Fabrication of dense anion exchange membranes by filling polymer matrix into quaternized branched polyethyleneimine @cellulose aerogel through in-situ polymerization

Abstract In order to construct interconnected three-dimensional ion transport structures within anion exchange membranes (AEMs), we proposed the idea of preparing AEMs by filling the aerogel three-dimensional network skeleton by in-situ polymerization. First, quaternized branched polyethyleneimine (QBPEI) with a large number of quaternary ammonium groups was cross-linked with cellulose to construct an aerogel with a three-dimensional network. Then, poly(4-vinylbenzyl chloride) (PVBC) was filled into the aerogel network through in-situ polymerization. Finally, dense AEMs with internal three-dimensional ion transport networks were prepared by hot pressing PVBC/QBPEI@cellulose. The prepared AEMs have low ion exchange capacity (IEC) values and high ionic conductivities, with the membrane with the best overall performance (IEC value of 1.58 meq./g) having a maximum hydroxide conductivity of 38.88 mS/cm at 80 °C. In addition, the optimized membrane has good chemical and dimensional stability, and the maximum power density of the fuel cell assembled based on it is 46.32 mW/cm2. Although the performance of the prepared composite membranes needs to be further improved due to the preparation process, the design idea in this work provides a feasible solution for the construction of continuous ion fast transport channels in AEMs. Graphical Abstract


Introduction
Currently, serious fossil fuel pollution and traditional energy crisis have prompted the search for green alternative energy sources.As recognized as one of the most promising emerging clean energy sources, hydrogen energy has attracted extensive attention worldwide.Nowadays, the large-scale consumption of hydrogen energy mainly relies on fuel cells, a power generation device that can efficiently and safely utilize hydrogen energy and directly convert the chemical energy of hydrogen into electrical energy. [1]Therefore, fuel cells are critical to the development of hydrogen as an emerging environmentally friendly energy source.
As an important category of fuel cells, proton exchange membrane fuel cells (PEMFCs) have been developed by leaps and bounds in the past few decades, but their high production costs have greatly limited the possibility of their widespread application. [2]In contrast, anion exchange membrane fuel cells (AEMFCs) that work in alkaline environments have received increasing attention in recent years because they do not require the use of precious metal catalysts and expensive perfluorosulfonic acid-based membranes. [3]As one of the core components of AEMFCs, anion exchange membrane (AEM) is responsible for the important functions of transporting OH -anions and isolating fuel and oxidant.However, so far, AEM is still one of the key technical barriers restricting the development of AEMFCs.The underlying reason is that the preparation of AEMs with efficient ionic conductivity and high stability remains a technical challenge due to the relatively low mobility of OH À ions and the degradation of polymer electrolytes in alkaline environments. [4]nder normal circumstances, increasing the ion exchange capacity (IEC) value of AEM can achieve high ionic conductivity.However, the paradox is that increasing the IEC value also brings serious negative effects to AEM, such as highwater absorption and large size swelling, which leads to a decrease in the mechanical properties and stability of AEM. [5] promising solution to this dilemma is to build interconnected ion-conducting channels in the AEM matrix.In this way, the matrix material can provide good mechanical performance guarantee for AEM, and at the same time, the internal continuous ion-conducting channels can ensure the high ionic conductivity of AEM.[6] According to previous reports, the construction of ion-conducting channels through hydrophilic/hydrophobic microphase separation is an effective way to obtain AEMs with excellent ionic conductivity and mechanical properties.[7][8] One of the simple ways to realize the microphase separation strategy is to introduce nanofillers into the polymer matrix.In this way, the microphase separation in AEM can be achieved through the inherent incompatibility between the polymer matrix and the nanofillers on the one hand, and the ionic conductivity can be improved by increasing the density of ionic functional groups in the membrane on the other hand.[9] Iyappan et al. successfully fabricated composite AEMs with significant microphase separation by incorporating thermally stabilized and high surface area graphene oxide (GO) into a polymer matrix.The composite membrane showed improved overall performance compared to the pristine membrane, particularly with a 3-fold increase in ionic conductivity to 106.83 mS/cm.[10] Similarly, Li et al.  found that the incorporation of graphitic carbon nitride nanomaterials into polymer matrix improved both the physicochemical and electrochemical properties of composite membranes.[11] The modified nanomaterials formed hydrophilic domains in the polymer matrix, causing the composite membranes to exhibit significant microphase separation and additional nanoscale confinement structures, which facilitated the construction of oriented hydrophilic channels.[12] Nevertheless, it is important to note that not all ion-exchange groups are effectively utilized due to the poor connectivity of hydrophilic ion channels as a result of insufficient microphase separation, which limits the ionic conductivity of AEM to some extent.
Cellulose is the most abundant natural polymer material with the characteristics of renewable, derivable, and environmental protection, which has a broad application prospect in the preparation of high-performance composite materials. [13]Recently, Christoph Weder et al. reported a simple method for preparing homogeneous polymer/nanocellulose composites based on nanocellulose self-assembly and polymer solution infiltration, which can form a continuous three-dimensional network of nanocelluloses inside the composite. [14]This method provides a new idea for the construction of uniformly distributed filler microstructures in polymer systems.However, considering that the ionic groups may adversely interfere with the self-assembly of nanocellulose, the construction of a similar ion-conducting network loaded with ionic groups in a hydrophobic polymer matrix has not been thoroughly investigated.Carla Vilela et al. prepared a series of nanocomposite membranes by insitu free radical polymerization of methacryloyl chloride in a swollen three-dimensional bacterial nanocellulose network. [15]However, since nanocellulose is only used as a reinforcing element in the system, the maximum ionic conductivity of the nanocomposite membrane is limited to 10.0 mS/cm at 94 � C.
Inspired by the above research ideas, and combined with the starting point of trying to build interconnected ion-conducting channels in AEM, we herein introduced the threedimensional network structure with ion-conducting groups into the polymer matrix by in-situ polymerization, and achieved compact filling of polymer matrix by hot pressing to prepare a dense AEM.Specifically, [Amim][Cl] is first used to dissolve cellulose, and the methods of cross-linking and solvent exchange are used to prepare the quaternized branched polyethyleneimine modified cellulose (QBPEI@cellulose) aerogel with a uniform three-dimensional network structure.Then, through the in-situ polymerization of 4-vinylbenzyl chloride (VBC) in the aerogel pores and subsequent hot-pressing, a hybrid membrane with QBPEI@cellulose as the main threedimensional ion transport channel and poly(4-vinylbenzyl chloride) (PVBC) as the matrix is prepared.Obviously, the hybrid membrane has a dual cross-linked network structure inside, one formed by cellulose and QBPEI, and the other formed by PVBC and divinylbenzene during the initiation of polymerization.Moreover, due to the absence of aryl ether bonds in the aliphatic backbone of PVBC, the resulting polymer matrix is theoretically highly alkaline stable.Finally, the chloromethyl groups that are not involved in cross-linking in PVBC are further quaternized with trimethylamine to form ion-conducting sites in the membrane together with QBPEI.A series of hybrid AEMs were prepared with different contents of PVBC and QBPEI, and their microphase separation morphology, ionic conductivity, mechanical properties, and physicochemical stability were systematically investigated in order to select the AEMs with the best overall performance and to validate the potential of the prepared AEMs for application in AEMFCs.

