Fuel Cell Electrocatalysts

The held to which the specific features of CNTs and CNFs could bring the most significant advancements is perhaps that of fuel cell electrocatalysis [125,187]. The main uses of CNTs or CNFs as catalyst support for anode or cathode catalysis in direct methanol fuel cells (DMFCs) or proton-exchange membrane fuel cells (PEMFCs) are covered in Chapter 12. In this section we summarize the main advantages linked to the use of nanotubes or nauofibers for these applications. 

More than a hundred articles have been published on the use of CNTs or CNFs as catalyst supports for DMFC and PEMFC. The most studied reaction is methanol oxidation (anode catalyst), followed by oxygen reduction (cathode catalyst) and to a lesser extent, hydrogen oxidation (anode catalyst). Platinum is 

The advances made by the use of CNTs and CNFs as supports for fuel cell applications are generally attributed to (1) the possibility of reaching high metal dispersion and high electroactive surface area values for Vulcan XC-72R the catalyst particles can sink into the microporosity, thus reducing the number of three-phase boundary active sites (2) the peculiar three-dimensional mesoporous network formed by these materials, which provides improved mass transport and (3) their excellent conducting properties, which improve electron transfer. 

The use of nanomaterials for enzyme immobilization may offer some advantages regarding enzyme stabilization, control of the pore size to protein molecule dimensions, multiple sites for interaction, and reduced mass-transfer limitations 

Even thongh large-scale industrial production of nanotubes or nanofibers is not now under way, the current synthesis processes for MCWNTs and CNFs permit 

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The change in the electronic properties of Ru particles upon modification with Se was investigated recently by electrochemical nuclear magnetic resonance (EC-NMR) and XPS [28]. In this work, it was established for the first time that Se, which is a p-type semiconductor in elemental form, becomes metallic when interacting with Ru, due to charge transfer from Ru to Se. On the basis of this and previous results, the authors emphasized that the combination of two or more elements to induce electronic alterations on a major catalytic component, as exemplified by Se addition on Ru, is quite a promising method to design stable and potent fuel cell electrocatalysts. 

Markovic NM, Ross PN. 2002. Surface science studies of model fuel cell electrocatalysts. Surf Sci Rep 45 121-229. 

Brankovic SR, Wang JX, Adzic RR. 2001b. Pt submonolayers on Ru nanoparticles—A novel low Pt loading, high CO tolerance fuel cell electrocatalyst. Electrochem Solid State Lett 4 A217-A220. 

MaiUard F, Lu GQ, Wieckowski A, Stimnting U. 2005. Ru-decorated Pt surfaces as model fuel cell electrocatalysts for CO electrooxidation. J Phys Chem B 109 16230-16243. 

Guerin S, Hayden BE, Lee CE, Mormiche C, Owen JR, Russell AE, Theobald B, Thompsett D. 2004. Combinatorial electrochemical screening of fuel cell electrocatalysts. J Comb Chem 6 149-158. 

Zhang, H., Wang, X., Zhang, J., and Zhang, J. Conventional catalyst ink, catalyst layer, and MEA preparation. In PEM fuel cell electrocatalysts and catalyst layers Fundamentals and applications, ed. J. Zhang. London Springer, 2008. 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

Maniguet, S. EXAFS studies of carbon supported fuel cells electrocatalysts. Ph.D. Thesis, University of Southampton, 2002. 

Combinatorial Synthesis and High-Throughput Screening of Fuel Cell Electrocatalysts... 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

Three techniques have been described in the literature to prepare combinatorial libraries of fuel cell electrocatalysts solution-based methods [8, 10-14], electrodeposition methods [15-17] and thin film, vacuum deposition methods [18-21]. Vacuum deposition methods were chosen herein for electrocatalyst libraries in order to focus on the intrinsic activity of the materials, e.g., for ordered or disordered single-phase, metal alloys. 

The complete primary screening workflow for the discovery of new fuel cell electrocatalysts is shown in Fig. 11.8. The individual components of this workflow are designed such that no bottleneck occurs. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

Strasser, P., Fan, Q., Devenney, M., Weinberg, W. H., Combinatorial Exploration of ternary fuel cell electrocatalysts for DMFC anodes — a comparative study of PtRuCo, PtRuNi and PtRuW systems, AIChE fall meeting, 2003, San Francisco. 

There are a number of different ways to prepare metal overlayers in practical nanoscale fuel cell electrocatalysts. One approach is schematically shown in Figure 3.3.13A, where a spherical alloy nanoparticle consisting of a noble metal (grey), platinum, and a nonnoble metal (red), such as copper, is subject to a corrosive electrochemical... 

Adzic, R. et al.. Low Pt loading fuel cell electrocatalysts, 271, 2006 Annual Merit-Review and Peer Evaluation Report, DOE Hydrogen, Fuel Cells and Infrastructure Technologies Program, Arlington, VA, May 16-19, 2006. 

Zhang, J. et al., Platinum and mixed platinum-metal monolayer fuel cell electrocatalysts design, activity and long-term performance stability, ECS Trans., 3, 31, 2006. 

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