Metal Organic Frameworks Derived Nickel,Iron And Carbon Based Nanomaterials For Electrocatalysis

Developing clean,efficient and sustainable energy is of significant importance to address the ever-increasing energy crisis and environmental pollution in the present-day world.Hydrogen energy has been reported as an ideal renewable energy source due to the advantages of abundance,clean,high efficiency,high energy density and environmental friendliness.The production and utilization of hydrogen energy are crucial for alleviating energy and environmental issues and have become a research hotspot.Electrolysis devices and hydrogen-oxygen fuel cells have attracted widespread attention due to the unique advantages and application prospects in the production and utilization of hydrogen.However,the sluggish kinetics of electrocatalytic reactions including oxygen evolution reaction?OER?,hydrogen evolution reaction?HER?,and oxygen reduction reaction?ORR?is one of the important bottlenecks for the development of these energy devices.Precious metals-based electrocatalysts exhibit excellent electrocatalytic activities,but the high cost and limited reserves hinder their large-scale commercialization.Exploring low-cost,earth-abundant,and high-efficient non-noble metal-based catalysts to replace precious metals has become one of the research hotspots in energy conversion field.Metal-organic frameworks?MOFs?,as an emerging inorganic-organic hybrid functional material,possess unparalleled advantages of traditional inorganic materials,such as large surface area,high porosity,diverse structures and easy functionalization,showing a broader application prospects in energy storage and conversion fields.In this thesis,based on the advantages of nickel and iron based Prussian blue analogues and ZIF-8,heteroatom incorporated Ni and Fe based nanocubes,Ni and Fe based phosphides,sulfide and selenide,as well as three dimensional nitrogen doped hierarchical porous carbon were synthesized by simply tuning the composition and morphology to improve electrocatalytic activities of the catalysts.The formation mechanism of the catalysts and the relationship between the structure and electrochemical performances were discussed preliminarily,providing new research ideas for constructing electrocatalysts with MOFs as precursors.The main achievements of the dissertation were as follows:?1?Heteroatom incorporated amorphous NiFe-based porous nanocubes?Ni-Fe-O-P,Ni-Fe-O-B,Ni-Fe-O-S?were successfully synthesized via a simple one-step calcination of Ni-Fe based Prussian blue analogues and heteroatom containing small molecules.The resulting three-dimensional?3D?porous cubes and amorphous structure effectively increase the number of active sites and promote fast mass transfer.Heteroatom incorporation can tune the electronic structure,which may lead to the formation of the higher average oxidative valence of Ni species as active sites during OER process.Compared with Ni-Fe-O materials,the resulting three catalysts all exhibit excellent OER electrocatalytic activities.?2?Porous structured nickel and iron based phosphides nanocubes were synthesized through phosphidation of Ni-Fe based Prussian blue analogue with sodium hypophosphite at low temperature.The catalysts prepared at the suitable phosphidation temperature inherit the nanocube morphology of the precursors with rougher and porous features on the surface,effectively increasing the contact area between the catalysts and electrolyte and exposing more active sites.Electrochemical measurements demonstrate that Ni-Fe-P-350 catalysts exhibit excellent electrocatalytic activities for HER and OER,and also show good activities and stability for two-electrode overall water splitting.?3?In order to further improve the electrocatalytic activity,nickel-iron based Prussian blue analogues were grown on nickel foam by in-situ chemical etching and deposition,which were then utilized for the synthesis of nickel foam supported nickel-iron phosphide nanocatalysts via low-temperature phosphidation with sodium hypophosphite.The interconnected nanosheet structure of Ni-Fe-P and macroporous network skeleton of conductive nickel foam effectively promote the adequate contact between catalysts and electrolyte,accelerate electron transport during electrocatalytic process,and slow down the abscission of the catalysts during long-term stability test.The obtained Ni-Fe-P/NF catalysts exhibit excellent catalytic activities toward HER and OER.When used in two-electrode water electrolysis devices,Ni-Fe-P/NF shows better electrocatalytic activity and stability than precious-metal based catalysts.?4?Based on the easy functionalization merits of MOFs,3D hierarchical porous sea-urchin-like Ni-Fe-S nanocomposites were synthesized by a facial hydrothermal treatment of nickel-iron based Prussian blue analogues and sulfur powder with post-calcination.Experimental results show that the hydrothermal temperature and time,the mass ratio of precursors as well as hydrazine hydrate have crucial effects on the morphology and composition of the catalysts.The 3D sea-urchin-like morphology and hierarchical porous structure of the prepared Ni-Fe-S_3:1-160 catalysts effectively shorten the ion diffusion distance and provide more active sites for electrocatalytic reaction.Besides,the formed carbon layer effectively prevents the aggregation of the nanosulfides and improves the electronic conductivity and electrocatalytic stability of the catalysts.Ni-Fe-S_3:1-160 exhibits excellent electrocatalytic activities and stability for OER.The Ni-Fe-Se catalysts were also prepared with the same synthesis method except replacing sulfur powder with selenium powder,and showed excellent OER electrocatalytic performance.?5?3D nitrogen doped hierarchical porous carbon materials were synthesized through a NaCl-assisted approach to change the carbonization behavior of ZIF-8 during pyrolysis process.The structure and morphology of the products are easily modulated by controlling the NaCl/ZIF-8 mass ratio and the calcination temperature.The prepared NHPC_1:3-900under the optimal conditions exhibits 3D structure with a large number of thin nanosheets interweaved together,and possesses large surface area with abundant micro-,meso-,and macro-porous,and an appropriate amount of pyridine N and graphitized N,which are favorable for enhancing the ORR electrocatalytic activities.NHPC_1:3-900 shows better ORR catalytic activities,stability and methanol resistance than Pt/C.Besides,The obtained composites by mixing NHPC_1:3-900 and Ni-Fe-O-P prepared in the second chapter exhibit excellent ORR and OER bifunctional catalytic activity and stability,and also show high peak power density and excellent discharging/charging stability when served as oxygen electrode in zinc-air battery.