Titanium Polar Mesh Used in Water Electrolysis

Producing hydrogen by electrolyzing water should be the simplest method. Water resources are abundant and the water only contains hydrogen and oxygen. The produced hydrogen and oxygen are of high purity. Both hydrogen and oxygen are important raw materials for industry. However, the traditional method of electrolyzing water to produce hydrogen consumes too much power and is costly. This method accounts for a small proportion in the hydrogen production industry. Therefore, scientists have been looking for new ways to electrolyze water to produce hydrogen. The overall goal is lower cost and simpler methods. The direction of the technical route is that the anode and cathode materials should play a catalytic role in the separation of water ions, and ion membrane technology is also used to allow ions to move in one direction.

ONE. Proton exchange membrane electrolysis of water for hydrogen production (PEM technology)

The traditional method of electrolyzing water to produce hydrogen has several characteristics. The anode and cathode are made of Ni-Mo alloy, which plays a catalytic role. The isolation membrane is made of asbestos, and the cathode side is made of alkaline water. The method is simple and the process is mature, but the cost is too high and the efficiency of hydrogen production is not high. In 2014, a Canadian company proposed ion exchange membrane technology (shorthand as PEM), which has excellent performance and was officially put into production, with an overall efficiency of 74% to 87%. In PEM technology, hydrogen ions in water pass through the proton exchange membrane and combine with electrons to become hydrogen atoms, which combine with each other to form hydrogen molecules. This technology can work at medium and high currents and high voltages, and can start and stop quickly and conveniently. This technology occupies an important position in industrial hydrogen production.

The following is a schematic diagram of the working principle of PEM technology.

The main components of the PEM water electrolyzer are, from inside to outside, the proton exchange membrane, cathode and anode catalytic layers, cathode and anode gas diffusion layers, cathode and anode end plates, etc. The diffusion layer, catalytic layer and proton exchange membrane constitute the membrane electrode, which is the main site for material transmission and electrochemical reactions in the entire water electrolyzer. The characteristics and structure of the membrane electrode directly affect the performance and life of the PEM water electrolyzer. Most of the proton exchange membranes currently used are perfluorosulfonic acid membranes, and the preparation process is complicated. The price of proton exchange membranes is as high as hundreds to thousands of dollars/m2. To reduce costs, people are trying to develop new materials. Exchange membranes are important to reduce costs. The current research directions are modified perfluorosulfonic acid proton exchange membranes, organic/inorganic nanocomposite proton exchange membranes and fluorine-free proton exchange membranes. It is also important to reduce the cost of electrocatalysts. Electrocatalysts have always been in a strong acidic environment and are prone to corrosion, agglomeration, loss and other problems. Nowadays, ‘palladium‘ (also known as ‘platinum‘) is generally used, and the development of non-precious metal hydrogen evolution catalysts that can adapt to acidic environments has become a research hotspot. Membrane electrodes are also important to reduce costs. The current mainstream preparation method is to directly coat the catalyst active components on both sides of the proton exchange membrane. Vigorously develop hydrogen production from renewable energy sources (wind, solar, hydropower), which is a green and low-carbon hydrogen production method. With a large amount of flexibly available hydrogen, not only the energy demand for hydrogen can be adjusted remotely, but also the fluctuation of power consumption in the grid can be adjusted.

The main features of high-temperature solid oxide electrolysis of water for hydrogen production are: using solid oxide as the electrolyte material, the operating temperature is between 800 and 1000°C, and the hydrogen production efficiency is significantly improved. However, this technology has not yet been marketed. The electrodes of this technology use non-noble metals with catalytic functions; the cathode material can be made of nickel to make porous cermets, which can effectively solve corrosion problems and catalytic problems; for example, the anode porous material can be calcium titanium oxide, which can have an antioxidant effect. ; The electrolyte material is mainly to support and separate the electrodes, separate hydrogen and oxygen, and also allow oxygen ions to pass through. For example, zirconium oxide mixed with rare earths can be used; the high temperature is to make it easier for water molecules to crack into hydrogen under the action of the catalyst. ions and oxygen ions, high temperature can improve the stability and output efficiency of the reaction; under the action of DC voltage and the above mechanism, the reaction that occurs is:

Cathode H2O + 2e → H2 + O2,

Anode O2 →2e + (1/2)O2.

