First-Principles Evaluation of the Catalytic Activity of Nanostructured Carbon for the Oxygen Reduction Reaction at Fuel Cell Cathodes

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
G. P. Hartmann and G. S. Hwang (University of Texas at Austin)
Carbon nanomaterials, including fullerenes, carbon nanotubes (CNTs), graphene, and their composites, have been actively studied as a potential candidate to replace platinum(Pt)-based ORR catalysts as fuel cell cathodes. These sp2-bonded carbon nanostructures are inherently constituted by largely extended p-conjugation systems, making them essentially unreactive toward molecular oxygen (O2) in their pristine forms. However, the conjugated p-systems can be severely disrupted with localized p electrons through dopant incorporation, defect creation, and/or local lattice distortions. Doped, defective and distorted carbon structures are currently thought to be responsible for O2 adsorption/activation and thus ORR, but the active sites and the exact molecular mechanisms are still uncertain.

First principles computational methods are well suited to study in detail reactions at the surface of a heterogeneous catalyst and through these methods we explored the mechanisms by which modification can activate graphene for the adsorption and reduction of oxygen, with an aim to define design guidelines for carbon ORR catalysts. Using density functional theory (DFT) calculations, we have identified a novel method for the activation of graphene while establishing a computational scheme for the study of electrocatalysis by carbon. An important component of our methodology is the application explicitly solvated system to model the ORR at the solid-liquid interface with aqueous solvent. Many previous DFT studies were modeled in the gas phase where O2 adsorption is consistently predicted to be endothermic on carbon. Our study clearly demonstrates that a more accurate description of interactions at the graphene-solvent interface is necessary to properly describe O2 adsorption, which appears strongly influenced by solvation. Ab-initio MD simulations were performed with the explicit solvent model to outline a full reaction mechanism including novel elementary reaction steps. Another unique approach taken in our studies is the use of detailed electronic structure analysis to determine the factors influencing overpotentials in the course of the ORR.

In this presentation, we will focus on introducing the computational scheme that we have developed for the investigation of molecular mechanisms underlying the ORR on carbon catalysts and the synergetic effects of doping, defect creation, and lattice distortions on their catalytic performance for oxygen reduction. We will also discuss the key guidelines for the rational design and development of high-efficiency, cost-effective carbon-based ORR catalysts for fuel cell applications.