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Review on research progress of fuel cell
2017/11/23 view:
I. Introduction TO Fuel Cells
1. Definition
The fuel cell (fuelcells) is a kind of electrochemical power generation device which does not need the Carnot cycle, and the energy conversion rate is high. Fuel and air are sent to fuel cells, and electricity is produced wonderfully. It looks to have positive and negative polarity and electrolyte, such as a battery, but in essence it can not "store electricity" but a "power plant". The fuel cell is considered as an environmentally friendly energy conversion device due to the lack of nitrogen and sulphur oxides that pollute the environment in the process of energy conversion. With these advantages, fuel cell technology is considered to be one of the new environmentally efficient power generation technologies in the 21st century. As research continues to break through, fuel cells have been used in power stations, micro-power sources and so on.
2. Basic structure
The basic structure of fuel cell is mainly composed of four parts, namely anode, cathode, electrolyte and external circuit. Usually the anode is extremely hydrogen electrode, the anion extremely oxygen electrode. Both anode and cathode need to contain a certain amount of electrical catalysts to accelerate the electrochemical reaction of the electrode, the electrolyte between the two electrodes.
3. Classification
There are many kinds of fuel cell, there are many kinds of classification methods. The following are broadly categorized by different methods:
(1) According to the operating mechanism to classify: can be divided into acidic fuel cells and alkaline fuel cells;
(2) According to the type of electrolyte to classify: there are acidic, alkaline, molten salts or solid electrolyte;
Review on research progress of fuel cell
Fig. 2 Detailed description of fuel cell classification
(3) classified by type of fuel: direct fuel cell and indirect fuel cell;
(4) According to fuel cell working temperature: Low temperature type (less than 200 ℃), Medium temperature type (200-750 ℃), high temperature (above 750 ℃).
4. Principle
The working principle of fuel cell is relatively simple, including fuel oxidation and oxygen reduction two electrode reaction and ion transmission process. The early fuel cell structure was relatively simple, requiring only the transfer of ionic electrolytes and two of solid-state electrodes. When hydrogen is the fuel and oxygen is oxidant, the anode reaction 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, the H2 through the diffusion of the anode, under the action of the catalyst was oxidized and e, since then, h through the electrolyte to reach the cathode, while the electrons through the external circuit to drive the load after work also reached the cathode, thus and O2 a reduction reaction (ORR).
Review on research progress of fuel cell
Figure 3 Schematic diagram of fuel cell
Two. Fuel cell application
Today, many types of fuel cells have been developed according to different application requirements. The types of conductive ions can be divided into acidic fuel cells, alkaline fuel cells, branded carbonate fuel cells and solid oxide fuel cells (SOFC). Acidic fuel cells can also be subdivided into pemfc, direct alcohol fuel cell and phosphoric acid fuel cell. All kinds of fuel cells have their working characteristics, operating temperature is low to -40°c, high to 1000°. Fuel cell types can be selected according to different requirements. Among them, PEMFC is the most concerned fuel cell in recent decades. Pemfc not only has the general characteristic of fuel cell, but also has the advantages of fast start-up and working at low temperature, no electrolyte loss, long life, higher specific power and energy than the other, it is considered to be the ideal alternative for the future internal combustion engine as a vehicle power supply.
Because of the characteristics of fuel cell modularization, wide power range and fuel diversification, it can be used in many situations: small to scooter power supply, mobile charging device, large-megawatt power station. In fact, the commercialization of fuel cells is as hot as tea. Data show that from 2008 to 2011, the world-wide fuel cell as a communications network equipment, logistics and airport ground back-up power market share increase 214%. The market value of fuel cells is expected to reach $19.2 billion trillion by 2020.
Review on research progress of fuel cell
Fig. 4. Fuel cell applications
Specific applications are briefly described as follows:
(1) Portable power supply
Portable Power Market sales of the year increase long attracted a lot of power technology, its products include: notebook computers, mobile phones, radios and other mobile devices need to power, for the convenience of personal portability, portable mobile power supply requirements usually require the power supply has a high ratio of energy, lightweight and small characteristics. The energy density of the fuel cell is usually 5 to 10 times times of the rechargeable battery, which makes it more competitive. In addition, the fuel cell does not require extra charge to allow it to adapt to a longer life in the wild. At present, direct methanol fuel cell (DMFC) and PEMFC have been used as military individual power supply and mobile charging device. The cost, stability and life span will be the technical problems that fuel cells apply to the skillful mobile power supply.
