An electrochemically induced dual-site adsorption composite film of Ni-MOF derivative/NiCo LDH for selective bromide-ion extraction

An electrochemically induced electrode with bromide ion (Br-) dual adsorption sites was obtained by in-situ growing of Ni-MOF derivative/NiCo layered double hydroxide (Ni-MOF(D)/NiCo LDH) composite film on the stainless-steel wire mesh via an electrodeposition-assisted solvothermal method, which exhibited excellent Br- selectivity from ionic mixture solutions of Br-, NO3– and I- in an electrochemically switched ion exchange (ESIX) system. The Br- extraction capacity was as high as 126.30 mg·g−1 and the separation factors of Br-/NO3– and Br-/I- were 8.29 and 4.83 respectively. Combining the simulation and experimental results, it was confirmed that the highly efficient selective extraction of Br- in neutral solution should be derived from the dual-site (i.e., electrochemically induced laminate metal coordination sites and interlayer anion exchange sites of NiCo LDH) adsorption. Furthermore, the electrode demonstrated superior adsorption–desorption performance and electrochemical stability because of the in-situ growth strategy, which enhanced the binding force between NiCo LDH and substrate and improved the electrical conductivity of NiCo LDH, and provided a storage space for Br-. This study served deep understanding of ions selective extraction mechanism of LDH-based materials in the ESIX process, which could be a novel reaction concept for the design of those electrodes with specific recognition sites in the applications of them in other fields such as supercapacitors, sensors and batteries.