Diamond-coated silicon wires for supercapacitor applications in ionic liquids

• We uniformly coated highly complex 3D nanostructures with diamond.
• We achieved an energy density of 84 μJ/cm2 and a power density of 0.94 mW/cm2.
• We found that diamond nanomaterial is suitable for high frequency supercapacitor.

For a long time sp2 carbon has been the dominating material for supercapacitor applications. In this paper a new concept of using boron-doped diamond for supercapacitors is proposed. Diamond surface enlargement is realized via bottom-up template-growth. In this method, silicon nanowire electrodes are coated with a thin (~ 100 nm) layer of nanocrystalline diamond (NCD) by microwave enhanced chemical vapor deposition (MWCVD). The quality of overgrowth is characterized by high resolution scanning electron microscopy which reveals a homogeneous coverage of diamond on Si nanowire surface. To enhance the potential window to 4 V, a room temperature ionic liquid is used as electrolyte. The dilution of the ionic liquid is investigated in terms of conductivity and specific capacitance. The capacitance as measured via cyclic voltammetry reaches 105 μF/cm2. An energy density of 84 μJ/cm2 and a high power density of 0.94 mW/cm2 are obtained in combination with good stability of over 10,000 charging/discharging cycles.