Asymmetric Electrode Configurations Enhance Operating Power Density and Energy Efficiency of the Aqueous, Electrode-decoupled Titanium-Cerium Redox Flow Battery

Abstract

Redox flow batteries (RFBs), with decoupled scaling in energy and power, are an attractive solution for grid scale energy storage. Given the low margins and extreme price sensitivity of electricity supply, it is critically important for RFBs to reduce capital and operating costs. Improving the operating power density and energy efficiency of the RFB is a pathway towards lowered costs but achieving simultaneous improvements in both parameters is hampered by the fact that they are typically inversely correlated. This study demonstrates a 50% improvement in operating power density of an aqueous, electrode-decoupled Titanium-Cerium RFB without loss of energy efficiency through electrode engineering driven by fundamental investigations of charge-transfer kinetics at the Ti and Ce electrodes. Exploiting the significant difference in reaction kinetics between the Ti and Ce actives, the interfacial area and surface functionalization (affecting electrode-electrolyte contact angles and kinetics of charge transfer) of the electrode were optimized to increase operating power while reducing overall cell resistance. This resulted in increasing operating current density of a Ti-Ce RFB from 100 mA cm-2 to 150 mA cm-2 while sustaining ~70% energy efficiency over 80 h and 100 cycles. Notably, this study shows the key role played by the rate limiting electrode and the effect of electrode surface area in improving its performance. Overall, this study offers a template to significantly improve the overall performance of kinetically limited aqueous RFBs without catalysts or electrolyte reformulation.