A Comparative Study of Electrolyte Types and Processing Parameters in the Electrochemical Oxidation of Carbon Fibers and Their Interfacial Interactions with Vinyl Ester Resin

The chemically inert surface of carbon fibers (CFs) limits their interfacial adhesion with vinyl ester resin (VER). In this study, electrochemical surface treatment was performed in four electrolyte systems with different acid–base characteristics, including H2SO4, NH4HSO4, NH4HCO3, and KOH. The effects of electrolyte type, concentration, current density, and treatment time on CF surface properties and CF/VER interfacial performance were evaluated. Acidic electrolytes produced stronger oxidative functionalization, increasing the surface oxygen content from 13 at.% to 27 at.% and 26 at.% for H2SO4- and NH4HSO4-treated CFs, respectively. All treatments significantly improved wettability, reducing the water contact angle from 116.1° to 30.2–49.4° and the VER contact angle from 121.9° to 29.4–50.4°. Consequently, the interfacial shear strength increased from 27.9 MPa to 42.8–48.4 MPa, while the single-fiber tensile strength remained at 3.1–3.3 GPa. The enhancement was attributed to the synergistic effects of chemical oxidation and mild physical etching. These results clarify the distinct roles of acidic and alkaline electrolytes and provide guidance for optimizing electrochemical surface treatment of CFs for high-performance CF/VER composites.