Electrochemical performance of K+ -intercalated MnO2 nano-cauliflowers and their Na-ion-based pseudocapacitors

•In situ synthesis of K+ pre-intercalated MnO2.•Enhanced electrochemical performance due to presence of K+ for Na- ion supercapacitors in basic electrolytes.•High specific capacitance (∼148F g−1) of K+-MnO2, i.e. ∼20 % greater than MnO2.•Pseudocapacitive nature of the fabricated energy storage device.•Asymmetric device MnO2-U//AC with energy density of ̴ 23 Wh Kg−1.