439758 First-Principles Approaches to Fuel Cell Catalyst Design

Sunday, November 8, 2015
Exhibit Hall 1 (Salt Palace Convention Center)
Luke T. Roling, Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, WI

Low-temperature proton-exchange membrane (PEM) fuel cells are a promising candidate to replace combustion engines in portable applications with renewable technologies. Their commercialization is limited primarily by the high cost of electrode catalysts, for which high loadings of Pt-based nanoparticles are presently required to yield suitable activity. My doctoral research has focused on obtaining a fundamental understanding of key aspects of low-temperature fuel cell catalysis through the use of highly accurate density functional theory (DFT) methods, providing atomic-scale insight into the nature of key components of PEM fuel cell catalysis.

One crucial aspect of improving fuel cell viability is the improvement of the oxygen reduction reaction (ORR) at the cathode. Although Pt is the best monometallic ORR catalyst, its expense and the requirement of a high overpotential to obtain significant catalytic activity make it unsuitable for large scale production. One strategy to address these points is to deposit Pt as conformal layers over a substrate such as Pd, which introduces compressive strain into the Pt layers and destabilizes adsorbed O and OH to improve catalytic activity. The Pt-mass activity and durability of such catalysts are simultaneously improved relative to pure Pt particles due to the core-shell templating. Novel DFT models are used to connect closely with experiments, rationalizing and predicting observed activity trends on a range of shape-selected nanocrystals toward optimal catalyst design.

Another significant aspect of fuel cell catalyst design is the identification of a viable chemical fuel to be oxidized at the fuel cell anode. Hydrogen oxidation is a clean process, but practical considerations (most notably, storage) limit its potential in portable applications. One promising candidate to replace hydrogen is dimethyl ether (DME), perhaps most notably due to its compatibility with existing fuel infrastructures for ease of transportation and storage. A detailed investigation of DME electro-oxidation, including determination of the thermodynamics and kinetics of a complete network of elementary reaction steps, on three Pt facets demonstrates the structure sensitivity of DME electro-oxidation and informs catalyst design for improving DME electro-oxidation.

In addition to engineering surface reactivity, DFT also provides critical insights into practical considerations governing the formation of catalysts. In one example, calculations show that Pt and Pd intermix during the layer-by-layer formation of core-shell alloy structures due to favorable substitution of Pt into the Pd surface. As a result, a small amount of Pd is dispersed into Pt layers as they are added. Calculations further show that this dispersion of Pd into the Pt layers is a necessary and sufficient condition for the formation of a new class of hollow Pt nanostructures with well-defined facets and excellent catalytic activity for reactions such as the ORR. A broader DFT study has applied these same principles to a number of transition metal surfaces, allowing the prediction of substitution and intermixing for dozens of permutations of adlayer and substrate metals.

My future work will extend these catalyst design principles to a wider range of chemical processes, including biomass conversion (both electrochemical and non-electrochemical) and other strategies for sustainable energy storage and production.

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