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Lamellar composite membrane with acid-base pair anchored layer-by-layer structure towards highly enhanced conductivity and stability
Journal of Membrane Science 602 (2020) 117978
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Lamellar composite membrane with acid-base pair anchored layer-by-layer structure towards highly enhanced conductivity and stability Jingtao Wang a, c, Yarong Liu a, Jingchuan Dang a, Guoli Zhou a, **, Yan Wang a, Yafang Zhang a, Lingbo Qu b, Wenjia Wu a, b, * a b c
School of Chemical Engineering, Zhengzhou University, Zhengzhou, 450001, PR China College of Chemistry, Zhengzhou University, Zhengzhou, 450001, PR China Henan Institute of Advanced Technology, Zhengzhou University, 97 Wenhua Road, Zhengzhou, 450003, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Lamellar composite membranes Acid-base pair Proton conductivity Water stability
Graphene oxide (GO) lamellar membranes have attracted intensive attentions in membrane separations due to the ultrafast molecule transport, but are seldom utilized as proton exchange membrane because of the inferior proton conduction and water stability. Herein, we demonstrate the fabrication of acid-base pair anchored lamellar composite membrane, by layer-by-layer assembling dopamine modified GO (DGO) nanosheets and phosphorylated bacterial cellulose (PBCn), for conquering the trade-off between proton conductivity and water stability. The abundant P– – O(OH)2 groups on PBCn form acid-base pairs with –NH2/ NH groups on DGO nanosheets. The acid-base pairs serve as low-energy-barrier proton transfer highways; meanwhile impose strong attractive force on adjacent GO layers. In this way, the lamellar composite membrane exhibits 244.9% enhancement of in-plane conductivity over GO membrane at 90 � C and 100% RH, and keeps stable in water medium even after 14 days. Moreover, the through-plane conductivity that determines fuel cell performance, gains 9.9 times augment (vs. GO membrane). Consequently, the transfer anisotropy coefficient significantly decreases from 15.5 to 4.9, thus offering a 128.9% enhancement in maximum power density (182.9 mW cm 2). Furthermore, the acid-base pairs impart membrane a high tensile strength of 203.5 MPa, surpassing most pre viously reported freestanding laminar membranes.
1. Introduction Graphene oxide (GO) lamellar membrane with ordered 2D transfer nanochannels, has attracted widespread interests in gas separation and water/solvent purification, due to the ultrafast and selective mass transport [1–4]. However, it is seldom utilized as proton exchange membrane (PEM), albeit the well-defined and ultra-stable transfer nanochannels, which is indispensable for superior and steady proton conduction performances. This is mainly because of the inferior con duction ability, originating from the lack of functional groups capable for proton hopping [3]. To this end, functionalized GO nanosheets with proton conduction groups have been prepared and vacuum-filtrated into lamellar mem branes for PEM [5–7]. The presence of functional groups can efficiently enhance the proton conduction of GO lamellar membrane. Ravikumar et al. obtained a proton conductivity of 40 mS cm 1 at 30 � C and 100%
RH for sulfonic acid functionalized GO lamellar membrane [6]. Gao et al. reported a 50% enhancement of proton conductivity (vs. GO membrane) by preparing ozone-oxidized GO lamellar membrane [7]. Unfortunately, these membranes still suffer from intrinsic shortcoming, i.e., the unacceptable stability in aqueous medium because of identical charge induced repulsive interactions between adjacent GO nanosheets. While water is essential for efficient proton conduction, whether for vehicle-type transfer or Grotthuss-type transfer [8,9]. Although this problem has been effectively settled by chemical reduction (e.g., HI, N2H4⋅H2O, HCl, etc.) or ion/covalent crosslinking (e.g., Al3þ, Mg2þ, glutaraldehyde, diamine, etc.) methods in other fields, these solutions are not applicable for PEM [10–14]. This is because, they remove or consume partial intrinsic conduction groups ( OH, C–O–C) on GO nanosheets, and insert proton insulated ions or molecules. These will lead to inferior proton conduction performance. In brief, a trade-off relationship exists between proton conductivity and water stability for
* Corresponding author. School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, PR China. ** Corresponding author. E-mail addresses: [email protected] (G. Zhou), [email protected] (W. Wu). https://doi.org/10.1016/j.memsci.2020.117978 Received 10 December 2019; Received in revised form 6 February 2020; Accepted 16 February 2020 Available online 17 February 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 602 (2020) 117978
grinding on a ball mill at 40 r min 1 for 6 h. Here, two kinds of PBCn, where n (n ¼ 2 or 4 h) represented different phosphorylation times, were synthesized as follows: 2.0 g BC powder was soaked in 100 mL DMF along with 40 g urea. After the mixture was stilled for 1 h, 20.0 g phosphoric acid was added dropwise. The mixture was put in an oil bath at 130 � C for phosphorylation. After cooling, the mixture was filtered and washed with a mixture of water and 1-propanol, then washed by 0.1 mol L 1 hydrochloric acid, and finally water. Ultimately, the resultant products were gained via lyophilization and ball-milling. The reaction equation was shown in Fig. S1.
GO lamellar membrane, which impedes the utilization as PEM. There fore, approaches that can simultaneously enhance the proton conduc tivity and water stability are urgently desired for GO lamellar membranes. A strategy of constructing acid-base pairs offers great potential to address these problems [15–20]. The presence of acid-base pairs can facilitate the protonation/deprotonation process and create a large number of proton defects, thus paving low-energy-barrier transfer pathways for proton hopping [21,22]. Ranjani et al. prepared acid-base paired composite membrane by incorporating sulfonated GO nanosheets into chitosan (CS) matrix, and found a 28.3% increase of proton con ductivity over the membrane with GO nanosheets [23]. Meanwhile, the strong electrostatic interactions among acid-base pairs can restrain the motion of adjacent polymer chains, thereby strengthening the water stability and mechanical property of mem branes. For instance, Li et al. fabricated dopamine-modified silica nanoparticles (DSiO2) and introduced them into poly(ether ether ke tone) (SPEEK) matrix. Compared with SPEEK-SiO2 membrane, the presence of acid-base pairs confers SPEEK-DSiO2 membrane 7.8% decrease in swelling ratio and 35.1% increase in tensile strength [24]. Thus, we conjecture that, constructing acid-base pairs in GO interlayers might be an effective approach in harvesting both proton conduction and water stability. However, to the best of our knowledge, relevant study is rarely reported. Herein, we demonstrate a fabrication of acid-base pair anchored lamellar composite membrane with simultaneously enhanced proton conduction and water stability, by layer-by-layer assembling dopamine modified GO (DGO) nanosheets and phosphorylated bacterial cellulose (PBCn). Bacterial cellulose (BC) was selected as medium, rather than common plant cellulose (tens of microns), due to its nanoscale dimen sion [25]. In addition, the rich surficial hydroxyl groups and large sur face area were favourable for subsequent phosphorylation and acid-base paired assembly. The formed acid-base pairs between PBCn and DGO nanosheet impart low-energy-barrier proton transfer pathways, and meantime effectively alleviate repulsive interaction between adjacent GO layers. As a result, proton conductivity and water stability of DGO@PBCn-X composite membrane are highly enhanced. Importantly, the through-plane proton conductivity of DGO@PBC4-3, which directly determines the fuel cell performance, shows a 9.9 times augment when compared with GO membrane. Therefore, the transfer anisotropy coef ficient (σk/σ?) remarkably decreases from 15.5 to 4.9, and the maximum power density is promoted by 128.9%. Furthermore, the membrane displays superior mechanical properties with tensile strength of 203.5 MPa, outperforming most previously reported freestanding laminar membranes. Collectively, our study may open a new avenue for devel oping advanced lamellar membranes by alleviating the intrinsic shortcomings.
