Overview of Research Progress on Fuel Cells

1. Definitions

FuelCells is an electrochemical power generation device that does not require the Carnot cycle and has a high energy conversion rate. Fuel and air are sent to fuel cells separately, and electricity is wonderfully produced. It looks at positive and negative poles and electrolytes from the outside, like a battery, but in essence it can not "store electricity" but a "power plant." Fuel cells are also considered to be an environmentally friendly energy conversion device because of the fact that there are almost no nitrogen and sulfur oxides that pollute the environment during the energy conversion process. 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 break through, fuel cells have begun to be used in power stations, micro-power supplies, and so on.

2. Basic structure

The basic structure of the fuel cell is mainly composed of four parts: anode, cathode, electrolyte and external circuit. Usually the anode is extremely hydrogen electrode and the cathode is extremely oxygen electrode. Both the anode and the cathode need to contain a certain amount of electric catalyst to accelerate the electrochemical reaction that occurs on the electrode. Between the two electrodes is the electrolyte.

3. Classification

At present, there are many types of fuel cells and there are many ways to classify them. Classified by different methods as follows:

(1) Classified by operating mechanism: can be divided into acidic fuel cells and alkaline fuel cells;

(2) Classified by type of electrolyte: acidic, alkaline, molten salt or solid electrolyte;

Overview of Research Progress on Fuel Cells

(3) Classified by type of fuel: there are direct fuel cells and indirect fuel cells;

(4) By operating temperature of fuel cell: low temperature (below 200 °C); Medium temperature (200-750 °C); High temperature type (above 750 °C).

4. Principle

The working principle of fuel cells is relatively simple, including fuel oxidation and oxygen reduction of two electrode reactions and ion transfer process. The early fuel cell structure was relatively simple, requiring only electrolytes and two solid electrodes to transmit ions. When hydrogen is used as fuel and oxygen is used as an oxidant, the anode reaction and the total reaction of the fuel cell are:

Anode: H2 → 2 H + 2 E-

Cathodes: 1/2 O2 + 2 H + 2 E-→ H2O

Total reaction: H2 +1 / 2 O2 → H2O

Among them, H2 reaches the anode by diffusion and is oxidized to and E-under the action of a catalyst. Since then, H+ reaches the cathode through the electrolyte, and the electrons also reach the cathode after the load is powered by the external circuit. This leads to a reduction reaction (ORR) with O2.

Overview of Research Progress on Fuel Cells

II. Fuel cell applications

Today, there are many types of fuel cells that have been developed according to different application needs. According to the conductivity ion category, it can be divided into acidic fuel cells, alkaline fuel cells, soldering carbonate fuel cells and solid oxide fuel cells (SOFC). Acidic fuel cells can also be subdivided into PEMFC, direct alcohol fuel cells and phosphate fuel cells. All types of fuel cells have their working characteristics, with operating temperatures as low as -40 °C and as high as 1000 °. Fuel cell types can be selected according to different needs. Among them, PEMFC is the fuel cell that has received the most attention in recent decades. PEMFC not only has the universal characteristics of fuel cells, but also has the advantages of rapid start and work at low temperatures, no electrolytic fluid loss, long life, specific power and higher specific energy. It is considered to be the ideal solution for replacing internal combustion engines as automotive power sources in the future.

Due to the characteristics of modular fuel cells, wide range of power, and fuel diversification, it can be applied to a variety of occasions: small to scooter power supplies, mobile charging devices, and up to megawatt power stations. In fact, the commercialization of fuel cells has gone on like fire. Data show that from 2008 to 2011, the world market share of fuel cells as backup power sources for communications network equipment, logistics and airport ground handling increased by 214. The market value of fuel cells is expected to reach $19.2 billion by 2020.

Overview of Research Progress on Fuel Cells

The details should be briefly described as follows:

(1) Portable power supply

The annual increase in sales of portable power supplies has attracted many power technologies. Its products include laptops, mobile phones, radios, and other mobile devices that require power supply for personal convenience. The basic requirements of portable mobile power supply usually require that the power supply has the characteristics of high specific energy, light and compact. The energy density of fuel cells is usually 5 to 10 times that of rechargeable batteries, making them more competitive. In addition, the fact that fuel cells do not require additional charges also allows them to adapt to longer wild life. At present, there are direct methanol fuel cells (DMFC) and PEMFC used as military individual power sources and mobile charging devices. Cost, stability and life will be the technical problems that fuel cells need to solve when applied to handy mobile power sources.

