Self‐Assembled Conjugated Hollow Fibers for Spongy Monoliths Toward Solar‐Driven Desalination and Hydrovoltaic Electricity Generation

ABSTRACT

This study presents a molecular design strategy for a self‐assembled conjugated hollow‐fiber monolith that enables simultaneous solar‐driven desalination and hydrovoltaic electricity generation. In this bottom‐up approach, aromatic monomers undergo metal‐free condensation to form cross‐linked hollow fibers that spontaneously entangle into a spongy monolith in a single step without templates. A gradient coating of polypyrrole subsequently yields a hierarchical aerogel that facilitates efficient light absorption, directional water transport, and interfacial solar evaporation through outer channels while suppressing bulk infiltration through a hydrophobic inner core. The developed system exhibits a high evaporation rate with strong salt resistance. The desalinated seawater produced under outdoor conditions meets World Health Organization drinking‐water standards and supports rice seed germination under natural sunlight, confirming its potential for potable water production and agricultural irrigation. Solar‐induced ion migration within the hollow channels generates hydrovoltaic outputs tunable by electrolyte salinity and device configuration, thereby enabling concurrent freshwater production and energy harvesting. Overall, the proposed system constitutes an integrated single‐material platform that addresses the global water–energy nexus through sustainable, low‐cost solar purification and electricity generation.

A self‐assembled conjugated hollow fiber monolith enables simultaneous solar‐driven desalination and hydrovoltaic electricity generation. Metal‐free polymerization produces cross‐linked hollow fibers that entangle into a porous monolith, followed by gradient polypyrrole coating for efficient light absorption and directional water transport. The system achieves high evaporation and stable electricity generation, offering a sustainable, integrated water–energy platform.

1. Introduction

Global warming has intensified freshwater scarcity by altering precipitation patterns and increasing drought frequency, ultimately rendering desalination, that is, the removal of salts from saline water, a vital means of managing water resources [1]. However, as conventional thermal and membrane‐based desalination methods suffer from high energy consumption and fouling issues [2], solar‐driven interfacial evaporation (SDIE) has emerged as a more sustainable alternative, offering low energy requirements, simplified device architectures, and environmentally benign operations [3]. To enable efficient SDIE, a broad range of photothermal materials has been investigated, as solar absorption and heat generation are critical to its performance. For instance, metal nanoparticles such as Ag, Au, and Pt, which generate heat via collective free electron oscillations, i.e., localized surface plasmon resonance, and carbon allotropes such as carbon black, carbon nanotubes, and graphene, which exhibit broad light absorption and efficient heat dissipation, have been extensively studied [4, 5, 6, 7, 8, 9, 10], alongside various conductive polymers and inorganic semiconductors [11, 12, 13, 14]. In most cases, these materials are embedded in functional matrices, often foams, hydrogels, or membranes, which are indispensable for achieving stable and efficient SDIE. Specifically, these matrices are engineered to (i) provide mechanical support for the photothermal components, (ii) preserve structural stability under prolonged exposure to aqueous environments, (iii) sustain continuous water transport from the bulk reservoir to the evaporation interface, and (iv) suppress conductive heat loss to the underlying water. Consequently, these matrices function not only as passive supports but also as active components that strongly dictate the overall efficiency, durability, and performance of SDIE systems [15, 16, 17].

From a design perspective, fiber‐based matrices are particularly attractive for SDIE owing to their high porosity, interconnected channels, and capillary‐driven water transport behavior, all of which render them versatile platforms for SDIE applications [18, 19, 20]. To achieve such anisotropic features, various materials have been employed in the fabrication of fibrous matrices, including natural fibers (e.g., cellulose, cotton, chitosan, and silk), synthetic polymers (e.g., polyacrylonitrile, polyacrylamide, poly(vinyl alcohol), and polyurethane), and inorganic solids (e.g., SiO₂ and Al₂O₃) [13, 21, 22, 23, 24, 25], using methods such as spinning, blowing, and freeze‐drying [26, 27]. Among these options, conjugated porous polymer networks, known as conjugated microporous polymers (CMPs), have garnered particular attention, as they can be processed into fibers with rigid π‐conjugated networks, high surface areas, well‐defined porosity, and excellent thermal stability. Notably, their synthetic tunability, stemming from modular aromatic precursors and post‐polymerization modification strategies, allows precise molecular‐ to macro‐scale control over fiber architectures, directly influencing SDIE performance. For instance, CMP‐type fibers with solid or hollow interiors and even spherical forms can be fabricated with or without using templates, enabling the formation of large‐scale monolithic constructs or functional composites with other substrates [28, 29, 30, 31]. Beyond establishing internal mass transport pathways [32, 33, 34, 35, 36], hybridization with photoactive components or elemental doping has also been demonstrated to enhance SDIE performance [37, 38, 39, 40, 41]. However, despite the intrinsic structural advantages of the resulting materials, they have been rarely explored for SDIE so far, and their evaporation efficiencies still remain relatively modest. Given this limitation, the broad diversity of available building blocks and synthetic strategies offers considerable potential to advance the molecular design of conjugated polymer‐based fibers. In parallel, the development of macroscopic architectures or integration with appropriate photothermal components could further improve desalination performance by promoting light harvesting, continuous water transport, and interfacial stability under solar exposure. Although CMP‐based materials have been explored for SDIE, most reported systems require a template during synthesis and rarely demonstrate SDIE performance under seawater desalination conditions. Only a few CMP nanotubes decorated with photothermal materials have been reported; however, these were not self‐standing and functioned merely as photoactive composite components. Moreover, the development of template‐free hollow microfibers and the integration of SDIE capability with ion‐transport–driven hydrovoltaic effects remain largely unexplored in CMP‐based fiber systems, as summarized in Table S1.

