Design Of Three-dimensional Electrode Material Based On Layered Hydroxide For High Performance Supercapacitors

Supercapacitors,also known as electrochemical capacitors,are a kind of special capacitors which store the charge through electrostatic ion adsorption/desorption process or by reversible faradic reactions at the electrode/electrolyte interface of high specific surface area materials.Supercapacitors have been extensively used in civilian and transportation fields due to their unique characteristics,such as high power densities,fast charge/discharge ability and excellent cycle stability.However,the energy density of supercapacitors is relatively low.One of the methods to solve this problem is selecting the electrode materials with high capacitance and constructing special nanostructure.In addition,assembling asymmetric capacitor to broaden the potential window is another effective strategy.Hydrotalcite-like layered hydroxides characterized with rich sources,redox active sites,simple synthesis,low cost and unique 2D layered structure,have attracted wide attention as electrode matrials for supercapacitors.Hydrotalcite-like layered hydroxides can simultaneously store the energy by means of the electrical double layer capacitance and pseudocapacitance.On the one hand,hydrotalcite-like layered hydroxide can be used as a template to synthesize porous carbon for electric double layer capacitors;On the other hand,it can be directly used as a supercapacitor electrode material.However,the low electrical conductivity limits its practical application.As a result,combining layered hydroxide with highly conductive materials can achieve excellent electrochemical performance due to their synergistic effect.Meanwhile,the electrode materials with three-dimensional structure can not only improve the utilization rate of the active sites,but also accelerate electrons and ions transmission speed.The main research contents in this paper foucus on the three-dimensional electrode materials around layered hydroxides,including two parts: firstly,three-dimensional porous carbon was synthesized by using the layered hydroxide as template;secondly,three-dimensional composite electrode materials were prepared by combining the layered hydroxide with conductive substrates.The supercapacitor behavior of the as-prepared materials were thoroughly studied to realize maximum energy density and power density.The details are as follows:?1?A template strategy for preparing three-dimensional?3D?porous carbon nanosheets?PCNs?with an oriented and interconnected nanostructure is proposed.Zinc layered hydroxide nitrate is used as a layered template and provides a nanospace to confine the carbonization process of the organic carbon precursor?gallic acid?.The unique nanostructure,large surface area?1138.8 m2 g?-1??and reasonable porous distribution of chemically activated PCNs?aPCNs?significantly shorten the ion transport length in low dimensions and improve the electrolyte wettability and ion accessible surface area for charge storage.The aPCNs exhibit excellent performance as demonstrated by their large specific capacitance?327 F g?-1? at a current density of 0.5 A g?-1??,superior rate capability?retaining 60.2% at 20 A g?-1??and stable cyclability.In particular,the assembled symmetric device based on aPCNs delivers an energy density as high as 10.2 Wh kg?-1? at a power density of 301 W kg?-1?.?2?An exfoliated MXene?Ti₃C₂??e-MXene?nanosheets/nickel-aluminum layered double hydroxide?MXene/NiAl-LDH?composite as supercapacitor electrode material is fabricated by in situ growth of NiAl-LDH on e-MXene substrate.The NiAl-LDH platelets homogeneously grown on the surface of the e-MXene sheets construct a three-dimensional?3D?porous structure,which not only leads to high active sites exposure of NiAl-LDH and facile liquid electrolyte penetration,but also alleviates the volume change of NiAl-LDH during the charge/discharge process.Meanwhile,the e-MXene substrate forms a conductive network to facilitate the electron transport of active material.The optimized MXene/NiAl-LDH composite exhibits a high specific capacitance of 1061 F g?-1? at a current density of 1 A g?-1?,excellent capacitance retention of 70% after 4000 cycle tests at a current density of 4 A g?-1? and a good rate capability with 556 F g?-1? retention at 10 A g?-1?.?3?NiMn-layered double hydroxide?NiMn-LDH?grown on MDC is constructed by in situ growth method using carbon derived MXene?Ti₃C₂??MDC?as conductive substrate.The resulting composite material?MDC/NiMn-LDH?displays a three-dimensional architecture with tunable Ni/Mn ratio and enlarged surface area.When Ni/Mn ratio is 5:1?MDC/Ni5Mn1-LDH?,MDC/Ni5Mn1-LDH exhibits superior supercapacitive performance.At a current density of 1 A g?-1?,the specific capacitance of MDC/Ni5Mn1-LDH is 1640 F g?-1?.The capacity retention is 69.5% at the charge-discharge current density of 10 A g?-1??1140 F g?-1??.After 4500 cycle tests at a current density of 8 A g?-1?,the capacitance retention is as high as 73.5%.In particular,we assembled asymmetric capacitor?MDC/Ni5Mn1-LDH//aPCNs?based on MDC/Ni5Mn1-LDH as positive electrode,aPCNs in our first work as negative electrode.MDC/Ni5Mn1-LDH//aPCNs delivers an energy density of as high as 54.5 Wh kg?-1? at a power density of 825 W kg?-1? in a wide potential window?0-1.65 V?,and energy density can matain 29.3 Wh kg?-1? even at a high power density of 16.5 kW kg?-1?.