**2. Materials and Methods**

#### *2.1. Materials*

VOSO4·3.5H2O (99%) was provided by Shenyang Haizhongtian Fine Chemical Co., Ltd.(Shenyang, China). The additives are listed in Table 2. HEDP (60% in water), ATMPA (50% in water), EDTMPS (98%), and DTPMP (50% in water) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China. HDTPMA (>98%) was obtained from Adamas-beta Co., Ltd.(Shanghai, China). Other chemicals used in the experiment are of analytical grade. The previous experiment shows that, under the experimental conditions selected in this study, the five additives are stable.

#### *2.2. Preparation of the V(V) Electrolyte Solution*

VOSO4·3.5H2O (99%) was dissolved ina3MH2SO4 solution to prepare 2 M VOSO4 solution, and the prepared V(IV) electrolyte solution was placed in a double-chamber electrolytic cell with 50 mL and 25 mL of positive and negative electrolyte tank capacity, respectively. The electrolyte was then charged (cut-off potential: 1.55 V, stepwise current

density: 200−10 mA cm−2) until the V(IV) ions in the positive electrolyte were converted to V(V) and the V(IV) ions in the negative electrolyte were converted to V(II). When the electrolysis was done, the total vanadium concentration in the positive electrolyte was determined by redox titration using a potentiometric titrator (PHS-3C, Shanghai Leici Co., Ltd., Shanghai, China).


**Table 2.** Molecular structure of the studied organic additives.

#### *2.3. Thermal Stability Test of V(V)*

To investigate the effect of various phosphonates additives on the long-term stability of the V(V) electrolyte, electrolyte samples containing 0.05 wt% additives and blank sample were stored in a sealed oven at 50 ◦C until measurable orange precipitation was observed. All thermal stability tests were performed without any agitation. Each sample was visually monitored more than twice a day during the test to record the V2O5 precipitation and the change in solution color. The samples were filtered and their equilibrium concentrations of vanadium were determined by redox titration again, at the end of the 30-day test.
