Oligomer redox-species RFB have been proposed to reduce the crossover of the electroactive species, while using  low cost  membranes. Such redox-active oligomers are known as redoxymers. One system uses organic polymers and a saline solution with a cellulose membrane. The prototype underwent 10,000 charging cycles while retaining substantial capacity. The energy density was 10 Wh/L. Current density reached 100 milliamperes/cm.
Another oligomer RFB employs viologen and TEMPO redoxymers in combination with low-cost dialysis membranes. Functionalized macromolecules (similar to acrylic glass or Styrofoam) dissolved in water are the active electrode material. The size-selective nanoporous membrane 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 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 water solubility is not trivial. So far, RFBs with oligomer redox-species have not demonstrated competitive area-specific power. It is not clear whether low operating current density is an intrinsic feature of large redox-molecules or not.