Towards Sustainable Energy Storage Via Incorporation of Organic Molecules on Carbon Spheres

Abstract

Large scale energy storage at lower cost using environmentally benign materials is becoming important due to increasing fraction of electrical energy generation from intermittent renewable energy sources such as wind, solar, and hydro, to name a few. Recently, our group reported on a novel grid-scale energy storage concept named electrochemical flow capacitor (EFC), which takes advantage of both supercapacitors (excellent cycle life, ultrafast charging/discharging) and flow batteries (scalable architecture).1 EFC utilizes the flowable/suspension electrodes composed of electrolyte, electronically conducting active material and, eventually, conductive additives which facilitate the charge transport throughout the suspension network.2–4 Due to the capacitive charge storage mechanism, EFC shows a lower energy density in comparison to batteries. To address this issue, we incorporated small organic redox molecule (hydroquinone), which is inexpensive, non-toxic and offers a highly reversible two-electron two-proton redox reaction.5 Addition of hydroquinone (HQ) molecules adds faradic contribution, which enhances the capacitance and energy density of the flowable electrodes.6We incorporated HQ by mixing HQ with carbon spheres, where HQ was physisorbed on the surface as well as inside the pores of the spheres and by using it as a redox mediator with varying concentrations. The incorporation of HQ was characterized via various microscopic and electrochemical techniques. Carbon spheres with physisorbed HQ molecules exhibited enhanced capacitance of 342 F/g, which was twice as high as that of the carbon slurry (160 F/g). We also observed the increase in capacitance when HQ was used as redox mediator, however this composition showed a poor cycle life. The best performing electrode in the static mode was further examined under intermittent flow mode, and a two-fold higher capacitance was observed.