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

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.