Design And Performance Studies Of Non-Noble Metal Based Electrocatalysts For Efficient H₂O₂ Synthesis

Hydrogen peroxide（H₂O₂）,as one of the most important chemicals,has been widely applied in the fields of industry,agriculture,medicine,energy and environment.Currently,the industrial production of H₂O₂predominately relies on the anthraquinone oxidation process,which suffers from complicated operation and serious secondary pollution.As an emerging alternative,electrochemical two-electrons oxygen reduction（2e-ORR）and water oxidation（2e-WOR）,which use O₂ or H₂O as raw materials and electricity as energy input,have been identified as green,safe,economical and efficient routes for H₂O₂ production.The key challenge is to design low-cost and abundant electrocatalysts with high activity,selectivity and H₂O₂ yield,which could selectively promote 2e^-pathways and hinder the competitive 4e^-processes.To address this challenge,this thesis focuses on the construction of high-performance non-noble metal based 2e-ORR and 2e-WOR electrocatalysts.The structures and compositions of electrocatalysts are rationally designed at both atomic and micron/nanoscale levels by considering the fundamental machnisms of 2e-ORR and WOR processes.Several crucial properties including the active sites exposure,adsorption/desorption ability of reactants,products and intermediates as well as the conductivity and stability have been optimized,realizing the efficient electrosynthesis of H₂O₂ with high selectivity and production rate.Our works will provide new insights into the development of novel2e-ORR/WOR electrocatalysts.The major conclusions of this thesis are as follows:1.Titanium-doped zinc cobalt sulfide hollow superstructures（Ti-Zn Co S HSS）have been constructed as efficient 2e-ORR electrocatalysts mainly via d-band engineering.Using hybrid metal-organic frameworks（MOFs）as precursors treated by a selective sulfidation process,hollow superstructures assembled by interconnected Ti-Zn Co S nanocages have been prepared.The polymetallic composition adjusts the d-band center of electrocatalysts toward optimized adsorption energy with oxygen intermediates,thus improving the selectivity and activity of 2e-ORR process.Furhermore,the hollow superstructure provides abundant active sites and promotes mass and electron transfer.Benefiting from the synergistic d-band center and superstructure engineering at both atomic and nanoscale levels,Ti-Zn Co S HSS shows excellent 2e-ORR performances with a high selectivity of 98%,onset potential of 0.77V vs.RHE and H₂O₂ production rate of 675 mmol h^-1 g^-1_cat.After 12 h cycling test,the catalyst maintained a high selectivity of～98%,indicating its outstanding stability.The development of polymetallic sulfides with complex nanostructure paves the way towards the design of advanced 2e-ORR electrocatalysts.2.Zinc/cobalt bimetallic zeolite imidazole framework（Zn Co-ZIF）based 2e-ORR electrocatalysts have been developed via facet engineering.By tuning the synthesis conditions such as surfactant concentration,cubic,truncated octahedral and rhombic dodecahedral Zn Co-ZIF nanocrystals have been synthesized with（001）,（001）/（110）and（110）crystal facets exposure,denoted as Zn Co-ZIF-C,-T and-R,respectively.Among them,Zn Co-ZIF-C exhibits the best 2e-ORR performances with H₂O₂selectivity of～100%and a production rate of～4.3 mol g^-1_cat h^-1 in 0.1 M KOH electrolyte.With the reduction of（001）crystal facet exposure ratio,the H₂O₂ selectivity and production rate of Zn Co-ZIF nanocrystals gradually decrease,implying that the（001）facet of Zn Co-ZIF is more favorable for the 2e-ORR process.In addition,the Zn Co-ZIF-C also shows superior stability with negligible current density and faraday efficiency decrease after 12 h test.Collectively,a facet dependent 2e-ORR process has been demonstrated for MOF electrocatalysts,the detailed mechanism is still under investigation.3.CaSnO₃@carbon composite fiber membrane（CaSn O₃@CF）has been prepared by electrospinning technique as self-standing electrodes for efficient 2e-WOR.In the hybrid electrocatalyst,high conductivity and stability of carbon fiber and superior activity and selectivity of Ca Sn O₃ nanocrystals are intergrated.In addition,the confinement effect of carbon substrate limits the agglomeration of Ca Sn O₃nanoparticles,introducing abundant oxygen vacancies.Density functional theory（DFT）calculations indicate the vital role of oxygen vacancy（O_v）in modulating the electronic structures of active sites（Sn）and optimizing adsorption free energy of oxygen intermediates（e.g.OH*,O*,and OOH*）,thus boosting the 2e-WOR selectivity and activity.Thanks to the synergy between hybridization strategy and O_v engineering,Ca Sn O₃@CF composite membrane shows a remarkable H₂O₂ selectivity of～90%,high production rate of 39.8μmol cm^-2 min^-1 and excellent stability（only～1%current decline after 12 h successive test）,offering new insights into the design of 2e-WOR composite electrocatalysts.4.Hollow carbon fiber membrane（ZnO@CF）embedded with tiny Zn O nanoparticles has been syntheized via a MOF engaged electrospinning-pyrolysis method as an efficient 2e-WOR electrocatalyst.The resultant Zn O@carbon composite fiber exhibits a foam-like hierarchical structure composed of interconnected hollow carbon nanocubes encapsulated with oxygen vacancy rich Zn O nanocrystals.Owing to the improved selectivity of Zn O,excellent conductivity of carbon fiber,promoted active site exposure and mass transfer of hollow structure,the free-standing membrane electrode shows superior 2e-WOR performances with a high selectivity（83.8%at 2.8V vs.RHE）,H₂O₂ generation rate(19.7μmol cm^-2 min^-1)and robust stability.With environmental friendness and cost-effectiveness,the developed Zn O@CF 2e-WOR elelctrocatalyst is of significance for the practical production of H₂O₂.