Electrospun PAN Membrane Loaded with AgInP2Se6 Nanosheets for Triboelectric Energy Harvesting
Filipa M. Oliveira, Jiri Sturala, Bing Wu, Latifah Alrabie, Vojtěch Kundrát, Jakub Zalesak, Ana I. S. Neves, Monica F. Craciun, Rui Gusmão, Zdeněk Sofer, Evgeniya Kovalska
TL;DR
This paper introduces a new flexible energy-harvesting membrane made by combining silver-indium-phosphorus-selenide nanosheets with a polymer, which also works as a humidity sensor.
Contribution
First demonstration of exfoliated AgInP2Se6 in a polymer matrix for triboelectric energy harvesting and humidity sensing.
Findings
The AgInP2Se6/PAN membrane achieves a peak power density of 480 mW·m−2 in triboelectric mode.
The device shows humidity-dependent electrical response and full recovery after high humidity exposure.
Exfoliated AgInP2Se6 maintains crystallinity and fiber integrity in the electrospun PAN matrix.
Abstract
To address the gap in the application of mixed‐metal phosphorus trichalcogenides (MIMIIIP2X6) in energy harvesting systems, this work investigates the incorporation of exfoliated silver‐indium‐phosphorus‐selenide (AgInP2Se6) into electrospun polyacrylonitrile (PAN) fibers for triboelectric nanogenerator (TENG) applications. This research marks the first‐time exfoliation of AgInP2Se6 (exf‐AgInP2Se6) and its integration into a polymer matrix, resulting in the first demonstration of a MIMIIIP2X6‐based polymer composite processed into a flexible nanogenerator via electrospinning. Comprehensive structural and morphological characterization confirms the successful incorporation of exfoliated AgInP2Se6 within the PAN fiber without compromising the crystallinity or fiber integrity. The exf‐AgInP2Se6/PAN membrane is tested as a TENG device, operating in contact‐separation mode, and demonstrates…
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Figure 7- —Royal Society10.13039/501100000288
- —Grantová Agentura České Republiky10.13039/501100001824
- —Czech Operational Programme Research
- —ERC‐CZ program
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Taxonomy
TopicsAdvanced Sensor and Energy Harvesting Materials · 2D Materials and Applications · Supercapacitor Materials and Fabrication
Introduction
1
Triboelectric nanogenerators (TENGs) were first reported in 2012^[^ 1 ^]^ and have emerged as a promising technology that converts mechanical energy into electrical energy from sources such as mechanical vibrations, human motion, water waves, and wind, thereby addressing the growing demand for sustainable and portable energy solutions.^[^ 2 ^]^ With the rapid expansion of the Internet of Things (IoT), wearable electronics, and sensors, there is a critical need for self‐powered systems that can operate without reliance on conventional batteries, which are limited by lifespan and environmental impact.^[^ 3 ^]^ TENGs offer a clean, renewable, and cost‐effective approach by utilizing the triboelectric effect. This phenomenon, coupled with electrostatic induction, is used to power small electronic devices and sensors.^[^ 4, 5 ^]^ Their inherent ability to harvest energy from various mechanical sources, including human biomechanics, acoustic sounds, and ambient forces, makes them highly attractive for next‐generation energy harvesting applications, extending their utility beyond traditional power generation to self‐powered sensing and human‐interface devices.^[^ 6, 7, 8 ^]^
The performance of TENGs is critically dependent on the selection and engineering of materials, which offer a broad range of triboelectric properties, flexibility, and cost‐effective processing.^[^ 9, 10, 11 ^]^ Polymers, including polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyamides, and polyvinylidene fluoride, are commonly chosen for their distinct electron affinities, which dictate their position in the triboelectric series and influence charge transfer upon contact.^[^ 12, 13 ^]^ Recent advancements have focused on significantly enhancing TENG output by incorporating nanomaterials into polymer matrices, forming composites that improve charge generation and transfer efficiency. Strategies such as physical and chemical surface and biological modifications are employed to increase surface charge density and overall energy harvesting efficiency.^[^ 11, 14 ^]^
Among the diverse strategies employed to enhance the performance of TENGs, two‐dimensional (2D) materials have emerged as promising candidates for enhancing TENG performance ^[^ 15, 16, 17, 18 ^]^ owing to their unique electrical properties, large surface area, flexibility, and optical transparency.^[^ 19 ^]^ Despite the broad variety within the family of 2D materials, most studies have predominantly focused on transition metal dichalcogenides (TMDs), such as MoS_2_ or WS_2_,^[^ 20, 21, 22 ^]^ as well as graphene.^[^ 23 ^]^ Beyond TMDs and graphene, other 2D materials, such as MXenes,^[^ 24 ^]^ black phosphorus,^[^ 25, 26 ^]^ layered metal–organic frameworks,^[^ 27, 28 ^]^ covalent organic frameworks,^[^ 29 ^]^ and h‐BN^[^ 30 ^]^ have also been explored for triboelectric applications. On the other hand, layered metal phosphorus trichalcogenides (MPX_3_),^[^ 31 ^]^ and, in particular, mixed‐metal M^I^M^III^P_2_X_6_ compounds, to the best of our knowledge, remain unexplored in this context.
M^I^M^III^P_2_X_6_ compounds, where M^I^ are mono‐ and trivalent cations respectively and X is a chalcogen, exhibit intermediate band gaps (1.3 to 3.5 eV), strong interlayer van der Waals interactions, and tunable electronic properties beneficial for charge generation and retention in TENGs.^[^ 32, 33, 34 ^]^ Within this family, silver indium phosphorous selenide (AgInP_2_Se_6_) combines structural stability and durability,^[^ 35 ^]^ ferroelectricity,^[^ 36, 37 ^]^ and high carrier mobility,^[^ 38 ^]^ making it an excellent candidate for efficient charge separation and transfer. To date, however, its potential for triboelectric energy harvesting has not been explored.
