The Moore group is actively contributing to the development of materials for the next generation energy storage systems. Our main projects are the preparation and study of new redox active molecules, electrolytes for non-aqueous media, and polymeric membranes and separators.
Electrochemically reversible fluids of high energy density are promising materials for capturing the electrical energy generated from intermittent sources like solar and wind. To meet this technological challenge there is a need to understand the fundamental chemistry and transport properties of redox-active molecules and macromolecules. Our research builds on the group’s expertise in organic, polymer, and colloid synthesis, as well as physical organic chemistry. We are
developing redox-active polymers (RAPs) and redox-active colloids (RACs) to overcome limitations of battery electrolytes for grid storage. The project is highly collaborative and is part of the Joint Center for Energy Storage Research, also known as JCESR. Moore group members collaborate closely with the Rodriguez-Lopez group, among others. An example of a current project on this topic is described below.
Fig. 1: Size exclusion based rejection of redox active polymers and colloidal particles while permitting counter ions to pass through
Widely available porous separators, currently used in non-aqueous Li-ion batteries, prevent energy-storing particles (RAPs) from crossing through their pores. On the other hand, the charge-balancing ions in the flow battery are significantly smaller than these RAP particles, so they can cross freely through the pores.
Non-aqueous flow batteries (NRFB) offer high energy density compared to aqueous flow batteries. However, NRFBs suffer from low ionic conductivity, which is one of the major hurdles to be overcome for their commercialization. Thus, fast and selective transport of the charge balancing ions and negligible crossover of the redox active material are highly desired. We proposed a size-exclusion strategy to transport the charge balancing ions while preventing the redox active material from crossover (Fig.1).
For the first time, redox active polymers (RAP 6-8) that are soluble in propylene carbonate up to 1 molar concentration (based on viologen repeat unit molecular weight) are reported. Viologen polymers also show reversible two electron reduction process, and ~ 98% of viologen units are electroactive in 21 kDa and 85 kDa polymers. Thus, high energy densities can be realized with these polymers. Electron self-exchange between viologen repeat units in polymers is observed. This will help to transport electrons into the core of the redox aggregated structures (such as films, and colloidlal particles).
Fig. 2: Monomer and redox active polymers (RAP’s) synthesized and studied in this work
Figure 3 shows the diffusion of electrolyte salt (LiBF4), redox active small molecule, and redox active polymers of different molecular weights (21, 85, 171 kDa) across two commercial off-the-shelf membranes (Celgard 2400, 2325) of two different pore sizes (43 and 28 nm) were studied using Permgear Side-Bi-Side cell. As expected, concentration of the electrolyte salt and monomer were found to be same in both the compartments within 24 h indicating fast diffusion of small molecules across the membrane; however , all the three polymers showed only ~15% crossover for 28nm pore size membrane. Molecular weight dependent crossover was observed for the 43nm pore size membrane i.e. the low MW 21 kDa polymer shows higher crossover compared to the high MW polymers.
Fig. 3: Polymers and LiBF4 diffusion across membranes of two different pore sizes.
To summarize, we have provided proof of concept demonstration for redox active polymers in which Coulombic efficiency of flowable electrodes is maintained by size-exclusion based mechanism of separation. Size-exclusion strategy will enable the use of COTS membranes in flow batteries, thus helps to bring down the cost, and increase the ionic conductivity and Coulombic efficiency of flow batteries. For the first time a redox active polymer that meets the requirements for use as anolyte in NRFBs is developed. A redox active polymer approaching the energy density requirements of anolytes in NRFBs is developed. Charge capacity of viologen polymers is ~52 amp.h/L; with lithium as the counter electrode it has the energy density ~100 W.h/L. Molecular design principles developed here are useful to develop redox active nanostructures and colloidal particles for use in non-aqueous colloidal flow batteries.
Autonomic Materials for Smarter, Safer, Longer Lasting Batteries
Gavvalapalli N.; Jingshu H.; Cheng, K.J.; Lichtenstein, T.; Shen, M.; Moore, J.S. and Rodríguez-López, J. "Impact of Redox Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Non-Aqueous Solvents" J. Am. Chem. Soc., 2014,136, 16309–16316 DOI: 10.1021/ja508482e