Deconvoluting the Effects of Surface Area, Functionalization, and Wetting on the Performance of Porous Carbon Electrodes in Aqueous Redox Flow Batteries
Authors: Greco Katharine Virginia , Bonesteel Jane K. , Brushett Fikile R. 
Published: February 2020

Redox flow batteries (RFBs) are a promising electrochemical technology for grid-scale energy storage, but further cost reductions are needed for ubiquitous adoption.1 Of particular importance are porous electrodes which are responsible for multiple critical functions in the flow cell including providing surfaces for electrochemical reactions, distributing liquid electrolytes, and conducting electrons and heat. However, there is limited knowledge on how to systematically design and implement these materials in emerging RFB applications, forcing the repurposing of available materials that are not tailored for this electrochemical system. Currently, high performance RFB reactors employ porous electrodes based on the fibrous carbon backing layers of fuel cell gas diffusion layers. While functional, these materials are not designed to meet all RFB requirements and are typically pretreated prior to use.2,3 Arguably the most widely-reported pretreatment strategy, thermal oxidation in air provides a means of introducing redox-enhancing oxygen functional groups to the electrode surface and improving hydrophilicity, albeit often at the expense of electrochemically active surface area (ECSA).4,5 Consequently, optimal performance is achieved by a balance of electrode properties rather than a maximization of each. Unfortunately, the potential- and temperature-dependence of underlying structure-property relationships remain poorly understood and, thus, electrode optimizations are typically performed via trial-and-error approaches.

In this presentation, we will investigate the interplay between surface functionalization, wetting, and surface area on the performance of porous carbon electrodes in aqueous RFBs. Specifically, we systematically vary pretreatment conditions (e.g., temperature, time, atmosphere) for select porous carbon electrodes, characterize their wettability, surface chemistry, and ECSA, and correlate these properties to electrochemical performance. To this end, we leverage model surface-sensitivity redox couples over a range of potentials in combination with diagnostic flow cell configurations to disaggregate coupled effects and identify dominant sensitivities. Overall, we seek to determine generalizable performance descriptors that can guide the design of next-generation electrodes specifically for RFB applications.

Acknowledgements

The authors acknowledge the financial support of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the United States Department of Energy. K.V.G acknowledges additional funding from the National Science Foundation Graduate Research Fellowship. The authors acknowledge the Center for Nanoscale Systems and the NSF’s National Nanotechnology Infrastructure Network (NNIN) for the use of Nanoscale Analysis facility for electrode property characterization.

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