Discipline: Technology and Engineering
Subcategory: Materials Science
Tasha Williams - University of South Florida
Co-Author(s): Ramakrishnan Rajagopalan, Pennsylvania State University, PA
In this investigation, we report the fabrication and use of gel electrolyte membranes in electrochemical double layer capacitors. Highly porous ultrathin polyvinylidene fluoride (PVDF) gel electrolyte membranes were fabricated using breath figure method. We report the fabrication and use of gel electrolyte membranes in energy storage devices. Processing conditions such as humidity, casting thickness, drying time and conditions have been systematically varied in order to produce membranes with porosity > 60% and high electrolyte uptake. These ultrathin membranes were then used in development of high energy density electrochemical capacitors. The ionic conductivity of gel membranes can be considerably higher than conventional porous polymer separators. Limited research has been conducted on these membranes with a solid process that works. Currently, these membranes are being explored in development of lithium ion batteries, electrochemical capacitors, solar cells, fuel cells and electro-chromic displays. This new area of research is important and allows us to look at membranes differently and further develop a process that works in an electrochemical capacitor. The objective of this research was to develop a fabrication process that can lead to highly porous ultrathin PVDF gel electrolyte membranes. We used a high humidity process that takes advantage of evaporation induced cooling to create water condensation on polymer films. Water being a non-solvent can help in templating the pore structure of the membrane. In this study we tested porosity, electrolyte uptake, and ionic conductivity to determine whether the membrane could perform well. We originally used PVDF/Acetone solution getting an average ionic conductivity of around 4×10-4 S/cm-1. We changed the solution to PVDF/Acetone/Ethanol and found that by adding a small amount of non-solvent (ethanol) to the solution for the membrane, we are able to increase the porosity and the ionic conductivity. The porosity for all of the membranes tested ranged from 18-49%. In the first part of the research, we tried to optimize the membrane conditions that include casting thickness and exposure time in the humid chamber. We did not find any strong correlation between our process parameters and ionic conductivity of the membrane. This may have been due to the fact that the membrane had to be cast initially outside the chamber prior to the exposure in high humid conditions. It would be very helpful to design an apparatus where the entire process is done inside the humid chamber. Considering the delay in exposure to high humidity, a solvent with slightly higher boiling point than acetone could be a better option. Our second approach was to add a small amount of non-solvent such as ethanol with acetone in the dissolved PVDF solution. When this mixture was casted in the high humid chamber, we saw significant improvement in both porosity and ionic conductivity. Our preliminary results are encouraging and warrants further investigation. We compared ionic conductivity of our membranes with a state-of-the-art commercial PVDF membrane which is 100 micron thick. The results are comparable. The developed technique could be used to make even thinner membranes in the order of 5 10 micron. The current gel electrolyte membranes have ionic conductivity that is still at least an order of magnitude or more lower than the liquid electrolytes. Hence, there is still lot of room to improve the ionic conduction.
Funder Acknowledgement(s): I would like to thank Ramakrishnan Rajagopalan for helping me in the field. This study was supported, in part, by a grant from NSF Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST), EEC #1160483.
Faculty Advisor: Ram Rajagopalan, firstname.lastname@example.org
Role: I conducted the all of the research with the guidance of my mentor.