Effects of Acid–Base Properties of Ammonium Salt Electrolyte on Electrochemical Surface Activation of Carbon Fibers

Surface activation is an essential process in carbon fiber preparation for surface inertness of carbon fibers. In this study, X-ray photoelectron, Raman spectroscopies, dynamic contact angle, monofilament tensile strength and interfacial shear strength (IFSS) measurements were used to investigate the activation effects of NH4HCO3, NH4H2PO4, and NH4HSO4 electrolytes on carbon fiber surfaces under the same conductivity conditions. The results showed that the surface structures and properties of carbon fibers after treatment with different ammonium salts differed significantly. Oxidation by active oxygen [O] was stronger under acidic conditions, i.e., with NH4H2PO4 and NH4HSO4, than that under alkaline conditions, i.e., with NH4HCO3. However, when the acidity was too high, oxidation by active oxygen [O] was weakened by the effect of H+ concentration. The surface graphitization degree (R) of carbon fibers decreased slightly after treatment with the three ammonium salts because of the effects of active oxygen [O] and anionic etching or stripping. As the electrolyte acidity increased, the R decreased more significantly, and the monofilament tensile strength increased more significantly. The dynamic contact angle between the activated carbon fibers and deionized water after treatment with NH4H2PO4 was the lowest. The IFSS was the largest and was 52.3% higher than that before treatment.