Real-time evaluation of charge transfer patterns at buried ionotronic interfaces

Abstract

Bionic and triboelectrification based ionotronics – an emerging field that bridges ions and electrons at an insulating–metallic interface – form an electric double layer (EDL), which determines charge desolation, deposition, and transfer patterns. Therefore, mechanistic evaluation of the EDL is imperative. However, the EDL forms at a unidirectional buried interface below the insulating layer, which impedes its real-time monitoring via electrochemical methods, probe microscopy, and spectral excitation. This study for the first time evaluates the EDL of ionotronics using in situ electrochemical impedance spectroscopy in conjunction with triboelectrification perturbation as an alternative to the dynamic reversible carriers of a conventional electrochemical cell. The charge dynamics at the insulating–metallic interface are elucidated through four patterns of charge transfer, charge recombination due to electrode-initiated polarity, precipitation of oppositely charged ions at the EDL, and hindered diffusion of delocalized charge. These patterns help explore neuromorphic interfaces, noise-cancellation and the intricate EDL of miniaturized robots, and enhance power sensitivity and longevity of self-powered sensors. The approach used in this work can be utilized to study a variety of buried interfaces.

---

Introduction

Ionotronics is an emerging field that focuses on the exploration of the intricate interplay between ions and electrons in ionic conductors. Unlike conventional electronics that primarily rely on electron movement, ionotronics leverages the dynamic interactions of mobile ions within chemically active media.¹ The approach of integrating ion transport and electronic conduction holds promise for the development of ionotronic devices such as transistors, memristors, and sensors, with applications in neuromorphic computing, wearable electronics, and artificial intelligence.² One of the key aspects of ionotronics is the exploration of complex charge dynamics, where the movements of ions and electrons interact to enable efficient charge transfer.

Charge dynamics entail intricate and often non-linear behavior involving temporal changes, spatial distributions, chemical compositions, and structural characteristics, which are studied via a combination of in situ techniques, which enable the direct investigation of phenomena within their natural or operational environment, without the need for sample extraction. This offers real-time analysis, thereby preserving the native conditions and providing valuable insights into complex dynamic behavior and interactions. Charge dynamics in ionotronics involves the integration of ions, ion transport, and ion manipulation in electronic devices to achieve new or enhance existing functionalities. Charge transfer occurs through various mechanisms, including ion migration, intercalation, redox reactions, and electrochemical processes based on which ionotronics are divided into four systems. Ionotronic devices utilize the movement of ions, either as charge carriers or for modulation purposes, to control electronic properties, such as conductivity, resistivity, and polarization. Table 1 presents a comprehensive summary of the intricate interfaces found in ionotronics systems, together with the associated charge carrier species and the corresponding transduction mechanisms evaluated via in situ methods. However, the charge dynamics of triboelectrification based ionotronics (TEIs) has never been studied.

Table 1 Comparison of charge dynamics in electrochemical and optoelectronic systems^a

System	Electrochemical	Electrodynamic	Electrostatic	Triboelectric
a Scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM).
Device design	Two electrodes mediated by conductive electrolytes	Two/three electrodes assisted with charge-transporting layers	Two electrodes separated by an insulator	One electrode buried in insulators
Charge carriers	Ions	Electron/hole pair	Delocalized charge	Induction of delocalized charge
Charge carrier mechanism	Intercalation/deintercalation, redox cycle, reaction	Conduction, radiation	Polarization/conduction	Polarization/induction
Charge dynamics evaluation techniques	SECM, EIS, in situ IR/Raman spectroscopy, XAS	Potential mapping & EIS	Potential mapping, in situ TEM	In situ-EIS
Examples	Battery, fuel cell, electrolyzer, corrosion cell	Diode, transistor, solar cell, thermoelectric generator	Capacitor, piezoelectric generator	triboelectric generator, ionotronics
Reference	21 and 22	23 and 24	25 and 26	This work

---

Ionotronic devices based on triboelectrification-induced polarization have a universal design paradigm that encompasses an insulating electrification layer, an ionically conductive induction layer, and an electron conducting electrode.³ The interface between the induction layer and electrode forms a stable electric double layer (EDL), exhibiting capacitive characteristics. The EDL has two domains: a metallic electrode and an ionic conductor. Suitable ionic conductors are either hydrogels or gels. Hydrogels are composed of hydrophilic polymer networks swollen with water or ionic aqueous solution, while gels have a similar structure but with comparatively reduced ionic crosslinking.^4,5 Their conductivity, strength, and durability are defined by the bonding nature of the ionic entities.¹ The sources of ionic entities are dissolved and/or dispersed ions, along with residual precipitated ions, oligomers, and reactive by-products. In these devices, the ionic conductor induces charges from the insulator layer and distributes them in a matrix via active ionic entities which then form the EDL with the electrode layer.⁶ The EDL mechanism is realized from the electrical output pattern of the device⁷ through numerical analysis⁸ of the interfaces and spectral techniques, such as X-ray scattering,⁹ infrared and Raman,^10,11 non-linear second-order sum frequency generation (SFG),¹² SFG evaluation under various pH conditions and electric fields,^13–15 and X-ray absorption spectroscopy.¹⁶ In the systems evaluated using these techniques, the EDL is formed either in an active redox cell, a conductive solid state cell, or a floating metallic layer, where the EDL hosting layer is easily accessible and its molecular, electrostatic, and morphological changes can be observed. However, the EDL in TEIs is formed at a unidirectional buried interface,^1,3,7 and thus the chemical and electrostatic processes cannot be directly monitored in real-time using probe microscopy. Furthermore, the insulating characteristic of the electrification layer imposes constraints on the applicability of optical and spectral excitation methods.¹⁷ The EDL's exclusive susceptibility to electrochemical evaluation arises from the absence of dynamically reversible carrier ions and the restriction to a single electrode. Consequently, real-time monitoring of ionotronics has been a challenging pursuit.