In this work, we pioneer the integration of exfoliated AgInP_2_Se_6_ (exf‐AgInP_2_Se_6_) into electrospun PAN fibers to fabricate composite membranes for TENG applications. Using liquid‐phase exfoliation (LPE) and electrospinning, we achieve homogeneous dispersion of nanosheets within PAN, combining the processability of the polymer with the unique electrical and ferroelectric properties of AgInP_2_Se_6_. Comprehensive structural and functional characterization demonstrates that the resulting devices deliver competitive electrical output, efficient humidity sensing, and stable cumulative energy harvesting. These findings establish M^I^M^III^P_2_X_6_ materials as a new platform for advancing TENG‐driven, self‐powered systems.
Results and Discussion
2
Structural and Morphological Characterization of Produced Materials
2.1
AgInP_2_Se_6_ was synthesized from the corresponding chemical elements as described in the Experimental section. To confirm the successful synthesis of this inorganic compound, X‐ray diffraction (XRD) was performed. The diffraction pattern (Figure 1A) displays sharp and well‐defined diffraction peaks corresponding to the hexagonal crystal system (space group P‐31c, No. 163). The peaks at 13.6°, 27.0°, 40.8°, and 55.5° correspond to the (002), (004), (006), and (008) planes, confirming the crystallinity and phase purity of the material.^[^ 32, 36 ^]^ The morphology and elemental composition of the bulk material were further examined using scanning electron microscopy (SEM) (Figure 1B) in combination with energy‐dispersive X‐ray spectroscopy (EDS) (Figure S1, Supporting Information). The SEM image (Figure 1B) exhibits the characteristic layered morphology typical of van der Waals solids. At the same time, EDS analysis (Figure S1, Supporting Information) confirms the presence of Ag, In, P, and Se elements, with ratios consistent with the expected stoichiometry of 1:1:2:6 (Table S1, Supporting Information).
A) XRD patterns of bulk AgInP2Se6, exf‐AgInP2Se6, and exf‐AgInP2Se6/PAN composite. B) SEM image of bulk AgInP2Se6.
After LPE, the structural integrity of exf‐AgInP_2_Se_6_ was first evaluated by XRD (Figure 1A). A digital photo of the vacuum filtered material is shown in Figure S2A (Supporting Information). The exfoliated material exhibits diffraction peaks similar to those of the bulk counterpart, indicating that the layered crystalline structure is preserved after LPE. The exf‐AgInP_2_Se_6_ sheets were further examined by scanning transmission electron microscopy (STEM) (Figure S2B,C, Supporting Information). Elemental mapping via EDS (Figure S2C, Supporting Information) confirmed the distribution of Ag, In, P, and Se elements, with stoichiometry consistent with the expected composition (Figure S2D, Table S1, Supporting Information). The lateral size of the exf‐AgInP_2_Se_6_ nanosheets was also measured, with an average estimated at 424 nm (Figure S2E, Supporting Information). Atomic force microscopy (AFM) provided height profiles of the layered material, determining sheet thicknesses ranging from ≈20 to 45 nm (Figure S3A, Supporting Information), and confirming the formation of few‐ to multi‐layered flakes. Raman spectroscopy (Figure S3B, Supporting Information) further characterizes the vibrational modes of exf‐AgInP_2_Se_6_, revealing distinct peaks at 150.2 cm^−1^ (E_1g_) and 213.5 cm^−1^ (A_1g_), both redshifted compared to the bulk material (152.9 and 216.2 cm^−1^, respectively), indicating structural and interlayer interaction changes after exfoliation. In addition, zeta potential (ζ) measurements (Figure S3C, Supporting Information) show a mean ζ‐potential of 74.7 mV, suggesting good colloidal stability of the exf‐AgInP_2_Se_6_ sheets. Transmission electron microscopy (TEM) analysis (Figure 2) further reveals the successful exfoliation into thinner, few‐layer nanosheets with a high surface area. A low‐magnification TEM image (Figure 2A) reveals large, transparent sheet‐like structures. A high‐resolution TEM (HRTEM) image (Figure 2B) displays well‐resolved lattice fringes, confirming the high crystallinity of the exfoliated material. The corresponding selected area electron diffraction (SAED) pattern (Figure 2C) shows a hexagonal symmetry, in agreement with the P‐31c space group confirmed by XRD (Figure 1A). Elemental mapping by TEM‐EDS (Figure 2D) shows a uniform distribution of Ag, In, P, and Se, further verifying the elemental composition of the exfoliated sheets.
TEM characterization of exf‐AgInP2Se6: A) TEM and B) high‐resolution TEM images of exf‐AgInP2Se6. C) corresponding SAED pattern. D) Low‐magnification TEM image and EDS elemental mapping of Ag, In, P, and Se elements across exf‐AgInP2Se6 nanosheets.
Following confirmation of the successful LPE, the exf‐AgInP_2_Se_6_ nanosheets were incorporated into a PAN polymer matrix via electrospinning to produce a composite membrane. First, XRD was performed to confirm the structural integrity of the exf‐AgInP_2_Se_6_/PAN composite membrane. The XRD pattern (Figure 1A) supports that there are no noticeable peak shifts or the appearance of new phases in the composite membrane, suggesting that the crystal structure of the inorganic filler is well maintained within the PAN polymer matrix after the electrospinning process.
The surface morphology of the electrospun membrane was then examined at different length scales. A macroscopic view of the membrane (Figure 3A) reveals a flexible, free‐standing structure with uniform coverage and a reddish‐brown color, maintaining the color of the incorporated exfoliated chalcogenide filler (Figure S2A, Supporting Information).
