Composition And Structure Modulation Of Nickel-based Electrodes Towards Performance Optimization

Nickel is located in the fourth period and group VIII of the periodic table and the valent electron configuration of nickel is 3d⁸4s²,which possesses active chemical properties.Nickel-compounds are indispensable electrode materials for alkaline Ni-Zn batteries and seawater electrolysis with the advantages of cheap price,abundant reserves,corrosion resistance and rich active sites.However,the activity and stability of nickel-based electrode in alkaline electrolytes still face the following challenges:（1）α-Ni（OH）₂ is easily converted toβ-Ni（OH）₂ under alkaline conditions,which affects the stability of the electrode.（2）The adsorption energy of the active sites to the reaction intermediates and the dissociation energy of the active sites to the reaction precursors are not satisfactory,and the reactivity of the electrode materials is low.（3）The corrosion of electrodes by Cl^-in seawater electrolyte and the occurrence of chlorine oxidation side reactions will reduce the stability of electrodes and the efficiency of electrolysis.（4）The number of active sites exposed at the electrode is inadequate,and the transfer rates of ions and electrons is slow.Therefore,the active sites and structures of nickel-based compounds need to be optimized by reasonable modulation to improve their performance for electrochemical applications.This thesis focused on solving the problems of low electrode activity and poor stability faced by alkaline Ni-Zn batteries and seawater splitting for hydrogen.The rational design of nickel-based compounds was made for high activity and stability nickel-based electrodes via composition modulation and structure modulation methods.The research contents and conclusions are divided into the following points.（1）The chemical environment of Ni in Ni Mn-LDH was regulated by the doping of Zn to improve the capacity and stability of the cathode electrodes of Ni-Zn batteries.In Ni-Zn batteries,the lower valence state of Ni ions can transfer more electrons during the oxidation process and delivers higher specific capacity.Therefore,in this thesis,by introducing Zn,which is less electronegative than Ni,in Ni Mn-LDH,the electron density around Ni²⁺was increased and the problem of low capacity of nickel-based anode was solved.In addition,the interaction betweenα-Ni（OH）₂ laminates and interlayer anions was enhanced by Zn doping,which solved the problem of instability ofα-Ni（OH）₂ in alkaline electrolyte.The results showed that Zn doping increases the capacity of the Ni Mn-LDH electrode from 191 m Ah g^-1 to 294 m Ah g^-1 and also improves the stability of the Ni Mn-LDH electrode.（2）Based on the design of electrode structure,Ni V-LDH/Co₃O₄hierarchical arrays electrode was synthesized by in situ growing Ni V-LDH nanosheets on Co₃O₄ nanorod arrays.The advantages of the hierarchical structure are:1)The hierarchical arrays structures accelerate the transfer of electrolyte ions and expose more active sites;2)The active materials are directly grown on nickel foam,which enhances the electron transfer process between substrate and active materials;3)The active materials are firmly loaded on the substrate,which can prevent the shedding of the active materials.The results of the study showed that the mass-loading of Ni V-LDH/Co₃O₄ electrode increased to 11.1 mg cm^-2.The electrode also exhibited a high area capacity of 1.98 m Ah cm^-2 and good stability.（3）NiCoFeP bifunctional electrode was prepared by the phosphating treatment of Ni Co Fe-LDH at high-temperature for alkaline seawater splitting.In addition,the components of electrolyte were regulated to achieve the triple production of hydrogen,oxygen and salt by continuous electrolysis of seawater.In this thesis,on the one hand,the phosphate protective layer formed on the surface of phosphate electrode after oxidation was used to improve the selectivity of oxygen evolution reaction and the electrode’s corrosion resistance to chloride ion.The problems of electrode corrosion and chloride oxidation side reactions caused by Cl^-in the electrolyte were solved.On the other hand,the electrolyte was optimized by"sodium common ion effect",and the saturated concentration of Na Cl in the electrolyte was reduced,which inhibits the corrosion of Cl^-on the electrode.The results showed that no obvious corrosion was detected on the NiCoFeP electrode after 100 hours of stability testing at a current density of 500 m A cm^-2,and pure Na Cl crystals could be precipitated during the electrolysis process.This study also scaled up the preparation of NiCoFeP electrodes and integrated a 1 k W seawater splitting equipment,and completed the performance evaluation of the electrodes in the electrolytic equipment.（4）NiCoP/Cr₂O₃ electrodes were prepared for alkaline seawater splitting for hydrogen by modulating the interfacial structure,and the stability of the electrodes was investigated through frequent start-stop tests.Cr₂O₃ has a strong adsorption energy for hydroxyl groups,which solved the problem of high energy barriers from water dissociation steps under alkaline conditions.In addition,Ni Co P and Cr₂O₃ have strong corrosion resistance to Cl^-,and this electrode also solved the problem of cathode’s corrosion from Cl^-in the interval of electrolysis reaction.Experimental results showed that the overpotential of the Ni Co P/Cr₂O₃ electrode at the current density of 100 m A cm^-2 was 33 m V lower than that of the Ni Co P electrode and no obvious corrosion was detected after 350 h of frequent start-stop tests.