Publications

Abstract Borophene, a monolayer of boron atoms, belongs to intensively studied two-dimensional beyond-graphene materials. The B36 borophene nanoflake is a finite size model system, containing a hexagonal vacancy similar to the ones present in (312 and chi 3 borophene sheets. The hydrogen binding performance of B36 decorated with various transition metal atoms is investigated using density functional theory and quantum theory of atoms in molecules. Hydrogen is considered to become one of the crucial energy sources in future, hence, a search for effective hydrogen storage materials is of urge importance. Obtained results suggest that B36 decorated with Co, Ni, Fe, and Cu possess strong affinity to bind the H2 molecule via formation of eta 2-dihydrogen bonds. Among them, the strongest H2 binding is found for Co- and Ni-decorated B36. Furthermore, B36 decorated with Sc and Ti behave like H-H bond breakers while B36 decorated with Zn possess only negligible affinity to bind H2 molecule. The stability of the B36 decorated with Co and Ni is verified by ab initio molecular dynamics. The presented data may also serve as a basis for reference in future large-scale computational studies of borophene-based materials.

Abstract Progress in electrochemical applications of ionic liquids builds on an understanding of electrical double layer. This computational study focuses on structure-determined quantities - maximum packing density, potentials, and capacitances - evaluated using a one-electrode electrical double layer model. Interfaces of the 40 studied ions are grouped into four distinct classes according to their characteristic packing at the model surface. The simulations suggest that the exact screening by a monolayer of counter-ions (preceding the crowding of ions) is unlikely for ions in known air- and water-stable ionic liquids within their electrochemical stability window. This work discusses how the assessed structure-determined quantities can guide the experimental tuning of (electro/mechano)chemical properties and characterize the structure of ionic liquid-electrode interfaces.

Abstract This study investigates the structural and dynamic properties of ternary mixtures composed of NaPF6, ethylene carbonate (EC), and the ionic liquid choline glycine (ChGly), with a focus on their potential as electrolytes for supercapacitors. The combination of NaPF6-EC, known for its high ionic conductivity, with the biodegradable and environmentally friendly ChGly offers a promising approach to enhancing electrolyte performance. Through molecular simulations, we analyze how the inclusion of small concentrations of ChGly affects key properties such as density, cohesive energy, and ion mobility. Our findings demonstrate that the NaPF6-EC-ChGly mixture exhibits a complex network of electrostatic interactions and hydrogen bonding, with the glycine anion significantly influencing the liquid structure. In mixtures with small additions of ChGly, we observed an optimal balance of diffusion and ionic mobility. These results highlight the potential of ChGly as a green additive to conventional electrolytes, paving the way for more sustainable and high-performance energy storage devices.

Abstract Magnetic ionic liquids (MILs) stand out as a remarkable subclass of ionic liquids (ILs), combining the desirable features of traditional ILs with the unique ability to respond to external magnetic fields. The incorporation of paramagnetic species into their structures endows them with additional attractive features, including thermochromic behavior and luminescence. These exceptional properties position MILs as highly promising materials for diverse applications, such as gas capture, DNA extractions, and sensing technologies. The present Review synthesizes key experimental findings, offering insights into the structural, thermal, magnetic, and optical properties across various MIL families. Special emphasis is placed on unraveling the influence of different paramagnetic species on MILs' behavior and functionality. Additionally, the Review highlights recent advancements in computational approaches applied to MIL research. By leveraging molecular dynamics (MD) simulations and density functional theory (DFT) calculations, these computational techniques have provided invaluable insights into the underlying mechanisms governing MILs' behavior, facilitating accurate property predictions. In conclusion, this Review provides a comprehensive overview of the current state of research on MILs, showcasing their special properties and potential applications while highlighting the indispensable role of computational methods in unraveling the complexities of these intriguing materials. The Review concludes with a forward-looking perspective on the future directions of research in the field of magnetic ionic liquids.

Abstract Electrolytes play a crucial role in enhancing the performance of energy storage devices, including batteries and supercapacitors. However, traditional electrolytes, such as aqueous solutions, organic solvents, and ionic liquids, exhibit inherent limitations and challenges. Deep eutectic solvents have recently emerged as promising alternatives due to their environmentally friendly nature and favorable properties. Despite their widespread applications in various domains, their potential as electrolytes remains relatively underexplored. This study investigates three distinct types of deep eutectic solvents derived from different isomers of butanediols combined with choline chloride. Ab initio molecular dynamics simulations are employed to analyze the microstructure of these deep eutectic solvents, focusing on non-covalent electrostatic interactions, hydrogen bonding patterns, and vibrational spectra. The results reveal significant differences in the structural configuration of hydrogen bond acceptors and hydrogen bond donors and their interactions within the deep eutectic solvents. Specifically, the positioning of functional groups in hydrogen bond donors significantly impacts the hydrogen bonding network and the interaction with monoatomic ions. Moreover, the vibrational spectra analysis highlights the existence of hydrogen bonds involving stretching modes of the OH group, as evidenced by redshift deviations. Overall, this study provides valuable insights into the unique features of deep eutectic solvents as potential electrolytes for energy storage applications. The comprehensive analysis of their microstructure and vibrational properties enhances our understanding of deep eutectic solvent utilization and opens avenues for further research in sustainable energy storage.

