Electrochemical Study of Platinum-Ruthenium-Copper Trimetallic Nanostructures for Methanol Oxidation

Abstract

Direct methanol fuel cells commonly utilize platinum based catalysts due to their efficiency for methanol oxidation (MOR). Platinum based electrocatalysts are limited by intermediates adsorbing onto the electrocatalyst’s surface.1 Methanol oxidation forms multiple intermediate species, which are integral in generating electrons that drive the fuel cell.2 The production of intermediate species can also slow down reaction kinetics, while specific intermediate species can strongly adsorb onto active sites along the platinum catalyst surface reducing the number of MOR reactive sites. In order to remove the adsorbed intermediates, the MOR onset potential will shift to a more oxidizing potential.1These limitations can be addressed by developing a durable electrocatalyst that resists CO poisoning, maintains efficiency for MOR, and reduces the onset potential of methanol oxidation. An electrocatalyst comprised of nanomaterials could address some of these limitations, due to their synthetically tunable properties. Nanomaterials can be tailored to possess the ideal composition, morphology, and surface roughness for the catalysis of specific reactions. Previous studies have shown that Pt-based nanomaterials are efficient electrocatalysts, and their activity can be attributed to energetically favorable surface facets, as well as synergistic properties of alloys, specifically electronic and geometric effects.3 Geometric effects allow for the contraction or expansion of the nanomaterial’s crystal lattice, altering its electrochemical activity for a specific fuel.4 Electronic effects occur when the center d-band shifts, weakening the bond to the adsorbed species. In the case of MOR, the oxidation of CO would become more energetically favorable.1 These effects improve the resistance of intermediate poisoning, as well as the activity of the electrocatalyst by freeing more reactive sites. These effects can also influence the activity of pair sites in platinum alloyed nanomaterials, through the bifunctional mechanism.1 , 3 , 5 Previous work by our group investigated the effect of anisotropic alloyed nanostructures electrochemical activity, which demonstrated that the platinum-copper nanodendrites can enhance the MOR activity, while maintaining the dendritic morphology of the nanostructures.1 Current research focuses on the development of multimetallic nanostructures with various morphologies, surface roughness, as well as investigating each nanomaterial’s electrocatalytic activity. Multimetallic nanostructures are formed by first synthesizing copper nanomaterials. Then platinum and ruthenium precursor salts can alloy and co-reduce onto the vertices and edges of the copper nanomaterials, through galvanic replacement and co-reduction mechanisms. The morphology, porosity, surface roughness, and composition of these ruthenium-platinum-copper nanomaterials can be synthetically tailored. Platinum and ruthenium were chosen as secondary alloying metals with copper due to their binding energy for species such as –CO, -OH.6 During the synthetic process pair sites between Pt, Cu, and Ru will form, which could improve resistance towards intermediate poisoning and activity for MOR. The activity of these supportless electrocatalysts will be evaluated by cyclic voltammetry and chronoamperometry. The electrochemical surface area, the charging double layer capacitance, surface diffusion, the efficiency for MOR, and the resistance for CO for each electrocatalyst will be evaluated. Additionally, the morphological and electrochemical durability of each multimetallic nanomaterial in electrolyte and analyte will also be studied in this work. As previously discussed the pair site can affect the electrocatalytic activity for MOR; this work will also study the influence of the nanomaterial's copper core on the electrocatalytic properties. 1. Chen, A.; Holt-Hindle, P., Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications. Chemical Reviews 2010, 110(6), 3767-3804. 2. Cohen, J. L.; Volpe, D. J.; Abruna, H. D., Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes. Physical Chemistry Chemical Physics 2007, 9(1), 49-77. 3. 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. 4. 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. 5. 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. 6. Rossmeisl, J.; Ferrin, P.; Tritsaris, G. A.; Nilekar, A. U.; Koh, S.; Bae, S. E.; Brankovic, S. R.; Strasser, P.; Mavrikakis, M., Bifunctional anode catalysts for direct methanol fuel cells. Energy & Environmental Science 2012, 5 (8), 8335-8342.