Proton and Electron Transports in a Typical Intercalation Compound: Graphite Hydrogenosulfate

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PROTON

AND

ELECTRON TRANSPORTS IN A TYPICAL INTERCALATION COMPOUND : GRAPHITE HYDROGENOSULFATE

AHMED HARRACH and ANDRE METROT Universit6 Reims Champagne Ardenne, Laboratoire d'Electrochimie et Chimie du Solide - BP 347 -

51062 Reims C6dex - France

ABSTRACT "Overcharging" of pure stage graphite hydrogenosulfate i.e charging of graphene layers with expulsion of H+ from the intercalate layer is modelized by transmission lines which agree quite well with A.C Impedance measurements. The meaning of the apparent Diffusivity Da = 12 /3 RICI is discussed as significant of the displacement of H+/e- holes pairs along the blocking interface separating graphene and intercalate layers. INTRODUCTION The electrochemical intercalation of an ionic specie M+ (or A-) in an lattice < Ho > can be described by the general equation


+ 6x M+ +

6x e

-




x + 6x

From a macroscopic point of view this reaction may be either monophasic, involving non-steochiometry, or biphasic when two phase domains are separated by a composition gap, with a finite value of 6x. During the well established processes of the intercalation of acids into graphite the two cases occur [1]. * biphasic situations during stage transformations, i.e intercalation of compact acid layers between previously empty graphite interlayers. The progression of this reaction appears clearly as the moving of an interface frontier. * monophasic situationsduring "overcharging" of pure stage compounds, i.e expulsion of H+ according to the equation < CY+yA-,

zHA >

< C(y++6 y)+ (y+6y)A-, (z-6y)HA > + 6y(H++e-)

(I)

Fig.l shows the Potential-Charge E(Q) or Potential-Composition E(y) curves observed for the intercalation of 18M sulfuric acid into Highly Oriented Pyrographite (HOPG)* : the potential plateaus correspond to two-phases equilibria, while capacitive-like slopes correspond to pure low stages overcharging, as identified by in-situ X-ray diffractometry. Plateaus and overcharging domains are mismatched at the beginning of intercalation (stages 8 to 4).

*

Kindly supplied by Dr A.W. MOORE, Union Carbide

Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society

374

C458 C48

c+ 88 STAGE N>3 S....................

2

c1

231 I•-------------------.......

23!

.. "'"

';

1.0

0.-

-J I

0.5 •//

CHARGE

y

I

Z I--

0 a.

0

0.03

0.02

0.01 1 1000

2000 CHARGE Q

Fig.l Steady-state Potential-Charge at 20'C.

0.04 I

1 3000 (A.SMOLE-1)

4000 -

E(Q) or E(y) curve for H 2 So

THE CAPACITIVE ASPECT OF PURE MODELLING BY A TRANSMISSION LINE

STAGES

4

18M - HOPG,

OVERCHARGING

AND

ITS

The " cristallographic and electronic structures of a stage n graphite hydrogenosulfate are well-established as regular stackings, along the c-axis, of one sulfuric acid and n graphene layers. Sulfuric layers are negatively charged (deficit of H+) while adjacent graphene layers are positively charged (electronic holes in the f1 band). This charge is directly available as the quantity of electricity used during anodic synthesis. The interface betw