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First author: Yiqiang Sun
Corresponding authors: Professor Xu Kun, Academician Xie Yi
Communication units: School of Chemistry and Chemical Engineering, Anhui University, Hefei National Research Center for Microscale Physical Sciences, University of Science and Technology of China
Thesis DOI: 10.1002/anie.202109116
Full text at a glance
In general, the introduction of heteroatoms into the electrocatalyst is considered one of the important ways to modulate its intrinsic electronic structure to improve catalytic activity. However, for transition metal-sulfur compounds (HS-TMC) with highly symmetrical crystal structures, the introduction of heteroatoms, especially heteroatoms with large atomic radii, often leads to large lattice distortions and vacancy defects, which can lead to structural phase transitions or restructuring problems in the doped material during the catalytic reaction. This unpredictable situation makes it difficult to explore the relationship between the eigen-electronic structure and catalytic activity of doped catalysts. In this paper, Professor Xu Kun of Anhui University and Academician Xie Yi of the University of Science and Technology of China took the thermodynamically stable cubic CoSe2 phase as an example to demonstrate that the light element nitrogen doping strategy can effectively regulate the intrinsic electronic structure with the structurally stable phase HS-TMC, thereby improving its electrocatalytic hydrogen evolution activity. For comparison, the authors also confirmed that the introduction of phosphorus (with a relatively large atomic radius) leads to structural evolution from cubic CoSe2 to orthogonal phases, and that the structural phase of phosphorus-doped CoSe2 is unstable during the HER process. This work provides new insights into the rational design of electrocatalysts with highly symmetric crystal structures based on doping strategies.
Background
Driven by global environmental issues and the energy crisis, the search for new energy storage and conversion technologies to replace traditional fossil fuels has become critical. Among them, the production of hydrogen (H2) by electrolyzing water has attracted much attention due to its high energy density and environmentally friendly characteristics. Therefore, the development of highly active and durable electrocatalysts that facilitate the cathodic hydrogen evolution reaction (HER) process is critical. Although precious metals (e.g., Pt, Ru) are considered the most advanced catalysts, their high cost and scarce reserves greatly limit large-scale industrial applications. It can be said that it is very urgent to develop an efficient, stable and highly abundant transition metal-based catalyst to improve the efficiency of H2 conversion.
In recent years, many transition metal-based materials, including transition metal chalcogens (THCs), phosphides, nitrides, and carbides, have been used as electrocatalysts for HER. Among them, TMC materials are widely studied due to suitable d-electron configurations. To further improve the electrocatalytic activity of HER, heteroatomic doping is used to modulate the electronic structure of TMC. However, the introduction of heteroatoms is prone to cause large lattice distortion and vacancy defects, resulting in the phenomenon of reconfiguration or phase transition of the catalyst in the alkaline HER process. Especially for HMCs with highly symmetrical crystal structures, doping can even directly induce structural phase transitions. Therefore, in recent years, there has been controversy about whether the catalyst is really active or only used as a "pre-catalyst", which makes it difficult to establish the relationship between the eigen-electronic structure of the doped catalyst and the catalytic activity.
In this paper, the authors successfully achieved phase-free modulation of the eigen-electronic structure of cubic phase CoSe2 by doping light element nitrogen, thereby increasing its electrocatalytic HER activity in alkaline dielectrics by 5.4 times. High-resolution transmission electron microscopy (HRTEM) and high-angle circular darkfield scanning transmission electron microscopy (HAADF-STEM) did not find significant lattice distortions and vacancy defects in the nitrogen-doped cubic phase CoSe2 (N-c-CoSe2). Extended X-ray Absorption Fine Structure (EXAFS) curve fitting analysis and density functional theory (DFT) calculations show that N-doped into c-CoSe2 can regulate the electronic and coordination structures. In addition, in situ Raman spectroscopy confirms that N-doped CoSe2 retains the cubic phase structure during the HER process, guaranteeing its true initial "active species" role. This light element doping strategy opens a new door for studying the relationship between the intrinsic electronic structure and catalytic activity of doped catalysts.
Graphic and text analysis
Figure 1. Structure and morphological characterization of N-c-CoSe2 nanowires: (a) geometric structure of cubic phase CoSe2 crystals; (b) XRD diffraction patterns for N-c-CoSe2 and c-CoSe2; (c) SEM plot; (d) HRTEM plot, inset TEM plot; (e) HAADF-STEM plot; (f) [1-2] cubic phase CoSe2 fitted crystal structure from the perspective of [1-2], where blue is Co and yellow is Se; (g-i) HadDF-STEM plot corresponding line sweep EELS element distribution curve and element map graph.
Figure 2. XPS and XAFS characterization of c-CoSe2 and N-c-CoSe2: (a,b) Co2p and N1s XPS spectra; (c) Co K-edge XANES spectra; (d) corresponding FT-EXAFS curves; and (e,f) wavelet transform spectra.
Figure 3. Basic HER electrochemical properties: (a) o-CoSe2, c-CoSe2, P-o-CoSe2, N-c-CoSe2, IR correction polarization curve of N-c-CoSe2 catalyst in 1 M KOH solution; (b) HER performance comparison; (c) Tafel curve; (d) Nyquist curve, illustration of fitted equivalent circuit; (e) Cdl linear fitting and calculation; (f) Cdl normalized HER polarization curve.
