We introduce an aqueous flow battery based on low-cost, non-flammable, non-corrosive and Earth-abundant elements. During charging, electrons are stored in a concentrated water solution of 2,5-dihydroxy-1,4-benzoquinone (DHBQ), which rapidly receives electrons with inexpensive carbon electrodes without the assistance of any metal electro-catalyst. Electrons are withdrawn from a second water solution of a food additive, potassium ferrocyanide (K4Fe(CN)6). When these two solutions flow along opposite sides of a cation-conducting membrane, this flow battery delivers a cell potential of 1.21 V, a peak galvanic power density of 300 mW/cm2 and a coulombic efficiency exceeding 99%. Continuous cell cycling at 100 mA/cm2 shows a capacity retention rate of 99.76%/cycle over 150 cycles. Various molecular modifications involving substitution for hydrogens on the aryl ring were implemented to block decomposition by nucleophilic attack of hydroxide ions in solution. These modifications resulted in increased capacity retention rates of up to 99.962%/cycle over 400 consecutive cycles, accompanied by changes in voltage, solubility, kinetics and cell resistance. Quantum chemistry calculations of a large number of organic compounds predicted a number of related structures that should have even higher performance and stability. Flow batteries based on alkaline-soluble dihydroxybenzoquinones and derivatives are promising candidates for large-scale, stationary-storage of electrical energy.