The construction of a sandwich structured Co₃O₄@C@PPy electrode for improving pseudocapacitive storage

Sandwich structured hybrids consisting of a Co₃O₄ nanowire as the core, amorphous carbon (C) as the inner shell and a polypyrrole (PPy) outer layer as the exodermis are synthesized via a hydrothermal method and constant current electropolymerization. The formation mechanism and growth stage of PPy on carbon surfaces is investigated and it was discovered that PPy layer thickness, corresponding to nucleation time of the polymer, as the dynamic factor, can influence the pseudocapacitive properties of the obtained composites. The carbon layer acts as both a network to increase the electric conductivity and a buffer agent to reduce volume expansion of Co₃O₄ during ion insertion/extraction to achieve higher capacitance and better cyclic stability. So for a capacitor, the Co₃O₄@C@PPy electrode delivers a higher areal capacitance of 2.71 F cm^-2 at 10 mA cm^-2 (1663 F g^-1 at 6.1 A g^-1) and improved rate capability compared to Co₃O₄ and Co₃O₄@C. An asymmetric device is assembled by the Co₃O₄@C@PPy hybrids as a cathode and a relatively high energy density of 63.64 W h kg^-1 at a power density of 0.54 kW kg^-1 is obtained, demonstrating that the sandwich structured Co₃O₄@C@PPy hybrids have enormous potential for high-performance pseudocapacitors.