Herein,high-valence-state Mn(V)oxide,barium manganate(V)(Ba_(3)(MnO_(4))_(2)),is examined as an anode electrocatalyst of a H^(+)-conducting solid oxide steam electrolysis cell(H-SOEC).Ba_(3)(MnO_(4))_(2)comprises C_(3...Herein,high-valence-state Mn(V)oxide,barium manganate(V)(Ba_(3)(MnO_(4))_(2)),is examined as an anode electrocatalyst of a H^(+)-conducting solid oxide steam electrolysis cell(H-SOEC).Ba_(3)(MnO_(4))_(2)comprises C_(3v)-symmetric MnO_(4)_(3)-oxo-anions with three long Mn-O bonds and one short Mn-O bond at room temperature.Ba_(3)(MnO_(4))_(2)caused a conductivity jump by one order of magnitude at approximately 600℃owing to the antiferromagnetic/paramagnetic phase transition,accompanied by a shape change of the tetrahedral MnO_(4)_(3)-anions from C_(3v)to Td symmetry,as confirmed by the electrical conductivity measurements and the extended X-ray absorption fine structure at an elevated temperature.Hence,the Ba_(3)(MnO_(4))_(2)base anode of the H-SOEC exhibited improved performance,with anode polarization resistances being lower than those of Sm0.5Sr0.5CoO_(3),a well-known H-SOEC anode material.Impedance analysis in terms of oxygen and water partial pressure revealed that the superior performance of the Ba_(3)(MnO_(4))_(2)base anode can be attributed to the extended reaction area.Since abundant unoccupied 3d states of the high-valence-state Mn5+cations are favorable for charge transfer interactions with water electron donors,thereby facilitating water adsorption,the oxygen evolution reaction could occur directly over the electrode surface,and thus the reaction sites were not limited to the gas-electrode-electrolyte triple phase boundary.展开更多
The pyrolysis of zeolitic imidazolate frameworks(ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts(SACs) on N-doped carbons(M-N-C).Understanding ...The pyrolysis of zeolitic imidazolate frameworks(ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts(SACs) on N-doped carbons(M-N-C).Understanding the structural evolution of M-N-C as a function of ZIF pyrolysis temperature is important for realizing high performance catalysts.Herein,we report a detailed investigation of the evolution of Zn single atom catalyst sites during the pyrolysis of ZIF-8 at temperatures ranging from 500 to 900℃.Results from Zn L-edge and Zn K-edge X-ray absorption spectroscopy studies reveal that tetrahedral ZnN4 centers in ZIF-8 transform to porphyrin-like ZnN4 centers supported on N-doped carbon at temperatures as low as 600℃.As the pyrolysis temperature increased in the range 600-900℃,the Zn atoms moved closer to the N4 coordination plane.This subtle geometry change in the ZnN4 sites alters the electron density on the Zn atoms(formally Zn2+),strongly impacting the catalytic performance for the peroxidase-like decomposition of H2 O2.The catalyst obtained at 800℃(Zn-N-C-800) offered the best performance for H2 O2 decomposition.This work provides valuable new insights about the evolution of porphyrin-like single metal sites on N-doped carbons from ZIF precursors and the factors influencing SAC activity.展开更多
基金supported by the JST MIRAI“Realization of a low carbon society,global issue”,No.JPMJM17E7the JSPS KAKENHI:Grant-in-Aid for Scientific Research(B),No.18H02066the“Nanotechnology Platform”program of the MEXT Japan,XAFS measurements were performed with the approval of SPring-8(No.2017B1288).
文摘Herein,high-valence-state Mn(V)oxide,barium manganate(V)(Ba_(3)(MnO_(4))_(2)),is examined as an anode electrocatalyst of a H^(+)-conducting solid oxide steam electrolysis cell(H-SOEC).Ba_(3)(MnO_(4))_(2)comprises C_(3v)-symmetric MnO_(4)_(3)-oxo-anions with three long Mn-O bonds and one short Mn-O bond at room temperature.Ba_(3)(MnO_(4))_(2)caused a conductivity jump by one order of magnitude at approximately 600℃owing to the antiferromagnetic/paramagnetic phase transition,accompanied by a shape change of the tetrahedral MnO_(4)_(3)-anions from C_(3v)to Td symmetry,as confirmed by the electrical conductivity measurements and the extended X-ray absorption fine structure at an elevated temperature.Hence,the Ba_(3)(MnO_(4))_(2)base anode of the H-SOEC exhibited improved performance,with anode polarization resistances being lower than those of Sm0.5Sr0.5CoO_(3),a well-known H-SOEC anode material.Impedance analysis in terms of oxygen and water partial pressure revealed that the superior performance of the Ba_(3)(MnO_(4))_(2)base anode can be attributed to the extended reaction area.Since abundant unoccupied 3d states of the high-valence-state Mn5+cations are favorable for charge transfer interactions with water electron donors,thereby facilitating water adsorption,the oxygen evolution reaction could occur directly over the electrode surface,and thus the reaction sites were not limited to the gas-electrode-electrolyte triple phase boundary.
基金supported by the Ministry of Business, Innovation and Employment Catalyst Fund (MAUX 1609)the University of Auckland Faculty Research Development Fund+1 种基金the MacDiarmid Institute for Advanced Materials and Nanotechnologya generous philanthropic donation from Greg and Kathryn Trounson。
文摘The pyrolysis of zeolitic imidazolate frameworks(ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts(SACs) on N-doped carbons(M-N-C).Understanding the structural evolution of M-N-C as a function of ZIF pyrolysis temperature is important for realizing high performance catalysts.Herein,we report a detailed investigation of the evolution of Zn single atom catalyst sites during the pyrolysis of ZIF-8 at temperatures ranging from 500 to 900℃.Results from Zn L-edge and Zn K-edge X-ray absorption spectroscopy studies reveal that tetrahedral ZnN4 centers in ZIF-8 transform to porphyrin-like ZnN4 centers supported on N-doped carbon at temperatures as low as 600℃.As the pyrolysis temperature increased in the range 600-900℃,the Zn atoms moved closer to the N4 coordination plane.This subtle geometry change in the ZnN4 sites alters the electron density on the Zn atoms(formally Zn2+),strongly impacting the catalytic performance for the peroxidase-like decomposition of H2 O2.The catalyst obtained at 800℃(Zn-N-C-800) offered the best performance for H2 O2 decomposition.This work provides valuable new insights about the evolution of porphyrin-like single metal sites on N-doped carbons from ZIF precursors and the factors influencing SAC activity.
