Graphene‐Nanowall‐Decorated Carbon Felt with Excellent Electrochemical Activity Toward VO2 +/VO2+ Couple for All Vanadium Redox Flow Battery

3D graphene‐nanowall‐decorated carbon felts (CF) are synthesized via an in situ microwave plasma enhanced chemical vapor deposition method and used as positive electrode for vanadium redox flow battery (VRFB). The carbon fibers in CF are successfully wrapped by vertically grown graphene nanowalls, which not only increase the electrode specific area, but also expose a high density of sharp graphene edges with good catalytic activities to the vanadium ions. As a result, the VRFB with this novel electrode shows three times higher reaction rate toward VO2 +/VO2+ redox couple and 11% increased energy efficiency over VRFB with an unmodified CF electrode. Moreover, this designed architecture shows excellent stability in the battery operation. After 100 charging–discharging cycles, the electrode not only shows no observable morphology change, it can also be reused in another battery and practical with the same performance. It is believed that this novel structure including the synthesis procedure will provide a new developing direction for the VRFB electrode.

SEM images of CF-G-2 (a, b), CF-G-3 (c, d) and CF-G-4 (e, f) samples. The area conductivity of these samples are tested by a equipment shown in Figure S3a. The sample with an area of 6.25 cm 2 was placed into the PVDF frame (2.5 * 2.5 cm) and pressed to 2 mm, then linear sweep voltammetry (LSV) technique was used to get the I-V curve ( Figure 1b). Area resistance (R s ) of the sample can be calculated from the equation R s = (E/I-R e )/S, where E is the voltage, I is the current, R e is the resistance of the equipment (~0.120 Ω) and S is the surface area of the sample. The R s values of 27.8,23.9 and 26.0 mΩ cm -2 for CF-G-1, CF-G-2, CF-G-3 and CF-G-4, respectively) are smaller than that of the CF (36.4 mΩ cm -2 ), demonstrating the enhanced conductivity of the CF-G electrodes.

Table S2
Electrochemical properties obtained from the cyclic voltammetry curves at the scan rate of 5 mV s -1 and EIS plots for different electrodes.  Figure S6 CV curves of CF electrodes under different scan rates range from 5 mv s -1 to 50 mv s -1 .
where Eo is the equilibrium potential, Epa is anodic peak potential, Epc is the cathodic peak potential, m is the total number of peaks.
The reversibility of the reaction can be estimated by the value of ) a ( Epc Ep  /n (n is the tansfer electron numer during oxidation/reduction processes). The reaction is irreversible if the value is larger than 57~63, as shown in figure S6, the values of ) a ( Epc Ep  /n (~300-800 mV varying with scan rate) are much larger than 57~63, suggesting the reaction system in our experiment is totally irreverbile. The following equation can be utilized to calculate reaction rate constant of totally irreversible reactions.
Where Ip is the peak current density, n is number of transfer electron during electrode reaction (n = 1), F is faraday constant (F = 96485 C/mol), A is surface area of the electrode (geometric area ~ 1 cm 2 ), C o * is the electrolyte concentration (0.1 M), K o is the reaction rate constant, α is the symmetry coefficient and Ep is peak potential. A plot of ln Ip vs.
determined at different scan rates have a slope of -αf and an intercept proportional to K o . The K o can be obtained from the intercept without using the α value.

Figure S8
The VRFB performance with the CF-G-1 as positive electrode within 200 cycles (change the membrane after 100 cycles).

Figure S10
XPS survey (a), C1s high-resolution (b) and O1s high-resolution spectra (c), and VRFB performance of CF-G-1 of after concentrated nitric acid treatment (d).

Figure S11
The proposed catalytic mechanism for CF-G materials during VO 2 + /VO 2+ redox reactions.

Figure S12
SEM images for CF-G-1 after 200 cycles (a) Homemade electrode stability test apparatus (b), SEM image after long structure stability test (c).