Ultrathin CoFe-layered Double Hydroxide For Ultrahigh Capacitance Supercapacitors

With the consumption of global fossil energy multiplication and the growing environmental pollution problems,renewable energy,such as wind energy,geothermal energy and solar energy,can be converted into electric energy without pollution,which has become the focus of attention.However,the intermittent of these energies can not meet the continuity of power supply,so the development of large capacity,good stability and safe power storage devices is urgently demanded.Meanwhile,the boom in portable electronics and electric vehicles has also required the replacement of traditional power storage devices,such as lead-acid batteries.As a new type of electrochemical energy storage devices,supercapacitors have become the focus of research because of their excellent power density,long cycle life,good rate performance,safety and environmental-friendly natures.Unfortunately,commercial supercapacitors dominated by carbon materials are still limited by the lower energy density.Therefore,supercapacitors with high energy density need to be developed urgently.Herein,we design the high-performance electrodes mainly from two aspects:electrode active material and nanostructure.Concretely,we combined the CoFe layered double hydroxide(LDH)with high conductance Cu3N used as active material,in which the Co element as typical pseudocapacitive material mixed with Fe element can exert maximum capacity,and Cu3N can effectively improve the conductivity of the system.In terms of nanostructure,we fabricated the ultrathin(2.5 nm)CoFe-LDH nanosheets on the porous Cu3N nanowires by electrochemical deposition,and the composite electrode Cu3N@CoFe-LDH has a unique embedded core-shell structure.This composite electrode has achieved excellent electrochemical performance.Additionally,we also design another composite electrode CNT@CoFe-LDH by deposited the ultrathin CoFe-LDH on carbon nanotube(CNT)arrays based on the above work.For this electrode,we widened the electrochemical potential window in the performance test,and proposed a concept of full potential range electrode,which exhibited excellent electrochemical performance in a wider potential window.The details is as follows:(1)First,the precursor of Cu(OH)2 nanowire arrays was prepared on a copper foam substrate by wet chemistry method.Then,the Cu(OH)2 nanowires was converted to porous Cu3N nanowires by annealing in H3N atmosphere.Finally,the Cu3N nanowire was placed in Co and Fe mixed salt solution,and the Cu3N@CoFe-LDH NWAs electrode was obtained by electrochemical constant voltage deposition.Notably,the effect of deposition time on the thickness of CoFe-LDH was explored and the optimal electrodeposition time was obtained.Furthermore,under a three-electrode test system,the prepared Cu3N@CoFe-LDH NWAs electrode materials possess a remarkable specific capacitance of 3078 mF cm-2 and remain 92.6%after cycling for 10000 cycles.After analysis,the excellent electrochemical performance is mainly due to the large specific surface area,good electrical conductivity and unique nanostructures.Meanwhile,an asymmetric supercapacitors device was assembled using the commercial activated carbon as negative electrodes,and the device also exhibit excellent electrochemical performance.(2)First,the carbon nanotubes were synthesized on a carbon cloth soaked in nickel salt solution by annealing in the mixed atmosphere of hydrogen and saturated ethanol vapor.After cooling,the carbon nanotubes were hydrophilic treatment in acid solution.Finally,the ultrathin CoFe-LDH was deposited on the carbon cloth supported carbon nanotubes,and form core-shell CNT@CoFe-LDH electrode.Unlike the above work,the as-prepared CNT@CoFe-LDH electrode have a winder potential window,so that the Fe elements also participate in the redox reaction and the electrode can exhibited Co and Fe redox peaks at the same time in the positive and negative potential respectively.In the case,the specific capacitance and potential of the electrode both have a great enhancement.Additionally,a full potential range symmetric supercapacitor device was assembled with CNT@CoFe-LDH as a positive and negative electrode,which showed high power density,good rate performance and excellent cycling stability in electrochemical measurements.