Electrocatalyst structural effects

Bogdanoff, R, I. Herrmann, M. Hilgendorff, I. Dorbrandt, S. Fiechter, and H. Tiib-utsch (2004). Probing structural effects of pyrolyzed CoTMPP-based electrocatalysts for oxygen reduction via new preparation strategies. J. New. Mat. Electrochem. Syst. 7, 85-92. 

Tributsch H. Probing structural effects of pyrolysed CoTMPP-based electrocatalysts for oxygen reduction via new preparation strategies. J New Mater Electrochem Syst 2004 7(2) 85—92. 

Abstract Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects. 

The importance of structural effects in gas diffusion electrodes was realized long before the development of the current generation of CLs for PEFCs. The basic theory of gas diffusion electrodes, including the interplay of reactant transport through porous networks and electrochemical processes at highly dispersed electrode I electrolyte interfaces, dates back to the 1940s and 50s [13, 14]. Later work realized the importance of surface area and utilization of electrocatalysts in porous electrodes [15]. A series of seminal contributions by R. De Levie opened... 

Dubau L, Hahn F, Coutanceau C, Leger JM, Lamy C. On the structure effects of bimetallic PtRu electrocatalysts towards methanol oxidation. J Electroanal Chem 2003 554 407-15. 

Dubau, L., Hahn, F., Coutanceau, C., et al. (2003). On the Structure Effects of Bimetallic PtRu Electrocatalysts Towards Methanol Oxidation, J. Electroanal. Chem., 554-555, pp. 407-415. Vigier, F., Coutanceau C., Hahn, R, etal. (2004), On the Mechanism of Ethanol Electro-oxidation on Pt and PtSn Catalysts Electrochemical and In Situ IR Reflectance Spectroscopy Studies,... 

Evaluation of CL performance requires a number of parameters that define the ideal electrocatalyst performance, allowing deviations from ideal behavior to be rationalized and quantified. Ideal electrocatalyst performance is achieved when the total Pt surface area per unit volume, Stot, is utilized and when reaction conditions at the reaction plane (or Helmholtz layer) near the catalyst surface are uniform throughout the layer. These conditions would render each portion of the catalyst surface equally active. Deviations from ideal behavior arise due to statistical underutilization of catalyst atoms, as well as nonuniform distributions of reactants and reaction rates at the reaction plane that are caused by transport effects. This section introduces the effectiveness factor ofPt utilization and addresses the hierarchy of structural effects from atomistic to macroscopic scales that determine its value. 

When considering the morphology of prepared electro-catalysts are different to each other especially to the commercial one, one can think that the structure of electrode which was optimized to the commercial catalyst may not be optimum. So, the for the better electrode structures was conducted by investigating the effect of NFP. Fig. 2 is a schematic of electrode which depicts the effect of Nafion content[9]. For the conventional electrocatalysts, the range of 30 35 % NFP is reported as optimum value[10]. 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

The phosphoric acid cell has been under research for a longer time than that of any other kind of fuel cell. Alloys of Pt with Cr, V, and Ti and other non-noble metals are better than Pt (Appleby, 1986). The particle size of the catalyst has been reduced to that of tens of atoms (Stonehart, 1993).10 Much attention has been given to the search for non-noble (hence cheaper) catalysts that are stable in hot acids. The best are the porphyrins, the formulas for which are shown in Fig. 13.20. They are applied to a base of graphite. These electrocatalysts are more effective in alkaline fuel cells than in those with acid electrolytes. Curiously, these substances are more stable and give better catalysis after pyrolysis in He at 800 °C, a process that would decompose the organic part of the structure. Perhaps the only active part of the porphyrin catalyst is the central... 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

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