Aqueous High-Voltage Zinc-Vanadium Redox Flow Battery with the High-Index Facets Polyhedron Ce_2/3TiO₃ Electrocatalyst

Aqueous redox flow batteries (RFBs) have been considered as the most promising energy storage systems which have several advantages of using inexpensive and nonflammable electrolytes, long lifetime. Most importantly, the independent and tunable power and capacity make it suitable technology for large energy storage systems. Although the reliable cycling performance and stability have been demonstrated, traditional aqueous RFBs still suffer from the limited voltage range and low energy density because of the limited operating voltage by the water splitting voltage of 1.23 V to avoid the hydrogen and oxygen evolutions. In addition, carbon-based electrode materials for RFBs such as carbon paper, graphite felt, and carbon felt were suffered from low electrolyte access to the electrode surface due to the hydrophobic nature which was ascribed to the high graphitization temperature.<br/>To overcome the narrow voltage of aqueous RFBs, we demonstrated the hybrid zinc-vanadium RFB with high operating voltage of 2.3 V, which was composed of two-membranes and three-electrolyte reservoirs. According to the Pourbaix diagram, the redox potential of the active materials was mainly affected by pH value. Therefore, we dissolved the zinc redox couple in alkaline solution (Zn/ [Zn(OH)₄]²⁻, −1.26 V vs standard hydrogen electrode, SHE) to exhibit a higher operating voltage than the zinc redox couple in neutral solution ((Zn/Zn²⁺, −0.76 V vs SHE), allowing the different types of electrolyte combinations. Furthermore, to fully utilize the high voltage of the hybrid zinc-vanadium RFB, we fabricated binary cerium titanium oxide (Ce_2/3TiO₃, hereafter referred to as CTO) with abundant defects and prepared the CTO catalyst ink with mixing Ketjen black (KB) as conductive agent and binder solution, then coated onto the carbon felt electrode for VO²⁺/VO₂⁺ redox couple. We firstly found that the CTO-KB electrocatalysts could improve the VO²⁺/VO₂⁺ redox reaction, which could be attributed to synergistic effect of conductive KB. Specifically, the CTO-KB composite electrocatalysts showed a significantly improved peak separation potential (ΔE) of 227 mV, which was 140 mV less than that of the pristine carbon felt electrode. In addition, the reference carbon felt electrode had only 140.95 and 131.08 mA cm⁻², whereas CTO-KB represented high oxidation and reduction peak current densities of 167.4 and 159.11 mA cm⁻², respectively, and the CTO-KB showed superior performance than other composite electrocatalysts due to the synergistic effect of KB nanoparticles. The abundant defects with edge dislocation, planar defects, and oxygen vacancies were observed in the microstructure of the CTO, which also gave rise to the high catalytic activity. The hybrid zinc-vanadium RFB showed excellent cell efficiency at a current density of 40 mA cm⁻² and recovered to initial state at the decreased current density. the composite electrocatalysts exhibited higher capacity characteristics than the cell to which pristine CF was applied even after 50 cycles due to low overpotential. As a result, we presented the possibility of driving next-generation energy storage devices with higher efficiencies and showed the possibility of using a high voltage energy storage device using a low potential zinc anode active material.<br/><br/>Acknowledgments<br/>This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2021R1C1C1008349, NRF-2021R1A4A1022198). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE)(20214000000140, Graduate School of Convergence for Clean Energy Integrated Power Generation). This work was supported by BK21 FOUR Program by Pusan National University Research Grant, 2021.