Ptn-Mn(II)Nx and Ptn-Mn(III)Nx are both winning combinations for the durability of these hybrid catalysts in PEM fuel cells: a deep insight into synergism between Pt clusters and MnNx/C sites

Abstract

In this work, we investigated the influence of a carbon substrate (graphene), doped or undoped, on the adsorption energy, and thus the stability, of Pt(n)-clusters/nanoparticles (NPs) grown on it. For substrate doping, we considered N-doping (graphitic or pyridinic), and carbon doping with MnN₍₂₊₂₎/C or MnN₍₄₊₂₎/C sites in the immediate vicinity of Pt(n)-clusters/NPs. These non-noble sites, containing Mn^(II) or Mn^(III), are also capable of oxygen reduction in fuel cells, similar to Pt(n) nanoparticles. The Pt(n)-graphene interaction is initiated by a Pt atom occupying a single carbon vacancy, forming three Pt–C bonds. The entire Pt(n)-cluster/NP is then built upon this atom. A more negative adsorption energy corresponds to stronger adhesion and enhanced cluster stability. This effect is particularly pronounced when the graphene substrate is doped with Mn^{(II or III)}N₍₄₊₂₎/C or Mn^{(II or III)}N₍₂₊₂₎/C sites. We further examined whether a Pt(n)-cluster/NP adjacent to a MnN_x/C site could stabilize the latter against demetallation in acidic PEM fuel cell environments. Our findings confirm this hypothesis: Pt(n) effectively stabilizes MnN_x/C against demetallation. This effect is especially significant for Mn^(II)N₍₄₊₂₎/C and Mn^(II)N₍₂₊₂₎/C sites, previously shown to undergo spontaneous Mn demetallation. In the presence of Pt(n), the formerly spontaneous demetallation becomes a thermodynamic equilibrium (in a closed thermodynamic environment), improving MnN_x/C stability. Despite this stabilization, MnN_x sites remain less durable than platinum under PEM fuel cell cathode conditions. This led us to examine what happens to Pt(n) adhesion after Mn demetallation. Our calculations show that the Pt–C bonding energy is minimally affected by the demetallated MnN_x sites. Thus, Pt(n) clusters should remain stably anchored even after Mn loss. Finally, beyond these stability insights, we reviewed literature regarding the catalytic performance of Pt(n)-MnN_x/C hybrid systems, highlighting how they combine the high activity of Pt with the complementary functionalities of non-noble molecular sites. These findings provide theoretical guidance for designing more robust and efficient hybrid electrocatalysts for fuel cell applications.