Current Status and Prospects of Key Technologies for Green Hydrogen Production in China

Abstract: This article introduces the system overview of traditional hydrogen production technology, focusing on the current mainstream key technologies for green hydrogen production. Based on this, the advantages and disadvantages of different advanced hydrogen production technologies are compared, and the bottlenecks and barriers in hydrogen production equipment, technology, economy, and standard system are analyzed in depth. At the same time, corresponding countermeasures to promote the orderly development of the hydrogen production industry are pointed out, and finally, the development prospects of green hydrogen production technology in China are pointed out.

Hydrogen energy is a secondary energy source that must be converted from the hydrogen element present in compounds through chemical processes. China is a major producer of hydrogen (H2), with a total hydrogen production of over 30 million tons in 2021. Currently, H2 is mainly used in the chemical industry in China, with fossil fuel production accounting for nearly 80% of H2, while the total amount of H2 produced by electrolysis of water and other green hydrogen production technologies is less than 1%. At present, China's energy central enterprises have regarded the construction of a hydrogen energy supply system as an important development direction. National energy giants such as State Power Investment Corporation, China National Petroleum Corporation, and China Petrochemical Corporation have chosen different technological routes based on their own advantages, and have successively begun to lay out hydrogen energy production and supply in China. China Shipbuilding Heavy Industry Corporation and some private enterprises have the conditions for commercial promotion of hydrogen production technology and equipment. In addition, Chinese enterprises and research institutes are actively exploring other new hydrogen production technologies or low-cost hydrogen production technologies, but there is still a long way to go before their industrial application.

Overview of Traditional Hydrogen Production Technology System

Among the traditional hydrogen production methods, hydrogen production from fossil energy such as coal and natural gas reforming is the mainstream of industrial hydrogen production today. The current fossil fuel hydrogen production process is mature and can be used for large-scale industrial production, with relatively low raw material prices. However, the hydrogen production process emits a large amount of CO2 and pollutants. Industrial by-product hydrogen is mainly distributed in chemical industry, metallurgy and other fields, of which hydrogen production from coal gasification (Figure 1) is large in scale, relatively mature in technology and low in cost, but it is faced with bottlenecks such as high pollution and low purity of hydrogen production. Hydrogen production from chlor alkali by-products has good potential for application due to its economic, simple operation, and high purity characteristics. However, there are also problems with small hydrogen production and scattered production capacity.

Hydrogen production from fossil energy has a high carbon emission, of which hydrogen production from coal has the highest carbon emission, and the carbon emission from producing 1kg H2 exceeds 20kg CO2. At present, most of China's electricity comes from thermal power, so carbon emissions are very high, even exceeding coal to hydrogen production. In recent years, with the development and improvement of fossil fuel hydrogen production coupled with carbon capture technology, carbon emission intensity will significantly decrease, but still higher than that of renewable energy hydrogen production, and it will bring higher carbon capture costs.

Research progress on key technologies for advanced green hydrogen production

2.1 Hydrogen production technology through electrolysis of water

At present, commonly used hydrogen production technologies through electrolysis of water include alkaline electrolysis, proton exchange membrane electrolysis, and solid oxide electrolysis.

2.1.1 Alkaline electrolysis of water for hydrogen production

The alkaline electrolysis water (AWE) hydrogen production device consists of an electrolytic cell and an auxiliary system, using KOH as the electrolyte and a porous membrane as the diaphragm. Under the stimulation of direct current, H2O is decomposed into H2 and O2. The advantages of alkaline electrolysis for hydrogen production are that it does not require precious metals as catalysts, has relatively low costs, mature equipment technology, good product durability, and a service life of up to 30 years. The disadvantages are that the required diaphragm is thick, the resistance is high, the working current for hydrogen production is low, and the equipment volume is large. In addition, due to the strong breathability of porous membranes, it is necessary to effectively ensure pressure balance on both sides of the electrolytic cell. More importantly, alkaline electrolytes react with CO2 in the air to form insoluble carbonates (such as K2CO3, Na2CO3, etc.).

