Pseudocapacitive Carbon Nanofibers Using Sodium Chloride (a.k.a common salt)

Pseudocapacitors have gained attention due to their higher achievable energy densities than electric double layer capacitors. The most studied pseudocapacitive materials are transition metal oxides such as RuO₂, MnO₂, etc. and conducting polymers such as polyaniline and polypyrrole. However, these materials present some important drawbacks such as they are expensive, operate in a low voltage window, cause decomposition of the electrolyte, or have poor stability. An alternative approach is the surface modification of carbon materials to include functional groups, which can participate in pseudofaradaic charge transfer reactions and maintain the high cyclability of supercapacitors.

A novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing carbon nanofibers using common salt (sodium chloride (NaCl)) is presented. NaCl enhances capacitance by modifying carbon surface functionalities, while still keeping intact the free-standing matrix of carbon nanofibers. The blend of sodium chloride 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 (CNF-13) compared to that without NaCl (CNF-0) (Fig.1). 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 (57% at 20 A g^-1) and high cyclability.

Fig. 1 Cyclic voltammograms of the treated carbon nanofibers in 1 M H₂SO₄ using a two-electrode test cell at a scan rate of 5 mV s^-1.

References:

1. R. Singhal and V. Kalra, J. Mater. Chem. A, 2015, 3, 377–385.