Ternary NiFeMn layered metal oxide (LDO) compounds for capacitive deionization defluoridation: The unique role of Mn

Fluoride (F–) contamination in drinking water or groundwater due to natural and human activities has been recognized as one of the major problems to human health. In this work, the novel capacitive deionization (CDI) defluorination was proposed and offered satisfactory results especially with the layered metal oxide (LMO) electrode. The NiFe-LMO, NiMn-LMO and NiFeMn-LMO electrodes were prepared and characterized. NiFeMn-LMO was used as the CDI anode, nitric acid treatment AC as the cathode, and the asymmetric module (H-AC//NiFeMn-LMO) was assembled for defluorination test. The NiFeMn-LMO defluoridation capacity can reach 16.7 mg/g at the initial NaF concentration of 500 mg/L and the voltage of 1.4 V, and it can keep 93.3% of the initial capacity after 30 cycles at the voltage of 1.0 V. CDI defluorination mechanism of LMO was analyzed. It was found that Mn tended to transform from low state to high state in the process of CDI defluorination and the introduction of Mn improved the pseudcapacitive behavior of materials, thus enabling the ternary NiFeMn-LMO to have excellent CDI defluorination performance. This provides a new idea for the development of high-performance CDI defluorinated electrode materials.