Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators.

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3 nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1 m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5 nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

Optimizing Membranes for Osmotic Power Generation.

The design of ion-selective membranes is the key towards efficient reverse electrodialysis-based osmotic power conversion. The tradeoff between ion selectivity (output voltage) and ion permeability (output current) in existing porous membranes, however, limits the upgradation of power generation efficiency for practical applications. Thus, we provide the simple guidelines based on fundamentals of ion transport in nanofluidics for promoting osmotic power conversion. In addition, we discuss strategies for optimizing membrane performance through analysis of various material parameters in membrane design, such as pore size, surface charge, pore density, membrane thickness, ion pathway, pore order, and ionic diode effect. Lastly, a perspective on the future directions of membrane design to further maximize the efficiency of osmotic power conversion is outlined.

Highly selective and high-performance osmotic power generators in subnanochannel membranes enabled by metal-organic frameworks.

The electric organs of electric eels are able to convert ionic gradients into high-efficiency electricity because their electrocytes contain numerous "subnanoscale" protein ion channels that can achieve highly selective and ultrafast ion transport. Despite increasing awareness of blue energy production through nanochannel membranes, achieving high-performance energy output remains considerably unexplored. Here, we report on a heterogeneous subnanochannel membrane, consisting of a continuous UiO-66-NH2 metal-organic framework (MOF) and a highly ordered alumina nanochannel membrane. In the positively charged membrane, the angstrom-scale windows function as ionic filters for screening anions with different hydrated sizes. Driven by osmosis, the subnanochannel membrane can produce an exceptionally high Br-/NO3 - selectivity of ~1240, hence yielding an unprecedented power of up to 26.8 W/m2 under a 100-fold KBr gradient. Achieving ultrahigh selective and ultrafast osmotic transport in ion channel-mimetic MOF-based membranes opens previously unexplored avenues toward advanced separation technologies and energy-harvesting devices.

Single Mesopores with High Surface Charges as Ultrahigh Performance Osmotic Power Generators.

Numerous studies on osmotic power generators with nanoscale pores are conducted. However, their performance output is limited because of the finite osmotic current and conductance from such tiny pores. Here, a proof-of-concept study demonstrating that the rectified mesopore (sub-micrometer-scale pore) with high surface charges can be applied in osmotic energy conversion is reported. A single conical mesopore of ≈405 nm in tip diameter, which can reach an osmotic conductance as high as 0.284 µS (corresponding to a current of 27.5 nA and voltage of 97 mV), enables a record-high power of 667 pW under a 1000-fold salinity gradient, more than doubling all of the state-of-the-art single-pore osmotic power generators reported. This work extends the knowledge of osmotic energy with solid-state pores from nanoscale to mesoscale and opens up a promising avenue toward ultrahigh performance osmotic power.

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores.

Ample studies have shown the use of nanofluidics in the ionic diode and osmotic power generation, but similar ionic devices performed with large-sized mesopores are still poorly understood. In this study, we model and realize the mesoscale ionic diode and osmotic power generator, composed of an asymmetric cone-shaped mesopore with its narrow opening filled with a polyelectrolyte (PE) layer with high space charges. We show that, only when the space charge density of a PE layer is sufficiently large (>1×106 C/m3), the considered mesopore system is able to create an asymmetric ionic distributions in the pore and then rectify ionic current. As a result, the output osmotic power performance can be improved when the filled PE carries sufficiently high space charges. For example, the considered PE-filled mesopore system can show an amplification of the osmotic power of up to 35.1-fold, compared to the bare solid-state mesopore. The findings provide necessary information for the development of large-sized ionic diode and osmotic power harvesting device.

Thermal Dependence of the Mesoscale Ionic Diode: Modeling and Experimental Verification.

