Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective

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Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective Proton Conductor for Construction of Nanocomposite Proton Exchange Membrane Xiang Qiu,† Mitsuru Ueda,†,‡ Huayuan Hu,† Yuqian Sui,† Xuan Zhang,*,† and Lianjun Wang*,† †

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Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, 200 Xiaolingwei, Nanjing 210094, Jiangsu Province, China ‡ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: To improve proton conduction properties of conventional sulfonated poly(ether ether ketone) (SPEEK), poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) was prepared to fabricate nanocomposite membranes, which then were further doped with phosphoric acid (PA). The ABPBI-GO was synthesized through the reaction of 3,4diaminobenzoic acid with the carboxyl acid groups present on the GO surface. The simultaneous incorporation of ABPBIGO and PA into SPEEK did not only improve the physicochemical performance of the membranes in terms of thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeation resistance but also relieve PA leaching from the membranes though acid−base interactions. The resulting composite membranes exhibited enhanced proton conductivities in extended humidity ranges thanks to the hygroscopic character of PA and the increased water uptake. Moreover, the unique self-ionization, self-dehydration, and nonvolatile properties of PA improved the high-temperature proton conductivities (σ) of PA-doped membranes. The PA-doped SPEEK/ABPBI-GO-3.0 delivered a σ of 7.5 mS cm−1 at 140 °C/0% RH. This value was fourfold higher than that of pristine SPEEK membranes. The PA-doped SPEEK/ABPBI-GO-3.0 based fuel cell membranes delivered power densities of 831.06 and 72.25 mW cm−2 at 80 °C/95% RH and 120 °C/0% RH, respectively. By contrast, the PA-doped SPEEK membrane generated only 655.63 and 44.58 mW cm−2 under the same testing conditions. KEYWORDS: proton exchange membrane, sulfonated poly(ether ether ketone) (SPEEK) nanocomposite membranes, poly(2,5-benzimidazole)-grafted graphene oxide, phosphoric acid doping, H2/air fuel cell

1. INTRODUCTION Proton exchange membranes (PEMs) have been extensively studied for their potential application in PEM fuel cells (PEMFCs), considered as one of the most promising clean energy devices.1,2 The currently commercialized perfluoro sulfonic acid based membranes include Nafion and aromatic ionomers, such as sulfonated poly(arylene ether sulfone) (SPAES),3 sulfonated poly(ether ether ketone) (SPEEK),4 and sulfonated polyphenylenes (SPP).5,6 However, aromatic ionomers could merely be used at low- or medium-temperature systems due to their greater humidity dependence.7,8 Besides, membranes fabricated from sulfonic acid-based polymers often possess extremely low proton conductivities at temperatures above 100 °C due to evaporation of water molecules.9−12 It has been reported that inorganic nanofillers could have the ability for water capture due to their specific porous architectures.13,14 The inter- or intramolecular interactions formed between inorganic and various alternative organic polymer matrixes could generate nanodomains at the interface © 2017 American Chemical Society

of polymer−filler systems. Therefore, tremendous efforts have been recently devoted to preparing inorganic−organic composite PEMs capable of operating at elevated temperatures.15,16 For instance, Wang and co-workers17 incorporated polydopamine-modified graphene oxide (DGO) into sulfonated poly(ether ether ketone) (SPEEK) to fabricate nanocomposite membranes. The resulting SPEEK/DGO-7.5 membrane showed enhanced proton conductivities reaching up to 3.26 mS cm−1, which is nearly threefold higher than SPEEK control membranes at 120 °C under anhydrous condition. In another study, Wang and co-workers18 prepared a series of 2-D materials based on Ti3C2Tx physically blended into SPEEK membranes. With use of appropriate fillers contents, the fabricated membranes exhibited improved proton conductivities reaching up to 1.69 mS cm−1 at 120 °C/0% RH. Overall, Received: June 1, 2017 Accepted: September 5, 2017 Published: September 5, 2017 33049