Herein, for the first time, we evaluated the real-time EDL formation, dilution, and electrode deterioration mechanisms of TEIs using in situ electrochemical impedance spectroscopy (in situ-EIS) in conjunction with triboelectrification perturbation as an alternative to dynamically reversible charge carriers, which induce delocalized charges in the matrix via a specific charge transfer pattern. Impedance measurements capture the transport of charges induced by triboelectrification facilitated by electrostatic delocalization, where the underlying circuitry is between the electrification layer and electron collector electrode. For illustrating EDL formation and the role of active ionic, polarized, and precipitated charged entities in EDL dilution or saturation, a series of ionic conductive solutions containing various active entities was studied. From this a novel mechanism is proposed to explain four distinct charge transfer patterns observed at the electrification induced interfaces. The charge transfer patterns concluded from this study include charge recombination in the electrification and induction layers, and at their interface. Furthermore, these patterns distinguish EDL dilution caused by electrode-initiated recombination from that caused by faradaic reaction-initiated recombination. This approach can quantitatively evaluate the precipitation of oppositely charged active complex ions from the induction layer to the EDL, which causes electrode corrosion,¹⁸ pores clogging,¹⁹ full electrode dissolution,²⁰ leading to performance fluctuations in ionotronics. These charge pattern dynamics can be further extrapolated in emerging robotics and bioelectronics to control tissue damage at neuromorphic interfaces, cancel selective noise by choosing desired delocalization patterns, boost power sensitivity and longevity of self-powered sensors, and define the intricate EDL of miniaturized robots. Furthermore, the compatibility of in situ-EIS with triboelectrification perturbation allows the study of a variety of buried interfaces of ionic cables, ionic membranes, and ionotronic devices.

Results and discussion

In situ-EIS is a technique used to analyze the electrochemical behavior of a process/material, such as charge transfer, mass transport, and dielectric properties. The technique involves applying an AC signal to the system under investigation and measuring the resulting voltage response. By varying the frequency of the AC signal over a wide range, the impedance of the system can be determined at different frequencies. The impedance change over time in the frequency domain is then used to analyze the electrochemical processes occurring within the material, its interfaces and the system. To verify the compatibility of in situ-EIS with insulating dielectric system the behavior of redox cells and photovoltaic cells is studied and compared with that of a TEI device. The ionic-conductive component of the TEI device is an induction layer located below the electrification layer and comprises a conductive medium based on conjugated bonds or ionic charges. In this regard, in situ-EIS evaluation is carried out for TEI devices with various induction layers including, acidic, basic, salt based, polymeric, carbonaceous, and metallic complexes. Accordingly, four charge transfer patterns are deduced, and the underlying mechanisms are explained in detail. For conceptual validation, the impact of charge transfer pattern on the electrical output of TEIs is evaluated by comparing two closely related polymeric systems, and the impact of changes in dimensions, vertical force, and lateral force is analyzed.

In situ-EIS for charge transfer dynamics of ionotronic devices

In ionotronics, the movement of ions and electrons occurs at nanoscale interfaces, which introduces intricate charge transfer dynamics. The interplay between different length scales and the associated processes, such as ion diffusion, interfacial reactions, and charge accumulation, contributes to the complexity of the charge dynamics. In situ-EIS has the potential to characterize ionic conductors and their interfaces as a function of charge transfer kinetics, ionic/electrical conductivity, degradation, and performance stability.

Therefore, three classes of ionotronics are selected including (i) optoelectronic (ii) electrochemical, and (iii) triboelectrification driven systems and their active interfaces and charge distribution patterns are studied (Fig. 1). In the optoelectronic class, at an interface where two layers of opposite polarity are involved, charges are generated upon illumination. These charges are carried by ionically conductive charge transporting layers to electronic conductor following the EIS charge pattern of two constant-phase capacitive components connected in series to electrodes at both ends.^21,22 The open circuit potential (OCP) shows negative inclination due to trap states, defects, and ion migration in the photosensitive layer,²³ as shown in Fig. 1a.

Fig. 1 The charge generation interface varies based on the charge source and transfer pattern, which defines the underlying circuitry. Comparison of charge dynamics in photovoltaic, electrochemical, and electrification-based ionotronics (via open circuit potential, Nyquist plot, and the corresponding transfer-dependent circuit) shows compatibility of triboelectrification perturbation with in situ-EIS, supporting the role of triboelectrification in delocalized charges with/out active carrier species in the matrix. (a) Stimuli activated positive-negative junction-based photovoltaic without active ions, (b) electrochemical device with active ions, (c) an electrode-based insulating buried interfacial behavior without and (d) with active species under triboelectrification perturbation.

Electrochemical ionotronics have the same polarity structure as optoelectronics except that ionic charge carriers are used to stimulate the charge transfer process. The EIS Nyquist curve depicts both charge transfer kinetics and diffusion-controlled behavior, which are represented by components such as Warburg impedance (W), charge transfer resistance (R_ct), double layer capacitance (C_dl), and sheet resistance (R_s). The device shows a stable OCP at positive potential due to intra-matrix coupling/decoupling of charge carrier species,^24,25 as shown in Fig. 1b.

Triboelectrification ionotronics involve a mechanically stimulated insulating electrification component and a buried electronic conductor that transduces all charges to an external shutter. Due to the insulating nature of the ionic/electrical interface and the requirement of mechanical input, electrochemical characterization, which is the only possible real-time monitoring tool for TEI, has never been used. Also, the absence of dynamic reversible carriers in TEIs presents another obstacle to its evaluation.³ Therefore, the charge dynamics at the insulating buried ionic/electrical interface are investigated in this study by coupling in situ-EIS with triboelectrification perturbation, which accelerates charges delocalization throughout the ionic conductor, acting as reversible charge carriers, Fig. 1c and d. Two types of triboelectrification driven devices were considered: a pristine ionic conductive layer with electrostatic charge carriers, and an ionic conductive layer with active species and electrostatic delocalizing charges. The pristine device upon slight finger tapping (0.05 N) of the electrification layer gives a prominent OCP response, indicating charge activation or delocalization. Negative voltage inclination indicates tribopositivity of the induction layer. The OCP of the active species based TEIs with a cyclic wave structure gives a large response upon tapping, as the triboelectrification input combines with intra-matrix charge coupling/decoupling. Unlike these devices, the circuitry of TEIs is between the electrification source and electronic component, as TEI is a single electrode system. Briefly, an insulating capacitance is developed at the skin/electrification interface C_d, which transfers charges to the ionic conductive layer due to bulk resistance R_d; the charges delocalize in the matrix, forming a faradaic capacitance C_F. The charges concentrate around the electronic conductor with resistance R_s forming a double layer capacitance at the ionic/electrical interface C_dl. The charges flowing to the ground are collected as the output of TEIs. When this system is incorporated with active species, the charges delocalize in the matrix via diffusion (a Warburg component W), and the EDL converts to a CCPE which fluctuates between capacitive and inductive behavior depending on the nature of active species. The fluctuation response in the OCP curve of TEIs and the change from charge transfer controlled kinetics to mass transfer-controlled diffusion in the presence of active species in the matrix validate the capability of triboelectrification triggered in situ-EIS to evaluate single electrode ionotronics.

