A Bifunctional Liquid Fuel Cell Coupling Power Generation and V 3.5+ Electrolytes Production for All Vanadium Flow Batteries.

All vanadium flow batteries (VFBs) are considered one of the most promising large-scale energy storage technology, but restricts by the high manufacturing cost of V 3.5+ electrolytes using the current electrolysis method. Here, a bifunctional liquid fuel cell is designed and proposed to produce V 3.5+ electrolytes and generate power energy by using formic acid as fuels and V 4+ as oxidants. Compared with the traditional electrolysis method, this method not only does not consume additional electric energy, but also can output electric energy. Therefore, the process cost of producing V 3.5+ electrolytes is reduced by 16.3%. This fuel cell has a maximum power of 0.276 mW cm -2 at an operating current of 1.75 mA cm -2 . Ultraviolet-visible spectrum and potentiometric titration identify the oxidation state of prepared vanadium electrolytes is 3.48 ± 0.06, close to the ideal 3.5. VFBs with prepared V 3.5+ electrolytes deliver similar energy conversion efficiency and superior capacity retention to that with commercial V 3.5+ electrolytes. This work proposes a simple and practical strategy to prepare V 3.5+ electrolytes.