Accessible Surface Area in Porous Carbon Electrodes and Its Impact on Redox Flow Battery Performance

Abstract

Carbon-fiber electrodes exhibit many desirable material properties including conductivity, porosity, and chemical stability, and, accordingly, are employed in a range of electrochemical technologies.1 Such electrodes are particularly important in redox flow battery (RFB) systems, as they serve multiple critical functions including providing surface area for electrochemical reactions, distributing liquid electrolytes, and conducting electrons and heat. While commercially available materials possess suitable permeability and conductivity, the performance of pristine substrates is limited, thus oxidative treatment prior to use is common practice.2,3 Partial oxidation of the electrode surface has been shown to add a range of oxygen functional groups, which improve hydrophilicity and, in some cases, catalyze redox reactions, as well as to increase available surface area for the electrochemical reactions.4,5 Both effects are posited to improve electrode performance but their relative importance and effectiveness remains unclear.6 Here, we critically assess the nature of the surface area generated by thermal pretreatment in air and its role on RFB electrode performance. Using binder-free Freundenberg H23 as a model substrate, we systematically vary pretreatment time and temperature to create different surface morphologies and develop structure-function relations. First, we use Brauner Emmanuel Teller gas adsorption (e.g., N2, Kr, Ar/CO2) and mercury porosimetry to estimate physical surface area and pore size distribution. In parallel, we use voltammetry and impedance spectroscopy to determine electrochemically active surface area with different electrolyte compositions. In general, we find that only a fraction of the physical surface area is accessible for electrochemical redox reactions due, in large part, to the nanoscopic dimensions of the added surface area. Second, we develop a simple mathematical model to assess the effectiveness of the available surface area of carbon fibers for Faradaic reactions. We find that, under typical RFB operating conditions, external mass transfer limitations govern performance, limiting the utilization of recessed areas generated during pretreatment. Ultimately, these results highlight the potential limits of performance improvement strategies based on increasing surface area of fibrous substrates and motivate new approaches for electrode development. 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. References M. H. Chakrabarti et al., Journal of Power Sources, 253, 150–166 (2014).T. J. Rabbow, M. Trampert, P. Pokorny, P. Binder, and A. H. Whitehead, Electrochimica Acta, 173, 17–23 (2015).N. Pour et al., The Journal of Physical Chemistry C, 119, 5311–5318 (2015).B. Sun and M. Skyllas-Kazacos, Electrochim. Acta, 37, 8 (1992).T. J. Rabbow and A. H. Whitehead, Carbon, 111, 782–788 (2017).K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Applied Materials & Interfaces, 10, 44430–44442 (2018).