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.