I. Introduction to Fuel Cells

Definition

Fuel Cells (Fuel Cells) is a need to go through the Carnot cycle of electrochemical power plant, high energy conversion rate. Fuel and air were sent to the fuel cell, the electricity was wonderful to produce it. It looks from the appearance of positive and negative and electrolyte, like a battery, but in essence it can not "storage" but a "power plant." As in the energy conversion process, almost no pollution of the environment containing nitrogen and sulfur oxides, fuel cells are also considered an environmentally friendly energy conversion device. As a result of these excellent, fuel cell technology is considered to be one of the 21st century new environmentally friendly and efficient power generation technology. With the continuous breakthrough in research, fuel cells have been in power stations, micro-power and other aspects of the application.

2. Basic structure

The basic structure of the fuel cell is mainly composed of four parts, namely the anode, cathode, electrolyte and external circuit. Usually the anode is hydrogen electrode, the cathode is oxygen electrode. Both the anode and the cathode need to contain a certain amount of electrocatalyst, used to accelerate the electrochemical reaction occurred on the electrode, between the two electrodes is the electrolyte.

Figure 1. Schematic diagram of the basic structure of the fuel cell

3. Classification

At present, many types of fuel cells, there are many kinds of classification methods. According to different methods are generally classified as follows:

(1) according to the operating mechanism to classify: can be divided into acid fuel cell and alkaline fuel cell;

(2) according to the type of electrolyte to classify: acidic, alkaline, molten salt or solid electrolyte;

Figure 2. Fuel cell classification details

(3) according to the type of fuel to classify: a direct fuel cell and indirect fuel cell;

(4) According to the fuel cell operating temperature points: a low temperature (less than 200 ℃); medium temperature (200-750 ℃); high temperature (higher than 750 ℃).

4. Principle

Fuel cell working principle is relatively simple, mainly including fuel oxidation and oxygen reduction two electrode reaction and ion transport process. The early fuel cell structure is relatively simple, only need to transmit the electrolyte and two solid-state electrodes. When the hydrogen as fuel, oxygen as oxidant, the fuel cell anode and cathode reaction and the total reaction were: 

Anode: H2 → 2H ++ 2e-

Cathode: 1/2 O2 + 2H ++ 2e- → H2O

Total reaction: H2 + 1 / 2O2 → H2O

Where H2 is oxidized to and from the catalyst by diffusion, and then H + reaches the cathode through the electrolyte, and the electrons reach the cathode through the external circuit and then reacts with O2. ORR).

Figure 3. Fuel cell schematic

2. Fuel cell applications

Today, many types of fuel cells have been developed according to different application requirements. According to the conductive ion category can be divided into acid fuel cells, alkaline fuel cells, branded carbonate fuel cell and solid oxide fuel cell (SOFC). Acid fuel cells can also be subdivided into PEMFC, direct alcohol fuel cells and phosphoric acid fuel cells. All types of fuel cells have their operating characteristics, operating temperature as low as -40 ° C, up to 1000 °. The fuel cell type can be selected according to different requirements. Where PEMFC is the most watched fuel cell in recent decades. PEMFC not only has the general characteristics of fuel cells, as well as low temperature can quickly start and work, no electrolyte loss, long life, higher than the power and high energy advantages, is considered the future alternative to internal combustion engine as the ideal vehicle power supply program.

Due to the modularization of fuel cells, wide power range and fuel diversification, it can be used in a variety of applications: small to scooter power, mobile charging devices, large to megawatt power stations. In fact, the commercialization of fuel cells is carried out like fire, such as tea. Data show that from 2008 to 2011, the world's fuel cell as a communications network equipment, logistics and airport ground standby power supply market share increased by 214%. It is expected that by 2020, the market value of fuel cells will reach $ 192.

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The specific application for a brief introduction is as follows:

(1) portable power supply

The increase in sales of the portable power market has attracted a lot of power technology. Its products include: laptops, mobile phones, radios and other mobile devices that require power. For the convenience of personal carrying, the basic requirements for portable mobile power are usually required to have a high power ratio Energy, light and small and so on. The energy density of the fuel cell is usually 5 to 10 times that of the rechargeable battery, making it more competitive. In addition, the fuel cell does not require additional charge characteristics also make it adapt to longer-term field life. At present, direct methanol fuel cells (DMFC) and PEMFC have been used as military single power and mobile charging devices. Cost, stability and longevity will be the technical problem that the fuel cell needs to solve when applying the fuel cell to the portable power supply.

