The storage system is based on a low-cost organic compound known as fluorenone (C13H8O), which is largely used as a precursor to synthesize a variety of organic electronic materials and is bright fluorescent yellow in color. “Alternative materials for flow batteries include organic molecules, which are far more available, more environmentally friendly and less expensive than vanadium,” the US group stated. “But organics haven’t held up well to the demands of flow-battery technology, usually petering out faster than required.”
In order to prevent fast degradation, the fluorenone compound was transformed into a redox reversible, water-soluble compound. Prior to this technical improvement, fluorenone molecules were not water-soluble enough and could not provide redox reversibility in aqueous solutions. The solubility of the compound is crucial in redox flow batteries and the academics claimed to have been able to increase that of fluorenone in water from almost 0 with pristine fluorenone up to 1.5 moles per liter.
The research team found that the ability of fluorenone to carry out reversible reactions is dependent on its concentration. “This is a great demonstration of using molecular engineering to change a material from one widely considered impossible for use into something useful for energy storage,” said research co-author Wei Wang. “This opens up important new chemical space that we can explore.”
The battery has a size of 10 cm2 and a power output of 500 milliwatts. Despite its small dimensions, its energy density is more than twice that of the vanadium batteries in use today, the scientists claim. “In laboratory testing that mimicked real-world conditions, the PNNL battery operated continuously for 120 days, ending only when other equipment unrelated to the battery itself wore out,” they further explained. “The battery went through 1,111 full cycles of charging and discharging — the equivalent of several years of operation under normal circumstances — and lost less than 3 percent of its energy capacity.”
The battery was presented in the paper “Reversible ketone hydrogenation and dehydrogenation for aqueous organic redox flow batteries”, published in Science.
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