Ionic Diode Research Articles

Hydrogel ionic diode with ultra-high rectification ratio for ionic circuit

Organisms transmit signals through complex ion systems, and signal transmission in artificial electronic circuits mainly depends on electrons. The development of rectifier ionic diode, grounded in ion transport principles, paves ways for artificial bionic circuit and seamless human–machine interaction. Among its pivotal performance indicators, the ion rectification ratio (IR) is of primal importance, facilitating enhanced ion selectivity, bolstered energy efficiency, and robust, high-performing operations. Here, hybrid hydrogel ionic diode based on chitosan (CS) doped with 5,10,15,20-tetrakis(4-aminophenyl)-21,23H-porphyrin (TAPP)/sodium polyacrylate (PAAs) doped with 5,10,15,20-tetrakis(4-sulfonatophenyl)-21,23H-porphyrin (TPPS) was constructed to achieve a high IR. This design achieved a remarkable feat by elevating the IR from 5.57 to an unprecedented 158.64, surpassing previous benchmarks set by hydrogel-based ionic diodes. Moreover, this study emphasizes the selectivity of polyelectrolytes to different porphyrin structures, as well as the charge properties of the groups carried by porphyrins. It delves into the fundamental mechanisms underpinning their interactions, providing a comprehensive understanding of their behavior. In order to demonstrate the excellent rectification characteristics and practical applicability of the CS-TAPP/PAAs-TPPS ionic diode, we successfully integrated it into the full-wave rectification and logic gate system. The full-wave rectifier can convert a 1.2 V sinusoidal voltage input to a 0.8 V full-wave voltage output. This shows the advantages of high IR ionic diodes. It offers insights for the development of high-performance hydrogel-based ionic diodes, and promotes the advancement of novel biomimetic devices and ionic circuits.

Optically Mediated Hydrogel-Based Ionic Diode.

Ionic diodes with environmentally modulated ion-rectifying characteristics have attracted much attention and show great promise in the construction of smart devices with environmental adaptability. One immediate challenge is to integrate stimuli responsiveness and ion rectification into one single ionic diode, which requires a close cooperation of chemical principles and device technologies. Herein, an ionic diode based on a photoresponsive hydrogel with optically mediated ion-rectifying performances is introduced. Relying on the photoresponsive concentration of proton in the hydrogel, the ionic current rectification can be prominently enhanced upon ultraviolet (UV) irradiation. A maximum ionic current rectification ratio of the optically mediated ionic diode about 4× 105 is achieved. Furthermore, the hydrogel-based diode can serve as an AND logic gate operated by UV light and voltage bias as two independent inputs. As a proof of concept, to use the optically mediated diode is achieved to modulate the feedback of a robot with logic behaviors. This work provides a novel and valuable strategy for designing functional hydrogel-based devices with the integration of stimuli-responsiveness and logic signal processing through chemical approaches.

Gradient p‐Polyanion/n‐Polycation Heterojunction Endows Ionic Diodes with Vastly Boosted Output Voltage, Power Density and Sensitivity

AbstractIonic diode‐based generators show great superiority in harvesting low‐frequency mechanical energy from human motions for various wearable applications. The major challenge is to improve their extremely limited output voltage for the miniaturization/simplicity of ionic diode‐based self‐powered devices as well as further improvement of mechanoelectric conversion performance. Herein, the construction of gradient polycation/polyanion heterojunction is demonstrated to vastly boost the output voltage, energy‐harvesting efficiency, and sensing performance of ionic diode. Besides an inherent potential at the polyanion/polycation heterojunction interface, the additional transmembrane potentials can be generated across gradient polyanion and polycation conductors of ionic diodes due to their gradient charge distribution. Consequently, the gradient ionic diode‐based generators produce much higher output voltage (306.53 mV) than previously reported homogeneous ones (4.1–135 mV). Meanwhile, the high output voltage imparts gradient ionic diode‐based generator with ultrahigh power density (50.16 µW cm−2 or 250.8 µW cm−3) and pressure‐perception sensitivity (43582 mV/MPa), which are dramatically increased by 162.76% and 128.71%, respectively, compare to those of homogeneous one. Impressively, the resulting diode‐based generators can generate ultralarge current density (≈0.52 mA cm−2) with ultralong discharge time (>400 s), thus enabling the continuous power generation under the low‐frequency pressure for various wearable and implantable applications.

