<p>Engineering redox-active compounds to support stable
multi-electron transfer is an emerging strategy for enhancing the energy
density and reducing the cost of redox flow batteries (RFBs). However, when sequential
electron transfers occur at disparate redox potentials, increases in
electrolyte capacity are accompanied by decreases in voltaic efficiency,
restricting the viable design space. To understand these performance tradeoffs
for two-electron compounds specifically, we apply theoretical models to
investigate the influence of the electron transfer mechanism and redox-active
species properties on galvanostatic processes. First, we model chronopotentiometry
at a planar electrode to understand how the electrochemical response and associated
concentration distributions depend on thermodynamic, kinetic, and mass
transport factors. Second, using a zero-dimensional galvanostatic charge /
discharge model, we assess the effects of these key descriptors on performance
for a single half-cell. Specifically, we examine how different properties (i.e.,
average of the two redox potentials, difference between the two redox
potentials, charging rate, mass transfer rate, and comproportionation rate) affect
the electrode polarization and voltaic efficiency. Finally, we extend the galvanostatic
model to include two-electron compounds in both half-cells, demonstrating
compounding voltage losses for a full cell. These results evince limitations to
the applicability of multi-electron compounds—as such, we suggest new
directions for molecular and systems engineering that may improve the prospects
of these materials within RFBs.<b></b></p>
Too Much of a Good Thing? Assessing Performance Tradeoffs of Two-Electron Compounds for Redox Flow Batteries