Synthesis of [Amim][Cl]
[Amim][Cl] was synthesized by a one-step method and the specific steps are as follows: Allyl chloride (91.8 g, 1.2 mol) was added dropwise to N-methylimidazole (82.10 g, 1 mol) under ice-water bath conditions.After the dripping was completed, the above mixed solution was condensed and refluxed at 55 � C for 24 h.After the reaction, the unreacted raw materials were extracted with ethyl acetate, and the residual ethyl acetate was removed by rotary evaporation at 80 � C. Finally, the product was dried in a vacuum oven at 100 � C for 12 h. [16]

Preparation of QBPEI@cellulose aerogel
First, 1.5 g of cellulose was added to 48.5 g of [Amim][Cl], and the mixture was mechanically stirred at room temperature for 12 h to prepare a 3 wt% cellulose/[Amim][Cl] solution.Afterwards, a small amount of NaOH and epichlorohydrin were added to the above solution, and the mixture was mechanically stirred for 1 h to ensure that they were completely dissolved.Subsequently, 100 g of 3 wt% BPEI/[Amim][Cl] solution was added to the above system, and the bubbles in the solution were removed ultrasonically after being fully miscible.Then, the reaction system was raised to 60 � C and left for 5 h to chemically crosslink cellulose and BPEI.After that, a certain amount of deionized water was added to the reaction mixture, and the solution was allowed to stand until a white solid precipitated.The white solid was washed with deionized water to remove unreacted BPEI, [Amim][Cl] and NaOH to obtain a BPEI modified cellulose (BPEI@cellulose) hydrogel.
The obtained BPEI@cellulose hydrogel was placed in absolute ethanol to replace the internal deionized water, and then reacted in a quaternization solution (the mass ratio of bromopropane to absolute ethanol is 5:1) at 80 � C for 24 h to ensure that the BPEI was quaternized to QBPEI.Finally, the QBPEI@cellulose hydrogel was placed in deionized water for 2 days, and then freeze-dried to obtain QBPEI@cellulose aerogel. [17]