(2) Fixed power supply

Fixed power sources include emergency backup power sources, uninterrupted power treatment, and independent power stations in remote areas. At present, fuel cells occupy about 70 megawatts of fixed power market each year, compared with traditional lead-acid batteries. Fuel cells have longer operating time (about 5 times that of lead-acid cells), higher density than energy, smaller volume, and better environmental adaptability. Independent power stations are considered to be the most economical and reliable way to supply electricity in remote areas where smart grids are difficult to reach and where emergencies occur. Fuel cells have been used as independent power stations in many disasters, which have played an important role in disaster relief. It should be noted that fixed power stations usually require a longer life (more than 80,000 hours), which is the biggest technical challenge for fuel cell technology to be applied to fixed power stations.

(3) Traffic power supply

Traffic power has been a major inducing factor in the development of clean energy technologies, as 17% of global greenhouse gas (CO2) is generated by fossil fuel-based transportation power, along with other air pollution problems such as haze. The H2 fueled PEMFC is considered to be the best alternative to the internal combustion engine. The main reasons are: (a) the exhaust gas has only water and no pollution discharge; (b) the fuel cell is extremely efficient (53%-59%), Almost twice as many as conventional internal combustion engines; (c) low-temperature rapid start-up, low operating noise and stable operation, many countries in the world are advancing fuel cell transportation power programs, and Japan is one of the most radical countries. Japan plans to build more than 1,000 ammonia-adding stations and operate 2 million fuel cell vehicles by 2025. In 2015, Toyota Motor Corporation of Japan began selling Mirai, the world's first PEMFC power source, marking a new era in fuel cell technology for automotive power.

Overview of Research Progress on Fuel Cells

III. Fuel cell research

1. Fuel cell development

The fuel cell is a self-running power plant. Its birth and development are based on electrochemistry, electrocatalysis, electrode process dynamics, material science, chemical process and automation. Since 1839, Grove has published the world's first report on fuel cells for more than 160 years. From a technical point of view, we realize that the emergence, development and improvement of new concepts is the key to the development of fuel cells. For example, fuel cells use gas as an oxidant and fuel, but the solubility of gas in liquid electrolytes is very small, resulting in a very low operating current density of the battery. For this purpose, scientists proposed the concept of a three-phase interface between porous gas diffusion electrodes and electrochemical reactions. It is the appearance of porous gas diffusion electrodes that makes fuel cells have the necessary conditions for practical application. In order to stabilize the three-phase interface, a two-hole structure electrode was used, and a water-repellent material, such as polytetrafluoroethylene, was added to the electrode to prepare a bonded water-repellent electrode. For fuel cells with a solid electrolyte as a diaphragm, such as proton exchange membrane fuel cells and solid oxide fuel cells, in order to establish a three-phase interface within the electrode, ion exchange resin or solid oxide electrolyte material is mixed into the electric catalyst in order to achieve the electrode. Three-dimensional.

Materials science is the foundation of fuel cell development. The discovery of a new high-performance material and its use in fuel cells will promote the rapid development of a fuel cell. The development of asbestos membranes and their successful application in alkaline batteries have ensured the successful use of asbestos membrane alkaline oxyhydrogen fuel cells for space shuttles. The successful development of a lithium metasilicate diaphragm in molten carbonate accelerated the construction of a MW-scale experimental power plant for molten carbonate fuel cells. The development of yttria-stabilized zirconia solid electrolyte membranes has made solid oxide fuel cells a hot research topic for future fuel cell decentralized power plants. The emergence of perfluorosulfonic acid-type proton exchange membranes has led to the revival of research on proton exchange membrane fuel cells, which has led to rapid development.

Before the 1960s, due to the rapid development and progress of hydroelectric power, thermal power generation and chemical batteries, fuel cells have been in the basic research stage of theory and application, mainly on the concept, materials and principles. The breakthrough in fuel cells relies mainly on the efforts of scientists. In the 1960s, due to the urgent need for manned spacecraft for high power, high specific power and high specific energy batteries, fuel cells have attracted the attention of some countries and military departments. It is in this context that the United States introduced Bacon's technology to successfully produce the main power source of the Apollo moon landing space, the Bacon-type medium-temperature hydrogen-oxygen fuel cell. Since the 1990s, human beings have paid more and more attention to environmental protection for the purpose of sustainable development, protecting the earth, and benefiting future generations. Based on the rapid advancement of proton exchange membrane fuel cells, various electric vehicles powered by them have been introduced. In addition to high cost, their performance is comparable to that of diesel locomotives. Therefore, fuel cell electric vehicles have become the focus of attention and competition of the US government and big car companies.

