| NSF Org: |
CBET Division of Chemical, Bioengineering, Environmental, and Transport Systems |
| Recipient: |
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| Initial Amendment Date: | July 24, 2018 |
| Latest Amendment Date: | May 19, 2021 |
| Award Number: | 1805103 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Carole Read
cread@nsf.gov (703)292-2418 CBET Division of Chemical, Bioengineering, Environmental, and Transport Systems ENG Directorate for Engineering |
| Start Date: | August 1, 2018 |
| End Date: | July 31, 2021 (Estimated) |
| Total Intended Award Amount: | $185,503.00 |
| Total Awarded Amount to Date: | $185,503.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
500 S LIMESTONE LEXINGTON KY US 40526-0001 (859)257-9420 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
500 S Limestone 109 Kinkead Hall Lexington KY US 40526-0001 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EchemS-Electrochemical Systems |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
This project focuses on a type of battery called non-aqueous redox flow batteries (RFBs) that are promising for large-scale, stationary energy storage applications. RFBs have advantages for electrical grid-scale energy storage options that would reduce overall energy consumption when linked with an electrical grid. Non-aqueous RFBs that contain organic electro-active species have the following unique features relative to other RFB designs: higher operating voltages, non-corrosive electrolytes, smaller size, and use of scalable organic active materials (which are more environmentally friendly and potentially lower cost). This collaborative project addresses fundamental research to support the design of electrolytes for non-aqueous RFBs with high energy density, better stability, and acceptable fluid flow properties. This project will not only establish the foundational knowledge necessary to design electrolytes for next-generation grid storage batteries but will also provide fundamental insights into other electrochemical technologies necessary for a sustainable energy economy. The Principal Investigators Brushett and Odom have worked extensively with underrepresented groups in STEM fields and with mentoring undergraduate and graduate students in both research groups. Further, the PIs will establish summer student exchange programs with each other's institutions. At MIT, Dr. Brushett will engage with the THINK program, which seeks to foster exceptional innovation, networking, and knowledge in high school students working on projects that benefit the community. At University of Kentucky (UK), Dr. Odom will focus on Mixing Art & Science, which will introduce non-scientists to concepts and issues in energy collection and storage by attracting them with an accessible activity and will continue to serve as a co-organizer for UK's Expanding Your Horizons Annual Conference.
Fundamental knowledge gaps exist both in (1) the molecular design of stable concentrated redox active solutions and (2) the electrochemical characterization of these concentrated electrolytes. At present, most investigations have focused on molecular discovery and electrochemical characterization under dilute conditions followed by direct integration into an unoptimized laboratory flow cell for preliminary cycling analysis. This approach has led to uneven advances in the field as, to date, most nonaqueous flow cells have shown poor performance and durability. It is unclear whether the observed results are due to fundamental instabilities of the redox organic materials, concentration-dependent changes in the physical and electrochemical properties of redox electrolytes, or failures in cell design and engineering. This collaborative research project focuses on the development of soluble and stable redox active molecules, based on substituted phenothiazines, as a platform chemistry for characterizing physical and electrochemical properties of solutions containing high concentrations of redox active materials and supporting salts in organic electrolytes - referred to as "redox electrolytes" - for use in nonaqueous flow batteries. The major scientific outcome of this research will be fundamental understanding of the role of chemical structure and surrounding electrolytes on the performance and durability of redox active organic materials at high concentrations in aprotic organic electrolytes. Further, new electrochemical methods will be developed to enable unambiguous characterization of concentrated nonaqueous redox electrolytes.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This collaborative research explored development of soluble and stable redox active molecules and characterization of physical and electrochemical properties of solutions containing high concentrations of redox active materials. Current techno-economic analyses suggest use of highly concentrated solutions of redox electrolytes for nonaqueous redox flow batteries could make them competitive with their aqueous counterparts.1-2 We focused on understanding factors that increase solubility of these redox active molecules across all states of charge. Introducing solubilizing groups on phenothiazine core helped to increase solubility in the uncharged state of the molecule (Figure 1).3-6 For e.g., addition of glycol chain at the N atom gave N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT)4, which is a liquid at room temperature and miscible with polar solvents. The systematic chemical modification of the phenothiazine core allowed us to access an array of compounds with varying solubilities in uncharged state.
