Extensive research into the oxygen reduction reaction (ORR) in both acidic and alkaline electrolyte has yielded robust guidelines for the development of highly active catalysts.1–3 For example, Lima et al. demonstrated a volcano-like dependence of ORR specific activity upon the calculated metal d-band center energies of extended polycrystalline electrodes specific to alkaline electrolyte.4 Such results revealed optimum activity for Pt monolayers atop Pd(111) extended electrodes. The challenge has been the translation of these guides, often developed on extended polycrystalline electrodes, into active, durable, and scalable catalysts.
Techniques for generating thin overlayers or shells of noble metal catalysts on cores include galvanic displacement, chemical processing/etching, or metal-ligand-mediated solution phase synthesis. We have developed an approach based on vapor deposition from metalorganic precursors on monolithic sacrificial templates to fabricate multicomponent and highly dispersed catalysts. Additionally, we have the ability to use subsequent heat treatments to derive specific structural features, such as increased porosity, phase segregation, or annealing.5–7 These structures have been used to mimic the ligand effects observed on extended electrodes but for relatively high surface area materials. Here we synthesize fractional overlayers of Pt on Pd nanotubes (PtPdNTs) to enhance the alkaline ORR reaction through optimized electronic effects.
Electrocatalysts were tested using a conventional three-electrode electrochemical cell in 0.1M KOH. Electrodes were made by drop casting supported or unsupported catalysts onto a glassy carbon rotating disk electrode and allowing the ink to dry slowly in a nearly saturated atmosphere. Polarization curves were obtained over the range 0.05 – 1.125 V vs. RHE at 10 mV/s under rotation. Potentiostatic electrochemical impedance spectroscopy (EIS) spectra were recorded from 200 kHz to 1 Hz at 0.6 V vs. RHE with a 10 mV sine perturbation amplitude under O2 mass transport limited conditions and were used to correct for cell resistance. Electrochemical active surface areas (ECSAs) were obtained using Cu-stripping.
Electrochemical results yielded significantly enhanced specific activity and exchange current density for the PtPd NTs vs. monometallic Pt and Pd NTs, as well as to monometallic Pt/C and Pd/C catalysts. Additionally, the highly porous nature of the nanotubes resulted in increased mass activity vs. the high surface area materials. XRD and HR-TEM were used to confirm the Pt and Pd segregation. These exceptional results demonstrate the power of vapor grown materials to translate the activity enhancements from extended electrodes to practical electrocatalysts.
(1) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886–17892.
(2) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552–556.
(3) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B Environ. 2005, 56, 9–35.
(4) Lima, F. H. B.; Zhang, J.; Shao, M. H.; Sasaki, K.; Vukmirovic, M. B.; Ticianelli, E. A.; Adzic, R. R. Catalytic Activity−d-Band Center Correlation for the O2 Reduction Reaction on Platinum in Alkaline Solutions. J. Phys. Chem. C 2007, 111, 404–410.
(5) Papandrew, A. B.; Chisholm, C. R. I.; Zecevic, S. K.; Veith, G. M.; Zawodzinski, T. A. Activity and Evolution of Vapor Deposited Pt-Pd Oxygen Reduction Catalysts for Solid Acid Fuel Cells. J. Electrochem. Soc. 2013, 160, F175–F182.
(6) Papandrew, A. B.; Atkinson III, R. W.; Unocic, R. R.; Zawodzinski, T. Ruthenium as a CO-Tolerant Hydrogen Oxidation Catalyst for Solid Acid Fuel Cells. J. Mater. Chem. A 2015.
(7) Papandrew, A. B.; Atkinson, R. W.; Goenaga, G. A.; Kocha, S. S.; Zack, J. W.; Pivovar, B. S.; Zawodzinski, T. A. Oxygen Reduction Activity of Vapor-Grown Platinum Nanotubes. J. Electrochem. Soc. 2013, 160, F848–F852.
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