(2) Fixed power supply
The fixed power supply includes emergency standby power supply, uninterrupted electric therapy, remote area independent power station and so on. At present, fuel cells account for about 70% of the world's MW fixed power market each year, compared to traditional lead-acid batteries, fuel cells have a longer operating time (about 5 times times as much as lead-acid batteries), higher than the king's density, smaller volume and better environmental adaptability. For remote areas and emergency locations where smart grids are inaccessible, independent power stations are considered the most economical and reliable way to supply electricity. In our country many times disaster, the fuel cell is used as the independent power station, has played the important role for the relief work. It is important to note that fixed power plants typically require longer lifespans (more than 80,000 hours), which is the biggest technical challenge for the application of fuel cell technology to fixed power plants.
(3) Traffic power supply
The transportation power supply has been the main inducement factor for the research and development of clean energy technology, because 17% of the global greenhouse gas (CO2) is generated by the transportation power based on fossil fuels, and other air pollution problems, such as haze, etc. H2-fueled Pemfc is considered the best alternative to internal combustion engines, the main reasons are: (a) The exhaust gas is only water, without any pollution emissions; (b) fuel cell efficiency is very high (53%-59%), almost twice times the traditional internal combustion engine; (c) Low temperature fast start, lower noise and stable operation, Japan is one of the most radical countries in the world to drive fuel cell traffic. Japan plans to build more than 1000 ammonia stations and run 2 million fuel-cell vehicles by 2025. In 2015, Toyota Motor Corp. began selling Mirai, the world's first PEMFC, as the main power source, marking the new era of fuel-cell technology used in automotive power.
Review on research progress of fuel cell
Figure 5. Toyota Fuel cell car Mirai photos
Three. Fuel cell research
1. Fuel cell development
The fuel cell is a power plant that runs automatically. Its birth, development is based on electrochemistry, electrocatalytic, electrode process kinetics, material science, chemical process and automation and other disciplines. It has been more than 160 years since the 1839 Grove published the World's first report on fuel cells. From the technical point of view, we realize that the emergence, development and perfection of the new concept is the key to fuel cell development. such as fuel cells with gas as oxidant and fuel, but the solubility of gases in the liquid electrolyte is very small, resulting in a very low working current density of the battery. The concept of the porous gas diffusion electrode and the three-phase interface of electrochemical reaction was proposed. It is the appearance of porous gas diffusion electrodes that make the fuel cells have the necessary conditions to become practical. In order to stabilize the three-phase interface, the two-hole structure electrode was used, and the material with hydrophobic properties was added to the electrode, such as PTFE, to prepare the bonding type of hydrophobic pole. For a fuel cell with a solid electrolyte diaphragm, such as a proton exchange membrane fuel cell and a solid oxide fuel cell, in order to establish a three-phase interface within the electrode, an ion exchange resin or solid oxide electrolyte material is mixed into the electric catalyst to achieve the three-dimensional electrode.
Material science is the foundation of fuel cell development. The discovery of a new material with excellent performance and its application in fuel cell 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 the asbestos membrane alkaline hydrogen-oxygen fuel cell was successfully used in the Space shuttle. The successful development of the lithium aluminate diaphragm in molten carbonate has accelerated the construction of a megawatt-level experimental power plant for molten carbonate fuel cells. The development of yttrium oxide stabilized zirconia solid electrolyte diaphragm makes the solid oxide fuel cell become the research hotspot of the future fuel cell decentralized power plant. But the appearance of the whole fluorine sulfonic type proton exchange membrane, also promoted the proton exchange membrane fuel cell research to obtain the revival, then the rapid development.
Before the 1960s, fuel cells were in the basic research stage of theory and application because of the rapid development and progress of hydroelectric, thermal power and chemical batteries, mainly on concepts, materials and principles. The breakthroughs in fuel cells depend mainly on the efforts of scientists. In the 60 's, due to the urgent need of the manned spacecraft for high-power, high specific power and higher than energy batteries, fuel cells have been paid great attention by some countries and military departments. It is in this context that the United States introduced the technology of Bacon, the successful Apollo Lunar lander on the main power supply-bacon-type medium temperature hydrogen oxygen fuel cell. Since the 1990s, mankind has paid more and more attention to environmental protection for the purpose of sustainable development, protecting the earth and benefiting future generations. Based on the high-speed progress of proton exchange membrane fuel cell, all kinds of electric vehicles with its power have been invented, besides the high cost, its performance can be comparable with diesel locomotive. So the fuel cell electric vehicle has become the focus of attention and competition of US government and big automobile company.
