Background:
With the rise of two-dimensional materials such as graphene, the relationship between carbon-based two-dimensional materials and new energy storage is becoming more and more close. Graphene has become an important material for charge storage due to its ultra-high specific surface area, so an in-depth understanding of the energy storage mechanism of graphene electrodes is essential for the design and development of next-generation energy storage devices.
However, there is currently a lack of ideal methods to study the interaction of electrolytes with material surfaces. The traditional experimental method (EWOD) is used to study the electrowettability of the surface by energizing the dielectric layer added to the surface, which is simple and efficient, but cannot accurately measure the electrowettability of energy storage materials. Therefore, it is important to develop new research methods to improve the relationship between electrowettability and energy storage.
To quantify the driving effect of these changes on electrowetting,Paola Carbone, Athanasios A., University of Manchester, UKPapaderakis and Robert Aw.dryfe teamA molecular dynamics method was used to directly calculate the solid-liquid surface tension (SL) of different concentrations of LiCl electrolyte solutions when they came into contact with neutral and charged graphene sheets.
To account for the electrode surface polarization effects caused by external voltage and electrolyte, the authors developed a new method that combines the polarization of graphene electrodes (modeled by quantum mechanics) and the long-term motion of the electrolyte in solution (modeled by molecular dynamics), and a simulation method that integrates quantum mechanics and molecular dynamics (QM MD) balances computational accuracy and cost for modeling the graphene electrolyte interface as well as the changes in the graphene electrode when charging in a conductive polymer solution.
Research Highlights:
Through the innovative QM MD theoretical model, the authors** electrowettability of graphene electrodes at different voltages and electrolyte solution concentrations. This research is the first in the field of electrowettability to achieve a perfect closed loop from theory to experimental verification, which is groundbreaking and forward-looking.
Through in-depth analysis, the authors found that the interfacial hydrogen bond network and the electric double-layer structure play an important role in electrical wettability as well as capacitance changes. On the surface of positively charged graphene, the electrowetting response increases with the increase of electrolyte concentration, resulting in the surface becoming highly hydrophilic, which provides a new idea and direction for the follow-up research on electrowetting and energy storage correlation.
In this paper, the behavior and mechanism of ionized materials on the electrode surface are studied, including the transport of ionized materials, the influence of electrode surface properties on transportation, and the application of molecular dynamics simulation in research, which provides valuable information for understanding the electrochemical properties and behavior of materials.
Calculation method:
In terms of quantum mechanical calculations, the DFTB+ software package was selected to calculate the electronic structure at the SCC-DFTB level based on the electro-wet effect between graphene and electrolyte. The first Brillouin zone was sampled at the point, and the convergence threshold of self-consistent charge optimization was set to 1 10 2 ha.
In the molecular dynamics simulation, the model was run under the NVT ensemble using Gromacs, and the Nose-Hoover thermostat was used to maintain a temperature of 298 K and set the relaxation time to 1 ps. Among them, the SETTLE algorithm takes TIP4P-200552 as a water model with rigid constraints, and the parameters of ions (Li+ and Cl) and carbon atoms are taken from the Madrid-2019 force field and Amber force field, respectivelyGromacs-LS post-processes the output trajectories to obtain the local stresses used to calculate the SL.
**Explained
The authors designed and developed a novel quantum mechanical-classical molecular dynamics (QM MD)-based model (Fig. 1) to study the physicochemical changes between graphene electrodes and electrolyte solutions, which can capture surface polarization dynamics and structural evolution while balancing accuracy and computational cost.
Fig.1 Structure of innovative QM MD waterborne graphene supercapacitors
In Figures 2a and b, the authors observed that in a pure water system, when the electrodes are negatively charged, the water molecules are closer to the electrode surface, while when the electrodes are positively charged, the water density distribution does not change, a difference due to the different orientations of the water molecules towards the surface when the surface is charged. In the case of being positively charged, the water molecules always face the surface towards the oxygen atoms.
The results show that when an externally applied electric field forces water molecules to align with surface-facing oxygen atoms, its wettability is greatly enhanced. Through these simulations and experimental results, the authors can gain an in-depth understanding of the dynamics and structural evolution of the electrode-electrolyte interface during electrolysis-induced surface polarization, and describe the normalized density distribution of ions (Fig. 3c-h).