2.1.2 Proton exchange membrane electrolysis of water for hydrogen production

The proton exchange membrane (PEM) used for hydrogen production through water electrolysis is very thin and has low resistance, which can withstand large currents while maintaining high efficiency. Therefore, the equipment volume and footprint are much smaller than alkaline electrolysis water equipment. At the same time, due to the use of impermeable membranes in PEM electrolyzed water, it can withstand greater pressure without strict pressure control on both sides, and can achieve rapid start stop. The amplitude and response speed of power regulation are also much higher than those of alkaline electrolyzed water. The current PEM hydrogen production technology abroad is relatively mature and has entered the early stage of market-oriented application. Representative companies such as Proton, Siemens, and ITM Power have successively distributed megawatt level PEN electrolyzed water system products, vigorously promoting their large-scale application. The development of China's PEM hydrogen production industry is relatively lagging behind. Although some enterprises have formed hydrogen production prototypes with a high degree of autonomy, there is still a bottleneck problem with key materials such as proton exchange membranes. Subsequent efforts should be intensified to tackle key technologies such as low-cost catalysts and gas diffusion layers, in order to improve the efficiency and lifespan of critical equipment.

2.1.3 Solid oxide electrolysis of water for hydrogen production

Solid oxide electrolyte (SOEC) electrolysis of water for hydrogen production is a high-temperature electrolysis technology with an operating temperature of 700-1000 ℃. Its structure consists of a porous hydrogen electrode, an oxygen electrode, and a dense solid electrolyte layer. Due to its high operating temperature, it can greatly increase the power of the reaction and significantly reduce electricity consumption. In certain specific situations, such as high-temperature gas cooled reactors, solar collectors, etc., SOEC electrolysis of water for hydrogen production technology has great potential for development. The SOEC electrolysis water hydrogen production technology has significant advantages in terms of electricity consumption, but there are still technical barriers such as high operating temperature, large investment, slow start stop, and low cycle life. It is still in the indoor verification stage and has not been commercially promoted. At present, in addition to solid oxide electrolysis of water, AWE and PEM hydrogen production have been widely applied.

2.2 Solar powered water splitting hydrogen production technology

At present, the existing solar water splitting hydrogen production includes three categories: photocatalytic hydrogen production, photoelectrochemical hydrogen production, and solid-state photothermal decomposition hydrogen production.

2.2.1 Photocatalytic hydrogen production

The principle of photocatalytic hydrogen production is to utilize the light absorption properties of photocatalysts to achieve the photocatalytic water splitting reaction. Photocatalysts can generate a certain number of photo generated electrons and holes under the action of light, which can reduce H2O molecules adsorbed on the catalyst surface to H2 (Figure 2). The special properties that optical conductor materials should possess include: ① a wide range of solar light response; ② High separation efficiency of electrons and holes; ③ Suitable surface reactive sites; ④ Strong durability, etc. Photocatalytic hydrogen production has the advantages of easy availability of photocatalytic materials, simple hydrogen production system, and low cost, and has broad application prospects. However, photocatalysts are still in the stage of demonstration research and development, and there are common problems such as low hydrogen production efficiency and easy recombination of photo excited electron hole pairs, which still have a long way to go before commercial application.

2.2.2 Photoelectrochemical hydrogen production

Photoelectrochemical (PEC) hydrogen production can generate a large number of charge carriers during the water splitting process, which can achieve long-term durability under strong light conditions and in strong electrolytes. So far, PEC hydrogen production photoelectrode materials that have been developed include GaAs, InGaN, MoS2, and metal selenides. MoS2 has the best hydrogen production effect due to its economic, simple synthesis process, and good photoelectric effect. Through extensive practice, it has been proven that modified MoS2 materials have better hydrogen production performance. By introducing high-performance carbon materials, the active sites on the surface of MoS2 can be significantly increased, and its electrical properties can be significantly improved.

2.2.3 Photothermal decomposition method for hydrogen production

As early as 1971, Ford et al. first reported the process of direct photothermal decomposition for hydrogen production. Its main principle is to achieve a system temperature of over 2000K under illumination, directly obtain H2 and O2 in one step, and finally use a separation device to obtain pure hydrogen. Therefore, the core of thermal decomposition hydrogen production (TWSC) lies in good temperature resistant materials and effective gas separation facilities. To significantly improve the efficiency and purity of TWSC hydrogen production, researchers have proposed hundreds of solar thermal chemical hydrogen production methods, including TWSC hydrogen production technologies such as HyS, Cu Cl, and S-I. Cu Cl hydrogen production has become the mainstream of TWSC hydrogen production due to its advantages of high hydrogen purity, low pollution, and economy. Pal established the Cu Cl hydrogen production model at the beginning of the 21st century and successfully applied it to the Algeria region with sufficient sunlight throughout the year. The on-site results showed that the solar energy utilization efficiency of the model was as high as 93%, and the annual hydrogen production exceeded 82 t/a.

