62nd Annual Report on Research 2017

Significance:

Polymer electrolyte membrane fuel cells (PEMFCs) and direct formic acid fuel cells (DFAFCs) are a highly promising and well-developed technologies^1-2 that will likely support a future hydrogen economy.³ A major strategy in this regard is the development and application of sub-5 nm diameter bimetallic catalysts instead of bulk platinum.^4-8

Background:

Clusters of nanoparticles (NPs) offer advantages over isolated NPs. An interparticle gap with decreasing nanoscale dimensions permits increased readsorbtion of chemical intermediates and hence more efficient power output.⁸ Furthermore, the potential drop in an interparticle gap approaches 300 mV, suggesting that fuel cell reactions are catalyzed with less overpotential and less interference from other ions.⁹ As the distance in neighboring Pt NPs approaches 1 nm, the specific activity of Pt NPs for the oxygen reduction reaction can be increased 6 fold compared to that of a commercial catalyst. Controlling the composition and particle arrangement in clustered NPs is challenging and hence the role of Pt-based clusters in catalysis is largely unknown.^9-10

Achievements:

Publications from this research are listed below.¹¹ This research project has established a versatile method for the preparation of a self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-PVP) block copolymer (BCP) template capable of metal ion-loading for NP synthesis (see TOC image). We have shown that selection of the BCP template determines the length scale of the array of NP clusters while the distribution of metal anions co-dissolved in a common loading bath specifies for the composition of the NPs.^11a Using this approach, our group has studied two metal NP systems and determined important structure-property relationships between the arrays of NP clusters and their electrocatalytic activity. In the case of PtAu, the BCP template synthesis yields PtAu with a nanostructure comprised of a Pt-rich shell located on a PtAu alloy core. The PtAu core-shell bimetallic NPs have a very high density of electrochemically active Pt surface sites. Accordingly, the electrocatalytic activity for the oxidation of methanol with core-shell PtAu NPs was ~ 2-4 fold that of a Pt benchmark catalyst (ETEK) and only 28% less than that of the PtRu bimetallic benchmark catalyst. The Au that is present in the PtAu NPs was beneficial to Pt-rich electrocatalysts, as the carbonaceous poisoning I_f/I_b metric was greatly enhanced relative to both benchmark materials (~ 2-3 fold increase). In the case of the PtIr system (see TOC image), the BCP template synthesis yields an array of alloyed PtIr NP.^11b Many of the bimetallic PtIr NPs are much higher in activity than typical industry/research standards. The most active array of ~Pt₈₈Ir₁₂ NPs had mass activities for formic acid oxidation that were four-fold higher that a PtRu industrial research standard. Overall, we anticipate that BCP template method of preparing highly active clusters of low-diameter, Pt-containing NPs will have a positive impact on several emerging areas in nanoscience such as energy devices, solar fuels, and heterogeneous catalysis. Current research is focused on the generation of electrocatalytically active and stable bimetallics NPs with non-precious metals. Recent work has focused on the synthesis of colloidal analogues with random copolymers of PS and PVP.^11c The colloidal analogues are capable of electrostatically loading metal-anions such as aurate and borate-capped silver NPs.  For the latter, we have studied the ability of organic molecules to interact with the surface of the NPs using Raman scattering spectroscopy.

Impact of Research on PI's Career and that of the participating students. To date, the research has allowed the PI to publish three full papers in highly-reputable, peer reviewed venues.¹¹ Two new collaborations have resulted from the work. The research has allowed three (3) different MS students to fulfill their requirements for their degree and, for each of them, to author their first publication.  Furthermore, eight (8) different undergraduates have participated in the published research.  Many of these students have gone onto pursue higher-level graduate work in the US and Canada. 

References:

(1)       Vielstich, W.; Lamm, A.; Gasteiger, H. A. Handbook for Fuel Cells: Fundamentals, Technology and Applications; Wiley: Chichester, England; Hoboken NJ, 2003.

(2)       Satyapal, S. Fuel Cell System Cost -2011, 2011.

(3)       The US Department of Energy (DOE).  http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/fuel_cells.pdf, 2012.

(4)       Adlhart, C.; Uggerud, E. Reactions of platinum clusters Ptn, n=1-21, with CH4: to react or not to react. Chem. Commun. 2006, 2581-2582.

(5)       Sun, Y.; Zhuang, L.; Lu, J.; Hong, X.; Liu, P. Collapse in crystalline structure and decline in catalytic activity of Pt nanoparticles on reducing particle size to 1 nm. J. Am. Chem. Soc. 2007, 129, 15465-+.

(6)       Yamamoto, K.; Imaoka, T.; Chun, W. J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi, A. Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions. Nat. Chem. 2009, 1, 397-402.

(7)       Wang, C.; Markovic, N. M.; Stamenkovic, V. R. Advanced Platinum Alloy Electrocatalysts for the Oxygen Reduction Reaction. ACS Catalysis 2012, 2, 891-898.

(8)       Fuhrmann, J.; Zhao, H.; Langmach, H.; Seidel, Y. E.; Jusys, Z.; Behm, R. J. The Role of Reactive Reaction Intermediates in Two-Step Heterogeneous Electrocatalytic Reactions: A Model Study. 11 2011, 501–510.

(9)       Nesselberger, M.; Roefzaad, M.; Hamou, R. F.; Biedermann, P. U.; Schweinberger, F. F.; Kunz, S.; Schloegl, K.; Ashton, S.; Hieiz, U.; Mayrhofer, K. J. J.; Arenz, M. The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters. Nat. Mater. 2013, 12, 919-924.

(10)     Speder, J.; Altmann, L.; Baumer, M.; Kirkensgaard, J. J. K.; Mortensen, K.; Arenz, M. The particle proximity effect: from model to high surface area fuel cell catalysts. RSC Advances 2014, 4, 14971-14978.

 (11) (a) Mikkelsen, K.; Cassidy, B.; Hofstetter, N.; Bergquist, L.; Rider, D. A., Block Copolymer Templated Synthesis of Core-Shell PtAu Bimetallic Nanocatalysts for the Methanol Oxidation Reaction. Chem. Mater. 2014, 26, 6928–6940.

(b) Taylor, A. K.; Perez, D. S.; Zhang, X.; Pilapil, B. K.; Engelhard, M. H.; Gates, B. D.; Rider, D. A. Block Copolymer Templated Synthesis of PtIr Bimetallic Nanocatalysts for the Formic Acid Oxidation Reaction J. Mater. Chem. A 2017, 5,  21514–21527.

(c) Curtis, T.; Taylor, A. K.; Alden, S. E.; Swanson, C.; Lo, J.; Knight, L.; King, A.; Gates, B. D.; Emory_, S. R.; Rider, D. A. Synthesis and Characterization of Tunable, pH-Responsive Polymer Microgels for Surface-Enhanced Raman Scattering (SERS) Detection. Macromolecules 2017, (article ID: ma-2017-02414e). ADDIN EN.REFLIST