(2) fixed power supply

Fixed power supply includes emergency backup power supply, uninterrupted electrotherapy, remote area independent power station. At present, the fuel cell occupies about 70% of the world's megawatt-class fixed power market every year, compared to the traditional lead-acid batteries, fuel cells have a longer running time (about 5 times the lead-acid battery), higher than the ratio Can be density, smaller size and better environmental adaptability. For remote areas where smart grids are difficult to reach and emergencies occur, standalone power plants are considered the most economical and reliable way of supplying power. In China's many times to show the disaster, the fuel cell is used as an independent power station, disaster relief work played an important role. It is important to note that fixed power plants typically require a longer life (greater than 80,000 hours), which is the biggest technical challenge for fuel cell technology to be used in fixed power plants.

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Traffic power has always been a major factor in the development of clean energy technologies because 17% of the world's greenhouse gases (CO2) are generated by fossil fuel-based traffic, along with other air pollution problems such as haze. (B) the efficiency of the fuel cell is extremely high (53% -59%), and the fuel cell is considered to be the best alternative to the internal combustion engine. The main reason is that: (a) the exhaust gas is only water and there is no pollution discharge; (C) low temperature fast start, low operating noise and stable operation, many countries in the world are pushing fuel cell traffic power program, Japan is one of the most radical countries. Japan plans to build more than 1,000 ammonia stations and run 2 million fuel cell vehicles by 2025. In 2015, Toyota Motor Corporation of Japan began selling the world's first PEMFC as the main power supply car Mirai, marking the fuel cell technology used in the new era of automotive power.

Figure 5. Toyota fuel cell car Mirai photo

3. Fuel cell research

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The fuel cell is an autorun power plant. Its birth and development is based on electrochemistry, electrocatalysis, electrode process dynamics, materials science, chemical process and automation and other disciplines. From 1839 Grove published the world's first report on the fuel cell has been more than 160 years of history. From a technical point of view, we realize that the emergence of new concepts, development and improvement is the key to the development of fuel cells. Such as fuel cells with gas as oxidant and fuel, but the gas in the liquid electrolyte solubility is very small, resulting in the battery current density is very low. For this reason scientists put forward the concept of porous gas diffusion electrode and electrochemical reaction three-phase interface. It is the emergence of porous gas diffusion electrode, so that the fuel cell has a practical conditions to the practical. In order to stabilize the three-phase interface, a double-pore structure electrode is started, and a material having hydrophobic properties such as polytetrafluoroethylene is added to the electrode to prepare a bonded hydrophobic electrode. In the case of a fuel cell having a solid electrolyte as a separator, such as a proton exchange membrane fuel cell and a solid oxide fuel cell, an ion exchange resin or a solid oxide electrolyte material is incorporated into the electrocatalyst to form a three-phase interface in the electrode, Electrode of the three-dimensional.

Material science is the basis for the development of fuel cells. The discovery of a new and excellent material and its use in fuel cells will promote the rapid development of a fuel cell. Such as the development of asbestos film and its successful application in alkaline batteries, to ensure that the asbestos membrane alkaline hydrogen fuel cell successfully used for space shuttle. The successful development of the lithium aluminate lithium diaphragm in the molten carbonate has succeeded in accelerating the construction of the molten carbonate fuel cell megawatt stage experimental power station. The development of yttria stabilized zirconia solid electrolyte membrane makes solid oxide fuel cell a research hotspot for future fuel cell distributed power plant. The emergence of perfluorosulfonic acid proton exchange membrane, but also to promote the proton exchange membrane fuel cell research revived, and then the rapid development.

Before the 1960s, due to the rapid development and progress of hydropower, thermal power and chemical batteries, fuel cells have been in the basic research stage of theory and application, mainly on the concept, materials and principles of research. Fuel cell breakthrough mainly by the efforts of scientists. Into the 60's, due to manned spacecraft for high-power, high specific power and high energy than the urgent needs of the battery, fuel cell was caused by some countries and military departments attach great importance. It is in this context, the United States introduced the Bacon technology, the success of the Apollo moon on the main power ship - Bacon medium temperature hydrogen fuel cell. Since the 1990s, human beings have paid more attention to environmental protection for the purpose of sustainable development, protection of the earth and the benefit of future generations. Based on the rapid progress of proton exchange membrane fuel cell, a variety of electric vehicles with its power has been available, in addition to high cost, its performance has been comparable with the diesel locomotive. So the fuel cell electric car has become the focus of the US government and the big car companies concerned with the competition.

