Charge and material transport in porous materials

1. Introduction to the definition and application of porous materials

A porous material is a material that forms a network structure from pores that are interpenetrated or closed, and the boundaries or surfaces of the pores are composed of pillars or plates. Typical pore structures are: a two-dimensional structure formed by a large number of polygonal holes gathered on a plane; it is called a "honeycomb" material because its shape is similar to the hexagonal structure of a honeycomb; more commonly, it is made up of a large number of A three-dimensional structure in which polyhedral shaped pores are aggregated in space, commonly referred to as a "foam" material. If the solids constituting the holes exist only at the boundary of the holes (that is, the holes are in communication), they are called open holes; if the holes are also solid, that is, each hole is completely separated from the surrounding holes, it is called closed hole. And some holes are semi-open and semi-closed.

Porous materials are a kind of material that is developing rapidly in the current material science, especially porous materials with pore size in the nanometer order. It has many unique properties and strong applicability, which has attracted the attention of the European and American scientific circles and the industrial and commercial circles. At the MRS conference in 1994, many companies reported on their new developments in the practical application of porous materials. The US Department of Energy has provided huge funding for further research on porous materials used to select membrane separation technologies. Porous materials have a wide range of research, and various inorganic aerogels, organic aerogels, porous semiconductor materials, and porous metal materials have been studied. The common features of these materials are low density, high porosity, large specific surface area, and selective permeation of gases.

Figure 1 Example of a porous material

2. Charge and mass transfer in the classification properties of porous materials

Mechanical properties

Parts made of porous materials can improve mechanical properties such as strength and stiffness while reducing density. According to estimates, aircraft made from porous materials will have a net mass reduction of half under the same mechanical properties. In addition, the porous material has high impact toughness and is used in the automotive industry to effectively reduce the damage that traffic accidents cause to passengers.

The effect of the porous structure on mechanical properties should be divided into direct and indirect effects. For example, accelerating (or slowing down) the diffusion process, the effect of the phase change, the indirect effect of such pores is that certain structures are formed. The direct effect of the stomata is expressed as the relationship between the properties of the stomata and the mechanical properties. The direct and indirect effects of the pores can be distinguished by the use of single-phase materials.

According to the literature on the mechanical properties of porous materials, the results of various industrial iron powders show that the minimum and maximum KIC correlations are toward lower porosity as the concentration of associated impurities decreases. The direction of the change, but can not successfully distinguish the impact of the above factors. For all porosity values, the fracture of the iron specimen is intragranular, and the maximum breaking rate on the fracture surface corresponds to the minimum crack resistance. The results obtained were interpreted. According to the assumption that the pores have a spherical shape and a uniform distribution, the cracks are purified and bent like elastic fibers when passing through the pores. Experiments have shown that it is possible to prevent the crack from expanding in the iron due to the pores. It has also been demonstrated that in addition to the general porosity, the properties of other porous structures must be considered, in this case for the same spherical pores.

2. Adsorption performance

The molecular diameter and thermal motion freedom of different gases or liquids are different. Therefore, the differential properties of different porous materials can be used to purify gases or liquids and can be reused for efficient porous adsorption. Purify the material. Different pore materials have different ability to adsorb nitrogen, and their applications in daily production and life are also different. In other words, different applications require different properties of the pore material. Microporous zeolite molecular sieves, mesoporous materials, multi-stage pore materials, and porous aromatic skeleton (PAF) materials have their own characteristics, and are also applied to various fields of daily life because of their different structures and characteristics. For example, microporous zeolite molecular sieves are widely used in petroleum catalysis, environmental protection, fine chemicals and other fields due to their regular pore structure and size, strong adsorption capacity and high catalytic performance. Mesoporous materials are good catalysts and catalyst carriers in macromolecular catalytic reactions based on their narrow pore size distribution, regular channel arrangement order and large specific surface area.

Figure 2 Preparation and application technology of porous oil carbon adsorption materials

The adsorption mechanism in the article "Application of Layered Silicate Minerals to Prepare Porous Materials and Their Adsorption Properties" illustrates the charge and mass transport in the adsorption properties of porous materials. The theory discusses the specific adsorption of heavy metal ions by oxide minerals. It is believed that the selective adsorption of heavy metal ions is the variable charge surface of minerals. The surface of amorphous hydrated oxides and hydroxides such as iron and aluminum is composed of metal ions and light bases, and has hydrophilicity and Lewis acid-base behavior. Light bases exposed on the surface are brought about by dissociation and association. There is a certain amount of surface charge, and the amount of charge varies with the value of the medium. The adsorption of heavy metal ions by variable charge surfaces is different from that of exchangeable adsorption. The heavy metal ions in the variable charge surface can exchange with the hydrogen-oxygen coordination group in the metal atom coordination shell of the mineral, through covalent bond or coordination. The bond is bonded to the solid surface, the bare aluminum alcohol, iron alcohol and silanol on the edge of the layered silicate mineral and the light base aluminum layer of the layered silicate mineral and the siloxane base surface are produced by the broken bond. Silanols are all such coordinating groups. In addition to obligate adsorption, there is also exchangeable adsorption. In the layered silicate structure, there is a wide variety of isomorphous substitutions with a certain amount of surface net negative charge. It adsorbs heavy metal ions by electrostatic action, to a certain extent, Its adsorption capacity has a closer relationship with exchange. Due to structural damage, the porous alumina material obtained by the study is immersed in activated alkali to introduce a large number of hydrogen-oxygen coordination groups, which can exchange adsorption with heavy metals, and may cause some heavy metal ions to form precipitates in the surface and pore structure. Increase the amount of adsorption.

