Electrochemical Liquid Phase TEM for CO₂ reduction: The Role of Liquid Flow Configuration

Recently energy transition, together with climate change issue, is raising a lot of interest in research due to the need to reduce the fossil fuels dependence in favor of more clean technologies. In this context are involved particular chemical reactions, allowing to convert abundant and waste gases into valuable products, as fuels or chemicals. These are mainly thermochemical or electrochemical processes and take place in the presence of a catalyst, which is a material containing active sites that weakly bind the reactants to provide them into the correct orientation to form the desired products. Among these reactions, CO₂ electro-reduction reaction in acqueous electrolyte is of particular interest because it allows the exploitation of a pollutant gas using green solvents.<br/>In order to study the catalytic activity, including the identification of active sites, the reaction intermediates and the chemical path of reactions, <i>in situ</i> approaches have been developed for different characterization techniques. <i>In-situ </i>liquid-phase TEM (LP-TEM) is gaining more and more attention, as it allows to observe the evolution of the morphology and crystalline structure of materials in liquid environment, under electrical or thermal stimulus¹.<br/>In particular, the miniaturized electrochemical cell used to perform the LP-TEM has some drawbacks especially when the applied potential of the experiments is strongly negative. Unfortunately, the potentials required to perform the majority of the electrochemical reactions, in particular CO₂RR, requires potential lower than -0.8 V vs RHE. At these voltages, the platinum forming the electrodes catalyzes the water splitting reaction, causing the production of gaseous hydrogen and oxygen, which aggregates forming bubbles. As the cell is miniaturized, the formation of these bubbles tends to fill the cell and de-wet the electrodes, thus blocking the electrochemical activity. The flow of electrolyte inside the cell is supposed to bring away these side products but it is too weak to force bubbles away, due to the non-optimal geometry of the today available commercial chips.<br/>In this paper we present a novel chip configuration, a prototype optimized to provide better liquid exchange inside the cell and expected to bring faster the gaseous products away. This chip has been validated first under the optical microscope observation with the aid of ad-hoc top glass chips, to confirm that the optimized microfluidic pattern can favor the gas bubbles removal from the electrode surface. With the validated configuration, LP-TEM measurements have been positively performed, also with increased flow rates, where a preferential path for bubbles has been assessed. This ensures that electrodes are continuously wetted by electrolyte during experiments, being able to perform electroreduction reactions for longer time, even when opening the potential window and going to more negative voltages. As a proof of concept, the electrodeposition of Zn has been performed, which was reported not being feasible with standard commercial chips². This experiment confirms the good performance of the modified chips, with conditions similar to those used in literature², (e.g. 500 nA) and points out that the new geometry could be effective also for different electrochemical applications.<br/>[1] Hwang, S. et al., <b>2020</b>, <i>Adv. Energy Mater</i>., 10, 1902105<br/>[2]Sasaki, Y. et al., <b>2021</b>, <i>J. Electrochem. Soc</i>., 168, 112511<br/><b>ACKNOWLEDGMENT</b><br/>The authors would like to sincerely thank Protochips for providing the prototype chips for experimental flow tests. This work has received funding partly from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192) and partly by the European Union – NextGenerationEU under the Project iENTRANCE (Project code: IR0000027, Concession Decree No. 128 of 21/06/2022 adopted by the Italian Ministry of Research, CUP: B33C22000710006).