Free Standing Hierarchically-Structured Electrodes for Energy Storage Devices

Fabrication of novel three-dimensional material architectures is essential for successful development of energy storage devices that allow high rate operation along with sufficient energy capacity. We will present our work on development of nanofiber-based hierarchically structured materials for applications in electric double layer capacitors and lithium-based batteries using a simple and scalable electrospinning methodology. As a first step, a comprehensive study on fabrication and structure-control of porous carbon nanofibers exhibiting specific surface areas of >1500 m²/g and multi-levels of pore sizes in the range of micro (<2 nm), meso (<50 nm) and macropores (>50 nm) was conducted. In particular, blends of polyacrylontritrle (PAN) and a sacrificial polymer in dimethyl formamide (DMF) were electrospun into non-woven nanofiber mats with diameters in the range of 200-300 nm.  Fast evaporation of solvent (~200 nl/s) and high elongational flow rate (~10⁵ s^-1) during electrospinning allowed us to prevent phase separation and develop a co-continuous morphology of PAN and the sacrificial polymer in the nanofibers. Electrospun nanofiber mats were subjected to stabilization and carbonization processes to obtain porous carbon nanofibers (CNFs) as PAN converted to carbon and the sacrificial polymer decomposed out to create intra-fiber pores (Figure 1). These materials exhibit specific surface area of up to 1600 m²/g without any activation process. We exhibit the tunability of the pore sizes within CNFs by varying material composition. These materials were tested as free-standing electrodes (without any binder) for electric double-layer supercapacitors. Our study shows that they exhibit a large specific capacitance (210 F/g and 80 F/cm³), possibly due to the presence of a large fraction of meso-pores (2-4 nm) and macropores (>50 nm) compared to state-of-the-art activated carbons (pores <2 nm), which leads to an increase in the accessible carbon surface, thereby improving specific capacitance.  These electrodes exhibit near-ideal rectangular CV curve even at a high scan rate of 2V/s (Figure 2). This again is attributed to the relatively larger pore size in these materials compared to most activated carbons, which allows faster ion transport and charge/discharge. Note that activated carbons often deviate from ideal behavior even at a scan rate of 100 mV/s scan rate.These materials retain 75% performance at a large current density of 20 A g¹indicating excellent power handling capability. Preliminary data on use of these hierarchically structured materials for high energy density lithium-based batteries – Lithium-sulfur and Lithium-air, will be presented.