Rational design of crystalline/amorphous nickel manganese phosphate octahydrate heterostructure for high-performance aqueous and all-solid-state asymmetric supercapacitors

•Nickel manganese phosphate grown in-situ on nickel foam by the hydrothermal method.•The Ni2Mn(PO4)2·8H2O exhibits a high specific capacitance of 2351.6 F g−1 at 1 A g−1.•Aqueous and all-solid-state asymmetric supercapacitor with excellent performance.•DFT calculations confirm the electron conductivity improvement.