Formation Of Electrical Double Layer Research Articles

Multi-cationic ionic liquid combination enabling 86-fold enhancement in frequency response and superior energy density in all-solid-state supercapacitors

Polymer ionic-conductor electrolytes with seamless electrode-electrolyte interface are indispensable for extracting the full potential of all-solid-state electrochemical double layer capacitors. While ionic liquids are among the best ionic conductors in liquid state and offer a wide potential window, translating their ionic mobility and activity to solid-state is important, yet challenging. In this direction, we report a mixed ionic liquid based solid electrolyte with an ionic conductivity of 1 mS/cm that delivers 86-fold higher scan-rate operability and 2.6 times lower relaxation time constant than their individual constituents. Specifically, a flexible polymeric matrix impregnated with a mixture of monocationic (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIm TFSI)) and a dicationic ionic liquid (1,4-di(vinylimidazolium) butane bisbromide ([DVIM]Br)) facilitates rapid and extensive formation of electrical double layer when integrated with carbon-based electrodes. The resulting all-solid-sate flexible supercapacitor achieves superior scan rate operability (15,000 mV/s) and ultralow relaxation time constant (7.8 ms), translating to high discharge current density (>1 mA/cm2), energy density (26.4 Wh/kg) and power density (61,127 W/kg). The improved rate capability (~83 %) and energy storage performance is attributed to a synergistic interplay of ionic strength, conductivity and ion mobility of the mixed ionic-liquid that unfold newer avenues towards solid-electrolyte engineering for energy storage devices.

Extensive Analysis on the Thermoelectric Properties of Aqueous Zn-Doped Nickel Ferrite Nanofluids for Magnetically Tuned Thermoelectric Applications.

Because of the ever-increasing consumption of energy, higher-efficiency thermoelectric materials are in more demand. In this regard, initially, ZnxNi(1-x)Fe2O4 nanoparticles were synthesized using a co-precipitation technique and investigated using Rietveld refinement and Brunauer-Emmett-Teller (BET) analyses. Then, they were dispersed in water to obtain stable 1 vol % of aqueous ZnxNi(1-x)Fe2O4 nanofluids and their viscous, electrical, thermal, and thermoelectric properties were analyzed in the absence and presence of a magnetic field. The Rietveld refinement revealed the formation of single phase spinel ferrite structures and cationic distribution of ZnxNi(1-x)Fe2O4 nanoparticles, whereas BET analysis revealed the surface area of the nanoparticles. The viscosity studies proved the pseudo-plastic shear thinning behavior and dipole-dipole interactions of ZnxNi(1-x)Fe2O4 nanofluids. The electrical conductivity, thermal conductivity, and Seebeck coefficient studies revealed that the maximum enhancement was observed for the Zn0.2Ni0.8Fe2O4 nanofluid, which was attributed to the enhanced electrical double layer formation and Brownian motion of nanoparticles in the nanofluid. The enhancement in the properties of synthesized nanofluids in the presence of a magnetic field was attributed to the formation of chain-like structures, which was substantiated through the magneto-viscosity studies. The thermoelectric energy conversion efficiency of ZnxNi(1-x)Fe2O4 nanofluids was calculated which showed that the maximum enhancement of 27% was observed in the Zn0.2Ni0.8Fe2O4 nanofluid at 770 G. The observed results proved that the synthesized nanofluids are magnetically tunable thermoelectric materials which are suitable for waste heat energy harvesting applications.

Microscopic Model for Cyclic Voltammetry of Porous Electrodes.

Cyclic voltammetry (CV) is a widespread experimental technique for characterizing electrochemical devices such as supercapacitors. Despite its wide use, a quantitative relation between CV and microscopic properties of supercapacitors is still lacking. In this Letter, we use both the microscopic "stack-electrode" model and its equivalent circuit for predicting the cyclic voltammetry of electric double-layer formation in porous electrodes. We find that the dimensionless combination ωτ_{n}, with ω the scan frequency of the time-dependent potential and τ_{n} the relaxation timescale of the stack-electrode model, governs the CV curves and capacitance: the capacitance is scan-rate independent for ωτ_{n}≪1 and scan-rate dependent for ωτ_{n}≫1. With a single fit parameter and all other model parameters dictated by experiments, our model reproduces experimental CV curves over a wide range of ω. Meanwhile, the influence of the pore size distribution on the charging dynamics is investigated to explain the experimental data.

