1. Basic structure of proton exchange membrane fuel cell
The proton exchange membrane fuel cell uses a polymer electrolyte membrane as the electrolyte, so it can also be called a polymer membrane fuel cell or simply a membrane fuel cell. The proton exchange membrane fuel cell is mainly composed of a proton exchange membrane, a membrane electrode group and a bipolar plate, as shown in Figure 1.
1) Proton exchange membrane
The proton exchange membrane is a key part of the proton exchange membrane fuel cell. Because proton-conducting polymers are used for electrolysis
The quality of the fuel cell is so named. The basic material that composes the proton exchange membrane is polyethylene, in which hydrogen is replaced by fluorine into polytetrafluoroethylene. The chemical bond between fluorine and carbon makes the membrane very durable and resistant to chemical reactions. Adding sulfurous acid can absorb H+ ions into the membrane. In the electrolyte, this material is made by DuPont.
When protons are conducted in a proton exchange membrane, they tend to be in the form of hydrated protons. Water plays an important role in the membrane. The conductivity of protons in the membrane increases with the increase of water content, which is approximately proportional to the change, while the change with temperature is a nonlinear relationship. Studies have shown that when the water content is high, the membrane is fully swollen, and the energy barrier to be crossed for proton transfer in the ion cluster is the same as the energy barrier in the channel: when the water content is low, the channel becomes narrow, and the proton transfer in the channel needs to be The jumped energy barrier is higher than the energy barrier in the cluster, which causes some protons to accumulate at both ends of the channel, forming a microcapacitance. At high frequencies, the capacitive impedance of the membrane is equivalent to pure resistance. When the temperature is high and the water is lacking, the impedance of the membrane will be greatly increased, causing it to fail to work properly. The main properties of polymer proton exchange membranes are as follows.
(1) Resistant to chemical reactions, with high thermal and chemical stability.
(2) They have good mechanical properties, sufficient strength and flexibility, and strong chemical bonds, so they can be used to make extremely thin films.
(3) It has a high water content and can absorb a large amount of water.
(4) It has high proton conductivity and electronic insulation. If the membranes contain sufficient moisture, the fluoride ions they absorb can pass through the membrane well.
2) Membrane electrode group
The performance of proton exchange membrane fuel cells depends to a large extent on the membrane electrode stack, because this is the core part of the fuel cell. The electrolyte membrane is sandwiched between the anode and cathode electrodes, and the electrodes include catalyst particles and a gas diffusion layer.
3) Bipolar plate
Bipolar plates (also known as flow field plates) are the major part of the mass and volume of proton exchange membrane fuel cells. It not only affects the performance of the PEM fuel cell, but also affects its cost. Its main function is to separate the oxidant and the reductant, collect the current, and guide the reaction gas to distribute evenly. Commonly used bipolar plate materials are non-porous graphite plate, surface-modified metal plate, composite bipolar plate, etc. In a fuel cell stack, the anode flow field plate and the cathode flow field plate are fabricated back-to-back, which is a bipolar plate.
The materials for making bipolar plates require low processing cost, light weight, thin plates, good mechanical properties, high surface and volume conductivity, low air permeability, and good corrosion resistance. Therefore, the selection of appropriate materials and preparation technology for bipolar plates can greatly improve the performance of proton exchange membrane fuel cells.
The bipolar plate structure of the proton exchange membrane fuel cell should have the following functions.
(1) Collect conduction current. Because the fuel cell has low voltage, high current, and great influence of internal resistance, it is necessary to use electrical conductors with good electrical conductivity to ensure high efficiency and low waste heat.
(2) The oxidant and the reductant are separated, and the reactants are evenly distributed throughout the electrode, and then sent to the electrode catalyst layer for electrochemical reaction.
(3) Remove the water produced in the electrochemical reaction and humidify the reaction gas.
(4) It can well conduct the heat generated in the reaction process and prevent the local temperature of the battery from being too high.
In order to satisfy the above functions at the same time, the bipolar structure plate of the proton exchange membrane fuel cell should have the following characteristics.
(1) The bipolar plate must be a good conductor of heat. The thermal management of the PEM fuel cell is very important to its performance. During the operation of the fuel cell, it is necessary to ensure that the temperature is stable and evenly distributed throughout the fuel cell, and the waste heat generated during the operation can be quickly discharged. In order to make the reaction gas, cooling water and reactants transfer faster and better, many flow channels are carved on the surface of the bipolar plate.