DOI: 10.1021/acsami.7b07777 ACS Appl. Mater. Interfaces 2017, 9, 33049−33058

Research Article

ACS Applied Materials & Interfaces

centrifuging, which then were washed several times with 5% HCl solution and deionized water. The final product was dried in a vacuum oven at 60 °C for 8 h. ABPBI-GO was prepared using a polycondensation reaction between DABA and carboxyl groups available on the GO surface. This was similar to the common synthesis method of polybenzimidazole.20,27 First, dried GO (250 mg) was added to 250 mL of DMAc under sonication to yield a homogeneous dispersion. Together with the GO dispersion, DABA (1.25 g) and PPA (10.0 g) were added to a 500 mL two-necked flask, and the mixture was refluxed at 160 °C for 2 h under a protective atmosphere of N2. Afterward, the mixture was poured into 500 mL of deionized water and separated by centrifugation. The obtained black product was repeatedly washed with deionized water and DMAc and then dried in a vacuum oven at 80 °C overnight. 2.3. Preparation of Sulfonated Poly(ether ether ketone) (SPEEK). SPEEK with a theoretical ion-exchange capacity of 1.70 mequiv g−1 was synthesized using a random copolymerization of MHQ, DFBP, and 6F-BPA followed by further demethylation and sulfonation. The detailed description of the synthesis process was similar to that reported previously,28 which was also indicated in the Supporting Information (Scheme S1 and Figure S1). 2.4. Preparation of Nanocomposite Membranes. A certain amount of ABPBI-GO was added to DMAc (1 mL) to form a uniform dispersion through sonication treatment. SPEEK was dissolved in DMAc at a concentration of 10% (w/v), and the solution was filtrated off to remove insoluble particles. The obtained transparent SPEEK solution was then uniformly blended with the ABPBI-GO dispersion, and directly cast onto a clean Petri dish. The obtained nanocomposite membranes were named as SPEEK/ABPBI-GO-X, where X (X = 1.0, 3.0) represents the weight percentage of the nanofillers (ABPBI-GO) to SPEEK. After drying in an oven at 80 °C for 12 h, the membranes were immersed in a 2 M HCl aqueous solution at 50 °C for 72 h to allow proton exchange, followed by washing with deionized water under the same conditions. For further doping of PA, the air-dried membranes with appropriate sizes were first placed in a vacuum oven at 120 °C for 2 h, weighed (W1), and then immersed in 400 mL of 45 wt % PA solution at 40 °C for 1 day. After blotting with dust-free paper, the PA-doped membranes were dried in a vacuum oven at 40 °C for 2 days and weighed again (W2). The PA content was considered as weight percent of absorbed PA in the membranes and calculated using eq 1.

despite the noticeable progress in development of PEMs membranes, some technical issues still require solutions. This includes insufficient efficiency of proton conduction under anhydrous conditions and the undesired compatibility between various components. Phosphoric acid (PA) is widely employed as proton conductor at high-temperature operation conditions owing to its unique self-ionization, self-dehydration, and nonvolatile properties.19−21 Because of the basic character of polybenzimidazole (PBI), which could yield strong acid−base interaction, PA-doped PBI membranes are considered as the most successful high-temperature PEMs over the past decades.22−25 Nevertheless, these properties might not directly be applied to other aromatic PEMs, such as SPAES, SPEEK, and SPP. The reason for this has to do with the neutral or even acidic character of the molecules. Overall, PA alone is not expected to be stabilized after immobilization, which leads to serious leaching phenomena. This, in turn, may reduce the performance of the resulting fuel cells. In this study, a facile synthetic approach was proposed for grafting poly(2,5-benzimidazole)s on graphene oxide. The resulting polymer-modified GO was utilized to prepare nanocomposite SPEEK membranes. The interfacial compatibility between the different systems was expected to improve thanks to the organic−organic hybrid approach. Meanwhile, the presence of benzimidazole groups should provide a feasible adsorption and retention of PA, making a broadened operating temperature range for SPEEK-based membranes, which contributes to potential applications of high-temperature PEMFCs. The properties of the resulting PEMs were investigated and discussed. This included the thermal stability, mechanical strength, PA content and retention, water uptake, dimensional change, proton conductivity, methanol permeability, and single-cell performance. In particular, the hightemperature performances of PA-doped SPEEK/ABPBI-GO membranes were systematically studied and discussed in this work, involving conductivity at 100−140 °C, and a single-cell test at 120 °C.

