As a new energy electric vehicle, fuel cell vehicles need only one or two minutes to fill up the fuel, the core component is the fuel cell, in which the conductivity of proton conduction membrane greatly affects the energy conversion efficiency of the fuel cell. Recently, Zhang Sheng, a professor at the School of Chemical Engineering of Tianjin University, and Sir AndreHeim, winner of the Nobel Prize in Physics at the University of Manchester, United Kingdom, and others, confirmed the proton conductivity of two-dimensional materials such as graphene and boron nitride, and further found that the high-temperature proton exchange membrane used by the widely used mica in nature for fuel cells is better than the current commercial membrane performance. more energy-saving and environmental protection.
The two studies were recently published in Nature Nano and Nature Communications.
Find thinner “films” to increase range
It is reported that compared with the current common domestic lithium-ion electric vehicles, fuel cell vehicles save a long charging time, only a minute or two to fill up the fuel. At the same time, fuel cell vehicles do not go through the thermal process, not limited by thermal cycle, energy conversion efficiency is very high, long-lasting, and fuel cell power generation process products only water, more environmentally friendly, so fuel cell vehicles have become one of the main development directions of future vehicles.
The fuel cell works by losing the electron to become a proton, and then passing through the proton exchange membrane to the cathode and the electron syndication to generate water, the proton spevened inside the battery and the electron steaming the outer circuit constitutes a current loop, so proton conduction performance is very critical for the fuel cell energy conversion efficiency. At present, commercial perfluorosulfonate proton conduction film thickness of at least 5 microns or more, need to be below 100 degrees C in a hydrated state to function, at this time the purity of hydrogen requirements are high. If the development of membrane materials that can conduct protons efficiently at more than 100 degrees C, it will help to improve the efficiency of fuel cells, reduce the requirements for hydrogen purity, simplify the water management system, and achieve the goal of reducing costs and pollution, which is of great significance to the commercial development of fuel cell vehicles.
“Finding an efficient high-temperature proton conduction membrane material is not easy. Zhang Sheng said, “This material not only requires thin, but also allows protons to pass through at high speed, but also to block the penetration of hydrogen.” Because the penetration of hydrogen produces side reactions that reduce the battery output voltage and affect the overall reaction efficiency of the fuel cell. It also needs to be resistant to high temperatures. “
2D materials such as graphene are ideal materials
Zhang Sheng first prepared a micron-scale single-layer graphene, boron nitride film with his collaborators. The thickness is about 0.3 nanometers (1 nanometer equals 0.001 microns), the film on both sides are placed in different concentrations of hydrochloric acid solution, due to the presence of the concentration gradient, the high concentration of the side of the ions will spread to the lower concentration side, the movement of the ions formed an electric current.
Based on the theory, they calculated that two-dimensional materials such as graphene and boron nitride with a hexadecimal mesh structure, due to their special physical structure, allowed only particles less than 10 piashes (1 piameter equal to one thousandth of a nanometer) to pass through. Hydrochloric acid consists of hydrogen and chloride ions, the proton radius is about 0.001 pitry, the radius of the chloride ions is about 180 pitry, so only smaller protons can pass through the film. It is proved that the current through the two-dimensional film in this experiment is all generated by proton conduction, while the slightly larger chloride ions do not contribute at all. “Through this experiment, graphene and boron nitride are only allowed to pass through protons, blocking the passage of other ions and molecules, including hydrogen, to meet the requirements of the fuel cell proton conduction membrane material,” Zhang said. But he also admitted that graphene and boron nitride, although thinner than commercial proton conduction membrane (a difference of 10,000 times), but because the structure is too dense, resulting in proton conduction resistance is greater than commercial film, energy conversion efficiency has not improved, not suitable for commercial promotion.
Mica membrane snares more application prospects than graphene
Based on the confirmation that two-dimensional materials such as graphene can be used as proton conduction materials, Zhang Sheng and his co-authors have actively explored another two-dimensional material, mica, which is more promising than graphene in the field of fuel cells.
“The mica is an extremely abundant and inexpensive mineral in the earth’s crust, with a sponge-like layer of aluminum silicate, and potassium ions, like water, in its pores. As an ion exchange reaction, potassium ions can be easily exchanged with protons, Zhang said. Because the radius of potassium ions is about 100 pitry, and the proton radius is about 0.001 pitry, which is much smaller, protons can be well transmitted in the pores in which the potassium ions are located.
It is found that the proton conductivity of the mica membrane after ion exchange processing has been greatly improved, and the application temperature can be extended from 100 degrees C to 500 degrees C, which is very promising. “We found that the coinecell proton conductivity increased by 100 times after the ion exchange reaction, ” Zhang said. At the same time, the thermal stability of the mica membrane is higher, and the reserves are abundant and the price is low. “The study also found that at temperatures of 150 degrees C, the masteria proton conductivity was more than twice the current commercial requirement, and that when used in fuel cells, the vehicle’s mileage would be significantly increased.
At present, Zhang Sheng is leading the research team to prepare large-scale mica film, using its high-efficiency proton conductivity and excellent heat resistance to improve the existing fuel cell technology to promote the development of fuel cell vehicles. In addition to fuel cells, Zhang also plans to use the proton conduction membrane materials for solar photolysis, marine blue energy extraction, and many clean energy technologies for the electrochemical conversion of carbon dioxide into chemical raw materials such as meth acid, ethanol and ethylene.