Mesoscale ionic diodes, which can rectify ionic current at conditions at which their pore size is larger than 100 nm and thus over 100 times larger than the Debye length, have been recently discovered with potential applications in ionic circuits as well as osmotic power generation. Compared with the conventional nanoscale ionic diodes, the mesoscale ionic diodes can offer much higher conductance, ionic current resolution, and power generated. However, the thermal response, which has been proven playing a crucial role in nanofluidic devices, of the mesoscale ionic diode remains significantly unexplored. Here, we report the thermal dependence of the mesoscale ionic diode comprising a conical pore with a tip opening diameter of ∼400 nm. To capture its underlying physics more accurately, our model takes into account the practical equilibrium chemistry reaction of functional carboxyl groups on the pore surface. Modeling results predict that in the mesoscale ionic diode prepared currents increase but the performance decreases with the increase of temperature, which is consistent with our experimental data and indicates that the ion transport properties apparently depend on the presence of highly mobile hydroxide ions. The results gathered can provide important guidance for the design of new mesoscale ionic diodes, enriching their applications in thermoelectric power and thermoresponsive chemical sensors.

Novel patterned sapphire substrates for enhancing the efficiency of GaN-based light-emitting diodes.

In this study, a novel patterned sapphire substrate (PSS) was used to obtain mesa-type light-emitting diodes (LED), which can efficiently reduce the threading dislocation densities. Silicon nitride (Si3N4) was used as a barrier to form the PSS, replacing the commonly used silicon dioxide (SiO2). The refractive index of Si3N4 is 2.02, which falls between those of sapphire (1.78) and GaN (2.4), so it can be used as a gradient refractive index (GRI) material, enhancing the light extraction efficiency (LEE) of light-emitting diodes. The simulation and experimental results obtained indicate that the LEE is enhanced compared with the conventional PSS-LED. After re-growing, we observed that an air void exists on the top of the textured Si3N4 layer due to GaN epitaxial lateral overgrowth (ELOG). Temperature-dependent PL was used to estimate the internal quantum efficiency (IQE) of the PSS-LED and that of the PSS-LED with the Si3N4 embedded air void (PSA-LED). The IQE of the PSA-LED is 4.56 times higher than that of the PSS-LED. Then, a TracePro optical simulation was used to prove that the air voids will affect the final luminous efficiency. The luminous efficiency of the four different structures considered is ranked as Si3N4 (PSN-LED) > PSA-LED > PSS-LED with SiO2 (PSO-LED) > PSS-LED. Finally, we fabricated LED devices with different thickness of the Si3N4 barrier. The device shows the best luminance-current-voltage (LIV) performance when the Si3N4 thickness is 220 nm.

Electrodiffusioosmosis-Induced Negative Differential Resistance in pH-Regulated Mesopores Containing Purely Monovalent Solutions.

Negative differential resistance (NDR) refers to a unique electrical property where current decreases with increasing voltage. Herein, we report experimental evidence showing that the NDR effect can be observed in mesopores that feature charged pore walls and are subjected to a KCl concentration gradient. NDR in our system originates from the solution and ion flows driven by the synergistic effects of electroosmosis [electroosmotic flow (EOF)] and diffusioosmosis, the so-called electrodiffusioosmosis. Experiments reveal that in addition to the ion current rectification, the mesopores considered here exhibit the NDR phenomenon that is dependent on the magnitude and direction of the salinity gradient and on pH. The NDR behavior can be observed only at conditions at which the EOF and diffusioosmosis occur in the opposite directions: diffusioosmosis fills the tip opening with a high concentration solution, while EOF brings a low concentration solution to the pore. All experimental findings are supported by our numerical model, which takes into account the interfacial site reactions of acidic and basic functional groups on the entire pore membrane surfaces. Our results provide an important insight into how liquid pH, salinity gradients, interfacial site reactions, and pore geometries can influence the current-voltage characteristics of mesopores, enriching transport modes that can be induced by voltage.

Unraveling the Anomalous Surface-Charge-Dependent Osmotic Power Using a Single Funnel-Shaped Nanochannel.

Nanofluidic osmotic power, which converts a difference in salinity between brine and fresh water into electricity with nanoscale channels, has received more and more attention in recent years. It is long believed that to gain high-performance osmotic power, highly charged channel materials should be exploited so as to enhance the ion selectivity. In this paper, we report counterintuitive surface-charge-density-dependent osmotic power in a single funnel-shaped nanochannel (FSN), violating the previous viewpoint. For the highly charged nanochannel, the performance of osmotic power decreases with a further increase in its surface charge density. With increasing pH (surface charge density), the FSN enables a local maximum power density as high as ∼3.5 kW/m2 in a 500 mM/1 mM KCl gradient. This observation is strongly supported by our rigorous model where the equilibrium chemical reaction between functional carboxylate ion groups on the channel wall and protons is taken into account. The modeling reveals that for a highly charged nanochannel, a significant increase in the surface charge density amplifies the ion concentration polarization effect, thus weakening the effective salinity ratio across the channel and undermining the osmotic power generated.

