Porous carbons with tailored heteroatom doping and well-defined porosity as high-performance electrodes for robust Na-ion capacitors

As one of the next-generation electric energy storage devices, new-born Na-ion capacitors gain more and more attention due to their merits inherited from batteries and capacitors. Herein, we propose an efficient method to prepare two tailored carbons as cathode and anode materials for high-performance Na-ion capacitors. As low-cost biowaste, fish scale (with hydroxyapatite inside) is specifically chosen as the precursor and pyrolyzed with potassium hydroxide. Due to the synergetic effect of potassium hydroxide and hydroxyapatite, well-defined hierarchical structures with interconnected pores can be realized. Moreover, the collagen within fish scale acts as precursor to form carbons with heteroatoms. By simply adjusting potassium hydroxide dosage and pyrolysis temperature, two N,O-doped hierarchical porous carbons are obtained and further employed as cathode and anode materials, respectively. The hierarchical porous carbon electrodes exhibit superior electrochemical performance in half cells due to the hybrid energy-storage features induced by rich heteroatoms and unique porosity. Consequently, the assembled Na-ion capacitor using these two hierarchical porous carbon electrodes delivers high energy density (103.2 W h kg(-1)), power density (15.9 kW kg(-1)) and long cycling lifetime (81.1% of the initial capacitance over 2500 cycles).