Electrocatalysts for ORR

In the acid medium of PEM fuel cells that may also contain some levels of fluoride derived from membrane degradation, Pt cannot be replaced with a non-noble metal or a metallic oxide, as both will corrode in such environment. This chapter describes some of the efforts that have been made over 40 years to obtain other non-precious metal electrocatalysts for ORR in acidic medium. They aU started with the discovery in 1964 that C0N4 phthalocyanine was capable of oxygen reduction in an alkaline solution. ... 

Pt-based catalysts are two necessary approaches at the current technology stage. It is believed that non-noble metal electrocatalysts is probably the sustainable solution for PEM fuel cell commercialization. In the past several decades, various nonnoble metal catalysts for ORR have been explored, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, transition metal chalcogenides, conductive polymer-based catalysts, metal oxides/carbides/nitrides/ oxynitrides/carbonitrides, and enzymatic compoimds. The major effort in non-noble metal electrocatalysts for ORR is to increase both the catalytic activity and stability. 

More recently. Ye et al. proposed a new sol—gel method combined with a supercritical drying technique for the synthesis of nanocomposite electrocatalysts for ORR. They claimed that their method had a structure-preserving ability and favored a nanoscale mixing of the constituents, and then could produce a remarkably homogeneous solution. The chemical, physical, and morphological properties of catalysts produced using this sol—gel... 

As mentioned above, the pretreatment of the carbon electrode surface can significantly affect its ORR activity, and even alter the ORR mechanism. Actually, the carbon surface can also be modified by some monolayer substances such as anthraquinones (AQ), and transition metal macrocycle complexes. Due to the conjugate structures of these molecules, they can irreversibly adsorb on the carbon surface to form monolayers that could serve as an electrocatalyst for ORR. This kind of electrode structure can be used to evaluate the electrocatalyst s ORR activity, particularly in acidic solution. 

Partial or complete replacement of platinum for ORR has attracted a lot of interest in order to enhance electrocatalytic activity and reduce materials costs. Therefore, some non-platinmn promising electrocatalysts for ORR have been developed, i.e., using car-bides, " nitrides,macroeycle materials, " and other preeious metals, such as Pd" " or Ru-based materials, " as well as non-precious metal chalcogenides. ... 

Typically, the membranes for low-temperature PEM fuel cells are made of perfluoiocaibon-sulfonic acid ionomers (PFSA). The best material known is Nafion produced by DuPont, though similar materials have been developed for commercial or development purposes by other manufacturers such as Asahi Glass (Flemion) or As Chemical (Aciplex). In the electrode part, the catalyst is generally dispersed as small particles on a conductive support (carbon powders). Despite many attempts to develop a non-noble-metal catalyst, platinum remains the best known electrocatalyst for ORR in PEM fuel cells. The most extensive limitations to large-scale commercial use of these materials arise from the fact that... 

Koslowski UI, Abs-Wurmbach I, Fiechter S, Bogdanoff P (2008) Nature of the catalytic centers of porphyrin-based electrocatalysts for ORR a correlation of kinetic current density with the site density of Fe-N4 centers. J Phys Chem C 112 15356-15366... 

The electrocatalysts for various catalytic processes contain noble metals. Due to high prices and limited resources of noble metals, reduction of their content in catalysts is needed. Core-shell catalysts offer an attractive choice since noble metals (active component) are restricted to the surface in the shell of a catalyst, while core can be made of less expensive metals. In this way ultimate reduction in noble metal loading and its complete utilization can be achieved, while core-shell interaction can change electronic properties of noble metal shell making it more active and durable. One of the most attractive types of core-shell catalyst is a Pt L electrocatalysts for ORR. 

Oxygen Reduction Reaction in Acid Solution, Table 2 Structural and electrochemical characteristics of the Pd/C and Pd-Fe/C electrocatalysts for ORR in 0.1 mol dm HClOa (From [33]) ... 

In this study. Iron phthalocyanine (Fe-PC) is used as iron/nitrogen/carbon source to synthesize Fe-Nx-C electrocatalysts for ORR by p3Tolyzation in an inert atmosphere [71]. Considering the high carbon content of the Fe-PC molecule, the... 

Platinum has long been the best electrocatalyst for ORR, and oxygen reduction over platinum and other Pt bi-metallic surfaces has been reviewed recently [70-72]. As discussed in these reviews, over the three low-index (100), (111), and (110) platinum surfaces in aqueous... 

---