3D Network V2O5 Electrodes in a Gel Electrolyte for High-Voltage Wearable Symmetric Pseudocapacitors.

A 3D network composed of V2O5 nanofibers was manufactured on a novel conductive printing paper [urea-LiClO4-PVA (ULP) deep eutectic solvent gel-doped graphite/printing paper, U-paper] for use as electrodes linked with a ULP neutral gel electrolyte for 3D network V2O5 wearable symmetric pseudocapacitors (WSSCs). The function of the ULP gel is not only that it can be doped into the conductive ink to decrease the resistance of the conductive printing paper but also that it increases the stability of V2O5-based electrodes. Moreover, 3D network V2O5 WSSCs containing the ULP gel can support high operating voltages of 4.0 V with great specific capacitance (160 F/g) and offer a high energy density (355 W h/kg at 0.2 kW/kg). The 3D network V2O5 WSSCs exhibit a superior cycling stability/durability after 5000 cycles (capacitance retention of ∼91%). Operando X-ray absorption spectroscopy experiments show the reversibility and pseudocapacitive properties of V2O5 from the ULP gel and offer the information of the oxidation states of vanadium during charge-discharge cycles. The 3D network V2O5 WSSCs with the ULP gel electrolyte show great potential prospective candidates for smarter 3D wearable energy-storage devices and Internet-of-Things applications.

Rectification of Concentration Polarization in Mesopores Leads To High Conductance Ionic Diodes and High Performance Osmotic Power.

Nanopores exhibit a set of interesting transport properties that stem from interactions of the passing ions and molecules with the pore walls. Nanopores are used, for example, as ionic diodes and transistors, biosensors, and osmotic power generators. Using nanopores is however disadvantaged by their high resistance, small switching currents in nA range, low power generated, and signals that can be difficult to distinguish from the background. Here, we present a mesopore with ionic conductance reaching µS that rectifies ion current in salt concentrations as high as 1 M. The mesopore is conically shaped, and its region close to the narrow opening is filled with high molecular weight poly-l-lysine. To elucidate the underlying mechanism of ion current rectification (ICR), a continuum model based on a set of Poisson-Nernst-Planck and Stokes-Brinkman equations was adopted. The results revealed that embedding the polyelectrolyte in a conical pore leads to rectification of the effect of concentration polarization (CP) that is induced by the polyelectrolyte, and observed as voltage polarity-dependent modulations of ionic concentrations in the pore, and consequently ICR. Our work reveals the link between ICR and CP, significantly extending the knowledge of how charged polyelectrolytes modulate ion transport on nano- and mesoscales. The osmotic power application is also demonstrated with the developed polyelectrolyte-filled mesopores, which enable a power of up to ∼120 pW from one pore, which is much higher than the reported values using single nanoscale pores.

Voltage-Induced Modulation of Ionic Concentrations and Ion Current Rectification in Mesopores with Highly Charged Pore Walls.

It is believed that ion current rectification (ICR), a property that assures preferential ionic transport in one direction, can only be observed in nanopores when the pore size is comparable to the thickness of the electric double layer (EDL). Rectifying nanopores became the basis of biological sensors and components of ionic circuits. Here we report that appreciable ICR can also occur in highly charged conical, polymer mesopores whose tip diameters are as large as 400 nm, thus over 100-fold larger than the EDL thickness. A rigorous model taking into account the surface equilibrium reaction of functional carboxyl groups on the pore wall and electroosmotic flow is employed to explain that unexpected phenomenon. Results show that the pore rectification results from the high density of surface charges as well as the presence of highly mobile hydroxide ions, whose concentration is enhanced for one voltage polarity. This work provides evidence that highly charged surfaces can extend the ICR of pores to the submicron scale, suggesting the potential use of highly charged large pores for energy and sensing applications. Our results also provide insight into how a mixture of ions with different mobilities can influence current-voltage curves and rectification.

Anomalous pH-Dependent Nanofluidic Salinity Gradient Power.