Influence of water on the electrochemical characteristics and nanostructure of Bi(hkl)│Ionic liquid interface

In electrochemical applications of ionic liquids (ILs), the presence of residual amounts of water can influence the performance of ILs as electrolytes when developing novel electrochemical devices. In this work, the influence of water additive on the electrical double layer formation and adsorption of ions from 1-ethyl-3-methylimidazolium trifluromethanesulfonate IL at semi-metallic bismuth single crystal electrodes has been studied via cyclic voltammetry, electrochemical impedance spectroscopy, and in situ scanning tunneling microscopy methods. Major differences in water-adsorbed IL mixtures are observed at bismuth single crystal electrodes within a wide potential region: increased current densities and anomalous capacitance maxima, especially at positive polarization conditions. The kinetics of adsorption layer formation significantly are strongly affected by the adsorption processes of both cations and anions under polarization conditions, shown by the characteristic relaxation time constant results. Interfacial distribution of ions and surface nanostructure is visualized at the atomic resolution under in situ scanning tunneling microscopy study. These results provide a detailed understanding of the Bi(hkl) surface structure and the ion adsorption processes dependence on electrode polarization.

Entropy characteristics in an electric double layer with off-centre charge ions. A statistical thermodynamics approach

Statistical thermodynamics methods are utilised to characterise aspects of the entropy of formation of a planar electric double layer. The model system consists of a primitive model electrolyte but with off-centre charge cations and centro-symmetric anions, next to a planar, uniformly charged electrode. An expression is developed for the entropy of off-centre charge ions. The theoretically predicted results of this entropy as a function of the electrode surface charge are compared with those from the corresponding Monte Carlo simulations. The entropy change with respect to the absolute value of the surface charge density is seen to be negative (or normal) if the displacement of the cation charge centre is zero or is independent of temperature. However, if the displacement is temperature dependent, the entropy can be positive (or anomalous). In each situation, the simulation data are well reproduced by the predicted results.

Theoretical Insights into Impact of Electrode and Electrolyte Over Li-Air Battery

The lithium-air battery is getting attention as a good technology for energy storage. Nevertheless, the design of such a device still faces some crucial limitations. The most challenging one concerns the selection of electrodes and solvents to provide good performance during the cell operation. Based on literature evidence, we have selected three system configurations of electrode/electrolyte to be evaluated via molecular dynamics simulation: gold/DMSO (dimethyl sulfoxide), graphene/DMSO, and graphene/TEGDME (tetraethylene glycol dimethyl ether) using lithium hexafluorophosphate—LiPF6. We show that the local concentration distributions for DMSO and Li+ are highly influenced by the electrode material. Furthermore, the Li+ solvation shell presents a more complex arrangement when in composition with TEGDME. Additionally, the electrical double layer capacitance in this solvent shows high impedance to the electrode surface. This investigation noticed that the electric field plays a role over the system, contributing to the solvent organization and the electrical double layer formation. Overall, this study strengthens the idea that theoretical findings may contribute to a complementary investigation, leading some light over experimental Li-air battery data reported in the literature.

Flexible and breathable all-nanofiber iontronic pressure sensors with ultraviolet shielding and antibacterial performances for wearable electronics

Fabricating flexible pressure sensors with comprehensive performances is highly desired but is still a critical challenge up to now. Herein, we present a breathable and flexible all-nanofiber iontronic pressure sensor with various merits of high-pressure sensitivity, excellent ultraviolet (UV)-shielding, self-cleaning, and antibacterial performances. The fabricated pressure sensor is assembled by three layers of breathable electrospun thermoplastic polyurethane (TPU) nanofiber films. The inner layer, which is constructed by TPU nanofibers loaded with ionic liquid, offers free ions to enhance the capacitance intensity because of the formation of interfacial electric double layer, thus increasing the pressure sensitivity of the sensor. The as-prepared iontronic flexible pressure sensor exhibits a high sensitivity of 6.21 kPa−1, wide working range from 23 Pa to 120 kPa, fast response time of 170 ms and recovery time of 135 ms, and excellent stability over 6000 loading/unloading cycles. More interestingly, nano-TiO2 is filled in the TPU nanofibers of the two outer layers, endowing the pressure sensors with excellent UV-shielding, self-cleaning, and antibacterial performances, which are the ideal merits for wearable electronics. By virtue of these remarkable performances, the sensor could be employed in human psychological signal monitoring and wearable encrypted information transmission system, exhibiting great potential in wearable electronics.

