Advanced adjustable ionic conductivity of polybenzimidazole membranes with arrayed two-dimensional AlOOH nanosheets for water electrolysis

Polybenzimidazole (PBI) membrane is promising but limited by its lower ion conductivity when used in the energy �eld. Two-dimension (2D) layered hydroxides with abundant hydroxyl groups could accelerate ion transport. However, the ionic conductivity cannot be adjusted when the 2D nanosheet is introduced by blending or spraying methods. Here, a series of novel arrayed 2D AlOOH-PBI composite membranes with adjustable ionic conductivity were prepared via the in-situ growing-etching method through the control of the thickness of nanosheets. The hydroxide ion conductivity of the proposed membrane was about 5.5-fold higher than that of pristine PBI membrane. The proton conductivity also showed about 1.5-fold enhancement. Meanwhile, the membrane electrode assembly with novel membranes showed superior voltage performance of 2.07 V at a current density of 1 A cm − 2 , and the long-term stability was con�rmed for over 200 h at a current density of 500 mA•cm − 2 in water electrolysis. These results look prospective for the preparation of new membranes for energy applications.


Introduction
The current world energy crisis and greenhouse effect originating from excessive combustion of fossil fuels have attracted increasing attention due to the urgency of the situation.To solve the above environmental problems, priority has been given to the development of renewable clean energy and storage technologies [1][2][3] .These include water electrolysis, fuel cells, redox ow batteries, and carbon dioxide reduction owing to their advantages in terms of simple processing, mild reaction conditions, easy adjustment of reaction processes, and mild to no environmental pollution.
High-performance ion conductive membranes (ICM) play a vital role in many renewable energy and electronic devices, which need to meet the criteria of high ion conductivity, suitable water uptake, longterm stability, and low price for industrial applications 4,5 .Among these, polybenzimidazoles (PBI) membranes are promising materials in the clean energy eld, such as fuel cells, ow batteries, and water electrolysis due to their high thermal resistance, good mechanical properties, and excellent chemical stability.Nevertheless, PBI still suffers from ionic conductivity that requires improvements.Since PBI is a class of aromatic heterocyclic polymers with the main chain containing imidazole moieties, regulating the cation conductive membrane (CCM) and anion conductive membrane (ACM) by acid or alkali doping could solve the above issues due to the amphoteric nature of the imidazole 4 .Thus, various modi cation methods have been so far utilized, including polymer structure modi cation, porous membranes, blending membranes, and the introduction of hydrophilic substances 5,6 .A great focus has also been paid to twodimensional (2D) layered materials for improving the ion conductivity, such as layered double hydroxide (LDH) 7 , metal-organic frameworks (MOFs) 8,9 , hydrogen-bonded organic frameworks (HOFs) 10,11 , covalent organic frameworks (COFs) 12 , transition metal carbides and nitrides (MXenes) 13,14 , graphene-based materials 15 , and ionic clays 16,17 .These strategies were found effective for enhancing the comprehensive performance of ICMs.
Layered hydroxides or layer double hydroxide (LDH) are a class of ionic clay-like compounds consisting of positively charged metal hydroxide layers with a structure containing mainly laminate and interlayer anions with water molecules stacked on top of each other.LDH shows excellent thermal and chemical stability, as well as outstanding ion conductivity in energy storage devices [18][19][20][21] .For instance, Hu et al. 7 studied MgAl-based LDH composite membranes via spraying method with fast and selective ion transport in alkaline zinc-based ow batteries and obtained high Coulombic and energy e ciencies.Wan et al. 22 fabricated a highly alkali-resistant PTFE/LDH composite membrane via the pore-lling method for excellent performance in alkaline water electrolysis.However, most reports have been focused on accelerating the transport of proton or hydroxide ions for speci c application elds, while controlling the ions transport process and the adjustability of ionic conductivity are often neglected but of great importance for many applications.
Herein, a series of pseudo-boehmite (γ-AlOOH) based PBI composite membranes were successfully prepared by in-situ growth-etching method.The adjustment of proton and hydroxide ion conductivities, as well as the conduction mechanism, were all studied.The results showed that the ionic conductivity of PBI-based 2D nanosheet composite membranes can be adjusted by controlling the thickness and surface morphology of γ-AlOOH nanosheet layers by acid or base etching in a highly directional manner based on the ion transport mechanism.The Grotthuss transport mechanism was veri ed as the main transport mechanism of proton and hydroxide ions in γ-AlOOH-PBI composite membranes.Finally, the practicability of γ-AlOOH-PBI composite membranes was evaluated in an alkaline water electrolysis system assembled with the novel membrane electrode assembly (MEA).