Abstract Technologies involving a solvent|surface interface, such as nanotechnology, electrochemistry, and energy storage applications, are actively pursuing ecologically responsible and sustainable development practices. In response to this pressing need, deep eutectic solvents have emerged as a promising solution to bridge the gap between technological requirements and environmental concerns. In this work, we present the results of a molecular dynamics simulation study of the interface between a monocrystalline gold surface and the deep eutectic solvent ethaline, where a molar ratio of 1:2 choline chloride:ethylene glycol was used for ethaline. The simulations covered a range of temperatures from 313 K to 343 K and applied charge values ranging from 0 to +/- 24 mu C cm-2. Several key interfacial properties were thoroughly analyzed, including among others, charge density profiles, radial distribution functions, hydrogen bond close contacts, and molecular orientation. Additionally, we examined how the differential capacitance varied upon the applied potential. Our findings reveal that, at neutral surfaces, all components of the solvent are present in the innermost layer, with ethylene glycol molecules being the most prevalent, followed by choline cations and a residual amount of chloride anions. For lower applied charges, this mixed composition at the boundary layer persists, despite the growing accumulation of ionic species with charges opposite to that of the electrode. As surface polarization increases, unique innermost boundary layers composed exclusively of one of the ionic species and the hydrogen bond donor molecules are observed, forming a multilayer structure, with subsequent layers enriched of paired counterions. Interestingly, even at higher applied charges, choline cations and ethylene glycol molecules tended to orient themselves in a parallel fashion toward the electrodes. Differential capacitance curves exhibited a camel-shaped behavior, suggesting a complex interplay of electrochemical processes at the DES|Au(100) interface. In summary, our study provides valuable insights into the interfacial properties of deep eutectic solvents on gold surfaces and their response to changes in temperature and potential, which are crucial for understanding and optimizing deep eutectic solventbased electrochemical systems.

Abstract Amino acid ionic liquids have garnered significant attention for their potential in electrochemical energy storage due to their wide electrochemical stability windows and inherent safety. The performance of ChGly as an electrolyte for supercapacitors has been compared to that of highly efficient conventional ionic liquids. However, a thorough understanding of the microstructural characteristics responsible for the enhanced properties of ChGly aqueous solutions remains largely unexplored. In this study, ab initio molecular dynamics simulations were employed to investigate the energetic, structural, transport and spectroscopic properties of ChGly-based pure and aqueous electrolytes. A comprehensive analysis of the cation-anion and water-ion hydrogen bonding was conducted for both electrolyte systems. Structural features were examined using radial and spatial distribution functions, while the vibrational power spectra were analyzed to identify significant differences in intermolecular interactions between pure and aqueous electrolytes, stemming from modified solvation shell structures. The findings presented in this work shed light on crucial structural and spectroscopic distinctions between pure and aqueous ChGly electrolytes, providing valuable insights for further advancements in the field.

Abstract Regardless of the many common properties shared by ionic liquids (ILs) and deep eutectic solvents (DES), these are two completely different classes of solvents. Unlike ILs, DES are not composed solely of ions, which adds an extra complexity to the systems. Indeed, DES possess a distinct nanostructure organization which is mainly due to the strong hydrogen network present in these solvents. For this reason ILs and DES cannot be treated as similar mixtures, having their own particularities and different promising applications. Nevertheless, both solvents are highly tunable owing to the infinite number of possible combinations of their components and so it is important to tailor their specific properties according to a specific task, by choosing the right components at the right molar ratios. The growing number of applications involving an IL/surface or DES/surface interface is increasing rapidly, but the upscaling of such technologies needs a profound knowledge of chemical compositions, structure and orientational arrangements at these interfaces, also known as the electrical double layer (EDL). In this work, we provide a summary of the progress made in the interfacial area involving ILs and DES, paying special attention to the EDL arrangements and dynamics. It is clear that classical EDL theories do not apply to neither of these solvents, being the one proposed by Kornyshev the most accepted for ILs. Yet, a multilayer organization of the mixtures components at the solvent|electrode interface is recognized by experimental and computational simulation communities for both electrolytes. Each system has its own characteristic differential capacitance curves, which vary greatly due to the different nature of adsorption/desorption species and their reorientation.

