Cellulose hydrogel with in-situ confined nanopores for boosting moist-electric conversion

Abstract

Hydrogels are promising for moist-electric generator, yet their performance is limited by microscale pores, low charge density, and unstable pore structures. Here, a delignified pomelo peel-confined carboxymethyl cellulose nanofluidic hydrogel is designed to address these limitations. Leveraging the hierarchical porous architecture of delignified pomelo peel, the nanofluidic hydrogel achieves sub-Debye-length nanopores with high stability and charge density. At 80% relative humidity, a single device unit exhibits an open-circuit voltage of 1.32 V and a short-circuit current density of 693.2 µA cm^-2, which are nearly triple and twenty times higher than delignified pomelo peel. The output voltage exceeds that of conventional hydrogel without nanopores by about 0.4 V. This enhanced performance is due to sub-Debye-length nanopores synergizing H⁺/Cu²⁺ gradient diffusion and Debye screening effect. Moreover, the integrated devices reach an ultrahigh output voltage exceeding 5000 V. We report the prototype of a moisture-stimulated negative air ion generator for efficient air purification. This work advances moisture energy harvesting through pore engineering and expands its applications.

Introduction

With the rising global energy demand and environmental challenges, advancing renewable energy has become critical. The emergence of hydrovoltaic technology that generates electricity through the interaction between nanostructured materials and water molecules has provided a new way for energy harvesting. Moist-electric generator (MEG) is representative that offers a sustainable energy harvesting solution by directly converting atmospheric humidity into electricity, independent of geographical or climatic constraints^1,2. The electricity generation mechanism primarily relies on coupled ion gradient diffusion and streaming potential. When water molecules are adsorbed by hygroscopic materials, hydrophilic groups dissociate and induce an asymmetric distribution of cations for the ion gradient diffusion. Simultaneously, capillary-driven water flow in nanochannels enables ion-selective transport: counterions migrate with the fluid along charged surfaces while co-ions are electrostatically repelled, generating a streaming potential at solid-liquid interfaces^3,4. This mechanism efficiently converts humidity’s chemical potential into electrical energy, with conversion efficiency governed primarily by channel dimensions and surface charge density⁵. Recent studies demonstrate that some hydrovoltaic devices based on porous materials (e.g., wood) have achieved enhanced performance through optimized nanopores and ionizable groups, which can synergistically improve water adsorption, ion dissociation, and sustained ion-selective transport^6,7,8. However, developing high-performance MEG still faces challenges in maximizing output performance and operational durability. The key to addressing these challenges lies in the rational design of hygroscopic materials.

Hydrogels, which are cross-linked polymer networks containing significant amounts of water, possess strong water retention and rapid ion transport capabilities through significant pore structures⁹. Their desirable moisture absorption properties, low cost, and pore customizability make hydrogels a promising platform for designing sustainable and high-performance MEG¹⁰. To develop high-performance MEG, the ion-selective transport serves as the cornerstone of energy conversion efficiency in hydroelectric systems, as it governs the directional migration of counterions while rigorously excluding co-ions. This process prevents charge neutralization caused by bidirectional ion diffusion, thereby sustaining stable ionic gradients and superior streaming potential outputs¹¹. Additionally, the ion diffusion driven by the moisture absorption and ionic gradients is also important contributing to the performance of MEG. Therefore, the efficient ion diffusion and ion-selective behavior driven by pore structure and surface charges are two critical factors. However, in currently reported hydrogels, the ion diffusion and ion-selective transport are severely limited owing to the absence of tailored transporting pores with uniform length¹², suitable pore size^13,14, and stability¹⁵, which are more beneficial for the diffusion of water and ions as well as the collection of charge. Additionally, the strong interaction of hydrogels with moisture is essential for enhancing MEG’s output performance¹⁶. But it is usually constrained by their pore structures (with pore radius ranging from a few to tens of microns), leading to insufficient specific surface area to contact with moisture. Although the chemical modification of polymer chains, that is polymer molecular engineering¹⁷, provides functional groups as adsorption sites for water molecules, they are still inadequate for achieving satisfactory improvements in MEG’s output performance.

Nanopore engineering represents a promising strategy. Specifically, nanopores can significantly increase the specific surface area contacting with water molecules, which can improve the carrier yield and accelerate the establishment of local ion concentration gradients, thus promoting the ion diffusion of MEG¹⁸. Furthermore, according to the Debye screening effect, when the pore size is reduced below the Debye length, an overlap of the electric double layer occurs in the confined spaces, leading to an exponential enhancement of the electric field strength generated by surface charges on the pore walls. This intensified local electric field enables surface charge density to dominate ion selectivity and precisely regulate ion transport pathways through the Stern layer, thereby achieving preferential counterion transport and complete exclusion of co-ions. Therefore, the design of nanopores smaller than the Debye length can significantly enhance selective ion transport behavior, which is crucial for streaming potential, as it facilitates effective charge separation while inhibiting counterion back-diffusion, thus maintaining the potential difference and significantly improving the streaming potential in MEG^19,20. It can be stated that the good ion gradient diffusion and streaming potential serve as the dominant contributors to achieving superior output performance of MEG. Nevertheless, the nanopore design has predominantly been confined to inorganic materials, such as carbon-based materials^{20,21,22,23,24}, silicon nanowire arrays¹², and metal oxide nanowires^25,26, which have significantly contributed to enhancing the streaming potential in some hydrovoltaic devices. It has been rarely reported in organic hydrogel materials, primarily due to several challenges. (1) Conventional approaches for creating nanopores in hydrogel substrates, including template-assisted techniques and self-assembly, often require complex chemical processes, posing challenges in achieving uniform nanopores with dimensions smaller than the Debye length^27,28. (2) The surface charge density within nanopores is often poor, greatly constraining the ion-selective transport. This restriction diminishes the efficiency of diffusion processes linked to water molecule dissociation, thereby significantly undermining the output performance of hydrogel-based MEGs⁵. (3) The dimensional stability of nanopores in hydrogels deteriorates under high-humidity conditions. The hygroscopic behavior leads to pronounced expansion of the pore size, disrupting the selective ion transport and leading to a marked reduction in the output performance of hydrogel-based MEGs.

Carboxymethyl cellulose (CMC) is a biomass-derived macromolecule obtained from natural cellulose²⁹. The three-dimensional network, ionized carboxyl groups, and exceptional hydration properties make CMC a promising feedstock for constructing hydrogels with highly charged and stable nanopores below the Debye length, which contributes to enhancing the selective ion transport behavior for developing high-performance MEG³⁰. Here, we proposed an in situ nano-confinement strategy to fabricate a delignified pomelo peel (DPP)-confined CMC nanofluidic hydrogel (PCNH). The CMC hydrogel was successfully confined within the DPP skeleton, resulting in a PCNH with uniform, highly charged, and stable nanopores smaller than the Debye length. Boosted by the Debye screening effect, the PCNH-MEG demonstrates satisfactory ion-selective transport behavior, whose open-circuit voltage (V_oc) is about 0.4 V higher than that of conventional CMCH-MEG without nanopores. After optimizing the nanopore architecture, a copper electrode-based PCNH-MEG achieved a high V_oc of 1.51 V and a short-circuit current density (J_sc) of 740.5 μA cm⁻² at 90% relative humidity. The enhanced performance is attributed to the designed nanopores, which offer a moist-electric conversion mechanism based on H⁺/Cu²⁺ gradient diffusion and the Debye screening effect. Notably, the strategy of nanopores is versatile and can be readily adapted to other hydrogel substrates. Beneficial from the moist-electric conversion mechanism coupled with additional redox reactions, the integrated PCNH-MEGs deliver output voltage exceeding 5000 V, a substantial advance over previous MEGs with active electrodes. Finally, we reported the prototype of a moisture-stimulated NAI generator driven by integrated PCNH-MEGs, realizing highly efficient air purification. This work offers an approach to developing high-performance hydrogel-based MEGs through rational nanopore engineering, with potential applications in sustainable energy harvesting and environmental remediation technologies.

