Membrane and Electrode Modifications for Improving Power Density in Aqueous Redox Flow Batteries

Abstract

Global electricity production from renewable sources has grown dramatically in recent years. While this has fostered lower dependence on carbon-based energy generation, the intermittent nature of these renewable energy resources has introduced new technical challenges to regional electrical grids. One approach to address intermittent energy generation and associated inefficiencies is to introduce grid-scale electricity storage using redox flow batteries. Redox flow batteries provide a reasonably stable, long-term means to store renewable energy. Vanadium redox flow batteries (VRBs) have been of particular interest due to their chemical stability, long life cyclability, and potential for high capacity electrical storage. VRBs also have a number of research challenges. One key factor limiting market penetration of redox flow batteries in energy storage portfolios is the high capital cost associated with a comparatively low power density technology. To this end, current research efforts focus on improving the energy density of VRBs to warrant this cost in the long term. This work focuses on four approaches to improve the power density of VRBs: the use of conductive membrane coatings, the generation of electroactive hydrophilic surface functional groups, use of drop-in electrocatalysts via direct reduction of metal salts, and chemical synthesis of lead oxide particles on the VRB anode. First, it is demonstrated that the rate capability of VRBs can be improved by electrospray deposition of a conductive carbon/binder mixture onto the surface of the ion-exchange membrane in a VRB. This work goes on to show dramatic rate improvements by hydrothermally treating graphite felt electrodes with various surface modifiers, most notably ammonium persulfate. Alternative approaches to improve the current density of a VRB involve the use of electrocatalysts on the surface of the anode. This work shows that the use of drop-in metal salts and the chemical synthesis of lead oxides are both facile routes to form catalytically-active nanoparticles on the surface of the VRB anode, which significantly accelerate the kinetics of otherwise sluggish anodic reactions in VRBs. These four methods each offer a commercially scalable and inexpensive pathway to increasing the capacity and power density of aqueous vanadium redox flow batteries.