Graphene materials have the advantages of excellent electron transmission, thermal conductivity, mechanical processing performance, high specific surface area, and high specific capacitance, and have wide application prospects in the field of capacitive deionization technology (CDI). However, there is a phenomenon of interlayer stacking in graphene, which results in the actual specific surface area and specific capacitance being much smaller than the theoretical value. The microporous electric double layer is also weakened due to overlap, resulting in a decrease in adsorption capacity, which limits the performance of graphene and its hydrophobic It also limits CDI desalination performance. The improvement of graphene-based electrode materials and the improvement of CDI desalination performance are mainly from the following aspects:
Optimize graphene electrode material design
The complete graphene surface is inert, not easy to interact with other substances, and there is a strong van der Waals force between the layers, which is easy to cause aggregation, which makes it difficult for graphene to integrate into water and other solvents. By using the active sites at the edges / defects of graphene, the introduction of hydroxyl, carboxyl, sulfonic acid groups and other hydrophilic groups can change the hydrophilic / hydrophobic properties of graphene. The study found that the sulfonation modification method, that is, a sulfonic acid group-containing functional group reacts with graphene / graphene oxide to form a bond to obtain a structurally stable composite material, can not only maintain the original performance of graphene, but also improve its water dispersibility.
Optimize the specific surface area / porosity of graphene electrode materials
The specific surface area is a key performance parameter of the electrode material. The larger the specific surface area, the larger the contact surface between the electrode and the solution, the more adsorption sites, and the higher the ion adsorption amount and adsorption rate. Considering the specific surface area and the different roles of pores in the process of CDI desalination, it is necessary to design a suitable pore structure, and only the reasonable distribution of pore sizes can exert the excellent performance of graphene electrode materials. There are two common methods:
Three-dimensional structured design: The graphene material is subjected to three-dimensional structured design to reduce the effect of "layer-to-layer" stacking between two layers of graphene material, increase the specific surface area, and improve the pore structure.
Porous carbon doping: combining graphene with porous carbon materials can effectively improve the specific surface area / pore structure of graphene. Common porous carbon materials are: activated carbon, mesoporous carbon, carbon aerogel, carbon nanotubes, activated carbon nanofibers, etc.
Optimizing the conductivity of graphene electrode materials
The CDI electrode should choose materials with higher specific capacitance. Under the same conditions, the higher the specific capacitance of the electrode material, the higher the electric double layer capacitance, the stronger the adsorption capacity for ions, and the better the desalination effect; meanwhile, the higher the Faraday capacitance, the stronger the conductivity of the electrode under the same conditions. The smaller the internal resistance potential consumed, the higher the electrode ion adsorption. For graphene materials, currently doping / loading conductive materials is commonly used to improve their conductivity. The main methods include doping of metal oxides, doping of conductive polymers, and nitrogen doping.
Graphene / metal oxide doping: The embedding of metal oxide nanoparticles such as TiO2, MnO2, Co3O4, Fe3O4, etc. can significantly improve the electrochemical performance of graphene, increase the capacitance, and effectively solve the problem of agglomeration.
Graphene / conductive polymer doping: conductive polymer is a polymer with extended conjugated π bond through chemical or electrochemical doping to show semiconductor or even conductor properties, graphene material and conductive polymer The inter-electron transfer can further enhance and expand the performance of electrode materials.
Nitrogen-doped graphene-based electrode: N-doping not only improves the conductivity of the material, but also introduces a large number of defect structures, resulting in more contact area and enhanced graphene material adsorption activity.
To promote the large-scale promotion and application of graphene-based electrode adsorption materials, the following issues should also be explored:
1) Explore the influence of the preparation process on graphene-based materials, optimize the performance of graphene-based materials under the conditions of process diversity;
2) Systematically study the microstructure construction of graphene-based materials, solve the problem of agglomeration, design and prepare materials with high hydrophilicity, high specific surface area, suitable pores, and high conductivity;
3) Strengthen the evaluation of the overall performance of graphene-based electrode materials. The desalination effect of CDI electrodes is comprehensively affected by electrical conductivity, specific surface area, pore size, electrical conductivity, and specific capacitance.