Hydrothermal Corrosion Of Ni Foam To Nickel-Based Binder-Free Electrodes And Their Electrochemical Properties

Compared with traditional electrode with polymer binder,binder-free electrode is a better mode to gain high performance electrochemical electrode.Economical,simple and convenient route to binder-free electrode attracts numerous attentions.Due to its low cost,various chemical states,and inherent catalytic properties,nickel-based compounds are widely used in numerous electrochemical fields related to energy and environment.However,their low conductivity and traditional fabrication mode of electrode with binder hinder the electrochemical properties.In this thesis,NiO,Ni₃S₂,and Ni?OH?₂ binder-free electrodes were obtained through the corrosion of nickel foam by oxalic acid and sodium sulfide aqueous solution,as well as water under hydrothermal condition,respectively.With the assistance of X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray photoelectron spectroscopy?XPS?,cyclic voltammetry curve,constant current charging-discharging curve,and current-time response curve,the formation mechanisms,pseudocapacitance properties and hydrazine electrooxidation properties of as-prepared binder-free electrodes were carefully investigated.The main contents and results could be summarized as following:?1?Hydrothermal corrosion of nickel foam by oxalic acid to NiO electrodes and their pseudocapacitance propertiesPorously polycrystal NiO micro-pillars consisting of nanoparticles were grown on the surface of Ni foam by the heating treatment of NiC₂O₄·2H₂O which were formed through hydrothermal corrosion of nickel foam by oxalic acid solution.Owing to the unique structure as well as morphology,which are good for mechanical stability,ion diffusion,electronic transport,and surface active sites,the NiO binder-free electrode shows the excellent pseudocapacitance properties,including high areal capacitance,good rate performance,and cycling stability.At 3 mA cm^-2,the areal capacitance reaches 1.533 F cm^-2.Its areal capacitance still keeps 1.18 F cm^-2 at a very high current density of 20 mA cm^-2.After 1000-time charge-discharge tests at an ultrahigh current density of 50 mA cm^-2,the areal capacitance attains 78.2%of its initial value.?2?Hydrothermal corrosion of nickel foam by sodium sulfide to Ni₃S₂ electrodes and their hydrazine electrooxidation propertiesNi₃S₂ binder-free electrodes were obtained through chemical corrosion of nickel foam by sodium sulfide under hydrothermal condition.The formation mechanism of the Ni₃S₂ layer was also investigated.Firstly,the nickel atoms at the surface of nickel foam form Ni²⁺ ions in solution and leave electrons on Ni foam through the "electric double-layer",and then the Ni²⁺,ionized S^2-,and metallic Ni atoms react to form the Ni₃S₂.At the same time,the accumulating electrons are consumed by oxygen and water.It was found that a Ni₃S₂ layer leads to enhanced property in hydrazine electrooxidation.The enhanced activity is mainly attributed to the highly conductive Ni₃S₂ layer which acts as the mediator shuttling electrons transfer between hydrazine and nickel foam.As the anode of direct hydrazine fuel cell,the obtained Ni₃S₂ electrode exhibits high current density,low onset potential and good durability.As the working electrode of electrochemical sensor toward hydrazine,its sensitivity reaches 14674.44 ?A mM^-1 cm^-2,and a good dependence between current signal and hydrazine concentration as well as selectivity were also observed.?3?Hydrothermal corrosion of nickel foam by water to Ni₃S₂ electrodes and their pseudocapacitance propertiesUsing ultrapure water as reaction media to corrode nickel foam under hydrothermal condition,single crystal Ni?OH?₂ nanosheets were deposited on the surface of nickel foam.The formation mechanism of Ni?OH?₂ can be attributed to oxygen-consuming corrosion or hydrogen evolution corrosion under hydrothermal condition.Through tuning the reaction time,the influence of reaction time on Ni?OH?₂ amount loaded on nickel foam,morphology of active material,areal capacitance,and specific capacitance was also investigated.It was found that the sample corroded for 14 h has the highest areal capacitance.At the current density of 5 mA cm^-2(4 A g^-1),electrode's areal capacitance is 0.863 F cm^-2,and the specific capacitance of the loaded active materials is 693 F g^-1.At 12.5 mA cm^-2,no attenuation of the capacitance is observed after 1000-time charge-discharge tests.