News Resource: Chemistry and Chemical Engineering

Editor: News Agency of BIT

Translator: Han Miaomiao, News Agency of BIT

Recently, the team of associate professor Feng Caihong from the School of Chemistry and Chemical Engineering of BIT has made important research progress in the regulation of anode oxygen evolution reaction catalyst materials for water electrolysis for produce hydrogen. The relevant results was published in the top international chemical journal Applied Catalysis B: Environmental (impact factor 24.319) under the title Construction dual-regulated NiCo₂S₄@Mo-doped CoFe-LDH for oxygen evolution reaction at large current density. The corresponding author of this article is Associate Professor Feng Caihong, and the first author is Shen Xueran (now a doctoral student in engineering) from the School of Chemistry and Chemical Engineering, BIT, whose research direction is industrial high-current electrolysis of water catalysis Material.

As a green and sustainable energy, hydrogen is expected to replace traditional fossil energy. Alkaline electrolysis of water to produce hydrogen is one of the ideal ways to deal with the global energy crisis and achieve the goal of carbon neutrality. However, due to the complex multi-proton coupling and multi-electron transfer process in the oxygen evolution reaction process of the anode in the process of water electrolysis, the actual potential is much higher than the theoretical potential. Therefore, non-precious metal electrocatalysts are developed to improve the efficiency of electrocatalytic oxygen evolution reaction. The main problem currently facing.

As a representative of binary metal sulfides, NiCo₂S₄has been shown to have excellent OER catalytic activity. At the same time, many researchers have modified it by interface engineering in order to further improve its catalytic activity (ACS Appl. Mater. Interfaces 2017, 9, 15364-15372), but these studies are mostly based on small current density (<100 mA cm -2), there are still many problems in industrial application in large-scale electrolysis of water. In addition, tuning the electronic structure by introducing high-valent metal ions such as Mo, V, and W into layered double metal hydroxides (LDHs) has been shown to be another effective means to improve the electrochemical performance of catalysts. Based on this, the advantages of interface engineering and element doping were effectively integrated, and a hierarchical structure catalyst with not only abundant heterointerfaces but also strong electronic interactions was designed and fabricated on a nickel foam substrate. The gas-repellent structure that tightly wraps the hollow nanotubes fully exposes the catalytically active area, which is a very suitable OER catalyst at high current density.

Fig. 1 Schematic diagram of the synthesis scheme of NiCo₂S₄@CoFeMox-LDH/NF

This work employs a three-step hydrothermal method to grow Mo-doped CoFe-LDH nanosheet-wrapped NiCo₂S₄nanotube-encapsulated hierarchical catalysts on nickel foam frameworks, and use them to catalyze OER. The nanotube arrays uniformly distributed on the surface of the nickel foam and the outer surrounding loose nanosheets greatly increase the catalytic surface area.

Fig. 2 (a) XRD patterns of NCS/NF and NCS@CFM0.075-LDH/NF. (b-g) XPS spectra of NCS, NCS@CF-LDH and NCS@CFM0.075-LDH. (h, i) XANES and EXAFE spectra of NCS@CF-LDH/NF and NCS@CFM0.075-LDH/NF at the Co-K side.

Through the XPS spectrum of NCS, NCS@CF-LDH and NCS@CFM0.075-LDH, it is clearly observed that with the recombination of CoFe-LDH and the introduction of Mo atoms, the pair of peaks for Co3+ shift towards low binding energy in the Co 2p XPS spectrum of NCS while in the S 2p XPS spectrum, the pair of peaks of S2 moves to the direction to gain electron, indicating that part of the electrons are transferred from Co to S, which means that the electronic structure of the material is dually regulated by interfacial effect and metal synergy. Afterwards, there is a strong interaction between NiCo₂S₄and CoFeM0.075-LDH. In addition, the higher oxidation state of Co is more beneficial to enhance the OER activity. The XAFS test further proved the changes in the local electronic structure and coordination environment after Mo doping. The XANES results were consistent with the XPS results, indicating that the oxidation state of Co was improved. The EXAFS test results showed that the Co-S/O bond length was increased, and the the coordination environment changes is expected to optimize the adsorption energy during the OER process.