Abstract <jats:p> Ionic liquids (ILs) are a unique class of electrolytes, which characteristics make them suitable for use in solar cells, supercapacitors, and fuel cells<jats:sup>1</jats:sup>. Due to the appealing properties such as good electrochemical stability, low vapour pressure, high concentration of ions and the lack of solvent, they have been under intense study since the early 2000s. Although numerous theoretical<jats:sup>2,3</jats:sup>, computational<jats:sup>4,5</jats:sup>, and experimental studies<jats:sup>6,7</jats:sup> have shed light on the interfacial properties of ILs, which differ noticeably from the aqueous electrolytes, multiple open questions remain. One such problem is how the interfacial capacitance is affected by the ambient temperature, as studies have shown both positive and negative temperature dependences<jats:sup>8,9</jats:sup>. Understanding the temperature dependence of interfacial capacitance is crucial as it is relevant for the description of energy storage and is one of the few quantities, which can be estimated both experimentally and computationally.</jats:p> <jats:p>In this study, we combine the density functional theory (DFT) calculations with molecular dynamics (MD) simulations of graphene (Gr) | EMImBF<jats:sub>4</jats:sub> IL interface to explain the effect of temperature on capacitance. MD simulations allow us to investigate the probable distribution of ions near the electrode’s surface and relate the changes of ILs structure to the capacitance using the interfacial bilayer model (IBL). We show that the increase of temperature affects the capacitance near the potential of zero charge by attenuating the overscreening without a notable change in the IL interfacial structure. The characteristic peaks and plateaus in the capacitance potential dependence are explained through the concepts of IL layering and saturation of the second IL layer described in the IBL. Using the DFT calculations, we estimate the impact of the quantum capacitance of Gr on the total interfacial capacitance and its temperature dependence. By accounting for the limiting quantum capacitance, the total interfacial capacitance was significantly altered in the case of the Gr electrode, as the effect of the temperature was dampened, and a V-shaped capacitance curve was obtained.</jats:p> <jats:p>Acknowledgements:</jats:p> <jats:p>This work was supported by the Estonian Research Council grant PSG249 and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011). The financial support from FCT/MCTES through the Portuguese national funds, project No. UID/QUI/50006/2021 (LAQV@REQUIMTE) is also acknowledged.</jats:p> <jats:p>References:</jats:p> <jats:p>1 D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon and C. A. Angell, <jats:italic>Energy Environ. Sci.</jats:italic>, 2014, <jats:bold>7</jats:bold>, 232–250.</jats:p> <jats:p>2 A. A. Kornyshev, <jats:italic>J. Phys. Chem. B</jats:italic>, 2007, <jats:bold>111</jats:bold>, 5545–5557.</jats:p> <jats:p>3 Z. A. H. Goodwin and A. A. Kornyshev, <jats:italic>Electrochem. Commun.</jats:italic>, 2017, <jats:bold>82</jats:bold>, 129–133.</jats:p> <jats:p>4 M. Salanne, <jats:italic>Phys. Chem. Chem. Phys.</jats:italic>, 2015, <jats:bold>17</jats:bold>, 14270–14279.</jats:p> <jats:p>5 I. V. Voroshylova, H. Ers, V. Koverga, B. Docampo-Álvarez, P. Pikma, V. B. Ivaništšev and M. N. D. S. Cordeiro, <jats:italic>Electrochim. Acta</jats:italic>, 2021, <jats:bold>379</jats:bold>, 138148.</jats:p> <jats:p>6 L. Siinor, K. Lust and E. Lust, <jats:italic>J. Electrochem. Soc.</jats:italic>, 2010, <jats:bold>157</jats:bold>, F83.</jats:p> <jats:p>7 V. Lockett, M. Horne, R. Sedev, T. Rodopoulos and J. Ralston, <jats:italic>Phys. Chem. Chem. Phys.</jats:italic>, 2010, <jats:bold>12</jats:bold>, 12499–12512.</jats:p> <jats:p>8 F. Silva, C. Gomes, M. Figueiredo, R. Costa, A. Martins and C. M. Pereira, <jats:italic>J. Electroanal. Chem.</jats:italic>, 2008, <jats:bold>622</jats:bold>, 153–160.</jats:p> <jats:p>9 M. Drüschler, N. Borisenko, J. Wallauer, C. Winter, B. Huber, F. Endres and B. Roling, <jats:italic>Phys. Chem. Chem. Phys.</jats:italic>, 2012, <jats:bold>14</jats:bold>, 5090–5099. </jats:p>

Abstract This work describes the effect of potential and temperature on the grapheneionic liquid (EMImBF4) interfacial structure and properties with the focus on a novel phenomenon of ionic saturation. We apply classical molecular dynamics simulations to reproduce well-known phenomena of overscreening, mono -layer formation, and temperature-induced smearing of the interfacial structure. Using quantum density functional theory calculations, we show how quantum capacitance dampens the influence of temperature and improves the agreement with the experimental data. Using a bilayer model, we study characteristic features of capacitance-potential dependence and relate them to the changes in interfacial structure. These insights are of fundamental and practical importance for the application of similar interfaces in electrochemical energy storage and transformation devices such as capacitors and actuators. (C) 2022 The Authors. Published by Elsevier B.V.