Design And Preparation Of Bifunctional Oxygen Electrocatalysts For The Application In Zinc-Air Battery

The serious energy crisis caused by the burgeoning energy demands of current society and increasingly prominent environmental problems compel us to pursue new energy technologies.Metal-air batteries,especially rechargeable zinc-air battery（ZAB）,are considered a good choice because of their high theoretical energy density of 1086Wh kg^-1,among many other advantages.However,the current ZAB suffer from low round-trip efficiency,poor cycling stability,severe carbon corrosion of catalysts,and inferior discharging/charging current density,among others.These problems are due to the lack of a high-efficiency and durable electrocatalyst that can accelerate both ORR and OER.Since ORR and OER have distinct pathways and rate-determining steps,which require different active sites.The design of catalysts with dual active sites provides an effective solution to tackle the inherent disadvantages of single-site catalysts.A common approach is to construct composite electrocatalysts that mechanically mix an ORR-active component（Pt/C）with an OER-active component（Ru O₂）.However,this strategy always results in inhomogeneous distribution of two components,rough interfacial structures,and insufficient electronic modulation effect,which inevitably lead to low activity and poor durability during long-term operations.Hence,the development of effective bifunctional electrocatalysts with excellent reactivity and durability has become one of the important research topics for high-performance ZABs.In light of the aforementioned considerations,we devote to design and construct a series of efficient bifunctional non-noble metal electrocatalysts by deliberate composition optimization,morphology/structure manipulation and heterointerface construction.The research contents of this dissertation are as follows:（1）Inspired by turning“waste”into treasure,a facile and scalable“delignification-impregnation-carbonization”strategy was designed to fabricate uniformly dispersed NiFe alloy nanoparticles anchored on bamboo stick-derived N-doped carbon fibers（NiFe@N-CFs）as an air cathode for flexible Zn-air batteries.Benefiting from the inherent cellulose fibers with abundant macropore structure in raw bamboo stick,the lignified carbon fibers possess interconnected porous structure.In addition to the high electric conductivity endowed by one-dimensional carbon fibers,this porous structure facilitates mass transfer in electrochemical reactions as well as exposing more active sites.Meanwhile,the mesopores evenly scattered on the wall of macropores effectively precludes nanoparticles from aggregation during cycling owing to the pore spatial confinement effect.Moreover,the strong metal-support interaction between NiFe nanoparticles and N-doped carbon and the synergistic effect may also contribute to the catalysis performance.Consequently,the resulting electrocatalyst has a reversible oxygen overpotential（（35）E）of 0.71 V,exhibiting impressive catalytic performance and stability.When employed as bifunctional air cathode in a liquid Zn-air battery,the NiFe@N-CFs based device can achieve peak power density of 102 m W cm^-2 and can be steadily cycled for 150 h at 10 m A cm^-2.In addition,a commercial red light-emitting diode and stopwatch can be powered using two quasi-solid-state ZABs by integrating the NiFe@N-CFs air electrode and alkaline polyvinyl alcohol gel electrolyte.（2）A novel bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon（NC）was prepared through a strategy of coordination construction-cation exchange-pyrolysis and used as a highly efficient bifunctional oxygen electrocatalyst.The preparation strategy is versatile,which allows hydrothermal coordination reaction for morphology construction and subsequent cation exchange for composition regulation.The formation of graphitic carbon is beneficial to improve the electrical conductivity and corrosion?resistance ability during electrocatalysis.Experimental characterizations and density functional theory（DFT）calculations reveal the formation of Co/CoFe heterostructure and synergistic effect between metal layer and NC support,leading to accelerated reaction kinetics and optimized adsorption energy for intermediates of ORR and OER.The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V.The aqueous ZAB constructed using this air electrode exhibits a slight voltage loss of only 60 m V after 550-cycle test（360 h,15 days）.We also prepared sodium polyacrylate hydrogel（PANa）with good alkaline-tolerance,stretchability and water-retention capability as a promising quasi-solid-state electrolyte to overcome the disadvantages of polyvinyl alcohol（PVA）polymer electrolytes used commonly,the assembled quasi-solid-state ZAB demonstrates excellent mechanical stability and cyclability under different bending states.