Preparation of Cobalt-based Electrode Materials and Their Electrocatalytic Performance Toward Water Splitting

Author:Xu Ning

Supervisor:wang ping


Degree Year:2019





Hydrogen energy is a clean and efficient secondary energy source.It is an ideal energy alternative due to its wide raw materials,high energy density and the pollution-free combustion product.Electrolytic water hydrogen production technology enables the conversion between electrical energy and chemical energy,and considered to be the most ideal green pollution-free hydrogen production technology among various advanced technologies.Water dissociation involves two half reactions:hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),which are both essential for improving the efficiency of overall water splitting.The development of highly active,cost-effective and highly stable HER and OER electrocatalysts to increase the electrolysis water reaction rate and reduce the overpotential is crucial to the development of water splitting.Currently,the state-of-the-art OER catalysts(RuO2,IrO2)and HER catalytic materials(Pt,Ru,Rh,etc.)remain to be noble metals.Unfortunately,the scarcity and high cost of noble metals severely limit their widespread application.For the oxygen evolution reaction,the metal Co and its alloy with low overpotential in an alkaline medium,low price,easy preparation and high corrosion resistance under anodic polarization conditions.The high-valent cobalt oxide CoOOH produced in the OER process is an active site for catalyzing the oxygen evolution reaction,and thus can be used as an ideal OER electrocatalyst.Moreover,for the hydrogen evolution reaction,the metal Co with moderate hydrogen adsorption energy,and the Co-based oxide or hydroxide can promote the dissociation of water,and thus be a promising HER electrocatalyst.However,the activity and stability of these catalysts are still far from the noble metals,so the materials need to be appropriately modified to improve the activity and stability of the catalyst.The oxygen evolution reaction(OER)is a multistep four-electron transfer oxidation reaction with sluggish kinetics and thus the bottleneck of water splitting.Herein,in the third chapter,we prepared a Co-Ni-B@NF catalyst using a simple electroless plating method.The introduction of Ni element changes the electronic structure of the Co-B composite and enhances the charge transfer.The results of XPS also indicate that electrons are transferred from B to the metals Co and Ni.In the OER process,the Co-Ni-B@NF catalyst will form a Co-Ni-B(core)/Co(Ni)OxHy(shell)structure,in which the metal core will facilitate electron transport and the oxide shell can provide active sites.Thereby,the electrocatalytic activity of the catalyst was improved,and the presence of CoOOH/NiOOH was also detected by XPS results.In addition,the Co-Ni-B@NF catalyst can be used as a bifunctional electrocatalyst to catalyze the OER,HER and total water-resolving reactions,all exhibiting excellent electrochemical activity and stabilityEfficient water-splitting reactions not only need to improve the reaction efficiency of OER,but also explore the high-efficient HER catalysts.For non-precious metal Co-based catalysts,the formed Co(OH)2 nanosheets on the surface of the catalysts under alkaline conditions can promote the dissociation of water,while the flourishing growth of Co(OH)2 may deteriorate the electrocatalytic activity and durability performance due to their low electrical conductivity.In the fourth chapter,a carbon-coated Co-Mo-O nanocatalyst was successfully developed.The catalyst grows in a certain direction into a bundled of rectangular cuboids,which self-assembled into three-dimensional hierarchical network structure.Coating the catalyst surface with carbon shells can provide a simple and effective method to moderate the growth of Co(OH)2 nanosheets,while increasing the conductivity of the material,thereby improving the HER activity and stability of the catalyst.The first-principles calculation results clearly suggested that the formed Co(OH)2 during the activation of HER process may work cooperation with the Co3Mo to provide the catalytic active site of HER,in which Co(OH)2 promotes the dissociation of water to form active Had species and the adjacent Co3Mo facilitates the Had adsorption and recombination.The synergy of these two parts increases the HER activity of the catalyst in an alkaline electrolyte.Co-Mo-O@C/NF catalyst exhibits excellent HER electrochemical activity and outstanding durability in an alkaline environment.After a 24 h stability test,the activity was only slightly floating.In addition,Co-Mo-O@C/NF catalyst can also be used as a bifunctional catalyst,showing superior activity and durability for both OER and overall water splitting systemsAlthough non-precious metal catalysts have developed very rapidly in recent years,the most active catalysts are still the precious metal catalysts for HER and OER.In the fifth chapter,the highly dispersed Pt nanoparticles were successfully deposited on the porous CoO/NF to obtain the Pt@CoO/NF catalyst with a self-assembled 3D flower-like nanostructure.The Pt@CoO/NF catalyst was synthesized by using H2PtCl6 as the precursor through a simple strong electrostatic adsorption method.Highly dispersed of Pt particles on the CoO surface with optimized utilization of Pt can expose more active sites,and the good interface contacts between Pt and CoO leading to the enhanced electric conductivity through the hydrogen spillover of Pt The cooperation with Pt and CoO exhibits higher intrinsic HER activity than Pt in alkaline electrolyte,in which CoO is effective for promoting water dissociation and Pt is active for the adsorption and recombination of active Had.Combination of these aspects simultaneously make the Pt@CoO/NF catalyst exhibits excellent HER electrochemical activity as well as outstanding durability in an alkaline solution.Subsequently,the highly dispersed IrO2@CoOOH/NF catalyst was also synthesized by SEA method and exhibit superior OER activity and satisfactory long-term stability.We assembled the IrO2@CoOOH/NF and Pt@CoO/NF electrocatalysts in a two-electrode cell under the alkaline environment.The cell has a current density of 10 mA cm-2 at a voltage of 1.637 V and also exhibits excellent stability.