Key technologies and development status of hydrogen energy utilization in the context of carbon neutrality

1. The role of hydrogen energy in the future green energy structure

At present, the world's energy production and consumption structure is forming a new future energy system under the joint action of two trends: the global carbon emissions are still intensifying, the proportion of renewable energy is increasing, and a variety of new energy sources coexist. However, the new system still faces many challenges, including the volatility brought about by the increase in renewable energy installations and the long distance between energy supply and demand.

The value of hydrogen energy is that it can provide solutions to various key energy challenges, that is, to provide solutions for the conversion of matter and energy between multiple energy sources. The role of hydrogen energy in the future energy mix is shown in Figure 1. The value of hydrogen energy in the European Hydrogen Roadmap is described as follows: First, hydrogen is the most realistic option for large-scale decarbonization of major carbon emitters such as transport, industry and buildings; Second, hydrogen plays an important role in the systematic regulation of renewable energy production, transportation and consumption, and can provide a flexible cross-field, cross-time and cross-place energy circulation system. Finally, hydrogen is used in a way that is more in line with the preferences and habits of current users. In the future energy system, hydrogen has the potential to replace traditional fossil fuels such as coal, oil and natural gas.

2. Main key technologies of hydrogen energy

Hydrogen as a raw material is widely used in industrial raw materials, direct combustion energy supply, household fuel cells and fuel cell vehicles and other fields is the main use and development direction of hydrogen energy, and related technologies have made great progress in recent years. However, the core of the development of new energy is to achieve cheap and efficient raw material sources and storage and transportation, and hydrogen energy development is also facing the same problem. Therefore, hydrogen production and hydrogen storage technology is the key to the efficient use of hydrogen, is an important bottleneck limiting the development of large-scale industrialization of hydrogen energy, and has become one of the key and difficult points in the current development of hydrogen energy industrialization.

2.1.1 Hydrogen source supply methods Hydrogen sources are very wide, the main hydrogen source supply methods include coal, natural gas and other fossil energy reforming hydrogen production, industrial by-product hydrogen production and electrolytic water hydrogen production, the future or large-scale hydrogen source supply potential of other ways include biomass hydrogen production, photothermal hydrogen production, photoelectric hydrogen production and nuclear hydrogen production. At present, more than 95% of hydrogen comes from fossil energy reforming hydrogen production and industrial by-production hydrogen, hydrogen from other sources is still very limited, however, the use of renewable energy electrolytic water hydrogen production, so that renewable energy through the "electricity - hydrogen - electricity (or chemical raw materials)" way to power, transportation, heat and chemical industry coupled together, to achieve the "green hydrogen" real efficient use. To realize the true role of hydrogen as an energy source. The key core technology of hydrogen production from renewable energy sources is the efficient hydrogen production technology from electrolytic water. Hydrogen production by electrolytic water is to dissociate water molecules into hydrogen and oxygen through an electrochemical process under the action of direct current, and precipitate them at the Yin and Yang poles respectively. Anode: H₂O→1/2O₂+2H++2e-1 Cathode: 2H++2e-→H₂ Total reaction: H₂O→H₂+1/2O₂ According to the difference in the electrolyte system, hydrogen production from electrolytic water can be divided into alkaline electrolytic water, proton exchange membrane (PEM) electrolytic water and solid oxide electrolytic water 3 kinds. The basic principle of the three is the same, that is, in the process of REDOX reaction, prevent the free exchange of electrons, and decompose the charge transfer process into the electron transfer of the external circuit and the ion transfer of the internal circuit, so as to achieve the generation and utilization of hydrogen.

2.1.2 Alkaline water electrolysis to produce hydrogen

Alkaline liquid electrolysis water technology is based on KOH, NaOH aqueous solution as electrolyte, using asbestos cloth as diaphragm, under the action of direct current water electrolysis, hydrogen and oxygen are generated, the reaction temperature is low (60~80℃). The produced hydrogen is about 99% pure and needs to be dealkali mist treatment. The main structural features of alkaline electrolyzer are liquid electrolyte and porous separator, as shown in Figure 2. The maximum operating current density of alkaline electrolyzers is less than 400mA/cm², and the efficiency is usually about 60%.

