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 3 ) (100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS 3 (100) surface emerges to be the best among TMPS 3 surfaces with optimal O 2 activation resulting in very low overpotential. The study reveals that ORR occurs on the MnPS 3 surface via 4e reduction associative pathway where the kinetically rate-determining step (RDS) is the formation of O*+H 2 O with an activation barrier of 0.66 eV. Additionally, high CO tolerance and easy desorption of H 2 O make MnPS 3 a robust catalyst. Substitution in half of the Mn sites of MnPS 3 (100) surface with Co considerably enhances the ORR activity. Mn 0.5 Co 0.5 PS 3 (100) surface exhibits an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H 2 O 2 on Mn 0.5 Co 0.5 PS 3 proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS 3 (100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity. Besides, Mn 0.5 Co 0.5 PS 3 (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 2 O desorption ability, and robust dynamic and thermal stability, this CO tolerant Mn 0.5 Co 0.5 PS 3 catalyst can be a potential alternative to Pt with encouraging practical viability.