【作者】 王晶；

【导师】 刘雄军；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2022， 博士

【摘要】 氢能被认为是二十一世纪最具发展前景的可再生能源。电解水制氢是一种绿色环保、可持续获取氢能的有效途径。然而,目前电解水制氢催化剂存在成本高、反应效率低、电能消耗大等问题,严重限制了其大规模应用。因此,开发高效低成本催化剂是电解水制氢工业化的关键。过渡族金属基纳米催化剂成本低,并显示出良好的电解水催化活性,然而其稳定性较差,无法满足工业应用大电流密度下长时间服役的要求。针对目前电解水催化剂普遍存在的活性与稳定性之间的矛盾,本文以开发低成本、高活性、高稳定性的碱性电解水催化剂为目的,围绕无序合金(即非晶合金和高熵合金)前驱体制备多孔金属的孔结构调控和电解水性能优化展开系统研究,获得了系列性能优异的多级结构多孔金属催化材料。主要研究内容和结论如下:(1)发现改变冷却速度和表面应力状态,可以实现对Ni40Zr40Ti20非晶合金脱合金处理后孔结构的调控,从而显著改善多孔Ni/非晶合金复合材料的催化活性和稳定性。分别采用Ni40Zr40Ti20非晶带材和丝材作为前驱体,经过脱合金处理后,非晶带材表面形成具有均匀纳米孔结构的Ni/非晶复合材料,而非晶丝材表面形成纳米孔和微米裂隙形成的多级孔结构的Ni/非晶复合材料。由于非晶丝材表面较大的残余应力可加速原子扩散,从而加快脱合金腐蚀速率,导致脱合金产物表面形成微米尺度裂隙。多级孔结构的Ni/非晶复合材料在碱性电解液中表面出较为突出的HER性能,其在电流密度为10 mA/cm2时仅需78 mV的过电位,Tafel斜率为42.4mV/dec。此外,由于微米级缝隙可使气泡及时溢出,在10 mA/cm2条件下连续工作24 h,孔结构仍然完整,性能没有明显衰减,而均匀孔结构的复合材料电极在该电流密度下工作12 h性能就急剧恶化。多级孔结构使复合材料的电解稳定性大幅提升。(2)采用合金化策略,基于Ni40Zr40Ti17Pt3非晶合金前驱体,成功制备出HER性能优异的Pt75(NiCo)25/非晶和OER性能优异的(PtIr)75Ni25/非晶多级孔结构电极。系统研究了 Co、Fe、Ir、Ru等元素合金化对Ni40Zr40Ti17Pt3非晶合金前驱体制备的多孔/非晶复合材料孔结构和性能的影响。结果表明,Co部分置换Ni后制备的多孔Pt75(NiCo)25/非晶复合材料具有优异的HER活性,驱动10 mA/cm2和100 mA/cm2电流仅分别需要16 mV和73 mV的过电位,且稳定性大幅提升,在50 mA/cm2下持续工作24 h性能无明显衰减,这是由于Co金化后能促进H2O的解离,提供更多碱性环境下缺乏的H质子,从而提升反应速率。Ir部分置换Pt后获得的(PtIr)75Ni25/非晶复合材料显示优异OER活性,驱动10 mA/cm2和100 mA/cm2的电流密度分别需要249 mV和309 mV的过电位,且显示出良好的全解水性能。合金化元素改变了 Pt的配位环境和电子结构,从而影响其催化性能。该结果表明,合金化是提高多孔材料催化活性和稳定性的有效策略。(3)利用相分离原理调控FeCoNiCu高熵合金前驱体的组织结构,制备出纳米孔与微米孔结合的多级孔结构高熵合金电极,其在碱性条件下显示出优异的电解水性能。采用真空电弧熔炼方法制备了不同Cu含量的Fe-Co-Ni-Cu-(Al)高熵合金。合金在凝固过程中形成微米尺度的相分离和纳米尺度的调幅分解组织,即由贫Cu相和富Cu相组成。通过调整脱合金工艺可以选择性去除贫Cu相或富Cu相,从而得到富Cu或贫Cu微米孔和纳米孔构成的多级孔结构高熵合金电极。贫Cu多级孔高熵电极表现出优异的OER活性,电流密度为10 mA/cm2仅需要269 mV的过电位,Tafel斜率为54 mV/dec,优于商业RuO2催化剂,这是由于脱合金之后高熵合金中微米尺度的相分离和纳米尺度的调幅分解组织中的富Cu相被选择性溶解,形成多级孔结构;另一方面,多孔材料表面由于金属原子高价氧化态的增加,对应于金属氧化物和金属氢氧化物的形成,这些物质改变了金属元素的电子结构,进而提高其催化反应速率。该研究表明,通过相分离调控高熵合金前驱体的组织可以有效地调控脱合金产物的多级孔结构,进而调控其催化性能。(4)利用选择性激光熔化结合脱合金方法设计制备出满足工业应用需求的多级结构FeCoNiCu高熵合金电极,在工业碱性电解槽中表现出优于商用催化剂的活性和稳定性。采用选择性激光熔化(SLM)自由设计制备了具有不同网格形状的FeCoNiCu高熵合金。由于SLM制备具有层层叠加和快速冷却的特点,导致合金凝固过程中形成纳米尺度片层状相分离组织。通过脱合金处理去除前驱体的富Cu相,形成微米网格孔与网格骨架表面的纳米阵列组成的多级结构的自支撑多孔高熵合金电极。由于微米孔有利于反应产生的气泡及时溢出,并且纳米阵列可以提供丰富的催化活性位点,制备的多级结构的高熵合金电极表现出优异的OER活性和稳定性,在电流密度为10 mA/cm2时仅需261 mV的过电位,Tafel斜率低至49 mV/dec,且在90000次循环伏安测试后性能没有明显的改变。优异的催化活性来源于纳米阵列表面包覆的高熵氧化物层,其包含价态更高的金属,在较低的电位下就能产生羟基氧化物M-OOH。OER反应能垒较低。此外,采用SLM直接设计制备出工业用大尺寸电极,其在工业用碱性电解槽中表现出优于商业雷尼镍的催化活性和稳定性,在施加电位小于2 V的电解槽中析氧瞬时电流密度可以达到320 mA/cm2,且在300 mA/cm2下持续工作200小时性能无明显衰减,显示出良好的工业应用前景。本论文基于非晶合金和高熵合金前驱体,通过调整合金成分和制备工艺,制备出高效低成本的多级孔结构自支撑电解水电极,并揭示其催化活性和稳定性大幅提升的机理,为这些新型催化材料的工业应用奠定了基础。更多还原

【Abstract】 Hydrogen energy is considered to be the most promising renewable energy source for the 21st century.Hydrogen production by water electrolysis is an effective way to obtain hydrogen energy in a green and sustainable way.However,electrolytic hydrogen production is currently facing problems such as high cost,low efficiency,and high electrical energy consumption,which seriously limit its largescale commercialization.Therefore,the development of low-cost and efficient catalysts is the key to the industrial application of electrolytic hydrogen production technology.Low-cost transition metal-based nanocatalysts show great catalytic activity for water splitting,however,their stability cannot meet the requirements of long service time at the high current density as required by industries.To address the tradeoff between the activity and stability of current electrolytic water catalysts,this thesis aims to develop low-cost,high-activity,and high-stability electrolytic water catalysts in