Liu Jing, Professor of Institute of science and chemistry, Chinese Academy of Sciences: a real-time hydrogen production technology with high safety and economy by aluminum water reaction triggered by liquid metal

The author of the paper: Xu Shuo has become an important mission of the world in recent decades to solve the growing energy demand and worsening environmental problems Hydrogen, with its high-quality energy density and clean and nontoxic combustion products, is praised as the ideal energy in the 21st century The United States, Europe, Japan and other countries have upgraded the development of hydrogen energy to the level of national energy strategy, and started to lay out the roadmap for the development of hydrogen energy technology As the largest hydrogen producing country in the world, China has also issued a series of favorable policies for hydrogen energy development in recent years, and various regions have launched plans for the development of hydrogen energy industry However, in the past two months, there have been three consecutive hydrogen explosion accidents in the world (Figure 1: South Korea May 23, the United States June 1, Norway June 10), which triggered deep reflection and discussion in the industry Hydrogen, with an explosion limit of 4% - 75.6%, has a wide range and is easy to cause hydrogen embrittlement and hydrogen corrosion With the development of hydrogen energy in full swing, the safety of hydrogen energy cannot be ignored In order to deal with this major challenge, the technology of hydrogen production that can be used immediately appears to have unique advantages Figure 1 hydrogen explosion accident site (source: top left, first finance and economics; top right, ABC 7news; bottom, China Science and technology network) We know that there are many classic hydrogen production methods (Figure 2) At present, about 95% of hydrogen in the market comes from the reforming or cracking of hydrocarbon fuel, such as coal, oil, natural gas, methanol, etc These methods are not only complex in process, but also can not be ignored in carbon emission Electrolyzed water is a promising way to produce hydrogen, but it needs very expensive catalyst and its energy efficiency needs to be improved Metal hydrolysis reaction is a traditional and unconventional hydrogen production method According to different metal reaction activities, its reaction triggering conditions and hydrogen production performance are different, so it can meet the needs of different occasions (Xu S., Liu J., metal based direct hydrogen generation as unconstrained high density energy Frontiersin energy 2019, 13, 27-53) As early as the 1960s, the military used the principle of rapid hydrolysis reaction of dense aluminum / magnesium metal fuel to generate a large number of reaction heat and hydrogen, providing power for underwater high-speed weapons Aluminum, as the most abundant metal element in the earth's crust, is an ideal energy material with low density However, aluminum in the air will immediately form a dense alumina passivation film, which will prevent the internal aluminum and water contact reaction At present, the commonly used method is to prepare aluminum alloy with low melting point metal alloy, such as gallium, indium, tin, bismuth, etc by high temperature melting or ball milling This method not only consumes a lot of energy, but also the alloy is easy to be aged in the air Fig 2 a variety of typical hydrogen production methods (source: Frontier in energy, 2019) In 2015, the joint team of Tsinghua University and Institute of physics and chemistry of Chinese Academy of Sciences led by Professor Liu Jing found that at room temperature, gallium based liquid metal can directly devour aluminum foil, and the liquid alloy formed can directly react with water to generate hydrogen (Zhang J, Et al Self - fueledbiomimetic liquid metal mollusk Advanced materials, 2015, 27, 2648-2655) Since then, the research on hydrogen production from aluminum water reaction triggered by liquid metal at room temperature has begun In 2016, the research team revealed the hydrogen production behavior mode of aluminum fed liquid phase GA – in alloy inside NaOH electronic International Journal of hydrogen energy, 2016, 41, 1453-1459), and studied the effect of different binary gallium based alloys on the hydrogen production performance of triggering aluminum water reaction (Tan s.c., et al Comparative study on activation of aluminum with our liquid metals to generate hydrogen in alkaline solution International Journal of hydrogen energy, 2016, 41, 22663-22667) In the experiment, the researchers also found by chance (Fig 3), the contact with metal substrate can significantly accelerate the hydrogen production rate of aluminum feed liquid phase GA alloy inside aqueous solution International Journal of hydrogen energy, 2016, 41, 6193-6199) Then, the research group broadened the aluminum materials used in the reaction from aluminum foil to aluminum plate At room temperature, the aluminum plate treated with liquid metal can react directly with seawater to generate hydrogen, and explored the influence of different reaction conditions on the hydrogen production performance (Lu J.R., et al Controlled room temperature hydrogen generation interaction of artistic seawater with aluminum plates activated byliquid Ga-In alloy, RSC Adv , 2017 , 7 , 30839–30844 ）。 