Design And Preparation Of High-efficiency Nickel-based Electrocatalytic Materials And Their Urea Oxidation Performance

Efficient and sustainable fuel production guarantees clean energy utilisation and its integration with energy storage and transport infrastructure,overcoming the intermittent nature of solar and wind energy and providing high energy density fuel for transportation.Hydrogen energy is a green and clean secondary renewable energy with great potential to become the primary fuel for a low-carbon economy.Hydrogen can only be obtained from hydrogen-containing water,coal,natural gas and other compounds.Among the existing hydrogen production technologies,hydrogen production by electrolysis of water has the advantages of simple operation,high product purity and no pollution,and a key idea for creating efficient and sustainable fuels.However,in the four-electron-proton coupling process of electrolysed water,the kinetics of oxygen evolution reaction(OER)is sluggish,resulting in large-scale hydrogen production requiring large consumption of electrical energy.It significantly limits the further development of electrolysis technology.The thermodynamic potential of Urea Oxidation Reaction(UOR)is 0.37 V,which is lower than the 1.23 V required for OER.Replacing the sluggish OER with a more thermodynamically favourable UOR is an effective strategy to reduce electrical energy consumption and obtain hydrogen energy more efficiently and economically.Simultaneously,the utilisation of urea and other nitrogen-containing products originated from agricultural wastewater and industrial production to produce chemical fuels will help improve environmental pollution.However,the current research on the catalytic reaction mechanism is not yet well-understood,limiting the further application of the urea oxidation reaction.This article intends to take nickel-based oxides as the research object and deeply study the mechanism of urea oxidation reaction from three aspects:crystal structure,electronic structure,and surface dynamics.The specific research content is as follows:(1)Preparation and performance of NiMoO₃S electro-catalyst:It has reported that the catalyst performance could be beneficially boosted by enhanced metal-oxygen covalency.In the NiMoO₄ catalyst,the Ni oxidation state could be effectively improved by the highly electronegative Mo⁶⁺,but it will reduce the covalency of Ni-O.Therefore,through the one-step vapour deposition method,the oxygen sites were partially replaced by S on the NiMoO₄/NF precursor,and a regular-shaped NiMoO₃S/NF nanorod array was obtained.Density functional theory calculations(DFT)and experimental results showed the optimised electronic structure of the electro-catalyst,the improved covalency of Ni-O,and the increased adsorption energy of the active sites.The NiMoO₃S/NF can achieve 10 m A·cm^-2 at a low potential of 1.340 V,and the Tafel slope is only 41.18 m V·dec^-1.After 100 hours of continuous operation at 10 m A·cm^-2,the catalyst still showed excellent catalytic performance.Kinetic experiments were conducted to investigate the difference in the Ni²⁺/Ni³⁺redox capacity of different samples.The weighted central of redox peaks has been used as the descriptor of the Ni²⁺/Ni³⁺redox capacity for better understanding the relationship between catalyst structure and catalytic activity.(2)Preparation and performance study of Li Ni O₂ electro-catalysts:It is an effective strategy to adjust the electronic structure of Ni sites to improve UOR performance,but studies have shown that the CO₂ desorption occurs on the O position adjacent to Ni.In this section,Li Ni O₂,composed of Ni O₆ octahedral layer and Li O₆ octahedral layer connected by oxygen atoms,is taken as the research object.The lattice stress engineering is induced by Li vacancies.TEM results showed the(003)spacing related to the interlayer spacing changes significantly,that is,the signal of lattice strain.The results of Rietveld refinement showed that the presence of lattice stresses acting significantly on the oxygen atoms.Combined with cyclic voltammetry,characterisations and DTF,it could be found that the highly oxidised Ni⁴⁺species were induced by enhanced Ni-O covalency,and the oxygen sites could be activated by the lattice stress.The overlap of tetravalent nickel with a non-binding oxygen band offered extra electrons,resulting in enhancing the kinetics.We further discussed the conventional adsorbate evolution mechanism and the lattice oxygen involved mechanism thoroughly.The results indicated that the optimized catalyst has lower barriers of proton desorption.Both catalysts spontaneously form carbon dioxide,indicating the LOM is the optimal pathway.Lastly,Electrochemical tests results showed that the catalytic performance could be improved by more than 7 times and the Tafel slope is only 37.84 m V·dec^-1,indicating excellent catalytic UOR performance.