Fuel cell

1. Why research fuel cells

Hydrogen fuel cells have the advantages of high fuel energy conversion, low noise and zero emission, and can be widely used in vehicles such as automobiles, aircraft, trains and fixed power stations. Since the application of fuel cells in manned spaceflight, underwater submarines, distributed power stations, fuel cells have been the concern of governments and enterprises, in the future, the proportion of coal electricity is relatively low, due to the increase in the scale of renewable energy technologies such as wind energy, solar energy, the entire upstream power structure will be more and more clean. Fuel cells have several advantages over the conventional combustion technology currently used in many power plants and passenger cars:

First, the power generation efficiency is as high as 50% to 60%, if it can be combined to form a cyclic power generation system, the power generation efficiency can be as high as 70% or more;

Second, compared with traditional thermal power generation, fuel cells are less polluting to the environment.

Third, because the fuel cell has fewer internal components, it will not produce large noise during operation, and the general noise is 50dB ~ 70dB.

2. Working principle and system composition

2.1 Working Principle

The power generation principle of a fuel cell is similar to that of a primary or secondary battery. Hydrogen oxidation and oxygen reduction reactions occur on both sides of the electrolyte diaphragm, and electrons work through the external circuit, and the reaction product is water (Figure 1). However, unlike the primary cell, the reactants in the fuel cell are not stored in the battery in advance, but the product is discharged after the reaction is passed into the fuel gas and oxidation gas, so the fuel cell is not an energy storage device but a conversion device, and its electrodes and electrolytes are not directly involved in the reaction during the reaction.

2.2 System Composition

Fuel cell power generation requires a relatively complex system (FIG. 2). In addition to fuel cell stacks, it also includes fuel supply subsystem, oxidizer supply subsystem, hydrothermal management subsystem and electrical management and control subsystem, etc. The main system components include air compressor, humidifier, hydrogen circulation pump, high-pressure hydrogen bottle, etc. These subsystems and fuel cell stacks (or modules) constitute the fuel cell power generation system. The complexity of fuel cell systems poses challenges for operational reliability.

Fuel cell stack

Fuel cell stack is the core of fuel cell power system. It produces direct current (DC) electricity through an electrochemical reaction in a fuel cell. A single fuel cell generates a current of less than 1v, so a single fuel cell is usually connected in series into a fuel cell stack, and a typical fuel cell stack may consist of hundreds of fuel cells. The amount of energy produced by a fuel cell depends on several factors, such as the fuel cell type, cell size, operating temperature, and gas pressure supplied to the cell.

Fuel processor

The fuel processor converts the fuel into a form that the fuel cell can use. Depending on the fuel and fuel cell type, the fuel processor can be a simple adsorbent bed that removes impurities, or a combination of multiple reactors and adsorbents.

Power regulator

Power regulation includes controlling current characteristics such as current (amperage), voltage, and frequency to meet the needs of the application. Fuel cells generate electricity in the form of direct current (DC). On a direct current circuit, electrons flow in only one direction. If a fuel cell is used to power a device that uses alternating current, direct current must be converted to alternating current.

Air compressor

The fuel cell performance increases with the increase of reactant gas pressure. Therefore, many fuel cell systems include an air compressor, which can increase the inlet air pressure to 2 to 4 times the ambient atmospheric pressure. For transportation applications, the efficiency of the air compressor should be at least 75%. In some cases, an expander is also included to restore power from high-pressure exhaust gases. The efficiency of the extender should be at least 80%.

humidifier

The core polymer electrolyte membrane of PEM fuel cells does not work well when dry, so many fuel cell systems have humidifiers for the air intakes. Humidifiers typically consist of a thin film that can be made of the same material as PEM. By flowing dry inlet air on one side of the humidifier and moist exhaust air on the other side, the water produced by the fuel cell can be recycled to keep the PEM well-hydrated.

2.3 Key materials and components

Polymer electrolyte membrane (PEM) fuel cell is a hot topic in the application research of fuel cell vehicle. PEM fuel cells are made of several different layers of materials. The main components of a PEM fuel cell are shown in Figure 3. The core of a PEM fuel cell is the membrane electrode Assembly (MEA), which includes the membrane, catalyst layer, and gas diffusion layer (GDLs). Hardware components for one meant to be incorporated into the fuel cell include gaskets, which provide a seal that is protected against leakage of the gas, and biphase steel plates, which are used to assemble the personal PEM fuel cell with the fuel cell stack and provide gas for the fuel and air channels.

catalyst (catalyst) is one of the key materials of fuel cell, its role is to reduce the activation energy of the reaction, promote the REDOX process of hydrogen and oxygen on the electrode, and improve the reaction rate. Due to the low exchange current density of oxygen reduction reaction (ORR), it is the control step of the total reaction of fuel cell. At present, the commonly used commercial catalyst in fuel cells is Pt/C, a supported catalyst consisting of Pt nanoparticles dispersed onto a carbon powder (such as XC-72) carrier.

