Experimental Section
Materials. Ethylene glycol monomethyl ether (EGME) was bought from Scharlau. Ethanolamine was obtained from Sigma-Aldrich. Benzimidazole, Zn2SO4∙7H2O, and MnSO4∙2H2O, polyvinylidene fluoride (PVDF), and N-methylpyrrolidone (NMP) were purchased from was provided by Aladdin. Carboxyl-multiwalled carbon nanotube was bought from Xianfeng nano-materials Co. Ltd.
Zn-based gel coating. 1g zinc acetate (Zn(Ac)2·2H2O)was dispersed in 12 mL ethylene glycol monomethyl ether (C3H8O2, EGME) and stirred at 60 ℃ for 0.5 h to get a white suspension. Then, 1 mL ethanolamine was dropwise added to the suspension, and zinc acetate was dissolved to get the transparent Zn-based sol. The solution was aged at room temperature for 10 hours. Zn foil (100 µm) was washed with ethanol and water and then soaked in a dilute H2SO4 solution to remove the extra zinc oxides on the surface. One side of zinc foil was protected by a Kapton tape in the following synthesis process. To form a homogenous Zn-based gel on a Zn foil surface, the zinc-based sol (0.5 mL) was dip-casted onto a 5 cm×5 cm zinc foil surface. After drying at 120 ℃ in the vacuum oven for 1 h, a uniform gel layer was tightly attached to the zinc foil surface.
Zn2(bim)4 MOF layer growing.0.7g benzimidazole (bim) powder was put in a Teflon-lined stainless steel autoclave, and zinc-based gel-coated zinc foil was cut into 3mm-wide pieces and vertically placed above the powder without contact. The autoclave was heat treated at 185 ℃ for 12 h for vapor deposition. After natural cooling, the gel was converted to Zn2(bim)4, and the Zn@Zn2(bim)4 was washed with methanol several times to remove the residual bim on the surface and vacuum-dried. The MOF layer can also be grown on the copper foil (Cu@Zn2(bim)4 by applying the same method.
MnO2/CNT cathode fabrication.MnO2/CNT nanorods were synthesized according to previously reported work[12a], and the weight fraction of MnO2 in the MnO2/CNT is determined to be ~74% by the thermo gravimetric analysis (TGA) test (Figure S3b, Supporting information). Then, the MnO2/CNT, carbon black, and PVDF binder were mixed with a weight ratio of 8:1:1 in NPM solution to form a homogenous ink. The ink was dipped into carbon paper and dried in a vacuum oven for 12 h, and cathodes with different mass loadings can be prepared.