From the investment point of view, before the development of fuel cell investment mainly by the government, and so far the company has become the development of fuel cells, especially fuel cell electric vehicle investment. All the big car companies and oil companies have been involved in the development of fuel cell vehicles, just a few years time, invested about 8 billion US dollars, the successful development of fuel electric vehicles to 41, of which 24 kinds of car station wagons, 9 inter-city buses, 3 light-load trucks. This year the United States also announced a $ 2.5 billion investment in the development of fuel cell electric vehicle plan, of which the state allocated 1.5 billion US dollars, the three major auto companies to invest 1 billion US dollars.

2. Alkaline fuel cell (AFC) research status

This battery with 35% to 45% KOH as the electrolyte, penetrate into the porous and inert matrix membrane material, the working temperature is less than 100 ℃. The advantage of this type of battery is that the electrochemical reaction rate of oxygen in the alkaline solution is larger than that in the acidic solution, so there is a large current density and output power, but the oxidant should be pure oxygen, the amount of the precious metal catalyst in the battery is larger Utilization is not high. At present, the development of such fuel cell technology has been very mature, and has been successfully applied in space flight and submarines. China has developed 200W ammonia - air alkaline fuel cell system, made 1kW, 10kW, 20kW alkaline fuel cell, the 20th century, the late 90s in the tracking development has made very valuable results. 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 the carbon dioxide component of 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 battery produced by the electrochemical reaction of water to be discharged in time to maintain water balance. Therefore, the simplified drainage system and control system is also the basic technology needed to solve the development of alkaline fuel cell.

2. Phosphate type fuel cell (PAFC) research status

This battery uses phosphoric acid as the electrolyte, the working temperature of about 200 ℃. The outstanding advantage is that the amount of precious metal catalyst is much lower than that of alkaline hydroxide fuel cell, and the purity requirement of reducing agent is greatly reduced, and the carbon monoxide content can be allowed to reach 5%. This type of battery is generally organic hydrocarbons as fuel, positive and negative electrodes made of polytetrafluoroethylene porous electrode, the electrode coated Pt as a catalyst, the electrolyte is 85% H3PO4. In the range of 100 ~ 200 ℃ stable performance, strong conductivity. Phosphate batteries are less expensive than other fuel cells and are close to civilian levels. At present, the international power of large practical fuel cell power stations are using this fuel battery. The United States will be phosphate-type fuel cell as a national key research projects for research and development, to the world to sell 200kW grade phosphoric acid fuel cell, Japan has created the world's largest (11MW) phosphoric acid fuel cell. By the beginning of 2002, the United States has installed and tested 235 sets of 200 kWPAFC power generation units worldwide, accumulating 4.7 million hours of electricity and 23 sets in 2001. In the United States and Japan have several sets of devices has reached a continuous power generation 10,000 hours of design goals; Europe has five sets of 200kWPAFC power generation devices in operation; Japan Fu Electric and Mitsubishi Electric has developed a 500 kWPAFC power generation system; China Wei Zidong and others Pt3 (Fe / Co) / C oxygen reduction electrocatalyst, and the anchoring effect of Fe / Co on Pt was proposed. Phosphorus-based fuel cell power generation technology has been developed at a high speed, but its development time is longer and the utilization of waste heat is low.

3. Research status of molten carbonate fuel cell (MCFC)

The low melting mixture of two or more kinds of carbonates is an electrolyte, such as alkali-carbonate low-temperature co-infiltrated into a porous matrix, the electrode is made of nickel powder, and the cathode powder contains Transition metal elements as stabilizers, mainly in the United States, Japan and Western Europe to study and use more. 2 ~ 5MW outside the public pipeline-type molten carbonate fuel cell has come out to solve the performance of MCFC attenuation and electrolyte migration has made a breakthrough. US fuel cell energy company is currently testing the laboratory 263kWMCFC power plant. Italy Ansaldo company and Spain Spanishcomp's cooperation to develop 100kWMCFC power plant and 500kWMCFC power plant. Japan Hitachi Corporation in 2000 to develop a MMCFC power plant, Mitsubishi Corporation in 2000 to develop 200kWMCFC power plant, Toshiba developed a low-cost 10kWMCFC power generation device. China has officially included in the MCFC "Ninth Five-Year" research program, has developed a 1 ~ 5kW molten carbonate fuel cell. MCFC cathode, anode, electrolyte diaphragm and bipolar plate is the basic research of the four major difficulties, the integration of these four components and the management of the electrolyte is MCFC battery and power plant module installation and operation of the technical core.

4. Research status of solid oxide fuel cell (SOFC)

Electrolyte in the battery is a composite oxide, in the high temperature (1000 ℃ below), there is a strong ion conductive function. It is due to calcium, ytterbium or yttrium mixed with ionic valence is lower than the valence of zirconium ions, so that some oxygen anion lattice vacated out and conductive. At present, all countries in the world are developing this