2.3. Preparation of DGO and DGO@PBCn membranes Graphite powders were oxidized to prepare GO based on the Hum mers’ method [27]. DGO nanosheets were fabricated according to the method in literature [28]. To make free-standing DGO membrane, 25 mL, 1 mg mL 1 DGO dispersion was filtered on the surface of filter membrane. After vacuum-dried at 30 � C for 12 h, the DGO membrane was obtained by peeling off from the filter membrane. The DGO@PBCn composite membranes were synthesized as following: for preparing stable PBC4 dispersion, 0.5 g PBC4 was dispersed in 500 mL water/ formamide solution (1:4 v/v) under ultrasonication at 300 W for 2 h. Next, 25 mL DGO dispersion (1 mg mL 1) was mixed with 2 mL PBC4 dispersion (1 mg mL 1) and then sonicated for 30 min. The resultant DGO@PBC4-1 was then obtained by filtrating the mixture and drying for 24 h at 35 � C. Using the above method, DGO@PBC4-X, X (X ¼ 1, 2, 3, and 4) denoted the weight percentage of PBC4, and DGO@PBC2-3 were synthesized as shown in Table S1. And the average thickness of theses membranes ranged from 13 to 23 μm. Besides, PBC2 and PBC4 mem branes were also prepared by filtrating 25 mL PBC2 and PBC4 dispersions (1 mg mL 1), respectively. 2.4. Characterizations of DGO, BC, PBCn, and DGO@PBCn The morphology and structure of DGO, BC, PBCn, and membranes were characterized by atomic force microscopy (AFM, Bruker Dimen sion FastScan), scanning electron microscopy (SEM, Auriga FIB, Zeiss, Germany), and transmission electron microscopy (TEM, Tecnai G2 F20, FEI, U.S.). Their chemical structure and element content were probed by fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos, UK), and energy dispersive spectroscopy (EDS) on SEM. X-ray diffraction (XRD) patterns were tested on a Bruker D8 Advance ECO. Thermogravimetric analysis (TGA) was performed by 50 SHIMADZU equipment under N2 atmosphere. The Brunauer-Emmett Teller (BET) surface area of membranes was detected by automated gas sorption analyzer (Quantachrome Ltd., America). Zeta potentials were measured on Nano ZS (4 mW, He–Ne laser). The Raman spectrum was collected by LABRAM þ HR þ EVO equipment. The mechanical property of membranes was conducted by an electronic tensile machine (Testo metric 350AX) at a stretching rate of 1 mm min 1. Dynamic contact angles were tested on a FACE instrumentation (model OCA 25, Germany). IEC values of membranes were tested through back-titration method. The water uptake and dimensional swelling of the membranes were measured at 30 � C according to the method in literature [29]. The membrane resistance (R) was performed by the same experi mental instrumentation in our previously published work [30]. In-plane proton conductivity of membrane under 100% RH was probed by two parallel platinum electrodes in a temperature-controlled chamber equipped with H2O and D2O vapor. Through-plane proton conductivity of membrane at different temperatures under 100% RH was tested on a MTS-740 testing device (regulating the temperature and humidity) with H2O vapor. Proton conductivity was calculated according to the rela tionship: σ ¼ l/AR, where A is the cross-section of the membrane, R is the resistance, and l is the length between two electrodes. Membrane electrode assemblies (MEAs) were fabricated in
2. Experimental 2.1. Materials and chemicals Graphite powder (45 μm) was purchased from Sigma-Aldrich. BC was provided by QiHong Technology Co., Ltd. Filter membrane (pore diameter, 0.4 μm) was supplied by Tianjin Jinteng Experimental equipment Co., Ltd. All other reagents and solvents were of reagent grade and were utilized as received. Deionized water was used throughout the experiment. 2.2. Preparation of BC and PBCn The purification and phosphorylation of BC were performed via similar procedure reported earlier [26]. In brief, the purchased hydrogel was washed and subsequently treated with 2% (w/v) NaOH solution for 24 h, and finally rinsed again until neutral. The purified BC was minced into microgel and freeze-dried for 3 days. Then the BC was obtained by 2
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Journal of Membrane Science 602 (2020) 117978
accordance with the method in literature [5,31]. Electrocatalyst ink was prepared by mixing Pt/C electrocatalyst (40 wt% Pt, JM), isopropanol, Nafion solution (D2020, DuPont), and deionized water. The Nafion ionomer content in catalyst ink was 20 wt%. Afterwards, the catalyst ink was sprayed onto the surface of both sides of as-prepared membrane to prepare MEAs, and the Pt loadings in cathodic and anodic catalytic layers were both 0.3 mg cm 2. Then, two pieces of carbon paper (2 cm � 2 cm) were attached to both sides of MEAs by hot-pressing at 132 � C and 4.0 MPa for 2.0 min. Finally, these MEAs were sandwiched between two graphite plates and assembled into single cells for testing. The perfor mances of fuel cells were tested at 60 � C and 100% RH. The flux rates of H2 and O2 were 80 and 120 mL min 1, respectively. In addition, to evaluate the stability of MEAs, the voltage of the fuel cells based on the membrane was measured for 24 h under the identical system conditions, as the constant current density was 30 mA cm 2.
phosphorylation of BC (Fig. S6). With the phosphorylated degree increasing, the char yield gradually reduces from 28.3 wt% for PBC2 to 21.1 wt% for PBC4. Next, the DGO nanosheets and PBCn were dispersed in, respectively, water and water/formamide solution, followed by mixing together as precursor solution. The obvious Tyndall Effect in Figs. 1a and S7 in dicates the uniform dispersion of DGO nanosheets and/or PBC4. Zeta potential results in Fig. 1b reveal that, the presence of –NH2/ NH groups confers DGO nanosheets high positive zeta potentials with the pH – O(OH)2 groups impart PBC4 negative ranging from 2 to 8, while the P– zeta potentials. The opposite charge permits a pre-assembly of DGO nanosheets and PBC4 in solution via acid-base electrostatic interactions, which can be verified by FTIR results of precursor solution (Fig. 1c). Compared with DGO, the incorporation of PBC4 gives two characteristic – O and P–OH groups peaks at 1204 and 973 cm 1, corresponding to P– [26,35]. While, the peak intensity related to –NH2/ NH groups (1607 cm 1) weakens sharply for DGO@PBC4-1 and DGO@PBC4-3 suggesting the generation of electrostatic interactions between PBC4 and DGO nanosheets [36]. This is favourable for the construction of layer-by-layer structure lamellar composite membrane (Fig. 1d). Here, four types of DGO@PBC4-X precursor solutions, where X (X ¼ 1, 2, 3, or 4) corre sponds to different PBCn contents, were prepared and filtrated for lamellar composite membranes (DGO@PBCn-X, Table S1), where X (X ¼ 1, 2, 3, or 4) corresponds to different PBCn contents, were prepared and filtrated for lamellar composite membranes (DGO@PBCn-X, Table S1). Cross-sectional SEM image in Fig. 2a shows that DGO membrane exhibits a compact lamellar structure. By comparison, the cross-sections of DGO@PBCn-X keep lamellar structure (Figs. 2b and S8). Surficial SEM images show that the surface of DGO@PBC4-3 becomes rough, and fibrous morphology is observed (Fig. S9). In AFM images (Figs. 2c, d, and S10), clear fibrous structures are observed on the surface of DGO@PBCn-X, and the number of visible fiber increases with PBCn loading. According to TGA results in Fig. S11, the PBC4 contents in DGO@PBC4-1, DGO@PBC4-2, DGO@PBC4-3, and DGO@PBC4-4 are calculated to be 9.1, 16.4, 31.9, and 44.8, respectively (Table S1). The PBC2 content of the reference of DGO@PBC2-3 is 32.7 wt%. The P elemental mapping in Fig. 2e suggests the homogeneous distribution of
3. Results and discussion DGO nanosheets were prepared by self-polymerization of dopamine on GO surface. GO shows a typical sheet structure with mean lateral size of ~3 μm and thickness of ~0.89 nm (AFM, Fig. S2a). After dopamine polymerization, the sheet structure and size of DGO are well maintained (TEM, Fig. S2b). The thickness of DGO increases to ~1.2 nm due to the introduced poly-dopamine layer (AFM, Fig. S2c). This can be further verified by FTIR, as shown in Fig. S3. A new peak of N–H stretching vibration is observed for DGO at 1591 cm 1 [32]. Two kinds of PBCn (n ¼ 2 or 4) with different phosphorylated degree were synthesized by controlling reaction time (Fig. S1). AFM image reveals that the length and diameter of BC are in micrometer scale and ~3.5 nm, respectively (Fig. S4a). PBC4 keeps its fibrous morphology after phosphorylation, and its diameter is ~3.7 nm, which is slightly larger than that of BC (AFM, Fig. S4b). FTIR displays that the phosphorylation brings four new peaks for PBCn at 2398.2, 1426.9, 898.4, and 812.7 cm 1, corresponding to – O, P–O–C, and P–OH, respectively [26,33,34]. Meanwhile, the P–H, P– intensity of these four peaks increases from PBC2 to PBC4, suggesting the elevated amount of phosphate groups (Fig. S5). The lower char yields of PBC2 and PBC4 (compared with BC) further confirm the successful
Fig. 1. (a) Digital photographs of DGO, PBC4, and DGO@PBC4-3 precursor solutions. (b) Zeta potential of DGO in water and PBC4 in water/formamide solution with pH of 2–10. (c) FTIR spectra of DGO in water and DGO@PBC4-1 and DGO@PBC4-3 in water/formamide solution. (d) Schematic of the preparation of DGO@PBCn-X. 3
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Journal of Membrane Science 602 (2020) 117978
Fig. 2. Cross-sectional SEM images of (a) DGO and (b) DGO@PBC4-3. AFM images of (c) DGO and (d) DGO@PBC4-3. (e) P elemental mapping of cross-sectional DGO@PBC4-3. (f) Raman spectra of DGO, DGO@PBC4-1, and DGO@PBC4-3. (g) XRD patterns of dry DGO, DGO@PBC4-1, and DGO@PBC4-3.