PA content (wt%) =

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents methoxyhydroquinone (MHQ), 4,4′difluorobenzophenone (DFBP), 4,4′-(hexafluoroisopropylidene) diphenol A (6F-BPA), and borontribromide (BBr3) (1 M in CH2Cl2) were purchased from TCI (Shanghai) and used as received. Toluene, N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and dichloromethane were utilized after dehydration. Both the 3,4diaminobenzoic acid (DABA, 98%) and polyphosphoric acid (PPA, 85%) were obtained from Amethyst-Chemicals (Beijing) and used without further purification. The other materials and chemicals like graphite powder, concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), sodium nitrite (NaNO2), hydrochloric acid (HCl), and hydrogen peroxide aqueous solution (H2O2, 30%) were purchased from Aladdin (Shanghai) and used as received. 2.2. Preparation of GO and ABPBI-GO. GO was synthesized using the modified Hummers method, considered as the most commonly used approach with graphite powder.26 In an ice bath, concentrated H2SO4 (46.0 mL) was added to a mixture of graphite powder (1.0 g) and sodium nitrate (1.0 g) dispersed in a 500 mL round-bottom flask. Next, KMnO4 (4.0 g) was slowly added under vigorous stirring at a controlled temperature below 20 °C for 1 h. The reaction was then heated at 35 °C for 1 h, and 60.0 mL of deionized water was slowly added to the mixture. The temperature of the suspension was raised to 90 °C and kept for another 30 min. Finally, deionized water (140 mL) and 30% H2O2 (20.0 mL) were added to the mixture. The GO sheets were separated from the mixture through

W2 − W1 × 100 W2

(1)

2.5. Characterizations. The chemical structures of GO and ABPBI-GO were probed by Fourier transform infrared (FTIR, Nicolet IS-10) spectroscopy at the frequency range from 500 to 4000 cm−1. The crystalline structures of GO and ABPBI-GO were characterized by X-ray diffractometry (XRD) using a Bruker D8 in the range of 5−80°. X-ray photoelectron spectroscopy (XPS) (PHI Quantera II, Japan) was utilized to determine the chemical compositions of GO and ABPBI-GO. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on an AVANCE III (Bruker, Germany) with a proton frequency of 500 MHz using DMSO-d6 as the solvent. Thermogravimetric analysis (TGA) was carried out using a SDT Q600 in N2, operating from 50 to 800 °C at a heating rate of 10 °C min−1. The number- and weight-average molecular weights (Mn and Mw) were measured using gel permeation chromatography (GPC) on a Waters 1515 HPLC system. The eluate was N,N-dimethylformamide (DMF) containing 0.05 M LiBr, the flow rate was set to 1.0 mL min−1, and the equipment was calibrated using standard polystyrene samples. The mechanical properties of the membranes were examined on a Shimadzu AGS-100NX using elongation rate of 2 mm min−1 at 25 °C and 70% relative humidity. The morphologies of the membranes were observed by transmission electron microscopy (TEM), where the membranes in acidic form were first ion-exchanged using 0.5 M lead acetate aqueous solutions for 24 h. They were then repeatedly rinsed with water and dried in an oven at 60 °C for more than 12 h. Next, the stained 33050


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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of Poly(2,5-benzimidazole)-Grafted Graphene Oxide (ABPBI-GO)

Figure 1. Characterization of GO and ABPBI-GO using various techniques: (a) FTIR spectra, (b) XRD patterns, (c) Raman spectra, and (d) TGA curves. membranes were embedded in epoxy resin, sectioned to 70 nm thickness with a Power-Tome-XL, and placed on copper grids. TEM images were taken on a Hitachi H-7650 at an accelerating voltage of 80 kV. 2.6. Water Uptake (WU) Measurements. WU was measured by keeping the membranes in thermo-controlled humid equipment under various relative humidity conditions at 80 °C for 2 h. The samples were then removed from the chamber and quickly weighed.28 WU was calculated using eq 2.

WU = (W1 − W0)/W0 × 100%

Δlc = (l − ls)/ls

where ts and ls refer to the thickness and length of the membranes measured at 30 °C/30% RH, respectively. t and l are the same parameters of the membranes under different RH conditions. 2.8. Ion Exchange Capacity (IEC). IEC was measured using the acid−base back-titration method. The acidified membranes were immersed in 20 wt % sodium chloride solutions for ion exchange of H+ at 50 °C for 3 days. Sodium hydroxide standard solutions were then used to neutralize the acid solutions by adding phenolphthalein as an indicator.28 The ion exchange capacity was calculated using eq 5.