Previous studies on nanofluidic salinity gradient power (NSGP), where energy associated with the salinity gradient can be harvested with ion-selective nanopores, all suggest that nanofluidic devices having higher surface charge density should have higher performance, including osmotic power and conversion efficiency. In this manuscript, this viewpoint is challenged and anomalous counterintuitive pH-dependent NSGP behaviors are reported. For example, with equal pH deviation from its isoelectric point (IEP), the nanopore at pH < IEP is shown to have smaller surface charge density but remarkably higher NSGP performance than that at pH > IEP. Moreover, for sufficiently low pH, the NSGP performance decreases with lowering pH (increasing nanopore charge density). As a result, a maximum osmotic power density as high as 5.85 kW m-2 can be generated along with a conversion efficiency of 26.3% achieved for a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Using the rigorous model with considering the surface equilibrium reactions on the pore wall, it is proved that these counterintuitive surface-charge-dependent NSGP behaviors result from the pH-dependent ion concentration polarization effect, which yields the degradation in effective concentration ratio across the nanopore. These findings provide significant insight for the design of next-generation, high-performance NSGP devices.

Salt gradient driven ion transport in solid-state nanopores: the crucial role of reservoir geometry and size.

Modern applications of nanotechnology such as salinity gradient power and ionic diodes usually involve the transport of ionic species in a system comprising a nanopore connecting two large reservoirs. The charge properties on the nanopore surface plays a key role, and they need to be estimated by fitting a mathematical model for the system to measurable quantities such as ionic current or conductance. This model can also be used to simulate the system behavior under various conditions. However, the large difference between the linear size of a nanopore and that of a reservoir makes relevant analyses difficult. Considering numerical efforts, the impact of the computational domain for the reservoir geometry and size on the system behavior is almost always overlooked in previous studies, where the computational domain for a reservoir is often assumed to have a relatively small size. Taking salinity gradient ionic current as an example, we show for the first time that the performance of a reservoir-nanopore-reservoir system is influenced appreciably by the computational domain for the reservoir geometry and size, especially when a voltage bias is not applied. Using the reported experimental data for the osmotic current in a single boron nitride nanopore, we show that its surface charge density can be estimated realistically by choosing an appropriate computational domain for reservoir geometry and size. Numerical simulation also reveals that choosing appropriate reservoir geometry and size is necessary; otherwise, the results obtained might be unreliable, or even misleading. To avoid this, we suggest that for the nanopore with the pore length smaller than 1000 nm, the size of the computational domain of a reservoir, (length × radius), with equal length and radius, should exceed 800 × 800 nm.

Highly Charged Particles Cause a Larger Current Blockage in Micropores Compared to Neutral Particles.

Single pores in the resistive-pulse technique are used as an analytics tool to detect, size, and characterize physical as well as chemical properties of individual objects such as molecules and particles. Each object passing through a pore causes a transient change of the transmembrane current called a resistive pulse. In high salt concentrations when the pore diameter is significantly larger than the screening Debye length, it is assumed that the particle size and surface charge can be determined independently from the same experiment. In this article we challenge this assumption and show that highly charged hard spheres can cause a significant increase of the resistive-pulse amplitude compared to neutral particles of a similar diameter. As a result, resistive pulses overestimate the size of charged particles by even 20%. The observation is explained by the effect of concentration polarization created across particles in a pore, revealed by numerical modeling of ionic concentrations, ion current, and local electric fields. It is notable that in resistive-pulse experiments with cylindrical pores, concentration polarization was previously shown to influence ionic concentrations only at pore entrances; consequently, additional and transient modulation of resistive pulses was observed when a particle entered or left the pore. Here we postulate that concentration polarization can occur across transported particles at any particle position along the pore axis and affect the magnitude of the entire resistive pulse. Consequently, the recorded resistive pulses of highly charged particles reflect not only the particles' volume but also the size of the depletion zone created in front of the moving particle. Moreover, the modeling identified that the effective surface charge density of particles depended not only on the density of functional groups on the particle but also on the capacitance of the Stern layer. The findings are of crucial importance for sizing particles and characterizing their surface charge properties.

Utilization of Calophyllum inophyllum shell and kernel oil cake for reducing sugar production.

