Electrochemical Studies and In Situ Electrochemical Nuclear Magnetic Resonance Investigations on Platinum-based Bimetallic Electrocatalysts for Direct Methanol Fuel Cells

Abstract

Two parallel objectives of this thesis research are 1) to investigate the electrocatalytic activities of Pt-based bimetallic nanoparticles (NPs) towards methanol (MeOH) electro-oxidation reaction (MOR), carbon monoxide (CO) tolerance and oxygen reduction reaction (ORR); and 2) to use electrochemical nuclear magnetic resonance (EC-NMR) techniques to investigate the electronic properties of the Pt-based bimetallic electrocatalysts.

For the first objective, instead of utilizing the traditional bimetallic alloy model, a surface modification strategy was adopted in this thesis work to prepare Pt-modified M (M = Ru, Au) bimetallic electrocatalysts. By experimentally controlling the Pt coverage on the metal substrates via a spontaneous deposition method, the resulting Pt-modified Ru or Au substrates indicated Pt coverage-dependent electrochemistry. Most importantly, it was observed that the inactivity of Pt towards MeOH oxidation with very low coverage and, the emerging and increasing activity with increasing coverage coincided with the phenomena predicted by the ensemble effect, a hypothesis that had not earlier been supported directly by experimental evidence. Later a one-pot wet chemistry method was developed to prepare Pt-decorated Ru nanoparticles with a submonolayer of Pt. The electrochemical characterization of the whole series of samples provided experimental evidence that strongly supported a bifunctional mechanism, rather than an electronic effect as the dominant factor contributing to the enhanced CO tolerance. Using a core(Au)/shell(Pt) model, a detailed investigation of electrocatalytic properties of Au@Pt nanoparticles was performed as a function of the Pt shell packing density and Au core size. It was observed that the electrochemical behavior of Pt deviated significantly from its bulk counterpart especially at low coverage. The data obtained so far indicated that the future of PtAu as an anode electrocatalyst for MOR is questionable.

In the second part of the thesis, a unique 195Pt NMR based in situ technique was developed to determine the local Pt concentration and electronic properties in Pt-based bimetallic systems with reasonable spatial resolution. Before this study, this was a challenging task in the area of Pt-based bimetallic systems, especially at the nanoscale. In combination with electrochemical characterization, this methodology opened a way to study, in much more specific terms and at higher resolution, the local elemental composition/electronic properties/catalytic activity relationship.