【作者】 杨洋；

【导师】 李俊；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2022， 博士

【摘要】 随着电力系统中风电与光伏等可再生能源占比持续增加,可再生能源强不可预测性、间歇性和波动性为电力系统的安全稳定运行带来了巨大挑战。电解水技术则为将可再生电力转化为可长期储存的氢能的理想技术。该技术包含析氧反应（OER）和析氢反应（HER）两类半反应,任一半反应进行缓慢都将恶化电解水能效。为解决此问题,合理构建高效稳定电解水电极至关重要。通常,二氧化铱与铂等贵金属分别具有最优异的电解水OER和HER电催化活性,但昂贵价格和稀缺储量限制了其规模化应用。此外,电解水系统OER过程涉及多步质子耦合电子转移且存在较高能垒,从而导致OER动力学过程缓慢与本征活性较低;且其高反应电位会不断氧化电极材料致使实际OER活性物种不明确。与此同时,电解水HER阴极由于气相产量两倍于OER阳极,气泡易粘附于HER电极表面,严重降低了活性位点利用率,进而制约了反应物供给传输,从而不断增大电解水系统能耗。因此,开发低成本、高活性和高稳定性且具有气泡行为调控特性的非贵金属电解水电极迫在眉睫。基于上述问题,本文主要内容包括:1)针对过渡金属化合物析氧电极实际催化活性物种争议问题,构建了非金属化合物电化学重构电极,通过物化特性表征、电化学表征、稳定性测试等方式,明确了该电极在OER反应电位下会使非金属元素氧化刻蚀从而发生不可逆结构重构,并证实了具有稳定OER活性的物质实际为镍基羟基氧化物;然后通过调节非金属类别调控了重构电极氧空位含量,并采用电化学测试明晰了氧空位含量与电解水性能的关系。2)基于氧空位广泛存在于过渡金属基重构电极,为进一步揭示氧空位缺陷效应,本文构建了钼硫双元素电化学刻蚀重构电极,并利用密度泛函理论和第一性原理模拟计算揭示了氧空位含量对电解水析氧过程中电子态密度和氧物种吸附自由能的作用规律及其对OER性能的耦联影响。3)通过叠层压制法和水热合成法制备了梯度孔隙气泡传输调控电极,揭示了电极孔隙结构对气泡脱附速率及脱附尺寸的影响规律,采用气泡力平衡明晰了气泡传输调控机理,并分析了电极孔隙结构对物质传输和HER性能的影响规律。4)基于电极结构能够强化气泡脱附并加速底物供给角度出发,本论文通过机械旋转法与溶剂热合成法进一步设计了开放式螺旋纤维气泡滑移电极,研究了电极结构对催化晶体生长的影响规律,可视化了电极结构对气泡脱附的影响,借助力平衡分析揭示了气泡定向滑移机制,再利用微粒子图像测速验系统示踪检测了电极附近液相流场分布,并表征了电极结构对HER性能和电化学活性位点利用率的影响。5)基于本文优异OER与HER催化剂制备策略,进一步构建了具有梯度疏水涂层及梯形泡沫金属结构的电解水装置,结合可视化分析与受力分析明晰了电解水电极浸润性、结构对气泡行为的影响机理,并利用电化学手段对比了梯度疏水梯形电极在电解水时不同电流与电压条件下对应的性能参数。研究所获得的主要结论如下:（1）通过水热合成法与气相沉积法制备得到了镍基非金属化合物OER预催化电极,采用电化学表面重构法明晰了具有优异OER性能的活性物种为无定形相Ni OOH,并通过调控预催化电极非金属类别构建了具有丰富活性氧空位的重构电极Ni₂P₂S₆/CC-SR。因此,双非金属元素化合物表面共刻蚀重构电极Ni₂P₂S₆/CC-SR展现出良好的OER催化活性,在达到10 m A cm^-2时仅需219 m V过电位。（2）通过水热合成法和硫化反应法构建的钼硫双元素电化学刻蚀重构电极Ni Mo₃S₄/CC-SR具有最高的氧空位含量（60.3%）;随后第一性原理模拟计算表明随重构电极对应Ni OOH（101）晶面模型氧空位含量提升,Ni 3d轨道价带中心随之朝正向偏移,且电极速控步骤吸附自由能随之下降,说明OER电极对氧物种的吸附能力得以优化。电化学测试结果表明,氧空位含量与电化学性能呈现正相关关系,且钼硫元素电化学重构电极Ni Mo₃S₄/CC-SR具有最优异的OER催化活性,仅需过电位207 m V即可获得电流密度10 m A cm^-2。（3）基于具有梯度孔隙结构的电极基底,构建了负载有多孔富纳米片边缘的Ni₃S₂/Mo S₂的HER电极。可视化与气泡传输调控机理分析结果表明,具有由内至外孔隙渐缩特性的SML/NF-HE梯度孔隙电极由于气泡粘附力由内至外逐渐减小从而不断发生气泡切割,因而展现出最小的气泡脱附尺寸,进而暴露出更多电极催化活性位点,最终实现高效电化学析氢,达到-10 m A cm^-2电流密度时仅需83 m V过电位。（4）通过机械旋转法与溶剂热合成法制备了具有开放螺旋纤维结构的Mo S₂⊥CFB析氢电极。该电极具备优异催化剂晶体生长微环境,电极内外均密集且均匀生长着纳米片催化剂;且Mo S₂⊥CFB析氢电极由于具有气泡定向滑移特性,展现出优异气泡脱附特性和液相底物供给能力,因而实现了在过电位为0.4 V时,析氢电流密度相比于平面电极Mo S₂⊥CC提升3.7倍。（5）构建了具有梯度疏水涂层及梯形泡沫金属结构的电解水装置。由于其存在梯度疏水结构与梯形结构,表面气泡将在双拉普拉斯压力差共同驱动下由端部向近PTFE夹具根部发生连续定向渗透迁移,从而实现了氢/氧气泡定向渗透、迁移与自动分离捕集;且达到10 m A cm^-2和100 m A cm^-2电流密度所需电解电压仅分别为1.52 V和1.68 V。更多还原

【Abstract】 With the continuous increase in the proportion of renewable energy,such as wind power and photovoltaic in the power system,the strong unpredictability,intermittency and volatility of renewable energy have brought great challenges to the safe and stable operation of the power system.Water electrolysis technology is an ideal technology to convert renewable electricity into long-storable hydrogen energy.Water electrolysis includes two kinds of half-reactions,hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,in which any sluggish half-reaction will decrease its overall energy efficiency.To overcome this barrier,it is extremely significant to rationally construct an electrode with high efficiency in water electrolysis.Generally,iridium dioxide and platinum noble metals have the most excellent OER and HER activity,respectively;but the high price of these catalysts limits their wild application.Moreover,the OER process involves multi-step proton-coupled electron transfer and high energy barrier,resulting in slow OER kinetic process and low intrinsic activity;and its high reaction potential will continuously oxidize the electrode material,resulting in the uncertainty of the actual OER active species.Meanwhile,the gas production in HER cathode is twice than that in OER anode,resulting in bubbles covering the electrode surface,which