Fig. 3. (a) High-resolution XPS of N 1s peak for DGO, DGO@PBC4-1, and DGO@PBC4-3. (b) BET surface area of DGO, DGO@PBC4-1, DGO@PBC2-3, and DGO@PBC4-3. (c) Full scan XPS spectra of DGO, DGO@PBC4-1, and DGO@PBC4-3 (insert: amplification of the P2p peak). (d) Schematic illustration of acid-base interactions in DGO@PBC4-3. (e) The mechanical properties of GO, DGO, DGO@PBC4-1, and DGO@PBC4-3. (f) Comparison in mechanical strength of GO and DGO@PBC4-3. 4
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Journal of Membrane Science 602 (2020) 117978
PBC4 in membrane. Together, these results demonstrate the successful fabrication of DGO@PBCn-X composite membrane. The layer-by-layer structure of DGO@PBCn-X was verified by Raman and XRD characterizations. Raman spectra in Fig. 2f show that all the membranes display two peaks, assigning to D band (1346.9 cm 1) and G band (1578.7 cm 1). Compared with the area ratio (ID/IG) of 0.97 for DGO membrane, it elevates to 1.09 and 1.35 for DGO@PBC4-1 and DGO@PBC4-3, respectively. This increase should arise from the inter calation of PBCn in interlayers of DGO membrane, which increases the defect density and disorder degree of DGO membrane [27,37]. The speculation can be confirmed by XRD results (Figs. 2g and S12). DGO membrane displays a d-spacing of 0.86 nm. Intercalating PBCn elevates the d-spacing of DGO@PBCn-X to 1.34 nm for DGO@PBC4-1. While it is interesting to find that, the d-spacing of DGO@PBCn-X decreases with the increase of the amount and phosphorylated degree of PBCn. For instance, the d-spacing values of DGO@PBC4-2 and DGO@PBC4-3 are 1.11 and 0.99 nm, respectively. These phenomena should result from the formation of acid-base interactions between NH2/ NH groups on – O(OH)2 groups on PBCn, which narrows the DGO nanosheets and P– distance between adjacent DGO layers. High-resolution XPS of N 1s peak and FTIR spectra of membranes further demonstrate the formation of acid-base interactions. Fig. 3a shows that the binding energy of pyrrolic N atoms53 increases from 400.5 eV for DGO to 400.7 and 401.5 eV for DGO@PBC4-1 and DGO@PBC4-3, respectively [38,39]. This energy shift indicates the change of electron distribution and the formation of acid-base interactions [40]. And acid-base pair formation also decreases the peak intensity of N–H groups in DGO@PBCn-X (1586.3 cm 1, Fig. S13) [31]. These findings indicate the acid-base pair anchored layer-by-layer structure in these lamellar composite membranes. The insertion of PBCn in DGO interlayers meanwhile increases the BET surface area of lamellar composite membrane. Figs. 3b and S14 show that DGO membrane obtains a BET surface area of 36.3 m2 g 1. By comparison, the BET surface area of DGO@PBC4-1 elevates to 48.7 m2 g 1. Besides, it increases with fiber amount for DGO@PBC4-X (Table S2). DGO@PBC4-3 and DGO@PBC2-3 with similar PBCn loading amount obtain comparable BET surface area (~102 m2 g 1). Mean – O(OH)2 group content of DGO@PBC4-X also increases while, The P– with PBC4 loading amount and shows positive correlation with its BET surface area. (Figs. 3c and S15). According to the peak area ratios, the P contents of DGO@PBC4-1, DGO@PBC4-2, DGO@PBC4-3, and DGO@PBC4-4 are calculated to be 1.6, 2.9, 6.3, and 8.6 wt%, respec – O(OH)2 group contents of 4.2, 7.6, 16.5, tively, correlating with P– and 22.5 wt% (Table S2). By comparison, DGO@PBC2-3 attains a much – O(OH)2 group content of 4.7 wt% than DGO@PBC4-3, due to lower P– the lower phosphorylated degree. Accordingly, the intercalation of PBCn – O(OH)2 groups would with high BET surface area and abundant P– confer DGO@PBCn-X continuous proton transfer networks and strong interlayer interactions (Fig. 3d). The mechanical properties in Figs. 3e and S16 reveal that GO membrane exhibits a tensile strength of 30.1 MPa and an elastic modulus of 3.8 GPa. By comparison, the tensile strength and elastic modulus of DGO membrane increase to 73.4 MPa and 6.2 GPa, respec tively. While the generation of strong acid-base interactions endows DGO@PBCn-X with highly enhanced mechanical property, and the me chanical property enhances with PBCn loading amount. The tensile strength and elastic modulus of DGO@PBC4-3 reach 203.5 MPa and 14.2 GPa, respectively, which are ~6.8 and ~3.7 times of that of GO membrane. Such performance surpasses most previously reported freestanding lamellar membranes including GO, montmorillonite (MMT), covalent organic framework (COF), etc (Table S3). Compared with DGO@PBC4-3, the tensile strength and elastic modulus of DGO@PBC2-3 are a litter lower (178.1 MPa, 11.0 GPa) owing to the relatively weaker acid-base interactions. Macroscopically, DGO@PBC4-3 with a thickness of 21 μm and a width of 13 mm is strong enough to support about 200 g mass (Fig. 3f), while GO and DGO membranes fail to do so (Fig. S17). The strong acid-base interactions between PBCn and DGO nanosheets
then confer DGO@PBCn-X highly boosted water stability. Figs. 4a and S18 show that, after immersing in water for 14 d, GO and DGO mem branes break into small fragments, while DGO@PBCn-X remains stable. After an additional treatment of stirring at a speed of 500 r min 1 for 5 min, only DGO@PBC4-3 still remains stable. These phenomena should result from the strong acid-base interactions between PBC4 and DGO layers, which effectively inhibit them from separating from each other. This speculation is also verified by XRD results. Compared with the sharply increased d-spacing of GO membrane from 0.75 nm under dry condition to 1.56 nm under wet condition, DGO membrane displays a smaller d-spacing change from 0.86 nm to 1.51 nm, due to the surface adhesion properties of poly-dopamine layer (Figs. 2g, 4b, and S19) [41]. Similarly, DGO@PBC4-1 also exhibits an increase in d-spacing (from 1.34 to 1.85 nm). While the increase degree (0.51 nm) is less than that of DGO membrane (0.65 nm), and especially, the d-spacing of DGO@PBC4-3 only increases by 0.02 nm. Considering the stronger hy drophilicity and higher water uptake of DGO@PBCn-X (Fig. S20 and Table S2), the less increase in d-spacing together with the lower swelling ratio further confirm the effect of acid-base interactions on enhancing water stability. The water uptake and swelling ratio (through-plane) of DGO membrane at 30 � C are 25.8% and 24.4%, respectively. In com parison, DGO@PBC4-3 attains an enhanced water uptake of 44.4% and a reduced swelling ratio of 13.2%. In addition, XRD results in Fig. 4c reveal that DGO@PBC4-3 can also remain stable in various conditions, including acidic solution, basic solution, as well as a series of solvents. The abundant acid-base pairs along fiber-sheet interfaces then serve as transfer highways, imparting lamellar composite membrane highly enhanced proton conduction. Figs. 5a and S21a show that the in-plane conductivities of all membranes increase with the increase of tempera ture (ranging of 30–90 � C) and exhibit a positive