Preparation of PVBC/QBPEI@cellulose hybrid AEMs
The preparation of PVBC/QBPEI@cellulose hybrid AEMs is as follows (Scheme 1): Firstly, according to the formula in Table 1, the polymerization solutions containing monomer (VBC), initiator (AIBN), crosslinking agent (DVB) and solvent (anhydrous ether) were firstly prepared.Then, QBPEI@cellulose aerogels were put into the above-mentioned polymerization solutions of different formulations, and left to stand for 3 days, so that the polymerization solution was evenly filled into QBPEI@cellulose aerogel.After that, the QBPEI@cellulose aerogel filled with the polymerization solution was taken out and the residual ether was removed, and the in-situ polymerization was carried out at 80 � C for 24 h under N 2 protection.After the polymerization was completed, the filling amount of PVBC and the mass ratio of PVBC to QBPEI were calculated according to the weight difference between the final PVBC-filled QBPEI@cellulose aerogel and the initial QBPEI@cellulose aerogel.
The PVBC-filled QBPEI@cellulose aerogel was ground with a ball mill, and the powder below 300 mesh was obtained after passing through a 300-mesh screen.Afterwards, the obtained powder was hot pressed for 15 min under the conditions of 140 � C and 20 MPa to obtain a PVBC/QBPEI@cellulose hybrid membrane with a thickness of 90-100 lm.According to the previous calculation of PVBC filling amount, the mass ratios of PVBC to QBPEI in the PVBC/QBPEI@cellulose hybrid membranes prepared after immersing in different polymerization solutions were 6:1, 5:1, 4:1, 3:1, 2:1, and 1:1, respectively.For simplicity, the resulting PVBC/QBPEI@cellulose hybrid membrane was abbreviated as PQx, where x is the mass ratio of PVBC to QBPEI (x ¼ 1-6).Subsequently, the PQx membrane was immersed in a 30% trimethylamine aqueous solution for 48 h at room temperature to quaternize the chloromethyl groups of PVBC.After that, the PQx membrane was washed with distilled water and vacuum dried at 40 � C, and then immersed in a 1 M N 2 -saturated KOH aqueous solution at 30 � C until the Br À and Cl À in the membrane were fully converted into OH À (The process was monitored by AgNO 3 and HNO 3 ).After the OH À replacement was completed, the PQx membrane was taken out and rinsed with N 2 -saturated deionized water to neutrality.Finally, the OH À type PQx hybrid membrane was dried again at 40 � C and stored in a desiccator until use.

Chemical structure analysis
The proton nuclear magnetic resonance ( 1 H NMR) spectrum was acquired with a 400 MHz Bruker spectrometer, using tetramethyl silane as an internal standard and DMSOd 6 or CCl 3 D as the solvent.The elemental contents of cellulose and modified cellulose aerogels were analyzed using an EA3000 elemental analyzer (Euro Vector, Italy).And, the Fourier transform infrared spectroscopy (FTIR) was recorded using a Bruker Tensor 27 FTIR spectrometer within the wave number range of 4000-650 cm À 1 .

Molecular weight determination
The gel permeation chromatography (GPC) test was performed on a Waters 1515 GPC instrument, using tetrahydrofuran as the eluent (flow rate: 1.0 mL/min) and polystyrene (PDI ¼ 1.02) as the standard.

Morphological confirmation
The scanning electron microscopy (SEM) measurements were carried out on a FEI Inspect F50 microscope equipped with an energy dispersive X-ray spectroscopy (EDS) detector, and the acceleration voltage was 5 kV.Before testing, all samples were sputtered with gold.The transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 microscope at an accelerating voltage of 200 kV.Prior to observation, the PQx hybrid membrane was immersed in 0.2 mol/L H 2 PtCl 6 aqueous solution for 48 h, then washed with deionized water and dried in a vacuum oven at 50 � C for 12 h.After that, the dyed PQx hybrid membrane was embedded in epoxy resin and cured, and then cut into slices about 100 nm thick.The atomic force microscopy (AFM) images were collected with a SEIKO SPA-400 microscope at room temperature.

