Preparation Of Molybdenum-Based Interstitial Compound Integrated Electrodes And Their Electrocatalytic Performance For Hydrogen Evolution Reaction

To meet the growing demand for energy and to solve the environmental problem caused by the use of fossil fuels,renewable and inexpensive green energy has gained great attention from the researchers around the world.Hydrogen energy,as the widely used secondary energy source,is considered as the most promising clean energy source in the 21st century due to its green,low carbon and abundant source.Hydrogen production through water electrolysis is more efficient,pure and environmentally friendly than the traditional hydrogen production from fossil fuels.Nevertheless,hydrogen evolution reaction（HER）often requires the use of a noble metal-based catalyst and the application of a higher-than-theoretical voltage to drive the HER.The development of efficient and inexpensive non-precious metal-based HER catalysts is an inevitable choice to reduce the cost and improve the efficiency of hydrogen production from electrolytic water.Molybdenum-based interstitial compounds exhibited excellent HER activity.The smaller radius of the quasi-metal atoms have the tendency to occupy the interstitial positions in the original close-packings.As a result,molybdenum-based interstitial compounds maintain the high electrical conductivity of the metal and provide the long-term corrosion resistance.However,most of the reported molybdenum-based interstitial compounds are in powder form,and the polymer binder is necessary to make these catalysts as the usable working electrodes.After use of the polymer binders,a large number of active sites were covered,resulting in the relatively low HER efficiency.The integrated electrodes without aid of the polymer binders accelerated the electron transfer and ensured their full contacts with the active sites.The integrated electrodes also provide large electrochemical surface areas and increase the adsorption capacity around the active sites,ultimately achieving excellent hydrogen reduction efficiency and long-term stability.This thesis focuses on the design of molybdenum-based interstitial compounds integrated electrodes and their HER performances.By designing the structures and components of catalytic active sites,the interactions among the active metals were adjusted to improve the catalyst activity and stability.The influence of catalyst structures were explored to reveal the structure-effect relationships.The research results provided the reasonable approach for the structural design and efficient synthesis of the catalysts.The main research contents are as follows.1.A nickel/molybdenum nitride heterostructure nanorod integrated electrode（Ni-MoN@NF）anchored on nickel foam was synthesised by simultaneous reduction of pyrolytic dicyandiamide and nitridation of NiMoO₄@NF precursor.Nickel/molybdenum nitride heterostructure nanorod arrays possess abundant active sites and good electron transfer capability.The construction of the heterogeneous Ni/MoN interface on the integrated electrode further resulted in much higher HER activity than that of the nickel foam under the same condition.The Ni-MoN@NF integrated electrode in alkaline medium exhibits excellent HER performance with overpotentials of 37 and 110 m V at 10 and 100 m A cm^-2,respectively.The HER activity of the Ni-MoN@NF integrated electrode is still remained over 100 hours of the continuous catalysis,dsemonstrating its long-term durability.In particular,the excellent activity is maintained at high current density(overpotential of only 283 m V at 1000 m A cm^-2).This work provides a positive strategy for the preparation and application of the molybdenum nitride-based electrocatalysts in the field of renewable energy.2.An efficient electrocatalyst based on MoB₂and carbon nanotubes（CNTs）heterostructure（Co,Ni-MoB₂@CNT/CC）was successfully constructed with the molten salt-assisted borothermal reduction of（NH₄）₄[NiH₆Mo₆O₂₄]·5H₂O（NiMo₆）,Co（NO₃）₂·6H₂O and 2-methylimidazoleon the CC.During the boron reduction process,NiMo₆formed MoB₂.At the same time,Co catalysed the carbon on the polyacid surface,allowing CNTs to grow in situ on the surface of MoB₂.Heterojunction surface engineering is one of the most effective methods to optimise the composition and structure of electrocatalysts.The pH range of the integrated electrode was broadened by the Co,Ni-MoB₂@CNT heterojunction with appropriately selected CC substrate materials.Co,Ni-MoB₂@CNT/CC exhibits remarkable HER performance with a low overpotential of 98.6,113.0 and 73.9 m V at 10 m A cm^-2in acidic,neutral,and alkaline electrolytes,respectively.Notably,even at 500 m A cm^-2,the electrochemical activity of Co,Ni-MoB₂@CNT/CC exceeds that of Pt/C/CC in alkaline solution,and maintains over 50 h.Theoretical calculations reveal that the construction of heterostructure is beneficial to both water dissociation and reactive intermediate adsorption,resulting in superior alkaline HER performance.This work further demonstrates that transition metal diborides represented by MoB₂not only perform well in acidic HER reactions,but also have excellent electrocatalytic HER performance in high pH electrolytes.3.A surface oxygen-enriched molybdenum boride integrated electrode（O-MoB@Mo）was synthesised in-situ by using a simple oxidation followed by boride reduction on the industrial-grade metallic molybdenum substrate.The stable structure between the in situ formed O-MoB catalyst and the molybdenum substrate enabled O-MoB@Moto exhibit high catalytic activity and excellent durability.O-MoB@Moretains the metallic properties of the substrate after boronisation,indicating its potential as a flexible electrode.This electrode featured efficient and stable catalytic HER in alkaline solutions,acidic solutions or the simulated alkaline seawater systems.The overpotentials of O-MoB@Moin the above electrolytes at 10m A cm^-2are only 133,145 and 130 m V,respectively.The overpotentials of the O-MoB@Mointegrated electrode at the industrial grade current density of 1000 m A cm^-2are also only 573,413 and 438 m V,respectively.O-MoB@Mofeatures stable and efficient catalysis performence in 1.0 M KOH and 0.5 M H₂SO₄ solution for 100hours at a high current density of 500 m A cm^-2.Theoretical calculations show that O-MoB possesses a wider d-energy band and lower ΔG_H*than MoB.The rich O on the surface favoured the modification of the MoB electron structure,thus enhancing the charge transfer and the HER activity.In addition,the cost of the metal molybdenum substrate is less than the carbon cloth.Thus,the metal molybdenum substrate is more suitable for the preparation of the cheap and efficient integrated electrodes.4.A bifunctional Mo₂FeB₂/FeB@IF electrocatalyst was prepared by boride reduction.During the electrocatalytic water splitting process,the Mo₂FeB₂/FeB heterostructure accelerated the electron transfer.The overpotentials of Mo₂FeB₂/FeB@IF were 123 and 323 m V,respectively,in 1 M KOH at current densities of 10 and 500 m A cm^-2.The HER activity of Mo₂FeB₂/FeB@IF was stil maintained for 100 h of continious catalysis at a current density of 100 m A cm^-2,demonstrating excellent stability.The surface self-reconfiguration process promoted the conversion from Mo₂FeB₂/FeB@IF to FeOOH@IF.The overpotentials at current densities of 10 and 500 m A cm^-2were 300 and 452 m V,respectively.Mo₂FeB₂/FeB@IF featured stable OER performance for 100 h at a current density of 100 m A cm^-2,showing excellent OER activity and stability.Moreover,the high-performance alkaline electrolyzer with Mo₂FeB₂/FeB@IF and FeOOH@IF as cathode and anode was assembled for total water splitting.This work provided the experimental basis for the development of bifunctional electrocatalytic water splitting catalysts with high activity,high stability and low cost.