Hydrogen Production From Water Electrolysis
Hydrogen production by water electrolysis means that water molecules are dissociated under the action of direct current to generate oxygen and hydrogen, which are precipitated from the anode and cathode of the electrolytic cell respectively. According to the different membrane materials of electrolyzers, water electrolysis for hydrogen production is usually divided into alkaline water electrolysis (AE), proton exchange membrane (PEM) water electrolysis and high temperature solid oxide water electrolysis (SOEC).
1. Hydrogen production by alkaline water electrolysis
The diaphragm of alkaline water electrolysis hydrogen production electrolyzer is mainly composed of asbestos, which plays the role of separating gas. The cathode and anode are mainly composed of metal alloys, such as Ni-Mo alloys, etc., which decompose water to produce hydrogen and oxygen. The electrolyte of industrial alkaline water electrolyzer is usually KOH solution, the mass fraction is 20%~30%, the operating temperature of the electrolyzer is 70~80℃, the working current density is about 0.25 A/cm2, and the gas pressure is 0.1~3.0 MPa. The efficiency is 62%~82%. The alkaline water electrolysis hydrogen production technology is mature, and the investment and operation costs are low, but there are problems such as loss of lye, corrosion, and high energy consumption. The development of hydrogen production equipment from water electrolyzers is a research hotspot at home and abroad for hydrogen production from alkaline water electrolysis.
2. Hydrogen production by PEM water electrolysis
Different from alkaline water electrolysis hydrogen production, PEM water electrolysis hydrogen production uses perfluorosulfonic acid proton exchange membrane with good chemical stability, proton conductivity and gas separation as solid electrolyte instead of asbestos membrane, which can effectively prevent electron transfer and improve Electrolyzer safety. The main components of the PEM water electrolyzer are proton exchange membrane, cathode and anode catalyst layers, cathode and anode gas diffusion layers, cathode and anode end plates, etc. from inside to outside. Among them, the diffusion layer, catalytic layer and proton exchange membrane constitute the membrane electrode, which is the main place for material transport and electrochemical reaction in the entire water electrolyzer. The characteristics and structure of the membrane electrode directly affect the performance and life of the PEM water electrolyzer.
Compared with AE hydrogen production, PEM water electrolysis hydrogen production has higher working current density (˃1 A/cm2), higher overall efficiency (74%~87%), higher hydrogen volume fraction (>99.99%), and gas production. With higher pressure (3~4 MPa), faster dynamic response, and adaptability to the volatility of renewable energy power generation, it is considered to be a promising water electrolysis hydrogen production technology. At present, PEM water electrolysis hydrogen production technology has been demonstrated and applied in the fields of on-site hydrogen production at hydrogen refueling stations, wind power and other renewable energy water electrolysis hydrogen production, and energy storage, and has been gradually promoted.
In the past five years, the cost of electrolyzers has dropped by 40%, but high investment and operating costs are still the main problems to be solved in PEM water electrolysis for hydrogen production. For this reason, reducing the material cost of catalysts and electrolyzers, especially the noble metal loadings of cathode and anode electrocatalysts, and improving the efficiency and life of electrolyzers are the research focus of the development of PEM water electrolysis hydrogen production technology.
3. Hydrogen production by high temperature solid oxide water electrolysis
Different from alkaline water electrolysis and PEM water electrolysis, high-temperature solid oxide water electrolysis uses solid oxide as the electrolyte material, and the working temperature is 800~1 000 °C. The electrochemical performance of the hydrogen production process is significantly improved and the efficiency is higher.
The electrode of SOEC electrolytic cell adopts non-precious metal catalyst, the cathode material adopts porous metal ceramic Ni/YSZ, the anode material adopts perovskite oxide, and the electrolyte adopts YSZ oxygen ion conductor. The all-ceramic material structure avoids the problem of material corrosion. The high temperature and high humidity working environment limits the selection of materials with high stability, good durability and attenuation resistance for electrolyzers, and also restricts the selection and large-scale promotion of SOEC hydrogen production technology application scenarios.
At present, SOEC hydrogen production technology is still in the experimental stage. In China, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Tsinghua University, and University of Science and Technology of China have carried out exploratory research. Internationally, SOEC technology research is concentrated in the United States, Japan and the European Union. The main institutions include Mitsubishi Heavy Industries, Toshiba, Kyocera, Idaho National Laboratory, Bloom Energy, Topsoe, etc. The research focuses on electrolytic cell electrodes, electrolytes, connectors and other key materials and components, as well as stack structure design and integration.
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