Page 311 - Handbook of Battery Materials
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10.3 Electrochemical Behavior 281
for the electrochemical reactions of oxygen and at the same time do not promote
carbon oxidation.
In redox flow batteries such as Zn/Cl 2 and Zn/Br 2 , carbon plays a major role
in the positive electrode where reactions involving Cl 2 and Br 2 occur. In these
types of batteries, graphite is used as the bipolar separator, and a thin layer of
high-surface-area carbon serves as an electrocatalyst. Two potential problems with
carbon in redox flow batteries are: (i) slow oxidation of carbon and (ii) intercalation
of halogen molecules, particularly Br 2 , in graphite electrodes. The reversible redox
potentials for the Cl 2 and Br 2 reactions (Equations 10.8 and 10.9)
−
Cl 2 + 2e → 2Cl − (10.8)
−
Br 2 + 2e → 2Br − (10.9)
are 1.358 and 1.066 V, respectively. These potentials are considerably higher than
the reversible potential for the C/H 2 O reaction (see Table 10.2), which suggests
that carbon is susceptible to oxidation at the redox potentials for the Cl 2 and Br 2
reactions.
In the Zn/Cl 2 battery, carbon is utilized in both electrodes, serving as a
flow-through positive electrode and a substrate for the zinc negative electrode.
The requirements are listed below. Chlorine, flow-through electrode:
• relatively narrow pore size distribution for uniform flow characteristics;
• uniform porosity and permeability for good electrolyte flow distribution;
• low resistivity to minimize IR drop in the electrode;
• capability to accept activation treatment;
• no distortion in flowing electrolyte;
• adequate physical strength to permit press-fitting of electrode into the intercell
busbar.
Graphite substrate for zinc deposit:
• low surface porosity and fine grain-size for attaining an adherent and uniform
zinc deposit;
• low exchange current for hydrogen evolution;
• good physical strength for press-fitting of the electrode into the intercell bus-bar;
• easily machined into thin electrodes, about 1 mm thick.
Jorne et al. [36] investigated the reactivity of graphites in acidic solutions that are
typically used for Zn/Cl 2 cells. The degradation of porous graphite is attributed
to oxidation to CO 2 . The rate of CO 2 evolution gradually decreased with oxidation
time until a steady state was reached. The decline in the CO 2 evolution rate is
attributed to the formation of surface oxides on the active sites.
A composite consisting of a mixture of carbon particles (e.g., carbon black
or graphite) and a polymer binder such as polyethylene or polypropylene with
a surface layer of a carbon-black or carbon-felt flow-through structure serves
as the Br 2 electrode in Zn/Br 2 batteries. Because of the low surface area of
the carbon-polymer surface, an additional layer of carbon is necessary to obtain
higher reaction rates. The mechanical deterioration of graphite-polymer composite