Surface Charge Density Research Articles

Unraveling Flow Effect on Capacitive Energy Extraction from Salinity Gradients.

The harvesting of salinity gradient energy through a capacitive double-layer expansion (CDLE) technique is directly associated with ion adsorption and desorption in electrodes. Herein, we show that energy extraction can be modulated by regulating ion adsorption/desorption through water flow. The flow effects on the output energy, capacitance, and energy density under practical conditions are systematically investigated from a theoretical perspective, upon which the optimal operating condition is identified for energy extraction. We demonstrate that the net charge accumulation displays a negative correlation with the water flow velocity and so does the surface charge density, and this causes a nontrivial variation in the magnitude of output energy when water flows are introduced. When high water flows are introduced in both the charging and discharging processes, the energy extraction can be significantly reduced by 47.69-49.32%. However, when a high flow is solely exerted in the discharging process, the energy extraction can be enhanced by 12.94-14.49% even at low operation voltages. This study not only offers a comprehensive understanding of the microscopic mechanisms of surface-engineered energy extraction with water flows but also provides a novel direction for energy extraction enhancement.

Understanding the Role of Surface Charge on Nanobubble Capillary Bridging during Particle-Particle Interaction.

The interactions between particles due to long-range hydrophobic forces have been extensively investigated. The hydrophobic force is likely a capillary force that arises from the formation of capillary bridges due to the merging of nanobubbles. In this study, we aim to investigate the impact of the nanobubble surface charge on the capillary bridge and, subsequently, the interaction between particles. The surface charge of the nanobubbles was altered in the presence of various surfactants (cationic, anionic, and nonionic) and salts (mono-, di-, and trivalent). The particle-particle interaction was quantified by measuring the aggregate size of the hydrophobized glass particles. Both experimental and theoretical findings confirm that the interaction between particles was enhanced when the surface potential of the nanobubble was around the neutral regime. This is probably because, when the surface potential was close to neutral, the interaction between two surface-deposited nanobubbles dominated over electrostatic repulsion, which was more conducive to the formation of the nanobubble capillary bridge. The estimation of the constrained Gibbs potential also showed the capillary bridge to be more stable when surface charge density along the bridge gas-liquid interface was minimal.

Water-Assisted Contact Electrification Properties of Selected Polymers and Surface Functionalization Molecules: A Computational Study.

Motivated by the requirements of performance stability in environments of variable humidity, the focus of this study is on the effects and role of humidity-induced water molecules and ions in the contact electrification (CE) mechanisms of triboelectric materials. In particular, the compatibility of direct charge transfer-based CE and other generally known or proposed water molecules or OH/H3O ion-facilitated CE mechanisms was assessed for a set of high-performance polymeric materials and functionalization molecules. The first set of test mechanisms included OH/H3O ion adsorption at the low-humidity limit. The adsorption resulted in physisorption or H transfer involving reactions that were not fully compatible with charge affinity-driven CE reactions on the considered contact surfaces for both ions in terms of the potential increase of the resultant density of surface charge. An alternative mechanism, which yielded compatibility at a large humidity limit, consisted of free energy-driven segregation and separation of the ions. Further test mechanisms included water adsorption-induced charge transfer and two mechanisms pertinent to charged material transfer: adsorption modulation due to formation of water monolayers and water solvation-induced separation of polymer fragments. According to the obtained results, both mechanisms could be verified as viable contributors to enhanced charge transfer. Consequently, the results allowed for conclusions regarding the general applicability of different, water-assisted CE mechanisms and the selection of particular pairs of contact materials of similar type for optimum performance in humid environments.

Analytical solutions for viscoelectric effects in electrokinetic nanochannels.

Understanding electrokinetic transport in nanochannels and nanopores is essential for emerging biological and electrochemical applications. The viscoelectric effect is an important mechanism implicated in the increase of local viscosity due to the polarization of a solvent under a strong electric field. However, most analyses of the viscoelectric effect have been limited to numerical analyses. In this work, we present a set of analytical solutions applicable to the physical description of viscoelectric effects in nanochannel electrokinetic systems. To achieve such closed-form solutions, we employ the Debye-Hückel approximation of small diffuse charge layer potentials compared to the thermal potential. We analyze critical parameters, including electroosmotic flow profiles, electroosmotic mobility, flow rate, and channel conductance. We compare and benchmark our analytical solutions with published predictions from numerical models. Importantly, we leverage these analytical solutions to identify essential thermophysical and nondimensional parameters that govern the behavior of these systems. We identify scaling parameters and relations among surface charge density, ionic strength, and nanochannel height.

