Towards Advanced Porous Carbon Electrodes for Redox Flow Batteries

The intermittency and unpredictability of renewable energy technologies has resulted in an increased need for large-scale grid storage. Redox flow batteries (RFBs) are a promising technology to satisfy these grid storage needs due to their ability to decouple power and energy, and their potentially low cost. Carbon fiber-based porous materials are typically used as electrodes and must fulfill several functionalities: high permeability and surface area, stability under operating conditions, electrochemical activity and selectivity, and stability under compressive forces. State-of-the-art RFB electrodes are re-purposed gas diffusion layers (GDLs) typically used in polymer electrolyte fuel cells and, while there is some overlap in desired properties, they have not been optimized for this specific application.

In this work, we systematically investigate various commercially available carbon substrates, specifically three carbon papers, a cloth, and a felt. In the first part, we characterize the pressure drop characteristic and electrochemically active surface area. Highly permeable electrodes, like carbon cloth, are found to experience a smaller pressure drop in a flow cell setup and would decrease pumping costs. Higher accessed surface areas, like that of SGL 39AA carbon paper, are found to be preferable for battery performance due to their increased wetted surface in contact with the electrolyte. In the second part, we present an approach for modifying porous electrodes based on attaching polymer films onto the carbon fiber surfaces. A hydrophilic monomer was grafted onto the carbon electrode leading to a large increase in the wettability of these electrodes with water and an increase in accessed surface area by a factor of more than 30 times. This might prove to be an easy and effective method to improve battery performance in aqueous vanadium flow batteries.