Facile Solid-State Synthesis of Supported Ptm-Nps for the Oxygen Reduction Reaction
Authors: Hopf Alexander , De Bellis Jacopo , Imhof Timo , Pfänder Norbert , Ledendecker Marc , Schüth Ferdi 
Published: July 2022

Supported bimetallic catalysts composed of platinum and transition metals are highly investigated electrocatalysts for the oxygen reduction reaction (ORR) because of their enhanced activity and stability. However, common routes to synthesize these materials are often laborious and not scalable, employing organic solvents or uneconomical metal deposition methods [1-2]. Here, we present a mechanochemistry-assisted, dry and scalable synthesis route towards supported bimetallic catalysts, which consists of only two steps: First, metal salts are dispersed on a carbon support in a planetary mill without further additives. Following, the powder is reduced with hydrogen and annealed to yield alloyed catalysts. With this process, we are able to synthesize PtM/C catalysts where M was Ni, Co or Ru. Both metal loading and ratio could be adjusted by changing the amount of metal salt. The average size of the PtM-NPs was similarly controlled by changing the annealing temperature.

With X-ray diffraction (XRD), we show that no metal or salt reflexes are present after milling and that the XRD pattern matches the carbon support. After reduction and annealing, clear reflexes of the targeted alloy composition are visible. The latter was confirmed by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDX). After milling, ionic metal species are present as sub-nanometer clusters evenly dispersed over the carbon support. After reduction and annealing, PtM nanoparticles have formed. Neither by XRD nor by STEM-EDX was the presence of unalloyed Pt or M detected. Optical emission spectrometry was used to confirm the targeted loading and bulk composition of the catalysts.

Due to the known high activity for catalyzing the ORR, PtNi/C and PtCo/C were subjected to electrochemical evaluation in a rotating disc electrode setup [2]. In both cases, specific activities surpassing 1 mA/cm2
Pt at 0.9 V were reached. The electrochemically active surface area (ECSA) and mass activity were also in the range expected for the composition and particle size. To assess the stability, the catalysts were cycled from 0.4 to 1.0 V (RHE) with a scan rate of 1 V/s for 10800 cycles. The loss in ECSA of approximately 5 % was also in line with expectations. Since iron impurities are known to reduce the lifetime of a proton exchange membrane fuel cell, the milling process was adapted to a Si3N4-mill instead of a steel mill, again demonstrating the flexibility of this synthesis route [3].

To conclude, the reported solid-state procedure allows the dry synthesis of supported bimetallic catalysts over a wide range of compositions. The materials show high activity and stability catalyzing the ORR. Due to its simplicity and flexibility, we expect this synthesis approach to be applied in other fields of catalysis within a short period of time.

References:

[1] K. Loza, M. Heggen, M. Epple, Adv. Funct. Mater., 2020, 30, 1909250.

[2] I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 6744.

[3] A. Collier, H. Wang, X. Z. Yuan, J. Zhang, D. P. Wilkinson, Int. J. Hydrog. Energy, 2006, 31, 1838–1854.

[4] J. De Bellis, M. Felderhoff, F. Schüth, Chem. Mater., 2021, 33, 6, 2037–2045.

Figure 1