Charge transfer pattern of triboelectrification based ionotronics

A universal design of TEIs consists of an outer insulator layer, an inner ionic conductor layer, and an electrical conductor electrode layer, as shown in Fig. S2 and S3.† The electrode and ionic conductor interface form a stable EDL which behaves as a capacitor. The EDL and the insulator represent two capacitors in-series, capacitively coupling the electrical signal from the electrode and transporting charge in the form of alternating current.³ Since the EDL is formed at a unidirectional buried interface, its chemical and electrostatic processes cannot be directly monitored in real-time using probe microscopy. Furthermore, the insulating nature of the electrification layer imposes constraints on the applicability of optical and spectral excitation methods. The exclusive susceptibility of the EDL to electrochemical testing arises from the absence of reversible dynamic carrier ions and the restriction to a single electrode. Therefore, an in situ-EIS approach in conjunction with triboelectrification perturbation is devised to enable real-time monitoring of the EDL.

One electrode is connected to the working electrode probe of the potentiostat and the other is inserted into the induction layer that is connected with the counter electrode. To complete the measurement circuit, triboelectrification perturbation is provided via slight hand tapping (0.05 N), as shown in Fig. 2a. This way, the underlying circuitry lies between the electrification layer (silicon rubber, SR) and electrode layer (Pt), mediated by the ionic conductive layer, as shown in Fig. 2c. When skin comes in direct contact with SR, the quantity of short circuit charge (Q_sc) is null with no open circuit voltage (V_oc) across the interface. However, as skin retrieves, V_oc and Q_sc become, V_oc = −σA/2C₀ and, Q_sc = −σA/2 respectively,^26,27 where σ is the density of electrostatic charges on the surface of SR, C₀ is the collective capacitance of the device, and A is the contact area between SR and skin. TEI is an electrostatic system that exhibits inherent capacitive behavior encompassing multiple capacitors. There are three capacitive layers between (i) skin and SR, (ii) SR and the ionic induction layer, and (iii) electrical double-layer capacitance between the ionic induction layer and Pt lead nodes.^28,29 Therefore, the performance of TEIs is determined by the releasing state of the charge transfer cycle, as charge induction is initiated on all interfaces at this stage.³

Fig. 2 Advanced analytical techniques based on morphology, potential distribution, surface functionality, and electrochemical response are unable to study charge dynamics at the ionic/electrical interface due to their occurrence in buried interface beneath insulating layers and the single probing electrode in the form of the electrical double layer EDL capacitive interface. This interface is studied via in situ-EIS along with triboelectrification perturbation which induces mobility of static charges regardless of carrier species and shows different charge transfer patterns as a function of the phase angle profile over a vast range of frequencies. (a) Setup for EIS measurement, (b) schematic illustration of the four types of charge transfer mechanisms, (c) Bode phase profile of ionotronics with various types of active species as indicated.

The induction layer generally comprises a conductor, which induces both electrostatic and dynamic charges. This includes polymers, carbonaceous materials, ionic liquids, ionogels, hydrogels, nanostructures suspension, and nanometer thick films. Depending on the fabrication process, each induction layer has different types of residual entities, such as anions, cations, monomers, radicals, salts, acids, and bases. If the residual entity is reactive, the induced charge density of the matrix decreases by either neutralizing the charges via recombination or decreasing the polarity of charges through bond formation.³⁰ This phenomenon is schematically illustrated in Fig. 2b along with four types of charge transfer patterns toward the EDL based on charge recombination, dilution, and precipitation mechanisms.

TEI in the releasing state drives the induction of electrification charges into the ionic conductive layer. These charges further delocalize throughout the matrix and concentrate around the electrical conductor Pt. To maintain charge neutrality, Pt generates the same number of charges with opposite polarity, which continuously flow to ground maintaining capacitance at the ionic vs. electrical interface, referred to as the true-EDL, as mentioned in Case I. When the overall matrix of the induction layer contains reactive species, the rate of charge mobility via recombination or irregular polarization within the matrix is reduced, as illustrated in Case II. The reactive residual species in the matrix cause EDL dilution by restricting charge mobility (Case II) or generating precipitated charges around the EDL (Case III). In the latter case, the active residuals agglomerate and move toward the opposite polarity region in the matrix, i.e., the electrode, where they reduce the density of opposite charges around Pt, thus diluting the EDL. Amphoteric active species carry the induced charges to the electrode layer and either intercalate or react with the electrode, lowering the electrode capacity to induce charge. This way the charges around the electrode are neutralized back in the matrix due to increased internal resistance of the electrode, as illustrated in Case IV. The types of induction mechanisms to ground following Case II–IV, are referred to as pseudo-EDL.

The four charge transfer patterns are further elaborated in the Bode phaser plot of EIS. The phase angle profile over a wide range of frequencies reveals the charge transfer pattern, as shown in Fig. 2c. Three regions are observed: 10⁻²–10⁰ Hz (low-frequency diffusion region), 10⁰–10² Hz (medium frequency, charge transfer region), and 10²–10⁶ Hz (high frequency, ohmic and inductive region). The activity in the low-frequency region is attributed to equivalent series capacitance and ion diffusion in the matrix. The medium frequency region shows electrostatic displacement of charges or dipole delocalization, which is characteristic of a double layer. The high-frequency region is attributed to ohmic or inductive resistance originating from the internal resistance of the electrode and the faradaic charge screening mechanism. While true-EDLs exhibit faradaic charge transfer, pseudo-EDLs exhibit a dominant ion diffusion triggered charge transfer pattern, typically observed in induction layers with active residuals such as acid, base, salt, multiwalled carbon nanotubes, oxidized metal, and oxidant species as shown in Fig. 2c. The four charge transfer patterns are further evaluated via Nyquist graphs (Fig. S4†) according to their equivalent circuit fit values listed in Table S1, as detailed in ESI Note 1.†

