Syntheses And Electrocatalytic Properties Of Fe-based Metal Organic-frameworks Derived Electrocatalysts

As a new type of porous materials,MOFs show many unique advantages such as high specific surface area,adjustable regular pore size structure,and rich metal/ligand combination forms.Meanwhile,MOFs can be used as precursors to prepare a series of derivatives.Carbon-based catalysts or metal compounds can also be obtained through high temperature treatment of MOFs precursors.MOFs derived nanomaterials can expose plenty of active sites and the carbon material obtained after calcination can further increase their conductivity.Hollow nanostructures have a large surface area,reduced mass transfer distance,and increased contact area between electrolyte and electrode.High surface area and open porous channels provide them large accessible active sites for catalytic reactions.The hollow structures can be obtained using traditional template methods.However,many steps are involved in the template methods and the removal of templates is sometimes difficult.MOFs are ideal precursors for deriving hollow nanomaterials.However,it is still a challenge to obtain MOFs-derived hollow structured materials with a simple one-step method.In this thesis,we demonstrate several simple one-step derivation methods using MOFs as precursors,to obtain corresponding electrocatalysts and systematically study their electrocatalytic properties.The main content is listed as follows:1.Hollow FeOOH hexagonal bipyramidal polyhedrons with uniform morphology and size have been fabricated by a simple one-step chemical etching method using MIL-53?Fe?particles as templates.Under mild alkaline conditions,we found that FeOOH was initiallyformed on the surface of MIL-53?Fe?and fixed on the surface of MOF particles,inheriting the morphologies of parent MOFs,and finally the MOFs were etched to hollow FeOOH structures.Considering that FeOOH particles have high catalytic OER activity but poor conductivity,we deposited a layer of dispersive Ni?OH?₂ nanosheets on their surface.The FeOOH/Ni?OH?₂ composites have a hollow honeycomb structure,high specific surface area,and good mass transfer channels,showing excellent electrocatalytic OER activity.The onset potential of these nanocomposites is 1.47 V,an overpotential of 310 mV to reach the current density of 10 mA cm^-2,and a Tafel slope of 70 mV dec^-1.We have employed chronoamperometry method?i-t?to test the stability of this material and no obvious decrease in current density was found under a constant applied potential for 12 h operation.We also studied the roles of FeOOH and Ni?OH?₂ in the composites through a series of control experiments.The results indicate that the enhanced OER performance of FeOOH/Ni?OH?₂ composites is owing to their unique hollow structure and the synergistic effect between FeOOH and Ni?OH?₂.2.We have developed two FeNiP bimetallic phosphides with different morphologies by simply controlling the phosphating temperature using FeNi-MIL-88nanorods as templates.We found that the phosphating reaction begins preferentially on the surface of MOF particles at low temperature.The sequentially formed phosphide gradually diffuses to the surface,leading to the formation of hollow FeNiP nanotube structure due to the kirkendall effect.When the phosphating reaction happens at high temperature,metal ions in MOFs will be rapidly phosphated and form porous metal phosphides locally.We have systematically studied the electrocatalytic properties of two Fe-Ni-P electrocatalysts and found that the crystallinity of particles is the decisive factor to their catalytic properties.Taking the advantage of tunable Fe/Ni ratio in the MOF nanorods,we have also prepared a series of Fe_xNi_y-MIL-88nanorods and derived them into corresponding FexNiy-P electrocatalysts and investigated the effects of Fe/Ni ratio on their electrocatalytic properties.When the Fe/Ni ratio is close to 1:2,the catalysts shows the best OER performance due to the optimal electronic structure of Fe-Ni-P interface.These composites were characterized by SEM,XRD,EDX,TEM and XPS.The electrochemical tests show that porous FeNi₂P exhibit the highest catalytic activity:the OER overpotential is 286mV to reach the current density of 10 mA cm^-2,and the Tafel slope is 70 mV dec^-1.Meanwhile,an overpotential of 170 mV@10 mA cm^-2 is needed for HER using the same materials as cathode.In an electrolytic cell,a small voltage of 1.63 V is able to deliver a current density of 10 mA cm^-22 by employing FeNi₂P coated Ni foams as both cathode and anode,implying the promising application of the prepared Fe-Ni-P electrocatalysts in overall water splitting.