With the increasing demand for sustainable clean energy in human society, multiphase photocatalytic decomposition of aquatic hydrogen systems has attracted more and more attention.The catalytic system can convert solar energy into hydrogen fuel, which has important value in many fields such as energy, environment and catalysis.However, in the current report on the system, the energy conversion efficiency is too low, and there is a big gap with the actual production and application requirements.At the same time, the high demand for pure water may also exacerbate the shortage of freshwater resources.Therefore, the development of new catalytic systems and the full use of the extensive marine resources of the earth have become a new challenge for scientists in this field.
On January 4, 2024, the Edman Tsang team at Oxford University, in collaboration with Wu Xinping team at East China University of Technology, published a study titled "Electrolyte-assisted polarization leading to enhanced charge separation and solar-to-hydrogen conversion efficiency of seawater."The results report a new strategy for hydrogen production from high-temperature decomposition of seawater driven by sunlight, achieving an energy conversion efficiency of up to 15.9%, exceeding that of similar systems reported so far.At the same time, the results clearly reveal the mechanism of electrolyte ions in seawater to promote the separation of photogenic carriers in the catalytic system.
Photocatalytic decomposition of aquatic hydrogen systems has received widespread and sustained attention in recent decades for its efficient conversion of solar energy into hydrogen energy.Hydrogen has the advantages of green and high calorific value, which means that it may be an important alternative to fossil energy and long-term energy storage.Similar to other multiphase photocatalytic systems, there are the following key steps in the system: (1) photo-catalysts excite and produce excited photo-carriers (electrons and holes), (2) photo-carriers separate and migrate to the catalyst surface.The energy conversion efficiency of solar hydrogen production is one of the important parameters to evaluate the performance of the system. At present, the energy conversion efficiency reported in the literature is less than 5%.This is mainly due to the low separation efficiency of photoelectrons and holes, and the rapid recombination of photoelectric carriers, which fails to participate in subsequent chemical reactions, making it difficult to use solar energy efficiently.In addition, because the oceans contain more than 95% of the Earth's water resources, more and more people are focusing on photocatalytic decomposition of seawater to reduce the pressure on the strategy's freshwater demand.However, due to the presence of a large number of electrolyte ions mainly sodium chloride in seawater, it may have a complicated effect on the photocatalytic process, which is still controversial in academia.It is important to develop stable and efficient hydrogen catalysts for photolysis of seawater and to explore the mechanism of electrolyte ions in seawater.
Edman Tsang and Wu's team build on previous work on high-temperature multiphase photocatalytic decomposition of pure water (Nat. Commun., , 2019, 10, 4421; Energy Environ.Sci., 2022, 15, 265-277), a surface-regulated nitrogen-doped titanium dioxide nanoparticle catalyst (N-TiO2) was constructed and applied to the hydrogen production reaction of high temperature photodegradable seawater.In order to elucidate the mechanism of electrolyte in photocatalytic water decomposition, various artificial seawater was prepared and natural seawater was collected.The catalytic test results show that the photocatalytic activity is positively correlated with the ionic strength of the solution, which fully demonstrates the important role of electrolyte ions in the catalytic process.
In order to explore the mechanism of electrolyte effect in depth, Edman Tsang's team used a variety of characterization techniques to investigate.The results of time-resolved photoluminescence spectra show that the lifetime of photogenic carriers is also positively correlated with the ionic strength in solution.Both scanning transmission electron microscopy and near atmospheric pressure X-ray spectroscopy show that photoelectrons migrate selectively to (101) crystal surface of N-TiO2 catalyst, while photoelectrons migrate to (001) crystal surface.This surface-selective carrier migration leads to positive and negative charges, while surfaces with different charges attract oppositely charged electrolyte ions, forming a local electric field near the surface of the catalyst.The local electric field will then react on the photocarrier inside the catalyst, prolonging its life.Ultimately, this electrolyte-assisted polarization effect greatly improves the catalytic hydrogen production activity and energy conversion efficiency.Wu Xinping's team constructed a model close to the actual photocatalytic reaction system, carried out the systematic density functional theory (DFT) calculation, and innovatively put forward electrolyte induced charge polarization energy as an index.
Quantum efficiency and other performance testing and mechanism discussion.
In summary, this work systematically demonstrates a new strategy for hydrogen production from photolysis of seawater at high temperature, clarifies the key role of electrolyte ions in the system.This work provides new ideas for the industrialization of photocatalytic decomposition water system, which is expected to promote the further development of the research field.