Au(111) in [Mppip][Tfsi] in Dependence of Water Content: Revealing the Initial Stages of Cathodic Corrosion Using in-Situ STM
Authors: Heubach Maren-Kathrin , Schuett Fabian M. , Abdelrahman Areeg , Kibler Ludwig A. , Jacob Timo 
Published: July 2022

Cathodic corrosion was first observed by Fritz Haber at the end of the 19th century,[1] and was nearly forgotten for a long time, but has recently returned as an important research topic.[2,3] The prospects to form nanoparticles of noble metals and to restructure metal surfaces, in particular, stirred researchers’ interest.[4,5] Moreover, cathodic corrosion may hinder other processes such as metal deposition that occur at highly negative potentials. As ionic liquids have desirable properties of being stable over a relatively wide potential window coupled with low ion mobility, these electrolytes are often used for studies of processes at very low electrode potentials, for example, the deposition of metals with low reduction potentials such as lithium or sodium.[6,7] Unfortunately, if an electrode is cathodically corroded at potentials positive of the onset of metal deposition, this is of little use. Therefore, it is crucial to understand and thus prevent the initial stages of cathodic corrosion in such systems.

In-situ scanning tunneling microscopy (STM) was used to study cathodic corrosion of a Au(111) surface in the hydrophobic ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([MPPip][TFSI]) with various water contents. It has been observed that the onset potential of cathodic corrosion is highly dependent on the presence and the amount of water. This notable role of water has been found already for cathodic corrosion of Au in aqueous methanolic alkali metal hydroxide electrolytes:[8] the higher the water content, the more positive the onset potential. Cathodic corrosion of Au(111) begins at the so-called elbows of the herringbone reconstruction, which can lead to a characteristic pattern of equidistant holes. In hydrophilic ionic liquids, a similar process has already been observed.[9] In contrast, for the hydrophobic ionic liquid [MPPip][TFSI], the pits did not merge with time, instead, the number of pits slowly increased. Furthermore, the number of pits formed during this process was observed to increase with the increase of water content. Further studies are planned to elucidate the dependence of preparation procedure, as well as experimental constituents and contaminants on the cathodic corrosion of Au(111) in the ionic liquid [MPPip][TFSI]. Such studies can help to understand if cathodic corrosion can be induced by the ionic liquid itself or only by contaminants such as water. It is known that adsorbed hydrogen plays a major role in the process[3] and it has been reported that in the absence of water cathodic corrosion is not taking place.[10] On the other hand, other studies proposed that the presence of water is not obligatory if hydrogen can be formed in another way than water reduction, for example by reduction of the IL cation.[11]

References

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[9] A. V. Rudnev, M. R. Ehrenburg, E. B. Molodkina, A. Abdelrahman, M. Arenz, P. Broekmann, T. Jacob, ChemElectroChem
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[10] M. M. Elnagar, J. M. Hermann, T. Jacob, L. A. Kibler, ECS Meet. Abstr.
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Figure 1