Controled Synthesis And Electrocatalytic Applications Of Nickel-based Nanomaterials

In recent years,the global fossil energy crisis has become increasingly serious,while the emission level of atmospheric pollutants and greenhouse gases have continued to rise.It becomes very urgent and important to reduce the utilization of fossil fuel and explore clean and sustainable energy.Hydrogen energy has received wide attention from industry and research field owing to its outstanding advantages of green,zero carbon emission and high energy density.This most promising energy utilization pattern involves that the hydrogen is first produced via water splitting using clean electricity obtained from solar,wind and tidal sources as well as the abandoned electricity from power grid,which can be then transferred into fuel cell device for daily or industrial energy demands.However,there are many barriers to be overcome before practical application for these application fields.Among these,the prominent factor is the design and development for non-precious metal electrocatalysts with low cost,high activity and excellent durability,particularly for the half-reactions of water oxidation（OER）and hydrogen oxidation（HOR）.Therefore,how to design efficient non-precious metal catalysts to improve the two oxidation reaction rates and replace the commercial precious metal catalysts is the key scientific issues,which possesses great scientific significances and practical values.This dissertation focuses on the design and synthesis of Ni-based transition metal nanomaterials and their electrocatalytic performance for alkaline water/hydrogen oxidation reactions.On the basis of theoretical guidances,we developed a series of synthesis strategies for Ni-based nanomaterials（oxides,alloys and heterojunction nanomaterials）,and achieved large-scale preparation of Ni-based nanomaterial electrocatalysts with the goal of practicality.Multiple adjustments have been made in the electrocatalytic performance,achieving high activity and stability in water/hydrogen electrocatalytic oxidation.Meanwhile,combined with advanced technologies such as operando Raman,synchrotron radiation measurements and density functional theory（DFT）calculations,we deeply investigated the electronic structure and active sites of our studied catalysts and analyzed the the mechanism of their high activity and stability.The main findings are summarized as follows:1.We developed a facile strategy for scale-up synthesis of amorphous multimetallic oxides that facilitates the alkaline OER efficiently.These novel amorphous Ni Fe Mo oxides can be obtained by rapid mixing of supersaturated metal precursor solutions at room temperature.This method is fast and easy to scale up,yielding up to 515 g in one batch.In addition,the amorphous catalyst exhibits superior OER elecrocatalysis than crystalline counterpart.The operando Raman characterization confirms that the amorphous Ni Fe Mo catalyst goes through faster surface reconstruction to form Ni Fe OOH layer with rich O-vacancies which acts as actual active sites for OER.Theoretical calculations show that the generation of oxygen vacancies can optimize the adsorption of intermediate species,reduce energy barrier,and thus improve the catalytic activity.This work deeply explores the scale-up synthesis and structural evolution of amorphous nanomaterials,which is expected to promote the further development of related research.2.We developed a class of nickel-based alloy catalysts with highly efficient alkaline HOR activity.First,microwave heating is used to quickly synthesize the precursors of the molybdenum/tungsten-doped nickel hydroxide nanosheets,and then the prepared precursors are reduced under a hydrogen/argon atmosphere to obtain a series of nickel-based alloy materials.Mo Ni₄ and WNi₄ alloys we prepared have excellent alkaline HOR activity,surpassing commercial Pt/C for the first time,and superior to all reported non-precious metal catalysts.In addition,the catalyst has very robust long-term stability and CO tolerance,which opens up a new direction for the development of non-noble metal anodes for alkaline fuel cell.Through ultraviolet photoelectron spectroscopy,CO stripping experiments and DFT calculations,we attribute this remarkable HOR reactivity to a synergistic effect between nickel and molybdenum/tungsten that offers optimized adsorption of hydrogen and hydroxyl intermediates.Meanwhile,the work provides an effective reference for the design and optimization of catalytic active sites,and this alloy optimization strategy is expected to expand to other application areas.3.We developed a nickel-based heterojunction catalyst with efficient and durable performance for alkaline HOR.Through hydrothermal synthesis and subsequent calcination,we prepared Ni/V₂O₃ heterojunction catalyst on a three-dimensional substrate.Compared to single Ni and V₂O₃,the HOR activity and stability have been greatly improved on the novel heterojunction catalyst with rich Ni/V₂O₃ interfaces.DFT calculations show that the sites near interface can weaken hydrogen adsorption which is too strong for Ni metal,and meanwhile increase the adsorption of hydroxyl groups,jointly facilitating the HOR process.In addition to its high activity,the catalyst also exhibits ultra-high stability.After 16 hours of uninterrupted testing,its activity has only decreased by 1.5%.Through in-depth characterization after stability testing,we find that the Ni sites away from the Ni/V₂O₃ interface are easier to be oxidized,which is consistent with the observation on Ni metal;Obviously,Ni sites adjacent to the interface still remain at low valence state.This shows that the interface structure can not only enhance the alkaline HOR performance of Ni-based catalysts,but also stabilize the metallic valence state of Ni and thus improve the catalytic stability.This interface optimization strategy provides a novel idea for the structural design of highly active and highly stable catalysts.