Pt Monolayer Shell on Hollow Pd Core Electrocatalysts: Scale up Synthesis, Structure, and Activity for the Oxygen Reduction Reaction

The rapidly increasing demand for fossil fuels used in transportation and power generation, and their detrimental environmental effects, have resulted in a wide-spread challenge for the development of renewable energy technologies. Fuel cells are one of the most promising clean energy technologies, particularly attractive for automobile applications, due to their high efficiency, high energy density, and low or zero emissions. The most challenging problem of their application is the slow kinetics of oxygen reduction reaction (ORR) at fuel cell cathodes, even on Pt - the best single element electrocatalyst, which causes a large loss of the cell voltage, resulting in significant efficiency loss. In addition, Pt dissolves under certain operating condition of fuel cells. To mitigate these two drawbacks, catalysts containing large amounts of expensive Pt are required. This is one of the main reasons for the slow commercialization of fuel cells.

To address these drawbacks, the use of Pt monolayer (Pt_ML) electrocatalysts was proposed to reduce the cost of Pt while attempting to enhance their ORR activity and stability [1, 2]. Such catalysts, consisting of a monolayer of Pt on a substrate of another material, minimize the amount of Pt while ensuring that all Pt atoms are available at the surface for catalytic activity [1, 2]. Additionally, through geometric and electronic interactions with the substrate [3, 4], a Pt_MLcan change its electronic properties and be more active and durable than pure Pt electrocatalysts.

Several single crystal surfaces, including Pd (111), Ru (0001), Ir (111), Rh (111), and Au (111) have been investigated as supports for Pt_ML catalysts for the ORR [5]. Variations in ORR activity could be accounted for by the oxygen binding energy, which must be tuned to intermediate strength. This leads to a classic catalysis ‘volcano plot’ of ORR activity vs. oxygen binding energy with Pt_MLon Pd(111) on the top of the volcano plot having higher ORR activity than Pt(111) [5].

Since oxygen binding energy appears to be the major descriptor of the ORR kinetics, DFT calculations suggests that further enhancement in ORR activity of Pt_ML on Pd could be accomplished by weakening the O and OH binding energies [5]. This could be achieved by additional contraction of the Pt_ML shell. A hollow core is an interesting structure to investigate geometric interaction between the Pt_ML shell and the core because a hollow core may induce a desirable lattice contraction of a Pt_ML, leading to improved ORR activity by reducing the oxygen binding energy.

In this contribution, the synthesis, characterization and kinetics of the ORR of a Pt_MLshell on Pd(hollow), or Pd-Au(hollow) core electrocatalysts are reported. Comparisons between the ORR catalytic activity of the electrocatalysts with hollow cores and those of Pt solid and Pt hollow nanoparticles were obtained using the rotating disk electrode technique. Hollow nanoparticles were made using Ni or Cu nanoparticles as sacrificial templates. The Pt ORR specific and mass activities of the electrocatalysts with hollow cores were found to be considerably higher than those of the electrocatalysts with solid cores, Fig. 1. This enhanced Pt activity is attributed to the smooth surface morphology and hollow-induced lattice contraction. In addition, the hollow particles have a mass-saving geometry.

Acknowledgment

Work at Brookhaven National Laboratory is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886. The work at BNL was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering, under Contracts No. DE-AC02-98CH10886. DB thanks the MICINN, Spain (MAT2010-20442 and MAT2011-28673-C02-01) for FPU grant.

References

1. R. R. Adzic, J. Zhang, K. Sasaki, M. B. Vukmirovic, M. Shao, J. X. Wang, A. U. Nilekar, M. Mavrikakis, J. A. Valerio, F, Uribe, Top. Catal., 46, 249 (2007).

2. J. Zhang, Y. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki, and R. R. Adzic, J. Phys. Chem. B, 108, 10955 (2004).

3. B. Hammer, J. K. Nørskov, Adv. Catal., 45, 71 (2000).

4. J. Greeley, J. K. Nørskov, M. Mavrikakis, Annu. Rev. Phys. Chem., 53, 319 (2002).

5. J. Zhang, M. B. Vukmirovic, Y. Xu, M. Mavrikakis, R. R. Adzic, Angew. Chem. Int. Ed., 44, 2132 (2005).