| In today’s tense situation of ever-growing excessive energy consumption and serious environmental pollution,hydrogen energy has become an important strategic target of sustainable development in the future due to its large energy density,environmental friendliness,and superior utilization efficiency.Among numerous hydrogen production strategies,water electrolysis as a major and renewable energy-driven hydrogen generation technology has sparked great interest.Typically,the water electrolysis process is divided into two critical half-reactions:one is the cathodic hydrogen evolution reaction（HER）,and the other is the anodic oxygen evolution reaction（OER）.Considerable efforts are devoted to developing efficient electrocatalysts for the respective half-reactions to decrease the large overpotential.Up to now,Pt,Ir and Ru-based catalysts are assessed as benchmark ones for the HER（e.g.Pt/C）and the OER（e.g.Ru O2 and Ir O2）.However,these commercial catalysts face great difficulty in realizing large scale practical applications owing to their valuableness and scarcity.On the other hand,these commercial catalysts also have the disadvantage of poor stability.Therefore,the preparation of electrocatalysts with high efficiency,cost effectiveness and desirable durability still remains a huge challenge.Various types of electrocatalysts toward water electrolysis have been developed by researchers.From the perspective of the architectures of the catalysts,they mainly include zero-dimensional（quantum dots,nanodots,etc.）,one-dimensional（nanofibers,nanorods,etc.）,two-dimensional（nanosheets,nanofilms,etc.）and three-dimensional（3D）structures.There is little research on three-dimensional materials in the field of electro-catalytic water decomposition.Recently,metal aerogel,perfectly inheriting the characteristics of aerogel（large specific surface area,3D self-supporting structure）and metal（excellent conductivity,remarkable catalytic activity and unique plasma characteristics）,has emerged as the most promising candidate toward water splitting application.These unique physical and chemical characteristics endow metal aerogel materials with the advantages of abundant catalytic active sites,robust structural stability,rapid proton/electron transport,presenting great potential in the field of electrocatalysis.Notably,early works mainly focused on the preparation routes and fabricating some noble metallic aerogel-based materials,while the investigation of their application mainly concentrated on alcohol oxidation reactions,and there are a few reports on electrocatalytic water splitting,which lefts great potential and development prospects.Therefore,a variety of water electrolytic catalysts were successfully fabricated by taking metal aerogel as the object.Then a series of electrocatalysts with high catalytic activities are achieved through the effective synergistic effect among different components and the structural advantages of metal aerogels.The main contents of this thesis are as follows:1.Two kinds of alloy aerogel materials were synthesized by directly reduction of precursors containing various metal salts via a one-step reduction strategy.Due to the effective synergistic effect among different metal components in the alloy system and the 3D porous structure of the aerogel,the prepared two types of multi-component alloy metal aerogel materials both presented outstanding electrocatalytic properties.a)The amorphous ternary FeCoNi alloy aerogel（FeCoNi AG）was obtained by directly reduction of precursor containing three metal salts of Fe,Co and Ni via a one-step process,where Na BH4was used as both the reducing and gelling agent.Meanwhile,we also prepared binary and monometallic alloy-based aerogels composed of Fe,Co or Ni,and systematically compared their OER performances.Moreover,the electrocatalytic performance of amorphous and crystalline FeCoNi alloy aerogels were further compared to explore the influence of amorphous structure on the catalytic activity of materials.By virtue of the structural advantages of amorphous and aerogel as well as the synergistic effect among the three metal components of Fe,Co and Ni,the prepared FeCoNi AG catalyst exhibited excellent OER catalytic activity and long-term stability in alkaline environment.To reach the current density of 10 m A cm-2,FeCoNi AG only needs an overpotential of 235 m V.After 5000 cycles and 30 h of i-t stability tests,the electrocatalytic performance of the sample can still be well maintained.b)Subsequently,a series of metal high entropy alloy aerogels composed of more metal elements（Co Ni Ru Ir Fe,CoNiRuIrMn and Co Ni Ru Ir Cu）were also fabricated via a one-step reduction route.Benefitting from the synergistic effect of various metals and structural advantages of metal aerogel（large surface area,high porosity and self-supported