2.1.3 PEM water electrolysis Hydrogen production PEM water electrolysis, also known as solidpolymerelectrolyte (SPE) water electrolysis, the working principle is shown in Figure 3. Water (2H₂O) produces a hydrolysis reaction on the anode, and splits into protons (4H+), electrons (4e-) and gaseous oxygen under the action of electric field and catalyst; 4H+ protons arrive at the cathode through the proton exchange membrane under the action of potential difference. 4e- electrons are conducted through the external circuit, producing 4H++4e- reaction on the cathode, precipitating hydrogen (2H₂), and realizing the separation of hydrogen and oxygen; In the cathode cavity, with the increase of hydrogen production, the pressure gradually increases until the predetermined pressure is reached. The operating current density of PEM electrolytic cell is usually higher than 1A/cm², at least 4 times that of alkaline electrolytic tank, with high efficiency, high gas purity, adjustable current density, low energy consumption, small size, lye free, green, safe and reliable, and can achieve higher gas production pressure, etc. It is recognized as one of the most promising electrolytic hydrogen production technologies in the field of hydrogen production.

The main components of a typical PEM hydro-electric solution pool include the anode and cathode extreme plate, the anode and cathode gas diffusion layer, the cathode and cathode catalytic layer and the proton exchange membrane. Among them, the yin-yang extreme plate acts as a fixed component of the electrolytic pool, guiding the transmission of electricity and the distribution of water and gas; The anode and cathode gas diffusion layer plays the role of collecting flow and promoting the transfer of gas and liquid. The core of the anode and cathode catalytic layer is a three-phase interface composed of catalyst, electron conduction medium and proton conduction medium, which is the core place of electrochemical reaction. As a solid electrolyte, the proton exchange membrane generally uses perfluorosulfonic acid membrane, which plays the role of isolating the cathode and cathode to generate gas, preventing electron transfer and transferring protons at the same time. At present, the commonly used proton exchange membranes are mainly from DuPont, AsahiGlass, AsahiChemicalIndustry, Tokuyama and other companies. PEM electrolytic water has high requirements on catalyst support. The ideal catalyst should have high specific surface area and porosity, high electron conductivity, good electrocatalytic performance, long-term mechanical and electrochemical stability, small bubble effect, high selectivity, cheap availability and non-toxic conditions. The catalysts that meet the above conditions are mainly precious metals/oxides such as Ir and Ru, as well as binary and ternary alloys/mixed oxides based on them. Due to the high price and scarce resources of Ir and Ru, and the current Ir consumption of PEM electrolyzers often exceeds 2mg/cm², it is urgent to reduce the amount of IrO₂ in PEM hydrocarbon-solution pools. The commercial Pt-based catalyst can be directly used for the hydrogen evolution reaction of PEM electrolytic water cathode, and the current Pt load of PEM electrolytic water cathode is 0.4~0.6mg/cm². Although the advantages of PEM electrolysis water hydrogen production technology and renewable energy coupling are obvious, in order to better meet the needs of renewable energy applications, it is also necessary to further develop in the following aspects: 1) improve the power of PEM electrolysis water hydrogen production, and match the needs of large-scale renewable energy consumption; 2) Improve current density and wide load variation capability, reduce system cost, achieve efficient consumption of renewable energy, and facilitate auxiliary power grid peak load, reduce power grid burden, and improve energy efficiency; 3) Increase gas output pressure, facilitate gas storage and transportation, reduce the demand for subsequent pressurization equipment, and reduce the overall energy consumption. 2.1.4 solid oxide electrolysis of water hydrogen production of high temperature solid oxide electrolytic cell (solidoxideelectrolysiscell SOEC) solid oxide fuel cell (SOFC) solidoxidefuelcell, adverse reaction. 

