Fe3O4 nanoparticles with similar to 10 nm diameters were synthesized by an extremely low-cost, scalable and relatively biocompatible chemical co-precipitation method. Magnetic measurements revealed that Fe3O4 nanoparticles have bifunctional superparamagnetic and ferromagnetic character with saturation magnetization (Ms) values of 64 and 71 emu g(-1) at 298 K and 10 K, respectively. Pseudocapacitive Fe3O4 nanoparticles were then integrated into hazelnut shells - an abundant agricultural biomass - by an energy efficient hydrothermal carbonization method. Presence of magnesium oxide (MgO) ceramic template or its precursor in the hydrothermal reactor allowed simultaneous introduction of pores into the composite structure. Hierarchically micro-mesoporous Fe3O4/C nanocomposite possesses a high specific surface area of 344 m(2) g(-1). Electrochemical properties of Fe3O4/C nanocomposite were investigated by cyclic voltammetry and galvanostatic charge-discharge measurements in a conventional three-electrode cell. The Fe3O4/C nanocomposite is able to operate in a large negative potential window in 1 M Na2SO4 aqueous electrolyte (-1.2-0 V vs. Ag/AgCl). Synergistic effect of the Fe3O4 and carbon leads to enhanced specific capacitance, rate capability and cyclability making Fe3O4/C nanocomposite a very promising negative electrode material for asymmetric supercapacitors. (C) 2016 Elsevier B.V. All rights reserved.