Conceptual validation

To evaluate the charge transfer mechanism and its impact on electrical output and device durability, a polyvinyl alcohol–polyaniline copolymer (PVA@Pan) induction layer was enclosed between two layers of SR film with edges sealed using uncured fresh SR gel. The air gaps and voids were removed by applying vacuum via a micro diameter needle tip of a hypodermic syringe (Fig. S1†). A metallic lead was inserted to establish connection with the induction layer. Skin owing to its stratified tissue structure, keratin, fatty acids, and cholesterol rich content with low water, was used as the positive electrification and perturbation layer.^31,32 In the EIS study, short-term triboelectric perturbation was applied through manual hand tapping. For long-term output assessment simulating human skin contact, ex vivo porcine epidermal tissue was mounted on a linear motor probe interfacing with the TEI. Both configurations demonstrated comparable output (Fig. S13†), validating the experimental approach for mimicking human tactile interactions.

Working mechanism. The TEI comprises three layers: a negative electrification layer (SR), an electrostatic induction layer (PVA@Pan), and an electrical conductor (Ag lead).³³ The working mechanism of the charge transfer cycle is illustrated in Fig. 3a. When the positive electrification layer (skin) comes in contact with SR, electrification occurs, resulting in the generation of the same amount of charge with opposite polarities at both contacting surfaces (Fig. 3a(i)). Since the opposite charges coincide at the same plane, no electrical potential difference is established between these surfaces. When the two surfaces begin to separate, the static charges residing on the surface of SR induce migration of ions in PVA@Pan to neutralize the static charges. This results in the formation of an excessive layer of positive ions at the interface between SR and PVA@Pan. Simultaneously, an EDL forms at the interface between PVA@Pan and Ag, leading to polarization of the EDL, thus generating negative charges on PVA@Pan and an equal number of positive charges on Ag. To establish the EDL, electrons flow from Ag to ground via the external load until all the accumulated static charges on SR are neutralized (Fig. 3a(ii)). This double layer at the Ag/PVA@Pan interface exhibits a high capacitance of ∼0.1 F m⁻² and a low voltage of ∼10⁻² V within a nanometer range.^7,34 Therefore, this interface does not undergo any electrochemical reaction since the voltage is less than 1 V.⁷ As long as the positive layer remains at a distance from SR, all charges remain in electrostatic equilibrium as shown in Fig. 3a(iii). When skin reapproaches SR, the electrostatic equilibrium is disrupted, and the entire process is reversed, with an electron flux in the opposite direction from ground to the Ag/PVA@Pan interface through the external load (Fig. 3a(iv)). This cycle of the skin approaching and retracting from SR generates alternating current signals.

Fig. 3 Based on the four charge transfer cases, an inductive layer-based triboelectrification device was further studied for the impact of charge transfer on the electrical output of ionotronics. A copolymer of polyvinyl alcohol and polyaniline PVA@Pan was used in two forms: one in the pristine state forming a true-EDL interface and the other containing active residuals forming a pseudo-EDL interface. Impact of the EDL charge transfer mechanism on electrical output. (a) Schematic of the working mechanism of the charge transfer cycle in TEI along with EDL formation at the releasing state as indicated. (b) Topography and potential mapping images (25 μm²) of the induction layers measured from the spin coated film of the induction layer on a conductive stub. (c) Electrode durability testing for both types of inductive matrix and their corresponding digital and optical images after 50 days of storage. The leaching of the electrode indicated a pseudo-EDL induction mechanism followed by active species. (d) Comparison of the charge transfer mechanism for both induction layers via the Bode phaser profile. The electrical output from hand tapping reveals the impact of true-EDL and pseudo-EDL transfer mechanisms on (e) transferred charge Q_sc, (f) open circuit potential V_oc, and (g) short circuit current density J_sc. (h) Power density profiles against external load resistance.

Charge dynamics in the induction layer. To demonstrate the impact of charge transfer mechanisms on the electrical output of ionotronic devices, two devices with the same inductive layer (PVA@Pan) were fabricated. Two kinds of PVA@Pan copolymers were synthesized: one using PVA and polyaniline precursor (forming a copolymer without active residuals), and the other using PVA and aniline precursor (with active residuals). The copolymer with active residuals is named PVA@Pan_{in situ}, and the other without active residuals is referred to as PVA@Pan_interfacial. The detailed preparation and spectral evaluation protocol are provided in ESI Note 2–3 and Fig. S5, S6, of the ESI.† The triboelectric properties of a material are described by the nature of its surface atoms and can be significantly altered.^35,36 The tuning of the triboelectric properties of PVA (Fig. S7†) with two types of Pan via copolymerization was evaluated using Kelvin probe force microscopy (KPFM), as shown in Fig. 3b. The (5 μm × 5 μm) PVA topography shows a high surface roughness (Ra: 14.8 nm), which significantly decreases after in situ (Ra: 6.97 nm) or interfacial (Ra: 5.66 nm) copolymerization with Pan, showing uniform copolymerization and low entanglement formation. The increase in surface potential after copolymerization is due to the presence and distribution of the more triboactive cationic chain of Pan with a potential drop ranging between 38–98 mV. For metallic lead durability, the PVA@Pan devices were stored for 50 days in an ambient environment. After storage, the metallic lead of the pseudo-EDL device was degraded at some points due to the presence of sulfate/sulfonic acid (SO₄²⁻/HSO₄⁻) and unreacted/partially oxidized aniline oligomer species in the induction layer, ESI Note 4 and Fig. S8.† On the other hand, the true-EDL device remained intact with a polished texture, as demonstrated in the digital and optical photographs in Fig. 3c. From the phase angle profile, it can be confirmed that the true-EDL device exhibits faradaic charge transfer, whereas the pseudo-EDL exhibits dominant ion diffusion-triggered charge transfer, Fig. 3d; a detailed explanation is provided in ESI Note 5 and Fig. S9.†

