Enhanced Atomic-Scale Imaging of PGM-Free Catalysts By Low-Voltage Scanning Transmission Electron Microscopy
Authors: Zachman Michael J. , Chung Dong Young , Holby Edward F. , Stamenkovic Vojislav R. , Cullen David A. 
Published: December 2020

Platinum group metal (PGM)-free catalysts are promising candidates to supplant costly precious metal catalysts in hydrogen fuel cells and enable low-cost, commercially viable devices [1]. Recent developments in PGM-free catalysts have enabled performance comparable to Pt catalysts by some metrics [2], but durability remains a significant challenge and limits practical application of these devices [3]. To improve the durability of PGM-free catalysts, understanding and controlling atomic-scale active site structure and the corresponding degradation mechanisms is imperative [1]. Active sites in these catalysts are commonly thought to be FeN4 complexes within a graphene lattice. Many potential active site structures have been predicted by computational methods, however, and a technique capable of directly validating these models is needed to reveal the true nature of these sites and enable increased control over degradation [1].

Scanning transmission electron microscopy (STEM) is capable of resolving a wide range of materials properties down to the atomic scale, such as structure, composition, and bonding information. The characteristics of PGM-free materials makes obtaining atomic-scale information about active site structure by STEM challenging, however. These materials are often defect-rich, containing a large number of dopants, holes, etc., and a large portion of proposed active sites exist at edges of the material, all of which increase susceptibility to beam damage. Conventional high-angle annular dark-field (HAADF) STEM imaging exacerbates this problem since only a small proportion of the incident beam electrons generate signal, and this signal is most sensitive to heavy elements. A large electron dose is therefore required to obtain a sufficient signal from light elements such as carbon and nitrogen, which can result in beam-induced atomic displacements at susceptible sites when using standard accelerating voltages. As a consequence, direct structural analysis of active sites is often not possible, impeding our understanding of degradation pathways.

In this talk, we will demonstrate advancements in STEM techniques that significantly enhance our ability to directly analyze atomic-scale active site structure in PGM-free materials. We utilize a recently developed dose-efficient imaging technique based on four-dimensional (4D)-STEM [4,5] to reduce the required electron dose and simultaneously increase sensitivity to light elements. To minimize beam damage at sensitive active sites, we perform this dose-efficient 4D-STEM technique in a low-voltage aberration-corrected instrument operated at 30 kV, producing an atomic-scale probe below the threshold energy for beam-induced atomic displacements at many sites. We perform these experiments on a model PGM-free catalyst that contains many monolayer regions, which is ideal for low-voltage 4D-STEM and enables experimental validation through simulations of proposed active site structures. This work demonstrates how low-voltage 4D-STEM will provide new atomic-scale insights into the structure and degradation pathways of PGM-free catalyst active sites, accelerating the development of low-cost hydrogen fuel cells.

References:

[1] U. Martinez et al., Adv. Mater., 31, 1806545 (2019).

[2] H. T. Chung et al., Science, 357, 479 (2017).

[3] Y. Shao et al., Adv. Mater., 31, 1807615 (2019).

[4] K. Müller et al., Nat. Commun., 5, 5653, (2014).

[5] Y. Jiang et al., Nature, 559, 343 (2018).

[6] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.