Fabrication Of Carbon Nanocomposites Based On Layered Hydroxide Precursors And Their Application In Electrocatalytic Water Decomposition

Hydrogen production from electrolytic water can effectively solve the two major problems of"energy crisis"and"environmental pollution",but its commercial application is hampered by the high overpotential and slow kinetics of hydrogen（HER）and oxygen（OER）evolution reactions at the cathode and anode,as well as the high cost and low durability of the precious metal-based catalysts used for these reactions.In recent years,carbon-coated transition metal-based nanomaterials（TM@C）have shown excellent competitiveness in electrocatalytic reactions.However,the preparation of TM@C catalysts with high purity,great dispersion,high BET area and splendid surface activity is extremely challenging,and the development of new technologies to achieve low-cost,scalable and controllable preparation of efficient and stable TM@C catalysts is urgent.Layered metal hydroxide（LMHs）,as a highly ordered layered functional material,has attracted people’s attention in the field of electrocatalysis due to its diverse composition and structure,environmental friendliness,low price and simple synthesis method.In this thesis,carbon coated metal/alloy nanocomposites with high dispersion,high purity and high activity were synthesized by one step pyrolysis in nitrogen atmosphere using transition metal hydroxides intercalated with organic anions as precursors.The specific contents and conclusions are as follows:（1）Carbon coated nickel nanocomposites（Ni@C/CNTs）were prepared by thermal decomposition in N₂atmosphere using nickel hydroxide（Ni-Sal LHSs）of salicylate anion intercalated by self-assembly in aqueous phase as precursor.The precursor,pyrolysis process and products were investigated in detail by SEM,TG,MS,XRD,FT-IR,XPS and other characterizations,and the results were as follows:The salicylate ions were arranged in a bilayer between the layers and had strong interactions with the lamellar.During pyrolysis,salicylate ion,releases reducing gases such as H₂and CO during in situ polymerization graphitization dehydrogenation and deoxygenation to realize the reduction of metal ions and produce a large number of uniform mesopores,while rich functional groups（C-O-C,C-OH,etc.）are formed on the surface of graphitic carbon.By adjusting the roasting temperature,the morphology and microstructure of the product can be precisely regulated.The final roasting products present a unique cluster morphology of carbon coated nickel nanoparticles modified by carbon nanotubes.The nanoparticles are about 5～10 nm with high crystallinity and good dispersity.There is a strong Ni-O-C interaction between Ni NPs and graphite carbon layer,which promotes electron transfer.Among them,the Ni@C/CNTs-700nanocomposites roasted at 700℃have a large specific surface area(114.9m²g^-1),enhanced hydrophilicity and adsorption to H*and OH^-,which greatly reduces the large water adsorption and dissociation energy barrier in alkaline HER reaction.The Ni@C/CNTs-700 nanocomposites can be used as efficient and stable HER catalysts in alkaline and acidic media.In 1 M KOH electrolyte,a current density of 10 m A cm^-2can be achieved with an overpotential of only 79 m V,with a tested stability of up to 75 h,showing catalytic activity and stability better than 20%Pt/C.The catalyst also exhibited good catalytic performance and corrosion resistance in 0.5 M H₂SO₄medium.In addition to the electrocatalytic performance of hydrogen evolution,Ni@C/CNT-700 catalyst showed excellent performance in the hydrogenation degradation of p-nitrophenol,rhodamine B and methylene blue.（2）The Co_xNi_y-Sal LDHs precursors with different metal ratios were prepared by adjusting the feed ratio of Co and Ni salts in the same method（x:y is the metal mole ratio）,and carbon-coated cobalt-nickel alloy nanocomposites（Co_xNi_y@C）were further synthesized.A series of characterizations such as SEM、TG、XRD,FT-IR and XPS show that the precursor has a regular one-dimensional fiber morphology with a diameter of 60 nm and a length of 15 um.The calcined products still maintain a one-dimensional fibrous shape,which is composed of a large number of orderly arranged carbon coated Co Ni alloy nanoparticles with a small size of about 5 nm,high crystallinity and good dispersion.Among them,the Co₅Ni₁@C-750 nanocomposite produced at a feeding ratio of 5:1 and a roasting temperature of 750°C has a large specific surface area(105.89 m²g^-1),and enhanced M-O-C and C-OH bonds,which greatly accelerates the electron transport during the reaction and reduces the adsorption energy of OH^-.Electrocatalytic tests showed that the Co₅Ni₁@C-750 catalyst exhibited better bifunctional HER and OER activity in 1 M KOH medium,requiring170 m V and 320 m V to achieve 10 m A cm^-2and maintain it for 40 h,respectively,which is better than the OER catalytic performance of Ru O₂.In the total hydrolysis test,the required cell voltage is only 1.55 V and can be stable at this voltage for 30 h,which is better than that of single metal Co@C-750 nanomaterial.In summary,Ni@C/CNTs and Co₅Ni₁@C nanocomposites with carbon-encapsulated structures were prepared by one-step pyrolysis using transition metal-based LMHs intercalated with organic salicylate as a single precursor without the use of carbon source,reducing agent and structure guide.The prepared materials have catalytic performance comparable to that of noble metal-based catalysts and are of positive significance for energy development and environmental remediation.