Formation Of Electrochemical Double Layer Research Articles

Deciphering the Decomposition Mechanisms of Ether and Fluorinated Ether Electrolytes on Lithium Metal Surfaces: Insights from CMD and AIMD Simulations.

The performance of lithium metal batteries can be significantly enhanced by incorporating fluorinated ether-based electrolytes, yet the solid electrolyte interphase (SEI) formation mechanism on lithium metal surfaces remains elusive. This study employs classical and ab initio molecular dynamics simulations to investigate the decomposition mechanisms of lithium bis(fluoromethanesulfonyl)imide (LiFSI) in 1,2-diethoxyethane (DEE) and its fluorinated analogues, F5DEE and F2DEE, when in contact with lithium metal. Our findings indicate that F5DEE-based electrolytes favor the formation of a FSI-rich primary solvation shell around Li+, while F2DEE-based electrolytes yield a solvent-rich environment. The normalized number density at the Li/electrolyte/Li interface shows a depletion of FSI anions in the electrochemical double layer (EDL) structure near the Li anode upon charging, with the distance between the first main peak of the FSI anion and Li anode following the order F5DEE < DEE < F2DEE. Analysis of the electronic projected density of states and charge transfer dynamics unveils the reductive dissociation pathways of FSI anions and fluorinated DEE solvents on the lithium metal surface, taking into account the influence of the EDL structure. DEE is identified as the most reduction-stable solvent, leading to the selective dissociation of FSI anions and the formation of an entirely inorganic SEI. In contrast, F2DEE displays a pronounced reduction tendency, forming an organic-rich SEI due to the solvent-dominated lowest unoccupied molecular orbital at the interface. F5DEE, competing with FSI anions for reduction, results in the formation of an inorganic-rich hybrid SEI with the highest LiF content. The simulation results correlate well with experimental observations and underscore the pivotal role of various fluorinated functional groups in the formation of EDL and SEI near the lithium metal surface.

Chemical synthesis of polyaniline and polythiophene electrodes with excellent performance in supercapacitors

The emergence of portable electronics in miniaturized and intelligent devices demands high-performance supercapacitors (SC) and batteries as power sources. For the fabrication of such energy storage devices, conducting polymers (CPs) have significant advantages due to their high theoretical capacitive performance and conductivity. In this work, we developed two CPs including polyaniline and polythiophene through a low-cost chemically synthesized approach and the film-by-spin coating method. The structural and morphological properties of the CPs are analyzed using Fourier-transform infrared spectroscopy (FTIR), contact angle measurement, and scanning electron microscopy (SEM). Based on these CPs, novel pristine polyaniline and polythiophene-based SCs (PASC and PTSC) are developed. The prepared CPs contribute to high electrochemical performances due to their high conductive nature of the electrode and conjugated polymer materials reaction. Hence both electrochemical double-layer formation and pseudocapacitance contributed to the energy-storing performances of the device. Electrochemical impedance spectroscopic analysis (0.1 Hz to 100 kHz) demonstrates faster ionic exchange and high capacitance of the PASC electrode as compared to PTSC in H3PO4 electrolyte. The PASC devices exhibit specific capacitance of 13.22 mF·cm−2 with energy and power densities of 1.175 μW·h·cm−2 and 4.99 μW·cm−2 at a current of 50 μA. Compared to PTSC (specific capacitance 3.30 mF·cm−2) the PASC shows four times higher specific capacitance due to its improved surface, structural and electrical properties. The electrochemical performance reveals the superior SC performance for this type of CP electrode.