(2)

IEC = C NaOHVNaOH/W0

where W1 and W0 refer to weights of the membrane samples at wet and dry conditions. 2.7. Dimensional Change (DC). The dimensional change was measured by putting the membranes in thermo-controlled humid equipment for at least 2 h at 80 °C under different relative humidity conditions.28 The dimensional change in membrane thickness direction (Δtc) and the plane direction (Δlc) were calculated using eqs 3 and (4). Δtc = (t − ts)/ts

(4)

(5)

where CNaOH and VNaOH refer to the concentration and consumed volume of NaOH solution, respectively. 2.9. Proton Conductivity (σ). The proton conductivity was estimated by means of an Hioki IM 3533-01 impedance analyzer at a frequency ranging from 1 to 106 Hz. The membranes were placed in a two-point probe conductivity cell equipped with two Pt plate electrodes. The cell was then kept in thermo-controlled humidity equipment at different testing conditions for 1 h before each measurement.28 σ was calculated using eq 6.

(3) 33051


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Figure 2. (a) XPS spectra for GO and ABPBI-GO and (b) deconvoluted XPS N 1s spectra of ABPBI-GO. σ = d /(tswR s )

The morphologies of GO and ABPBI-GO were observed by TEM, and the results are shown in Figure S2. Both samples appeared as two-dimensional sheets with many wrinkles. Besides, no significant differences could be noticed between both samples. The chemical structures of GO and ABPBI-GO were determined by FTIR spectroscopy (Figure 1a). The spectrum of GO depicted five characteristic peaks at 3408, 1727, 1630, 1060, and 983 cm−1, assigned to the O−H stretching vibration, CO stretching vibration, CC stretching vibration, C−O stretching vibration, and C−O−C stretching vibration, respectively.32−35 For ABPBI-GO, the appearance of three additional new absorption peaks at 1572 (CN stretching vibrations), 1440 (benzimidazole ring) and 1284 cm−1 (imidazole breathing mode) was consistent with ABPBI prepared by the same condition,20 confirming the successful modification of GO. To easily distinguish benzimidazole and imidazole peaks, the spectrum of ABPBI-GO was separately provided in the Supporting Information as Figure S3. To further investigate the nanostructures of GO and ABPBIGO, the samples were analyzed by XRD and the data are gathered in Figure 1b. GO displayed a diffraction peak at 2θ = 10.9°, indicating an average interlayer spacing of 0.81 nm (calculated by Bragg’s law: λ = 2d sin θ). After the successful modification with poly(2,5-benzimidazole), ABPBI-GO showed a relatively weak diffraction peak at 2θ = 9.8°, corresponding to an enhanced interlayer distance of 0.90 nm. This enhancement was probably caused by the polymer chains grafted on the surface of GO between layers, although the high reaction temperature removed some oxygen-containing functional groups which usually cause partial restacking and decrease interlayer distance.36 ABPBI-GO also revealed another new peak at 2θ = 25.8°, consistent with the XRD pattern of PBI reported in the literature and may arise from a convolution of crystalline and amorphous scattering.37 Figure 1c illustrates the Raman spectra of GO and ABPBIGO. Both samples exhibited two characteristic peaks at 1334 and 1570 cm−1, generally referred to as the D-band and Gband. The defects involved in carbon lattice of graphene materials can be presented by the D-band, while the G-band indicates the presence of sp2 hybridized carbons.38 The calculated I(D)/I(G) values of GO and ABPBI-GO were 1.12 and 1.17, respectively. This indicated further surficial modification of GO with DABA and PPA, which created more defects in the carbon lattice. The thermal behavior of GO and ABPBI-GO was evaluated by TGA analysis under both nitrogen and air atmosphere, and the results are presented in Figure 1d. The GO showed two obvious weight loss stages in N2. The first one ranged from 150

(6)