In-situ growth of γ-AlOOH
As shown in Fig. 1a, γ-AlOOH consisted of a 2D layered material with a dioxygen octahedron structure, as well as a class of hydrated alumina with incomplete crystallization 16 .The crystalline system of γ-AlOOH appeared orthogonal with a laminar structure consisting of many AlO 6 octahedra within every single structural layer.The oxygen atoms were arranged in cubic dense stacks at the apex of the octahedron, while aluminum atoms were located at the center of the octahedron to form a bilayer structure.
Meanwhile, the hydroxyl groups were distributed on the lamellar structure surface, and all layers were connected by hydrogen bonds.The SEM images representing surface morphologies of γ-AlOOH at 50°C, 70°C, and 95°Care provided in Fig. 1b, Fig. 1c, and Fig. 1d, respectively.Of note, the nanostructure did not form at 50°C (Fig. 1b).As the temperature rose to 70°C, the 2D nanosheets grew perpendicular to the aluminum substrate (Fig. 1c), while a more pronounced and dense 2D nanosheet structure was formed at 95°C (Fig. 1d).At temperatures from 50°C to 95°C, the nanosheet structure changed from non-existent to visible and from lax to dense.The dioxygen octahedral structure of γ-AlOOH with strong in-plane covalent bonds and weak out-of-plane chemical bonds enabled exfoliation into 2D nanosheets with huge surface areas and ultra-high aspect ratios 23,24 .The hydrogen bond network was formed due to the presence of out-of-plane hydrogen atoms, which played an important role in the ionic conductivity of the electrolytes.
The FT-IR spectra of γ-AlOOH membranes at different temperatures are gathered in Fig. 2a.The peaks at 1069 cm − 1 corresponded to the bending vibration of Al-OH, while those at 3291 cm − 1 and 3096 cm − 1 were caused by the stretching vibrations of V as (Al)O-H and V s (Al)O-H, respectively 25 .The peaks near 753 cm − 1 , 627 cm − 1 , and 476 cm − 1 were assigned to Al-O twisting, stretching, and bending vibrations, respectively 26,27 .All peaks were characteristic of γ-AlOOH.Compared to γ-AlOOH-70℃, the intensity of the bending vibration peak of Al-OH for AlOOH-95℃ increased.Also, the twisting, stretching, and bending vibration peaks of Al-O revealed the same variation, suggesting pure nanosheets of AlOOH-95℃ containing more hydroxyl groups.Therefore, FT-IR con rmed the formation of γ-AlOOH.
The hydrogen-bond interactions in γ-AlOOH-70℃ and γ-AlOOH-95℃ were con rmed by XPS.The highresolution core line spectra (O1s) of γ-AlOOH-70℃ and γ-AlOOH-95℃ are displayed in Fig. 2c and Fig. 2d.The O1s spectra can be divided into three characteristic peaks: oxygen atoms bonded to metal (530.6 eV for Al-O-Al), hydroxyl groups or their surface-adsorbed oxygens (532.0 eV for Al-O-H), and minor amounts of adsorbed water (533.0eV for H-O-H) 28,29 .The Al-OH peak intensity was enhanced at 95°C, indicating the existence of more hydroxyl groups on the surface in the Pseudo-boehmite, conducive to the transport of proton or hydroxide ions.Compared to γ-AlOOH-75℃, the membrane γ-AlOOH-95℃ demonstrated a higher Al-O-H ratio, indicating Al-O-H with more hydroxyl groups or surface-adsorbed oxygen.Such hydroxyl groups or surface-adsorbed oxygens originated from the hydrogen bond network among hydroxyl groups in Pseudo-boehmite, interlayer OH − , and water molecules.The formed hydrogen bond network would endow the membrane with fast OH − transport behavior and high ionic conductivity.