Incorporating Electrolyte Correlation Effects into Variational Models of Electrochemical Interfaces.

We propose a way for obtaining a classical free energy density functional for electrolytes based on a first-principle many-body partition function. Via a one-loop expansion, we include coulombic correlations beyond the conventional mean-field approximation. To examine electrochemical interfaces, we integrate the electrolyte free energy functional into a hybrid quantum-classical model. This scheme self-consistently couples electronic, ionic, and solvent degrees of freedom and incorporates electrolyte correlation effects. The derived free energy functional causes a correlation-induced enhancement in interfacial counterion density and leads to an overall increase in capacitance. This effect is partially compensated by a reduction of the dielectric permittivity of interfacial water. At larger surface charge densities, ion crowding at the interface stifles these correlation effects. While scientifically intriguing already at planar interfaces, we anticipate these correlation effects to play an essential role for electrolytes in nanoconfinement.

Feroxyhyte - from synthesis and characterization to application.

Feroxyhyte (δ-FeOOH) was synthesized and characterized using X-ray diffractometry (XRD), simultaneous thermal analysis (STA), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), and low-temperature nitrogen adsorption-desorption measurements. Its potential application as adsorbent of an anionic and cationic dyes such as C.I. Acid Violet 1 (AV1) and C.I. Basic Blue 3 (BB3) was investigated by determining the adsorption capacities based on the Langmuir (36.6 mg/g for AV1 and 187 mg/g for BB3), Freundlich and Dubinin-Radushkevich isotherm models. Adsorption of AV1 and BB3 by δ-FeOOH drops with the presence of additives such as cationic and anionic surfactants (CTAB, SDS) and ionic polymers (PAA, PEI). The surface and electrokinetic properties of examined suspensions were also described. They include determination of the solid surface charge density and the zeta potential, as well as values of point of zero charge and isoelectric point of feroxyhyte particles without and with adsorbed layers of organic substances. Their analysis made possible to propose the most probable structure of electrical double layer formed at the iron mineral/aqueous solution interface.

Quantifying Dielectric Material Charge Trapping and De-Trapping Ability Via Ultra-Fast Charge Self-Injection Technique.

Recently, utilizing the air breakdown effect in the charge excitation strategy proves as an efficient charge injection technique to increase the surface charge density of dielectric polymers for triboelectric nanogenerators (TENGs). However, quantitative characterization of the ability of dielectric polymers to trap reverse charges and the effect on the startup time of secondary self-charge excitation (SSCE) are essential for extensive applications. Here, an ultra-fast charge self-injection technique based on a self-charge excitation strategy is proposed, and a standard method to quantify the charge trapping and de-trapping abilities of 23 traditional tribo-materials is introduced. Further, the relationship among the distribution of dielectric intrinsic deep, shallow trap states, and transportation of trapped charges is systematically analyzed in this article. It shows that the de-trapping rate of charges directly determines the reactivation and failure of SSCE. Last, independent of TENG contact efficiency, an ultra-high charge density of 2.67 mC m-2 and an ultra-fast startup time of SSCE are obtained using a 15µm poly(vinylidene fluoride-trifluoroethylene) film, breaking the historical record for material modification. As a standard for material selection, this work quantifies the charge trapping and de-trapping ability of the triboelectric dielectric series and provides insights for understanding the charge transport in dielectrics.