Results

Design concept and performance of PCNH-MEG

To develop hydrogels featuring nanopores with high surface charge and dimensions below the Debye length, we employ sustainable raw materials including delignified PP (DPP), CMC derived from residual PP, and citric acid (CA). The specific design principles are as follows: 1) The hierarchical porous structure of DPP offers an ideal platform for the infiltration of CMC solution, enabling the construction of CMC hydrogel within the DPP. Additionally, delignification treatment can improve hydrophilicity by exposing more hydroxyl groups of cellulose, thereby promoting interfacial interactions between CMC and the cellulose framework in DPP. Meanwhile, the pore connectivity and the specific surface area of PP can also be enhanced. This effectively reduces the infiltration resistance of CMC and facilitates uniform CMC distribution within the interconnected pores of DPP. 2) The DPP features a micrometer porous structure, which not only provides a framework for in-situ hydrogel synthesis but also constrains the intrinsic swelling behavior of the hydrogel matrix, thus confining the pore structure within the nanoscale size, smaller than the Debye length³¹. 3) Highly substituted CMC is prepared from PP scraps through multiple etherification steps (Supplementary Figs. 1, 2). Beneficial from the carboxyl and hydroxyl groups, the obtained CMC (DS 1.34, Supplementary Note 1) is expected to enhance the hygroscopicity and facilitate the dissociation and diffusion of free H⁺, thus supporting selective ion transport. 4) CA not only crosslinks CMC polymer chains via ester and hydrogen bonds to form the hydrogel (Supplementary Fig. 3) but also its abundant carboxyl groups are expected to further increase the surface charge density of the hydrogels. The dual functionality synergistically improves cation-selective ion transport.

With these natural feedstocks and design principles, we propose an in-situ nano-confinement strategy which is shown in Fig. 1A and Supplementary Fig. 1. The three-dimensional porous structure of DPP is used as a confining skeleton. With the aid of sufficient permeation time and vacuum assistance, the capillary effect drives the permeation and filling of the CMC solution with the natural pores in DPP. The CMC polymer chains crosslink after subsequent immersion in a CA solution, in situ forming a CMC hydrogel matrix within each pore of the DPP. Interestingly, the micro-sized pore structure of DPP confines the volume of CMC hydrogel, thereby obtaining the confined nanopore architecture within each pore unit of DPP (as evidenced by Supplementary Figs. 4, 5). Finally, we obtained a DPP-confined CMC nanofluidic hydrogel (PCNH) with high-density negatively charged nanopores smaller than Debye length. When moisture flows through the negatively charged nanopores in PCNH, an electric double layer consisting of a stern layer and a diffusion layer enriched with free ions, forms at the solid-liquid interface between the nanopore walls and the aqueous solution. The Debye length (λ_d), defined as the thickness of the diffusion layer, serves as a critical parameter for evaluating the Debye screening effect (the typical values for λ_d are shown in Supplementary Table 1). Specifically, when the pore size is Beyond λ_d, the electric field decays exponentially to negligible levels, whereas within λ_d, the surface charge-induced local electric fields dominate ion transport (Debye screening effect). This effect generates ultrahigh ion selectivity, enabling selective counterion permeation. Therefore, the sub-Debye-length nanopores in PCNH can induce the Debye screening effect where diffusion layers from opposing walls merge, leading to a strong local electric field throughout the nanopores, preserving the distinctive ion-selective transport behavior^32,33,34. Compared to conventional CMC hydrogel lacking engineered pores, the PCNH-MEG (0.25 cm²) continuously generates a 1.51 V for over 180 h at 90% RH, demonstrating its excellent output performance and stability (Fig. 1B, C). This can be ascribed to the engineered nanopores, which enable H⁺/Cu²⁺ gradient diffusion and selective ion transport induced by the Debye screening effect²⁶. Notably, the V_oc (1.51 V), J_sc (740.5 μA cm⁻²), and integrated voltages (5030 V) delivered by integrated PCNH-MEG are much higher compared with currently reported MEGs based on the active electrode (Fig. 1D)^{13,14,25,35,36,37,38,39,40,41,42}. Additionally, PCNH-MEG demonstrates advantages in both material cost and environmental sustainability (Supplementary Table 2).

Fig. 1: Design, performance, and concept of PCNH-MEG.

PCNH and MEG are defined as delignified pomelo peel-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel and moist-electric generator, respectively. A Schematic illustration of the preparation process and nanopore structure of PCNH, which exhibits better cation selectivity owing to the stronger double electric layer (EDL) overlap induced by sub-Debye length nanopores. The figure on the right represents the overlapping thickness of the EDL (consisting of the stern layer and the diffusion layer), also known as the Debye length. B The structure of a PCNH-MEG unit with asymmetric-moisture penetration layers. C A continuous open-circuit voltage (V_oc) of a PCNH-MEG unit. Inset is a comparison of V_oc with conventional CMC hydrogels. Data represents the mean ± standard deviation (n = 3). D A comprehensive comparison between this work and other reported MEGs based on the active electrode in terms of V_oc, short-circuit current density (J_sc), integrated voltage, cost level, and eco-friendliness (EF). The cost level and EF are distinguished by the size and color of the dots respectively. E Schematic diagram of air purification system enabled by the moisture-driven NAI generator (MDNG) powered by integrated PCNH-MEGs. NAI refers to the negative air ion.

The electric eel is a naturally evolved bio-organism with abundant nanofluidic channels for power generation from ionic gradients, which has thousands of multiple organ stacks in series to enable huge potentials of over 600 V. Inspired by electric eel, we integrated multiple PCNH-MEG units on a large scale to mimic the power generation behavior of electric eels, achieving a high output voltage of 5030 V under moisture stimulation. Such a high voltage is sufficient to be combined with carbon fibers to develop a highly efficient NAls generator. The generated NAls, which refers to the negatively charged air molecules formed when atmospheric air gains electrons, is capable of adsorbing the fine particulate matter (PM) in the environment, allowing the PM to settle and thus purify the air, potentially enhancing air quality and protecting public health in a sustainable and large-scale integrated manner (Fig. 1E).