Based on the above systematic research, the research team applied for a series of invention patents and obtained the authorization Fig 3 differences in hydrogen production performance of liquid metals on different substrates (source: International Journal of hydrogen energy, 2016) In 2018, in a long review entitled "hydrogen production by aluminum water reaction triggered by liquid metals at room temperature" (Xu s, Et al Liquid metal activated aluminum water reaction for direct hydrogen generation at room temperature Renewable and sustainable energy reviews, 2018, 92, 17-37) The research team systematically interpreted the reaction activation mechanism, elaborated the latest research progress, and summarized the influencing factors of the reaction Due to rehbinder effect, liquid gallium can destroy the passivation film on the surface of aluminum, and gallium atoms diffuse into the interior of aluminum along the grain boundary, forming gallium (aluminum) solid solution The intermetallics formed by indium, tin and other low melting point metals are also distributed on the aluminum grain boundary, which promotes the negative electrode potential shift of aluminum and facilitates the hydrolysis reaction Moreover, due to the different electrode potential, aluminum and liquid metal will form a micro corrosion cell pair, accelerating the anodic corrosion of aluminum At room temperature, activated aluminum reacts with neutral water to produce hydrogen and aluminum hydroxide The product has no pollution, and by-product aluminum hydroxide can be recycled At the end of the reaction, the liquid metal can also be separated from the reaction products and reused The experiment shows that the loss is very small In addition, a series of unique movement behaviors of liquid metal triggered by hydrogen production are shown (Fig 4), which provides unconventional application ideas for aluminum water reaction triggered by liquid metal to generate hydrogen Fig 4 a series of motion behaviors caused by aluminum water reaction triggered by liquid metal (source: renewable and sustainable energy reviews, 2018) However, considering comprehensively, the cost of using aluminum as the material for hydrogen production is still difficult to obtain market acceptance at present According to the survey data released by relevant departments, the annual output of global primary aluminum in 2018 is as high as 64.3 MT, accounting for about 50% in China However, the recycling of used aluminum is not in place In 2017, China produced 3.65 MT of aluminum foil, but most of the used aluminum foil was directly landfilled In fact, waste aluminum can be reused completely and hydrogen can be produced by hydrolysis reaction Recently, the research group has conducted life cycle assessment on the hydrogen production process from aluminum scrap (Figure 5), highlighting the advantages of hydrogen production from aluminum scrap in terms of energy loss and carbon emission, and its energy output and input ratio is about nine times that of electrolytic hydrogen (Xu S., et al Liquid metal activated hydrogen production from waste for power supply and its life cycle assessment
International Journal ofHydrogen Energy , 2019 DOI:
10.1016/j.ijhydene.2019.05.176 ） 。 In the study, the feasibility of hydrogen production from waste aluminum was further proved In order to save cost, the best material ratio of liquid metal to aluminum and the reusability of liquid metal were also explored Figure 5 conceptual model of hydrogen production from waste aluminum (source: International Journal of hydrogenenergy, 2019) Of course, there are still many shortcomings in this technology In the future, more efforts will be made in strengthening the precise control of reaction, adjusting and controlling the separation of by-products in real time, reducing the amount of liquid metal, optimizing the recovery of liquid metal, etc In general, aluminum water reaction hydrogen production triggered by room temperature liquid metal is expected to achieve instant on-demand hydrogen production, slow down the safety hazards in the process of hydrogen transportation and storage, convenient operation, environmentally friendly, and will play an important role in special occasions If the waste aluminum is recycled, the use cost of this technology will be greatly reduced, and more possible ways will be provided for the coming of "hydrogen economy" era!