An Investigation of Structure−Catalytic Activity Relationship for Pt−Co/C Bimetallic Nanoparticles toward the Oxygen Reduction Reaction

Abstract

Carbon-supported Pt−Co alloy nanoparticle catalysts of nominal atomic composition but with different alloying extents were prepared via a modified Watanabe process by employing microwave heating. Their structure was studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. Transmission electron microscopy (TEM) images indicated that the in-house-prepared Pt−Co alloy nanoparticles (sample-1 and sample-2) were well dispersed on the surface of the carbon support with narrow particle size distribution which is consistent with XRD grain size values. The catalyst composition obtained from XAS was nearly the same as that of the nominal value (1:1). The alloying extent values of Pt calculated from XAS measurements for sample-2 is similar to that of the commercial E-tek Pt−Co/C. This observation demonstrates the practical viability of our preparation protocol for Pt−Co/C catalysts. A comparative study was made for the oxygen reduction reaction (ORR) using a thin-film rotating disk electrode method to evaluate the catalytic behavior of Pt−Co/C and Pt/C catalyst with similar metal loadings and particle sizes. As compared to the Pt/C catalyst, the bimetallic Pt−Co/C of sample-2 exhibited an enhancement factor of 3 in mass activity at 0.95 V toward ORR. The enhancement in activity of sample-2 is similar in magnitude to that of the commercial E-tek Pt−Co/C catalyst. It is found that that the samples possessing a high alloying extent of Pt in the cluster enhances the activity of the bimetallic Pt−Co/C toward ORR. This observation confirms that the alloying extent of Pt is an important parameter by which one can have control over the fine-tuning of the catalytic activities of bimetallic nanoclusters. This activity enhancement may originate from the favorable electronic effects of a well mixed alloy underneath a thin “Pt-rich skin” structure of the Pt−Co bimetallic nanoparticles. This kind of thin “Pt-rich skin” is created by the dissolution of Co oxide on Pt−Co bimetallic nanoparticles while washing in acidic electrolyte before being subjected to ORR.

Chemical Dealloying Mechanism of Bimetallic Pt–Co Nanoparticles and Enhancement of Catalytic Activity toward Oxygen Reduction

“…Concerns like reducing the usage of noble metals have been considered to be major challenges in promoting the technology. To overcome such issues, some studies have reported that Pt-based alloys with transition metals (Co, [6] Ni, [7] Cr, [8] etc.) are helpful to improve the kinetics of the oxygen reduction reaction (ORR).…”

“…It was demonstrated that the formation of Pt-skin alloys is caused by the dissolution of the base metal from Pt alloy under acidic conditions. [6,8,[15][16][17][18][19] Stamenkovic et al [19][20][21] have shown that the polycrystalline Pt 3 Co nanocrystal with a Pt-skin layer is more active than the polycrystalline Pt and bimetallic Pt 3 M surfaces (M = Fe, Ni, V, and Ti), because of the unusual electronic properties of the Abstract: The chemical dealloying mechanism of bimetallic Pt-Co nanoparticles (NPs) and enhancement of their electrocatalytic activity towards the oxygen reduction reaction (ORR) have been investigated on a fundamental level by the combination of X-ray absorption spectroscopy (XAS) and aberration-corrected scanning transmission electron microscopy (STEM). Structural parameters, such as coordination numbers, alloy extent, and the unfilled d states of Pt atoms, are derived from the XAS spectra, together with the compositional variation analyzed by line-scanning energy-dispersive X-ray spectroscopy (EDX) on an atomic scale, to gain new insights into the dealloying process of bimetallic PtCo NPs.…”

Carbon-supported PtCo2Ni2 alloy with enhanced activity and stability for oxygen reduction

“…In the case of ORR, the coverage of the adsorbed hydroxyl species (OH ad ) on the surface active sites will inhibit the O 2 adsorption and hinder the reaction. Upon the cyclic voltammetry (CV) curves of the catalysts, the Pt oxide oxidation peaks (0.8−1.0 V) have positive shifts relative to the Pt/C catalysts, suggesting that the PtCo 2 Ni 2 /C catalysts could promote the desorption of OH ad during ORR [52]. C600 catalyst shows the highest electrochemical surface area (ECSA) compared with other samples (Fig.…”

Relating Structural Aspects of Bimetallic Pt3Cr1/C Nanoparticles to Their Electrocatalytic Activity, Stability, and Selectivity in the Oxygen Reduction Reaction

“…[13] The mass activity i m was then obtained via calculation of i k [Eq. (5)] and normalized with the weight of Pt used [Eq.…”

“…The number ofmetals in the alloy and the increase in particle size with respect to pure Pt. [13] The former problem occurs because the true alloy composition involving the 3d transition metal, which is difficult to reduce during synthesis, remains below the designed composition.…”