Apparatus and method for regenerating electrolyte of a redox flow battery

A battery system comprising a redox flow battery and a regenerating cell for regenerating an electrolyte of said redox flow battery, said regenerating cell (16), comprising:

• a negative electrolyte chamber (22) containing a positive electrolyte of said redox flow battery (1) introduced from a positive electrolyte tank (3) of said redox flow battery (1) through an inlet pipe, said negative electrolyte chamber (22) having a negative electrode (26) immersed in said positive electrolyte, a positive electrolyte chamber (20) connected to said negative electrolyte chamber through a membrane (18), said positive electrolyte chamber containing a formic acid solution and having a positive electrode (24) immersed in said formic acid solution, voltage applying means for applying a voltage between said positive and negative electrodes (24, 26) to emit electrons from said formic acid solution for changing a portion of metal ions contained in said positive electrolyte and having a high valence state, to metal ions having a lower valence state, and an outlet pipe for returning, to said positive electrolyte tank, said positive electrolyte containing low valence state metal ions produced by an application of a voltage by said voltage applying means to said positive and negative electrodes (24, 26).