The basic features of PCNH’s nanopores

It is well known that characterizing the pore structures of hydrogels in their hydrated state is particularly challenging. Conventional approaches rely on scanning electron microscopy (SEM) to examine the pore morphology of freeze-dried hydrogels. However, the freezing and sublimation processes introduce mechanical stresses that can cause pore collapse or deformation, failing to accurately represent the microstructure of hydrogels in the hydrated state⁴³. To reliably assess the effectiveness of the in-situ nano-confinement strategy, we utilize the Cryo-SEM which adopts ultra-rapid freezing technology to avoid the formation of ice crystals to precisely observe the true pore structures of hydrated PCNH⁴⁴. CA-crosslinked CMC hydrogel (CMCH) is employed as a control sample. The PP specifically refers to the spongy mesocarp in the middle layer of the pomelo peel, as indicated by the red dashed box in Supplementary Fig. 6A. This layer is primarily composed of parenchyma cells which exhibit irregular polyhedral or nearly spherical shapes as marked by yellow dashed lines in Supplementary Fig. 6B. After delignification, the white PP sheets exhibit three-dimensional interconnected microporous structures with pore sizes ranging from 20 to 325 μm as depicted in Fig. 2A and Supplementary Fig. 7. These pore units not only serve as in-situ reaction spaces for the CA-induced crosslinking of CMC polymer chains to form hydrogels but also constrain the volume of the hydrogel matrix within each unit, thereby controlling the pore structure at the nanoscale. As expected, compared to the micron-sized pores of CMCH lacking DPP constraints, PCNH demonstrates an interconnected nanofluid network with abundant nanopores ranging from 90 to 210 nm (Fig. 2B, C, and Supplementary Fig. 7). These nanopores are expected to function as ion transport channels during the moist-electric conversion. The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy of PCNH demonstrates that the CMC hydrogel within the DPP micron-sized pore units is cross-linked through ester and hydrogen bonds provided by CA. Furthermore, extensive hydrogen bonding between the CMC hydrogel and DPP sheets was observed (Fig. 2D and Supplementary Fig. 8)⁴⁵. This chemical structure of PCNH was further confirmed by the X-ray Photoelectron Spectroscopy (XPS, Supplementary Fig. 9). These molecular interactions may also ensure the formation and structural stability of the confined nanopores to some extent.

Fig. 2: The basic features of PCNH’s nanopores.

PCNH is defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel. Photographic and Cryo-SEM images of A DPP, B CMCH, and C PCNH. D ATR-FTIR spectra, E Zeta potential, and F I-V curves of DPP, CMCH, and PCNH. G The schematic illustration of different pore morphology between PCNH and CMCH. H The Cryo-SEM images and corresponding pore-size distribution of PCNH samples (PCNH₁, PCNH₂, and PCNH₃) prepared by filling different amounts of CMC. The letter “D” represents the average pore diameter of the PCNH samples. I A comparison of the swelling ratio between PCNH (yellow square) and CMCH₁ (green square). The inset displays the swelling behavior of these two hydrogels at different swelling time, wherein the images above and below represent the PCNH and CMCH₁, respectively. CMCH₁ is defined as CMC hydrogel prepared with a higher CMC solution concentration. J The morphological and microscopic changes of PCNH and CMCH₁ after being exposed to moisture for about 48 h.

After obtaining confined nanopores, we focus on analyzing the surface charge characteristics of PCNH. As shown in Fig. 2E, Supplementary Table 3, and Supplementary Note 2, PCNH exhibits a zeta potential of −94.6 mV and a surface charge density of −6.82 mC m⁻², owing to the abundant carboxyl groups in the molecular structures of CMC and CA. These values are significantly higher than those of DPP (−45.8 mV, −3.3 mC m⁻²), which primarily contains hydroxyl groups in the molecular structure. Interestingly, despite sharing the same molecular structure as PCNH, CMCH lacking the confinement of DPP exhibits relatively lower zeta potential and surface charge density, measured at −85.6 mV and −6.17 mC m⁻², respectively. We hypothesized that the relatively loose porous structure of CMCH (Fig. 2B), maybe due to the absence of confinement, results in a lower surface charge density. The surface charge determines the efficiency of selective ion transport. We then evaluated the selective ion transport properties of PCNH by I-V curves through a lab-made device proposed in our previous work (Supplementary Fig. 10)⁴⁶. The voltage and current correspond to the ion selectivity and ion transport efficiency of the material, respectively⁴⁷. As illustrated in Fig. 2F, compared to DPP and CMCH, PCNH exhibits the highest voltage and current, indicating that confined nanopores with higher surface charge density indeed enhance the cation-selective behavior and reduce the ion transport resistance⁴⁸. The better cation selectivity of PCNH is further supported by its stronger adsorption capacity for potassium ions (Supplementary Fig. 11).

The size of nanopores in PCNH can be efficiently tuned by adjusting the loading of CMC within the DPP micropores. As shown in Fig. 2H, Supplementary Fig. 12, and Supplementary Table 4, increasing the CMC loading significantly reduces the average pore size of PCNH from 1.96 μm to 63.6 nm. Samples with different pore sizes are designated as PCNH₃, PCNH₂, and PCNH₁, respectively. This reduction can be attributed to the confined microstructure of DPP, which limits the volume of CMC hydrogel within each pore unit. Higher CMC content results in more pronounced interfacial compression between the hydrogel matrix and the pore units. Consequently, the nanopores of PCNH become progressively smaller and more densely packed. Notably, compared with conventional CMC hydrogel, the nanoporous morphology of PCNH displays a more uniform orientation which may be conducive to improving the ion transport efficiency. This may be caused by the in-situ nano-confinement effect of DPP, which promotes a more ordered arrangement of CMC hydrogels under interfacial extrusion (Fig. 2G). In addition, the size stability of the PCNH’s nanopores is investigated under extreme humidity conditions. As shown in Fig. 2I, after 48 h of immersion, CMCH₁ (with a higher CMC solution concentration, Supplementary Table 5) exhibits a significant swelling ratio (SR) of 88%, and the average pore size increases from 251.3 nm to 1.36 μm (Supplementary Fig. 13). In contrast, the swelling of PCNH is notably suppressed, with the average pore size slightly changing from 93.0 nm to 158.3 nm (Supplementary Fig. 14). The spatial confinement imposed by the DPP micropores contributes to maintaining the excellent dimensional stability of PCNH’s nanopores under high-humidity conditions, preventing their collapse or expansion (Fig. 2J, Supplementary Fig. 15). The excellent pore size stability of PCNH ensures the remained ion-selective transport behavior during long-term moist-electric conversion.

Moist-electric conversion mechanism enabled by PCNH’s nanopores

After successfully constructing nanopores with high surface charge and excellent dimensional stability, we first measured the capability of PCNH to convert moisture into electrical energy. As shown in Fig. 3A, under 80% RH, the PCNH-MEG with copper electrodes achieves a V_oc of 1.32 V and a J_sc of 693.2 µA cm⁻², representing nearly 3 and 20 times increases, respectively, compared to the DPP-MEG based on the copper electrodes. Under identical conditions using copper as the active electrode, the enhanced output performance of PCNH-MEG may be attributed to the excellent hygroscopic capacity, high surface charge density, and nanoscale effects of nanopores. Therefore, it is necessary to study the roles and underlying mechanisms of these three factors in moist-electric conversion.

Fig. 3: The H⁺/Cu²⁺ gradient diffusion mechanism enabled by PCNH’s nanopores.

PCNH is defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel. A The performance comparison of DPP-MEG and PCNH-MEG at 80% relative humidity (RH) in terms of open-circuit voltage (V_oc) and short-circuit current density (J_sc). Data represents the mean ± standard deviation (n = 3). MEG is defined as the moist-electric generator. B Schematic of the power-generation principle based on ion gradient diffusion. C The comparison of moisture uptake capability between PCNH and the pure DPP at 80% RH. D The potential change of PCNH’s nonwetting side acquired from the KPFM test. The test humidity in the KPFM measurement was 80% RH and the scan range is 5 × 5 μm. E The comparison of Nyquist plot of the electrochemical performances between PCNH and the pure DPP. F The produced Cu²⁺ from the redox reaction of copper electrode flows from the top to the bottom of PCNH. G The EDS mapping of Cu on the top and bottom surfaces of PCNH. H Electrical performance includes V_oc and J_sc of PCNH-MEG with different top electrodes. Data represents the mean ± standard deviation (n = 3).