(2) The bipolar plate must have good corrosion resistance. Since the working condition of the proton exchange membrane fuel cell is an acidic electrolyte, in order to ensure that it can maintain long-term stable performance, the bipolar plate must have the ability to resist electrochemical and chemical corrosion.
(3) The bipolar plate material should have the characteristics of light weight, high strength and good toughness, and be suitable for batch processing.
(4) The bipolar plate should be able to distribute gas and liquid water well. Under different operating conditions, the flow field design of the bipolar plate is the key, its purpose is to ensure that the gas distribution and water management meet the requirements of the application.
(5) There is a heating plate or a cooling plate. Heating or cooling plates can be used to heat or cool the fuel cell, which ensures that the PEM fuel cell remains near the optimum operating temperature. Heating plates typically use ohmic (resistive) heat, and cooling plates are used when air cooling is insufficient, using liquid (such as water) circulation to cool the stack.
2. The working principle of proton exchange membrane fuel cell
The basic working principle of the proton exchange membrane fuel cell is equivalent to the reverse process of the electrolysis of water. The polymer electrolyte exchange membrane is sandwiched between two electrodes (anode and cathode). The exchange membrane only allows hydrogen protons to migrate without providing flow for electrons. aisle. As the electrochemical reaction continues, electrons rapidly generate a concentration difference on both sides of the exchange membrane, and these electrons flow from the anode to the cathode through a closed loop connecting the two electrodes, forming a continuous current.
Under the action of the anode catalyst, the hydrogen gas is decomposed into hydrogen ions and electrons. Hydrogen ions in the form of hydrated protons H+are transferred from one sulfur radical to another in the proton exchange membrane, and finally reach the cathode to achieve proton transfer.
The reaction formula is
The proton exchange membrane only allows ions H” to pass through, while the electrons are collected by the load of the external circuit to form a current that can be used to do effective work, and finally reach the cathode to combine with the oxygen ions and oxygen diffused through the exchange membrane to form a product. water. The reaction formula is
The overall battery reaction formula is
The reaction product water should be removed to prevent the “water flooding” of the battery from affecting normal operation. In addition, both the unused hydrogen and oxygen are exhausted through the anode and cathode outlets of the cell, respectively, as shown in Figure 2.
For this reaction to continue, electrons from the anode must flow through an external circuit to the cathode, while protons travel through the proton exchange membrane by electromigration. A single fuel cell reaction produces an output voltage of approximately 0.7V. In practical applications, it is necessary to connect multiple fuel cells in series to form a fuel cell stack, and the required voltage can be obtained by adding the voltages of each other in series.
3. Characteristics of proton exchange membrane fuel cells
The characteristics of proton exchange membrane fuel cells are mainly manifested in the following aspects.
(1) High energy conversion efficiency. The chemical energy is directly converted into electrical energy through hydrogen oxidation, not through the heat engine process, and is not limited by the Carnot cycle.
(2) Zero emissions can be achieved. The only discharge is pure water, there is no pollutant discharge, and it is an environmentally friendly energy source.
(3) Low operating noise and high reliability. The proton exchange membrane fuel cell stack has no mechanical moving parts, and only flows of gas and water during operation.
(4) Easy maintenance. The internal structure of the proton exchange membrane fuel cell is simple, and the battery module presents a natural “building block”
The structure makes the assembly and maintenance of the battery pack very convenient, and it is easy to realize the “maintenance-free” design.
(5) Hydrogen comes from a wide range of sources. Hydrogen is the most abundant element in the world, and the sources of hydrogen are extremely wide, making it a renewable energy resource. Hydrogen can be produced by reforming oil, natural gas, methanol, methane, etc.; hydrogen can also be obtained by electrolysis of water, photolysis of water, and biological hydrogen.
(6) The technology is mature. The technologies for hydrogen production, storage, transportation and use are now very mature, safe and reliable.
(7) High hydrogen purity requirements. Such batteries require pure hydrogen because they are highly susceptible to contamination by carbon monoxide and other impurities.
(8) The working temperature of proton exchange membrane fuel cells is low, the starting speed is relatively fast, and the power density is high, so it is very suitable for use as a new generation of vehicle power.