Characterization of exf‐AgInP2Se6/PAN: A) digital photo of the flexible membrane. B) SEM image and C,D) confocal microscopy images at different magnifications showing the fibrous morphology. E) EDS spectrum. F) TGA curves of pure PAN and exf‐AgInP2Se6/PAN measured under Ar‐O2 atmosphere.
The average membrane thickness was measured to be 34.7 ± 1.4 µm, confirming the production of a uniform fibrous composite. A high‐magnification SEM image (Figure 3B) reveals a dense interconnected fibrous network, with the elemental mapping (Figure S4A, Supporting Information) confirming the presence and homogenous distribution of Ag, In, P, and Se elements throughout the fibrous membrane. The fibers exhibit an average fiber diameter of 1.24 µm and a relatively narrow size distribution (inset, Figure S4B, Supporting Information). The corresponding scanning STEM‐high‐angle annular dark‐field (HADF) micrograph (Figure S4B, Supporting Information) highlights the embedded flakes of exf‐AgInP_2_Se_6_, clearly distinguished by contrast and confined within the fiber walls, confirming their successful incorporation into the PAN matrix. Localized aggregates of exf‐AgInP_2_Se_6_ sheet appear to cause minor fiber diameter expansion, but without compromising the overall structural integrity or leading to bead formation. The fibers remain continuous and cylindrical, indicating effective integration of the exf‐AgInP_2_Se_6_ within the polymeric fiber walls, which preserves a continuous filament with no apparent mechanical failure or fracture initiation.
To further assess the membrane's structure and fiber arrangement on larger scales, confocal microscopy was employed. The wide‐area image in Figure 3C demonstrates excellent macroscopic uniformity and compact fiber packing, and a zoomed‐in view (Figure 3D) shows randomly orientated, well‐separated fibers. The electrospun fibers appear straight and continuous, free of bead defects, indicating stable jet formation and optimized electrospinning conditions. These confocal images support the SEM observations (Figure 3B; Figure S4B, Supporting Information) and confirm the three‐dimensional fibrous structure of the composite. EDS elemental mapping (Figure 3E; Figure S4B, Supporting Information) confirms the homogeneous distribution of Ag, In, P, and Se elements throughout the composite membrane, with the stoichiometry being consistent with the theoretical and corresponding bulk and exf‐AgInP_2_Se_6_ (Table S1, Supporting Information).
To investigate the dispersion and structural integration of the exfoliated flakes within the composite fibers, cross‐sectional SEM and TEM analysis of individual exf‐AgInP_2_Se_6_/PAN fibers was performed (Figure 4). A focused ion‐beam (FIB) was used to section a selected fiber (Figure 4A) of the exf‐AgInP_2_Se_6_/PAN membrane chunk. The selected chunk was transferred to a FIB grid and thinned using the standard lamella preparative procedure (Figure 4B). The TEM analysis (Figure 4C,D) was performed in a cryo‐holder at ≈−165 °C, ensuring that the polymer matrix remained relatively stable at moderate electron doses. The polymer matrix was found to be porous, as is typical of some types of electrospun fibers.^[^ 39, 40 ^]^ The gold cover deposited on the surface of the fiber showed the strongest contrast (dark ring in Figure 4C). The arrows highlight the well‐embedded exf‐AgInP_2_Se_6_ sheets. In some regions of the analyzed sample, the 2D material chunks extended to the fiber surface. The occurrence of the embedded material was irregular. The pores in the polymer matrix did not show any coverage with the 2D material. However, it is reasonable to assume that the 2D material lumps extend into the pores. The several‐layered crystal of exf‐AgInP_2_Se_6_ was observed in detail in Figure 4D, which shows the layered structure in the cross‐section. The corresponding FFT analysis shown in the inset of Figure 4D indicates an interlayer distance of ≈0.67 nm, which is in good agreement with the interlayer spacing of 0.65 nm obtained from the XRD pattern of the exf‐AgInP_2_Se_6_‐PAN membrane. The cross‐sectional analysis provided structural insight into the exf‐AgInP_2_Se_6_‐PAN interface, revealing close attachment between the two phases.
Cross‐sectional SEM and TEM analysis of an individual AgInP2Se6‐PAN nanofiber: A) SEM image of a selected and gold‐coated nanofiber with the marked position of the FIB cut. B) SEM image of lifted up and lamella showing porous cross‐section of the nanofiber. C) TEM imaging of the cross‐section from B showing materials contrast of PAN (brighter) and exf‐AgInP2Se6 (dark plates embedded in the PAN fiber) phases. Dark ring on the nanofiber's surface is a sputtered gold coating. D) detailed TEM image of the layered plate of exf‐AgInP2Se6 phases marked by a yellow rectangle in image C. The inset in image D) displays the FFT analysis of the exf‐AgInP2Se6 with a corresponding interlayer distance of 0.68 nm.
