Research progress and application prospects of fuel cells

Energy issues have been receiving widespread attention in recent years. With the continuous use of the three major fossil energy sources, energy reserves, over-exploitation, and environmental problems are becoming more and more serious. According to a survey by the World Energy Organization, mineral energy including coal, oil, and natural gas will be exhausted in the next 100 to 200 years, and new energy utilization technologies will be continuously developed and utilized. Fuel cells are a new energy source with great potential.

1 Overview of Fuel Cells

A fuel cell is a device that uses fuel to generate electrical energy through chemical reactions. The fuels used include pure hydrogen, methanol, ethanol, natural gas, and gasoline, which is the most widely used now. The most common is the proton exchange membrane fuel cell that uses hydrogen and oxygen as fuel. Because the fuel is cheap, has no chemical hazards, and does not pollute the environment, it generates pure water and heat after power generation, which is currently impossible for all other power sources.

Since the amount of electricity generated by fuel cells is small and cannot provide a large amount of electricity instantly, it can only be used for stable power supply. At present, some laptops have begun to study the use of fuel cells. Using fuel cells as the power of automobiles has been recognized as an inevitable trend in the 21st century.

Fuel cells use combustible fuels to react with oxygen to generate electricity. Usually, combustible fuels such as gas, gasoline, methane, ethanol, hydrogen and other combustible materials need to be burned to heat water, so that the water boils to produce water vapor and drives the turbine to generate electricity. Most of the energy in this conversion method is usually converted into useless heat energy, and the conversion efficiency is quite low, only about 30%; fuel cells use special catalysts to react fuel with oxygen to produce carbon dioxide and water. Because there is no need to drive turbines and other power generation equipment, there is no need to heat water into water vapor and then dissipate heat to turn it back into water, so the energy conversion efficiency is as high as about 70%, which is 40% higher than the general energy utilization method, and the carbon dioxide emissions are much lower than the general method. Water is a harmless product, so it is also a low-pollution energy.

2 Classification of fuel cells

(1) According to the operating mechanism of fuel cells, they can be divided into acid fuel cells and alkaline fuel cells.

(2) According to the type of electrolyte, fuel cells can be divided into alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, proton exchange membrane fuel cells, etc. Among fuel cells, phosphoric acid fuel cells and proton exchange membrane fuel cells can be cold started and quickly started, and can be used as mobile power sources to meet the requirements of special situations, making them more competitive.

(3) According to the type of fuel, there are gas fuels such as hydrogen, methane, ethane, butylene, butane and natural gas; organic liquid fuels such as methanol, toluene, gasoline and diesel. Organic liquid fuels and gas fuels must be "reformed" into hydrogen by a reformer before they can become fuel for fuel cells.

(4) According to the operating temperature of fuel cells, there are low-temperature types with operating temperatures below 200°C; medium-temperature types with operating temperatures between 200 and 750°C; and high-temperature types with operating temperatures above 750°C.

Fuel cells that work at room temperature, such as proton exchange membrane fuel cells, require precious metals as catalysts. Most of the chemical energy of the fuel can be converted into electrical energy, only a small amount of waste heat and water is generated, and no nitrogen oxides that pollute the atmospheric environment are generated. No waste heat energy recovery device is required, and the volume is small and the weight is light. However, the catalyst platinum will react with carbon monoxide in the working medium to produce "poisoning" and fail, which will reduce the efficiency of the fuel cell or completely damage it. In addition, the price of platinum is very high, which increases the cost of the fuel cell.

Another type is a fuel cell that works at high temperature (600~1000℃), such as molten carbonate fuel cells and solid oxide fuel cells. This type of fuel cell does not require precious metals as catalysts. However, due to the high working temperature, a composite waste heat recovery device is required to utilize waste heat. It is large in size and heavy in weight, and is only suitable for use in high-power power plants.

3 Research progress of fuel cells

3.1 Domestic research status

As early as the 1950s, China carried out research on fuel cells and made many breakthroughs in the innovation of key materials and key technologies for fuel cells. The government attaches great importance to the research and development of fuel cells, and has successively developed 30 kW hydrogen-oxygen fuel electrodes, fuel cell electric vehicles, etc. Fuel cell technology, especially proton exchange membrane fuel cell technology, has also developed rapidly, and proton exchange membrane fuel cell stacks of various specifications such as 60 kW and 75 kW have been developed one after another. The development of fuel cell engines with a net output of 40 kW for electric cars and 100 kW for city buses has made China's fuel cell technology enter the ranks of advanced countries in the world.

