Effect of the Components of the Electrode on the Morphological and Electrochemical Performance of Manganese Dioxide-Based Electrode for Application in Hybrid Electrochemical Capacitor 1

There are two classes of electrochemical capacitors categorized by their energy storage mechanism: electrical double layer capacitors (EDLCs) exclusively made with high specific surface area carbon-based materials and pseudocapacitors including generally conducting polymers or transition-metal oxides. In transition metal oxide-based electrodes such as manganese oxide, the reversible redox processes occurring at or near the surface of the electrode material leads to higher charge storage  properties than carbon-based electrode EDLCs.² However, due to the low potential stability window of MnO₂-based electrode of about 1V, a hybrid electrochemical capacitor using electrodes with different potential range has been proposed.³ Such hybrid electrochemical capacitor can consist of a carbon negative electrode, a manganese dioxide-based positive electrode and a mild neutral electrolyte.⁴

Manganese dioxide is characterized by a theoretical specific capacitance of about 1233 F/g, which is far from being experimentally attainable.⁵ For example, in the case of thicker composite electrode and higher loading of MnO₂ the electrochemically addressable material is commonly in the 10 to 20 % range. ^{6, 7} To maximize the utilization of MnO₂, more fundamentals studies are needed to understand the role of all components of a composite electrode.

Composite electrode based on manganese dioxide, a binder (poly(tetrafluoroethylene, PTFE) and a carbon additive (Acetylene black or high surface area Black Pearls 2000) were characterized by scanning electron microscopy and nitrogen gas adsorption. The electrochemical performances of the MnO₂-Carbon-PTFE composite electrodes materials are evaluated by cyclic voltammetry. Brunauer–Emmett–Teller (BET) surface area measurements indicate that the addition of the PTFE binder does not block access to the porous network of MnO₂ and the two carbon powders. Only acetylene black appears to slightly adversely affect the mesoporous surface, presumably because of its larger particle size compared to Black Pearls 2000. The electrochemical utilization of MnO₂ is similar whether acetylene black or Black Pearls 2000 is used as carbon additive. This suggests that the porosity of Black Pearls, which could perhaps act as a reservoir of ionic species, does not appear to play a significant role as demonstrated by similar specific capacitance at slow scan rate. On the other hand, the effect of additional conductive agent such as acetylene black induces a slight increase of the specific capacitance at high scan rate. Finally, a plot of Coulombic efficiency as a function of the upper positive potential limit showed that using a high surface carbon support with MnO₂ can cancel the effect of the larger potential window of electroactivity of MnO₂because of its smaller electrochemical potential stability range.

References

 

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