mV, impurity level is quite without influence. This is not surprising, if we bear in mind that all the most common organic substances commence to be really oxidized on platinum at 600 mV.21 If the greater part of impurities consist of organic substances of low carbon atomicity, as is to be expected after distillation from permanganate and pre-electrolysis, which both may rupture large molecules without complete oxidation to C02, it is reasonable to expect that the adsorption of such impurities is rather difficult at sufficiently positive potentials. However, the bigger and more difficultly oxidable impurities may still affect results at a long time even at rather high positive potentials. Also, from Fig. 2 one can observe that curve c, obtained at t = 3 min and then without impurity effect, is an evolution of curve b towards the steady-state curve d, at least for potentials higher than 800 mV. In conclusion, the results show that the actual capacitance of platinum at E > 800 mV (he) is approached by steady-state measurements, but slight poisoning effects due to impurities should always be considered.
Capacitance/potential Acknowledgements-This Italy.
curves for platinum
1469
research was sponsored by the Consiglio Nazionale delle Ricerche, Rome, REFERENCES
.I. electrochem. Sot. 111, 114 (1964). 1. M. C. BANTA and N. HACKERMAN, 2. 3. 4. 5. 6. 7. I10: 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
E. GILEADI,B. T. RUBINand J. O’M. BOCKRIS, J. phys. Chem. 69, 3335 (1965). W. H~LAND, E. G~LEADIand J. O’M. BOCKRIS.J. ahvs. Chem. 70. 1207 (1966). . P. V. POPATand N. HACKERMAN, J.&x Chek. 82,‘1198 (1958).* E. G~LEADI,J. electroanal. Chem. 11, 137 (1966). A. N. FRUMKIN, Dokl. Akad. Nauk SSSR 154,1432 (1964). M. Basrrsa, H. KAMMERMAIER and C. A. KNORR,Z. Elektrochem. 60, 37 (1956). J. P. HOARE,Nature, Lond. 204,71 (1964). H. A. LAU~NENand C. G. ENKE,J. electrochem. Sot. 107,773 (1960). W. D. ROBERTSON, .I. electrochem. Sot. 100,194 (1953). J. N. SARMOUSAKIS and M. J. PRAGER.J. electrochem. Sot. 104.454 11957). , , , S. GILMAN,Electrochim. Acta 9, 1025 11964). S. Ta~s~rr~, Electrochim. Metall. 1,267 (1966). J. LLOPISand F. COMM, An. R. Sot. esp. Fis. Quim. 524,233 (1956). M. W. BREITER, J. electroanal. Chem. 7, 38 (1964). S. TRASATTI, Electrochim. Metall., submitted for publication. G. BIANCHI,A. BAROSI,G. FAUNAand T. MUSSMI.J. electrochem. Sot. 112.921 (1965). _ , I . S. GIrxu,>.phys. Chkm. 66,2657 (1962). T. B. WARNERand S. SCHULD~NER, J. phys. Chem. 69,404s (1965). S. SCHULDINER and T. B. WARNER,Electrochim. Acta 11, 307 (1966). L. FORMARO and S. TRASA~, Chimica Znd. 48,706 (1966). M. PROSKURNIN and A. FRUMKIN,Trans. Faraday Sot. 31,110 (1935). W. G. FRENCHand T. KUWANA,J. phys. Chem. 68,1279 (1964). V. E. KAZARINOV and N. A. BALA~HOVA, Dokl. Akad. Nauk SSSR 157,1174 (1964). N. A. SHUMILOVA, G. V. ZHUTAEVA, M. R. TARASEVICH and R. KH. Buasrrrsr~, Zh. Fir. Khim. 39, 1012 (1965).