correlation with IEC and water uptake as listed in Table S2. The in-plane conductivities of GO and DGO membranes display in-plane conductivities of 62.4 and 72.2 mS cm 1 under 90 � C and 100% RH, respectively. By comparison, DGO@PBC4-3 with higher IEC value and water uptake gains an elevated in-plane conductivity, reaching 215.2 mS cm 1, 244.9% higher than that of GO membrane. Apart from the higher IEC value and water up take, such huge conduction enhancement should arise from the formed acid-base pairs (e.g., –PO–3⋅⋅⋅þ3HN–) along fiber-sheet interfaces, in which the de-protonation/protonation process is significantly promoted by the strong electrostatic attractions between acid and base groups [21, 42]. As a result, the transfer energy barrier is obviously reduced. To verify this speculation, DGO@PBC2-3 with similar structure to – O(OH)2 groups was prepared. And it DGO@PBC4-3 but reduced P– displays a reduced in-plane conductivity of 133.3 mS cm 1 under identical condition due to the decreased number of acid-base pairs (Fig. S21b). Direct evidence is provided by the conduction activation energy (Ea, Fig. S21c), which was calculated from a linear fitting of temperature-dependent in-plane conductivities. It shows that the Ea value obviously decreases from 11.2 eV for GO membrane to 4.89 eV for DGO@PBC4-3 (Fig. 5a). Then, deuterium-related experiments were performed and the results are shown in Fig. 5b. All the membranes display decreased conductivity under D2O vapor (vs. H2O vapor), due to the involvement of heavier deuterium atom (vs. proton) in Grotthuss-type migration [43]. And DGO@PBC4-3 attains a more distinct conductivity reduction of 61.2% under D2O vapor when compared with the 34.4% for GO membrane and 39.9% for DGO membrane. This suggests that the constructed acid-base pairs in nano channels enable more proportion of Grotthuss-type migration in DGO@PBC4-3. Note that excessive PBC4 loading amount would aggre gate in DGO@PBC4-4, causing a decline of in-plane conductivity (170.2 vs. 215.2 mS cm 1 for DGO@PBC4-3, Fig. S21a). Next, the through-plane conductivities of membranes, which deter mine the full cell performance, were measured at different temperature (Figs. 5c and S22). The through-plane conductivities of GO and DGO membrane are, respectively, 4.03 and 6.60 mS cm 1 at 90 � C and 100% RH (Figs. 5c and S22a). In comparison, DGO@PBC4-3 acquires almost 5
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Fig. 4. (a) Photographs of GO and DGO@PBC4-3 after being immersed in water for 14 d and then stirred at a speed of 500 r min 1 for 5 min. (b) The d-spacing of GO, DGO, DGO@PBC4-1, and DGO@PBC4-3 under dry and wet conditions (The values are calculated from XRD). (c) XRD patterns of DGO@PBC4-3 after treatment in different solutions for 14 d (The concentrations of HCl and NaClO solution are 2 mol L 1 and 300 ppm, respectively).
Fig. 5. (a) Temperature-dependent in-plane conductivities (σk) at 100% RH. (b) In-plane conductivities under H2O and D2O vapor at 90 � C. (c) Temperaturedependent through-plane conductivities (σ?) at 100% RH. (d) In-plane conductivities (σk), through-plane conductivities (σ?), and transfer anisotropy coefficient (σk/σ?) at 90 � C and 100% RH. (e) Weight losses and through-plane conductivities of DGO@PBC4-3 in 30 � C water over time. (f) Performance comparison between DGO@PBC4-3 and various previously reported membranes.
10 times’ higher through-plane conductivity than GO membrane, reaching 43.9 mS cm 1. Notably, such conductivity enhancement at vertical direction is more striking than that at parallel direction (a 2.4fold increase). Moreover, the Ea value for proton transfer at vertical direction declines by 76.9%, from 40.3 kJ mol 1 for GO membrane to 9.30 kJ mol 1 for DGO@PBC4-3. By comparison, the Ea value at parallel direction only decreases by 56.3% (from 11.2 to 4.89 kJ mol 1, Figs. 5a and S22b). As a result, the transfer anisotropy coefficient (σk/σ?) de scends to 4.9 for DGO@PBC4-3 from the 15.5 for GO membrane at 90 � C and 100% RH (Fig. 5d). Importantly, the excellent water stability brings acceptable conduction stability for these lamellar composite mem branes. DGO@PBC4-3 was immersed in water at 30 � C, and meanwhile its through-plane proton conductivity and weight loss were recorded consecutively for 90 h. Fig. 5e reveals that there is no significant weight loss during the entire measurements. And the through-plane proton conductivity of DGO@PBC4-3 is constant over days. This stable con ductivity should be due to the presence of strong acid-base interactions, which improves membrane structural (e.g., anti-swelling property) and
chemical stabilities. Collectively, the constructed acid-base pairs be tween adjacent DGO layers permit lamellar composite membrane simultaneously enhanced stability and conductivity. The performance of DGO@PBC4-3 in this work is superior to many reported membranes, including GO-based lamellar membranes, hybrid membranes, and block copolymer membranes (Fig. 5f and Table S4). The as-prepared membranes were assembled into MEAs for measuring the hydrogen fuel cell performance at 60 � C and 100% RH (Fig. 6a and b). The open circuit voltage of all membranes is above 0.9 V, suggesting an acceptable gas barrier property (Fig. 6a) [44]. GO mem brane gains the maximum power density of 79.9 mW cm 2 and current density of 305.3 mA cm 2. In comparison, DGO membrane shows enhanced maximum power density (109.7 mW cm 2) and current density (394.4 mA cm 2), because of the improved proton conductivity. Furthermore, the formed acid-base pairs in interlayers of DGO@PBCn-X give highly promoted proton conduction and thus fuel cell perfor mances. For instance, DGO@PBC4-3 achieves the maximum power density and current density of 182.9 mW cm 2 and 674.8 mA cm 2, 6
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Fig. 6. (a) Single cell performances of the membranes at 60 � C and 100% RH. (b) The stability of the MEAs at 30 mA cm
respectively, which are enhanced by 128.9% and 121.0% as compared to GO membrane. The stability of MEAs based on GO membrane and DGO@PBC4-3 in single cell was evaluated at a current of 30 mA cm 2 for 24 h. As shown in Fig. 6b, the voltage of fuel cell based on GO membrane declines ~4.2%, deriving from structure deformation under operating condition [5]. By comparison, the excellent structure stability confers the MEA based on DGO@PBC4-3 with almost no voltage loss, holding potential for practical application.
2
for 24 h.
& editing. Acknowledgments This work is supported by the National Natural Science Foundation of China (21576244), Natural Science Foundation of Henan Province (182300410276), Training Plan for Young Backbone Teachers in Uni versities of Henan Province (2018GGJS003), and Fok Ying Tung Edu cation Foundation (161065). Center for advanced analysis and computational science, Zhengzhou University is also highly acknowledged.
