Adsorption of Organic Molecules at Solid-Electrolyte Interface – the Effect of the Electrolyte and Electrode Structure

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Understanding processes at the molecular level at the solid-liquid interface is essential to improving and developing novel technologies in electrochemistry, molecular electronics, catalysis, corrosion inhibition, electrodeposition of metals, formation of composite layers, and energy storage devices. Self-assembled monolayers (SAMs) play a significant role in many of those applications and their improvements [1-2]. At the same time, the characteristics of SAMs depend on many aspects, like the characteristics of individual molecules forming the monolayer, the interactions between these molecules, SAM interactions with the electrolyte, the crystalline surface of the substrate, etc.The adsorption of bipyridines resulting in the formation of various nanostructures at the electrode surface has been studied in both aqueous electrolytes [2-5] and, more recently, in IL medium [6-8]. The adsorption of bipyridines is of interest as it offers insight into the interactions which drive the formation of self-assembled monolayers. This is due to aromatic nitrogen-containing heterocycles, which facilitate the possibility of interacting with the surface or other molecules through nitrogen lone electron pair, π-π stacking or hydrogen bonds.To this day, the universal understanding that the adsorption of organic molecules results in the formation of an ordered monolayer is based on the phenomenon observed in mainly aqueous solutions. However, the conception might not be straightforwardly transferrable to the organic additive + ionic liquid | electrode interface. For example, the existence of an interfacial multilayer structure of pure IL ions in contrast with an aqueous electrolyte has been previously shown in both experimental and computational studies. Furthermore, it has been shown that in ILs, if the ions form rigid layers on the electrode, it is necessary to apply an overpotential for interfacial restructuring.This work compares the adsorption of 4,4′-BP and 2,2′-BP at the interfaces between various metal single crystal electrodes (Sb, Bi, Au, Cd) and aqueous or ionic liquid electrolytes. The adsorption process has been studied by applying scanning tunnelling microscopy, electrochemical impedance spectroscopy and cyclic voltammetry methods. Based on the comparison, similarities and differences between aqueous and ionic liquid as electrolytes will be pointed out. Additionally, the influence of the physical and chemical properties of the various metal single crystal electrodes will be discussed.[1] D. R. MacFarlane, et al., Energy Environ. Sci. 7, 232–250 (2014)[2] N.V. Plechkova et al., Chem. Soc. Rev. 37, 123–150 (2008)[3] Pikma et al., Electrochem. Commun. 2015, 61 (Supplement C), 61–65.[4] H. Ers et al., Electrochim. Acta 2022, 421, 140468.[5] H. Ers et al., J. Electroanal. Chem. 2021, 903, 115826.[6] L. Siinor et al., Electrochem. Commun. 2013, 35, 5–7.[7] L. Siinor et al., J. Electroanal. Chem. 2014, 719, 133–137.