Performance test of AEM
The thermogravimetric analysis (TGA) was performed on a TGA Q50 analyzer under the protection of nitrogen atmosphere, with a temperature range of 30-600 � C and a heating rate of 10 � C/min.The tensile test was carried out on an Instron 5567 universal tensile testing machine at room temperature.Prior to testing, the specimens were cut into rectangular strips (approximately 4.0 cm � 1.0 cm), thereafter, the test was performed at a velocity of 10 mm/min.
The water uptake (WU) and swelling ratio (SR) were tested by soaking the PQx hybrid membranes (approximately 4.0 cm � 1.0 cm) in deionized water at 20-80 � C for 48 h.After that, the swollen membrane was taken out, the wet surface of membrane was wiped dry with filter paper, and the wet weight and length (W w and L w ) were recorded.Subsequently, the swollen membrane was vacuum dried at 80 � C for 24 h, and the dry weight and length (W d and L d ) were measured.The water uptake and swelling ratio of the PQx hybrid membrane were calculated by WU ¼ The ion exchange capacity (IEC) values of the specimens were measured by immersing the OH À type PQx hybrid membranes in 0.01 M HCl (30 mL) aqueous solution for 48 h at 80 � C. Afterwards, the above HCl solution was titrated by 0.01 M NaOH aqueous solution.Finally, the IEC value was acquired by IEC ¼ (M 0 -M 1 )/W, where M 0 is the initial molar amount of HCl, M 1 is the molar amount of HCl obtained by NaOH titration, and W is the dry weight of the OH À type PQx hybrid membrane.In addition, the hydration number (k) of the PQx hybrid membrane was calculated by k¼ (10 � WU)/(IEC � molar mass of water), which is defined as the number of water molecules absorbed by each quaternary ammonium group.The alkaline stability of the OH À type PQx hybrid membranes was investigated by the residual ionic conductivity after immersing them in 1 M NaOH solution at 80 � C for different times.The oxidative stability of the OH À type PQx hybrid membranes was evaluated by the remaining weight after soaking them in Fenton's reagent (3 wt% H 2 O 2 with 4 ppm FeSO 4 ) at 80 � C for different times.
The ionic conductivities of the OH À type PQx hybrid membranes (approximately 4 cm � 0.5 cm) were tested by the electrochemical impedance method.The test was performed on a CHI650E electrochemical workstation with the frequency between 0.1 Hz and 100 kHz and the test temperature range of 20-80 � C. The ionic conductivity of the OH À type PQx hybrid membrane was obtained by r ¼ D/(A � R), where D is the distance between the two platinum electrodes, A refers to the cross-sectional area of the OH À type PQx hybrid membrane, and R represents the measured electrochemical impedance value of the membrane.
In addition, since the temperature dependence of ionic conductivity conforms to the Arrhenius equation, the apparent activation energy (E a ) for ion transmission can be obtained by E a ¼À b � R, where R is the gas constant and b is the slope of the lnr vs. 1000/T curve of the OH À type PQx hybrid membrane.
The OH À type PQx hybrid membrane was used to fabricate membrane electrode assembly (MEA) to evaluate the performance of single H 2 /O 2 fuel cell.First, the ionomer solution (3 wt%) of the hybrid membrane and the Pt/C catalyst (40 wt%) were uniformly mixed in isopropanol to obtain a 2 wt% catalyst ink (the weight ratio of Pt/C catalyst to ionomer was 4:1).Then, the above catalyst ink was coated on carbon paper, and the Pt loading on the carbon paper was controlled to be 0.5 mg/cm 2 .After the two gas diffusion electrodes were vacuum dried at 80 � C for 12 h, the OH À type PQx hybrid membrane was sandwiched in the middle, and then hot-pressed to get MEA.Afterwards, the prepared MEA was tested in a Hephas Mini-L100 fuel cell station at 80 � C, where the H 2 /O 2 flow rate was 400 mL/min, the humidity was 100%, and the back pressure was 0.2 MPa. [18]

Synthesis and structure characterization
Cellulose is difficult to dissolve in typical organic solvents due to molecular hydrogen bonding.However, 1-allyl-3methylimidazole chloride ([Amim][Cl]), an ionic liquid, can quickly dissolve cellulose and maintain its properties, making it an ideal solvent for cellulose. [19]Herein, the successful preparation of [Amim][Cl] was confirmed by 1 HNMR, and the purity of the prepared [Amim][Cl] was 99.21% (see Figure S1 in the Supplementary Information).As shown in Figure S2, [Amim][Cl] dissolves cellulose by breaking the hydrogen bonds in the cellulose molecular chains, and thus the process is only a physical dissolution and does not involve chemical reactions.
To verify the formation of PVBC polymer, PVBC was synthesized under the same conditions as PVBC-filled QBPEI@cellulose aerogel.The 1 HNMR results showed that VBC was successfully polymerized into PVBC (as shown in Figure S3), and the molecular weight of the as-prepared PVBC was determined by GPC to be 127.6 kDa (see Figure S4 in the Supplementary Information).
The chemical structures of the PQx membranes were confirmed by FT-IR analysis, using cellulose, QBPEI@cellulose aerogel and PVBC polymer as controls. This is consistent with the elemental analyses of cellulose and QBPEI@cellulose (see Table S1 in the Supplementary Information).For PVBC, the absorption peaks at 1502 and 1479 cm À 1 are caused by the -C ¼ C-stretching vibration of the benzene ring. [24]In contrast, the PQ3 and PQ4 hybrid membranes show the characteristic peaks of both the benzene ring in PVBC and the hydroxyl and quaternary ammonium groups in QBPEI@cellulose aerogel.The above results indicate that PVBC can be successfully introduced into the PQx membrane by the method of filling QBPEI@cellulose aerogel by in-situ polymerization.

