Ti3C2Tx Filler Effect on the Proton Conduction Property of Polymer

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Ti3C2Tx Filler Effect on the Proton Conduction Property of Polymer Electrolyte Membrane Yahua Liu, Jiakui Zhang, Xiang Zhang, Yifan Li, and Jingtao Wang* School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China S Supporting Information *

ABSTRACT: Conductive polymer electrolyte membranes are increasingly attractive for a wide range of applications in hydrogen-relevant devices, for instance hydrogen fuel cells. In this study, two-dimensional Ti3C2Tx, a typical representative of the recently developed MXene family, is synthesized and employed as a universal filler for its features of large specific surface area, high aspect ratio, and sufficient terminated −OH groups. The Ti3C2Tx is incorporated into polymer matrix to explore its function on membrane microstructure and proton conduction property. Both phase-separated (acidic Nafion and sulfonated poly(ether ether ketone)) and non-phase-separated (basic chitosan) polymers are utilized as membrane matrixes. The microstructures, physicochemical properties, and proton conduction properties of the membranes are extensively investigated. It is demonstrated that Ti3C2Tx generates significant promotion effect on proton conduction of the composite membrane by facilitating both vehicle-type and Grotthuss-type proton transfer, yielding several times increased proton conductivity for every polymer-based composite membrane under various conditions, and the composite membrane achieves elevated hydrogen fuel cell performance. The stable Ti3C2Tx also reinforces the thermal and mechanical stabilities of these composite membranes. Since the MXene family includes more than 70 members, this exploration is expected to open up new perspectives for expanding their applications, especially as membrane modifiers and proton conductors. KEYWORDS: MXene, universal active filler, organic−inorganic composite membrane, hydrogen fuel cell, proton conduction property

1. INTRODUCTION Two-dimensional (2D) materials, such as graphene and its derivatives, have garnered tremendous attention in the past decades.1−3 The morphologies of high aspect ratios and atomic thicknesses render 2D materials large electrochemically active surfaces and excellent mechanical properties, bestowing on them versatile applications.4−6 Despite the accessible advantages, there are only relatively few freestanding monolayer solids obtained in experiment except for graphene, such as montmorillonites, hexagonal boron nitrides,7 transition metal dichalcogenides,8 and metal oxides.9 In 2011, Barsoum’s group reported on a new class of 2D transition metal carbides and/or carbonitrides labeled MXenes, which augmented 2D materials and received renewed research interests.10 MXenes are mainly produced by selectively etching A from MAX phases that include a big family with more than 70 members.11 The MAX phases have a general formula of Mn+1AXn, where “M” is the early transition metal, “A” is mainly a group IIIA or IVA (i.e., group 13 or 14) element, “X” is C and/or N, and n = 1, 2, or 3.12 Although a set of MXenes have © XXXX American Chemical Society

been obtained and their physical and chemical properties were studied experimentally or predicted theoretically since then,13−15 the comprehensive exploration of their properties and applications is not comparable to that of graphene-based materials. Within the MXene family, Ti3C2Tx is prevalently recommended to be the typical representative and has been controllably mass produced.16,17 Generally, the exfoliated Ti3C2Tx layers are uniformly terminated by −OH, −O−, and/or −F groups, where T and x represent the terminating groups and their numbers, respectively.18 Different from graphene, the complex layered materials of MXenes composed of more than one element are more potential to offer new opportunities for specific properties.19 Particularly, the features of atomic-scale thicknesses, high elastic moduli along basal plane, and hydrophilic surface endow Ti3C2Tx with a host of Received: April 22, 2016 Accepted: July 19, 2016

A

DOI: 10.1021/acsami.6b04800 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

were conducted on Ti3C2Tx and composite membranes, and the influence of Ti3C2Tx on microstructure and physicochemical property of the membrane was explored. The proton conductivity and proton transfer mechanism through the membrane were evaluated under both high humidity and low humidity within the temperature range of 30−120 °C to elucidate the effect of Ti3C2Tx on proton conduction of composite PEM.