Impact of charge dynamics on electrical output. The pseudo-EDL based TEI was connected to ground to measure the output against skin as the positive counterpart under a small force of 0.3 N. The TEI shows a charge transfer of 74 nC, a V_oc of 132 V and a J_sc of 103 mA m⁻², Fig. 3e–g. For the reactive species-free PVA@Pan induction layer, the transferred charge, V_oc, and J_sc increase to 88 nC, 168 V, and 132 mA m⁻², respectively, owing to the true-EDL induction mechanism. However, in both cases, the output is significantly enhanced compared to the pristine PVA system with a transferred charge of 32 nC, a V_oc of 67 V, and a J_sc of 24 m Am⁻², as shown in Fig. S10.† Furthermore, the power density of TEI was measured over a range of external resistances (10² to 10⁶ Ω) and assessed using P = I²R/A, where I represents the output current over a cycle at resistance R for an active contact area A. The current output decreases gradually when the load R is below 1 MΩ. With increasing resistance above 1 MΩ, a rapid decrease in current was observed owing to substantial charge usage by the resistor (Fig. S11†). The related power density depicts a considerable increase with load resistance, where peak power densities of 4.19 W m⁻² and 7.44 W m⁻² were obtained at a 5 MΩ resistor for the pseudo-ETL and true-EDL mechanism devices, respectively (Fig. 3h). Therefore, the true-EDL induction mechanism based TEI device was further studied for output scaling, durability, and use in powering electronics.

The electrical performance disparities between true-EDL and pseudo-EDL devices were quantified through ionic conductivity, charge transfer time constants (τ), and interfacial resistance derived from electrochemical impedance spectroscopy (EIS), Table S2.† True-EDL systems exhibit 2.3× higher ionic conductivity (0.023 S m⁻¹vs. 0.01 S m⁻¹ for pseudo-EDLs), attributed to efficient charge delocalization and minimized reactive species interference, whereas pseudo-EDL systems suffer from charge recombination at residual aniline monomers and acidic byproducts (e.g., H₂SO₄), as confirmed by XPS-detected sulfur accumulation (S 2p peak at 168.2 eV). Charge transfer time constants (τ = RC) reveal a significantly faster response in true-EDL devices (12 ms vs. 47 ms for the pseudo-EDLs), calculated using interfacial resistance (R = 12 000 Ω for true-EDLs vs. 47 000 Ω for pseudo-EDLs) and nominal capacitance (C = 1 μF for both). This disparity stems from true-EDL's low interfacial resistance and stable double-layer formation, contrasting with pseudo-EDL's slower kinetics due to faradaic reactions (e.g., N–metal bonds) and Warburg impedance from proton diffusion. Mechanistically, the true-EDL operates via non-faradaic storage with delocalized charges at the PVA@Pan interface, while the pseudo-EDL combines redox processes and ionic migration losses. These findings align with the literature on EDL-pseudocapacitive systems, where bimodal ion size effects enhance packing density in the true-EDL, and slower pseudocapacitive responses arise from faradaic steps. Collectively, true-EDL's superior performance (higher conductivity, lower resistance, and faster charge transfer) underscores the critical role of material purity and interface engineering in optimizing ionotronic device efficiency. The induction layer maintains stable output over prolonged cycling, and the device performance remains intact even after extended storage as demonstrated in ESI Note 6 and Fig. S12.†

Stability of charge dynamic under variable vertical and lateral forces. The contacting force is crucial for inducing deep charge dynamics in the induction layer, and thus the output was studied using a tapping force ranging from 0.3 N to 1.5 N with the device placed on a force sensor as shown in the inset of Fig. 4a. A significant enhancement in J_sc was observed with increasing force from 132 mA m⁻² to 213 mA m⁻² (0.8 N) and 303 mA m⁻²(1.5 N). The current I_R and power density at load resistance follow the same pattern (Fig. 4c and d) with a maximum power density reaching 15.5 W m⁻², outperforming all reported TEIs as shown in Table S3.† To accommodate various application scenarios, scaling of the output was studied with sizes of 1 × 1, 2 × 2, 3 × 3, and 3.5 × 3.5 cm² (Fig. 4e–g). The magnitude of the electrical output is proportional to the device size, where V_oc and I_sc linearly increased from 190 V and 303 μA to 518 V and 120 μA, as the device size increased from 1 to 12.25 cm². In general, V_oc is considered independent of device size; however, herein V_oc increased with increasing size of the air exposed edges.^35,37,38 This voltage enhancement phenomenon demonstrates consistent behavior across diverse induction layers in TEI devices, confirming that the geometric scaling principle, where a decreasing perimeter-to-area ratio reduces edge-induced charge leakage, is independent of material composition as shown in ESI Note 7 and Fig. S14.† Furthermore, to demonstrate the TEI's capability as a motion sensor, the electrical output was measured during finger touch and sliding motion, corresponding to outputs between 57 and 92 mA m⁻² and 82 to 103 V, as shown in Fig. 4h–j and S15.† When the TEI was stretched diagonally and coaxially, and even when the rolled device was stretched, the output remained intact, indicating mechanical endurance and a leakage-free device design (Fig. 4k, Movie S1†).

Fig. 4 As the charge transfer pattern determines the electrical output, the stability of charge transfer is assessed by changing the vertical and tangential axial input force, and the size of the TEI device. (a) Schematic of TEI under various tapping forces, (b) J_sc enhancement, (c) corresponding current, and (d) power density profile against external load. (e) Schematic of device active area and (f, g) corresponding increase in output. Demonstration of harvesting human kinetic energy via (h) one finger touch, (i) finger diagonally sliding on the device, and (j) two fingers touching. (k) The output voltage response of the stretching and rolling device. A 9 cm² TEI is used as a power source for lightning (l) 500 LEDs of 0.06 W each, and (m) 104 LEDs of 0.5 W each via hand tapping. The circuit is shown in the inset of (l).

