Molecular metallocycle electrocatalysts like metalloporphyrins and metallophthalocyanines were found to be effective for oxygen reduction reaction(ORR)due to their M-N_(4) active sites and large conjugated elec-tronic...Molecular metallocycle electrocatalysts like metalloporphyrins and metallophthalocyanines were found to be effective for oxygen reduction reaction(ORR)due to their M-N_(4) active sites and large conjugated elec-tronic molecular structures.Herein,the“substituents optimization”strategy combined with“push effect”modification was innovatively employed to target a single Co-N_(4) active site in three substituted phthalo-cyaninato cobalt complexes:tetranitrophthalocyaninato cobalt(CoTNPc),tetra(4-nitrophenoxy)phthalo-cyaninato cobalt(CoTPNPc),and tetraphenoxy phthalocyaninato cobalt(CoTPPc)electrocatalyst,also with 4-phenylpyridine axial coordination on Co-N_(4) unit.Through substituents screening,the half-wave poten-tial(E_(1/2))for ORR increases in the order of CoTPNPc(0.75 V)<CoTPPc(0.80 V)<CoTNPc(0.83 V)along with decreased electron-withdrawing ability of their substituents from-OC_(6) H_(4)-NO_(2),-OC_(6) H_(5) to-NO_(2) in the three cobalt phthalocyanine derivatives.CoTNPc with the weakest electron-withdrawing substituent exhibits the best ORR performance among the three compounds.This is attributed to its higher elec-tron delocalization and lifted HOMO energy level with the lower energy barrier in the rate-determining step relative to the other two compounds,which facilitate the electron transfer and reduction of oxy-gen as evidenced by XPS,UPS,and DRS analysis combined with DFT calculations.Further coordination of 4-phenylpyridine shifts the E_(1/2) up to 0.78,0.82,and 0.85 V for CoTPNPc,CoTPPc,and CoTNPc.DFT calcu-lations demonstrate that the introduction of the electron-donating phenylpyridine ligand into the cobalt phthalocyanines breaks the symmetry of the Co-N_(4) center and also raises the electron density of Co sites,which promotes O_(2) adsorption and improves ORR performance.After comparing the two strategies,the substituents on metallophthalocyanine are more determined by the electroactivity than the axial group,which directly regulates the coordination environment and then the activation barrier of the ORR pro-cess.This work provides theoretical and experimental guidance by two coupling strategies for the design of highly active molecular CoPc-based ORR electrocatalysts in the practical application.展开更多
基金supported by the Key Program of the National Natural Science Foundation of China(No.22133006)National Nat-ural Science Foundation of China(No.21771192)+2 种基金Program for Tais-han Scholar of Shandong Province(No.ts201712019)NSF of Shan-dong Province(No.ZR2017ZB0315)Shandong Energy Group 2019 Science and Technology Program(Nos.YKKJ2019AJ11JG-R66 and YKKJ2019AJ05JG-R60).
文摘Molecular metallocycle electrocatalysts like metalloporphyrins and metallophthalocyanines were found to be effective for oxygen reduction reaction(ORR)due to their M-N_(4) active sites and large conjugated elec-tronic molecular structures.Herein,the“substituents optimization”strategy combined with“push effect”modification was innovatively employed to target a single Co-N_(4) active site in three substituted phthalo-cyaninato cobalt complexes:tetranitrophthalocyaninato cobalt(CoTNPc),tetra(4-nitrophenoxy)phthalo-cyaninato cobalt(CoTPNPc),and tetraphenoxy phthalocyaninato cobalt(CoTPPc)electrocatalyst,also with 4-phenylpyridine axial coordination on Co-N_(4) unit.Through substituents screening,the half-wave poten-tial(E_(1/2))for ORR increases in the order of CoTPNPc(0.75 V)<CoTPPc(0.80 V)<CoTNPc(0.83 V)along with decreased electron-withdrawing ability of their substituents from-OC_(6) H_(4)-NO_(2),-OC_(6) H_(5) to-NO_(2) in the three cobalt phthalocyanine derivatives.CoTNPc with the weakest electron-withdrawing substituent exhibits the best ORR performance among the three compounds.This is attributed to its higher elec-tron delocalization and lifted HOMO energy level with the lower energy barrier in the rate-determining step relative to the other two compounds,which facilitate the electron transfer and reduction of oxy-gen as evidenced by XPS,UPS,and DRS analysis combined with DFT calculations.Further coordination of 4-phenylpyridine shifts the E_(1/2) up to 0.78,0.82,and 0.85 V for CoTPNPc,CoTPPc,and CoTNPc.DFT calcu-lations demonstrate that the introduction of the electron-donating phenylpyridine ligand into the cobalt phthalocyanines breaks the symmetry of the Co-N_(4) center and also raises the electron density of Co sites,which promotes O_(2) adsorption and improves ORR performance.After comparing the two strategies,the substituents on metallophthalocyanine are more determined by the electroactivity than the axial group,which directly regulates the coordination environment and then the activation barrier of the ORR pro-cess.This work provides theoretical and experimental guidance by two coupling strategies for the design of highly active molecular CoPc-based ORR electrocatalysts in the practical application.
