(301g) Conductive Membrane Coatings for Improving Current Density in Redox Flow Batteries

Global electricity production from renewable sources has grown dramatically in recent years. Some of these forms of electricity production are intermittent in nature, and have proven difficult to incorporate into regional electrical grids. Grid-scale electricity storage methods are a promising solution to the inefficiencies of unpredictable electricity generation. Redox flow batteries have demonstrated the ability to be used as a reasonably low-cost, long-term, grid-scale electrical storage method. Specifically, vanadium redox flow batteries (VRBs) have been of special interest due to their chemical stability, long life cyclability, and potential for high electrical capacity compared to other redox chemistries. Accompanying these advantages are some research challenges. There have been a number of efforts to improve the electrical conductivity of VRBs by physically or chemically altering the electrodes of the cell. Most VRBs use a carbon or graphite felt as porous electrodes in the battery – the large surface area from the felt is used to increase the number of reaction sites at which the redox reaction can occur. Many modifications to the electrodes have been investigated, including using various highly conductive carbons, metal catalysts, and covering the carbon felt with hydrophilic surface groups. Less attention has been spent improving the membrane surface properties to alter the electrical and ionic conductivity in the cell. A new approach to improve the rate capability of VRBs involves improving the electrical conductivity of the surface of the membrane itself using air-assisted electrospray deposition of conductive carbons.

A conductive coating of carbon nanotubes (CNTs) and Nafion dispersion in water was used to coat a Nafion 117 membrane via air-assisted electrospraying to improve the rate capability and cycling performance of VRBs. It was found that electrospraying a highly conductive coating directly onto the surface of the membrane allowed for stable cycling performance at nearly double the current density that was afforded by the pristine Nafion membrane. A templating technique was used during the electrospraying process to allow for alternating domains of coated and uncoated membrane surface, which helped reduce the restriction of proton transport across the membrane, further improving rate capability and capacity retention of the VRB. The interfacial resistance between the membrane and the electrode was greatly diminished with the addition of a very small mass (<0.01 mg/cm²) of CNTs. This method has shown to be a fast, simple, and scalable technique for improving the rate capability of vanadium redox flow batteries.

This technique was subsequently extended to a quinone-based aqueous redox flow battery system. While the results were more modest when compared to the VRB system, improvements in current density were still seen with the use of a membrane with a conductive coating.