Controling Of Electrocatalytic Performance In Ni-based LDHs Composites For Flexible And Soild Zn-air Battery

Owing to the advantages of low cost,high security and high energy density,rechargeable Zn-air batteries are considered as an ideal energy storage systems for electric vehicles,portable power supply and power grid systems.However,the sluggish kinetics of oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?during discharge and charge process,leading to low round-trip efficiency and poor cycle life of rechargeable Zn-air battery.Hence,it is necessary for developing high properties and cost-efficient catalysts to accelerate the OER/ORR processes,finally reducing the overpotential and increasing the energy efficiency of Zn-air batteries.Researches find that layered double hydroxides?LDHs?have emerged as extremely fascinating candidates as nonprecious electrocatalysts due to their low cost and remarkable OER catalytic activity in alkaline media.However,the applications of LDHs in rechargeable Zn-air batteries are seriously restricted by their low electrical conductivity,less active area and poor ORR performance,especially employing as air-cathode materials for flexible and solid-state Zn–air battery devices.In order to solve these problems,here,we have designed and developed several Ni-based LDHs hybrid materials as electrocatalysts for Zn-air batteries via material components design,interfacial modulation as well as tuning the valence of elements in this thesis.Both ORR and OER of LDHs-based materials can be greatly improved due to the synergetic effects of hybrid components.We also disclose the mechanism of the improving of ORR and OER performances.Importantly,the thesis establish the qualitative relationships of“material synthesis-electronic structure-electrocatalytic activity”by studying the morphologies,surface element valences and electrocatalytic activity of materials.Besides,we find an unique phenomenon:the“electron absorption/donor effects”could trigger the“seesaw”rule between OER and ORR of LDHs-based electrocatalysts.The above discoveries can further guide the design and development of advanced LDHs-based bifunctional electrocatalysts.Finally,by using these types of electrocatalytic materials,it can achieve low overpotentials,high round-trip efficiency and excellent long-term stability of the developed aqueous-based Zn-air batteries and flexible solid-state Zn-air batteries.The main contents are as followed:?1?We fistly presented a core/shell design based on porous NiCo₂O₄ nanowires as core and ultrathin NiMn LDH nanosheets as shell by an efficient chemical co-deposition method,which forms a tight spinel oxide/LDH interface.Benefiting from the high active surface area,rapid mass/charge transport,high electron conductivity as well as strong synergistic effect,the core/shell NiCo₂O₄@NiMn LDH materials deliver a rather low OER overpotential of 255 mV at 10.0 mA cm^-2 with good stability in alkaline media.The overpotentials of OER process are outperforming all of NiCo₂O₄ electrodes reported to date,and also superior to those of commercial Pt/C and Ir/C electrocatalysts.The primary Zn-air batteries based on NiCo₂O₄@NiMn LDH materials exhibit a high specific capacity(722 mAh g _Zn^-1)and energy density(866 Wh kg^-1),superior reversibility?initial round-trip efficiency of 63.5%?.Remarkablly,the rechargeable Zn-air batteries achieve an excellent stability:the voltage gap increased only about 45 mV(5 mA cm^-2)and 20 mV(20 mA cm^-2)after500 continous cycles,which are much better than those of commercial Ir/C catalyst.Furthermore,the developed flexible solid Zn-air battery also displays very good mechanical properties,long cycle life and outstanding round-trip efficiency.?2?We successfully construct the Co₃O₄@NiFe LDH vertical core/shell nanoarrays on Ni foam and carbon cloth by two-step hydrothermal method.The morphologies,material properties and electrochemical performance of Co₃O₄@NiFe LDH can be controlled via adjusting the growth time of NiFe LDH.We find that the valence states of Co?Ni and Fe ions are adjusted by the“electron absorption/donor effects”,finally synergetically improving the OER and ORR performances.By this core/shell design,the maximum ORR current densities of Co₃O₄@NiFe LDH increase to 3-7 mA cm^-2,almost an order of magnitude increases compared to pure NiFe LDHs(0.45 mA cm^-2).Meanwhile,the Co₃O₄@NiFe LDH also exhibits enhanced oxygen evolution reaction?OER?activity with an ultralow overpotential of 226 mV at 35 mA cm^-2,which is much less than that of NiFe LDH?266 mV?.This value is comparable to that of current advanced OER catalysts,and much better than those of commercial Ir/C and Pt/C catalysts.The rechargeable Zn-air batteries can deliver excellent stability of 1250 cycles of continuous testing?>200 h operations?,low voltage gap of⁰.8 V(15 mA cm^-2)and high round-trip efficiency of⁶0%,with voltage fluctuation of only 30 mV.The Co₃O₄@NiFe LDH on carbon cloth is firstly employed as flexible air-cathode of solid-state flexible Zn-air battery.The flexible solid-state battery can cycle more than120 cycles with operation time of 20 h.A battery pack in series with three cells is able to light a LED panel.As well,the flexible sandwich-type Zn-air battery also shows good mechanical stability under synchronously cyclic bending experiment and electrochemical testing.Beyond,our work fill the gap of the synchronous balance and optimization in the OER and ORR performance of bifunctional electrocatalysts.It can also provide quite a few inspire for the design and control of the bifunctional electrocatalytic properties by interfacial design.?3?By utilizing the differences of surface charges of the colloid particles,We develop a universal synthetic method based on“electrostatic adsorption effect”to construct the hybrid electrocatalysts.Herein,graphene quantum dots?GQDs?with negatively charged surfaces are firstly introduced into the Ni-based LDHs system to synthesize a series of Ni-based LDHs@GQDs nanosheet arrays.Impressively,we demonstrate that the obviously“electron absorption effect”between Ni-based LDHs and GQDs can promote the valences of metal cations in Ni-based LDHs tended to a higher valence,which markedly enhances OER activities of Ni-based LDHs.The optimal NiFe-LDH@GQDs electrocatalyst displays a rather low OER overpotential of189 mV at 10 mA cm^-2(NiFe LDH:218 mV at 10 mA cm^-2).The overpotential is comparable to that of current advanced OER catalysts,and much better than those of commercial Ir/C and Pt/C catalysts.Benefiting from the prominent OER performance,the charge potential(1.92 V at 10 mA cm^-2)of Zn-air battery with NiFe LDH@GQDs is obviously lower than that of the Zn-air battery with NiFe LDH(1.95 V at 10 mA cm^-2).Moreover,the rechargeable Zn-air battery displays very good stability of 120 h?500cycles?of continuous charge/discharge operations,with almost no voltage fluctuation.In summary,our work successfully develops the general synthetic methods for LDHs-based electrocatalysts,and studys the oxygen catalytic behavior of these electrocatalysts and corresponding Zn-air battery performance.From our research,we demnonstrate that the charge transfer in LDHs and oxide or LDHs and nanocarbon guided by the“electron absorption/donor effects”,and establish the relationship of“charge transfer modulation and bifunctional electrocatalysis chemistry”.This work provides significant insight and offers a new and easy strategy to design high-performance LDHs catalysts for Zn-air batteries.