The intrinsic impact of dielectric constant on output generation of triboelectric nanogenerators

Triboelectric Nanogenerators (TENG), which convert mechanical energy into electrical pulses, are envisioned to lead future energy harvesting and self-powered sensing applications in wearable and IoT sectors. While optimisation of most parameters related to TENG is well understood, the impact of dielectric constant, which is fundamental to their output generation, is still a highly debated topic. The intrinsic impact of dielectric constant on TENG performance is extremely difficult to assess experimentally, as it closely associates with other parameters like surface charge density that can alter TENG outputs. Herein, for the first time, we present a study to fully characterise the intrinsic impact of dielectric constant on TENG outputs. First, theoretical methods are used to investigate TENG outputs against increasing dielectric constant. A novel experimental technique is introduced using an array of liquid filler materials, providing a unique pathway to systematically tune the dielectric constant without impacting other TENG parameters. This study, for the first time, unveils the load-dependent behaviour of TENG output generation against increasing dielectric constant, along with several other previously unreported output trends. By tuning the dielectric constant, we demonstrate up to 122% (theoretical) and 28% (experimental) increments in TENG power output, paving the way for enhanced TENG optimisation.

CsPbI3-PVDF Composite-Based Multimode Hybrid Piezo-Triboelectric Nanogenerator: Self-Powered Moisture Monitoring System.

For several decades, the development of potential flexible electronics, such as electronic skin, wearable technology, environmental monitoring systems, and the internet of Things network, has been emphasized. In this context, piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) are highly regarded due to their simple design, high output performance, and cost-effectiveness. On a smaller scale, self-powered sensor research and development based on piezo-triboelectric hybrid nanogenerators have lately become more popular. When a material in the TENG is a piezoelectric material, these two distinct effects can be coupled. Herein, we developed a multimode hybrid piezo-triboelectric nanogenerator using the CsPbI3-PVDF composite. The addition of CsPbI3 to PVDF significantly enhances its electroactive phase and dielectric property, thereby enhancing its surface charge density. 5 wt % CsPbI3 incorporation in poly(vinylidene difluoride) (PVDF) results in a high electroactive phase (FEA) value of >90%. Moreover, CsPbI3-PVDF composite-based PENGs were fabricated in three modes, viz., nanogenerators in contact-separation mode (TECS), single electrode mode (TESE), and sliding mode (TES), and the output performance of all the devices was investigated. The fabricated TECS, TESE, and TES reveal peak output powers of 3.08, 1.29, and 0.15 mW at an external load of 5.6 MΩ. Through analysis of the contact angle measurement and experimental quantification, the hydrophilicity of the composite film has been identified. The hydrophobicity and moisture absorption capacity of CsPbI3-PVDF film make it an attractive option for self-powered humidity monitoring. The TENGs effectively powered several low-powered electronic devices with just a few human finger taps. This study offers a high-performance PTENG device that is reliant on ambient humidity, which is a helpful step toward creating a self-powered sensor.

A sharp immersed method for electrohydrodynamic flows accompanied by charge evaporation

AbstractThis article presents a sharp immersed method for simulating electrohydrodynamic (EHD) flows that involve charge evaporation. This well‐known multi‐scale, multi‐physics problem is widely used in various fields, including industry and medicine. The method adopts a fully sharp model, where surface tension and Maxwell stress are treated as surface forces and free charges are concentrated on the zero thickness liquid‐vacuum interface. Incorporating charge evaporation imposes strict restrictions on the time‐step, as the rate of evaporation sharply increases with surface evolution. To overcome this challenge, an iterative algorithm that couples the electric field and surface charge density is proposed to obtain accurate results, even with significantly large time‐steps. To mitigate the numerical residuals near the interface, which may introduce parasitic flows and cause numerical instability, an immersed interface method‐based iterative projection method for the Navier–Stokes equations is proposed, in which a traction boundary condition involving multiple surface forces is imposed on the sharp interface. Numerical experiments were carried out, and the results show that the method is splitting‐error‐free and stable. The sharp immersed method is applied to simulate the electric‐induced deformation of an ionic liquid drop with charge evaporation. The results indicate that charge evaporation can suppress the sharp development of Taylor cones at the ends of the drops. These findings have significant implications for the design and optimization of EHD systems in various applications.