Demonstration of ionotronics as a power source. Furthermore, TEI was explored as a power source by converting its AC output to DC via a bridge rectifier and connecting it to a load of 500 LEDs (0.06 W each). When the device was tapped, the LEDs illuminated as shown in Fig. 4l and Movie S2.† The inset of Fig. 4l shows the electric circuit of the setup. The same setup was used for lighting 104 high-power LEDs (0.5 W each) as shown in Fig. 4m and Movie S3.† Furthermore, wireless power transmission was demonstrated via a simple circuit setup with two inductive coils (inductance: 50 μH, diameter: 52 mm), with one coil connected to TEI via a bridge rectifier and the other connected to a source meter or LED as shown in Fig. S16.†^39,40 Owing to the high power density and low internal resistance, a complete output current charging pattern was observed and the LED illuminated when the TEI was tapped as shown in Movies S4 and S5.† The application of TEI as a sensor and as a direct/wireless power source demonstrates its potential for next generation flexible and wearable electronics.

Extended applications in future electronics. The real-time evaluation of charge transfer patterns at buried ionotronic interfaces enables transformative applications across next-generation electronics through precise control of interfacial charge dynamics. Neuromorphic computing systems could leverage stable charge delocalization (Case I) and transient diffusion-controlled responses (Case II) to mimic synaptic plasticity, enabling energy-efficient (<1 fJ per event) brain-inspired circuits for wearable edge computing. In healthcare, corrosion-resistant true-EDL mechanisms underpin implantable sensors capable of continuous metabolite monitoring (e.g., glucose/lactate) with wireless data transmission and self-diagnosis via real-time EIS, extending operational lifetimes in physiological environments. Industrial and consumer applications benefit from frequency-selective impedance matching (Cases II/IV), enabling adaptive noise cancellation through triboelectric-mechano coupling to attenuate low-frequency vibrations (1–500 Hz) or target specific noise bands (e.g., 100 Hz machinery hum) via AI-tuned charge transfer patterns. Soft robotics utilize edge-sealed TEIs to maintain actuator performance underwater through ionic leakage suppression, while sustainable energy infrastructure leverages high power density (15.5 W m⁻²) for broad-frequency harvesting. Addressing challenges like miniaturization-induced edge leakage (requiring fractal electrodes) and environmental instability (>90% RH operation via hydrophobic coatings) will be critical, alongside AI/ML integration to predict EDL failure modes through phase profile analysis, ultimately advancing adaptive, self-sustaining systems across healthcare, robotics, and energy domains.

Conclusions

Ionotronics is the study of interfaces where ions interplay with electrons forming complex charge dynamics at insulating/metallic interfaces, leading to the formation of EDLs. The complex charge dynamics at these interfaces involve phenomena such as ion accumulation or depletion, formation of space charge regions, charge transfer across the interface, and the modulation of electronic properties. The EDL forms at a unidirectional buried interface below the insulating layer, making their real-time monitoring via probe microscopy, optical, and spectral excitation a daunting challenge. In this study, we evaluate the EDL formation, dilution, and electrode deterioration mechanisms using in situ-EIS, relying on triboelectrification perturbation as an alternative to the utilization of dynamic reversible carriers of conventional electrochemical cells. The EDL capacitive response displayed different charge patterns for active, polarized, and precipitated charge entities in the ionic conductive matrix. Therefore, the charge transfer pattern is divided into four standard cases. For conceptual validation, an ionotronic device with an ionically conductive layer was studied in pristine and active species loaded forms. The EDL of the pristine device harvested 44% more energy supporting the presumptions. Furthermore, the charge transfer patterns defined in this study work for the TEI system can differentiate charge recombination at various interfaces and define electrode clogging, corrosion and dissolution processes. This approach can be used to identify changes in the EDL mechanism of robotics while moving from large scale to miniaturized design. Also, the compatibility of in situ-EIS measurement with triboelectrification perturbation can be universally used for the evaluation of buried electrically conductive interfaces, such as ionic cables, ionic membranes, and ionotronic devices, such as artificial muscles, skins, axons, luminescent devices, liquid crystal devices, artificial eels, and triboelectric nanogenerators.

Experimental

Fabrication of triboelectrification based ionotronic devices

A template of the arrays of 1 cm² Al foil was stuck on glass with a 1 cm gap from adjacent units on both sides as shown in Fig. S1.† Polysiloxane elastomer (SR, Ecoflex 00-50) with a 1 : 1 weight ratio of base and cure was mixed and spread on the template to dry at room temperature. Each 1 cm² unit with a concave mold was spun with uncured SR at the edges and 0.3 mL viscous solution of PVA@Pan was placed in the center. A second SR mold was placed on the top, and the device was dried overnight to fully cure both the inside and outside SR. Two types of metallic leads for charge collection were used: conductive tape or metallic wire. For conductive tape, the lead was first placed on the SR mold before spinning the uncured SR while for metallic wire the lead was inserted into the fully sealed device from the side until it reached the induction layer Fig. S5.† The most important step is the removal of air trapped in the device. A very thin hypodermic syringe knob was inserted from the side and the trapped air was sucked out leaving the induction layer uniformly spread over the entire area. This approach is much better than plasma treatment which lasts only for a few hours and causes a dramatic drop in the device performance. The device has final dimensions of 20 × 20 × 1 mm³ with an enclosed 10 × 10 mm² electrically active induction layer. A screw gauge was used to measure the thickness of the device.

In situ-EIS and triboelectrification perturbation

A two-electrode measurement setup was adopted for all types of devices where one electrode of the device is connected to the working electrode (WE) and working sense electrode (WS) cables and another electrode is connected to the counter electrode (CE) and reference electrode (RE) cables, depicted in Fig. 2d. Potentiostatic testing mode was adopted. Triboelectrification perturbation was applied to the device by finger tapping, using the finger as a source of tribopositive layer against the top silicone tribonegative layer of the device placed on the force sensor for calibration of the input force. To ensure consistent tapping, the operator underwent standardized training to ensure that taps were applied within a strict ±5% force deviation range, with the timing carefully controlled using a stopwatch. Statistical reproducibility was confirmed through the execution of multiple measurement cycles (n = 50), revealing a variation in V_oc of less than 5%. For evaluation of charge transfer mechanism in different media; a fully enclosed square shaped silicone pouch was made and filled with various conductive/active solutions such as brine, acidic, basic, carbonaceous, and polymeric gels with and without active residuals.

