hydrogen fuel cell is an electrochemical device whose individual cells consist of two electrodes, positive and negative, and an electrolyte. The positive and negative electrodes of a fuel cell do not contain active materials themselves; they serve only as catalytic converters. From an energy perspective, a fuel cell's function is to charge external energy into the cell, allowing it to generate electricity and be reused repeatedly. Its capacity is determined by the size and weight of the electrodes. During fuel cell operation, hydrogen fuel stored in a "hydrogen storage tank" outside the cell is supplied to the electrodes via pumps, piping, and other system components. An electrochemical reaction occurs at the electrodes, simultaneously outputting electrical energy. During this process, the fuel cell electrodes do not undergo significant changes. Theoretically, as long as hydrogen (the anode reactant) and an oxidant (the cathode reactant, such as air or O2) are continuously supplied to the fuel cell, electricity can be generated continuously. However, in reality, due to factors such as aging and degradation of the electrode components, the lifespan of a fuel cell is limited. The principle of hydrogen fuel cell power generation can be simply summarized as the electrochemical reaction between oxygen and hydrogen to produce water. During this reaction, the chemical energy stored in the hydrogen is converted into electrical energy and heat is released. The hydrogen-oxygen reaction is divided into two half-electrode reactions, as shown in Equations (2-1) and (2-2).

Anode

Cathode：

After hydrogen enters the cell, it dissociates into electrons and protons under the action of a catalyst. Electrons flow through an external circuit to the oxygen side, generating an electric current and performing external electrical work, while protons flow through the cell's internal proton conductor to the oxygen side. After oxygen enters the cell, it combines with the electrons and protons conducted from the hydrogen side under the action of a catalyst to form water, releasing a large amount of heat. Since the hydrogen side undergoes an electron-losing oxidation reaction, it is the anode for the cell, while the oxygen side undergoes an electron-gaining reduction reaction, making it the cathode. From the perspective of external electrical work, since electrons flow from the hydrogen side to the oxygen side through the external circuit, in the opposite direction of the external current flow, the oxygen side has a higher potential and thus serves as the positive electrode, while the hydrogen side serves as the negative electrode. This analysis of the principles reveals that the normal operation of a fuel cell involves physical and chemical processes such as reactant gas transport, electrode reactions, electron and proton conduction, product water discharge, and heat generation and dissipation. The PEMFC structure must not only facilitate these processes but also optimize overall performance, cost, and durability. A battery consisting of one PEMFC is called a single cell. Figure 2-1 is a physical picture of a single cell assembled in the laboratory using a fixture. The PEMFC stack actually installed on a vehicle is composed of multiple fuel cells stacked in series.

A single cell primarily consists of a current collector plate, a flow field plate (also known as a bipolar plate, BP), and a membrane electrode assembly (MEA). The current collector plate conducts the current generated by the electrochemical reaction to the external circuit. The bipolar plate transports reactant gases and product water, conducts electricity, and transports coolant. The flow field plate has flow channels (GCs) on the side closest to the MEA to evenly distribute the oxygen and hydrogen required for the electrochemical reaction, achieving a uniform current density and improving cell output. These channels also function to promptly discharge liquid water generated by the reaction, reducing oxygen transfer resistance and concentration polarization losses. Coolant channels are located on the other side of the MEA to control cell operating temperature. The MEA is a key component for cell hydrothermal management, and its structural design and material selection remain a hot topic of research. The MEA is the most critical core component of PEMFC power generation. It consists of two single-sided electrodes, the anode and cathode, separated spatially by a proton exchange membrane (PEM). The PEM is the core component of the MEA, not only isolating the anode and cathode gas reactants but also transporting protons and insulating electrons. Common PEMs include perfluorosulfonic acid proton exchange membranes, partially fluorinated polymer proton exchange membranes, non-fluorinated polymer proton exchange membranes, and composite proton exchange membranes. PEM performance is closely related to water content. To prevent performance degradation caused by membrane water loss, the operating temperature of PEMFCs is mostly below 100°C. Each single-sided electrode includes a gas diffusion base (GDB), a microporous layer (MPL), and a catalyst layer (CL). In addition, there is a junction area at the intersection of each layer. The structure is shown in Figure 2-2.

The following is a brief introduction to each layer: (1) GDB is composed of porous materials with a pore size range of 1 to 100 μm. It has the functions of supporting and protecting CL and PEM, as well as transferring charges and transporting water vapor. A GDB with good performance generally has low gas mass transfer resistance, can evenly distribute the reaction gas to the catalyst layer, and can also promptly discharge accumulated water, with a long service life. (2) MPL is a dense porous carbon particle layer deposited on the surface of GDB, mainly composed of carbon powder and hydrophobic agent polytetrafluoroethylene (PTFE), which plays the role of flattening the surface, reducing interfacial impedance, improving pore structure and enhancing drainage performance. (3) CL is a porous structure composed of catalyst and ion polymer, which is the place where the electrode reaction occurs. The active site on the catalyst surface is the reaction site, the ion polymer is used to transfer protons, and the pores are used to transport the reaction gas. The junction of the three is the three-phase interface. CL is the core reaction area, which involves almost all physical and chemical processes such as material transfer, chemical reaction and phase change in PEMFC. Its structural design, preparation process and catalyst synthesis have always been the current research focus.