Preparation Of Ni-and Fe-based Catalysts And Electrocatalysis For Urea And Water Decomposition

With the shortage of fossil fuels and the aggravation of environmental pollution,people pay more and more attention to the development of new energy sources.Among them,urea fuel cell and electrochemical hydrolysis hydrogen production technology have attracted extensive research interest due to their unique advantages.However,the slow kinetics of anode reaction greatly hindered the rapid development of these two technologies.In a urea fuel cell,the anode releases energy by oxidizing urea molecules to CO₂,N₂,and H₂O through a complex 6e^-process,while the cathode releases hydrogen through reduction reaction.In the water electrolysis device,the anode oxidizes H₂O molecules to O₂ through 4e^-process,while the cathode reduces H₂O molecules to hydrogen.It can be seen that the development of urea fuel cell not only facilitates life,but also provides an effective method for the decomposition of urea produced in life and industrialization.And the product of electrochemical water decomposition is H₂ and O₂ with the environment pollution-free,and hydrogen is a kind of high energy density fuel(120 MJ kg^-1,three times for traditional gas),so the development of urea oxide fuel cells and electrochemical water decomposition technique to realize national advocated the construction of the homeland have the great significance.The development of anodic catalyst has become the bottleneck for the rapid development of these two technologies,a large number of studies have shown that noble metal catalysts（such as Ru-,pt-base,etc.）have very high electrocatalytic performance for urea oxidation and water decomposition.However,the scarcity and high price of precious metals greatly limit their scale application.Therefore,it is an urgent problem to search non-noble metal alternative catalysts with high catalytic performance,abundant reserves and low cost to develop the urea oxidation electrochemical batteries and electrochemical water decomposition technology.The electro-oxidation reaction of urea provides fuel energy directly at the anode and releases hydrogen at the cathode during the electrolysis process;Urea oxidation also provides an effective method for the urea decomposition produced by life and industrialization.The products of water decomposition are H₂ and O₂,which are pollution-free to the environment.Besides,hydrogen energy is a fuel with high specific energy density,so urea oxidation and water decomposition have become a promising new energy source due to their high energy efficiency and clean harmlessness.Studies have shown that noble metals catalysts such as ruthenium and platinum group have high catalytic performance for urea oxidation and water decomposition.However,high cost and shortage limit their large-scale applications.Therefore,it is an urgent problem for people to find low-cost and abundant catalysts on earth for urea oxidation and water decomposition.In this paper,the preparation of NiCr₂O₄-CNTs、β-Ni（OH）₂-CNTs and Fe₉-Co₁/NF three kinds of catalyst,using scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray powder diffraction（XRD）,X-ray spectrometer（EDS）and X-ray photoelectron spectroscopy（XPS）and so on technologies through physical and chemical methods to demonstrate the catalyst morphology,structure and chemical valence;and the electrocatalytic properties of urea oxidation（DUFC）and water decomposition（OER）were studied under alkaline conditions.The research work includes the research progress of anode catalytic materials for DUFC and OER and experiments:First part:The advantages and disadvantages of DUFC and OER,working principles and reaction mechanism in alkaline media are summarized,and the classification of anode catalysts and carriers are briefly described.At the same time,the purpose and content of this paper are also described.Second part:As the experiment contents of the article,the first part describes temperature effect on crystallinity and chemical states of nickel hydroxide as alternative superior catalyst for urea electrooxidation;the second part explores the hierarchical NiCr₂O₄-CNTs composites formed by different Ni/Cr contents for urea electrooxidation;the third part researchs different microstructure of Fe-Co composites and its application in electrocatalytic water decomposition,the three research contents are as follows:（1）、This work represents a simple one-step hydrothermal method using Ni（NO₃）₂×6H₂O as nickel source and CNTs as carrier to synthesize a series of Ni（OH）₂-CNTs composites with lamellar structures under different temperature conditions,where theβ-Ni（OH）₂-CNTs are formed in the range of 20 to 140°C to study the effect of temperature for properties and structure.XRD shows that the crystal structures of Ni（OH）₂ synthesized at 20,80 and 140°C are perfectly consistent with the standard model of hexagonalβ-Ni（OH）₂（JCPDS:14-0117）,XPS results show that the lamellar structures ofβ-Ni（OH）₂-CNTs（80°C）catalyst contains ca.54.2%Ni（III）species in the hybrid catalyst;The electrochemical studies find that theβ-Ni（OH）₂-CNTs（80°C）catalyst exhibits the largest ESA value(105.5 m² g^-1),highest catalytic activity(98.5 mA cm^-2)and stability for urea electro-oxidation.The excellent performance ofβ-Ni（OH）₂-CNTs（80°C）catalyst is mainly contributed to the unique lamellar structures,high content of Ni（III）species and uniform distributed ofβ-Ni（OH）₂2 on CNTs support in the hybrid catalyst.（2）、NiCr₂O₄-CNTs（1/2）were synthesized by simple hydrothermal method with NiCl₂×6H₂O,CrCl₃×6H₂O)as nickel and chromium sources and CNTs as carrier.For better comparison,NiCr₂O₄-CNTs catalysts with Ni/Cr molar ratios of 1:1,1:3,2:1 and 3:1 were prepared which the total weight of the metal ions is fixed.The ICP result indicates that the molar ratio of Ni/Cr was consistent with the theoretical value;XRD shows the crystal structure of NiCr₂O₄-CNTs（1/2）is consistent with the standard model of NiCr₂O₄（JCPDS:23-1271）;XPS demonstrate that the binding energies of Ni 2p_3/2/2 of NiCr₂O₄-CNTs（1/2）exhibits a negative shift 0.25 eV,Cr 2p_3/2exhibits positive shift 0.18 eV,suggesting electron density shift from Cr to Ni center of NiCr₂O₄-CNTs（1/2）;The electrochemical studies show that the NiCr₂O₄-CNTs（1/2）catalyst exhibits the largest ESA value(50.7 m² g^-1),highest catalytic activity(115.6 mA cm^-2)and permanent stability for urea electro-oxidation.The prominent performance of NiCr₂O₄-CNTs（1/2）catalyst is mainly due to the electron density shift from Cr to Ni center,which having synergistic effects and enhancing catalytic performance.（3）、Using FeSO₄×7H₂O and Co（NO₃）₂×6H₂O as Iron and cobalt sources,nickel foam（NF）as carrier,the bimetallic Fe_x-Co_y/NF catalyst was prepared by electrodeposition method.XRD results show that Fe and Co species of Fe₉-Co₁/NF were Fe（OH）₃ and Co（OH）₂,which were consistent with the standard Fe（OH）₃（JCPDS:38-0032）and Co（OH）₂（JCPDS:30-0443）.SEM and XPS results indicate that Fe₉-Co₁/NF has unique structure liking velvet flowers and higher content ratio of Co（III）（ca.41.9%）in CoOOH and Fe（III）（ca.72.1%）in FeOOH.The electrocatalytic activity find that Fe₉-Co₁/NF demonstrate much smaller overpotential(222 mV at 50 mA cm^-2),lower Tafel slope(43.0 mV dec^-1),and remarkable long-term stability for running 12 h with 20 mA cm^-22 and 50 mA cm^-2,respectively.In two electrode device,the potential is 1.54 V to deliver the current density with 20 mA cm^-22 in water splitting.The outstanding catalytic performance of Fe₉-Co₁/NF could attribute to the unique morphology,synergistic effect between Co（III）and Fe（III）,good electrical conductivity and exposed rich active sites,which are beneficial superior performance for OER.