ON SOME DEBATED ASPECTS OF THE CAPACITANCE/ POTENTIAL CURVES FOR PLATINUM* Laboratory
L. FORMARO and S. TRASATTI of Electrochemistry and Metallurgy, University of Milan, Milan, Italy
Abstract-Two points are discussed here: (a) the actual value of the capacitance in the potential range where no adsorption of hydrogen or oxygen takes place; (b) the general shape of the capacitance curves in the oxygen-adsorption region. Resultsin the literature can be explained by taking into account the time elapsing between electrode cleaning and capacitance reading. The effect of impurities is studied and discussed by making use of two solutions at different purity level. Only within a narrow range of anodic potentials from 400 to 500 mV (he) are platinum electrodes free of adsorbed hydrogen and oxygen; at this potential the capacitance value is 34 pF/cma (real surface area). For E > 400 mV, capacitance falls with time; the possible reasons for this phenomenon are examined and discussed. The capacitance curves obtained by the technique adopted in the present work are compared with steady-state curves. R&um&-On discute deux points: (a) la valeur reelle de la capacitC dans le domaine de potentiel oti il n’y a ni hydrog&ne ni oxyg&ne adsorbes; (b) la forme generale des courbes de capacit6 dans le domaine de l’adsorption d’oxygkne. On montre que les r&.ultats trouvables en litterature peuvent dtre expliques en considtrant le temps qui s’&coule entre l’activation de 1’6lectrode et la mesure de la capacit6. On discute l’effet des impure& en employant dew solutions avec de diffkrents degrCs d’impuret6s. Seulement autour de 400-500 mV (he) l’blectrode de platine est libre d’hydrogkne ou d’oxygkne adsorb&; il ce potentiel-ill la capacitC a une valeur de 34 pF/crn* (r&f&&e il la surface r&lle). Pour des potentiels > 400 mV la capacit6 d&-o& dans le temps, et l’on discute les causes possibles de ce phenom&ne. Enfin l’on compare et analyse les courbes de capacite obtenues en condition stationnaire et non-stationnaire. Zusammenfaw-In der vorliegenden Arbeit werden zwei Punkte diskutiert: (a) der wirkliche Wert der Kapazitgt impotentialbereich, in dem keinewasserstoffoder Sauerstoff-Adsorption erfolgt ; (b) die allgemeine Form der Kurven im Sauerstoffadsorptionsbereich. Es wird gezeigt, dass die in der Literatur auffmdbaren Ergebnisse gedeutet werden kSnnen, wem die zwischen der Elektroden-Reinigung und der KapazitBtsmessung abgelaufene Zeit beriicksichtigt wird. Die Wirkung von Verunreinigungen wurde durch die Verwendung von zwei verschieden reinen LBsungen untersucht. Die Platinelektrode ist frei von adsorbiertem Wasserstoff und Sauerstoff erst in einem eingebei diesem Potential betrlgt die Kapazitlt schr&kten Bereich von 400-500mV (WE) aufwlrts; 34 pF/cma (unter Bezugnahme auf die reale Fllche). Bei Potentialen oberhalb 400 mV nimmt die Kapazitlt mit der Zeit ab; die Urachen von derartigen Phlnomenen werden diskutiert. Schliesslich werden die durch instationlre Messungen erlangten Ergebnisse mit denjenigen in stationken Zustand verglichen. INTRODUCTION
STUDIES on the differential capacitance of platinum electrodes were numerous in past times1 but in the most recent years they have been less frequent; furthermore, in the measurements are few recent papers2s3 where this subject is dealt with, capacitance used only to support conclusions derived by other experimental methods. A probable reason for this is that the field has appeared less fruitful than expected, for poor agreement is normally observed between the experimental data of different authors, which leads to a difficult interpretation of measurements in terms of double layer structure. Whilst in the case of mercury the knowledge of the double layer l Manuscript received 3 December 1966. 1457 6
1458
L. FORMARO and S. TRASA~
structure is in an advanced stage, in the case of platinum even the correct experimental procedure is not yet set up. Attempts to explain platinum capacitance by comparison with the behaviour of mercur>A led to conclusions not so evident as to be conclusive. The determination of the potential of zero charge on platinum, by exploiting the relationship found on mercury between the pzc and the potential of maximum adsorption for uncharged organic molecules2*6 must be considered in the light of Frumkin’s point,s that the platinum behaviour is strongly affected by the adsorption of hydrogen and oxygen. Several authors did not refer their capacitance values to electrode real surface areas, and this has also contributed to discrepant experimental data, because the roughness factor of a platinum electrode depends on several factors (material manufacture, electrode preparation, electrode treatment etc). Leaving out the theoretical interpretation of capacitance data, we shall deal in particular with the results as regards experimental technique. From published work one sees that two points are debated: (a) the actual value of the