promising applications, especially as a reinforcer for polymer composites.20 The hydrogen fuel cell, converting chemical energy directly into electronic energy, is considered as a promising power device due to its high efficiency, compactness, and environmental friendliness.21,22 After hydrogen being oxidized at anode, the resultant protons must be transported through a conducting membrane (usually assuming polymer electrolyte membrane, PEM) to cathode for completing energy conversion. The PEM, as the pivotal component of hydrogen fuel cell, should possess sufficient proton conductivity and adequate structural stability for high cell performance and long-term operation.23 Perfluorosulfonic acid polymer, such as Nafion, is the state-of-the-art PEM for its natures of high hydrated proton conductivity and chemical/mechanical durability. Under the desirable operation condition of elevated temperature and low humidity, however, like most of the polymer membranes, Nafion suffers from poor proton transfer ability and degraded structural stability. Among various strategies to overcome these deficiencies, polymer−inorganic compositing has been demonstrated to be a facile and effective approach by fusing the excellent stability of inorganic filler.24−27 In addition, if the embedded filler is proton conductive, it will trigger synergy effect between polymer matrix and inorganic filler for proton conduction. In such a way, the creation of additional pathways along filler surface and the connection of “dead ends” within polymer matrix would significantly facilitate proton migration through PEM. It is generally recognized that proton transfer through PEM obeys the two mechanisms of vehicle mechanism (proton diffusing via hydronium) and Grotthuss mechanism (proton hopping from one carrier site to a neighboring one via hydrogen networks).28,29 Accordingly, the presence of conductive fillers will generate extensive hydrogen networks and shorten the proton hopping distance via Grotthuss-type means. Meanwhile, the fillers with hydrophilic groups can sorb and bound numerous water molecules to provide more proton carriers for vehicle-type proton transport. Although diverse inorganic materials have been successfully employed, 2D sheet fillers own intrinsic peculiarities of large specific surface area and high aspect ratio to construct wide and long-range proton transfer pathways within membrane when compared with other dimensional materials (0D, 1D, and 3D).30−34 These features inspire us that MXenes might be one kind of promising 2D conductive fillers for their inherent conductivities and the potential to form extensive hydrogen networks for the presence of hydroxy groups on their surfaces. Moreover, the terminated −F groups with strong electronegative nature might assist the H+ to transfer from polymer to MXenes surface.35,36 Additionally, the MXenes are anticipated to reinforce the structural and thermal stabilities of composite membrane, similar to the function of most inorganic fillers. Herein, ultrathin Ti3C2Tx nanosheets were employed as universal 2D fillers for organic−inorganic composite PEMs for the first time. Two typical polymers (Nafion and chitosan, (CS)) were selected as representatives, the former of which is a commercial acidic polyelectrolyte with phase-separated structure whereas the latter is a low-cost basic polyelectrolyte with non-phase-separated structure. Ti3C2Tx nanosheets were welldispersed into Nafion or CS matrix via solution casting method within the two series of composite membranes. Synchronously, sulfonated poly(ether ether ketone) (SPEEK) was also selected as an analogue to Nafion to further verify the function of Ti3C2Tx on polymer membrane. Systematic characterizations

