Preparation Of Novel Carbon-based Electrocatalysts Towards Oxygen Reduction Reaction In Zn-Air Batteries

The ever-increasing energy crisis and environmental pollution have become common issues that need to be urgently addressed.Promoting the supply of renewable energy and optimizing the structure of energy consumption are much critical for the evolution of our society.It is proven that rational design and efficient use of electrochemical energy storage technology is the key to achieving the goal of energy development.From the perspective of energy density,power density,cost and safety,zinc-air batteries（ZABs）are regarded as one of the most promising advanced energy storage system.Oxygen reduction reaction（ORR）,as an indispensable part of ZABs,always suffers from sluggish kinetics,thus determining the overall battery performance.Therefore,the exploration of low-cost and high-performance catalysts to accelerate the ORR kinetics is the hotpot in the filed of ZABs.Among numerous candidates,carbon materials attract extensive attentions due to its high conductivity,large specific surface area,excellent stability and other intriguing meri ts.However,most of carbon materials are inert,and have limited active sites,thus giving rise to unsatisfactory catalytic performance.How to adjust the active sites and improve the catalytic activity is the major task for the design of high-performance carbon-based catalysts.In this dissertation,carbon materials were used as the research subject,and reasonable screening of precursors and careful design of preparation path were adopted to build high-performance carbon-based electrocatalysts.During this process,the morphology,structure and composition of carbon-based electrocatalysts were effectively regulated,and the active sites and electron/mass transfer behaviors can be well optimized.By systematical charaterization and analysis,the structure-activity relationship was established,which provides some new perspectives into the design of advanced ZABs.The specific results were listed as follows.（1）A direct pyrolysis strategy to prepare 3D carbon nanotubes/mesoporous carbon composites（CNTs/MC）for ORR in ZABs.In this work,a facile and scalable one-step pyrolysis method was developed to convert biomass to 3D CNTs/MC hybrid with the aid of metal salt and melamine.By systematical characterization and analysis,it is confirmed that accurately regulating the feed of precursors and pyrolysis temperature is the key to successful preparation of biomass-derived CNTs/MC hybrid.Benefiting from the unique 3D architecture and integration of individual merits of CNTs and MC,the as-synthesized 3D CNTs/MC hybrid exhibits remarkable electrochemical performance for ORR and ZABs.（2）A cooperative protection strategy to synthesize high-performance Fe-N-C catalyst towards ORR in ZABs.Herein,abundant Fe-N_x moieties and hyperfine Fe₃C nanodots co-decorated N-doped carbon framework（Fe-N_x/Fe₃C@NC）was successfully fabricated using yeast,Fe Cl ₃ and melamine as precursors.The selected precursors not only offer the necessary iron,nitrogen and carbon sources,but boost uniformly dispersion of metal species and formation of highly dense active sites（Fe₃C and Fe-N_x）.Thanks to the synergistic effect of Fe₃C and Fe-N_x,the obtained Fe-N_x/Fe₃C@NC catalyst shows excellent electrocatalytic performance in both acidic and alkaline electrolytes.When employed as the cathod e catalyst for ZABs,Fe₃C/Fe-N_x@NC exhibits comparable performance to that of commercial Pt/C catalyst.（3）A boric acid（H₃BO₃）-assisted pyrolysis strategy to prepare high-loading single-atom catalysts（SACs）for ORR in ZABs.In this work,single Co atom catalyst（Co-N-B-C）was synthesized by only employing biomass,cobalt salt and H ₃BO₃ as precursors.The results show that H₃BO₃ can be converted to molten-state B₂O₃ during the pyrolysis process,thus playing an important role in the synthesis of Co-N-B-C.The acquired Co-N-B-C catalyst not only presents hierarchical porous structure,large specific surface area and abundant carbon edges,but also possesses high-loading single Co atom（4.2 wt.%）.Benefiting from these intriguing features,Co-N-B-C catalyst shows excellent performance and versatile application toward ORR in ZABs.Furthermore,our proposed strategy is also demonstrated as a universal way to produce other SACs.（4）An in-situ self-template strategy to synthesize coordination unsaturated single Cu atom catalyst for efficient electrocatalytic reaction in ZABs.Herein,we for the first time successfully synthesized hollow N-doped octahedron carbon decorated with high-density unsaturated Cu-N₂ configurations（Cu-N₂@HNOC）by a delicate in-situ self-template method by using Cu O octahedron and polyrrole monomer as initial precursors.The results show that the unsaturated linear coordinated Cu-N₂sites can deliver outstanding intrinsic catalytic activity,which is demonstrated by the DFT calculation and related experiments.Besides,it is verified by molecular dynamics simulation that Cu-N₂ sites can be stable even at the high temperature of 2000 K without any aggregation.Owing to dense high-activity Cu-N₂ sites and unique hollow octahedron structure,the obtained Cu-N₂@HNOC catalyst exhibit promising electrochemical activity and durability for ORR.The coordination unsaturated yet thermodynamic stable Cu-N₂ active sites provide more possibility for the commercial application in ZABs.（5）A polyvinylidene fluoride（PVDF）-assisted pyrolysis strategy to prepare corrugated plate oxygen electrocatalysis nanoreactor for oxygen electrocatalysis in ZABs.Herein,PVDF-assisted pyrolysis strategy was proposed to fabricate 2D cobalt,nitrogen,fluorine tri-doped carbon nanosheets decorated with high-density cobalt nanoparticles（Co/Co NFC-NS-700）by using 2D ZIF-67 as initial precursor.Both of the introduced PVDF and pyrolysis temperature have a great impact on the morphology,structure and chemical composition of final products.As a result,a novel corrugated plate-like bifunctional electrocatalyst was obtained,which simultaneously realizes rapid mass transfer and efficient bifunctional active sites towards oxygen reduction/evolution reaction.Benefiting from all these merits,it shows appreciable electrocatalytic performance for rechargeble ZABs.