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The lithium amalgam electrode in dimethylformamide
SHORT COMMUNICATION
THE LITHIUM AMALGAM ELECTRODE IN DIMETHYLFORMAMIDE J.S.DUNNEXT*,D. GEAREYand I. A. MENZIES~ University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 lQD, England (Receiued 13 September 1979)
INTRODUCTION
exchange current, derived from fl at the two Lower frequencies,
lithium electrode is of interest because of its use in high energy batteries and solid electrodes have been studied in several non-aqueous solvents [I-3]. However at such elcctrodes the reaction is always complicated by the electrocrystallization step. We report here a measurement of the exchange current at the dropping mercury electrode, where such effects are absent, using the Faradaic impedance method
i, = 0.19 mAcmvZ.
The
Cdl.
An approximate correction for the double layer potential was made using Peover’s measurements of the electrode charge in tetrabutyl-ammonium perchloratedimethylformamide [7]. Thus at -2.41 V us see, comsponding to the half
XI? ohm cm2
EXPERIMENTAL
The cell, electrodes, measuring system and solvent purification have been described previously [S]. Tbe reference electrode consisted of a cadmium amalgam coated with a paste of cadmium chloride-dimethylformamide [6] ; its potential was 0.98 V us aq. s.c.e. The supporting electrolyte was 0.3 mol/dm’ t&methyl-ammonium perchlorate in dimethylformamide. 0.1 mmol/dm’ lithium ion was added as the perchlorate. All measurements were made at 25 + 0.2 C.
Measurements of the cell admittance were made at frequencies of 2500, NO0 and 10,000rad s-l over a range of potentialsaround the lithium halfwavepotential (- 1.43 V us Cd,/CdCI,). The admittances were transformed to theequivalent impedances by the usual inversion method and the solution resistance, determined at potentials well removed from the lithium wave, was subtracted to give the electrode impedances. These are plotted in the complex plane in Fig. 1. Also shown is a segment of the circle, diameter 135 n cm*, with centre on the real axis (R,) at 67.5acm’. The peak impedances lie on this circle showing that the reaction is under complete activation control at the frequencies used. To obtain the best estimates of the peak charge transfer resistance, 8, plots of the real component of the electrode admittance against potential were drawn. The values thus obtained were the same at 2000 and NYJOrad s-l where, 0 = 135 Rem*. At 10,ooO rad s-l, the real part of the electrode admittance is a minor part of the total impedance and the scatter in the plot was too great to give a useful result. Hence the apparent
Fig. 1. The real (R,) and imaginary (X,) partsof theelectrode impedance during the reduction of lithium ion (1oOnmol dm-‘) in dimethylformamide at a dropping mercury electrode, (A) at 2500, (B) at 4ONl and (C) at lO,OlWrads-’ ; balances on (+) anodic and (0) cathodic sides of peak.
* Present address : 74, Holmdale Road, Chisleburst, Kent, U.K. t Present address: Department of Materials Technology, University of Technology, Loughborough, U.K. B.A.25/J--9
729
SHORT COMMUNICATION
730
wave potential oflithium, the charge on the electrode is 15 PC cnb2 and the diffuse layer potential, b2 = 99mV. Heoa
the “true” exchange current, assuming that OL= 0.5, iOl= 0.31 mA cm-2. REFERENCES
1. R. J. Jasinski, High Energy Batteries, Plenum Press, New York (1967).
2. R. T. Lacey, Ph.D. Thesis, Manchester (1975). 3. J. 0. Resenhard, G. Eichinger, J. e&roan& Chem. interfwid Electrochem. 68, 1 (1976). 4. M. Sluyters-Rehbach and J. H. Sluytors, Electrounul. Chem. 4, 1 (1970). 5. J. S. Dunnett and D. Gearey, Electrochim. Acra 19, 907 (1974). 6. L. W. Maple, Analyr. Chem. 39, 844 (1967). 7. M. E. Peover, Disc. Foroday Sot. 45, 184 (1968).