platinum capacitance in the potential range where no adsorption of hydrogen or oxygen takes place; (b) the general shape of the capacitance/potential curves in the oxygen-adsorption region. The results of the various authors are in agreement in the hydrogen adsorption region; here, the double layer capacitance is masked’ by a very high faradaic capacitance due to H+ + e z Hads. Data at present available on the two points under discussion are the following: (1) The actual value of platinum capacitance without hydrogen or oxygen adsorbed on the surface has hitherto been considered to be 18 pF/cm2,8*11and this value was even recommended4 and used2 for the determination of the real surface area of platinum. However, Gilman12 proved that a platinum electrode in a solution has the surface partly covered with organic impurities contained in the solution itself’, and concluded that the value of 18 pF/cm2 is too small, and due to the poisoning of the electrode surface. On the basis of these results, TrasattP carried out capacitance measurements, examining in particular the effect of the time elapsed after electrode surface cleaning; he found that the capacitance falls with time and that the decrease rate depends on stirring and temperature, the whole process being controlled by mass transfer in the solution. (2) Some authorss*Q.14stated that in the oxygen-adsorption region the platinum capacitance rises nearly proportionally to the amount of adsorbed oxygen. Gilman12 and BreiteP showed on the contrary that platinum capacitance rises steeply from 400 mV to 800 mV (he) and then drops when the surface commences to oxidize. In Gihnan’s and Breiter’s opinion the impurity effects account for the behaviour observed by others. However, TrasattP observed that at potentials ranging between 600 and 1000 mV (he) the decrease in capacitance, just after electrode cleaning, is much more marked than one could expect only from the impurity effect; he suggested that a slow rearrangement of the surface state takes place in this potential range. Topics (a) and (b), briefly considered by Trasatti in his previous paper,is are taken up and investigated in the present work, by using two solutions of different purity in respect of organic pollution. Thus, a clear separation of impurity effects from any other surface phenomenon is possible.
Capacitance/potential curves for platinum EXPERIMENTAL
1459
TECHNIQUE
Generalities All measurements were made in 1 M HClO, solutions prepared by dilution of analysed reagent-grade 60% perchloric acid. Experiments were carried out at 2X, temperature beingregulated to within fO*l”C by means of an air-thermostat. 99.999 % nitrogen (with traces of rare gases) was used when necessary. The gas line was made up with either copper or glass or Teflon, plastic and rubber tubes being excluded. No grease was used in any part of the apparatus employed for these experiments. The bridge for capacitance measurements has been described.13 Test electrode The electrode was prepared by melting the end of a 99.98 % pure platinum wire to a small sphere in a Hs-0, flame and then sealing the wire into a soft glass tubing. The electrode was then introduced into a vacuum furnace and kept at 500°C for 3 h to anneal the metal and eliminate all gases absorbed in it. Before each experiment the platinum bead and the glass tubing were cleaned in cont. H$O,, then washed and boiled in triply distilled water. All capacitance values are reported on the basis of the real surface area of the electrode (O-0493 cm2) as determined by cathodic charging curves.16 Electrode potentials were measured using a type 610-A Keithley electrometer and are referred to the hydrogen electrode (he) in the same 1 M HClO, solution, as described by Bianchi et a1.l’ Solutions Solutions were prepared using triply distilled water, the last distillation being made from alkaline permanganate in a nitrogen atmosphere. The resistivity of the water was 2 x lo6 s2 cm, measured in air. Since the main purpose of the present work was to separate impurity effects from any other surface phenomena, two kinds of solution were used, the content in organic impurities being controlled and defined by cathodic charging curvePr (see later). (i) Solutions “A”. The 1 M HClO, solutions were pre-electrolysed in a separate vessel under a nitrogen stream between platinized platinum electrodes for at least 24 h at a current density of about 0.1 mA/cm2 (referred to real surface). The solutions were then introduced into the test cell for experiments. The impurity contents of the solutions “A” were defined as about 2.3 per cent.ls (ii) Solutions “B”. Since the main cause of the presence of impurities in solutions in the contact between these and the atmosphere,ls solutions “A” were introduced into the cell and pre-electrolysed again for 20 h. The pre-electrolysis was carried out at controlled potential, not at controlled current. The two electrodes used for this purpose were kept at about 60 mV and 1500 mV, respectively, by means of a potentiometer directly connected to them. The voltage drop across the potentiometer divided between the two electrodes so as to fulfill the equality between the rates of anodic and cathodic processes. The cell voltage can thus be kept at such a value as water does not decompose; since in these conditions any detectable evolution of oxygen as well as hydrogen is quite absent, any nitrogen flowing through solutions, which may cause further contaminations, can be avoided. For higher effectiveness and rapidity of the pre-electrolysis, the polarites of the two electrodes were reversed every 30 s with
