The power generation process of proton exchange membrane fuel cell does not involve hydrogen-oxygen combustion, with high energy conversion rate, pollution-free power generation process, modular power generation unit, high reliability, convenient assembly and maintenance, and no noise when working. Therefore, proton exchange membrane fuel cell is a clean and efficient green power supply. Inside the fuel cell, the proton exchange membrane provides a channel for the migration and transportation of protons, so that protons can reach the cathode from the anode through the membrane, and form a loop with the electron transfer of the external circuit to provide current to the outside world. Therefore, the performance of proton exchange membrane plays a very important role in the performance of fuel cells, and its performance directly affects the service life of the cells.
principle of operation
In principle, a proton exchange membrane fuel cell is equivalent to a "reverse" device for electrolyzing water. Its single cell consists of anode, cathode and proton exchange membrane with catalyst coating. The anode is where hydrogen fuel is oxidized and the cathode is where oxidant is reduced. Both electrodes contain catalysts to accelerate the electrochemical reaction of the electrode, and proton exchange membranes are used as electrolytes. When working, it is equivalent to a DC power supply, the anode is a negative power supply and the cathode is a positive power supply. Its working principle is shown in the figure and video.
Hydrogen is delivered directly to the negative electrode and oxygen is delivered directly to the positive electrode. Hydrogen is transported to the negative electrode in the form of molecules and decomposed into H+ ions (protons) in the presence of a catalyst. Electrons (e-) of hydrogen atoms transmitted through external circuits generate electricity for work. Then, these same electrons are sent to the positive electrode, and the H+ ions returned through the membrane react with oxygen at the positive electrode in the presence of a catalyst to generate water and heat.
Fuel cell stack
A single fuel cell itself is of little use because the electromotive force it produces is less than1v. Fuel cells used in automobiles are usually composed of hundreds of fuel cells into a fuel cell stack, as shown in the figure. In this arrangement, fuel cells are connected in series, and the total voltage of this stack is the sum of the voltages of each individual cell. The fuel cells in the battery pack are connected end to end, and the fuel cell pack in the car contains about 400 cells.
The total voltage of a fuel cell stack is determined by the number of cells that make up the stack. However, the power generation capacity of the battery pack is determined by the surface area of the electrodes. Because the output power of fuel cell stack is related to voltage and current, increasing the number of cells or increasing the surface area of cells can improve the output power. Some fuel cell vehicles use multiple battery packs according to the output power and space constraints required by the vehicle.
Methanol fuel cell
When hydrogen is used as fuel for fuel cells, the cost and safety of high-pressure gas cylinders needed to store hydrogen are not ideal. Therefore, another improved PEM fuel cell method is to replace hydrogen with liquid methanol, as shown in the figure.
The most common method to make methanol is to synthesize methanol from natural gas, and the chemical formula of methanol is CH3OH. It has higher energy density than gaseous hydrogen, because it exists in liquid form at room temperature, and there is no need to use compressors or other high-pressure equipment. It will be simpler to refuel a fuel truck with liquid fuel instead of high-pressure gas, which is almost similar to refueling a fuel truck, as shown in the figure.
However, methanol itself is corrosive and cannot be stored in the existing fuel tank, so a special device is needed to treat and store methanol alone. In addition, in the methanol fuel cell, methanol passing through the membrane device will reduce the working performance of the cell. A large number of catalysts are needed in the structure of direct methanol fuel cell, which leads to the increase of its cost.