New materials to achieve hydrogen purification "in one step"

Ceramic membrane is an asymmetric membrane formed by inorganic ceramic materials prepared by a special process. Because of its good stability, high strength, and high efficiency, it can be widely used in many production and life fields such as food, beverage, plant (medicine) deep processing, biomedicine, fermentation, and fine chemicals.

Recently, Jiang Heqing, a researcher at the Qingdao Institute of Bioenergy and Processes, Chinese Academy of Sciences, cooperated with the University of Hannover in Germany to develop a new type of titanium-based two-phase mixed conductor oxygen permeable membrane. Compared with the chemical instability of traditional iron-based dual-phase membranes, titanium-based dual-phase membrane materials are treated for 100 hours in an atmosphere containing water vapor and high-concentration hydrogen, and still maintain the original phase structure and microscopic morphology, and are stable against reduction The sex is very good.

The researchers said that this new material can produce high-purity hydrogen without carbon monoxide (CO) in one step, which can be used as a fuel directly in a hydrogen fuel cell. Related results were published in "Germany Applied Chemistry" a few days ago.

Ceramic oxygen permeable membrane has obvious advantages

Ceramic oxygen permeable membrane is a kind of ceramic membrane with mixed conductivity of oxygen ions and electrons. It has 100% selective permeability to oxygen, which has obvious advantages compared with traditional technology.

"In the case of Air Products in the United States, the company uses membrane technology to produce oxygen. Compared with the traditional cryogenic technology, the investment cost is reduced by 25% to 30%, and the energy consumption is reduced by 35% to 60%." Jiang Heqing said.

In addition, in chemical production, membrane reactors can be used to combine the two independent processes of reaction and separation. “For example, the oxygen supply method on one side of the oxygen permeable membrane can be used to efficiently convert natural gas on the other side of the membrane into high-value chemicals such as synthesis gas, ethane, and ethylene, so as to realize the resource utilization of natural gas.” Jiang Heqing explained.

Based on this, the US Department of Energy formulated the Gas To Liquid (GTL) plan as early as 1992, which envisaged the use of membrane reactor technology to convert methane into synthesis gas, and successively established research led by Argonne National Laboratory and Air Products. group.

As a renewable energy source, hydrogen is widely used in ammonia synthesis, petroleum refining, semiconductor production and fuel cell industries. The use of abundant industrial by-product hydrogen as the hydrogen source of fuel cells is conducive to solving the high cost and large-scale storage and transportation problems of fuel hydrogen.

However, the industrial by-product hydrogen contains trace amounts of CO and other impurities, which will poison and deactivate the fuel cell electrode and seriously affect its operational stability. There is an urgent need to develop a new and efficient hydrogen production system to overcome the industrial by-product hydrogen separation and purification process. The process is complicated and the hydrogen purity is low.

"The preparation cost of this mixed conductor membrane is low, the process is simple, the process of chemical production is strengthened, the complicated and costly separation and purification process is avoided, and it has broad application prospects in the field of hydrogen separation and preparation." Jiang Heqing said.

Make hydrogen separation technology more economical

“At present, hydrogen production can be divided into four main streams in terms of process and technology: hydrogen production from electrolysis of water, hydrogen production from fossil fuels, hydrogen production from industrial by-products, and hydrogen production from biomass.” Jiang Heqing introduced, “hydrogen production from electrolysis water can be directly Obtaining fuel hydrogen without CO, but its high cost is not conducive to the widespread promotion of this technology. Although the cost of the three types of hydrogen production technologies such as fossil fuel hydrogen production is relatively low, the hydrogen produced cannot be directly used as fuel hydrogen. The reason is that CO is inevitably produced during the production process, and it must go through a subsequent separation and purification process before it can be used in fuel cells."

The Ministry of Science and Technology recently released the "Renewable Energy and Hydrogen Energy Technology" 2020 key special assessment indicators, clearly stated that the hydrogen CO content in the fuel cell system should not exceed 0.2ppm, which also puts forward new requirements for hydrogen production technology.

According to the author’s understanding, at present, pressure swing adsorption is usually used in the industry to separate and purify hydrogen. However, this method is complicated in process, energy-consuming, and needs to provide higher pressure during the purification process, which raises questions about the safety of the entire hydrogen production process. Very demanding.

However, Jiang Heqing pointed out: "The use of membrane separation technology can couple reaction and separation, and directly obtain CO-free hydrogen in one step, which is easy to integrate later, and has a smaller investment and floor space. It is a more economical and more promising solution. Hydrogen separation technology."

