Page 308 - Handbook of Battery Materials

278 10 Carbons

sufficiently electrocatalytically active for oxygen evolution to occur simultaneously
with carbon corrosion at potentials corresponding to charge conditions for a bi-
functional air electrode in metal/air batteries. In this situation, oxygen evolution
is the dominant anodic reaction, thus complicating the measurement of carbon
corrosion. Ross and co-workers [30] developed experimental techniques to over-
come this difficulty. Their results with acetylene black in 30 wt% KOH showed
that substantial amounts of CO in addition to CO 2 (carbonate species) and O 2 ,are
produced at 550–600 mV (vs Hg/HgO reference electrode) and temperatures up
◦
to 65 C. Evidence for the formation of an organic species (appearance of a deep
◦
reddish-brown color in the solution) was found at 65 C but the composition was
not identified. However, Thiele [31] and Heller [32] concluded that the organic
species was probably a humic acid.
The major oxidation reactions of acetylene black in an alkaline electrolyte (30 wt%
KOH + 2 wt% LiOH) are strongly dependent on the potential (vs Hg/HgO) and
temperature [30]:
◦
• 500 mV and <50 C: carbonate formation (CO 2 ) is the dominant reaction;
◦
• 500–600 mV and <50 C: carbonate formation and O 2 evolution rates are com-
parable;
◦
• 600 mV or >60 C and >450 mV: O 2 evolution and CO formation are the
dominant reactions.
Other experiments by Ross and co-workers [30] clearly indicate that the common
metal (Co, Ni, Fe, Cr, Ru) oxides that are used for oxygen electrocatalysts also
catalyze the oxidation of carbon in alkaline electrolytes.
The surface structure has a strong influence on the corrosion rate of carbon in
both acid and alkaline electrolytes. Studies by Kinoshita [33] clearly showed that
the specific corrosion rate (milliamperes per square centimeter of carbon black
◦
in 96 wt% H 3 PO 4 at 160 C) was affected by heat treatment. A similar trend in
the corrosion rate in alkaline electrolyte was observed by Ross [30c], as shown in
Figure 10.4. It is evident that the corrosion rates of the nongraphitized carbons are
higher than those of the corresponding graphitized carbons. Their study further
indicated that some types of carbon blacks (e.g., semi-reinforcing furnace blacks)
showed a larger decrease in the corrosion rate after graphitization than others that
were evaluated. The decrease in the corrosion rate is attributable to the change in
the surface microstructure after heat treatment. The surface layers rearrange to
form a graphitic structure with basal planes that are exposed to the electrolyte. This
surface is more resistant to corrosion than the edge plane sites, and experiments
by Ross [30c] indicate that the nongraphitic surface sites, which are capable of
adsorbing iodine from solution, are the likely corrosion sites.

10.3.5
Electrocatalysis
Carbon shows reasonable electrocatalytic activity for oxygen reduction in alkaline
electrolytes, but it is a relatively poor oxygen electrocatalyst in acid electrolytes.