As shown in Fig. 3B, the hydrophilic carboxyl groups in PCNH can dissociate upon moisture, releasing a significant amount of H⁺. They can induce the ion gradient diffusion by the highly charged nanopores of PCNH-MEG, thus driving H⁺ migration to trigger charge separation and generate electrical energy⁴⁹. Accordingly, the enhanced moisture uptake capability and induced ion gradient diffusion are two critical factors for the elevated performance of PCNH-MEG⁵⁰. Compared to natural DPP, PCNH exhibits a better hygroscopic capacity of up to 33% at 80% RH (Fig. 3C). This is primarily attributed to the high specific surface area and excellent hygroscopic properties of nanopores in PCNH, which enable the capture of a significant amount of water molecules from the surrounding humidity. This is sufficient for establishing a satisfactory ion gradient. As confirmed by Kelvin Probe Force Microscopy (KPFM) in Fig. 3D, when wetted at 80 RH% from 0 to 60 min, the unwetted surface of PCNH exhibits an increased surface potential from 197 to 656 mV, indicating the presence of H⁺ gradient diffusion behavior governed by the negatively charged nanopores³⁷. Therefore, compared with DPP without nanoporous structures, PCNH decorated with nanopores is more conducive to the rapid conduction of ions, showing enhanced ionic conductivity (163.1 mS m⁻¹) (Fig. 3E and Supplementary Fig. 16). Moreover, we acknowledge that the gradient diffusion of Cu²⁺ generated by the redox may also contribute to the enhanced performance of PCNH (Fig. 3F). After 72 h of continuous power generation, the concentration of Cu on the top surface of PCNH is significantly higher than that on the bottom, indicating the Cu²⁺ diffusion from top to bottom within the hydrogel matrix to generate electrical energy (Fig. 3G). These results suggest that the high hygroscopicity and H⁺/Cu²⁺ gradient diffusion of nanopores is one of the key mechanisms during the moist-electric conversion of PCNH-MEG. Additionally, we investigate the contribution of the redox reaction of copper electrode to the high output performance of PCNH-MEG (Supplementary Fig. 17). As shown in Fig. 3H, when the electrode is silver, the redox reaction is minimal⁵¹, resulting in a V_oc of 1.11 V and J_sc of 152 μA cm⁻² at 80% RH. However, when the copper electrode is used, the redox reaction leads to an increase in V_oc to 1.32 V and J_sc to 693.2 μA cm⁻². The redox reaction occurring at the copper electrode enhances ion exchange between the electrode and PCNH, thereby facilitating ion transport and increasing the output current density. The additional redox reaction offers the potential for large-scale integration of multiple PCNH-MEG units to achieve high output voltages, as discussed in the following section.

Apart from ion gradient diffusion, the Debye screening effect induced by nanopores may also play a critical role in the high output performance of PCNH-MEG. To validate this potential mechanism, we conducted a series of experiments. As shown in Fig. 4A, under 80% RH, the V_oc of PCNH-MEG with copper electrode is nearly 0.4 V higher than that of CMCH-MEG based on copper electrode. Notably, even with carbon electrode, the V_oc of PCNH-MEG is still 0.41 V higher than that of CMCH-MEG with microporous structures (Supplementary Figs. 18–20). These findings suggest that the voltage enhancement of PCNH-MEG predominantly originates from the design of highly charged nanopores, rather than the redox reactions. Thus, beyond the ion gradient diffusion, we believe that the nanopores, with dimensions below the Debye length, exhibit excellent selectivity for counter-ions (Debye screening effect). This can further amplify the streaming potential and contribute to the high output performance of PCNH-MEG^4,52. Therefore, we provide a schematic illustration to elucidate the Debye screening effect (Fig. 4B). When moisture flows through the negatively charged nanopores in PCNH, which are smaller than the Debye length, the Debye screening effect is triggered. The selective transport of dissociated H⁺ and active Cu²⁺ is enhanced, leading to a significant streaming potential. We provide direct evidence supporting this mechanism. As indicated by the following equation⁵, the streaming potential is primarily governed by the transmembrane pressure difference (ΔP) and zeta potential of PCNH.

$${V}_{{{{\rm{str}}}}}=\frac{{{{{\rm{\varepsilon }}}}}_{0}{{{{\rm{\varepsilon }}}}}_{{{{\rm{r}}}}}{{{\rm{\xi }}}}\Delta {{{\rm{P}}}}}{{{{\rm{\sigma }}}}{{{\rm{\eta }}}}}$$

(1)

where ε_r, η, and σ are the dielectric constant, viscosity, and conductivity of the fluid, respectively, ε₀ is the electric permittivity of vacuum, ζ is the zeta potential of PCNH, and ΔP is the transmembrane pressure difference which is a function of PCNH’s pore size modeled by Poiseuille’slaw (Supplementary Note. 3)⁵³.

Fig. 4: The Debye screening effect mechanism induced by PCNH’s nanopores.

PCNH is defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel. A A comparison of open-circuit voltage (V_oc) at 80% relative humidity (RH) between PCNH-MEG and CMCH-MEG. Data represents the mean ± standard deviation (n = 3). B The proposed mechanism of streaming potential involves ion diffusion along the negatively charged surface of nanopores in PCNH. C The transmembrane pressure difference (ΔP) of PCNH samples (PCNH₄, PCNH₂, PCNH, PCNH₁) with varying pore sizes. The ΔP data represents the mean ± standard deviation (n = 3). D The schematic of an electrical double layer and electric potential profile normal to the negatively charged wall in PCNH. E A custom test device wherein different concentrations of KCl moisture are passed into one side of the PCNH. F The ion concentration affects the generated V_oc of PCNH-MEG and the Debye length in PCNH. The V_oc data represents the mean ± standard deviation (n = 3). G The PCNH possesses satisfactory ion selectivity permitting only counter-ions to pass. H The pore size and genarated V_oc of PCNH-MEG prepared by filling different volumes of CMC. The V_oc data represents the mean ± standard deviation (n = 3). I The generated V_oc of CMCH-MEG at original, dried, and swelled state. The inset is the corresponding schematic picture and morphology of CMCH. J The Zeta potential comparison of PCNH samples (PCNH_0.80, PCNH_1.12, PCNH_1.34) prepared by filling different DS of CMC. K The variation of surface charge density and generated V_oc of PCNH-MEG with CMC’s DS. The V_oc data represents the mean ± standard deviation (n = 3).

We measured the ΔP values of PCNH samples with different average pore radius using a custom-built set-up (Supplementary Fig. 21). These samples, designated as PCNH₁ (31.8 nm), PCNH (78.5 nm), PCNH₂ (184.2 nm), and PCNH₄ (1.35 μm), were prepared by turning the CMC loading in DPP micropores (detailed discussion in Section “The basic features of PCNH's nanopores”). Subsequently, if we assume that the concentration of cation (H₃O⁺) in DI water is 10⁻⁷mol L⁻¹ (M) in nanopores, the temperature is 298 K, Debye length of 962 nm could be estimated in this case¹². As shown in Fig. 4C, when the pore radius of PCNH decreases from 1.35 µm to 184.2 nm (below the Debye length), the resulting ΔP exhibits a sharp increase from 2.2 to 7.9 kPa. However, further reducing the pore size from 184.2 to 31.8 nm, the ΔP increases slowly from 7.9 to 10.5 kPa. This finding demonstrates that the enhancement of ΔP in PCNH is primarily governed by the Debye screening effect, which becomes pronounced when the nanopore size is reduced below the Debye length, thereby amplifying the streaming potential.

