Study On Mechanical Design Principles And Efficient Electrocatalytic Materials For Flexible Metal-air Batteries

Flexible metal-air batteries have attracted a lot of attention from researchers due to their high theoretical energy density.However,there are still several key challenges before practical application.Problems such as slow ORR（oxygen reduction reaction）/OER（oxygen evolution reaction）kinetics,high overpotential and slow mass transfer rate of air electrodes.This topic focuses on the cathode and proposes to design catalysts with appropriate binding energy to effectively catalyze ORR/OER,reduce the overpotential of the battery and enhance the energy efficiency and cycling performance of the battery.It has been found that transition metal phosphides have better potential for electrochemical reactions of rechargeable metal-air battery because of their abundant reserves,low cost and high catalytic activity.In general,the excellent redox activity,the good electrical conductivity and the fast electron transport caused by low electronegativity of phosphorus in metal centers are responsible for their excellent electrocatalytic performance.However,the real active substance remains to be studied in the process of alkaline OER due to structural evolution,and the influence of structural evolution on catalytic performance also remains to be explored.Apart from transition metal phosphides,single-atom catalysts have become a hot topic of current researches because of their catalytic activity centers and tunable electronic structure,maximum atom utilization,which can confer superior ORR/OER performance to electrocatalysts.However,due to the fact that single-atom catalysts have the large surface energy,stabilizing single atoms is usually difficult and the loading capacity is rather low.In order to reduce the overpotential of metal-air batteries,enhance the stability of metal-air batteries as well as to solve the above-mentioned catalyst material problems and to explore the potential of electrocatalyst materials for the utilization in flexible devices.The following research works have been done in this paper.（1）The mechanical parameters and fatigue resistance of common porous collector such as nickel foam and carbon cloth were tested,and then a"sandwich"type of flexible metal-air battery was constructed,which was simulated by ABAQUS under bending and twisting conditions in terms of force and displacement.（2）A doping and phosphorisation strategy was used to prepare(Ni_1-xCr_x)₂P（0≤x≤0.15）with an overpotential of only 293 mV at 10 mA cm^-2 when used in alkaline OERs;the constructed rechargeable Zn-Air battery can be cycled stably at 5 mA cm^-2for over 208 h with negligible voltage degradation.The results also show that the typical Ni₂P group is unstable and undergoes in situ oxidation in the initial OER or during CV activation,eventually converting to（oxy）hydroxide as the true Ni₂P active species.DFT calculations and experimental results also clarify that surface engineering and electronic tuning by doping with Cr ions and in situ electrochemical phase transitions can contribute significantly to the performance of the Zn-Air battery.（3）The Co₃O₄@Ni₂P leaf-type heterojunction electrocatalysts with rich interfaces were prepared using a facile secondary growth strategy.The structural and electronic co-regulation of the heterogeneous interfacial sites was shown to be advantageous compared to Co₃O₄ or Ni₂P.The corresponding reaction mechanism of the ORR was proposed by DFT simulations.Electron localization function（ELF）results also elucidate that highly delocalized electrons can promote ORR/OER.Using the Co₃O₄@Ni₂P heterostructure as an electrocatalyst at the air electrode,the developed Zn-air battery can provide 183.8 mW cm^-2 at a current density of 10 mA cm^-2 with a high power density and a low charge/discharge voltage gap of 0.772 V（after 177 h cycles of continuous testing）.（4）The NiCoP/NiO core-shell heterojunction nanowire arrays with predominant O vacancies were prepared using a secondary growth and sintering strategy for electrocatalysts in high-performance Zn-air batteries.DFT was used to systematically investigate the geometrical electronic properties of NiCoP/NiO core-shell heterostructured electrocatalysts.The unique NiCoP/NiO core-shell heterostructured electrocatalysts show highly efficient electrocatalytic performance for ORR and OER by comparison with NiCoP nanowire arrays and NiO nanosheet arrays.To better explain the’structure-activity’relationship,the NiCoP/NiO core-shell heterostructures were used as electrocatalysts for air-breathing electrodes,and the developed Zn-air batteries could provide a high specific capacity of 742.44 mAh g _Zn^-1 at 5 mA cm^-2.It also shows good stability in flexible Zn-air batteries.（5）Inspired by the"pitaya fruit"configuration,we have designed a"pitaya fruit"-style bifunctional electrocatalyst,a maximum loading of 6.82 wt%of Ru single atoms was obtained by modulating the concentration of precursors.By using the optimized Ru_0.3 SAs-NC（Ru single-atom riveted on nitrogen-doped porous carbon）as an electrocatalyst,the triggered Li-O₂ battery can provide a cut-off capacity of 1000 mA h g^-1 with a minimum overpotential of the ORR as low as 0.17 V.The in-situ DEMS results quantify an e^-/O₂ of only 2.14 for the entire cycle of the Li-O₂ battery,indicating its excellent electrocatalytic performance.Furthermore,the high activity and high loading of the single-atom catalyst facilitates the formation/decomposition of Li₂O₂ on the flower-like nanoscale in the electrochemical reaction.In this thesis,a series of electrocatalyst materials is constructed in situ on the flexible collector in order to enhance the electrochemical performance of the flexible metal-air battery.In particular,the electrocatalytic activity of transition metal phosphide precatalysts was enhanced by doping and heterojunction interfacial charge transfer strategies,from which the electrochemical corrosion mechanism was revealed,and the"microstructure-active structure-catalytic performance"relationship of catalyst materials was established.The structure-property-performance relationship has been established by the controlled synthesis of highly loaded single-atom catalysts through spatial domain-limiting effects and ion substitution strategies.This work can provide some references for the design,preparation and performance study of catalyst materials,provide new understanding to reveal the real active material of catalysts,and provide some concepts and foundation for flexible energy storage technology.