Preparation Of Supported Nanoarrays And Their Applications In Catalytic Hydrogen Production And Electrochemical Oxygen Evolution

Increasing attention has been paid to the “green fuel”—hydrogen due to its high calorific value,extensive sources and environmental-friendly features.Specially,as the new and rapidly developed energy in 21 century,the production,storage and release of hydrogen are becoming the main study for researchers.There are various methods for the production of hydrogen,including electrochemical water splitting and hydrolysis of hydrogen storage materials,which are more environmental and economic than traditional water methods.However,catalysts are necessary for both water splitting and hydrogen hydrolysis reactions and transition metal nanoarrays have been widely used in catalyzing electrochemical water splitting reaction and hydrolysis of Na BH4 and NH3BH3,owing to their low-cost,high surface area and high activities.In this Ph.D.thesis,we summarized our works in this area: self-supported cobalt carbonate hydroxide(CCH)nanoarray,cobalt oxide(Co3O4)nanoarray and cobalt phosphate(Co P)nanoarray were prepared and applied in catalyzing Na BH4 and NH3BH3 hydrolysis reactions.Furthermore,self-supported copper oxide(Cu O)nanoarray was also prepared and used in catalyzing the methanolysis of NH3BH3;CCH and Co P nanoarrays were also transformed into CCH@Co-Pi core-shell nanoarray and Co P@Co-Bi-Pi core-shell interconnected network,respectively,which were studied in the application of catalyzing oxygen evolution reaction of water splitting in neutral and near neutral electrolytes respectively.Details are as follow:1.CCH nanoarray was prepared through a one-step hydrothermal process,transforming into a nanostructured amorphous Co Bx active phase during catalyzing the hydrolysis of Na BH4 alkaline solution.This sef-supported nanoarray shows high catalytic activity to the hydrolysis reaction with a high hydrogen generation rate(4000 m L min-1 gcat-1)and low activation energy(Ea = 39.78 k J mol-1),mainly due to its high surface area,exposing more active sites.2.Based on the above work,Co3O4 nanoarray was prepared through a thermal annealing treatment of CCH nanoarray in the air,which also shows high catalytic activity to the hydrolysis of Na BH4(1940 m L min-1 gcat-1,Ea = 59.84 k J mol-1).Specially,Co3O4 nanoarray behaves more stable than that CCH nanoarray in hydrolysis process and it can be due to that the nanoarray becomes more stable after thermal annealing treatment and the active phase Co Bx was only derived on the surface of nanowire without the destruction of nanoarray.3.Furthermore,Co P nanoarray was obtained through a low-temperature phosphide process of CCH nanoarray,showing high catalytic activity to Na BH4 hydrolysis reaction(6500 m L min-1 gcat-1,Ea = 41 k J mol-1).It is found that Co P nanoarray also shows excellent catalytic activity and stability to NH3BH3 hydrolysis reaction(4000 m L min-1 gcat-1,Ea = 40.9 k J mol-1).4.Methanolysis of ammonia borane for the production of hydrogen is also an important way of hydrogen economy.There are several advantages by using methanol instead of water as the solvent of NH3BH3,including zero generation of NH3,recycling by-products and better low temperature resistance.Thus,a self-supported Cu(OH)2 nanoarray was prepared and transformed into a self-supported Cu O nanoarray.It is found that this Cu O nanoarray shows high catalytic activity to the methanolysis reaction of NH3BH3(4629 m L min-1 g Cuo-1,Ea = 38.97 k J mol-1).Compared to traditional powder catalysts,which are often deposited on substrates in real applications,the above self-supported catalysts has higher surface area and thus higher catalytic activity to hydrolysis reactions.Furthermore,these self-supported catalysts can be easily separated from fuel solutions and thus can be used in controlled release of hydrogen.Our work provide cost-effective,robust and reusable monolithic integrated catalysts for on-demand hydrogen generation from Na BH4 and NH3BH3 as fuel feed to cells in portable devices.Electrochemical water splitting,which can be divided into hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),is another common way for hydrogen production.However,OER is more kinetically sluggish,becoming the bottle of water splitting.Electrochemical water splitting in neutral and near neutral electrolytes has attracted increasing attention due to its mild environment.Thus,it is highly attractive but still a key challenge to design and OER catalyst electrodes for highly efficient and stable water oxidation under neutral or near neutral conditions.We have also done some works based on self-supported nanoarray catalyst.Details are as follow:5.We also demonstrate the in-situ electrochemical formation of self-supported Co-Pi nanowire array integrated on Ti mesh(Co-Pi NA/Ti)as a 3D oxygen evolution reaction(OER)catalyst electrode in neutral electrolyte.The Co-Pi as the active phase forms in situ from CCH nanoarray upon its oxidative polarization in neutral phosphate-buffered solution(p H 7.0),forming a CCH@Co-Pi core-shell nanoarray.Catalytic onset occurs at overpotential of 220 m V and only overpotential of 460 m V is required to drive a geometrical catalytic current density of 10 m A cm-2,outperforming all reported Co-Pi and other non-noble-metal OER catalysts in neutral electrolytes.This catalyst also demonstrates high long-term electrochemical stability.6.Based on work 5,we also demonstrate the development of an interconnected network of core-shell Co P@Co-Bi-Pi via anodic polarization of Co P nanoarray in potassium borate aqueous electrolyte(KBi).This 3D Co-Bi-Pi@Co P exhibits super high catalytic activity for water oxidation at p H 9.2 and needs overpotential of only 410m V to drive a geometrical catalytic current density of 10 m A cm-2.Remarkably,this catalyst also demonstrates high long-term electrochemical stability with its activity being maintained for at least 27 h in KBi.It can be seen that these self-supported nanoarray catalysts have great potential in catalyzing OER in neutral and near neutral conditions.This can be mainly due to its 3D nanoarray structure with high surface area,exposing more active site.Our works not only offer attractive materials for efficient and stable water oxidation electrocatalysis under near neutral conditions,but would open up new horizons for in situ electrochemical fabrication of 3D catalyst electrodes for applications.