Based on this background, this study presents a molecular design strategy for fabricating conjugated hollow fiber‐based hierarchical monoliths tailored for solar‐driven desalination and hydrovoltaic applications. In this strategy, selected aromatic monomers are subjected to condensation polymerization under metal‐free conditions to yield a nitrogen‐doped conjugated network that self‐assembles into hollow fibers. The resulting fibers autonomously intertwine to form a monolithic structure in situ through a one‐pot synthetic process. Concurrently, the fibrous monolith facilitates the formation of a gradient polypyrrole (PPy) coating that extends from the surface to the interior. This coating imparts photothermal characteristics with broadened light absorption and enhances the water wettability of the fibers. The PPy‐enriched hollow channels near the interface promote water transport and solar‐driven evaporation, while the inner hydrophobic fibers function under aqueous conditions to direct water outward, analogous to the xylem and phloem of plants, respectively. Consequently, the overall system enables effective desalination without salt accumulation, as well as the purification of dye‐contaminated wastewater under solar exposure. In particular, the desalinated seawater generated under outdoor conditions satisfies global World Health Organization (WHO) standards for potable water and is subsequently used to support rice seed germination. Furthermore, the monolith demonstrates a salt‐gradient‐driven hydrovoltaic effect under solar exposure, functioning as a hydrovoltaic electricity generator. Taken together, the scalability and processability of this system enable concurrent freshwater production and energy harvesting within a single material platform, thereby supporting integrated water–energy nexus applications.

2. Results and Discussion

Figure 1 illustrates the overall synthetic procedure for fabricating the self‐assembled hollow‐fiber spongy monolith, along with its post‐polymerization modification process. Generally, the molecular design strategy for this class of monoliths involves (i) the formation of a conjugated cross‐linked structure through metal‐free condensation between selected monomers, (ii) self‐assembly with anisotropic growth into hollow fibers during cross‐linking, and (iii) spontaneous fiber entanglement to create the monolith—all occurring in a one‐pot synthesis, as demonstrated previously [28, 29, 30, 42, 43]. In this study, to initiate this synthesis protocol, triacetyl quaterphenyl (tQP) and terephthalaldehyde (TPA) were subjected to a Chichibabin‐type cyclo‐condensation reaction under acidic conditions at 120 °C, at a total monomer concentration of 0.24 m in the presence of ammonium acetate, yielding a fully conjugated pyridinic network. Owing to skeletal rigidity, favorable bond angles, interchain interactions, and interfacial reactivity, the network spontaneously assembled into elongated hollow fibers while undergoing covalent cross‐linking. The sequential entanglement of these hollow fibers then created the hollow‐fiber monolith (HF). In particular, the fibers created internal channels within the HF structure and contributed to its low density (ρ, 0.87 g cm⁻³), allowing a cylindrical HF sample (diameter, 2.4 cm; height, 2 cm) to rest delicately atop a cherry blossom (Figure 1a). The resulting HF was subsequently functionalized with PPy through immersion in a pyrrole solution followed by in situ oxidative polymerization to yield a PPy‐coated conductive monolith (HF‐PPy) (Figure 1b). In particular, the hollow and compact arrangement of component fibers led to gradient PPy encapsulation in HF‐PPy, with the PPy concentration progressively increasing from the interior toward the exterior. Consequently, the outer surface became enriched with PPy and appeared dark, as its greater accessibility to pyrrole and the catalyst promoted pyrrole–Fe³⁺ complexation and subsequent PPy formation during polymerization. In the middle layer, where accessibility was limited, the lower pyrrole concentration yielded a PPy coating with visible flaking, resulting in a lighter coloration. In contrast, the innermost region, where pyrrole diffusion was most constrained, remained free of PPy, retaining the original smooth yellow fibers.

FIGURE 1.

Schematic illustration of the design strategy for the functional monolithic aerogel and its application in solar‐driven evaporation: (a) synthetic route for the self‐assembled conjugated HF structure derived from tQP and TPA under metal‐free conditions; (b) sequential functionalization with PPy, yielding the target aerogel (HF‐PPy) with a gradient PPy distribution across the monolith (right); (c) solar‐driven water evaporation performance of HF‐PPy, originating from the synergistic integration of internal channels and PPy coating layers.

