Synergistic regulation of porosity and wettability of waste coffee residue-based carbon materials via chemical activation for high-performance flow-electrode capacitive mixing

Flow-electrode capacitive mixing (F-CapMix) has shown great potential to harvest salinity gradient energy as an alternative to fossil fuel-driven non-renewable energy. In the F-CapMix process, active materials are chiefly responsible for determining its power density by providing capacitive charge storage and a conductive pathway. Hence, this study evaluated the feasibility of waste coffee residue-based carbon materials (WCR-CM) as cost-effective active materials for the F-CapMix process by improving their porosity and wettability via chemical activation with three different types of alkali agents (i.e., NaOH, NaHCO 3 , and Na 2 CO 3 ). NaOH-activated WCR-CM (WCR-CM NO ) exhibited superior porosity and wettability to WCR-CM, NaHCO 3 -activated WCR-CM (WCR-CM NH ), and Na 2 CO 3 -activated WCR-CM (WCR-CM NC ). These phenomena led to a greater contribution of capacitive behavior in the WCR-CM NO -based F-CapMix process (94.6%) to charge storage than in other WCR-CMs (32.5%–72.8%). Moreover, the high viscosity of WCR-CM NO -based flow-electrodes, along with excellent porosity and wettability, facilitated the formation of a charge percolation network that improved charge distribution across the interfaces of ion-exchange membranes and reduced electronic and ionic transfer resistance. Consequently, the WCR-CM NO -based F-CapMix process possessed a considerably higher power density (0.55 W m -2 ) compared with that of WCR-CM (0.14 W m -2 ), WCR-CM NH (0.32 W m -2 ), and WCR-CM NC (0.38 W m -2 ). In addition, this surpassed other F-CapMix processes that utilized carbon materials previously reported in the literature. Based on these observations, WCR-CM NO might effectively enhance the viability of the F-CapMix process for producing salinity gradient energy to meet energy demand.