Deciphering the Role of Substitution in Transition-Metal Phosphorous Trisulfide (100) Surface: A Highly Efficient and Durable Pt-free ORR Electrocatalyst.

The rational design and development of earth-abundant, cost-effective, environmentally benign, and highly robust oxygen reduction reaction (ORR) electrocatalysts can circumvent the obstacles associated with the large-scale commercialization of fuel cells. Here, using first-principles-based density functional theory (DFT), we have computationally screened the potential and feasibility of transition-metal phosphorous trisulfides (TMPS ) (100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS (100) surface emerges to be the best among TMPS surfaces with optimal O activation resulting in very low overpotential. The study reveals that ORR occurs on the MnPS surface via 4e reduction associative pathway where the kinetically rate-determining step (RDS) is the formation of O*+H O with an activation barrier of 0.66 eV. Additionally, high CO tolerance and easy desorption of H O make MnPS a robust catalyst. Substitution in half of the Mn sites of MnPS (100) surface with Co considerably enhances the ORR activity. Mn Co PS (100) surface exhibits an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H O on Mn Co PS proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS (100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity. Besides, Mn Co PS (100) surface obtains the apex of the volcano plot due to its optimal position of the d-band center which further justifies the improved ORR activity. With Pt-like onset potential, facile H O desorption ability, and robust dynamic and thermal stability, this CO tolerant Mn Co PS catalyst can be a potential alternative to Pt with encouraging practical viability.