1460
L. FORMARO and S. TRASAW
a relay; in this way, alternately, the electrodes oxidized anodically the impurities adsorbed during the cathodic working. The aim of such a pre-electrolysis was exclusively to oxidize organic impurities; also, since no detectable evolution of oxygen took place at the anode, residual oxygen in solution was completely reduced at the cathode. The impurity content of solutions “B” was found to be go.7 % (see later). During the course of the pre-electrolysis, solutions were stirred by means of a magnetic stirrer. Since our experiments were carried out in differently impure solutions and the impurity contents were scarcely reproducible and could not be corrected at all, it was necessary to perform a single run of measurements in the solution “B”, whilst a certain amount of the solution “A” was stored under nitrogen atmosphere. Cells The cell previously13 described was used to perform experiments in solutions “A”. These were degassed in the cell for 0.5 h before each experiment, using a nitrogen stream. A closely similar cell was used for experiments in solutions “B”, but in this case the auxiliary electrode for alternating current consisted of two semi-cylindricalshaped platinum sheets which were employed as pre-electrolysis electrodes. At the end of the pre-electrolysis, the two platinum electrodes were connected to the same point of the impedance bridge to form the auxiliary single electrode. The solutions were carefully degassed by nitrogen before the pre-electrolysis in this case as well. During the pre-electrolysis and successive measurements the cell was kept hermetically sealed by means of Teflon stopcocks. Electrode pre-treatment
A platinum electrode, kept either in the air or in a solution for a long time, is partly covered with impurities. 12*13Therefore, it is necessary to remove them to get a clean surface and reproducible as well as reliable capacitance values. Before each measurement the electrode surface was cleaned by the potentiostatic sequence shown in Fig. 1. Such a kind of pre-treatment was first introduced by Gilman.ls The rationale behind the various potential steps is as follows. Anodization at 1500 mV (he) removes impurities largely and forms a layer of adsorbed or combined oxygen. Solutions were kept stirred so that traces of molecular oxygen anodically evolved and residuals of the impurity oxidation could be swept away. At 1200 mV the oxygen layer is retained13 and hinders re-adsorption of impurities. Solutions were kept stirred for 20 s so as to restore normal conditions of concentration at the interface. Solutions “A” were stirred by gas bubbling and solutions “B” by a magnetic stirrer. Final results were found to be not affected by using either stirring. Gas or stirrer was then turned off to allow solutions to become quiescent, so as to get reproducible conditions in the solution side as well. At 60 mV the oxygen layer is reduced within the first few ms. The duration of this step was chosen as 2 s, a sufficiently long time for any oxygen adsorbedlD to be reduced, and a sufficiently short one for successive measurements to be unaffected by any hydrogen adsorption.20 At the potential E each capacitance measurement was made after the potentiostat (type-557 Amel) had been disconnected by switching to a potentiometric circuit, which allowed ac to be superimposed. The duration of this step is afterwards denoted as t and the main
Capacitance/potential
*With
curves for platinum
1461
stirring
FIG. 1. Potential/time
sequence used to clean the electrode.
purpose of this work was to investigate the influence of the time of measurement on the capacitance value. RESULTS
The capacitance/potential curves obtained by the above technique appear in Fig. 2. Curves b and c were obtained with solutions “B” with t = 30 s and t = 3 min, respectively. The electrode is still impurity-free within this time period (see later). Curve a was obtained by Gilman12 by the poten ti a 1-ste p method with t = 1 s using a similar pre-treatment. Curve d was obtained in solutions “A” under steady conditions, ie measuring the capacitance only after its variation with time became negligible. x I =30 This
work
120 o Gilman” 2 %
A I ‘3
{.
s (S0l.B) min
(Sol.B)
Steady-stale /=
(Sol.
