Fuel cell is a chemical device that directly converts the chemical energy of fuel into electrical energy, also known as electrochemical generator. It is a relayHydropower, thermal power generation andAtomic power generationThe fourth generation technology after that.Because the fuel cell iselectrochemical reactionThe Gibbs free energy in the chemical energy of the fuel is partially converted into electrical energy, which is not affected byCarnot cycleThe effect is limited, so the efficiency is high.In addition, fuel for fuel cell andoxygenAs the raw material, there is no mechanical transmission component, so the discharged harmful gas is very little, and the service life is long.It can be seen that fuel cell is the most promising power generation technology from the perspective of energy conservation and ecological environment protection[1]。
Chinese name
fuel cell
Foreign name
fuel cell
Essence
Device for converting chemical energy into electrical energy
Fuel cell is a kind ofFuelAndOxidantA power generation device in which the chemical energy of is directly converted into electrical energy through electrochemical reaction.In theory, fuel cells can operate at nearly 100% thermal efficiency, with high economy.Due to the limitations of various technical factors, and considering the energy consumption of the whole device system, the total conversion efficiency of various fuel cells currently in operation is mostly in the range of 45%~60%, for example, the heat removal utilization can reach more than 80%.In addition, the fuel cell device does not contain or contains few moving parts, works reliably, requires less maintenance, and is quieter than the traditional generator set.In addition, the chemical energy is directly converted into electrical energy in an isothermal electrochemical manner without going through a thermal engine process, which is not limited by the Carnot cycle. Therefore, the energy conversion efficiency is high, the reaction is clean and complete, and few harmful substances are produced. It is becoming an ideal way of energy utilization.At the same time, as the fuel cell technology continues to mature, and the West East Gas Transmission Project provides sufficient natural gas sources, all of these make the fuel cell regarded as a promising energy power device, and there is a broad prospect for the commercial application of fuel cells[2-3]。
Principle and composition
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Fuel cell is a kind of energy conversion device. Its composition is the same as that of ordinary batteries. It is based on the electrochemical principle, that is, the working principle of the primary cell. It directly converts the chemical energy stored in fuel and oxidant into electrical energy isothermal, so the actual process is redox reaction.Take the hydrogen oxygen fuel cell as an example to illustrate the fuel cell. The reaction principle of the hydrogen oxygen fuel cell is the reverse process of water electrolysis.The electrodes shall be:
The fuel cell is mainly composed of three parts, namely electrode, electrolyte diaphragm and external circuit (collector).
fuel cell
1. Electrode
Fuel gas and oxidation gas are respectively fed into the anode and cathode of the fuel cell.The electrode can be mainly divided into two parts, one is anode, the other is cathode, and the thickness is generally 200-500mm;Its structure is different from the flat electrode of ordinary batteries. The electrode of fuel cell is porous structure, so the main reason for the design of porous structure is that the fuel and oxidant used in fuel cell are mostly gases (such as oxygen, hydrogen, etc.), and the solubility of gas in electrolyte is not high. In order to improve the actual working current density of fuel cell and reduce polarization,Therefore, porous electrodes have been developed to increase the surface area of the electrodes involved in the reaction, which is one of the key reasons why the fuel cell was able to move from the theoretical research stage to the practical stage.The key to the electrode performance of fuel cell is the performance of catalyst, electrode materials and electrode manufacturing process. At present, the electrode of high-temperature fuel cell is mainly made of catalyst materials, such as the Y of solid oxide fuel cell (SOFC)twoOthree－stabilized－ZrOtwo(YSZ for short) and nickel oxide electrode of molten carbonate fuel cell (MCFC for short), while low-temperature fuel cell is mainly composed of gas diffusion layer supporting a thin layer of catalyst material, such as platinum electrode of phosphoric acid fuel cell (PAFC for short) and proton exchange membrane fuel cell (PEMFC for short)[4]。
2. Electrolyte diaphragm
The main function of the electrolyte diaphragm is to separate oxidant and reducing agent, and conduct ions, so the thinner the electrolyte diaphragm, the better, but also need to consider the strength. As far as the current technology is concerned, its thickness is generally about tens of millimeters to hundreds of millimeters.In order to prevent the short circuit in the battery caused by the mixing of two gases, the electrolyte is usually compact[3]。As for materials, there are mainly two development directions at present. First, asbestos film, silicon carbide SiC film, and lithium aluminate (LiAlOthree）The membrane and other insulating materials are made into a porous membrane, and then immersed into molten lithium potassium carbonate, potassium hydroxide and phosphoric acid to make it adhere to the membrane hole. The other is to use perfluorinated sulfonic acid resin (such as PEMFC) and YSZ (such as SOFC).In addition, since the electrolyte film used is solid, electrolyte corrosion can be avoided.
