Electrocatalyst supports particle-size distribution

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

Particle size distribution of pyrolyzed and plasma treated cobalt TMPP-based electrocatalysts supported on Black Pearls. (Source Iris Herrmann, Helmholtz Centre Berlin for Materials and Energy, Germany.)... 

Recently, taking advantage of the very narrow size distribution of the metal particles obtained, microemulsion has been used to prepare electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) Catalysts containing 40 % Pt Ru (1 1) and 40% Pt Pd (1 1) on charcoal were prepared by mixing aqueous solutions of chloroplatinic acid, ruthenium chloride and palladium chloride with Berol 050 as surfactant in iso-octane. Reduction of the metal salts was complete after addition of hydrazine. In order to support the particles, the microemulsion was destabilised with tetrahydrofurane in the presence of charcoal. Both isolated particles in the range of 2-5 nm and aggregates of about 20 nm were detected by transmission electron microscopy. The electrochemical performance of membrane electrode assemblies, MEAs, prepared using this catalyst was comparable to that of the MEAs prepared with a commercial catalyst. 

The results obtained with bulk alloys can be also translated to more practical carbon supported PtM catalysts. In general, a certain increase in the mass activity (A/gpt) for the ORR reaction was found in PtM (M = Cr, V, Mn, Ni, Co) as compared to Pt/C. Again, it is difficult to extract accurate conclusions to explain the role of the foreign metal on the activity of carbon supported bimetallic (or polymetallic) electrocatalysts for the ORR since features such as particle size and size distribution, or metal loading cannot always be eliminated by normalization procedures. Paulus et studied... 

The rare noble metal Pt is the most commonly used metal in fuel cell catalysts. A Pt-based catalyst supported on carbon has both high intrinsic activity and great stability [7-10]. It is generally accepted that the catalytic activity of a Pt-based catalyst is highly dependent on the dispersion and the size distribution of the Pt crystallites [11, 12]. Therefore, much attention has been foeused upon the preparation of highly dispersed Pt catalysts. Nanosized platinum particles dispersed on high-surfaee-area carbon substrates have been used as an electrocatalyst for both anode and cathode. 

A major challenge in the synthesis of CNTs and CNFs as supports for Pt electrocatalysts is to control the size and distribution of the Pt nanoparticles. Since the dispersion and particle size of Pt on the support material can strongly affect its utilization and catalytic activity [2], the synthesis of Pt nanoparticles supported by CNTs and CNFs are of fundamental and practical importance [63, 68, 76-79]. In a recent review, Lee et al. [80] described the various synthesis methods of the Pt electrocatalyst using CNTs and CNFs as flie support of the Pt catalyst in the PEM fuel cell applications. The deposition, distribution, and crystalline size of Pt nanoparticles supported on CNTs and CNFs are significantly affected by factors including the synthesis method, oxidation treatment of CNTs and the Pt precursors. In this section, we will focus on the synthetic methods of depositing Pt nanoparticles onto CNTs and their morphology. 

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