Fuel cells and all-vanadium flow batteries are both important energy storage and conversion technologies, which can be widely used in a variety of fields, such as renewable energy storage and electric vehicles. Separator materials play a key role in the performance of these cells, as they must have high ionic conductivity and good mechanical stability, while preventing electron penetration at both ends of the electrolyte.
Diaphragm materials can be broadly divided into the following categories:
- Perfluorosulfonic acid membranes (e.g., Nafion™): These materials typically have excellent chemical stability and excellent electrical conductivity, but are expensive at a higher cost.
- Non-fluorinated membranes (e.g., sulfonated polyetheretherketone, SPEEK): These are more environmentally friendly options and are relatively inexpensive, but may be slightly inferior in performance to perfluorosulfonic acid membranes.
- Porous membranes (e.g., porous polyvinylidene fluoride, PVDF): have specific pore structures that effectively support electrolyte flow and ion exchange.
- Water treatment membranes (mainly polyolenes): Although primarily used in water treatment, they can also find applications in specific battery technologies, such as supporting certain types of ion exchange.
Source: Aggregate energy storage
The preparation methods and mechanisms of each material are different, which affects their application in laboratory preparation and to larger-scale production processes, and today share the diaphragm/proton membrane preparation method. (If there is any error, please add a group to correct)
The manufacturing process of separator is mainly as follows:
1. Symmetrical membranes (homogeneous membranes)
1.1 Preparation method of dense film
Solution Casting Method:
This method is commonly used in laboratories. Specifically, it refers to the polymer solution configured into a certain mass fraction, after heating and stirring, deaeration treatment, and then cast in the mold, when the polymer solution is gradually solidified, a proton membrane is formed. The mold can be selected according to the polymer and solvent, such as glass plate, stainless steel plate, petri dish, etc. (as shown in the figure below).
For example, to prepare common perfluorinated proton exchange membranes, perfluorosulfonic acid resin is usually dissolved with a high boiling solvent (or using a water-alcohol system), and a relatively uniform casting film solution is obtained by high-speed centrifugation or microwave shaking deaeration treatment, and then the casting film solution is poured on a flat glass plate or stainless steel plate (if it is a water alcohol system, it needs to be slowly baked at low temperature, and then annealed at high temperature), and then a special scraper can be used to spread it into a uniform coating with a certain thickness, as shown in the following figure (thickness adjustment depends on the type of scraper adjusted empirically, In the case of a precisely adjustable scraper, it can be calculated from the solid content of the cast film and the gram weight of the proton film according to the law of conservation of matter), and then transfer it to a specific environment to allow the solvent to completely volatilize, and finally form a uniform film.
Melt film formation method (melt extrusion):
The polymer is heated above the melting point and melted through a specific mold or extruder die to form a film. The advantage of this method is that the equipment is simple and easy to operate, but the thickness and uniformity of the membrane are not easy to control. In addition, the amount of polymer used in the laboratory stage is large, and this method is mostly used in engineering scale-up. Generally, organic polymers cannot match suitable solvents to prepare casting film solutions, and this method will be used.
1.2 Preparation method of microporous membrane (symmetrical membrane)
Stretching method:
When the polymer is in a semi-crystalline state, there is a crystalline and amorphous region inside, the mechanical properties of the two regions are different, when the polymer is subjected to tensile force, the amorphous region is overstretched and causes local fracture to form micropores, and the crystal region is preserved as the skeleton of the microporous region to form a stretched semicrystalline film.
Key points: 1. The formation of semi-crystalline polymers is the key to the tensile method; 2. The drafting factor and drafting speed are the key to forming parameters such as target micropore size and porosity.
The steps are broadly divided into polymer melt extrusion→ forming parallel arrangement of microcrystallites along the extrusion direction→ heat treatment→ cold stretching, pore making→ heat setting. This method is mostly used for polyethylene, PTFE and other membrane materials.
Nuclear Track Etching:
The polymer film is bombarded vertically by isotope fission fragments or charged particles emitted by heavy particle accelerators, and the long chains of polymer molecules are broken. Due to the ability to form a highly active chemical reaction at the fracture, it can be preferentially dissolved by chemical etchants and form holes that are etched. The size of the membrane pores is controlled by the degree of erosion.
