Preparation And Electrochemical Properties Of Heteroatom-doped MXene

Fuel cell can directly convert chemical energy into electric energy,which is an efficient and environmentally friendly energy conversion system.But the oxygen reduction reaction in the cathode is the core of energy conversion and the slow oxygen reduction reaction kinetics has become the key reason restricting the development of fuel cells.Catalysts based on platinum,iridium,ruthenium and other noble metals have high catalytic activity,but low reserves and high cost limit their large-scale application in practice.The development of non-precious metal catalysts to replace precious metal catalysts is the ideal solution,however,common carbon carrier materials like carbon black,carbon nanotubes,graphene,etc.have problems such as easy corrosion and dissolution.MXene as an emerging two-dimensional layered metal carbon/nitride material brings a turnaround in catalyst design,and its high electrical conductivity,excellent mechanical properties,large specific surface area,unique atomic structure,etc.The characteristics bring new opportunities for the design and application of high performance electrocatalysts.In this paper,we reasonably designed a new oxygen reduction catalyst based on MXene structure,and synthesized a catalyst with high efficiency oxygen reduction catalytic activity through heteroatom doping strategy,regulation and optimization of various experimental parameters.Specific research content and innovation points are as follows:（1）Through the polymeric-pyrolysis strategy,PPy polymeric film was formed on the surface of MXene substrate and Fe-N_x active site was formed after pyrolysis,thus improving the stability and electrocatalytic activity of the catalyst.The obtained MXene/PPy/Fe catalyst has excellent catalytic effect on ORR.Combining the advantages of MXene and PPy,the polymerized-pyrolyzed PPy was carbonized on the MXene surface to construct high-speed charge transport channels and form Fe-N_x active sites.The optimized MXene/PPy/Fe-900exhibited superior ORR performance with onset and half-wave potentials of 0.984 V and 0.852V（vs.RHE）,and excellent durability and resistance to methanol poisoning.MXene/PPy/Fe-900was tested as the cathode catalytic material to assemble zinc vacancy cells,which reached a peak power density of 227.3 m W cm^-2,and exhibited a charge-discharge cycle stability that was better than Pt/C catalysts in the constant current discharge-charge cycle test.（2）The co-doped Ti₃C₂T_x-MXene-based Co and N catalysts were designed and synthesized by the strong chelation effect between the effective groups of chitosan and metal ions.By sol-gel method and heat treatment,cobalt and nitrogen atoms were successfully introduced into the carbon protective layer,and a novel nanostructured catalyst with MXen-based Co and N co-doped carbon skeleton was successfully prepared.The lamellar structure of MXene was still maintained inside the catalyst,which provided favorable support for improving the electrical conductivity and catalytic activity of the catalyst,and the formation of the external carbon layer could effectively prevent the oxidation of MXene.After optimization of the composition ratio and pyrolysis temperature,the MXene/Co NC-900 catalyst was obtained with a half-wave potential of 0.843 V vs.RHE.The MXene/Co NC-900 sample was used as cathode catalytic material to assemble Zinc-Air battery.The specific capacity was up to 795 m Ah g^-1 and the peak power density was 231 m W cm^-2.The zinc-air battery based on MXene/Co NC-900 catalyst shows better charge-discharge cycle stability than Pt/C catalyst in the constant current discharge-charge cycle test,which proves the potential of MXene/Co NC-900 in practical energy devices.