Structure-oriented Design Of Transition Metal Compounds And Application In Electrocatalysis

With the development of society,environmental pollution and the energy crisis have become increasingly serious,forcing people to look for new energy supply systems.The development and utilization of new energy has become one of the most concerned issues in today's society.Molecular hydrogen?H₂?is regarded as a promising new energy carrier because of its high energy density,cleanness and pollution-free.Among the many methods for producing hydrogen,water splitting to produce hydrogen or coupling the hydrogen evolution reaction with the electro-oxidation reaction?such as urea oxidation reaction?of high-energy chemical compound to attract hydrogen has attracted much attention.Direct liquid fuel cells to replace traditional non-renewable energy sources have also attracted widespread interest.Direct hydrazine fuel cells?DHFC?have attracted much attention due to their high energy density,pollution-free final products,and wide applicable pH range.Electrolytic hydrogen production and hydrazine fuel cells are both limited by the low efficiency of key chemical reactions.During the electrochemical reaction,their kinetics are slow,which severely restricts the energy conversion efficiency.Catalytic technology is the key way to realize the preparation and conversion of new energy.Therefore,in this paper,the author designs and synthesizes a series of high-efficiency and stable electrocatalysts through defect engineering,optimization of active sites,design of crystal structure,and synergy of composite structure to improve the chemical reaction rate.This paper has a good guiding role for the design of future electrocatalysts.This thesis mainly discusses from the following three aspects:1.The author found the effect of the platinum counter electrode in HER catalysis,especially long-term HER operation,has drawn substantial attention,which involves the electrochemical dissolution of platinum on the Pt counter electrode and subsequent redeposition of Pt nanocrystals on the working electrode.The redox Pt dissolution-redeposition can bring in significantly enhanced HER activity of the cathode material,and further,pH-universal catalytic activity could be realized benefitting from the as-deposited Pt species,i.e.,atoms,clusters,or nanocrystals.The author uses the defect structure engineering of the base surface of molybdenum disulfide nanosheets to introduce additional active sites for the hydrogen evolution reaction,combining rich defect sites with a large surface area,which can provide rich reaction sites for adsorption and anchoring of platinum nanocrystals.Benefiting from the synergy between the defect-rich molybdenum disulfide nanosheets and platinum nanocrystals,the defect-rich nanosheet-platinum composite catalyst significantly enhances the hydrogen evolution reaction activity in both acidic and alkaline environments,and is even better than the benchmark platinum/carbon catalyst.The defect-rich nanosheet-platinum?DR-MoS₂-Pt?composite catalyst shows a large cathode current density,a small Tafel slope,and excellent electrochemical stability,showing excellent electrocatalytic hydrogen evolution reaction performance.This method of modifying platinum-rich molybdenum disulfide nanosheets by electrochemical dissolution-redeposition of platinum reduces the amount of platinum used and achieves highly active hydrogen evolution over a wide pH range.This work provides experience for the design of new electrocatalysts and has important guiding significance.2.The author designed a hierarchical copper-incorporated?-Ni?OH?₂ nanoarray catalyst using nickel foam as the substrate,which exhibits robust electrocatalytic performance towards urea oxidation,hydrogen evolution and oxygen evolution.The unique 1D-2D-3D hierarchical structure not only provides abundant reactive sites for the generation of catalytically active species,but also guarantees a robust and conductive skeleton for facile charge transport and offers effective buffering space for the Ni²⁺-to-Ni³⁺pre-oxidation process.Besides,the presence of local Ni³⁺ions and the optimal Cu incorporation further promote the electrooxidation behaviors such as the UOR and OER,and the in situ reduction of Cu²⁺during HER operation effectively boosts electron transfer and leads to enhance HER activity,therefore synergistically resulting in efficient and stable trifunctional electrocatalysis.Thence,the hierarchical copper-incorporated?-Ni?OH?₂ nanoarray catalyst displays excellent performance in coupled urea electrolysis and overall water splitting,providing a promising material candidate for industrial wastewater treatment and clean energy production.This work provides a model for synergistic realization of clean energy production and wastewater treatment,and has important inspiration for the preparation of future electrocatalysts.3.The authors used the dual-metal lanthanum-incorporated zeolitic-imidazolate framework?La/ZIF?as the precursors to prepare a composite electrocatalyst,in which nitrogen-doped porous carbon is embedded with hybrid nanoparticles based on metallic Co and amorphous LaCoO_x,which exhibit high activity for the electrocatalytic hydrazine oxidation reaction?HzOR?.The hybrid nanoparticles based on metallic cobalt and amorphous LaCoO_x could provide abundant active sites for HzOR catalysis,while the highly conductive and porous N-doped carbon could act as both robust skeleton for anchoring the active hybrid nanoparticles and facile charge transport pathway for HzOR process,thereby resulting in enhanced HzOR activity.With the synergistic merits of enriched active sites,large surface area and enhanced charge transfer ability,promoted HzOR performance with high activity and stability was realized for the optimized catalyst,which shows ultralow onset potential of-0.17 V vs.RHE,high HzOR current density of 69.2 mA cm^-2 at 0.3 V vs.RHE and superior stability for 20 h continuous catalysis,making the catalyst a promising electrode material for hydrazine-assisted hydrogen production.This work provided guidance for the optimization of the performance of the HzOR catalyst in the future,and inspired the design of the HzOR catalyst in the future.