Role of Ionic Liquids As Modifiers of Ru-Based Catalysts for Water Electrolysis
Authors: Johny Jacob , Spanos Ioannis , Antil Neha , Louis Anandaraj Savarithai Jenani , Bordet Alexis , Tesch Marc Frederic 
Published: February 2024

The generation of hydrogen through electrocatalytic water splitting has attracted tremendous attention in the past years [1,2]. Although water electrolysis has been already established in industrial settings, a large amount of research is still being carried out to further improve the catalytic efficiency and stability to exploit the full potential of this technology. One approach in this endeavor is the utilization of ionic liquids (ILs) as modifiers of electrocatalysts and electrode surfaces [3, 4]. The special physicochemical properties of the IL can affect and ideally improve the characteristics of the electrocatalyst, e.g., in terms of mass and electron transport [3]. Furthermore, the hydrophilic nature of certain ILs can in addition influence gas bubbles removal on the working electrode and improve the stability of the catalysts (e.g., Ru) by acting as a monomer-like binder [4].
In this study, we compared the effect of four different ILs that are based on the same cation chain (1-butyl-3-(4-methylphenethyl)-1H-imidazol-3-ium) but different anions (-OAc, -NTf2, -Br, and -MeSO3) on Ru nanoparticles as catalysts for hydrogen and oxygen evolution reactions (HER and OER). To this end, carbon nanotubes were functionalized using different ILs and subsequently decorated with 1-2 nm Ru nanoparticles (Ru NPs) following an established organometallic approach [5]. We analyze how the catalytic activity/stability for HER, as well as OER, is influenced by the metal-ILs combination by established electrochemical methods utilizing a rotational disc electrode setup, with a particular focus on the different hydrophilic natures (depending on the anion) of the four ILs. To obtain a more comprehensive picture of how the different ILs affect Ru dissolution during the reaction, the IL/catalyst systems are additionally investigated by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in a flow cell setup, as well as by electrochemical quartz crystal microbalance (EQCM) measurements.
In this work, we discuss the use of ILs as modifiers for catalyst/electrode systems in water-splitting reactions, highlighting the importance of careful IL selection as well as the necessity for more future research in this field to fully explore the potential of IL/NPs-systems in electrochemistry.

References

[1]. Zhu, Jing, et al. "Recent advances in electrocatalytic hydrogen evolution using nanoparticles." Chemical reviews 120.2 (2019): 851-918.
[2]. Sun, Hainan, et al. "Electrochemical water splitting: Bridging the gaps between fundamental research and industrial applications." Energy & Environmental Materials (2022): e12441.
[3]. Zhang, Chenyun, et al. "Ionic Liquids as Promisingly Multi-Functional Participants for Electrocatalyst of Water Splitting: A Review." Molecules 28.7 (2023): 3051.
[4]. Spanos, Ioannis, et al. "Poly (ionic liquid) binders as ionic conductors and polymer electrolyte interfaces for enhanced electrochemical performance of water splitting electrodes." Sustainable Energy & Fuels 2.7 (2018): 1446-1451.
[5]. Bordet, Alexis, and Walter Leitner. "Metal nanoparticles immobilized on molecularly modified surfaces: versatile catalytic systems for controlled hydrogenation and hydrogenolysis." Accounts of Chemical Research 54.9 (2021): 2144-2157.

Figure 1