The Influence of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage but further cost reductions are needed for ubiquitous adoption of this technology [1]. While current state-of-the-art RFB systems employ aqueous chemistries, transitioning to nonaqueous electrolytes offer a new pathway to reduced cost through increased energy density. While research efforts have primarily focused on molecular discovery [2], there has been significantly less attention paid to the development of other critical system components. Of particular importance are the porous electrodes used in the RFB’s electrochemical stack. Today’s electrodes largely draw from the fuel cell material set, but, within a RFB, the porous electrode must perform a range of different roles including serving as active surfaces for electrochemical reactions, enabling excellent liquid electrolyte distribution, and maintaining low pressure drops [3]. Thus, to enable the development of advanced electrodes tailored for RFB applications, it is necessary to quantify the performance-limiting factors for the present materials set. However, unambiguous analysis is challenging in an operating RFB due to the complex coupling of transport and reactions which varies as a function of state of charge during cell cycling. Deconvoluting the role of electrode properties on flow battery performance requires the development of diagnostic techniques that enable electrode characterization under well-controlled but application-relevant conditions. To this end, we systematically compare the operando performance of RFBs containing selected carbon paper, felt, and cloth electrodes using the single-electrolyte cell configuration (Figure 1a) [4] and a model organic redox couple (TEMPO/TEMPO+) [5]. Using polarization and electrochemical impedance spectroscopy, we quantify the impact of electrode microstructure on battery performance (Figure 1b). We find that, depending on the electrode choice and flow conditions, current densities as high as 450 mA cm-2 can be achieved at an overpotential of 0.3 V with a cell area specific resistance as low as 0.7 Ω cm2. This result suggests that, through appropriate cell engineering and materials selection, high power performance is possible in nonaqueous flow batteries. Finally, a simple convection-diffusion mass transport model is developed to explain the observed experimental behavior and provide guidance for the bottom-up design of next generation RFB electrodes. Darlin et al., Energy Environ. Sci., 7, 3459 (2014).A. Kowalski et al., Curr. Op. Chem. En., 13, 45 (2016).Weber et al., J. Appl. Electrochem., 41(10), 1137 (2011).Darling et al., J. Electrochem. Soc., 161, A1381 (2014).D. Milshtein et al., J. Power Sources, 327, 151 (2016). Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. Figure 1