AbstractAll vanadium flow batteries (VFBs) are considered one of the most promising large‐scale energy storage technology, but restricts by the high manufacturing cost of V3.5+electrolytes using the current electrolysis method. Here, a bifunctional liquid fuel cell is designed and proposed to produce V3.5+electrolytes and generate power energy by using formic acid as fuels and V4+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 V3.5+electrolytes is reduced by 16.3%. This fuel cell has a maximum power of 0.276 mW cm−2at 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 V3.5+electrolytes deliver similar energy conversion efficiency and superior capacity retention to that with commercial V3.5+electrolytes. This work proposes a simple and practical strategy to prepare V3.5+electrolytes.