Micromorphology characterization
In order to verify the feasibility of filling QBPEI@cellulose aerogel by in-situ polymerization of VBC, the PVBC-filled QBPEI@cellulose aerogels with different PVBC contents were prepared according to the formulas in Table 1, and their microscopic morphologies were characterized by SEM.As shown in Figure 2, the pure QBPEI@cellulose aerogel has abundant porous networks due to the high-density physical and chemical cross-linked network formed by QBPEI and cellulose, which is similar to the research findings of Yu et al. [25] This interconnected network structure is the basis for the construction of connected three-dimensional ion transport channels.Compared to pure QBPEI@cellulose aerogel, PVBC-filled QBPEI@cellulose aerogel has fewer and smaller pores.Moreover, this trend becomes more evident as the amount of PVBC filling in the aerogel increases, resulting in the absence of visible pores in the PVBC-filled QBPEI@cellulose aerogel prepared according to formula 1.Therefore, the PVBC polymer matrix can be successfully filled into the QBPEI@cellulose aerogel's three-dimensional network via in-situ polymerization of VBC. [26]y adjusting the concentration of VBC in the polymerization solution, the amount of VBC monomer infiltrated into the pores of QBPEI@cellulose aerogel can be controlled, and thus the amount of PVBC introduced into QBPEI@cellulose aerogel can be well adjusted.Furthermore, the macroscopic and microscopic morphologies of the OH À type PQx hybrid membranes were studied, and the influence of different PVBC content on them was evaluated.According to the macroscopic morphologies of OH À type PQx hybrid membranes with different PVBC contents (as shown in Figure S5), it can be known that the PQx hybrid membranes are light yellow and transparent, which exhibit different flexibility due to the difference in PVBC content.Obviously, there are many dark yellow spots in the PQ1 membrane, which may be caused by the agglomeration of QBPEI@cellulose aerogel.In contrast, the PQ3 and PQ4 membranes with moderate PVBC content are smooth and flexible, without obvious aggregation, indicating that PVBC can be evenly distributed in the QBPEI@cellulose three-dimensional network in appropriate proportions.However, continuing to increase the PVBC content results in the final PQ5 membrane being very brittle.This is due to the rigid benzene ring structure inside the PVBC, which reduces the overall flexibility of the PQ5 membrane. [27]heoretically, the prepared PQx membranes have an interpenetrating three-dimensional network structure because not only the internal QBPEI@cellulose aerogel has a three-dimensional cross-linking network, but also the in-situ polymerized PVBC can form a cross-linked network under the action of the cross-linking agent divinylbenzene. [28]herefore, to reveal the internal structural characteristics of the OH À type PQx hybrid membranes, SEM, TEM and AFM were subsequently performed.The cross-sectional SEM images of the PQ3 (Figure 2k) and PQ5 (Figure 2l) membranes show that the thickness of the PQx membrane prepared by the hot-pressing method is about 100 lm.Moreover, both membranes show a relatively uniform and smooth cross-section, indicating the uniform dispersion of QBPEI@cellulose in the PQx membrane under the optimal composition ratio range.Figure 2f-h show the surface SEM images of the OH À type PQx hybrid membranes with different PVBC contents.Except for the many protrusions on the surface of the PQ1 membrane, the surfaces of other PQx membranes prepared under the optimized ratio range are smooth, uniform, and dense.In addition, the EDX spectra of Br and Cl elements on the surface of the unalkalized PQ3 membrane show that the Br element in QBPEI and the Cl element in PVBC are evenly distributed in the PQ3 membrane, indicating that QBPEI @cellulose aerogel can be evenly distributed in the PVBC matrix under the optimal composition ratio range, which is conducive to the formation of subsequent connected ion transport paths. [29]ompared to PVBC-filled QBPEI@cellulose aerogel (Figure 2a-d), the PVBC/QBPEI@cellulose membrane prepared under high pressure for a short time is transparent, smooth, and compact, meeting the structural requirements of AEMs, which is consistent with the findings of Krishna et al. [30] Additionally, the hot pressing method used to prepare PQx membranes allows for the production of ultra-thin and heterotropic membranes using different molds to fulfill various production needs. [31]s shown in Figure 2m-n, the PQx membranes present obvious dark hydrophilic areas and light hydrophobic areas.The dark areas are hydrophilic channels formed by QBPEI@cellulose aerogel and quaternary ammonium groups in PVBC, while the light-colored areas correspond to the hydrophobic PVBC backbone. [32]And, a hydrophilic network structure with good connectivity can be formed in the PQ3 membrane, which is conducive to the rapid conduction of OH À in the membrane.However, when the content of QBPEI@cellulose aerogel continues to decrease (PQ5), part of the network channels formed by QBPEI@cellulose aerogel begin to appear poorly connected.In general, when the composition ratio is optimized, the QBPEI@cellulose aerogel can be evenly distributed in the PVBC matrix, which facilitates the construction of connected three-dimensional ion transport paths. [33]Similarly, Figure 2o-p show the AFM phase diagrams of the PQ3 and PQ5 membranes, which are also characterized by a distinct microphase separation structure, in which the hydrophilic phase formed by BPEI@cellulose aerogel and quaternary amino groups is dark brown, and the hydrophobic phase formed by the PVBC main chain is bright yellow. [34]In addition, comparing the AFM phase diagrams of PQ3 and PQ5 membranes, it is found that with the increase of PVBC content, the hydrophobic areas in the PQx membranes become larger, further indicating that the ion transport path in the membrane can be optimized by adjusting the ratio of the components. [35]

Thermal stability
Alkaline anion exchange membrane fuel cells (AEMFCs) are generally used at 60-80 � C, so the thermal stability of the prepared AEM needs to meet this operating temperature. [36]s shown in Figure 3, the QBPEI@cellulose aerogel begins to thermally decompose at 235 � C, which is caused by the degradation of the main chains of cellulose and QBPEI. [37]n comparison, as the temperature increases, the PQx membranes have three stages of weight loss.The weight loss in the first stage (0-120 � C) is due to the evaporation of a small amount of bound water in the membrane.The weight loss in the second stage (120-350 � C) is mainly due to the degradation of the QBPEI and cellulose backbone in the QBPEI@cellulose aerogel, as described in previous studies. [38]The weight loss in the third stage (350-500 � C) is attributed to the degradation of the PVBC backbone.As the content of QBPEI@cellulose aerogel decreases, the thermal stability of the PQx membrane increases. [39]In particular, the thermal decomposition curve of PQ1 membrane is obviously different from other PQx membranes.This is mainly due to the high content of QBPEI@cellulose aerogel in the PQ1 membrane, which results in the membrane exhibiting thermal stability similar to QBPEI@cellulose aerogel. [40]verall, all PQx membranes exhibit high thermal stability, enabling them to maintain the normal operation of alkaline AEMFCs at 80 � C.

