Synthesis Of MOFs-Based Composite And Metal Hydroxide Materials For Supercapacitor Electrode

This thesis aims at the synthesis of materials based on metal-organic frameworks and their application on supercapacitor electrodes. On one hand, a new composite consisted of Co-MOF and reduced graphene oxide(rGO)(which was considered to have good electrical conductivity and large specific surface area) was prepared by a two-step method. On the other, a novel method was explored to synthesize Co(OH)2using a Co-based metal-organic framework as a template via a solid-solid conversion process in alkali solution. And both of these two materials were studied as supercapacitor electrodes.First of all, a new composite Co-MOF/rGO was prepared using Co(NO3)2·6H2O,H2 BDC and graphene oxide(GO) via a two-step method. And Vitamine C sodium salt was introduced as the reductant. The x-ray diffraction(XRD) peaks of Co-MOF/rGO composite were similar with Co-MOF crystal, demonstrating the presence of Co-MOF structure in the composite. Field emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM) showed that the irregular Co-MOF sheets and lumps with distinct sizes were loaded onto r GO. All these above proved the combination of Co-MOF and r GO. The X-ray photoelectron spectra(XPS)demonstrated the reduction of GO to rGO. The electrochemical behaviors of Co-MOF/r GO composite and Co-MOF were investigated in three electrode system.The specific capacitance of Co-MOF/r GO composite was 267.2 F g-1, which was about twice of bare Co-MOF(126.9 F g-1), at the scan rate of 10 mV s-1. Additionally,the composite exhibited better rate property. Notably, the composite underwent a transformation to Co OOH under the effects of electric field and LiOH solution with the cycles of CV test, and achieved a pretty high specific capacitance(which was about six times of the initial capacitance).Next, Co(NO3)2·6H2O and Na2(bpdc) were used to synthesize a Co-MOF, then the Co-MOF was treated with NaOH solution and converted to Co(OH)2 via a solid-solid conversion process. Fourier transform infrared(FT-IR) spectroscopy and X-raydiffraction(XRD) showed the products were Co(OH)2 instead of Co-MOF. This material was proved to have a loose structure via field emission scanning microscopy(FE-SEM) and transmitting electron microscopy(TEM). Moreover, the N2absorption-desorption isotherm exhibited that this material was full of mesoporous.This kind of structure could increase the contact area between materials and electrolytes, provide good electrolyte penetration and shorten the ion transfer distance.As a result, remarkable electrochemical properties were obtained. The MOF-derived Co(OH)2 showed a high specific capacitance(604.5 F g-1 at the current density of 0.1A g-1), excellent rate capability, as well as good cycling performance. Then an asymmetric supercapacitor was fabricated with activated carbon(AC) as the cathode and the MOF-derived Co(OH)2 as the anode. It also showed superior property.