Somaye Rasouli

On the Degradation of PtNi nanocatalysts for PEM Fuel Cells: An Identical Location Aberration-corrected STEM Study

A thorough knowledge of the atomic structure and composition of electrocatalyst nanoparticles is paramount to the development of advanced materials for proton exchange membrane fuel cells (PEMFC), one of the most promising energy conversion devices for automotive and stationary applications. Pt nanoparticles (NPs) are currently used as the catalyst to promote the kinetics of the hydrogen oxidation and oxygen reduction reactions in the anode and cathode of the fuel cell, respectively. However, Pt-based alloys are being investigated to replace Pt on the cathode as a way to improve the efficiency of the fuel cell, and reduce cost [1]. Although the enhancement in the ORR activity of Pt alloys is well established, the durability of the catalysts remains the main issue for their commercialization.

Understanding the Stability of Nanoscale Catalysts in PEM Fuel Cells by Identical Location TEM

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising energy conversion devices due to their high energy density, low operating temperature, high efficiency, and ultimate cleanness—no carbon dioxide emission. Yet, a critical factor which significantly influences the performance of PEMFC is the stability of platinum group metal catalysts, which consists of Pt or Pt-alloy nanoparticles (2–5 nm in diameter) supported on the surface of carbon particles (40–100 nm in diameter) during fuel cell cycling. In fact, the Pt or Pt-alloy catalysts typically dissolve and/or grow in size with the number of cycles. In order to reveal the degradation mechanisms of these nanocatalysts, we have developed an experimental setup which replicates on a TEM grid the effect of voltage cycling on the cathode of an MEA. Using this approach, it is possible to track the behavior of a single nanoparticle at different stages of voltage cycling at the nano/atomic scale. Through these direct observations, we demonstrated that due to carbon corrosion the defects appear at the carbon/nanoparticle interface, which in turn result in particle migration and consequently coalescence. We also revealed the mass transfer mechanisms during the coalescence of nanoparticles. In addition, we revisited the commonly held view on the mechanism of particle dissolution and deposition. Thus, during the later stages of cycling, when the concentration of dissoluble Pt reaches a critical amount, single atoms and atomic clusters appear on the carbon support, which consequently move toward other particles and re-deposit on their surface. Furthermore, we investigated the atomic surface evolution of Pt-Ni nanoparticles under the effect of voltage through advanced spectroscopy technique such as EDS.

On the Study of PEM Fuel Cells by Transmission Electron Microscopy

The long-term efficiency of proton exchange membrane fuel cells (PEMFC) is largely restricted by the instability of catalyst nanoparticles during fuel cell operation. Due to their large surface area-tovolume ratio, Pt and Pt-alloy nanoparticles have a strong tendency to grow in size over short time scales, which lead to a reduction in their electrochemically-active surface area, and consequently to an undesired catalyst deactivation and reduction in cell performance after several cycles [1]. Yet, it is still unclear what the main degradation mechanism is, particularly whether modified Ostwald ripening or coalescence is predominant for particle growth within the cathode.

A Precise Description of Inorganic Nanoparticles in HRTEM Micrographs

Nanoparticles (NPs) play an important role in a number of technologies, and many of their properties show a strong dependence on size and shape (i.e., their morphology) [4], [5], [9],[11]. There are numerous analytical methods used to characterize the morphology of NPs. Among these, transmission electron microscopy (TEM) represents a highly attractive option, primarily because it is the only analytical technique that directly allows for real space visualization of NPs [7]. However, subsampling presents a large source of uncertainty for this method of studying NP size, as many particle size distributions (PSDs) often represent data from a sample size on the order of 100 [8]. The primary reason for subsampling is that the complexity of TEM micrographs often precludes automated segmentation and sizing of NPs [10]. The need to manually segment NPs in TEM micrographs represents a bottleneck that must be overcome to address the crucial problem of subsampling. Sadly, there has been little work to date on this subject in the microscopy literature.

Aberration-Corrected STEM Study on Pt 0.8 Ni De-alloyed Nanocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for transport and stationary applications. Pt nanoparticles are currently used as the catalyst to promote the kinetics of the hydrogen oxidation and oxygen reduction reactions in the anode and cathode of the fuel cell, respectively. However, alloys of Pt with base metals are being investigated to replace Pt on the cathode as a way to improve the efficiency of the fuel cell, and reduce cost [1].