Construction Of Carbon-Based Oxygen Electrocatalysts Based On The Synergistic Effect Of Multi-Metal For Zinc-Air Batteries

Rechargeable Zn-Air batteries（RZABs）combine the advantages of both electrochemical batteries and fuel cells.The RZABs have semi-open battery structure,enabling the continuous supply of oxygen from the air for the cathodic reaction.In theory,the RZABs exhibit attractive advantages in terms of high energy density,stable discharge voltage,intrinsic safety,and eco-friendliness.The emergence of RZABs provides a promising solution for next-generation medium-and long-term energy storage systems with high specific energy density and reliable safety.However,the development of RZABs still faces enormous challenges,such as the limited energy efficiency,low electrode utilization efficiency,unsatisfactory power density,and poor operation lifespan.These drawbacks have severely impeded the large-scale commercialization of RZABs.The main reasons for hindering the advancement of RZABs are the slow kinetics and high overpotential of the oxygen reduction reaction（ORR）and the oxygen evolution reaction（OER）associated with the battery discharge and charge process,respectively.These disadvantages lead to unavoidable energy losses due to the polarization effects.Therefore,the development of highly efficient oxygen electrocatalysts for ORR and OER is crucial to promote the development of RZABs.Currently,commercial Pt/C and Ru O₂ catalysts are state-of-the-art catalysts for ORR and OER,respectively.But these precious-metal catalysts have extremely high cost and unsatisfactory long-term durability.It encourages developing precious-metal-free oxygen electrocatalysts with highly efficiency and low-cost.Taking advantage of the short-range or long-range synergistic effects among multiple transition-metal elements,the charge distribution,the band structure and the adsorption modes of oxygen species can be also optimized for active sites,thus effectively enhancing the overall ORR and OER electrocatalytic activities for precious-metal-free oxygen electrocatalysts.Additionally,the synergistic effects play a crucial role in enhancing the long-term stability.The exploration of the mechanisms behind synergistic effects will shed light on the further development of highly efficient precious-metal-free oxygen electrocatalysts,thereby overcoming the bottlenecks of RZABs.Base on abovementioned issues,the thesis was sequentially conducted on the aspects of materials preparation,structural optimization,mechanism exploration and practical application.Firstly,carbon materials with multi-transition-metal core/shell structure and nitrogen-doped carbon nanosheets with transition-metal dual-atom active sites were prepared.These as-prepared catalysts were intensively investigated.The synergistic effects among transition-metal sites were uncovered with the help of systematical physical and chemical characterizations,in-situ electrochemical characterizations and theoretical calculations.The relationships between the electronic structure,the d-band structure,the oxygen adsorption mode,and the free energy barrier for oxygen electrocatalysis were comprehensively discussed.Finally,the as-prepared oxygen electrocatalysts were applied to RZABs,showing the promising potential of carbon-based materials with multiple transition metal species in energy storage and conversion applications.The detailed research contents are listed as follows:1.A facile and scalable method was developed to prepare Co Cu Ni/Zn Mn₂O₄core/shell structure supported on nitrogen-doped carbon materials（Co Cu Ni/Zn Mn₂O₄-NC）as highly efficient oxygen electrocatalyst.Taking advantages of the difference in electronegativity between Co,Cu,Ni,and Zn,Mn,the unique Co Cu Ni/Zn Mn₂O₄ core/shell structure was successfully constructed by one-step pyrolysis.The electrochemical measurements demonstrated that the Co Cu Ni/Zn Mn₂O₄-NC exhibited remarkable electrocatalytic performance towards both ORR and OER.The ORR half-wave potential of Co Cu Ni/Zn Mn₂O₄-NC was0.942 V,while the OER overpotential was only 450 m V to reach the current density of 10 m A cm^-2.The performance of Co Cu Ni/Zn Mn₂O₄-NC outperformed commercial Pt/C-Ru O₂ catalyst.Moreover,the Co Cu Ni/Zn Mn₂O₄-NC displayed excellent long-term stability