4. Conclusions In summary, we demonstrate a simultaneous enhancement of proton conductivity and water stability related to GO-based lamellar mem branes, by preparing a layer-by-layer structured lamellar composite membrane from DGO nanosheets and PBCn. The constructed acid-base pairs between PBCn and DGO nanosheets act as low-energy-barrier proton transfer pathways. As a result, DGO@PBC4-3 obtains in-plane conductivity of 215.2 mS cm 1, 244.9% improvement over GO mem brane. Moreover, it acquires a more obvious augment in through-plane conductivity by almost 10 times of that of GO membrane. As a conse quence, the transfer anisotropy coefficient remarkably decreases, and hydrogen fuel cell performances are enhanced by over 120%. Further more, the acid-base pairs effectively, making the lamellar composite membrane show no breakdown or delamination in water, acid, and basic solutions even after 14 d. Besides, the lamellar composite membrane achieves a high tensile strength of 203.5 MPa. Such mechanical property outperforms most previously reported freestanding lamellar mem branes, including GO, MMT, COF etc. The construction of layer-by-layer anchored structure may pave a way to designing advanced lamellar membranes with highly enhanced performances for many transport and separation applications.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Jingtao Wang: Conceptualization, Methodology, Investigation, Writing - original draft. Yarong Liu: Investigation, Data curation, Visualization, Writing - original draft. Jingchuan Dang: Methodology, Investigation, Visualization. Guoli Zhou: Resources, Methodology, Su pervision, Writing - review & editing. Yan Wang: Resources, Software. Yafang Zhang: Software, Visualization. Lingbo Qu: Writing - review & editing. Wenjia Wu: Conceptualization, Visualization, Writing - review 7
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Lamellar composite membrane with acid-base pair anchored layer-by-layer structure towards highly enhanced conductivity and stability Jingtao Wang a, c, Yarong Liu a, Jingchuan Dang a, Guoli Zhou a, **, Yan Wang a, Yafang Zhang a, Lingbo Qu b, Wenjia Wu a, b, * a b c
School of Chemical Engineering, Zhengzhou University, Zhengzhou, 450001, PR China College of Chemistry, Zhengzhou University, Zhengzhou, 450001, PR China Henan Institute of Advanced Technology, Zhengzhou University, 97 Wenhua Road, Zhengzhou, 450003, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Lamellar composite membranes Acid-base pair Proton conductivity Water stability
Graphene oxide (GO) lamellar membranes have attracted intensive attentions in membrane separations due to the ultrafast molecule transport, but are seldom utilized as proton exchange membrane because of the inferior proton conduction and water stability. Herein, we demonstrate the fabrication of acid-base pair anchored lamellar composite membrane, by layer-by-layer assembling dopamine modified GO (DGO) nanosheets and phosphorylated bacterial cellulose (PBCn), for conquering the trade-off between proton conductivity and water stability. The abundant P– – O(OH)2 groups on PBCn form acid-base pairs with –NH2/ NH groups on DGO nanosheets. The acid-base pairs serve as low-energy-barrier proton transfer highways; meanwhile impose strong attractive force on adjacent GO layers. In this way, the lamellar composite membrane exhibits 244.9% enhancement of in-plane conductivity over GO membrane at 90 � C and 100% RH, and keeps stable in water medium even after 14 days. Moreover, the through-plane conductivity that determines fuel cell performance, gains 9.9 times augment (vs. GO membrane). Consequently, the transfer anisotropy coefficient significantly decreases from 15.5 to 4.9, thus offering a 128.9% enhancement in maximum power density (182.9 mW cm 2). Furthermore, the acid-base pairs impart membrane a high tensile strength of 203.5 MPa, surpassing most pre viously reported freestanding laminar membranes.
1. Introduction Graphene oxide (GO) lamellar membrane with ordered 2D transfer nanochannels, has attracted widespread interests in gas separation and water/solvent purification, due to the ultrafast and selective mass transport [1–4]. However, it is seldom utilized as proton exchange membrane (PEM), albeit the well-defined and ultra-stable transfer nanochannels, which is indispensable for superior and steady proton conduction performances. This is mainly because of the inferior con duction ability, originating from the lack of functional groups capable for proton hopping [3]. To this end, functionalized GO nanosheets with proton conduction groups have been prepared and vacuum-filtrated into lamellar mem branes for PEM [5–7]. The presence of functional groups can efficiently enhance the proton conduction of GO lamellar membrane. Ravikumar et al. obtained a proton conductivity of 40 mS cm 1 at 30 � C and 100%
RH for sulfonic acid functionalized GO lamellar membrane [6]. Gao et al. reported a 50% enhancement of proton conductivity (vs. GO membrane) by preparing ozone-oxidized GO lamellar membrane [7]. Unfortunately, these membranes still suffer from intrinsic shortcoming, i.e., the unacceptable stability in aqueous medium because of identical charge induced repulsive interactions between adjacent GO nanosheets. While water is essential for efficient proton conduction, whether for vehicle-type transfer or Grotthuss-type transfer [8,9]. Although this problem has been effectively settled by chemical reduction (e.g., HI, N2H4⋅H2O, HCl, etc.) or ion/covalent crosslinking (e.g., Al3þ, Mg2þ, glutaraldehyde, diamine, etc.) methods in other fields, these solutions are not applicable for PEM [10–14]. This is because, they remove or consume partial intrinsic conduction groups ( OH, C–O–C) on GO nanosheets, and insert proton insulated ions or molecules. These will lead to inferior proton conduction performance. In brief, a trade-off relationship exists between proton conductivity and water stability for
* Corresponding author. School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, PR China. ** Corresponding author. E-mail addresses: [email protected] (G. Zhou), [email protected] (W. Wu). https://doi.org/10.1016/j.memsci.2020.117978 Received 10 December 2019; Received in revised form 6 February 2020; Accepted 16 February 2020 Available online 17 February 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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grinding on a ball mill at 40 r min 1 for 6 h. Here, two kinds of PBCn, where n (n ¼ 2 or 4 h) represented different phosphorylation times, were synthesized as follows: 2.0 g BC powder was soaked in 100 mL DMF along with 40 g urea. After the mixture was stilled for 1 h, 20.0 g phosphoric acid was added dropwise. The mixture was put in an oil bath at 130 � C for phosphorylation. After cooling, the mixture was filtered and washed with a mixture of water and 1-propanol, then washed by 0.1 mol L 1 hydrochloric acid, and finally water. Ultimately, the resultant products were gained via lyophilization and ball-milling. The reaction equation was shown in Fig. S1.
GO lamellar membrane, which impedes the utilization as PEM. There fore, approaches that can simultaneously enhance the proton conduc tivity and water stability are urgently desired for GO lamellar membranes. A strategy of constructing acid-base pairs offers great potential to address these problems [15–20]. The presence of acid-base pairs can facilitate the protonation/deprotonation process and create a large number of proton defects, thus paving low-energy-barrier transfer pathways for proton hopping [21,22]. Ranjani et al. prepared acid-base paired composite membrane by incorporating sulfonated GO nanosheets into chitosan (CS) matrix, and found a 28.3% increase of proton con ductivity over the membrane with GO nanosheets [23]. Meanwhile, the strong electrostatic interactions among acid-base pairs can restrain the motion of adjacent polymer chains, thereby strengthening the water stability and mechanical property of mem branes. For instance, Li et al. fabricated dopamine-modified silica nanoparticles (DSiO2) and introduced them into poly(ether ether ke tone) (SPEEK) matrix. Compared with SPEEK-SiO2 membrane, the presence of acid-base pairs confers SPEEK-DSiO2 membrane 7.8% decrease in swelling ratio and 35.1% increase in tensile strength [24]. Thus, we conjecture that, constructing acid-base pairs in GO interlayers might be an effective approach in harvesting both proton conduction and water stability. However, to the best of our knowledge, relevant study is rarely reported. Herein, we demonstrate a fabrication of acid-base pair anchored lamellar composite membrane with simultaneously enhanced proton conduction and water stability, by layer-by-layer assembling dopamine modified GO (DGO) nanosheets and phosphorylated bacterial cellulose (PBCn). Bacterial cellulose (BC) was selected as medium, rather than common plant cellulose (tens of microns), due to its nanoscale dimen sion [25]. In addition, the rich surficial hydroxyl groups and large sur face area were favourable for subsequent phosphorylation and acid-base paired assembly. The formed acid-base pairs between PBCn and DGO nanosheet impart low-energy-barrier proton transfer pathways, and meantime effectively alleviate repulsive interaction between adjacent GO layers. As a result, proton conductivity and water stability of DGO@PBCn-X composite membrane are highly enhanced. Importantly, the through-plane proton conductivity of DGO@PBC4-3, which directly determines the fuel cell performance, shows a 9.9 times augment when compared with GO membrane. Therefore, the transfer anisotropy coef ficient (σk/σ?) remarkably decreases from 15.5 to 4.9, and the maximum power density is promoted by 128.9%. Furthermore, the membrane displays superior mechanical properties with tensile strength of 203.5 MPa, outperforming most previously reported freestanding laminar membranes. Collectively, our study may open a new avenue for devel oping advanced lamellar membranes by alleviating the intrinsic shortcomings.
