Super-capacitors are important energy storage devices complementary to rechargeable batteries. While super-capacitors possess high power density, their energy density is usually low which does not allow them to store energy as much as batteries. To enhance the energy density, asymmetric supercapacitors (ASCs) consisting of a carbon-based negative electrode and a battery-type positive electrode have been proposed and developed. However, the commonly used carbon-based materials only show comparatively low capacitance when used in negative electrodes, which limit the overall performance of ASCs.
Recently, the Nanomaterials for Electrochemical Energy Storage and Conversion (NESC) group demonstrated that cobalt phosphide (CoP) nanocrystals supported on carbon nanofibers (CoP/CNF) can serve as high-performance negative electrode materials in ASCs and show outstanding specific capacitance 748 F g−1 at 2 A g−1 and superior long-term cycle stability, outperforming conventional carbon-based and nearly all metal phosphide based negative electrode materials reported previously. The researchers have fabricated a binder-free mille-crêpe-like negative electrode by loading CoP/CNF onto an ultrathin, interconnected carbon nanotube (CNT) film current collector, followed by folding the film multiple times, which exhibits a high total-mass (active materials and current collector) based specific capacitance of 200.0 F g−1 at 1 A g−1, due to the lightweight of CNT film collector. Furthermore, they also demonstrated an all-phosphide ASC (APASC) using CoP/CNF/CNT as the negative electrode and the NiP/CNF/CNT the group developed last year (ACS Nano) as the positive electrode, which delivers a specific capacitance of 163.8 F g-1 and shows excellent long cycle stability up to 50000 cycles. The all-phosphide asymmetric supercapacitors the NESC developed hold substantial promise for use in the next-generation energy storage systems. A paper entitled “Mille-crêpe-like metal phosphide nanocrystals/carbon nanotube film composites as high-capacitance negative electrodes in asymmetric supercapacitors” has been published in ACS Applied Energy Materials. Research Fellow Nan Zhang is the first author of this paper. The research work was performed in collaboration with researchers from the Institute of Physics, Chinese Academy of Sciences.