Applications Of Carbon-based Hybrid Materials In Hydrogen And Oxygen Electrode Reactions

Growing energy demand and an imminent energy crisis have spurred massive research into renewable energy conversion and storage techniques.Development of earth-rich,efficient and stable catalysts for oxygen reduction(ORR),hydrogen evolution(HER)and oxygen evolution(OER)are at the heart of key renewable energy technologies such as fuel cells and water splitting.Bifunctional HER and OER catalysts can be used in electrochemical water splitting systems to replace Pt as a cathode and replace Ir O2/Ru O2 as an anode;and bifunctional OER and ORR catalysts can be used as cathodes for rechargeable metal-air batteries.The slow ORR is the rate determining step in the fuel cell,and Pt-based compound can effectively catalyze this process.However,the scarcity,high cost and electrochemical instability of precious platinum have severely hampered its large-scale commercial applications.As a reverse reaction of ORR,OER plays an important role in energy storage,such as water decomposition to produce hydrogen.The most stable metal oxide in the acidic electrolyte should be iridium oxide(Ir O2).Increasing the intrinsic activity of Ir O2 is a concern in its practical application.One of the effective strategies for improving the intrinsic activity of Ir O2 is to reduce the size of the nanoparticles,which allows more Ir atoms to be exposed on the surface and increase the unsaturated Ir atoms on the surface.Therefore,in this thesis,metal active centers are supported on carbon supports(such as carbon nanotubes,graphene,etc.)by different synthesis methods,and these electrocatalytic materials are used in ORR,OER and HER,respectively.The details are as follows:1.Electrocatalytic performance of ultrafine iridium oxide supported on carbon nanotubes in oxygen electrode reactionUltrafine iridium oxide is successfully introduced into multi-walled carbon nanotubes(MWCNTs)by a simple impregnation method and subsequent hydrogen reduction strategy.The Ir O2@CNT hybrid material exhibits efficient and stable water oxidation performance.The effects of different contents of Ir O2 on the morphology and electrocatalytic properties of hybrid materials are evaluated by XRD,XPS,Raman,TEM,HAADF-STEM and N2 adsorption techniques.Electrochemical test results show that the initial TOF value of Ir O2@CNT hybrid material can reach 3.48s-1.In addition,2.0% Ir O2@CNT requires only 217 and 272 m V of overpotential to provide a current density of 10 m A cm-2 in 1.0 M KOH and 0.5 M H2SO4,respectively;and exhibits good stability in both alkaline and acidic electrolytes.In addition,2.0% Ir O2@CNT shows excellent electrocatalytic performance for ORR reaction(onset potential: ~0.91 V vs.RHE,half-wave potential: ~0.796 V vs.RHE,diffusion limiting current density: ~4.6 m A cm-2).Excellent electrochemical performance can be attributed to the synergy between Ir O2 and MWCNT.2.Atomically dispersed Sc-N co-doped graphene catalysts for oxygen reduction reactionThe novel catalyst is synthesized by simple pyrolysis.The catalyst consists of a single Sc atom uniformly dispersed on a high surface area nitrogen-doped graphene carrier.The effects of different calcination temperatures on the morphology and electrocatalytic properties of hybrid materials are evaluated by XRD,Raman,SEM,TEM and XPS techniques.Electrochemical test results show that Sc@NG-750 has excellent ORR electrocatalytic performance(half wave potential: ~0.89 V vs.RHE and ~0.72 V vs.RHE in 0.1 M KOH and 0.5 M H2SO4,respectively).The excellent electrochemical performance is due to the presence of an active site Sc-Nx embedded in the N-doped graphene.3.Co/Co Ox nanoparticles embedded on nitrogen-doped carbon-graphite as trifunctional catalyst for HER,OER,and ORRThe Co/Co Ox nanoparticles are embedded on nitrogen-doped carbon graphene by a simple one-pot synthesis strategy.The Co@NC-G catalyst can be used as a highly efficient trifunctional electrocatalyst for HER,ORR and OER.The effects of different calcination temperatures and graphene contents on the morphology and electrocatalytic properties of hybrid materials are evaluated by XRD,SEM,TEM,XPS,FT-IR,UV/Vis and N2 adsorption techniques.Electrochemical test results show that on the Co@NC-G-700 hybrid material for HER,the overpotential required to deliver 10 m A cm-2 is 140 and 136 m V in 0.5 M H2SO4 and 1.0 M KOH,respectively;and the potential of the OER in 1.0 M KOH is only 322 m V.In addition,Co@NC-G-700 has good ORR performance in 0.1 M KOH(half-wave potential:~0.83 V vs.RHE,diffusion limited current density: ~5.2 m A cm-2).In addition to its good HER,ORR and OER activities,it also exhibits good electrochemical stability in these catalytic reactions.Co-Nx sites play a major role in efficient HER and ORR performance,while Co-O-Co,Co-N-C and Co-O-C sites synergies may play a key role in OER.