## 1 Introduction
The important materials needed to be investigated and developed to decrease voltage losses are the membrane and electrode. Cation exchange membranes and anion exchange membranes are the two types of membranes that are commonly used [19; 23]. The membrane's primary roles are to separate vanadium ions from the cathode and anode sides, as well as to transport protons to complete the electrical circuit [24]. To complete its functions, a membrane should acquire good ion conductivity, high ion selectivity, low permeation rate of vanadium ions, high chemical stability, and high performance [25; 26]. Nafion is a perfluorosulfonic acid proton exchange membrane that consists of a hydrophobic part and a hydrophilic part, and it is made by co-polymerizing unsaturated perfluoroalkyl sulfonyl fluoride with tetrafluoroethylene in varying quantities. Nafion has many types that have different equivalent weights (EW) [27]. EW represents grams of dry Nafion per mole of sulfonic acid groups that affect proton transfer and thickness of membrane related to ohmic loss in the polarization curve. An electrode provides an active area for a redox reaction; thus, it should deliver high electrochemical activity, be electrochemically stable in the various potential windows, have high electrical conductivity to increase kinetics in terms of charge transfer rate, and have chemical stability to prevent corrosion [28]. Conventionally, the flow field of a flow battery can be classified into two major types: flow-through and flow-by- by [6]. A carbon felt used as an electrode is assembled as the flow-through structure, while carbon papers are applied for the flow-by structure. It is worth noting that the carbon felt and carbon paper electrodes can be utilized for the flow-through design. By stacking electrodes in the right frame, the flow-through structure has the ability to increase the electrode surface area, improve electrolyte homogeneity, and decrease pressure drops. The electrodes' large surface areas contribute to the high voltage efficiency of VRFBs [29]. Additionally, compared to the energy loss in the flow-by structure, a pumping system with a flow-through structure exhibits reduced energy loss. In terms of cell design, two flow patterns--horizontal and vertical--of electrolyte fed into a VRFB stack are briefly described for a basic understanding. Single cells of VRFBs are connected electrically in a series, and electrolytes are injected into the stack in a parallel direction to a stack structure (Figure 1a). This horizontal pattern establishes a parallel flow pattern leading to electrical shunt current that causes corrosion problems, loss of energy efficiency, and nonuniform distribution of current to the cell [30; 31]. The vertical pattern (Figure 1b) occurs when single cells are assembled in a series and the electrolytes are vertically fed into the stack. This stacking feature was designed to prevent electrolyte block, resulting in a current bypass, and to protect water electrolysis. Furthermore, the demand for pump power is lower than parallel feeding. In this context, the aim of the present work is to offer an extensive study of the use of polarization curves to investigate the performance of a charging process (electrolyte preparation from vanadyl sulfate (VOSO4 \(\cdot\)xH2O)) and a discharging process. The focus of this study was put primarily on the effects of charging voltage on the performance of the charging step (electrolyte preparation). The performance was evaluated by cell current, moles of electron, and reaction time. In the step of discharging, the electrolyte feeding rate was optimized, since this factor directly relates to the function of carbon electrodes in terms of mass transport. All studies were carried out by operating hand-tailored single-cell VRFBs installed with flow frame design for the carbon paper electrode. Materials selection of main components as an electrode and a membrane was concerned for further design of VRFBs.