The Controllable Synthesis And Electrocatalytic Performance Of Silver-based Materials

The demand for energy is increasing with the population growth and the development of the industrial society.The heavy use of fossil fuels has aggravated pollution of the environment.So it is crucial for meeting these challenges to search and develop clean,effective and reproducible energy.Fuel cells have become attractive for the high energy efficiency,pollution-free and portability of it.Platinum?Pt?-based materials are the major catalysts for oxygen reduction reaction?ORR?on cathode of fuel cell.However,Pt is scarce and expensive,and the poor methanol tolerance and electrochemical stability.The development of non-Pt catalysts with the higher efficiency for ORR is very important to promote the large-scale commercial application of the fuel cells.Among the various non-Pt electrocatalysts for ORR in fuel cells,sliver?Ag?is regarded as one of the most promising candidate for replacing Pt-based electrocatalysts owing to its relatively low price,abundant reserves,high conductivity and electrochemical stability in alkaline medium.It is of great significance to develop efficient Ag-based catalysts to replace Pt for ORR for promoting the large-scale commercial development of fuel cell.Great efforts have been made to improve the electrocatalytic activity of Ag-based catalysts for ORR.However,there is still a gap between the catalytic performance of the Ag-based catalysts and Pt.We synthesize different Ag-based catalysts by the electrodeposition,nanocasting,and hydrogen reduction method.The electrocatalytic activity of Ag-based material is improved not only by adjusting the morphology,size and electronic structure of Ag catalysts but also by utilizing the interaction between Ag and carrier,improving the dispersion of Ag,and increasing specific surface area of catalysts.The influence of structure,morphology and the interaction between components of metallic silver,silver nanoalloy,and supported silver-based catalyst on electrocatalytic activity are studied systematically.The research contents and results are as follows:1.Silver nanodendrites?AgNDs?are prepared on glassy carbon electrode?GCE?by electrodeposition method using 7,7,8,8-tetracyanoquinodimethane?TCNQ?as the inducer.The AgNDs/TCNQ/GCE prepared at the optimized conditions are investigated as electrocatalysts for ORR and hydrogen peroxide?H₂O₂?detection.In 0.1 M KOH,the ORR onset potential of AgNDs/TCNQ/GCE is 0.98 V_RHE,which is the same as that over Pt/C-JM catalyst(0.98 V_RHE).AgNDs/TCNQ/GCE exhibits electrocatalytic performance toward H₂O₂ detection with a sensitivity of 450?AmM^-1cm^-2 and a detection limit of 0.47?M in a range from 10?M to 17 mM.The AgNDs/TCNQ/GCE possesses higher electrocatalytic activity for ORR and H₂O₂ detection than the similar Ag-based electrocatalysts reported in the literature.The dendritic morphology exposes more active sites and the existence of TCNQ enhances the electron transportation capacity of interface between AgNDs and GCE,which facilitate the higher electrocatalytic activity for ORR and H₂O₂ detection.2.Mesoporous carbon?tri-OMC?loaded Ag-Fe alloy nanoparticle?AgFe/tri-OMC?are synthesized by the hydrogen reduction method.The characterization methods of TEM,STEM-Mapping,XPS and XRD have proved the existence of AgFe alloy.In 0.1M KOH,the ORR onset potential and half-wave potential of AgFe/tri-OMC are 0.96V_RHE and 0.80 V_RHE,which are 60 mV and 20 mV positive than Ag/tri-OMC and 20mV and 20 mV negative than Pt/C.The half-wave potential of AgFe/tri-OMC is 70 mV positive than Ag₇₅Fe₂₅5 nanoparticles reported in the literature.The electrocatalytic activity of AgFe/tri-OMC is highest among the AgFe alloy catalysts.The introduction of Fe adjusts the d-band center of Ag,which enhances the electrocatalytic activity of Ag.Meanwhile,the method of preparing AgFe alloy has an important influence on the electrocatalytic activity for ORR.3.Mesoporous carbon?tri-OMC?loaded Ag-Cu alloy nanoparticle?AgCu/tri-OMC?are synthesized and characterized by the methods similar to AgFe/tri-OMC.The existence and composition of AgCu alloy are determined.In 0.1 M KOH,the ORR onset potential of AgCu/tri-OMC is 1.00 V_RHE,which are 20,100,and 40 m V positive than Pt/C,Ag/tri-OMC,and AgFe/tri-OMC?the previous chapter?,respectively.The half-wave potential of AgCu/tri-OMC is the same as that over Pt/C-JM catalyst(0.82V_RHE),which are 40 mV and 20 mV positive than Ag/tri-OMC and AgFe/tri-OMC?the previous chapter?.The Tafel curves prove that the kinetic reaction rate of AgCu/tri-OMC is faster than Ag/tri-OMC,Pt/C,and AgFe/tri-OMC?the previous chapter?.The AgCu/tri-OMC exhibits higher current density at low overpotential.The results fully testify that different transition metals have different effects on electrocatalytic activity of Ag-based alloy catalysts.The d-band center should be adjusted closely to the value which makes O-O bond split and desorption of the oxygenated intermediates achieve the best balance.4.The ordered mesoporous silver-nitrogen doped graphitic carbons?Ag-N-GC?is synthesized by wet-impregnation nanocasting method with silver nitrate and 2,2-pyridine complexes as the precursor and SBA-15 as hard template.In alkaline medium,the ORR onset potential and half-wave potential of Ag-N-GC-900 are 1.00 V_RHEHE and0.82 V_RHE.The onset potential is 20 mV higher than Pt/C(0.98 V_RHE)and the half-wave potential is the same as that over Pt/C(0.82 V_RHE).Ag-N-GC-900 shows higher electrocatalytic activity in acidic medium than the similar materials reported in the literature.Tafel curves further convince the kinetics reaction rate of Ag-N-GC-900 is faster than that of Pt/C.The Ag-N-GC synthesized by nanocasting method possess the active sites with high catalytic activity,high density of active sites,and strong capacity of electron transfer and material transfer,which facilitate the reaction rate of ORR.5.The iron-nitrogen-carbon supported silver nanoparticles materials?Fe-N-C/AgNPs?is synthesized by incorporating AgNPs onto the mesoporous Fe-N-C frameworks using NaBH₄ as reductant and sodium citrate as protective agent.The morphology of materials is characterized by XRD,SEM,TEM,and N₂ stripping absorption curve.The characterization results show well-ordered hexagonal arrays of mesopore channels,the well dispersed AgNPs,and large specific surface area.In 0.1M KOH,the ORR onset potential and half-wave potential of Fe-N-C/AgNPs-2 are 1.03V_RHEHE and 0.88 V_RHE,which are positive than Pt/C(0.98 V_RHEHE and 0.82 V_RHE).The detection limit of Fe-N-C/AgNPs-2 for nitrobenzene is 0.33?M in a range from 1 ppm to 110 ppm.The electrocatalytic activity of Fe-N-C/AgNPs-2/GCE for ORR and NB detection are higher than the catalysts reported in the literature.The result of XPS and Tafel curves explain the synergistic effect between AgNPs and Fe-N-C.The AgNPs as cocatalyst enhance the electrocatalytic activity of Fe-N-C for ORR by inducing a more negative charge on the C adjacent to N.