Using common salt to impart pseudocapacitive functionalities to carbon nanofibers†
A novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing carbon nanofibers using common salt (sodium chloride) is presented. The blend of sodium chloride (NaCl) and polyacrylonitrile is electrospun together, followed by pyrolysis and mild acid treatment to obtain functionalized free-standing (binder-free) carbon nanofibers. The synthesized materials have a low surface area of only 24 m2 g−1, however the electrochemical studies show a five-fold increase in specific capacitance on incorporation of NaCl compared to that without NaCl. The XPS characterization demonstrates that the presence of NaCl leads to enhanced oxygen on the surface of carbon nanofibers, particularly in the form of carboxyl groups. These carboxyl groups then facilitate the adsorption of sulfur functional groups on acid treatment. A high specific capacitance of 204 F g−1, areal capacitance of 1.15 F cm−2, and volumetric capacitance of 63 F cm−3 in 1 M H2SO4 are obtained, which are attributed to the surface functional groups participating in the pseudocapacitive redox reactions. The fabricated nanofibers demonstrate good capacitance retention at high current densities and high cyclability.