【作者】 谢俊峰；

【导师】 谢毅；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2014， 博士

【摘要】 电化学能源转换与存储是应对二十一世纪能源危机的一种重要能源解决途径,在这个领域中,设计高性能的电催化剂和能源存储材料是重中之重。近年来,围绕着二维晶体的研究热潮逐渐兴起,为能源领域的材料设计带来了新的希望。较之于传统的体相材料,二维晶体通常显现出独特的电学、光学以及磁学性质,并有着自身独特的结构优势,这为探索新型电催化剂和能源存储材料指引了方向,也为功能导向性的材料设计与性能优化提供了一个崭新的平台。本论文旨在通过对材料电化学性能制约因素的分析,实现二维晶体的功能导向性设计,以期实现更为高效的电化学能源转换与存储性能。作者通过缺陷工程、元素掺杂、无序结构调控、优势晶面设计以及层层组装杂合等多种方法,对非铂析氢反应电催化剂和超级电容器电极材料的电化学性能进行了系统化和导向性的优化提升,获得了高性能的析氢反应电催化剂并实现了柔性能源存储器件的组装。本论文中对二维晶体进行功能导向性设计的优化策略将为未来设计和制备高性能能源转换与存储材料提供新的思路。本论文主要包括以下几方面的内容：1.基于对二硫化钼析氢反应电催化剂活性位点位于类石墨烯片层边缘的认识,作者首次功能导向性地设计并制备了能够额外暴露活性边缘位点的富缺陷二硫化钼超薄纳米片。材料富缺陷的结构导致纳米片的表面形成许多微小的裂纹,进而使活性边缘位点得到额外暴露。经过缺陷工程设计,这种新型的富缺陷二硫化铝超薄纳米片显示出很低的析氢反应起始过电位(120mV),并兼具了较高的交换电流密度和阴极电流密度,实现了电催化性能的显著提升。此外,富缺陷二硫化钼超薄纳米片显示出很小的塔菲尔斜率(50mV decade-1),进一步证实了缺陷结构设计对析氢反应催化活性的增强效应。此外,富缺陷二硫化钼超薄纳米片的准周期性晶畴排列方式保证了材料具有良好的电化学稳定性。这种通过缺陷工程增加催化剂活性位点的思路为未来催化剂的设计与性能优化开辟了一条新的道路。2.作者首次设计并制备了氧掺杂的二硫化钼超薄纳米片,并实现了可控的无序结构调控,进而实现了二硫化钼电催化剂活性位点与导电性的协同调控。理论计算分析表明,氧掺杂可以有效减小二硫化钼的带隙,从而提高其本征导电性。此外,氧掺杂二硫化钼可以在较小的过电位下实现更高的氢原子覆盖率,从而大幅度地提高其催化活性。通过对一系列具有不同无序度和氧掺杂量的二硫化钼电催化剂析氢反应活性的研究,作者发现具有适度无序度的电催化剂显示出最高的催化活性,其起始过电位约为120mV,塔菲尔斜率为55mV decade-1,同时显现出极高的阴极电流密度和交换电流密度。实验证明,优化后的产物具有最小的电荷转移电阻,这是无序结构决定的畴间导电性和氧掺杂量决定的本征导电性之间优化平衡的结果。另外,优化后的产物因为无序结构的存在具有较多的不饱和硫原子作为活性位点,因此显示出最优的催化活性。本工作报导的这种协同调控活性位点与导电性的策略为进一步提高现有催化剂的性能创造了可能,并对新型电催化剂的设计与优化具有较高的借鉴意义。3.作者通过液相剥离法首次制备了具有原子级厚度的金属性氮化钼超薄纳米片,并对其析氢反应机理进行了深入的探究。理论计算分析表明,原子级厚度的氮化钼纳米片具有金属特性,其较高的导电性有利于电催化过程中电子的快速传导。而对氮化钼晶体结构的分析则显示,原子级厚度的氮化钼纳米片具有极高的表面钼原子暴露率,这种独特的结构特征为研究钼基电催化剂的析氢反应机理提供一个理想的模型。实验结果显示,较之于体相氮化钼电催化剂,原子级厚度氮化钼纳米片的析氢反应活性显示出大幅度的提高,其起始过电位约为100mV,且具有极大的交换电流密度。经过对剥离前后的氮化钼电催化剂的析氢反应活性的归一化比较,最终得到了纳米片表面钼原子为析氢反应活性位点的结论,为未来设计高性能钼基电催化剂以及探究析氢反应机理创造了条件。4.基于对双电层电容电极材料与赝电容电极材料自身优缺点的理解,作者首次功能导向性地设计并制备了具有层层组装结构的氢氧化镍/石墨烯杂合纳米片,并组装了首个柔性全固态薄膜赝电容器。这种氢氧化镍/石墨烯杂合纳米片独特的层层组装结构使得高导电性的石墨烯组分与赝电容性质的氢氧化镍组分实现电化学性能的协同优化,赋予了这种新型的能源存储材料较高的比电容以及优越的电化学稳定性。以氢氧化镍/石墨烯杂合纳米片作为电极活性材料的柔性全固态薄膜赝电容器显示出660.8F cm-3的体积比电容,且经过2000次充放电循环性能未发生明显衰减。同时,杂合纳米片超薄的二维结构赋予器件良好的机械性能,为柔性电子产品的开发提供了一种可行的能源供给方式。更多还原

【Abstract】 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.更多还原