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Aqueous Nafion solution (5 wt %, DE520) was supplied by Shanghai Hesen 17 Electric Co., Ltd. CS with the deacetylation degree of 91% was provided by Golden-Shell Biochemical Co. (Zhejiang, China) and used as received. Poly(ether ether ketone) (Victrex PEEK, grade 381G) was purchased from Nanjing Yuanbang Engineering Plastics Co., Ltd., which was completely dried under vacuum prior to the sulfonation. Ethanol, concentrated sulfuric acid (98 wt %), aqueous hydrogen peroxide solution (30 wt %), and N,N-dimethylformamide (DMF) were purchased from Kewei Chemistry Co., Ltd., and used without further purification. N,N-Dimethylacetamide (DMAC) was obtained from Bailingwei Technology Co., Ltd., and used without further purification. Deionized (DI) water was utilized throughout the whole experiment. 2.2. Synthesis of Ti3C2Tx. Ti3C2Tx was synthesized by selectively etching out the Al layer from Ti3AlC2, and the details concerning the synthesis procedure were given in the Supporting Information. 2.3. Preparation of Membranes. Nafion-based membranes: The solvent in the obtained Nafion solution was removed by drying at 60 °C in a vacuum oven. The obtained Nafion resin was then redissolved in DMAC and stirred for 4 h at room temperature to obtain a 5 wt % of Nafion/DMAC homogeneous solution. Simultaneously, a specific amount of Ti3C2Tx was dispersed into DMAC with alternant agitation and ultrasonic treatment for 24 h. Afterward, the Ti3C2Tx/DMAC was introduced into the Nafion/DMAC and treated with vigorous agitation and ultrasonic for another 24 h. The resultant homogeneous solution was cast onto a clear glass plate and dried at 80 °C in a vacuum oven for 12 h, and then at 120 °C for another 4 h. The membrane was peeled off from the glass plate in water and then pretreated by standard procedures at 80 °C each in H2O2 (3 wt %), DI water, H2SO4 (1 M), and DI water, and finally dried under vacuum at 60 °C for 8 h. The obtained composite membrane was named as Nafion/Ti3C2Tx-X, where X (X = 1, 2, 5, 10, or 20) represents the weight percentage of Ti3C2Tx to Nafion. The recast Nafion was also fabricated for comparison and designated as recast Nafion. The thicknesses of the membranes fell in the range 60−110 μm. CS-based membranes and SPEEK-based membranes were prepared in a similar way with Nafion-based membranes, and the detailed process is descripted in the Supporting Information. And the SPEEK was self-prepared with the degree of sulfonation of 66.49%. 2.4. Characterization of Ti3C2Tx and Prepared Membranes. A field emission scanning electron microscope (SEM, Philips XL30SFEG) was utilized to observe the microstructures of the produced Ti3C2Tx and membranes, and energy dispersive spectroscopy (EDS) on the same equipment was performed to inspect the distribution of component within Ti3C2Tx. Transmission electron microscopy (TEM, Tecnai G2 20 STWIN) performed at 200 kV was used to obtain the morphology of Ti3C2Tx. Their chemical structures were determined by Fourier transform infrared (FTIR, Nicolet MAGNA-IR 560) spectroscopy with a resolution of 4 cm−1 in the range 4000−400 cm−1. The phase separation and crystalline structure of the membrane were probed by small-angle X-ray scattering (SAXS) and wide-angle X-ray diffractometry (WXRD) with a RigakuD/max2500v/Pc (Cu K 40 kV, 200 mV) at the rate of 0.1 cm−1 (in the range of 0.10°−3.00°) and 2 cm−1, respectively. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA-50 Shimadzu) heating from 30 to 800 °C under a nitrogen atmosphere at a rate of 10 °C min−1. Differential scanning calorimetry (DSC) was conducted by a 204 F1 NETZSCH. The mechanical property of the membrane (1.0 cm × 4.0 cm) was collected by an Instron mechanical tester (Testometric B


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Figure 1. (a) SEM and (b) TEM images of Ti3C2Tx. (c) WXRD patterns of Ti3C2Tx and Ti3AlC2. (d) XPS spectrum and elemental maps of Ti3C2Tx: (e) titanium, (f) carbon, (g) oxygen, and (h) fluorine. 350AX) with an elongation rate of 2 mm min−1 at room temperature. The compositional analysis and chemical bond determination of Ti3C2Tx were analyzed by X-ray photoelectron spectroscopy (XPS) (Escalab250Xi, Thermo Fisher Scien-tific, U.S). 2.5. Measurement. Water uptake and area swelling of the membrane were determined as followed. A rectangular-shaped membrane sample was dried in an oven at 60 °C for 24 h. Its weight (Wdry, g) and area (Adry, cm2) were measured. Then the sample was immersed in DI water for 24 h at a defined temperature, and its weight (Wwet, g) and area (Awet, cm2) were remeasured quickly after removing the surface water. The values of water uptake and area swelling were the averages of three measurements respectively and calculated by the equations