2.3. Preparation of DGO and DGO@PBCn membranes Graphite powders were oxidized to prepare GO based on the Hum mers’ method [27]. DGO nanosheets were fabricated according to the method in literature [28]. To make free-standing DGO membrane, 25 mL, 1 mg mL 1 DGO dispersion was filtered on the surface of filter membrane. After vacuum-dried at 30 � C for 12 h, the DGO membrane was obtained by peeling off from the filter membrane. The DGO@PBCn composite membranes were synthesized as following: for preparing stable PBC4 dispersion, 0.5 g PBC4 was dispersed in 500 mL water/ formamide solution (1:4 v/v) under ultrasonication at 300 W for 2 h. Next, 25 mL DGO dispersion (1 mg mL 1) was mixed with 2 mL PBC4 dispersion (1 mg mL 1) and then sonicated for 30 min. The resultant DGO@PBC4-1 was then obtained by filtrating the mixture and drying for 24 h at 35 � C. Using the above method, DGO@PBC4-X, X (X ¼ 1, 2, 3, and 4) denoted the weight percentage of PBC4, and DGO@PBC2-3 were synthesized as shown in Table S1. And the average thickness of theses membranes ranged from 13 to 23 μm. Besides, PBC2 and PBC4 mem branes were also prepared by filtrating 25 mL PBC2 and PBC4 dispersions (1 mg mL 1), respectively. 2.4. Characterizations of DGO, BC, PBCn, and DGO@PBCn The morphology and structure of DGO, BC, PBCn, and membranes were characterized by atomic force microscopy (AFM, Bruker Dimen sion FastScan), scanning electron microscopy (SEM, Auriga FIB, Zeiss, Germany), and transmission electron microscopy (TEM, Tecnai G2 F20, FEI, U.S.). Their chemical structure and element content were probed by fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos, UK), and energy dispersive spectroscopy (EDS) on SEM. X-ray diffraction (XRD) patterns were tested on a Bruker D8 Advance ECO. Thermogravimetric analysis (TGA) was performed by 50 SHIMADZU equipment under N2 atmosphere. The Brunauer-Emmett Teller (BET) surface area of membranes was detected by automated gas sorption analyzer (Quantachrome Ltd., America). Zeta potentials were measured on Nano ZS (4 mW, He–Ne laser). The Raman spectrum was collected by LABRAM þ HR þ EVO equipment. The mechanical property of membranes was conducted by an electronic tensile machine (Testo metric 350AX) at a stretching rate of 1 mm min 1. Dynamic contact angles were tested on a FACE instrumentation (model OCA 25, Germany). IEC values of membranes were tested through back-titration method. The water uptake and dimensional swelling of the membranes were measured at 30 � C according to the method in literature [29]. The membrane resistance (R) was performed by the same experi mental instrumentation in our previously published work [30]. In-plane proton conductivity of membrane under 100% RH was probed by two parallel platinum electrodes in a temperature-controlled chamber equipped with H2O and D2O vapor. Through-plane proton conductivity of membrane at different temperatures under 100% RH was tested on a MTS-740 testing device (regulating the temperature and humidity) with H2O vapor. Proton conductivity was calculated according to the rela tionship: σ ¼ l/AR, where A is the cross-section of the membrane, R is the resistance, and l is the length between two electrodes. Membrane electrode assemblies (MEAs) were fabricated in
2. Experimental 2.1. Materials and chemicals Graphite powder (45 μm) was purchased from Sigma-Aldrich. BC was provided by QiHong Technology Co., Ltd. Filter membrane (pore diameter, 0.4 μm) was supplied by Tianjin Jinteng Experimental equipment Co., Ltd. All other reagents and solvents were of reagent grade and were utilized as received. Deionized water was used throughout the experiment. 2.2. Preparation of BC and PBCn The purification and phosphorylation of BC were performed via similar procedure reported earlier [26]. In brief, the purchased hydrogel was washed and subsequently treated with 2% (w/v) NaOH solution for 24 h, and finally rinsed again until neutral. The purified BC was minced into microgel and freeze-dried for 3 days. Then the BC was obtained by 2
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accordance with the method in literature [5,31]. Electrocatalyst ink was prepared by mixing Pt/C electrocatalyst (40 wt% Pt, JM), isopropanol, Nafion solution (D2020, DuPont), and deionized water. The Nafion ionomer content in catalyst ink was 20 wt%. Afterwards, the catalyst ink was sprayed onto the surface of both sides of as-prepared membrane to prepare MEAs, and the Pt loadings in cathodic and anodic catalytic layers were both 0.3 mg cm 2. Then, two pieces of carbon paper (2 cm � 2 cm) were attached to both sides of MEAs by hot-pressing at 132 � C and 4.0 MPa for 2.0 min. Finally, these MEAs were sandwiched between two graphite plates and assembled into single cells for testing. The perfor mances of fuel cells were tested at 60 � C and 100% RH. The flux rates of H2 and O2 were 80 and 120 mL min 1, respectively. In addition, to evaluate the stability of MEAs, the voltage of the fuel cells based on the membrane was measured for 24 h under the identical system conditions, as the constant current density was 30 mA cm 2.
phosphorylation of BC (Fig. S6). With the phosphorylated degree increasing, the char yield gradually reduces from 28.3 wt% for PBC2 to 21.1 wt% for PBC4. Next, the DGO nanosheets and PBCn were dispersed in, respectively, water and water/formamide solution, followed by mixing together as precursor solution. The obvious Tyndall Effect in Figs. 1a and S7 in dicates the uniform dispersion of DGO nanosheets and/or PBC4. Zeta potential results in Fig. 1b reveal that, the presence of –NH2/ NH groups confers DGO nanosheets high positive zeta potentials with the pH – O(OH)2 groups impart PBC4 negative ranging from 2 to 8, while the P– zeta potentials. The opposite charge permits a pre-assembly of DGO nanosheets and PBC4 in solution via acid-base electrostatic interactions, which can be verified by FTIR results of precursor solution (Fig. 1c). Compared with DGO, the incorporation of PBC4 gives two characteristic – O and P–OH groups peaks at 1204 and 973 cm 1, corresponding to P– [26,35]. While, the peak intensity related to –NH2/ NH groups (1607 cm 1) weakens sharply for DGO@PBC4-1 and DGO@PBC4-3 suggesting the generation of electrostatic interactions between PBC4 and DGO nanosheets [36]. This is favourable for the construction of layer-by-layer structure lamellar composite membrane (Fig. 1d). Here, four types of DGO@PBC4-X precursor solutions, where X (X ¼ 1, 2, 3, or 4) corre sponds to different PBCn contents, were prepared and filtrated for lamellar composite membranes (DGO@PBCn-X, Table S1), where X (X ¼ 1, 2, 3, or 4) corresponds to different PBCn contents, were prepared and filtrated for lamellar composite membranes (DGO@PBCn-X, Table S1). Cross-sectional SEM image in Fig. 2a shows that DGO membrane exhibits a compact lamellar structure. By comparison, the cross-sections of DGO@PBCn-X keep lamellar structure (Figs. 2b and S8). Surficial SEM images show that the surface of DGO@PBC4-3 becomes rough, and fibrous morphology is observed (Fig. S9). In AFM images (Figs. 2c, d, and S10), clear fibrous structures are observed on the surface of DGO@PBCn-X, and the number of visible fiber increases with PBCn loading. According to TGA results in Fig. S11, the PBC4 contents in DGO@PBC4-1, DGO@PBC4-2, DGO@PBC4-3, and DGO@PBC4-4 are calculated to be 9.1, 16.4, 31.9, and 44.8, respectively (Table S1). The PBC2 content of the reference of DGO@PBC2-3 is 32.7 wt%. The P elemental mapping in Fig. 2e suggests the homogeneous distribution of
3. Results and discussion DGO nanosheets were prepared by self-polymerization of dopamine on GO surface. GO shows a typical sheet structure with mean lateral size of ~3 μm and thickness of ~0.89 nm (AFM, Fig. S2a). After dopamine polymerization, the sheet structure and size of DGO are well maintained (TEM, Fig. S2b). The thickness of DGO increases to ~1.2 nm due to the introduced poly-dopamine layer (AFM, Fig. S2c). This can be further verified by FTIR, as shown in Fig. S3. A new peak of N–H stretching vibration is observed for DGO at 1591 cm 1 [32]. Two kinds of PBCn (n ¼ 2 or 4) with different phosphorylated degree were synthesized by controlling reaction time (Fig. S1). AFM image reveals that the length and diameter of BC are in micrometer scale and ~3.5 nm, respectively (Fig. S4a). PBC4 keeps its fibrous morphology after phosphorylation, and its diameter is ~3.7 nm, which is slightly larger than that of BC (AFM, Fig. S4b). FTIR displays that the phosphorylation brings four new peaks for PBCn at 2398.2, 1426.9, 898.4, and 812.7 cm 1, corresponding to – O, P–O–C, and P–OH, respectively [26,33,34]. Meanwhile, the P–H, P– intensity of these four peaks increases from PBC2 to PBC4, suggesting the elevated amount of phosphate groups (Fig. S5). The lower char yields of PBC2 and PBC4 (compared with BC) further confirm the successful
Fig. 1. (a) Digital photographs of DGO, PBC4, and DGO@PBC4-3 precursor solutions. (b) Zeta potential of DGO in water and PBC4 in water/formamide solution with pH of 2–10. (c) FTIR spectra of DGO in water and DGO@PBC4-1 and DGO@PBC4-3 in water/formamide solution. (d) Schematic of the preparation of DGO@PBCn-X. 3
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Fig. 2. Cross-sectional SEM images of (a) DGO and (b) DGO@PBC4-3. AFM images of (c) DGO and (d) DGO@PBC4-3. (e) P elemental mapping of cross-sectional DGO@PBC4-3. (f) Raman spectra of DGO, DGO@PBC4-1, and DGO@PBC4-3. (g) XRD patterns of dry DGO, DGO@PBC4-1, and DGO@PBC4-3.
