The intermittent nature of renewable sources can be alleviated by grid-scale energy storage technologies, such as redox flow batteries (RFBs). However, these systems are currently not cost-competitive for widespread deployment. The system costs are linked to stack performance that is limited by kinetic, ohmic and mass-transfer overpotentials as well as materials stability. Carbon fiber-based porous electrodes, ubiquitous in electrochemical reactors, strongly influence these limiting phenomena as they facilitate redox reactions on their surface, conduct electrons, and provide flow paths for reactant transport. Unfortunately, the inherently hydrophobic and inert carbon surface is not optimal for aqueous electrolytes with kinetically sluggish redox couples such as vanadium and iron¹. In these systems, controlling the surface chemical state of the electrode is vital to attain significant performance gains. Popular strategies to modify surface chemistry are thermal and acid treatments with a general aim to increase the heteroatom content and number of functional groups of the carbon surface². These functional groups increase the surface energy of inherently hydrophobic carbon electrode and can serve as active sites for redox-reactions³. However, these treatment strategies can cause embrittlement of the electrode⁴, mass-loss⁵, and the formed functional groups, especially in the case of oxygen, can manifest in multiple chemical forms, hindering proper structure-property relationships to be established. Thus, to correlate the surface chemical state to the battery performance, there is a need to develop methodologies to synthesize homogenous, conformal, and stable interfaces onto three-dimensional porous electrodes⁶.

Here we propose electrografting as a surface modification strategy for carbon fiber-based RFB electrodes with a model molecule, taurine. Electrografting is the electrochemical analogue of chemical grafting where covalent bonds are formed between species and the conductive substrate⁷, with an added advantage that the charge transfer reactions responsible for bond formation can be controlled with applied voltage. We selected taurine as a model molecule to graft on carbon cloth electrodes as its amine group can undergo oxidative electrografting and we hypothesize its sulfonic acid group to be beneficial for electrode wetting, allowing coverage of the electrode surface with a thin layer of desired functional groups. We performed diagnostic studies on glassy carbon electrodes by hydrodynamic voltammetry, where the kinetic rate for the reduction of Fe³⁺ on taurine treated electrodes is revealed to be an order of magnitude faster than untreated electrodes (1.23 x 10^-4 vs 1.84 x 10^-5 cm s^-1). In-situ flow cell studies in single cell configuration revealed improved performance for mixed Fe^2+/3+ electrolyte, especially at lower flow rates (Figure 1). Also, for the first time, we visualized wetting dynamics of porous RFB electrodes in flow cells using neutron radiography. We find that treated electrodes imbibe the acidic electrolyte instantaneously even at low flow rates, which indicates the electrode interfaces feature hydrophilic character. Finally, we performed extended in-situ stability tests under mixed Fe^2+/3+ electrolyte flow at open-circuit and applied voltage conditions. Impedance spectroscopy revealed performance degradation with pristine electrodes but not with taurine treated electrodes, confirming the formation of a stable interface with electrografting of taurine. In summary, we show that electrografting of taurine is a facile and environmentally benign approach to functionalize porous electrodes for aqueous redox flow batteries. Beyond this specific application, the possibility to extend the molecular library makes electrografting a suitable approach to engineer interfaces for next-generation electrochemical devices.

References

1. P. Chen and R. L. McCreery, Anal. Chem., 68, 3958–3965 (1996).
2. K. Jae Kim et al., Journal of Materials Chemistry A, 3, 16913–16933 (2015).
3. R. H. Bradley and P. Pendleton, Adsorption Science & Technology, 31, 113–133 (2013).
4. L. Yue, W. Li, F. Sun, L. Zhao, and L. Xing, Carbon, 48, 3079–3090 (2010).
5. K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Appl. Mater. Interfaces, 10, 44430–44442 (2018).
6. A. Forner-Cuenca and F. R. Brushett, Current Opinion in Electrochemistry, 18, 113–122 (2019).
7. D. Bélanger and J. Pinson, Chem. Soc. Rev., 40, 3995–4048 (2011).