INVESTIGATION OF THE PROTON-EXCHANGE PROCESSES AT THE ICE - METAL INTERFACE

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INVESTIGATION OF THE PROTON-EXCHANGE PROCESSES AT THE ICE - METAL INTERFACE

N. Khusnatdinov, V. Petrenko, A. Zaretskii

To cite this version:

N. Khusnatdinov, V. Petrenko, A. Zaretskii. INVESTIGATION OF THE PROTON-EXCHANGE

PROCESSES AT THE ICE - METAL INTERFACE. Journal de Physique Colloques, 1987, 48 (C1),

pp.C1-105-C1-108. �10.1051/jphyscol:1987115�. �jpa-00226259�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment a u no 3, Tome 48, mars 1987

INVESTIGATION OF THE PROTON-EXCHANGE PROCESSES AT THE ICE - ^METAL

INTERFACE

N.N. KHUSNATDINOV, V.F. PETRENKO and A.V. ZARETSKII

Institute of Solid State Physics, The USSR Academy of Sciences, Chernogolovka 142432, USSR, Moscow District

Resume - La presence dtune atmosphere dqhydrog&ne exerce une grande influence sur les courants continus de volume et de surface dans le cas d*un 6chantillon de glace maintenu entre des hlectrodes de Pd et d'alliages de Pd. La charge automatique de Pd et de ses alliages est peut-stre une possibilite pour produire une 6lectrode ohmique protonique permanente.

Abstract - Strong influence of the hydrogen atmosphere on steady volume and surface currents has been observed in ice samples with Pd and Pd alloys electrodes. Possible mechanisms of this phenomenon have been discussed.

Automatic charging of Pd and its alloy electrodes may be one of the ways of producing qteternalqt ohmic proton electrodes.

1.

INTRODUCTION

Production of ohmic electrodes for ice is an extremely important problem. Some attempts have been made to solve it (1 - 4). Hydrogen saturated Pd electrodes are most often used for this purpose

But these electrodes having many advantages also reveal an obvious limitation - the time of their work is restricted by the amount of the hydrogen incorporated. It is tempting to create "eternal" proton ohmic electrodes for ice. This being the case, it is of interest to try the electrodes made of Pd and of its alloys with automatic charging of protons from H2 atmosphere or from cold hydrogen plasma.

2.

EXPERIMENTAL PROCEDURE

The experimental procedure of growing ice and producing samples is similar to that described in (4). Thin foils (10 - 50 microns) of highly pure Pd, Au, Pt, A1 or of Pd-Ru with specially treated activated surface served as electrodes (5). In some cases metals were sprayed on ice in vacuum at 80 K.

Fig. I presents the scheme of the measurements

G A t~ ELECTROMETER

B 'Yo

Fig.1 S c h e m e of the m e a s u r e

-

m e n t s .

With the power source switched off we can measure the potential difference between the electrod and guard ring (and the corresponding currents).

The sample was placed in a chamber which could be filled with different gases. A discharge in a hydrogen atmosphere at 400 V and H2 pressure P 103 Pa produced a plasma, the negative potential being delivered to the

electrode.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987115

C1-106 JOURNAL

DE

3. RESULTS

3.1. Effect of H7 on steady volume currents in ice

Fi5.

shows the influence of the hydrogen-nitrogen atmosphere - (0.1 H2 + 0.9 N2) 10 Pa - on steady volume currents in the ice with Pd and Pd-Ru electrodes. A supply of nitrogen under the similar conditions did not lead to any doticeable change in the current. When H2 was delivered, no change in the current passing in the samples with Au, Pt and A1 electrodes was observed.

0 2 0 40 6 0 T I M E , m i n 0 5 0 VOLT'AGE,V

Fig. 2 The influence of the H2 Fig. 3 Voltage-current characteristics atmosphere on steady volume currents. of (Pd-Ru - ^Ice - (Pd-Ru) system.

P(H2) = lo4 Pa. Thickness and area P(H2)= 10 1 ^Pa.

of the sample 1 = 1.3

S = 25 mm2. The temperature dependence of SST is shown in an insertion (see text).

Fig. 3 depicts voltage-current characteristics (VCC) of (Pd-Ru) - ^ice - ^(Pd-Ru)

system in a hydrogen atmosphere. At low voltages,V <40 V, the VCCs were linear. And the value of current observed is retained for a long time. One might thus have assumed the conductivity GT determined from VCC to be the static conductivity of ice (the temperature de endence of GT is indicated in an insertion of Fig. 3).

d'

Howevgr the values of ST have proved to be lower than the actual conductivity of ice SST, determined from the ac - measurements.

