Comparison of the Influence of Oxygen Groups Introduced by Graphene Oxide on the Activity of Carbon Felt in Vanadium and Anthraquinone Flow Batteries

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.


Randles-Sevčik Analysis
To estimate the comparative surface area of the pristine and modified felts, a Randles-Sevčik was performed on the voltammograms with the knowledge of the reported diffusion coefficient.For an irreversible system the relevant equation is: The Randles-Sevčik equation, at 25ºC and summarizing the constants, can be expressed as follows: Where: n -number of electrons per molecule oxidized or reduced  -transfer coefficient a n -number of electrons involved in rate-determining step  -scan rate (V•s -1 ) For the reaction: 2 2 2 n=1, αna=0.5, (generally considered in literature) A plot of ip vs.  1/2 should give a straight line with slope proportional to D0.
We can calculate the active surface area of the felt used as working electrode: The results for the oxidation peaks are 56.64 cm 2 and 60.93 cm 2 for the pristine felt and GFD+rGO-PEG 5 steps respectively.If the same calculation is repeated with the reduction peaks the values are 66.19 cm 2 for the pristine felt and 35.80 cm 2 for GFD+rGO-PEG 5 steps.

Figure S1 .
Figure S1.High resolution XPS spectra of C1s for the pristine carbon felt and for the felt with different impregnation steps: 2, 5, 7, 10 and 12.

Figure S2 .
Figure S2.High resolution XPS spectra of O1s for the pristine carbon felt and for the carbon felt with different impregnation steps: 2, 5, 7, 10 and 12.

Figure S5 .
Figure S5.Capture of water drops falling on the surface of both the pristine felt (left) and a rGO/PEG-modified felt by 5 impregnation steps (right).

Figure S6 :
Figure S6: Cyclic voltammetry of the 2,7-AQDS electrolyte with pristine and modified carbon felt (0.05 M 2,7-AQDS in 1.0 M H2SO4, scan rate 5 mV s -1 ).Modified felt was a different batch.For the oxidation peak: Ip,pristine is 91.8 mA and Ip,5-steps is 84.3 mA, the latter value being 8.1 % lower than the first.

Figure S7 .
Figure S7.Relation between the oxidation and reduction peaks current and the square root of the scan rate from 2 to 200 mV s -1 .6.Cell Voltage vs.Time Plots

Figure S9 .
Figure S9.Effect of current density on the on the cell voltage vs. time profiles of the 2,7-AQDS organic flow battery.Current density values: 50, 100 and 150 mA/cm 2 .

Table S1 .
Percentage of the different carbon and oxygen species determined by XPS for the felts (from C1s and O1s deconvolution).