Page 304 - Handbook of Battery Materials

274 10 Carbons

Nitrogen is usually present in minor amounts, but sulfur can be present in high
concentrations, >1%, depending on the precursor that is used to manufacture
the carbonaceous material. Besides sulfur that is bonded to carbon, other forms
such as elemental sulfur, inorganic sulfate, and organosulfur compounds may be
present. The carbon–sulfur surface compounds on carbon blacks are relatively
stable, but they desorb as H 2 S when carbon is heat-treated in H 2 between 500 and
◦
1000 C.
The surface oxide groups on carbon play a major role in its surface properties;
for example, the wettability in aqueous electrolytes, work function, and pH in
water are strongly affected by the presence of surface groups on the carbonaceous
material. Typically, the wettability of carbon blacks increases as the concentration
of surface oxides increases [16]. The pH of an aqueous slurry of carbon decreases
as the volatile or oxygen content of the carbon increases [17]. The work function of
carbon blacks shows a minimum at a pH near 6 [18].
The physicochemical properties of carbonaceous materials can be altered in a
predictable manner by different types of treatments. For example, heat treatment of
soft carbons, depending on the temperature, leads to an increase in the crystallite
parameters, L a and L c and a decrease in the d(0 0 2) spacing. Besides these physical
changes in the carbon material, other properties such as the electrical conductivity
and chemical reactivity are changed. A review of the electronic properties of graphite
and other types of carbonaceous materials is presented by Spain [3].

10.3
Electrochemical Behavior

10.3.1
Potential

Several significant electrode potentials of interest in aqueous batteries are listed in
Table 10.2; these include the oxidation of carbon and oxygen evolution/reduction
reactions in acid and alkaline electrolytes. For example, for the oxidation of
o
carbon in alkaline electrolyte, E at 25 Cis −0.780 V vs SHE (standard hydrogen
◦
electrode) or −0.682 V (vs Hg/HgO reference electrode) in 0.1 mol L −1 CO 2− at
3
pH = 14. Based on the standard potentials for carbon in aqueous electrolytes, it is
thermodynamically stable in water and other aqueous solutions at a pH less than
about 13, provided no oxidizing agents are present.
The typical products that form during oxidation of carbon in acid and alkaline
electrolytes are CO 2 and carbonate species, respectively. Additional details of
the thermodynamic stability of carbon in aqueous electrolytes, and the electrode
potentials for reactions involving carbon, are presented in the review by Randin [19].
The standard oxidation potentials suggest that carbon has a limited stability
domain in aqueous electrolytes. As noted in Table 10.2 the oxidation (corrosion)
of carbon should occur at potentials much lower than the reversible potential for
oxygen evolution/reduction. To illustrate this point further, take the example of an