MOF-Derived Cu-N-C And Cu-Fe-N-C Electrocatalyst:Design,Preparation And Electrocatalytic Reduction

Energy and environment are the two most concerned global issues.Electrochemistry is an efficient energy conversion for oxygen and carbon dioxide reduction.On the one hand,fuel cells and Zn-air cell,which can electrochemical conversion of O₂ into H₂O,are considered desirable technology solutions as they can electrochemically convert energy to electricity and effectively alleviate the gap of human energy demand in the future.On the other hand,electrochemical conversion of CO₂into chemical fuels and the energy is transferred to these clean,high energy density fuels to be stored for recycling.Meanwhile,carbon cycle through electrochemical method is of great significance for solving greenhouse effect and energy crisis.However,the slow kinetics of oxygen reduction reaction(ORR)requires the use of rare and expensive noble metal catalysts,and electrochemical reduction of carbon dioxide is limited by the high overpotential of kinetic reaction,serious side reaction of hydrogen evolution and low chemical selectivity.This calls for improved catalysts with high efficiency,selectivity,and durability for both the ORR and CO₂RR.At present,transition metal(M)and nitrogen(N)co-doped carbon based electrocatalysts(M-N-C)are one of the most promising for the ORR and CO₂RR because of their unique electronic properties and structural features.Resulting from the unique features of ultrahigh surface area,intrinsically porosity and diverse composition/structures,metal-organic frameworks(MOFs)have been most ideal precursors.Copper(Cu),an earth-abundant element,is unique in some aspects.It has multiple easy-tuning valences(e.g.Cu(0),Cu(I),Cu(II)even Cu(III)),an outstanding water stability as well as an excellent electronic conductivity(only after Ag),all of which render it compatible with different electrocatalytic reduction.Therefore,in this paper,non-noble metal Cu was selected as the research object and to better understand the effects of the morphology,composition,structure and oxygen reduction activity of Cu-N-C and Cu-Fe-N-C catalysts derived from MOF precursor with the Cu doping.The dissertation is mainly divided into the following three parts:(1)ZIF-8-derived Cu(OH)₂@ZIF-8 is used as the precursor of Cu@Cu-N-C electrocatalysts.Cu(OH)₂ encapsulated in ZIF-8 gradually evolves into the Cu₂O phase as the temperature increases.The finally resulted Cu₂O not only serves as the Cu source but also a template tuning the morphology,and the optimal Cu@Cu-N-C catalyst features a hollow polyhedral morphology.To gain more insight into the unique Cu(0)-Cu(II)-N_x active sites in Cu@Cu-N-C,acid etching and probe experiment have been carried out.And the current work demonstrates that the synergistic interactions between Cu(0)and Cu(II)-N_x are beneficial to attain high ORR and CO₂RR performance.In alkaline electrolyte,it shows the ORR performance,selectivity and long-term stability comparable to Pt/C,and the assembled Zn-air battery displays an excellent durability with the open circuit voltage(1.23 V)decaying only by ca.4.2%in 80 hrs at 20 m A cm^-2.For CO₂RR,it also delivers an outstanding catalytic performance converting CO₂into CO with a high Faradaic efficiency(90%at-0.5 V vs RHE).(2)This paper present a highly active ZIF-8-derived Fe,Cu-codoped electrocatalyst(Cu-Fe-N-C)toward ORR in both alkaline and acidic electrolytes,which is pyrolyzed from the precursor of Cu(OH)₂-Fe(OH)₃@ZIF-8 followed by acid treatment.Meanwhile,we have made a deep investigation on the impact of the increasing Cu dopant on the morphology,composition,structure and oxygen reduction activity of the resultant Cu-Fe-N-C materials based on various measurements.The best-performance Cu-Fe-N-C exhibits a higher half-wave potential(E_1/2)of 0.88 V(vs.RHE)(30 m V higher than the commercial Pt/C)in alkaline electrolyte,and a half-wave potential of 0.79 V(vs.RHE)comparable to Pt/C in acidic electrolyte.Moreover,an enhanced durability in both alkaline(?E_1/2=5 m V after 10 000 cycles)and acidic media(?E_1/2=20 m V after 10 000 cycles)is also observed with Cu-Fe-N-C compared to Fe-N-C and Cu-N-C.The zinc-air battery based on Cu-Fe-N-C as cathode catalyst exhibits a peak power density of 140 m W cm^-2 superior to the reference Pt/C catalyst(74 m W cm^-2),and an impressive durability with only ca.4.3%decay in the output voltage for 50 hrs at 20 m A cm^-2.H₂-O₂ proton exchange membrane fuel cell(PEMFC)with Cu-Fe-N-C as the cathode catalyst delivers a maximum power density of 262 m W cm^-2.(3)A highly efficient Cu/Fe-N-C electrocatalyst for ORR and CO₂RR performance has been prepared by pyrolyzing the precursor of a nonporous eight-fold interpenetrated metal-organic framework(Cu/Fe/Zn-TTPA).The effects of Cu/Fe molar ratio and temperature on the morphology,composition,structure and oxygen reduction activity of the catalysts were investigated.The results show that the 1:1 Cu/Fe molar ratio of Cu/Fe-N-C catalyst has the best ORR and CO₂RR catalytic performance at 1000 ^oC.In alkaline electrolyte and acidic electrolyte,the Cu/Fe-N-C exhibits a half-wave potential(E_1/2)of 0.85 V and 0.70 V(vs.RHE),respectively,comparable to Pt/C.In addition,Cu/Fe-N-C delivers an outstanding catalytic performance converting CO₂into CO with a high Faradaic efficiency(91%at-0.5 V vs RHE).