Overall, the hierarchical PPy coating layers not only altered the surface properties of the entire aerogel but also imparted light absorption and photothermal effects, thereby enabling its use in a solar‐driven evaporation system (Figure 1c). In particular, the PPy‐enriched outer surface primarily induced solar heating while simultaneously providing amphiphilic channels for efficient water transport. Meanwhile, the hydrophobic inner region suppressed bulk water infiltration and redirected water outward. Consequently, solar‐driven evaporation occurred at the aerogel interface under localized heating, mimicking the structural configuration of plant xylem.

An overview of the HF‐PPy structure derived from the cylindrical HF is presented in Figure 2a. As depicted, the monolith turned black upon PPy incorporation without a noticeable change in overall volume (Figure 2a, left). The gradient distribution of PPy was visually confirmed in the aerogel cross‐section, which exhibited a distinct color variation from the surface to the core (Figure 2a, right). Specifically, the outermost layer (∼2 mm depth) appeared black, followed by a ∼5 mm khaki middle layer with progressively reduced PPy content, and finally a ∼5 mm yellow region at the core. This color gradient was further quantified along a straight transverse line from the surface to the core using the CMYK color model (Figure 2b). Notably, the K value, indicative of black coloration, was highest at 98 in the outer layer and progressively declined to 17 at the core, while the Y value (indicating yellow coloration) increased from 8 to 40, reflecting the surface‐enriched PPy distribution.

FIGURE 2.

Structural visualization and characterization of HF‐PPy. (a) Photograph of (left) the aerogel monolith and (right) cross‐sectional view showing three distinct regions: outer layer (2 mm from the surface), middle layer (5 mm), and inner region (5 mm). (b) Profiles of the Y and K values from the CMYK color model along a linear transverse path from the surface to the core. (c) Change in Raman intensity measured at 2120 cm⁻¹ along a transverse line from the surface to the core of HF‐Ppy, as described in the inset. (d–f) SEM images and (g–i) TEM images of fibers from each layer, along with corresponding densities: the hollow fibers in (d,g) the inner region, (e,h) the middle layer, and (f,i) the outer layer.

Raman spectra were collected from the cross‐sectional surface of cylindrical HF‐PPy to quantitatively analyze PPy formation (Figure 2c). Owing to the conjugated backbone of HF, strong fluorescence was observed, resulting in a broad intensity distribution across the entire Raman shift range (Figure S1a). At the surface of HF‐PPy, fluorescence intensity was significantly suppressed due to PPy, and no distinct Raman peaks were observed (Figure S1b). With the PPy concentration gradually decreasing from the surface toward the core, the fluorescence intensity correspondingly increased, reaching its maximum in the inner core region. Change in the normalized Raman intensity at 2120 cm⁻¹ allowed quantitative distinction of each layer, revealing an outer layer (∼2 mm), a middle layer (∼5 mm), and an inner core (∼5 mm).

The overall fiber morphology of each layer was examined using scanning electron microscopy (SEM). In the hydrophobic yellow inner core, fibers exhibited smooth surfaces, with an average diameter of 0.6 ± 0.1 µm (Figure 2d). The inset of Figure 2d clearly displays the hollow interiors of the fibers, as visualized using a cooling cross‐section polisher, with a wall thickness of 160 ± 26 nm. Upon PPy coating, the fiber diameter increased. Specifically, the average diameters reached 1.1 ± 0.1 µm in the middle layer (Figure 2e) and 1.3 ± 0.2 µm in the outer layer (Figure 2f); both these layers also exhibited submicron flakes and particles. This morphological evolution was further validated by transmission electron microscopy (TEM), which confirmed the persistence of the hollow fiber structure (Figure 2g–i). Additional low‐magnification SEM images are shown in Figure S2. Specifically, the lumen diameter of the hollow fibers located in the inner core was 0.26 ± 0.02 µm, comparable to those in the middle and outer layers, while the wall thickness increased by approximately 1.5 fold following PPy incorporation. These findings confirm that the hollow structure was preserved during PPy surface coating, without clogging in the HF‐PPy structure. Following this morphological analysis, the density of each layer was determined using the buoyancy method and is presented alongside the TEM images. The inner core exhibited the lowest density (ρ) of 0.87 g cm⁻³, which increased to 0.98 g cm⁻³ in the middle layer and 1.08 g cm⁻³ in the outer layer, owing to PPy incorporation (overall ρ, 0.99 g cm⁻³).

The elemental composition of internal regions in the HF‐PPy structure was analyzed using X‐ray photoelectron spectroscopy (XPS) (Figure 3a). In the C 1s spectrum of the inner core, two deconvoluted peaks appeared at 283.6 and 284.3 eV, corresponding to sp² carbon in benzene and pyridine rings, respectively [44, 45]. Notably, the latter peak broadened with increasing PPy content, as evidenced by an increase in its full width at half maximum from 0.96 in the inner core to 1.68 in the outer layer. A comparable trend was observed in the N 1s spectra: a single peak for pyridinic nitrogen was detected at 399.3 eV in the inner core, while an additional peak at 400.6 eV, attributed to pyrrolic nitrogen, emerged following PPy incorporation, with its intensity increasing with rising PPy concentration. In parallel, the N/C atomic ratio increased from 0.12 in the inner core to 0.17 in the middle layer and 0.23 in the outer layer (top, Figure 3b). This trend was further supported by energy‐dispersive X‐ray spectroscopy (EDS), which confirmed that the N/C ratio at the outer coating surface was nearly twice that in the inner core. Additionally, elemental mapping revealed the presence of both N and C signals in individual HF‐PPy fibers (bottom, Figure 3b), whereas only C signals were detected in HF (Figure S3). Taken together with the observed weight change during PPy coating, these findings indicate that the PPy content in HF‐PPy reached approximately 44 wt%.