Unipolar Ionic Diode Nanofluidic Membranes Enabled by Stepped Mesochannels for Enhanced Salinity Gradient Energy Harvesting.

Developing ionic diode membranes featuring asymmetric structures is in high demand for salinity gradient energy harvesting. These membranes offer benefits in mitigating ion concentration polarization, thereby promoting ion permeability. However, most reported works focus on the role of heterogeneous charge-based bipolar ionic diode membranes for ion concentration polarization suppression, with comparatively less attention given to maintaining ion selectivity. Herein, unipolar ionic diode nanofluidic mesoporous silica membranes featuring stepped mesochannels were developed via a micellar sequential oriented interfacial self-assembly strategy as a salinity gradient energy harvester. Due to the asymmetric mesochannels and unipolar structure (both sides carry negative charge), the ionic diode membranes exhibit a strong rectification ratio of ∼15.91 to facilitate unidirectional ion transport while maintaining excellent cation selectivity (cation transfer number of ∼0.85). Besides, the vertically aligned mesochannels significantly reduce ion transport resistance, generating a high ionic flux. Consequently, the unipolar ionic diode nanofluidic membranes demonstrate a power output of 5.88 W/m2 between artificial sea and river water. The unipolar feature gives notable enhancements of 296% and 144% in power output compared to the symmetric membrane and bipolar ionic diode membrane, respectively. This work opens up new routes for designing ionic diode membranes for salinity gradient energy harvesting.

Ultrathin Ionic Diodes with Electrostatically Heterogeneous Hybrid Interfaces of Nanoporous SiO2 Nanofilms and Polymer Layer-by-Layer Multilayers.

Nanofluidic ionic diodes have attracted much attention due to their unique functions as unidirectional ion transportation ability and promising applications from molecular sensing, and energy harvesting to emerging neuromorphic devices. However, it remains a challenge to fabricate diode-like nanofluidic systems with ultrathin film thickness <100nm. Herein the formation of ultrathin ionic diodes from hybrid nanoassemblies of nanoporous (NP) SiO2 nanofilms and polyelectrolyte layer-by-layer (LbL) multilayers is described. Ultrathin ionic diodes are prepared by integrating polyelectrolyte multilayers onto photo-oxidized NP SiO2 nanofilms obtained from silsesquioxane-containing block copolymer thin films as a template. The obtained ultrathin ionic diodes exhibit ion current rectification (ICR) properties with high ICR factor = ≈20 under low ionic strength and asymmetric pH conditions. It is concluded that this ICR behavior arises from effective ion accumulation and depletion at the interface of NP SiO2 nanofilms and LbL multilayers attributed to high ion selectivity by combining the experimental data and theoretical calculations using finite element methods. These results demonstrate that the hybrid nano assemblies of NP SiO2 nanofilms and polyelectrolyte LbL multilayers have potential applications for (bio)sensing materials and integrated ionic circuits for seamless connection of human-machine interfaces.

Engineering Multiscale Heterostructure as Ionic Diode and Light-Driven Ion Pump for Osmotic-Solar Energy Harvesting

Engineering Multiscale Heterostructure as Ionic Diode and Light-Driven Ion Pump for Osmotic-Solar Energy Harvesting

In-Situ Gas Permeation-Driven Ionic Current Rectification of Heterogeneously Charged Nanopore Arrays.

