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 (TMPS3) (100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS3(100) surface emerges to be the best among TMPS3 surfaces with optimal O-2 activation resulting in very low overpotential. The study reveals that ORR occurs on the MnPS3 surface via 4e reduction associative pathway where the kinetically rate-determining step (RDS) is the formation of O*+H2O with an activation barrier of 0.66 eV. Additionally, high CO tolerance and easy desorption of H2O make MnPS3 a robust catalyst. Substitution in half of the Mn sites of MnPS3(100) surface with Co considerably enhances the ORR activity. Mn0.5Co0.5PS3(100) surface exhibits an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H2O2 on Mn0.5Co0.5PS3 proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS3(100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity. Besides, Mn0.5Co0.5PS3(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 H2O desorption ability, and robust dynamic and thermal stability, this CO tolerant Mn0.5Co0.5PS3 catalyst can be a potential alternative to Pt with encouraging practical viability.