In terms of investment, before the investment in the development of fuel cells mainly depends on the government, but so far the company has become the main investment in the development of fuel cells, especially fuel cell electric vehicles. All major car 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 U.S. dollars. There are 41 types of fuel electric vehicles that have been successfully developed, including 24 passenger cars and buses. 9 kinds of buses and 3 types of light trucks. This year, the United States announced a plan to invest 2.5 billion U.S. dollars in the development of fuel-cell electric cars, of which the state allocated 1.5 billion U.S. dollars and the three major car companies invested 1 billion U.S. dollars.

2. Research status of alkaline fuel cell (AFC)

The battery uses 35% to 45% KOH as an electrolyte and penetrates into a porous, inert matrix membrane material at an operating temperature of less than 100 °C. The advantage of this kind of battery is that the electrochemical reaction speed of oxygen in the alkali liquid is larger than that in the acidic liquid, so there is a large current density and output power, but the oxidant should be pure oxygen, and the amount of precious metal catalyst in the battery is large, and the utilization rate is not high. At present, the development of such fuel cell technology is very mature and has been successfully applied in space flight andspecials. A 200W ammonia-air alkaline fuel cell system has been developed in China, and 1kW, 10kW, and 20kW alkaline fuel cells have been fabricated. In the late 1990s, very valuable results were achieved in tracking development. The core technology for the development of alkaline fuel cells is to avoid the destruction of alkaline electrolyte components by carbon dioxide, whether it is a part of the carbon dioxide in the air or the carbon dioxide contained in the hydrocarbon reforming gas. Removal 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 discharged in time to maintain the water balance. Therefore, simplifying the drainage system and control system is also the core technology that needs to be solved in the development of alkaline fuel cells.

3. Research status of phosphoric acid fuel cell (PAFC)

This battery uses phosphoric acid as an electrolyte and has an operating temperature of about 200 °C. The outstanding advantage is that the amount of precious metal catalyst is greatly reduced compared with the alkaline hydroxide fuel cell, the purity of the reducing agent is required to be greatly reduced, and the carbon monoxide content can be allowed to reach 5%. Such batteries generally use organic hydrocarbons as fuel, positive and negative electrodes are made of porous electrodes made of polytetrafluoroethylene, electrodes are coated with Pt as a catalyst, and electrolytes are 85% of H3PO4. It has stable performance and strong conductivity in the range of 100 to 200 °C. Phosphoric acid batteries are cheaper to manufacture than other fuel cells and are close to being available for civilian use. At present, the fuel cell power station with high power in the world is using the battery of this fuel. The United States has listed phosphoric acid fuel cells as national key scientific research projects for research and development, and sold 200kW grade phosphoric acid fuel cells to the world. Japan has produced 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 worldwide, with a cumulative power generation of 4.7 million hours and sold 23 sets in 2001. In the United States and Japan, several sets of devices have reached the design goal of 10,000 hours of continuous power generation; five sets of 200kW PAFC power generation units are currently in operation in Europe; Japan's Furi Electric and Mitsubishi Electric have developed 500kW PAFC power generation systems; China's Wei Zidong and others conducted Pt3 (Fe/Co)/C oxygen reduction electrocatalyst research, and proposed the anchoring effect of Fe/Co on Pt. Phosphoric acid fuel cell power generation technology has been rapidly developed, but its development time is slowed down due to development delays such as long startup time and low waste heat utilization value.

