Photocatalytic total water lysis to hydrogen production with close commercial efficiency

On January 5, 2023, Beijing time, Yonezawada’s research group at the University of Michigan published a new study entitled “Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting” in the journal Nature.

The infrared heat effect generated by high-intensity focused sunlight not only promotes the positive water decomposition reaction but also inhibits the reverse hydrogen-oxygen recombination reaction through the infrared heat effect generated by high-intensity focused sunlight in the photocatalytic total water lysis process on the InGaN/GaN surface, and the strategy makes InGaN nanowires show ultra-high photocatalytic total water lysis efficiency.

The corresponding author of the paper is Yonezawa Da; The first author is Zhou Peng.

As a new type of clean energy, hydrogen is widely used in fuel cells and petrochemical industries, but the current hydrogen is mainly obtained through water gas conversion reaction, which is not only prone to produce a large amount of carbon emissions, but also needs to consume a lot of heat energy. Hydrogen production from water based on photocatalysis is an environmentally friendly and sustainable technology that only consumes sunlight and water and does not produce any carbon emissions, so the technology has attracted widespread attention. However, the current photocatalytic total water lysis hydrogen production technology limits its practical application due to its low solar to hydrogen (STH).

At present, the factors restricting the efficiency of photocatalytic water splitting mainly include the following four aspects: First, the band gap of the photocatalyst semiconductor directly determines its theoretical maximum STH, although 40% of sunlight is located in the visible spectrum (400-700 nm), which can theoretically contribute 24% of STH in photocatalytic total hydrolysis water, but the light response range of the currently reported visible light responsive photocatalyst is usually limited to 400-485 nm, so its energy conversion efficiency is limited. In addition to ultraviolet and visible light, the infrared content in the solar spectrum is as high as 50%, but infrared light cannot directly photoexcite the photocatalyst to produce electrons and holes with sufficient energy to drive water decomposition, which also limits the maximization of photocatalytic total water lysis efficiency. In addition, although the theoretical maximum STH will increase with the decrease of band gap, too small band gap can easily lead to the semiconductor edge potential can not meet the redox potential requirements of water decomposition, so a strategy that can effectively use the whole solar spectrum to improve STH is required. Second, after the semiconductor photocatalyst is excited by light, the generated electron-hole pairs are prone to recombination, resulting in a decrease in the number of effective photogenerated electron-hole pairs, which causes the efficiency of the photocatalytic reaction to decrease. Third, when the photogenerated electron-hole pair reaches the surface of the semiconductor photocatalyst, it needs to react with water through the surface catalytic process to generate hydrogen and oxygen (2H2O → 2H2 + O2), but the high barrier of the surface catalytic reaction greatly reduces the formation rate of hydrogen and oxygen. Fourth, the hydrogen and oxygen produced are easy to undergo a composite reaction (2H2O ← 2H2 + O2) on the surface of the photocatalyst, which regenerates water, which greatly reduces the efficiency of photocatalytic water splitting.

Recently, Yonezawada’s research group prepared an InGaN/GaN nanowire photocatalyst with high crystallinity and wide visible light response range (<632 nm) on commercial silicon wafers by molecular beam epitaxial growth technology, and under the irradiation of high-intensity focused sunlight (3800 mW cm-2), the water decomposition efficiency of the nanowire showed obvious temperature-dependent characteristics, and a STH efficiency of 9.2% was observed at the optimal reaction temperature (70°C), close to the efficiency required for commercialization (10%) ), and can be maintained for 74 hours. The optimal reaction temperature (70°C) can be generated directly by the infrared heat effect of high-intensity focused sunlight without the need for an additional energy supply.

Figure 1: Structural characterization. (a) FESEM image of InGaN/GaN nanowires. (b) XRD plot of InGaN/GaN nanowires. (c) STEM images of InGaN/GaN heterostructures. (d) HRTEM of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires. Illustration: FESEM of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires. (e) STEM and elemental distribution of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires.

The mechanism study found that with the photocatalytic total hydrolysis reaction, the hydrogen and oxygen produced by water decomposition will remain unchanged after reaching a certain concentration, which is because the hydrogen and oxygen generation reaction and the composite reaction reach equilibrium at this time point. Further dark reaction tests showed that the contents of hydrogen and oxygen gradually decreased by a stoichiometric ratio of 2:1 and approached the equilibrium state, which confirmed the existence of hydrogen-oxygen recombination reaction, which was considered to be one of the main factors restricting the photocatalytic total hydrolysis to achieve maximum STH efficiency. However, the hydrogen-oxygen recombination reaction test at different temperatures showed that the equilibrium concentration of hydrogen and oxygen showed a strong dependence on temperature, especially the highest hydrogen-oxygen equilibrium concentration at 70°C, indicating that this temperature condition had the best inhibitory effect on hydrogen-oxygen recombination. In addition, DFT simulation confirmed that Rh is the main active center of hydrogen-oxygen recombination, and showed that the reaction was exothermic, so an appropriate increase in thermodynamics could inhibit the hydrogen-oxygen recombination reaction at the Rh site. However, when the reaction temperature exceeds 80 °C, the hydrogen-oxygen recombination trend increases, which is due to the further increase of temperature leads to the increase of the diffusion coefficient of hydrogen and oxygen, which accelerates the mass transfer in water, and this mass transfer dominates in hydrogen-oxygen recombination, so appropriate increase of temperature can inhibit the photocatalytic total hydrolysis reaction of hydrogen and oxygen recombination, and the optimal reaction temperature is 70 °C.

Figure 2: Performance evaluation and mechanism analysis. (a) Temperature dependence of the STTH efficiency of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires. (b) Stability test of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires under 3800 mW cm-2 high-intensity focused sunlight. Each cycle: 1 hour. (c) Temperature-dependent hydrogen-oxygen recombination. (d) Free energy change curve of hydrogen-oxygen recombination reaction on cocatalysts Co3O4, Rh and Cr2O3.

Figure 3: Temperature-dependent mechanism of photocatalytic total hydrolysis of water.

In order to confirm the wide practicability and feasibility of this technology, photocatalytic total hydrolysis tests were carried out between tap water and simulated seawater as water sources, and it was found that InGaN/GaN nanowires still had high STH efficiency (~7%). In addition, higher light intensity (~16,070 mW cm-2) is used in large-scale outdoor reaction systems, and high light intensity can greatly reduce the cost of photocatalyst materials per unit natural light area. The test results show that the InGaN/GaN nanowires on 4 cm × 4 cm commercial silicon wafers can not only exist stably under high light intensity and high temperature conditions, but also exhibit an energy conversion efficiency of 6.2% from natural light to hydrogen, which is the highest efficiency of similar natural photocatalytic total water lysis reaction systems to date, and also provides the possibility for the industrial application of photocatalytic total water lysis devices.

Figure 4: Practical and large-scale applications. STTH efficiency of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires in (a) tap water and (b) simulated seawater. (c) Image of outdoor photocatalytic total water lysis system and (d) STH efficiency of Rh/Cr2O3/Co3O4-InGaN/GaN nanowires at high intensity focused natural light outdoors (~16,070 mW cm-2). Each cycle: 10 minutes.

(Source: Science Network)

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