due to the Zn Mn₂O₄ shell with strong alkaline-resistance,which protected the Co Cu Ni alloy from electrochemical corrosion in alkaline environments.Systematic characterizations in conjoint with theoretical calculations confirmed that the existence of favorable synergistic interactions within the core/shell structure.The construction of Co Cu Ni/Zn Mn₂O₄ core/shell structure enabled long-range electronic interactions between the Co Cu Ni alloy core and the Zn Mn₂O₄ shell.Firstly,the magnetic exchange interactions were enhanced,endowing the core/shell structure with ferromagnetism.Moreover,the spin-states of Mn sites in Zn Mn₂O₄ shell were enhanced,thus further optimizing the adsorption energy of oxygen intermediates.Using the Co Cu Ni/Zn Mn₂O₄ as the catalyst layer of the air cathode,the liquid RZABs exhibited high power density(244.4 m W cm^-2),large specific capacity(802.5m Ah g _Zn^-1@100 m A cm^-2)and excellent long-term cycling stability(>450 h@50 m A cm^-2),which were superior to the liquid RZABs using commercial Pt/C-Ru O₂.Furthermore,the quasi-solid-state pouch RZABs using mass-produced Co Cu Ni/Zn Mn₂O₄-NC also showed excellent performance.This work not only shed light on the transition-metal core/shell structures with improved ORR and OER activities,but also provided a profound comprehension on regulating the spin-states based on long-range multi-metal synergistic interactions.2.A straightforward method was developed to prepare nitrogen-doped carbon nanosheets with Fe Co-N₆ dual-atom active sites（Fe Co-NCNS）using a sacrificial template in combination with cascade anchoring strategy.The structure of Fe Co-NCNS was investigated by employing various physical characterizations and in-situ electrochemical scanning microscopy techniques.The as-prepared Fe Co-NCNS oxygen electrocatalyst possessed hierarchical porous structure and large specific surface area,thus effectively enhancing the mass transfer capability.Moreover,the optimized porous structure provided ample anchoring sites for dual-atoms.The active site density of Fe Co-NCNS was up to 2.5×10¹⁹ sites g^-1.Furthermore,the short-range synergistic effect in hetero-diatomic Fe Co-N₆ active sites was comprehensively investigated by theoretical calculations in combination with comparative electrochemical experiments,including X-ray photoelectron spectroscopy（XPS）,X-ray absorption spectroscopy（XAS）,and in-situ electrochemical surface-enhanced Raman spectroscopy（EC-SERS）.In an alkaline electrolyte,the Fe Co-N₆ active sites in Fe Co-NCNS were spontaneously to a unique O-bridging structure（denoted as Fe Co-N₆-O）via a thermodynamically favorable process.Compared to the pristine dual-atom structure,the synergistically dynamic transformation significantly influenced the interfacial charge distribution,promoting the increase of charge polarization and the downshift of d-band center relative to the Fermi level for the metal reaction center（the Fe sites in the dual-atom structures）.Eventually,the dynamic structural evolution towards Fe Co-N₆-O contributed significantly to lower free energy barriers,thus enhancing the O₂ electroreduction kinetics.Based on the above results,the structure-activity relationships applicable to various dual-atom and single-atom active sites were established.Benefiting from the enhanced intrinsic activity and mass transfer capability,the as-prepared Fe Co-NCNS exhibited excellent ORR performance in electrochemical measurements,with an ORR half-wave potential of 0.898 V,a mass-specific kinetic current density of 8.02 A g^-1,and a turnover frequency of 2.08 e^-site^-1 s^-1.The ORR performance of Fe Co-NCNS surpassed that of commercial Pt/C catalysts.The Fe Co-NCNS also exhibited OER electrocatalytic activity comparable to commercial Ru O₂.Meanwhile,the Fe Co-NCNS showed excellent long-term durability.The power density,specific capacity,and charge-discharge cycle lifespan of RZABs using Fe Co-NCNS were superior to those of counterparts using commercial Pt/C and Ru O₂ composite catalysts.This work not only provided a systematic protocol to in-situ monitor the dynamical structural transformation at the atomic level,but also gained valuable insight into the essential short-range synergistic effect and the structure-activity relationships for dual-atom electrocatalysts.