CHEMICAL SCIENCE

Scientists have realized efficient and stable electrolysis of natural seawater to produce hydrogen


On January 30, 2023, Professor Ling Tao of Tianjin University and the team of Professor Qiao Shizhang of the University of Adelaide in Australia published a research result entitled “Direct seawater electrolysis by adjusting the local reaction environment of a catalyst” in the journal Nature Energy.

By introducing hard Lewis acid materials on a series of common catalyst surfaces, this achievement constructs a local alkaline reaction microenvironment on the catalyst surface, and achieves efficient and stable electrolysis hydrogen production in near-neutral natural seawater without purification/desalination treatment and without adding strong alkalis. This achievement provides a general strategy for regulating the reaction microenvironment and is applicable to a variety of catalyst systems.

The corresponding authors of the paper are Ling Tao and Qiao Shizhang; The first authors are Guo Jiaxin and Zheng Yao.

Electrolysis of water is the most promising “green hydrogen” technology and is essential to achieving the global goal of “zero carbon”. At present, the mainstream electrolysis water hydrogen production technology adopts high-purity water, and the production of 1 kg of hydrogen requires about 10 kg of water, and the large-scale application in the future is bound to aggravate the shortage of fresh water resources. Seawater accounts for 96.5% of the earth’s water reserves, is rich in resources and is a natural electrolyte. In the early 70s of the 20th century, scientists put forward the idea of direct electrolysis of seawater to produce hydrogen, which attracted the attention and research of a large number of scholars in the fields of energy, materials and chemistry, but after nearly 50 years of research, direct electrolysis of seawater to produce hydrogen still faces great challenges: (1) natural seawater is nearly neutral (pH~8), and the catalyst activity is poor; (2) Natural seawater contains a large amount of Cl−, which will inhibit the oxygen evolution reaction of the anode and corrode the catalyst; (3) Natural seawater also contains a large number of metal cations such as Mg2+ and Ca2+, which will produce precipitation during electrolysis and block the electrode. At present, seawater treatment processes such as purification/desalination are mainly used internationally to remove Cl− and metal cation impurities in natural seawater, and strong alkali is added to purified seawater to improve the efficiency of hydrogen production. However, this can significantly increase the cost of direct electrolysis of seawater hydrogen production systems, hindering large-scale adoption.

In this work, Professor Ling Tao of Tianjin University and Professor Qiao Shizhang of the University of Adelaide in Australia built a new catalytic material system, introduced hard Lewis acid materials on the surface of the existing catalyst, and constructed a favorable local alkaline reaction microenvironment on the surface of the catalyst, so as to achieve efficient and stable direct natural seawater electrolysis hydrogen production without purification/desalination treatment and without adding strong alkali. This in-situ constructed local alkaline reaction microenvironment promotes the hydrogen production activity of the catalyst by electrolysis of water, effectively inhibits the harmful Cl−-related reaction on the surface of the catalyst, and alleviates the problem of precipitation forming blocked electrodes.

Mechanism of local alkaline reaction microenvironment formation. The results of in situ infrared spectroscopy showed that the introduction of hard Lewis acid material on the catalyst surface could promote the dissociation of water molecules (H2O →OH− + H+), and the generated OH− was enriched on the catalyst surface to form a local alkaline reaction microenvironment. In situ UV-Vis spectroscopy and isotope labeling proved that the mechanism of hydrogen production from seawater electrolysis of catalysts changed from neutral to alkaline in the local alkaline microenvironment, which significantly improved the activity of hydrogen production by electrolysis in natural seawater.

Figure 1: Source of OH- in the local alkaline microenvironment on the catalyst surface.

The local alkaline reaction microenvironment promotes the efficiency of hydrogen production by electrolysis of natural seawater. Without purification/desalination and without adding strong alkalis, the natural seawater electrolyzer constructed by this work obtains a hydrogen evolution current density of 1.0 A cm-2 at 1.87 V to meet the needs of industrial applications. This performance is much higher than the existing natural seawater and alkalized seawater electrolysis hydrogen production performance, and close to the existing proton exchange membrane electrolyzer using precious metals.

Figure 2: Performance of flow-based natural seawater electrolyzers.

The locally alkaline reaction microenvironment inhibits chlorine chemical reactions. After the introduction of hard Lewis acid material on the catalyst surface, due to the strong combination of OH− and hard Lewis acid material, it is preferentially enriched on the catalyst surface, thereby inhibiting the diffusion of Cl− from seawater to the catalyst surface.

Figure 3: Oxygen evolution activity and chemical analysis of chlorine of catalysts in natural seawater.

The locally alkaline reaction microenvironment alleviates the problem of precipitation clogging of the electrode. Due to the strong combination of OH− and hard Lewis acid material, OH- generated in the cathodic hydrogen evolution reaction can be slowed down and rapidly diffused into seawater under the drive of electric field, thereby avoiding the rapid rise of seawater pH value and generating a large number of precipitates.

Figure 4: Analysis of hydrogen evolution activity and precipitation generation of catalysts in natural seawater.

The results provide a general strategy for regulating the reaction microenvironment, which can be applied to a variety of catalyst systems. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41560-023-01195-x



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