where d is the distance between the two Pt plates, ts and ws are the thickness and width of the membrane film, and R is the measured resistance. 2.10. Evaluation of Fuel Cell Performance. The membrane electrolyte assembly (MEA) was performed according to our previous method.29 Briefly, commercial Pt/C (30.6 mg, 60 wt %) (Hispec4000, Johnson and Matthey) was first soaked in 0.6 mL deionized water under sonication for 10 min. The suspension was then mixed with 257.3 mg Nafion isopropyl alcohol solution (5 wt %). Excess isopropyl alcohol was added to control the total solid content to 1 wt %. After 30 min of further sonication at 5 °C, the well-dispersed catalyst ink was sprayed on the membrane surface as conventional catalyst materials using electrostatic spraying equipment with a platform heating temperature of 80 °C (SP 201, Kunshan Sunlaite New Energy Co. Ltd.). The active surface area of the MEA was measured as 9.0 cm2 (3.0 cm × 3.0 cm) and the loading amounts of Pt on each side were 0.5 mg cm−2. The single-cell performance was investigated using an in-house fuel cell station (HTS-125, Shanghai Hephas Energy Co. Ltd.) under ambient pressure. At medium temperature testing, the cell temperature was set to 80 °C, and the gas humidifying temperatures were both controlled at 78.5 °C for the anode and cathode, which corresponded to 95% RH. At high-temperature testing, the cell temperature was set to 120 °C without gas humidification on both sides. During both tests, the H2 gas and air flow rates were fixed to 200 mL min−1 and 500 mL min−1, respectively.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of GO and ABPBI-GO. Poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) was synthesized using the two-step procedure shown in Scheme 1. First, exfoliated GO was obtained by a violent oxidation of graphite powder with a 4 weight equivalent amount of the strong oxidant KMnO4, followed by ultrasonic treatment. A large amount of carboxyl acid groups formed on the basal plane and edges of GO after previous oxidation.30,31 These functional groups were essential to subsequent modification of GO with DABA by the polycondensation reaction. Similar to conventional synthesis of poly(2,5benzimidazole), initial benzimidazole groups were formed by the reaction between − COOH on the GO surface and the diamine groups provided by DABA. Under these circumstances, the grafted chains were able to be further extended from the tail groups. However, some free poly(2,5-benzimidazole)s would be inevitably formed due to self-polymerization of DABA, and their molecular weights were controlled by limiting the reaction time to less than 2 h.20 After that, most of the unattached chains could easily be removed through rinsing with DMAc. 33052


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ACS Applied Materials & Interfaces Table 1. Physico-chemical Properties of the Membranes IEC (mequiv g−1)

a

WUa (%)

σa (mS cm−1)

mechanical propertiesb

code

theo.

titr.

50%

95%

50%

95%

Y (GPa)

S (MPa)

E (%)

PA content (%)

PM c (×10−7 cm2 s−1)

SPEEK SPEEK/ABPBI-GO-1.0 SPEEK/ABPBI-GO-3.0 Nafion 212

1.70

1.72 1.71 1.69 0.91

16.2 14.5 12.9 4.1

40.1 39.0 37.9 15.4

17.8 22.4 24.9 22.1

141.6 143.5 156.1 142.0

0.83 1.17 1.23 0.04

25.31 30.15 20.34 17.0

7.7 9.6 4.7 320

19.3 22.0 26.4

1.51 1.20 0.76 4.75

At 80 °C, SPEEK-based membranes in PA-doped forms, Nafion 212 in PA-free form. bAt 25 °C, 70% RH. cMethanol permeability.

Figure 3. Percentage weight change of PA-doped SPEEK membranes as a function of time at (a) 80 °C/50% RH and (b) 80 °C/95% RH.

to 300 °C, and was mainly ascribed to the decomposition of oxygen-containing functional groups. The weight loss recorded between 300 and 800 °C was attributed to the decomposition of the GO backbone. By comparison, ABPBI-GO displayed a gradual weight loss curve instead of several apparent weight loss stages under nitrogen atmosphere. With a total weight loss of only 32%, ABPBI-GO exhibited better thermal stability. This might be due to the presence of less oxygen-containing functional groups in ABPBI-GO and the outstanding thermal stability of ABPBI itself. The degradation rate of ABPBI-GO was accelerating after 700 °C, as confirmed by the decline tendency at the end range of the curve. However, this was somewhat different from that of GO. Note that GO sample was completely burned off in air atmosphere at 620 °C,39 whereas the temperature was found to increase to 710 °C for ABPBIGO, which again suggested improved thermal property of the nanocomposite. To gain further understanding of the composition, XPS was conducted and the obtained profiles are displayed in Figure 2. Both GO and ABPBI-GO showed two characteristic peaks at 532.0 and 284.3 eV (In Figure 2a), corresponding to O 1s and C 1s, respectively. A new peak also appeared in ABPBI-GO at the binding energy of 400.1 eV attributed to N 1s, with the content as high as 4.7%, confirming the successful modification of GO. As a result of the spin−orbit coupling,40 three subpeaks deconvoluted from N 1s spectra of ABPBI-GO were shown in Figure 2b. The two peaks located at the binding energy of 398.4 and 399.3 eV were referred to pyridinic-N (=N−) and pyrrolicN (−N−) in benzimidazole groups, respectively.41 The other peak at 400.2 eV may be assigned to the nitrogen of the amino group (−NH−).42 3.2. Characterization of Nanocomposite Membranes. In inorganic−organic hybrids, the compatibility between the different components plays a crucial role in the resulting physicochemical properties. The interfacial compatibility between the SPEEK matrix and ABPBI-GO nanofillers indicated by the morphology of membrane was investigated

