Metal-Organic Frameworks-Derived Carbon-Based Nanocomposites And Their Electrocatalytic Properties

With increased energy and environment crises,it is of great significance to develop and utilize a clean and renewable energy.Hydrogen energy(H2)is regarded as an ideal new energy due to its high energy density and zero-carbon product.Electrochemical water splitting and H2-O2 fuel cells have become research hotspots because they can achieve clean and efficient H2 production and conversion,respectively.Ammonia(NH3)is another hydrogen energy carrier,not only served as clean fuel but vital chemical material,but its source mainly depends on the industrial Haber-Bosch process with high-energy consumption and serious pollution.Electrochemical ammonia synthesis is an emerging nitrogen fixation technology,which is capable of achieving the electricitydriven N2-to-NH3 conversion at atmosphere.In order to enable these electrocatalytic reactions proceed efficiently,the development of highly-active,selective and durable electrocatalysts is extremely crucial.Metal-organic frameworks(MOFs)derived nanocomposites are endowed with great potential as ideal electrocatalysts,because of their unique ability to achieve precise tuning of morphology,composition,and structure via molecular design,coupled with their high porosity,high conductivity,and corrosion resistance.In this thesis,a series of advanced electrocatalysts were developed from ZIF8 and MOF-5 precursors via phosphorization,pyrolysis,freeze-drying and acid-etching process et al.,and their electrocatalytic properties for hydrogen evolution(HER)and oxidation reaction(HOR),and nitrogen reduction reaction(NRR)were further modified through morphological control,electronic tuning,defect engineering et al.The main research results and conclusion are listed below:(1)Water electrolysis can produce high-purity H2 from water,and it is thus significant to develop efficient and low-cost electrocatalysts for hydrogen evolution and oxygen evolution reactions.Traditional MOFs materials always suffer from poor electrical conductivity,concerning this issue,NiMOF nanoarrays were in-situ grown on conductive nickel foam by hydrothermal process(NiMOF NAs/NF),followed by phosphating treatment with sodium hypophosphite,and was conformally transformed into a carbon-confined Ni2O electrocatalyst(Ni2P@C NAs/NF).Compared to NiMOF NAs/NF,the well-designed Ni2P@C NAs/NF exhibits increased electrical conductivity,mass transport and electroactive surface area.Simultaneously,the implantation of carbon layer into Ni2P can modulate the electronic structure around metallic Ni,which promotes the sharp improvement on its intrinsic catalytic activity.At a large current density of 100 mA cm-2,Ni2P@C NAs/NF only demanded an OER overpotential of 297 mV,much lower than NiMOF NAs/NF(357 mV)and Ni2P NAs/NF(325 mV).The structural evolution of catalyst electrode during electrolysis test was also investigated systematically via Raman,TEM,LS V and other characterizations.This work provides crucial theoretical guideline for the rational construction of highly active 3D porous Nibased catalyst materials.(2)H2-O2 fuel cell is an efficient energy conversion device from chemical energy in H2 into electrical energy.For anodic hydrogen oxidation reaction(HOR),Pt is regarded as the benchmarking catalyst,but its HOR activities remains to be improved in alkaline solutin and Pt sites are susceptible to poisoning by CO molecules.In this work,single-atom Mo functionalized Pt nanocatalyst(Mo-Pt/NC)was successfully fabricated by immersion and pyrolysis processes.Structurally,single-atom modification strategy enables the minimum blocking on Pt surface,and endows more available catalytic centres for HOR.Mechanistic analyses indicated that the implanted Mo species exhibited as a bifunctional promoter,could not only tune the electronic microenvironment around Pt,but act as a favorable H2O*-trapping agent,which made the H*binding energy(ΔG0app)closer to zero for increased integral HOR kinetics.MoPt/NC showed the extremely high kinetics current density(1584 mA mg-1Pt at 25 mV overpotential)in alkaline medium,with the 11-fold and 4-fold mass-specific activity of commercial Pt/C and Pt/NC,respectively.Moreover,Mo-Pt/NC catalysts became more CO tolerance on the basis of the Mo-modified electronic effect.This work verifies the significance of engineering the catalyst’s surface at atomic scale and contributes to the in-depth understanding of the HOR mechanism at alkaline media.(3)In order to further enhance the electrochemical stability of HOR catalyst,on the basis of easy functionalization feature and spatial confinement of MOFs,ZIF-8 was employed as a template to adsorb phosphomolybdic acid(PMA)and ruthenium source.Sub-nanometer RuMo catalyst(below 2 nm)anchored on N,P co-doped hollow carbon framework was synthesized via freeze-drying and pyrolysis treatment.The HOR activity of designed Sub-RuMo catalyst was increased to 22.6 and 9.4 times relative to those of commercial Ru/C and Pt/C,respectively.Combined the electronic tuning of Ru surface by Mo element with the confined effect from heteroatom-doped carbon support,the formation rate of soluble RuOx species was effectively suppressed during HOR running process,making Sub-RuMo catalyst more resistant to oxidation and electrochemically durable in alkaline solution.This work proposed a feasible approach to improve the electrochemical stability of Ru-based catalysts.(4)NH3 is another clean hydrogen energy carrier,which can be obtained from electrocatalytic NRR at ambient environment,but its conversion efficiency and yield are extremely low,seriously hindering its application in the ammonia industry.Herein,the acid-etching strategy was employed to construct numerous vacancies on the surface of PdZn catalyst,to prepare rich-defect PdZn catalyst(etched-PdZn/NHCP).The etched-PdZn/NHCP exhibited superior NRR catalytic activity relative to PdZn/NHCP in neutral PBS.Notably,etched-PdZn/NHCP achieved a high Faradic efficiency of 16.9%,much higher control samples and other reported Pd-based catalysts(<10%).Mechanism studies revealed that the creation of vacancy defects in PdZn catalyst could induce sufficient localized electrons to strengthen the chemisorption and hydrogenation of N2 molecules,which could facilitate the N≡N triple bond broken to generate NH3.This work proposed a facile and effective strategy to improve the N2-to-NH3 conversion efficiency and ammonia yield.