Thermal decomposition kinetics and mechanism of poly(ethylene 2,5-furan dicarboxylate) Nanocomposites for food packaging applications

Poly(ethylene 2,5-furan dicarboxylate) (PEF) based nanocomposites containing different nanoparticles like Ag, TiO2, ZnO, ZrO2 Ce-Bioglass, have been synthesized via in-situ polymerization techniques targeting food packaging applications. Zeta potential measurements showed an increase in the negative zeta potential value due to an increase in the surface charge density of the nanocomposites. Thermogravimetric analysis results proved that, except PEF-ZnO nanocomposite, all the other nanocomposites exhibited good resistance to thermal degradation without serious mass loss until 330 °C. Thermal decomposition kinetic analysis and the dependence of activation energy on the degree of conversion (α), indicated that the presence of ZnO nanoparticles influences, the degradation mechanism of PEF. In contrast, the presence of Ce-Bioglass nanoparticles leads to a slower degradation process, contributing to the enhanced resistance to thermal degradation of the PEF-Bioglass nanocomposite. The thermal degradation mechanism of PEF nanocomposites analyzed by pyrolysis‒gas chromatography/mass spectrometry (Py-GC/MS) indicated that the primary thermal degradation mechanism for the studied nanocomposites was β-hydrogen bond scission, while to a lesser extent, α-hydrogen bond scission products were noted in PEF-TiO2 and PEF-ZrO2 nanocomposites.

The effect of sulphur supplementation on cadmium phytotoxicity in wheat and lettuce: changes in physiochemical properties of roots and accumulation of phytochelatins.

Intensive sulphur fertilisation has been reported to improve the nutrient balance and growth of Cd-exposed plants, but the reasons of this phenomenon and the role of sulphur compounds in the resistance to cadmium are unclear. We investigated sulphur supplementation-induced changes in the surface properties of roots and the level of thiol peptides (PCs) in Cd-stressed Triticum aestivum L. (monocots clade) and Lactuca sativa L. (dicots clade) grown in nutrient solution. The combination of three sulphur (2mM S-basic level, 6 or 9mM S-elevated levels) and four cadmium (0, 0.0002, 0.02 or 0.04mM Cd) concentrations was used. The physicochemical parameters of the roots were determined based on the apparent surface area (Sr), total variable surface charge (Q), cation exchange capacity (CEC) and surface charge density (SCD). In Cd-exposed plants supplied with sulphur, a different character and trend in the physicochemical changes (adsorption and ion exchange) of roots were noted. At the increased sulphur levels, as a rule, the Sr, CEC, Q and SCD values clearly increased in the lettuce but decreased in the wheat in the entire range of the Cd concentrations, except the enhanced Sr of wheat supplied with 6mM S together with elevated (0.0002mM) and unchanged (0.02, 0.04mM Cd) value of this parameter at 9mM S. This indicates a clade-specific and/or species-specific plant reaction. The 6mM S appears to be more effective than 9mM S in alleviation of the cadmium's toxic effects on roots. It was found that at 0.02 and 0.04mM Cd, the use of 6mM S limits the Cd accumulation in the roots of both species in comparison with the basic S fertilisation. Moreover, PC accumulation was much more efficient in wheat than in lettuce, and intensive sulphur nutrition generally induced biosynthesis of these chelating compounds. Physicochemical parameters together with quantitative and qualitative assessment of thiol peptides can be important indicators of the efficiency of root system functioning under cadmium stress. The differences between the species and the multidirectional character of the changes are a result of the involvement of a number of multi-level mechanisms engaged in the defence against metal toxicity.

Mismatch and mix: Making use of electrokinetic aspects of spacers for intensified electrodialysis

Electrodialysis (ED) processes are limited in terms of practical current densities due to the emergence of concentration boundary layers. A great deal of research has focused on methods to promote mixing of these boundary layers, particularly through developing new spacers. Spacers are crucial to the performance of electrodialysis stacks, forming the flow channels between membranes and leading to hydrodynamic mixing that reduces mass transport limitations. Conventional analysis of spacers in ED has focused on characterizing the hydrodynamic mixing resulting from spacer geometry and assessing how this impacts the overall stack resistance. In this work, we explore the electrokinetic aspect of spacer performance: the electric-field bending induced by polymeric spacers and resulting electro-osmotic mixing. We carried out an experimental study to characterize the ED performance of a lab-scale stack using spacers with different surface charge densities, using polyelectrolyte adsorption on the spacers. Our results show that substantial increases in the current density or reductions in power consumption can be achieved when making use of this enhanced mixing due to electro-osmosis. We also explain this enhancement through the use of a simplified theoretical model to highlight the potential of utilizing electrokinetic mixing from spacers in electrodialysis.