To further validate the contribution of the Debye screening effect induced by nanopores to the output voltage of PCNH-MEG, we use a classical Gouy–Chapman–Stern model to describe the EDL⁵⁴. As illustrated in Fig. 4D, the diffusion of ion solution along with the negatively charged surface of PCNH creates an EDL region wherein the potential decays exponentially. A characteristic length is defined as the Debye length (λ_d), which can be extracted by the following equation¹².

$${\lambda }_{d}=\sqrt{\varepsilon {{{{\rm{\varepsilon }}}}}_{0}{kT}/2{n}_{{bulk}}{z}^{2}{e}^{2}}$$

(2)

Where ε and ε₀ are relative permittivity and the permittivity of vacuum, respectively. k is the dielectric constant of water, T is the absolute temperature, ${n}_{{bulk}}$ represents the bulk ion concentration, z is the valence of the ions, and e is the charge of an electron.

As shown in Eq. (2), the Debye length can be modulated by varying the ion concentration. By systematically changing the Debye length, we try to validate that the voltage enhancement in PCNH is governed by the relationship between the pore radius and Debye length. Therefore, a device we specially designed is used to evaluate the electrical behaviors of PCNH-MEG with varying Debye lengths. As shown in Fig. 4E, the carbon and copper electrodes are attached to the left side of the solid electrolyzer and the right side of the hollow electrolyzer, respectively. The PCNH is positioned between them. Once the moisture is introduced into the right side, an asymmetric moisture gradient is established. To control the Debye length, KCl solutions of varying concentrations are introduced into a humidifier, which then supplies the moisture to the hollow electrolyzer. The resulting electrical performance of PCNH-MEG under different Debye length conditions is systematically evaluated. As the solution concentration increases from 10⁻⁷M to 10⁻¹M, its Debye length decreases significantly from 962 nm to 1 nm. With the increase of Debye length, the nanopore radius of PCNH gradually becomes substantially larger than Debye length, leading to a reduction in V_oc from 1.34 V to 0.87 V (Fig. 4F). When the Debye length is extended from 1 nm to 30.5 nm by adjusting the KCl solution concentration while remaining below the average nanopore radius of PCNH (78.5 nm), a gradual increase in the V_oc of PCNH-MEG is observed. Notably, once the Debye length increases from 30.5 nm to 96.2 nm (exceeds the average nanopore radius of PCNH), the V_oc increases sharply to 1.32 V. This result is consistent with the variation of measured ΔP, providing further evidence that the voltage enhancement in PCNH-MEG is primarily driven by the Debye screening effect of nanopores. It becomes pronounced when the pore size is reduced to below the Debye length, the PCNH possesses better ion selectivity permitting only counter-ions to pass (Fig. 4G). Meanwhile, the finding suggests PCNH-MEG may possess exceptional electrical output performance in high-salinity environments, such as the sea (Supplementary Fig. 22). Additionally, the output performance of PCNH-MEG can be customized by adjusting the nanopore size, we have compared the voltage performance of several PCNH samples (PCNH₁, PCNH, PCNH₂, PCNH₃, PCNH₄) with different pore size (Supplementary Fig. 23) which are prepared by adjusting the filling amount of CMC in DPP. As displayed in Fig. 4H, a low V_oc is obtained when the pore radius of PCNH samples (PCNH₃, PCNH₄) is larger than the Debye length of 962 nm, whereas a higher V_oc of about 1.33 V is generated by PCNH (PCNH₁, PCNH, PCNH₂) with the pore radius smaller than the Debye length, further demonstrating that the Debye screening effect enabled by the pore size smaller than Debye length is the key factor to improve its voltage performance.

To further isolate the influence of the Debye screening effect from redox reactions associated with the active electrode, we substituted the copper electrode with a carbon electrode and assessed the output voltage of CMCH-MEG with varying pore structures (Fig. 4I and Supplementary Fig. 24). The pore structure of CMCH is adjusted by controlling its swelling state. The original CMCH-MEG exhibits a V_oc of 0.67 V with a pore radius of 1.01 μm (Supplementary Fig. 25). Upon drying, the pore size of CMCH decreases to 117.3 nm (Supplementary Fig. 26), which is substantially smaller than the Debye length of 962 nm, thereby activating the Debye screening effect and resulting in a generated V_oc of 1.07 V. However, following the hygroscopic absorption of CMCH, the pore size increases to 1.04 μm (Supplementary Fig. 27), leading to a subsequent decrease in V_oc to 0.66 V due to the disappearance of the Debye screening effect. The Debye screening effect is similarly evident in widely studied hydrogels, including polyacrylamide hydrogel (PAMH) and polyacrylic hydrogel (PAAH). Notably, their output voltage increases significantly as the pore size is reduced below the Debye length through drying (Supplementary Figs. 28, 29). These observations provide strong evidence for the strategy of realizing the Debye screening effect through pore engineering. Besides, we discover that the surface charge density of PCNH’s nanopores contributes to the enhancement of the Debye screening effect. As illustrated in Fig. 4J, with increasing CMC’s DS, the Zeta potential of PCNH increases from −75.5 mV to −94.2 mV, contributing to the dramatically enhanced surface charge density (Supplementary Table 6). This may be conducive to the selective transport of cations in PCNH’s nanopores. As anticipated, the output voltage of the PCNH-MEG shows a positive correlation with the surface charge density (Fig. 4K).

Regulation of PCNH-MEG’s output performance

After understanding the moist-electric conversion mechanism of the nanopores, the output performance of PCNH-MEG was regulated. As shown in Fig. 5A, the V_oc and short-circuit current (I_sc) of PCNH-MEG initially increases and then decreases with the further increment of PCNH’s thickness. Specifically, as the thickness increases to 2.5 mm, both V_oc and I_sc of PCNH-MEG initially increase due to the enhanced streaming potential and improved ion gradient diffusion within the nanopores. This facilitates the efficient separation and transport of H⁺ and Cu²⁺. However, further increasing the hydrogel thickness leads to the reduction of I_sc because of the increased ion diffusion resistance, while the V_oc remains stable as the streaming potential and ion transport resistance reach equilibrium⁵⁰. Furthermore, based on the binding energy of carboxyl groups in CMC and CA with water molecules (Fig. 5D and Supplementary Table 7), the output performance of PCNH-MEG is enhanced by independently increasing the DS of CMC and CA concentrations (Fig. 5B, C). This improvement is attributed to the increased surface charge density of the nanopores (Supplementary Fig. 30 and Tables 8 and 9), which facilitates selective cationic transport. Additionally, increasing the CMC solution loading within the DPP micropore structure can reduce the size of the nanopores to below the Debye length (detailed discussion in Section “The basic features of PCNH’s nanopores”), thereby amplifying the Debye screening effect of PCNH. Accordingly, the V_oc and I_sc of PCNH-MEG are further enhanced (Supplementary Fig. 31). Based on these results, the optimal PCNH with a thickness of 2.5 mm is obtained by infiltrating 120 mL of CMC solution (DS = 1.34) into the microporous structure of DPP, followed by crosslinking with 40% CA.

Fig. 5: Regulation of PCNH-MEG’s output performance.

