The Influence of Electrode and Channel Configurations on Flow-Battery Performance

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Redox flow batteries (RFBs) have attributes that make them attractive for grid-scale energy storage [1]. Energy is stored and released by changing the oxidation states of ionic species that are dissolved in electrolyte solutions. The electrolyte solutions are stored in inexpensive storage vessels and fed to electrochemical reactors where the active ions are oxidized and reduced. This architecture lends itself to large, multi-hour discharge applications where relatively low energy densities can be tolerated. Reactor cost constitutes a large fraction of total battery cost [2]. Therefore, maximizing the power density of the reactor at economically-viable energy efficiencies is a path to lower capital costs [3]. Historically, RFB cells have been constructed with porous flow-through carbon electrodes [4, 5]. However, recent work has shown that combining porous-carbon electrodes with flow fields that cause forced convection within the electrodes can yield substantially higher cell performance [6, 7].The focus of this work is on both the performance and pressure drop of flow-battery cells with different flow-field designs. Both experimental and modeling results will be used to provide a comprehensive comparison of different flow-field and electrode options. Tests intended to measure the electrical resistances and pressure drops of the various combinations of electrodes and flow fields will be presented. Electrochemical tests are done with a mixture of VO2+ and VO2 + in H2SO4 that mimics the positive electrolyte of a conventional vanadium-redox flow battery (VRFB), but yield quasi steady-state results [8]. Contributions to cell polarization will be isolated and discussed. Mathematical modeling of flow distributions will be related to the experimental results in order to provide fundamental understanding of the differences in both the pressure drop and polarization of the different cell types.This paper will specifically focus on flow-through (FT) electrodes, parallel flow fields (PFF), and interdigitated flow-field (IDFF) designs. The majority of the cells tested exhibited qualitatively similar behavior, with the exception of the sole PFF cell, which had much lower performance than all of the FT and IDFF cells. The poor performance of the PFF cell can be attributed to low rates of mass transfer to the carbon fibers in the electrode in the absence of forced convection through the porous-carbon electrode.The performance of RFB cells with an IDFF adjacent to porous-carbon electrodes can equal, or even surpass, that of cells with FT carbon electrodes. Kinetic, ohmic, and mass-transport losses can all make important contributions to the performance of both FT and IDFF cells. However, the FT-electrode configuration requires thicker electrodes in order to yield manageable pressure drops, which results in relatively large ohmic losses. Thinner carbon-paper electrodes can be readily used with IDFF cells, provided reaction kinetics are acceptably fast, resulting in substantially lower ohmic losses. Scaling to larger active area yields smaller pressure drop increases with IDFF than it does with FT, indicating that the IDFF configuration is well suited for large RFB cells. Acknowledgements The authors would like to thank their flow-battery project colleagues at UTRC and, specifically, Mike Fortin who built all cells that will be discussed in this presentation. This work was funded, in part, by the Advanced Research Projects Agency - Energy (ARPA-E), U.S. Department of Energy (DOE) under Award Number DEAR0000149.