Finally, thermogravimetric analysis (TGA) was performed under Ar‐O_2_ atmosphere to evaluate the thermal stability and filler content of the exf‐AgInP_2_Se_6_/PAN composite, in comparison to pure PAN (Figure 3F). Both samples exhibit a three‐step degradation process. For pure PAN, the first weight loss of ≈20% has an onset at 303 °C, corresponding to the elimination of residual solvent (N, N‐Dimethylformamide (DMF)), adsorbed moisture, low‐molecular‐weight oligomers, and oxidation reactions. In contrast, the exf‐AgInP_2_Se_6_/PAN membrane shows an earlier onset of weight loss at 290 °C. This shift may be attributed to interactions between PAN and the incorporated exf‐AgInP_2_Se_6_ nanosheets, as well as to the higher oxygen content (2.7 at.%, Table S1, Supporting Information) detected in the exfoliated material, which likely contributes to the presence of more hydroxyl groups and promotes earlier thermal degradation. In the second degradation event (≈300–450 °C), both materials exhibit thermal decomposition associated with the crosslinking of the PAN chains. The composite begins to decompose slightly earlier, likely due to interactions between the polymer matrix and the embedded exf‐AgInP_2_Se_6_ nanosheets. On the other hand, in the third and last stage (≈450–700 °C), the composite displays a thermal degradation shift toward higher temperature (onset temperature at 509 °C) compared to the pure PAN (onset temperature at 495 °C), suggesting improved thermal stability resulting from the synergistic interaction between the exf‐AgInP_2_Se_6_ and the polymer. At 1000 °C, the composite retains a residual mass of 16.78 wt.%, which corresponds to the inorganic exf‐AgInP_2_Se_6_. Based on this TGA residue (it is assumed that formed N_2_, CO_2_, and SeO_2_ escaped, and the rest formed stable oxides except Ag), the estimated atomic composition of the composite was calculated as 0.78% Ag, 0.78% In, 1.57% P, 4.71% Se, 69.12% C, and 23.04% N. The values for C and N are attributed to the PAN matrix, confirming the presence of both organic and inorganic compounds in the exf‐AgInP_2_Se_6_/PAN composite. These results further validate the successful incorporation and thermal stability of exf‐AgInP_2_Se_6_ within the PAN matrix, which could expand its application in areas requiring high temperatures.
These combined structural and morphological characterizations confirm the preservation of the crystalline structure of AgInP_2_Se_6_ after exfoliation and during polymer composite fabrication, as well as the successful integration and dispersion of the exf‐AgInP_2_Se_6_ nanosheets within the PAN fibrous membrane. Building on the high surface area and structural and surface characteristics of the exf‐AgInP_2_Se_6_ composite membrane, we subsequently evaluated its performance for triboelectric energy harvesting.
Fabrication and Functional Performance of Electrospun exf‐AgInP2Se6/PAN‐TENG
2.2
The exf‐AgInP_2_Se_6_/PAN‐TENG is designed as a double electrode device operating in contact‐separation (CS) mode (Figure 5A). We fabricated a 1.5 × 2.5 cm device comprising exf‐AgInP_2_Se_6_/PAN membrane and polyimide as the triboelectric layers, each attached to a copper (Cu) electrode to facilitate charge transfer during mechanical movements. The enlarged optical image of electrospun exf‐AgInP_2_Se_6_/PAN emphasizes the fibrous morphology and surface texture. Figure 5B illustrates the triboelectric series of the exf‐AgInP_2_Se_6_/PAN obtained in CS mode with various materials, including PTFE, polyimide, polyethersulfone (PES), polyimide textile, polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), MoS_2_–PDMS, graphene–textile, Cu textile, silver (Ag) textile, polyvinyl chloride (PVC), PAN, and nitrile. The devices were tested in the TENG setup^[^ 7 ^]^ using a cyclic physical stimulus with a horizontal moving stage at a frequency of 1 Hz and under ambient conditions. A detailed analysis of the voltage output is demonstrated in Figure S5 (Supporting Information), confirming that the exf‐AgInP_2_Se_6_/PAN generates the highest electrical output when paired with PTFE and polyimide, which are the most electronegative materials in the series. The triboelectric characteristics of exf‐AgInP_2_Se_6_/PAN fall between those of polyimide textile and PDMS. Despite PTFE yielding the highest TENG output, polyimide was selected for further testing due to its lower cost and smaller environmental footprint, whereas PTFE poses high production energy demands and contributes to persistent PFAS “forever chemicals” contamination in ecosystems.^[^ 41 ^]^ The TENG output is influenced by the difference in electronegativity between the two contacting materials,^[^ 2 ^]^ indicating that exf‐AgInP_2_Se_6_/PAN functions as a positively charged triboelectric layer, while polyimide serves as a strongly negatively charged counterpart. The detailed electrical output performance of Cu–polyimide | AgInP_2_Se_6_/PAN–Cu configuration is demonstrated in Figure S6A–C (Supporting Information). At the same time, Figure S6D–F (Supporting Information) illustrates corresponding measurements after 5 months of storage, demonstrating the device's relatively long‐term stability and reliable operational performance. The schematic illustration in Figure 5C shows the working principle mechanism of this double‐electrode TENG prior to materials contact, during contact, and after separation. When polyimide and exf‐AgInP_2_Se_6_/PAN undergo contact, electron transfer toward polyimide due to its higher electron affinity, resulting in opposite surface charges. During separation, a potential difference develops between two triboelectric materials, initiating charge carrier migration and inducing an electric field that drives current through an external circuit via electrostatic induction.
Exf‐AgInP2Se6/PAN‐TENG device. A) Schematic image of the TENG device, its components, and an enlarged optical image of electrospun exf‐AgInP2Se6/PAN. B) TENG series of the tested triboelectric materials and the position of exf‐AgInP2Se6/PAN within it. C) Schematic illustration of the TENG working mechanism: initial stage, contact mode, and separation mode.
To evaluate the device's functionality under mechanical stresses, we tested the exf‐AgInP_2_Se_6_/PAN‐TENG in various deformation states (Figure 6A; Figure S7A, Supporting Information), including a parallel TENG, where two electrodes and corresponding triboelectric layers were assembled on flat electrode holders, and a curved TENG, in which the device components were either bent or compressed. The triboelectric performance of the exf‐AgInP_2_Se_6_/PAN‐TENG was compared in four configurations: flat‐I, bent, compressed, and flat‐II (Figure 6B; Figure S7B–D, Supporting Information). Flat‐I indicates the initial performance of the parallel TENG in its unaltered state. Bent and compressed refer to the device's response under deformation; the insets show how the exf‐AgInP_2_Se_6_/PAN layer is positioned during these tests. Finally, flat‐II denotes the re‐evaluation of the parallel TENG after it has undergone mechanical deformation.