FIGURE 3.

(a) XPS spectra of C 1s (top) and N 1s (bottom) electrons collected from each region of HF‐PPy. (b) (top) N/C atomic ratios in each region estimated by XPS and EDS analyses and (bottom) corresponding SEM image obtained in secondary electron mode (left) and EDS elemental maps of carbon (middle) and nitrogen (right). (c) XRD patterns of all HF‐PPy regions with assigned Miller indices; the inset presents an expanded view at high 2θ angles. (d,e) Photographs of (d) water and (e) n‐hexane droplets deposited on each region of HF‐PPy.

The crystalline structure of HF‐PPy was characterized using X‐ray diffraction (XRD) (Figure 3c). All examined regions of HF‐PPy exhibited characteristic peaks at 2θ = 5.1°, 9.1°, 10.4°, 13.7°, and 15.6°, corresponding to the Miller indices of (100), (110), (200), (210), and (300) planes, respectively, in agreement with a hexagonal packing unit cell (lattice parameter: 19.9 Å). Additionally, a broad reflection near 24°, attributed to π–π stacking between conjugated backbones, appeared in all regions, confirming that the crystalline ordering of the self‐assembled fibers was preserved after PPy coating (inset, Figure 3c).

Beyond structural integrity, the surface wettability was notably altered following PPy incorporation (Figure 3d). The hydrophobic inner core without PPy exhibited a water contact angle exceeding 140°, whereas the PPy‐containing middle and outer layers completely absorbed water droplets. Furthermore, all samples exhibited rapid absorption of n‐hexane droplets (Figure 3e). Collectively, these findings indicate that PPy embedment imparted amphiphilic character to the fibers, thereby enabling their function as water transport channels within HF‐PPy during solar evaporation.

The thermal stability of HF‐PPy was evaluated using thermogravimetric analysis (Figure S4). Owing to their crosslinked conjugated architectures, the inner core and middle layer exhibited high thermal stability, with 10% weight loss (T _d10) near 400 °C and a char yield of approximately 70% at 800 °C under a nitrogen atmosphere. In contrast, the outer layer containing a higher PPy loading displayed a lower T _d10 of 240 °C and a reduced char yield of 55%, reflecting the intrinsically low thermal stability of PPy [46].

From a holistic perspective, the specific surface areas and pore‐size distributions of HF and HF‐PPy were analyzed using nitrogen adsorption–desorption isotherms (Figure 4a). Both samples exhibited Type II Langmuir isotherms, indicative of multilayer adsorption within macropores [47]. However, the specific surface area of HF (13.4 m² g⁻¹, determined using the Brunauer–Emmett–Teller method) decreased to 6.8  m² g⁻¹ after PPy incorporation, nearly half the original value. Correspondingly, some micro‐ and mesopores evident in HF, which were located on the surface of individual fibers, vanished upon PPy coating, as inferred from nonlocal density functional theory (NLDFT) (inset, Figure 4a).

FIGURE 4.

(a) Nitrogen adsorption–desorption isotherms of HF (orange) and HF‐PPy (purple) recorded at 77 K, where closed and open circles represent adsorption and desorption, respectively; the inset presents the corresponding pore size distribution calculated using NLDFT. (b) Compressive stress–strain curves of HF (orange) and HF‐PPy (purple) recorded during loading–unloading cycles. (c) Fluorescence spectra of HF (orange) and HF‐PPy (purple) under 365 nm excitation. (d) Variation in the electrical resistance of HF (orange) and HF‐PPy (purple) as a function of density under applied loads ranging from 200 to 2000 kgf. (e) Time‐dependent absorption behavior of HF‐PPy, and (f) comparison of absorption capacities of HF‐PPy (purple) and HF (orange) as a function of solvent density.

The pore structure of both monoliths was further investigated by mercury intrusion porosimetry (Figure S5). Upon PPy incorporation, the total intrusion volume decreased from 19.74 mL g⁻¹ (HF) to 10.83 mL g⁻¹ (HF‐PPy). Meanwhile, the volume‐based median pore diameter increased from 41 to 98 µm, with a notable reduction of meso‐ and macropore contributions. These results support preferential filling of the internal channels or voids during PPy polymerization, leading to a shift of the accessible pore volume toward larger pores.

