1. Introduction to fuel cells 1. Definition Fuel cells (FuelCells) are an electrochemical power generation device that does not need to go through the Carnot cycle and has a high energy conversion rate. Fuel and air are fed into the fuel cell respectively, and electricity is wonderfully produced. It looks like a battery with positive and negative electrodes and electrolytes, but in fact it cannot "storage electricity" but is a "power plant". Because during the energy conversion process, almost no nitrogen and sulfur oxides that pollute the environment are produced, the fuel cell is also considered an environmentally friendly energy conversion device. Due to these advantages, fuel cell technology is considered to be one of the new environmentally friendly and efficient power generation technologies in the 21st century. As research continues to make breakthroughs, fuel cells have begun to be used in power stations, micro power supplies, etc. 2. Basic structure The basic structure of a fuel cell is mainly composed of four parts, namely anode, cathode, electrolyte and external circuit. Usually the anode is a hydrogen electrode and the cathode is an oxygen electrode. Both the anode and the cathode need to contain a certain amount of electrocatalyst to accelerate the electrochemical reaction that occurs on the electrodes. There is an electrolyte between the two electrodes. 3. Classification There are currently many types of fuel cells, and there are many ways to classify them. They are roughly classified according to different methods as follows: (1) Classified according to the operating mechanism: they can be divided into acid fuel cells and alkaline fuel cells; (2) Classified according to the type of electrolyte: acidic, alkaline, molten salt or solid electrolyte ; (3) Classified according to the type of fuel: there are direct fuel cells and indirect fuel cells; (4) According to the operating temperature of the fuel cell: there are low-temperature type (less than 200℃); medium-temperature type (200-750℃) ; High temperature type (above 750℃). 4. Principle The working principle of the fuel cell is relatively simple, mainly including the two electrode reactions of fuel oxidation and oxygen reduction and the ion transport process. The structure of early fuel cells was relatively simple, requiring only an electrolyte to transport ions and two solid electrodes. When hydrogen is used as fuel and oxygen is used as oxidant, the cathode and anode reactions and the total reaction of the fuel cell are: Anode: H2→2H++2e-Cathode: 1/2O2+2H++2e-→H2O Total reaction: H2+1/2O2→H2O Among them, H2 passes through Diffusion reaches the anode and is oxidized to e- under the action of the catalyst. After that, H+ reaches the cathode through the electrolyte, while the electrons drive the load through the external circuit and then reach the cathode, thereby causing a reduction reaction (ORR) with O2. 2. Fuel Cell Applications Today, many types of fuel cells have been developed according to different application requirements. According to the type of conducting ions, it can be divided into acid fuel cells, alkaline fuel cells, molten carbonate fuel cells and solid oxide fuel cells (SOFC). Acid fuel cells can also be subdivided into PEMFC, direct alcohol fuel cells and phosphoric acid fuel cells. Each type of fuel cell has its own operating characteristics, with operating temperatures as low as -40°C and as high as 1000°C. Fuel cell types can be selected according to different needs. Among them, PEMFC is the fuel cell that has attracted the most attention in recent decades. PEMFC not only has the common characteristics of fuel cells, but also has outstanding advantages such as rapid startup and operation at low temperatures, no electrolyte loss, long life, and high specific power and specific energy. It is considered to be the most ideal solution to replace internal combustion engines as automotive power sources in the future. Due to the modularity, wide power range and fuel diversity of fuel cells, they can be used in a variety of situations: as small as scooter power supplies and mobile charging devices, as large as megawatt power stations. In fact, the commercialization of fuel cells is proceeding at a rapid pace. Data show that from 2008 to 2011, the market share of fuel cells as backup power supply for communication network equipment, logistics and airport ground handling increased by 214% worldwide. It is expected that by 2020, the total market value of fuel cells will reach US$192. The specific applications are briefly introduced as follows: (1) Portable power supply The year-by-year increase in sales of the portable power supply market has attracted many power supply technologies. Its products include: notebook computers, mobile phones, radios and other mobile devices that require power supply. In order to facilitate personal portability, The basic requirements for portable mobile power supplies usually require that the power supply has the characteristics of high specific energy, light weight and compactness. The energy density of fuel cells is usually 5 to 10 times that of rechargeable batteries, making them highly competitive. In addition, the fact that fuel cells do not require additional charging also makes them suitable for longer outdoor life. Currently, direct methanol fuel cells (DMFC) and PEMFC are used as special power supplies and mobile charging devices. Cost, stability and lifespan will be the technical issues that need to be solved when fuel cells are used in portable mobile power sources. (2) Fixed power supply Fixed power supply includes emergency backup power supply, uninterrupted electrotherapy, independent power stations in remote areas, etc. At present, fuel cells account for about 70% of the world's megawatt stationary power supply market every year. Compared with traditional lead-acid batteries, fuel cells have a longer operating time (about 5 times that of lead-acid batteries) and a higher specific energy consumption. High density, smaller