water uptake (%) = (Wwet − Wdry)/Wdry × 100%

(1)

area swelling (%) = (A wet − Adry )/Adry × 100%

(2)

resultant membranes under both hydrated and anhydrous conditions were tested. As for the former, the membrane sample was immersed in water for 48 h prior to measurement at room temperature. And as for the latter, the proton conductivity was tested using dry air, and the system was equilibrated at the desired temperature for 48 h. The proton conductivity (σ, S cm−1) of the sample was calculated by the equation σ = l /AR

where l (cm), A (cm2), and R (Ω) are the membrane thickness, membrane area, and membrane resistance, respectively. The performance of the single cell was carried out with an in-house single fuel cell test setup (CHINO Fuel Cell Testing System (FC5100 series)) at 90 °C. Membrane electrode assemblies (MEAs) with the active area of 4 cm2 (2.0 cm × 2.0 cm) were prepared by hot pressing the anode and cathode together (4.0 MPa at 140 °C for 2.0 min). Both anode and cathode possessing 0.8 mg cm−2 of Pt as the catalyst were prepared with an air-spraying method. H2 and O2 were fed into anode and cathode at the flow rate of 150 and 300 mL min−1, respectively. Prior to the measurement, the single cell was first activated using hydrous H2 (humidification temperature, 80 °C) for 4 h and followed by keeping under the operation conditions for at least 4 h. All single cell tests were conducted three times, and the results were presented as the average values.

The ion exchange capacity (IEC) value of the membrane was evaluated via acid−base titration. The dry and preweighted sample (Wd) was immersed in 2.0 M NaCl solution at 30 °C for 24 h to completely liberate H+ by exchanging with Na+. The liberated H+ was then titrated by 0.01 M NaOH solution using phenolphthalein as an indicator. The IEC (mmol g−1) was calculated by the equation

IEC = 0.01 × 1000 × VNaOH/Wd

(4)

(3)

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Ti3C2Tx. The Ti3C2Tx is synthesized via the common liquid etching by concentrated hydrofluoric acid (as illustrated in Scheme S1 of the Supporting Information). It can be seen that the pristine Ti3AlC2 is compact (see Figure S1a), whereas the removal of Al atoms results in the formation of exfoliated Ti3C2Tx which

where VNaOH (L) and Wd (g) are the volume of NaOH solution consumed in the titration and the weight of the dry membrane, respectively. The IEC value is the average value of three samples. The resistance value (R) of the membrane was measured in a conductivity cell by the ac impedance spectroscopy method. The membrane resistance was probed by a frequency response analyzer (FRA, RST5000F11) with an oscillating voltage of 0.02 V over a frequency range of 1 MHz−10 Hz. Proton conductivities of the C


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Figure 2. SEM images of cross sections of (a) recast Nafion, (b) Nafion/Ti3C2Tx-10, (c) CS, and (d) CS/Ti3C2Tx-5. SAXS curves of Nafion-based membranes under (e) dry state and (f) wet state.

ture of the Membranes. The Ti3C2Tx nanosheets are incorporated into two typical polymers of Nafion and CS to fabricate composite membranes: Nafion/Ti3C2Tx-X and CS/ Ti3C2Tx-X. The inner morphologies of prepared membranes and the dispersion of Ti3C2Tx are examined by SEM. As shown in Figures 2a and 2c, the cross sections of recast Nafion and CS are dense, uniform, and defect-free. By comparison, the incorporation of Ti3C2Tx renders composite membranes rough cross sections with some sheet wrinkles (Figure 2b,d), like the observation in other sheet-filled composite membranes.27,38 For another, the whole cross sections of composite membranes are uniform without clearly visible Ti3C2Tx nanosheet, which is owing to the good compatibility between Ti3C2Tx and polymer matrix originating from the hydrophilic nature of Ti3C2Tx surface. This good compatibility also allows composite membranes to display similar FTIR spectra with plain membrane (see Figure S2a). For instance, all Nafionbased membranes exhibit the characteristic bands at 1127 (C− F), 1053 (OSO), and 981 cm−1 (C−O−C), and the characteristic bands at 3260 (hydroxyl group), 1621 (amide I group), and 1525 cm−1 (amide II group) appear in the spectra of all CS-based membranes (see Figure S2b). These close FTIR spectra infer that Ti3C2Tx nanosheets are physically mixed with Nafion or CS matrix without forming chemical bond.

