Electrochemically generated hydrated electrons

Eleetroanalytical Chemistry andlnterfacialElectrochemistry, 42 (1973) App.ll-App.15

App.ll

© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

PRELIMINARY NOTE E l e c t r o c h e m i c a l l y generated h y d r a t e d electrons

A. BEWICK,B.E. CONWAY*and A.M. TUXFORD

Department of Chemistry, The University, Southampton S09 5NH (England) (Received 4th August 1972)

Introduction In recent years there has been a renewed discussion of the possible involvement of the hydrated electron in cathodic reductive processes. This was stimulated by the attempt of Walker ~-3 to measure the concentration of hydrated electrons near a silver cathode using an optical method. Although positive results were claimed for these experiments, the interpretation of the data is far from conclusive. There are substantial arguments, which have been reviewed by Conway4, against the hydrated electron as a significant species in these reactions. The original claims t' 2 are continuing to be advanced3 without further experimental support. Up to the present, the case based on Walker's results relies mainly on the measurement at a single wavelength of the change in reflectivity of a.silver electrode in response to modulation of the potential under uncontrolled electrochemical conditions. The experiment required a laser light source and thus a wavelength scan leading to an actual diagnostic spectrum was not possible. In view of the continued controversy, we now present results obtained using modulated specular reflectance spectroscopy under controlled electrochemical conditions.

Experimental The experimental technique has already been briefly described s . For the present experiments, the potential of a silver electrode in a three-electrode cell was modulated with a square wave profile using a programmed potentiostat with a fast response. The variation in reflectivity of the electrode caused by the modulation was measured over the wavelength range 250 to 700 nm using a single reflection of parallelpolarised light. In orde[ to cover the longer wavelength region a photodiode type 9715QB (EMI) was used in place of the photomultiplier originally described. The limiting sensitivity of the method with the phase sensitive detection system employed corresponded to a relative change in reflectivity, AR/R, of + 2 x 1 0 -6 . *Commonwealth Visiting Professor, University of Southampton, 1969-70.

App. 12

PRELIMINARYNOTE

Results and discussion Figure 1 shows the wavelength dependence of AR/R using a 0.25 M solution of Na2 SO4. Two potential ranges were covered; (a) switching between -400 mV and -1 V (Ag/AgC1) at 30 Hz, and (b) switching between -0.55 V and -1.20 V at 30 Hz. The latter regime was the most cathodic range that could be employed without interference fromhydrogen evolution. For comparison, Fig. 2 shows the electroreflectance spectrum of silver6 and the absorption spectrum of the hydrated electron 7. -22 -2C -18 -16

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Wovelength (nm) Fig. 1. Wavelength dependence of A R / R for silver electrode in 0.25 M sodium sulphate solution. (a) 30 Hz square pulse from - 0 . 4 V to - 1 . 0 V, (b) 30 Hz squares pulse from - 0 . 5 5 V to - 1 . 2 V (Ag/AgCI).

It is clear that the results of Fig. 1 are of the correct sign and form to be attributed predominantly to the electroreflectance spectrum of silver. Even at the most cathodic potentials employed, there is no sign of the superposition of the absorption spectrum of the hydrated electron. Absorption of light by hydrated electrons would produce either positive~r values for AR/R or diminution of the negative values. It is possible to calculate an upper limit to the possible amount of hydrated electron species that could have been present without being detected by the present measurements. A positive going peak at about 700 nm superimposed on the negative plateau shown on Fig. 1 would have been discernible if its amplitude exeeded 3 x 10 "s AR/R. * T h r o u g h o u t this paper, & R / R is calculated as (R 2 - R 1 ) / R ~ where R ~ is the reflectivity at the m o r e cathodic potential and R 2 that at the more anodic potential.

PRELIMINARY NOTE

App. 13

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Fig. 2. The elect'roreflectance spectrum of silver 6 and the absorption spectrum of the hydrated electron 7 .

For an absorbing species such as the hydrated electron with an extinction coefficient of 1.5 x 104 M "1 cm "1 at 700 nm, this corresponds to about 10 "12 mol cm "2 of species in the light path (either on the electrode surface or in a reaction layer). This is the maximum amount of the species that could have been present at -1.20 V. It is necessary to attempt a comparison with Walker's results. This cannot be made with much precision because of the uncertainty in the electrode potentials used by him. One important difference is, however, immediately apparent. The sign of the reflectivity change obtained by Walker is opposite to that reported here. This suggests that the potential ranges employed in the two sets of experiments were very different. It is possible to estimate the approximate value of the potential that the silver electrode would have taken up in Walker's experiments. The silver and platinum electrodes had areas of about 300 cm z and 3 cm 2 respectively and Fig. 3 is an equivalent circuit of the cell for the 0.25 M sodium sulphate electrolyte. CAg and Cpt are the double layer capacitances for the two electrodes, the values shown being calculated using values of 20/~F cm -2 , Re is the electrolyte resistance and Z p t is the impedance of an electrode process taking place on the platinum electrode when it is driven away from the double layer region of potential. At the modulation frequency used by Walker, the reactances CAg and Cpt are 0.3 S2 and 30 fZ respectively and typical experimental parameters were given as 1.1 V RMS cell voltage and 38 mA RMS cell current. A simple calculation gives the amplitude of modulation of the potential of the silver electrode. This is very small being only 32 mV peak to peak. A similar value was calculated by McIntyre 8 on the basis of the relative surface area of the two electrodes. Establishing the average potential of the silver electrode is more difficult. However, a good estimate