Fabrication of γ-AlOOH-PBI composite membrane
The pristine PBI membrane with a thickness of 75 ± 5 µm and porosity of 84.47% was used as a support (or substrate) to prepare γ-AlOOH-PBI composite membranes.SEM images of cross-sections and surfaces of the two kinds of microstructure porous membranes are illustrated in Fig. S1 of the Supporting Information.A single dense skin layer surface and spongy-like cross-section morphology were formed on the support.The dense nature of the surface of the support, as well as the porous nature of the main structure, would be conducive to low gas crossover and high ionic conductivity.Using isopropyl alcohol as a non-solvent, the slow exchange rate between DMAc and IPA resulted in a very slow phase transition of isopropyl alcohol, contributing to the formation of a dense epidermal layer and a homogeneous honeycomb porous structure 30 .In addition, unlike the pristine PBI, the γ-AlOOH-PBI composite porous membrane exhibited a distinct layer of nanosheet structure on the bottom surface.
The FT-IR spectra of the pristine PBI and γ-AlOOH-PBI composite porous membrane are provided in Fig. 2b.The characteristic peak at 1630 cm − 1 was attributed to C = N bond stretching vibration of the imidazole ring 31 .In PBI spectrum, an absorption band was observed at 1169 cm − 1 and assigned to Ar-O-Ar characterizing the PBI backbone 32 .The peaks located at 1420 cm − 1 and 1280 cm − 1 were related to inplane deformation and stretching vibration of the imidazole rings, respectively 33

Adjustment of morphology and ionic conductivity
The ionic conductivity is crucial in ion transport membrane, mainly affecting the internal resistance.Therefore, the proton or hydroxide ion conductivities of both pristine PBI and γ-AlOOH-PBI membranes were evaluated to clarify the effect of the γ-AlOOH nanosheet layer on PBI composite membranes.Notably, the morphologies of the nanosheets could be regulated by acid or alkali etching at different stages, leading to adjustable ionic conductivity.
Compared to pristine PBI, the cross-section of γ-AlOOH-PBI composite membrane contained a nanosheet structure with a thickness of 600 nm (Fig. 3B).Furthermore, the bottom-surface of PBI membrane showed a dense structure (Fig. 3a), while that of γ-AlOOH-PBI composite membrane was covered with a layer of nanoparticles with a striped shape (Fig. 3b).Also, the nanosheets were grown vertically on the membrane surface to yield an array of 2D nanosheets (Fig. 3B-3G).Also, the proton and hydroxide ions followed a network of hydrogen bonds perpendicular to the membrane surface.Unlike other 2D nanosheet composite membranes, the hydrogen bond network structure perpendicular to the membrane surface played a key role in ion transport.The γ-AlOOH-PBI composite membranes contained strong hydrogen bond networks upon the introduction of γ-AlOOH nanosheets and exhibited a better ionic conductivity than the pristine PBI membrane (Fig. 4b).Several reasons contributed to these features.The rst had to do with the large network of hydrogen bonds between hydroxyl groups, interlayer ions, and water molecules in γ-AlOOH nanosheet, which ensured e cient ion transport.The second consisted of arrayed 2D AlOOH nanosheets grown vertically on the membrane surface, providing longitudinal transport channels along which ions can be rapidly delivered.
The hydroxide ion conductivity of the composite membrane can effectively be adjusted by alkali regulation.As shown in Fig. 3, the thickness of arrayed 2D AlOOH nanosheets deposited on PBI membrane surface gradually decreased as a function of the regulation time.The change in the content of Al element in EDS spectroscopy was consistent with this result (Fig. S3).Besides, the nanosheet layer structure became gradually exposed, from the stripe shape of the initial nanoparticles to the layered nanosheet.The hydroxide conductivity values at different KOH regulation times and temperatures of 25-60°C are given in Fig. 4a.An increasing tendency was observed as a function of the decrease in thickness of γ-AlOOH nanosheet.γ-AlOOH-PBI-B14 exhibited the highest hydroxide conductivity reaching 43.62 mSŸcm − 1 at 60°C, a value much larger than that of PBI-B3 membrane (18.38mS↔cm− 1 ).This value was also superior to those of Na on115, Na on212, and FAA-3-50 membranes (Fig. 4b).This can be attributed to the thinner nanosheet thickness, which can accelerate ion mobility, shorten the hopping distance, and reduce transport resistance.Meanwhile, the surface of the alkali-modulated AlOOH-PBI-B14 composite membrane showed a most complete arrayed 2D AlOOH nanosheet layer structure (Fig. 3e and Fig. 3E).However, the ionic conductivity of γ-AlOOH-PBI-B21 composite membrane decreased, which may be due to the dissolution and collapse of some nanosheets.
Meanwhile, the proton transport characteristics of γ-AlOOH-PBI composite membranes were studied under different temperatures and acid modulation days.The changing trend of the nanosheet thickness as a function of the regulation time looked similar to that of alkali regulation (Fig. S4).As shown in Fig. 4c, the proton conductivity showed rst an increasing trend followed by a decline as the thickness of γ-AlOOH nanosheet layer became thinner, similar to the trend of hydroxide conductivity.Unlike the alkali regulation, the proton conductivity of γ-AlOOH-PBI-A3 composite membrane signi cantly increased when compared to PBI-A3 membrane, while γ-AlOOH-PBI-A7 composite membrane showed a small ampli cation than γ-AlOOH-PBI-A3 composite membrane.As shown in Fig. 4d, the proton conductivity of γ-AlOOH-PBI-A7 composite membrane reached 61.80 mS•cm − 1 at 60°C, a value much higher than those of Na on115 (55.58 mS•cm − 1 ), Na on212 (36.03 mS•cm − 1 ), and pristine PBI membrane (39.60mS•cm − 1 ).

