Reusing Waste Coffee Grounds as Electrode Materials: Recent Advances and Future Opportunities.

Coffee industry produces more than eight million tons of waste coffee grounds (WCG) annually. These WCG contain caffeine, tannins, and polyphenols and can be of great environmental concern if not properly disposed of. On the other hand, components of WCG are mainly macromolecular cellulose and lignocellulose, which can be utilized as cheap carbon precursors. Accordingly, various forms of carbon materials have been reportedly synthesized from WCG, including activated carbon, mesoporous carbon, carbon nanosheets, carbon nanotubes, graphene sheet fibers (i.e., graphenated carbon nanotubes), and particle-like graphene. Upcycling of various biomass and/or waste into value-added functional materials is of growing significance to offer more sustainable solutions and enable circular economy. In this context, this review offers timely insight on the recent advances of WCG derived carbon as value-added electrode materials. As electrodes, they have shown to possess excellent electrochemical properties and found applications in capacitor/supercapacitor, batteries, electrochemical sensors, owing to their low cost, high electrical conductivity, polarization, and chemical stability. Collectively, these efforts could represent an environmentally friendly and circular economy approach, which could not only help solve the food waste issue, but also generate high performance carbon-based materials for many electrochemical applications.