The Design,synthesis And Applications Of Co-based Hybrid Catalysts For Oxygen Reduction/Evolution Reaction

Owing to their high energy densities,low cost,high safety,and environmental-friendly,recharageable zinc-air batteries（RZABs）are regarded to be one of the most promosing candidate of the next-generation clean energy,especially as the power source of the wearable electronics.Currently,the development of RZABs is significantly hampered by the lack of high efficient,low cost,and high stability oxygen reduction/oxygen evolution reaction（ORR/OER）bi-functional catalysts.Among the alternative non-precious-metal-based catalysts,Co-based hybrid catalysts are receiving discernable attention.It is originated from their outstanding merits of low price,natural abundance,and relatively high activity and durability in alkaline electrolytes.Therefore,we introduced multiple facile and simple synthesis strategies to fabricate serveal Co-based catalysts with optimized/excellent ORR/OER performance through modifying their morphology,composition,and structure in this thesis.Concurrently,the structure-function relationship between morphology-composition-structure and performance of as-prepared Co-based hybrid catalysts was deciphered and deeply discussed.Firstly,following the principle of confinement reacation,a sovent-thermal method was adpted to anchor Co₃O₄quantum dots on N-doped carbon nanotubes with rich oxygen functional groups（o-NCNT）.The as-prepared one-dimensional Co₃O₄/o-NCNT hybrid catalysts exhibited good ORR/OER acitivity（E=0.95 V）.It was found that the enhanced ORR/OER performance of the Co₃O₄/o-NCNT hybrid catalysts is rooted on their modified electronic structure induced by N doping and the quantum dot effect,which could strongly increase the exposed active-sites number.The flexible sandwich-like RZABs with the Co₃O₄/o-NCNT demonstrated good charge-discharge performance and can power up various electronics,indicating their favourable practical application value.This work provides a new idea for the preparation of electrocatalytic materials with quantum dot effect.Scecondly,an in-situ coupling strategy was developed to synthesize two-dimensional atomically thin mesoporous Co₃O₄/N-doped reduced graphene oxide（N-rGO）hybrid nanosheets with total thickness of 1.8 nm.The as-synthsized hybrid naosheets delivered excellent ORR/OER bi-functional activity（E=0.93 V）and stability than that of commercial Pt/C and Ru O₂.The strong coupling effect caused by Co–N–C and Co–O–C coordination between Co₃O₄ nanosheets and N-rGO substrates was strongly proved by X-ray absorption fine structure（XAFS）spectra.The synergistic effect induced by the coupling effect,the optimized electronic structure induced by N doping as well as the merits of the hybrid nanosheets（i.e.,atomic layer thickness,mesoporous property）concurrently promote their outstanding ORR/OER performance of the Co₃O₄/N-rGO hybrid nanosheets.In addition,a simple,rapid and continuous method based on low-cost raw materials was explored for preparing a truly fiber-shaped RZABs with a diameter of 1 mm.Also,their length is adjustable.The fiber-shaped RZABs based on the hybrid nanosheets displayed outstanding resistance to the external failure and superior charge-discharge performance.It can be knitted into clothes to power up different electronics,and even charge a smart phone.This work provides a significant guidance for propelling the ORR/OER performance of Co₃O₄,concurrently promotes the development of fiber-shaped RZABs effectively.Then,considering single-atom catalysts is a perfect platform for probing the catalytic reaction mechanism,a facile strategy was developed to fabricate Co single-atoms anchored on nitrogen-doped hollow graphene-nanocages composites（Co SAs/N-HC）without using template.Interestingly,the strategy can be extended to prepare other3d–5d transition metal single-atom catalysts M SAs/N-HC（M=Mn,Fe,Ni,Cu,Zr,Ru,Ir,Pt）.The as-prepared Co SAs/N-HC demonstrated outstanding ORR activity(E_1/2=0.87 V),cycling stability and methanol toxicity resistance,which is much better than those of commercial Pt/C.Also,the Co SAs/N-HC exhibited comparable OER activity to that of commercial Ir O₂.It was found that the outstanding ORR performance of the Co SAs/N-HC is rooted on its high specific surface area and hierarchical pore structure induced by three-dimensional hollow structure,especially the atomically dispersed Co–N₄ moieties.Meanwhile,DFT calculation was employed to further demonstrate the key role of Co–N₄ moieties in the electrocatlytic ORR process.This work not only provides a new thinking for the preparation of single-atom catalysts with hollow structure,but also points out the direction for the design and preparation of high performance ORR catalysts.Finally,following the results obtained in the former chapater,CoIr@N-C hybrid catalysts with three-dimensional morphology were prepared through a simple grinding-calcination strategy.The CoIr nanoparticles（NPs）are coated by 5–8 carbon layers,forming a core-shell structure.The strategy can be scaled up.The as-prepared CoIr@N-C hybrid catalysts hold merits of hierarchical pore structure,large specific surface area（518 m²/g）,and high porosity（0.55 cm³/g）.It was demonstrated that the formed CoIr NPs by Ir doping hold much stronger electron adsorption than that of Co NPs.which induced a significant increase of the M–N_x moieties of the CoIr@N-C.In addition,the electronic structure of the C matrix also optimized by N doping and the generated M-N_x moieties.All of above characterizations imbue the as-prepared CoIr@N-C hybrid catalysts demonstrated unparalleled ORR/OER activity（ΔE=0.82 V,0.72 V after IR correction）and stability.Furthermore,the CoIr@N-C hybrid catalysts could guarantee the home-made liquid RZABs stably operate 300 cycles,suggesting the as-made CoIr@N-C hybrid catalysts hold great practical application potential.This work provides meaningful guidance for the scale-up preparation ORR/OER bi-functional catalysts with high activity,high stability and low cost.