PCNH and MEG are defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel and moist-electric generator, respectively. Performance optimization of PCNH-MEG in terms of open-circuit voltage (V_oc) and short-circuit current (I_sc) with different A Thickness, B CMC’s DS, and C CA’s concentration. D The calculated binding energy values of the two pairs of components in PCNH’s polymer networks based on the density functional theory (DFT) optimized structures. The curves of E V_oc and F I_sc versus time at different RH conditions. G The charging and discharging behavior of PCNH-MEG at 70% relative humidity (RH). H Continuous output voltage of PCNH-MEG over a long duration for over 180 h at 90% RH. The inset is an illustration of the PCNH-MEG device (left) and a graph of data collected from 174 h to 178 h (right). I The V_oc of PCNH-MEG before and after turning over the upper side of the PCNH. J Output voltage (orange curve decorated with the rhombus), current density (green curve decorated with the ball), and power density (cyan curve) of PCNH-MEG at 90% RH with external resistances varied from 10⁰ to 10⁸ Ω (the inset is an equivalent circuit diagram). K The performance comparison of representative MEGs based on the active electrode: the power density with the variation of RH.

To evaluate the humidity-dependent performance of PCNH-MEG, we subject it to a sealed humidity control system ranging from 30% to 90% (Supplementary Fig. 32). Both the V_oc and I_sc of PCNH-MEG increase with enhanced humidity, owing to the effective water absorption, high ion dissociation, and rapid ion diffusion in the material (Fig. 5E, F, and Supplementary Fig. 33)¹⁷. At 90% RH, PCNH-MEG achieves a maximum V_oc of 1.51 V and an I_sc of 185.1 μA (J_sc of 740.5 μA cm⁻²), outperforming most previously reported MEGs based on active electrode (Supplementary Fig. 34 and Supplementary Table 10). Notably, even at 30% RH, our MEG maintains a stable V_oc of 0.73 V and a J_sc of 146.4 μA cm⁻², demonstrating its capability for keeping power flowing around the clock. This advantage stems from the in-situ nanoconfinement strategy, which constructs sub-Debye length, and highly charged nanopores to enhance ion transport and charge separation. The resulting moist-electric conversion mechanism, driven by ion gradient diffusion and Debye screening effect, provides a special approach for MEG design. Furthermore, this strategy supports scalable integration and offers advantages in sustainability, cost, and practical applications.

Moreover, robust self-charging capability is critical for practical application. To assess the charge-discharge behavior of PCNH-MEG, cyclic V-I tests are conducted at 70% RH. As illustrated in Fig. 5G, the V_oc of PCNH-MEG, connected to an external resistor, decreases to 0.70 V after 1 h of continuous discharge, but fully recovers to its original V_oc after 1 h of self-charging. This demonstrated the robust charge-discharge cycling capability of PCNH-MEG. More importantly, the PCNH-MEG device maintains its peak performance of 1.48 V (98% of the peak value) even after 180 h of continuous operation, demonstrating its excellent working stability (Fig. 5H). Additionally, the output performance of PCNH-MEG is influenced by the electrode configuration and the direction of moisture flow (Fig. 5I and Supplementary Fig. 35)²⁸. Notably, the maximum output power density of PCNH-MEG at a load resistance of 10 kΩ reaches 101.1 μW cm⁻² (Fig. 5J), much higher than that of most previously reported MEGs with the active electrode (Fig. 5K and Supplementary Table 11)^{13,14,25,36,38,40,50,55,56}.

Large-scale integration of PCNH-MEG for air purification

The power output of PCNH-MEG can be improved through a straightforward serial or parallel configuration. To demonstrate this, we develop an efficient and scalable integration method (Supplementary Fig. 36). Figure 6A, B displays the voltage of the integrated PCNH-MEGs with serial numbers 765–4369 and 18–765, respectively. The voltage exhibits a linear increase with the number of PCNH-MEG serial units (Fig. 6C). In a practical demonstration, 154 PCNH-MEG units are integrated and result in a high voltage of 206.5 V (Fig. 6D and Supplementary Movie 1), which is sufficient to power a lamp bulb (Fig. 6E and Supplementary Movie 2). Under ambient conditions (84–89% RH, 20  ±  5 °C), 4369 PCNH-MEG units exhibit an exceptional voltage output of 5030 V. This enhanced performance is attributed to the combined effects of ionic gradient diffusion and Debye screening effect of the nanopores, as well as the contributions from redox reactions. Notably, the output voltage is much higher than that of most integrated MEGs with active electrodes reported to date (Fig. 6F and Supplementary Table 12)^{23,25,35,36,37,50,51,55,56,57,58,59,60,61,62,63,64,65}. Additionally, the excellent structural stability of nanopore allows the integrated device to maintain over 80% of its initial output voltage after nearly 17 h (Supplementary Fig. 37). Enhancing the hygroscopic area of PCNH significantly improves the current output of the MEG. When four MEGs, each with a surface area of ~65 cm², are connected in parallel, the current increases markedly, reaching ~80 mA (Fig. 6G, H). Such integrated configuration exhibits excellent linearity in performance (Fig. 6I). This suggests that enlarging the device area to amplify current output could offer a more practical and efficient strategy compared to parallel integration of multiple PCNH-MEG units within confined spaces.

Fig. 6: The large-scale integration of PCNH-MEG.

PCNH and MEG are defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel and moist-electric generator, respectively. A The stable voltage of integrated PCNH-MEGs with serial numbers ranging from 765 to 4369. B The stable voltage of the integrated device with serial numbers ranging from 18 to 765. C The plot of voltage related to the serial numbers ranging from 765 to 4369 of integrated PCNH-MEGs. Inset is an enlargement of serial numbers ranging from 18 to 765. D Photograph of integrated devices generating a voltage of 206.5 V. E A lamp bulb (0.5 W) driven by the integrated PCNH-MEGs. F The performance comparison of reported integrated MEGs based on the active electrode. G The stable current curves of integrated PCNH-MEGs with different parallel group numbers. H The stable current curves of integrated PCNH-MEGs with different parallel areas. I The plot of current related to the parallel group numbers. Inset is an enlargement of each group’s parallel area ranging from 6.5 to 65.2 cm².

The negative air ion (NAI) generator based on corona discharge has been widely used in air purification, wherein the generated NAIs can not only sink particulate matter in the air but also have a therapeutic effect on the human body⁶⁶. After confirming the large-scale integration capability of PCNH-MEG, we develop an efficient moisture-driven NAI generator (MDNG) by combining a negative ion electrode (carbon fiber) with integrated PCNH-MEGs (Fig. 7A and Supplementary Fig. 38), which can provide a continuous high voltage at ambient environment without any rectifier or booster module to generate NAIs for dust removal. As depicted in Fig. 7B, when the negative ion electrode is discharged by the integrated PCNH-MEGs, the corona formed at the tip of carbon fiber brushes rapidly releases electrons, which are promptly captured by airborne molecules such as O₂, N₂, and CO₂, leading to the formation of NAIs. The negatively charged dust particles are then attracted to a dust collection plate, while additional dust particles and NAIs cluster together and settle due to gravity⁶⁷. Building on this foundation, the air purification potential of integrated PCNH-MEGs is highly promising, with the capability to transform polluted, smog-laden environments into clear air by harnessing ubiquitous atmospheric moisture (Supplementary Fig. 39). As illustrated in Fig. 7C and Supplementary Movie 3, the integrated PCNH-MEGs efficiently convert ambient moisture into electricity, successfully powering the NAI generator to rapidly produce NAIs with a density of 138 × 10⁶ ions cm⁻³, demonstrating the effectiveness of MDNG in generating NAIs. Due to the insufficient sampling rate of commercial air ion counters, we designed a testing platform for accurately measuring the total NAIs (Supplementary Fig. 40). As schematically shown in Fig. 7D, a carbon fiber is secured in an open glass chamber with a fan on the left and a conductive silver mesh on the right. Once the carbon fiber is connected to the integrated PCNH-MEGs, the fan-driven NAIs are directed to a silver mesh, which collects the charged ions from the air via the generated current. According to the formula in Supplementary Note 4, the total charge collected can be equated to the NAIs quantity⁶⁸. The current corresponding to collected air ions increases significantly when the voltage exceeds the threshold (2000V) for effective NAIs production by the carbon fiber electrode (Fig. 7E). Notably, the integrated PCNH-MEGs generate a voltage of 5030 V, producing a current of approximately 4 μA (Fig. 7G), corresponding to the theoretical generation of 6.4 × 10¹³ negative air ions (NAIs) per second. This equates to an ion concentration of 6.4 × 10⁵ ions cm⁻³ in a 100 m³ space, establishing favorable conditions for efficient dust removal.

