Catalytic Performance And Mechanism Of Noble Metal/Carbon Electrocatalvsts For Hydrogen Evolution Reaction

As a clean and renewable energy source,hydrogen(H2)is expected to be an alternative to traditional fossil fuels.Especially,the cathodic reaction of water splitting-hydrogen evolution reaction(HER)has attracted much attention due to the high purity of produced hydrogen(>99.7%),fast kinetics and high energy efficiency.Although platinum-based metals(Pt,Ru,Pd and Ir,etc)catalysts display superb HER activity,the shortage of resources brings high cost problems,which limits the industrial applications of water electrolysis.In addition,high loading is needed to make up for the loss of activity during the long-term reaction process.It is a long-term goal to design low cost Pt-based catalyst with high activity and stability.On the basis of ensuring high catalytic activity,there are two basic strategies to cut the catalyst cost via reducing the metal loading:1)Increasing the atomic utilization and enhancing the intrinsic catalytic activity of exposed metal atoms(controling the size and morphology of catalyst,adjusting the elelctronic state of active sites).2)Multi active sites catalyze the reaction synergistically.Based on this,this thesis aims to reduce the usage of noble metals and further improve the electrocatalytic performance by designing the microstructure and and controling the electronic structure of the catalysts.Furthermore,this dissertation deeply studied the origin of improved HER performance by DFT calculation from atomic level.This paper provides guidance to the development of efficient and low-cost noble metal catalytic system.The main content of this paper is as follows:(1)A novel electrocatalyst,Pt nanoparticles(NPs)anchored on Ni/nitrogen-doped graphene nanotubes(Pt/Ni@NGNTs),is designed.Pt NPs preferentially distribute around the protective graphene shells of enclosed metal Ni,due to the enriched electron density and structure defects induced by Ni and N dopants.The electronic interactions between Pt and Ni@NGNTs tunes electronic properties and greatly boosts catalytic performances of anchored Pt NPs.Impressively,Ni@NGNTs increases the mass activity of Pt metal by 30.1 times in acidic electrolyte relative to commercial 20 wt%Pt/C,and the durability reaches up to 50 h.Moreover,Pt/Ni@NGNTs also exhibits comparable activity to Pt/C under alkaline conditions.(2)A feasible "preadsorption-in situ reduction" strategy was developed for the synthesis of stable Pt clusters trapped by structure defects on carbon nanotubes(CNTs).The Pt clusters are demonstrated to be superior active for hydrogen evolution reaction(HER)with a 100-fold increase over the mass activity of the benchmark 20 wt%Pt/C and a much lower overpotential.Besides.The developed catalyst shows excellent thermal stability(600 ?)and chemical stability(oxidation resistance).The DFT calculations results suggested that vacancy defective sites stabilize the Pt cluster from migration and oxidation,and optimize the electronic densities of state.These results provide new insight into the effects of structural defects on the active phases,and afford a facile and competitive strategy to synthesize stable and highly active catalysts.(3)Generally,the HER kinetic of Pt metal under alkaline condition is two or three orders of magnitude slower than that of acidic medium due to the sluggish water dissociation.In the fourth chapter,highly activity Pt nanosheets is fabiricated under HER alkaline conditions.Stable adsorbed PtCly ions on CNTs are in-situ electrochemically reduced into a unique Pt nanosheet structure enclosed by high-index(311)and low-index(200)and(111)facets during HER process.Experimental results and density functional theory(DFT)calculation disclose the function mechanism between these unique structures and reactants.The adsorbed H2O and reactive species act as capping agents protecting the(311)facet where the dissociation of water molecule is highly promoted,and the produced H*intermediates favorably combine and release on the nearby low-index Pt sites.The joint collaborations of these active sites afford Pt nanosheets comparable activity to 20 wt%Pt/C and a 12.7-fold over mass activity.These findings provide novel insight into the synthesis of heterogeneous catalysts with high specificity.(4)To succeed in real applications,hydrogen evolution reaction(HER)demands an efficient,durable and cheap catalyst.The cheap price,high activity and similar hydrogen binding energy(65 kcal/mol)make ruthenium as a potential alternative to platinum for effective HER if effective strategy improves its stability,and further optimizes the activity.Here,surface modified Ru nanoparticles with transition metal are fabricated via simple thermal activation.In 1.0 M KOH solution,the obtained catalyst exhibits a low overpotentials of only 2 mV at 10 mA/cm2,far lower than that of Pt/C(39 mV).Moreover,it also shows comparable activity to Pt/C in 0.5 M H2SO4.excellent stability under both alkaline and acid solution is demonstrated by the 10,000 cycles without any activity loss.Density functional theory(DFT)calculations reveal that the formed Ru-Co sites act as higher active sites for water dissociation than Ru.Moreover,doped Co atoms enhance the oxidation resistance of metallic Ru,leading to the improved stability.In summary,highly active and low-cost Pt and Ru catlysts are fabricated by enhancing the intrinsic activity via simple,efficient strategies.The relationship between the activity and the reaction mechanism is deeply explored by combining the experimental phenomena with theoretical calculations,which provides theoretical guidance for the design of transition metal catalysts with high activity and high stability in the future.