No blood for platinum: cheaper alternatives for fuel cell catalysts
By Christopher MooreFuel cell devices directly convert chemical energy into electricity by electrochemical reactions, and have received recent interest due to their lack of moving parts and relatively clean operation. Various types of fuel cells are currently discussed in the scientific literature, though polymer electrolyte fuel cells (PEFC) show characteristics that would make them suitable for automobile engines that produce zero emissions. (For a discussion on fuel cells in general, see this article.)
Several problems currently exist that limit the commercial viability of PEFCs, the largest being the cost of catalyst material. Though Pt has historically been the optimum cathode electrocatalyst for fuel cells, [1] the use of expensive Pt and Pt-based catalysts in fuel-cell electrodes makes their cost prohibitive. Since Pt requirements scale with fuel-cell size, large-scale fuel-cell costs can not be reduced through efficient production and economies of scale. If the transition to a hydrogen economy is to be realized, reductions in fuel cell costs are necessary. Otherwise the familiar rallying cry “no blood for oil” may be replaced by “no blood for platinum”. Recent studies have been conducted on means to reduce the amount of platinum necessary for successful catalysis, as well as studies to eliminate platinum from the process altogether. Specifically, in this article I examine studies of platinum monolayers, palladium alloys, and cobolt-polypyrrole composite catalysts.
As mentioned, means of reducing Pt loading in fuel cell cathodes have been researched with the caveat that reductions in Pt must not significantly slow O2 reduction kinetics. Decreases in Pt particle size (nanoparticles) and attempts at alloying Pt have resulted in little success, [2,3] though Zhang et al. report that deposition of monolayer amounts of Pt on suitable metal nanoparticles may significantly reduce Pt loading while increasing reduction kinetic rates.[4] Specifically, the kinetics of O2 reduction was studied for two types of systems: Pt monolayers deposited on Pd(111) surfaces and carbon-supported Pd nanoparticles. The method used to form these Pt monolayers is discussed in reference 5. Using the rotating disk-ring electrode technique, the authors show O2 reduction enhancement at Pt monolayers on Pd(111) and Pd nanoparticle surfaces compared with reaction on Pt(111) and Pt nanoparticles. The authors attribute this increase in catalytic activity to a decrease in the formation of PtOH, as well as enhanced atomic scale surface roughness and low coordination of some atoms. Coupled with increased catalytic activity is a reduction in Pt loading, with a mass-specific activity for the Pt/Pd/C electrode 5-8 times higher than that of Pt/C electrocatalysts. In other work, Zhang et al. demonstrate reduced Pt-loading and increased catalytic activity for Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co systems.[6] So it is possible to reduce the amount of platinum without degrading performance.
Though significant reductions in Pt loading have been achieved via monolayer deposition, the extreme high cost of Pt has led some researchers to investigate catalytic activity of alloys that contain no Pt. Using a high throughput screening method based on scanning electrochemical microscopy (SECM), [7] Fernandez et al. report that Pd-Co-Au and Pd-Ti show essentially equal or slightly better catalytic performance compared to traditional Pt catalysts.[8] The authors produced an array of bi- or tri-metallic catalyst spots with different compositions on a glassy carbon support. A microelectrode tip is used to generate oxygen at a constant current as the tip is scanned across the surface. Each spot within the array will register a current proportional to its rate of oxygen reduction, so varying compositions could be screened as to catalyst efficacy. This allowed the researchers to determine which composites would be effective without the sacrifice of vast amounts of time. The performance of effective membranes (found via SECM) in proton exchange membrane fuel-cells (PEMFCs) was studied, with steady-state polarizations curves showing equal or better performance than that of commercial Pt catalysts at the same loading.
Although these alloys would eliminate the necessity of Pt and reduce overall cost, they still consist of precious and/or semi-precious metals. Very recenty, studies have been conducted on catalysts that rely on non-precious metals such as cobalt and iron. The pyrolysis of various porphyrins has been shown to produce MeNx (Me = non-precious metal) type species demonstrating active oxygen reduction reaction sites, [9,10] though most of these materials offer reduced catalytic activity and poor stability. Bashyam and Zelenay report a class of metal composite catalysts that require relatively cheap cobalt and a heteroatomic polymer, and are synthesized via a simple chemical method.[11] Fuel-cell performance plots for these cobalt-polypyrrole-carbon (Co-PPY-C) catalysts show favorable comparison with other non-Pt cathode catalysts, specifically the Pd-Co-Au and Pd-Ti alloys discussed above.[8] Current-density vs. time plots also demonstrate the relative long-term performance and stability of Co-PPY-C catalysts, with no appreciable drop in current density after 100 hours of operation. Although fundamental electrochemistry studies are still necessary to understand the O2 mechanism, the potential is demonstrated for the making of a variety of non-precious metal composite materials with high catalytic activity and stability.
Fuel-cell catalyst costs will potentially be reduced, allowing for the broad commercialization of fuel-cell powered devices. Our cars may one day be powered by cheap hydrogen fuel cells made from (relatively) common and cheap materials, thus preventing wars over valued platinum mines. The research has focused on decreasing Pt loading and developing composites of precious and non-precious metals that demonstrate favorable catalytic activity and stability.
* This article was adapted from a talk given in the Department of Chemistry at Virginia Commonwealth University on Oct. 31st 2006.
Refrences
[1] B.C.H. Steele, A. Heinzel, Nature, 414, 345-352 (2001).
[2] T. Toda, H. Igarashi, M.J. Watanabe, J. Electroanal. Chem., 460, 258 (1999).
[3] U.A. Paulus, A. Wokaun, G.G. Scherer, T.J. Schmidt, V. Stamenkovic, N.M. Markovic, P.N. Ross, Electrochim. Acta, 47, 3787 (2002).
[4] J. Zhang, Y. Mo, M.B. Vukmirovic, R. Klie, K. Sasaki, R.R. Adzic, J. Phys. Chem. B, 108, 10955-10964 (2004).
[5] S.R. Brankovic, J.X. Wang, P.N. Ross, R.R. Adzic, Surf. Sci., 477, L173-L179 (2001).
[6] J. Zhang, H.B. Lima, M.H. Shao, K. Sasaki, J.X. Wang, J. Hanson, R.R. Adzic, J. Phys. Chem. B, 109, 22701-22704 (2005).
[7] J.L. Fernandez, D. Walsh, A.J. Bard, J. Am. Chem. Soc., 127, 357-365 (2005).
[8] J.L. Fernandez, V. Raghuveer, A. Manthiram, A.J. Bard, J. Am. Chem. Soc., 127, 13100-13101 (2005).
[9] D. Villers, X. Jacques-Bedard, J.P. Dodelet, J. Electrochem. Soc., 151, A1507-A1515 (2004).
[10] B. Wang, J. Power Sources, 152, 1-15 (2005).
[11] R. Bashyam, P. Zelenay, Nature, 443, 63-66 (2006).
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