Fig. 7: The integrated PCNH-MEGs for air purification.

PCNH and MEGs are defined as delignified pomelo peel (DPP)-confined carboxymethyl cellulose (CMC) nanofluidic hydrogel and moist-electric generators, respectively. A A three-dimensional schematic of a moisture-driven NAI generator (MDNG) that consists of integrated PCNH-MEGs and carbon fiber electrode array layer. NAI refers to the negative air ion. B The mechanism of NAIs production and air purification. C Time-dependent variation of NAIs density monitored by an ion air counter (inset) when connected with the integrated PCNH-MEGs. D Schematic of the testing platform for measuring the quantity of NAIs. E The relationship between the average collected ion current and the negative voltage generated by integrated PCNH-MEGs. F The smog-removing performance of MDNG with and without connection with the integrated PCNH-MEGs. G The collected current when receiving an ultrahigh voltage of nearly 5030 V from the integrated PCNH-MEGs. H Comparative measurements of the dynamic variation in PM 2.5 levels when connected and not connected with the integrated PCNH-MEGs.

To evaluate the practical air purification performance of the MDNG, the negative ion electrode was placed inside a sealed glass container with a volume of 5595 cm³ (Fig. 7F). The exhaust process was assessed by introducing smoke into the container and monitoring its behavior. Upon activation of the MDNG, the smoke is completely cleared within 48 s, restoring the container to its initial transparency. In comparison, without MDNG, the smoke persists, showing no significant change over an extended duration (Supplementary Fig. 41 and Supplementary Movie 4). Additionally, we further evaluate the air purification capacity of MDNG by monitoring PM 2.5 levels before and after operation. As illustrated in Fig. 7H, Supplementary Fig. 42, and Supplementary Movie 5, the PM 2.5 concentration quickly increases to an overload level of 999 μg m⁻³ upon smoke injection and remains stable in the absence of MDNG operation. In contrast, activation of the carbon electrode connected to the integrated PCNH-MEGs induces a rapid decline in PM 2.5 levels, reaching 0 μg m⁻³ within 70 s. These results demonstrate the excellent air purification efficiency of MDNG, highlighting its considerable promise for applications in pollution mitigation and environmental health.

Discussion

We developed an in situ nano-confinement strategy to create PCNH with highly charged and stable nanopores smaller than the Debye length. The engineered nanopores enabled PCNH-MEG to synergically display the moist-electric conversion mechanism based on continuous ion gradient diffusion and Debye screening effect. The hydrophilic carboxyl groups in PCNH dissociated in the presence of moisture, releasing a large number of H⁺. The highly charged nanopores then induced ion gradient diffusion, driving the migration of H⁺ to trigger charge separation and generate electrical energy. Compared to natural DPP, PCNH had a better hygroscopic and ion diffusion capacity due to the highly charged nanopores, which enabled it to demonstrate more excellent H⁺/Cu²⁺ gradient diffusion. Moreover, the Debye screening effect induced by the nanopores is another crucial factor. When the pore size of PCNH was reduced below the Debye length, the Debye screening effect became pronounced to enhance the selective transport of dissociated H⁺ and active Cu²⁺, leading to a significant streaming potential. Experiments and theoretical analyses demonstrated that adjusting the Debye length by changing the ion concentration can directly affect the output voltage of PCNH-MEG, providing strong evidence for the role of the Debye screening effect. Taking advantage of these mechanisms, the PCNH-MEG delivered a high V_oc of 1.51 V, a J_sc of 740.5 μA cm⁻², and a power density of 101.1 μW cm⁻² at 90% RH. Notably, boosted by the Debye screening effect, the PCNH-MEG achieved enhanced V_oc which was about 0.4 V higher than that of conventional CMCH-MEG without nanopores. Additionally, redox at copper electrodes enabled integrated PCNH-MEGs to deliver an ultrahigh output voltage exceeding 5000 V. Finally, we reported the prototype of a moisture-stimulated NAI generator driven by integrated PCNH-MEGs, which can generate 6.4 × 10¹³ NAIs per second for reducing the PM 2.5 concentration from 999 to 0 μg m⁻³ within 70 s. This work provides a generalized ion-selection mechanism for developing high-performance hydrogel-based MEGs by nanopore engineering and would inspire future innovations in clean energy and air purification technologies.

Methods

Materials

Pomelo peel (PP) was collected from the local market in Fujian, Guangxi, and Thailand. Sodium hydroxide (NaOH, ACS, 97%), sodium chlorite (NaClO₂, AR, 95%), chloroacetic acid (AR, 98%), and 37% hydrochloric acid (HCl) were supplied by Sigma-Aldrich. Glycerol (ACS, 99.5%), citric acid (CA, AR, 99.5%), acrylamide (AM, AR, 99%), acrylic acid (AA, AR, 99.5%), ammonium persulfate (APS, AR, 98%), and N, N’-methylenebisacrylamide (MBA, AR, 99%) were obtained from Macklin Biochemical Co. Ltd (Shanghai, China). Organic solvents including ethanol (AR, 99.5%), isopropanol (ACS, 99.5%), and dimethyl sulfoxide (DMSO, AR, 99.8%) were all purchased from Macklin Biochemical Co. Ltd (Shanghai, China). Deionized (DI) water was used for all the experiments.

Preparation of highly substituted CMC

The CMC was obtained from PP scraps via cellulose extraction and multi-step etherification. Briefly, the powdered PP was treated in 12% NaOH solution at 100 °C for 3 h. Then the sample was further treated in 2 wt% NaClO₂ solution (pH = 4.6) at 80 °C for 30 min, followed by a thorough washing in a mixture of ethanol and DI water (v/v = 1:1), resulting in the PP cellulose. Afterward, the obtained cellulose was alkalized at 35 °C for 60 min where aqueous NaOH (3.24 mol/AGU; 30% w/v) and dimethyl sulfoxide (90 mL) were added. Subsequently, the ethanol solution with chloroacetic acid was added. When the reaction time was 30 min at the set temperature of 70 °C, 25 mL of ethanol solution with 2 g of NaOH was added again and magnetically stirred for another 2 h. After the system was cooled to room temperature, it was adjusted to neutral by dropping dilute hydrochloric acid. Then, the separated products were alternately washed with 75% alcohol and 95% alcohol three times, respectively. Notably, single-step carboxymethylation gave only low substituted products. Highly substituted CMC can be prepared by performing multiple steps of carboxymethylation for the cellulose. In each step, the prepared CMC of the previous step was purified with 80% aqueous ethanol solution and then carboxymethylated with a fresh addition of NaOH and chloroacetic acid. Finally, the synthesized crude CMC samples with different DS were purified by 80% ethanol solution, and the CMC samples were obtained by drying in an oven at 60 °C for 12 h. (The preparation process is shown in Supplementary Fig. 1, and the DS of CMC is measured by the method in Supplementary Note 1).

