Multi-phase Multi-scale Electrocatalysts For The Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is an important electrochemicalreaction, which is widely applied in the fuel cells, metal-air batteries,chlor-alkali industry and corrosions. However, the sluggish kinetics of theORR resulted from the high overpotential, complicated process and smallexchange current density, limits the practical applications. As a result, weshould introduce some electrocatalysts to lower the overpotential and improvethe reaction efficiency of the ORR process. Carbon supported platinum (Pt/C)is now developed as the catalysts for the ORR process, but the high cost andlack of stability highly limit the applications as the commercial catalysts forthe devices. Thus, the design of new electrocatalysts with high catalyticactivity and stability is quite a demanding task for the investigations of theORR process.This thesis mainly focused on the supported electrocatalysts for the ORRprocess, applying metal, metal alloy, metal-carbide, and non-metalmetal-sulfide nanoparticles supported on various carbon materials in order toobtain the multi-phase multi-scale electrocatalysts, which could promote thereaction rates and efficiency of the ORR process. The influences of the support material sizes, various compositions of the catalytic nanoparticles, andthe synergetic effects between the support materials were investigated byanalyzing the activity and mechanism of the multi-scale multi-phaseelectrocatalysts for the ORR process. The controlled synthesis ofelectrocatalysts was accomplished, and some principles for the design of highefficiency electrocatalysts towards the ORR process were also investigated.The main achievements are listed as below:(1) Platinum (Pt) nanoparticles supported on carbon materials with variousmorphologies and structures, such as carbon spheres, carbon nanotubes,carbon fiber mats and hierarchical porous carbons fabricated frombio-materials were synthesized and applied as the electrocatalysts for the ORRprocess. High graphitization of the carbon materials could improve theregularity of the graphite sheets, and catalytic activities for the ORR process interms of current densities. The surface state of support materials alsoinfluenced the catalytic properties of the electrocatalysts. The hierarchicalporous carbons (HPC) fabricated from the bio-materials possessed not onlyhigh surface areas, but also large amounts of surface functional groups, whichcould make the nanoparticles highly dispersed. Some nitrogen atoms insertedinto the frameworks of the carbons and improved the catalytic activities of theHPC supported electrocatalysts. The principles of the support materials for thehighly active electrocatalysts were high graphitization and surface area;certain amounts of surface functional groups or nitrogen atoms doped on the surface carbon frameworks; hierarchical porous structures which couldpromote the mass transportation.(2) Several carbon materials with similar graphitization but different sizesand morphologies were selected as the support materials for the Ptnanoparticles. The multi-scale effect of the support materials to the ORRprocess was further analyzed by investigating the mechanism of the reaction,including calculating the numbers transferred during the process and theexchange current densities. The results indicated that the one-dimensionalcarbon materials in macroscopic and microscopic scales, such as carbonnanotubes and carbon fibrous mats, could help the charge transformationduring the catalytic process, which made the corresponding electrocatalystsexhibit high catalytic activity, as well as good selectivity for the ORR process.The promotion effects of three-dimensional HPC support material mainly liedin the influence of the hierarchical porous structures to the mass transportationduring the ORR process.(3) The carbon nanotube supported Pt-gold (Au) nanoparticles, includingcore-shell, alloy and segregated, were synthesized and their catalyticproperties were characterized. The result indicated that the Pt-Au core-shellstructure exhibited better catalytic activity than the alloy and segregatedstructures. Through the analysis of the ORR mechanisms, we found that thecore-shell structures could improve the catalytic activity through thesynergetic effects between the two metals. In the case of Pt-Au alloy, the surface structures of the nanoparticles changed, and the affinity of the oxygenwas also strengthened. The synergetic effects between Pt and Au was quiteweak in the Pt-Au segregated structures, thus the ORR catalytic process wasmainly accomplished by the Pt nanoparticles supported on the Au particles. Inorder to further analyze the crystalline structure effects to the electrocatalyticactivity for the ORR process, Pt-Ion (Fe) alloy nanoparticles supported oncarbon nanotubes were synthesized and heat-treated at different temperatures.As the temperature increased, the crystalline structures of Pt-Fe alloytransformed from face centered cubic solid solution to the intermetalliccompounds of FePt3or FePt. The result indicated that FePt compound washighly active for the ORR process, but the highest activity was obtained withthe mixture of solid solution and FePt3structure.(4) In order to further reduce the cost and improve the stability of theelectrocatalysts, the TiC and WC fibrous mats were compound with Ptnanoparticles and applied as the electrocatalysts for the ORR process. The TiCand WC particles were insert into the carbon sheets to form the fibers andinterwoven to form the fibrous mat. TiC particles partially inserted in thecarbon sheets with the diameter of~50nm while the WC particles fullyinserted with the diameter of~5nm. Pt nanoparticles could be highlydispersed on the surface of the fibers, and the catalytic activities wereimproved through the surface charge transformation. The enhancement of TiCfibers was more significant than the WC fibers due to the direct contact between the Pt and TiC particles instead of interconnect though the carbonatoms. Furthermore, the TiC and WC fibers also exhibited certain catalyticactivities in the alkaline solutions through the four-electron pathway, whichmight be developed as an alternative electrocatalysts of the metal materials.(5) The non-metal cobalt-sulfur compounds supported on the carbonnanotubes were synthesized after sensitizing and activating the surface of thesupport materials. Through the appropriate heat-treatment, the cobalt-sulfurparticles changed to the mixture of mainly Co9S8and Co3S4, and the catalyticactivities and mechanism before and after the heat-treatment were compared.The results indicated that the carbon nanotube supported cobalt-sulfur particlemainly catalyzed the ORR proceed through the four-electron pathway and theCo9S8could highly improve the catalytic activities compared with the othercompounds.