Probing the surface electrical properties of clay minerals with electrostatic force microscopy and kelvin probe force microscopy

Many surface processes of clay minerals require in-depth understanding of their surface electrical properties, such as surface charge density, surface potential distribution, etc In this paper, electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) were used to study the surface charge densities, surface potentials, electric field intensities, and electric field force gradients of three common clay minerals: kaolinite, montmorillonite, and illite. The properties were directly imaged, and the average surface permanent charge densities of kaolinite, montmorillonite, and illite were obtained to be −0.0060, −2.136, and −5.456 μC m−2, respectively. In addition, a good linear relationship was found between the surface charge densities obtained by KPFM and the layer charges calculated from the mineral chemical formulas of three clay minerals.

Surface charge and surface current densities at material boundaries

In electromagnetism, materials with a polarization density P→ or a magnetization density M→ are known to exhibit a bound surface charge density σb=P→·n̂ or a surface current density κ→b=M→×n̂, respectively, where n̂ is the unit vector perpendicular to the material boundary surface, directed outward. These expressions can be obtained from volume integrations for the electric potential V, or the magnetic vector potential A→, in which the integrals are restricted to the material volumes delimited by their respective boundaries. In that case, applying the divergence theorem leads to surface integrals on material boundaries and to the above-mentioned surface quantities. In this paper, a simple derivation is presented, which shows that both σb and κ→b are included in the expressions for the volume charge or current densities, provided that the divergence and curl operators are evaluated at the boundary so as to account for discontinuities at interfaces.

Electrical Responses of Modified Mineral Surfaces as Observed With Spectral Induced Polarization and Atomic Force Microscopy

AbstractAtomic force microscopy (AFM) and spectral induced polarization (SIP) are widely used to investigate the electrical properties of mineral surfaces at vastly different scales of measurement. We compare AFM and SIP measurements made on two different materials (glass beads and silica gel) subjected to etching, deposition of iron oxide particles, and inclusion of calcite grains. We found that the treatments produced qualitatively consistent behaviors in the AFM and SIP data. Direct AFM measurements of surface charge density for silica and calcite surfaces were quantitatively compared to values estimated from the SIP results using a grain polarization model. No statistically significant difference (at a 95% confidence level) was found between the surface charge density of silica estimated by AFM (2.3 ± 6.6 mC/m2 for glass beads and 1.6 ± 0.1 mC/m2 for silica gel) versus SIP (5.4 ± 4.4 mC/m2 for glass beads and 1.6 ± 0.5 mC/m2 for silica gel). The surface charge density for calcite determined by AFM (43.5 ± 12.9 mC/m2) was approximately 19 times higher than that found for silica. While the charge density of calcite surfaces determined by SIP was also generally higher than that found for silica, different treatments produced significantly different values between 4.7 and 258 mC/m2 (with a maximum 95% CI of ±8.7 mC/m2). Several possible explanations exist for the range of the observed SIP measurements, including aging of the calcite surfaces. Overall, this study suggests the potential for the complementary use of AFM and SIP measurements to constrain future investigations of polarization mechanisms in porous media.

Carbon fabric coated with nanostructured zinc oxide layers for use in triboelectric self-powered touch sensors

An environmentally friendly, low-cost, and lightweight biocompatible textile triboelectric material was made by in situ coating carbon fabric (CF) with nanostructured zinc oxide (ZnO) layers using the automatic Successive Ionic Layer Adsorption and Reaction (SILAR) method. Depending on the deposition mode, we created triboelectric CF/ZnO textiles with multidirectionally intergrown short ZnO nanorods or with arrays of ZnO nanosheets. The Raman spectra confirmed the hexagonal wurtzite structure of both types of ZnO layers and the unique a-axis texture of the nanosheets. In the developed triboelectric CF/ZnO/PET/ITO sensors, the upper tribonegative part was made of a polyethylene terephthalate film coated with a thin layer of indium-tin oxide, and the lower tribopositive part was made of CF/ZnO textile. In tests with repeated hand tapping at low frequency 1.3–13 Hz and a force of ∼ 5 N (pressure of ∼ 33 kPa), the open-circuit voltage pulses were ∼ 15 V for short ZnO nanorods and ∼ 30 V for ZnO nanosheets, their duration did not exceed a few milliseconds. Due to the nanosheet morphology of the ZnO layer, the maximum touch-induced surface charge density for the corresponding triboelectric textile (0.7 µC/m2) was almost twice than for the CF/ZnO textile with intergrown ZnO short nanorods (0.4 µC/m2). The touch sensor with ZnO nanosheets showed an output voltage of 3.6 V, a current density of 1.47 µA/cm2, and a power density of 1.8 µW/cm2. It can be used as a dual-mode sensor due to ability to recognize the hardness of objects by analyzing the output current peaks.

