MnO2/thermally reduced graphene oxide composites for high-voltage asymmetric supercapacitors

•MnO2/rGO composites synthesized by an easy hydrothermal method.•rGO synthesized directly by using thermal exfoliation/reduction of graphite oxide.•No decrease of capacitance after 5000 cycles at 2.0 V for an asymmetric system.•A capacitance decrease of only 8% after 5000 cycles at 2.1 V was observed.•An outstanding energy density value of 24 Wh kg−1 at 100 W kg−1 was achieved.

We report on the facile fabrication method of MnO2/reduced graphene oxide (MnO2/rGO) composites for supercapacitor applications using the hydrothermal approach. The rGO obtained by thermal exfoliation/reduction of graphite oxide with negligible amount of oxygen (0.8 wt%) served as a highly stable, conductive support for porous MnO2-nanoneedle-like particles, providing a proper rate capability to the MnO2/rGO electrode. The MnO2/rGO (3:1) was used as the positive electrode in an asymmetric supercapacitor operating at an outstanding cell voltage of 2.1 V in 1 mol L−1 Na2SO4. Activated carbon was used as the negative electrode. The calculated energy densities reached remarkable values of 21 and 24 Wh kg−1 (at 100 W kg−1) for the asymmetric device charged up to 2.0 and 2.1 V, respectively. The cyclability test followed by post-mortem scanning electron microscopy (SEM) studies of the electrode material revealed excellent electrochemical stability of the MnO2/rGO electrode after cycling up to 2.0 V, as reflected by no loss of capacitance after 5000 cycles and preservation of the pristine morphology of the electrode material. The small decrease in capacitance (8%) after 5000 cycles of charge/discharge for the asymmetric system with an operating cell voltage of 2.1 V was the result of the MnO2 morphology transformation.

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