Transient Plasmonic Imaging of Ion Migration on Single Nanoparticles and Insight for Double Layer Dynamics

AbstractSingle‐nanoparticle electrochemistry offers electrochemical behaviors of individual entities beyond the ensemble system. An electric double layer (EDL) exists on any charged particle–liquid interface because of counter‐ion accumulation, while direct measuring of the interfacial ion migration remains a challenge. Herein, a plasmonic‐based transient microscopic method, with a temporal resolution of 1–2 μs, was demonstrated to directly track the ion migration dynamics on single charged nanoparticles. We found that the dynamics of EDL formation might deviate significantly from the prediction made by using the classical resistance–capacitance (RC) model under nanoscale and transient conditions. Under ultrafast charging, due to the limit migration rate of ions in the solution, the actual time scale of the EDL formation could be up to 5 times slower than the predicted value from the RC model. We then proposed a new theoretical model to describe the transient dynamics of EDL formation. These results may expand our current knowledge about nano‐electrochemistry and transient electrochemistry.

Transient Plasmonic Imaging of Ion Migration on Single Nanoparticles and Insight for Double Layer Dynamics.

Single-nanoparticle electrochemistry offers electrochemical behaviors of individual entities beyond the ensemble system. An electric double layer (EDL) exists on any charged particle-liquid interface because of counter-ion accumulation, while direct measuring of the interfacial ion migration remains a challenge. Herein, a plasmonic-based transient microscopic method, with a temporal resolution of 1-2 μs, was demonstrated to directly track the ion migration dynamics on single charged nanoparticles. We found that the dynamics of EDL formation might deviate significantly from the prediction made by using the classical resistance-capacitance (RC) model under nanoscale and transient conditions. Under ultrafast charging, due to the limit migration rate of ions in the solution, the actual time scale of the EDL formation could be up to 5 times slower than the predicted value from the RC model. We then proposed a new theoretical model to describe the transient dynamics of EDL formation. These results may expand our current knowledge about nano-electrochemistry and transient electrochemistry.

Probing the Dynamics of Electric Double Layer Formation over Wide Time Scales (10–9–10+5 s) in the Ionic Liquid DEME-TFSI

Probing the Dynamics of Electric Double Layer Formation over Wide Time Scales (10^–9–10⁺⁵ s) in the Ionic Liquid DEME-TFSI

Anomalous large capacitances of porous carbons based on protium adsorption.

The capacitances of porous carbon anodes were determined using a Ni(OH)2 cathode. We found that the capacitances were 300-700 F g-1 and above 3 times those of the carbon anodes prepared by electrical double layer formation, revealing the large capacitances based on protium H adsorption in the presence of highly concentrated KOH solution.

Graphene electrode functionalization for high performance hybrid energy storage with vanadyl sulfate redox electrolytes

Hybrid energy storage devices face the challenges of pairing suitable redox chemistries with stable electrodes to simultaneously realize the energy storage mechanisms of batteries and supercapacitors. Functionalization of electrodes enables improved catalytic activity and long-term cycling stability by introducing desired functional groups. Herein, we demonstrate novel hybrids using reversible vanadium (V) redox couples along with functionalized graphene electrodes. Redox couples of V(III)/V(IV) and V(IV)/V(V) are formed upon initial charging of the V(IV) electrolyte present in both half-cells, leading to a Galvani potential difference during operation. Moreover, functionalized electrodes with high surface area facilitate both facile redox reactions and electric double layer formation. The unique chemistry offers a straightforward device regeneration step which significantly enhances the cycling stability. Optimized hybrids exhibit a battery-like specific capacity of 700 mAh g−1 at 0.5 A g−1, a supercapacitor-like capacitance of 850 F g−1 over 10,000 cycles at 10 A g−1, and a very low self-discharge rate. This corresponds to power densities of 0.3 kW kg−1 and 9.2 kW kg−1 and energy densities of 363.8 Wh kg−1 and 24.9 Wh kg−1, respectively, outperforming similar aqueous redox hybrid systems.