2.3 Biomass hydrogen production technology

At present, biomass hydrogen production technology mainly includes two categories: thermochemical method and biological method.

2.3.1 Thermochemical hydrogen production

The current mainstream thermochemical hydrogen production technologies include biomass catalytic gasification, biomass reforming, and biomass pyrolysis for hydrogen production. The process flow is shown in Figure 3. The research focus of biomass catalytic gasification for hydrogen production is to improve the purity of H2 in the product, as H2S is also produced during the gasification process HCl、 Trace impurities such as alkali metals need to be treated with adsorbents in the reactor. The hydrogen production through bio oil reforming was first reported by NREL in the United States in 1997. It obtains bio oil through biomass pyrolysis and then combines it with steam reforming to achieve hydrogen production. After years of innovation and development, it has become a crucial hydrogen production technology. Compared with the former, the development of biomass pyrolysis for hydrogen production has reached a relatively mature level of technology, and there are currently multiple commercially operated biomass pyrolysis units worldwide. Compared with other hydrogen production technologies, thermochemical hydrogen production has significant advantages, but there are also certain technical bottlenecks, such as high cost of thermochemical hydrogen production, low hydrogen content in mixed products, and a large amount of impurities such as CO, H2S, and tar. These impurities can cause certain damage to fuel cells, so the mixed products are suitable as fuel or industrial raw materials, and are not suitable for high-purity hydrogen applications such as fuel cells.

2.3.2 Biogenic hydrogen production

The biological hydrogen production system includes anaerobic fermentation, photosynthetic organisms, and their coupling for hydrogen production. The fermentation of hydrogen by anaerobic bacteria is achieved through the decomposition of organic matter by anaerobic bacteria under the action of hydrogenase, thereby obtaining H2. This process can achieve hydrogen production without light energy. Photosynthetic biohydrogen production uses light energy as the reaction condition, which is conducive to the decomposition of aquatic hydrogen by photosynthetic microorganisms such as microalgae. The energy used for hydrogen production in this technology includes both bioenergy and light energy, so the efficiency of light fermentation for hydrogen production is generally higher than that of dark fermentation. The coupling technology of photosynthesis fermentation can combine the advantages of dark fermentation and photosynthetic biological hydrogen production. It can not only reduce the demand for light energy to a certain extent, but also significantly increase the production of H2, which is the main development direction of biological hydrogen production.

2.4 Nuclear hydrogen production technology

There are multiple ways to convert nuclear energy into hydrogen energy, including using nuclear power to electrolyze water for hydrogen production, or using the heat generated by nuclear reactors for hydrogen production. Nuclear power generation for hydrogen production is the same as ordinary electrolysis of water for hydrogen production technology, and using nuclear reactors to generate heat for hydrogen production is a hydrogen production technology with broad prospects for future applications. The hydrogen production principle is shown in Figure 4. Methane steam reforming (SMR) is the main hydrogen production method in industry. When using the heat generated by nuclear reactors as the heat source for steam reforming, it can significantly reduce the amount of methane required and the cost of the process. But this technology still belongs to fossil fuel hydrogen production, which will generate a large amount of greenhouse gases, which is not conducive to promoting the process of carbon neutrality. High temperature electrolytic hydrogen production uses high-temperature steam generated by nuclear reactors as raw material, and the power consumption can be reduced to 2.8 kWh/m3, which is much lower than traditional hydrogen production. However, it still faces barriers such as immature technology and high costs. The thermochemical cycle decomposition of water for hydrogen production utilizes the heat generated by nuclear reactions to directly produce hydrogen. However, since the reaction needs to be carried out at high temperatures above 2500 ℃, it is difficult to apply in practice. Therefore, how to use thermal cycles to control the reaction temperature within a suitable range is the main focus of this field in the future.

