Taurine Electrografting onto Porous Electrodes Improves Redox Flow Battery Performance

Abstract

The surface properties of porous carbonaceous electrodes govern the performance, durability, and ultimately the cost of redox flow batteries (RFBs). State-of-the-art carbon fiber-based electrode interfaces suffer from limited kinetic activity and incomplete wettability, fundamentally limiting the performance. Surface treatments for electrodes such as thermal and acid activation are a common practice to make them more suitable for aqueous RFBs; however, these treatments offer limited control over the desired functional properties. Here, we propose, for the first time, electrografting as a facile, rapid, and versatile technique to enable task-specific functionalization of porous carbonaceous electrodes for use in RFBs. Electrografting allows covalent attachment of organic molecules on conductive substrates upon application of an electrochemical driving force, and the vast library of available organic molecules can unlock a broad range of desired functional properties. To showcase the potential of electrografting for RFBs, we elect to investigate taurine, an amine with a highly hydrophilic sulfonic acid tail. Oxidative electrografting with cyclic voltammetry allows covalent attachment of taurine through the amine group to the fiber surface, resulting in taurine-functionalized carbon cloth electrodes. In situ polarization and impedance spectroscopy in single-electrolyte flow cells reveal that taurine-treated cloth electrodes result in 40% lower charge transfer and 25% lower mass transfer resistances than off-the-shelf cloth electrodes. We find that taurine-treated electrode interfaces promote faster Fe3+ reduction reaction kinetics as the electrochemical surface area normalized current densities are 2-fold and 4-fold higher than oxidized and untreated glassy carbon surfaces, respectively. Improved mass transfer of taurine-treated electrodes is attributed to their superior wettability, as revealed by operando neutron radiography within a flow cell setup. Through demonstrating promising results for aqueous systems with the model molecule taurine, this work aims to bring forth electrografting as a facile technique to tailor electrode surfaces for other RFB chemistries and electrochemical technologies.

Engineering Lung-Inspired Flow Field Geometries for Electrochemical Flow Cells with Stereolithography 3D Printing

“…Although the proposed method is effective in manufacturing conductive structures, the use of costly coatings (such as Ag and Pt) is incompatible with large scale manufacturing. Our goal is to demonstrate the potential of these novel flow field geometries, and future work should be devoted to the development of coatings with lower electronic resistivity or readily conductive resins, which are cost effective. , We acknowledge that the kinetic rate constant of the Fe 2+ /Fe 3+ redox reaction on the surface of the platinum coating might be different compared with that on the carbon electrode surfaces. − However, we do not anticipate the Pt coating to largely change the overall performance since the redox reaction is an outer sphere, and kinetic rate constants in the literature do not differ significantly. Furthermore, the area of the carbonaceous porous electrode is significantly larger than the area of the flow field coated with platinum.…”

A Viewpoint on X-ray Tomography Imaging in Electrocatalysis

“…Neutron CT has been employed in previous studies to investigate water production and removal in fuel cells, water/ice formation during subzero operating conditions, 97 and the distribution of water in flow channels of fuel cell stacks. 98 Figure 10 shows a typical neutron CT study 99 on redox flow battery electrode and electrolyte, with data acquired at Swiss Spallation Neutron Source. The carbon felt cathodes and anodes with different treatments were separated by thick PTFE membrane and injected with mixture electrolytes of ferrous chloride and ferric chloride.…”

“…(Bottom) The corresponding plot of water thickness/velocity versus time. [Reprinted from ref . Open access under Creative Commons CC BY license.…”

Understanding the Role of Electrode Thickness on Redox Flow Cell Performance**

“…The decrease in activation overpotential is however dependent on the redox chemistry used. Electrolytes featuring sluggish kinetics, such as vanadium (reaction rate constant � 10 À 6 cm s À 1 in H 2 SO 4 [89] ), benefit more of an increased internal surface area compared to kinetically facile electrolytes such as Fe 2 + /Fe 3 + electrolytes (reaction rate constant � 10 À 3 cm s À 1 in HCl and with thermally treated carbon electrodes [90] ) because of the high exchange current density. Furthermore, the mass transfer overpotential is decreased upon electrode stacking as a result of evaluating at a constant electrolyte velocity, resulting in higher mass flow rates and thus enhanced reactant supply for thicker electrodes.…”