alkaline,and systematically investigate the pore structure modulation and electrolytic water performance optimization of porous metals prepared from disordered alloys,i.e.,metallic glass(MG)and high-entropy alloys(HEAs)precursors.The main content and conclusion are as follows:(1)By varying the cooling rate and surface stress state,the pore structure of the dealloyed Ni40Zr40Ti20 MG was successfully modulated,by which the catalytic activity and stability of porous Ni/MG composites were significantly improved.Both Ni40Zr40Ti20 MG ribbons and wires were used as precursors for dealloying.After dealloying,nanoporous Ni/MG composites with a homogeneous nanopore structure were fabricated on the surface of the MG ribbon,whilst porous Ni/MG composites with a hierarchical structure consisting of nanopores and micron-scale silts were prepared on the surface of the MG wire.The formation of the hierarchical structure is due to the fact that the gradient of residual stress in the MG wire can act as a driving force for atomic diffusion,resulting in high etch rates.Meantime,the rapid dealloying process prevented the complete release of residual stress in the MG wire,leading to the creation of silts on its surface.The porous Ni/MG composite exhibited outstanding HER performance in alkaline electrolytes,only requiring an overpotential of 78 mV at a current density of 10 mA/cm2 and a Tafel slope of 42.4 mV/dec.In addition,the pore structure remained intact for 24 h at 10 mA/cm2 with insignificant degradation in performance.The significant increase in the electrolytic stability is due to micron-scale silts that allowed bubbles to escape in time,whereas the performance of the composite with a uniform nanopore structure deteriorated dramatically at 10 mA/cm2 for 12 h.(2)With the addition of different alloying metals,hierarchical-pore-structure Pt75(NiCo)25/MG with outstanding HER performance and(PtIr)75Ni25/MG with extraordinary OER performance were prepared based on the Ni40Zr40Ti17Pt3 MG.Effects of the addition of Co,Fe,Ir,and Ru on the pore structure and catalytic properties of porous metal/MG composites prepared from Ni40Zr40Ti17Pt3 MG precursors were systematically investigated.The results show that the porous Pt75(NiCo)25/MG composites prepared by partial replacement of Ni by Co have shown prominent HER activity,requiring an overpotential of 16 and 73 mV to drive 10 and 100 mA/cm2 current density,respectively.Its durability was greatly enhanced,with no significant decay in continuous operation at 50 mA/cm2 for 24 h.The(PtIr)75Ni25/MG composite obtained by partial replacement of Pt with Ir showed promising OER activity,requiring overpotentials of 249 and 309 mV to drive currents of 10 and 100 mA/cm2,respectively.Our analyses indicated that the alloying elements altered the atomic coordination and electronic structure of Pt,thus improving its catalytic performance.The results suggest that alloying is an effective strategy to improve the catalytic activity and stability of porous materials.