At present, the economic development model based on the consumption of coal, oil and natural gas has led to the increasingly prominent problems of environmental pollution and greenhouse effect. In order to realize the sustainable development of human society, establish a harmonious relationship between man and nature, and develop green energy such as wind energy, hydropower, solar energy, biomass energy, geothermal energy and marine energy, it has become a topic of great concern all over the world. Hydrogen fuel cell is the key carrier to realize the conversion of hydrogen energy into electric energy. After the goal of carbon neutralization and carbon peak has been put forward, it has received new high attention at the level of basic research and industrial application.
Hydrogen fuel cell is a power generation device that directly converts the chemical energy of hydrogen into electric energy through electrochemical reaction. The single cell is composed of positive and negative electrodes (hydrogen fuel electrode, oxidant electrode) and electrolyte. Hydrogen oxidation reaction and redox reaction occur on both sides of the electrolyte diaphragm respectively, and the electrons do work through the external circuit to generate electric energy. As long as hydrogen fuel and oxidant (pure oxygen or air) are continuously input, the fuel cell will continuously generate electric energy from the energy source. Such working characteristics determine that hydrogen fuel cell has the characteristics of both ordinary chemical cell and internal combustion engine. It has the advantages of high energy conversion efficiency and no emission of environmental pollutants.
Hydrogen fuel cell is different from common lithium battery, and the system is more complex. It is mainly composed of electric stack and system components (air compressor, humidifier, hydrogen circulating pump and hydrogen bottle). The stack is the core of the whole battery system, including each battery unit composed of membrane electrode and bipolar plate, as well as collector plate, end plate, sealing ring and so on. The key materials of membrane electrode are proton exchange membrane, catalyst and gas diffusion layer. The durability (and other properties) of these components and materials determine the service life and working condition adaptability of the stack. In recent years, the research on hydrogen fuel cell technology has focused on electric stack, bipolar plate, control technology, etc. the hydrogen fuel cell technology system and some related cutting-edge research are shown in the figure.
Hydrogen fuel cell technology system
Hydrogen fuel cell technology system
Components with different materials and structures have different working characteristics, which have a decisive impact on the output characteristics and working life of hydrogen fuel cells.
membrane electrode
In the hydrogen fuel cell system, membrane electrode (MEA) is the core component, which is usually composed of cathode diffusion layer, cathode catalyst layer, electrolyte membrane, anode catalyst layer and anode gas diffusion layer, which directly determines the power density, durability and service life of hydrogen fuel cell. According to different electrolytes in MEA, common hydrogen fuel cells are divided into:
Alkaline fuel cell (AFC);
Molten carbonate fuel cell (MCFC);
Phosphoric acid fuel cell (PAFC);
Solid oxide fuel cell (SOFC);
Proton exchange membrane fuel cell (PEMFC);
Various types of fuel cells have corresponding fuel types, mass specific power and area specific power performance. Among them, proton exchange membrane fuel cell has become a research hotspot and the first choice for hydrogen fuel cell vehicles to enter the commercialization process with short start-up time (~ 1min), low operating temperature (< 100 ℃), compact structure and high power density. Mea assembly process includes hot pressing method (PTFE method), gradient method, CCM (catalyst coated membrane) and ordering method. Hot pressing is the first generation technology; At present, the second generation CCM method is widely used, including transfer printing, spraying, electrochemical deposition, dry powder spraying, etc., which has the advantages of high platinum utilization and durability; The ordering method can make mea have the maximum reaction active area and pore connectivity, so as to achieve higher catalyst utilization. It is the frontier direction of the new generation of MEA preparation technology.
Proton exchange membrane
Proton exchange membrane is mainly used to transfer protons (hydrogen ions) and isolate fuels and oxidants. Therefore, proton exchange membrane needs to have good proton conductivity, chemical stability and mechanical stability. Therefore, ideal membrane materials are those that show high ionic conductivity and prevent the intersection of electron transmission with hydrogen and oxygen.
The types of proton exchange membranes used at home and abroad include perfluorosulfonic acid PEM, partially fluorinated sulfonic acid PEM and new non fluorinated polymer PEM. Perfluorosulfonic acid PEM has high chemical stability and is most widely used at present. For example, Toyota Mirai, honda clarity, Hyundai NEXO and domestic Xinyuan all adopt Gore mechanical reinforced composite membrane, which is mainly composed of ePTFE (reinforced expanded polytetrafluoroethylene) + new fluorinated ion polymer. At present, the proton exchange membrane of hydrogen fuel cell is basically monopolized and controlled by foreign countries, and domestic enterprises or research units are still in the stage of research.
Types, advantages and disadvantages of proton exchange membrane
Types, advantages and disadvantages of proton exchange membrane
Bipolar plate
Bipolar plates (BPS) in hydrogen fuel cells, also known as flow field plates, play the role of separating reaction gas, removing heat and discharging chemical reaction products (water); It shall meet the requirements of high conductivity, good thermal conductivity and gas compactness, excellent mechanical and corrosion resistance, etc. Bipolar plate is not only the decisive factor of the volume and mass density of hydrogen fuel cell, but also an important factor affecting the manufacturing cost. At present, there are mainly several types of porous graphite bipolar plates, metal bipolar plates and composite bipolar plates on the market. Graphite bipolar plates have good conductivity, but they are brittle and difficult to be produced in large quantities and used for a long time. Metal bipolar plates have high strength, good toughness, good conductivity and thermal conductivity, but they are easy to be corroded in PEMFC working environment, Composite bipolar plate combines the advantages of graphite plate and metal plate. It has the advantages of corrosion resistance, small volume, light weight and high strength. It has developed into the mainstream technology.
