Overview of Hydrogen Fuel Cell Growth

During the reaction process of the fuel cell stack, the proton exchange membrane needs to maintain a certain humidity to ensure a high reaction efficiency. Therefore, the reaction medium is required to carry a certain amount of water vapor into the stack. This step is usually realized by a humidifier. . This article analyzes the fuel cell humidifier from six aspects: hydrogen fuel cell principle, basic principle of water transfer, humidifier selection and application requirements, humidifier model and parameters, membrane material and hollow fiber tube structure, and internal humidification technology introduce.

1. Principle of hydrogen fuel cell

H2 passes through the anode carbon fiber diffusion layer in a gaseous state, and is separated into H protons and electrons at the catalytic layer. H protons (in the state of H3O+) pass through the proton exchange membrane and combine with O ions at the cathode catalytic layer to form water.

Theoretically, the proton exchange membrane can only pass protons. There are many sulfonate groups on the membrane material, and only when it is wet can it have a high proton conductivity. Under normal circumstances, both the anode hydrogen and the cathode air must be humidified, and the reaction on the cathode side produces water. Under the gradient difference of water concentration on both sides, the water will migrate to the other side through the membrane.

2. The basic principle of water transfer

1. Principle of water transfer

Electromigration: Hydrogen usually does not exist in the state of bare atomic nuclei during the conduction process, but migrates through hydrogen bonds and water molecules to form hydronium ions, causing water molecules to migrate from the anode to the cathode with protons. The amount of electromigrated water is related to the current density and related to proton hydration number;

Back-diffusion: Water is formed at the cathode, driven by the water concentration gradient on both sides of the membrane, water is transferred from the cathode to the anode, and the amount of water is proportional to the concentration gradient of water and the diffusion coefficient of water in the membrane, and inversely proportional to the thickness of the membrane.

Pressure difference migration: Driven by the pressure difference on both sides of the membrane, water flows from the high pressure side to the low pressure side, and the amount of water is proportional to the pressure gradient and the permeability coefficient of water in the membrane, and inversely proportional to the viscosity of water in the membrane. The effect is minimal.

2. How does the water content affect the performance of the proton exchange membrane?

A. Cathode air humidity: The relative humidity of the air increases, resulting in the suppression of the migration of water generated at the reaction interface to the cathode diffusion layer-flow channel interface, thereby promoting the migration of water to the anode side.

B. Cathode air dew point temperature: When the air dew point temperature rises, the water generated by the reaction migrates to the anode, which increases the water content in the membrane, enhances the proton conductivity of the membrane, and increases the output potential of the battery. If the air dew point temperature is too high, the absolute amount of water in the cathode is too much to be taken away in gaseous form, resulting in flooding. At the same time, the oxygen concentration decreases, the reaction rate decreases; the mass transfer resistance increases, the membrane ohmic resistance increases, and the battery performance decreases.

C. Stack temperature: When the temperature of the stack increases, the saturation pressure of water vapor increases, which promotes the evaporation of water in the anode diffusion layer, promotes the migration of water concentration, improves the proton conductivity of the membrane, and improves the performance of the stack.

D. Crossover effect: Under relatively dry reaction conditions, the electrode will accelerate the degradation rate of the membrane electrolyte, resulting in damage to the membrane and allowing gas to permeate to the other electrode side.

E. Membrane metal ion effect and catalyst poisoning: Excessive moisture will increase the chance of impurities contaminating the MEA. Harmful components such as metal ions, CO, and S from the environment, and metal ions produced in the battery will diffuse with excess water. To the surface of the electrode and the membrane, causing metal ions and catalyst poisoning of the membrane.

3. Humidifier selection and application requirements

The selection of humidifier mainly considers its dew point close to temperature, flow resistance, temperature and pressure resistance, maximum transmembrane pressure difference, etc.

1. The performance and reliability of the stack require water content

By testing the influence of the stack on the output power of the stack under different air humidity (water content), determine the optimal air humidity into the stack; at the same time, the influence of different water content on the life of the stack should also be considered.

2. The dew point of the humidifier is close to the temperature as the reason for evaluating its humidification ability

Humidifiers for fuel cells are gas-humidifying types, and are usually given a wet gas that is close to saturation on the wet side (initial dew point on the wet side) to see how humid the dry air can be (final dew point on the dry side). The difference between the initial dew point on the wet side and the final dew point on the dry side is defined as the dew point approach temperature, which can basically evaluate the humidification performance of the humidifier. It can also be evaluated by the membrane water transmissibility g/(min.cm2).

3. Allowable medium temperature and transmembrane pressure difference: membrane material and membrane structure

Generally, the temperature resistance of membrane materials is above 100°C. In DOE requirements, the transmembrane pressure difference must be >75kpa, and it is difficult to achieve this level for unsupported ultra-thin hollow fiber tubes.

4. Reliability: performance, leakage

For general reliability tests, the dew point can be compared with the temperature before and after durability; the film damage rate can also be judged by the bubble method.

4. Humidifier model and parameters

(1) Perma Pure, DuPont exclusively authorizes Nafion hollow fiber tubes;

(2) KOLON, polysulfone homogeneous hollow fiber tube;

(3) NOK, polyphenylsulfone hollow fiber membrane, nanoporous;

(4) Dpoint, using sandwich composite flat membrane Gore+PFSA.

5. Membrane material and hollow fiber tube structure

1. Polysulfone series, polyimide, fluorine-containing sulfonic acid membrane

Polysulfone has excellent mechanical properties, chemical stability, good heat resistance, biodegradation resistance, high internal porosity and stable microporous structure, and is often used as a substrate for gas separation membranes. However, it is a hydrophobic membrane material.

Polysulfone, polyethersulfone, and polyphenylsulfone have similar properties. If they are to be used in fuel cells, their hydrophilicity can generally be improved by yellow flower treatment.

Polyimide has high air permeability, selectivity, good heat resistance, high mechanical strength, chemical stability, and good solvent resistance, and can be made into a self-supporting asymmetric hollow fiber membrane with high permeability coefficient. Poor hydrophilicity, need sulfonation treatment.

Polyimide is also being studied extensively as a proton exchange membrane with good prospects in the future.

Perfluorosulfonic acid PFSA, as a proton exchange membrane, has the function of water transfer under concentration difference, and can also be used as a humidifier membrane. Fluorine-containing membranes also include Gore's ePTFE expanded polytetrafluoroethylene and Ballard's BAM3G partially fluorinated proton exchange membrane. The price is too expensive.

2. Polysulfone series, polyimide, fluorine-containing sulfonic acid membrane

Hollow fiber tube membranes are mainly divided into porous membranes, epidermal membranes, and homogeneous membranes. According to their characteristics, they can be made into ultrafiltration membranes, forward/reverse osmosis membranes, gas separation membranes, hemodialysis membranes, etc. The hollow fiber membrane is characterized by a large surface area under the same volume.

The hollow fiber tube preparation process is mainly divided into solution spinning method and melt spinning method. The solution spinning method requires a porogen to produce micropores on the membrane, and generally the pore size is slightly larger, which is more commonly used; the melt spinning method produces micropores by stretching, and the technical requirements are high.

The flat membrane is composed of a thin PFSA interlayer in the center and porous layers on both sides. The surface area is relatively small.

6. Internal humidification technology

At the heart of humidification is water management. Toyota does not need an external humidifier through temperature control and anode water circulation. Internal humidification also has high requirements for the stack and higher requirements for control strategies. In addition, water exchange is also performed through porous carbon plates on the collector end plate, and water exchange is performed by adding a module similar to a single stack in the middle of the stack.