Function-Oriented Design And Electrochemical Properties Of Two-Dimensional Crystals

Electrochemical energy conversion and storage is one of the most important pathways to solve the energy crisis in21th century. In this research field, design of high-performance electrocatalysts and energy storage materials is the most vital issue. During the past few years, the emergence of two-dimensional (2D) crystals brings in huge promise for the material design with energy-related applications, In sharp contrast with the traditional bulk materials,2D crystals usually exhibit unique electronic, magnetic and optical properties and inherent structural benefits, which not only offer the opportunity for us to pursue novel electrocatalysts and energy storage materials, but also provide a new platform to achieve function-oriented material design and property optimization.The goal of this dissertation is to realize function-oriented design of2D crystals based on the analyses of the restrictive factors of electrochemical properties, in order to achieve efficient electrochemical energy conversion and storage. In this dissertation, the author highlights the systematic and function-oriented design of the non-platinum hydrogen-evolution electrocatalysts and the electrode materials for supercapacitor by means of defect engineering, elemental incorporation, disorder engineering, crystal facet engineering and layer-by-layer assembly. The optimization strategy by function-oriented design of2D crystals will shed new light on the design and fabrication of high-performance energy conversion and storage materials. The details of this dissertation are summarized briefly as follows:1. The active sites for hydrogen evolution reaction (HER) of MoS2electrocatalysts are located on the edges of the graphene-analogous layers. Based on this fact, we put forward the function-oriented design and fabrication of the defect-rich MoS2ultrathin nanosheets with additional active edge sites for the first time. The defect-rich structure can lead to the formation of tiny crackles on the surface of the nanosheets, which causes the additional exposure of active edge sites. With the merits of the defect engineering, the novel defect-rich MoS2ultrathin nanosheets exhibit low HER onset overpotential of120mV as well as large exchange current density and cathodic current density, revealing the significant enhancement of the electrocatalytic performance. Moreover, the defect-rich MoS2ultrathin nanosheets possess small Tafel slope of50mV decade-1, further proving the enhancement effect of the HER activity by defect engineering. Furthermore, the quasi-periodic arrangement of the nanodomains in the defect-rich MoS2ultrathin nanosheets ensures the excellent electrochemical stability. The strategy of enriching active sites by defect engineering in this work will pave a new pathway for design and optimization of advanced catalysts in the near future.2. For the first time, we proposed the design and preparation of the oxygen-incorporated MoS2ultrathin nanosheets with controllable disorder engineering, and achieve synergistic modulation of active sites and conductivity for HER catalysis. Theoretical calculations revealed that oxygen incorporation can effectively reduce the bandgap of MoS2, and thus enhance the intrinsic conductivity. Besides, oxygen-incorporated MoS2can achieve a higher hydrogen coverage under a smaller overpotential, which can significantly enhance the HER activity. Through the comprehensive investigation of the HER performance of a series of oxygen-incorporated MoS2ultrathin nanosheets with different degrees of disorder, an optimized catalyst with moderate degree of disorder was identified, which shows a low onset overpotential of120mV, small Tafel slope of55mV decade-1, as well as extremely large exchange current density and cathodic current density. The optimized catalyst possesses the lowest charge-transfer resistance, which results in the balance of the disorder-determined inter-domain conductivity and the oxygen incorporation-determined intrinsic conductivity. Furthermore, the disordered structure of the novel catalyst offers rich unsaturated sulfur atoms as the active sites, which is also responsible for the superior HER activity. The strategy of synergistic modulation of active sites and conductivity will provide the opportunity for enhancing the catalytic activity of catalysts, and offer new insight in the design and optimization of novel electrocatalysts.3. The authors synthesized the atomically-thin8-MoN nanosheets and investigate the HER mechanism for the first time. With the aid of the theoretical calculations, the metallic behavior of the atomically-thin8-MoN nanosheets is revealed, which can facilitate the electron transport during the electrocatalytic process. The atomically-thin8-MoN nanosheets possess extremely high exposure of surface Mo atoms, which provides an ideal structural model for investigating the HER mechanism of Mo-based electrocatalysts. Experimental results indicated that the atomically-thin8-MoN nanosheets exhibit a much higher HER activity than the bulk counterpart, which shows a low onset overpotential of100mV and dramatically high exchange current density. By fairly evaluating the HER activity of δ-MoN nanosheets and the bulk by means of normalization with electrochemical surface area, the conclusion that surface Mo atoms are HER active sites was identified. The novel HER mechanism will provide the opportunity to design highly efficient Mo-based HER catalysts and further investigate the HER mechanism in the near future.4. Based on the understanding of the benefits and drawbacks of the electrode materials with double-layer capacitance and pseudocapacitance, we proposed the first function-oriented design and synthesis of layer-by-layer P-Ni(OH)2/graphene hybrid nanosheets and fabricated the first flexible all-solid-state thin-film pseudocapacitor. The unique layer-by-layer structure of the β-Ni(OH)2/graphene hybrid nanosheets can synergistically optimize the electrochemical behavior between the highly conductive graphene and the pseudocapacitive P-Ni(OH)2, thus guaranteeing the high specific capacitance as well as the excellent stability of this novel energy storage material. By employing the β-Ni(OH)2/graphene hybrid nanosheets as the active electrode material, the flexible all-solid-state thin-film pseudocapacitor exhibits a high volumetric specific capacitance of660.8F cm-3, and negligible degradation occurs even after2000charge/discharge cycles. Meanwhile, the ultrathin configuration of the hybrid nanosheets endows superior mechanical property for the as-fabricated nano-device, which can be regarded as a feasible energy supply for the exploitation of flexible electronics.