A)
Is
100 -
I 0.3
I 0.5
Electrode
I 0.7
I 0.9
potential,
I I.1
I I.3
I I.5
V (rhe)
FIG. 2. Capacitance/potential
curves obtained as indicated. 5 KHz.
At 400 mV (he) curves a, b and c merge and the capacitance has a value of about 34 ,uF/cm2. At potentials ranging between 500 and 1000 mV (he) the capacitance drops with time, so that b and c are below a. However, we observed that curve b was slightly displaced upward in this potential range when measurements at t = 30 s were taken in rapid succession without keeping the electrode at the potential E for a too long time between two successive measurements. The above drop in capacitance appears in any case. On the contrary, at 400 mV (he) the duration of previous measurements had no effect on the capacitance value. For E > 1000 mV (he) as well,
L. FORMAROand S. T~~sarrr
1462
the capacitance was relatively little affected in this sense; curves a, b and c are considerably closer to one another. As regards curve d, it is remarkably lower than curve c throughout the potential range investigated and tends to merge with it only at E = 1500 mV (he). The capacitance variation with time in solutions “A” is shown in Fig. 3. At 400 mV (he) the capacitance behaviour just after pre-treatment is markedly different from that at any other potential. At 500 mV an initial decrease in capacity can
1
I 1 I 12345
I
I
I
I
I
7
9
I2
Time,
16
min
FIG. 3. Time-dependence of capacitance at the various potentials. Solutions “A”. 5 KHz.
indeed be seen. At 700 and 1000 mV the decrease is still marked, yet the more positive the potential, the less marked is the decrease. A comparison between solutions “A” and “B” in respect of the phenomenon of decrease in capacitance with time is shown in Figs. 4-8. At 400 mV (he), curves merge at t = 30 s, but for longer times they are very divergent ; capacitance decreases much more in solutions “A” (less pure) and tends to a value as low as 17 pF/cm* (curve d, Fig. 2). The capacitance at 400 mV appears to be constant after 2 min in solutions “B”; it decreases less than one per cent in the third minute. At 500 mV (he) (Fig. 5) the behaviour is very similar to that above for long times, but just after pre-treatment a marked fall in capacitance can be seen, similar in both kinds of solution. At 800 mV (he) (Fig. 6), the curves are even more similar. There is an initial large fall in capacitance in both solutions; for intermediate times the curve obtained in solutions “B” remains slightIy above the other one, but they merge for very long
1463
Capacitance/potential curves for platinum
II
I
2
I
3
1 5I
4
I
I
7
9
Time,
I
12
I
16
min
FIG. 4. Time-dependence of capacitance at 400 mV (he).
I I I I I 12345
I
I
7
9
Time, FIG.
I
12
I
I6
min
5. Time-dependence of capacitance at 500 mV (he).
times. This last fact may be related to a lower starting value of capacitance for the curve in solutions “B”. At potentials where a large fall in capacitance occurs, capacitance values were in fact found to be reproducible within 5 per cent; at 400 mV, on the contrary, the reproducibility was better than one per cent. At 1000 mV (he) (Fig. 7) the difference in shape between curves “A” and 73” is even less marked. At 1300 mV (Fig. 8) the two curves are practically parallel; an equal starting value of capacitance would cause the two curves to merge.
L. FORMAROand S. TRASA~-~I
1464
min
Time,
FIG. 6. Time-dependence of capacitance at 800 mV (he).
I I I 12345
I
I
I
7
Time,
I
9
I 12
I
16
min
FIG. 7. Time-dependence of capacitance at 1000 mV (he). DISCUSSION
The purpose of this work was to examine (a) the actual value of platinum capacitance in the potential range where no adsorption of hydrogen or oxygen takes place (b) the general shape of the curves in the oxygen-adsorption region. The two points will be separately discussed, though for the discussion of each reference to the other must be made.
1465
Capacitance/potential curves for platinum
In the following discussion the time t is chiefly taken into consideration, and we attempt to show that a different duration of the electrode at the potential of measurement can account for the observed disagreement between the previous results. Actid
value of capacitance
The general shape of the curves in Fig. 2 suggests that the electrode is free of either hydrogen or oxygen species only in a very narrow range of potential from
I I I 12345
I
I
I
I
I
I
7
9
12
I6
Time,
min
FIG. 8. Time-dependence of capacitance at 1300 mV (he).