Metal‐organic chemical vapor deposition of anatase titania on multiwalled carbon nanotubes for electrochemical capacitors

AbstractIn practice, the capacitance from the electrochemical double layer formation on porous carbon‐based electrodes is still below preferred values, limiting their use in electrochemical capacitors. The current drive is to innovate ways that generate additional capacitance in the electrochemical double layer capacitive nature of carbon nanomaterials towards both a high specific energy density (Es) and power density (Ps). Herein we report the use of metal‐organic chemical vapor deposition (MOCVD) to coat multiwalled carbon nanotubes (MWCNTs) with anatase titanium dioxide (TiO2) to induce pseudocapacitive charge storage characteristics on a carbon‐based electrode. The study shows that MWCNTs were coated in bundles, and targeted TiO2 loadings were successfully attained, though the TiO2 agglomerates also increased with TiO2 wt.%. The 10 wt.% TiO2 TiO2‐MWCNT material displayed the best capacitive behavior with associated specific discharge capacitance (Cd), Es, and Ps values of 907 F kg−1, 55.56 Wh kg−1, and 2.78 W kg−1 at 0.1 A g−1, respectively, due to the synergistic effect of the two components of the composite. Additionally, the integral capacitance (Cs) of the 20 wt.% TiO2 material was enhanced more than 5000‐fold relative to that of the 5 wt.% TiO2 TiO2‐MWCNT composite at higher scan speeds of 100 and 200 mV s−1. Electrochemical measurements further demonstrated the possible positive tuning of capacitive characteristics (charge/discharge rates, Cd and Cs) with TiO2 wt.% control. The MOCVD synthesis method imparted the TiO2‐MWCNT composites with suitable traits that showed high potential in improving physicochemical processes favorable in electrical energy storage.

Simulation of Electrochemical Double Layer Formation with Complex Geometries

A novel method is introduced to simulate the formation of electrochemical double layers on complex electrode particle geometries. Electrochemical double layers play the most crucial role in electrical energy storage of supercapacitors and in capacitive deionization (desalination) devices. The double-layer region usually spans 20 to 50 nanometers, whereas other significant length scales, e.g., particle size or inter-particle space in electrodes, are in tens to hundreds of microns. Thus, a direct numerical simulation in the continuum scale must resolve the ionic concentration and potential gradients in spatial scales across 3 ∼ 4 orders of magnitude difference and is highly challenging. In this paper, we use the smoothed boundary method that defines complex microstructures with a continuous domain-parameter function to reformulate the Nernst-Planck-Poisson equations and solve the reformulated governing equations on adaptively refined meshes. The method allows for accurate simulations of arbitrarily complex geometries with resolutions spanning from nanometers in the double layer to micron/millimeters in the particle/electrode scales. The results show that ions first rapidly adsorb onto or are repelled from the particle surfaces, followed by long-distance diffusion to alleviate the concentration gradient until the system reaches the steady state. The concentration and potential evolutions highly depend on the particle geometries.

Template-free synthesis of Se-nanorods-rGO nanocomposite for application in supercapacitors

AbstractBoth selenium and reduced graphene oxide have low specific capacitance due to their chemical nature. Nevertheless, their specific capacitance could be enhanced by hybridizing Se nanomaterials with reduced graphene oxide via formation of electrochemical double layer at their interfacial area. Therefore, novel Se-nanorods/rGO nanocomposite was successfully synthesized by template free hot reflux route starting with graphene oxide and selenium dichloride. The composite of rGO decorated by Se-nanorods is characterized using X-ray diffractometry (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption– desorption. The unique architecture of the composite exhibits high specific capacitance of 390 F/ g at 5 mV/s scan rate in 1.0 M KOH solution with ~ 90% cyclic stability after 5000 cycles making it very promising electrode material for supercapacitor applications.

Textile-Based Potentiometric Electrochemical pH Sensor for Wearable Applications.