Ionic transport mechanism in γ-AlOOH-PBI composite membrane
Since the ionic conductivities of all membranes exhibited Arrhenius-type behavior, the activation energy (Ea) of the proton or hydroxide ion conduction process can be obtained by tting the data to the Arrhenius relationship between conductivity σ and temperature.This can be summarized by Eq. (2):

RT
The Arrhenius plots of different membranes are gathered in Fig. 5a and 5b.The γ-AlOOH-PBI composite membranes showed lower E a values than PBI membrane, suggesting composite membranes possessing better ion-exchange e ciency.The γ-AlOOH-PBI-B14 composite membrane exhibited the lowest E a of 11.31 kJ/mol, while γ-AlOOH-PBI-A7 composite membrane illustrated the lowest value of 2.43 kJ/mol, consistent with the ionic conductivity data.Moreover, all results suggested proton and hydroxide ion transport controlled by the Grotthuss mechanism (≤ 0.4 eV) 34 , with proton and hydroxide ion hopping between hydrogen bond networks in the 2D nanochannels.Generally, both proton and hydroxide ions transport followed the Grotthuss mechanism 34,35 , as well as the vehicular mechanism (standard diffusion) [36][37][38] , considered dominant transport mechanisms for proton and hydroxide transport through cation or anion transport membranes.
The hydroxide ions and proton transport mechanism in γ-AlOOH-PBI composite membranes are provided in Fig. 5c and 5d.The unique structure of γ-AlOOH facilitated the construction of ion channels.The proton or hydroxide ion conductivity of γ-AlOOH-PBI composite membranes improved with the incorporation of 2D nanosheets and may be due to several reasons.Firstly, the Grotthuss mechanism was responsible for the rapid proton and hydroxide ion conduction in γ-AlOOH-PBI composite membrane 39,40 , mainly due to the unique strong hydrogen bonding network structure of the arrayed 2D AlOOH nanosheet layer, as well as hydrogen bonding between 2D AlOOH nanosheet and the imidazole ring.Here, the transport of hydroxide ions could be considered as a reverse transport of protons in nanocon ned 2D nanosheet layers.Secondly, the introduction of 2D nanosheets increased the hydrophilic groups (Fig. S6), and stimulated the formation of ionic channels on the membranes, thereby facilitating the transport of proton or hydroxide ions according to vehicle mechanism.Thirdly, proton or hydroxide ions diffused through the hydrogen-bonded network of water molecules, and the formation/dissociation of covalent bonds combined with the diffusion/migration transport process (vehicular mechanism) 41 .
Besides, the EDS results showed an increase in content of elemental K in the composite membranes as a function of days of alkali conditioning then eventually remained stable, indicating membranes doped with a certain amount of KOH until saturation (Fig. S3).Hence, the combination between K + and -NH-took place in the imidazole ring of PBI owing to neutralization or interaction, thereby further stimulating hydroxide ions transport in KOH environment 42 .Similar to alkali regulation, the content of S increased gradually with regulation time (Fig. S5), and the presence of S element indicated the incorporation of a certain amount of H 2 SO 4 into the membrane.Thus, the proton hopping also included the exchange between protonated and unprotonated nitrogen of the polymer 43 .The amorphous structure of membranes and exible polymer chains facilitated the proton transition.

