Assignment of energy loss contributions in redox flow batteries using exergy destruction analysis

Various microscopic processes are responsible for the inefficiencies with which redox flow batteries (RFBs) operate. We introduce theory presently to enable the systematic analysis of energy losses in RFBs using exergy destruction rates derived from irreversible thermodynamics. We apply this analysis to RFBs for the first time by performing simulations with a transient, 2D model using a homogenized Poisson-Nernst-Planck formulation including multicomponent hydrodynamic dispersion, Donnan exclusion in ion exchange membranes, and Marcus-Hush-Chidsey kinetics. In the limit of low Wagner number, we map charge capacity utilization and cell polarization in the space of pore-scale Damköhler number (a non-dimensional parameter for characteristic pore-scale mass transfer resistance) and salt Damköhler number (a non-dimensional parameter for characteristic ohmic polarization) by varying the applied current density and electrode fiber diameter. Exergy destruction rates are analyzed from (1) pore-scale mass transfer, (2) reaction kinetics, (3) irreversible tank mixing, (4) bulk species transport, and (5) electronic conduction. For high coulombic efficiencies the sum of these exergy destruction contributions balances with the energy lost during a given cycle. This method of energy loss assignment to specific mechanisms at specific instants in time and locations in space provides guidance for the development of energy-efficient, high-rate RFBs in the future.