The cathode material is generally Ni/YSZ porous cermet, the anode material is mainly perovskite oxide material, and the intermediate electrolyte is YSZ oxygen ion conductor. The water vapor mixed with a small amount of hydrogen enters from the cathode (the purpose of mixing hydrogen is to ensure the reduction atmosphere of the cathode and prevent the cathode material Ni from being oxidized), and the electrolytic reaction occurs at the cathode, decomposing into H₂ and O²-, O²- through the electrolyte layer to the anode at a high temperature environment, where the anode loses electrons and generates O². Due to the good thermal stability and chemical stability of solid oxides, the entire system at high temperature electrolysis voltage is low, resulting in less energy consumption, the hydrogen production efficiency of the system can be as high as 90%. However, at present, in terms of technology, the stability of anode and cathode materials under high temperature and high humidity conditions and the rapid decay of the reactor system under long-term operation are still problems to be solved. Therefore, SOEC technology is still in the technology development stage, with some small demonstration projects in places such as Karlsruhe, Germany, supported by projects such as HELMETH. 2.2 Hydrogen storage technology Compared with other fuels, hydrogen has a large mass energy density, but a small volume energy density (1/3000 of gasoline). Therefore, a major prerequisite for building a hydrogen energy storage system is to store and transport hydrogen at a high volume energy density. Especially when hydrogen is used in the field of transportation, it also requires a high mass energy density. At present, hydrogen storage methods mainly include high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage, organic liquid hydrogen storage, porous materials and metal alloys and other physical solid hydrogen storage. For the large-scale storage and transportation of hydrogen, although a variety of technologies and means have been developed so far, the most feasible industrial hydrogen storage is still only high-pressure gaseous hydrogen storage and cryogenic liquefied hydrogen storage. High pressure gaseous hydrogen storage is the most common and direct way of hydrogen storage, the hydrogen in the high pressure container is stored in the gaseous state, and the storage amount is proportional to the pressure. High pressure hydrogen storage technology commercial generally use gas storage cylinders that can withstand 20MPa hydrogen pressure, hydrogen storage pressure of about 15MPa, because the hydrogen density is low and the hydrogen storage tank itself is heavier, the hydrogen mass fraction is generally less than 3%. In order to improve the hydrogen storage density, the researchers developed a carbon fiber fully wound high-pressure hydrogen cylinder with aluminum inner liner molding and high fatigue resistance, which can withstand 35~70MPa high pressure and a mass concentration of 19~39g/L. 

Toyota's Mirai hydrogen fuel cell vehicle hydrogen storage system uses a polyamide wire plus lightweight metal high-pressure hydrogen storage tank, which can withstand 70MPa high pressure. Low temperature liquefied hydrogen storage is a practical way to store hydrogen. Because the density of liquid hydrogen at normal temperature and pressure is 845 times that of gaseous hydrogen, low temperature liquefied hydrogen storage has the advantages of high hydrogen storage density and small storage container volume, and its mass concentration is about 70g/L, which is higher than that of high pressure gaseous hydrogen storage (about 39g/L under 70MPa). However, the hydrogen liquefaction process requires multiple stages of compression and cooling, and the hydrogen temperature is reduced to 20K, which will consume a lot of energy, and the energy consumed by liquefaction accounts for about 30% of the hydrogen energy. In addition, in order to avoid the evaporation loss of liquid hydrogen, the thermal insulation performance of liquid hydrogen storage vessels is demanding, and thermal insulation materials with good thermal insulation performance are needed. The design and manufacture of low temperature hydrogen storage tank and the selection of materials have always been costly problems, resulting in the complexity of liquefaction process and hydrogen storage tank technology, and the cost increase. Low-temperature liquefied hydrogen storage technology is mainly used in military and aerospace fields, and its commercial research and application have just begun. However, due to its advantages in large-scale and long-distance storage and transportation, it may be complementary with high-pressure gaseous hydrogen storage in the future. 2.3 Hydrogen Transport Technology 2.3.1 Container Transport Hydrogen can be transported in containers as a compressed gas, liquid or stored in hydrides. Short-distance hydrogen transport is mainly carried out by long-tube trailers. Intercontinental hydrogen could be transported in liquid form in ship containers, similar to today's liquefied natural gas shipments. Liquid hydrogen is much less dense than natural gas and therefore more expensive to transport. In addition, there are other safety issues in the intercontinental transport of hydrogen, such as container leaks, accidents during hydrogen filling and unloading, and ship collisions. 2.3.2 Pipeline Transportation Another major mode of hydrogen transportation is pipeline transportation. Due to the similar nature of hydrogen and natural gas, hydrogen is also transported in a pipeline in a very similar way to natural gas. In fact, when using steel materials and welding processes to transport natural gas, the transport pressure can reach up to 8MPa, which can also achieve the transport of hydrogen in the pipeline, and the test methods used today are sufficient to control the transport risk of hydrogen and natural gas at the same level. However, the pipeline transportation of hydrogen also needs to solve some problems, such as the diffusion loss of hydrogen is about 3 times that of natural gas, the brittleness of the material after adsorption of hydrogen, the need to increase a large number of gas monitoring instruments, the need to install outdoor emergency empouting equipment, etc., which will increase the cost of transportation. At present, the cost of hydrogen transportation pipeline is about $630,000 /km, the cost of natural gas pipeline is only about $250,000 /km, and the cost of hydrogen pipeline is about 2.5 times that of natural gas pipeline.

