A fuel cell is an energy conversion device that directly converts the chemical energy of the supplied fuel into electrical energy through an electrode reaction. It is a power generation device that can continuously obtain electricity by continuously supplying fuel. The biggest feature of this device is that since the reaction process does not involve combustion, its energy conversion efficiency is not limited by the “Carnot cycle”, and the efficiency of most types of fuel batteries is as high as 50% to 60%. Secondary utilization, the total efficiency can be as high as 80%~85%. As a power source, its actual energy utilization efficiency is 2~3 times that of ordinary internal combustion engines. It has been actively developed in recent years due to its advantages of high power generation efficiency, adaptability to various fuels, and good environmental characteristics.
The hydrogen-oxygen fuel cell device is essentially a “reverse” reaction device of water electrolysis. Hydrogen and oxygen are electrochemically reacted to produce water, and electricity is released. The basic structure of a fuel cell is mainly composed of four parts: anode, cathode, electrolyte and external circuit. The anode is a hydrogen electrode, the cathode is an oxygen electrode, and an electrolyte is between the two electrodes. Typically, both the anode and the cathode contain a certain amount of catalyst to accelerate the electrochemical reactions taking place at the electrodes. Its basic structure is shown in Figure 1.
Depending on the type of electrolyte used, fuel source, etc., fuel cells can be divided into five types: Alkaline Fuel Cells (AFC), Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), Solid Oxide Fuel Cells Fuel cells (SOFC) and proton exchange membrane fuel cells (PEMFC). Various fuel cells have different operating temperatures in particular. The phosphoric acid fuel cell (PAFC), which was first developed, operates at a temperature of about 200°C. In contrast, molten carbonate fuel cells (MCFCs) and solid oxide fuel cells can both be used in power plants using fossil fuels as basic fuels and can be used as power sources. High-temperature fuel cells can use high-quality exhaust gas for turbine power generation due to the high temperature of the exhaust gas, so they can also be called fuel cells that can combine power generation.
Proton exchange membrane fuel cell (PEMFC) is a new type of fuel cell developed relatively late, and it is currently the most widely used. The electrolyte is a solid organic membrane that conducts protons when humidified. It generally uses platinum as a catalyst, and the working environment temperature is 60~80 ℃, which is a low temperature fuel cell .
The proton exchange membrane fuel cell is mainly composed of membrane electrode, sealing ring and flow field plate with air channel. The membrane electrode is the core part of the proton exchange membrane fuel cell . In the middle is a very thin membrane – a proton exchange membrane (PEM). This membrane does not conduct electrons and is an excellent conductor of hydrogen ions. It not only acts as an electrolyte to provide hydrogen ions The channel also acts as a diaphragm to isolate the bipolar reactive gases. On both sides of the membrane are gas electrodes, which are composed of carbon paper and catalyst, the anode is a hydrogen electrode, and the cathode is an oxygen electrode. Flow field plates are usually made of graphite. Proton exchange membrane fuel batteries use hydrogen as fuel and air or pure oxygen as oxidant. Multiple cells can be connected in series or in parallel as required to form battery packs (also known as stacks) of different powers. Its basic working principle is as follows.
①Hydrogen reaches the anode of the cell through the gas conduction channel on the bipolar plate, and reaches the proton exchange membrane through the diffusion layer on the electrode.
②Hydrogen is decomposed into 2 hydrogen ions (ie protons) under the action of the anode catalyst, and 2 electrons are released.
③ At the other end of the cell , oxygen or air reaches the cell through the gas-conducting channel on the bipolar plate, and the cathode of the cell reaches the proton exchange membrane through the diffusion layer on the electrode. At the same time, hydrogen ions pass through the electrolyte to the cathode, and electrons also reach the cathode through an external circuit.
④ Under the action of the cathode catalyst, oxygen reacts with hydrogen ions and electrons to form water.
⑤ At the same time, the electrons form a current under the connection of the external circuit, and the electrical energy can be output to the load through proper connection. The generated water is discharged with the reaction exhaust gas through the electrode.
The main advantages of proton exchange membrane fuel cell :
①High energy conversion efficiency. Through hydrogen oxidation, chemical energy is directly converted into electrical energy, without the heat engine process, and is not limited by the Carnot cycle.
②Zero emission can be achieved. Its only discharge is pure water (and water vapor), no pollutants are discharged, and it is an environmentally friendly energy source.
③ Low operating noise and high reliability. The PEMFC battery pack has no mechanical moving parts, and only flows of gas and water during operation.
④Easy to maintain. The internal structure of PEMFC is simple, and the cell module presents a natural “building block” structure, which makes the assembly and maintenance of the cell pack very convenient; it is easy to realize the “maintenance-free” design.
⑤ The power generation efficiency is little affected by load changes, so it is very suitable for use as a decentralized power generation device (as the main unit), and also as a “peak-shaving” generating unit (as an auxiliary unit) for the power grid.
⑥Hydrogen is the most abundant element in the world. Hydrogen comes from a wide range of sources. It is a renewable energy resource that is inexhaustible and inexhaustible. Hydrogen can be produced from petroleum, natural gas, methanol, methane, etc.; hydrogen can also be obtained by electrolysis of water, photolysis of water, and biological hydrogen.
⑦Hydrogen production, storage, transportation and use technologies are now very mature, safe and reliable.
Therefore, the fuel cell is considered to be one of the most potential batteries and one of the main partners of the photovoltaic power generation system in the future.