Fabrication of PCNH

The sliced PP pieces with different thicknesses were treated in an aqueous solution of 2 wt% NaClO₂ (pH = 4.6) at 80 °C for 1 h to slightly remove the lignin of PP, resulting in the delignified PP (DPP), which was dehydrated in ethanol solution for 1 day to widely open the internal pores of DPP. Then the above-obtained DPP was immersed in the CMC solution (with 10% glycerol added) under vacuum conditions for 6 h to remove the possible bubbles and maintain the efficient infiltration of CMC into the DPP. Subsequently, the CMC-infiltrated DPP was transferred to a plastic container with different volumes of CMC solution, which was heated in a heating plate at 80 °C overnight to complete the further infiltration of CMC in the DPP. Finally, the PCNH was prepared by immersing in CA solution at 120 °C for 1 h. The parameters comprising the thickness of DPP, CMC concentrations, volumes of CMC, CA concentration, and DS of CMC were varied individually to study their effect on hydrogel properties (as shown in Supplementary Tables 4, 6, 8, 9).

Fabrication of CMCH, PAMH, and PAAH

The CA crosslinked CMC hydrogel (CMCH) was prepared by an esterification-crosslinking mechanism. Briefly, an aqueous solution containing 2% CMC (DS 1.34) and 10% glycerol was prepared using a mechanical stirrer (the specific addition ratio is shown in Supplementary Table 5). The solution was kept overnight to remove the entrapped air bubbles, which were cast into 9 cm diameter Petri dishes and dried at 80° overnight. The resulting film was crosslinked by being immersed in 40% CA solution at 120 °C for 1 h to obtain the CMCH, which was washed with DI water and isopropyl alcohol for 1 h to remove untreated entities. Furthermore, the PAMH and PAAH were synthesized by free radical polymerization, wherein the monomer of AM/AA, crosslinker of MBA, and initiator of APS were dissolved in DI water to obtain a mixed solution (the specific addition ratio is shown in Supplementary Table 5). Subsequently, the mixed solution was degassed for 1 h and then treated in a centrifuge with a rotation speed of 2744 g for 10 min to attain a homogeneous solution without bubbles, which was placed into appropriate glass molds according to shape requirements. Then, the uniform solution was heated at 70 °C for 40 min to initiate gelation and the acquired pre-gel was dried at 80 °C overnight to further complete the gelation. Finally, we selected some of the hydrogels prepared above to be further dried in an 80 °C oven for 8 h to make the pore structure of hydrogels shrink to nanoscale, which is cooled to room temperature to be ready for testing together with the initially synthesized hydrogels.

Fabrication of PCNH-MEG, CMCH-MEG, PAMH-MEG, and PAAH-MEG

The prepared hydrogel as a hygroscopic layer was connected with the copper (Cu) and carbon (C) as the top and bottom electrodes, respectively. The dimensions of the C electrodes were standardized as 1 × 1 cm² active area with 0.5 mm thickness, while Cu electrodes were implemented as 1-mm-diameter wire. Notably, to ensure the electrode-hydrogel interfacial stability under high humidity, the close contact of electrode-hydrogel under humid conditions was achieved through a carefully designed strategy: the C electrode was first secured to an acrylic substrate using conductive tape with the negative terminal connected, followed by laminating the highly adhesive PCNH hydrogel onto the electrode surface. The Cu wire serving as the positive electrode was then installed on top of the hydrogel and reinforced with additional tape. For other electrodes (Ag, C), the same method was adopted. This approach capitalizes on both the intrinsic adhesion properties of the PCNH hydrogel and supplementary mechanical fixation to guarantee long-term interfacial stability.

Large-scale integration of PCNH-MEG

In brief, six 30*30 cm acrylic panels were used as the substrate of PCNH-MEG units. The flexible graphite sheet and copper electrode were attached to the substrate one by one to serve as the bottom and top electrodes, respectively. Finally, the PCNH samples are placed between the bottom and top electrodes one by one, ensuring tight contact, thus enabling the series integration of multiple devices.

Characterization and electrical measurements

The chemical structure of the samples was analyzed by Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR, BrukerVector33, Germany) spectra. The pore structure of samples was observed using a cryogenic scanning electron microscopy (Cryo-SEM) system, wherein supercooled liquid nitrogen (<−196 °C) was used as the freezing medium, achieving an ultra-high freezing rate (typically 10⁴–10⁵K/s). This rapid freezing process minimized ice crystal nucleation and growth by transitioning water into a glassy state within microseconds, thereby avoiding the formation of large crystalline structures. The Zeta potential (ζ potential) data was obtained with a Zetasizer nano-ZS apparatus (Malvern, UK). The ion transport behavior of hydrogels was evaluated by measuring the I-V curves through a lab-made device and a Keithley 2450 source meter (Keithley Instruments, Cleveland, OH). The range of the sweeping voltage was from −0.2 V to 0.2 V with a step voltage of 0.05 V. Specifically, the as-prepared samples were cut into small pieces (10 mm × 5 mm) and sandwiched between two PI membranes which were employed to control the effective membrane area (0.03 mm²), and the entire system was mounted between two chambers of the custom electrochemical cell which contains the KCl aqueous solution (pH ~5.7) with different concentrations. Then two Ag/AgCl electrodes were fixed separately in each chamber to apply transmembrane voltages and a pair of salt bridges were used to eliminate the imbalanced redox potential on the electrode/electrolyte interface (Supplementary Fig. 10). The cation selectivity of hydrogels was further demonstrated by comparing the amount of K and Cl characterized in an energy-dispersive X-ray spectroscopy (EDX) analyzer (SuperXG2). Tapping-mode KPFM (Bruker Dimension Icon, Germany) was conducted on a Scanning Probe Microscope. Hydrogel samples were first freeze-dried to remove bulk water, followed by further dehydration in an 80 °C oven to eliminate residual adsorbed moisture. The pre-dried samples were then exposed to a controlled 80% RH environment, with surface potential measurements conducted at precisely timed intervals (0, 30, and 60 minutes) of humidity exposure.

The electrochemical impedance spectra (EIS) of PCNH-MEG was performed in a frequency range of 1–100 kHz, with 5 mV a.c. amplitude. The electrical performance of one MEG unit was detected by a Keithley 2450 source meter (Keithley Instruments, Cleveland, OH), and the humidity was controlled by adjusting the flow rate of nitrogen (as shown in Supplementary Fig. 32). The integrated voltage of the PCNH-MEG was measured by connecting with a matched resistance, wherein the ultrahigh voltage can be indirectly calculated by the measured current. The NAIs were quantified by an air ion counter (Priesen Society, 3003H), and we have designed a testing platform for further quantitatively monitoring the NAIs as shown in Supplementary Fig. 40. An air quality monitor (Amazon) was used for PM 2.5 testing, wherein the software platform can realize real-time data acquisition, control, and analysis.