Standard Article

Macrocycles

Electrocatalysis

The oxygen reduction/evolution reaction

  1. J. H. Zagal

Published Online: 15 DEC 2010

DOI: 10.1002/9780470974001.f205042

Handbook of Fuel Cells

Handbook of Fuel Cells

How to Cite

Zagal, J. H. 2010. Macrocycles. Handbook of Fuel Cells. .

Author Information

  1. Universidad de Santiago de Chile, Departamento de Química de los Materiales, Facultad de Química y Biología, Santiago, Chile

Publication History

  1. Published Online: 15 DEC 2010

Abstract

The catalysis of the reduction of oxygen on supported metal macrocyclics is discussed in terms of electronic interactions of the dioxygen molecules with metal centers that lead to the weakening of the O[BOND]O bond and to a lowering of activation energy. Correlations between electronic properties of the macrocyclic complexes and activity are also discussed, such as the influence of the redox potential of the metal center in the electrocatalysis. The overall 2- versus 4-electron reduction mechanisms on metallo-macrocyclics are discussed including the different approaches used in the literature to achieve 4-electron reduction mechanisms by designing face-to-face binuclear complexes, polymerized metal complexes or macrocycles substituted with peripheral redox groups. The systems that are more promising for practical applications, such as the heat-treated carbon supported catalysts, are discussed in detail and with explanations on how these pyrolyzed catalysts have higher activity and better stability compared with other supported macrocyclic systems. The requirement for the existence of the “burned-in” M[BOND]N4 moiety also known as the “super-site” in heat-treated catalysts is emphasized. Heat-treated carbon supported macrocyclics catalysts are not poisoned by methanol and in principle they can be suitable for low-temperature acid electrolyte fuel cells. The methods of preparation of supported catalysts include: (a) modified electrodes by adsorption or surface modification; (b) heat-treated carbon supported catalysts; (c) influence of carbon surface modification and electrodes prepared by polymerized macrocyclics; and (d) methanol tolerance and comparison of the activities with platinum-based electrocatalysts.

Keywords:

  • heat-treated macrocyclics;
  • pyrolysis temperature;
  • face-to-face porphyrins;
  • methanol tolerance;
  • redox catalysis;
  • phthalocyanines;
  • porphyrines;
  • tetra-aza-annulenes;
  • Schiff bases;
  • N-4 chelates