Free standing hollow carbon nanofiber mats for supercapacitor electrodes

Abstract

Free standing hollow carbon nanofiber (CNF) mats with high graphitic content have been fabricated through co-axial electrospinning followed by high temperature pyrolysis. A blend of polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA) in different weight ratios is used as the shell polymer whereas PMMA is used as the core polymer. The sacrificial PMMA template is removed during carbonization, creating a hollow core along with pores in the shell. In order to establish the best base case electrode, no further chemical or physical activation procedures or addition of metal oxide particles were employed. The structural and electrochemical properties of hollow CNFs as supercapacitor electrodes are systematically studied and compared with those of porous (PAN : PMMA weight ratios 2 : 1, 1 : 1, 1 : 2, 1 : 5) and solid CNFs. The nanofiber mats have been used directly as electrodes without binder and conductive additives owing to their good conductivity and mechanical stability. The hollow CNFs with the precursor polymer ratio of 1 : 5 (PAN : PMMA) in the shell, exhibited the highest specific surface area of 812.6 m² g⁻¹ with a large percentage of mesopores, delivering a capacitance of ∼185 F g⁻¹ at 5 mV s⁻¹, which was almost two orders of magnitude higher than the solid CNFs (1.2 F g⁻¹). In addition, the hollow CNFs exhibited an excellent charge/discharge capability delivering a capacitance of ∼105 F g⁻¹ at a current density of 2 A g⁻¹, with a capacitance retention of ∼80% after 3000 cycles. This study establishes the electrospun hollow CNFs as potential supercapacitor electrodes that can be easily modified further with the addition of functional materials.