The Design,Fabrication Of 3D Hierarchically Nanostructured Nickel-based Electrocatalysts And Study Of Their Catalytic Properties For Hydrazine Electrooxidation

Fuel cells（FCs）is an efficient and clean power generation technologies with broad application prospects.Among various types of FCs,direct hydrazine fuel cell（DHFC）has received considerable attention thanks to its intriguing features of high standard equilibrium cell voltage,high energy density,high efficiencies,strong environment affinity,and more importantly,the allows for the use of non-noble transition metals as anode electrocatalysts.At present,the development of anode electrocatalysts with high activity,selectivity,stability and low material cost is one of the core tasks for the development of DHFC technology.With years of unceasing efforts,a serious of non-noble metal alloys or compounds have been successfully developed as high performance electrocatalysts for hydrazine oxidation reaction（Hz OR）by optimizing the active phase components to improve its intrinsic activity,and utilizing the nanostructure engineering strategy to maximize the number of active sites and enhance the mass transfer characteristics of the catalyst.But restricted by the factors of lacking efficient mechanism investigation and understanding of the hydrazine electrooxidation reaction processes,the construction of catalytic materials are still the results of empirical attempts,and often associated with problems of low catalytic activity and poor durability.In view of this research status,this thesis focuses on the anodic hydrazine oxidation reaction of DHFC,with the aims of developing anode electrocatalysts with low cost and high-performance for DHFC.By regulating the components and optimizing the structures of the Ni-based materials,several nickel-based alloys and compounds with high activity and good stability towards Hz OR were prepared by simple and feasible methods.On the basis of the characterized phase/structure and evaluated catalytic performance of the catalysts,the correlation between their composition/structure and properties were preliminarily explored.The obtained research results are as follows:（1）A series of Ni foam（NF）supported Ni–Co alloy catalysts（Ni–Co/NF）with hierarchical nanostructure were prepared using a hydrothermal method combined with two-step calcination.The thus-prepared Ni–Co/NF catalyst exhibited extraordinarily high activity and good durability as well as nearly 100%selectivity toward the alkaline Hz OR following a four-electron pathway under ambient conditions,it possessed a current density of1213 m A cm^-2 under 0.3 V（vs.RHE）,a retention rate of 82.7%after 1000 CV cycles,outperforming most of the existing anode electrocatalysts of DHFCs.The results of structural characterization and theoretical calculations revealed that,the exceptionally high performance of the Ni–Co/NF towards Hz OR was contributed by the merits of improved intrinsic activity of Ni–Co alloy,the 3D hierarchical structure with abundant exposed active sites and facilitated mass transfer,as well as the good electrical conductivity of the Ni-Co alloy.This work offers a platform for the rational design and synthesis of highly efficient and cost-effective self-supported electrocatalysts as potential anode electrode for DHFC application.（2）On the basis of previous work on Ni-Co alloy,a series of NF supported bimetallic phosphides（（Ni,Co）x P）with varied chemical compositions and similar urchin-like morphology were fabricated using a hydrothermal method followed by reductive calcination and further phosphatization treatments.The effects of Ni/Co molar ratio on the phase structure and the catalytic performance of the（Ni,Co）x P catalysts towards Hz OR were systematically studied.It was found that the Ni Co P/NF catalyst with Ni/Co molar ratio of 1:1 had the best performance,it exhibited a current density as high as 1511 m A cm^-2 at 0.30 V（vs.RHE）,which reached the top level of Hz OR catalyst in terms of area-specific activity.Besides,it also exhibited excellent stability with a current decay of only 3%after 100 h constant current(10 m A cm^-2)test.But meanwhile,the（Ni,Co）x P catalysts were found with non-faradaic hydrazine decomposition activity,and the decomposition rate was positively correlated with the Co content in a certain range.This work revealed the correlation between composition,phase structure and catalytic performance of（Ni,Co）x P in a wide composition range,which has important reference value for the development of metal phosphides as electrocatalysts for Hz OR.（3）Building on the work of the previous chapter,the experimental study was conducted with the focus on the phase structure evolution of nickel phosphides（Nix P）and the correlation between phase structure and catalytic performance.A series of Nix P/NF catalyst with nanoflower-like morphology was prepared by a simple hydrothermal method combined with a phosphatization treatment.The effects of phosphating temperature and the amount of phosphorus source on the phase structure of phosphatization product were preliminarily explored,and besides,the controllable synthesis of Ni2P,Ni5P4 and Ni P2 phases was achieved by carefully regulating the phosphatization parameters.It was found that the intrinsic activity and electrical conductivity of Nix P for Hz OR was increased with the increase of Ni/P ratio.The best performing Ni2P/NF catalyst showed a current density of 1038 m A cm^-2 at 0.30 V（vs.RHE）;possessed an activity retention rate of 92.7%after 1000 CV cycles;exhibited a potential increment of 16 m V after 120-hour constant current(10 m A cm^-2)test;besides,Ni2P catalyst possessed a nearly 100%faradaic efficiency for Hz OR via four-electron pathway,and basically demonstrates no catalytic activity for the non-faradaic decomposition of hydrazine.（4）Based on the research of Ni-based alloy and phosphides,the research object is further expanded to transition metal nitrides.Nanoflower-like Ni-Fe bimetallic nitride catalysts（（Ni,Fe）x N/NF）with different chemical compositions were constructed by a hydrothermal method combined with nitridation treatment.It was found that the feed ratio of Ni/Fe in the hydrothermal process had an important influence on the phase structure of the target catalysts and their intrinsic catalytic performance toward Hz OR.Among them,the Ni2Fe2N/NF catalyst with a Ni/Fe molar ratio of 1:1 had the best Hz OR performance.It possessed a current density of 1017 m A cm^-2 at 0.3 V（vs.RHE）,which was superior to the reported transition metal nitride catalysts for Hz OR.At the same time,Ni2Fe2N/NF also has good stability and selectivity,it presented an activity retention of 90.8%after 1000 CV cycles,possessed a nearly 100%selectivity for the faradaic hydrazine decomposition following four-electron pathway,and what’s more,Ni2Fe2N can effectively inhibit the non-faradaic decomposition of hydrazine when compared with previously studied Ni3N and Co3N.This work reveals the potential of bimetallic nitrides as high-performance Hz OR electrocatalysts,and has certain value for further expanding the research scope of Hz OR electrocatalysts.