Image courtesy of Argonne National Laboratory
Comparing the performance of palladium clusters of different sizes for water oxidation; (inset) calculated structure of the Pd6O6 cluster (red = oxygen, grey = palladium), a catalytic site for the reaction.
Researchers have discovered through experiments that palladium (Pd) clusters are among the most active oxidation catalysts for electrochemical water splitting reactions, demonstrating that a critical cluster size greatly increases the catalytic activity. Calculations suggest the effect is due to the arrangement of the bridges between Pd atoms that are maximized in certain larger clusters, which are still smaller than a nanometer (billionth of a meter) in size.
The combined results of computational studies and experiments suggest a promising new approach for the rational design and synthesis of improved water oxidation electrocatalysts for an electrochemical system that produces hydrogen fuel through water splitting or for similar solar-to-fuels applications.
Water oxidation is a key catalytic step for fuel generation with electrochemical processes, such as the production of hydrogen fuel by water splitting reactions. This reaction, which evolves oxygen gas and provides four electrons to produce hydrogen in the water reduction portion of the electrolysis cell, has been the rate limiting step for fuel generation. Recently, significant progress has been made in synthesizing electrocatalytic materials with greater efficiencies and increased reaction rates, both key parameters to enable their commercial use in water electrolysis or related solar-to-fuels applications. The complexity of both the catalytic materials and the water oxidation reaction makes understanding how the catalytic site functions critical to improving the process. Researchers at Argonne National Laboratory and their collaborators have recently studied water oxidation under alkaline conditions using size-selected clusters of Pd (e.g., Pd4, Pd6, and Pd17 clusters) to probe the relationship between cluster size and the water oxidation reaction. They find that while the smaller Pd4 cluster shows no increase in reaction rate over the bare support material, Pd6 and Pd17 clusters are among the most active (in terms of the number of oxygen molecules produced per second per Pd atom) catalysts known. Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites (which are only present in larger, three-dimensional clusters) are active for the water oxidation reaction. The ability to experimentally synthesize size-specific clusters allowed direct comparison to theory. The system (Pd4, Pd6, or Pd17 clusters on an ultrananocrystalline diamond-coated silicon electrode) showed stable electrochemical potentials over several cycles, and synchrotron studies of the electrodes showed no evidence for evolution or dissolution of either the electrode support material or the clusters. The results of this study illuminate a potential pathway to the development of enhanced electrocatalysts that could utilize theoretical calculations to predict the catalytic activity of metallic nanoclusters with varied compositions.
Argonne National Laboratory
DOE, Office of Science, Basic Energy Sciences program; X-ray scattering work was carried out at the Advanced Photon Source; J.G. was supported through a DOE Early Career Award; R.L.J. and C.J.H. acknowledge the University of Birmingham (U.K.) North America fund for travel support, and Argonne's Center for Nanoscale Materials for computer resources.
G. Kwon, G. A. Ferguson, C. J. Heard, E. C. Tyo, C. Yin, J. DeBartolo, S. Seifert, R. E. Winans, A. J. Kropf, J. Greeley, R. L. Johnston, L. A. Curtiss, M. J. Pellin, and S. Vajda, ACS Nano, 7, 5808 (2013); [DOI: 10.1021/nn400772s]
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