3. External circuit (collector)
Also known as Bipolar Plate, it has the functions of collecting current, separating oxidant and reducing agent, and channeling reaction gas. The performance of collector mainly depends on its material characteristics, flow field design and processing technology.
The fuel gas emits electrons on the anode, and the electrons pass through the external circuit to the cathode and combine with the oxidation gas to generate ions.Under the action of electric field, ions migrate to the anode through electrolyte, react with fuel gas, form a circuit, and generate current.At the same time, due to its own electrochemical reaction and the internal resistance of the cell, the fuel cell will also generate some heat.In addition to conducting electrons, the cathode and anode of the battery are also used as catalysts for redox reaction.When the fuel is hydrocarbon, the anode requires higher catalytic activity.In addition, only the fuel cell body can not work, and there must be a set of correspondingAuxiliary system, including reagent supply systemHeat removal system、drainage systemElectrical performanceControl systemAnd safety devices.
Fuel cell system
A fuel cell is usually composed of an electrolyte plate that forms an ionic conductor, fuel electrodes (anode) and air electrodes (cathode) configured on both sides of it, and gas flow paths on both sides. The role of the gas flow path is to enable fuel gas and air (oxidant gas) to pass through the flow path.In practical fuel cells, due to different working electrolytes, the types of ions related to the electrolyte and reaction are also different.
1. PAFC and PEMFC react with hydrogen ion (H+）Relevant, the reaction occurred is:
Fuel pole: Htwo==2H++2e-（1）
Air pole: 2H++1/2Otwo+2e-==HtwoO（2）
All: Htwo+1/2Otwo==HtwoO（3）
In the fuel pole, H in the supplied fuel gastwoDecomposition into H+And e-，H+O moved to electrolyte and supplied to air pole sidetwoReact.e-Through the external load circuit, it returns to the air pole side and participates in the reaction of the air pole side.A series of reactions contributed to-Through the external circuit uninterruptedly, power generation is formed.It can be seen from the above equation (3) that HtwoAnd OtwoGenerated HtwoO，There is no other reaction, HtwoThe chemical energy has been converted into electrical energy.However, in fact, there is a certain resistance with the reaction of the electrode, which will cause the generation of some heat energy, thus reducing the proportion of electrical energy converted.The set of batteries that cause these reactions is called a module, and the voltage generated is usually less than one volt.Therefore, in order to obtain large output, it is necessary to adopt the method of multi-layer superposition of components to obtain high voltage reactor.For the electrical connection between assemblies and the separation between fuel gas and air, components called diaphragms with gas flow paths in the upper and lower sides are used. The diaphragms of PAFC and PEMFC are made of carbon materials.The output of the stack is determined by the product of the total voltage and current, and the current is proportional to the reaction area in the battery.The electrolyte of PAFC isConcentrated phosphoric acidPEMFC electrolyte is a membrane of proton conducting polymer system.Carbon porous body is used as electrode. In order to promote reaction, Pt is used as catalyst. CO in fuel gas will cause poisoning and reduce electrode performance.Therefore, the CO content in fuel gas must be limited in the application of PAFC and PEMFC, especially for PEMFC working at low temperature.
2. The basic composition and reaction principle of phosphoric acid fuel cell are as follows: the fuel gas or city gas is sent to the modifier after adding water vapor to convert the fuel into Htwo, CO and water vapor mixture, CO and water are further converted intoHtwoAnd COtwo。After such treatment, the fuel gas enters the negative pole (fuel pole) of the fuel stack, and at the same time, it transmits oxygen to the positive pole (air pole) of the fuel stack for chemical reaction, and quickly generates electric energy and heat energy with the help of catalyst.
3. Compared with PAFC and PEMFC, high-temperature fuel cell MCFC and SOFC do not need catalyst. Coal gasification gas with CO as the main component can be directly used as fuel, and it is also easy to use its high-quality exhaust gas to form combined cycle power generation.
