Preparation And The Catalytic Performance For Oxygen Reduction Reaction Of Porous Doped Graphene-based Metal-free Catalysts

As high-efficiency energy conversion devices,fuel cells and metal-air batteries have attracted widespread attention.However,the inherent slow oxygen reduction reaction?ORR?kinetics in their cathodes severely limit their development in commercial applications.Typically,platinum?Pt?based materials are considered to be one of the most effective ORR electrocatalysts.However,Pt-based catalysts also face serious challenges in practical applications due to scarcity,high cost,low stability and low methanol tolerance.Therefore,it is important to study non-precious metal catalysts?NPMCs?as an effective alternative to ORR.Due to its excellent charge conductivity,large surface area and rich doping characteristics,graphene exhibits excellent performance in electrocatalytic applications.Graphene-based N-doped holey carbon nanosheets?GNHCNs?and 3D nitrogen and phosphorous co-doped holey graphene foams?N,P-HGFs?catalysts were designed and prepared.Then the performance of the catalysts was analyzed by physical characterization,ORR activity and zinc-air batteries?ZABs?performance testing.First of all,GNHCNs were prepared by growing small particle size ZIF-8 on graphene sheets structure using a two-step heat treatment method.The complex of graphene oxide?GO?with ZIF-8 is pyrolyzed,and then the resulting product is immersed in a saturated urea solution for further pyrolysis to increase its nitrogen content and specific surface area and obtain GNHCNs_Urea.The excellent electrical conductivity of graphene can enhance the electron conductivity of the electroactive center of ZIF-8 surface,and ZIF-8attached to the surface of graphene can prevent the aggregation of graphene sheet structure.As a result,the obtained GNHCNs_Urea exhibited excellent long-term stability and methanol resistance,and its half-wave potential was 0.86 V,which was comparable to a commercial Pt/C catalysts.In addition,the zinc-air battery using GNHCNs_Urea as the ORR catalysts also exhibited excellent discharge performance and long-term stability with an open circuit potential of 1.45 V,a peak power density of 126 mW cm^-2,and a specific capacity of 677 mAh g_Zn^-1.It is proven that GNHCNs_Urea catalysts have potential applications in green clean energy.The experimental results show that the ORR activity of the catalyst is derived from the total doping content of pyridine nitrogen and graphite nitrogen,the number of defect sites,and the specific surface area.In the three catalysts of NGNHCNs_Urea,GNHCNs_MF and GNHCNs_DICY,the addition of urea can cause NGNHCNs_Urea to have relatively high defect sites,large specific surface area and layered porous nanostructures.These factors work together to give NGNHCNs_Urea the highest ORR activity.In addition,a controllable 3D N,P-HGFs catalyst was designed and prepared by hydrothermal and subsequent pyrolysis processes.The obtained graphene oxide?GO?is used as the carbon source,and the melamine and the phytic acid are respectively used as the nitrogen source and the phosphorus source.The hydrogel which is obtained by hydrothermal treatment is washed and freeze-dried,then uniformly ground with urea,followed by heat treatment at 1000? to prepare N,P-HGFs-1000.The obtained N,P-HGFs-1000 showed a half-wave potential of 0.865 V,demonstrating that N,P-HGFs-1000has a higher ORR activity than the commercial Pt/C.In addition,the Tafel slope of N,P-HGFs-1000 is 77.55 mV decade^-1,the number of transferred electrons is close to 4.And N,P-HGFs-1000 exhibits a high electrochemical activity area,excellent long-term stability and methanol resistance.Furthermore,ZABs using N,P-HGFs-1000 as an ORR catalyst exhibited excellent discharge performance and long-term stability.The experimental results and DFT calculations show that the concentration of Graphicic N-C-P has the largest correlation coefficient with the ORR activity of the catalyst in N,P-HGF catalyst,indicating that the main active site of the catalyst is Graphicic N-C-P.In addition,Graphic N-C-P has the highest occupied molecular orbital?HOMO?energy and turnover frequency?TOF?,showing its high ORR intrinsic activity.The charge density distribution shows that when the doped P atom is bonded to the carbon atom adjacent to the graphite N,the charge density of the carbon atom adjacent to the graphite N is further increased,thereby causing the O₂ molecule adsorption to be more easily performed,and thus the ORR process is accelerated.In summary,in the N,P-HGF catalyst,the Graphite N-C-P structure plays a leading role in the ORR process,in which the positively charged C atom in the Graphite N-C-P structure is the main ORR active site.This work sheds a light on the rational design and controllable synthesis of the metal-free carbon-based ORR catalysts,which further facilitates the development of the electrocatalysts in practical applications.