Ionic diodes provide ionic current rectification (ICR), which is useful for micro-/nanofluidic devices for ionic current-mediated applications. However, the modulation of ICR is not fully developed, and current challenges include limited active control and localized modulation for further multiplexing of micro-/nanofluidic ionic diodes. Herein, a microfluidic device integrated with particle-assembly-based ionic diodes (PAIDs) and a gas-flow channel above them is presented. Exploiting in-situ gas permeation through a polymeric film, precise control over the physiochemical conditions of the nanopores within the PAIDs, leading to the modulation of ICR is demonstrated. The investigation not only characterizes the rectification properties of the PAIDs but also unveils their capacitor-like behavior and the ability to actively modulate ICR using various gas flows. Furthermore, the reversible modulation of ICR through dynamic switching of gas-dissolved solutions, enabling ion-signal amplification is showcased. This pioneering approach of in situ gas-permeation offers programmable manipulation of ion transport along PAIDs, thereby positioning ionic diodes as versatile nanofluidic components. Looking ahead, the development of multiplexed PAIDs in an addressable manner on a chip holds promise for practical applications across diverse fields, including ion signaling, ion-based logic, chemical reactors, and (bio)chemical sensing.

Molecular Structure Effects on Ionic Diode Performance in Desalination: Ultrahigh Rectification in Butylated Intrinsically Microporous Polyamine (PIM‐EA‐TB)

AbstractIonic diodes are components in ionic circuits for AC‐electricity driven desalination and ion extraction processes. Independent of the ionic diode type/mechanism, achieving ionic current rectification at high ionic strengths is challenging but important, for example in seawater desalination and treatment of brine. Here, the butylation of a molecularly rigid (glassy) polymer of intrinsic microporosity (PIM‐EA‐TB) is shown to give anionic diodes with ultrahigh rectification effects (associated with interfacial resistivity) even in high ionic strength aqueous 2 M NaCl solution. The effect is rationalised based on polymer structure affecting ion transport.

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields.

Improving the ionic rectification in nanochannels enables versatile applications such as biosensors, energy harvesting, and fluidic diodes. While previous work mostly focused on the effect of channel geometry and surface charge, in this work via a series of molecular dynamics simulations, we find a striking phenomenon that the ionic current rectification (ICR) ratio in Janus graphene oxide (GO) channels can be tremendously promoted by lateral electric fields. First, under a given axial electric field, an additional lateral electric field can improve the ICR ratio by several times to an order, depending on the channel symmetry. The symmetric channel has an obviously greater ICR ratio because it maintains a more pronounced ion transport disparity at opposite axial fields. The underlying mechanism for the function of the lateral electric field is that it promotes the lateral migration of ions and thus amplifies the ion-residue electrostatic interaction at opposite axial fields, enlarging the ion dynamical difference. Furthermore, for different axial electric fields, the ICR ratio can always be improved by lateral electric fields (up to two orders), suggesting that the ICR improvement is universal. Our results demonstrate that applying a lateral electric field could be a new method to improve the rectification performance of nanochannels, providing valuable guidance for the design of efficient ionic diode devices.

Study on the Rectification of Ionic Diode Based on Cross-Linked Nanocellulose Bipolar Membranes.

Nanocellulose-based membranes have attracted intense attention in bioelectronic devices due to their low cost, flexibility, biocompatibility, degradability, and sustainability. Herein, we demonstrate a flexible ionic diode using a cross-linked bipolar membrane fabricated from positively and negatively charged cellulose nanofibrils (CNFs). The rectified current originates from the asymmetric charge distribution, which can selectively determine the direction of ion transport inside the bipolar membrane. The mechanism of rectification was demonstrated by electrochemical impedance spectroscopy with voltage biases. The rectifying behavior of this kind of ionic diode was studied by using linear sweep voltammetry to obtain current-voltage characteristics and the time dependence of the current. In addition, the performance of cross-linked CNF diodes was investigated while changing parameters such as the thickness of the bipolar membranes, the scanning voltage range, and the scanning rate. A good long-term stability due to the high density cross-linking of the diode was shown in both current-voltage characteristics and the time dependence of current.