by cross-sectional TEM, and the obtained data are shown in Figure S4. The image corresponding to SPEEK control membrane revealed uniform and defect-free features. By comparison, several obvious dark lines, which is the crosssection of ABPBI-GO nanosheets, appeared inside the composite for SPEEK/ABPBI-GO-1.0 membrane. The introduction of the polymer-modified GO into SPEEK matrix maintained the structural integrity of the membrane, which benefited from the organic−organic collaborative blending. Even after 3.0 wt % ABPBI-GO incorporation, the composite membrane still exhibited a relatively homogeneous arrangement over the entire scanning area without any apparent defects. The thermal stabilities of all the membranes were examined by TGA under a nitrogen atmosphere after sufficient water absorption and being air-dried (Figure S5). A total of four weight-loss stages located at 50−150, 200−300, 380−520, and >520 °C were recorded for membranes. These stages were respectively attributed to evaporation of absorbed water molecules, deterioration of sulfonic acid groups, decomposition of aliphatic side chains, and burnout of the polymer backbone. After incorporation, the SPEEK/ABPBI-GO-1.0 and SPEEK/ ABPBI-GO-3.0 exhibited a somewhat better thermal stability than the SPEEK membrane, which is probably due to the outstanding thermal stability of the ABPBI-grafted GO nanofillers. The tensile strength, Young’s modulus, and elongation of all the PA-free membranes were investigated as the representatives of the mechanical properties listed in Table 1. In general, the introduction of inorganic materials as nanofillers into membranes should reinforce the material and improve the mechanical strength.43,44 The SPEEK/ABPBI-GO-1.0 composite exhibited larger tensile strength and Young’s modulus than the pristine SPEEK membrane, indicating its enhanced rigidity thanks to the incorporated ABPBI-GO. Meanwhile, the elongation at break of this membrane slightly increased, which could obtain a reasonable explanation from the 33053


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Figure 4. (a) Water uptake of all the membranes before and after PA doping as a function of relative humidity. Dimensional changes of (b) PA-free membranes and (c) membranes after PA doping at 95% RH and 80 °C.

gested the excellent PA retention ability of the ABPBI-GO composite membrane. The nanocomposite membranes exhibited less PA leakage under the hydrous condition, indicating that the acid−base interaction between the benzimidazole groups of ABPBI-GO and phosphoric acid exerted great influence on the PA retention. Meanwhile, the presence of ABPBI-GO in the polymer limited the mobility of main chains, confirmed by the reduced dimensional changes in the following section, which provided a higher obstacle for PA leaching. 3.4. Water Uptake and Dimensional Changes. Water uptake (WU) is extremely crucial for proton migration through Grotthuss and Vehicle mechanisms in which hydrogen-bonding networks are provided.45 The water uptake before and after immersion in PA solution was separately measured under different relative humidity conditions at 80 °C, and the results are shown in Figure 4a. The ABPBI-GO incorporated membranes exhibited lower water uptakes than the pristine SPEEK membrane resulting from the relative hydrophobicity of ABPBI-GO. The acid−base interaction between sulfonic acids in SPEEK matrix and benzimidazoles in ABPBI-GO, which may restrict the chain motion and reduce the free volume,46 can be another possible reason for the WU decrease. However, after adsorption of phosphoric acid, the water uptake of the membranes tremendously increased. The hygroscopic character of phosphoric acid endowed PA-doped membranes with an important role in water uptake.22 For instance, the water uptake in SPEEK/ABPBI-GO-3.0 membrane rose from 22.7% before immersion in PA solution to 37.9% in PA-doped form. The improvement in water uptake by doped PA was also recorded in several published studies.10,22 The dimensional changes (DC) in the membranes were evaluated together with WU under the same testing conditions. Figure 4b shows the swelling behavior of the membranes without PA content along in-plane and through-plane