size and better environmental adaptability. For remote areas and emergency areas that are difficult to reach with smart grids, independent power stations are considered the most economical and reliable way to provide power. In many geological disasters in our country, fuel cells have been used as independent power stations and played an important role in disaster relief efforts. It should be noted that fixed power stations usually require a long life (greater than 80,000 hours), which is the biggest technical challenge for applying fuel cell technology to fixed power stations. (3) Transportation power supply Transportation power supply has always been the main inducement for clean energy technology research and development, because 17% of the world’s greenhouse gases (CO2) are produced by transportation power based on fossil fuels, and are also accompanied by other air pollution problems. , such as smog, etc. PEMFC fueled by H2 is considered to be the best alternative power for internal combustion engines. The main reasons are: (a) the exhaust gas is only water and does not emit any pollution; (b) the fuel cell has extremely high working efficiency (53%-59%). It is almost twice as powerful as a traditional internal combustion engine; (c) it starts quickly at low temperature, has low operating noise and is stable in operation. Many countries in the world are promoting fuel cell transportation power solutions, and Japan is one of the most radical countries. Japan plans to build more than 1,000 ammonia filling stations and operate 2 million fuel cell vehicles by 2025. In 2015, Japan's Toyota Motor Corporation began selling the Mirai, the world's first car with PEMFC as the main power source (parameter | picture), marking a new era in the application of fuel cell technology to automotive power. 3. Fuel cell research 1. Development of fuel cells A fuel cell is an automatically operating power plant. Its birth and development are based on disciplines such as electrochemistry, electrocatalysis, electrode process dynamics, materials science, chemical process and automation. It has been more than 160 years since Grove published the world's first report on fuel cells in 1839. From a technical point of view, we realize that the generation, development and improvement of new concepts are the key to the development of fuel cells. For example, fuel cells use gas as oxidant and fuel, but the solubility of gas in liquid electrolyte is very small, resulting in extremely low operating current density of the battery. For this reason, scientists have proposed the concepts of porous gas diffusion electrodes and electrochemical reaction three-phase interfaces. It is the emergence of porous gas diffusion electrodes that gives fuel cells the necessary conditions for practical use. In order to stabilize the three-phase interface, dual-pore structure electrodes began to be used, and then materials with hydrophobic properties, such as polytetrafluoroethylene, were added to the electrodes to prepare adhesive hydrophobic electrodes. For fuel cells with solid electrolytes as separators, such as proton exchange membrane fuel cells and solid oxide fuel cells, in order to establish a three-phase interface in the electrode, ion exchange resin or solid oxide electrolyte materials are mixed into the electrocatalyst in order to achieve Three-dimensionalization of electrodes. Materials science is the basis for fuel cell development. The discovery of a new material with excellent properties and its application in fuel cells will promote the rapid development of a fuel cell. For example, the development of asbestos membrane and its successful application in alkaline batteries ensured that asbestos membrane alkaline hydrogen-oxygen fuel cells were successfully used in space shuttles. The successful development of lithium metaaluminate separators stable in molten carbonate has accelerated the construction of megawatt-scale experimental power stations for molten carbonate fuel cells. The development of yttria-stabilized zirconia solid electrolyte separators has made solid oxide fuel cells a research hotspot for future fuel cell decentralized power stations. The emergence of perfluorosulfonic acid-type proton exchange membrane has promoted the renaissance of research on proton exchange membrane fuel cells, and then developed rapidly. Before the 1960s, due to the rapid development and progress of hydropower, thermal power and chemical batteries, fuel cells had been in the basic research stage of theory and application, mainly research on concepts, materials and principles. The breakthrough of fuel cells mainly relies on the efforts of scientists. In the 1960s, due to the urgent need for high-power, high-specific power and high-specific energy batteries for manned spacecraft, fuel cells attracted great attention from some countries and special departments. It was against this background that the United States introduced Bacon's technology to produce the Bacon-type medium-temperature hydrogen-oxygen fuel cell that was the main power source on the Apollo lunar landing spacecraft. Since the 1990s, humans have paid increasing attention to environmental protection for the purposes of sustainable development, protecting the earth, and benefiting future generations. Based on the rapid progress of proton exchange membrane fuel cells, various electric vehicles powered by them have been launched. In addition to their high cost, their performance is comparable to that of internal combustion locomotives. Therefore, fuel cell electric vehicles have become the focus of attention and competition from the U.S. government and major automobile companies. From an investment perspective, investment in the development of fuel cells previously relied mainly on the government, but now the company has become the main investor in the development of fuel cells, especially fuel cell electric vehicles. All the major automobile companies and oil companies in the world have been involved in the development of fuel cell vehicles. In just a few years, they have invested about 8 billion US dollars and successfully developed 41 types of fuel electric vehicles, including 24 types of sedans and station wagons. There are 9 types of inter-city buses and 3 types of light-duty trucks. This year, the United States announced a US$2.5 billion investment plan to develop fuel cell electric vehicles, of which the state allocated US$1.5 billion and the three major automobile companies invested US$1 billion. 