Operando Electrochemical Liquid-Cell Scanning Transmission Electron Microscopy (EC-STEM) Studies of Evolving Cu Nanocatalysts for CO₂ Electroreduction

Abstract

The design and synthesis of nanocatalysts with welldefined sizes, compositions, and structures have revolutionized our accessibility to tunable catalyst activity and selectivity for a variety of energy-related electrochemical reactions. Nonetheless, establishing structure-(re)activity correlations requires the understanding of the dynamic evolution of pristine nanocatalysts and the identification of their active states under operating conditions. We previously communicated the operando observation of Cu nanocatalysts evolving into active metallic Cu nanograins for CO 2 electroreduction (Yang et al. Nature 2023, 614, 262−269). Here, we expand our discussion to the technical capabilities and further research applications of operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM), which enables quantitative electrochemistry while tracking dynamic structural evolution of sub-10 nm Cu nanocatalysts. The coexistent H 2 bubbles, often disruptive to operando spectroscopy, are an effective approach to create a thin-liquid layer that significantly improves spatial resolution while remaining electrochemically accessible to Cu nanocatalysts. Operando four-dimensional (4D) STEM in liquids provides insights into the complex structure of active polycrystalline metallic Cu nanograins. With continuous technical developments, we anticipate that operando EC-STEM will evolve into a powerful electroanalytical method to advance our understanding of a variety of nanoscale electrocatalysts at solid/liquid interfaces.

The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure–property relationships of Pt-based fuel cells electrocatalysts

“…Surprisingly, a relatively high number of studies concerning Cu-based catalysts for CO 2 reduction using in situ TEM have been reported, such as. 159–161…”

Reducing Ensemble Averaging for Mechanistic Understanding of Electrocatalysis in Energy Conversion Reactions

“…For example, Yang and co-workers 14 used this approach to track the structural evolution of a monolayer of 7 nm Cu nanoparticles, which were found to be highly mobile, forming metallic nanograins, the boundaries of which were determined to serve as active sites for C−C coupling. A following study by the same group 12 improved the technique by exploiting the electrogenerated H 2 bubbles to create a thin-liquid layer, improving the spatial resolution further. The authors were able to visualize the transition of the self-assembled pristine 7 nm Cu nanoparticle ensemble (Figure 1d) to the active polycrystalline Cu nanograin phase.…”

“…(e) False-color image showing different crystal domains on Cu nanograins at −0.8 V vs RHE obtained by operando electrochemical 4D-STEM diffraction imaging. Adapted with permission from ref . (f) Experimental (black) and DFT-calculated (purple, blue, and red) SERS spectra of Ni­(tpyS) 2 with relevant identified vibrational modes highlighted.…”

“…An approach that has garnered increased attention to provide nanometer spatial resolution in electrocatalysis is liquid cell (LC)- or EC-STEM. ,, While the LC variant of STEM does not allow for full exploitation of the spatial resolution as is possible in an ex situ experiment, LC-STEM has been correlated to high-resolution X-ray spectroscopy to investigate the origin of the C 2+ product selectivity of the CO 2 RR on nanoscale Cu. For example, Yang and co-workers used this approach to track the structural evolution of a monolayer of 7 nm Cu nanoparticles, which were found to be highly mobile, forming metallic nanograins, the boundaries of which were determined to serve as active sites for C–C coupling.…”

“…Structure–activity relationships, characterization of active sites and local environments, identification of intermediates, etc., are all complementary descriptors used in the electrocatalytic design cycle. For this, ex situ characterization techniques, such as conventional scanning and transmission electron microscopies (SEM and TEM, respectively), provide important information on the initial and final states of the catalyst but overlook extrinsic factors, such as the interaction of the solvent and electrolyte molecules with active sites and intermediates, which have recently been shown to affect catalytic performance. − In situ and operando techniques, on the other hand, can overcome these limitations by enabling measurements under operating conditions. Recently, in situ/operando spectroscopic techniques, such as X-ray spectrocopies, − infrared spectroscopy, ,, surface-enhanced Raman spectroscopy (SERS), ,,, and/or scanning probe microscopic techniques, including scanning transmission electron microscopy (STEM), , atomic force microscopy (AFM), and scanning electrochemical cell microscopy (SECCM), − and many others have been employed to obtain valuable insight into catalytic processes.…”