The development and research of various components of proton exchange membrane fuel cells in China have made great progress. Among them, in terms of catalysts: researchers from Tsinghua University have developed a new platinum/carbon electrode catalyst. The carbon carrier is placed in carbon monoxide for activation treatment before use, that is, the carbon carrier is placed in flowing carbon monoxide gas and heated to 350~900 ℃, activated for 1~12 hours, and then Pt is loaded on the carbon carrier by precipitation method to obtain Pt/C catalyst; Changchun Institute of Applied Chemistry has developed a nano-scale high-activity electrocatalyst for use as an anode catalyst. The catalyst has uniform particle size, with a particle diameter of about (4±0.5) nm, and its electrochemical performance is better than similar international products. Fudan University uses a precipitation method to prepare a nano-loaded wall platinum/carbon catalyst in the presence of a surfactant, and the effect of this catalyst is very good. In addition, when studying catalysts, Chinese researchers generally add powdered activated carbon to a chloroplatinic acid solution, and then add excess formaldehyde to reduce it. In the reaction, palmitic acid, stearic acid or silicone oil is used as a surfactant, and the doping component is one of the metal elements or non-metallic substances such as Pd, Ir, Ru, etc.

In terms of electrode assembly: Beijing Century Fuyuan Fuel Cell Co., Ltd. has developed a horizontal plate coating method to make a fuel cell with multiple membrane electrodes on a piece of proton exchange membrane. Multiple membrane electrodes are composed of a piece of proton exchange membrane, multiple catalyst layers and multiple diffusion layers, and multiple power generation units are composed of multiple membrane electrodes and multiple guide plates; Beijing Solar New Technology Co., Ltd. has developed a ceramic inorganic composite thick membrane electrode, the mass percentage of the components in the material is: graphite 25%~30%, Ag 25%~30%, PbO 30%~35%, BO 6%~8%, SiO22%~4%, metal or non-metal and conductive powder and other oxides composed of inorganic binders are mixed, screen printed, sintered, to form a microscopic network conductive channel.

In terms of proton exchange membrane: Tsinghua University has developed polyvinylidene fluoride grafted polystyrene sulfonic acid PEM. Polyvinylidene fluoride is dissolved in methyl pyrrolidone solvent, and the polymer solution is heated to the boiling temperature of methyl pyrrolidone, refluxed at this temperature for 0.5~5 hours, and the temperature is reduced to 90 ℃, and then an initiator is added to the solution. After keeping it at 90 ℃ for 1~5 hours, it is reduced to room temperature, and chloroform is added to the solution until all insoluble solids are precipitated. The solid is taken out, the initiator is added, and then the proton exchange membrane is obtained after treatment.

In terms of bipolar plates: Tianjin Power Supply Research Institute has developed a utility model bipolar plate, which includes a metal plate gas reaction area, a gas inlet, and a gas outlet. Grooves are provided around the upper and lower gas reaction areas of the metal plate, and dark channels are provided between the gas inlet, the gas outlet and the gas reaction area. This design improves the sealing of the battery pack, prolongs its life and improves performance; the bipolar plate developed by Dalian Institute of Chemical Physics is composed of three layers of thin metal plates, with a separator in the middle that conducts electricity but does not allow gas to pass through liquid, and guide plates with strip grooves on both sides, which account for 50% to 80% of the entire working area. This novel design improves the utilization rate of the reaction gas, thereby improving battery performance.

In terms of electrolytes: Jilin University has developed a solid composite electrolyte, which is synthesized from the matrix material Ce1-xRexO2-d and metal compounds of Ni, Al, Co, Na, Ca, K or NiAl compound additives, and is made through mixing, grinding, sintering, cooling, crushing, grinding and other processes. It is directly pressed into thin sheets using a mold, and the strength after sintering can reach 10 MPa. It can be used as a PEMFC electrolyte, and a variety of fuels such as methanol, ethanol, methane and ethane can be used; Shanghai Jiaotong University has developed a new electrolyte-polyaryletherketone with sulfonate side groups and carboxylate side groups, which can be used as a cationic component of PEM.

3.2 Research status abroad

Developed countries have made the development of large-scale fuel cells a key research project, and the business community has also invested heavily in the research and development of fuel cell technology. Now many important results have been achieved, making fuel cells about to replace traditional generators and internal combustion engines and widely used in power generation and automobiles. It is worth noting that this important new power generation method can greatly reduce air pollution and solve the problems of power supply and grid peak regulation. 2 MW, 4.5 MW and 11 MW complete sets of fuel cell power generation equipment have entered commercial production, and fuel cell power plants of various levels have been built in some developed countries.

The high efficiency, pollution-free, short construction period, easy maintenance and low cost of fuel cells will ignite the green revolution of new energy and environmental protection in the 21st century. Today, in North America, Japan and Europe, fuel cell power generation is rapidly entering the stage of industrial-scale application with a rapid momentum of catching up, and will become the fourth generation of power generation after thermal power, hydropower and nuclear power in the 21st century. The rapid development of fuel cell technology abroad must attract our sufficient attention. Now it has become a topic that the energy and power industries have to face up to.

Since alkaline fuel cells often use air as an oxidant in actual use, they will be poisoned by CO2 and greatly reduce efficiency and service life. Therefore, people believe that alkaline fuel cells are not suitable for automotive power and other aspects, and have shifted their research focus to proton exchange membrane fuel cells. Only a few institutions are still studying alkaline fuel cells. In order to solve the problem of alkaline fuel cells, a lot of research work has been carried out abroad.