Proton exchange membrane is a polymer electrolyte membrane, which plays an important role in conducting protons, isolating cathode and anode reactants in fuel cells, and is also used as a catalyst support in the preparation of CCM membrane electrodes, which is the core device of fuel cells and a key component that determines the performance, life and cost of fuel cells. In practical applications, the proton exchange membrane is required to have high proton conductivity and good chemical and mechanical stability.

membrane electrode assembly MEA (membrane electrode assembly MEA) is a combination of membrane, catalytic layer and diffusion layer, and is also one of the core components of fuel cells. At present, 3 generations of MEA technology routes have been developed internationally (Figure 4). Among them, the first and second generation technology has been basically mature, and domestic new source power, Wuhan New Energy and other companies can provide membrane electrode products. The third generation of ordered membrane electrode technology is still in the research stage at home and abroad.

The function of bipolar plates in fuel cells is to conduct electrons, distribute reactive gas and assist in discharging generated water, which requires that the bipolar plate material is a good conductor of electricity and heat, has a certain strength and gas density. In terms of performance stability, the bipolar plate is required to have corrosion resistance in the fuel cell acidic (pH=2 ~ 3), potential (~ 1.1V), wet and hot (gas-water two-phase flow, ~ 80℃) environment, and is compatible with other fuel cell components and materials without pollution, and has a certain hydrophobicity to assist the discharge of water generated by the battery. From the aspect of productization, the bipolar plate material is required to be easy to process and low cost. The bipolar plate materials commonly used in fuel cells include hard carbon plate, composite bipolar plate and metal bipolar plate.

The Fuel Cell Stack is the core of the fuel cell power generation system. Usually in order to meet certain power and voltage requirements, the stack is usually formed by hundreds of single batteries in series, and the reaction gas, water, refrigerant and other fluids are usually in parallel or in a specially designed way (such as series parallel) through each single battery. The uniformity of fuel cell stack is an important factor restricting the performance of fuel cell stack.

3. Main types of fuel cells

Normally, fuel cells can be divided into phosphate fuel cells, solid oxide fuel cells, alkaline fuel cells, proton exchange membrane fuel cells, lysocarbonate fuel cells, etc., as shown in Table 1. In recent years, with the deepening of the research on fuel cells, direct carbon fuel cells, microbial fuel cells, direct methanol fuel cells, glucose /O2 enzyme fuel cells and so on have been gradually born. Among the above categories, the earliest fuel cells to be developed are phosphoric acid fuel cells and alkaline fuel cells, also known as the first generation of fuel cells, which have developed more mature technologies. The second generation fuel cell is a molten carbonate fuel cell, and the third generation fuel cell is a solid oxide fuel cell.

4. Current status and future R&D direction

China has a layout in the vehicle, system and stack, but there are still fewer relevant enterprises in the parts and components, especially the most basic key materials and components, such as proton exchange membrane, carbon paper, catalyst, air compressor, hydrogen circulation pump, etc. Although relevant domestic enterprises have begun to intervene, there is still a large gap in reliability and durability compared with international advanced products, and most of the key components and key materials are still dependent on imports.

Although fuel cell vehicles are developing rapidly, from the perspective of commercialization requirements, there is still a certain gap in China's automotive fuel cell technology, and the future needs to strengthen the layout of the following aspects:

1) Improve fuel cell stack performance and specific power. At present, the power level of domestic fuel cell vehicle stack is still generally low. Domestic automotive fuel cell piles are mainly 30 ~ 50 kW, which is far from the fuel cell power level of about 100 kW for international premium vehicles.

2) Improve the durability of the fuel cell. Improving the durability of fuel cell stacks and systems is a prerequisite for fuel cell commercialization. At present, improving the system control strategy is one of the effective ways to improve the durability of fuel cell vehicles.

3) Reduce the cost of fuel cells. It is recommended to develop low-cost materials and components, such as low Pt catalysts and membrane electrodes, low cost bipolar plates and system components, and achieve mass production to reduce the cost of stacks and systems.

4) Strengthen mass production technology of key materials and core components. This seriously restricts the independent and controllable development of China's hydrogen fuel cell industry, strengthens the technological transformation of the above-mentioned key materials and core components, accelerates the formation of batch preparation technology with fully independent intellectual property rights and the establishment of product production lines, and fully realizes the localization and mass production of key materials and core components.