【作者】 李海龙；

【导师】 郭晓辉；

【作者基本信息】 西北大学 ， 无机化学， 2020， 博士

【摘要】 当前社会,能源消耗日益增加,环境污染越来越严重,因此寻找并开发新的清洁能源迫在眉睫。氢能源被认为是现阶段化石能源最具潜力的替代品,其中,电解水制氢被认为是世界上最清洁的制氢方法之一,因其没有多余的副产物污染,能量转化效率高,解决了化石能源的枯竭和环境污染问题,但是电催化分解水高的过电位和缓慢的动力学导致电能利用率较低。目前商业上主要使用贵金属催化剂（如:铂、铱、钌等）,但是其丰度含量低,成本高,而且催化剂的稳定性差,限制了它们的规模化应用。因此开发可宏量制备的非贵金属电催化剂成为大家关注的焦点问题之一。其中,过渡金属（铁、钴、镍）基纳米复合材料被证实具有较高的电解水活性,因而在电解水制氢方面有潜在的应用前景。这里通过缺陷工程中的阴离子空位、杂原子掺杂等手段调控钴基金属和基底的相互作用来调节电子结构、活性位点,氧空位等来提升电催化剂性能,从而替代或者减少贵金属在工业中的使用,本文的主要研究内容如下:1.通过简单的湿化学法结合后续惰性气氛热解处理,将Co原子掺杂到Mo O₂纳米棒（NRs）中以促进氧空位的产生从而增强Mo O₂NRs的HER催化活性。优化后的Co-Mo O₂-0.01 NRs表现出十分优异的电催化活性,其在10 m A cm^-2时过电位低至26 m V,Tafel斜率约为30.9 m V dec^-1,远优于纯Mo O₂NRs的制氢活性,甚至优于商用Pt/C催化性能。此外,Co-Mo O₂-0.01 NRs还表现出优异的长期稳定性和耐久性,经过3000次循环后催化活性没有明显下降。研究结果表明:Co原子掺杂到Mo O₂NRs中不仅促进催化剂中氧空位的产生,而且增强了其导电性。氧空位可以激活其周围氧原子作为活性位点,增加催化剂表面的活性位点数量,而导电性增强使得电荷迁移加快,从而大大提高其电催化制氢效率。该工作通过合理的缺陷工程设计为开发资源丰富的高效HER电催化剂提供了一种新的有希望的合成策略。2.通过一种简便的电化学沉积方法及随后的阴离子交换过程,首次合成了二维Co O（OH）_1-xS_x纳米片。所制备的Co O（OH）_0.75S_0.25纳米片在HER中10 m A cm^-2时具有166m V的低过电位和93.4 m V dec^-1的Tafel斜率,其在OER中30 m A cm^-2时也具有378 m V的低过电位和106.3 m V dec^-1的Tafel斜率,表明Co O（OH）_1-xS_x纳米片是一种很有应用前景的全水解电催化剂。研究结果发现:由于八面体Co-O_x的t_2g轨道局部改变,导致了电导率增加以及活性位点更多地暴露,赋予了Co O（OH）_1-xS_x优异的全解水特性。此外,X射线吸收精细结构分析和第一性原理计算进一步证明,Co OOH中硫杂原子的取代可能是由于未被占据的t_2g轨道状态引起的,因此有助于提高电导率并减少反应势垒,其有利于电催化活性的提升。该工作为合成新型过渡金属氧化物硫化物基电催化剂提供了一条可行的合成思路。3.通过简单的水热反应和退火处理,先使用硝酸镍做为镍源,以不同比例的阳离子掺杂,合成出一系列Ni_xCo_1-x（OH）₂纳米片,再以次亚磷酸钠为磷源,经过退火磷化成功制备出Ni_xCo_1-xP双金属磷化物六角形纳米片催化剂。所制备Ni_xCo_1-xP纳米片在OER中10 m A cm^-2时其过电位为320 m V,Tafel斜率为110.0 m V dec^-1,并且在24小时的测试后活性没有明显降低。研究发现,磷化退火处理使催化剂形成了多孔结构,而多孔的Ni_xCo_1-xP暴露出更多的催化活性位点,增加了催化剂和反应物的接触面积,同时由于镍的掺杂使得产物的阻抗得到很大程度的减小,加快了电荷转移速率,从而使该催化剂的性能有了极大提升。该工作为开发其它高效双过渡金属基析氧电催化剂提供了研究思路。更多还原

