Size-controlled Ag quantum dots decorated on binder-free hierarchical NiCoP films by magnetron sputtering to boost electrochemical performance for supercapacitors

Abstract

This paper reports novel binder-free and self-supported electrodes of hierarchical nickel–cobalt phosphide (NiCoP) films decorated with size-controlled Ag quantum dots by magnetron sputtering (Ag/NiCoP). Ag quantum dots with an average particle size of 7.90 nm uniformly distribute over the nanosheet-assembled architecture of NiCoP films. Benefitting from the good ohmic contact in the interfaces between Ag quantum dots and NiCoP nanosheets, Ag/NiCoP exhibits an ultrahigh specific capacitance of 6150 mF cm⁻² (3050 F g⁻¹ at 1 A g⁻¹) higher than the 3445 mF cm⁻² (1722 F g⁻¹ at 1 A g⁻¹) of bare NiCoP at 2 mA cm⁻². The specific areal capacitance has been increased by 78.5% after introducing Ag quantum dots. 34% capacitance retention rate is achieved while the current density increases from 2 to 30 mA cm⁻². The cycling stability displays a remarkable capacitance retention of 73% for 4000 cycles at 30 mA cm⁻². These boosted electrochemical performances are mainly attributed to the synergistic effects of enough electroactive sites, high electronic conductivity, and easy electrolyte ion diffusion. An asymmetric supercapacitor is fabricated using hierarchical Ag/NiCoP as the positive electrode and activated carbon as the negative electrode. The supercapacitor delivers an energy density of 0.254 mW h cm⁻² (1.81 mW h cm⁻³) at a power density of 1.88 mW cm⁻² (13.4 mW cm⁻³). At a power density of 18.8 mW cm⁻² (134 mW cm⁻³), an energy density of 0.115 mW h cm⁻² (0.82 mW h cm⁻³) can still be maintained. This study provides an avenue to design a novel generation of supercapacitors for energy storage devices.