(1) Main components of MCFC: containing electrolyte related to electrode reaction (usually carbonate mixed with Li and K) and two electrode plates (fuel electrode and air electrode) connected with it up and down, as well as gas chamber and electrode clamp for fuel gas and oxidant gas flowing outside the two electrodes respectively. Electrolyte is liquid in molten state at the operating temperature of MCFC of about 600~700 ℃,An ionic conductor is formed.The electrode is a nickel based porous body, and the gas chamber is formed by corrosion resistant metal.Operating principle of MCFC: O of air poletwo(Air) and COtwoCombined with electricity to generate COthree2-（Carbonate ion）, electrolyte will change COthree2-Move to the fuel pole side, and compare with H as fuel supply+Combine and release e-, and generate H at the same timetwoO and COtwo。The chemical reaction formula is as follows:
Fuel pole: Htwo+COthree2-==HtwoO+COtwo+2e-（4）
Air pole: COtwo+1/2Otwo+2e-==COthree2-（5）
All: Htwo+1/2Otwo==HtwoO（6）
In this reaction, e-As in the case of PAFC, it is discharged from the fuel pole and returned to the air pole through the external circuit-The uninterrupted flow in the external circuit enables fuel cell power generation.In addition, the biggest feature of MCFC is that it must have CO that is conducive to reactionthree2-Therefore, the oxidant gas supplied must contain carbonic acid gas.In addition, catalyst is filled in the battery, so that CH, as the main component of natural gasfourIt is modified inside the battery, and H is directly generated inside the batterytwoThe method of has also been developed.When the fuel is gas, its main components are CO and HtwoO reaction generates HtwoTherefore, CO can be used as fuel equivalently.In order to obtain greater output, diaphragms are usually made of Ni and stainless steel.
(2) SOFC is mainly composed of ceramic materials, and ZrO is usually used as electrolytetwo(Zirconia）, which constitutes O2-Conductor Y oftwoOthree（Yttrium oxide）It is used as stabilized YSZ (stabilized zirconia).In the electrode, the fuel electrode uses Ni and YSZ composite porous body to form a cermet, and the air electrode uses LaMnOthree(lanthanum manganese oxide).Diaphragm adopts LaCrOthree(Lanthanum chromium oxide)。In order to avoid cracks caused by different shapes of batteries and thermal expansion differences between electrolytes, SOFCs operating at lower temperatures were developed.In addition to the flat type like other fuel cells, the cell shape has also developed a cylindrical type to avoid stress concentration.The reaction formula of SOFC is as follows:
Fuel pole: Htwo+O2-==HtwoO+2e-（7）
Air pole: 1/2Otwo+2e-==O2-（8）
All: Htwo+1/2Otwo==HtwoO（9）
Fuel pole, HtwoMoving through electrolyte, and O2-Reaction generates HtwoO and e-。Air pole from OtwoAnd e-Generate O2-。Like other fuel cells, HtwoAnd OtwoGenerate HtwoO。In SOFC, the main component CH of natural gas can be directly transferred internally without the action of other catalysts because it is of high temperature operation typefourModified to HtwoAnd CO, the main component of gas, can be directly used as fuel.
advantage
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Fuel cell is a device that directly converts the chemical energy of fuel into electric energy. Unlike ordinary batteries, active substances are stored inside the battery, which does not limit the capacity of the battery. Its positive and negative electrodes do not contain active substances, but are just catalytic conversion elements. Therefore, fuel cell is worthy of its namechemical energyConvert toelectric energyEnergy conversion machine.When the battery is working, the fuel and oxidant are supplied externally for reaction. In principle, as long as the reactants are continuously input and the reaction products are continuously discharged, the fuel cell can continuously generate electricity. It has been known as the fourth generation power generation technology after hydropower, thermal power and nuclear power[5]。
High power generation efficiency
Fuel cell power generation is not limited by Carnot cycle.Theoretically, its power generation efficiency can reach 85%~90%. However, due to the limitations of various polarizations during operation, the energy conversion efficiency of fuel cells is currently about 40%~60%.If the combined heat and power supply is realized, the total utilization rate of fuel can reach more than 80%[3]。
Small environmental pollution
When fuel cells use hydrogen rich gases such as natural gas as fuel, carbon dioxide emissions are reduced by more than 40% compared with heat engine processes, which is very important to alleviate the greenhouse effect of the earth.In addition, since the fuel gas of the fuel cell must be desulfurized before reaction, and electricity is generated according to the electrochemical principle, there is no high-temperature combustion process, so nitrogen and sulfur oxides are almost not emitted, reducing the pollution to the atmosphere[3]。
High specific energy
The specific energy of liquid hydrogen fuel cell is 800 times that of nickel cadmium battery, and the specific energy of direct methanol fuel cell is more than 10 times higher than that of lithium ion battery (rechargeable battery with the highest energy density).At present, although the actual specific energy of fuel cells is only 10% of the theoretical value, it is still much higher than the actual specific energy of ordinary cells[3]。
Less radiation
The fuel cell has simple structure, less radiation and less loss.Even near an 11MW fuel cell power plant, very little radiation is measured[3]。
Wide fuel range
For fuel cells, as long as the substances containing hydrogen atoms can be used as fuel, such as natural gas, petroleum, coal and other fossil products, or biogas, alcohol, methanol, etc., fuel cells are very suitable for the needs of energy diversification and can slow the depletion of mainstream energy[3]。
High reliability
When the load of the fuel cell changes, it will respond quickly.It can withstand overload operation above rated power or operation below rated power with little change in efficiency.Due to its highly reliable operation, fuel cells can be used as various emergency power supplies and uninterruptible power supplies[3]。
Easy to build
The fuel cell has an assembled structure, which is easy to install and maintain, and does not need many auxiliary facilities.The design and manufacture of fuel cell power stations are quite convenient[3]。
application
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Alkaline fuel cell (AFC) is the earliest developed fuel cell technology, which was successfully applied in the field of space flight in the 1960s.