(3)Using the principle of phase separation to regulate the microstructure of FeCoNiCu HEA precursors,a hierarchical-pore-structure electrode with a combination of nanopores and micropores was prepared,which showed outstanding water splitting performance under alkaline conditions.Micron-scale phase separation of Cu-poor and Cu-rich phases and nanoscale chemical fluctuation occurred during the solidification of the Fe-Co-Ni-Cu-(Al)HEAs with different Cu contents.Both the Cu-poor and Cu-rich phases can be selectively removed by adjusting the dealloying electrolyte,resulting in Cu-rich or Cu-poor electrodes with a hierarchical pore structure consisting of micron-and nano-pores.The Cu-poor HEA electrode with a hierarchical pore structure exhibited prominent OER activity,requiring an overpotential of only 269 mV at a current density of 10 mA/cm2 and a Tafel slope of 54 mV/dec,superior to that of the commercial RuO2 catalysts.On one hand,the selective dissolution of the Cu-rich phase in the micron-scale phase separation and nanoscale modulation decomposition microstructure in the HEA after dealloying results in a hierarchical pore structure.The multi-scaled pores not only enhance electron transport kinetics but also provide a fast channel for gas release;on the other hand,the surface of the porous material corresponds to the formation of metal oxides and metal hydroxides due to the increase in the high-valent oxidation state of the metal atoms,which change the electronic structure of the metal elements and thus increase their catalytic reaction rates.This study shows that modulation of the microstructure of the HEA precursors by phase separation is effective in the manipulation of pore structure and the catalytic performance.(4)Self-supported HEA electrodes with a hierarchical structure with micronscale pores and nanoarrays were prepared by 3D printing,which showed superior performance to the commercial Raney Ni under industrial water splitting conditions.FeCoNiCu HEAs with different pore structures were prepared using the selective laser melting(SLM)technique.Due to the layer stacking and rapid cooling characteristics of the SLM technique,nanoscale phase separation occurs during the solidification of the HEA in the form of nanoscale lamellae.The Cu-rich phase in the precursor was removed by dealloying to form a self-supporting,hierarchical porous HEA electrode consisting of micron grid pores with nanoarrays on the surface of the grid ligament.As the micron pores facilitate the timely escape of reaction-generated bubbles and the nanoarrays provide abundant catalytic active sites,the prepared HEA electrode thus exhibited promising OER activity and stability,requiring an overpotential as low as 261 mV at a current density of 10 mA/cm2 and a Tafel slope as low as 49 mV/dec.Importantly,no significant degradation in performance after 90,000 cycles of cyclic voltammetry testing.The superior catalytic activity is derived from the high entropy oxide layer encapsulated on the surface of the nanoarray,which contains high valence metals and produces an M-OOH at low potentials.In addition,an industrial large-size electrode prepared by SLM showed better catalytic activity and stability than commercial Raney Ni in the industrial alkaline electrolyzer,with an instantaneous current density of 320 mA/cm2 for OER under an applied potential of less than 2 V.Also,no significant degradation in performance was observed after working at 300 mA/cm2 for 200 hours,showing great potential for industrial application.In this thesis,by adjusting the composition and preparation process of the MG and HEA precursors,the high-efficiency,low-cost,and self-supported electrolytic electrodes with hierarchical pore structure for water electrolysis were fabricated.The underlying mechanisms responsible for the significantly enhanced catalytic activity and stability were revealed,which lays the foundation for the industrial application of these new catalytic materials.更多还原