Iron vacancy accelerates biogas slurry-derived Fe₃O₄/mesoporous carbon for water purification

Fe₃O₄ is a promising transition metal oxide for ion removal owing to its high theoretical capacity, hydrophilicity, and non-toxicity, but its structural instability during ion insertion-extraction limits practical application. Here, Fe₃O₄ was integrated with mesoporous carbon derived from biogas slurry to enhance conductivity and sustainability, followed by alkaline etching to introduce abundant iron vacancies (VFO). The resulting VFO-C composite exhibits accelerated charge transfer, numerous intercalation-active sites, and superior electrochemical stability. At 1.6 V, the material achieved a desalination capacity of 126 mg g^-1 and retained 96.6% of its initial capacity after prolonged cycling. This performance surpasses conventional Fe₃O₄ electrodes, highlighting the synergistic benefits of defect engineering and waste-derived carbon. The strategy not only advances high-efficiency and durable capacitive deionization but also broadens potential applications in energy storage systems such as supercapacitors and batteries.