3. Permeability

In the preparation process of the material, by controlling the structural characteristics such as the size, direction, pore shape and arrangement of the pores, combined with the inherent properties of the porous material, such as good heat resistance and high structural stability, a porous molecular sieve, a high temperature gas separation membrane and the like can be prepared. .

It is reported in the literature that when the porosity of the porous material ranges from 57 to 95%, both the viscous permeability coefficient and the inertial permeability coefficient increase significantly with the increase of porosity; when the size of the porous material ranges from 10 to 40 PPI, the viscous permeability coefficient and The inertial permeability coefficient is also significantly increased as the pore size increases. When the pore size is decreased, the volume specific surface area is increased, the fluid resistance is increased, and the viscosity permeability coefficient is decreased. When the pore diameter is decreased, the flow rate of the fluid through the porous material is decreased under the same pressure, the fluid inertial energy loss is reduced, and the inertial washing is reduced.

4. Photoelectric performance

Porous silicon emits visible light under laser irradiation, and is considered to be an ideal material for new optoelectronic components. At the same time, porous materials are also considered to be the first choice for electrode materials in new hybrid fuel cells for hybrid vehicles. In recent years, studies have shown that the pore size, pore structure, pore distribution and pore wall thickness of the electrode active material can greatly affect the electrolyte infiltration, ion transport and the diffusion of ions in the active material crystal, thus affecting the electrode. The overall performance, the application of porous materials in electrochemical energy conversion and storage has become an emerging topic and has attracted widespread attention.

Fig. 3 Schematic diagram of photoelectrocatalytic mechanism of porous structure and powder structure BiVO4 photoanode material

Taking supercapacitors as an example, a reasonable pore size distribution is important for improving the overall performance of carbon-based supercapacitors such as energy storage capacity, rate stability, and cycle stability. In general, an ideal supercapacitor carbon material should have multiple types of pores, namely micropores, mesopores, and macropores. These different types of holes play different roles in electrochemical electric double layer capacitors. Micropores are mainly used for charge storage, so the more micropores, the stronger the energy storage capacity and the higher the energy density; the mesopores are mainly responsible for the transport of electrolytes, affecting the rate stability and cycle stability of electric double layer capacitors; As the ion pool, sufficient electrolyte ions are provided for mesopores and micropores, which also affects rate stability and cycle stability.

Figure 4. Application of porous materials in supercapacitors

For the types of anode materials for lithium ion batteries , the types can be mainly classified into carbon material anodes and non-carbon material anodes. Carbon anode materials mainly include graphite anode materials, soft carbon materials, hard carbon materials and some carbon composite materials. These anodes have lower cost and better cycle stability, and have been well applied in industry, but the theoretical specific capacity is better. Small, the conductivity is not very good. In order to increase the specific capacity of the anode material, many studies have shifted to some metal oxides, as well as some lithium alloys. These materials have a higher specific capacity than graphite, but during the charging and discharging process, the material itself will undergo serious volume expansion, causing damage to the material structure, and the cycle stability of the battery is very poor, hindering its industrial production. Yu et al. prepared a novel electrode material (MTO/3D-GN) by inlaying porous titanium dioxide microspheres into a graphene-based porous material. This material has excellent electrochemical performance with a reversible specific capacity of up to 124 mAh/g at a current density of 20C. The cycle performance and rate performance of this material are also superior to those of pure titanium dioxide. The reaction mechanism here can be explained as: the three-dimensional graphene-based porous material (3D-GN) provides a large contact area for the electrode material and the electrolyte, accelerates the transport speed of electrons and lithium ions, and is a titanium dioxide during the electrochemical reaction. The volume change provides dual protection, the most important being the improved conductivity of the entire electrode material during the electrochemical process. Cao et al. prepared a lithium ion battery electrode material that does not require a binder by depositing molybdenum disulfide on the surface of the porous graphene skeleton. This material has excellent electrochemical properties. In addition, the interpenetrating pore structure provided by the graphene-based porous material can facilitate the diffusion and transfer of electrons and lithium ions, effectively reducing the transfer path, thus eliminating the need to provide other conductive agents.

Figure 5. Application of porous materials in lithium batteries

For the sensor, there is a strong synergistic effect between the porous material and the chromophore group, and its high specific surface area can increase the contact area with the detected substance, which makes it have great advantages in the application of the sensor field. . In addition, the graphene-based porous material has a very low electrical resistivity, and the electrons travel very fast on a continuous and interpenetrating backbone, which greatly increases the sensitivity of the sensor.

Figure 6. Application of porous materials in sensors