In this work, we present a potentiometric pH sensor on textile substrate for wearable applications. The sensitive (thick film graphite composite) and reference electrodes (Ag/AgCl) are printed on cellulose-polyester blend cloth. An excellent adhesion between printed electrodes allow the textile-based sensor to be washed with a reliable pH response. The developed textile-based pH sensor works on the basis of electrochemical reaction, as observed through the potentiometric, cyclic voltammetry (100 mV/s) and electrochemical impedance spectroscopic (10 mHz to 1 MHz) analysis. The electrochemical double layer formation and the ionic exchanges of the sensitive electrode-pH solution interaction are observed through the electrochemical impedance spectroscopic analysis. Potentiometric analysis reveals that the fabricated textile-based sensor exhibits a sensitivity (slope factor) of 4 mV/pH with a response time of 5 s in the pH range 6–9. The presented sensor shows stable response with a potential of 47 ± 2 mV for long time (2000 s) even after it was washed in tap water. These results indicate that the sensor can be used for wearable applications.

Application of electrochemical impedance spectroscopy as a technique to characterize phase diagrams of surfactant solutions

Characterizing the phase behavior of surfactants in solution is essential to their application on different industrial and research fields. Nowadays, the methodology to determine surfactant structural changes in solutions relies on measurements of bulk-solution properties, which frequently fail when working at highly low surfactant solution. Therefore, alternative strategies are required to accomplish such evaluations. Here, we demonstrate that electrochemical impedance spectroscopy (EIS) can be easily applied to characterize surfactant-phase behavior by monitoring structural modifications at the surfactant solution-electrode interface. A graphical analysis of EIS measurements in form of Bode-type diagrams allows obtaining two representative electrochemical parameters: the surfactant solution resistance and the characteristic time constant associated with the electrochemical double-layer formation, as a function of both temperature and surfactant concentration. These parameters are found to be correlated with the change on the surfactant phase and therefore, its detection is accurately performed even at low surfactant concentrations.

Amphicharge‐Storable Pyropolymers Containing Multitiered Nanopores

AbstractIn this study, hierarchically nanoporous pyropolymers (HN‐PPs) including numerous redox‐active heteroatoms are fabricated from polyaniline nanotubes by heating with KOH. In the large operating voltage range 1.0–4.8 V versus Li+/Li, HN‐PPs store amphicharges by a pseudocapacitive manner of Li‐ion (mainly &lt;3.0 V) and electrochemical double layer formation of anion (primarily &gt;3.0 V). Through these surface‐driven charge storage behaviors, HN‐PPs achieve a significantly high specific capacity of ≈460 mA h g−1 at 0.5 A g−1, maintaining specific capacities of 140 mA h g−1 at a high specific current of 30 A g−1 and 305 mA h g−1 after 2000 cycles at 3 A g−1. Furthermore, asymmetric energy storage devices based on HN‐PPs deliver a high specific energy of 265 W h kg−1 and high specific power of 5081 W kg−1 with long‐term cycling performance.

Investigation on the applicable pore size of nanoporous carbon for electrochemical double-layer formation at different current densities

Considering the wide application of nanoporous carbon material in electrochemical double-layer capacitors (EDLCs), the investigation of the structure-performance relationship is critical to constructing suitable nanostructure for high performance EDLCs. Herein, we focus on an important but neglected issue of different applicable pore sizes for electrochemical double-layer (EDL) formation at different current loadings. Small-sized pores with different sizes/distributions are introduced for energy storage while a developed 3D large meso/macroporous structure is constructed for guaranteeing rapid mass transfer. An interesting result is obtained that the applicable pore size greatly depends on the current loadings. For instance, pores larger than 0.59nm are responsible for EDL formation at a low current density of 0.1Ag−1 in aqueous electrolyte. As increasing the current density to 1 and 10Ag−1, the most effective pore size increases to larger than 0.73 and 0.93nm, respectively, which is attributed to the insufficient time of solvent shell removing, squeezing behavior of solvated ions and charge arrangement at higher current densities. These encouraging results may raise concerns about constructing suitable pore size more precisely to realize a target oriented use, particularly those targeting higher power EDLCs applications.

