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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
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