Study On Metal-Organic Frameworks Derived Hierarchically Porous Transition Metal Phosphides And Their Electrocatalytic Performance

With the excessive consumption of carbon-based fuels and aggravation of environmental problems,it is necessary to develop sustainable and regenerative alternative energy sources.Electrochemical water splitting is a promising technology to produce hydrogen,which is a high energy density,renewable,secure,and environmentally friendly alternative for fossil fuels.However,the two core half-reactions,oxygen evolution reaction?OER?and hydrogen evolution reaction?HER?,need efficient electrocatalysts to expedite sluggish processes.To date,the state-of-the-art electrocatalysts are ruthenium/iridium-based oxides and platinum-based noble-metal electrocatalysts for OER and HER,respectively,but high cost and scarce earth abundance of noble-metals greatly restrict its large-scale applications.Therefore,it is of great significance to develop efficient OER and HER electrocatalysts with earth-abundant materials,such as transition-metal phosphides,sulfides,oxides,carbides,nitrides,and many other transition metal compounds.Among them,transition metal phosphides?TMPs?have been considered as promising alternatives to replace noble-metal electrocatalysts due to their unique charged natures,high catalytic activity,and earth abundance.In this thesis,a series of single and bimetallic metal–organic frameworks?MOFs?hybrid materials with uniform morphology and size have been synthesized.Then the MOFs-based TMPs with perfect dispersity and high Brunauer-Emmett-Teller?BET?surface area are prepared via a facile phosphorization strategy.Furthermore,a microfluidic flow reaction system is used for ultrafast and continuous synthesis of nitrogen-doped phosphonate MOFs CUP-1-Ni for the first time and then the MOFs derived nitrogen,phosphorus-codoped hierarchically porous Ni₂P nanocrystals?Ni₂P NPHPN?are obtained by one-step pyrolysis.The Ni₂P NPHPN exhibit excellent electrocatalytic performance and is comparable to the noble-metal electrocatalysts.Moreover,the synthesis could lead to gram-scale preparation of the electrocatalysts without obvious variation of catalytic performance.These results not only presents important guidelines for the rational design of multicomponent doping transition metal phosphides electrocatalyst but also highlight the proof-of-concept of the practical application of transition metal phosphides.Firstly,using MOF-74-Ni as the precursor,we synthesized a regular three-dimensional hierarchically porous nickel phosphide polyhedron with a single phase of Ni₂P?Ni₂P polyhedron?via an in-situ conversion strategy of a one-step calcination at low temperature.The Ni₂P polyhedron well inherited the regular morphology and porosity of MOF-74-Ni,which had a high BET surface area and hierarchical pores properties.Besides,benefiting from the high metal contents and uniform distribution of the metal center in MOF-74-Ni morphology,the as-prepared Ni₂P polyhedron exhibited perfect dispersity and the active sites were highly exposed.With all these properties,the Ni₂P polyhedron showed excellent electrocatalytic performances and durability for HER.Secondly,the MOF-74-Ni/graphene oxide with a sandwich-type structure was synthesized via a facile one-step room-temperature reaction and the Ni₂P/rGO with ultrasmall particle sizes?average about 2.6 nm?was obtained via an in-situ conversion strategy of a one-step calcination at low temperature.Due to the specific interfacial effect between MOFs and graphene oxide,the carbon/graphene surface tends to form a homogeneous carbon during the phosphorization calcinations,which could connect into a carbon network and function as an electron highway to obtain satisfactory electronic conductivity and enhanced charge-transfer efficiency.As a result,the Ni₂P/rGO exhibited excellent catalytic performance for both HER and OER in alkaline.And the electrolyzer employing Ni₂P/rGO/NF as a bifunctional catalyst in both the cathode and anode generated 10 mA cm^-2 at a voltage of 1.61 V with excellent stability,which is comparable to the integrated Pt/C and RuO₂ counterparts.Then,due to the unique advantage of MOF-74 that coordinative metal sites can be replaced without influencing the underlying framework structure of the MOFs crystals,a series of nickel–cobalt mixed metal phosphide nanotubes were synthesized.The incorporation heteroatom can significantly enhance their electrocatalytic performance due to a synergetic effect,such as an improvement in charge transfer between the dopants and the host metal atoms and the optimization of their electronic structures to reduce the kinetic energy barriers during the catalytic processes.Benefiting from the uniform distribution of metal center,the obtained Co4Ni1P nanotubes exhibited remarkable catalytic performance for both OER and HER in an alkaline electrolyte,and the electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalysts in alkaline solutions achieved a current density of 10 mA cm^-2 at a voltage of 1.59 V,which is superior to the integrated Pt/C and RuO₂ counterparts.Finall,a microfluidic flow reaction system is used for ultrafast and continuous synthesis of nitrogen-doped phosphonate MOFs CUP-1-Ni for the first time and then the metal–organic frameworks derived nitrogen,phosphorus-codoped hierarchically porous Ni₂P nanocrystals?Ni₂P NPHPN?are obtained by one-step pyrolysis.And the controllable synthesis strategy makes it a reality that synthesizing multi-component doping Ni₂P electrocatalyst via simple change the pyrolysis temperature and time of a single MOFs.In addition,in situ technology was used to gain insights into the formation mechanism and the probable mechanism was given.The Ni₂P NPHPN exhibit excellent catalytic performance for both OER and HER in an alkaline electrolyte.Moreover,the synthesis could lead to gram-scale preparation of the electrocatalysts without obvious variation of catalytic performance.It is greatly meaningful for the practical commercialization of transition metal phosphides in large-scale water splitting.