Metal-organic Frameworks Derived Carbon Based Nanomaterials And Their Applications In Electrocatalysis

To date,increasing efforts have been devoted to the development of cost-effective and efficient electrocatalysts for sustainable energy conversion processes and technologies such as water splitting and metal-air battery.Despite progress over the past few years in developing efficient low-cost catalysts,precious metals such as Pt or Ir are still regarded as the best catalysts among various electrocatalytic reactions including hydrogen evolution reaction(HER),oxygen evolution reaction(OER),and oxygen reduction reaction(ORR).However,the natural scarcity and high cost greatly hamper their large-scale commercialization.Carbon based material,especially for graphene,is regarded as a potential alternative for metals due to its unique structural and electronic properties.Even though pristine graphene exhibits inert activity as electrocatalyst,its electronic properties can be tuned by introducing heteroatoms such as nitrogen atoms.However,the catalytic activity of N-doped graphene is far away from precious metal counterpart.It was found that the combination of graphene shell with metal core could further tune the electronic structure of carbon active sites.Besides,incorporating single metal atoms(such as Fe,Co and Ni)into graphene framework is also benefical for improving electrochemical performance.Therefore,nitrogen doped graphene encapsulated metal or alloy(M@NG)and single metal atom electrocatalysts incorporated in graphene framework are emerging as novel and fascinating carbon based material to replace the state-of-art precious electrocatalysts.Metal-organic frameworks(MOFs)possess great potentials to serve as templates and reactive precursors to fabricate carbon-based nanomaterials via direct annealing process,owing to their high surface areas,large pore volumes,abundant and tunable metal nodes and organic linkers.During the annealing process under inert atmosphere,metal ions in the precursor will form alloy nanoparticles or coordinated single metal atoms after post treatment.Meanwhile,the organic linker will serve as carbon and nitrogen sources for the formation of nitrogen doped graphene.By modulating the metal ion centers and treatment process,nitrogen doping levels,metal types and coordinated environment can be initiatively engineered,providing perfect opportunity to study their relationships with electrochemical activities.Therefore,we design various MOFs precursors as promising templates to fabricate M@NG and single metal atoms electrocatalysts via thermolysis and post treatment.The HER,OER and ORR activity of our materials as well as corresponding mechanism are investigated through combining experiments with theoretical calculations.The dissertation is discussed in details from the following aspects:1.Non-precious metal based catalysts are emerging as the most promising alternatives to Pt-based ones for hydrogen evolution reaction(HER)due to its low cost and rich reserves.However,its low efficiency and stability due to inherent corrosion and oxidation in acid media are the main barriers blocking sustainable hydrogen production.Metal-organic frameworks,with both designable metal ion centers and organic ligands,are promising precursors for the one-step synthesis of metal/alloy@carbon composites for HER.Herein,we synthesized FeCo alloy nanoparticles encapsulated in highly nitrogen-doped(8.2 atom%)graphene layers by direct annealing of MOFs nanoaprticles in N2.The catalyst shows low onset overpotential(88mV)and an overpotential of only 262 mV at 10 mAcm-2.Besides,it exhibits an excellent long-term durability performance even after 10000th cycles due to the protection of graphene layers.Our density function theory calculations reveal that the nitrogen dopants can provide adsorption sites for H*and the proper increasing of nitrogen will decrease ?GH*for HER.Besides,the unique structure of metal and graphene composites derived from MOFs can also decrease the ?GH*thereby promoting catalytic activity.2.Electrochemical water splitting is considered as the most promising technology for hydrogen production.Considering overall water splitting for practical applications,the catalysts for oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)should be performed in the same electrolyte,especially in alkaline solutions.However,designing and searching high active and inexpensive electrocatalysts for both OER and HER in basic media still remains a significant challenge.Herein,we report a facile and universal strategy to synthesize non-precious transition metals,binary alloys as well as ternary alloys encapsulated in graphene layers by direct annealing of metal-organic frameworks.Density functional theory(DFT)calculations prove that by increasing freedom degrees of alloys or altering metal proportions in FeCoNi ternary alloys,the electronic structures of materials can also be tuned intentionally through changing number of transferred electron between alloys and graphene.The optimal materials alloys FeCo and FeCoNi exhibited remarkable catalytic performance for HER and OER in 1.0 M KOH,reaching a current density of 10 mA cm-2 at low overpotentials of 149 mV for HER and 288 mV for OER,respectively.What's more,as an overall alkaline water electrolysis,they were comparable to that of Pt/RuO2 couple,along with long cycling stability.3.Unlike metals with incomplete d-shells such as Pt,Fe and Co,Copper(Cu),with a filled d-electron shell is generally regarded as not sufficiently active for oxygen reduction reaction(ORR).However,laccase and other copper enzymes could catalyze ORR efficiently with a relative low onset potential.In this work,atomically dispersed Cu-Nx cofactors were incorporated within the graphene frameworks by direct annealing of MOFs with post etching process.The electrocatalysts exhibit excellent ORR activity and zinc-air battery performance,even superior than commercial 20wt%Pt/C.Density function theory calculations suggest that when Cu atoms are coordinated with surrounding N atoms,valence electrons of Cu atoms will transfer to nitrogen atoms,simultaneously tuning electronic structure of Cu and enabling fast ORR kinetics and zinc-air battery performance4.Manganese(Mn)is generally regarded as not sufficiently active for oxygen reduction reaction(ORR)compared to other transition metals such as Fe and Co.However,in biology,manganese-containing enzymes could catalyze oxygen-evolving reactions efficiently with a relative low onset potential.In this work,atomically dispersed O and N atoms coordinated Mn active sites are incorporated within the graphene frameworks to emulate both the structure and function of Mn cofactors in heme-copper oxidases superfamily.Unlike previous single metal catalysts with general M-N-C structures,in this work,we prove that coordinated O atom can also play a significant role in tuning intrinsic catalytic activities of transition metals.The biomimetic electrocatalyst exhibits superior performance for ORR(onset potential 0.94,half-wave potential 0.86 V)and zinc-air batteries under alkaline conditions which is even better than commercial Pt/C.The excellent performance can be ascribed to the abundant atomically dispersed Mn cofactors in graphene frameworks confirmed by HAADF-STEM and XAFS analysis.Theoretical calculations reveal that the intrinsic catalytic activity of metal Mn can be significantly improved via changing local geometry of nearest coordinated O and N atoms.Especially,graphene framework containing Mn-N3O1 cofactor demonstrates the fastest ORR kinetics due to the tuning of d electronic states to a reasonable state.