Flexible deformable micro-nano superstructure can efficiently electrocatalyze oxygen evolution reactions

On September 7, 2022, Beijing time, Professor Wang Tie’s team at Tianjin University of Technology and the Institute of Chemistry of the Chinese Academy of Sciences published a new study entitled “Variable nanosheets for highly efficient oxygen evolution reaction” in the journal Chem, in which the research group designed and prepared a flexible, deformable micro-nano metastructure material. The material can improve the mass transfer efficiency of the catalytic interface, thereby increasing the electrocatalytic oxygen evolution reaction rate.

The corresponding author of the paper is Professor Wang Tie; The first author of the thesis is Dr. Qiao Xuezhi (currently an associate professor at Shandong University).

The transformation of the energy system is critical to achieving the goal of carbon peaking and carbon neutrality, and hydrogen as a clean, sustainable energy source is an ideal alternative to fossil fuels. Among the various water hydrogen production technologies, electrolyzed water to hydrogen plays a key role in the decarbonization efforts of the national economy. Among them, the Oxygen Evolution Reaction (OER) is a more energy-intensive reaction, and the high activity and stable OER is the key to electrochemical water decomposition. As an heterogeneous catalytic reaction, gas separation and reactant mass transfer at the reaction interface greatly affect the OER efficiency. The gas product adheres to the surface of the solid catalyst and gradually grows into bubbles, covering a large area of catalytically active sites. The limited mass transfer rate greatly hinders the diffusion of liquid phase reactants and affects the reaction rate.

In view of the above key scientific issues, Professor Wang Tie’s team from Tianjin University of Technology and the Institute of Chemistry, Chinese Academy of Sciences conducted a preliminary study (J. Am. Chem. Soc., 2020, 142, 9408-9414.) found that the atomic coordination and binding force on the surface of the nanosheet are weaker than those of the bulk material, so the mechanical properties of the material are negatively correlated with the thickness. In this study, by regulating the synthesis conditions of the material, a micro-nano superstructure material with a small Young’s modulus, flexible and deformable was prepared to accelerate the gas separation on the electrode surface and mass transfer at the interface, thereby increasing the rate of OER.

By regulating the reaction time, two metal organic framework (MOF) templates NF-MOF1h and NF-MOF5h of different thicknesses and morphologies were grown on the surface of nickel foam (NF). Aqueous phase unstable MOFs are converted into more stable cobalt-nickel sulfides (NF-CoNiS1h and NF-CoiS5h) by solvothermal vulcanization reactions. Scanning electron microscopy and transmission electron microscopy images show that the morphology of the nanosheets remains consistent during vulcanization. Powder X-Ray Diffraction (PXRD) and Raman spectroscopy confirm that the crystal structure of the synthesized MOF and cobalt-nickel sulfide is consistent with the crystal structure of Co-Ni MOF and NiCo2S4 (JCPDF: 20-0782).

Figure 1: Material characterization

Atomic Force Microscope (AFM) height images show that the thickness of cobalt-nickel sulfide nanosheets in NF-CoNiS1h is 34.9±2.1 nm, while the thickness in NF-CoNiS5h is 5.4±1.0 nm. The NF-CoNiS1h estimated to have a Young’s modulus (E) of 3.55±0.36 GPa, while the modulus of NF-CoNiS5h is only 0.17±0.02 GPa, similar to the modulus of soft, deformable materials such as DNA, rubber, and polymers. The deformation of the nanosheets was verified by in situ TEM, and the thin stacked nanosheets of NF-CoNiS5h are flexible, can expand and shrink, and exhibit periodicity, while NF-CoNiS1h nanosheets do not exhibit this expansion property. Using COMSOL Multiphysics, the deformation of materials of different thicknesses and Young’s modulus is simulated, indicating that the thinner the thickness, the smaller the modulus, and the greater the offset angle and offset distance. Stacked nanosheets with a thickness of 5 nm and a Young’s modulus of 0.3 GPa bend under the action of a simulated constant electric field, and the deflection angle and deflection displacement show periodic changes.

Figure 2: Mechanical properties of the material

In the chemical process of electrochemical oxygen production, the mass transfer efficiency of the electrode surface significantly affects the electrocatalytic rate. The periodic contraction and expansion of the material improves the mass transfer of reactants and products around the material, which is mainly reflected in two aspects. First, the separation of bubbles is accelerated; Second, the forced convection caused by deformation reduces the concentration gradient of reactants and products. Bubble detachment experiments of NF-CoNiS1h and NF-CoNiS5h during in situ monitoring of water electrolysis showed that the bubble behavior on the NF-CoNiS5h sample was significantly different from that of NF-CoNiS1h, in NF-CoNiS1h, the gas product adhered to the surface and continued to be produced, forming larger bubbles, and when the material adhesion was not enough to adhere to the bubbles, it was separated from the electrode surface. However, in NF-CoNiS5h, the bubbles are smaller and the separation is faster. Bubbles adhering to the solid electrode prevent water from coming into contact with the catalyst, forming inactive catalytic sites and reducing the reaction rate. Integrating the concentration field of reactants and products during the electrochemical reaction into the simulation of material motion under the applied electric field indicates that the consumption of deformable nanomaterial reactants and the rate of product generation increase, which is mainly due to forced convection caused by deformation. Even under low Reynolds number conditions, liquids can be pumped with irreversible kinetics through this forced convection. At the same time, flexible nanosheets exhibit chemotaxis, i.e., an electric field drives its active deformation to a region with higher concentrations of reactants to facilitate further catalytic reactions.

Figure 3: Bubble behavior indicated by the catalyst.

Compared with MOF, cobalt-nickel sulfides with similar morphology exhibited better electrocatalytic activity. It is worth noting that compared with NF-CoNiS1h, NF-CoNiS5h with a deformable nanosheet stacking structure shows further increased OER activity, manifested by a significant decrease in overpotential and a significant decrease in the slope of the Tafir. This is attributed to the deformation stacked three-dimensional nanosheet superstructure exposing a large number of surfactive centers and accelerating the separation of bubbles in the material indication, thereby facilitating rapid charge transfer at the electrolyte and electrode interfaces. In summary, NF-CoNiS5h, a micro-nano metastructure material with stacked structure, is a stable and practical electrocatalyst for OER.

Figure 4: Electrocatalytic activity.

This work was supported by the National Natural Science Foundation of China (21925405, 22104141, 22104142, 22004122), the National Key Research and Development Program of China (2018YFA0208800), the Chinese Academy of Sciences (XDA23030106, YJKYYQ20180044), and the China Postdoctoral Science Foundation (2020M680676, 2021T140680). (Source: Science Network)

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