App. 14

PRELIMINARY NOTE

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6OJJF 7 ~ Fig. 3. Equivalent circuit for the cell with 0.25 M sodium sulphate electrolyte. can be made using the following arguments. The predominant contribution to the cell impedance comes from the platinum electrode. Therefore in the absence of a significant shunting impedance Zpt , most of the applied 1.1 V RMS would appear at this electrode. Such a potential span exceeds the double layer region by a considerable margin and it is clear that the platinum electrode would be driven alternately into the hydrogen adsorption region, where the pseudo capacity is about 1000 #F cm -z , and into the oxide region where similar amounts of charge are involved in layer formation; a contribution from Zpt cart also be seen to be necessary in order that the cell current should reach 38 mA. It can be concluded, therefore, that the silver electrode would be at a potential within the platinum-double-layer region, and probably more towards the anodic end due to the relatively slow time scale for platinum oxide formation i.e. at about +600 to +700 mV w.r.t, hydrogen. This is about 1.2 V more anodic than the highest potential used in the present experiments and for which an upper limit of 10 "12 tool cm "2 was calculated for the amount of hydrated electrons. At the estimated potential of Walker's experiments this amount would be equivalent to between 10 -32 tool cm "2 and 1 0 -22 tool cm "2 depending upon whether Nernstian conditions are maintained at the electrode surface or the electron transfer is rate determining. These values are, of course, well beyond the resolution of the technique employed and a factor of 10 "19 to 10 -9 less than the amount calculated by Walker from the observed reflectivity change. The most likely explanation for the effect observed by Walker is that it is due to a combination of the electroreflectance effect 6 and changes in concentration of ions in the double layer. The magnitude and sign of the' double layer effect depends upon the position of the electrode 'potential with respect to that at which the double layer has a minimum refractive index 9 . Walker's observation, that the reflectivity change did not occur if a diode was connected in the circuit so that only anodic current could flow through the silver electrode, could then be explained by the shift in average potential that would take place when the diode was reversed; the magnitude of the double

PRELIMINARY NOTE

App. 15

layer effect on the reflectivity could well be markedly different at the two potentials. The electrode potentials will be very different for the situations with and without the diode in circuit. In the former case, charge will build up on CAg until a potential is reached at which a faradaic process allows charge to leak away in the form of a product. These processes must be hydrogen evolution and oxygen evolution for the cathodic and anodic regimes respectively and, in fact, gas evolution was reported for the cases with the diode in circuit 1, 2. It can be concluded from this that the difference in electrode potentials between the pure a.c. experiment and either of those involving rectification is so large that these experiments cannot be meaningfully compared; thus there is no evidence on which to assign the reflectance change to either the anodic or cathodic cycle of the a.c. modulation as was attempted by Walker and hence there is no evidence for the detection of the hydrated electron in these experiments. Postl and Schindewolf l° also found no evidence for the formation of hydrated electrons at a silver electrode. They employed electrodes of equal area but did not control nor measure the electrode potential. The difficulties with the interpretation of the results o f Walker do not, of course, apply to the photoelectron emission experiments o f Heyrovsky 11 and Barker 12, where the electrochemical electron emission into solution is assisted by a quantum of light. REFERENCES 1 D.C. Walker, Can. J. Chem., 45 (1967) 807. 2 D.C. Walker, Anal Chem., 39 (1967) 896. 3 G.A. Kenney and D.C. Walker in A.J. Bard,(Ed.), Electroanalytical Chemistry, Vol. 5, Marcel Dekker, New York, 1971. 4 B.E. Conway and D.J. MacKinnon, J. Phys. Chem., 74 (1970) 3363; B.E. Conway, Modern Aspects ofElectrochemistrv, Vol. 7, Plenum, New York, 1972, chap. 2. 5 A. Bewick and A.M. Tuxford, Syrup. Faraday Soc., 4 (1970) 114. 6 J2D.E. Mclntyre in R.H. Muller (Ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 9, Academic Press;(to be published)J. Feinleib, Phys. Rev. Lett., 16 (1966) 1200; R. Garrigos, R. Kofman, A. Jolivet and J. Richard, Nuovo Cimento, 8B (1972) 242. 7 J.P. Keane, Radiat. Res., 22 (1964) 1. 8 J.D.E. Mclntyre, private communication. 9 M. Stedman, Chem. Phys. Lett., 2 (1968) 457; M. Stedman, Symp. Faraday Soc., 4 (1970) 64; A. Bewick, P.R. Cantrill, F.A. Hawkins and A.M. Tuxford, paper presented at the Electrochemical Society Meeting, Houston, May 1972, Abstract No. 97. 10 D. Postl and U. Schindewolf, Ber. Bunsenges. Phys. Chem., 75 (1971) 662. 11 M. Heyrovsky and R.G.W. Norrish, Nature, 200 (1963) 880; M. Heyrovsky, Nature, 206 (1965) 1356; M. Heyrovsky, Proc. Roy. Soc., A301 (1967) 411. 12 G.C. Barker, A.W. Gardner and D.C. Sammon, J. Electrochem. Soc., 13 (1966) 1183; Trans. Faraday Soc., 66 (1970) 1498 and 1509, G.C. Barker, Electrochim. Acta, 13 (1968) 1221.