400 mV (he). At E > 400 mV the initial fall in capacitance will be later shown to be probably due to a surface rearrangement with respect to adsorbed oxygenated species. At 400 mV (Fig. 4) the capacitance remains constant as long as the electrode surface remains clean; the purer the solution, the longer the time during which capacitance does not vary. This fact shows that after pre-treatment the electrode surface is thoroughly clean and the successive variations, at this potential, are not due to rearrangements connected with the previous pre-treatments. Our results show that just after pre-treatment, the electrode reaches a steady capacitance at 400 mV. The subsequent decrease in capacitance is due to impurities reaching the electrode surface at a rate depending on their concentrations; the purer the solution, the less marked the decrease in capacitance, so that the steady value is a function of the purity of the solution. This suggests also that a quasi-equilibrium exists between adsorbed impurities and those in solution, as in the case of most common organic substances.2*5*21A value of 17 ,uF/cm2 for capacitance was found in solutions “A” in agreement with those obtained by other authors4*8-11J4 working under steady conditions; this is the capacitance of a platinum electrode the surface of which is partly covered with impurities. The agreement on this value of the results of various authors
around
1466
L. FORMARO and S. T~~s~rrr
indicates that the solution purity was approximately the same in all cases. A higher purity degree may lead to values a little higher,s but steady-state measurements are entirely reliable only when made in absolutely pure solutions in which the platinum capacitance must remain indefinitely constant at 400 mV (he) after pre-treatment. The constancy of the capacitance at 400 mV (he) proves the double layer structure to be not affected by pre-treatment or, more probably, the pre-treatment produces so labile a modification that it disappears within a few ms.12 Capacitance measurements performed by the above technique are fully reliable for real surface area determinations,16 since the whole electrode surface is in the same condition. Capacitance measurements on mercury show why the true capacitance of platinum can be determined only just after electrode pre-treatment. On mercury the capacitance is measured with a dropping electrode precisely to avoid impurity effects, as shown by Proskurnin and Frumkin. 22 With platinum it is obviously not possible to work in the same manner, but it is reasonable to choose a pre-treatment which, in a sense, can create a new surface for each measurement. TABLE 1. CAPACITANCEAND HYDROOEN ADSORPTIONAS A FUNCTION OF ELECTRODEPOLARIZATION TIME IAT 4OOmV(he)
t S
Solutions “A”
Solutions “B”
c
,uF/cma
-
223 223 223 223 223 222 219 218 212
-
223 223 223 223 223 223 223 223 221.5 212
-
33.2 32.5 31.6 29.8
0.3 1.0 1.9 3.7
5 10 20 30 45 60 90 120 180 600
33.5 33.5 33.5 33.5 33-l 30.0
0 0 0 0.4 3.5
* qa,* is 223 pC/cnP,
AH
&/cm2
5 10 20 30 45 60 90 120 180
33.5 -
4a
AC
pc Pcm= 0 0 0 0 1 4 5 11 0 0 0 0 0 0 0 1.5 11
AC/C
Aqa/qa,s*
-
-
0009 0.030 0.057 0.110
0 0 0 0 0.005 0018 0.023 0.050
-
-
0 0 0 0.012 @104
0 0 0 0 0 0 0 0.007 0.050
the value for monolayer hydrogen adsorption.
