Trimetallic Platinum-Ruthenium-Copper Nanotubes for Methanol Oxidation

     Platinum materials are traditionally utilized as the standard electrocatalysts in direct methanol fuel cells; however, platinum is hindered by CO poisoning, which is an intermediate of methanol oxidation.¹ CO adsorption to platinum active sites causes slow kinetics for methanol oxidation (MOR), and requires higher onset potential for the removal of adsorbed CO and commencement of methanol oxidation.¹  Bimetallic Pt-based nanomaterials can function as efficient electrocatalysts due to the synergistic properties of alloys, specifically electronic and geometric effects.² Electronic effects occur when the center d-band is shifted upwards, weakening the bond to the adsorbed CO, allowing for easier oxidation of CO.¹ The geometric effects allow for the contraction or expansion of their crystal lattice, resulting in the alternation of the electrochemical activity for a specific fuel.³ When platinum is alloyed with either copper or ruthenium, the electrocatalysts can become more tolerant to CO adsorption.^{2, 4} In addition, the bifunctional alloys can enhance electrocatalytic activity for MOR. An oxygenated species (-OH) favorably adsorb onto the additional metal, aids in the oxidation of CO adsorbed on platinum.¹

     Anisotropic nanostructures can increase the electrochemical activity while maintaining the stability of catalysts.  Additionally, nanomaterials possess tunable surface area, morphologies, and surface facets that enable the formation of a catalyst with enhanced electrocatalytic activity.^5,6 We have demonstrated that the platinum-copper nanodendrites can enhance the MOR activity and maintain the dendritic morphology of the nanostructures.⁷

     In this work, we synthesize binary and ternary metallic nanotubes and study their activity for MOR. Bimetallic nanostructures are formed first by alloying platinum precursor salts with copper nanowires through the galvanic replacement and co-reduction mechanisms. This process forms hollow platinum-copper nanotubes with controllable surface roughness. To further improve the activity for MOR, we alloy ruthenium into platinum-copper nanostructures. The morphology, surface roughness, and composition of these ruthenium-platinum-copper nanotubes can be synthetically tuned by controlling the precursor ratio, reaction time, and reaction temperature. The electrochemical activity for MOR will be evaluated by cyclic voltammetry and chronoamperometry to characterize the electrochemical surface area, the efficiency for MOR, the tolerance for CO, and the stability of the electrocatalysts in acidic media. The enhanced activity of these multimetallic nanostructures as supportless electrocatalysts for MOR will be discussed.

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2.             Lee, H.-Y.; Vogel, W.; Chu, P. P.-J., Nanostructure and Surface Composition of Pt and Ru Binary Catalysts on Polyaniline-Functionalized Carbon Nanotubes. Langmuir 2011, 27 (23), 14654-14661.

3.             Wang, J. X.; Ma, C.; Choi, Y.; Su, D.; Zhu, Y.; Liu, P.; Si, R.; Vukmirovic, M. B.; Zhang, Y.; Adzic, R. R., Kirkendall Effect and Lattice Contraction in Nanocatalysts: A New Strategy to Enhance Sustainable Activity. Journal of the American Chemical Society 2011, 133 (34), 13551-13557.

4.             Sun, X.; Li, D.; Ding, Y.; Zhu, W.; Guo, S.; Wang, Z. L.; Sun, S., Core/Shell Au/CuPt Nanoparticles and Their Dual Electrocatalysis for Both Reduction and Oxidation Reactions. Journal of the American Chemical Society 2014, 136 (15), 5745-5749.

5.             Koenigsmann, C.; Zhou, W.-p.; Adzic, R. R.; Sutter, E.; Wong, S. S., Size-Dependent Enhancement of Electrocatalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires. Nano Letters 2010, 10 (8), 2806-2811.

6.             Li, M.; Liu, P.; Adzic, R. R., Platinum Monolayer Electrocatalysts for Anodic Oxidation of Alcohols. The Journal of Physical Chemistry Letters 2012, 3 (23), 3480-3485.

7.             Taylor, E.; Chen, S.; Tao, J.; Wu, L.; Zhu, Y.; Chen, J., Synthesis of Pt–Cu Nanodendrites through Controlled Reduction Kinetics for Enhanced Methanol Electro-Oxidation. ChemSusChem 2013, 6 (10), 1863-1867.