FeCl₃-ZnCl₂ engineered graphitized hierarchical porous carbon from loofah sponge for high-performance inverted...

Inverted capacitive deionization (i-CDI) is an effective strategy for mitigating the anodic degradation and co-ion expulsion commonly encountered in conventional capacitive deionization, but the development of cost-effective, high-performance electrode materials for i-CDI remains a hurdle. Herein, a FeCl₃-ZnCl₂ dual-salt-mediated one-step carbonization-activation strategy was proposed to engineer graphitized hierarchical porous carbon with rich oxygen functionalities from Loofah sponge. FeCl₃ primarily promoted catalytic graphitization while the dual-salt treatment helped preserve/expose oxygen-containing surface functionalities and meanwhile generates a high surface area and well-developed pore structure. The combined activation between FeCl₃ and ZnCl₂ resulted in a hierarchical porous network with improved structural and electrochemical properties compared to single salt activation. The optimal sample denoted LSCFCZC800 exhibited an ultra-high specific surface area of 2549 m² g^-1, oxygen-rich functional groups and well-defined hierarchical porous carbon framework. Surface analysis further showed that dual-salt treatment produced an oxygen-enriched carbon surface containing abundant carboxyl-related species and improved wettability, which are favorable for ion adsorption in i-CDI. When used as an active electrode in an asymmetric i-CDI configuration paired with commercial activated carbon as the counter electrode, the device delivered a substantial salt adsorption capacity of 48 mg g^-1, a rapid adsorption rate of 2.2 mg g^-1 min^-1 and 71% capacity retention after 100 cycles (500 mg L^-1 NaCl 1.2 V). Real groundwater tests further confirmed its practical applicability, reducing conductivity from 2215 to approximately 608 μS cm^-1 while converting very hard water into soft water. The superior desalination performance is associated with coupled effects of hierarchical porosity, enhanced conductivity, good wettability, and favorable surface charge characteristics. This work demonstrated a simple and effective route for designing biomass-derived carbon for efficient i-CDI desalination of saline groundwater.