Given the growing demand for new materials for supercapacitor applications, high entropy alloys (HEAs) are being extensively investigated. They are an efficient alternative to existing energy sources due to their synergistic contribution from individual element. We demonstrate the development of nanostructured HEA (FeCoNiCuMn) as a cathode material with specific capacitance (C _s) of ~388 F g ^-1 (5 mV s ^-1). As anode material, green graphene (rice straw biochar) synthesized using pyrolysis shows a maximum C _s of ~560 F g ^-1 at similar scan rate (5 mV s ^-1). A hybrid asymmetric liquid state device was assembled using the FeCoNiCuMn nanostructured HEA and green graphene as electrodes. Utilizing the green source, the device provided a high C _s of 83.22 F g ^-1 at 2 A g ^-1. The specific energy of the device was 33.4 Wh kg ^-1 and specific power of 1.7 kW kg ^-1. The electrochemical behavior of each element in the high entropy composition was studied through post X-ray photoelectron spectroscopy and scanning electron microscopic analysis. The chemical behavior of FeCoNiCuMn is further investigated using DFT studies. The enhanced electrochemical properties and synergistic contribution of each element of the HEA is studied via of d-band theory. The current work can be utilized to develop asymmetric hybrid supercapacitors as environmental friendly energy source.