Study On Electrocatalytic Activity And Reaction Mechanisms Of Carbon-Supported Metal Phthalocyanine Nano-Catalysts For Oxygen Reduction Reaction

Metal-N4macrocylces such as Fe-and Co-centered phthalocyanines (MPc) complexes are considered the most promising oxygen reduction reaction (ORR) catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalyst. While, very few are concerned related to other metal phthalcyanines (Ni, Cu), since in strong acidic conditions, such as in proton exchange membrane fuel cells (PEMFCs), the requirements for both good acitivity and stability are difficult to be achieved.The carbon-supported MPc/C (M=Cu, Ni, Zn, Co) nano-catalysts for oxygen reduction reaction (ORR) are synthesized via a combined solvent-impregnation along with the high temperature treatment. The electrocatalytic activity of MPc/C and CoPc-M/C (CoPc/C modified with the first-low transition metal salts, M=Fe, Co, Ni, V) catalysts are screened by CV, LSV employing a rotating disk electrode (RDE) technique to quantitatively obtain the ORR kinetic constants and the reaction mechanisms. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and Thermogravimetry (TG) are used to identify the catalyst structure and composition. The main points are summarized as follows:(1) Heat-treament can significantly improve the ORR activity of the MPc/C (M=Cu, Ni, Zn) catalyst, and the optimal heat-treated temperature is around800℃, under which, an onset potential of0.10V and a half-wave potential of-0.05V are achieved in alkaline electrolyte. Besides the ORR kinetic rate is increased, the ORR electron transfer number is also increased from2.2to3.6after heat-treated at800℃. These trends are also observed in NiPc/C catalyst. The ORR activity of NiPc/C catalyst varies with the heat-treated temperature, an onset potential of0.05V and a half-wave potential of-0.15V are achieved after the catalyst was heat treated at800℃. In addition to an increase in ORR kinetics the number of electrons transferred for ORR also increased from2.2to2.9. (2) XRD, TEM and SEM results demonstate that after heat-treatment the distribution of CuPc/C and NiPc/C catalysts are not unifom and the particles sizes are more larger than that of without heat-treatment. What’s more, due to the formation of metal particle (metallic Cu or Ni), the quantity of ORR active sites would be reduced, resulting in a less ORR activity. TG results show that Ni and Cu species may prevent phthalocyanine from thermal decomposition and leads to higher stability which is helpful to form more active sites. XPS anlysis indicate that center metal and pyridinic-N and graphitic-N may be responsible for the enhanced ORR activity.(3) The CoPc-M/C catalysts were prepared by modified with the simple metal salts (where M=Fe, Co, Ni, V). The oxygen reduction analysis results indicate that the degree of the enhancement is dependent on the kind of the metal ions and the enhancement was only observed of Fe. In addition, the catalytic activity of CoPc/C is improved with5wt%Fe loading was demonstrated by the observed positive shift of more than90mV for the onset potential and near110mV for the half-wave potential. The transfer electron number (n) on CoPc-5%Fe/C electrode is2.6in acid solution and3.2in alkaline solution compared to pure with the value of n is2in acid solution and2.8in alkaline solution. XRD, TEM and XPS analysis revealed that dispersion of the particle size of these catalysts is highly improved after modification of5%Fe, thus the enhanced catalytic acitivities due to the increase in active centers which might be from the synergistic effect of Co-Nx and Fe-Nx.(4) The electrocatalytic activity order of MPcs varies with heat-treatment temperatures. In the case of CoPc/C catalyst, the optimal heat-treated temperature is around600℃, while, for MPc/C (M=Cu, Zn, Ni) catalysts, the optimal heat-treated temperatures are all around800℃.(5) The membrane electrode assembly (MEA) fabricated with MPc/C (M=Co, Cu, Zn, Ni) catalyst and the Tokuyama membrane gave the intial power densities of12.6,6.8,6.1and6mW cm-2at room temperature using H2and O2gases. These initial results are promising for the application in alkaline anion exchange membrane fuel cells (AAEMFCs).