3.2. Effect of H7 on the current source METAL - ICE SURFACE - ^METAL

There exists a potential difference, VT, between electrode

and the guard ring G at temperatures exeeding 240 K, and as a result current IT flows. The current is likely to flow over the surface of ice. Its temperature dependence proves this statement.

The potential difference exists between any pair of electrodes, and between electrodes

the same material as well (in the latter case VT is small). At a constant temperature, the surface current IT depends mainly on VT, the characteristic inner resistance RT being of the order of lo8 -

^Ohms.

We have studied the influence of the H2 supply on VT, IT and on RT, using different combinations of materials for electrodes A and G.

Fig. 4 presents typical diagram illustrating the dependence of H2 supply on VT. All

the potentials were measured with respect to the Au electrode.

0 5 10 T I M E , m i n

Fig. 4 The influence of H2 on VT.

P(H2) =

Pa.

1

-

8 - i

-

^f

I

260K

---

A u

1.

2 21. 21.

P d

-

i I I I

0 5 10 T I M E , m i n

Fig. 5 The influence of direct H2 supply through the Pd electrode on VT.

P(H2) =

Pa.

Hydrogen affects most strongly the Pd - Au and Pt - Au pairs. In all the cases RT changes only slightly and variation of IT is defined by variation of VT, as the ambient atmosphere changes. Experiments with the direct hydrogen supply to the Pd electrode have been performed to elucidate the role of the H2 atmosphere

Fig.

3.3. Influence of plasma on VCC.

The method employed for producing hydrogen plasma, alongside the automatic charging of Pd electrodes, caused marked heating of samples and a consequent temperature dependent drift of measured quantities. Application of a high-frequency discharge as in (3) also leads to warming up of the samples. Similar methods of producing ohmic proton contacts are by far tempting, but the problem of generating hydrogen plasma at a temperature below 270 K is still a barrier to it.

4.

DISCUSSION

The effect of the H2 atmosphere on steady volume currents and on the potential difference VT, observed in our work, may be thought of as the principal experimental result. The H2 atmosphere may give rise to different phenomena. It is evident that the currents described in 3.2 are caused by chemical reactions at the electrodes. In this case, hydrogen may be released

2H20 H30+ + ^{IH- (volume}

2H30+ + ^2el- H20 + H2 f _(cathode

I A

&OH- 2H20 + 4el- + O2 (anode

An increase in the external H2 presure, leads, according to the Le Chatelier

principle, to slowing down of the corresponding reactions and hence, to variations

in VT and IT.

C1-108 JOURNAL D E PHYSIQUE

The results of experiments shown in Figs. 2 and 5 suggest that supply of H2 causes penetration of protons into the region adjacent to the electrode. Let us consider the following model

the protons of an electrode entering into ice increase the density of H30+ defects at the interface n, (x = 0). Owing to diffusion, the excess density of H30+ will fall down to the equilibrium value n+0 (at the distance of the Debye screening length ID).

The proton diffusion into ice causes the change of the potential difference VT - VT (H2) which is given by

where e is the proton charge and is the Boltzmann constant. The total number of protons per unit area, N+, which penetrated into ice can easily be calculated by the relation

Assuming that VT - ^VT(H2)

100 mV and ncO(T = 260 K) 10'9 m-3

evaluated from the measurements similar to that described in (6)), we have N+ C 1021 m-2. The characteristic time of hydrogen penetration, t, turns out to be of order of

-

300 sec. In our experiments it is thus possible to generate currents more than 10-10 A.

Acknowledgements

We are indebted to Dr. A.P. Mishchenko from the Topochiev Institute of Petrochemical Synthesis of the USSR Academy of Sciences for numerous helpful advices and for providing us with Pd-Ru foil. We also thank Mr. E.I. Potapov for technical assistance.

References

1.

Engelhardt H., Riehl N., Phys. Lett., 14 (19651, 20, Phys. Kondens. Matts., 5,

(19661, 73.

2. Durand M., Deleplanque A., Kahane

Sol. St. Comm., 5, (19671, 159.

3. Auvert G., Kahane A., in "Physics and Chemistry of Icew, edited by E. Whalley, S.J. Jones, L.W. Gold, Ottawa, Royal Soc. of Canada, p. 271, (1973).

4. Petrenko V.F., Whitworth R.W., Glen J.W., Phil. Mag., B,

(1983). 259.

5. Roshan N.R., Mishchenko A.P., Polyakova V.P., Parfenova N.L., Savitsky E.M., Voitekhova E.A., Gryaznov V.M., Sarylova M.E., J. Less Common Metals, 89, (19831, 423.

6. Zaretskii A.V., Petrenko V.F., Ryzhkin I.A., Trukhanov A.V., this volume.