A fuel cell is a power generation device that directly converts the chemical energy of fuel into electrical energy. The principle of a fuel cell is an electrochemical device, and its composition is the same as that of a general battery. Its single cell is composed of two positive and negative electrodes (the negative electrode is the fuel electrode and the positive electrode is the oxidant electrode) and an electrolyte. The difference is that the active materials of general batteries are stored inside the battery, therefore, the battery capacity is limited. The positive and negative electrodes of the fuel cell do not themselves contain active substances, but are just catalytic conversion elements. Therefore, the fuel cell is truly an energy conversion machine that converts chemical energy into electrical energy. When the battery is working, fuel and oxidant are supplied from the outside and react. In principle, as long as the reactants are continuously input and the reaction products are continuously eliminated, the fuel cell can continuously generate electricity. Here we take a hydrogen-oxygen fuel cell as an example to illustrate the hydrogen-oxygen fuel cell reaction principle. This reaction is the reverse process of electrolyzing water. The electrodes should be: Negative electrode: H2 2OH-→2H2O 2e- Positive electrode: 1/2O2 H2O 2e-→2OH- Battery reaction: H2 1/2O2==H2O In addition, only the fuel cell body cannot work, and a set of corresponding auxiliary equipment must be provided System, including reactant supply system, heat removal system, drainage system, electrical performance control system and safety devices, etc. A fuel cell usually consists of an electrolyte plate that forms an ion conductor, a fuel electrode (anode) and an air electrode (cathode) arranged on both sides of it, and gas flow paths on both sides. The function of the gas flow path is to make the fuel gas and air (oxidant) Gas) can pass through the flow path. In practical fuel cells, depending on the working electrolyte, the types of ions that pass through the electrolyte and are related to the reaction are also different. The reactions of PAFC and PEMFC are related to hydrogen ions (H), and the reactions that occur are: Fuel electrode: H2==2H 2e- (1) Air electrode: 2H 1/2O2 2e-==H2O (2) Overall: H2 1/ 2O2==H2O (3) Hydrogen-oxygen fuel cell composition and reaction cycle diagram In the fuel electrode, H2 in the supplied fuel gas is decomposed into H and e-, and H moves to the electrolyte to react with O2 supplied from the air electrode side. e-passes through the external load circuit and then returns to the air pole side to participate in the reaction on the air pole side. A series of reactions contribute to the uninterrupted flow of e- through the external circuit, thus constituting electricity generation. And it can be seen from the reaction formula (3) in the above formula that the H2O generated from H2 and O2 has no other reactions except that the chemical energy of H2 is converted into electrical energy. But in fact, there is a certain resistance accompanying the reaction of the electrode, which will cause some heat energy to be generated, thus reducing the proportion of conversion into electrical energy. The group of cells that causes these reactions is called a module, and the voltage produced is usually less than one volt. Therefore, in order to obtain large output, it is necessary to use multi-layer stacking of components to obtain a high-voltage stack. The electrical connection between components and the separation between fuel gas and air are made of components called separators, which have gas flow paths on the upper and lower sides. The separators of PAFC and PEMFC are both made of carbon materials. The output of the stack is determined by the product of the total voltage and the current, which is proportional to the reaction area in the cell. The electrolyte of PAFC is a concentrated phosphoric acid aqueous solution, while the electrolyte of PEMFC is a proton conductive polymer-based membrane. The electrodes are all made of carbon porous bodies. In order to promote the reaction, Pt is used as a catalyst. The CO in the fuel gas will cause poisoning and reduce the electrode performance. For this reason, the amount of CO contained in the fuel gas must be limited in PAFC and PEMFC applications, especially for PEMFCs operating at low temperatures. The basic composition and reaction principle of the phosphoric acid fuel cell is: water vapor is added to the fuel gas or city gas and then sent to the reformer to convert the fuel into a mixture of H2, CO and water vapor. The CO and water are further processed in the shift reactor. The catalyst is converted into H2 and CO2. The fuel gas thus treated enters the negative electrode (fuel electrode) of the fuel pile, and at the same time, oxygen is transported to the positive electrode (air electrode) of the fuel pile for chemical reaction, and electrical energy and heat energy are quickly generated with the help of the catalyst.
Compared with PAFC and PEMFC, high-temperature fuel cells MCFC and SOFC do not require catalysts. Coal gasification gas with CO as the main component can be directly used as fuel, and it also has the characteristics of being easy to use its high-quality exhaust gas to form combined cycle power generation. Main components of MCFC. It contains the electrolyte related to the electrode reaction (usually a carbonate mixed with Li and K) and the two electrode plates (fuel electrode and air electrode) connected above and below it, as well as the gas that circulates fuel gas and oxidant gas outside each of the two electrodes. Chamber, electrode clamp, etc., the electrolyte is a molten liquid at the operating temperature of MCFC of about 600~700°C, forming an ionic conductor. The electrode is a nickel-based porous body, and the gas chamber is formed of a corrosion-resistant metal. MCFC working principle. O2 (air) and CO2 in the air electrode combine with electricity to generate CO32- (carbonate ions). The electrolyte moves CO32- to the fuel electrode side, combines with H supplied as fuel, releases e-, and generates H2O and CO2 at the same time. . The chemical reaction formula is as follows: Fuel electrode: H2 CO32-==H2O CO2 2e-(4) Air electrode: CO2 1/2O2 2e-==CO32-(5) Overall: H2 1/2O2==H2O(6) Here In a reaction, e- is the same as in PAFC. It is released from the fuel electrode and returns to the air electrode through the external circuit. The uninterrupted flow of e- in the external circuit realizes the fuel cell power generation. In addition, the biggest characteristic of MCFC is that there must be CO32- ions that contribute to the reaction, so the supplied oxidant gas must contain carbonic acid gas. In addition, a method has been developed in which a catalyst is filled inside the battery to modify CH4, which is the main component of natural gas, inside the battery, and H2 is directly generated inside the battery. When the fuel is coal gas, its main component CO reacts with H2O to generate H2. Therefore, CO can be equivalently utilized as fuel. In order to obtain greater output, the partitions are usually made of Ni and stainless steel. SOFC is mainly composed of ceramic materials. The electrolyte usually uses ZrO2 (zirconia), which constitutes the conductor Y2O3 (yttrium oxide) of O2- and is used as stabilized YSZ (stabilized zirconia). In the electrode, the fuel electrode is made of Ni and YSZ composite porous body to form a cermet, and the air electrode is made of LaMnO3 (lanthanum manganese oxide). The separator is made of LaCrO3 (lanthanum chromium oxide). In order to avoid cracks caused by different shapes of batteries and differences in thermal expansion between electrolytes, SOFCs that operate at lower temperatures have been developed. In addition to the flat-plate battery shape like other fuel cells, a cylindrical type has been developed to avoid stress concentration. The reaction formula of SOFC is as follows: Fuel electrode: H2 O2-==H2O 2e- (7) Air electrode: 1/2O2 2e-==O2- (8) Overall: H2 1/2O2==H2O (9) Fuel electrode, H2 moves through the electrolyte and reacts with O2- to generate H2O and e-. The air pole generates O2- from O2 and e-. Like other fuel cells, H2O is generated from H2 and O2. In SOFC, because it is a high-temperature working type, CH4, the main component of natural gas, can be directly modified internally into H2 for utilization without the action of other catalysts, and CO, the main component of coal gas, can be directly used as fuel.