Rational Design Of Metal-organic Frameworks As Effective Catalysts For Oxygen Electrocatalysis

Enrivonmental pollution and energy crisis call for the development of clean energy in order to reduce our dependence on fossil fuels.Hydrogen-oxygen fuel cell is a device that consumes hydrogen and oxygen to generate eletricity with only water and heat as the by-products.Unfortunately,hydrogen does not purely exist in nature,and its production highly relies on energy input.Water electrolysis represnets a promising strategy as it can produce highly pure hydrogen and be easily integrated with other renewable energy sources.No matter in fuel cells or electrolyzers,compared with the reactions on hydrogen electrode,the ones on oxygen electrode,namely oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),are more kinetically sluggish,thus requiring high loading of platinum-group metals(PGMs)as catalysts.The use of PGMs significantly increases the host hurdle of the devises,hampering them from large-scale implementation.Therefore,PGM-free catalysts with catalytic performance comparable or even superior to PGMs are urgently desirable.Metal-organic frameworks(MOFs)are low-cost materials,assembled from transition metals and organic linkers.MOFs possess high surface area,tunable structures and accessible metal sites,thus promising as oxygen electrocatalysts.In this work,we synthesized a series of MOFs and subsequently used them for oxygen electrocatalytic study.The detailed contents are as follows:1.Low-cost transition metal ions(Fe,Co and Zn)were coordinated with organic linkers(carboxylic-based or nitrogen heterocyclic-based)to generate MOFs(such as Materials from Institut Lavoisier,MILs or zeolitic imidazolate frameworks,ZIFs)with high surface area and level of crystallization.Transition metal ions(Fe)were also coordinated with organic linkers(carboxylic-based)to generate metal-organic gels(MOGs)with great inclusion properties.MOFs could be used as electrocatalysts in pristine phases,by supporting transition metal oxides,or by integrated with carbon materials.In addition,MOFs could be used as self-sacrificial precursors to generate MOFs-derived catalysts through pyrolysis and subseqent post-treatments.2.A mixed metal-ion MOF(Fe/Co)was prepared and investigated for its catalytic performance towards ORR/OER.However,the main shortcoming of using pure MOFs as electrocatalysts is their poor electronic conductivity,which cause high overpotential and energy consumption.Therefore in the following work,biomass-derived highly porous carbon materials was used as ZIF-67 support,and the hybrid material showed enhanced bifunctional activity and durability.3.Mn02 has been widely studied due to its promising electrocatalytic peroformance.Unfortunately,the high surface energy usually leads to strong agglomeration during either preparation or reaction,resulting in reduced acitve site exposure.To solve the problem,MOFs were used as supports to increase the dispersity of Mn02,such as MOF(Fe)as the support of ?-MnO2 and MIL-101(Cr)as the support of a-Mn02.In ?-MnO2/MOF(Fe),?-MnO2 nanorods protruded from MOF(Fe)surface,while in ?-MnO2/MIL-101(Cr),?-Mn02 nanoparticles were embedded in the MOF matrix.In either case,MOFs-supported Mn02 afforded better catalytic activity and durability than pure Mn02.4.MOF(Fe/Co)or MOG(Fe)was pyrolized to generate carbon-surpported metal nanoparticles.High temperature pyrolysis enables MOFs transformed into charge conductive materials,and therefore showing enhanced catalytic performance over pure MOFs.Additionallly,a mixture of MIL and ZIF was pyrolized to synthesize Fe-N-C for fuel cell and metal-air battery applications.The impacts of side-chain substitutions of organic linker to the final catalyst structure as well as performance were also investigated in details.The results indicate that the side-chain subsitutions of organic linker altered the rigidity of ZIFs and the decomposition mechansim during high-temperature pyrolysis,and eventually the nitrogen contents,surface area,carbon graphitization level in the final catalysts.5.The materials prepared through this work were characterized by conventional characterizations(such as infrared spectroscopy,N2 adsorption-desorption,X-ray diffraction spectroscopy,X-ray photoelectron spectroscopy,X-ray energy spectroscopy and electron microscopy)and synchrotron characterizations(X-ray absorption spectroscopy)to study their compositions and structures.Furthermore,the active site models were proposed based on the physical characterizations.For instance,in Fe-N-C case,X-ray absorption spectra confirmed the Fe-N coordination construction to be isolated Fe atoms ligated by four or two+two pyridinic-or pyrrolic-N,concurrently along with a small amount of Fe-Fe3C nanoparticles.This work combined experimental and theoretical methodology and investigated each proton-and charge-transfer mechanism.For example,in Fe-N-C case,Fe-N-C active sites were along with a small fraction of Fe-Fe3C nanoparticles.Fe loading in the optimal catalyst was determind by thermogravimetry to be 5.7 wt%,and the preferred exposed crystallographic planes of Fe and Fe3C were observed by electron microscopy.Therefore,three active site structures were proposed,including Fe-N4-C,Fe-N4-C@Fe3C(210)and Fe-N4-C@Fe(110).DFT calculation results suggest Fe-N4-C showed excellent oxygen affinity(absorption energy was ?E=-1.92 eV),which could be enhanced on Fe-N4-C@Fe3C(210)(?E=-3.36 eV).However,on these two active sites,OH*desorption showed energy uphill as the rate-determining step.Once Fe-N4-C was associated with Fe(110),such step was promoted and showed energy downhill.6.The electrocatalytic performance evaluation was done in half cells and full cells.In half cell,the best ORR catalyst achieved half-wave potential as higi as 0.79 V(vs.reversible hydrogen electrode,RHE)in acidic electrolyte and 0.93 V(vs.RHE)in alkaline electrolyte;the best OER catalyst delivered an overpotential of 353 mV when the current density reached 10 mA cm-2.In full cell,the best ORR catalyst reached a peak power density of 0.76 W· cm-2 in PEMFC and delivered a discharging(charging)capacity of 8,749 mAh·g-1(8,540 mAh·g-1)in an aprotic Li-air battery.Rational design of MOFs opens new pathway for future activity and durability improvement as well as cost reduction for large-scale implementation of the related energy storage and conversion devices.