When measuring capacitance for qualitative purposes, even steady-state measurements may be performed, as shown elsewhere,21 but the results must always be examined in the light of possible poisoning of the electrode surface. Our results and views are in full agreement with Gilman’s.12 At 400 mV (he) our capacitance and Gilman’s have practically the same value, ca 34 pF/cm2. This shows the effectiveness of pre-treatment and supports the validity of the charging-curve method16 for the determination of the real surface area of the electrode. To display fully the results obtained, a comparison is presented in Table 1 between the rate of decrease in capacitance at 400 mV (he) and the rate of decrease in qH, qH denoting the charge consumed during a cathodic charging curve to cover every available site with one hydrogen atom. Experimental details for determining qH are
Capacitance/potential curves for platinum
1467
described elsewhere.13 The drop in qH (AqH) is directly proportional to the fraction of surface covered with adsorbed impurities. Table 1 shows that the electrode remains clean after pre-treatment for about 45 s in solutions “A” and for 25-3 min in solutions “B”. Also, AC is, in per cent, about twice Aqn, in agreement with Gilman’s findings, l2 for not very high coverages, namely at not too long times. The Table cotirms that cathodic charging curves can be used to define conventionally the purity of a solution, as regards organic contamination, through the percentage of electrode surface covered by impurities after a chosen time.ls Oxygen adrorption region For E > 400 mV (he) our results are also in agreement with Gilman’s,12 but only qualitatively, which suggests that in this potential range the time t plays an important role. We think Gilman found such high capacitance values (Fig. 2) mainly because he performed his measurements at very short times. Figure 6 clearly shows that the capacitance decreases with time at 800 mV (he), independent of impurity effects, since such a decrease can also be observed in solutions “B” where the electrode remains clean for about 3 min after pre-treatment (Table 1). Gilman also observed a marked decrease in capacitance within the first few ms, but this decrease was no longer detected between 100 ms and 1 s. In our opinion, the decrease rate may be too low at these times to be detected over such a short time interval. On the other hand, the above discrepancies do not seem to be due to different frequency effects when measuring capacitance, since we worked with a bridge at 5 KHz and Gihnan employed the potential-step method with double layer charging times corresponding to an ac frequency of 5-50 KHz. Nevertheless, the possibility of an additional faradaic capacity may be taken into consideration. In the very potential range where a maximum in capacitance is observed, 750-950 mV (he), platinum shows, according to Breiter,ls a faradaic capacitance due to Pt + H,O e Pt-OH
+ H+.
The amount of the faradaic capacitance may be dependent on the amplitude of potential step. Gihnan employed a negative pulse of 6 mV; we worked with an alternating voltage across the cell never exceeding 5 mV (peak to peak). This may also partly account for the high capacitance values found by Gilman. Finally, we must bear in mind the above-mentioned slight ageing-effect of the electrode in this very potential range, consisting in a lowering in capacitance values when the pre-treatment is preceded by a lengthened polarization at not very positive potentials. This effect is not due to impurities, because it is absent at 400 mV. It must be, therefore, a surface phenomenon of platinum, and it affects the reaction of formation of oxygenated species. Thus our results agree qualitatively with Gilman’s, but they show that platinum capacitance, over a wide potential range where surface oxidation occurs, must be examined in the light of both the type of measurement and the time elapsed between pre-treatment and measurement. The fact that the decrease in capacitance appears, more or less strikingly, over the whole potential range from 500 to 1500 mV (he), leads us to believe it to be connected with the presence of oxygenated species on the electrode surface; moreover, it suggests that formation of such species, most probably adsorbed OH radicals, takes
1468
L. FORMAROand S. TRASAI-H
