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
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