*2.4. BET Surface Area Analysis'*

For the BET surface area test, a total of 10 g felt was cut into pieces to form the sample. ASAP2020 micromeritics® (Micromeritics Instrument Corp., Norcross, GA, USA) was used as the measuring tool. The process of isothermal absorption line condition started from the degas process, followed by a measuring process set from relative pressure 0.1 to 1 under 77 K. The desorption process operated under reverse, at room temperature. The results were transferred to surface area data using the Brunauer-Emmett-Teller (BET) calculation model.

### *2.5. CV (Syclic Voltammetry) Analysis*

For the cyclic voltammetry test, the felt was cut to 1 mm<sup>2</sup> size and 6.5 mm thick as the sample, and Autolab® (swiss) was used as the current provider and data collector. The three-electrode system was built for a positive test and negative test.

In the positive electrode test, 0.2M VOSO<sup>4</sup> (purchased from Alfa Aesar, Echo Chemical Co. Ltd., Miaoli, Taiwan) solution was used as electrolyte, saturated calomel electrode (SCE) as the reference electrode (RE), and 1 mm thick platinum wire as the working electrode (WE) and counter electrode (CE). The starting voltage was autodetected by the tool, the voltage range was set from −0.5 V to 1.5 V, and a scan rate of 5 mV s−<sup>1</sup> was chosen for the data collection.

In the negative electrode test, the electrolyte had a concentration of 0.17 M vanadium and a valance of 2.8–3, and a 2 M sulfuric acid diluted solution was made from the electrolysis reaction of the VRFB single-cell test. For the electrodes, the SCE was used as the RE, and 2 mm of thick glassy carbon electrode was used as the WE and CE to prevent the fast evolution of hydrogen within the analysis process. The starting voltage was also autodetected by the tool, the voltage range was set from −1.5 V to 0.5 V, and a scan rate of 5 mV s−<sup>1</sup> was selected.
