Multifunctional polymeric surface coatings of carbon fibre electrodes for enhanced energy storage performance

We report a proof-of-concept study on the development of multifunctional surface coatings for carbon fibre (CF) electrodes in structural energy composites, which for the first time addresses three key properties of CF: capacitance, tensile properties and interfacial adhesion, simultaneously. Multifunctional coatings have been designed by covalently grafting conductive and redox active poly(o-phenylenediamine) (PoPD) on CF via surface electro-initiated polymerisation using different diazonium salts. This has resulted in improvements in specific capacitance, tensile strength, tensile modulus and interfacial shear strength (IFSS) up to 30 F g⁻¹, 4.58 GPa, 276 GPa and 43.5 MPa, respectively, all of which are higher than the values reported for pristine CF. To further improve the IFSS, a bilayered polymeric coating has been designed by electro-grafting polyacrylamide on top of the conductive PoPD layer, which led to specific capacitance, tensile strength, tensile modulus and IFSS (up to 9 F g⁻¹, 4.28 GPa, 256 GPa and 55.6 MPa, respectively). The non-conductive polyacrylamide layer on top of the PoPD layer is thought to be the reason for depressed capacitive performance, though this does come at a trade-off for improved fibre–matrix adhesion. Thus, a means to tailor this material based on end user priorities is presented herein. In contrast to the conventional methods of surface activation for improving the capacitance of CF, which often result in a trade-off between electrochemical and mechanical/interfacial properties of CF, this method offers a means to simultaneously enhance the electrochemical, mechanical, and interfacial performance of CF with great tunability.