An organic flow battery having a positive electrode electrolyte containing organic compounds with extended conjugation and/or cyclic side chains is provided. The flow battery includes a positive electrode and a positive electrode electrolyte including first solvent and a first redox couple. The positive electrode electrolyte flows over and contacting the positive electrode. The first redox couple includes a first organic compound and a reduction product of the first organic compound. The flow battery also includes a negative electrode and a negative electrode electrolyte including a second solvent and a second redox couple. The negative electrode electrolyte flows g over and contacts the positive electrode. Typically, an ion exchange membrane is interposed between the positive electrode and the negative electrode Characteristically, the first organic compound resists crossover through the ion exchange membrane.