A redox flow battery system having decreased cross-over of active species and decreased hydrogen generation, which is particularly important with less expensive polyethylene or polypropylene membranes. The redox flow battery system comprises at least one rechargeable cell comprising a positive electrolyte, a negative electrolyte, and a separator positioned between the positive electrolyte and the negative electrolyte. The positive electrolyte is in contact with a positive electrode, and the negative electrolyte is in contact with a negative electrode. The positive and negative electrolytes comprise water and a metal precursor, and the concentration of the metal precursor in the negative electrolyte is greater than the concentration of the metal precursor in the positive electrolyte. The metal in the metal precursor comprises iron, copper, zinc manganese, titanium, tin, silver, vanadium, or cerium.