Alkaline water electrolysis and long-term stability
To further evaluate the practical applications of γ-AlOOH-PBI composite membranes in alkaline water electrolysis, the membranes based on commercial FAA-3-50, PBI, and γ-AlOOH-PBI-B14 were selected for electrolysis testing.A zero-gap single-cell alkaline water electrolysis was constructed for that purpose (Fig. 6a).In addition, the thermal and mechanical properties of the composite membranes were evaluated prior to testing.The novel composite membranes showed improved thermal and mechanical stabilities ((Fig.S7), conducive to hydrogen production from water electrolysis.
The polarization curves obtained for the commercial FAA-3-50, pristine PBI, and γ-AlOOH-PBI-B14 composite membranes are gathered in Fig. 6b.The performance of γ-AlOOH-PBI-B14 composite membrane was superior to those of FAA-3-50 and PBI membranes.The PBI membrane exhibited a voltage above 2.52 V at the current density of 500 mA•cm − 2 , while γ-AlOOH-PBI-B14 and FAA-3-50 membranes illustrated voltages of 1.74 V and 2.43 V at room temperature, respectively.Furthermore, a high current density of 1 A•cm − 2 was obtained at 2.07 V for the γ-AlOOH-PBI-B14 composite membrane.By comparison, FAA-3-50 membrane revealed a voltage greater than 2.52 V.Although the ionic conductivity of FAA-3-50 membrane was similar to that of γ-AlOOH-PBI-B14 composite membrane at room temperature, the electrolytic performance of γ-AlOOH-PBI-B14 composite membrane looked signi cantly better than that of FAA-3-50 membrane.This can be explained by two main reasons.On the one hand, the ordered γ-AlOOH nanosheet structure on γ-AlOOH-PBI-B14 composite membrane surface enhanced the catalyst utilization rate and improved the interface bonding with the catalyst.On the other hand, the better water uptake and alkali uptake of γ-AlOOH-PBI-B14 composite membrane reduced the resistance of ionic transport (Table S1).
The long-term electrolytic performances of FAA-3-50 and γ-AlOOH-PBI-B14 composite membranes were evaluated at a constant current of 500 mA•cm − 2 and room temperature (Fig. 6c).After 200h of zero-gap single-cell, the voltage of γ-AlOOH-PBI-B14 composite membrane was maintained at about 1.75V.However, the voltage of FAA-3-50 decreased signi cantly, probably due to the local strong alkaline environment formed on the membrane surface, which degraded the membrane under the attack of OH − , resulting in the membrane breakdown phenomenon.Therefore, the γ-AlOOH-PBI-B14 composite is promising as ACM membrane for alkaline water electrolysis applications.Additionally, γ-AlOOH-PBI-B14 exhibited superior performance, better than most currently reported PBI membranes (Fig. 7 and Table S2).

Conclusions
Arrayed 2D γ-AlOOH nanosheets and a series of novel γ-AlOOH-PBI composite membranes with adjustable ion conductivity were rstly constructed by in-situ growing-etching method.The results showed that the introduction of 2D γ-AlOOH nanosheet structures with highly targeted modulation by acid or base etching could signi cantly improve the ionic conductivity of γ-AlOOH-PBI membranes through the construction of strong hydrogen-bond networks and ion transport channels.The transport mechanism of proton and hydroxyl ions studied by activation energies calculations from Arrhenius plots suggested proton and hydroxide ions transport mainly controlled by the Grotthuss mechanism.The novel composite membranes also showed better thermal and mechanical stability.Meanwhile, the evaluation of γ-AlOOH-PBI-B14 composite membrane for alkaline water electrolysis revealed superior performance (1 A•cm − 2 at 2.07 V at room temperature) and long-term durability (at 500 cm − 2 for over 200 h).In sum, superionic ion transport and regulation of 2D γ-AlOOH nanosheet may improve the membrane properties, and composite membranes look promising for energy applications, including fuel cells, vanadium redox ow batteries, and carbon dioxide reduction.

Preparation of γ-AlOOH by in-situ growing method
The γ-AlOOH nanosheets were prepared by in-situ growing method.The process consisted of cutting the aluminum substrate to a certain size followed by soaking in 2% sodium hydroxide solution for 1-2 min to remove the impurities.Deionized water was then used for washing in water bath for 40 min at 50℃, 70℃, and 95℃, respectively.