Integrated In-Sensor Computing Optoelectronic Device for Environment-Adaptable Artificial Retina Perception Application.

With the development and application of artificial intelligence, there is an appeal to the exploitation of various sensors and memories. As the most important perception of human beings, vision occupies more than 80% of all the received information. Inspired by biological eyes, an artificial retina based on 2D Janus MoSSe was fabricated, which could simulate functions of visual perception with electronic/ion and optical comodulation. Furthermore, inspired by human brain, sensing, memory, and neuromorphic computing functions were integrated on one device for multifunctional intelligent electronics, which was beneficial for scalability and high efficiency. Through the formation of faradic electric double layer (EDL) at the metal-oxide/electrolyte interfaces could realize synaptic weight changes. On the basis of the optoelectronic performances, light adaptation of biological eyes, preprocessing, and recognition of handwritten digits were implemented successfully. This work may provide a strategy for the future integrated sensing-memory-processing device for optoelectronic artificial retina perception application.

Fabricating Silicon Resonators for Analysing Biological Samples.

The adaptability of microscale devices allows microtechnologies to be used for a wide range of applications. Biology and medicine are among those fields that, in recent decades, have applied microtechnologies to achieve new and improved functionality. However, despite their ability to achieve assay sensitivities that rival or exceed conventional standards, silicon-based microelectromechanical systems remain underutilised for biological and biomedical applications. Although microelectromechanical resonators and actuators do not always exhibit optimal performance in liquid due to electrical double layer formation and high damping, these issues have been solved with some innovative fabrication processes or alternative experimental approaches. This paper focuses on several examples of silicon-based resonating devices with a brief look at their fundamental sensing elements and key fabrication steps, as well as current and potential biological/biomedical applications.

Reducing the Self-Discharge Rate of Supercapacitors by Suppressing Electron Transfer in the Electric Double Layer

For supercapacitors, high self-discharge rate is an inevitable issue that causes fast decay of cell voltage and loss of stored energy. Designing supercapacitors with suppressed self-discharge for long-term energy storage has been a challenge. In this work, we demonstrate that substantially reduced self-discharge rate can be achieved by using highly concentrated electrolytes. Specifically, when supercapacitors with 14 M LiCl electrolyte are charged to 0.80 V, the open circuit voltage (OCV) drops to 0.65 V in 24 h. In stark contrast, when the electrolyte concentration is reduced to 1 M, the OCV drops from 0.80 to 0.65 V within only 0.3 h, which was 80 times faster than that with 14 M LiCl. Decreased OCV decay rate at high electrolyte concentration is also confirmed for supercapacitors with different electrolytes (e.g., LiNO3) or at higher charging voltages (1.60 V). The slow self-discharge in highly concentrated electrolyte can be largely attributed to impeded electron transfer between the electrodes and electrolyte due to the formation of hydration clusters and reduced amount of free water molecules, thereby faradaic reactions that cause fast self-discharge are reduced. Our study not only supports the newly revised model about the formation of electric double layer with the inclusion of electron transfer, but also points a direction for substantially reducing the self-discharge rate of supercapacitors.

Stripping away ion hydration shells in electrical double-layer formation: Water networks matter

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl- hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

Flexible Transparent Electric-Double-Layer Junctionless Thin Film Transistors with Low Operating Voltage

Flexible transparent junctionless thin film transistors gated by microporous SiO2 dielectric with electric-double-layer (EDL) effect are fabricated onto PET substrates at room temperature. These devices exhibit high-performance n-type transistor characteristics operating in enhancement mode with low operating voltage of 1.5 V due to the huge gate specific capacitance (0.44 μF cm−2 at 40 Hz) related to EDL formation. The electron field-effect mobility of 28.4 cm2 V−1 s−1, current on/off ratio and the subthreshold swing are found to be 5 × 106 and 0.1 V decade−1, respectively. These achievements demonstrate that flexible transparent junctionless thin-film transistors are promising for the next-generation low-power flexible transparent electronics applications.

