Optimising surface chemistry for second-life recycled carbon fibres: gains in composite performance at the expense of energy storage

Surface modification of recycled carbon fibres (rCFs) enables the development of high-value second life applications, by improvement of mechanical or multifunctional properties. This work investigates the potential of second-life applications in structural composites and energy storage by the translation of the bis-aryldiazomethine surface modification on reclaimed (recycled) non-woven CF fabrics. Bis-aryldiazomethines, which are capable of generating highly reactive diarylcarbenes upon thermal activation, can spontaneously tether onto the CF surface, irrespective of the surface chemistry or morphology of the fibre surface. Based on our previously reported study with pristine CFs, an optimal 5-mM concentration was employed to surface modify 200 g m–2 non-woven textiles. Composite laminates consisting of 15 layers of such non-woven fabrics infused within an epoxy system reported statistically significant improvements of 10.07%, and 10.84% for flexural modulus and fracture toughness respectively in comparison to the untreated control. We then explored a non-traditional use for these modified fibres as free-standing anodes within lithium-ion batteries. A broader range of concentrations, i.e. 0.1, 0.5, 1, 5 and 10 mM were employed for surface modification of 60 g m–2 non-woven fabrics, which were subsequently assembled into half-cell configurations. Cycling performance studies indicated that with the increase in bis-aryldiazomethine concentration, the attainable specific capacity reduced both in rate capability testing and over long-term cycling. In fact, the control (untreated) fibres reported an initial specific discharge capacity of 235.64 ± 9.63 mAh g−1 whereas the modified fibres reported relatively lower values of 207.21 ± 24.7, 179.82 ± 54.46, 82.59 ± 2.80, 148.54 ± 29.82 and 102.09 ± 10.41 mAh g−1 for 0.1-, 0.5-, 1-, 5- and 10-mM modified fibres respectively. This is an important observation that shows that attempts to tune the surface chemistry of these fibres come at the cost of capacity. Electrochemical impedance spectroscopy revealed increasing interfacial and charge transfer resistance upon increase in grafting concentration. Overall, this work depicts the versatility of bis-aryldiazomethine driven thermal surface modification as a potential pathway towards the use of rCFs within multifunctional second-life applications. However, it also highlights the importance of balancing the effective concentrations employed depending on the targeted end-application, as an improvement in one property often comes at the expense of another.