Highly-stable gradient poly(anion-π) hydrogels with superior salt-tolerant property for giant osmotic energy harvesting

Harvesting ocean salinity energy has attracted large attentions based on different kinds of asymmetric ionic membranes. Gradient polyelectrolyte hydrogels demonstrated superior power generation capacity due to their abundant charged groups and rectification behaviors. However, they generally suffer from the electrostatic screening and dramatic swelling, leading to unsatisfactory energy conversion performance. Here, an anti-swelling, salt-tolerant gradient polyanion hydrogel is reported for high-efficiency osmotic energy conversion. The gradient polyanion hydrogel is prepared by ultraviolet (UV) induced copolymerization of anionic monomer and aromatic monomer. Benefiting from the mutual electrostatic interaction and π-π conjugation interaction, the gradient polyanion hydrogel presents the outstanding anti-electrostatic screening and anti-swelling performance. As a result, they can maintain excellent cation selectivity and ionic diode effect for providing an ultrahigh power density of 16.4 W/m2 when mixing artificial seawater and river water. This new salt-tolerant gradient polyelectrolyte hydrogel provides a promising way to recover energy from seawater and wastewater.

Ion Concentration Polarization Tunes Interpore Interactions and Transport Properties of Nanopore Arrays

AbstractBiological processes require concerted function of many channels embedded in the cell membrane. While single solid‐state nanopores are already designed to mimic properties of individual biological channels, it is not yet known how to connect the pores to achieve biomimetic ionic circuits with interacting components. To identify fundamental processes that control interactions between nanopores embedded in the same membrane, a model system of minimal arrays consisting of two and three nanopores in silicon nitride films is designed. The constituent nanopores have an opening diameter &lt;10 nm, and the interpore spacing is tuned between 15 and 200 nm. The experimental and modeling results reveal that nanopores in an array interact with each other via overlapping depletion zones created by the process of concentration polarization. The interactions can be further controlled by salt concentration and voltage. These results showcase a possibility of tuning interactions between nanopores and transport properties of arrays by chemical modification of the pore walls. Arrays consisting of nanoporous ionic diodes feature depletion zones with higher concentrations, and lower current suppression than homogeneously charged pores. These experiments and modeling provide the first steps to leave the constraints of single nanopores and to design biomimetic ionic circuits.

Ion concentration polarization causes a nearly pore-length-independent conductance of nanopores.

There has been a great amount of interest in nanopores as the basis for sensors and templates for preparation of biomimetic channels as well as model systems to understand transport properties at the nanoscale. The presence of surface charges on the pore walls has been shown to induce ion selectivity as well as enhance ionic conductance compared to uncharged pores. Here, using three-dimensional continuum modeling, we examine the role of the length of charged nanopores as well as applied voltage for controlling ion selectivity and ionic conductance of single nanopores and small nanopore arrays. First, we present conditions where the ion current and ion selectivity of nanopores with homogeneous surface charges remain unchanged, even if the pore length decreases by a factor of 6. This length-independent conductance is explained through the effect of ion concentration polarization (ICP), which modifies local ionic concentrations, not only at the pore entrances but also in the pore in a voltage-dependent manner. We describe how voltage controls the ion selectivity of nanopores with different lengths and present the conditions when charged nanopores conduct less current than uncharged pores of the same geometrical characteristics. The manuscript provides different measures of the extent of the depletion zone induced by ICP in single pores and nanopore arrays, including systems with ionic diodes. The modeling shown here will help design selective nanopores for a variety of applications where single nanopores and nanopore arrays are used.

Efficient and cold-tolerant moisture-enabled power generator combining ionic diode and ionic hydrogel.