A Tutorial on Electrochemical Impedance Spectroscopy and Nanogap Electrodes for Biosensing Applications

Electrochemical Impedance Spectroscopy (EIS) is a powerful measurement technique used by scientists and engineers to characterize and understand the interfacial behavior of liquid-solid interfaces. Such investigations are important in numerous applications and fields of research, including electrochemical synthesis and deposition of materials, such as metals and conducting polymers, corrosion protection analysis, investigation of molecule-surface interactions, and research on batteries and fuel cells. In recent years, advances in nanotechnology have led to a rise in the use of EIS in chemical and bio-sensing. Areas such as molecular diagnostics and healthcare can greatly benefit from this novel technology, which includes low-cost, rapid response measurements that can be done by non-experts at the point of care. For new entrants, the depth of knowledge and breadth of EIS and its many applications can be rather daunting. This article therefore serves as a tutorial on basic EIS theory, whilst highlighting new applications in advanced bio-sensing. The tutorial begins with a theoretical overview, including electrical AC impedance, formation and structure of electrical double layers at liquid solid-interfaces and equivalent circuit models used to represent interfacial phenomena. This is followed by an introduction to bio-sensing and a review of recent research highlights on resistive and capacitive biosensors based on nanogap electrodes, especially those having novel applications from DNA detection to nanoparticle capturing. Lastly, the article highlights a recent new breakthrough on how the combination of nanogap electrodes with label-free methods, such as aptamer functionalized surfaces, can make capacitive sensors with extremely high sensitivity and specificity.

Highly mesoporous carbons derived from corn silks as high performance electrode materials of supercapacitors and zinc ion capacitors

An agricultural waste, corn silks, is used as raw material to prepare mesoporous carbons for electrochemical energy storage applications. The natural hollow tubular structure of corn silks leads to adequate mixing of the KOH activating reagent with the carbon precursor and results in highly mesoporous carbons. When tested as active materials of both electrical double layer capacitors (EDLCs) and zinc ion capacitors (ZICs), the mesoporous carbons exhibit very encouraging energy storage properties. The EDLC cell using 1 M TEABF4/PC (propylene carbonate) electrolyte delivers an energy density of 25 Wh kg−1 even at a high power density of 23,070 W kg−1, while the ZIC cell with 1 M Zn(CF3SO3)2/DMF (N, N-Dimethylformamide) electrolyte displays a high specific energy of 38.5 Wh kg−1 at 14,741 W kg−1. The high specific surface area and large mesopore volume of the corn silks-derived mesoporous carbons might have provided more adsorption sites for the formation of electrical double layer and also facilitated the ion diffusion kinetics, which have synergistically improved the energy-power outputs in both EDLCs and Zn-ion capacitors.

In Situ Electrochemical Impedance Spectroscopic Method for Determination of Surface Roughness and Morphological Convexity.

A novel in situ electrochemical impedance spectroscopy (EIS) method is developed for the determination of RMS roughness (h), electroactive roughness factor (Rc*), and morphological convexity (H̅*) of the electrode surface. Our method uses the angular frequency of maximum phase (ωM) in anomalous Warburg impedance to extract in situ RMS roughness (h). The compact electric double layer (C-EDL) formation frequency (ωH) is used to extract the electroactive roughness factor and morphological convexity. The theory unravels the inverse square root dependence of h on ωM through an elegant equation, , where D is the diffusion coefficient of electroactive species. Similarly, the equation for the electroactive roughness factor is Rc* = ωH0/ωH and ωH0 is the smooth electrode C-EDL formation frequency. These equations are validated for the nanoparticles deposited and mechanically roughened Pt electrodes. Finally, this in situ method is applicable for both low and high roughness electrodes which transcend the limitations of contemporary methods.