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Carbon black flow-electrode enhanced electrochemical desalination using single cycle operation.

Flow-electrode electrochemical desalination (FEED) processes (e.g., flow-electrode capacitive deionization, FCDI), which use flowable carbon particles as the electrodes, have attracted increasingly attentions, holding the promise for continuous operation and high desalting efficiency. While it is generally believed that carbon particles with abundant micropores and large specific capacitance (e.g., activated carbon, AC) should be ideal candidates for FEED electrodes, we provide evidence to the contrary, showing that highly conductive electrodes with low specific surface area can outperform microporous AC-based electrodes. This study revealed that FEED using solely high surface area AC particles (~2000 m2 g-1, specific capacitance of ~44 F g-1, average salt adsorption rate of ~0.15 μmol cm-2 min-1) was vastly outperformed by electrodes based solely on low surface area carbon black (CB, ~70 m2 g-1, ~0.5 F g-1, ~0.75 μmol cm-2 min-1). Electrochemical impedance spectroscopy (EIS) results suggest that the electrode formed by CB particles led to more effective electronic charge percolation, likely contributing to the improved desalination performance. In addition, we propose and demonstrate a novel operation mode, termed single cycle (SC), which greatly simplified the FEED cell's configuration and enabled simultaneous charging and discharging. Using SC mode with CB flow-electrodes delivered an increased average salt removal rate relative to the more traditional short-circuited closed cycle (SCC) mode, achieving up to 1.13 μmol cm-2 min-1. Further investigations demonstrate that up to 50% of energy input would be avoided when using CB flow-electrodes operated under SC mode as compare to that of AC flow-electrodes operated under SCC mode. In summary, the FEED process presented in this study provided an innovative and promising approach towards high-efficient and low-cost brackish water desalination.