Despite our effort to adopt simple and high-yielding synthetic routes, designing and synthesizing new redox active compounds with improved solubilities is time-demanding, often involving a trial-and-error approach. Mixtures of solids are known to undergo the phenomenon of melting point depression which, in turn, is associated to an overall increase in solubility of the mixture. We also studied binary and ternary mixtures of phenothiazines chosen from the active materials library developed in our group as higher-solubility alternatives to the pure constituents of such mixtures. We developed a protocol which allows to identify the mixtures and ratios of phenothiazines that could potentially exhibit an enhancement in overall solubility through the detection of melting point depression or eutectic melts by differential scanning calorimetry.
For use as a concentrated electrolyte, a redox active compound needs to be soluble at all concentrations in all states of charge. Design principles that work for enhancing solubility of uncharged form does not follow the same trend for charged form. For the charged form (radical cation salt), we studied effect of anion identity on the solubility of the phenothiazine radical cation. MEEPT-X (X = BF4, ClO4, PF6, SbCl6, SbF6, OTF, FSI, TFSI, PFSI, FTFSI) were synthesized via chemical oxidation to produce crystalline salts; their solubility was determined using NMR (Figure 2). Replacing the BF4 anion with the bis(trifluoromethanesulfonyl)imide (TFSI) anion in the charged state of MEEPT enhances the solubility by three times. A correlation between solubility and melting point is observed with most organic anions, with lower melting salts leading to increased solubility. We were able to enhance the solubility of charged forms by changing the chemical environment or optimizing the electrolyte (Figure 1) being used for electrochemical analysis.
The increase in concentration of redox active molecules linearly increase the energy density, however it is currently unclear whether highly concentrated electrolytes will have the required physical properties to favor their efficient cycling. In the dilute concentration regime, the conductivity increases linearly with concentration of charge carriers, however beyond dilute conditions the increase in viscosity and decrease in ionic mobility lowers the conductivity. Towards the end of the project, we studied and compared the change in physical properties and stability of the redox couples with concentration to better understand cycling at high concentration.6 We studied the concentration dependent flow cell cycling of MEEPT and MEEPT-TFSI redox couple to figure out at what point increased viscosity and lowered conductivity and diffusivity matter with efficient flow cell cycling of concentrated electrolytes. We also explored the effect of temperature to overcome some of these challenges.
References:
1. Darling, R. M.; Gallagher, K. G.; Kowalski, J. A.; Ha, S.; Brushett, F. R., Pathways to Low-Cost Electrochemical Energy Storage: A Comparison of Aqueous and Nonaqueous Flow Batteries. Energy Environ. Sci. 2014, 7, 3459-3477.
2. Dmello, R.; Milshtein, J. D.; Brushett, F. R.; Smith, K. C., Cost-driven Materials Selection Criteria for Redox Flow Battery Electrolytes. J. Power Sources 2016, 330, 261-272.
3. Kaur, A. P.; Holubowitch, N. E.; Ergun, S.; Elliott, C. F.; Odom, S. A., A Highly Soluble Organic Catholyte for Non-aqueous Redox Flow Batteries. Energy Technol. 2015, 3, 476-480.
4. Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries. Energy Environ. Sci. 2016, 9, 3531-3543.
5. Attanayake, N. H.; Kowalski, J. A.; Greco, K.; Casselman, M. D.; Milshtein, J. D.; Chapman, S. J.; Parkin, S. R.; Brushett, F. R.; Odom, S. A., Tailoring Two-Electron Donating Phenothiazines to Enable High Concentration Redox Electrolytes for Use in Nonaqueous Redox Flow Batteries. Chem. Mater. 2019, 31, 4353-4363.
6. Attanayake, N. H.; Liang, Z.; Wang, Y.; Kaur, A. P.; Parkin, S. R.; Mobley, J. K.; Ewoldt, R. H.; Landon, J.; Odom, S. A., Dual function organic active materials for nonaqueous redox flow batteries. Materials Advances 2021, 2 (4), 1390-1401.
Last Modified: 11/29/2021
Modified by: Aman Preet Kaur
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