Page 307 - Handbook of Battery Materials

10.3 Electrochemical Behavior 277

area, structure, and volatile content (oxygen surface groups) influence the electronic
conductivity of the composite structure. Typically the electronic conductivity is sig-
nificantly lower when less than about 30 wt% carbon black is incorporated in the
composite structure. Carbon blacks with higher surface area, and usually smaller
particle size, are desirable because the interparticle distance is shorter and electron
tunneling can occur more easily. The structure of carbon blacks is conventionally
defined by the amount of adsorption of dibutyl phthalate (DBP); higher adsorption
means a higher structure. For example, acetylene black has DBP absorption of
3
200–250 cm g −1 and is a high-structure black, whereas a low structure has a DBP
3
−1
absorption of <100 cm g . A higher oxygen or volatile content is not desirable
because its presence renders the carbon black less conductive. Further details are
available in the publication by Kinoshita [1].
10.3.3
Electrochemical Properties

A comprehensive review which discusses the surface properties and their role in
the electrochemistry of carbon surfaces was written by Leon and Radovic [26].
This review provides a useful complement to the following discussion on the
role of carbon in aqueous batteries. Four key parameters that are important for
carbonaceous materials in batteries, which were identified by Fischer and Wissler
[24], are:
1) chemical purity
2) crystalline structure
3) particle size distribution, and
4) porosity.

These parameters are critical to the operation of alkaline batteries. Evaluation of
graphite additives in the positive electrode for Cd/NiOOH cells by Veres and Csath
[27] showed that the state of oxidation of graphite and the level of impurities strongly
influence the electrode capacity. The capacity of the active material decreased with
an increase in the amount of impurities and the degree of oxidation of the graphite.
The studies by Biermann et al. [28] indicate that the carbon blacks used as the
conductive matrix in Leclanch´ e cells remain chemically inert, that is, they do not
undergo oxidation during storage or discharge of the cell. However, Caudle et al.
[29] found evidence that the ion-exchange properties of carbon black, which exist
because of the presence of surface redox groups, are responsible for electrochemical
interactions with MnO 2 . The extent of MnO 2 reduction to MnOOH depends on
the carbon black (e.g., furnace black > acetylene black).

10.3.4
Electrochemical Oxidation
In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at poten-
tials where carbon corrosion occurs. However, in alkaline electrolytes carbon is