The ionic diode structure has become one of the attractive structures in the field of moisture-based power generation. However, existing devices still suffer from poor moisture trapping, low surface charge, and inefficient ion separation, resulting in low output power. Moreover, water freezes at low temperatures (<0 °C), limiting the ionic diode structure to generate electricity in cold environments. In this paper, a moisture-enabled power generator has been designed and fabricated, which assembles a negatively charged ionic hydrogel film and a positively charged anodized aluminum oxide (AAO) film to construct a heterojunction. The hydrogel polymer network is modified with a large number of sulfonate groups that dissociate to provide nanoscale pores with high surface charge to improve the rectification ratio. And the lithium chloride (LiCl) salt with high hydration ability is added to the hydrogel as a moisture-trapping and anti-freezing component. Usually salt ions reduce the Debye length, so that the ion transport is finally not controlled by the electric double layer (EDL) and the rectification fails. Interestingly, due to the natural affinity of the hydrogel polymer network for LiCl, LiCl is locked on the hydrogel side and does not easily enter the AAO pores to change the distribution of EDL within the nanochannel. As a result, the device rectification ratio is almost independent of the amount of LiCl addition, demonstrating an excellent balance of high output power and high freeze resistance. Ultimately, the device exhibits excellent power generation performance in the -20 °C to 60 °C temperature range and 15% to 93% RH humidity range. Typically, under high humidity (93% RH) at room temperature (25 °C), it provides an open-circuit voltage of 1.25 V and a short-circuit current of 300 μA cm-2, with an on-load output power of up to 71.35 μW cm-2. Under medium humidity (50% RH) at low temperature (-20 °C), it provides an open-circuit voltage of 1.11 V and a short-circuit current of 15 μA cm-2.

A Switchable and Facile Ionic Diode Modulated by Polyethylene Glycol

We introduce a switchable ionic diode modulated by PEG, enabling dynamic control of ion transport and reversible ion flow switching. This system achieves tunable current rectification over two orders of...

Fiber‐Based Flexible Ionic Diode with High Robustness and Rectifying Performance: Toward Electronic Textile Circuits

AbstractIn response to the growing demand for wearable devices designed for seamless integration with 3D bio‐surfaces, fiber‐based devices have gained prominence in textile‐based wearable electronics due to their flexibility and unique structure. In particular, though diodes with rectifying properties are crucial for a range of electronic systems, there has been scant reporting on diodes with a fiber structure. This study introduces a fiber‐based flexible ionic diode that exhibits a rectification ratio of 2773 and an output current of 28.2 mA at 3 V. The diode is composed of a double helical Zn‐based fiber anode, a Ti‐based fiber cathode on Au nanoparticle‐based flexible fiber electrodes, and a LiCl hydrogel electrolyte. By modulating the double helical design and the ionic conductivity of the hydrogel, the electrical performance of the diode to achieve varying rectification ratios and output currents can be tailored. Owing to the remarkable flexibility and stability of the fiber electrodes, the fiber‐based ionic diode consistently upholds its rectifying capabilities, even under washing procedures and significant bending deformation. Furthermore, this diode seamlessly integrates into various electronic circuits, including half‐wave rectifiers, capacitor–diode filters, and logic gate systems.

Modeling the Transient Behavior of Ionic Diodes with the Nernst–Planck–Poisson Equations

AbstractIonic circuitry formed from soft charged polymers promises to enable a new kind of analog computing and usher in a new age of human–computer interaction, but devices remain in their infancy. Ionic diodes, created by the interface between two oppositely charged polymers, form one of the most fundamental elements of ionic circuitry. However, it is currently not well understood how the boundary conditions and geometry of a diode influence its transient response and performance. A consistent set of metrics with which to analyze these diodes is first developed, then a continuum model based on the Nernst–Planck–Poisson equations is used to study how device construction and three types of boundary conditions affect performance. This model is solved using the finite volume method and gives insight into transient diode behavior not described by previous analyses. It is demonstrated how different parts of a diode's construction, thickness, and operating voltage act as the limiting factors for different regimes. To conclude, it is demonstrated how blocking electrodes limit both diode lifetime and rectification behavior and how neutral bath boundary conditions play an important role in the performance of some diodes, with recommendations for building higher performance devices in the future.

