The assembled cell consists of two silicon-based chips and two O-rings that are sandwiched together to enclose the electrolyte flow path, as described in detail before . One chip (electroactive chip) contains patterned electrodes, while the other (spacer) contains patterned posts that dictate the spacing between the electroactive chip and the spacer. Both chips have an electron beam transparent SixNy window aligned with each other. The chip side with patterned electrodes is laid facing the spacing posts on the spacer chip, thus creating an enclosed liquid-cell. In the present study the electroactive chip was patterned with four separate electrodes that could be wired in the desired configuration. Three of the electrodes were made of platinum and one was made of amorphous indium tin oxide (ITO). The ITO electrode was used as the working electrode in all experiments. This electrode was patterned partially over the SixNy window, which is key for performing TEM analysis on the copper particles. The sample holder was assembled with the electroactive chip on the side closest to the electron gun inside the TEM, which allows obtaining optimal resolution in STEM mode. In this orientation, the STEM electron probe reaches the sample at its smallest diameter before crossing and scattering through the thick column of electrolyte liquid. The cell is mounted on a liquid sample holder (Hummingbird Scientific, USA). The sample holder has four leads that establish electrical connection with up to four electrodes of the top, electroactive chip. The chip electrodes can be wired in any desired configuration. Amorphous ITO was used as the working electrode (WE), while the counter (CE) and pseudo-reference electrodes (RE) were made of platinum to ensure good conductivity. A three-electrode configuration was used in all experiments. Amorphous ITO film is inert for CO2 ER and HER, which ensures that all the detected activity was from the copper catalyst deposited in situ for the experiments reported here (Figure ) . The only exception to this configuration was for bulk copper corrosion analysis, where the WE was a copper electrode prepared by the supplier through ebeam deposition during chip manufacture.