seriously reduces the utilization rate of active sites,thus restricting the supply and transmission of reactants,thereby increasing the energy consumption of the water electrolysis system.Therefore,it is highly desirable to develop non-noble metal electrocatalysts for water electrolysis with low-cost,high activity,high durability and bubble regulation.Based on the above problems,the main contents of this paper include:1)In view of the dispute over the actual catalytic active species of the transition metal compound oxygen evolution electrode,an electrochemical reconstruction electrode of non-metallic compounds was constructed.By means of the physicochemical characterization,electrochemical characterization and stability test,it was clear that the electrode would oxidize and etch non-metallic elements at the OER reaction potential,resulting in irreversible structural reconstruction.It was confirmed that nickel based hydroxyl oxide was the actual species with stable OER activity.Then,the oxygen vacancy content of the reconstructed electrode was adjusted by the non-metallic category.Besides,the relationship between the oxygen vacancy content and the performance of water electrolysis was also clarified by electrochemical tests.2)Based on the fact that oxygen vacancies widely exist in transition metal based reconstruction electrodes,in order to further reveal the effect on oxygen vacancy defect,a surface reconstruction electrode of molybdenum/sulfur two-element electrochemical etching was constructed.Moreover,the interaction mechanism of oxygen vacancy content on electronic density of states and adsorption free energy of oxygen species in the process of water electrolysis and its coupling effect on OER performance were revealed by the density functional theory and first principles simulation calculation.3)The gradient pore bubble transport regulating electrode was prepared by lamination pressing method and hydrothermal synthesis method.The effect of electrode pore structure on bubble desorption rate and desorption size was revealed.The bubble force balance was used to clarify the bubble transport regulating mechanism.The effect of electrode pore structure on material transport and HER performance was also analyzed.4)Based on the view that the electrode structure can strengthen bubble desorption and accelerate the supply of substrate,an open-structured spiral fiber electrode with the bubble sliding behavior was designed by mechanical rotation method and solvent thermal synthesis method.Studying the influence of electrode structure on the growth of catalytic crystal.visualizing the influence of electrode structure on bubble desorption.Revealing the bubble directional sliding mechanism with the help of force balance analysis.The particle image velocimetry system was used to trace and detect the distribution of liquid flow field near the electrode;and the influence of electrode structure on HER performance and the utilization of electrochemical active sites was also characterized.5)Based on the excellent preparation strategy of OER and HER catalysts in this paper,a water electrolysis device with gradient hydrophobic coating and trapezoidal foam metal structure was further constructed.Combined with visualization analysis and force analysis,the influence mechanism of wettability and structure for the water electrolysis electrode on bubble behaviors was clarified.The corresponding performance parameters of gradient hydrophobic trapezoidal electrode under different current and voltage conditions were compared by electrochemical means.The main conclusions of the study are as follows:（1）Nickel based