MXene‐enhanced environmentally stable organohydrogel ionic diode toward harvesting ultralow‐frequency mechanical energy and moisture energy

AbstractWith the accelerating advancement of distributed sensors and portable electronic devices in the era of big data, harvesting energy from the surrounding environment to power electrical devices has become increasingly attractive. However, most mechanical energy harvesters often require high operating frequencies to function properly. Moreover, for practical applications, the survivability of devices in harsh operating environments is a vital issue which must be addressed. Besides, the single‐stimulus responsiveness limits their further applications in complex external environments. Here, a pressure and moisture dual‐responsive ionic diode consisting of two organohydrogels with opposite charges as an energy harvester is proposed. The organohydrogel ionic diode utilizes the migration of cations and anions to form the depletion zone and followed by an enhancement of the built‐in potential along the depletion zone as a result of mechanical stress or humidity, converting ultralow‐frequency mechanical energy or moisture energy into electrical energy. Meanwhile, this mechanism is further confirmed by the finite element analysis. With the increased rectification ratio due to the introduction of MXene, the ionic diode exhibits a relatively large output current (∼10.10 μA cm−2) and power density (∼0.10 μW cm−2) at a mechanical pressure of 0.01 Hz, outperforming most currently available mechanical energy harvesters. More impressively, the incorporation of ethylene glycol provides the hydrogel ionic diode with excellent temperature tolerance and long‐term environmental stability. The organohydrogel ionic diode can also be applied as a moisture‐driven power generator and self‐powered humidity sensor. This study presents promising prospects for the efficient collection of renewable and sustainable energy and the practical application of hydrogel‐based energy harvesters in extreme environments.

Patterned Coating of Ionic Diode Arrays Toward Flexible Moist‐Electric Generators to Power Wireless Sensor Nodes

AbstractUbiquitous moisture is attractive for developing sustainable mobile power sources. However, how to endow moisture‐electric generators (MEGs) with fine design, mass customization, high power output, and foldability is still a largely unsolved problem. In this work, based on the Patterned Coating method, use is made of modulated carbon nanotube, nano‐Al2O3, and liquid metal inks to design and fabricate MEGs with properties required for their applications. A single MEG of the ionic diode type thus fabricated can generate an open‐circuit voltage (VOC) of 1.03 V and a short‐circuit current density (JSC) of 47.77 µA cm−2. Through large‐area integrated fabrication, an array containing 160 MEGs can generate a VOC of 154.1 V, and an array containing 100 MEGs can produce a short‐circuit current of 1.95 mA. In addition, the excellent wearability of MEG is verified by analyzing its electrical output performance after subjecting it to 800 cycles of bending deformations. The simple and cost‐saving fabrication technology issued from the work opens new perspectives for MEG applications and paves a path to their industrialization. For example, the customizable ionic diode arrays can meet the on‐demand power supply requirements of wireless sensor network nodes in the Internet of Things.

A Guide for Cross‐Linking Modulation: The Record Rectifying Ratio of Hydrogel‐Based Ultra Flexible Ionic Diodes

AbstractThis study focuses on the effect of cross‐linking mechanisms in the development of hydrogel‐based flexible ionic diodes (HBIDs) with desirable features for ionic devices. The research explores one p‐type hydrogel and a series of n‐type hydrogels with varying cross‐linking ratios, investigating their ion‐conducting properties, morphological and electronic structures, as well as the performance of flexible HBIDs constructed using these hydrogels. The results indicate that the cross‐linking ratio significantly influences ionic conductivity, bulk resistance, and capacitance. Additionally, the HBIDs' ability to maintain their electrical performance under repeated mechanical bending is studied concerning the cross‐linking ratio. Notably, the HBID with the highest cross‐linking ratio (HG/12) exhibits relatively higher current densities (up to 8.9 mA cm−2) and considerably higher rectification ratios (up to 1269). Even after 500 cycles of deformation, the HG/12 diode maintained a rectification ratio of 987, demonstrating its excellent performance and durability.