Changes in physicochemical properties of both aerogel monoliths were systematically investigated. Uniaxial compressive testing revealed that the elastic modulus of HF was 1.17 ± 0.02 MPa, which increased to 2.4 ± 0.02 MPa in HF‐PPy owing to fiber reinforcement by the PPy coating (Figure 4b). The light absorption characteristics of HF and HF‐PPy, arising from their fully π‐conjugated backbones, were assessed using UV–vis spectroscopy (Figure S6). Under 350 nm excitation, HF exhibited green fluorescence emission at 430 nm; however, the emission intensity was markedly quenched in HF‐PPy under identical conditions (Figure 4c), likely owing to unfavorable energy transfer between the conjugated backbone and embedded PPy [48, 49, 50]. Beyond optical effects, the synergistic combination of the spongy structure and conductive PPy network endowed HF‐PPy with strain‐responsive electrical conductivity (Figure 4d). Consequently, upon compression, the resistance of cylindrical HF‐PPy progressively decreased, attributable to the formation of internal electrical pathways through the PPy coating as the inter‐fiber spacing narrowed under applied stress. This behavior was absent in bare HF. At 74% compressive strain (ρ, 1.49 g cm⁻³), the resistance reached 2.2 Ω, corresponding to an electrical conductivity of 0.7 S cm⁻¹, comparable to values reported for conductive polymers such as poly(3,4‐ethylenedioxythiophene) and poly(3‐hexylthiophene). In the uncompressed state, however, HF‐PPy remained non‐conductive, regardless of the presence of PPy [51, 52].

The designed HF‐PPy structure is a spongy aerogel composed of amphiphilic hollow fibers and is therefore capable of offering efficient mass transport and high absorption capacity for a wide range of liquid absorbates. Accordingly, HF‐PPy rapidly absorbed low‐viscosity solvents, including water (0.3–1.1 mPa·s), diesel (5.0 mPa·s), and silicone oil (9.7 mPa·s), upon immersion at room temperature, reaching equilibrium within 10 s (Figure 4e). The equilibrated absorption capacity of HF‐PPy was proportional to the density of the absorbates, indicating that liquid uptake occurred efficiently both within the fibers and in the inter‐fiber voids (Figure 4f). In contrast, while HF displayed a comparable absorption profile to HF‐PPy, it did not absorb water owing to its intrinsic hydrophobicity. These results underscore the functional role of gradient PPy coatings and the potential of HF‐PPy for application in solar desalination and hydrovoltaics, as discussed below.

HF‐PPy exhibited a high optical absorption efficiency of 96% across a broad wavelength range over 280–2500 nm (Figure 5a), which was determined from absorptance, reflectance, and transmittance, as detailed in Figure S7. This high efficiency could be attributed to the strong broadband light absorption induced by the PPy coating, in contrast to the far lower value for HF (20.7%). Upon exposure to simulated solar irradiation at 1.0 kW m⁻² using a xenon lamp, its surface temperature rapidly increased to 152°C within 5 min, as recorded by a thermal infrared (IR) camera (Figure S8). To further evaluate its photothermal characteristics, alternating “on–off” solar irradiation cycles (5 min each) were applied, inducing a stable and reversible photothermal response that enabled light‐driven thermal modulation (Figure 5b). The inset of this figure presents IR images capturing time‐dependent surface temperature changes during the “on” state. In the context of solar desalination, effective light absorption can be paired with efficient interfacial evaporation, as the latter directly governs freshwater production rates. To assess this, HF‐PPy was placed on an insulated seawater reservoir to ensure continuous water uptake and was subjected to varying irradiation intensities. Consequently, the mass loss of seawater increased steadily over time and was further enhanced as the irradiation intensity rose from 0.3 to 1.0 kW m⁻² (Figure 5c). The evaporation rate was further evaluated under increasing light intensities, showing a monotonic rise and reaching a maximum of 3.56 kg m⁻² h⁻¹ at 1.0 kW m⁻² (Figure S9a). Despite the elevated surface temperature, HF‐PPy achieved a high solar‐to‐vapor energy conversion efficiency of 94.6%, estimated by Equation 1. This efficiency was determined using the equivalent evaporation enthalpy of water in the HF‐PPy network, as measured by differential scanning calorimetry (Figure S10). Such high performance indicates efficient utilization of the absorbed solar energy for interfacial evaporation. To further elucidate the energy distribution, the heat loss mechanisms were quantitatively analyzed (Figure S11). The radiative, convective, and conductive heat losses were calculated to be 4.7%, 2.64%, and 5.6%, respectively [53, 54]. Notably, the combined contribution of the evaporation efficiency and heat losses may appear to slightly exceed 100%. This is attributed to the 3D hollow‐fiber architecture of HF‐PPy, which enables auxiliary energy harvesting from the ambient environment. When the active evaporation cools the surface, the 3D structure facilitates thermal energy transfer from the surrounding air to the evaporation interface, compensating for a portion of the heat losses [55, 56]. The exceptional performance of HF‐PPy is further supported by the synergistic effects of enhanced photothermal conversion and its hierarchical design. As confirmed by integrating‐sphere‐assisted UV–vis–NIR measurements, the PPy coating imparts strong broadband solar absorption. Simultaneously, the fibrous network provides a large interfacial area and continuous capillary‐driven water supply, while the aerogel‐type architecture effectively localizes thermal energy at the interface, thereby maximizing solar energy conversion for efficient vapor generation. Furthermore, HF‐PPy sustained a high evaporation rate even in concentrated saline solutions (1–11 wt% NaCl) (Figure S9b). No surface salt crystallization was observed, thereby avoiding blockage of solar absorption during evaporation. Long‐term solar desalination tests were further conducted under highly saline conditions (20 wt% NaCl) for 9 days to assess the durability of HF‐PPy. During the extended operation, the evaporation rate remained stable without noticeable degradation (Figure S12a), while no apparent salt crystallization or surface blockage was observed (Figure S12b), confirming the sustained photothermal performance and structural robustness of HF‐PPy under harsh desalination conditions. Compared with recent solar evaporators, HF‐PPy exhibited superior evaporation rates and solar‐to‐vapor conversion efficiency (Figure 5d), positioning it among the most promising photothermal materials for solar‐driven desalination [53, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68].