Aqueous chemimemristor based on proton-permeable graphene membranes

Memristive devices, electrical elements whose resistance depends on the history of applied electrical signals, are leading candidates for future data storage and neuromorphic computing. Memristive devices typically rely on solid-state technology, while aqueous memristive devices are crucial for biology-related applications such as next-generation brain-machine interfaces. Here, we report a simple graphene-based aqueous memristive device with long-term and tunable memory regulated by reversible voltage-induced interfacial acid-base equilibria enabled by selective proton permeation through the graphene. Surface-specific vibrational spectroscopy verifies that the memory of the graphene resistivity arises from the hysteretic proton permeation through the graphene, apparent from the reorganization of interfacial water at the graphene/water interface. The proton permeation alters the surface charge density on the CaF2 substrate of the graphene, affecting graphene's electron mobility, and giving rise to synapse-like resistivity dynamics. The results pave the way for developing experimentally straightforward and conceptually simple aqueous electrolyte-based neuromorphic iontronics using two-dimensional (2D) materials.

A fast electron-injection strategy for enhancing triboelectric surface charge density of polymers

Surface charge density regulation has been used for diverse range of applications. As a newly emerged high-efficiency energy converter, triboelectric nanogenerator (TENG) is sensitive to the surface charge density, making triboelectric polymers with high charge density crucial for expanding the range of TENG applications. Direct charge injection is an efficient strategy to boost output performance of TENG. However, it traditionally requires a high voltage (about 10 kV) and an injection period over minutes. The fast (<1 s) and low-voltage (< 3 kV) electron-injection process in this study for a contact-separate nanogenerator yields an output charge density of 252 μC·m−2, which is twice as large as that of the pristine device. The provided strategy can be applied to various polymers, providing opportunities for investigations in multi-science fields, not just limited to TENGs.

Role of Outer Surface Probes on Bullet-Shaped Asymmetric Solid-State Nanochannels for Lysozyme Protein Sensing.

Artificial solid-state nanochannels featuring precise partitions present a highly promising platform for biomarker detection. While the significance of probes on the outer surface (POS) has been relatively overlooked in the past, our research highlights their crucial role in biosensing. Furthermore, the contribution of POS on the bullet-shaped asymmetric nanochannels has not been extensively explored until now. Here, we fabricated a series of bullet-shaped nanochannels, each featuring a distinct asymmetric structure characterized by different tip- and base-pore diameters. These nanochannels were further modified with explicit distributions at the inner wall (PIW), the outer surface (POS), and their combination (POS + PIW) for lysozyme sensing. The impact of diameters, structural asymmetry, and surface charge density on the sensing efficacy of POS and PIW was thoroughly examined through experimental investigations and numerical simulations. POS demonstrates great individual sensing performance for lysozyme within a broad concentration range, spanning from 10 nM to 1 mM. Furthermore, it improves the sensitivity when combined with PIW, particularly within the nanochannels featuring the smaller base-pore diameter, resulting in a 2-fold increase in sensing performance for POS + PIW compared to PIW at a concentration of 10 nM. These findings are substantiated by numerical simulations that closely align with the experimental parameters. The contributions of POS are notably amplified in the presence of smaller base pores and a higher degree of asymmetry within the bullet-shaped nanochannels. These findings elucidate the mechanism underlying the role of POS within bullet-shaped asymmetric nanochannels and open up new avenues for manipulating and enhancing the sensing efficiency.