Microwave assisted synthesis of metallic nanoparticles is an attractive way for the elaboration of electrocatalysts using energy efficient processes. This method consists in the use of a weak reducing agent that acts as solvent with high boiling point at the same time. Among the possible solvents, diethylene glycol is one of the most suitable molecules for microwave absorption and heating. Since microwave energy leads to a more uniform heat-treatment than a conventional heating process, the overall activation energy for metal ion reduction decreases under microwave exposure. The carbon supported copper catalyst described herein is a good candidate for electrocatalytic processes due to the fact that copper is an abundant and effective transition metal for a number of reactions.

Observations were conducted in a Tecnai G2 fitted with a FEG, an EDX spectrometer and GIF Quantum. STEM-HAADF was the primary technique of image recording. Scan speed was kept high in order to follow the kinetic of the evolution of the system. In this experiment, we use a sealed TEM cell recently developed by Protochips , i.e. liquid/bias TEM specimen holder (Poseidon 510). This system permits to study materials under electrochemical environment in TEM instrument. The sample (Copper nanoparticles on carbon) that is inside the sealed TEM cell (liquid gap is from 500 nm to 3000 nm) is immersed in the liquid electrolyte introduced by micro-fluidic system (here aqueous electrolyte KCl/H2O at 0.01ML) and then electrochemically cycled using designed working electrode (glassy carbon) on transparent SixN1-x windows. For electrochemical measurement, the applied currents were between 100 nA and 500 nA. Voltammogram was taken to ensure the correct oxidation and reduction of the copper atoms during a cycle. A strong corrosion of the supporting carbon is then observed. Experiments at constant current (chronopotentiometry) show the coalescence of large Cu grains and their disappearance depending on the applied current (See figure 1 and 2). Gas bubble formation is also observed (Figure 3). In these specific liquid experiments, optimized conditions remain to be found for TEM observation and in-situ cycling, especially for electrocatalysis which is particularly demanding in terms of control of the experimental condition.

Figures:

Fig 1 : STEM HAADF image of Cu nanoparticles on carbon in the sealed TEM cell just after switching off the current in a chronopotentiometry experiment.

Fig 2 : STEM HAADF of the same area one minute later. The growth of the Cu particles comes from absorption of particles in the liquid. A small bubble can be observed under the particles.

Fig 3 : A larger view with the same area in the middle, 3 minutes after switching off the current. The bubble has grown and the Cu particles shrinked slightly.

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-observations-of-nanoparticles-of-carbon-supported-copper-electrocatalyst-during-electrochemical-cycling/