One flow-battery concept is based on redox active, organic polymers employs viologen and TEMPO with dialysis membranes. The polymer-based redox-flow battery (pRFB) uses functionalized macromolecules (similar to acrylic glass or Styrofoam) being dissolved in water as active material for the anode as well as the cathode. Thereby, metals and strongly corrosive electrolytes – like vanadium salts in sulfuric acid – are avoided and simple dialysis membranes can be employed. The membrane, which separates the cathode and the anode of the flow cell, works like a strainer and is produced much more easily and at lower cost than conventional ion-selective membranes. It retains the big "spaghetti"-like polymer molecules, while allowing the small counterions to pass. The concept may solve the high cost of traditional Nafion membrane but the design and synthesis of redox active polymer with high solubility in water is not trivial.