Fig. 3. (a) High-resolution XPS of N 1s peak for DGO, DGO@PBC4-1, and DGO@PBC4-3. (b) BET surface area of DGO, DGO@PBC4-1, DGO@PBC2-3, and DGO@PBC4-3. (c) Full scan XPS spectra of DGO, DGO@PBC4-1, and DGO@PBC4-3 (insert: amplification of the P2p peak). (d) Schematic illustration of acid-base interactions in DGO@PBC4-3. (e) The mechanical properties of GO, DGO, DGO@PBC4-1, and DGO@PBC4-3. (f) Comparison in mechanical strength of GO and DGO@PBC4-3. 4
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PBC4 in membrane. Together, these results demonstrate the successful fabrication of DGO@PBCn-X composite membrane. The layer-by-layer structure of DGO@PBCn-X was verified by Raman and XRD characterizations. Raman spectra in Fig. 2f show that all the membranes display two peaks, assigning to D band (1346.9 cm 1) and G band (1578.7 cm 1). Compared with the area ratio (ID/IG) of 0.97 for DGO membrane, it elevates to 1.09 and 1.35 for DGO@PBC4-1 and DGO@PBC4-3, respectively. This increase should arise from the inter calation of PBCn in interlayers of DGO membrane, which increases the defect density and disorder degree of DGO membrane [27,37]. The speculation can be confirmed by XRD results (Figs. 2g and S12). DGO membrane displays a d-spacing of 0.86 nm. Intercalating PBCn elevates the d-spacing of DGO@PBCn-X to 1.34 nm for DGO@PBC4-1. While it is interesting to find that, the d-spacing of DGO@PBCn-X decreases with the increase of the amount and phosphorylated degree of PBCn. For instance, the d-spacing values of DGO@PBC4-2 and DGO@PBC4-3 are 1.11 and 0.99 nm, respectively. These phenomena should result from the formation of acid-base interactions between NH2/ NH groups on – O(OH)2 groups on PBCn, which narrows the DGO nanosheets and P– distance between adjacent DGO layers. High-resolution XPS of N 1s peak and FTIR spectra of membranes further demonstrate the formation of acid-base interactions. Fig. 3a shows that the binding energy of pyrrolic N atoms53 increases from 400.5 eV for DGO to 400.7 and 401.5 eV for DGO@PBC4-1 and DGO@PBC4-3, respectively [38,39]. This energy shift indicates the change of electron distribution and the formation of acid-base interactions [40]. And acid-base pair formation also decreases the peak intensity of N–H groups in DGO@PBCn-X (1586.3 cm 1, Fig. S13) [31]. These findings indicate the acid-base pair anchored layer-by-layer structure in these lamellar composite membranes. The insertion of PBCn in DGO interlayers meanwhile increases the BET surface area of lamellar composite membrane. Figs. 3b and S14 show that DGO membrane obtains a BET surface area of 36.3 m2 g 1. By comparison, the BET surface area of DGO@PBC4-1 elevates to 48.7 m2 g 1. Besides, it increases with fiber amount for DGO@PBC4-X (Table S2). DGO@PBC4-3 and DGO@PBC2-3 with similar PBCn loading amount obtain comparable BET surface area (~102 m2 g 1). Mean – O(OH)2 group content of DGO@PBC4-X also increases while, The P– with PBC4 loading amount and shows positive correlation with its BET surface area. (Figs. 3c and S15). According to the peak area ratios, the P contents of DGO@PBC4-1, DGO@PBC4-2, DGO@PBC4-3, and DGO@PBC4-4 are calculated to be 1.6, 2.9, 6.3, and 8.6 wt%, respec – O(OH)2 group contents of 4.2, 7.6, 16.5, tively, correlating with P– and 22.5 wt% (Table S2). By comparison, DGO@PBC2-3 attains a much – O(OH)2 group content of 4.7 wt% than DGO@PBC4-3, due to lower P– the lower phosphorylated degree. Accordingly, the intercalation of PBCn – O(OH)2 groups would with high BET surface area and abundant P– confer DGO@PBCn-X continuous proton transfer networks and strong interlayer interactions (Fig. 3d). The mechanical properties in Figs. 3e and S16 reveal that GO membrane exhibits a tensile strength of 30.1 MPa and an elastic modulus of 3.8 GPa. By comparison, the tensile strength and elastic modulus of DGO membrane increase to 73.4 MPa and 6.2 GPa, respec tively. While the generation of strong acid-base interactions endows DGO@PBCn-X with highly enhanced mechanical property, and the me chanical property enhances with PBCn loading amount. The tensile strength and elastic modulus of DGO@PBC4-3 reach 203.5 MPa and 14.2 GPa, respectively, which are ~6.8 and ~3.7 times of that of GO membrane. Such performance surpasses most previously reported freestanding lamellar membranes including GO, montmorillonite (MMT), covalent organic framework (COF), etc (Table S3). Compared with DGO@PBC4-3, the tensile strength and elastic modulus of DGO@PBC2-3 are a litter lower (178.1 MPa, 11.0 GPa) owing to the relatively weaker acid-base interactions. Macroscopically, DGO@PBC4-3 with a thickness of 21 μm and a width of 13 mm is strong enough to support about 200 g mass (Fig. 3f), while GO and DGO membranes fail to do so (Fig. S17). The strong acid-base interactions between PBCn and DGO nanosheets
then confer DGO@PBCn-X highly boosted water stability. Figs. 4a and S18 show that, after immersing in water for 14 d, GO and DGO mem branes break into small fragments, while DGO@PBCn-X remains stable. After an additional treatment of stirring at a speed of 500 r min 1 for 5 min, only DGO@PBC4-3 still remains stable. These phenomena should result from the strong acid-base interactions between PBC4 and DGO layers, which effectively inhibit them from separating from each other. This speculation is also verified by XRD results. Compared with the sharply increased d-spacing of GO membrane from 0.75 nm under dry condition to 1.56 nm under wet condition, DGO membrane displays a smaller d-spacing change from 0.86 nm to 1.51 nm, due to the surface adhesion properties of poly-dopamine layer (Figs. 2g, 4b, and S19) [41]. Similarly, DGO@PBC4-1 also exhibits an increase in d-spacing (from 1.34 to 1.85 nm). While the increase degree (0.51 nm) is less than that of DGO membrane (0.65 nm), and especially, the d-spacing of DGO@PBC4-3 only increases by 0.02 nm. Considering the stronger hy drophilicity and higher water uptake of DGO@PBCn-X (Fig. S20 and Table S2), the less increase in d-spacing together with the lower swelling ratio further confirm the effect of acid-base interactions on enhancing water stability. The water uptake and swelling ratio (through-plane) of DGO membrane at 30 � C are 25.8% and 24.4%, respectively. In com parison, DGO@PBC4-3 attains an enhanced water uptake of 44.4% and a reduced swelling ratio of 13.2%. In addition, XRD results in Fig. 4c reveal that DGO@PBC4-3 can also remain stable in various conditions, including acidic solution, basic solution, as well as a series of solvents. The abundant acid-base pairs along fiber-sheet interfaces then serve as transfer highways, imparting lamellar composite membrane highly enhanced proton conduction. Figs. 5a and S21a show that the in-plane conductivities of all membranes increase with the increase of tempera ture (ranging of 30–90 � C) and exhibit a positive correlation with IEC and water uptake as listed in Table S2. The in-plane conductivities of GO and DGO membranes display in-plane conductivities of 62.4 and 72.2 mS cm 1 under 90 � C and 100% RH, respectively. By comparison, DGO@PBC4-3 with higher IEC value and water uptake gains an elevated in-plane conductivity, reaching 215.2 mS cm 1, 244.9% higher than that of GO membrane. Apart from the higher IEC value and water up take, such huge conduction enhancement should arise from the formed acid-base pairs (e.g., –PO–3⋅⋅⋅þ3HN–) along fiber-sheet interfaces, in which the de-protonation/protonation process is significantly promoted by the strong electrostatic attractions between acid and base groups [21, 42]. As a result, the transfer energy barrier is obviously reduced. To verify this speculation, DGO@PBC2-3 with similar structure to – O(OH)2 groups was prepared. And it DGO@PBC4-3 but reduced P– displays a reduced in-plane conductivity of 133.3 mS cm 1 under