Carbon Electrode Surface Treatments and Novel Electrode Materials for Energy-Efficient, High-Power Vanadium Redox Flow Batteries.

Efficient storage and rapid delivery of clean energy are critical issues facing our energy grid. Vanadium redox flow batteries (VRFBs) offer a solution to these challenges through long-term, large-scale energy storage and a decoupling of power and energy. VRFBs store energy in vanadium-ion redox couples that are soluble in aqueous electrolytes. Spatial separation of the electrolyte and electrode after charging minimizes self-discharge and capacity loss over time. Charging or discharging of the battery occurs when the liquid electrolytes flow past or through porous electrodes. Consequently, the electrode dictates VFRB efficiency and power by the reversibility and rate of redox reactions at its surface as well as mass transport of electrolyte within its volume. VRFB electrodes have been limited to carbon-based materials like carbon paper and graphite felt because of their stability in the acidic electrolyte. However, vanadium redox reactions are slow on carbon surfaces, and carbon’s hydrophobicity hinders mass transfer of the aqueous electrolyte. To overcome these shortcomings, surface treatment is a widespread practice to introduce functional groups and defects that increase redox kinetics and aid mass transfer. In this work, we explored atomic layer deposition (ALD) as a novel method of carbon electrode surface treatment. With an example of aluminum oxide, we illustrated the ability of ALD to coat thin films of electrocatalyst onto the carbon fibers within carbon paper. We then quantified vanadium redox reaction reversibility on ALD-modified carbon paper with cyclic voltammetry and described mass transport within these electrodes with contact angle measurements. These findings were compared to a baseline of thermal oxidation as a surface treatment. Finally, we contrasted cyclic voltammograms of surface-treated carbon paper to novel electrode materials like porous copper to understand if carbon materials are the most optimal VRFB electrodes. The knowledge gained from this work will guide the design of electrodes for more efficient and more powerful VRFBs. Advancing this energy storage system will importantly enable the widespread use of clean energy and reduce greenhouse gas emissions through effective grid-scale energy storage and fast delivery of clean energy for off-production use.