![]() ![]() ![]() In summary, some research results on additives of the positive electrolyte are summarized in Table 1. Through the above studies, it is found that both phosphate and –OH have a good effect on stabilizing pentavalent vanadium. Besides, the research confirmed in two ways (the Job plot and the Benesi–Hildebrand plot methods) that HEDP interacts with VO 2 + in a 1:1 binding stoichiometry, which is the reason for the enhancement in the stability of the electrolyte. selected 1 wt% HEDP as an electrolyte additive and confirmed that it can improve the electrolyte thermal stability and battery cycle efficiency of VRFB. reported some organic additives containing hydroxyl (–OH), such as sorbitol, mannitol, glucose, and fructose, and elaborated their stabilizing mechanism, indicating that these organic additives can clad the hydrated V(V) species and thus inhibit the formation of precipitation. investigated the effect of Na 3PO 4 as an electrolyte additive and found that it indeed delayed the V 2O 5 precipitation, but the VOPO 4♲H 2O precipitation was detected on the positive graphite mat after several cycle tests. Nonetheless, its electrochemical cycling performance was optimized. In addition, none of the new precipitation was proved to have been generated in the electrolyte. used 0.05 M sodium pyrophosphate as an additive in the positive electrolyte with 2.0 M V(V) and 4.0 M H 2SO 4, and the long-term stability of electrolyte was improved compared with the blank solution. It was concluded that the electrolyte stabilization mechanism by phosphoric acid at high temperatures could be attributed to the interaction between them and V(V) monomers or dimers forming two phosphate-containing substances, thus retarding the V 2O 5 precipitation. investigated the mechanism of precipitation retarding by phosphate in the vanadium electrolyte using nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering (DLS). The effects of sodium triphosphate and sodium hexametaphosphate as additives were also studied they both retarded the precipitation to a certain extent. ![]() The results show that phosphate anions enhance the stability of V(V) compounds at high temperatures, whereas for ammonium cations, the opposite is true-ammonium cations stabilize the negative half-cell electrolyte at low temperatures. used phosphoric acid and ammonium phosphate as additives. Owing to their being cost-effective and not interfering with electrolyte performance, they have been investigated widely nowadays. In order to improve the solubility of vanadium compounds in the sulfuric acid electrolyte, the introduction of additives is commonly performed. The precipitation process of V 2O 5 is irreversible, which is mainly responsible for the loss of battery capacity. This study provides a new approach to improve the stability of the positive electrolyte for vanadium redox flow battery. It is verified that the thermal stability and electrochemical stability of the electrolyte are significantly improved by the combination of ATMPA + HEDP or ATMPA + HDTMPA. It was found that most of the additives enhanced the electrochemical activity of the positive electrolyte, and the diffusion coefficients, exchange current densities, and reaction rate constants of V(IV) species became larger with the addition of these additives. The electrochemical performance of the electrolyte was further investigated by cyclic voltammetry, steady state polarization, and electrochemical impedance spectroscopy tests. HEDP and HDTMPA extended the stability time of the pentavalent vanadium electrolyte at 50 ☌ from 5 days (blank sample) to 30 days and 15 days, respectively. With 0.5 wt% addition, most of the selected additives were able to improve the thermal stability of the electrolyte. The effects of five phosphate containing additives (including 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexamethylene diamine tetramethylene phosphonic acid (HDTMPA), amino trimethylene phosphonic acid (ATMPA), sodium ethylenediamine tetramethylene phosphonate (EDTMPS), and diethyl triamine pentamethylene phosphonic acid (DTPMP)) on the thermal stability and electrochemical performance of the positive electrolyte of vanadium redox flow battery were investigated. The poor operational stability of electrolytes is a persistent impediment in building redox flow battery technology choosing suitable stability additives is usually the research direction to solve this problem. ![]()
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