Exploring Performance Limiting Factors in Non-Aqueous Redox Flow Batteries

J.D. Milshtein, R.M. Darling, F.R. Brushett
MIT,
United States

Keywords: non-aqueous redox flow battery, flow cell, area specific resistance, polarization, electrochemical impedance spectroscopy

Summary:

Redox flow batteries (RFBs) are promising electrochemical devices are suitable for storing energy over multiple hours, and RFBs offer several advantages over enclosed batteries including independent scaling of power and energy, long service life, improved safety, and simplified manufacturing. While aqueous RFB chemistries have been more widely studied, a recent techno-economic assessment has indicated non-aqueous RFBs offer an alternative pathway to low-cost energy storage [1]. Transitioning from aqueous to non-aqueous electrolytes offers a wider window of electrochemical stability that enables operation at higher cell voltages (>4 V) [1]. Further, a greater selection of redox materials may be available due to the wider solvent stability window and the variety of non-aqueous solvents. Together these benefits promise to reduce the cost of energy and to potentially enable high-energy small-footprint storage devices. As a nascent field, most research efforts have focused on active materials discovery and proof-of-concept validation in either static cells or un-optimized flow cells [2, 3]. Less consideration has given to the engineering challenges associated with developing the high-performance flow cells necessary to realize low system costs. To this end, we employ recently developed flow cell diagnostics to understand and overcome the performance limiting factors in non-aqueous flow cells [4]. Using a single electrolyte configuration (Figure 1A), where a model redox compound is oxidized and reduced in a continuous loop through the reactor, we systematically evaluate the role of cell architecture, constituent components, and electrolyte properties on cell performance with minimal confounding factors (e.g., reactant crossover). By leveraging cell polarization and electrochemical impedance spectroscopy, we can quantify resistive losses and unambiguously correlate these losses to kinetic, ohmic, or transport processes. Guided by this approach, we have been able to reduce cell resistance by an order of magnitude and begin to approach performance levels necessary for economic viability (Figure 1B & 1C) [5]. In this presentation, we will discuss the role of cell design, electrolyte selection, and flow rate on non-aqueous flow cell performance and will propose a set of design criteria for developing high performance electroreactors in a chemistry-agnostic fashion.