Synthesis Of Transition Metal/nitrogen Doped Carbon-based Nanomaterials And Their Applications In Rechargeable Zinc Air Batteries

Metal-air batteries have bcome emerging energy storage devices.Among them,zinc-air battery(ZAB)is the most promising due to its safety,low cost,and high theoretical energy density.However,sluggish dynamics of cathodic reactions in the air electrode including oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)have resulted in inferior energy conversion efficiency,unacceptable performances and durability for ZABs.Therefore,efficient catalysts are needed to reduce reaction energy barriers and accelerate the reaction rate.Currently,Pt/C and RuO₂/IrO₂ are respectively used as ORR and OER catalysts,but their high cost,poor stability,and limited reserves have seriously restricted the development of ZABs.Thus,developing highly efficient,low-cost,and stable non-precious metal catalysts is significant to promote the development of ZABs.Recently,as a novel bifunctional non-precious metal catalyst,transition metal-nitrogen-doped carbon-based nanomaterials(M-N-C)have achieved excellent results in catalyzing ORR and OER.This study mainly includes three parts as below:(1)With the aim of preventing the encapsulation of active sites caused by the stacking of graphene-based 2-dimensional(2D)materials,graphene oxide was covered by metal hydroxides with melamine as nitrogen source,and 0D carbon nanospheres and1D carbon nanotubes were grafted on the graphene surface to form hierarchical FeCoNi-N-rGO by high-temperature pyrolysis.The unique hierarchical structure effectively suppresses the overlap of 2D graphene sheets,exposes more active sites and facilitates mass transfer between the reactant/catalyst/electrolyte multiphase interfaces during the electrocatalytic reaction.The ORR onset potential,half-wave potential and limting current density of FeCoNi-N-rGO are very close to those of Pt/C,and its OER overpotential is 440 m V,a better OER catalytic performance than RuO₂,This indicates that FeCoNi-N-rGO can serve as non-precious metal bifunctional catalyst.Moreover,FeCoNi-N-rGO-based ZAB exhibits improved performance and stability compared to noble metal catalysts,with a peak power density of 152.5 m W/cm~2,a specific capacity of 766 m A/g Zn,and an excellent cycling stability of more than 200 h.(2)In order to synthesize non-precious metal catalysts with higher ORR activity and compensate for the slightly lower ORR catalytic activity of FeCoNi-N-rGO than Pt/C,white sugar was chosen as the carbon source and g-C₃N₄ as the soft template and nitrogen source to obtain folded graphene nanosheets(denoted as N-GC)with excellent ORR performance by one-step high-temperature pyrolysis.As an important parameter of ORR performances,the electron transfer number(n)reflects the electron transfer process of the catalyst.However,the results by using the Koutecky-Levich(K-L)equation(n K-L)and the rotating ring-disk electrode RRDE(n RRDE)method are often inconsistent.To address this problem,N-GC,GC and Pt/C were selected as comparison samples to test their n values in both 0.1 M KOH and 0.1 M HClO₄.In 0.1 M KOH,the carbon matrix(GC)has a certain ORR activity and its electron transfer process is 2-electron process,while there is a decreasing trend of n for N-GC as the reaction proceeds.In 0.1 M HClO₄ solution,GC has almost no ORR catalytic activity and no participation in ORR,and the n value of N-GC is stable for 4 electrons.And it was found that the RRDE method can represent the electron transfer process of the samples more comprehensively and intuitively than the K-L method.In addition,the DFT calculations also verify the conclusions obtained from the experiments.(3)Based on the above experiments,glucosamine was chosen as the ligand to ligate with Fe³⁺and Ni²⁺,and urea was chosen as the nitrogen source to pyrolyze and obtain the N and Fe,Ni co-doped carbon nanotubes(denoted as glu-Ni Fe).Abundant M-N-C on the nanotubes and Fe Ni alloy inside the nanotubes can respetively work as the ORR and OER electrocatalytic active sites,and the highly graphitized carbon nanotubes as the channels for electron transfer.Thus,glu-Ni Fe demonstrates the ORR half-wave potential of 0.85 V and the OER potential of 1.67 V at 10 m A/cm~2,which are better than those of Pt/C and RuO₂ catalysts.Moreover,the glu-Ni Fe based ZAB has good performances with a peak power density of 127 m W/cm~2 and good cycling stability over240 h.In addition,the performances of glu-Ni Fe based all-solid-state ZAB is also better than those of Pt/C-RuO₂ with the same process.