Sintering method: the powdered polymer or metal powder is evenly heated, the temperature and pressure are controlled, so that there are certain pores between the powder particles, and only the surface of the powder particles is melted but not completely dissolved, so as to bond with each other to form a porous thin layer or tubular structure. The size of the membrane pore size can be controlled by the particle size and sintering temperature of the raw powder, and this method is mostly used for membrane materials such as metal powder.
The dissolution method, also known as the solution phase separation method:
It usually refers to the mixing of some soluble polymers or other soluble solid additives in the film-making polymer, and after the film is formed, the membrane body is put into a water bath or some undesirable solvents, and the blended substances are leached out to make pores, such as PEG, alcohols, esters, etc., as shown in the figure below.
2.1 Phase conversion method
The phase conversion method is the process of polymer precipitation into a solid from a homogeneous liquid state to two liquid states (also known as liquid-liquid phase) to form the concentrated phase and dilute phase of the polymer, the concentrated phase finally forms the membrane body, and the dilute phase is converted into pores. This method is mostly used in the field of water treatment.
In recent years, with the introduction of the principle of "pore size screening" into the field of flow batteries, such as the porous membrane series of Zhang Huamin and Li Xianxian's team of Dalian Institute of Chemical Physics, most of them use phase conversion method to prepare films. The film-forming mechanism involves the transformation process of the casting film solution (composed of polymer solution and solvent) under specific conditions, and finally forms a polymer separation film with specific structure and performance. This process consists of two main stages: the phase splitting process and the phase conversion process.
Splitting process:
When the cast film solution is immersed in the gel bath, the solvent and non-solvent diffuse each other through the liquid film/gel bath interface. At this stage, when the exchange between solvents and non-solvents reaches a certain level, the casting film becomes a thermodynamically unstable system, so phase separation occurs. This step is the key to determine the pore structure of the film, and the research mainly focuses on the thermodynamic properties and mass transfer kinetics of the cast liquid system.
Phase Conversion Process:
After phase separation, solvents and non-solvents are further exchanged, and the condensation of film pores, interphase flow, and polymer enriched solidification into a film occur. This stage has a great influence on the structure and morphology of the base film, and the research mainly focuses on the structural control (gel kinetics process) during the transition from phase splitting to the liquid phase of the cast film.
The phase conversion method uses the cast film liquid and the surrounding environment to exchange solvent and non-solvent mass transfer, and the original steady-state solution becomes unstable and produces the liquid-liquid phase to two phases: that is, the polymer rich phase that finally forms the film and the polymer poor phase that forms the pores, and finally solidifies to form the membrane structure. This method is simple to operate and can be used to prepare various forms of membranes, so it is the most commonly used film making process. The phase conversion coating is characterized by the fact that the cortex and the support layer are of the same material, and the cortex and the support layer are prepared and formed simultaneously.
In addition, the effects of film-forming conditions such as the type, concentration, viscosity of film-forming solution, coagulation bath concentration, temperature, etc., on the structure and properties of cellulose membranes have also been studied in depth. These conditions determine the structure and properties of the membrane by influencing the diffusion velocity. For example, when the polymer solution precipitates slowly, a sponge-like structure (RO membrane) is obtained, and when a gel is formed rapidly, a finger-like structure (UF membrane) is obtained, as shown in the figure below.
2.2 Thermally induced phase separation:
The polymer is mixed with other solvents and heated above the melting point of the polymer to melt and separate from other solvents. The polymer is then cooled to form a solid film. This method results in a thinner film with high uniformity.
2.3 Chemical cross-linking:
The polymer solution is mixed with a crosslinker, and the crosslinking reaction is initiated by heating or a catalyst to form a water-insoluble cross-linked polymer film. The membrane performance obtained by this method is good, but the type and concentration of crosslinker have a great influence on the membrane performance.
The above are several manufacturing processes for proton exchange membranes, and different processes are suitable for different polymers and applications. In actual production, the appropriate process should be selected to prepare the diaphragm according to the specific needs.
资料来源：液流电池flow battery