As a green, low-carbon and clean energy, green hydrogen doesn’t emit greenhouse gases during the process of electrolyzing water, only consumes renewable energy. Especially in terms of carbon emissions, it has obvious advantages compared to traditional petrochemical fuel H2 production. With the in-depth development and performance improvement of key materials, proton exchange membrane (PEM) water electrolysis for hydrogen production has become an important method for manufacturing green hydrogen.

Common H2 preparation methods by water electrolysis

1）Alkaline electrolyzer（AEL）

Alkaline water electrolysis is the earliest commercialized hydrogen production technology. The main body of the electrolyzer is composed of bipolar plates, anode and cathode electrodes, ion exchange membranes and sealing components, which are pressed together through fastening screws and end pressure plates. Generally use potassium hydroxide and sodium hydroxide as electrolytic solutions. In the early days, asbestos was used as separator materials to separate gases. However, due to the damage to human lungs, asbestos fibers are abandoned gradually, PPS materials are now more frequently used. Nickel based alloy materials can be used for anode and cathode electrodes, basically it is an economical and mature H2 preparation route because no need use to precious metals. Currently there are megawatt level hydrogen production applications in China. The disadvantages include low efficiency, high energy consumption, strong corrosiveness, and the de alkaline treatment for gas produced by electrolysis is necessary.

2）Solid oxide electrolyzer（SOEL）

The significant feature of the high-temperature solid oxide electrolysis technology for H2 production is the use of solid oxide as electrolyte material. the anode materials are made of perovskite oxide, the cathode materials are made of metal-ceramic composite. The all ceramic structure is not only corrosion-resistant, but also can withstand working temperatures above 800 ℃, effectively enhance the activity of catalyst, reduce the energy consumption of water decomposition, and achieve hydrogen output efficiency of up to 90%. The major technical bottlenecks for SOEL solution focuses on energy consumption and service life issues under high-temperature working conditions. In addition, the hot and humid environment has strict limit for deciding electrolyzer materials, with high initial investment and maintenance costs in the later stage, the commercialization level lags behind AEL and PEMEL.

3）Proton exchange membrane electrolyzer（PEMEL）

Also known as polymer membrane electrolysis technology, the solid electrolyte uses a very thin perfluorosulfonic acid membrane, which has good proton conductivity and can help obtain higher current density than both AEL and SOEL. Compared with asbestos membranes, PEM has obvious advantages in gas permeability , and there will be no cross permeation of gas in order to produce higher purity of hydrogen gas. In addition, the compact structure of PEMEL can not only effectively reduce the impedance of stack, but also withstand a certain pressure. It has been gradually applied and promoted in many fields such as energy storage, wind power, hydrogen refueling stations, etc. The future research mainly focuses on reducing initial and operating costs, reducing the load of precious metal catalysts, and so on.

Structure and principle of PEMEL

The stack components mainly include membrane electrodes (MEA), bipolar plates at both ends, seal parts, etc. MEA consists of diffusion layers, catalytic layers and a proton exchange membrane(PEM). PEM is the core component of the membrane electrode, which is responsible for conducting protons and isolating hydrogen and oxygen. The porous gas diffusion layer allows for smooth material transmission between catalytic layer and bipolar plate, high transportation efficiency can also reduce catalyst consumption. The bipolar plate not only provides transmission channels for gas, liquid and electrons, but also supports the membrane electrode, maintain the stabe structure. The operation process of PEMEL is exactly opposite to that of PEMFC. Firstly, water is supplied to anode end through a water pump, and apply direct current to decompose the water, which undergoes an oxidation reaction to generate O² and H⁺, known as oxygen evolution reaction (OER). Then, O² leaves the device, and H⁺ pass through the ion exchange membrane to reach the negative electrode end, undergo a reduction reaction to generate H₂, known as hydrogen evolution reaction (HER). Due to the large amount of H⁺ generated on anode end to form strong acids, There are high requirements for corrosion resistance of materials. The hydrogen production system generally uses multiple stacked single batteries to build water electrolyzer with large capacity, the series connection between different batteries can be realized by connecting cathode and anode from different bipolar plates. If use a single stage structure, also can be arranged in parallel.

Prospects of PEM water electrolyzers for Hydrogen Production

At present, the PEM electrolysis water technology in China is in industrialization stage, and the usage scenarios of hydrogen are becoming increasingly diverse. With the landing of more and more hydrogen refueling stations, energy storage and other large-scale hydrogen energy demonstration projects, the overall costs of PEMEL hydrogen production have been reduced, and the hydrogen energy equipment level has greatly improved. Many core materials and manufacturing processes can directly rely on fuel cell technology, which is an important reason why the cost of electrolytic cells can be significantly reduced. In recent years, emerging energy sources such as wind, water, light have developed rapidly, The introduction of surplus renewable energy into power generation can help hydrogen energy market achieve larger-scale development. The PEMEL solution with advantage of fast start stop can also compensate for the fluctuation problem of renewable energy power generation. Despite the huge potential of electrolyzing water for hydrogen production, the overall layout of hydrogen energy industry chain in the future still needs to fully consider the coordinated development of transportation, storage and new energy supply, combine with the distribution of new energy resources and transmission channel conditions, to minimize operating costs as possible. In addition, the technology research and upgrading on stack systems are equally important. For example, precious metal catalysts which are widely used in electrode materials are very expensive. By reducing the loading of precious metals or finding substitutes to reduce usage, the raw material cost of MEA can be greatly reduced. In the development of membrane technology, it’s necessary to seek a balance among conductivity, gas permeability and mechanical properties, and reduce the thickness of membrane on this basis. Microscopic research and optimization on pore amount and pore size of gas diffusion layers can improve the overall efficiency of the electrolytic cell. Future research of proton exchange membrane (PEM) water electrolysis technology in hydrogen preparation will focuses on high cost-components to get greater development space.