On October 26, 2022, the Xue Junmin research team of the Department of Materials at the National University of Singapore, in collaboration with Dr. Xi Zhibo of the Institute of Chemistry, Energy and Environmental Sustainability, Agency for Science, Technology and Research, Singapore, Dr. Yu Zhigen of the Supercomputing Institute of the Singapore Science and Technology Agency, and Professor Wang Hao of the Department of Mechanical Engineering, National University of Singapore, published an article entitled “Pivotal role of reversible NiO6 geometric conversion in oxygen” in the journal Nature evolution”.
This achievement is the first to report a novel phototriggered OER mechanism (COM), in which metal acts as the redox center during deprotonation and oxygen acts as the redox center when oxygen-oxygen is bonded. Therefore, the COM mechanism can break through the drawbacks of the traditional OER mechanism and further improve the catalytic performance.
The corresponding authors of the paper are Xue Junmin, Wee Siang Vincent Lee, Wang Hao, Xi Zhibo and Yu Zhigen; The first author is Wang Xiaopeng.
Electrolysis hydrogen production is a promising energy storage technology, which is widely used to store intermittent energy such as wind energy and solar energy. It consists of two electrochemical reactions, hydrogen evolution (HER) at the cathode and oxygen evolution (OER) at the anode. Driving OER requires a higher overpotential compared to HER, which stems from its slow four-electron-proton transfer process. Therefore, improving the electrocatalytic activity of OER to reduce the voltage required for electrocatalytic oxygen production reaction is the direction of many scientific researchers. At present, there are two mechanisms of OER reaction: 1) the adsorption mechanism (AEM) of metal as a redox center when the electronic state near the Fermi level expresses metal properties; 2) When the electronic state near the Fermi level is oxygen, the lattice oxygen mechanism (LOM) of oxygen as the redox center is shown in Figure 1. Up to now, the OER mechanism reported in the literature is based on these two mechanisms. However, when the OER reaction follows the AEM mechanism (i.e., metal as redox center), oxygen-oxygen bonding is very difficult; When the OER reaction follows the LOM mechanism (i.e., oxygen as a redox center), the deprotonation process is slow, both of which directly restrict the further development of OER catalysts. In order to improve the electrocatalytic activity of OER, it is urgent to innovate at the theoretical level and break through the existing OER mechanism.
Figure 1: Two existing OER catalytic mechanisms. (a) The adsorption mechanism of metal as a redox center (AEM) when the electronic state near the Fermi level expresses metal properties; (b) When the electronic state near the Fermi level is oxygen, the Lattice oxygen oxidation mechanism (LOM) as the redox center is the oxygen mechanism. (Source: Nature)
In this work, the researchers found their previously reported strain-stabilized nickel hydroxide nanoribbon (NR-NiOOH), (Nature Communications. 11, 4647 (2020)； Energy & Environment Science. 13, 229, (2020)), the performance will be greatly improved when exposed to light, and stabilize after about 4 hours; When the lighting is turned off, performance gradually returns to the state it was in the dark, as shown in Figure 2. Further experiments show that the performance improvement is not due to the temperature increase, resistance change, grain growth and photocatalysis caused by light.
Figure 2: Electrochemical performance test. (Source: Nature)
Using in situ synchrotron radiation absorption spectroscopy, the researchers found that the valence state of nickel in NR-NiOOH gradually decreases when exposed to light, as shown in Figures 3a and b. This suggests that under light, the metal acts as the redox center for the catalytic reaction. Through further analysis of nickel electrons, coordination numbers and bond length during illumination, the researchers found that the valence state of nickel was reduced by the phase transition of the octahedron to the planar tetrahedron triggered by light. At the same time, the researchers found that when the octahedral to planar tetrahedral phase transition occurs, non-bonded oxygen is produced, suggesting that oxygen may also act as the redox center of the catalytic reaction when exposed to light. To test this hypothesis, the researchers performed TMA molecular probes and 18O isotope tests, which showed that oxygen also acts as a redox center for catalytic reactions when exposed to light.
Figure 3: (a) and (b) in situ synchrotron radiation absorption spectra; (c) TMA molecular probe test and corresponding synchrotron radiation absorption spectrum data; (d) 18O isotope test. (Source: Nature)
Based on these experimental results, the researchers proposed a new Coupled oxygen evolution mechanism (COM) for light triggering. When the catalytic reaction is based on this mechanism, the deprotonated metal is the redox center and oxygen is the redox center when O-O is bonded, as shown in Figure 4. Therefore, in principle, the COM mechanism can break through the drawbacks of the existing OER mechanism and further improve the catalytic performance. Subsequent theoretical calculations further proved this. At the same time, the researchers found that the key to the COM mechanism lies in the reversible deformation of the octahedron to the planar tetrahedron triggered by light, which is achieved by the transport of light-excited oxygen energy to the metal dz2 orbital electrons.
Figure 4: COM mechanism. (Source: Nature)
Next, the researchers conducted an in-depth study of the electron transport of light-excited oxygen energy to the metal dz2 orbital, and the results showed that this type of electron transport is closely related to crystal structure distortion. When the degree of crystal structure distortion is relatively low, such as NiOOH, its unoccupied dz2 orbital is completely coincident with the a1g* orbital. Since the a1g* band is formed by the hybridization of metal 4s and O2p orbitals, according to the Ni atomic orbital filling order, oxygen energy cannot be transported to the metal dz2 orbital electrons. This also explains why NiOOH does not have any current increase when illuminated. When the degree of crystal structure distortion is relatively high, such as NR-NiOOH, NiFeOOH and other doping systems, there is a non-coincident region between the unoccupied dz2 orbital and the a1g* orbital, so the electron transport of oxygen energy to the metal dz2 orbital can be realized, as shown in Figure 5.
Figure 5: Principles of the COM mechanism. (Source: Nature)
This study is the first to report a new photo-triggered OER mechanism (COM), which can break through the drawbacks of traditional OER mechanism and further improve catalytic performance. This work provides new ideas for OER research and is expected to promote the further development of the OER field, which is currently in a “dead end”. At the same time, the new mechanism of action of photocatalysts reported by the institute provides a new way to use solar energy, and provides new understanding and research strategies for more effective use of solar energy. (Source: Web of Science)
Related Paper Information:https://doi.org/10.1038/s41586-022-05296-7