【Abstract】 In recent years,energy consumption and environmental pollution have become the two focal issues all over the world,so developing renewable and clean energy sources is getting more and more attention.Hydrogen energy is considered to be the most potential substitute for fossil energy in the present stage by virtue of its attractive advantages of high energy density and no carbon emission.Electrocatalytic water splitting is considered as a promising and green strategy to produce hydrogen,in which no other toxic by-products are produced.And only water forms after hydrogen combustion,thus create a carbon-free and environment-friendly economy.However,high energy barrier and sluggish kinetics in the hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）during electrocatalytic water splitting result in an inefficient electricity conversion.Therefore,highly efficient electrocatalysts are required to accelerate the reaction rate,reduce reaction energy barrier and thus improve the energy conversion efficiency.Currently,noble metal-based catalysts such as platinum（Pt）,iridium（Ir）,ruthenium（Ru）are the most effective catalysts for HER and OER.Unfortunately,these catalysts has the disadvantages of low abundance,high cost and poor stability,which greatly limited their large scale application in practical production.Therefore,it is very essential to design and develop non-noble catalysts with high electrocatalytic activity and excellent stability for HER and OER.Recently,transition metal-based materials（for example,iron,cobalt,nickel,etc.）have been proved with considerable electrolytic catalytic activity for water splitting,and on the other hand,it is possible to enhance the catalytic activity through different strategies,such as constructing heterostructures,adjusting catalyst morphology,and defect engineering.In this work,defect engineering including anion vacancies and heteroatomic doping,is used to control the interaction between cobalt（Co）compounds and other substrates,which can adjust electron density,increase active sites,and induce oxygen vacancies and thus improve the electrochemical performance of Co-based catalysts and replace noble metal-based catalysts.The main research contents are included as follows:1.Co-doped Mo O₂ nanorods was successfully prepared through a facile wet chemistry method followed by annealing treatment process.Co doping can not only produce more oxygen vacancies in Mo O₂,but also improve the electrical conductivity of catalyst during HER process.Oxygen vacancies can activate the adjacent oxygen atoms as active sites,thereby increasing the number of active sites on the catalyst surface,and enhancing charge transfer,and greatly improving its electrocatalytic hydrogen production efficiency.Therefore,the optimized Co-Mo O₂-0.01 nanorods（NRs）exhibited an extraordinary electrochemical activity with a very low overpotential of 26 m V at 10 m A cm^-2 and a small Tafel slope of 30.9m V dec^-1,which are much better than that of pure Mo O₂ NRs and even superior to that of the commercial Pt/C catalyst.In addition,the Co-Mo O₂-0.01 NRs also display excellent long-terms stability and durability without obvious catalytic activity degradation after 3000cycles.This work provides a new promising synthetic strategy for developing earth-abundant and highly efficient HER electrocatalysts through rational defect-engineering design.2.Two-dimensional cobalt oxy-hydrate sulfide(Co O（OH）_1-xS_x)nanosheets were synthesized via a facile electrochemical deposition method followed by an anion-exchange process.The obtained Co O（OH）_0.75S_0.25 nanosheets exhibited a low overpotential of 166 m V and a small Tafel slope of 93.4 m V dec^-1 in HER process,while the Co O（OH）_0.75S_0.25 catalyst delivers an extremely small overpotential of 378 m V(at 30 m A cm^-2)and a suppressed Tafel slope of 106.3 m V dec^-1 for OER.It denotes that the specific Co O（OH）_1-xS_x nanosheets can be a promising catalyst for overall water splitting.Benefiting from the modified t_2g orbitals of octahedral Co-O_x,enhanced electrical conductivity and abundant active sites,Co O（OH）_1-xS_xdelivered high catalytic activity toward overall water splitting in alkaline electrolyte.X-ray absorption fine structure analysis and first-principles calculation denotes that The substitution of sulfur heteroatoms in Co OOH could result in the unoccupied t_2g orbit state and hence contribute to enhanced electrical conductivity and reduce the reaction barrier,all of which favor electrocatalytic functions in water splitting.This work provides a facile route to synthesize new transition-metal oxide sulfides for electrocatalytic water splitting.3.Through simple hydrothermal reaction and annealing treatment,employing nickel nitrate as nickel source,a series of Ni_xCo_1-x（OH）₂ nanosheets were firstly prepared by doping with different ratios of cations,then using sodium hypophosphite as phosphorus source,hexagonal Ni_xCo_1-xP bimetallic phosphide nanoplates catalyst was successfully synthesized after annealing and phosphating.The overpotential of the prepared Ni_xCo_1-xP nanosheets was320 m V at 10 m A cm^-2 for OER,Tafel slope was 110.0 m V dec^-1.Meanwhile,the catalytic activity of Ni_xCo_1-xP catalyst almost did not decrease after 24 hours testing.It can be found that the phosphating annealing treatment makes the catalyst form a porous structure,which endows the Ni_xCo_1-xP catalyst more exposed catalytic active sites and increases the contact area between the catalyst and the electrolyte.In addition,nickel doping significantly reduces the resistance of Ni_xCo_1-xP catalyst and accelerates charge transfer during the electrochemical reaction,thereby greatly improving the catalytic performance of the catalyst for OER.This work provides new research ideas for the development of other high-efficiency double transition metal-based oxygen evolution electrocatalysts.更多还原