2.5 Seawater Hydrogen Production Technology

Due to the complex composition of seawater and the lack of effective catalysts, direct electrolysis of seawater can lead to side reaction competition, catalyst deactivation, membrane blockage, and other issues during the production of H2. Based on this, many experts and scholars have proposed different indirect seawater hydrogen production technologies. Researchers have used solid oxide electrolysis technology to electrolyze seawater, first converting it into high-temperature steam and then electrolyzing it. Most impurities in seawater do not come into contact with the electrolysis device, so the electrolysis efficiency is relatively good. However, due to the delayed development and poor economic efficiency of solid oxide electrolysis technology, its activity is relatively low globally. Desalination electrolysis hydrogen production is the mainstream technology for seawater hydrogen production today. It first desalinates seawater through treatment technology and then combines mature freshwater hydrogen production technology to produce H2. From multiple perspectives, seawater desalination for hydrogen production has obvious advantages, but due to limitations in relevant technical conditions, this technology is still in the laboratory stage and has a long way to go before practical application.

3 Evaluation and Countermeasures

3.1 Comparison of Characteristics of Different Green Hydrogen Production Technologies

In recent years, countries around the world have conducted extensive research on processes such as electrolytic water hydrogen production, biomass hydrogen production, and nuclear hydrogen production. Green hydrogen production methods are developing towards diversification, and various new hydrogen production technologies are becoming popular, playing an important role in promoting the utilization of global hydrogen resources. Different green hydrogen production technologies have different applicable conditions, application effects, and cost inputs, and their respective process characteristics are shown in Table 1.

3.2 Challenges faced by the industrialization of green hydrogen production

Although the new green hydrogen production technology has shown certain advantages in many aspects, due to the limitations of relevant technical conditions, there will inevitably be many practical problems in the application process.

(1) Firstly, in terms of hydrogen production equipment and technology, although relevant domestic enterprises have carried out corresponding technology research and development, they are still in the stage of small-scale trial production and have not yet formed mature hydrogen production lines. The maturity of related core technologies is low, and the degree of localization of system equipment is not high.

(2) Secondly, in terms of economy, high input costs are still the biggest factor limiting the development of some green hydrogen production technologies. In addition, most hydrogen production processes require the addition of subsequent hydrogen purification technologies to obtain high-purity H2. How to effectively reduce hydrogen production costs is the main development direction in the future hydrogen production field.

(3) Finally, in terms of the standard system, the current hydrogen production industry in China is single and scattered, with a lack of key technical indicators and few mandatory national standards, making it difficult to meet the standardization needs of the hydrogen production industry.

3.3 Strategies for promoting the orderly development of the hydrogen production industry

Strengthen the research and development of core hydrogen production technologies, optimize the efficiency of hydrogen production technology, and effectively improve the durability of key materials such as photocatalysts and reactors, thereby maximizing H2 production.

Focusing on the improvement of key technologies such as low-cost catalysts and gas diffusion layers, we aim to enhance the efficiency and lifespan of hydrogen production facilities while accelerating the realization of low-cost hydrogen production, purification, and maximizing cost reduction and efficiency improvement.

Breaking the traditional standardization mode of hydrogen production, establishing a systematic and complete hydrogen production process industry chain, quickly filling the gap between hydrogen production standardization work and technological development, and narrowing the gap with mature standard systems in countries such as the United States and Japan.

4 Outlook

Although the new green hydrogen production has significant advantages in many aspects, due to limitations in energy consumption, cost, and other aspects, some technologies have not yet been applied in actual production. It is not easy to truly achieve green and low-carbon hydrogen production. Suggest starting from different perspectives and taking multiple measures to jointly promote the orderly development of China's green hydrogen production industry. Firstly, enhance international cooperation and actively carry out research and development of transformative hydrogen production technologies that are suitable for China's national conditions, taking into account the current situation of China's green hydrogen production industry. Secondly, deep interdisciplinary integration of multiple disciplines such as reaction kinetics, thermodynamics, gas separation, and material durability endows hydrogen production processes with targeted and efficient objectives. Thirdly, we will combine theory and practice, strengthen the practical evaluation and economic feasibility analysis of advanced hydrogen production technologies, and focus on both indoor verification and market practice, providing strong support for promoting the large-scale application of green hydrogen production processes in China.

Article source: Modern Chemical Industry

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