Controlled Preparation And Electrolytic Water Properties Of Three-Dimensional Transition Metal/Carbon Nanocomposites

Electrocatalytic hydrogen cracking technology is one of the key technologies that promise to achieve a clean and sustainable hydrogen energy framework in the future.Currently,noble metal-based catalysts exhibit excellent catalytic activity for electrolytic water,but suffer from shortage of resources,high price and poor stability.Therefore,the development of inexpensive and efficient non-precious metal-based catalysts is the key to realize the widespread application of electrolytic water hydrogen production.In recent years,transition metal/carbon nanocomposites have received wide attention due to the advantages of low raw material cost and good electrical conductivity.However,it is still a great challenge to synthesize high-quality transition metal/carbon nanocomposites in a controlled manner to achieve efficient electrocatalytic hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）under alkaline environment so as to meet the needs of industrialization.In this thesis,three-dimensional Ni/C and Ni Co Fe/C nanocomposites were prepared by one-step solid-state pyrolysis using supramolecular layered transition metal hydroxides as precursors and used for electrolytic water catalytic reactions（HER,OER and overall water splitting）in alkaline environment.And were characterized in detail to explore the correlation between their compositional structures and electrolytic water performance.The specific studies are as follows:（1）Three-dimensional layered nickel hydroxides（Ni-Sal LMHs）with salicylate intercalation were synthesized by co-precipitation using inexpensive water-soluble organic and inorganic salts as raw materials,and nickel/carbon nanocomposites（Ni/C）were prepared by pyrolysis using them as single precursors.The samples were investigated using XRD,TEM,FT-IR,XPS and other characterization methods.The results showed that:salicylate released reducing gas during the pyrolysis process to form abundant mesopores,which achieved the reduction of interlayer metals,inhibition of metal ion aggregation and functionalization of graphitic carbon surface in one step.The Ni/C nanocomposites maintained the three-dimensional morphology after roasting,and had a large specific surface area(112.69 m²·g^-1).Ni NPs on the surface of the samples are 10-15 nm in size,with high purity and high dispersion.The electrocatalytic test results show that Ni/C-500 has excellent HER catalytic activity and stability in alkaline media:The overpotential required to reach a current density of 10 m A·cm^-2in 1 M KOH electrolyte is 90 m V,Tafel slope is119 m V·dec^-1,and it has a charge transfer resistance of 6.72Ωand exhibits a long-term stability of 60 h,catalytic activity comparable to commercial Pt/C.Combined with material characterization and performance tests,the outstanding HER activity of Ni/C is attributed to high electrical conductivity,strong interfacial interactions,large specific surface area,excellent hydrophilicity and good adsorption of electrolytic water reaction species and intermediates in alkaline media.（2）Further,ternary nickel-cobalt-iron alloy carbon（Ni Co Fe/C）nanocomposites with different metal ratios were prepared by modulating the composition and ratio of metals.The characterization results showed that:Co and Fe successfully entered the precursor laminate structure,and no single metal phase existed in the product Ni Co Fe/C.The material still maintained a three-dimensional morphology with a large specific surface area（159.67m²·g-1）.The formed alloy nanoparticles were uniformly distributed with high purity and high dispersion;In addition,Ni Co Fe/C nanocomposites have good wettability,strong interfacial interactions and a large number of C-H,C-OH and C-O-C functional groups.Electrocatalytic tests showed that the metal ratio has a significant effect on the electrocatalytic performance of the material;The Ni Co Fe/C-500 obtained from the metal Ni Co Fe ratio of 4.8:1.2:1 has excellent bifunctional activity of both HER and OER,with overpotentials of129 m V and 329 m V,respectively,at a current density of 10 m A·cm^-2,both of which can work continuously for 40 h;Further,Ni Co Fe/C-500 was tested as a bifunctional catalyst for total hydrolysis,and a current density of 10 m A·cm^-2was achieved at a low overpotential of 1.53 V,which was lower than that of Ru O₂|Pt/C（1.6 V）under the same conditions,and a stability test of 60 h could be performed.The results show that the multi-active sites formed by the interaction between ternary transition metal alloys and carbon synergistically promote the electrocatalytic reaction,resulting in excellent simultaneous HER and OER bifunctional activity and overall water splitting performance.In summary,in this paper,three-dimensional Ni/C and Ni Co Fe/C nanocomposites with low cost,high dispersion,high purity and high crystallinity were controllably synthesized by one-step solid-state pyrolysis using supramolecular layered metal hydroxides as a single precursor without additional carbon source,reducing agent,structural guiding agent and special equipment.Among them,the monolithic Ni/C has HER performance comparable to that of the noble metal Pt/C;Ternary Ni Co Fe/C not only has excellent bifunctional activity of HER and OER at the same time,but also exhibits better overall water splitting performance than Ru O₂|Pt/C,which has potential applications in the field of alkaline electrolytic water.