Effect of the exf‐AgInP2Se6/PAN‐TENG configurations on performance. A) Schematic illustration of parallel and curved TENG designs. B) Comparison of the electrical output of the TENG measured in flat (I), bent, compressed, and flat (II) configurations. The results are demonstrated in the bar charts showing open‐circuit voltage (V oc, blue), short‐circuit current (I sc, red), and open‐circuit charge (Q oc, green). “Flat (I)” refers to the initial test of the parallel TENG. “Bent” and “compressed” correspond to the measurements taken with the curved TENG; inserts demonstrate the positioning of the exf‐AgInP2Se6 layer. “Flat (II)” illustrates the retest of the parallel TENG after deformation. C) Cumulative electrical output performance (V oc and I sc) of the exf‐AgInP2Se6‐TENG over time in the parallel configuration.
The results are demonstrated in the bar charts (Figure 6B) showing open circuit voltage (*V_oc_ *, blue), short circuit current (*I_sc_ *, red), and open circuit charge (*Q_oc_ *, green). As shown, the initial flat‐I state delivers the highest electrical output among all configurations, with *V_oc_ * = 3 V, *I_sc_
- = 340 nA, and *Q_oc_
- = 15 nC. In contrast, bending the exf‐AgInP_2_Se_6_/PAN‐TENG significantly reduces performance (*V_oc_
- = 1.9 V, *I_sc_ * = 9 nA, *Q_oc_
- = 0.2 nC) due to the loss of effective contact between fibers in the electrospun structure. When the device is compressed, enhanced fiber‐to‐fiber interaction improves triboelectric coupling, resulting in a higher voltage output of 3.6 V, although current and charge generation increase only modestly (*I_sc_
- = 11 nA, *Q_oc_
- = 1.27 nC). Upon returning to the flat‐II configuration, the voltage remains stable (≈3.5 V), but the current (32 nA) and charge (1.14 nC) stay lower than in flat‐I, indicating deformation‐induced changes ^[^ 42 ^]^ in the contact interface and reduced charge transfer efficiency.^[^ 43 ^]^ This was further confirmed by Raman measurements (Figure S8A, Supporting Information), which showed a 0.70 cm^−1^ blueshift of the A_1g_ mode, corresponding to out‐of‐plane vibration. The shift indicates mechanical strain induced by bending or compression and an increased defect density resulting from irreversible membrane deformation, both of which contribute to exf‐AgInP_2_Se_6_/PAN's structural disturbance and a reduction in triboelectric performance. Additional evaluation using SEM‐EDS (Figure S8B, Supporting Information) revealed no detectable changes in the overall membrane morphology, confirming that the observed strain primarily affects the localized regions within the fiber‐to‐fiber network.
In addition to mechanical deformation, we demonstrate the long‐term energy harvesting behavior of the exf‐AgInP_2_Se_6_/PAN‐TENG. We observed that prolonged operation of the device in CS mode resulted in a cumulative energy harvesting response, showcasing a progressive increase in electrical output over time (Figure 6C). In particular, the initial voltage output, *V_oc_ *, of ≈10 V increased more than fourfold to ≈45 V after 37 min of continuous operation. Similarly, the current output, *I_sc_ *, increased from ≈0.1 to 0.3 µA, while the transferred charge amplified from ≈5 to 16 nC, as shown in Figure S9 (Supporting Information). The observed cumulative enhancement in output performance during prolonged CS cycling can be attributed to a synergistic interplay of several mechanisms within the exf‐AgInP_2_Se_6_/PAN composite membrane. First, the electrospun PAN matrix and embedded nanosheets provide abundant trap states at defect sites and interfaces, enabling progressive charge accumulation that increases the effective surface charge density over time. Second, the intrinsic ferroelectric nature of AgInP_2_Se_6_ facilitates gradual dipole alignment under repeated mechanical stimulation, enhancing charge separation efficiency and retention. Third, continuous operation promotes improved conformal contact between the fibrous membrane and the electrodes, reducing interfacial resistance and facilitating more efficient charge transfer. Finally, the insulating character of the PAN fibers helps suppress charge leakage to the environment, thereby allowing the stored charges to build cumulatively rather than dissipate. Together, these synergistic effects underpin the time‐dependent rise in *V_oc_ *, *I_sc_ *, and transferred charge, highlighting the charge accumulation capacity and dynamic optimization of the exf‐AgInP_2_Se_6_/PAN‐TENG during operation, thus making it highly promising for enduring self‐powered sensors and low‐frequency wearable electronics.^[^ 44, 45 ^]^
Atmospheric humidity, or other practical settings in environmental conditions, can be a decisive factor affecting the performance of electronic devices. Electrospun TENG‐driven technology offers a promising self‐powered solution for humidity sensing, as such devices are highly responsive to the triboelectric material's reversible interactions with water molecules, leading to measurable changes in electrical output.^[^ 46, 47, 48 ^]^ In our study, the exf‐AgInP_2_Se_6_/PAN layer acted as the sensing element and was exposed to various humidity levels ranging from 20% to 70% relative humidity (RH, Figure 7A).
TENG‐driven applications. A) Schematic illustration of the exf‐AgInP2Se6/PAN‐TENG's sensing performance evaluation in contact‐separation mode across various humidity levels. B) Maximum output voltage, V oc (green plot) and current, I sc (blue plot) measured at 20–70% humidity as a function of time. The grey selections correspond to the TENG output measured under ambient (room) humidity conditions during and after the device had been exposed to 70% humidity. C) Schematic of the power density measurement circuit. The power density is maximum when the load resistance (Zload) equals the internal impedance of the TENG (Zint). D) Energy harvesting test showing dependence of the maximum output current, Isc, and power density, Pd, of the exf‐AgInP2Se6/PAN‐TENG on the resistance of the external load (1.6 Ω–10 GΩ). E) Schematic of the capacitor charging circuit. F) Energy supply test of the exf‐AgInP2Se6/PAN‐TENG for 1 and 10 µF capacitors, showing the capacitor voltage as a function of time.