3. Ideas and cases of hydrogen energy utilization in Europe

The Paris Agreement sets the goal of "keeping the global average temperature rise in the 21st century below 2 ° C and limiting the global temperature rise to 1.5 ° C above pre-industrial levels." To achieve this, the EU needs to significantly increase the amount of electricity generated from renewable sources and increase the electrification rate of end-users. In the future, wind and solar power will account for 30-60% of the total electricity generation in the EU, and the electrification rate will increase to 50-65% by 2050. This requires that the future energy supply system can meet the needs of different industries in the trend of low carbon, can withstand the impact of large-scale renewable energy on the smooth operation of the grid, and can efficiently transport energy from the supply center to the demand center, and the use of hydrogen energy to meet these challenges has been recognized as the most feasible solution in Europe.

According to the EU's hydrogen energy utilization program, in terms of hydrogen production, mainly through PtG technology to maximize the use of renewable energy and transport problems in Europe. PtG technology uses surplus renewable energy to electrolyze water, convert electric energy into hydrogen, and realize the utilization and long-term storage of renewable energy in the form of chemical energy. The hydrogen obtained by electrolysis can be directly diversified in the fields of transportation, industrial utilization or gas power generation, and can also be mixed into the natural gas network for storage and transportation, in addition, hydrogen and carbon dioxide can be combined, converted into methane and then input into the natural gas network.

4. Ideas and cases of hydrogen energy utilization in Japan

Japan's power system is dominated by centralized generation, and the Fukushima nuclear accident has exposed the fragility of the current system. Due to the heavy dependence on overseas energy supplies and the stagnation of nuclear power development, Japan's energy self-sufficiency rate fell from 20% in 2010 to about 8% in 2016. The realization of a self-sufficient distributed energy system has become the direction of Japan's energy transition [38-39]. It has been considered an effective, economical and safe way to construct hydrogen energy supply system and use it near the place of consumption. Especially for Japan, which is prone to natural disasters, the multiple utilization ways of hydrogen energy are suitable for distributed energy development and large-scale centralized power generation, which greatly enriches the flexibility of the energy system. According to the goal of Japan's "hydrogen society" national strategy, hydrogen energy will eventually form a new secondary energy supply structure together with electric energy and heat energy, and be popularized and utilized in the whole society. Japan's "White Paper on hydrogen Energy" predicts that by 2030, Japan's hydrogen energy market will reach 1 trillion yen, and hydrogen fuel power generation will account for 5% of the country's total power generation.

