【作者】 蒋玲；

【导师】 李永军；

【作者基本信息】 湖南大学 ， 化学， 2023， 博士

【摘要】 过度使用化石燃料（煤、石油和天然气）造成的全球变暖或环境污染正日益成为人类关注的问题,寻求化石燃料的替代能源成为当务之急。氢能因其燃烧热值高和绿色无污染,被认为是化石燃料的理想替代品。目前,生产氢气一般采用甲烷重整或煤气化的方法,不可避免地会排放污染物,这不是最优的方法。如果用于水分解的电力来自可再生的太阳能或风能,则水分解电催化析氢被认为是一种“绿色”技术。然而,要实现析氢反应（HER）的商业化,必须解决H⁺还原电位过高的问题。迄今为止,铂（Pt）仍然是最有效的HER催化剂,但由于其成本高和储量有限,限制其大规模的应用。因此,开发高效的无Pt电催化剂成为电催化研究的热点。二硫化钼（MoS₂）是一种化学性质稳定、无毒、成本低廉的材料,广泛应用于固体润滑剂、晶体管、电催化剂等领域。特别是MoS₂的边缘表现出优异的HER电催化活性。然而,MoS₂的基面对HER是化学惰性的,同时,导电性能差也限制了MoS₂在电化学中的应用。为了解决这些问题,本论文开发了相变、缺陷和异质结工程策略来设计MoS₂基电催化剂来促进HER性能的提升。希望这些研究能够缩小MoS₂和Pt在HER催化活性上的差距。具体研究内容如下:（1）Se-MoS₂纳米材料的设计构筑及其电催化析氢性能研究本章采用一步水热法制备了硒和氧共嵌入的MoS₂（Se-MoS₂）纳米片。硒和氧的共嵌入在MoS₂基面形成大量的S缺陷,从而增加了活性位点的数量。此外,硒和氧的共嵌入可以诱导MoS₂从2H相转变为1T相（约60%为1T-MoS₂）,从而使Se-MoS₂表现出更优异的导电性。优化后的Se-MoS₂不仅具有较好的电催化HER活性(在电流密度为10 m A cm^-2时的过电位为108 m V,Tafel斜率为47 m V dec^-1),而且还继承了原始MoS₂优异的稳定性。密度函数理论（DFT）计算表明,Se-MoS₂催化性能的提升得益于在MoS₂的费米能级附近出现了新的能带结构,提高了电子传递速率,降低了氢吸附的吉布斯自由能,且Se-MoS₂优异的HER性能可以用Volmer-Heyrovsky机制解释。（2）MoS_2-x纳米材料的设计构筑及其电催化析氢性能研究本章提出了一种缺陷预设计策略,通过先制备Se掺杂的MoS₂（Se-MoS₂）,然后再去除Se-MoS₂中的Se,从而获得具有单原子S空位的MoS₂（SV-MoS₂）。S空位的生成源于掺杂Se原子的气化,这也使得S空位的形成具有很高的可控性,为精确调节S空位的浓度提供了很好的可能性。结果表明,S空位浓度可控制在～7.46%～13.54%范围内。其中,S空位浓度为12.10%时的MoS_1.76表现出优异的HER性能(在电流密度为10 m A cm^-2时的过电位为100 m V,Tafel斜率为49 m V dec^-1),证实了关于S空位最佳浓度的理论预测。DFT计算进一步揭示了MoS₂催化性能的提升可能本质上源于S空位对MoS₂能带结构和态密度的调节,从而优化了氢吸附吉布斯自由能。这种缺陷预设计的策略降低了团聚S空位形成的概率,为理解S空位如何影响MoS₂的催化性能提供参考。（3）Au^δ+/MoS_1.76纳米材料的设计构筑及其电催化析氢性能本章采用界面工程、相变工程和S空位工程相结合的策略,设计并合成了莫特-肖特基异质结Au^δ+/MoS_1.76电催化剂。其中,金纳米颗粒可以与MoS₂形成莫特-肖特基异质界面,异质界面的存在一方面可以为催化反应提供活性位点;另一方面可以很好的稳定MoS₂的1T相（1T相的占比约64%）。同时,S空位的存在进一步增加了活性位点的数量。因此,Au^δ+/MoS_1.76展现出较低的过电位（～90 m V）和Tafel斜率(～47 m V dec^-1)。DFT计算表明莫特-肖特基异质界面的形成可以赋予MoS₂更多的电子,有效的稳定MoS₂的金属1T相,同时也调节了MoS₂的电子结构,优化了H*吸附的吉布斯自由能。（4）Mo₂C-MoS_1.76/G纳米材料的设计构筑及其电催化析氢性能研究本章提出了一种S空位诱导Mo₂C纳米晶在MoS₂面内生长的方法,通过在MoS₂表面一侧生长Mo₂C纳米晶,成功制备了Mo₂C-MoS_1.76/G异质结结构。Mo₂C纳米晶体的形成可归因于MoS₂表面S空位的活化。Mo₂C纳米晶不仅可以为催化反应提供丰富的界面催化活性位点,还可以增强材料的导电性。此外,石墨烯衬底的加入不仅增强了材料的导电性,还在一定程度上抑制了MoS₂聚集体的形成,保证了活性位点的充分暴露。电化学结果表明,Mo₂C-MoS_1.76/G具有良好的催化性能(在10 m A cm^-2时过电位为82 m V,Tafel斜率为46 m V dec^-1)和循环稳定性。（5）P-MoS_1.76/G纳米材料的设计构筑及其电催化析氢性能研究本章采用磷原子填充硫空位的策略,制备了具有双活性中心的P-MoS_1.76/G催化剂。磷原子的填充一方面可以作为催化活性位点,另一方面可以诱导MoS₂从2H相转变为1T相（1T相占比约74%）,从而使P-MoS_1.76/G展现出更高的导电性。磷原子和硫空位作为双活性中心,为提高HER活性提提供了可能。电化学结果表明P-MoS_1.76/G展现出良好的析氢催化性能(在电流密度为10 m A cm^-2时的过电位为78 m V,Tafel斜率为45 m V dec^-1)和循环稳定性。DFT计算表明,P-MoS_1.76/G良好的催化性能得益于在硫空位中引入的磷原子有效地改变了MoS₂的电子结构,优化H*吸附的吉布斯自由能。更多还原

【Abstract】 Global warming or environmental pollution caused by excessive use of fossil fuels（i.e.,coal,oil,and natural gas）is increasingly becoming a human concern.It is urgent to seek alternative energy sources to replace fossil fuels.Hydrogen energy is considered as an ideal substitute for fossil fuels because of its high calorific value and“green”feature.Now,hydrogen gas is generally produced through methane reforming or coal gasification with the inevitable emission of pollutants,which is not an optimal way.Electrocatalytic hydrogen evolution from water splitting is considered to a“green”technique if the electricity for water splitting originates from renewable solar or wind energy.However,to