Density Functional Theory (DFT) and Experimental Evidences of Metal–Support Interaction in Platinum Nanoparticles Supported on Nitrogen- and Sulfur-Doped Mesoporous Carbons: Synthesis, Activity, and Stability

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Abstract

In this paper, we report a comprehensive investigation of Pt nanoparticles (NPs) deposition on nitrogen- and sulfur-doped or codoped mesoporous carbons (N-MC, S-MC, and N,S-MC) to develop active and durable oxygen reduction catalysts for fuel cells. N-MC, S-MC, and N,S-MC were prepared by employing mesoporous silica as hard template and suitable organic precursors. Pt NPs were deposited by solid-state reduction of platinum acetylacetonate under N2/H2 flow on the three different supports. Pt NPs resulted to be well-dispersed over the doped MC supports with size distributions (from 1.8 nm to 3.5 nm) that are dependent on the type of doping heteroatom (N, S, or N and S). The influence of nitrogen and/or sulfur incorporated into the carbon matrix on the nucleation and growth of Pt NPs was also rationalized based on density functional theory (DFT) simulations. They highlighted that both nitrogen and sulfur increase the interactions between Pt and carbon support, but the interaction decreases as the nitrogen and sulfur functional groups become closer. The effect of sulfur content on the size and activity of Pt NPs was also evaluated. Electrochemical measurements in 0.5 M H2SO4 electrolyte allowed us to investigate the behavior of Pt NPs and to assess the relationship with electrochemical activity and stability. The Pt/S-MC showed mass activity and specific activity comparable with the state-of-the-art commercial standard Pt/C Tanaka (Pt 46% on Vulcan XC72), and the highest catalytic activity, with respect to Pt/N-MC and Pt/N,S-MC, was associated with a stronger interaction between Pt NPs and a thiophenic-like group, as proven by DFT calculations and X-ray photoelectron spectroscopy (XPS) analysis. Pt/S-MC was incorporated in a membrane electrode assembly and tested as cathode material in a PEM fuel cell, while accelerated degradation tests up to 10 000 voltammetric cycles were carried out in 0.5 M H2SO4: the influence of the doped support on the durability of the catalyst under harsh operational conditions has been highlighted.

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