Understanding the Effects of Different Engineered Carbon Supports (ECS) Supports on the Sintering Processes of Pt Nanoparticles

Abstract

Polymer Electrolyte Membrane Fuel Cell (PEMFC) technology is an advantageous solution for clean energy applications due to its zero-emission, high efficiency, low maintenance cost, and high energy density. The maturity of this technology has been the product of considerable scientific and industry efforts devoted to developing highly efficient and reliable systems increasingly adopted in the transportation and energy generation sectors. PEMFCs produce electricity by converting the electrochemical reactions of hydrogen oxidation and oxygen reduction into electrical energy. However, fabricating catalysts materials with high performance at scale is still a challenge that needs to be tackled to enable a marketable increase in the energy generated from this technology. For this reason, tunability of the catalyst materials at the core of this technology is more than essential to meet certain energy demands. Here we investigate commercially available catalysts based on platinum nanoparticles (Pt NPs) supported on Engineered Carbon Supports (ECS)TM produced by Pajarito Powder, LLC.Heat treatments (HT) were performed on base catalyst materials containing 30 wt% Pt supported on two different types of ECS, labeled as 3701 and 4601 (both made with similar materials and procedures, 4601 has more pore formers). The treatments were performed at temperatures ranging from 400 to 1000 °C in 7% H2/N2, N2, He, and Ar atmospheres for 1hr. The main goal of this study is to understand the effects and the properties of these types of ECS (having different degrees of porosity) on the stability of the Pt NPs, to get a clearer vision of how those different environments and temperatures affect the sintering processes taking place on the Pt NPs which can be correlated to the catalyst performance.Our preliminary results by X-ray diffraction (XRD) have shown an increase of the Pt crystallite size as temperature increases and 1000 °C in an atmosphere of 7% H2/N2 appears to have a stronger effect on Pt crystallite growth when compared to N2 or Ar, while such growth seems to have the same trend in both N2 and Ar atmosphere. Additionally, CO chemisorption showed a reduction of the active metallic surface area with increasing temperature, verifying the trend measured by XRD. For every treated sample, Raman characterization did not show a dramatic change in the ratios between the G-mode peak (representing crystalline sp2 hybridized carbon), and the D-mode peak (attributed to disordered sp3 diamond-like carbon). This can shed a light on the amorphization or graphitization processes taking place under these conditions and the degree of graphitization of the carbon support. Transmission Electron Microscopy (TEM) will be performed to further clarify any conclusion. X-ray Photoelectron Spectroscopy (XPS) surface characterization revealed that there is a reduction in the Pt NPs oxidation from 600 to 1000 °C. Treatments in N2, the O 1s spectra were different than the other conditions possibly due to the lower presence of atmospheric oxygen during this treatment. On the other hand, the C 1s spectra show a minor shift to higher binding energy in the main peak as a result of the treatments, indicating a slight increment in the amount of sp3 and a decrease in the amount of sp2 carbon nature.This research will be expanded with TEM imaging and physisorption data to get a more quantitative and qualitative interpretation of the phenomena taking place in the mobility and sintering processes of Pt NPs on the ECS surfaces and the role of the porosity and carbon morphology. Finally, direct measurements of the mass transport from which surface diffusivity can be determined during the process of Pt sintering will be also performed using in-situ TEM, simulating the furnace conditions using a methodology that would allow us to scrutinize how the nanoscale characteristics of these powders influence the coalescence and sintering processes in these nanoparticles.