（3）A facile and versatile“coordination construction-thermal decomposition”strategy was developed to construct Co/MnO heterointerface embedded in N-doped carbon nanowires by employing nitrilotriacetic acid as chelating agents to stabilize different metal ions and construct the nanowire structure.The difference in electronegativity of the two metals in nanowire precursors is favorable for forming metallic Co/MnO heterostructures instead of conventional bimetal oxides.The in-situ generated Co nanocrystals not only facilitate the formation of graphitic carbon as the conductive agent,but also improve the corrosion-resistance ability during electrocatalysis.DFT calculations suggest that the heterointerface engineering can moderate the adsorption free energies of oxygen-containing intermediates in OER and ORR,which effectively boosts the intrinsic activity of Co/MnO@NC.As a result,Co/MnO@NC exhibits prominent electrocatalytic properties with a minimized oxygen overpotential of 0.66 V.When equipped in liquid ZABs,the as-prepared Co/MnO@NC endows the ZABs with high peak power density of 146 m W cm^-2 and impressive discharge/charge stability for 400 h/600 cycles at 20 m A cm^-2.The quasi-solid-state ZAB also exhibits outstanding mechanical flexibility in addition to high battery performance.This strategy can be extended to prepare Fe/MnO@NC and Ni/MnO@NC electrocatalysts with other iron group elements,which paves a new way for development of efficient and robust electrocatalysts for energy conversion and storage.（4）The heterostructured FeNi₃/FeNi₃N electrocatalyst of hollow nanotubes is designed and fabricated by a facile and controllable self-templated synthetic strategy,which involves MIL-88A（Fe）as a sacrificial template followed by etching the MOF interior through an ion(Ni²⁺)exchange reaction.The heterostructured FeNi₃/FeNi₃N is obtained by facile and scalable nitridation treatment with well-preserved nanotube structure.Benefiting from the unique compositional and structural merits and the intrinsic metallic property endowed with high electric conductivity,the FeNi₃/FeNi₃N exhibits remarkable bifunctional activities towards ORR and OER with excellent long-term stability.DFT calculations suggest a synergistic effect between inner FeNi₃/FeNi₃N and in-situ developed surface Fe（Ni）OOH to reduce free energy for the rate-limiting O--O bond formation of OER.When used as the air-cathode for ZAB in liquid electrolyte,it exhibits high power density of 157 m W cm^-2 and impressive durability for at least 590 cycles.By integrating with hydrogel electrolyte,the flexible,quasi-solid-state ZAB with FeNi₃/FeNi₃N can power a digital timer and red LED screen under different bending angles.These results clearly demonstrate the promising application potentials of as-prepared FeNi₃/FeNi₃N in a variety of wearable and portable electronics.（5）A facile self-template synthetic strategy is developed to fabricate a bifunctional electrocatalysts with three-dimensional hierarchical spherical hollow structure that consists of heterostructure Co N/MnO nanoparticles anchored an ultrathin porous carbon nanosheet（denoted as Co N/MnO@NC）.Co N possesses excellent catalytic ability in the OER due to its d band center is close to the energy level of the OER.In addition,the intrinsic metallic nature makes up for the semi-conductive property of MnO,which improve the electron transfer rate and speed up the sluggish ORR kinetics.In addition,hollow structures possess distinguishable interior voids,low mass density,and highly porous shells,characteristics that lead to a large electrolyte-electrode contact area,numerous active sites for ion adsorption and interfacial reactions,efficient electron and mass transport paths on porous shells,and shortened diffusion distances by means of hollow structure.By taking advantages of structural and compositional benefits,the cathode exhibits superior electrocatalytic activity toward ORR and OER with a low overpotential of 0.69 V.The as-assembled rechargeable ZAB performs a high peak power density of 153 m W cm^-2and excellent durability of 720 cycles at 10m A cm^-2.Furthermore,the Co N/MnO@NC and PANa hydrogel electrolyte are integrated into a flexible quasi-solid-state ZAB,demonstrating an excellent cycling performance and a good round-trip efficiency even under bending states.In summary,we focus on the structure design strategy and active site regulation of precious-metal-free bifunctional oxygen electrocatalysts for ZAB.Special emphasis will be put on the interface engineering,which leads to redistribution of electrons on the catalyst surface and optimization of chemical adsorption,thus promoting the performance of catalysts.This dissertation reports reasonable and effective design strategies for bifunctional ORR/OER electrocatalysts,which provides new insights into the elaborate design and facile fabricate advanced electrocatalysts for sustainable energy systems,and is expected to guide targeted optimization of electrocatalysts and in-depth exploration of emerging candidates.