Common types of bipolar plates on the market
Common types of bipolar plates on the market
Foreign Toyota Mirai and honda clarity use titanium bipolar plates to improve power density by optimizing fluid channel and improving gas diffusion. At present, it has reached 3.1kw/l. Domestic companies such as Beijing hydrogen Pu and Shanghai Shenli have developed metal plate hydrogen fuel cells with a power density of more than 2kW / L.
catalyzer
Catalyst is the material to accelerate the electrochemical reaction of hydrogen fuel cell. It can reduce the activation energy and improve the reaction rate. The combination of transition metal elements and platinum alloys is often used at home and abroad to enhance the stability and corrosion resistance of catalysts, such as Pt Co / C and Pt Ni / C. The most common catalysts for hydrogen fuel cells are platinum alloys, nanostructured thin film alloys, graphitized catalysts, etc. due to the limitations of catalyst basic material technology and preparation process, the platinum load of Japan's advanced proton exchange membrane hydrogen fuel cells is only 1 / 3 ~ 1 / 4 of that in China, and the platinum load of domestic hydrogen fuel cell stacks is about 1g / kW, The hydrogen fuel cell carried by Toyota Mirai is only 0.30g/kw. This leads to the high cost of domestic stacks, so the domestic research on hydrogen fuel cell catalysts focuses on how to reduce the platinum load.
Air compressor
Fuel cells usually require a good air supply to function. Air compressors are used to push air into fuel cell stacks. The fuel cell extracts oxygen from the incoming air to react at the cathode. The development and application of special air compressor for hydrogen fuel cell abroad is more mature than that in China. There are many air compressors used in hydrogen fuel cell system, including turbine air compressor, centrifugal compressor, screw air compressor and so on. Specific applications include DOE and author Little cooperates to design the turbine air compressor applied to 50KW, the screw air compressor applied to Mercedes Benz A-class hydrogen fuel cell by Daimler, and the centrifugal compressor applied to 65kW hydrogen fuel cell system studied by Tongji University. Screw air compressor has high quality and noise; Therefore, the efficiency and noise of centrifugal air compressor are the best development direction in the future.
Hydrogen storage tank
At present, the storage methods of pure hydrogen mainly include high-pressure hydrogen storage, liquid hydrogen storage, activated carbon adsorption of hydrogen, metal hydrogen storage and carbon nano material hydrogen storage. High pressure hydrogen storage method is mainly selected for vehicle hydrogen storage, so a safe and large amount of hydrogen storage tank is particularly important. The main types of on-board hydrogen cylinders include metal cylinder, metal liner circumferential winding cylinder, metal liner full winding cylinder, plastic liner full winding cylinder, etc. Due to its physical characteristics, metal cylinder has low hydrogen storage pressure and high quality. At the initial stage in China, it is used to store 35MPa high-pressure hydrogen. In order to improve its disadvantages, the types of metal liner circumferential wound cylinder, metal liner fully wound cylinder and plastic liner fully wound cylinder are gradually produced, and the hydrogen storage pressure is increased to 70MPa.
For example, the 70 MPa hydrogen storage tank on Toyota's third generation Mirai hydrogen fuel cell vehicle adopts a three-layer structure, and the inner layer is a resin lining that seals hydrogen; The middle layer is a carbon fiber reinforced resin layer to ensure the compressive strength; The surface layer is a glass fiber reinforced resin layer to protect the surface. Special winding processes and methods are adopted to strengthen and lighten the hydrogen storage tank, such as the fiber winding process of applying tension to the carbon fiber impregnated with resin to make it roll up and stack; The circumferential winding, high angle spiral winding and low angle spiral winding methods strengthen the cylinder, edge and bottom of hydrogen storage tank respectively, so that the quality efficiency is increased by 20% compared with the original, reaching the highest level of 5.7% in the world.
In recent years, China's basic research on hydrogen fuel cell technology is more active, and it has the conditions to "match" with developed countries in some technical directions; However, on the whole, the core technology level and comprehensive technology system mastered are not as good as those in leading countries. At present, China has put forward the development vision of achieving carbon peak in 2030 and carbon neutrality in 2060. Actively developing hydrogen energy and guiding the transformation of high carbon emission hydrogen production process to green hydrogen production process is an important measure for energy innovation and development to achieve carbon peak and carbon neutralization.
The research shows that hydrogen energy and hydrogen fuel cell technology are expected to be widely used in the fields of automobiles, portable power generation and fixed power stations. Hydrogen energy will be a strategic emerging industry in China's energy field, and hydrogen fuel cell technology will also be a prerequisite for realizing the utilization of hydrogen energy. In the future, hydrogen fuel cells will be further improved in battery life and density, cold start performance, fuel storage equipment and thin film materials, and application range.
