NaHCO3 activation as a green strategy to reinforce structural properties of coffee biochar-based electrodes for advancing flow-electrode capacitive deionization: Effects of two-stage thermal decomposition

초록

Flow-electrode capacitive deionization (FCDI) has received remarkable interest as a potential desalination technique for producing fresh water to address the ever-growing water scarcity. Nevertheless, the requirement for high-quality activated carbons with high costs is necessary to ensure high desalting performance of the FCDI system, thereby weakening the pragmatic feasibility. In this study, coffee biochar-based electrodes (CBE) were proposed as alternatives to high-quality activated carbons by improving their structural properties via greener alkali agent-based activation, including sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3). Porous structures and surface functionalities of NaHCO3-activated CBE (CBENaH) were superior to those of CBE and Na2CO3-activated CBE (CBENaC) because two-stage thermal decomposition induced by NaHCO3 activation generated more redox reactants than Na2CO3 activation. Accordingly, the highest electrochemical performance was observed from the FCDI cell with CBENaH (specific capacitance = 0.37 F/g), followed by CBENaC and CBE (specific capacitance = 0.09-0.23 F/g). These phenomena resulted in the higher average salt removal rate of the FCDI system with CBENaH than CBE and CBENaC. Electrosorption primarily controlled the desalination behavior of the FCDI system with CBENaH over tested conditions, where electrodialysis was minorly supported. Despite repeating saline water desalination, a slight decrease was found for the desalting performance of the FCDI system with CBENaH, suggesting good cyclic stability. Hence, CBENaH was a promising substitute for high-quality activated carbons to increase the feasibility of the FCDI system capable of securing alternative water resources.

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