Based on Faraday junction, efficient photocatalytic dark reaction hydrogen production is realized

On March 29, 2023, the team of Sun Zhengming/Ruan Qiushi of Southeast University and Jiang Chaoran, associate researcher of Sinopec Beijing Research Institute of Chemical Industry, published a research result entitled “Stored photoelectrons in a faradaic junction for decoupled solar hydrogen production in the dark” in the journal Chem.

The dark reaction process in natural photosynthesis can store photogenerated charges and use them in dark conditions, which is a decoupling solar energy utilization strategy. In this study, Faraday junctions are used to simulate the process of natural photosynthesis in the dark reaction, and the photocatalytic reaction achieves continuous and efficient hydrogen production under intermittent sunlight.

The corresponding authors of the paper are Ruan Qiushi, Jiang Chaoran and Sun Zhengming; The first authors are Ruan Qiushi and Xi Xufeng.

Faraday junction is a heterojunction with capacitive properties, first proposed by Professor Luo Wenjun of Nanjing University (iScience 23, 100949, 2020; Chem. Sci. 11, 6297, 2020)。 Unlike conventional p-n junctions, z-type heterojunctions, and type II heterojunctions, when the photogenerated charge passes through the Faraday junction interface, the charge is stored in a redox reaction. Since this electrochemical charging process loses almost no energy, the stored electrons/holes have a redox capability similar to the initial excited state. In addition, Faraday junctions have the characteristics of photovoltage memory effect, that is, the dark-state output voltage after turning off the light can record the photovoltage. It is used in photoelectric catalysis, solar cells and photoelectric energy storage (Nat. Commun. 12, 6363, 2021;Nat. Commun.13, 2544, 2022;Natl. Sci. Rev. DOI: 10.1093/nsr/nwac249, 2022)。

Based on the Faraday junction concept, the authors designed a new Faraday junction formed by titanium oxide (TiOx) and carbon nitride (CN). For the first time, it was found that TiOx/CN Faraday junctions can not only increase the yield of hydrogen production from methanol reforming under light, but also release hydrogen by proton reduction within 30 minutes after the illumination stops, and its dark hydrogen production is as high as 2.4 mmol/g (Figure 1). In a 6-minute cycle lasting one hour of light and dark alternation, hydrogen production is 16% higher than that of continuous light for 30 minutes. This means that the TiOx/CN Faraday junction can store excess photogenerated electrons to improve the efficiency of photocatalytic reactions, and provide an effective way for the decoupling of sunlight from photocatalytic reactions.

Figure 1: Hydrogen production performance of light/dark reaction of TiOx/CN Faraday junction

Combined with in situ XPS, ESR, UV-VIS and other characterization methods, the authors proved that photogenerated electrons flow from CN to TiOx under illumination conditions, and Ti4+ was reduced to Ti3+, thereby proving the storage of photogenerated electrons in the Faraday node, and using photoelectrochemical methods to confirm the isoenergy transport process of photogenerated electrons in the Faraday node. The addition of deuterated methanol found that deuterium could penetrate deep into the TiOx/CN Faraday junction under illumination, and determined that TiOxH was an intermediate product of photogenerated electron storage. Further theoretical calculations describe the storage and release process of photogenerated electrons in the TiOx/CN Faraday junction, and the results show that the formation of TiO2H makes the stored electrons have higher energies than the TiO2 conduction band, which provides a basis for the isoenergy transport of photogenerated electrons, and the stored electrons are inert to methanol, which means that methanol will not spontaneously consume these electrons, because dark hydrogen production becomes feasible.

In summary, combined with experimental and theoretical calculation results, the authors propose the mechanism of TiOx/CN Faraday junction dark hydrogen production (Figure 2). That is, photogenerated electrons are stored in the TiOx/CN Faraday junction interface in the form of TiOxH through the isoenergy transport process under illumination, during which Ti4+ is reduced to Ti3+. In the dark state, electrons are released through a reversible reaction that combines with protons to produce hydrogen. It is worth mentioning that the electrons stored in the Faraday junction will be inactivated when exposed to electronic sacrifices such as O2, so extending the storage time of electrons is the focus of the team’s next research.

Figure 2: Schematic diagram of the mechanism of hydrogen production in the dark state of Faraday junction

The research results deepen the knowledge and understanding of the storage and release mechanism of photogenerated electrons in the composite interface, and provide an experimental basis and theoretical basis for the development of efficient and controllable photocatalytic dark reaction hydrogen production technology and solving the problem of photocatalytic continuous hydrogen production under intermittent sunlight. This strategy can be extended to photocatalytic carbon dioxide reduction, nitrogen fixation, methane conversion and other fields. (Source: Science Network)

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