Liis Siinor

Porphyrin adlayer formation in ionic liquids (IL) is a complex process that can have various applications in various fields: as a catalyst in either hydrogen evolution reactions or in CO₂ reduction reactions to photovoltaics, photocatalysts and corrosion inhibitors^1,2. Compared to aqueous solutions, IL-s allow us to study the formation and effect of an organic adlayer at larger electrochemical windows (EW). Porphyrins are known to be easily modifiable and form organic self-assembled layers^3,4. Tetraphenylporphyrin (TPP) and its derivates have shown promising results in battery-related applications⁵. Bi (111) crystal surface was chosen as a working electrode as it is reliably electrochemically polished to achieve a clean bilayer surface. Thus, when studying these complex systems methodically, we can start to understand the effects of porphyrin adlayers in IL-s⁶.

This study addresses the limited understanding of how time-dependent experimental factors affect electric double layer (EDL) stability and capacitance behavior in highly viscous ionic liquids (IL). The interface between the glassy carbon (GC) electrode and 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) methods. The aim was to determine how different experimental protocols (number of cycles vs holding time, and EIS time delay period) influence the EDL formation in EMImBF₄ at a GC interface. The capacitance values have been shown to systematically depend on the polarization time and number of conducted EIS cycles. Furthermore, the influence of EIS time delay, i.e., the duration of applied fixed potential, was demonstrated. The results highlight the importance of standardizing the design and protocol of experiments to facilitate better comparability of collected experimental results. Results show that stable capacitance values can be achieved either through 14 days of resting times or within 8 h of continuous cycling, and that EIS time delay affects measured capacitance by up to 30%. This work contributes to a novel methodological control study in IL-based electrochemistry and has implications for applications such as supercapacitors and electrocatalytic interfaces, where precise EDL characterization is essential.

The chemical extraction of rare-earth elements (REEs) from Estonian graptolite-argillite (GA) and phosphate rock (Phosphorite, PH) samples has been conducted and analyzed. For the initial leaching process, HCl and HNO₃ with different concentrations were used to extract REEs from GA and PH. Different extraction agents, including ionic liquids, were examined for the extraction of REEs from acidic aqueous solutions in the liquid–liquid extraction step. After leaching and extraction, all samples were characterized using the inductively coupled plasma mass spectrometry method (ICP-MS/MS). The highest REE extraction efficiencies from GA were established with 1-ethyl-3-methyl imidazolium diethyl phosphate (EMImDEPO₄) and from PH using bis(2-ethylhexyl) phosphate (D2EHPA).

Classical electrochemical and microscopy methods were used to characterize the interfacial processes of the adsorption of chloride ions from ionic liquids at the Bi(111) single crystal electrode. The mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium chloride was electrochemically characterized by using cyclic voltammetry and electrochemical impedance spectroscopy. In situ scanning tunneling microscopy images showed the formation of superstructures at the electrode's surface over an extended period of time. The specific adsorption of chloride ions reaches an equilibrium state in a more viscous ionic liquid medium slower than in aqueous and organic solvents. Capacitance values increase considerably (also depending on alternative current frequency) at the potential region, where the specific adsorption of chloride ions with partial charge transfer occurs.

Estonian phosphorite ore contains trace amounts of rare earth elements (REEs), many other d-metals, and some radioactive elements. Rare earth elements, Mo, V, etc. might be economically exploitable, while some radioactive and toxic elements should be removed before any other downstream processing for environmental and nutritional safety reasons. All untreated hazardous elements remain in landfilled waste in much higher concentration than they occur naturally. To resolve this problem U, Th, and Tl were removed from phosphorite ore at first using liquid extraction. In the next step, REE were isolated from raffinate. Nitrated Aliquat 336 (A336[NO₃]) and Bis(2-ethylhexyl) Phosphate (D2EHPA) were used in liquid extraction for comparison. An improved method for exclusive separation of radioactive elements and REEs from phosphorite ore in 2-steps has been developed, exploiting liquid extraction at different pH values.

Influence of chemical composition of ionic liquids and their mixtures on the electrical double layer characteristics of the cupercapacitors (EDLC) has been studied by cyclic voltammetry, electrochemical impedance spectroscopy, constant current charge-discharge and constant power discharge methods. It has been shown that addition of 1-ethyl-3-methylimidazolium iodide (EMImI), EMImBr or EMImCl to EMImBF₄ increases differential capacitance. D-glucose and TiC have been used for preparation of the carbon powders. Additional thermal pyrolysis and/or carbon dioxide activation steps were applied for activation of the electrode materials. The EDLCs based on EMImBF4 + 5 wt% EMImI demonstrated nearly 50% higher stored charge than those with pure EMImBF₄. Noticeable increase in specific capacitance up to 245 F g^-1 at 1.0 V, specific energy (36.7 Wh kg^-1) and specific power (30 kW kg^-1), and minor increase in characteristic relaxation time constant (from 1.45 s to 1.48 s) has been established for EDLCs containing EMImI.

Electrochemical characterization of the interface between Bi(111) surface and ionic liquid mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate with 1-ethyl-3-methylimidazolium bromide (EMImBF₄ + x% EMImBr) has been evaluated by using the electrochemical impedance spectroscopy, cyclic voltammetry methods and density functional theory calculations. Dependence of the experimental data on the bromide concentration and on the electrode potential has been analyzed. Comparison of adsorption data with Bi(111) | EMImBF₄ + x% EMImI interface shows that adsorption activity of halide ions from EMImBF₄ follows a similar trend as from the aqueous and organic electrolyte solutions. The adsorption activity of anions increases in order Cl⁻ < Br⁻ < I⁻, despite the fact that the solvation properties of relevant media are significantly different. We discuss this trend in the light of a possible application of the RTIL mixtures as an electrolyte for electrochemical energy storage devices like supercapacitors.