place even at 500 mV. Hence, the platinum electrode is certainly free of hydrogen or oxygen on the surface only in a really narrow potential range between 400 and 500 mV (he). The present results do not lead to a full explanation of what occurs on the platinum surface just after pre-treatment ; however, we may suggest probable reasons for the observed phenomena. The steep rise in capacitance with increase of potential in the range 400-800 mV is attributed by Gilman12 to a change in the structure of the double layer. The fall in capacitance with time cannot, however, be explained in this way because ionic equilibrium in the double layer is reached rapidly. We think that the formation, by the pre-treatment, of an unstable surface layer of atoms and their subsequent recrystallization23 is also to be excluded, because such a phenomen should affect the capacitance at 400 mV as well. Popat and Hackerman observed a similar decrease in capacitance, which they explained on the basis of slowness of ion adsorption-desorption processes. Ion adsorption can actually lead to a decrease in capacitance in the potential range referred to,15 but in our case other considerations seem to exclude that this may be the main reason. In fact, sulphate ions were found to adsorb on platinum beginning from O-17V;24 even considering that ClO,- ions adsorb on platinum less strongly than SO%,15 we conclude that a decrease in capacitance should be observed at 400 mV as well, if ion adsorption is the real cause. We prefer to explain the decrease in capacitance with time on the basis of a rearrangement of the electrode surface state, which might be due either to a variation in surface concentration of oxygenated species or, more likely, to a modification in the nature of the platinum-oxygen bond (a strengthening).25 Finally, the phenomenon of the decrease in capacitance enables us to re-consider to a some extent the steady-state measurements as regards the oxygen adsorption region. In Fig. 2, curve d, obtained with steady-state measurements and resembling the data reported by Laitinen and Enke,9 is the lowest. This curve was re-interpreted by Gilman12 exclusively on the basis of impurity effects. However, our results show that this effect is determinin g for E < 800 mV, but no longer for E > 800 mV, even though it may still slightly affect the results. In fact, Figs. 5-8 show the decrease in capacitance with time to be practically independent of the impurity level in the solution even at 800 mV. At 1300 mV, impurity level is quite without influence. This is not surprising, if we bear in mind that all the most common organic substances commence to be really oxidized on platinum at 600 mV.21 If the greater part of impurities consist of organic substances of low carbon atomicity, as is to be expected after distillation from permanganate and pre-electrolysis, which both may rupture large molecules without complete oxidation to C02, it is reasonable to expect that the adsorption of such impurities is rather difficult at sufficiently positive potentials. However, the bigger and more difficultly oxidable impurities may still affect results at a long time even at rather high positive potentials. Also, from Fig. 2 one can observe that curve c, obtained at t = 3 min and then without impurity effect, is an evolution of curve b towards the steady-state curve d, at least for potentials higher than 800 mV. In conclusion, the results show that the actual capacitance of platinum at E > 800 mV (he) is approached by steady-state measurements, but slight poisoning effects due to impurities should always be considered.
Capacitance/potential Acknowledgements-This Italy.
curves for platinum
1469
research was sponsored by the Consiglio Nazionale delle Ricerche, Rome, REFERENCES
.I. electrochem. Sot. 111, 114 (1964). 1. M. C. BANTA and N. HACKERMAN, 2. 3. 4. 5. 6. 7. I10: 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
E. GILEADI,B. T. RUBINand J. O’M. BOCKRIS, J. phys. Chem. 69, 3335 (1965). W. H~LAND, E. G~LEADIand J. O’M. BOCKRIS.J. ahvs. Chem. 70. 1207 (1966). . P. V. POPATand N. HACKERMAN, J.&x Chek. 82,‘1198 (1958).* E. G~LEADI,J. electroanal. Chem. 11, 137 (1966). A. N. FRUMKIN, Dokl. Akad. Nauk SSSR 154,1432 (1964). M. Basrrsa, H. KAMMERMAIER and C. A. KNORR,Z. Elektrochem. 60, 37 (1956). J. P. HOARE,Nature, Lond. 204,71 (1964). H. A. LAU~NENand C. G. ENKE,J. electrochem. Sot. 107,773 (1960). W. D. ROBERTSON, .I. electrochem. Sot. 100,194 (1953). J. N. SARMOUSAKIS and M. J. PRAGER.J. electrochem. Sot. 104.454 11957). , , , S. GILMAN,Electrochim. Acta 9, 1025 11964). S. Ta~s~rr~, Electrochim. Metall. 1,267 (1966). J. LLOPISand F. COMM, An. R. Sot. esp. Fis. Quim. 524,233 (1956). M. W. BREITER, J. electroanal. Chem. 7, 38 (1964). S. TRASATTI, Electrochim. Metall., submitted for publication. G. BIANCHI,A. BAROSI,G. FAUNAand T. MUSSMI.J. electrochem. Sot. 112.921 (1965). _ , I . S. GIrxu,>.phys. Chkm. 66,2657 (1962). T. B. WARNERand S. SCHULD~NER, J. phys. Chem. 69,404s (1965). S. SCHULDINER and T. B. WARNER,Electrochim. Acta 11, 307 (1966). L. FORMARO and S. TRASA~, Chimica Znd. 48,706 (1966). M. PROSKURNIN and A. FRUMKIN,Trans. Faraday Sot. 31,110 (1935). W. G. FRENCHand T. KUWANA,J. phys. Chem. 68,1279 (1964). V. E. KAZARINOV and N. A. BALA~HOVA, Dokl. Akad. Nauk SSSR 157,1174 (1964). N. A. SHUMILOVA, G. V. ZHUTAEVA, M. R. TARASEVICH and R. KH. Buasrrrsr~, Zh. Fir. Khim. 39, 1012 (1965).