commercialize the hydrogen evolution reaction（HER）,we have to address the problem of high overpotential of H⁺electroreduciton.To date,platinum is still the most effective catalyst for HER but impossible to be used on a large scale owing to its high cost and limited reserve.Therefore,developing efficient Pt-free electrocatalysts becomes a hot topic in the HER research.MoS₂,as a chemically stable,non-toxic and low-cost material,has been widely used as solid lubricant,transistor,and elecrocatalyst.Especially,MoS₂ edges show excellent HER elecrocatalytic activity.However,the basal plane of MoS₂ is completely inactive to HER,and the poor conductivity also limits the application of MoS₂ in electrochemistry.To address these issues,this thesis develops phase transition,defect and heterojunction engineering strategies to design MoS₂-based electrocatalysts for boosting HER.We hope these studies can narrow the gap between MoS₂ and Pt in the HER catalytic activity.Specific research contents are as follows:（1）Design and construction of Se-MoS₂ nanomaterials and electrocatalytic hydrogen evolution propertiesSe-and O-co-inserted MoS₂（Se-MoS₂）microspheres composed of intertwined nanosheets were fabricated by a hydrothermal strategy.Co-insertion of Se and O creates abundant S defects on the basal plane of MoS₂ and thus increases the number of active sites.Additionally,Co-insertion of Se and O is apt to induce the transition of MoS₂ from 2H to 1T phase（～60%1T MoS₂）,thereby exhibiting much high electric conductivity.Furthermore,optimized Se-MoS₂ not only shows electrocatalytic HER activity(an overpotential of 108 m V at 10 m A cm^-2and a Tafel slope of 47 m V dec^-1),also inherits the outstanding electrocatalytic durability of pristine MoS₂.Density function theory（DFT）calculations reveal that the excellent HER performance of optimized Se-MoS₂ can be well explained with Volmer-Heyrovsky mechanism,which is intrinsically attributed to the emergence of new band structures near the Fermi energy level of MoS₂,increasing the electrical conductivity and reducing the Gibbs free energy of hydrogen adsorption.（2）Design and construction of MoS_2-x nanomaterials and electrocatalytic hydrogen evolution propertiesA defect-pre-designed strategy was proposed to produce MoS₂ with single-atomic S vacancies（SV-MoS₂）simply by preparing Se-doped MoS₂（Se-MoS₂）and subsequent removing the Se of Se-MoS₂.S vacancies originate from the vaporization of the doped Se atoms.This strategy has a high selectivity and raises a good possibility for precisely modulating the concentration of S vacancies.The results show that the concentration of S vacancies can be controlled over the range from～7.46%to 13.54%.MoS_1.76 with～12.10%of S vacancies exhibits outstanding HER performance:an overpotential of100 m V at 10 m A cm^-2 and a Tafel slope of 49 m V dec^-1,corroborating the theoretical prediction about the optimum concentration of S vacancies.Density functional theory calculation further reveals that the activation of MoS₂ basal planes may intrinsically originate from the modification of S vacancies to band structure and density of state of MoS₂,optimizing the hydrogen adsorption energy.This defect-pre-designed strategy reduces the probability that the aggregates of S vacancies are formed,and will be more helpful for understanding how S vacancies affect the properties of MoS₂.