Application Of Single-Atom Metals And Nitrogen Codoped Graphene And Metal Oxides For Hydrogen And Oxygen Electrode Reactions

With the massive combustion of fossil fuels,the issues on energy and environment have been growing serious,which did damage to the sustainable developments of human beings.To solve the energy and environmental challenges,sustainable energy devices（water electrolyzers,H₂/O₂ fuel cells,metal-air batteries,etc.）as very effective and promising technologies have been extensively developed.Nevertheless,the hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,and oxygen reduction reaction（ORR）in these energy conversion technologies are facing sluggish kinetics and serious electrode polarization phenomenon.It is necessary to fabricate efficient and stable electrocatalysts to lower the overpotential,realizing high-efficiency energy conversion and storage.It is well known that Pt and Ru O₂/Ir O₂ have high electrocatalytic properties for HER,OER and ORR,respectively,but the scarcity,high price,instability and single functionality have greatly hindered their large-scale applications.In this paper,some electroactive materials with excellent properties are studied for HER,OER,and ORR,whose catalytic performances and mechanisms are discussed.This paper mainly includes three parts as follows:（1）Single atom catalysts（SACs）have shown their superiority in catalytic systems due to the unique properties including maximum atom utilization efficiency,low coordination environment,quantum size effect and controllable metal-support interaction.The introduction of isolated metal atoms into N-doped carbon has been also proved to be an effective strategy to improve their electrocatalytic activity.Using a simple high-temperature pyrolysis strategy,Ir atoms can be uniformly dispersed on the N-doped graphene substrate.We found that Ir@NG-750 showed good electrocatalytic properties for HER,OER and ORR.For HER,the Ir@NG-750catalyst was highly active and stable.In 0.1 M KOH and 0.5 M H₂SO₄ electrolytes,the required overpotentials to reach 10 m A cm^-2 were 114 m V and 25 m V,respectively.In 1 M KOH,0.1 M KOH and 0.5 M H₂SO₄,the OER overpotentials at 10 m A cm^-2reached 273 m V,340 m V and 371 m V,respectively.For the ORR,Ir@NG-750 also exhibited the half-wave potentials of 0.77 V and 0.865 V in 0.1 M KOH and 0.5 M H₂SO₄,respectively.At a battery voltage value of 1.7 V,the current density of 10 m A cm^-2 was provided during the whole water-splitting process.In the Ir@NG-750-based Zn-air battery,the peak power density of 56.8 m W cm^-2 was achieved at 0.62 V.Density functional theory（DFT）calculations further verified that the production of Ir N₄-G active centers was responsible for the excellent HER,OER,and ORR performances on Ir@NG-750.（2）Atomic Co,N codoped graphene catalyst was successfully constructed by pyrolysis approach.From the OER performance measurements,it was demonstrated that the resultant single-atom cobalt-based catalyst exhibited the best electrocatalytic performance when metal loading reached 0.7 wt%and calcination temperature was750°C.The alkaline OER can be effectively catalyzed by 0.7-Co@NG-750,with a low onset overpotential of～210 V.DFT calculations showed that the theoretical overpotentials of OER on Co-N₄ and Co-N₂C₂ sites reached 0.41 and 0.53 V,respectively,implying that both Co-N₄ and Co-N₂C₂ were the most possible active sites of 0.7-Co@NG-750 for the electrocatalytic water oxidation.（3）Transition metal oxides/hydroxides can accelerate the slow OER kinetics,which play a crucial role in practical applications.By sol-gel synthesis strategy,an amorphous Fe-Co-Sn oxide（Fe Co Sn O_x）was obtained,which can be used to catalyze OER.The overpotential of 270 m V was required to reach 10 m A cm^-2 in 1 M KOH with the catalysis of Fe Co Sn O_x.By the chronoamperometric measurement,it was found that an insignificant current density decay was kept for more than 25 hours at1.45 V,indicating that Fe Co Sn O_x was stable.ICP-AES analysis showed that Fe/Co/Sn leaching was not found in the electrolyte after OER stability test,further confirming the good stability/durability of Fe Co Sn O_x.