Figure 4. Structural stability tests: (a) IR correction polarization curve before and after 5000 CV cycles; (b) timing current curve for 24 h at 130 mV overpotential; (c) XRD diffraction pattern and (d) HRTEM plot of N-c-CoSe2 catalyst after HER test; (e) in situ Raman spectra of N-c-CoSe2 and (f) P-o-CoSe2 at various potentials in HER.
Figure 5. Schematic diagram of the structural evolution of the doped CoSe2 catalyst in the alkaline HER process.
Figure 6. Theoretical calculations of c-CoSe2 and N-c-CoSe2: (a) PDOS of Co; (b,c) the calculated change in herr free energy at the Se site of the Co site; and (d) the calculated adsorption energy of the water.
Summary and outlook
In this paper, the authors demonstrate that light element nitrogen doping can effectively regulate the intrinsic electronic structure of cubic phase CoSe2, thereby improving its catalytic activity and structural stability during alkaline HER processes. HaADF-STEM results show that nitrogen-doped cubic CoSe2 has few vacancy defects. XRD, HRTEM and in situ Raman tests after HER testing all showed that the cubic CoSe2 phase remained unchanged during the HER process (i.e., there was no phase transition in the metal Co structure). In addition, first-principles calculations confirm that the catalytic activity of nitrogen-doped cubic phase CoSe2 is higher than that of pure cubic phase CoSe2 due to the optimized electronic structure. This light element doping strategy provides a new idea for exploring the structure-activity relationship of catalysts of various metal chalcogenetic compounds.
Corresponding Author Introduction
Xu Kun, Professor, graduated from the Department of Chemistry of Anhui University with a bachelor's degree in chemistry in July 2010 and graduated from the Department of Chemistry of the University of Science and Technology of China with a doctorate degree in science in June 2015. From June 2015 to June 2017, he was engaged in postdoctoral research at the University of Science and Technology of China, and from June 2017 to June 2020, he was engaged in postdoctoral research at Nanyang Technological University in Singapore. In June 2020, he went to work in the Department of Chemistry of Anhui University. Mainly engaged in the regulation of the electronic structure of low-dimensional inorganic solid functional materials and its application in the field of electrocatalysis, a series of innovative research results have been made in related fields. Has been named as the (co-) first author/corresponding author in J. Am. Chem. Soc.，Angew. Chem.，Adv. Mater.，Nano Lett.，ACS Energy Lett.， ACS Catal.， ACS Mater. Lett., Chem. Sci.，J. Mater. Chem. A，Appl. Catal. B: Envirion. and other important international academic journals have published more than 20 papers.
Xie Yi was born in July 1967 in Fuyang, Anhui Province. He is a member of the Communist Party of China and a professor at the School of Chemistry and Materials Science and the National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China. He graduated from the Department of Chemistry of Xiamen University in 1988 with a bachelor's degree. After receiving his Ph.D. in Applied Chemistry from the University of Science and Technology of China in 1996, he stayed on to teach, and has since gone to the State University of New York at Stony Brook and Penn State University for postdoctoral research and visits. In 1998, he was promoted to professor after being awarded the National Outstanding Youth Fund, in 2000 he was selected as the third batch of Yangtze River Distinguished Professors of the Ministry of Education, in 2003 he became the academic leader of the Innovation Group Fund of the National Foundation of China, and since 2009, he has been the director of the Academic Committee of the School of Chemistry and Materials Science. In 2013, he was elected as an academician of the Chinese Academy of Sciences. In 2015, he was elected a fellow of the Academy of Sciences for Developing Countries (TWAS). Mainly engaged in inorganic solid state chemistry research. He has established a method for preparing non-oxide materials by solvatonic heat, proposed a variety of binary feature structure synergistic strategies, and realized the construction of functional nanomaterials with a series of complex structures; proposed the use of rich phase transition behavior in inorganic solids and the two-dimensional ultra-thin structure of semiconductors to achieve synchronous modulation of electrical and acoustic transportation, and obtained efficient thermoelectric materials; developed inorganic graphene chemistry, solved the problem that its ultra-thin structure cannot give accurate atomic position; revealed the fine structure of the two-dimensional ultra-thin structure of a series of semiconductors. The regulation law between the electronic structure and the basic properties of thermoelectric and optoelectronics. The above work was published as a corresponding author including Nature Commun., J. Am. Chem. Soc.、Angew. More than 300 SCI papers, including Chem. and Adv. Mater., and 1 Science as the first author. It has been cited more than 14,000 times by SCI, and the H factor is 66 according to the corresponding authors. The relevant work has twice been selected as a major achievement in the major scientific and technological infrastructure of the Chinese Academy of Sciences.
Literature source
Yiqiang Sun, Xiuling Li, Tao Zhang, Kun Xu,* Yisong Yang, Guozhu Chen, Cuncheng Li, Yi Xie, Nitrogen-Incorporated Cobalt Diselenide with Cubic Phase Maintaining for Enhanced Alkaline Hydrogen Evolution. Angew. Chem. Int. Ed. 2021, DOI: 10.1002/anie.202109116.
Literature links: https://doi.org/10.1002/anie.202109116