Functional Nanomaterials Electrocatalysts For Efficient Electrochemical H₂O₂ Production By Two-electron Oxygen Reduction Reaction In Acids

Hydrogen peroxide(H2O2)is a potential energy carrier and an important commodity chemical with high industrial value,which is extensively utilized in almost all industrial areas and various medical and other applications.Compared to the traditional energyintensive anthraquinone oxidation process and the method directly converted from H2 and O2 with potential hazard of explosion,electrochemical oxygen reduction reaction via the two-electron pathway to H2O2 represents a promising route to enable its on-site,ondemand production for direct consumer use.Its practical viability,however,hinges on the development of advanced electrocatalysts.Carbon-based materials exhibit appreciable two-electorn oxygen reduction reaction(2e-ORR)activity and selectivity in alkaline solution but generally very poor performance in neutral or acidic solution.For the reason of less stable in bases(especially at pH>9)and the stronger oxidation ability and more widely used in acidic media than basic media,running this reaction in acids is practically relevant and scientifically challenging.At present,the state-of-the-art 2e-ORR electrocatalysts in acids are Pt-Hg and Pd-Hg alloys,but the incorporation of toxic Hg raises serious health and environmental concerns.Au is a typical 2e-ORR electrocatalyst with high H2O2 selectivity but suffers from low activity in acids.For this,we have developed some materials and investigated their electrocatalytic performance systematically.We have solved some problems of catalytic system in various respects and revealed the inherent factors for the high catalytic perfomance of these materials.The focus of this dissertation is to design and explore noble-based,non-noble-metalbased and molecular catalysts for electrochemical H2O2 production via 2e-ORR in acidic conditions.In this study,stepwise solution growth and liquid phase exfoliated method were used to prepare functional nanomaterials in a well-controlled conditions.The physical and chemical characters of materials were reveald by a series of characterization techniques.The structure-function relationships of electrocatalysts was expolred by rigorous electrochemical measures.The catalytic process of the catalysts in atomic scale was also analyzed by density functional theory calculation.The main research contents and results are as follows:1.Based on the characteristic of high H2O2 selectivity and low catalytic activity for pure Au catalyst,we try to promote the activity whlie keeping high selectivity by alloying Au with Pd.We have prepared bimetallic PdAu nanoframes by stepwise solution growth and etching under well-controlled conditions.The product exhibits a great performance for H2O2 production in 0.1 M HClO4(0.56 V positive onset potential,>90%H2O2 selectivity and>12 h electrochemical stability),which are considerably improved over pure Au and other Au-based alloys.Theoretical simulations indicate that the incorporation of dilute Pd atoms enhances the adsorption of OOH*as the key intermediate,and thereby promotes the reaction activity and selectivity.Our catalyst can also be used for the electroFenton reaction to catalyze the degradation of organic dyes in minutes.2.Due to the inherent problem of prohibitive costs for noble-metal-based electrocatalysts,it is in great need to explore the high-efficiency non-noble-metal-based electrocatalysts for 2e-ORR.We have prepared mono-few lyaerd 2H phase MoTe2 nanoflakes with zigzag edges via liquid phase exfoliated method.We show that MoTe2 nanoflakes have a great performance in 0.5 M H2SO4:～140 mV onset overpotential and large mass activity of 27 A g-1 at 0.4 V vs.RHE,high H2O2 selectivity of 93%and decent stability,surpass the non-noble-metal-based conpetitors in acids.Theoretical simulations evidence that the high activity and selectivity of 2H-MoTe2 arise from the proper binding energies of OOH*and O*reation intermediate at its zigzag edges.3.Molecular catalysts with well-defined active centers and adjustable ligand structures are great model catalytic materials,but have rarely been studied in 2e-ORR.Based on previous reports,we choosed the combination of cobaltporphyrin-based catalysts and Ketjen black(KB)to explore the influences for H2O2 production via 2eORR in acids of catalysts contents and electron-drawing/electron-donating group in cobaltporphyrin.We found that the 2e-ORR catalytic activity was improved with the increase of catalysts ontents when the ontents<5wt%in 0.1 M HClO4.More cobaltporphyrin-based catalysts ontents(>5wt%)would reduce the H2O2 selectivity.Besides,we compared the 2e-ORR catalytic activity and H2O2 selectivity of cobaltporphyrin-based catalysts under 5wt%catalysts contents and found that CoP4COOH/KB has the highest acticity of 0.65 V onset potetial and commercial CoP/KB has the highest H2O2 selectivity(>90%).4.Based on above studies,we perform the high-throughput computational screening of 32 different metalloporphyrins by comparing their adsorption free energies towards key reaction intermediates.Cobalt porphyrin is revealed to be the optimal candidate with a theoretical overpotential as small as 40 mV.Guided by the computational predictions,we prepare hydrogen-bonded cobaltoporphyrin frameworks in order to promote the solution accessibility of catalytically active sites for H2O2 production in acids.The catalysts exhibit a great performance for 2e-ORR in 0.1 M HClO4:a positive onset potential at 0.68 V,high H2O2 selectivity of>90%,excellent turnover frequency of 10.9 s-1 at 0.55 V and good stability of>30 h.The combination of which clearly renders it stand out from existing competitors for this challenging reaction.