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Capacitive deionization (CDI) is an environmentally friendly, energy efficient, and low cost water purification technique in comparison with other conventional techniques, and it has attracted considerable attention in recent years. Here, we use biomass byproduct okara as the starting material to fabricate a boron and nitrogen codoped hierarchically porous carbon (BNC) with ultrahigh heteroatom contents and abundant in-plane nanoholes for CDI application. With the interconnected hierarchical porous structure, the BNC not only exhibits a large surface area (647.0 m3 g–1) for the adsorption of ions but also offers abundant ion transport channels to access the entire internal surface. Meanwhile, the ultrahigh dopants’ content of B (11.9 at%) and N (14.8 at%) further gives rise to the increased surface polarity and enhanced capacitance for BNC. Owing to these favorable properties, BNC exhibits top-level salt adsorption capacity (21.5 mg g–1) and charge efficiency (59.5%) at the initial NaCl concentration of ∼500 mg L–1. Moreover, we performed first-principle simulations to explore the different effects between N-doping and N,B-codoping on the capacitive property, which indicate that the boron and nitrogen codoping of carbon can largely increase the quantum capacitance over the double layer capacitance. The results of this work suggest a promising prospect for the BNC material in practical CDI application.

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