Preparation of polyaniline

Polyaniline (Pan) was prepared via interfacial polymerization of aniline at the aqueous-organic interface. The organic gasoline phase contained 0.1 M aniline monomers while the aqueous phase contained dissolved 0.35 M ammonium persulfate oxidant and 1 M H₂SO₄ dopant. The aqueous phase was stirred for 10 min to form a uniform dispersion, and then the organic phase was added carefully alongside the wall of the beaker containing the aqueous phase. The upper layer was formed by an organic aniline solution and the lower layer was formed by an aqueous APS solution. The polymerization was carried out without any disturbance for 24 h. The product was repeatedly washed with acetone and water until the washings were colorless. The products were dried at room temperature for 24 h.

Preparation of the polyvinyl alcohol and polyaniline copolymer

To check the detailed impact of the presence of active residuals in polymeric induction layers, two types of copolymers were synthesized: one using polyvinyl alcohol (PVA) and aniline precursor (without residuals), and the other using PVA and aniline precursor (with residuals). (i) For the Pan precursor copolymer, 3 g of PVA (M_w: ≈61 000) powder was fully dissolved in 30 mL of Milli-Q water (18.2 MΩ cm) at 80 °C with 700 rpm for 3 h and cooled to 50 °C. 0.01 g of Pan was dispersed in 5 mL of dimethyl formamide (DMF) by sonication for 5 min and a cooled PVA solution was added. The PVA/Pan mixture was stirred at 50 °C for 40 min to form a viscous solution ready for device fabrication. For further material characterization, the solution was solvent cast in a 90 mm plastic Petri-dish and was subjected to thermal treatment at 60 °C for 30 min. (ii) For the aniline precursor, chemical oxidative polymerization was carried out to form a copolymer; 3 g PVA powder was dissolved in 1 M H₂SO₄ acidified solution and stirred for 3 h at 80 °C. The solution was cooled down and maintained at 60 °C, followed by the addition of 0.045 mL aniline dissolved in 5 mL DMF. 0.68 g APS dissolved in 3 mL of DI water was added slowly drop by drop to the solution to initiate the polymerization of aniline monomers. After 20 min stirring, the reaction was stopped, and a viscous solution was used for device fabrication while the film was formed under the same film cast conditions mentioned above. A pristine PVA solution and film were also prepared for comparison.

Materials characterization

The absorbance and transmittance of the film and device were measured with a Shimadzu 2600 UV-Vis spectrophotometer equipped with a 60 mm integrating sphere. The chemical structure and composition of the samples were analyzed using an infrared spectrophotometer (Shimadzu, Japan) in attenuated total reflectance (ATR) mode with a resolution of 4 cm⁻¹ and a scanning range of 400–4000 cm⁻¹. A Gamry reference 3000 ZRA potentiostat/galvanostat was used to perform electrochemical impedance spectroscopy. The electrode durability was checked by keeping the electrodes inserted in TEI for 50 days and observed using an optical microscope (Carl ZEISS A2m, Germany). The PVA@Pan thin film was cut into dimensions of 10 mm × 30 mm × 0.2 mm for the tensile compression test, conducted on a Mechanical tester (Instron 34SC-05, USA), at a strain rate of 60 mm min⁻¹, under ambient conditions. Kelvin probe force microscopy (Bruker Dimension Icon, Germany) measurements were conducted in amplitude-modulated mode using Pt-coated probes (30 nm tip width) at a 0.98 Hz scan rate, 70 nm lift height, and 75 kHz resonance frequency. Samples were spin coated on a conductive stub, which was then connected to the ground of the AFM chuck holder, and the data were processed using Bruker Nanoscope software (v1.9).

Electrical characterization

The output performance of the TEI was assessed using an aluminum test chamber equipped with a LinMot linear motor system that delivered controlled periodic compressive forces in the range of 0.5–1.5 N at an actuation velocity of 1 m s⁻¹. The force variation was achieved by adjusting the acceleration and deceleration rates of the contact-separation cycle. A FlexiForce A201 sensor (Tekscan) was used to continuously monitor the applied force. To mimic human skin tapping, ex vivo porcine epidermal tissue (2 mm thickness, sanitized) was affixed to the head of the linear motor in contact with the TEI. The electrical output was evaluated utilizing a Keithley 6514 electrometer (input resistance = 200 teraohm) for measuring transferred charges, an oscilloscope (Tektronix, MDO 3024, impedance = 10 megohms) for measuring the output voltage, and a low-noise current preamplifier (Stanford Research Systems, model SR570, impedance = 4 ohms) for measuring short circuit current. The software platform was developed based on LabVIEW. All electrical output evaluations were conducted under ambient conditions (T = 22 ± 1 °C; % RH = 50 ± 3%).

Data availability

The data provided in the manuscript and ESI are available upon reasonable request.†

Author contributions

I. F. and M. F. contributed equally to this work. Conceptualization: I. F., W. A. D., methodology: I. F., M. F., investigation: I. F., visualization: M. F., formal analysis: I. F., M. F., J. S. C. L., supervision: W. A. D., writing – original draft: I. F., I. F., writing – review & editing: W. A. D., C. S. K. L.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was supported by a grant from the National Natural Science Foundation of China (grant no. 22072125) and Research Grants Council, Collaborative Research Fund (grant no. C1105-20G).

