Quantifying Mass Transfer Rates in Redox Flow Batteries

J.D. Milshtein, K. Tenny, J. Barton, J. Drake, R.M. Darling, F.R. Brushett
Massachusetts Institute of Technology,
United States

Keywords: flow batteries, mass transfer, diagnostics

Summary:

Redox flow batteries (RFBs) have emerged as attractive grid-scale energy storage devices for improving the efficiency of the existing fossil fuel infrastructure, the stability of the electric grid, and the penetration of intermittent renewables [1,2]. In these rechargeable devices, energy is stored and released by alternately reducing and oxidizing electroactive species, dissolved in liquid-phase electrolytes, which are housed in large external tanks and pumped through an electrochemical reactor [2]. Designing the electrochemical reactor to deliver sufficiently high power is a critical consideration towards achieving low battery prices and enabling a variety of grid services [3]. Generally, RFB reactor performance losses are attributed to one of three areas: ohmic losses through the membrane, charge transfer losses due to sluggish reaction kinetics, or mass transport losses due to inadequate delivery of active species to the electrode surface. Membrane and charge transfer losses tend to be chemistry specific challenges, whereas mass transport losses apply to all RFB chemistries. Mass transfer losses can be mitigated by improving the rate of convective mass transfer in the RFB’s porous electrodes. Several recent studies have demonstrated significant performance gains for RFBs by varying flow field type and electrode geometry [3–5]. While such reports represent excellent engineering efforts to improve RFB power density, increases in mass transfer rates are rarely quantified. Additionally, a recent report by Perry & Darling suggests that the reactor performance improvements, achieved by increasing electrolyte flow rate, varies with flow field type [3]. Thus, in general, the improvements in mass transfer rates have yet to be quantified or systematically studied. Here, we couple a single electrolyte diagnostic technique [3] with one-dimensional porous electrode modeling to quantify average mass transfer rates for typical RFB flow fields: flow through, serpentine, parallel, and interdigitated. Experimentally, we utilize a model redox electrolyte and measure cell polarization at various flow rates and active species concentrations. Computationally, we use the porous electrode model to calculate the overpotential drop across a flow battery electrode accounting for ohmic losses in the electrolyte, Butler-Volmer kinetics, and mass transfer. The porous electrode model is fitted to the experimental data to extract exchange current density and average mass transfer coefficient as a function of flow field type and operating condition. This work enables a quantitative investigation of the mass transfer enhancements afforded by modifying flow field type and flow rate in RFBs. Acknowledgements We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research, the NSF Graduate Research Fellowship Program, and the MIT Summer Research Program. References 1. I. Gyuk et al., Grid Energy Storage, US Department of Energy, Washington DC, (2013). 2. A. Z. Weber et al., J. Appl. Electrochem., 41, 1137 (2011). 3. R. M. Darling & M. L. Perry, J. Electrochem. Soc., 161, A1381 (2014). 4. J. Houser et al., J. Power Sources, 302, 369 (2016). 5. C. R. Dennison et al., J. Electrochem. Soc., 163, A5163 (2016).