FIGURE 5.

(a) UV–vis–near‐infrared absorption spectrum of HF‐PPy (gray) compared with the solar irradiation spectrum (red). (b) Temperature profile of HF‐PPy during five consecutive on–off solar irradiation cycles at 1.0 kW m⁻²; the inset displays thermal infrared images of HF‐PPy under illumination. (c) Mass change of seawater under solar irradiation (0.3, 0.6, and 1.0‐kW m⁻²). (d) Evaporation rate and photothermal conversion efficiency of HF‐PPy compared with reported values under 1.0 kW m⁻² irradiation. (e) Change in the concentrations of four major ions in natural seawater before and after solar desalination; the dotted lines indicate the WHO drinking water standards for Na⁺ ion. (f) UV–vis spectral change of model wastewaters containing organic dyes before and after purification by solar evaporation, with corresponding photographs shown in the inset.

To further confirm its desalination capability, HF‐PPy was used to treat natural seawater collected from the Korean coast, and the concentrations of four major ions (Na⁺, K⁺, Ca²⁺, and Mg²⁺) were analyzed before and after solar evaporation at 1.0 kW m⁻² for 6 h. Figure 5e reveals that the ion concentrations were markedly reduced after purification, dropping below the WHO‐specified limits for safe drinking water, thereby verifying the practical efficiency of HF‐PPy in seawater desalination. Furthermore, the purification performance of HF‐PPy was examined using model wastewater samples containing methyl orange (MO) and methylene blue (MB) at various concentrations (50–200 ppm). It exhibited efficient purification as determined by UV–vis spectrometry (Figure S13). For instance, the characteristic absorption peaks of both dyes (50 ppm) almost disappeared under identical conditions (Figure 5f), accompanied by a distinct visual transition from colored to transparent, as illustrated in the inset. These findings demonstrate that the HF‐PPy‐based system is not limited to salt rejection but is also effective in removing organic contaminants, thereby extending its applicability to a broader range of wastewater purification processes. Accordingly, HF‐PPy is anticipated to serve as a viable platform for solar‐driven production of potable water, as discussed below.

To facilitate the scalable industrial deployment of HF‐PPy, evaluating its desalination performance under conditions that closely replicate real‐world environments is essential. Accordingly, the freshwater generation performance of HF‐PPy was evaluated under natural sunlight exposure using a custom‐built setup, operated from 9:00 a.m. to 6:00 p.m. on five consecutive days (March 24 to March 28, 2026) on the rooftop of Jeonbuk National University in Jeonju, Korea (35.8469 °N, 127.1316 °E) (Figure 6a). Throughout this experiment, ambient temperature, relative humidity, wind speed, and solar irradiation intensity were recorded, along with the corresponding evaporation rate of HF‐PPy (Figure 6b; Figure S14). The outdoor evaporation behavior of HF‐PPy was governed by temporal variation in solar irradiation intensity, indicating that solar evaporation under natural sunlight follows the same mechanism demonstrated under laboratory conditions. Despite fluctuations in ambient temperature, relative humidity, and wind speed throughout the day, the evaporation rate exhibited a positive correlation with irradiation intensity, reaching its maximum near solar noon. This observation suggests that the photothermal conversion efficiency and interfacial heat localization characteristics of HF‐PPy are preserved under realistic outdoor environments. Moreover, the stable evaporation performance observed during repeated daily operation over multiple days highlights the robustness of the HF‐PPy architecture against environmental perturbations, supporting its feasibility for practical outdoor desalination applications. To explore the potential of the system beyond potable water production, the agricultural applicability of the purified seawater, collected directly from the outdoor desalination setup, was examined through rice seed germination tests. Rice was selected here as a model crop owing to its global importance as a staple food and its high sensitivity to water quality and salinity, rendering it a reliable product for assessing irrigation suitability [55]. As illustrated in Figure 6c, rice seeds irrigated with untreated seawater failed to germinate, whereas those irrigated with either tap water or HF‐PPy‐purified seawater germinated successfully. Furthermore, seedlings irrigated with the purified seawater exhibited comparable growth rates and final heights to those watered with tap water (Figure 6d). Together, these results demonstrate that the HF‐PPy‐based system not only enables efficient freshwater production under outdoor conditions but also delivers water suitable for sustainable agriculture.

