Studies On Active Sites Of Multimetal MOFs And Their Derivatives In Electrocatalysis

Hydrogen-oxygen fuel cells and metal-gas batteries,as the two most popular new energy devices in the current stage,can convert chemical energy in fuel or air into electrical energy with high efficiency and low pollution.However,the source of raw materials and the high over-potential in the reaction process also restrict its application in actual production and life.As the most likely technology at this stage to realize green and pollution-free hydrogen production,electrolysis of water reaction can solve the raw material problem for hydrogen-oxygen fuel cells.However,both the entire process of water electrolysis and the cathode reaction of fuel cells and metal-air batteries face significant energy barriers in terms of kinetics,resulting in a decrease in energy utilization efficiency.Therefore,it is necessary to develop highperformance electrocatalysts to reduce the overpotential of the reaction.As a new type of porous material,metal organic frameworks(MOFs)have attracted wide attention due to its variable structure,easily adjustable composition and clear topological structure.When the second or more other metals are introduced into the MOFs structure,its physical and chemical properties can be obviously changed.Based on this,this work focused on the research of multi-metal MOFs and its derivatives,prepared a series of multi-metal MOFs and its derivatives,and deeply explored its source of activity and catalytic mechanism in the electrocatalytic process.1.Considering that the frame structure can provide higher active specific surface area and higher mass transfer efficiency,at the same time,the multi-metal component can also provide the material with multi-functional properties.In chapter two,by using Co-Fe PBA as a template,a simple method is used to prepare the hollow 3D frame of Co-Fe bimetallic phosphide,which serves as an excellent catalyst for electrochemical water splitting.For HER,the Co0.6Fe0.4P-1.125 nanometer frame requires only 97 mV of overpotential to achieve a current density of 10 mA cm-2 in 0.5 M H2SO4 and 133 mV for 10 mA cm-2 in 1.0 M KOH.For OER,the overpotential required to reach 10 mA cm-2 in alkaline medium is only 297 mV.When the Co0.6Fe0.4P-1.125 nanoframe is used as the dual-functional catalyst in both the cathode and the anode,it can provide a current density of 10 mA cm-2 under a voltage of 1.57 V and a stability of more than 60 h,which is close to the benchmark Pt/C and RuO2.Further analysis of the active sites and the characterization of the catalyst after electrolysis proved that the partially oxidized Co0.6Fe0.4P is the main active site of HER,while the in-situ formed Co-Fe metal oxide/hydroxide is the catalytic OER activity site.2.Since the metal phosphide is easily converted into the corresponding hydroxide in situ during the OER process,it may even damage the morphology of the material.Encapsulation of the material by the carbon material can protect the material to a certain extent,and the conversion of metal elements into carbides can also improve the stability of the material.In Chapter 3,the S and N co-doped carbon cubes embedded with Co-Fe carbides were synthesized by pyrolyzing the Co-Fe Prussian blue analogue(PBA)coated with methionine.Due to the strong metal-sulfur interaction,the methionine coating provides a strong sheath that can maintain the cubic form of PBA during pyrolysis,which is very beneficial to promote the specific surface area and exposure of active sites,and it can also enhance the bifunctional activity of ORR and OER.Further studies on the origin of activity and the material composition after electrolysis show that the high activity of ORR is mainly derived from the S and N co-doped carbon shell and the uniformly dispersed metal-nitrogen-carbon structure.For OER,the embedded Co-Fe carbides are converted into layered(hydrogen)oxides in situ,which act as actual active sites and promote water oxidation.3.The high-temperature calcination process destroys the structure of MOFs,which hinders the exploration of catalytic active sites.The uncalcined MOFs material with a clear topological structure can better explain the catalytic mechanism.By adjusting the Ni/Co ratio and using HITP(HITP=2,3,6,7,10,1 1-hexaiminotriphenylbenzene)as a ligand,a series of bimetallic coordination polymers were synthesized to explore the role of metal center in regulating the activity of oxygen reduction.Comparing Co3HITP2 and Ni3HITP2,we found that the unpaired 3d electrons in Co3HITP2 lead to reduced coplanarity and more free radical characteristics of the molecular structure.Although reduced crystallinity and conductivity,Co3HITP2 achieved ORR activity equivalent to 20%Pt/C,indicating that the 3d orbital configuration of the metal center is of great help in promoting ORR activity.Experiments and DFT studies show that the ORR path changes from the four electrons of Co3HITP2 to the two electrons of Ni3HITP2.The rechargeable zinc-air battery using Co3HITP2 as the air cathode catalyst has excellent energy efficiency and stability.4.On the basis of Chapter 3,we synthesized highly dispersed CoRu nanoparticles supported in a sulfur and nitrogen co-doped hollow carbon framework,and used them as a cathode catalyst to reduce the overpotential of electrochemical decomposition of Li2CO3.After introducing a certain amount of O2,our Li-CO2 battery shows a high discharge voltage of 3.05 V and a low charge voltage of 4.08 V at a current of 100 mA g-1 as well as good cycle stability(at a capacity of 500 mAh g-1 for 550 h).It has also been found that O2 plays a vital role during the discharge process in rechargeable Li-CO2 batteries.In a pure CO2 environment,the discharge capacity of Li-CO2 batteries is negligible.After introducing O2 into CO2,the Li-CO2 battery can provide a high capacity of 21875 mA h g-1.