The TENG output performance was measured under a cyclic physical stimulus in CS mode at 1 Hz (Figure 7B; Figure S10, Supporting Information) across this humidity range. The highest electrical output was observed under the driest conditions (20% RH), reaching 63 V, 0.4 µA, and 23.1 nC. The output showed a nearly linear decrease as humidity increased, with performance at 70% RH reduced to 2 V, 0.03 µA, and 0.9 nC (Figure 7A; Figure S10A–C, Supporting Information). When tested at ambient room humidity (30% RH), the device produced ≈50 V, 0.3 µA, and 17.9 nC. Notably, similar performance was observed when the device was re‐measured at 30% RH after completing the high‐humidity (70% RH) tests, indicating full recovery of its output. This reversible humidity‐dependent modulation occurs from the adsorption/desorption dynamics of water molecules on the exf‐AgInP_2_Se_6_/PAN layer. The fibrous PAN morphology offers a high surface area for moisture uptake, while the 2D AgInP_2_Se_6_ provides additional active sites (e.g., surface defects, edge sites, and functional groups such as ─OH) for water interaction. These synergistic features enable rapid response/recovery of the TENG with changing RH, maintaining consistent performance without any permanent degradation.
Assessing output power provides further critical insights into the device's maximum performance capabilities. To evaluate the power density of the exf‐AgInP_2_Se_6_/PAN‐TENG, the output current (I) was measured across different external load resistances (R) from 1.6 Ω to 10 GΩ, using the circuit configuration illustrated in Figure 7C. The output power density (*P_d_ *) was calculated as P_d_ = RI^2^/A (where A = 1.5 × 2.5 cm^2^ is the area of the device), reaching a maximum *P_d_
- of 480 mW·m^−2^ at a 1 GΩ load (Figure 7D). Leveraging electrospun PAN as a less common yet effective triboelectric matrix integrated with novel 2D materials, the device achieves performance comparable to other electrospun triboelectric energy harvesters,^[^ 47 ^]^ demonstrating competitive output at higher load resistances. Finally, energy storage was demonstrated by charging 1 and 10 µF capacitors through a bridge rectifier (Figure 7E). The charging process is shown as the capacitor voltage, *V_capacitor_ *, versus time in Figure 7F. The voltage across each capacitor was monitored using a Keysight 34470A multimeter, with 1 and 10 µF capacitors reaching saturation at ≈0.8 V, ≈200 and 300 s, respectively. Such characteristics enable powering electronic devices for high‐resistance environments and for applications where low current is essential, such as sensors, wireless transmitters, and low‐power IoT systems.
Conclusion
3
In this work, we successfully fabricated a flexible triboelectric nanogenerator (TENG) by incorporating exfoliated AgInP_2_Se_6_ (exf‐AgInP_2_Se_6_) nanosheets into electrospun polyacrylonitrile (PAN) membranes, marking the first application of a M^I^M^III^P_2_X_6_‐type material in a polymer‐based triboelectric device. The exf‐AgInP_2_Se_6_ nanosheets were uniformly embedded within the PAN fibers, as confirmed by microscopy and elemental analysis, which also revealed a homogeneous distribution of Ag, In, P, and Se. XRD analysis further confirmed that the crystalline structure of AgInP_2_Se_6_ remained intact after exfoliation and electrospinning processes. TGA measurements supported the integration of the inorganic 2D compound into the polymer matrix, confirmed its elemental stoichiometry, and its thermal stability. The exf‐AgInP_2_Se_6_/PAN membrane was employed in a TENG device operating in contact‐separation mode, showing reliable mechanical durability and electrical output across flat, bent, and compressed configurations. A peak power density of 480 mW·m^−2^ was achieved at 1 GΩ, and the device demonstrated efficient capacitor charging and prolonged operation under repeated cycles. Notably, the TENG exhibited a reversible and sensitive response to humidity variations (20–70% RH), retaining its performance even after exposure to high moisture levels. These findings establish the multifunctionality and robustness of the exf‐AgInP_2_Se_6_/PAN‐based TENG, highlighting its promise for sustainable energy harvesting and self‐powered‐sensing applications.
Experimental Section
4
Chemicals
Selenium (99.999%), red phosphorus (99.999%), iodine (≥99.9%), and metals silver and indium in powder form (99.9%, <100 mesh) were obtained from STREM, Germany. Dimethylformamide (DMF) and acetonitrile (ACN) were purchased from Penta, Czech Republic. Polyacrylonitrile (PAN) was purchased from GoodFellow, England. All chemicals were used as received.
Synthesis and Exfoliation of AgInP2Se6
Elemental selenium, red phosphorus, silver, and indium were weighed in stoichiometric amounts corresponding to 10 g and placed into a quartz glass ampoule (25 × 180 mm; wall thickness 2 mm), which was then sealed under high vacuum (below 5 × 10^−3^ Pa) using a hydrogen/oxygen welding torch. An excess of 1 at. % phosphorus and selenium were used relative to the stoichiometric ratio, along with the addition of 250 mg of iodine to facilitate vapor transport. The ampoule was placed in a muffle furnace and heated at 650 °C for 240 h, with heating and cooling rates of 0.5 and 1 °C min^−1^, respectively. After synthesis, the ampoule was opened, and silver indium phosphorus selenide (AgInP_2_Se_6_) was stored in a glove box under an oxygen‐free atmosphere.