FIGURE 6.

(a) Representative photograph of the outdoor solar desalination system employing HF‐PPy, showing the overall setup including the dome structure and the surrounding white insulation layer used to minimize non‐photothermal evaporation. (b) Temporal profiles of ambient temperature, solar irradiation intensity, relative humidity, wind speed, and evaporation rate during the outdoor desalination test. (c) Rice seed germination after irrigation with (left) untreated seawater, (middle) tap water, and (right) HF‐PPy‐purified seawater. (d) Growth height of rice seedlings over time under irrigation with the three water sources.

To expand its functionality beyond water purification, the potential of HF‐PPy as a hydrovoltaic electricity generator under solar irradiation was further examined (Figure 7a). Upon solar irradiation at 1.0 kW m⁻², HF‐PPy generated a measurable voltage output proportional to NaCl concentration, confirming the presence of a salt‐gradient‐driven hydrovoltaic effect (Figure 7b). Specifically, a single HF‐PPy cell generated an open‐circuit voltage of approximately 84 mV in deionized water, which increased to approximately 154 mV in 17 wt% NaCl solution. This dependence on electrolyte salinity suggests that (i) ionic conductivity and charge separation at the solid–liquid interface jointly contributed to the hydrovoltaic effect in HF‐PPy and (ii) elevated salt concentrations enriched the reservoir of mobile ions, thereby enhancing ion migration during solar‐driven evaporation. This enhancement could arise from ion‐selective transport occurring through the hollow fibers of HF‐PPy; these internal channels restricted the movement of larger Cl⁻ ions due to steric effects and the presence of PPy, while the transport of smaller Na⁺ ions along with water flow established a stronger ionic gradient, thereby amplifying the output voltage [69].

FIGURE 7.

(a) Schematic illustration (left) and photograph (right) of the hydrovoltaic electricity generation system based on HF‐PPy. (b) Voltage output as a function of NaCl concentration under solar irradiation. (c) Voltage stability over 12 h of continuous irradiation in a 17 wt% NaCl solution; the inset shows the absence of salt deposition on HF‐PPy after prolonged operation. (d, e) (d) Voltage output as a function of the number of HF‐PPy cells connected in series and (e) current output with cells connected in parallel. (f) Comparison of voltage and current generation in various electrolyte solutions at 17 wt% and in dye‐containing wastewaters without added salt.

To provide direct evidence for this ion‐selective migration, the surface charge characteristics of HF‐PPy were investigated via zeta potential measurements. HF‐PPy exhibited negative zeta potentials over a broad pH range (pH 5–9), indicating a negatively charged surface primarily arising from the deprotonation of nitrogen atoms within the polypyrrole (Figure S15) [70]. Notably, under seawater‐relevant conditions (pH ≈ 8), HF‐PPy maintained a stable negative surface potential, which favors the selective migration of cations while suppressing anion transport at the solid–liquid interface. These results provide direct evidence that the observed hydrovoltaic output originates from concentration polarization governed by the HF‐PPy surface, preventing salt accumulation during evaporation. Furthermore, HF‐PPy maintained stable performance during extended operation in highly saline environments. Further, the output voltage of approximately 154 mV was stably sustained for up to 12 h in 17 wt% NaCl solution, with suppressing salt deposition observed on the HF‐PPy surface (Figure 7c). This salt‐resistant behavior is especially advantageous for practical applications, as conventional photothermal materials typically experience rapid salt accumulation and performance degradation under prolonged saline exposure.

To assess scalability, the hydrovoltaic system was tested by integrating multiple HF‐PPy cells. When these cells were connected in series, the output voltage increased proportionally; conversely, parallel connection enhanced the current output (Figure 7d,e). Five HF‐PPy cells connected in series generated an output voltage of approximately 764.2 mV, whereas the parallel connection of five cells yielded a current output of approximately 232.51 µA. This modular system could also be fine‐tuned by adjusting the number, connection mode, or geometry of HF‐PPy cells, enabling adaptation to varying power requirements. To further evaluate the long‐term operational stability of the scaled hydrovoltaic system, extended multi‐cycle tests were conducted using the series‐ and parallel‐connected HF‐PPy configurations. Specifically, five HF‐PPy units connected in series or in parallel were continuously operated for three consecutive cycles of 12 h under saline evaporation conditions. As shown in Figure S16, both configurations maintained stable voltage and current outputs throughout repeated operation cycles without noticeable performance degradation, indicating robust electrical durability even after system‐level integration. Of note, no output fluctuation associated with ion depletion or interfacial instability was observed during the prolonged operation. Moreover, surface inspections of all HF‐PPy units after the multi‐cycle tests revealed no discernible salt crystallization or structural damage, confirming that the hollow‐fiber architecture effectively suppresses salt accumulation and preserves ion‐transport pathways during the hydrovoltaic operation. These results demonstrate that HF‐PPy enables not only scalable power output through modular integration but also reliable long‐term operation, which is essential for practical hydrovoltaic energy harvesting under saline environments.