architecture）,the resultant CoNiRuIrMn catalyst presents remarkably higher electrocatalytic activity than other control samples toward OER,also significantly surpassing the commercial Ru O2and Ir O2 catalysts.The fabricated CoNiRuIrMn catalyst delivers an ultra-low overpotential of 167 m V at a current density of 10 m A cm-2.Furthermore,a high mass activity of 376.2 A g-1is achieved,demonstrating its exceptional intrinsic activity.The catalyst shows no evident deactivation or decomposition after 20 h continuous operation,illustrating satisfactory stability of materials.2.Galvanic replacement engineering is a primary way to construct nano alloymaterials.According to the difference of reduction potential between different metals,one metal can be replaced by another metal to form an alloy.Two types of non-precious/precious alloy aerogels were acquired by the replacement of the pre-synthesized monometallic（Fe,Co and Ni）aerogels with Ir or Rh salts.Owing to the effective synergistic effect of different metal components in the alloy and the 3D porous structure of the aerogel,the prepared alloy metal aerogel materials display excellent catalytic performance and stability.a)The pre-synthesized monometallic aerogels（Fe AG,Co AG and Ni AG）were further reacted with Rh salt via a galvanic replacement engineering to produce Fe Rh,Co Rh and Ni Rh alloy aerogels,respectively.We systematically investigated their HER electrocatalytic performances in three different p H electrolytes（1 M KOH,1 M PBS and 0.5 M H2SO4）.Thanks to the optimized electronic structure induced by the formation of Co Rh alloy and the unique physical and chemical properties of the metal aerogel,the prepared Co Rh alloy electrocatalysts exhibited superior HER catalytic activity and stability over a wide p H range compared with commercial Pt/C catalysts.b)After that,the Ni Ir alloy aerogel was fabricated with a similar preparation procedure.Originating from the optimized electronic structure induced by the formation of Ni Ir alloy and the unique structructral characteristic of the aerogel materials,the optimized Ni Ir alloy aerogel（Ni-Ir0.5）presents remarkable electrocatalytic properties for both HER and OER in 1 M KOH.To reach a current density of 10 m A cm-2,the overpotentials of Ni-Ir0.5 for HER and OER in are 22 m V and 257 m V,respectively.A prototype device for overall water electrolysis coupled with the Ni-Ir0.5 catalyst affords a low voltage of 1.516 V at 10 m A cm-2 under alkaline condition,exceeding the commercial Pt C||Ir O2 device（1.611 V）.Furthermore,the water splitting device also features an extraordinary durability of 40 h.3.Interface engineering is an effective strategy to improve the electrocatalytic activity of hybrid nanomaterials by means of the synergistic effect of double or even multiple active sites.In order to further expand the diversity of metal aerogel materials,we have designed and prepared two kinds of metal aerogel material systems with heterostructure,and investigated their electrocatalytic water decomposition performance.a)A metal aerogel with Ru/Ru O2heterostructure was prepared by partially oxidized pre-synthesized Ru aerogel.Density functional theory reveals that at the Ru/Ru O2interface,electrons transfer from Ru to Ru O2,which effectively regulates the electronic structure between the interfaces,increases the adsorption energy of intermediate reactants,and significantly improves the catalytic activity.The optimized Ru-30catalyst exhibits an exceptional HER performance,which is even superior to the benchmark Pt/C catalyst,and an outstanding OER efficiency that is better than commercial Ru O2 in both alkaline and acidic solutions.Furthermore,a two-electrode configuration based on the Ru-30 catalyst as both anode and cathode for the overall water splitting exhibits an ultra-low working voltage of 1.467 V in 1 M KOH and 1.468V in 0.5 M H2SO4at 10 m A cm-2.b)A further study has demonstated the preparation of Ru aerogel with a little content of Ir.Then the pre-synthesized Ru Ir alloy was partially oxidized to construct a metal aerogel material with high valence Ir/Ru/Ru O2 heterostructure.Due to the effective synergy among the heterogeneous components,the optimized Ru98Ir2-350 catalyst showed excellent HER activity and stability under alkaline conditions,and even reached the requirements of industrial production.At a high current density(1 A cm-2),the Ru98Ir2-350 catalyst only needs an overpotential of 121 m V,which is far superior to the HER performance of commercial Pt/C catalyst.In addition,the Ru98Ir2-350 catalyst delivers a very small Tafel slope of 8.3 m V dec-1and a high mass activity of 550 A g-1（overpotential:100 m V）.After 10000 cycles and 500 h of i-t stability tests at a high current density of 1 A cm-2,the electrocatalytic performance of the sample can still be well maintained. |
