Hydrogen fuel cell technology architecture and key technologies are sorted out

With the rapid development of the global economy caused by the environmental and energy problems become increasingly prominent, greenhouse gases and toxic and harmful substance emissions caused by the high attention of various countries, reduce the use of fossil energy for vehicles is an important measure to solve these problems, in recent years, the rapid development of the new energy automobile industry, causing new changes in the global automotive industry.

Hydrogen energy and hydrogen fuel cells are important ways to achieve clean utilization of energy and optimize the structure of energy consumption in mainland China. Hydrogen energy has the advantages of wide range of sources, renewable, stable storage, and rapid replenishment. China's hydrogen energy industry is entering a stage of rapid development with the support of the government, and its industrialization process is accelerating. Driven by the new round of energy revolution, countries around the world attach great importance to hydrogen fuel cell technology to support a low-carbon and clean development model. Developed countries or regions have actively developed the "hydrogen energy economy" and formulated development plans such as the Comprehensive Energy Strategy (the United States), the EU Hydrogen Energy Strategy (EU), and the Hydrogen Energy / Fuel Cell Strategic Development Roadmap (Japan) to promote the research and development, demonstration and commercial application of fuel cell technology. The mainland has also actively followed up the hydrogen energy development strategy, and national policy documents such as the Energy Technology Revolution Innovation Action Plan (2016-2030) and the Medium- and Long-term Development Plan for the Automobile Industry (2017) have clearly proposed to support the development of fuel cell vehicles. This paper focuses on the research and development and application status of key materials and core components of hydrogen fuel cell technology at home and abroad.

Key components and materials for hydrogen fuel cell stacks

1. Membrane electrode assembly

Membrane electrodes are the core of the stack, similar to the CPU in a computer, and determine the upper limits of the stack's performance, life, and cost. Membrane electrode assemblies consist of a proton exchange membrane, a catalyst, and a gas diffusion layer (gas diffusion layer). Its main performance indicators include output power per unit surface area (power density), precious gold use (platinum consumption per unit power output), life and cost. Membrane electrode production currently uses the second generation of production technology catalyst coating (CCM) technology, with roll-to-roll (Roll-to-Rioll) continuous high-speed production capacity.

(1) Proton exchange membrane (PEM)

Proton exchange membrane is the core component of the proton exchange membrane fuel cell (PEMFC), is a polymer electrolyte membrane, the current mainstream trend of proton exchange membrane is perfluorosulfonic acid enhanced composite membrane, proton exchange membrane gradually tends to thin, there are tens of microns reduced to more than ten microns, reduce the ohmic polarization of proton delivery, in order to achieve higher performance.

(2) Catalyst

In the stack of hydrogen fuel cells, the oxidation reaction of hydrogen on the electrode and the reduction reaction of oxygen are mainly controlled by the catalyst. Catalyst is the main factor affecting the activation polarization of hydrogen fuel cells, is regarded as the key material of hydrogen fuel cells, the current commonly used catalyst in fuel cells is Pt/C, that is, the nanoparticles of Pt dispersed to the toner (such as XC-72) carrier-mounted catalyst.

(3) Gas diffusion layer

The gas diffusion layer (GDL) consists of a carbon fiber base layer and a carbon microporous layer, located between the flow field and the membrane electrode, and its main role is to provide a transmission channel for the gas involved in the reaction and the water produced, and to support the membrane electrode. Therefore, GDL must have good mechanical strength, suitable pore structure, good electrical conductivity, and high stability.

2. Bipolar plate

Bipolar plate is the core structural component of the stack, playing a role in uniform distribution of gas, drainage, heat conductivity, conduction, accounting for about 60% of the weight and nearly 20% of the cost of the entire fuel cell, its performance directly affects the output power and service life of the battery. Bipolar plate materials are divided into two categories: carbon-based and metal-based materials, and carbon-based plates are divided into two categories: graphite plates and composite film pressed carbon plates.