Phosphoric acid fuel cell (PAFC) is also the first generation fuel cell technology, which is the most mature application technology at present. It has entered commercial application and batch production. Due to its high cost, it can only be used as a regional power station to supply power and heat on site.
Molten carbonate fuel cell (MCFC) is the second generation fuel cell technology, which is mainly used for equipment power generation.
Solid oxide fuel cell (SOFC), with its all solid state structure, higher energy efficiency and wide adaptability to gas, natural gas, mixed gas and other fuel gases, has developed fastest and been widely used, becoming the third generation fuel cell[6]。
As a high-tech product in the 21st century, fuel cells have been used in the automotive industry, energy generation, shipbuilding industry, aerospace, household power supply and other industries, and have attracted the attention of governments around the world[3]。
Several fuel cells
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SOFC
Solid oxide fuel cell (SOFC) is a kind of all solid state energy conversion device that directly converts the chemical energy in fuel gas and oxidation gas into electrical energy. It has the structure of general fuel cell.Solid oxide fuel cell uses dense solid oxide as electrolyte, operates at high temperature of 800~1000 ℃, and the reaction gas does not contact directly[7]Therefore, a higher pressure can be used to reduce the volume of the reactor without the risk of combustion or explosion.
RFC
Hydrogen fuel cell uses hydrogen as fuel, and generates electricity through proton exchange membrane after electrochemical reaction with oxygen.Hydrogen reacts with oxygen to generate water, which does not discharge pollutants such as hydrogen carbide, carbon monoxide, nitride and carbon dioxide, and is pollution-free with high power generation efficiency.At present, the power generation thermal efficiency of hydrogen fuel cell can reach 65%~85%, the weight energy density is 500~700 Wh/kg, and the volume energy density is 1000~1200 Wh/L, and the power generation efficiency is higher than that of solid oxide fuel cell[8]。The hydrogen fuel cell operates at 30~90 ℃. The startup time is very short. It can reach full load within 0~20 seconds. The service life can reach 10 years. There is no vibration, no exhaust emissions, and the cost of mass production can be reduced to 100~200 dollars/kW[9]。When hydrogen fuel cells are used in electric vehicles, compared with fuel vehicles, they are superior to existing vehicles in all aspects except cost.As long as the cost is further reduced, it is expected that practical electric vehicles will be available soon.
There are two main reasons why hydrogen fuel cells have not yet been fully industrialized.First, how to make hydrogen.There are various ways to produce hydrogen, which can be obtained by chemical methods such as reforming, decomposing, photolysis or hydrolysis of compounds, or by electrolysis of water, or by fermentation or photosynthesis of hydrogen producing microorganisms.Among them, hydrogen production by electrolysis of water is a completely clean hydrogen production method, but this method has a large energy consumption, and its application in on-site hydrogen production is limited. At present, it is still in further research and development.The biological hydrogen production method uses organic waste as raw material to produce hydrogen through photosynthesis or bacterial fermentation.However, the hydrogen production mechanism of this method is not well understood at present, and many problems such as strain cultivation, bacterial metabolic pathway, and bacterial hydrogen production conditions still need to be studied. In general, it is not yet mature.At present, the main large-scale hydrogen production method is to heat coal, oil and natural gas as raw materials to produce hydrogen, which requires 800 ℃. This high temperature, reformer and other equipment need special materials, and is not suitable for small-scale hydrogen production.Recently, methanol steam conversion has been developed to produce hydrogen. This hydrogen production method has low reaction temperature (260~280 ℃), mild process conditions, and energy consumption of about 50% of the former[10]Therefore, the conversion of methanol to hydrogen has become a research hotspot in this field.In addition, the research on metal hydride hydrogen storage and adsorption hydrogen storage technology also provides a way for on-board hydrogen storage and hydrogen production[11]。
DMFC
Proton exchange membrane fuel cells (PEMFCs) that directly use methanol as fuel are usually called direct methanol fuel cells (DMFCs).The membrane electrode is mainly composed of methanol anode, oxygen cathode and proton exchange membrane (PEM).The anode and cathode are respectively composed of stainless steel plate, plastic film, copper current collection plate, graphite, gas diffusion layer and porous catalytic layer.The gas diffusion layer plays the role of supporting the catalytic layer, collecting current and conducting reactants, and is composed of carbon paper or carbon cloth with conductive function;The catalytic layer is the place for electrochemical reaction, and the commonly used anode and cathode electrode catalysts are PtRu/C and Pt/C respectively.
