(583c) Structure-Activity-Stability Investigation of Transition Metal Antimonate Oxynitride and Oxysulfide Oxygen Reduction Nanoparticle Electrocatalysts

Hydrogen fuel cells (FCs) present a promising prospect for transitioning away from carbon-intensive fuels for energy needs. FC cathodes perform the oxygen reduction reaction (ORR), and the kinetics of this reaction is a large source of inefficiency. Commercial FC cathodes contain costly platinum-based catalysts so identifying highly active, non-precious ORR catalyst formulations could accelerate the adoption of fuel cells. Theory calculations predict first-row transition metal antimonates (MSbO_x, M=Mn,Fe,Cr,Ni) have desirable ORR performance characteristics with previous work demonstrating the viability of nanoscale ternary and quaternary systems in this family. In this work, we highlight (oxy)sulfides and (oxy)nitrides of manganese (MnSbO_xS_y and MnSbO_xN_z, respectively) as an extension of this class of materials that further enhance activity compared to previous antimonate-based materials. Colloidal synthesis of these nanoparticle catalysts is followed by high temperature surface modification under various atmospheres (air,H₂S,NH₃). We tune the degree of oxidation, nitridation, or sulfidation to synthesize oxide nanocrystals with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride and demonstrate their reliably enhanced electrochemical ORR activity in alkaline and acidic conditions. Converting the oxide (MnSb₂O₆) into oxysulfide or oxynitride promotes significant changes to crystallinity and the formation of additional phases as seen in X-ray diffraction patterns. X-ray photoelectron spectroscopy demonstrates shifts in Mn and Sb peaks that are indicative of changing average oxidation state in the near-surface. Transmission electron micrographs reveal polycrystalline oxide nanocrystals of ~50 nm diameter with a shell of ~4 nm after nitridation or sulfidation that is likely crucial to contextualizing the activity enhancement. Energy dispersive X-ray spectroscopy enables nanometer-resolution elemental mapping of element distribution before and after catalysis to understand how degradation affects activity trends. The combination of these characterization techniques provides a more complete picture of how manganese antimonate (oxy)sulfides and (oxy)nitrides catalyze the ORR and informs the development of more advanced non-precious FC cathode catalysts.