Modified Electrodes for Improved Performance of Redox Flow Batteries

Thursday, 5 October 2017: 11:30
National Harbor 8 (Gaylord National Resort and Convention Center)
R. K. Emmett and M. E. Roberts (Clemson University)
Redox Flow Batteries (RFBs) are an emerging electrochemical energy storage technology well-suited for intermittent energy sources, such as wind and solar. Integrated RFBs into grid-level energy systems would provide the storage necessary for alternative energy with only slight infrastructure changes. Currently, RFBs are limited by high material costs – from the use of precious metals – and/or low electrochemical activity. This work aims at improving power and energy of low-cost RFB electrolyte systems by doping carbon electrodes to facilitate Faradaic charge transfer at the interface, thereby decreasing costs and increasing accessibility of RFB systems. Carbon electrodes are modified with redox mediators to decrease the voltage drop and to increase charge transfer. Simulated half-cell experiments are conducted using carbon modified electrodes soaked with redox electrolytes s that are separated from pure base electrolytes by alkaline modified Nafion. Full cell testing is performed with modified Nafion separating the catholyte (0.7M Fe(CN)6 •3H2O and 1M NaOH) and the anolyte (0.35M Fe2SO4, 0.4M NaCl, Triethanolamine, and 3M NaOH). These electrolytes provide a test bed for evaluating all Iron RFBs as a cheaper alternative to Vanadium, therefore decreasing raw material cost and environmental impact.

Incorporating redox mediators into carbon electrodes improves kinetics elevating voltage efficiency and allowing higher current densities without ebbing discharge voltage. Power is modeled with current and voltage by Ohm’s Law (P=IE). Electrolytes with low inherit voltage efficiency display increased energy density due to fully capturing the redox behavior at raised current densities. Our alkaline discharge voltage difference has a maximum of 1.5 V with our modified electrodes. These electrodes showed a ~10x increase in power with catholyte half-cell testing (305 to 32W/Kg) and a ~4x increase in power with the anolyte (990 to 227Wh/Kg). Energy density improvements were observed for the catholyte (232%) and the anolyte (41%), lowering the volume of electrolyte needed. Long term stability experimentation revealed consistent, coulombic and voltage efficiencies for 1400 cycles (~4 years of daily use). These adaptations to RFB electrodes could also be applied to current RFBs to increase their performance, thereby decreasing material costs and improving cheaper alternatives.