The present invention relates to a rechargeable redox flow battery (RFB) comprising an electrochemical cell with at least one positive electrode in a positive half-cell and a negative electrode (anode) in a negative half-cell, wherein the negative half-cell comprises the Zn2+/Zn0 redox couple, at least methane sulfonic acid or its anion as an electrolyte for conductivity; and the positive half-cell comprises the CrIII/CrVI redox couple, the Ce4+/Ce3+ redox couple as a electrochemically reversible electron mediator at least methane sulfonic acid or its anion as an electrolyte, and at least one solvent.