Importance of the conductivity and specific surface area of electrode particles to the desalination performance and energy efficiency of flow-electrode capacitive deionization

Different from electrodialysis for desalination, flow-electrode capacitive deionization (FCDI) removes ions via electrosorption on the surface of charged flow-electrode particles. In this study, the exact effects of the conductivity and surface area of carbon electrode particles on the desalination performance and energy efficiency of the FCDI were investigated under the constant voltage (CC) and constant current (CV) power-supply modes. The low-cost activated carbon (AC) with a large specific surface area and carbon black (CB) with a high conductivity were used as the model electrode particles. In the CC mode, increasing the CA content from 0.5 wt% to 5 wt% achieved a limited enhancement of the desalination rate and efficiency (50.2%-54.5% in 2 h), but dramatically reduced the energy consumption from 1.39 kWh/kg to 0.73 kWh/kg. In the CV mode at 1.2 V, doubling the AC content from 5 wt% to 10 wt% or mixing a small amount of CB in AC (AC:CB = 9:1 at 5 wt%) significantly improved the desalination efficiency from 65.6% to over 90%, but the energy consumption remained similarly at 0.64–0.68 kWh/kg. The conductivity of the electrode particles greatly affects the energy efficiency in the CC mode and the desalinization rate in the CV mode. The CB addition can effectively enhance the electric charge transfer between the current collectors and electrode particles. The surface area of electrode particles largely regulates the FCDI capacitance, as well as the back diffusion during the process. Accordingly, the flow-electrode property can be optimized, e.g., CB is mixed with AC at an appropriate ratio, to ensure both a high conductivity (to facilitate electron transfer) and sufficient surface area (to increase the capacitance and alleviate back diffusion) for cost-effective desalination.