Assessment of Nanostructured Carbons As Electrocatalyst Supports for Solid Acid Fuel Cells

Abstract

Hydrogen fuel cells utilizing CsH2PO4 (CDP) as a solid proton conductor have tremendous promise as inexpensive devices for distributed generation. These systems known as solid acid fuel cells (SAFCs) demonstrate good activity for hydrogen oxidation and oxygen reduction, but require rather high platinum loadings. As part of an effort to decrease platinum content and increase platinum utilization we have developed a series of methodologies that use nanostructured carbons as a structure-directing electrode template. The current state-of-the-art SAFC utilizes thin films of platinum nanoparticles on a porous framework of CDP electrolyte.1 An enhancement in the platinum utilization, electrode surface area, and electrical interconnectivity may be best achieved by using carbon supports for the electrocatalyst and homogenizing framework with CDP. This approach should increase the number of catalytically active platinum sites for a given loading and may also allow for inclusion of nano-sized CDP particles, which will reduce the electrode overpotential. This reduction is due in part to an increase in the number of electrolyte-catalyst-gas triple-phase boundary points and should reduce the platinum loading in the electrocatalyst.We have investigated a variety of nanostructured carbon architectures including nanotubes and single-walled nanohorns2 as well as oxidized and surface functionalized derivatives of the above. The type of carbon support and its surface properties were shown to impart a pronounced effect on the fuel cell performance. We also propose a series of strategies to reduce the CDP particle size and increase surface area, thereby improving fuel cell performance and will discuss methods by which nanocomposite electrodes were created by infiltration of CDP into the pores or the interior of the carbons. The nature of the carbon nanostructure and the methodology employed to fabricate the composite electrodes was shown to dramatically impact the carbon corrosion during fuel cell operation. The loss of carbon by corrosion from the electrode will impact the electrical conductivity and may contribute to electrocatalyst nanoparticle coarsening. Developing a detailed understanding of these mechanisms and mitigation of the losses is an important consideration and as such we will suggest methodologies to ameliorate these effects. We will present a detailed structure-activity relationship using A.C. impedance spectroscopy, XRD, SEM, TEM, Raman microscopy, BET surface area, and measurements of proton conductivity. We also discuss how dispersion affects the homogeneity and percolation of the catalyst-CDP composites. Finally, we will discuss how functionalization of the nanostructured carbon materials affects the cell performance. (1) Papandrew, A. B.; Chisholm, C. R. I.; Elgammal, R. A.; Özer, M. M.; Zecevic, S. K. Chem. Mater. 2011, 23, 1659–1667. (2) Geohegan, D. B.; Puretzky, A. A.; Rouleau, C.; Jackson, J.; Eres, G.; Liu, Z.; Styers-Barnett, D.; Hu, H.; Zhao, B.; Ivanov, I. In Laser-Surface Interactions for New Materials Production: Tailoring Structure and Properties; Castillejo, M.; Ossi, P.; Zhigilei, L., Eds.; Springer Berlin Heidelberg, 2010; Vol. 130, pp. 1–17.