Page 305 - Handbook of Battery Materials

10.3 Electrochemical Behavior 275

Table 10.2 Standard potentials for reactions of carbon
materials in batteries containing aqueous electrolytes.

Electrochemical reaction Standard potential (V vs SHE) Electrolyte

+
C + 2H 2 O → CO 2 + 4H + 4e − 0.207 Acid
−
C + 6OH → CO 2− + 3H 2 O + 4e − –0.780 Alkaline
3
O 2 + 4H + 4e → 2H 2 O 1.229 Acid
+
−
+ −
O 2 + 2H + 2e → H 2 O 2 0.682 Acid
−
+
H 2 O 2 + 2H + 2e → 2H 2 O 1.776 Acid
O 2 + 2H 2 O + 4e → 4OH − 0.401 Alkaline
−
−
O 2 + H 2 O + 2e → HO + OH − –0.076 Alkaline
2
−
HO + H 2 O + 2e − → 3OH − 0.878 Alkaline
2
air electrode (for instance in a metal/air battery) that utilizes carbon. In an acid
electrolyte, for instance, a typical potential at which oxygen reduction occurs is
about 0.7 V, whereas in alkaline electrolyte the reaction may take place at about
0.1 V. At these operating potentials, the overpotential for carbon oxidation is high in
both electrolytes (i.e., 0.5 V in acid and 0.9 V in alkaline electrolytes). Furthermore,
the overpotential is much higher in alkaline electrolyte, which suggests that carbon
oxidation should be much greater at high pH. In rechargeable alkaline metal/air
batteries that utilize carbon in the bifunctional air electrodes, corrosion during
charge is a major problem that has not been resolved satisfactorily. The net result
is that practical rechargeable metal/air batteries are not available because of their
limited cycle life.
10.3.2
Conductive Matrix

Perhaps the first practical application of carbonaceous materials in batteries was
demonstrated in 1868 by Georges Leclanch´ e in cells that bear his name [20].
Coarsely ground MnO 2 was mixed with an equal volume of retort carbon to form
the positive electrode. Carbonaceous powdered materials such as acetylene black
and graphite are commonly used to enhance the conductivity of electrodes in
alkaline batteries. The particle morphology plays a significant role, particularly
when carbon blacks are used in batteries as an electrode additive to enhance the
electronic conductivity. One of the most common carbon blacks which is used
as an additive to enhance the electronic conductivity of electrodes that contain
metal oxides is acetylene black. A detailed discussion on the desirable properties
of acetylene black in Leclanch´ e cells is provided by Bregazzi [21]. A suitable carbon
for this application should have characteristics that include: (i) low resistivity in
the presence of the electrolyte and active electrode material, (ii) absorption and
retention of a significant amount of electrolyte without reduction of its capability
of mixing with the active material, (iii) compressibility and resilience in the cell,