Turning waste into resource: Metal framework-scale composite cathode overcomes limitations of low efficiency and...

Despite years of development, electrochemical water softening continues to face challenges in achieving high softening efficiency and maintaining long-term cathode stability. To address these issues, this study builds upon the characteristics of membrane-free electrochemical water softening and prior research by employing a large-pore stainless steel filter as the cathode. During extended operation, a fluffy, porous scale layer gradually forms on the cathode surface, transforming the stainless-steel filter into a metal framework-scale composite (MF-S) cathode. This composite cathode enhances OH⁻ enrichment and extraction, improving water softening efficiency. Additionally, the soft scale deposited on the cathode's pores and surface can be partially removed through simple backflushing, extending system's operational lifespan. Experimental results indicate that using a stainless-steel cathode with 15×10 mm pore size, the effluent pH exceeds 11.0 after 18 h of operation, with a Ca²⁺ hardness removal rate of over 97 %. To prevent clogging of the cathode pores during extended operation, backflushing is conducted every 25 h to remove scale. Remarkably, after 700 h of continuous operation, there is no observed decline in hardness removal efficiency, and the cathode remains functional, allowing the water softening process to continue. Electrochemical tests and finite element simulations reveal that the composite cathode significantly outperforms the stainless-steel filter cathode in generating and enriching OH⁻. The proposed composite cathode demonstrates strong practical potential, offering a new perspective for applying membrane-free, high-efficiency electrochemical water softening processes.