Design and Characterization of Electrospun Mats with Tailored Morphologies for Enhanced Active Layer Performance in Energy Conversion and Energy Storage Applications

The goal of this research was to utilize the morphological control inherently imparted by the electrospinning process to improve the active layer performance in energy conversion devices as well as to better understand the relationship between morphology and performance in energy storage devices. Discrete control of the active layer morphology can promote exciton dissociation in organic photovoltaic cells (OPVs), whereas developing efficient ion diffusion pathways and beneficial polymer-ion interaction in polymer-gel electrolytes is demonstrated to result in enhanced battery performance.

We demonstrate the ability to develop unique morphologies in Poly(3-hexafluoro propylene) (P3HT) films with energy storage applications using various electrospinning techniques. Electrospinning in a solvent-saturated atmosphere allows for the design of ribbon architectures with polymer domains on the order of 5-10 um. These ribbon structures form what appear to be bi-continuous films, which could then be filled with an acceptor / fullerene type material to create a bulk heterojucton for OPV devices. Dropping chloroform onto the electrospinning needle during the spinning process results in P3HT fibers with porous surfaces. These fibers have diameters of ~ 2 um. Using a coaxial needle to electrospin a P3HT solution in the core, and a CHCl3 sheath solution created hybrid ribbon-fiber structures. These structures have even smaller domain sizes than the ribbons created using a solvent saturated atmosphere. Cospinning P3HT with sacrificial polymers results in P3HT fiber morphologies upon removal of the sacrificial template polymer. Additionally, introducing P3HT into an established fiber matrix results in fibrous P3HT architectures after the template fibers are removed.

Developing hybrid polymer-gel electrolytes using crosslinked PEO electrospun fibers results in membranes with high affinity for liquid electrolyte components. These electrospun PEO fiber mats exhibit excellent ionic conductivities at room temperature (12 mS/cm) exceeding an electrospun PVDF control. Furthermore, the PEO fiber mats can absorb nearly three times as much liquid electrolyte as the PVDF control. PEO has been show to interact with lithium salts to aid in dissociation and diffusion during battery cycling. Although the ionic conductivity data suggest PEO to be a superior electrolyte, pulsed-field-gradient NMR shows that lithium diffusion is faster in PVDF samples. From coin cell discharge experiments, PEO is believed to interact strongly with Li+ ions, inhibiting them from diffusing rapidly during fast charge/discharge rates. However, PEO/PETA fiber electrolytes show nearly 100% theoretical capacity discharge at C/100 and a capacity retention of ~ 35% at a C/5 discharge rate in contrast to a glass fiber separator which shows only a capacity that is approximately 85% of the theoretical value.

The unique mechanical properties of PEO/PETA electrospun mats could lead to interesting artificial skin and wound healing applications. Upon crosslinking at elevated temperatures (~40 degrees C), the fiber mats exhibit improved tensile strength and much higher ultimate stress at break. The porous nature of the materials lend to easy oxygen diffusion for wound healing, and the hydrophilicity promotes continued adhesion to existing tissue making these mats possible adhesive-less bandages.