References

1. C. Yang and Z. Suo, Nat. Rev. Mater., 2018, 3, 125–142.
2. P. Peng, H. Qian, J. Liu, Z. Wang and D. Wei, Int. J. Smart Nano Mater., 2024, 15, 198–221.
3. I. Firdous, M. Fahim, F. Mushtaq and W. A. Daoud, Nano Energy, 2023, 116, 108817.
4. K. Kato, Y. Ikeda and K. Ito, ACS Macro Lett., 2019, 8, 700–704.
5. J. A. Burdick and W. L. Murphy, Nat. Commun., 2012, 3, 1269.
6. F. Yi, X. Wang, S. Niu, S. Li, Y. Yin, K. Dai, G. Zhang, L. Lin, Z. Wen, H. Guo, J. Wang, M.-H. Yeh, Y. Zi, Q. Liao, Z. You, Y. Zhang and Z. L. Wang, Sci. Adv., 2016, 2, e1501624.
7. C. Keplinger, J.-Y. Sun, C. C. Foo, P. Rothemund, G. M. Whitesides and Z. Suo, Science, 2013, 341, 984–987.
8. J. Shao, M. Willatzen, Y. Shi and Z. L. Wang, Nano Energy, 2019, 60, 630–640.
9. P. Fenter, L. Cheng, S. Rihs, M. Machesky, M. J. Bedzyk and N. C. Sturchio, J. Colloid Interface Sci., 2000, 225, 154–165.
10. K.-i. Ataka, T. Yotsuyanagi and M. Osawa, J. Phys. Chem., 1996, 100, 10664–10672.
11. M. Fleischmann, P. J. Hendra, I. R. Hill and M. E. Pemble, J. Electroanal. Chem. Interfacial Electrochem., 1981, 117, 243–255.
12. Y. R. Shen and V. Ostroverkhov, Chem. Rev., 2006, 106, 1140–1154.
13. L. Zhang, C. Tian, G. A. Waychunas and Y. R. Shen, J. Am. Chem. Soc., 2008, 130, 7686–7694.
14. V. Ostroverkhov, G. A. Waychunas and Y. Shen, Phys. Rev. Lett., 2005, 94, 046102.
15. J. Sung, L. Zhang, C. Tian, Y. R. Shen and G. A. Waychunas, J. Phys. Chem. C, 2011, 115, 13887–13893.
16. J.-J. Velasco-Velez, C. H. Wu, T. A. Pascal, L. F. Wan, J. Guo, D. Prendergast and M. Salmeron, Science, 2014, 346, 831–834.
17. I. López, J. Morey, J. B. Ledeuil, L. Madec and H. Martinez, J. Mater. Chem. A, 2021, 9, 25341–25368.
18. H. R. Zhou, J. Huang, M. Chen, Y. Li, M. Yuan and H. Yang, Colloids Surf., A, 2021, 631, 127657.
19. X. Fan, S. Liu, Z. Jia, J. J. Koh, J. C. C. Yeo, C.-G. Wang, N. E. Surat'man, X. J. Loh, J. Le Bideau, C. He, Z. Li and T.-P. Loh, Chem. Soc. Rev., 2023, 52, 2497–2527.
20. S. Boudesocque, L. Viau, H. Nouali and L. Dupont, Sep. Purif. Technol., 2023, 322, 124285.
21. D. Klotz, G. Tumen-Ulzii, C. Qin, T. Matsushima and C. Adachi, RSC Adv., 2019, 9, 33436–33445.
22. Y. Feng, J. Bian, S. Wang, C. Zhang, M. Wang and Y. Shi, J. Mater. Chem. A, 2019, 7, 8294–8302.
23. X. Chen, Y. Shirai, M. Yanagida and K. Miyano, J. Phys. Chem. C, 2019, 123, 3968–3978.
24. A. Parejiya, R. Amin, M. B. Dixit, R. Essehli, C. J. Jafta, D. L. Wood III and I. Belharouak, ACS Energy Lett., 2021, 6, 3669–3675.
25. P. Vadhva, J. Hu, M. J. Johnson, R. Stocker, M. Braglia, D. J. L. Brett and A. J. E. Rettie, ChemElectroChem, 2021, 8, 1930–1947.
26. S. Niu and Z. L. Wang, Nano Energy, 2015, 14, 161–192.
27. S. Niu, Y. Liu, S. Wang, L. Lin, Y. S. Zhou, Y. Hu and Z. L. Wang, Adv. Funct. Mater., 2014, 24, 3332–3340.
28. J. Y. Sun, C. Keplinger, G. M. Whitesides and Z. Suo, Adv. Mater., 2014, 26, 7608–7614.
29. X. Pu, M. Liu, X. Chen, J. Sun, C. Du, Y. Zhang, J. Zhai, W. Hu and Z. L. Wang, Sci. Adv., 2017, 3, e1700015.
30. D. Gao and P. S. Lee, Science, 2020, 367, 735–736.
31. F. Pirot, E. Berardesca, Y. N. Kalia, M. Singh, H. I. Maibach and R. H. Guy, Pharm. Res., 1998, 15(3), 492.
32. J. Crowther, A. Sieg, P. Blenkiron, C. Marcott, P. Matts, J. Kaczvinsky and A. Rawlings, Br. J. Dermatol., 2008, 159, 567–577.
33. K. Parida, G. Thangavel, G. Cai, X. Zhou, S. Park, J. Xiong and P. S. Lee, Nat. Commun., 2019, 10, 2158.
34. K. H. Lee, M. S. Kang, S. Zhang, Y. Gu, T. P. Lodge and C. D. Frisbie, Adv. Mater., 2012, 24, 4457–4462.
35. I. Firdous, M. Fahim, L. Wang, W. J. Li, Y. Zi and W. A. Daoud, Nano Energy, 2021, 89, 106315.
36. L. Lapčinskis, A. Linarts, K. Mālnieks, H. Kim, K. Rubenis, K. Pudzs, K. Smits, A. Kovaļovs, K. Kalniņš, A. Tamm, C. K. Jeong and A. Šutka, J. Mater. Chem. A, 2021, 9, 8984–8990.
37. X. Yin, D. Liu, L. Zhou, X. Li, C. Zhang, P. Cheng, H. Guo, W. Song, J. Wang and Z. L. Wang, ACS Nano, 2019, 13, 698–705.
38. I. Firdous, M. Fahim and W. A. Daoud, Nano Energy, 2021, 82, 105694.
39. Y. Chen, Y. Cheng, Y. Jie, X. Cao, N. Wang and Z. L. Wang, Energy Environ. Sci., 2019, 12, 2678–2684.
40. H. Wu, S. Wang, Z. Wang and Y. Zi, Nat. Commun., 2021, 12, 5470.

---

Footnotes

† Electronic supplementary information (ESI) available: TEI fabrication, schematic of ionotronic interfaces, KPFM images, electrical output of control TEI, current output@ resistive load, ESI note on spectral excitation assignment of Pan and PVA@Pan, material characterization of PVA@Pan, ESI note on in situ-EIS analysis, EIS comparison Nyquist plot, Bode modulus, and capacitance (−1/2πfZimag) graphs, biomechanical energy harvesting graphs, wireless power transmission, and TEI performance comparison tables. Other supporting materials for this manuscript include Movies S1 to S5. See DOI: https://doi.org/10.1039/d5ta01326g

‡ These authors contributed equally.

---