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.