Direct anodic exfoliation of graphite onto high-density aligned graphene for large capacity supercapacitors

Vertically oriented graphene nanosheets (VOGNs) fabricated on conductive substrates with a large amount of edge planes and open channels are ideal for electrochemical double-layer (EDL) capacitor electrodes. However, preparation of such a structure with high-density of graphene nanosheets is challenging. Herein, a facile, environment-friendly, and economical technique to prepare high-quality VOGNs directly on conductive graphite plates with a high mass loading is described. The VOGNs are obtained by electrochemical anodization of graphite and a large amount of aligned reduced graphene oxide (rGO) is produced and adheres strongly to the graphite substrate (G@rGO). The symmetrical supercapacitors composed of the G@rGO electrodes exhibit a high volumetric capacitance of 3.9Fcm−3 and energy density of 0.66WhL−1 (based on the volume of the whole electrode) at a current density of 7.5mAcm−3 in 6M KOH. The rate performance and long-term cycling stability are very good. The outstanding capacitive performance can be attributed to the unique structure of the G@rGO electrode which facilitates transportation of ions between the electrolyte and graphene surface, minimizes the distributive nature of charge storage, expedites the formation of EDL, and enhances the electrochemical utilization of graphene and stability by avoiding restacking and aggregation of graphene nanosheets.

Electric field-controlled magnetization switching in Co/Pt thin-film ferromagnets

A study of dynamic and reversible voltage controlled magnetization switching in ferromagnetic Co/Pt thin film with perpendicular magnetic anisotropy at room temperature is presented. The change in the magnetic properties of the system is observed in a relatively thick film of 15 nm. A surface charge is induced by the formation of electrochemical double layer between the metallic thin film and non-aqueous lithium LiClO4 electrolyte to manipulate the magnetism. The change in the magnetic properties occurred by the application of an external electric field. As the negative voltage was increased, the coercivity and the switching magnetic field decreased thus activating magnetization switching. The results are envisaged to lead to faster and ultra-low power magnetization switching as compared to spin-transfer torque (STT) switching in spintronic devices.

In Situ Solid-State NMR Spectroscopy of Electrochemical Cells: Batteries, Supercapacitors, and Fuel Cells

Electrochemical cells, in the form of batteries (or supercapacitors) and fuel cells, are efficient devices for energy storage and conversion. These devices show considerable promise for use in portable and static devices to power electronics and various modes of transport and to produce and store electricity both locally and on the grid. For example, high power and energy density lithium-ion batteries are being developed for use in hybrid electric vehicles where they improve the efficiency of fuel use and help to reduce greenhouse gas emissions. To gain insight into the chemical reactions involving the multiple components (electrodes, electrolytes, interfaces) in the electrochemical cells and to determine how cells operate and how they fail, researchers ideally should employ techniques that allow real-time characterization of the behavior of the cells under operating conditions. This Account reviews the recent use of in situ solid-state NMR spectroscopy, a technique that probes local structure and dynamics, to study these devices. In situ NMR studies of lithium-ion batteries are performed on the entire battery, by using a coin cell design, a flat sealed plastic bag, or a cylindrical cell. The battery is placed inside the NMR coil, leads are connected to a potentiostat, and the NMR spectra are recorded as a function of state of charge. (7)Li is used for many of these experiments because of its high sensitivity, straightforward spectral interpretation, and relevance to these devices. For example, (7)Li spectroscopy was used to detect intermediates formed during electrochemical cycling such as LixC and LiySiz species in batteries with carbon and silicon anodes, respectively. It was also used to observe and quantify the formation and growth of metallic lithium microstructures, which can cause short circuits and battery failure. This approach can be utilized to identify conditions that promote dendrite formation and whether different electrolytes and additives can help prevent dendrite formation. The in situ method was also applied to monitor (by (11)B NMR) electrochemical double-layer formation in supercapacitors in real time. Though this method is useful, it comes with challenges. The separation of the contributions from the different cell components in the NMR spectra is not trivial because of overlapping resonances. In addition, orientation-dependent NMR interactions, including the spatial- and orientation-dependent bulk magnetic susceptibility (BMS) effects, can lead to resonance broadening. Efforts to understand and mitigate these BMS effects are discussed in this Account. The in situ NMR investigation of fuel cells initially focused on the surface electrochemistry at the electrodes and the electrochemical oxidation of methanol and CO to CO2 on the Pt cathode. On the basis of the (13)C and (195)Pt NMR spectra of the adsorbates and electrodes, CO adsorbed on Pt and other reaction intermediates and complete oxidation products were detected and their mode of binding to the electrodes investigated. Appropriate design and engineering of the NMR hardware has allowed researchers to integrate intact direct methanol fuel cells into NMR probes. Chemical transformations of the circulating methanol could be followed and reaction intermediates could be detected in real time by either (2)H or (13)C NMR spectroscopy. By use of the in situ NMR approach, factors that control fuel cell performance, such as methanol cross over and catalyst performance, were identified.