Observations show that the Cl anion is always repelled by the neutral surface, regardless of ion concentration, which is consistent with the results of previous simulations. On the other hand, Li+ cation is 042 and 0Two adsorption peaks were exhibited at 72 nm, both of which were located outside the inner Helmholtz plane (IHP) extending from the graphene surface along the z-axis no more than one layer of water molecules.
When the electrode is negatively charged, the position of the Li+ adsorption peak remains the same, but the first peak is at 0The height at 42 nm is increased. The enhancement of surface polarization increases the Coulomb force attraction between Li+ and the surface of the material, but this enhancement does not exceed the contribution of the dehydration free energy, so the equilibrium between the two is not broken.
Both anions and cations are located within the outer Helmholtz plane (OHP) and extend from the graphene surface along the z-axis no more than the first ion adsorption layer, and the density peaks overlap significantly as the electrolyte concentration increases from 1 m to 2 m (see also Figure 2D,F). Through these analyses, the authors gained a deeper understanding of the interaction between the electrolyte and the electrode and the effect of electrolyte concentration on the ion distribution, which will support further research on the electrowettability and energy storage performance of the electrodes.
Fig.2 Volume normalized number densities n(z) of water, Li+, and Cl at the negative (left) and positive (right) electrodes
Figure 3 is based on pure water and three ion concentrations as a function of applied surface charge density, with the aim of depicting the electrode electrolyte interfacial tension as a function of pure water and three ion concentrations. To gain insight into the contribution of ion pairs to the observed -sl trend, the authors performed a meticulous analysis. The authors pay particular attention to van der Waals forces and the Coulomb interaction between water molecules and electrodes, and deduct these effects from -sl to examine the contribution of ions separately. Figures 3b and c illustrate how the ion's contribution to -sl varies with ion concentration.
Differences in critical concentration values may arise from the different ion species and electrode materials used in the study. To gain a deeper understanding of the variation of surface tension with ion concentration and surface charge, the authors calculated the difference between the cohesion energy (the sum of the water-water, water-ion, ion-ion, van der Waals and Coulomb energy contributions) and the adhesion energy (the sum of the water-graphene and ionic van der Waals and Coulomb energy contributions). As can be seen from the results in Figure 4d, this difference decreases as the ion concentration increases, but remains negative. This further confirms that the presence of ion species makes the surface more hydrophilic.
Fig. 3 The -sl values and their standard deviations for different charged surfaces as a function of ion concentration, electrode surface charge, cohesion energy, and adhesion energy
Figure 4A shows that there is a clear correlation between the number of hydrogen bonds (HBS) and the surface tension in a pure water system: the interface with the lowest surface tension (the water in contact with the positive surface) has the highest amount of HBS. Consistent with the case of pure water, in an electrolyte solution, the number of HBS in interface water in contact with a positively charged surface (the most hydrophilic) is always higher than that of a negatively charged and neutral surface, and decreases as the ion concentration increases.
This correlation holds true even after taking into account that the amount of interfacial water decreases with increasing salt concentration and the number of hydrogen bonds (HBS) per water molecule is calculated (Figure 4B). In Figures 4c and d, the authors show the distribution of the dipole moment angle of the water molecule relative to the electrode surface, which is a function of distance from the surface. An angle of 90° indicates a parallel arrangement, and angles of 0° and 180° indicate a vertical arrangement, with the water tail (hydrogen atoms) and the water head (oxygen atoms) pointing to the surface, respectively. This shows the changes in the number of hydrogen bonds and hydrogen bond density between the graphene surface and the water molecules at different ion concentrations, and the number and density of hydrogen bonds between the graphene surface and the water molecules gradually increase with the increase of ion concentration, indicating that the increase of ion concentration is conducive to enhancing the hydrophilicity of the graphene surface.
Fig.4 Analysis of the number of hydrogen bonds and the direction of rotation of water molecules at the interface
Bibliographic information
wei, z., elliott, j. d., papaderakis, a. a., dryfe, r. a., carbone, p. (2023). relation between double layer structure, capacitance, and surface tension in electrowetting of graphene and aqueous electrolytes. journal of the american chemical society.