α-MnO2 composite with gold nanoparticles on carbon cloth modified with MOFs-derived porous carbon for flexible and activity-enhanced sodium-ion supercapacitors

Improving the catalytic activity of electrodes in a limited space is crucial to energy storage devices such as supercapacitors. Herein, the surface activity of the flexible electrode (Au-MnO2/CPCN@CC) is enhanced by combining gold particles (AuNPs) modified α-MnO2 with conductive porous carbon nanoflakes (CPCN) derived from the metal-organic framework (MOF) on carbon cloth (CC). The Au-MnO2/CPCN@CC electrode exhibits a higher specific capacity of 503.7 F/g at 0.125 mA/cm2 and retains 87.68% capacity after 10,000 cycles in 1 M Na2SO4, while the structure without AuNPs achieves 242.6 F/g and 61.07% capacity retention after 10,000 cycles. The high stability of Au-MnO2/CPCN@CC stems from the good pseudocapacitance ratio of 83.80% at 1 mV/s compared to 78.43% of MnO2/CPCN@CC and 40.88% of Au/CPCN@CC. The supercapacitor assembled with Au-MnO2/CPCN@CC as the positive electrode and activated carbon (AC) as the negative electrode in 1 M NaPF6 shows an excellent power density of 79.96 W/kg at 71.48 Wh/kg as well as 81.93% capacitance retention after 10,000 cycles. Density-functional theory (DFT) calculations of Au-MnO2/CPCN composite reveal a sodium-ion adsorption energy of 3.744 eV and a large DOS near Fermi level. Hence, Au nanoparticles (AuNPs) and MOFs-derived CPCN lead to enhanced surface electron transport of MnO2 and ultimately superior performance.