THE LITHIUM AMALGAM ELECTRODE IN DIMETHYLFORMAMIDE J.S.DUNNEXT*,D. GEAREYand I. A. MENZIES~ University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 lQD, England (Receiued 13 September 1979)
INTRODUCTION
exchange current, derived from fl at the two Lower frequencies,
lithium electrode is of interest because of its use in high energy batteries and solid electrodes have been studied in several non-aqueous solvents [I-3]. However at such elcctrodes the reaction is always complicated by the electrocrystallization step. We report here a measurement of the exchange current at the dropping mercury electrode, where such effects are absent, using the Faradaic impedance method
i, = 0.19 mAcmvZ.
The
Cdl.
An approximate correction for the double layer potential was made using Peover’s measurements of the electrode charge in tetrabutyl-ammonium perchloratedimethylformamide [7]. Thus at -2.41 V us see, comsponding to the half
XI? ohm cm2
EXPERIMENTAL
The cell, electrodes, measuring system and solvent purification have been described previously [S]. Tbe reference electrode consisted of a cadmium amalgam coated with a paste of cadmium chloride-dimethylformamide [6] ; its potential was 0.98 V us aq. s.c.e. The supporting electrolyte was 0.3 mol/dm’ t&methyl-ammonium perchlorate in dimethylformamide. 0.1 mmol/dm’ lithium ion was added as the perchlorate. All measurements were made at 25 + 0.2 C.
Measurements of the cell admittance were made at frequencies of 2500, NO0 and 10,000rad s-l over a range of potentialsaround the lithium halfwavepotential (- 1.43 V us Cd,/CdCI,). The admittances were transformed to theequivalent impedances by the usual inversion method and the solution resistance, determined at potentials well removed from the lithium wave, was subtracted to give the electrode impedances. These are plotted in the complex plane in Fig. 1. Also shown is a segment of the circle, diameter 135 n cm*, with centre on the real axis (R,) at 67.5acm’. The peak impedances lie on this circle showing that the reaction is under complete activation control at the frequencies used. To obtain the best estimates of the peak charge transfer resistance, 8, plots of the real component of the electrode admittance against potential were drawn. The values thus obtained were the same at 2000 and NYJOrad s-l where, 0 = 135 Rem*. At 10,ooO rad s-l, the real part of the electrode admittance is a minor part of the total impedance and the scatter in the plot was too great to give a useful result. Hence the apparent
Fig. 1. The real (R,) and imaginary (X,) partsof theelectrode impedance during the reduction of lithium ion (1oOnmol dm-‘) in dimethylformamide at a dropping mercury electrode, (A) at 2500, (B) at 4ONl and (C) at lO,OlWrads-’ ; balances on (+) anodic and (0) cathodic sides of peak.
* Present address : 74, Holmdale Road, Chisleburst, Kent, U.K. t Present address: Department of Materials Technology, University of Technology, Loughborough, U.K. B.A.25/J--9
729
SHORT COMMUNICATION
730
wave potential oflithium, the charge on the electrode is 15 PC cnb2 and the diffuse layer potential, b2 = 99mV. Heoa
the “true” exchange current, assuming that OL= 0.5, iOl= 0.31 mA cm-2. REFERENCES
1. R. J. Jasinski, High Energy Batteries, Plenum Press, New York (1967).
2. R. T. Lacey, Ph.D. Thesis, Manchester (1975). 3. J. 0. Resenhard, G. Eichinger, J. e&roan& Chem. interfwid Electrochem. 68, 1 (1976). 4. M. Sluyters-Rehbach and J. H. Sluytors, Electrounul. Chem. 4, 1 (1970). 5. J. S. Dunnett and D. Gearey, Electrochim. Acra 19, 907 (1974). 6. L. W. Maple, Analyr. Chem. 39, 844 (1967). 7. M. E. Peover, Disc. Foroday Sot. 45, 184 (1968).