Mechanical properties
AEM is easily affected by tension and pressure, causing problems such as cracking or deformation.Therefore, the mechanical performance of AEM is one of the key factors affecting the safety and life of fuel cells. [41]As shown in Figure 4, as the content of QBPEI@cellulose aerogel decreases, the mechanical strength of PQx membrane first increases and then decreases.When the content of QBPEI@cellulose aerogel is too high, the agglomeration of QBPEI@cellulose in the hybrid membrane (verified by SEM and TEM) leads to stress defect areas in the membrane, so the tensile strength of the PQ1 membrane is only 10.85 MPa.When the content of QBPEI@cellulose aerogel is reduced to an appropriate range, the mechanical strength of the PQx membranes is improved.This is mainly because PVBC and QBPEI@cellulose aerogel form a relatively complete fully interpenetrating polymer network structure, thereby enhancing the intermolecular interaction in the membrane. [42]In particular, both PQ3 and PQ4 membranes exhibit excellent mechanical properties, which can meet actual application requirements.When the content of QBPEI@cellulose aerogel is too low, the poor mechanical properties of the PQx membranes are dominated by the rigid PVBC polymer. [43]It is worth mentioning that hot pressing not only increases the adhesion between the components and improves the film density, [44] but also promotes the homogeneous distribution of PVBC in the aerogel and improves the mechanical properties of the composite membrane, as further confirmed in Figure S5. [45]In general, by optimizing the mass ratio of QBPEI@cellulose aerogel and PVBC, a PQx membrane with good mechanical properties can be prepared to meet its subsequent use requirements.

Swelling rate and water absorption of AEM
The conduction of OH À is influenced not only by the conduction groups but also by water molecules, which act as ion transport mediators.However, excessive water absorption can cause the size expansion of AEM, reducing mechanical property and diluting ion concentration.Therefore, an ideal AEM should have appropriate water absorption and swelling rate. [46]As shown in Figure 5, PQx membranes have good consistency in water uptake and swelling ratio at different temperatures.As the content of QBPEI@cellulose aerogel decreases, the water uptake and swelling ratio of PQx membranes gradually decrease and become stable.When the content of QBPEI@cellulose aerogel is too much, the water uptake and swelling ratio of the PQx membranes are large.For example, the water uptake and swelling ratio of PQ1 at 80 � C are about 75% and 30%, respectively.The reasons for this situation are as follows: on the one hand, QBPEI@cellulose aerogel contains a large number of quaternary ammonium groups and hydroxyl groups, which will cause the membrane to be more hydrophilic when used as the main component of the PQx membrane; [47] On the other hand, the three-dimensional network constructed by QBPEI@cellulose aerogel makes it easier for water molecules to enter the inside of the membrane, which also causes the PQx membrane to have strong water absorption. [23]When the content of QBPEI@cellulose aerogel is within an appropriate range, PVBC and QBPEI can form a more complete interpenetrating polymer network structure, and the structural stability of the PQx membrane is enhanced, which prevents the water absorption and expansion of the PQx membrane to a certain extent. [48]When the content of QBPEI@cellulose aerogel continues to decrease, the hydrophobic PVBC polymer is the main component of the PQx membrane, and the cross-linking structure formed by p-divinylbenzene and PVBC increases the interaction between the molecular chains in the membrane.The above two facts both prevent water molecules from entering the PQx membrane, so the water uptake and swelling ratio of the PQx membrane are further reduced and stabilized. [49]In a similar study by Rao et al., hydroxyl-rich polyvinyl alcohol (PVA) and PEI were combined to create a PVA/PEI membrane with a water absorption rate of 164%, approximately 11 times that of PQ3.Thanks to the structure of PVBC-filled aerogel, the contact sites of water molecules are enveloped in PVBC layers.Moreover, after hot pressing of PQx, a dense structure is formed inside the AEM, which effectively prevents the penetration of water molecules, again proving the denseness of the PQx membrane (Figure 2). [50]In addition, the water uptake and swelling ratio of the same PQx membrane increase with the increase of the test temperature, similar to the phenomena observed by Li et al. [51] This phenomenon is because the increase of temperature accelerates the movement speed of water molecules and molecular chains in the membrane, thereby greatly improving the probability of contact between water molecules and the hydrophilic groups in the PQx membrane. [52]

