Graphyne As a Promising Metal-Free Electrocatalyst for Oxygen Reduction Reactions in Acidic Fuel Cells: A DFT Study

Graphyne, a new two-dimensional periodic carbon allotrope with a one-atom-thick sheet of carbon built from triple- and double-bonded units of two sp- and sp(2)-hybridized carbon atoms, has been shown in recent studies to have the potential for high-density hydrogen and lithium storage. We report here a density functional theory (DFT) study of an oxygen reduction reaction (ORR) involving graphyne and demonstrate that graphyne is a good, metal-free electrocatalyst for ORRs in acidic fuel cells. We optimized the geometrical structure, calculated the charge densities on each carbon atom in the graphyne, and simulated each step of the ORR reaction involving graphyne. The simulation results indicate that the distribution of the charge density at each carbon atom on the graphyne plane is not uniform and that a large number of positively charged carbon atoms, which are beneficial to the adsorption of O-2 and OOH+ molecules, can behave as catalytic sites to facilitate ORRs. When H is introduced into the system, a series of reactions can occur including the formation of an O-C chemical bond between oxygen and graphyne, breakage of the O-O bond, and the creation of water molecules. The results also indicate a decrease in the energy of the system and a positive value of the reversible potential for each reaction step on the graphyne surface. In addition, a spontaneous electron transformation process occurs during the ORR along a four-electron pathway. The results presented here should lead to an improvement in the catalytic efficiency of carbon nanomaterials and provide a theoretical framework for the analysis of their catalytic activity. This paper highlights the urgent need for new experimental syntheses for graphyne.