Exploring the Impact of Oligomerization on Redox Flow Cell Performance

Abstract

The redox flow battery (RFB) is a promising stationary energy storage technology due to its inherent decoupling of energy and power, long service lifetime, and potential for low-cost manufacturing.1 While current embodiments are constrained by high upfront cost, nonaqueous RFBs may offer a pathway to cost-competitive energy storage systems through higher energy densities;2 however, this concept is at an early stage and several challenges must be overcome. In particular, the development of stable, high-voltage redox chemistries in conjunction with low-cost, selective membranes is needed to mitigate difficult-to-reverse capacity fade and reduced efficiencies. As such, a range of redox-active organic macrostructures, including oligomers (RAOs),3 polymers (RAPs),4 and colloids (RACs),5 have been described in the peer-reviewed literature. Of specific interest are RAOs, which, like the other macrostructures, can be paired with size-exclusion membranes but exhibit electrochemical and fluid dynamic properties that more closely align with monomers. While this proof-of-concept has been demonstrated,6,7 to our knowledge, few studies have systematically evaluated how the molecular properties of RAOs influence their performance in flow cells.In this presentation, we will describe our efforts to understand how the molecular properties of different oligomers influence their behavior in redox flow cells. To this end, we synthesize and characterize a set of model compounds, ranging from monomers to tetramers, that utilize TEMPO as the redox-active unit. We then assess the thermodynamic, kinetic, and transport properties of these compounds using a suite of voltammetric techniques. Subsequently, we characterize the performance and durability of these compounds in symmetric flow cells via electrochemical impedance spectroscopy, DC polarization, and durational cycling. We perform post-test characterization after the cycling experiments to disaggregate molecular- and cell-level sources of capacity fade. We find that, in general, oligomers with increased size exhibit lower diffusivities while maintaining the facile charge transfer characteristics afforded by the appended monomer. These size-dependent variations in mass transport rates directly translate to differences in cell polarization and cycling performance. Time-dependent capacity fade observed during cycling is confirmed by post-mortem analysis. Broadly, these results align with previously developed relationships between molecular size, electrochemical properties, and flow cell performance; we posit that the principles for functionalization and testing utilized here can be applied to other RAOs with improved stability. Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. The authors thank Jeffrey Kowalski and Katharine Greco of the Brushett Group as well as Yu Cao and Jeffrey Moore of the Moore Group at the University of Illinois for their contributions to conceptualizing the framework and preliminary testing for this work.