Controlled Synthesis And Electrocatalytic Performance Of Metal-based Catalysts

The use of hydrogen as energy source is a clean, efficient and sustainable energy system towards hydrogen economy. Water splitting and fuels cells are used to generate and consume hydrogen, respectively, which rely on hydrogen evolution (HER) and oxygen reduction reactions (ORR).Development of highly active and stable electrocatalysts for the ORR and HER is the key to these electrochemical applications. Platinum-based nanostructures are the most efficient commercial electrocatalysts for the ORR and HER;however, their large-scale implementation has been limited by highmaterial costs. Therefore, thecritical issue in this field is to improve electrocatalytic activity and reduce platinum usage. In this dissertation,we focus on the development of HER and ORR electrocatalysts towards the goal of improving the electrocatalytic performance, and as a result, a series of high-performance electrocatalysts have been obtained by controlling microscopic structure and electronic structure. The major achievements are listed as follows:1. Controlled synthesis of trimetallic triStar nanostructures and their electrocatalytic hydrogen evolution performance. To lower the material cost and tune the electrocatalytic performance, there is a major trend to develop Pt-based alloy materials. Among the alloy materials, late transition metals such as Fe and Co are popularly used to form highly electrocatalytically active alloys with Pt. In this communication, we have developed a series of PtFeCo alloy nanostructures in a TriStar shape with tunable Fe and Co contents for electrocatalytic hydrogen evolution reaction (HER). The HER performance shows interesting volcano-type behavior depending on the ratio of Co to Fe. Not only the surface structures but also the composition control of the TriStar samples contribute to the HER performance enhancement. It demonstrates the excellent durability of our samples during long-term cycling, while the Pt/C shows relatively weak stability. The Pt81Fe28Co10sample well combines the surface design with the composition engineering, achieving current density up to 1325 mA cm-2 at potential of-400 mV. This synergistic working mechanism, which reduces the usage of Pt and boosts HER performance by forming alloys, opens up new possibilities for designing low-cost, high-performance electrocatalysts. We envision that it would provide fresh insights into rationally designing the alloy electrocatalysts from a different perspective.2. Immobilization of Pt nanoparticles in porous carbon nanorods andapplication in electrocatalytic hydrogen evolution reaction.Based on previous works, porous carbon nanorods were obtained via the pyrolysis of metal-organicframework (MOF) in an inert atmosphere. Increasing the roughness of the nanorods by removing oxides could improve theirelectrocatalytic performance. In this work, Pt ions were introduced into the MOF cavity through a double-solvent approach, and then the ohained MOF precursors were calcinated under nitrogen atmosphere to produce Pt/nano porous carbon(NPC) catalysts. The formation and structure of the synthesized Pt/NPC nanostructures were systematically characterized, suggesting thatPt at small sizeswere evenly dispersed in the carbon nanostructures. In NPC, the carbon layer did not prevent the access of reactants to the active sites of platinumnanoparticles; instead it protected the active metallic cores from oxidation and aggregation. We built a highly active and stable catalyst through controlling the microscopic structures.3. Synthesis of carbon-encapsulated PtCo nanoparticles and their improvedoxygen reduction reaction performance. Pt-based nanostructures have been the most active catalyst for ORR to date; however, their practical applications are hindered by high cost, low reserve and limited durability. Therefore, recent studies have aimed to enhance the activity of catalysts towardsORR and to reduce the Pt usage for minimal cost. In this work, the nanostructures where PtCo was encapsulated in the nitrogen-doped carbon derived from MOFwere designed, based on the excellent electrocatalytic performance of carbon-encapsulated metal nanoparticles. Pt@ZIF-67 was obtained by an in-situ synthetic method. Then the ZIF-67 and Pt@ZIF-67 were calcinated in nitrogen atmosphere to produce Co@Nitride Doped Carbon(NDC) and PtCo@NDC catalysts towards the ORR, respectively. We preliminarily solved the common problem that surface coating on metal is too thick, and limited the aggregation of metal nanoparticles. The superb electrocatalytic performance originated from the protection by the surface carbon layer.4. Synthesis of hollow cobalt sulfide structuresand their application in electrocatalysis. Hollow nanostructures with complex interior structures offer great advantages for designing advanced catalysts. In this work, uniform zeolitic imidazolate framework (ZIF-67)@amorphous CoS were synthesized by utilizing ZIF-67 nanocrystals as templates. By precisely tuning the reaction time and temperature, a variety of hollow nanostructures such as amorphous CoS nanocages (a-CoS NCs) and ZIF-67@α-CoS yolk-shell nanostructures could be fabricated. The yolk-shell nanostructures were then transformedinto Co nanoparticle-embedded carbon@Co9Sg nanocages by thermal annealing in N2 flow at 600℃. Similarly, the aqueous solution of ammonium tetrathiomolybdate was added into the uniform suspension of ZIF-67 nanoparticles to obtain the bimetallic precursor. Afterwards, thermal annealing was applied in inert gas to convert the amorphous bimetallic precursor to the mixture of MoS2 and CoS, whose electrocatalytic performance has been assessed.