A Multiscale Physical Model of Electrochemical Energy Storage Devices

A general multiscale physical modeling framework is presented to simulate the transient operation and mechanisms at multiple scales in electrochemical storage systems, such as lithium air batteries, lithium ion batteries and supercapacitors. This approach extends the one developed by my group and myself since 10 years for PEM Fuel Cells and Water Electrolyzers. The framework consists on detailed modular descriptions of the elementary kinetic reactions within the electrodes that can be based on ab initio-extracted parameters and the description of the electrochemical double layer formation under nonequilibrium conditions, micro and mesoscale structurally-resolved description of charges transfer across the porous media and cell potential calculation. This general approach allows accounting for the explicit transient description of materials degradation mechanisms and their impact on the instantaneous cell performance decay rate and durability. The specific application of this model to the case of lithium-air batteries is illustrated.

A Computational Study of the Interfacial Structure and Capacitance of Graphene in [BMIM][PF6] Ionic Liquid

A combination of graphene-like electrodes and ionic liquid (IL) electrolytes has emerged as a viable and attractive choice for electrochemical double layer (EDL) capacitors. Based on combined classical molecular dynamics and density functional theory calculations, we present the interfacial capacitance between planar graphene and [BMIM][PF6] IL, with particular attention to the relative contributions of the electric double layer capacitance at the graphene/IL interface and the quantum capacitance of graphene. The microstructure of [BMIM][PF6] near the graphene electrode with varying charge densities are investigated to provide a molecular description of EDLs, including BMIM/PF6 packing and orientation, cation-anion segregation, and electrode charge screening. Although the IL interfacial structures exhibit an alternative cation/anion layering extending a few nanometers, the calculated potential profiles provide evidence of one-ion thick compact EDL formation. The capacitance-potential curve of the EDL is convex- or bell-shaped, whereas the quantum capacitance of graphene is found to have concave- or U-shaped characteristics with a minimum of nearly zero. Consequently, we find that the total interfacial capacitance exhibits a U-shaped trend, consistent with existing experimental observations at a typical carbon/IL interface. Our work highlights the importance of the quantum capacitance in the overall performance of graphene-based EDL capacitors.

Multiscale Modeling of Electrochemical Double Layer Formation on Pt Electrodes Covered by Nafion

not Available.

An Electric Field–Induced Change in the Selectivity of a Metal Oxide–Catalyzed Epoxide Rearrangement

The rearrangement of cis-stilbene oxide catalyzed by Al(2)O(3) was studied in the presence of interfacial electric fields. Thin films of Al(2)O(3) deposited on Si electrodes were used as the opposing walls of a reaction vessel. Application of a voltage across the electrodes engendered electrochemical double layer formation at the Al(2)O(3)-solution interface. The aldehyde to ketone product ratio of the rearrangement was increased by up to a factor of 63 as the magnitude of the double layer charge density was increased. The results support a field-dipole effect on the selectivity of the catalytic reaction.

An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction

The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × √3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.