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Electrosorption-based capacitive deionization (CDI) has become a viable process for brackish water desalination and defluoridation. In this study, activated Douglas fir biochar is used as a low-cost electrode material 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, titanium isopropoxide, and p-phenylenediamine treatment, respectively, which introduced sulfonate, titanium dioxide and amine functional groups to the electrode surface. These modification methods are versatile and can be easily performed without sophisticated laboratory environment. Modified biochar electrodes were characterized by TEM, SEM-EDX, XRD, and XPS. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed to analyze the electrochemical properties of the electrodes. The salt adsorption capacity (SAC) was evaluated in a 3D-printed capacitive deionization flow cell using a chloride and fluoride ion sensor. It was found that functionalized biochar electrodes had increased SAC and charge efficiency in asymmetrical setup due to reduced co-ion effect. For example, the asymmetrical CDI cell with Nafion cathode and amine biochar anode improved NaCl removal capacity by 54% over the activated biochar symmetrical cell (identical anode and cathode), with SAC 6.01 mg NaCl/g biochar at the symmetrical cell and 9.25 mg/g for the asymmetrical cell. The charge efficiency also increased by ≈ 67% from symmetric AcB cell to asymmetric TB-05 cathode and AmB anode. This work shows that biochar can be engineered and explored broadly as an inexpensive sustainable electrode material for asymmetrical capacitive deionization.