Grid-scale energy storage is anticipated to play a critical role in the decarbonization of the electric power sector by enabling the reliable integration of intermittent renewable sources and increasing the efficiency of non-renewable energy processes.¹ Redox flow batteries (RFBs) are a promising electrochemical device for such applications due to their independent energy and power scaling and long operational lifetime.² Aqueous vanadium RFBs (VRBs) have been the most successful chemistry, due to their high power density and recoverable capacity from crossover, but further cost reductions are needed for ubiquitous adoption. Vanadium redox reactions have sluggish kinetics on the carbon electrodes typically employed in VRBs which motivates the implementation of pretreatment strategies to increase electrochemically active surface area and introduce catalytically active functional groups onto the electrode surface.³ While many pretreatments have been successfully demonstrated most studies are empirically-driven.⁴ Few have focused on systematically correlating changes in electrode structure and surface chemistry to shifts in electrochemical performance. To this end, we utilize a suite of ex-situ characterization techniques, coupled with flow cell testing, to explore structure-performance relations of thermally pretreated carbon paper electrodes. Specifically, we vary thermal pretreatment temperature from 400 °C, a commonly used temperature for VRB electrode treatments^3,4, to 500 °C, to quantify changes in the physicochemical properties of the electrode, and link these changes to shifts in VRB performance. We leverage microscopic, spectroscopic, and analytical techniques to determine how microstructural, morphological, and chemical properties of electrodes are impacted by thermal pretreatment. We then evaluate the effect of these properties on flow cell performance using polarization, electrochemical impedance spectroscopy, and cell cycling. We find that many factors contribute to the observed electrode performance, including hydrophilicity, electrochemical surface area, and surface chemistry, and that not all of these factors improve with increasing temperature. Consequently, while the best overall performance is achieved with pretreatment at 475 °C, a 26% increase in maximum power density over the base case of pretreatment at 400 °C, this enhancement is based on a balance of critical properties rather than a maximization of all.

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. A.F.C. acknowledges financial support from the Swiss National Science Foundation (P2EZP2_172183). The authors acknowledge the Center for Nanoscale Systems and the NSF’s National Nanotechnology Infrastructure Network (NNIN) for the use of Nanoscale Analysis facility to collect X-ray photoelectron spectroscopy data. We thank Michael L. Perry and Robert M. Darling for guidance in experimental conditions and critical evaluation of results.

References

1. Z. Yang, J. Zhang, M. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon, J. Liu, Chem. Rev., 111, 3577–3613 (2011).
2. M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011).
3. B. Sun and M. Skyllas-Kazacos, Electrochim. Acta, 37, 1253–1260 (1992).
4. M. H. Chakrabarti et al., J. Power Sources, 253, 150–166 (2014).