Ionic conductivity of AEM
As shown in Figure 6a, as the content of QBPEI@cellulose aerogel decreases, the k value of the PQx membrane gradually decreases, which is directly related to the water absorption and swelling degree of the hybrid membrane.In addition, as the QBPEI@cellulose aerogel content decreases, the IEC value of the PQx membrane first increases and then decreases, and PQ3 has the maximum IEC value of 1.58 meq./g.When the content of QBPEI@cellulose aerogel is too much, the agglomeration of QBPEI@cellulose causes a large amount of QBPEI to be embedded in the agglomerate (this is confirmed by TEM), resulting in that many quaternized groups cannot be effectively utilized.When the content of QBPEI@cellulose aerogel is within an appropriate range, QBPEI@cellulose aerogel and PVBC can form a relatively complete interpenetrating network structure, which is conducive to the formation of connected three-dimensional ion transmission paths.When the content of QBPEI@cellulose  aerogel continues to decrease, part of QBPEI@cellulose aerogel is wrapped by PVBC polymer, and the internal ion transmission path begins to appear discontinuous.
Consistent with expectations, the ionic conductivity of the PQx membrane increases with increasing temperature. [53]This is because as the temperature rises, the water uptake and swelling ratio of the PQx membrane increase, which can promote the formation of water channels that transport OH À in the membrane.Meanwhile, the movement of molecular chains in the PQx membrane is more active, thereby reducing the energy barrier of OH À transmission.In addition, the PQ3 membrane has the highest ionic conductivity at each  test temperature compared with other membranes, and its ionic conductivity is 38.88 mS/cm at 80 � C.This is because when the content of QBPEI@cellulose aerogel is within an appropriate range, the two extreme ion transport barriers mentioned earlier can be avoided: the massive embedding of QBPEI in QBPEI@cellulose aggregates and the encapsulation of QBPEI@cellulose by PVBC substrates.Furthermore, the curve of lnr and 1000/T of the PQx membrane drawn by the Arrhenius equation was used to calculate the apparent activation energy of ion transport in the membrane (Figure 6b).The apparent activation energy of PQx membranes is between 10-20 kJ/mol, which is slightly lower than the previously reported AEMs derived from the NafionV R precursor (about 18-33 kJ/mol).
Based on previous experimental data, a possible ion conduction model in PQx membranes is proposed.When the content of QBPEI@cellulose aerogel is within an appropriate range, the PQx membrane has a relatively uniform and highly interconnected interpenetrating polymer network structure (as shown in Figure 6c), establishing a hydrophilic channel with better connectivity in the membrane, thereby reducing barriers to OH À ion transport. [54]In comparison, when the content of QBPEI@cellulose aerogel is too high or too low, QBPEI@cellulose cannot build a continuous hydrophilic channel in the PVBC matrix, which hinders the rapid transmission of OH À inside the membrane.
In this study, the fast OH À transport channel constructed by QBPEI@cellulose aerogel is restricted by the cross-linked PVBC molecular chains as well as being confined to imperfect membrane molding methods, resulting in a maximum conductivity of only 38.88 mS/cm in purified water at 80 � C. Nevertheless, this value is comparable to other reported PVBC-based AEMs (Table S2).For example, in Li et al.'s study, the corresponding ionic conductivity of as-prepared PVBMPy-CL-25%PSF was only 20 mS/cm at 80 � C, and its IEC value was similar to that of the PQ3 membrane. [55]Liang et al. prepared the crosslinked PVBC/Montmorillonite-QPEI membrane with an ionic conductivity of only 14.5 mS/cm (80 � C) by casting method. [56]Prakash et al. found that trimethylammonium quaternary ammonium PVBC exhibited relatively low conductivity (only 0.52 mS/cm at 25 � C). [57] Compared to these research works, the ionic conductivity of PVBC-based PQx hybrid AEM is nearly doubled, which fully demonstrates the effectiveness of utilizing QBPEI@cellulose aerogel to construct OH À fast transport channels in PVBC matrix.Meanwhile, the establishment of better interconnected ionic transport channels is crucial for further improving its ionic conductivity.