bioinspired toughening mechanism. When a load was applied to the membrane, the hidden length of polymer chains could be released, benefiting from the breaking of sacrificial bonds (mainly acid−base coordination).16 However, the elongation at break of SPEEK/ABPBI-GO-3.0 was smaller than that of the SPEEK membrane even though the rigidity confirmed by the Young’s modulus was further enhanced. In other words, the plasticity of the polymer began to be destroyed by the inorganic fillers, and further increase of the incorporating amount would make membranes too fragile to meet the fabrication requirement of membrane electrode assembly (MEA). 3.3. Phosphoric Acid Doping and Retention. Phosphoric acid (PA) could be used as proton conductor because it provides extra hopping sites for proton transfer, which is prevalently applied in high-temperature PEMs accompanied by polybenzimidazole derivatives relying on its nonvolatile property.20 Thus, the membranes were immersed in a 45% phosphoric acid solution for 1 day, and the PA uptakes are summarized in Table 1. The PA content of the composite membranes obviously increased as more dosage of nanofillers was incorporated. This probably was related to the acid−base interaction between the PA and basic imidazole groups present in ABPBI-GO. To verify the PA-retention ability of the membranes, shortterm durability tests were conducted at 80 °C/50% RH and 80 °C/95% RH, and the data are illustrated in Figure 3. Under such testing conditions, PA was drained away via water molecules instead of evaporation. The SPEEK/ABPBI-GO-1.0 and SPEEK/ABPBI-GO-3.0 kept respectively 92.0% and 94.6% of their original weights under 50% RH, while the weight loss of the pristine SPEEK membrane reached ca. 10% after 48 h exposure. Under high-humidity conditions (95% RH), the weight loss of SPEEK reached 15.4%, whereas only 7.8% was recorded for SPEEK/ABPBI-GO-3.0 membrane. This sug33054


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Figure 5. (a) Proton conductivities of the membranes at relative humidity cycles ranging from 50% to 95% at 80 °C. (b) High-temperature proton conductivities of the membranes before and after PA doping.

Figure 6. An assumed cross-sectional configuration of multicomposite membranes (left) and possible low-barrier proton transfer pathway inside (right).

Notably, PA-doped SPEEK membranes possessed high proton conductivities reaching 17.8 mS cm−1 under 50% RH during the first cycle, while the pristine SPEEK displayed only a low value of 2.6 mS cm−1, according to our previous study.28 As expected, ABPBI-GO incorporated membranes held more proton conductors due to their higher PA contents, thus exhibiting elevated proton conductivities at every relative humidity cycle. Although the ABPBI-GO doped membranes have slightly lower IEC values, the acid−base interactions described above may connect isolated sulfonic acid groups in dead paths to form continuous proton migration networks, hence lowering the proton transfer barrier through the assistance of protonation−deprotonation sequences.17 On the basis of the mentioned considerations, the distribution of ABPBI-GO nanofillers and PA inside the SPEEK matrix was proposed and schematically illustrated in Figure 6. The loss of PA at high relative humidities reduced the proton conductivities during the following cycles of the membranes at variable degrees. The PA-doped SPEEK membrane possessed a proton conductivity of just 98.3 mS cm−1 under 95% RH after the third cycle, which was much lower than that of PA-free SPEEK recorded previously.28 Taking into account water uptake and dimensional changes in PA-doped forms, one possible reason is the serious volumetric expansion of SPEEK induced by higher water absorption. Consequently, the membrane did not recover its original shape under continuous operation. Although PA was removed along with water moisture, the occupied pore spaces left in the polymer matrix

