Pt-Mn/C and Pd-Mn/C Nanoparticles As ORR Electrocatalysts

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode remains as one of the main challenges for commercial viability of alkaline exchange membrane fuel cells (AEMFCs). It is therefore, of great interest to explore the development of electrocatalysts with superior activity and stability relative to the conventional carbon supported Pt nanoparticles (NPs). In this work we have studied the effects and evolution of structure and surface composition of Pt-Mn and Pd-Mn NPs on their electrocatalytic activity for the ORR in alkaline media. Both Pt-Mn and Pd-Mn NPs were synthesized using solvothermal methods and were supported on C via impregnation. In the case of Pt-Mn an ordered intermetallic phase was obtained by heat treatment, while for Pd-Mn NPs heat treatment did not lead to an ordered alloy. We have investigated the crystalline structure of these NPs using X-ray diffraction (XRD) and their size, morphology and composition using TEM/STEM, EELS and EDX. Cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry were used to assess their electrocatalytic activity and stability. The NPs were electrochemically dealloyed in acidic media, through potential cycling, leading to the formation of Pt and Pd shell on PtMn and PdMn core. The dealloyed nanocatalysts exhibited over 400% increase in mass activity and 500% increase in specific activity as compare to Pt/C. The stability of the electrocatalyst was tested after 4,000 potential cycles in alkaline media following the DOE protocol (0.6-1.0 V vs reversible hydrogen electrode). In alkaline media, Mn surface segregation ensued, leading to a lower electrocatalytic activity when compared to the freshly dealloyed catalyst.