A first principles study on aluminium-sulfur co-doped graphene as an electro-catalyst for oxygen reduction reaction in a PEM fuel cell

Abstract

High cost, sluggish reaction, and instability of platinum catalysts are the key hindrances that have slowed down extensive use and production of polymer electrolyte membrane fuel cell (PEMFC). To address these challenges, there is a need for development of stable, low-cost, and highly efficient oxygen reduction reaction (ORR) catalysts to lower platinum catalyst usage. Doping graphene with hetero-atoms has been found to promote the ORR electro-catalytic activity. In this work, aluminum and sulfur co-dopants’ effects on graphene's structural, electronic, and electro-catalytic activity are studied using first-principles density functional theory study (DFT). The bond lengths of Al-C, C-S, and Al-S elongated to 1.618 A, 1.619 A, and 1.952 A, respectively, after the geometry optimization task. The electronic properties of graphene are altered by introducing Al and S dopants, as shown in the calculated band structures and density of states (DOS). Pure graphene band structure has the conduction and valence bands touching at the K point with a bandgap of 0 eV, indicating its zero-gap character. When graphene is co-doped with both Al and S atoms, a bandgap of 1.059 eV is observed. The obtained DOS indicates a shift near the Fermi level and an increase in the number of occupied states with the dopants’ introduction. Further, the ORR process is evaluated based on the dissociative mechanism involving four intermediate steps. The obtained Gibbs free energy of the ORR intermediates; O, OH, and OOH at zero cell potential (U=O) are 2.24 eV, 1.197 eV, and 3.91 eV, respectively. The Gibbs free energy diagram shows a downhill progression, indicating that all the ORR intermediates on the Al-S co-doped graphene catalyst are exothermic. The DFT simulation results in this work suggest that the proposed Al-S co-doped graphene would be an effective electro-catalyst for the ORR process in fuel cells.