directions. For all membranes, the recorded dimensional changes in thickness were larger than those estimated along the plane direction, indicating their anisotropic nature. It is worth noting that the hybrid membranes exhibited considerably reduced swelling compared to the original ones, which was possibly due to the excellent structural stability of graphene and the acid−base interaction between sulfonic acids in SPEEK matrix and benzimidazoles in ABPBI-GO. In PA-doped forms, a similar tendency was also observed for all the membranes compared to their PA-free states shown in Figure 4c. 3.5. Proton Conductivity. In such multicomposite systems consisting of PA, ABPBI-GO, and SPEEK, proton conductivity could be influenced by various factors. To gain a clear view of the effect of ABPBI-GO doping and PA absorption on the resulting membranes properties, proton conductivities were measured during relative humidity cycles ranging from 50% to 95% at 80 °C. As depicted in Figure 5a, the conductivities of all membranes during the first run reached their maximum values at different RH. This was related to the elevated water uptake in the presence of PA. Furthermore, although the capability of PA in accepting and donating protons was hardly comparable to that of sulfonic acid groups due to their differences in acid dissociation constants (pKa),47 the location of sulfonic acids was fixed in the side chains of the SPEEK polymer. However, phosphoric acid is governed by only acid−base interactions and could freely migrate in the matrix, which shorten the distance between the hopping sites and form continuous proton transfer channels. 33055


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Figure 7. Fuel cell performance of SPEEK and SPEEK/ABPBI-GO composite membranes at (a) 80 °C/95% RH and (b) 120 °C/0% RH. The gas feed was fixed to 200 and 500 mL min−1 for H2 and air, respectively.

membranes based on the participation of ABPBI-GO and PA in proton migration. The SPEEK/ABPBI-GO-3.0 membranes showed the highest proton conductivities during hydrous cycles and under high-temperature operation. The SPEEK/ ABPBI-GO-3.0 membrane achieved a peak power density of 831.06 mW cm−2 at 80 °C/95% RH and 72.25 mW cm−2 at 120 °C/0% RH, indicating its potential application even in high-temperature fuel cells. Overall, these findings proposed a novel way for enhancing the electrochemical properties of conventional SPEEK ionomers at high temperatures.

induced an enlargement in the proton transfer distance. Thanks to the remarkable dimensional stability and lower PA leakage, the SPEEK/ABPBI-GO-3.0 membrane retained most of its original proton conductivity after the third cycle, which enables it to be applied in practical fuel cell operation. Figure 5b represents the proton conductivities of all the membranes at high temperatures ranging from 100 to 140 °C under 0% RH. Relying on the hydrogen bonding between water and sulfonic acid groups to open up the proton transfer channels, PEMs derived from sulfonic acid groups exhibited extremely unsatisfactory proton transfer capabilities under lowhumidity or high-temperature conditions.6 This was confirmed by the recorded low proton conductivities of PA-free membranes. By contrast, the PA-doped membranes showed elevated proton conductivities owing to self-ionization and selfdehydration of phosphoric acid.48 These proton conductivities simultaneously increased as temperature rose. 3.6. Fuel Cell Performances. Considering the impact of PA leaching and contribution from PA doping on the proton conduction, two typical single-cells testing conditions were conducted at 80 °C/95% RH and 120 °C/0% RH, and the polarization curves are displayed in Figure 7. Under both operating conditions, the measured open-circuit voltages (OCVs) ranged from 0.97 to 0.98 V for all the membranes, indicating the absence of significant gas permeation after incorporation of ABPBI-GO.49 Under 80 °C/95% RH, SPEEK/ABPBI-GO-3.0 achieved a peak power density of 831.06 mW cm−2, which was much higher than that of SPEEK (655.63 mW cm−2) and SPEEK/ABPBI-GO-1.0 (693.72 mW cm−2) membranes. The elevated and stable proton conductivity of SPEEK/ABPBI-GO-3.0 membrane at high relative humidity should be a reasonable explanation for this outstanding performance. As PA content rose, the ABPBI-GO incorporated membranes demonstrated elevated performances at 120 °C/0% RH, consistent with proton conductivities recorded at high temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07777. Synthesis process of SPEEK, TEM of graphene derivatives, FTIR spectra of ABPBI-GO, cross-section TEM of membranes, TGA curves of membranes, water retention availability, and methanol permeability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (L.W.). ORCID

Xuan Zhang: 0000-0001-8149-4580 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (21406117), Natural Science Foundation of Jiangsu Province (BK20140782), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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4. CONCLUSIONS A novel strategy to improve proton conduction and relieve the PA leaching of PA-doped SPEEK/ABPBI-GO membranes was proposed. The ABPBI-GO incorporated SPEEK membranes showed improved PA content and retention, better thermal properties, and enhanced dimensional stabilities when compared to the PA-doped SPEEK membrane. After absorption of PA, the water uptake of all PA-doped SPEEK/ ABPBI-GO membranes considerably increased. A low-barrier proton transfer pathway was constructed in the composite

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Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective

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