Phosphoric Acid Fuel Cell (PAFC) is a medium-temperature fuel cell that uses concentrated phosphoric acid as the electrolyte and noble metal-catalyzed gas diffusion electrodes as the positive and negative electrodes. Can work at 150~220℃. It has the advantages of stable electrolyte, concentrable phosphoric acid, low water vapor pressure, and the anode catalyst is not easily poisoned by CO. It is a civilian fuel cell that is close to commercialization.

　　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.

　　Anodic reaction: H2+2e- →2H+

　　Cathodic reaction: 1/2O2+2H+ → H2O+2e-

　　Total reaction: 1/2O2 +H2 → H2O

　　The output voltage of the battery body (that is, a single battery) is approximately 1 [V] under no load. To increase the current density, the design is usually rated at 0.6-0.7 [V/single battery]. The difference between the no-load state and the actual voltage is released as heat energy. Furthermore, the power generation efficiency of the battery body is not determined by the battery area, so the fuel cell is essentially highly efficient even if it has a low capacity. In actual use, dozens or even hundreds of battery bodies are connected in series to form a battery assembly (stack) according to the output requirements.

　　As a medium-low temperature (operating temperature 180-210°C) fuel cell, PAFC not only has high power generation efficiency, cleanliness, adaptability to a variety of fuels, no noise, low operating costs, few installation site restrictions, easy operation at atmospheric pressure, and excellent safety , good part load characteristics and other characteristics, and can also recover most of the heat in the form of hot water. The following table gives the main technical specifications of the advanced ONSI PC25C 200kW PAFC. PAFC was originally developed to control the peak and valley power balance of power plants, but more recently it has been focused on as an on-site centralized power system that provides electricity and heat to apartments, shopping malls, hospitals, hotels, etc.

　　PAFC is used in power plants in two situations: decentralized power plants, with a capacity between 10-20MW, installed in distribution stations; central power station-type power plants, with a capacity of more than 100MW, which can be used as medium-scale thermal power plants. Compared with ordinary power plants, PAFC power plants have the following advantages: even when the power generation load is relatively low, they still maintain high power generation efficiency; due to the modular structure, on-site installation is simple and time-saving, and the power plant capacity expansion is easy.

　　Influenced by the world oil crisis in 1973 and the research and development of PAFC in the United States, Japan decided to develop various types of fuel cells. PAFC was developed by the New Energy Industry Technology Development Organization (NEDO) as a large-scale energy-saving power generation technology. Since 1981, research and development of 100kW on-site PAFC power generation equipment have been carried out. In 1986, the development of a 200kW on-site power generation device was carried out to be suitable for PAFC power generation devices in remote areas or commercial use.

　　Fuji Electric Co., Ltd. is currently the largest PAFC battery stack supplier in Japan. As of 1992, the company had supplied 17 sets of PAFC demonstration devices at home and abroad. Fuji Electric completed the operation study of distributed 5MW equipment in March 1997. A total of 88 types of equipment of 50kW, 100kW and 500kW have been put into use as field equipment. The table below shows the operation status of power generation units delivered by Fuji Electric Company. As of 1998, some of them had exceeded their target life span of 40,000 hours.

　　Our country is still in the early stages of research in this area, and there is still a big gap between it and developed countries, and we need to catch up.

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