Chemical stability of AEM
The chemical stability of AEM is an important factor affecting the life of AEMFCs.The oxidation stability of PQx membranes was measured by their residual mass after oxidation experiments.As shown in Figure 7a, with the prolong of the oxidation time, the residual mass of all PQx membranes decreases rapidly at first, and the downward trend gradually becomes flat after 5 days.Among them, the PQ1 membrane has the worst oxidation resistance, which is mainly due to two reasons: 1) The PQ1 membrane has the largest water swelling property, which makes it easier for oxygen free radicals to enter the membrane, resulting in rapid degradation of the PQ1 membrane; [58] 2) Compared with other PQx membranes, the QBPEI@cellulose aerogel content in the PQ1 membrane is the largest, and the glycosidic bonds in cellulose are easily attacked by oxygen free radicals to undergo degradation reactions. [59]As the content of QBPEI@cellulose aerogel decreases, the antioxidant capacity of PQx membranes increases.Due to the three-dimensional network and compact membrane structure constructed by both hot pressing and chemical bond crosslinking, the water-absorbing swelling property of the membrane is reduced, making it difficult for oxygen free radicals to enter the membrane.Additionally, PVBC, the main component of the membrane, has few sites that can be attacked by oxygen free radicals. [55]It is worth noting that after the oxidation experiment, the PQ3 membrane with the highest ionic conductivity can maintain about 65% of its initial mass, showing good oxidation stability.
Furthermore, the alkali resistance of the PQ3 membrane with good various properties was evaluated by the residual ionic conductivity after the alkaline stability test.As the immersion time increases, the ionic conductivity of the PQ3 membrane first rapidly and then slowly decreases (Figure 7b).This is because in the initial stage of the PQ3 membrane immersed in the alkaline solution, the OH À ions in the solution can quickly enter the membrane through the internal hydrophilic channel, causing the quaternary ammonium groups to degrade.As the soaking time increases, the water-absorbing swelling of the PQ3 membrane reaches equilibrium.Due to the electrophilic nature of the quaternary ammonium groups on the QBPEI molecule chain, they are easily degraded by attack from nucleophilic groups such as OH À . [60]For the PVBC matrix, two contradictory aspects are exhibited: on the one hand, the regular arrangement of PVBC can hinder the further erosion of OH À , thus slowing down the overall degradation rate of the ion-conducting functional groups in the membranes; on the other hand, the electron-withdrawing effect inherent in the benzyl group of PVBC can promote the attack of OH À on the quaternary ammonium groups, thus accelerating the degradation rate of the quaternary ammonium groups. [61]Overall, the PQx membranes show sufficient durability in strongly alkaline environments due to the crosslinked structure of PVBC, the crosslinked network within QBPEI@cellulose aerogel, and the compact structure formed by hot pressing. [62]After 20 days of alkaline stability testing, the PQ3 membrane retains about 70% of its initial ionic conductivity, indicating that it has good alkali resistance.

Fuel cell applications
As a demonstration, the PQ3 membrane with both high ionic conductivity and excellent chemical stability was used as electrolyte and separator to assemble MEA, and the resulting H 2 /O 2 single cell was operated at 80 � C and 0.2 MPa back pressure to evaluate its performance.As shown in Figure 8, the open circuit voltage of the single cell assembled from the PQ3 membrane is 0.90 V, and the maximum power density reaches 46.32 mW/cm 2 when the current density is 97.67 mA/cm 2 , indicating that the PQ3 membrane meets the air tightness requirement of the separator.Nonetheless, what is unsatisfactory is that compared with the previously reported AEMs, the ionic conductivity and maximum power density of the PQ3 membrane are relatively low.This may be due to the fact that in order to prepare a relatively flat PQx membrane, the PVBC-filled QBPEI@cellulose aerogel was crushed by a ball mill before the hot pressing, resulting in partial destruction of the three-dimensional ion conduction network originally formed by the aerogel.Simultaneously, the three-dimensional ionic conduction network was not fully utilized due to the uniform distribution of aerogel in PVBC, resulting in the partial obstruction of hydroxide conduction in the PQx membranes.Follow-up work will focus on the above issues and consider improved membrane-forming processes and alternative matrix polymers to further improve the overall performance of such AEMs.

Conclusion
For the purpose of constructing an interconnected threedimensional ion-conducting structure, in this work, a series of PQx AEMs containing three-dimensional ion-conducting paths and interpenetrating cross-linked network structures were prepared by filling the PVBC matrix into QBPEI@cellulose aerogel by in-situ polymerization.With the optimized ratio of each component, the QBPEI@cellulose aerogel in the PQx hybrid membrane is evenly distributed, and the three-dimensional ion transmission path can be successfully constructed.Benefiting from the relatively uniform ion-conducting network constructed by the QBPEI@cellulose aerogel skeleton and the interpenetrating polymer network, the ionic conductivity of the PQ3 hybrid membrane with an IEC value of 1.58 meq./g is 38.88 mS/cm at 80 � C.Moreover, the reinforcing effects of interpenetrating cross-linked network and QBPEI@cellulose aerogel endow the PQ3 hybrid membrane with good mechanical performance, thermal and chemical stability, meeting the needs for assembly and operation in single fuel cells.The maximum power density of single fuel cell assembled with PQ3 hybrid membrane can reach 46.32 mW/cm 2 .Although the ionic conductivity of the PQ3 hybrid membrane and its power density of the single fuel cell are not satisfactory due to the molding method, this type of hybrid AEM proposes the idea of constructing a three-dimensional ion transmission pathway by using a unique aerogel network model, which also provides an attempt to the application of natural cellulose in the field of AEMs.

Scheme 1 .
Scheme 1. Schematic diagram of the preparation procedure and structure of PVBC/QBPEI@cellulose hybrid AEMs.

Figure 4 .
Figure 4. Mechanical properties of the PQx membranes in hydrated state.

Figure 6 .
Figure 6.The relationship between the ionic conductivity (a) and Arrhenius diagram (b) of the PQx membranes with temperature; (c) schematic diagram of the internal structure of PQx membranes.

Figure 5 .
Figure 5.The WU (a) and SR (b) of the PQx membranes at different temperatures.

Table 1 .
Formulation of in-situ polymerization solutions.