A novel strategy to synthesize hierarchical rod like MnCO3 on the reduced graphene oxide (RGO) sheets by a facile and cost-effective hydrothermal method is demonstrated. The chelating action of citric acid facilitates the formation a complex intermediate of Mn2+ and citrate ions, which finally results a 3D MnCO3/RGO (MRGO) composite with high electrical conductivity (∼1056 S m−1), good surface area (59 m2 g−1) and high pore volume (0.3 cm3 g−1). The specific capacitance (SC) of the MRGO composite is ∼1120 F g−1 at a current density of 2 A g−1 in three electrode system. An asymmetric device has been designed with MRGO as positive and sono-chemically reduced RGO (SRGO) as negative electrode material. The asymmetric device (MRGO//SRGO) shows the SC of ∼318 F g−1 (at 2 A g−1) and energy density of ∼113 W h kg−1 (at 1600 W kg−1). The true energy density (1.7 W h kg−1) has been calculated considering the total weight of the device. The MRGO//SRGO device can power a wall clock for ∼13 min after full charging. The Nyquist plot of the asymmetric cell has been simulated with Z-View software to measure the solution resistance, charge-transfer resistance and Warburg elements.