4. Research Status of Molten Carbonate Fuel Cell (MCFC)

The battery uses a low-melting mixture of two or more carbonates as an electrolyte, such as an alkali-carbonate low-temperature eutectic, which is infiltrated into a porous substrate, and the electrode is fired from nickel powder, and the cathode powder contains a large amount. Transition metal elements are used as stabilizers, mainly in the United States, Japan and Western Europe. A 2 to 5 MW external common pipeline type molten carbonate fuel cell has been introduced, and breakthroughs have been made in solving the performance degradation and electrolyte migration of MCFC. The US fuel cell energy company is currently testing 263kW MCFC power plants in the laboratory. Italy's Ansaldo cooperated with Spain's Spanishcomp's to develop a 100kW MCFC power plant and a 500kW MCFC power plant. Japan's Hitachi, Ltd. developed the 1MMCFC power generation unit in 2000, Mitsubishi developed the 200kW MCFC power generation unit in 2000, and Toshiba developed a low-cost 10kW MCFC power generation unit. China has officially listed MCFC in the national “Ninth Five-Year Plan”, and has developed a 1 to 5 kW molten carbonate fuel cell. Cathodes, anodes, electrolyte membranes and bipolar plates in MCFC are the four major difficulties in basic research. The integration of these four components and the management of electrolytes are the technical core of the installation and operation of MCFC battery packs and power station modules.

5. Research Status of Solid Oxide Fuel Cell (SOFC)

The electrolyte in the battery is a composite oxide, which has a strong ion conducting function at high temperatures (below 1000 ° C). It is because the ionic state of the mixed ions such as calcium, barium or strontium is lower than that of zirconium ions, and some oxygen anion lattice spaces are vacated to conduct electricity. At present, countries all over the world are developing such batteries, and there have been substantial progress, but there are disadvantages: high manufacturing cost; too high temperature; dielectric cracks; large resistance. Solid oxide fuel cells formed by various structures such as tubular, flat and corrugated have been developed, and such fuel cells are called third generation fuel cells. Several companies in the US and Japan are developing 10kW planar turbine SOFC power plants. Germany's Siemens-Westinghouse Electric is testing a 100kW SOFC tubular working reactor, and the United States is testing a 25kW SOFC working reactor. Most of the domestic countries are in the basic research stage of SOFC. The operation of SOFC at high temperatures also brings a range of materials, sealing and structural problems, such as sintering of the electrodes, chemical diffusion of the interface between the electrolyte and the electrodes, and matching between materials with different coefficients of thermal expansion and bipolar plate materials. Stability and so on. These also restrict the development of SOFC to a certain extent and become a key aspect of its technological breakthrough.

6. Research Status of Proton Exchange Membrane Fuel Cell (PEMFC)

PEMFC is the fifth-generation fuel cell that is rapidly developing after AFC, PAFC, MCFC, and SOFC. It is the lowest temperature, highest energy ratio, fastest start, longest life, and most widely used. It is forspecial and military power. Developed. In the results of the social survey of Time magazine, it was listed as the top ten new technology in the 21st century. Domestic research and development is representative of the use of AFC technology to accumulate a comprehensive PEMFC research; Extensive work has also been done on the preparation, characterization and analysis of PEMFC and Pt/C-electrocatalysts using polystyrene sulfonate membranes as electrolytes. A number of companies in the United States, Japan, Sanyo, Mitsubishi and other companies have also developed portable PEMFC power generation reactors. Electric Systems Canada, in collaboration with EBARA of Japan, has developed 250kWPEMFC power generation equipment and 1kWPEMFC portable power generation systems. Germany built a 250kWPEMFC experimental reactor in Berlin. The core technology of proton exchange membrane fuel cell is the preparation technology of electrode-membrane-electrode three-in-one component. In order to spread to the gas, proton conductors are added to the electrode, and the contact between the electrode and the membrane is improved. The electrode, membrane, and electrode are pressed together by means of hot pressure to form a three-in-one electrode-membrane-electrode assembly., The technical parameters of proton exchange membrane directly affect the performance of the tri-in-one component and therefore affect the operating efficiency of the entire cell and battery. The price of PEMFC also restricts its commercialization process. Therefore, improving its necessary component performance and reducing operating costs are important directions for the development of PEMFC.

7. Research status of direct carbon fuel cells

Compared with the direct combustion of carbon, direct carbon fuel cells have low pollution and high energy utilization, which is an ideal carbon utilization method. The research report on DCFC first appeared in 1896. Jacques uses coal as the negative electrode, iron as the positive electrode, and a battery system with molten NaOH as the electrolyte, and 100 cells to form the battery stack. When the operating temperature of the battery stack is 400~500 °C, the total output power is 1.5kW, current density up to 100mA? Cm-2. Direct carbon fuel cells have a wide range of raw materials and have the potential to realize the utilization of carbonaceous waste, but still face the problem of impurities in the fuel causing failure of the electrodes and electrolytes.

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