The direct methanol fuel cell does not need an intermediate conversion device, so the system has simple structure, high volume energy density, short starting time, good load response characteristics, high operation reliability, normal operation in a large temperature range, and convenient fuel supplement.The application field is very wide, mainly including: (1) portable mobile power supply for field operation or military field;(2) 50~1000kW fixed power generation equipment;(3) Future electric vehicle power source;(4) Mobile communication equipment power supply[3]。
In recent years, micro DMFC and military fuel cell have been close to practical use, but there are still many technical problems in terms of poor anode catalyst activity, lack of reasonable methanol and carbon dioxide bypass channels in the anode catalyst layer, and prevention of methanol penetration from anode to cathode[12]。In view of these problems, some solutions are also proposed.In terms of catalyst activity, noble metal binary and ternary alloy catalysts are used to improve the ability to resist CO poisoning or non noble metal catalysts are searched to improve the catalyst activity.For the problem that some CH3OH passes through PEM and reacts with O2 directly without generating current, methanol diffusion can be reduced by reducing the diffusion coefficient of CH3OH in PEM, improving or developing new PEM, and the cell efficiency can be improved[13]。With the improvement of fuel conversion efficiency, power density, reliability and cost reduction of DMFC, DMFC will become an ideal fuel cell in the future[3]。
Research and development
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Fuel cell research in China began in 1958, when the former Ministry of Electronics IndustryTianjin Power Supply Research InstituteThe earliestMCFCResearch.In the 1970s, with the promotion of the aerospace industry, China's fuel cell research showed its first climax.between whichChinese Academy of SciencesTwo types of alkaline asbestos membrane hydrogen oxygen fuel cell systems (kW level AFC) developed by Dalian Institute of Chemical Physics have passed the routine aerospaceEnvironmental simulation test。1990Changchun Institute of Applied Chemistry, Chinese Academy of SciencesHe undertook the research task of PEMFC of the Chinese Academy of Sciences, and began the research of direct methanol proton exchange membrane fuel cell (DMFC) in 1993.In 1991, Harbin Power Station Complete Equipment Research Institute of the Ministry of Electric Power Industry developed a MCFC principle battery composed of seven single batteries.During the Eighth Five Year Plan period, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Shanghai Institute of Silicate, Institute of Chemical MetallurgyTsinghua UniversityMore than ten domestic units have conducted research on SOFC.By the mid-1990s, China had entered the second climax of fuel cell research due to the promotion of the Ministry of Science and Technology of the People's Republic of China and the Chinese Academy of Sciences to include fuel cell technology in the Ninth Five Year Plan.In China, scientists have made a lot of progress in basic research and single technology of fuel cells, and accumulated some experience.However, due to the small amount of investment in fuel cell research over the years, there is still a big gap with developed countries in terms of the overall level of fuel cell technology.The relevant departments and experts in China attach great importance to fuel cellsxiangshan science conference The panel discussed the development of fuel cell technology in China, emphasizing the importance and necessity of independent research and development of fuel cell systems.In recent years, China has strengthened its efforts inPEMFCResearch efforts in.In 2000, Dalian Institute of Chemical Physics and Institute of Electrical Engineering of Chinese Academy of Sciences completed all the tests of 30kW fuel cells for vehicles.Vice Minister of the Ministry of Science and TechnologyXu GuanhuaIt was announced at the 16th EVS Conference that China will install the first fuel cell in 2000Electric vehicle。On July 10, 2020, the famous journal Science was published to the scientific research team of China University of Geosciences (Wuhan)Academic papers, announced that the proton was confined to the heterogeneous interface through the electronic state characteristics of semiconductor heterogeneous interface, and designed and constructed a proton channel with low migration barrier.High ionic conductivityelectrolyteDevelopment is the key to solve the current application of fuel cells.The research of the scientific research team of China University of Geosciences (Wuhan) is like building a highway for protons, that is, using semiconductor heterogeneous interface fields to induce metal states and boost the super protons to achieve fast and good "run up", so as to obtain excellent conductivity.
