Functionalized biochar electrodes for asymmetrical capacitive deionization

Abstract

Electrosorption-based capacitive deionization (CDI) has become a viable process for brackish water desalination. In this study, activated Douglas-fir biochar is used as a low-cost carbon-based alternative electrode with adsorption capacity comparable to activated carbon obtained from biomass precursors. Adding functional groups to the activated biochar enhanced salt removal capacity, providing cation and anion selectivity. The functionalized electrodes were prepared by Nafion and p-phenylenediamine treatment, respectively, which introduced sulfonate and amine functional groups to the electrode. These modification methods are versatile and can be easily performed without sophisticated laboratory environment. These modified biochar electrodes were characterized by SEM-EDX and XPS. Cyclic voltammetry and galvanostatic charge/discharge measurements were performed to analyze the electrochemical properties of both types of electrodes and the electrosorption capacity was evaluated in a 3D-printed capacitive deionization flow cell using a chloride ion-selective electrode. It was found that an asymmetrical cell with Nafion cathode and amine biochar anode increased removal capacity by 54% over the activated biochar symmetrical cell (identical anode and cathode), with 6.01 mg NaCl/g biochar at the symmetrical cell and 9.25 mg g−1 for the asymmetrical cell. This work shows that biochar can be engineered and explored broadly as an inexpensive sustainable electrode material for asymmetrical capacitive deionization.