Graphite bipolar plate is generally non-porous graphite plate or carbon plate as the substrate, and the use of CNC machine tools for runner processing, domestic graphite bipolar plate technology has developed very rapidly in recent years, the technical level is comparable to foreign countries, but the thickness is usually more than 2mm. Composite membrane carbon plate has broken through the 0.8mm thin plate technology in foreign countries, with the same volume power density as the metal plate. Graphite and metal bipolar plate performance pairs are shown in Table 1.

Table 1 Graphite and metal bipolar plate performance comparison

Hydrogen fuel cell system

In order to maintain the normal operation of the stack, the hydrogen fuel cell system also needs the synergy of external auxiliary subsystems such as hydrogen supply system, water management system, and air system, and the corresponding system components include hydrogen circulation pumps, hydrogen cylinders, humidifiers, and air compressors. Fuel cells will produce a large amount of water in the working state, too low water content will produce "dry film" phenomenon, hindering proton transmission; too high water content will produce "flooding" phenomenon, hindering the diffusion of gas in porous media, resulting in low stack output voltage. The accumulation of impurity gas (N2) penetrating from the cathode side to the anode hinders the contact of hydrogen with the catalyst layer, causing local "hydrogen starvation" and causing chemical corrosion. Therefore, the balance of water is of great significance to the stack life of PEM hydrogen fuel cells, and the solution is to introduce hydrogen circulation equipment (circulation pump, injector) into the stack to achieve functions such as gas purge, hydrogen reuse, and humidification hydrogen.

The durability of the fuel cell system is limited by the durability of the battery stack on the one hand, and the performance decay of the battery stack is accelerated due to the poor control effect of the working conditions of the fuel cell stack in the vehicle environment. At this stage, fuel cell durability test has many interference factors, long test cycle, high cost difficulties, fuel cell stack attenuation mechanism and life prediction modeling, fuel cell stack accelerated aging test and rapid evaluation, fuel cell health status online office and fault diagnosis and other technologies are unable to meet the needs of rapid development of the industry, need to carry out key research, in order to establish a fuel cell system durability technology system.

Mainland hydrogen fuel cell vehicle phased development goals

From 2020 to 2024, the commercial application of hydrogen fuel cell vehicles will be initially realized, with a scale of 8,000-10,000 units and more than 100 hydrogen refueling stations in operation. By 2025, accelerate the promotion and application of hydrogen fuel and fuel cell vehicles, optimize the structural design of fuel cell systems, accelerate the industrialization of key components, and reduce manufacturing costs, resulting in the market ownership of fuel cell vehicles reaching 50,000-100,000 vehicles. By 2030, realize the large-scale promotion and application of hydrogen fuel cell vehicles, realize the integration of hydrogen production, storage, transportation and application, and establish a sound hydrogen energy and fuel cell industry chain, with fuel cell vehicles reaching 800,000-1 million.

At present, the mainland has initially formed an industrial chain in the field of hydrogen fuel cell stacks and their key materials, but the gap in technological maturity is large. In terms of technical route, most of the hydrogen fuel cell stacks for vehicles have chosen proton exchange membrane systems, and metal plate electrodes and graphite plate electrodes coexist. The hydrogen fuel cell system of commercial vehicles uses graphite plate or composite plate, and has not yet adopted a metal bipolar plate with high power density. Graphite plate stack in recent years in the performance of a greater improvement, but there is still a significant difference with the metal plate stack, although the current cost is low, but from the processing convenience and material development trend analysis, the cost of metal plate will be lower than graphite plate. From the perspective of development stage, the mainland currently has the application conditions for small-scale promotion of hydrogen fuel cell vehicles.

Based on the 2035 fuel cell development goals, the key lies in the improvement and innovation of fuel cell stacks and key components, to achieve the market demand of fuel cells with good performance, high durability and low cost, greatly strengthen the technology research and development of fuel cells, and establish a supply system that can independently produce fuel cells and key components.