Liquid‐phase exfoliation (LPE) was employed to exfoliate AgInP_2_Se_6_. Initially, 1 g of the bulk material was subjected to shear force exfoliation for 1 h in 120 mL of DMF using an IKA T 18 digital Ultra Turrax with a stainless‐steel foot (S18–19 G) at 20 000 rpm, with the system continuously purged under an argon atmosphere. To prevent overheating and potentially boiling the solvent, water cooling was used (≈10 °C). After exfoliation, the suspension was divided into two separate phases, with the top 75% of the dispersions being collected for vacuum filtration using a nylon filter (47 mm Ø, 0.45 µm pore size, Ahlstrom‐Munksjo Germany GmbH), which contained the exfoliated AgInP_2_Se_6_ (exf‐AgInP_2_Se_6_). The exf‐AgInP_2_Se_6_ was then vacuum‐dried at 40 °C for structural and morphological characterization and the electrospinning process.
Preparation of Exf‐AgInP2Se6‐PAN Composite Membrane by Electrospinning
The exf‐AgInP_2_Se_6_‐PAN composite membrane was fabricated via electrospinning using a Bench‐Top NanoFiber Electrospinning Machine System (AME‐HZ‐12, Shandong AME Energy Co., Ltd., China). DMF was used as a solvent to dissolve PAN powder and disperse the exf‐AgInP_2_Se_6_ nanosheets, facilitating the formation of a homogeneous electrospinning solution. The homogeneous solution was prepared by dissolving PAN and exf‐AgInP_2_Se_6_, at a weight ratio of 2: 1, respectively, in 5 mL of DMF. The mixture was stirred overnight at 60 °C and 500 rpm to ensure complete dissolution and uniform dispersion. The resulting solution was loaded into a 5 mL syringe equipped with a 19G needle and fed using a syringe pump at a flow rate of 1 mL h^−1^ and speed rate of 2 mm s^−1^, with a tip‐to‐collector distance of 15 cm under an applied voltage of 17 kV. After the electrospinning, the composite membrane was carefully removed from the aluminum drum collector. No further treatments were applied, and the fiber membrane was used directly for structural and morphological characterization, as well as for evaluation of triboelectric nanogenerator (TENG) performance.
Materials Characterization: XRD Characterization
X‐ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance instrument (Bruker AXS GmbH). The instrument was configured in Bragg–Brentano parafocusing geometry and employed a CuKα radiation source with λ = 0.15 418 nm, operating at U = 30 kV and I = 10 mA. The XRD patterns were collected at room temperature, covering 2θ values from 5 to 60°. Data analysis was conducted using the HighScore Plus 4.9 software package.
Materials Characterization: Scanning Electron Microscopy
The morphology of the bulk AgInP_2_Se_6_ was observed using scanning electron microscopy (SEM) with a Tescan MAIA3 Triglav dual‐beam microscope with a field emission gun (FEG) at 5 kV acceleration voltage. An energy‐dispersive X‐ray spectroscopy (EDS) X‐MaxN 150 detector from Oxford Instruments was used for element analysis, operating at an acceleration voltage of 20 kV and controlled by AZtecEnergy software. The powder sample was drop‐cast on carbon tape and analyzed.
To investigate the morphology of exf‐AgInP_2_Se_6_, 5 µL of a 0.5 mg mL^−1^ dispersion was drop‐cast onto a Cu/C grid (TED PELLA, Inc.) and dried overnight at room temperature. Scanning electron microscopy (STEM) and EDS elemental maps were conducted at 30 kV.
The morphology of the exf‐AgInP_2_Se_6_/PAN prepared membrane was observed by SEM in LE‐BSE mode at 10 kV. EDS analysis was carried out at 20 kV. The sample was prepared by placing a small section of the membrane onto C tape. The same procedure was employed for analyzing the exf‐AgInP_2_Se_6_/PAN membrane after bending/compressing tests.
For the morphology of a single fiber of the exf‐AgInP_2_Se_6_/PAN prepared membrane, a small section of the membrane was placed on top of a Cu/C grid (TED PELLA, Inc.), and 5 µL of 0.5% Nafion in ethanol was drop cast. The grid was then dried overnight at room temperature. STEM micrographs and EDS elemental mapping were acquired under the same instrumental conditions as described for exf‐AgInP_2_Se_6_.
The morphology of the exf‐AgInP_2_Se_6_/PAN membrane was also observed using a S lynx optical profilometer microscope (Sensofar, Spain) operated by MountainsMap 7.4 version (Digital Surf, France), and a 3D light digital microscope (VHX‐7000 series, 4K high accuracy) to assess its surface morphology and structural uniformity. The thickness of the membrane was measured using a digital thickness micrometer (Mitutoyo 156‐101, Mexico).
Materials Characterization: Transmission Electron Microscopy
The exf‐AgInP_2_Se_6_ was observed by transmission electron microscopy (TEM) using a JEOL JEM‐1010 instrument at an accelerating voltage of 200 kV. Pictures were taken by a SIS MegaView III digital camera (Soft Imaging Systems) and analyzed by AnalySIS v. 2.0 software. Elemental maps were acquired using an SDD detector (X‐MaxN 80 TS) from Oxford Instruments (England). The exfoliated material was drop‐casted on a TEM grid (Cu, 200 mesh, Formvar/carbon from TED PELLA, Inc.) and dried overnight in an oven at 50 °C. High‐resolution TEM performed further observations on the interface between exf‐AgInP_2_Se_6_ and PAN from an individual nanofiber of the exf‐AgInP_2_Se_6_/PAN membrane. The membrane sample was primarily gold‐coated to improve its conductivity. Then, it proceeded to a cross‐sectional TEM lamella preparation, and a Thermo Fisher Helios 5 FX dual‐beam microscope was used. Amorphous carbon was selected for the deposition of a protective layer. The lamella was lifted out using a micromanipulator and subsequently transferred to a copper FIB grid. The thinning and polishing procedure was performed with FIB accelerating voltages ranging from 30 to 2 kV and beam currents ranging from 2 nA to 26 pA.
