Synthesis,Doping And Electrochemical Properties Of Nickel-cobalt-based Transition Metal Compounds

The ever-increasing environmental issues and current energy crisis demand call for not only the urgent development of alternative clean energy but also advanced energy storing systems and conversion devices.Especially,nickel and cobalt(Ni-Co)compounds have drawn intensive attention in electrode materials of supercapacitors and fuel cells for their high theoretical capacitance and cost effectiveness.Nevertheless,because of the gradual decay of electroactive species during cycling process,the cycling stability and conductivity of Ni-Co based compounds are greatly weaker compared to those of carbon materials.This is a major drawback of Ni-Co based materials for practical applications.At this point,the interaction between nickel-cobalt compounds and heteroatomic species such as C,N and S should go far towards solving this problem.Meanwhile,the nickel-cobalt-based compounds also have great development space in non-precious metal electrocatalysts.In this paper,the nickel-cobalt compounds are modified by fluorine doping,which improves its corresponding electrocatalytic properties.Here,some novel ideas are proposed to overcome the challenges about the Ni-Co based compounds,and the application,morphology as well as heteroatomic species doping are studied.Specific contents are as follows:The carbon-rich Ni-Co based hydroxycarbonates were synthesized by one-step solvothermal reaction of Ni(NO3)2,Co(NO3)2,urea and glucose which dissolved into distilled water/ethylene glycol mixture.During the reaction process,different carbon contents originated from glucose may affect the morphology of the compounds.Using a three-electrode system,it's proved that Ni-Co based compounds(Ni2Co1-C2)with 3D analogous carnation-like architecture can exhibit an enhanced specific capability of 1666 F/g(0.5 A/g),when the feeding mass ratio of glucose and cobalt nitrate is 2:1.In addition,Ni2Co1-C2 possesses a fairly high 89.9%capacitance retention at 5 A/g after 1000 cycles.On the basis of the above observation,the carbon-rich Ni-Co based hydroxycarbonates with N,S dual doping were synthesized by one-step hydrothermal reaction of thiourea,urea,glucose,Ni(NO3)2 and Co(NO3)2.In the case of a certain amount of carbon content,taking the different doping quantity and study the influence of doping quantity on the compounds'morphology.By electrochemical testing,the obtained Ni2Co1C2-S1 can maintain good carnation-like shape and show a maximum specific capability of 1958 F/g(0.5 A/g).Meanwhile,it can still retain a high 98.5%capacitance retention(after 1000 cycles,5 A/g).Based on the feeding quantity of Co(NO3)2,N and S co-doped Ni0.75Co0.25(CO3)0.125(OH)2/C(NSH)with 3D carnation,like structure was acquired by taking 2 times glucose and 1 times thiourea.Moreover,a series of compared samples were prepared using similar experimental procedures.To further evaluate the practical application,the NSH//reduced graphene oxide hydrogel based supercapacitor device was assembled,which can offer high energy densities of 44.4 Wh/kg and 11.7 Wh/kg at power densities of 460 W/kg and 9.8 kW/kg,respectively.Through a simple one-step hydrothermal reaction of Ni(NO3)2,Co(NO3)2,urea and NH4F(fluoride source)which dissolved into distilled water/ethylene glycol mixture,F doped columnar cobalt and nickel compounds were synthesized.Furthermore,the relationship between particle size and the amount of fluorine dopants have been preliminarily discussed.When the feeding quantity ratio of NH4F and Co(NO3)2 is 1:1,the experimental results show that the obtained sample presents uniform columnar morphology(h:0.48 ?m,?:1.70 ?m),and shows an overpotential of 457 mV at 10 mA/cm2 as the Tafel curve slope is only 56 mV/decade for oxygen evolution reaction(OER).