The structure of the single form of this technology is on the left side of the figure below, and the combined structure of the monomer is on the right side of the figure below:

The preliminary test results of this technology are still satisfactory, and the cost can be reduced to half of the cost of traditional electrolysis of water to produce hydrogen. However, this technology is still in the stage of further exploration. Scientists in China and other countries are rushing to solve the following problems:

1) Better anode and cathode electrolyte materials. Recently, some people use nickel mixed with manganese, iron, and phosphorus to make it into a foam, and then wrap graphene on the surface to make the cathode material. Some people use spinel (metal oxide) to make it. Anode material further enhances the catalytic effect and anti-corrosion effect,

2) The life of the electrolytic cell,

3) Waste heat utilization problem,

4) Further explore better structures. People hope to further reduce costs and make it commercially available as soon as possible.

THREE. Photoelectrochemical hydrogen production

There is a weak current hydrogen production technology, which uses the electric energy output from multi-junction solar cells to directly electrolyze water to produce hydrogen. This is a weak current hydrogen production method. The key to this technology is that the catalyst must be efficient. Some scientists use cobalt metal and phosphorus as catalysts, and use indium and tin oxides as electrodes. The electrodes and catalysts are placed in water, and rely on the weak current generated by the output of multi-junction solar cells. Under electrocatalysis, water is decomposed into hydrogen and oxygen. The highlights of this technology are that firstly, electrodes and catalysts are cheap and easy to obtain; secondly, it is carried out at normal temperature; thirdly, water is neutral before and after being electrocatalyzed, and will not corrode the electrodes like acidic (or alkaline) water.

There is also a weak electricity hydrogen production technology that works hard on the fabrication structure. The atomic deposition method is used to deposit titanium dioxide atoms onto the surface of the semiconductor electrode (in other words, the semiconductor electrode is covered with a titanium dioxide nano-protective layer), and then the titanium dioxide is covered with a molybdenum disulfide nano-layer that enhances the catalytic function. The catalyst layer is molybdenum disulfide and The protective layer of titanium dioxide forms an interlocking structure. Using this ‘electrode plus catalyst‘ method, hydrogen is produced with weak electricity. The highlights of this technology are that, firstly, it has opened up a new direction in making stable photoelectrode materials; secondly, the catalyst molybdenum disulfide is cheap and easy to obtain, which reduces the cost of water splitting.

There is also a weak electricity hydrogen production technology, which works hard to increase the emission of ultraviolet light. As we all know, ultraviolet light can decompose water, so some scientific and technological workers have conducted experiments using TiO2 as a catalyst and irradiating it with ultraviolet light to decompose water. However, visible light contains a small amount of ultraviolet light, so some scientific and technological workers have made materials that use visible light to excite ultraviolet light, so that visible light can produce more ultraviolet light, thus achieving the purpose of visible light decomposition of water. In sunlight, the wavelength of ultraviolet light is <400 nm, the wavelength of visible light is >400 nm, and the wavelength of infrared light is >760nm. Only ultraviolet light can decompose water. The question now is, what wavelength of ultraviolet light should be used to split water? This problem is easy to solve. Suppose we want to decompose pure water. People can measure the peak spectral frequency of ultraviolet light of pure water. This frequency of ultraviolet light is the most suitable frequency for decomposing water, so we design and excite ultraviolet light according to this frequency. materials will have significant effects.

The key to electrolyzing water is to achieve low cost. Low-cost water splitting has great practical prospects, is likely to become a commonly used technology, and is likely to become one of the mainstays of green energy. The reasons why this technology will definitely become popular are as follows:

1. Solar energy or other "weak current power supply devices" can be used in combination with "weak current hydrogen production devices" to produce hydrogen and oxygen around the clock.

2. If the hydrogen produced by electrolyzing water is a mixed gas of hydrogen and oxygen, the current ‘ion membrane technology‘ and hydrogen storage material technology can easily separate hydrogen and oxygen.

3. The electrode and catalyst are unified and can be made into a porous form or covered with nanomaterials that are more effective in catalysis. The production process is mature and the production price is low.

4. Both theoretical research methods and device manufacturing methods already have mature technical support. The remaining problem is that more talented engineers are needed to implement these practical devices. Based on this, it can be said that practical devices for electrocatalytic water are not far away from being launched on the market.

As we all know, hydrogen is widely used. Hydrogen can be used as an energy material. It can not only be directly burned to produce energy, but can also be used in fuel cells to produce current. In addition, hydrogen is a commonly used chemical intermediate that can produce nitrides through different industrial processes. Alcohols, methane, etc.; hydrogen has a wide range of applications. If the technology of electrocatalytic water becomes practical, clean energy will be inexhaustible, and people are looking forward to this day.