（3）Design and construction of Au^δ+/MoS_1.76 nanomaterials and electrocatalytic hydrogen evolution propertiesMott-Schottky heterojunction Au^δ+/MoS_1.76 electrocatalyst was designed by integrating interface engineering,phase transition,and S-vacancy creation.Gold nanoparticles adhering on MoS₂ produce a Mott-Schottky hetero-junction.The existence of hetero-junctions can not only provide active sites for catalytic reaction,also is capable of stabilizing 1T-phase MoS₂（～64%1T MoS₂）.Meanwhile,the presence of S vacancies increases the number of active sites.Thus,Au^δ+/MoS_1.76exhibits a low overpotential（～90 m V）and Tafel slope(～47 m V dec^-1).DFT calculation shows that the formation of Mott-Schottky hetero-junctions can endow MoS₂ with more electrons,effectively stabilize the metal 1T phase of MoS₂,adjust the electronic structure of MoS₂,and optimize the Gibbs free energy of H*adsorption.（4）Design and construction of Mo₂C-MoS_1.76/G nanomaterials and electrocatalytic hydrogen evolution propertiesMo₂C-MoS_1.76/G heterojunction structures are successfully prepared by growing Mo₂C nanocrystals in one side of MoS₂ surface is proposed,and.The formation of Mo₂C nanocrystals can be ascribed to the activation of S vacancy to the surface of MoS₂.Mo₂C nanocrystals can not only provide abundant interfacial catalytic active sites for catalytic reactions,but also enhance the conductivity of materials.In addition,the addition of graphene substrate not only enhances the conductivity of the material,but also inhibits the formation of MoS₂ aggregates to a certain extent,ensuring the full exposure of active sites.The electrochemical results show that Mo₂C-MoS_1.76/G exhibits good catalytic performance(an overpotential of 82 m V at 10 m A cm^-2and a Tafel slope of 46 m V dec^-1)and cyclic stability.（5）Design and construction of P-MoS_1.76/G nanomaterials and electrocatalytic hydrogen evolution propertiesP-MoS_1.76/G catalyst with double active centers was prepared by filling sulfur vacancy with phosphorus atoms.Phosphorus atoms can act as the catalytic active site,and is capable of inducing the transformation of MoS₂ from 2H to 1T phase（～74%1T MoS₂）,making P-MoS_1.76/G with a higher conductivity than pristine MoS₂.Phosphorus atoms and sulfur vacancies together work as active centers,offering a strong possibility for improving the HER activity.The electrochemical results show that P-MoS_1.76/G exhibits excellent hydrogen evolution catalytic performance(an overpotential of 78 m V at 10 m A cm^-2and a Tafel slope of 45 m V dec^-1)and cyclic stability.The DFT calculation shows that the excellent catalytic performance of P-MoS_1.76/G is due to the fact that the phosphorus atom introduced into the sulfur vacancy effectively changes the electronic structure of MoS₂ and optimizes the Gibbs free energy of H*adsorption.更多还原