identical condition due to the decreased number of acid-base pairs (Fig. S21b). Direct evidence is provided by the conduction activation energy (Ea, Fig. S21c), which was calculated from a linear fitting of temperature-dependent in-plane conductivities. It shows that the Ea value obviously decreases from 11.2 eV for GO membrane to 4.89 eV for DGO@PBC4-3 (Fig. 5a). Then, deuterium-related experiments were performed and the results are shown in Fig. 5b. All the membranes display decreased conductivity under D2O vapor (vs. H2O vapor), due to the involvement of heavier deuterium atom (vs. proton) in Grotthuss-type migration [43]. And DGO@PBC4-3 attains a more distinct conductivity reduction of 61.2% under D2O vapor when compared with the 34.4% for GO membrane and 39.9% for DGO membrane. This suggests that the constructed acid-base pairs in nano channels enable more proportion of Grotthuss-type migration in DGO@PBC4-3. Note that excessive PBC4 loading amount would aggre gate in DGO@PBC4-4, causing a decline of in-plane conductivity (170.2 vs. 215.2 mS cm 1 for DGO@PBC4-3, Fig. S21a). Next, the through-plane conductivities of membranes, which deter mine the full cell performance, were measured at different temperature (Figs. 5c and S22). The through-plane conductivities of GO and DGO membrane are, respectively, 4.03 and 6.60 mS cm 1 at 90 � C and 100% RH (Figs. 5c and S22a). In comparison, DGO@PBC4-3 acquires almost 5
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Fig. 4. (a) Photographs of GO and DGO@PBC4-3 after being immersed in water for 14 d and then stirred at a speed of 500 r min 1 for 5 min. (b) The d-spacing of GO, DGO, DGO@PBC4-1, and DGO@PBC4-3 under dry and wet conditions (The values are calculated from XRD). (c) XRD patterns of DGO@PBC4-3 after treatment in different solutions for 14 d (The concentrations of HCl and NaClO solution are 2 mol L 1 and 300 ppm, respectively).
Fig. 5. (a) Temperature-dependent in-plane conductivities (σk) at 100% RH. (b) In-plane conductivities under H2O and D2O vapor at 90 � C. (c) Temperaturedependent through-plane conductivities (σ?) at 100% RH. (d) In-plane conductivities (σk), through-plane conductivities (σ?), and transfer anisotropy coefficient (σk/σ?) at 90 � C and 100% RH. (e) Weight losses and through-plane conductivities of DGO@PBC4-3 in 30 � C water over time. (f) Performance comparison between DGO@PBC4-3 and various previously reported membranes.
10 times’ higher through-plane conductivity than GO membrane, reaching 43.9 mS cm 1. Notably, such conductivity enhancement at vertical direction is more striking than that at parallel direction (a 2.4fold increase). Moreover, the Ea value for proton transfer at vertical direction declines by 76.9%, from 40.3 kJ mol 1 for GO membrane to 9.30 kJ mol 1 for DGO@PBC4-3. By comparison, the Ea value at parallel direction only decreases by 56.3% (from 11.2 to 4.89 kJ mol 1, Figs. 5a and S22b). As a result, the transfer anisotropy coefficient (σk/σ?) de scends to 4.9 for DGO@PBC4-3 from the 15.5 for GO membrane at 90 � C and 100% RH (Fig. 5d). Importantly, the excellent water stability brings acceptable conduction stability for these lamellar composite mem branes. DGO@PBC4-3 was immersed in water at 30 � C, and meanwhile its through-plane proton conductivity and weight loss were recorded consecutively for 90 h. Fig. 5e reveals that there is no significant weight loss during the entire measurements. And the through-plane proton conductivity of DGO@PBC4-3 is constant over days. This stable con ductivity should be due to the presence of strong acid-base interactions, which improves membrane structural (e.g., anti-swelling property) and
chemical stabilities. Collectively, the constructed acid-base pairs be tween adjacent DGO layers permit lamellar composite membrane simultaneously enhanced stability and conductivity. The performance of DGO@PBC4-3 in this work is superior to many reported membranes, including GO-based lamellar membranes, hybrid membranes, and block copolymer membranes (Fig. 5f and Table S4). The as-prepared membranes were assembled into MEAs for measuring the hydrogen fuel cell performance at 60 � C and 100% RH (Fig. 6a and b). The open circuit voltage of all membranes is above 0.9 V, suggesting an acceptable gas barrier property (Fig. 6a) [44]. GO mem brane gains the maximum power density of 79.9 mW cm 2 and current density of 305.3 mA cm 2. In comparison, DGO membrane shows enhanced maximum power density (109.7 mW cm 2) and current density (394.4 mA cm 2), because of the improved proton conductivity. Furthermore, the formed acid-base pairs in interlayers of DGO@PBCn-X give highly promoted proton conduction and thus fuel cell perfor mances. For instance, DGO@PBC4-3 achieves the maximum power density and current density of 182.9 mW cm 2 and 674.8 mA cm 2, 6
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Fig. 6. (a) Single cell performances of the membranes at 60 � C and 100% RH. (b) The stability of the MEAs at 30 mA cm
respectively, which are enhanced by 128.9% and 121.0% as compared to GO membrane. The stability of MEAs based on GO membrane and DGO@PBC4-3 in single cell was evaluated at a current of 30 mA cm 2 for 24 h. As shown in Fig. 6b, the voltage of fuel cell based on GO membrane declines ~4.2%, deriving from structure deformation under operating condition [5]. By comparison, the excellent structure stability confers the MEA based on DGO@PBC4-3 with almost no voltage loss, holding potential for practical application.
2
for 24 h.
& editing. Acknowledgments This work is supported by the National Natural Science Foundation of China (21576244), Natural Science Foundation of Henan Province (182300410276), Training Plan for Young Backbone Teachers in Uni versities of Henan Province (2018GGJS003), and Fok Ying Tung Edu cation Foundation (161065). Center for advanced analysis and computational science, Zhengzhou University is also highly acknowledged.
4. Conclusions In summary, we demonstrate a simultaneous enhancement of proton conductivity and water stability related to GO-based lamellar mem branes, by preparing a layer-by-layer structured lamellar composite membrane from DGO nanosheets and PBCn. The constructed acid-base pairs between PBCn and DGO nanosheets act as low-energy-barrier proton transfer pathways. As a result, DGO@PBC4-3 obtains in-plane conductivity of 215.2 mS cm 1, 244.9% improvement over GO mem brane. Moreover, it acquires a more obvious augment in through-plane conductivity by almost 10 times of that of GO membrane. As a conse quence, the transfer anisotropy coefficient remarkably decreases, and hydrogen fuel cell performances are enhanced by over 120%. Further more, the acid-base pairs effectively, making the lamellar composite membrane show no breakdown or delamination in water, acid, and basic solutions even after 14 d. Besides, the lamellar composite membrane achieves a high tensile strength of 203.5 MPa. Such mechanical property outperforms most previously reported freestanding lamellar mem branes, including GO, MMT, COF etc. The construction of layer-by-layer anchored structure may pave a way to designing advanced lamellar membranes with highly enhanced performances for many transport and separation applications.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Jingtao Wang: Conceptualization, Methodology, Investigation, Writing - original draft. Yarong Liu: Investigation, Data curation, Visualization, Writing - original draft. Jingchuan Dang: Methodology, Investigation, Visualization. Guoli Zhou: Resources, Methodology, Su pervision, Writing - review & editing. Yan Wang: Resources, Software. Yafang Zhang: Software, Visualization. Lingbo Qu: Writing - review & editing. Wenjia Wu: Conceptualization, Visualization, Writing - review 7
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