Template-directed strategy synthesis of CoNi-layered double hydroxide nanosheet coated with polypyrrole for enhanced capacitive deionization

Recently, faradic material layered double hydroxide (LDH) has been widely used as an anode for capacitive desalination (CDI), due to its reversible ion insertion/deintercalation process and high anion storage capacity. Unfortunately, the issues of poor conductivity and self-stacking of LDH limit its application in the CDI. Herein, CoNi-LDH coated with polypyrrole (PPy@CoNi-LDH) was meticulously designed and synthesized using ZIF-67 as the precursor by template-directed strategy and in-situ polymerization. The CoNi-LDH has large interlayer distance of 0.91 nm, which is beneficial for ion insertion. Introduced PPy conductive layer improved the interface transfer rate of charges and increased the ion storage capacity of composite. The experimental test results indicate that PPy@CoNi-LDH1:1 electrode has a higher specific capacitance of 223.1 F g- 1 and lower resistivity of 62.33 Omega & sdot;cm compared to CoNi-LDH electrode (35.2 F g- 1 and 3.39 x 104 Omega & sdot;cm). Obtained PPy@CoNi-LDH1:1 also has abundant mesoporous and excellent hydrophilicity, which can promote ions transfer. Meanwhile, the specific surface area of PPy@CoNi-LDH1:1 with the coating structure is 1.78 times that of CoNi-LDH, effectively suppressing the self-stacking of CoNi-LDH and exposing more active sites. When employed PPy@CoNi-LDH1:1 as an anode and the activated carbon as a cathode, the PPy@CoNi-LDH1:1//AC cell exhibited a high Cl- removal capacity (60.20 mg g- 1), a swift Cl-removal rate (11.45 mg g- 1 min- 1) and good cycling stability (the retention rate is 89.67 % after 50 cycles) in NaCl solution of 500 mg L- 1 and 1.4 V applied voltage. Additionally, the adsorption mechanism indicated that surface charge attraction, ligand exchange, and ion insertion jointly promote the Cl- removal. This study provides a feasible solution about successful design of high adsorption capacity anode materials for CDI.