Materials Characterization: Thermogravimetric Analysis
Thermogravimetric analysis was carried out using a Themys TGA (SETARAM instrument) over the temperature range of 30–1000 °C and at a heating rate of 10 °C min^−1^. The instrument was purged with an argon/oxygen mixture (flow rate 64/16 mL min^−1^) for one hour before the start of measurement to stabilize the atmosphere and temperature. ≈2 mg of the sample was used for the analysis.
Materials Characterization: Raman spectroscopy
An InVia Raman microscope (Renishaw, England) was used for Raman spectroscopy measurements of bulk and exf‐AgInP_2_Se_6_ in backscattering geometry with a CCD detector. An Nd:YAG laser (532 nm, 50 mW), 10% power, and a 20× objective lens were used for the measurements at room temperature. The structure of the exf‐AgInP_2_Se_6_/PAN membrane was also analyzed by Raman spectroscopy after the mechanical deformation test. The membrane sample was bent and compressed three times each, then placed on a glass microscope slide and focused under the objective. Single‐point Raman measurements were performed at three focus positions: a random location within the membrane, on an AgInP_2_Se_6_ flake, and at a cross‐fibre junction. Each scan was carried out using a 532 nm laser excitation at 10% power with a 50× objective lens.
Materials Characterization: Atomic Force Microscopy
AFM measurements were performed using a NanoSurf Core atomic force microscope (NanoSurf AG, Switzerland) operated in tapping mode under ambient conditions. The exf‐AgInP_2_Se_6_ was dispersed in ACN and deposited onto a freshly cleaved mica sheet by spin coating. The obtained topographic images were analyzed using the Gwyddion analysis software.
Materials Characterization: Zeta Potential
The zeta potential was measured using a Zetasizer Nano system (Malvern Instruments Ltd., UK). The exf‐AgInP_2_Se_6_ was dispersed in DMF, which has a refractive index of 1.430, viscosity of 0.920 cP, and dielectric constant of 36.7. Measurements were conducted at 25 °C using a clear disposable zeta cell, with a count rate of 8.3 kcps and an attenuator setting of 8. Each reported value represents the average of 100 runs.
TENG Design and Device Characterization: Design
The exf‐AgInP_2_Se_6_/PAN‐TENG with a working area of 1.5 × 2.5 cm is designed as a double electrode TENG containing two copper (Cu) electrodes and two triboelectric layers: exf‐AgInP_2_Se_6_/PAN and polyimide film (Kapton HN thermal insulating film, 304 mm × 200 mm × 0.075 mm). The Cu electrodes were fabricated from double‐sided adhesive Cu tape (3 m 1182 conductive metallic tape, 25 mm × 16 m), which was attached to the aluminum electrode holders in the TENG setup. The exf‐AgInP_2_Se_6_/PAN layer was then attached to the Cu electrode on one side and to the polyimide film on the other.
TENG Design and Device Characterization: Triboelectric Performance
The triboelectric performance of the aforementioned TENG configuration was assessed in vertical contact‐separation (CS) mode using a controlled motion TENG setup. This setup comprises a motion controller with a linear motor driven by a voice coil actuator, connected to the aluminum moving stage. Consequently, the moving electrode was mounted on this stage, and the fixed electrode was placed on the opposite stationary side; both parts were secured to an aluminum holder on an insulating plate. The electrical output of the exf‐AgInP_2_Se_6_/PAN‐TENG was measured using a Keithley 6514 source meter, recording the open‐circuit voltage (*V_oc_ *), short‐circuit current (*I_sc_ *), and open‐circuit charge (*Q_oc_ *) under cyclic loading at a 40 N force, 1 Hz frequency, and in ambient conditions.
TENG Design and Device Characterization: Mechanical Deformation Test
To evaluate the mechanical stability and suitability for flexible electronics, the exf‐AgInP_2_Se_6_/PAN‐TENG was subjected to mechanical deformation tests. The device was tested in four configurations: an initial flat state (flat‐I), bending, compression, and a final flat state after deformation (flat‐II). For the flat‐I and flat‐II tests, the device was placed in a parallel TENG setup, with electrodes and triboelectric layers mounted on flat holders. The bending and compression tests were performed using a curved TENG set‐up. Electrical outputs for all configurations were measured utilizing a Keithley 6514 source meter and under a cyclic physical stimulus of a 40 N force at 1 Hz frequency in ambient conditions, following the methodology of the triboelectric performance assessment.
TENG Design and Device Characterization: Humidity Sensing Test
To evaluate the device's operability in different humidity conditions, a humidity sensing test was performed. For this, the device was assembled in the TENG set‐up used for triboelectric performance testing and enclosed within a custom‐made plastic glove box. A humidifier and hygrometer were placed inside the glove box to control and monitor humidity levels, respectively. Humidity was regulated by purging the plastic chamber with nitrogen gas. The TENG response was measured under cyclic physical stimulus in CS mode at 1 Hz frequency across a range of humidity levels from 20% to 70%.
TENG Design and Device Characterization: Power Density Test
The performance of the exf‐AgInP_2_Se_6_/PAN‐TENG was analyzed by varying the external load from 1.6 Ω to 10 GΩ. The output power density (*P_d_ *) of the device was evaluated by measuring the maximum short‐circuit current across different external resistances within this range. The power density (*P_d_ *) was calculated using the formula: *P_d_ = I^2^·R·A^−1^ *, where I is the current, R is the external load resistance, and A is the active area of the device.
Conflict of Interest
The authors declare no conflict of interest.
Supporting information
Supporting Information
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