Relentless exploration for safety

Prior to this, the Jiangheqing team had carried out a lot of research on related technologies for hydrogen production by oxygen permeable membranes. Based on the coupling strategy proposed by Jiang Heqing, the team coupled the hydrogen production by water splitting and the catalytic conversion of low-carbon alkanes on both sides of the membrane. One side obtained CO-free hydrogen, and the other side obtained high additives such as syngas and ethylene. Value product, avoiding the complicated and costly separation and purification process.

However, in the test process of membrane materials, the hydrogen separation performance gradually decreases with time.

"In order to reveal the reason, we have carried out a comprehensive characterization of the membrane material after the test, and found that the membrane surface has undergone serious corrosion damage, especially the variable Co ions are deeply reduced and precipitated on the membrane surface, making the membrane structure It was severely damaged.” Team member Jia Lujian said that in the process of water splitting and hydrogen production driven by the combustion reaction based on the oxygen permeable membrane, both sides of the oxygen permeable membrane are in a harsh and strong reducing atmosphere, so the membrane material is required to have higher The chemical stability.

Jia Lujian explained that Co and Fe-based mixed conductor oxygen permeable membrane materials have received extensive attention and research due to their high oxygen permeability, but they have poor stability under low oxygen bias or reducing atmosphere, mainly due to Co4+ and Fe4+ for a long time. Under low oxygen bias or reducing atmosphere, it will be over-reduced and precipitate from the perovskite structure, and long-term operation will cause the membrane to fail. "This is also our original intention to explore and develop Ti-based film materials that can operate stably in harsh atmospheres."

In addition, the team also encountered certain challenges in the sealing of membrane materials. According to Jia Lujian, in order to achieve a better sealing effect, the team has successively adopted sealing materials such as glass powder, ceramic powder, silver wire, and gold wire. Because the quality of the seal directly affects the purity of hydrogen on the water splitting side, screening and optimizing different sealing materials is a very critical step for preparing CO-free hydrogen.

By comparing different materials and optimizing the sealing conditions, the team found that the use of silver wire can achieve a better sealing effect, isolate the leakage and diffusion of impurity gases on the low-purity hydrogen side, and at the same time avoid the risk of leakage of high-risk gases such as hydrogen, and ensure the membrane reactor. safety.

One-step preparation of high-purity hydrogen

Jiang Heqing stated that the newly developed Ti-based oxygen permeable membrane material solves the problem of poor stability of traditional Co and Fe-based oxygen permeable membrane materials in the coupling process of reaction and separation. Therefore, the Ti-based membrane material is used to construct membrane reactors in industry. In the process of water splitting hydrogen production driven by the combustion of by-product hydrogen, CO-free hydrogen can be produced efficiently and at low cost.

"The Ti-based membrane material we developed can produce CO-free hydrogen in one step." Zhang Yan, a team member and associate researcher at the Qingdao Institute of Bioenergy and Processes, Chinese Academy of Sciences, said that a dense ceramic membrane with oxygen ion-electron mixed conductivity Oxygen has 100% selective permeability. The high-temperature water decomposition reaction and the industrial by-product hydrogen combustion reaction are coupled on both sides of the ceramic oxygen permeable membrane reactor. The combustion of low-purity hydrogen can promote the water decomposition on the other side of the ceramic membrane to generate oxygen. The in-situ removal of the carbon dioxide can promote the efficient decomposition of water and obtain CO-free hydrogen, which can be directly used in the hydrogen fuel cell.

Since oxygen is transmitted through oxygen vacancies in the form of oxygen ions, the oxygen permeable membrane has an excellent interception function for other gases. This characteristic determines that the purity of hydrogen obtained from the water splitting side can theoretically reach infinitely high.

"Ti-based membrane materials exhibit excellent stability in a strong reducing atmosphere. We designed and synthesized Ti-based dual-phase oxygen-permeable membrane materials based on the feature that Ti ions do not undergo deep reduction in a reducing atmosphere, which solves the problem of traditional Co and Fe-based mixed conductor film is unstable in a reducing atmosphere. This research shows the unique advantages of Ti oxygen-permeable film materials in the field of hydrogen production." Zhang Yan added.

Based on the Ti-based membrane material developed, Jiangheqing team members further used the phase inversion method and the extrusion molding process to prepare hollow fiber membranes and tubular membranes, respectively. Compared with the sheet-like membrane, the membrane area and hydrogen gas permeability are significantly improved.

At present, the team is actively carrying out the installation and commissioning of hollow fiber membranes and tubular membrane modules. "It is expected to be used in the purification and separation of industrial by-product hydrogen in the future, and promote the commercial application of membrane separation hydrogen production technology." Jiang Heqing said.