Abstract
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.