Moreover, HF‐PPy consistently generated electricity in various electrolyte solutions at 17 wt%, unlike in pure water. Notably, smaller cations (Li⁺ < Na⁺ < K⁺) produced higher voltage and current outputs under identical conditions, likely owing to enhanced charge separation and ionic conductivity within the internal channels (left, Figure 7f) [71, 72]. In wastewater containing 50 ppm of MB or MO, HF‐PPy exhibited only marginal electrical output in the absence of salts; however, the performance remained comparable to that in pure water (right, Figure 7f). These findings underscore the versatility of HF‐PPy, suggesting that it can function not only in saline water but also in unconventional media such as industrial wastewater or during desalination. From a broader perspective, the combined freshwater production and renewable power generation capabilities of HF‐PPy position it as a promising single‐material platform for addressing pressing global issues such as water scarcity and sustainable energy supply.

3. Conclusion

In summary, we developed a metal‐free molecular design strategy to construct conjugated hollow fibers that spontaneously assemble into a spongy monolith and to subsequently form a gradient PPy coating, thereby producing a hierarchical hollow‐fiber aerogel (HF‐PPy) capable of simultaneous solar‐driven desalination and hydrovoltaic electricity generation. This unique architecture, featuring PPy‐enriched outer channels that enhance light absorption and water transport alongside a hydrophobic inner core that prevents bulk infiltration, enables rapid interfacial solar evaporation (3.56 kg m⁻² h⁻¹ at 1.0 kW m⁻²), resists salt accumulation, and purifies wastewater. The HF‐PPy system effectively purifies natural seawater to meet WHO‐specified drinking‐water standards and supports rice seed germination, demonstrating its practical feasibility for both potable water production and agricultural irrigation under natural sunlight. Concurrently, solar‐induced ionic gradients across the hollow channels generate hydrovoltaic outputs that are tunable by electrolyte salinity and cell configuration, providing a proof‐of‐concept for coupling freshwater production with renewable electricity generation in a single material platform. While this study validates the fundamental synergy of the HF‐PPy system, further engineering and optimization are required to advance this platform toward practical water–energy applications. In particular, systematic investigation of long‐term structural durability and scalable fabrication strategies would facilitate the translation of this design concept into large‐scale real‐world applications.

4. Experimental Section

4.1. Synthesis of HF

Terephthalaldehyde (96 mg, 0.72 mmol, 1.0 equiv) and tQP (619 mg, 1.43 mmol, 2.0 equiv) were dissolved in acetic acid (7 mL), followed by the addition of ammonium acetate (1.7 g, 21.5 mmol, 30 equiv) dissolved in acetic acid (2 mL). The reaction mixture was stored at 120 °C for 24 h. After that, the resulting monolithic product was removed from the vessel and sequentially washed with MeOH, DCM, and acetone. The monolithic product was immersed in a 1:1 MeOH–DCM solution (v/v, 100 mL) under gentle stirring for 24 h for purification. After drying under vacuum at 25°C, a light yellow cylindrical solid was obtained. Yield: 73%.

4.2. Synthesis of HF‐PPy

HF monolith (550 mg) was immersed in a 1:1 pyrrole–ethanol solution (v/v, 10 mL) at 4 °C for 5 min. The pre‐soaked sample was then transferred into a solution containing iron(III) chloride (1.2 g, 7.4 mmol) and 0.3 M HCl (0.14 mL) in 1:1 water–ethanol (v/v, 30 mL) and maintained at 4 °C for 24 h. After the reaction, the obtained monolith was sequentially washed with water and ethanol, followed by vacuum drying to afford the desired black monolithic product (980 mg; quantitative yield).

4.3. Solar Vapor Generation, Seawater Desalination, and Water Purification Experiments

Solar‐driven water evaporation experiments were conducted under illumination from a xenon lamp (DY ZL‐1000, DY Tech, Republic of Korea). The mass change of the water was continuously monitored using an electronic balance connected to a computer. Control experiments in the absence of light were performed to determine the dark evaporation rate. The energy conversion efficiency (η _e ) was calculated according to the following equation:

$$Energyconversionefficency\left({\eta }_e,\%\right)=m{h}_{LV}/P_{in}$$ (1)

where m is the net evaporation rate, h_LV represents the vaporization enthalpy of the water inside the HF‐PPy, and P_in is the solar energy input (1 KW m⁻²).

4.4. Hydroelectric Generation Experiments

The evaluation of the hydroelectric performance of the hydrogel was carried out using aluminum foil electrodes (10 mm in width and 100 mm in length). These electrodes were placed on the bottom and top surfaces of a cubic container (40 mm × 40 mm × 40 mm) to ensure direct contact with the HF‐PPy inside. The vertical separation between the two electrodes was fixed at 20 mm. Electrical measurements, including open‐circuit voltage and short‐circuit current, were recorded with a precision electrometer (Keithley 6514B, Keithley, USA).

Funding

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (RS‐2025‐25404226) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2020R1A5A8018367). This research was also supported by the Korea Institute of Marine Science & Technology Promotion (KIMST), funded by the Ministry of Oceans and Fisheries, Korea (2023‐00254781), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. RS‐2024‐00345266).

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll73654‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.