Promising Trade-Offs Between Energy Storage and Load Bearing in Carbon Nanofibers as Structural Energy Storage Devices

Structural energy storage materials refer to a broad category of multifunctional materials which can simultaneously provide load bearing and energy storage to achieve weight reduction in weight-sensitive applications. Reliable and satisfactory performance in each function, load bearing or energy storage, requires peculiar material design with potential trade-offs between them. Here, the trade-offs between functionalities in an emerging class of nanomaterials, carbon nanofibers (CNFs), are unraveled. The CNFs are fabricated by emulsion and coaxial electrospinning and activated by KOH at different activation conditions. The effect of activation on supercapacitor performance is analyzed using two electrode test cells with aqueous electrolyte. Porous CNFs show promising energy storage capacity (191.3 F g(-1) and excellent cyclic stability) and load-bearing capability (sigma(f) > 0.55 +/- 0.15 GPa and E > 27.4 +/- 2.6 GPa). While activation enhances surface area and capacitance, it introduces flaws in the material, such as nanopores, reducing mechanical properties. It is found that moderate activation can lead to dramatic improvement in capacitance (by >300%), at a rather moderate loss in strength (<17%). The gain in specific surface area and capacitance in CNFs is many times those observed in bulk carbon structures, such as carbon fibers, indicating that activation is mainly effective near the free surfaces and for low-dimensional materials.