non-metallic compound OER pre-catalytic electrode was prepared by hydrothermal synthesis and vapor deposition.The active species with excellent OER performance was identified as amorphous Ni OOH by electrochemical surface reconstruction method,and the reconstructed electrode Ni₂P₂S₆/CC-SR with rich active oxygen vacancies was constructed by adjusting the non-metallic category of pre-catalytic electrode.Therefore,the bimetallic compound surface etching reconstructed electrode Ni₂P₂S₆/CC-SR also showed good OER catalytic activity,requiring only 219 m V overpotential at 10 m A cm^-2.（2）An surface reconstruction electrode（Ni Mo₃S₄/CC-SR）of the electrochemical etching on molybdenum and sulfur elements constructed by hydrothermal synthesis and sulfuration reaction had the highest oxygen vacancy content（60.3%）.Subsequently,the first principle simulation calculation exhibited that with the increase of oxygen vacancy content in Ni OOH（101）crystal plane model corresponding to the reconstructed electrode,the valence band center of Ni 3d orbit shifted to the positive direction and the adsorption free energy of electrode rate determining step decreased,indicating that the adsorption capacity of OER electrode for oxygen species was optimized.The electrochemical test results showed that there was a positive correlation between the oxygen vacancy content and the electrochemical performance;besides,the reconstruction electrode（Ni Mo₃S₄/CC-SR）with molybdenum/sulfur element had the highest OER catalytic activity.The current density of 10 m A cm^-2 could be obtained only at the overpotential of 207 m V.（3）Based on the electrode substrate with gradient pore structure,a Ni₃S₂/Mo S₂electrode for HER loaded with porous sufficient nanosheets was constructed.The results of visualization and bubble transport control mechanism analysis showed that the bubble in the SML/NF-HE gradient pore electrode with the decreased pores from the inside to the outside would be continuously cut due to the reduction of bubble adhesion from the inside to the outside,thus showing the smallest bubble desorption size,thereby exposing more electrode catalytic active sites,and finally realizing high-efficiency electrochemical hydrogen evolution.Only 83 m V overpotential was required to reach the current density of-10 m A cm^-2.（4）Mo S₂⊥CFB hydrogen evolution electrode with an open-structured spiral fiber was prepared by mechanical rotation and solvothermal synthesis method.This electrode had an excellent microenvironment for catalyst crystal growth,and nanosheet catalysts were densely and uniformly grown inside and outside the electrode.Moreover,Mo S₂⊥CFB hydrogen evolution electrode showed the most excellent bubble desorption characteristics and liquid substrate supply capacity due to its bubble unidirectional sliding characteristics.Therefore,when the overpotential is 0.4 V,the current density for hydrogen evolution is 3.7 times higher than that of the plane electrode Mo S₂⊥CC electrode.（5）An water electrolysis device with the gradient hydrophobic coating and trapezoidal foam metal structure was constructed.Due to the existence of gradient hydrophobic structure and trapezoidal structure,the bubbles above the electrode surface would undergo continuous oriented infiltration and migration from the end to the root of the PTFE clamp under the joint driving of the double Laplace pressure difference,thus realizing the oriented infiltration,migration and automatic separation/capture of hydrogen/oxygen bubbles.The electrolytic voltage required to achieve the current densities of 10 m A cm^-2 and 100 m A cm^-2 were only 1.52 V and 1.68 V,respectively.更多还原