Fig. 3 (a) OER polarization curves of NCS@CFMx (x = 0, 0.025, 0.05, 0.075 and 0.1)-LDH/NF. (b, c) OER polarization curves and corresponding Tafel plots of different samples. (d) Comparison chart of different samples. (e, f) double-layer capacitance(Cdl) and EIS plots of different samples.

Fig. 4 (a) Amount of O2 time-dependent experimentally measured versus time for NCS@CFM0.075-LDH/NF at 1 A. . (b) Stability of NCS@CFM0.075-LDH/NF at a current density of 1000 mA cm-2. (c) OER polarization curves for NCS@CFM0.075-LDH/NF before and after 1000 CV cycles. (d-i) XPS spectrum of NCS@CFM0.075-LDH after long-term OER test.

The effect of different Mo doping ratios on the overpotential ofelectrocatalytic activity was studied. The results show that the NCS@CFM0.075-LDH/NF catalyst exhibits the best OER catalytic activity, achieving 1000 mA cm-2 with only an overpotential of 332 mV, which is superior to the commercial catalyst IrO2/NF. At this high current density, it has nearly 100% Faradaic efficiency of electrocatalytic oxygen evolution reaction, and can work stably for more than 100h. This indicates that the NCS@CFM0.075-LDH/NF catalyst has great potential for industrialized large-scale water electrolysis. The characterization of the catalyst after long-term OER work shows that the microstructure is relatively stable, and the surface of the catalyst is reconstructed to form CoOOH, which is generally the real active site of the OER catalyst.

Fig. 5 Density functional theory calculations. (a-b) DOS of NCS, CFM-LDH and NCS@CFM-LDH. (c) Charge difference at the NCS@CFM-LDH interface. (d) Simulated OER reaction pathways for NCS@CFM-LDH. (e) Gibbs free energy diagram for the four steps of OER on NCS@CFM-LDH, NCS@CF-LDH and CFM-LDH.

The results of the catalysts DOS calculation show that the heterostructure and the introduction of Mo atoms leads to a higher density of carriers near the Fermi level, that is, faster electron transfer capability. The differential charge test results of NCS@CFM-LDH and NCS@CF-LDH also confirm that the introduction of Mo atoms plays a significant role in the regulation of the electronic structure. Finally, the Gibbs free energies of the OER four-electron reaction of NCS@CFM-LDH, NCS@CF-LDH and CFM-LDH as well as the reconstructed NiCo₂S₄@CoMoOOH, NiCo₂S₄@CoOOH and CoMoOOH catalysts were calculated. Strengthen the adsorption of the key intermediate O*, and appropriately reduce the value of DG2 to accelerate the OER reaction kinetics and improve the OER catalytic activity.

The above research work was supported by the National Natural Science Foundation of China.

Paper link:https://doi.org/10.1016/j.apcatb.2022.121917

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Feng Caihong, associate professor and doctoral supervisor of the School of Chemistry and Chemical Engineering, BIT. With the research direction of nanomaterials and applications, new energy materials and green catalysis. She received her Ph.D. degree from Beihang University in 2008. In 2012, she worked as a visiting scholar at Lawrence Berkeley National Laboratory and University of California, Davis. She has been working at BIT since June 2017, publishing more than 30 SCI papers In Nat. Commun., Appl. Catal. B: Environ., Chem. Eng. J., J. Mater. Chem. A, ACS Appl. Mater. Interfaces, J. Power Sources, ACS Sustain. Chem. Eng. , Mater. Chem. Front. and other important domestic and foreign journals.. And she has presided over and participated in a number of National Natural Science Foundation of China projects and enterprise horizontal projects.