From the investment point of view, the development of fuel cell investment in the past mainly by the Government, and so far the company has become the development of fuel cells, especially fuel cell electric vehicle investment body. All of the world's big auto companies and oil companies have been involved in the development of fuel cell vehicles, in just a few years, to invest about 8 billion U.S. dollars, the development of a successful fuel electric vehicle to reach 41 kinds, of which, 24 kinds of car wagon, urban bus 9, light truck 3 species. This year, the United States announced a plan to invest $2.5 billion in fuel-cell electric vehicles, with the state allocating $1.5 billion and three big auto companies investing in $1 billion.
2. Basic fuel cell (AFC) research status
This kind of battery uses 35%~45%koh as electrolyte, permeate in porous and inert matrix diaphragm material, working temperature is less than 100 ℃. The advantage of this kind of battery is that the electrochemical reaction rate of oxygen in lye is larger than that in acidic solution, so there is a larger current density and output power, but the oxidant should be pure oxygen, the amount of precious metal catalyst in the battery is larger and the utilization rate is not high. At present, the development of such fuel cell technology has been very mature, and has been successfully used in space flight and submarine. A 200W ammonia-air alkaline fuel cell system has been developed in China, which is made up of 1kW, 10kW and 20kW alkaline fuel cell, and has achieved very valuable results in tracking development in the late the 1990s. The core technology of developing alkaline fuel cell is to avoid the damage of carbon dioxide to alkaline electrolyte composition, whether it is the carbon dioxide in the air or the use of hydrocarbon reforming gas, it must be removed and treated, which undoubtedly increases the overall cost of the system. In addition, the battery will be produced by electrochemical reaction of the water should be discharged in time to maintain water balance. Therefore, simplifying the drainage system and the control system is also the core technology which needs to be solved in the development of alkaline fuel cell.
3. Research status of phosphoric acid fuel cell (PAFC)
This kind of battery uses phosphoric acid as electrolyte, working temperature is about 200 ℃. Its outstanding advantage is that the amount of precious metal catalyst is much less than that of alkaline hydroxide fuel cell, and the purity of reducing agent is reduced, and the carbon monoxide content can be allowed up to 5%. This kind of battery generally uses the organic hydrocarbon as the fuel, the positive and negative electrode uses the porous electrode which is made by the PTFE, the electrode is coated PT as the catalyst, the electrolyte is 85% $literal. In the 100~200℃ range of stable performance, strong conductivity. The phosphoric acid battery is less expensive than other fuel cells and is close to being available for civilian use. At present, the international high-power utility fuel cell power station uses this kind of fuel cell. The United States has developed the world's largest (11MW) phosphoric acid fuel cell by making phosphoric acid fuel cells a national key research project and selling 200kW-grade phosphoric acid fuel cells to the world. By the beginning of 2002, the United States had installed 235 sets of 200kWPAFC power generation devices around the world, accumulating electricity for 4.7 million hours and selling 23 sets in 2001. In the United States and Japan, several sets of devices have reached the design target of 10,000 hours of continuous power generation; 5 sets of 200kWPAFC power generation units are in operation in Europe, and Japan's Fu-ri Electric and Mitsubishi have developed 500kWPAFC power generation systems; Wei Zidong and others in China Pt3 (FE/CO) The study of C-oxygen reduction catalysts and the anchoring effect of Fe/co on PT were proposed. The production technology of phosphoric acid fuel cell has been developed at high speed, but its start-up time is long and the development obstacle of waste heat utilization value is slow.
4. Research status of molten carbonate fuel cell (MCFC)
This battery is electrolyte with two or more low melting mixtures of carbonate, such as alkali-carbonate low-temperature melt infiltration into porous matrix, electric extremely nickel powder fired, the cathode powder contains a variety of transition metal elements as stabilizers, mainly in the United States, Japan and Western Europe research and use more. 2~5MW External Utility pipeline type molten carbonate fuel cell has been introduced, which has made a breakthrough in solving the MCFC performance attenuation and electrolyte migration. U.S. fuel-cell energy companies are currently testing 263kWMCFC power plants in laboratories. The Italian Ansaldo Company is cooperating with Spanish Spanishcomp ' s to develop 100kWMCFC power generation devices and 500kWMCFC power generation devices. Hitachi, Japan, developed a 1MMCFC power generation device in 2000, Mitsubishi developed a 200kWMCFC power generation device in 2000, and Toshiba developed a low-cost 10kWMCFC power generation device. China has formally included MCFC in the national "95" project, has developed a 1~5kw molten carbonate fuel cell. MCFC Middle Yin
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