Controlled Synthesis Of Metal Organic Frameworks (MOFs) Derived Carbon-based Heterostructures For Capacitive Deionization

The shortage of freshwater resources has become one of the critical challenges to technological,economic,and social developments due to the emerging global energy crisis,environmental pollution as well as the increasing withdrawal and uneven distribution of freshwater.Desalination of seawater or brackish water resources has become an effective strategy to address the shortage of freshwater resources.Among the previously reported desalination technologies,capacitive deionization(CDI)technique has become one of the most promising seawater desalination methods due to its high desalination efficiency,low energy consumption,simple regeneration process,and no secondary pollution.As the core part of the CDI system,the electrode materials play a key role in achieving efficient and rapid CDI desalination performance and excellent cycling stability.At present,most electrode materials still suffered from low desalination and poor cycling stability,which greatly limit the development of CDI.Based on this,the study focuses on the construction of high-performance CDI electrode.In order to improve the desalination performance and cycling stability of CDI,nitrogen doped carbon@graphite carbon/carbon nanotubes(NC@GC/CNTs),N,P doped graphite carbon/MXene(N,P-GC/MXene),and nitrogen doped carbon/molybdenum carbide composite(NC/MoC)were designed and synthesized,and their electrochemical and CDI properties were thoroughly studied.The main research contents of this study are as follows:(1)By using two-dimensional(2D)ZIF-L as the template,the ZIF-L@ZIF-67 heterostructure with a core-shell structure was synthesized by in situ growth of 3D ZIF-67 on its surface.Subsequently,ZIF-L@ZIF-67 was thermally carbonized to obtain nitrogen doped carbon@graphitic carbon/carbon nanotubes(NC@GC/CNTs)with high specific surface area and suitable pore size distribution,and remarkable conductivity.After carbonization of the outer layer ZIF-67,a highly graphitized GC/CNTs structure was formed,successfully suppressing the possible aggregation or collapse of the internal ZIF-L during the carbonization process.When the NC@GC/CNTs were used as the CDI electrodes,the high salt adsorption capacity(SAC)was 56.30 mg g-1 in 1000 mg L-1 NaCl solution at 1.6 V,and the retention rate of SAC was 93.5%after 50 cycles.(2)The ZIF-67 dodecahedrons were in situ assembled onto the MXene nanosheets to synthesize ZIF-67/MXene.After carbonization and phosphating processes,the N,PGC/MXene was obtained with good structural characteristics,high electrochemical conductivity and pseudocapacitance contribution.The ZIF-67 derived N,P-GC nanoparticles effectively inhibits the possible oxidation of MXene nanosheets and improves the structural stability of MXene nanosheets.The hybrid CDI cell was assembled with N,PGC/MXene and activated carbon as electrode materials,which showed good desalination performance.In 1000 mg L-1 NaCl solution,a high SAC of 55.3 mg g-1 was achieved at 1.4 V,and the retention rate of SAC was 90.2%after 200 cycles.(3)Considering that Zn(MIm)42-in ZIF-8 has a similar four connected structure to MoO42-in Na2MoO4,the ZIF-8/MoO4 was obtained through an ion exchange method.After carbonization,the nitrogen doped carbon/molybdenum carbide(NC/MoC)composites were prepared,and NC/MoC possessed high specific surface area,hierarchical micro-mesoporous structure,and excellent conductivity were prepared through carbonization.When NC/MoC was used as the CDI electrode,the maximum SAC reached 123.84 mg g-1 at 1.6 V in the 500 mg L-1 NaCl solution,and the retention rate of SAC was 91.2%after 200 cycles.