E. Gulzow et al. found that when the electrode is prepared by a special method, it can operate normally under conditions of high CO2 content without being poisoned. In the preparation of the electrode, the catalyst material is mixed with PTFE (polytetrafluoroethylene) fine particles at high speed, and the small PTFE particles with a particle size of less than 1 mm are covered on the catalyst surface, which increases the strength of the electrode and prevents the electrode from being completely submerged by the electrolyte, reducing the possibility of carbonate precipitation blocking the micropores and causing mechanical damage to the electrode. In addition, gas is allowed to enter the electrode to form a three-phase zone in the area where the electrochemical reaction occurs; S. Rahman et al. combined the dry method and wet method of conventional electrode preparation and proposed a filtration method to optimize the performance of the electrode by controlling the PTFE content and grinding time. Studies have shown that when the PTFE content is 8% (mass fraction) and the grinding time is 60 s, the electrode performance is the best. Through the new electrode preparation method, alkaline fuel cells can withstand higher CO2 concentrations; E. Gulzow et al. added 5% CO2 to oxygen and conducted experiments on alkaline fuel cell electrodes for 3,500 consecutive hours. No effect of CO2 on the life and performance of the electrode was found, indicating that the new electrode preparation method can solve the problem of CO2 poisoning of the electrode.

In addition, some people have proposed using ammonia as a hydrogen source to avoid the problem of CO2 poisoning. Ammonia can be liquefied at room temperature at only 8~9 MPa, does not require high energy consumption, and is low in price. There is a relatively complete production and transportation system, while the use of hydrogen requires a long time for infrastructure construction. Ammonia has a strong pungent smell and its leakage is easy to detect. Compared with other fuels, ammonia is cleaner and will not cause damage to the land. The explosion range of ammonia is relatively small, only 15%~28% (volume fraction), which is relatively safe. In the use of alkaline fuel cells, only one reformer needs to be added to the fuel inlet to decompose NH3 into N2 and H2. Therefore, NH3 is expected to be widely used in alkaline fuel cells and has a good development prospect.

3.3 The latest achievements of fuel cells

Mobion fuel cells use MTIMicro's Mobion patented chip to simplify the complex structure inside the fuel cell, which can also effectively reduce the cost of the fuel cell. This chip integrates a fluid energy module, which allows the fuel cell to be used under temperature conditions between 0~40 ℃ and any humidity conditions. The Mobion chip uses a 100% methanol design and passive direct methanol fuel cell technology. The volume of the entire chip is only 9 cm3, which can be easily embedded in various portable electronic products. MTIMicro said that the Mobion fuel cell can provide 0.050 W/cm2 and 1.4 (W·h)/cm3 of energy, and the weight of the entire chip is less than 29 g.

4 Application prospects of fuel cells

The characteristics of fuel cells determine that it has broad application prospects. It can be used as a small power generation device; as a long-lasting battery; and used in electric vehicles.

Fuel cells are used as power generation devices because their prices may compete with general power generation devices. However, the commercial application prospects of fuel cells in electric vehicles are long-term, because cars need generators, and the price of generators is much cheaper than fuel cells. Therefore, in the short term, fuel cell cars are difficult to compete with other cars in terms of price.

At present, fuel cell research and development focuses on four aspects: (1) electrolyte membrane; (2) electrode; (3) fuel; (4) system structure. Japanese, American and European manufacturers develop fuel cells for portable electronic devices, especially focusing on material research and development in aspects (1) to (3). The fourth research topic is the system structure of the fuel cell. The first three aspects are necessary preparations for the construction of the fuel cell, while the system structure is the final result of the fuel cell.

The development, research and commercialization of fuel cells, especially solid oxide fuel cells, is an important means to solve the world's energy conservation and environmental protection problems. It has received widespread attention from many countries in the world, including the United States, Europe, Japan, Australia, and South Korea. Although there are still some problems in the application of solid oxide fuels, such as electrode materials, manufacturing costs, and high operating temperatures, the advantages outweigh the disadvantages. Accelerating the development of solid oxide fuel cells is bound to be the general trend of world energy development.

Lowering the battery operating temperature and miniaturization are the development trends of solid oxide fuel cells. The material preparation of its key components has become a bottleneck restricting the development of solid oxide fuel cells. The key technologies that need to be broken through are: (1) high-performance electrode materials and their preparation technology; (2) preparation technology of new electrolyte materials and electrode-supported electrolyte membranes; (3) battery structure optimization design and its preparation technology; (4) research on battery structure, performance and characterization.

5 Conclusion

The research and development of fuel cells has brought a profound revolution to portable electronic devices, and will also affect the centralized power supply systems in the automotive industry, residential and all aspects of society. It will bring people from centralized power supply to a new era of decentralized power supply. Although solar power can replace some energy sources, it is restricted by weather and climate, and the use of nuclear energy has safety issues. Fuel cell power supply does not emit carbon dioxide, which solves the problem of global environmental pollution caused by thermal power generation. It is a pure green and clean energy.

As the bottlenecks of key fuel cell technologies are resolved and new technologies are developed, researched and commercialized, the developing fuel cell technology will surely accelerate the pace of my country's economic construction and sustainable development.