Similar to Europe and the United States and other countries, Japan has officially carried out the demonstration and verification of PtG projects according to the planning of the "Hydrogen energy and fuel cell Strategic Roadmap". Among them, the "Fukushima Hydrogen Energy Research Area (FH2R)" project aims to build the world's largest "hydrogen society" demonstration base and smart community for hydrogen production, storage, transportation and use of renewable energy, and construct and operate a 10MW hydropower electrolysis plant in Namie, Fukushima Prefecture. In order to show the world the results of hydrogen energy development, the Japanese government also spent $350 million to build an underground pipeline for the Tokyo Olympic Games, and directly input the Fukushima hydrogen energy into the Olympic Village, so that at least 100 hydrogen fuel cell buses and training facilities, athletes dormitory and other more than 6,000 Olympic village buildings are all powered by hydrogen fuel.

Compared with the European Union and the United States, Japan has set the world's highest standard technical indicators and cost targets for PtG systems, including achieving investment costs of 50,000 yen /kW by 2020; Japan's renewable energy fixed price purchase system (FIT) will officially enter the power generation trading market by 2032.

In addition to the "Fukushima" project, Japan has also carried out the development and demonstration of hydrogen direct combustion power generation technology. In April 2018, Japanese companies Obayashi Group and Kawasaki Heavy Industries took the lead in the world to use 100% hydrogen as the fuel of 1MW gas turbine units. During the test period, 1.1MW of electricity and 2.8MW of heat were supplied to four adjacent facilities in PortLand, an artificial island in the central ward of Kobe City (Kobe City Medical Center General Hospital, Kobe Island Sports Center, Kobe International Exhibition Center, and Hong Kong Island Sewage Treatment Plant). With the support of a government grant, the company supplies the PortLand area's hotels, convention centers and other energy at market rates, and currently provides half of the area's annual electricity and heat demand, with Kansai Electric Company supplementing the shortfall.

In order to achieve large-scale hydrogen power generation, the experiment and demonstration of gas turbine co-firing power generation technology containing 20% hydrogen natural gas hybrid fuel has also been promoted in PortLand since 2018, and the detailed design experiment of 500MW class gas turbine has been carried out. With the breakthrough of technical problems such as reducing NOx value and improving power generation efficiency, large-scale hydrogen power generation will be possible. According to the goals of Japan's "hydrogen energy and fuel cell strategic roadmap", hydrogen power generation will be commercialized in 2030, the power generation cost is less than 17 yen/(kW×h), the hydrogen power consumption reaches 300,000 t per year, and the power generation capacity is equivalent to 1GW; The ultimate goal is to generate electricity at a cost of less than 12 yen/(kW×h), to remain competitive with LNG thermal power generation, taking into account environmental value, and to use 5 million to 10 million tons of hydrogen power per year, generating capacity equivalent to 15 to 30GW.

5. Hydrogen cost analysis

If hydrogen energy is to be widely accepted as an emerging energy source and occupy a place in the future energy structure, the cost factor will always play a decisive role. In the world, the hydrogen industry chain is not mature at this stage, especially the high price of hydrogen, and the cost still restricts the long-term development of hydrogen energy. Taking logistics vehicles, a typical scene of domestic hydrogen fuel cell vehicles, as an example, two popular hydrogen fuel cell logistics vehicles are selected to compare with traditional diesel logistics vehicles. The maximum load capacity of the two hydrogen fuel cell logistics vehicles is 3t, while the fuel consumption of 3t diesel logistics vehicles on the market is about 15L for 100 kilometers. The parameters of the two hydrogen fuel cell logistics vehicles are shown in Table 3. With reference to the current market price, assuming that No. 0 diesel is 6 yuan /L, the crossover point of the use cost of hydrogen and diesel is obtained. According to estimates, the crossover point of the use cost should be below 30 yuan /kg, that is, the price of hydrogen below this price in order to occupy the advantage in the market, and the current price of domestic hydrogen stations is 60 to 80 yuan /kg. Therefore, how to reduce the cost of hydrogen supply is an unavoidable problem for the current industrial development.