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Graphical abstract Highlights • Ce x Zr 1−x O 2 /GF (x = 0.8, 0.5, or 0.2) composite electrodes are fabricated for VFB application. • All Ce x Zr 1−x O 2 /GF show improved wettability and activity for VO2+/VO 2 + and V2+/V3+ couples. • Ce 0.8 Zr 0.2 O 2 /GF exhibits significantly enhanced voltage efficiency in Nafion membrane-based VFB. • Ce 0.8 Zr 0.2 O 2 /GF electrode combined with SPEEK membrane demonstrates superior VFB performance. • An efficient and inexpensive VFB is achieved. Abstract Vanadium flow battery (VFB) is one of the most promising technologies for grid scale energy storage because of its unlimited capacity and ultralong lifespan. Improving energy efficiency (EE) and reducing cost are critical to the practical application of VFB. The efficiency and cost of VFB mainly depend on its key materials such as electrode and proton-conducting membrane. Herein, we report the use of highly active ceria–zirconia mixed oxide decorated graphite felt (Ce x Zr 1−x O 2 /GF) electrodes in combination with a high proton-selective sulfonated poly(ether ether ketone) (SPEEK) membrane to achieve efficient and inexpensive VFB. Ce x Zr 1−x O 2 electrocatalysts can improve the electrolyte wettability and electrochemical activity of the GF electrode towards VO2+/VO 2 + and V2+/V3+ redox couples, leading to remarkably enhanced voltage efficiency (VE) of VFB. On the other hand, SPEEK membrane can effectively suppress the crossover of vanadium ions, resulting in greatly improved coulombic efficiency (CE) of VFB. As a result, the Ce x Zr 1−x O 2 /GF-SPEEK combination demonstrates superior battery efficiencies (CE, VE and EE) and cycling stability to the traditional GF-Nafion combination. Meanwhile, since hydrocarbon-based SPEEK membrane is much cheaper than commercial Nafion membrane, the cost of VFB is also significantly reduced. [ABSTRACT FROM AUTHOR]

Developing low-cost and high selectivity membranes is critical for vanadium flow batteries (VFB) energy storage system. Among several potential candidates, sulfonated poly(ether ether ketone) (SPEEK) membrane is most promising, yet its proton conductivity and durability are still insufficient to meet the needs of practical applications. Here we report the preparation of a mussel-inspired polydopamine coated carbon nanotube embedded SPEEK mixed matrix membrane (S/CNT@PDA MMM) and its successful application in VFB. The acid-base interface interaction between the SPEEK matrix and the CNT@PDA filler is beneficial to construct additional proton transfer pathway as well as improve the mechanical property of the S/CNT@PDA MMM. Compared to pristine SPEEK membrane, the S/CNT@PDA MMM demonstrates higher proton conductivity, superior proton to vanadium ion selectivity and significantly enhanced stability. As a result, the VFB assembled with the S/CNT@PDA MMM exhibits greatly improved rate performance compared with the benchmark Nafion 115 membrane, and reaches the peak power density of 580 mW cm −2 at 650 mA cm −2 . Moreover, the S/CNT@PDA MMM shows super long-term stability over 1000 cycles at 200 mA cm −2 , and wide temperature adaptability from − 20 °C to 50 °C, indicating the great potential of this MMM for practical application in VFB. [ABSTRACT FROM AUTHOR]

We report a one-pot solvothermal method to prepare a carbon dots decorated graphite felt (CDs/GF) electrode. The resulting CDs/GF shows excellent wettability and greatly enhanced activity towards vanadium redox couples. VFB with the CDs/GF electrode exhibits outstanding rate performance, superior cycling stability, and broad temperature adaptability from −20 to 60 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • The concept of V/H ratio is introduced to explore the vanadium concentration impact on VFB performance. • The physicochemical/electrochemical properties and battery performance of the electrolytes are studied. • Reducing the V/H ratio is beneficial to obtain higher efficiency and better cycle stability over wide temperature. • A lower V/H ratio can achieve higher electrolyte utilization and deliver higher capacity during long-term operation. • Techno-economic assessment reveals that a relatively low vanadium concentration (1.0–1.2 M) is more suitable. Fast capacity decay and narrow temperature window still hinds the practical application of vanadium flow batteries (VFB). Optimization the electrolyte composition is an effective strategy to realize the stable operation of VFB. Herein, we introduce the concept of V/H ratio (vanadium ion to proton concentration ratio) in electrolyte to explore the vanadium concentration impact on overall performance of VFB. Theoretical analysis and experimental results indicate that reducing the V/H ratio is beneficial to obtain higher efficiency and better cycle stability over wide temperature (-15 °C ~ 55 °C). Meanwhile, during long-term operation, a lower V/H ratio electrolyte can achieve higher electrolyte utilization rate and deliver higher capacity despite the lower theoretical capacity density. Unlike previous studies that focused on increasing the vanadium concentration to obtain a higher theoretical capacity density, our techno-economic assessment clearly demonstrates that a relatively low vanadium concentration (i.e. 1.0–1.2 M) is more suitable for VFB, because safer, cheaper and more stable operation is particularly important for large-scale energy storage. [ABSTRACT FROM AUTHOR]

Vanadium flow battery (VFB) is one of the most promising large-scale energy storage technologies because of super long lifespan and unlimited capacity. In depth study of the all-climate (wide temperature) properties of VFB is essential to the practical application of VFB around the whole globe. In this work, we established a wide temperature (−20–50 °C) research platform for VFB single cell evaluation. A series of typical cation exchange membranes (i.e. the benchmark Nafion 115 and Nafion 212, the Selemion CMV, and the promising candidate SPEEK) and anion exchange membranes (i.e. the Selemion AMV and APS) are studied in terms of their VFB efficiencies, long-term cycling stability and capacity fading mechanism in a temperature range of −20–50 °C. Moreover, the desirable membrane for VFB application under different temperature conditions is also discussed and suggested. [ABSTRACT FROM AUTHOR]