This study is aimed at fully utilizing fruit biomass of Calophyllum inophyllum for reducing sugar production. The effects of pretreatment conditions and post reaction wash on the lignin removal and enzymatic hydrolysis of shell were investigated. The oil cake was also subjected to solvent extraction followed by enzymatic hydrolysis. The results showed that the sequential acid/alkaline pretreatment of C. inophyllum shell resulted in better delignification than alkaline or acid only pretreatment. The reducing sugar yields obtained from sequential acid/alkaline pretreated shell and solvent extracted oil cake were 0.24g/g and 0.66g/g, respectively. The results suggested that the shell and oil cake of C. inophyllum could also be feedstocks for reducing sugar production.

Gate modulation of proton transport in a nanopore.

Proton transport in confined spaces plays a crucial role in many biological processes as well as in modern technological applications, such as fuel cells. To achieve active control of proton conductance, we investigate for the first time the gate modulation of proton transport in a pH-regulated nanopore by a multi-ion model. The model takes into account surface protonation/deprotonation reactions, surface curvature, electroosmotic flow, Stern layer, and electric double layer overlap. The proposed model is validated by good agreement with the existing experimental data on nanopore conductance with and without a gate voltage. The results show that the modulation of proton transport in a nanopore depends on the concentration of the background salt and solution pH. Without background salt, the gated nanopore exhibits an interesting ambipolar conductance behavior when pH is close to the isoelectric point of the dielectric pore material, and the net ionic and proton conductance can be actively regulated with a gate voltage as low as 1 V. The higher the background salt concentration, the lower is the performance of the gate control on the proton transport.

Deformability-Based Electrokinetic Particle Separation.

Deformability is an effective property that can be used in the separation of colloidal particles and cells. In this study, a microfluidic device is proposed and tested numerically for the sorting of deformable particles of various degrees. The separation process is numerically investigated by a direct numerical simulation of the fluid⁻particle⁻electric field interactions with an arbitrary Lagrangian⁻Eulerian finite-element method. The separation performance is investigated with the shear modulus of particles, the strength of the applied electric field, and the design of the contracted microfluidic devices as the main parameters. The results show that the particles with different shear moduli take different shapes and trajectories when passing through a microchannel contraction, enabling the separation of particles based on their difference in deformability.

Electrophoresis of pH-regulated nanoparticles: impact of the Stern layer.

A multi-ion model taking into account the Stern layer effect and the surface chemistry reactions is developed for the first time to investigate the surface charge properties and electrophoresis of pH-regulated silica nanoparticles (NPs). The applicability of the model is validated by comparing its prediction to the experimental data of the electrophoretic mobility of silica NPs available from the literature. Results show that if the particle size is fixed, the Stern layer effect on the surface charge properties of the NP is notable at high pH and background salt concentration; however, that effect on the particle mobility is significant when pH is around neutrality and the salt concentration is medium high (ca. 0.07 M) because of the double-layer polarization effect. Moreover, if pH and the background salt concentration are fixed, the Stern layer effect on the zeta potential and electrophoretic mobility of the NP becomes more significant for smaller particle size. Neglecting the Stern layer effect could result in the overestimation of the zeta potential, surface charge density, and electrophoretic mobility of a NP on the order of several times.

Ion transport and selectivity in biomimetic nanopores with pH-tunable zwitterionic polyelectrolyte brushes.

Inspired by nature, functionalized nanopores with biomimetic structures have attracted growing interests in using them as novel platforms for applications of regulating ion and nanoparticle transport. To improve these emerging applications, we study theoretically for the first time the ion transport and selectivity in short nanopores functionalized with pH tunable, zwitterionic polyelectrolyte (PE) brushes. In addition to background salt ions, the study takes into account the presence of H(+) and OH(-) ions along with the chemistry reactions between functional groups on PE chains and protons. Due to ion concentration polarization, the charge density of PE layers is not homogeneously distributed and depends significantly on the background salt concentration, pH, grafting density of PE chains, and applied voltage bias, thereby resulting in many interesting and unexpected ion transport phenomena in the nanopore. For example, the ion selectivity of the biomimetic nanopore can be regulated from anion-selective (cation-selective) to cation-selective (anion-selective) by diminishing (raising) the solution pH when a sufficiently small grafting density of PE chains, large voltage bias, and low background salt concentration are applied.