Preparation of walnut shell carbon electrodes with N/P co-doped for capacitive deionization in desalination

The escalating global water crisis has heightened the importance of seawater desalination technologies, with capacitive deionization emerging as a promising alternative due to its cost-effectiveness and high efficiency. This study investigates the development of a layered porous biochar electrode derived from walnut shells, utilizing potassium hydroxide activation combined with nitrogen and phosphorus doping to enhance desalination performance. Optimal CDI performance was observed in the electrode designated NPWAC-2-2, prepared with a mass ratio of (NH₄)₂HPO₄ to biochar of 2:1 and a holding time of 2 h. This electrode demonstrated remarkable characteristics, including a specific surface area of 2334.03 m² g^-1, a pore volume of 0.982 cm³ g^-1, a specific capacitance of 385.73 F g^-1 at 1 A g^-1, and a minimal charge transfer resistance of 3.38 Ω. In desalination trials, the NPWAC-2-2 electrode exhibited a maximum adsorption capacity of 23.38 mg g^-1 at a voltage of 1.6 V, a flow rate of 10 mL min^-1,and an initial salt concentration of 500 mg L^-1. It also demonstrated impressive cycling stability, retaining 93.0 % efficiency after 50 adsorption/desorption cycles at 1.6 V. Furthermore, the electrode's performance in imitate real seawater was evaluated, revealing effective electro-adsorption of various metal ions, including Ca²⁺, Zn²⁺, and Fe³⁺. The findings of this research facilitate the reuse of waste, conserve resources, and meet the dual objectives of carbon peak and carbon neutrality. They also represent a significant advancement in enhancing the efficiency of capacitive deionization, contribute to the economic feasibility of seawater desalination, and lay the groundwork for its broader implementation in addressing global water scarcity.