Electrochemistry at low temperatures down to 80 K further evidence for nuclear tunnelling in electron transfer reactions

J. Elecrroanal. Chem., 205 (1986) 343-346 Elsewei Sequoia S.A., Lausanne Prmted

343 in The Netherlands

Preliminary note ELECTROCHEMISTRY FURTHER

EVIDENCE

TRANSFER

ATSUSHI Department Bunkyo-ktr. KENICHI

FOR NUCLEAR

DOWN

TUNNELLING

TO 80 K

IN ELECTRON

REACTIONS

MATSUNAGA.

KIMINORI

of Synthetrc Chemrst?, Tokyo I13 (Japan)

ITOH and AKIRA

Facu1t.p ofEngmeermg,

FUJISHIMA

the Unrcerq~

of Tokyo, Hongo,

HONDA

Dewsron of Molecular Kyoto 606 (Japan) (Received

AT LOW TEMPERATURES

Engmeenng,

10th December

Graduate School of Engmeermg,

Kyoio Unrversrt): Salqwku.

1985: in revised form 3rd April 1986)

INTRODUCTION

In electrochemistry, few studies have been performed so far at temperatures as low as 80 K or below. Interesting phenomena should be observed in this temperature region; for instance, quantum mechanical effects are expected to appear. Stimming and Schmickler [l] recently carried out such a measurement at 120 K using glassy solutions containing HClO,, and suggested the importance of a nuclear tunnelling effect in electron transfer reactions at lower temperatures. This nuclear tunnelling process in electron transfer is little known in electrochemistry but is said to be important in biological systems [2]. We therefore decided to reexamine their results, and have obtained electrochemical parameters at temperatures down to 80 K. We present here further evidence of nuclear tunnelling in the electron transfer reaction of the Fe2+/Fe3+ couple in a solid medium at lower temperatures. We show also that the tunnelling process appears at different temperatures, depending on the ratio between H,O and HClO,. EXPERIMENTAL

Figure 1 shows schematically the experimental arrangement. A two-electrode cell (40 cm long) was immersed in a Dewar vessel filled with liq. N,. The working electrode was a Pt rod with a surface area of 3 x lo-* cm’. or SnO, prepared by vacuum deposition and/or spray pyrolysis with a surface area of 1 cm* and a thickness of ca. 200 nm. A Pt wire served as a counter electrode. The H,O-HClO, system was used as an electrolyte solution because it has a relatively high conductivity even in the frozen state [l]. The redox couple tested was 10 mM Fe2+/Fe3+, 0022-0728/86/$03.50

0 1986 Elsevier Sequoia

S.A.

344

Vi

Vi

:

dC

Voltage

Cl : condenser

A cell T.M. : transient 0.s

memory

oscilloscope

Fig. 1. Scheme of the experimental

arrangement

was the same as the one Stimming and Schmickler used [l]. The coulostatic pulse method [3-51 was employed to measure the kinetic parameters. This simple technique eliminates disturbance due to the ohmic drop and the double-layer charging. Stimming and Schmickler used the double-pulse galvanostatic method [l] for the same reasons. In the present experiment a condenser with a capacity of 200-560 pF was used as C, in Fig. 1 and the dc voltage I’, was set to be 0.1-10 V. Our procedure of setting temperatures was the following; the electrolytic system was cooled down to the boiling point of liquid nitrogen, and was then heated slowly. which

RESULTS

AND

DISCUSSION

Figure 2 shows an Arrhenius plot for the standard heterogeneous rate constant of 10 mM Fe2+/Fe3+ in 1 M HClO,. Figure 3 shows a similar plot in HClO, + 5.5 H,O. Here we succeeded in measuring the kinetic parameters down to 80 K by using the coulostatic pulse method. Potential decay curves were measured with overpotentials less than 20 mV. Careful shielding was needed to diminish the electrical noise due to the relatively large resistance of the system. Our plot in 1 M HClO, has the same tendency as that of Stimming and Schmickler [l] over the range of measuring temperatures they employed. Imperfect uniformity in this solvent system (1 M HClO,) below 273 K, may cause the relatively large difference between their results [l] and ours. The plots in Figs. 2 and 3 can be divided into four regions (region I corresponds to the liquid phase for each system). As shown in Figs. 2 and 3, the activation energy in region IV of each solvent system (0.7-0.9 kJ/mol) is much smaller than that in the other regions. We believe that this confirms the presence of nuclear tunnelling in the electron transfer in region IV as suggested by Stimming and Schmickler [l]. Region III in HClO, + 5.5 H,O is located at lower temperatures than that in 1 M HClO,. This is probably caused by partial crystallization of H,O in 1 M HClO, in region III, making the electron transfer reaction slow. One can see in Fig. 3 that regions II and III intersect at

345

3

I

I

I

4

5

6

I

I

7 8 lO'.T“/'K-'

I

I

I

I

9

10

11

12

1

Fig. 2. The Arrhemus plot for the standard heterogeneous rate constant of 10 mM Fe’+/Fe3+ HC104 from 293 K to 80 K. (0) Pt; (0) SnO?; (- - -) the result of Stlmmmg and Schmlckler

around 228 K. corresponding to the melting point of the HClO, [6]. The rate constants for SnO, electrodes gave a plot similar were two or three orders of magnitude smaller than those for decrease in the rate constant for SnO, is most likely due to

m 1 M [I].

+ 5.5 H,O system to that for Pt, and Pt electrodes. This electron tunnelling

T/K -2 -3 'i 2-4 " y-5 0" J-6

-8

3

4

5

6

7 8 9 lO'.T"/ K-'

Fig 3. The Arrhemus plot for the standard HCIO, +5.5 H,O from 293 K to 80 K.

10

heterogeneous

11

12

13

rate constant

of 10 mM

Fe’+/Fe’+

m

346

through the space-charge layer of the semiconductor electrode with donor density of ca. 10” cmP3. We also found that the resistance of the medium and the double-layer capacitance take on a constant value below ca. 170 K. Details about this phenomenon will be reported elsewhere. REFERENCES 1 2 3 4 5 6

U. Stimming and W. Schmickler. J. Electroanal. Chem. 150 (1983) 125. D. Devault and B. Chance, Biophys. J., 6 (1966) 825. H.P. van Leeuwen. Electrochm. Acta. 23 (1978) 207. P. Delahay, J. Phys. Chem., 66 (1962) 2204. J. Kudlrka and C.G. Enke. Anal. Chem., 44 (1972) 614. T. Iwaslta, S. Rottgermann and W. Schmickler, J. Electroanal. Chem., 196 (1985) 203