Liquid-phase electron microscopy studies of Cu/Cu₂O nanocatalyst evolution during CO₂ electroreduction

Abstract text (incl. figure legends and references)

Recent advancement in the design of novel catalysts for CO₂ reduction reaction (CO₂RR) increases demand for standardized methods for assessment of the catalyst's efficiency in CO₂ conversion to valuable hydrocarbons. Among the catalysts for CO₂RR, Cu nanocatalysts (Cu NCs) have attracted considerable attention due to copper's ability to produce a wide range of C₂₊ products and facet-dependent selectivity of Cu nanocrystals towards certain hydrocarbons¹. However, Cu NCs undergo rapid restructuring upon CO₂RR which results in their deactivation. Therefore, a considerable amount of research is dedicated to discerning the degradation mechanisms of Cu NCs². In this regard, in situ electron microscopy (in situ EM) has great potential as it allows the imaging of samples in a liquid medium under applied bias. Nevertheless, commercially available electrochemical chips with CO₂RR compatible glassy carbon (GC) working electrodes demonstrate a relatively narrow inert potential range in the cathodic region³. As a result, the competing hydrogen evolution reaction (HER) takes lead causing gas bubble formation, delamination, and electrical disconnection of GC electrodes within the short time of the reaction. In this work, we present correlative in situ transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) extended time studies of 40 nm Cu/Cu₂O NCs under CO₂RR conditions.

Samples for in situ EM experiments were prepared by drop-casting Cu/Cu₂O NCs on the membrane region of plasma-treated electrochemical chips. Thereafter, assembled in a holder the cell was filled with CO₂ saturated 0.1 M KHCO₃ electrolyte. Linear sweep voltammetry and chronoamperometry were utilized to drive CO₂ electroreduction. TEM images were acquired at an electron dose rate of 42 e^-nm^-2s^-1, and in ESEM experiments, a beam current was 36 pA. Energy-filtered TEM was used to enhance the contrast of catalyst particles in the liquid electrolyte.

In situ EM results indicate that in-house fabricated GC electrodes have a longer stability time as compared to values from the previous studies⁴: up to 4 min in TEM and 10 min in ESEM at a cathodic potential of -0.8 V_RHE. The shorter time in TEM is hypothesized to stem from an order of magnitude higher beam energy used in TEM. Nonetheless, utilizing TEM we were able to monitor the unique restructuring pathway of core-shell Cu/Cu₂O NCs inclusive of the facet selective core etching while the shell remained intact. The results were reproduced in ESEM, showing similar hollow cube particles after 10 min of CO₂RR. However, these observations diverge from bulk cell results, which points towards further studies needed to elucidate this different behavior.

In conclusion, in situ TEM and ESEM experiments are complementary for providing structural and morphological information on Cu NCs dissolution driven by CO₂RR at the nano- and meso-scale.

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