Alkaline Benzoquinone Aqueous Flow Battery for LargeScale Storage of Electrical Energy

The replacement of fossil fuel energy with renewable sources has been increasing as the cost of solar and wind energy falls rapidly. Recent reports show that from 2008 to 2015, the cost of wind generation fell by 41%, rooftop solar photovoltaic installations by 54%, and utility-scale photovoltaic installations by 64%. The cost of solar panels now takes up less than 30% of a fully installed solar electricity system.[1] Although the cost of electricity from wind and sunlight has dropped dramatically, their widespread adoption is impeded by the inherent intermittency of these renewable energy sources. Safe, lowcost, efficient, and scalable energy storage could solve this problem. A number of energy storage options are available, such as pumped hydro, flywheels, compressed air, supercapacitors, solid-electrode batteries, and redox-flow batteries (RFBs).[2] In RFBs, the redoxactive species are separately stored in electrolytes in external tanks, and react reversibly in a device similar to a fuel cell when they are pumped past the electrodes (Figure 1A). This design offers significant advantage over solid electrode batteries, by decoupling energy and power output: the former is determined by the tank size and electrolyte concentration, the latter by electrode area.[3] Moreover, aqueous RFBs eliminate the safety issues posed by flammable organic solvents and, due to low electrolyte resistance, enable high current densities. The all-vanadium RFB, which has been the most heavily commercialized, is restricted by the low earth-abundance and the high and fluctuating cost of vanadium.[4] Aqueous organic redox-flow batteries (AORFBs) exploiting water-soluble organic and organometallic redox-active molecules that are composed of only earth-abundant elements[5] have been the subject of recent research. Organic charge-storage materials offer structural diversity, tunable redox potential, and optimizable solubility.[6] High aqueous solubility, well-separated reduction potentials barely avoiding water splitting, stability, safety, and low cost at mass-production scales constitute the most critical attributes for novel aqueous organic electrolytes. Small molecule-based AORFBs can be run at acidic,[5a] neutral,[3a,5e] or basic pH.[5c,d,7] Substantially higher cell potentials An aqueous flow battery based on low-cost, nonflammable, noncorrosive, and earth-abundant elements is introduced. During charging, electrons are stored in a concentrated water solution of 2,5-dihydroxy-1,4-benzoquinone, which rapidly receives electrons with inexpensive carbon electrodes without the assistance of any metal electrocatalyst. Electrons are withdrawn from a second water solution of a food additive, potassium ferrocyanide. 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 cm−2, and a coulombic efficiency exceeding 99%. Continuous cell cycling at 100 mA cm−2 shows a capacity retention rate of 99.76% cycle−1 over 150 cycles. Various molecular modifications involving substitution for hydrogens on the aryl ring are implemented to block decomposition by nucleophilic attack of hydroxide ions. These modifications result in increased capacity retention rates of up to 99.96% cycle−1 over 400 consecutive cycles, accompanied by changes in voltage, solubility, kinetics, and cell resistance. Quantum chemistry calculations of a large number of organic compounds predict 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.