Valorization of lignin-derived carbon nanofibers into Ca-MOF-Based hierarchical electrodes for enhanced fluoride electrosorption.

Valorization of renewable lignin into high-performance electrochemical materials offers a sustainable route for advanced water treatment, yet achieving efficient fluoride removal together with long-term electrode stability remains challenging. Herein, we report a hierarchical Ca-MOF/lignin-derived carbon nanofiber (CMDNF) electrode fabricated via in situ solvothermal growth of a Ca-based metal-organic framework on electrospun lignin nanofibers, followed by controlled carbonization. This strategy converts biomass-derived nanofibers into a conductive and hierarchically porous carbon framework integrated with Ca-containing surface environments favorable for fluoride electrosorption. Structural characterization reveals that Ca species are uniformly distributed throughout the carbon architecture, while the lignin-derived framework provides structural robustness, efficient electron transport, and improved ion-accessible pathways. The optimized CMDNF electrode exhibits a specific capacitance of 163F··g-1 at 2 mV·s-1 and retains 91.27% of its initial capacitance after 5000 charge-discharge cycles, demonstrating good electrochemical stability. In fluoride electrosorption experiments, it delivers a maximum adsorption capacity of 26.81 mg·g-1 and reduces the fluoride concentration from 10 mg·L-1 to 1.15 mg·L-1 under model-solution conditions, below the WHO guideline value of 1.5 mg·L-1. XPS and EPR analyses suggest that the enhanced fluoride removal arises from the combined contribution of Ca-containing oxygen-coordinated surface environments, defect-rich carbon surfaces, and electric double-layer electrosorption. This work demonstrates a feasible strategy for converting lignin into high-performance CDI electrodes and provides useful insight into the rational design of biomass-derived hierarchical materials for fluoride removal at electrode-electrolyte interfaces.