Identification of Highly Active Catalytic Sites for Oxygen Reduction Reaction in Carbon Nanostructures from First-Principles Investigation

Abstract

Proton-exchange membrane (PEM) fuel cells are considered to be a promising technology for use as alternative energy sources for transport, stationary and portable applications. They use hydrogen and oxygen to generate electricity through an electrochemical process which includes the oxygen reduction reaction (ORR) at the cathode. Currently, platinum (Pt)-based catalysts are commonly used to speed up the sluggish ORR that limits the efficiency of low temperature PEM fuel cells. However, Pt is expensive and scarce, drawing much interest in developing non-Pt catalysts. Carbon nanomaterials, including fullerenes, carbon nanotubes (CNTs), graphene, and their composites, have been actively studied as a potential candidate to replace Pt-based ORR catalysts. These sp 2-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. The 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. We have theoretically 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. Our effort has been focused on identification of active sites that allow favorable O2 adsorption and subsequent reduction based on extensive first-principles quantum-mechanical calculations. In this talk, we will present some of our recent findings. Using density functional theory (DFT) calculations, we have identified a novel method for the activation of graphene. This modification creates quasi-localized electron states in the graphene π-bonding system which are vulnerable to form bonds with O2. The result is favorable O2 adsorption, which despite being necessary for ORR activity is not predicted for previously proposed active sites. In this work, we applied an 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. The rate limiting step of the ORR in this system is predicted to be desorption of final reduction products. DFT allows us to study this desorption separately from the accompanying electron injection. We predict that there is a small but surmountable activation barrier that must be overcome for the reduction product to be removed. Excitingly, the predicted maximum overpotential for any elementary reaction step of the modified carbon structures that we have found is approximately 0.6 eV, which can be competitive when compared to standard carbon supported Pt (Pt/C) catalysts. Our study provides insight into molecular mechanisms underlying the superior ORR activity of nanostructured carbon materials and also guidelines for the rational design and development of high-efficiency, cost-effective carbon-based ORR catalysts for fuel cell applications.