2. Research status of alkaline fuel cell (AFC). This kind of battery uses 35% to 45% KOH as electrolyte, which penetrates into the porous and inert matrix separator material, and the operating temperature is less than 100°C. The advantage of this type of battery is that the electrochemical reaction speed of oxygen in alkaline solution is greater than that in acidic solution, so it has greater current density and output power. However, the oxidant should be pure oxygen, and the amount of precious metal catalyst in the battery is larger, and The utilization rate is not high. At present, the development of this type of fuel cell technology is very mature and has been successfully used in aerospace and special applications. China has developed a 200W ammonia-air alkaline fuel cell system and produced 1kW, 10kW, and 20kW alkaline fuel cells. In the late 1990s, very valuable results were achieved in the follow-up development. The core technology of developing alkaline fuel cells is to avoid the damage of carbon dioxide to the components of the alkaline electrolyte. Whether it is a few parts per million of carbon dioxide in the air or the carbon dioxide contained in the reformed gas of hydrocarbons, it must be carried out. Eliminate processing, which undoubtedly increases the overall cost of the system. In addition, the water generated by the electrochemical reaction of the battery needs to be drained in time to maintain water balance. Therefore, simplifying the drainage system and control system is also a core technology that needs to be solved in the development of alkaline fuel cells. 2. Research status of phosphoric acid fuel cell (PAFC). This type of battery uses phosphoric acid as the electrolyte and has an operating temperature of about 200°C. Its outstanding advantages are that the amount of precious metal catalysts is greatly reduced compared with alkaline hydroxide fuel cells, the purity requirements of the reducing agent are greatly reduced, and the carbon monoxide content is allowed to be up to 5%. This type of battery generally uses organic hydrocarbons as fuel. The positive and negative electrodes are porous electrodes made of polytetrafluoroethylene. The electrodes are coated with Pt as a catalyst, and the electrolyte is 85% H3PO4. It has stable performance and strong conductivity within the range of 100~200℃. Phosphoric acid batteries are cheaper to produce than other fuel cells and are close to being available for civilian use. At present, all practical fuel cell power stations with higher power in the world use this fuel cell. The United States lists phosphoric acid fuel cells as a national key scientific research project for research and development, and sells 200kW phosphoric acid fuel cells to the world. Japan has manufactured the world's largest (11MW) phosphoric acid fuel cell. By the beginning of 2002, the United States had installed and tested 235 sets of 200kW PAFC power generation devices around the world, generating a total of 4.7 million hours of power generation. In 2001, 23 sets were sold. There are several devices in the United States and Japan that have reached the design goal of continuous power generation for 10,000 hours; there are currently five sets of 200kW PAFC power generation devices in operation in Europe; Japan's Furi Electric and Mitsubishi Electric have developed 500kW PAFC power generation systems; my country's Wei Zidong and others conducted Pt3 Research on (Fe/Co)/C oxygen reduction electrocatalyst, and proposed the anchoring effect of Fe/Co on Pt. Phosphoric acid fuel cell power generation technology has been developed rapidly, but development obstacles such as its long start-up time and low waste heat utilization value have slowed down its development. 3. Research status of molten carbonate fuel cell (MCFC). This kind of battery uses a low-melting mixture of two or more carbonates as the electrolyte. For example, an alkali-carbonate low-temperature blend is used to penetrate into the porous matrix and electrode. It is fired from nickel powder. The cathode powder contains a variety of transition metal elements as stabilizers. It is mainly researched and used in the United States, Japan and Western Europe. 2~5MW external pipeline type molten carbonate fuel cells have been launched, and breakthroughs have been made in solving the performance degradation and electrolyte migration of MCFC. American Fuel Cell Energy is currently testing a 263kW MCFC power generation device in the laboratory. Italy's Ansaldo Company cooperates with Spain's Spanishcomp's to develop 100kWMCFC power generation devices and 500kWMCFC power generation devices. Hitachi Corporation of Japan developed a 1MMCFC power generation device in 2000, Mitsubishi Corporation developed a 200kWMCFC power generation device in 2000, and Toshiba developed a low-cost 10kWMCFC power generation device. my country has officially included MCFC in the national "Ninth Five-Year Plan" research plan, and has developed 1 to 5kW molten carbonate fuel cells. The cathode, anode, electrolyte separator and bipolar plate in MCFC are the four major difficulties in basic research. The integration of these four components and the management of the electrolyte are the technical core of the installation and operation of MCFC battery packs and power station modules. 4. Current research status of solid oxide fuel cells (SOFC). The electrolyte in the battery is a composite oxide. At high temperatures (below 1000°C)

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