New research and development direction of hydrogen energy materials revealed

New materials are the cornerstone and precursor of hydrogen energy, and are at the top of each industrial chain, with the highest technical barriers. They will provide a solid material foundation for the new round of scientific and technological revolution and industrial revolution. The hydrogen energy industry chain mainly includes hydrogen preparation, hydrogen separation and purification, hydrogen storage, hydrogen energy conversion and other links, and puts forward more and more urgent demand for new materials.

1、New materials for hydrogen production

For a long time, hydrogen is mainly obtained by reforming non-renewable fossil resources such as natural gas and coal, which is not sustainable and environmentally friendly. Using renewable energy (such as solar energy) to produce hydrogen by electrolysis of water or photolysis of water is an ideal way to achieve green and sustainable hydrogen production. Among them, the core requirement of new materials for hydrogen production by electrolysis of water is to develop efficient and stable non-noble metal catalytic materials, while the core requirement of new materials for hydrogen production by photolysis of water is to develop efficient and stable broadband optical absorption semiconductor materials.

At present, the catalysts for hydrogen production from electrolytic water are mainly precious metals (such as platinum, iridium oxide, ruthenium oxide, etc.). Their scarcity of resources and high prices make it impossible to apply them in large-scale industrial hydrogen production. Therefore, it is urgent to develop non-noble metal catalytic materials with rich crust content, low cost, simple preparation method and excellent catalytic activity.

There are many kinds of non-precious metal electrocatalytic materials, among which the materials with great development potential are mainly composed of transition metal elements such as iron, cobalt, nickel, molybdenum, tungsten, and non-metallic elements such as oxygen, sulfur, selenium, nitrogen, phosphorus, and carbon. Among all kinds of new catalytic materials, transition metal alloys, transition metal (hydrogen) oxides, transition metal chalcogenides, transition metal nitrides and phosphides have attracted much attention.

The research and development of hydrogen production catalysts is mainly aimed at reducing or replacing precious metal catalysts, constantly exploring potential new catalytic materials, and increasing the number of active sites of catalysts as much as possible and improving the activity of individual active sites through optimizing the composition, morphology and substance equivalence strategy, and ultimately improving the overall catalytic activity and stability. At the same time, it is a research focus to explore new atomic-scale materials (such as sub-nanometer or single-atom catalytic materials) to obtain catalytic properties comparable to noble metals.

2、New materials for hydrogen separation and purification

In the process of hydrogen production, other impurity gases will inevitably be mixed, such as a certain amount of CO2 and CO in methane steam reforming hydrogen production, and oxygen in electrolytic water and photolysis water hydrogen production (especially in powder photocatalytic hydrogen production). Therefore, it is necessary to separate and purify hydrogen before its utilization.
The separation and purification technologies of hydrogen mainly include pressure swing adsorption, fractionation/low-temperature distillation and membrane separation. Among them, membrane separation technology is the most promising hydrogen separation technology due to its advantages of low energy consumption, continuous operation, low cost and easy operation.

Membrane materials are the basis and core of membrane separation technology, mainly including organic membrane, inorganic membrane and organic-inorganic hybrid membrane. The typical representative of organic membrane is polymer membrane, such as pure phase polymer membrane, multiphase polymer membrane and polymer mixed matrix membrane. This kind of material is the first membrane material put into commercial application, and also the mainstream gas separation material in the market at present. It has the advantages of low cost and easy preparation, but has the disadvantages of poor high temperature resistance and corrosion resistance. The research focus of polymer membrane materials is to control the pore size and structure of the membrane and improve its separation performance by optimizing the membrane forming process.

Inorganic membranes mainly include carbon-based membrane materials, silicon-based membrane materials, metal-based membrane materials and zeolite-based membrane materials. The research focuses on optimizing the preparation parameters, regulating the pore size and pore structure of the membrane, and obtaining the properties that match the target screening gas. Compared with polymer membrane materials, inorganic membrane has better high temperature resistance and corrosion resistance, but its composition and structure are relatively fixed, and the degree of freedom of regulation is relatively low.
The typical representative of organic/inorganic hybrid membrane is the metal-organic framework membrane material, which is self-assembled by organic ligands and metal units to form a periodic network structure, with a variety of pore structures, and can be flexibly adjusted according to specific application scenarios.

Because there are many combinations between organic ligands and metal central ions, their derivatives are many, and it is easy to realize many functions. The key problems to be solved in the application of hydrogen separation and purification membrane materials are to improve their stability during service, achieve large area and high quality controllable preparation, and reduce maintenance costs.

3、New hydrogen storage materials

Hydrogen has the characteristics of low density, easy combustion and easy diffusion at normal temperature and pressure, which brings great challenges to its storage. How to realize safe, reliable and efficient hydrogen storage is one of the technical problems to be solved urgently.

At present, hydrogen storage methods mainly include high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage and solid hydrogen storage. In the medium and long term, high-pressure gaseous hydrogen storage is mainly used, and the ultimate goal is to achieve efficient solid hydrogen storage. High-pressure gaseous hydrogen storage is the most widely used hydrogen storage method. Its technical core is the inner liner material, outer carbon fiber material and its winding technology.

After high-pressure gaseous hydrogen storage and low-temperature liquid hydrogen storage, the use of solid materials and organic liquid materials for hydrogen storage has gradually developed into a potential hydrogen storage method. Although the research on hydrogen storage materials has been nearly half a century, it is still in the exploration stage, and there is no large-scale application. This is mainly due to the lack of cheap, efficient and long-life new hydrogen storage materials. An ideal hydrogen storage material needs to meet a series of harsh conditions at the same time, such as high hydrogen storage density, fast hydrogen storage and release rate, mild working conditions, good reversible cycle performance, long service life, etc.

At present, many kinds of materials have been used for hydrogen storage research, mainly including inorganic materials and organic materials. Among them, inorganic hydrogen storage materials mainly include metals and metal alloys, coordination hydrides and carbon-based materials, while organic

materials mainly include organic framework compounds, organic liquids and porous polymers. From the current research hotspot, hydrogen storage materials have gradually changed from traditional metals and alloys to light element hydrides (such as coordination hydrides) and porous adsorption materials (such as metal-organic frame structures).

4、New materials in hydrogen energy conversion

At present, hydrogen is mainly used as industrial raw material in the fields of petroleum, chemical industry, fertilizer and metallurgy, such as synthetic ammonia, petroleum refining and methanol production, accounting for about 90% of the total, while its application as clean fuel and other fields only accounts for 10%. The most attractive application of hydrogen as fuel is fuel cell (used for power generation, new energy vehicles, etc.). Its final product is water, which can achieve true zero pollution.

However, at present, noble metal platinum and its alloys are mainly used as catalysts for oxygen reduction and hydrogen oxidation in fuel cells, and the cost is still high. Generally speaking, the cost of precious metals in fuel cells accounts for about 40% of the total cost of fuel cells. Similar to the development goals and trends of new materials for hydrogen production, while keeping the catalytic performance

unchanged, reducing the consumption of precious metals by a large margin is the bottleneck that needs to be broken through at present. The medium and long-term goal is to partially replace precious metals, while the long-term goal is to use new and efficient catalytic materials that do not contain precious metals and are cheap.