Synthesis Of Cobalt/nickel-carbon Composite Materials And Their Performance For Oxygen Reduction/evolution Reaction

Over the past decade,the surging interest for higher-energy-density,cheaper,and safer battery technology has spurred tremendous research efforts in the development of improved rechargeable metal-air batteries.Current metal-air batteries suffer from poor energy efficiency and cycle life,owing mainly to the poor rechargeability of air electrodes.Both oxygen reduction?ORR?and evolution?OER?reactions involve four elementary reaction steps,these reactions suffer from slow kinetics.Therefore,there is a need to design new and improved ORR/OER electrocatalyst with high activity and stability to improve the air electrode reaction efficiency to increase battery energy efficiency and lifetime.Carbonaceous materials have received widespread attention for their high electrical conductivity and good stability.However,carbonaceous materials have weaker oxygen catalytic activity.Transition metals with 3d orbitals have various valence states and exhibit excellent oxygen catalytic activity,but they have poor electrical conductivity.Compounding transition metals with carbonaceous materials can achieve complementarity and further improve the material's oxygen catalytic activity and stability.In this dissertation,the transition metal?Ni,Co?and heteroatom?P,Fe,N?doped carbonaceous materials are combined to prepare efficient and stable oxygen catalysts,and the electrocatalytic performance of these materials is studied.The main research contents are as follows:?1?Using natural holly leaves as a template and carbon source,first holly leaves with a three-dimensional micro/nano structure were calcined to 800°C in nitrogen to obtain self-supporting biochar as a substrate for electroless deposition.After depositing Ni-P for 30 min,the obtained amorphous Ni-P/carbonized blade?Ni-P/CL?composite exhibited excellent electrocatalytic performance for OER.Thanks to the high electronic conductivity and catalytic activity of Ni-P and the effective mass transfer behavior in the three-dimensional structure,a low overpotential of 300 m V was required to deliver a current density of 40 m A cm^-2 for Ni-P/CL with an electroless deposition time of 30 min.In addition,it also shows good stability.After constantly performing at 1.6 V vs.RHE for 12 h,a relatively high current retention of 85.7%was tested.This should be attributed to the amorphous and P-doped corrosion resistance of Ni-P,and the structural rigidity of the three-dimensional CL skeleton.?2?Duck blood containing heme?a type of ferriporphyrin?was carbonized and activated by KOH to obtain Fe,N doped carbon?a-CB?with an extremely high specific surface area of2655 m² g^-1.Then,Co3O4 nanoparticles were loaded on the efficient substrate via hydrothermal method.The as obtained Co3O4/a-CB composite exhibited good ORR catalytic performance,with an onset and half-wave potential of 0.95 and 0.77 V vs.RHE,respectively.These values are comparable to that of commercial Pt/C?20 wt%?catalyst.What's more,the composite had better durability and methanol tolerance than commercial Pt/C,making it a promising electrocatalyst for ORR.The excellent performance should be attributed to the synergy between the active site Co3O4 and the high specific surface area and high porosity Fe,N-doped carbon matrix.?3?Using ordered mesoporous carbon as the carbon matrix,preparing precursors by introducing functional groups and electrostatic adsorption methods,and then calcining at different temperatures to synthesize mesoporous carbon materials with both Co N and Co Ox active centers?Co N-Co O/Co3O4/MC?.The obtained Co N-Co O/Co3O4/MC catalyst exhibits excellent electrocatalytic activity and stability.For ORR in alkaline media,The catalyst shows a high half-wave potential close to that of commercial Pt/C?20 wt.%?catalyst,and presents enhanced electrochemical stability and methanol tolerance.When applying the catalyst as an air cathode for primary zinc-air batteries,it shows high performance comparable to that of benchmark Pt/C.The Co N-Co O/Co3O4/MC cathode demonstrated a reasonably high open-circuit voltage?1.428 V?and power density(109 m W cm^-2).Moreover,it delivers a specific capacity of 814.3 m Ah g Zn^-1 and an energy density of 950.3 Wh kg Zn^-1 at a current density of10 m A cm^-2.The excellent performance should be attributed to the enhanced activity of the active site by Co N and Co Ox adsorption on the mesoporous carbon.Simultaneously,the catalytic site density is increased due to the high surface area of the carbon matrix.More importantly,the porous 3D structure of MC promotes the diffusion of O2 and reactive ions in the electrode.