Page 312 - Handbook of Battery Materials

282 10 Carbons

electrodes (e.g., 50 wt% high-density polyethylene, 35 wt% graphite, 15 wt% carbon
black) in Br 2 -containing electrolytes was investigated by Futamata and Takeuchi
[37]. The intercalation of Br 2 in graphite and the reaction of Br 2 with polyethylene
resulted in mechanical degradation of the composite electrode.
Another type of redox flow battery that utilizes carbon electrodes and soluble
reactants involving vanadium compounds in H 2 SO 4 is under evaluation [38, 39]:
+
+
−
Positive electrode (discharge): VO + 2H + e → VO 2+ + H 2 O (10.10)
2
Negative electrode (discharge): V 2+ → V 3+ + e − (10.11)
Electrodes consisting of carbon-reinforced graphite or carbon fibers were inves-
tigated with the redox reactions of soluble vanadium ions. The former material
−1
showed evidence for the intercalation of H 2 SO 4 at concentrations >5mol L ;how-
ever, a similar reaction was not observed with the carbon fibers. Skyllas-Kazacos
and co-workers [39] noted that the electrochemical activity of graphite-polymer
composite electrodes in the vanadium redox battery was enhanced by a chemical
activation treatment involving strong inorganic acids (H 2 SO 4 ,HNO 3 ). The increase
in electrochemical activity is attributed to the increase in the concentration of sur-
face functional groups containing C–O and C=O, which could behave as active
sites.
Activation by electrochemical or gas-phase oxidation can alter the performance
of carbon electrodes for redox reactions. The two major changes that occur to
the carbon electrodes as a result of these treatments are an increase in the surface
area of the carbon and the formation of surface functional groups on the surface.
Jorne and Roayaie [40] reported that electrochemical activation (applying a current
◦
density of 33 mA cm −2 for 5 h in 0.975 mol L −1 H 2 SO 4 at 40 C) of porous
graphite electrodes produced an increase in the surface area of nearly an order of
−
magnitude, and this is mainly responsible for the improved kinetics of the Cl /Cl 2
redox reaction. On the other hand, gas-phase oxidation of highly oriented pyrolytic
◦
graphite in air at 600 C is reported to enhance the surface area and form acidic
surface oxides which help to increase the kinetics of the redox reactions involving
3+
both Cr /Cr 2+ and Fe /Fe 2+ [41].
3+
10.3.6
Intercalation
−
Highly ordered graphite serves as a host for intercalation of ions such as HSO ,
4
−
ClO , and BF − in aqueous electrolytes. Graphite intercalation compounds in
4
4
H 2 SO 4 containing HNO 3 have shown some encouraging results [42]. In lead–acid
batteries, graphite in the positive electrode is beneficial because the formation of an
intercalation compound C n HSO 4 ·2.5H 2 SO 4 expands the electrode structure [43].
This expansion increases the porosity and the amount of electrolyte available in the
electrode to improve the discharge performance. More recently, carbon has played
a pivotal role in the success of Li-ion batteries, serving as the host material for
lithium storage in the negative electrode. In this application, the high electronic
conductivity of carbon and its ability to intercalate and/or adsorb lithium ions are