Abstract text (incl. figure legends and references)
Introduction

Establishing a direct link between the morphology of a catalyst and its associated properties is a non-trivial challenge. It is especially complicated in electrocatalytic reactions where the catalysts can re-structure and further adopt thermodynamically unfavorable motifs due to kinetic limitations. In addition, since most property measurement techniques (such as voltammetry and gas chromatography) average over the entire catalyst ensemble, a detailed accounting of the catalyst morphologies and their relative distributions at different points of the reaction is needed to establish such correlation. Electrochemical cell transmission electron microscopy (EC-TEM) is one method that can provide us with such detailed information.

Objectives

The aim of our research is to reveal the restructuring dynamics of catalysts during reaction and to correlate these dynamics to their impact on the catalytic properties. In particular, I will focus on my group's EC-TEM studies of Cu-based catalysts for the electrochemical reduction of carbon dioxide (CO₂RR).

Materials & Methods

We adopted catalyst synthesis strategies that allow us to tune the catalyst morphology, size, and loading in a controlled manner while also being applicable to different support geometries such that we have near identical electrocatalysts in different setups. The in situ experiments are conducted inside a 300 kV Titan TEM from Thermo Fisher in STEM mode using a Hummingbird Scientific Bulk Liquid Electrochemistry holder and with an electrolyte of CO₂-saturated 0.1 KHCO₃. The samples were also characterized ex situ after reaction. The activity and selectivity of the catalysts were measured on the benchtop using standard techniques, such as gas chromatography.

Results

I will discuss two examples of our work, one using electro-deposited Cu₂O cubes and the other using lithographically patterned Cu-island arrays. Our studies using Cu₂O cubes with controlled size and loading showed that drastic reconstructions occurred in the first few minutes of potential application [1]. More importantly, the catalyst ensemble is made up of two motifs, fragmented cubes, and re-deposited nanoparticles. Then, we established the impact of the observed dynamics on catalytic performance by comparing the behavior of differently sized cubes with their time-resolved changes in product selectivity.

Next, Cu-islands were used to follow how iodide species in the electrolyte cause the formation of CuI pyramids and how these pyramids restructure under reaction conditions [2]. While the CuI pyramids also restructure drastically like the Cu₂O cubes, the catalyst morphology is different, taking the form of long fragmented filaments. Furthermore, our EC-TEM observations suggest that the presence of residual iodide stabilizes Cu⁺ species that have a known beneficial effect on hydrocarbon selectivity in CO₂RR.

Conclusions

Our results show how the re-structuring of Cu-based catalysts during CO₂RR can be very complex and how EC-TEM can provide valuable insight into the morphologies present during reaction.

References:

Grosse, P. et al. Dynamic Transformation of Cubic Copper Catalysts during CO2 Electroreduction and its Impact on Catalytic Selectivity. Nat. Commun. 12, 6736 (2021). Yoon, A., Poon, J., Grosse, P., Chee, S. W. & Roldan Cuenya, B. Iodide-mediated Cu catalyst restructuring during CO₂ electroreduction. J. Mater. Chem. A 10, 14041–14050 (2022).