Manipulation Of Pore Structures And Water Splitting Performance Of Porous Metals Prepared With Disordered Alloy Precursors

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.