Preparation of pristine PBI and γ-AlOOH-PBI composite membranes
The pristine PBI porous membranes were prepared by Nonsolvent Induced phase Separation (NIPS) method using DMAc as solvent and IPA as a coagulation bath.The polymer concentration was set to approximately 10 wt%, and the dissolved PBI solution was uniformly cast onto a clean glass substrate and then immersed into IPA to form the PBI membrane.
The γ-AlOOH-PBI composite membranes were manufactured by in-situ growing-etching method.Firstly, the treated aluminum sheets were dipped in a water bath for 40min at 95℃.The PBI solution was then uniformly cast on an aluminum plate containing nanolayers and put into IPA.Lastly, aluminum sheets were directly etched in 1-2% hydrochloric acid solution.The process is described in Scheme 1.
To regulate the ionic conductivity, the γ-AlOOH-PBI composite membranes were etched separately by acid or base.Each γ-AlOOH-PBI was immersed in 1MKOH and 3MH 2 SO 4 solution for 3, 7, 14, and 21 days at ambient temperature, respectively.By comparison, the pristine PBI was soaked for 3 days only.The pristine PBI and γ-AlOOH-PBI composite membranes were labeled as PBI-XY and γ-AlOOH-PBI-XY, where X represents A or B, A is acid-regulated, B refers to base-regulated, and Y is the immersing days.

Preparation of MEAs for alkaline water electrolysis
The MEAs used for alkaline water electrolysis were prepared through the catalyst-coated substrate (CCS) technique.The cathode catalyst layer was fabricated using 60% Pt/C, Na on ionomer, H 2 O, and isopropanol.The loading of Pt/C catalysts was set to ~ 0.5 mg •cm − 2 .The cathode catalyst slurry was sprayed onto the surface of C-GDL, and non-noble metals were utilized as anode catalyst.The preparation of self-supported anodic catalytic electrodes can be found in the previous report 44 .Firstly, Ni-GDL platforms (3cm⊆4cm) were immersed in 1MH 2 SO 4 for 1 h to remove residual impurities from the surface.Iron nitrate nonahydrate (Fe (NO 3 ) 3 •9H 2 O), Potassium bromide (KBr), and Cobaltous chloride (CoCl 2 ) were then dissolved in 50 mL deionized water at concentrations of 20 mM, 10 mM, and 5 mM, respectively.Subsequently, the Ni-GDLs with catalyst were immersed in the above solution for 12 h at room temperature under a continuous ow of oxygen above the liquid surface.After deposition, the obtained Ni-GDLs with catalyst were thoroughly rinsed with deionized water and then dried in air.
The ionic conductivity (σ) of each membrane was characterized by electrochemical impedance spectroscopy (EIS) at temperatures from 25°C to 70°C.The EIS measurements were performed with an Autolab PGSTAT 302N over the frequency range of 1 H Z to 1 MH Z .The membrane conductivity was determined by resistance measurements (R) of the real axis intercept of the Nyquist diagram at high frequencies.The ionic conductivity was calculated according to Eq. ( 1 . Compared to PBI, γ-AlOOH-PBI composite porous membrane showed a characteristic absorption band of Al-O near 547 cm − 1 , Al-OH around 1069 cm − 1 , and Vas (Al)O-H and Vs (Al)O-H around 3291 cm − 1 and 3096 cm − 1 , respectively.All peaks were typical characteristics of γ-AlOOH.Hence, FTIR con rmed the successful introduction of γ-AlOOH to the surfaces of composite membranes.EDS linear scanning was used to analyze the elementary compositions of the nanosheet layers, as well as the successful introduction of γ-AlOOH on the surface of the composite membrane.As shown in Fig. S2, γ-AlOOH-PBI membrane contained C, O, and Al elements.The O/Al atomic ratio was estimated to 44.73/24.44= 1.83, a value close to that of O/Al = 2 in γ-AlOOH.Moreover, the elemental contents of different parts of the nanosheets are presented.The O/Al atomic ratios in the middle of γ-AlOOH nanosheet structure and at the interface with the membrane looked much higher than the actual O/Al ratio in γ-AlOOH.Also, excess O and C elements were observed in PBI polymers.The detection of PBI elements in the middle of γ-AlOOH nanosheet structure may indicate the possible incorporation of PBI casting solution into the vertical nanosheet structure gaps during the preparation of γ-AlOOH-PBI membrane.Such a phenomenon may well promote the cross-bonding of PBI and 2D γ-AlOOH nanosheet structures. Figures

Figure 6 a
Figure 6