Porous Functionalized Self-Standing Carbon Fiber Paper Electrodes for High-Performance Capacitive Energy Storage

A facile and cost-efficient approach to functionalize raw carbon fiber paper (CFP) used for a self-standing capacitive electrode has been proposed here. Benefiting from the improved specific surface area and surface functional groups, the functionalized CFP (F-CFP) showed much enhanced capacitive performance, 3 orders of magnitude higher than that of the raw CFP. It delivered the areal capacitance of 1275 mF cm^-2 at 5 mA cm^-2 with a rather wide voltage window of 1.4 V (-0.4 to 1 V vs Ag/AgCl) in 0.5 M H₂SO₄. However, in a neutral 1 M Na₂SO₄ aqueous solution, although the areal capacitance of 1115 mF cm^-2 at 3 mA cm^-2 is slightly smaller, the potential window is much wider (2 V, -1 to 1 V vs Ag/AgCl), indicating a high overpotential of hydrogen evolution. The areal capacitance was still as high as 722 mF cm^-2 at a very fast charge-discharge current density of 50 mA cm^-2, and about 66% of the initial capacitance (at 3 mA cm^-2) was remained in Na₂SO₄, indicating considerable rate capability.