The effect of Mn addition on the promotion of oxygen reduction reaction performance for PtCo/C catalysts

The effect of Mn addition on the promotion of oxygen reduction reaction (ORR) performance for PtCo/C catalysts is investigated. The structures, surface compositions, chemical compositions, local structural parameters including coordination numbers and bond distances, morphologies and electrochemical properties of prepared catalysts are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-atomic emission spectrometer (ICP-AES), extended X-ray absorption fine structure (EXAFS), high resolution transmission electron microscopy (HRTEM) and rotating disk electrode (RDE) technique, respectively. For PtCo/C catalysts modified with 0.4 and 3.2 wt% of Mn, the Pt/Co atomic ratio is 3.5 and the size is about 3.0 nm with the same face centered cubic (fcc) structure. Moreover, Mn-modified PtCo/C catalysts have the same surface composition, which is about Pt/Co/Mn of 62/32/6, and their ORR activity are both higher than that of PtCo/C and commercial Pt/C catalysts. After accelerated durability test (ADT) of 1700 cycles, Mn-2 with 3.2 wt% Mn addition still presents the best ORR mass activity, which is about 420% of commercial Pt/C. In the Mn-modified PtCo/C system, due to the existence of the neighboring Co and/or Mn oxide, there is OH repulsion between Pt-OH and these non-noble metal hydroxides or oxides, decreasing the OH coverage on Pt and increasing the number of free Pt active sites. Besides, based on the XAS analysis, the unfilled d-orbital values and extent of hybridization of Pt and Co/Mn of the PtCo and Mn-modified samples are very similar. Therefore, the promotional effect on ORR performance of Mn-2 is attributed to the most enhanced Pt-skin structure, highest Pt usage, and comparable electron transfer number during ORR instead of changes in d-orbital vacancy and alloying degree. (C) 2013 Elsevier Ltd. All rights reserved.