exhibits a layered and well-aligned architecture with the lateral size up to several micrometers (Figure 1a). After intercalation as seen from TEM image in Figure 1b, the Ti3C2Tx is delaminated into individual sheets with the thickness of about several nanometers. The nanosheet structure is subsequently probed by WXRD, as shown in Figure 1c. Although Ti3C2Tx holds almost all the characteristic bands for the pristine Ti3AlC2, their intensity becomes weak or even vanishes after removing Al layer, in agreement with the observation in the literature.37 Furthermore, the bands for Ti3C2Tx at (002) and (004) downshift to lower angles, inferring an enlarged c lattice parameter. The terminated functional groups (i.e., −OH and/ or −F) on the surface of Ti3C2Tx are detected by XPS (Figure 1d). The presence of signals of O 1s (21.65%) and F 1s (3.82%) suggests the coexistence of −Ti−O− and −Ti−F, and the molar ratio of terminated oxygenic groups (mainly −Ti− OH) to fluorine (−Ti−F) is about 6.7. The FITR spectrum of Ti3C2Tx in Figure S1b exhibits the peaks at 492 and 957 cm−1 corresponding to the stretching vibrations of Ti−O and hydroxyl group, respectively. Additionally, these groups are dispersed uniformly and randomly as proven by their EDS maps in Figure 1e−h. 3.2. Microstructure and Physicochemical Properties of Nafion- and CS-Based Membranes. 3.2.1. MicrostrucD


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Figure 3. DSC curves of (a) Nafion-based membranes and (b) CS-based membranes. Stress−strain curves of (c) Nafion-based membranes and (d) CS-based membranes.

values of composite membranes and plain membrane. Figure S 2d depicts the WXRD curves of CS-based membranes, and the hydrogen-bonding interaction among CS chains affords semicrystalline character with the bands at 12.0°, 18.6°, and 22.8° to all CS-based membranes. The composite membranes display slightly dilated and depressed bands due to the interference of Ti3C2Tx with CS chain stacking as a result of interfacial interaction and steric effect. 3.2.2. Thermal and Mechanical Properties of the Membranes. The structure alteration also affects the thermal and mechanical properties of composite membranes. As illustrated in Figure S2e,f, composite membranes present analogous thermal degradation mechanisms to that of plain membrane because of the inexistence of covalent bond between Ti3C2Tx and Nafion or CS. Nevertheless, the interfacial interaction and stable Ti3C2Tx let the onset temperature of decomposition of polymer chains shift to slightly higher values for composite membranes, along with higher char yields at 800 °C. All the membranes are thermally stable up to 200 °C, adequate for practical application. Furthermore, the improved thermal stabilities of composite membranes can be clearly proved from the results of DSC as depicted in Figure 3a,b. Close to the value in the literature,39 recast Nafion attains the glass transition temperature (Tg) of 126 °C. By comparison, composite membranes display higher Tg in the range of 126− 132 °C upon Ti3C2Tx incorporation. Like the trend in Nafionbased membranes, the increase of transition temperature (Td) value is also noted for CS-based membranes (Figure 3b): 219, 225, and 234 °C for CS, CS/Ti3C2Tx-2, and CS/Ti3C2Tx-10, respectively. The generated hydrogen-bonding interaction should be responsible for such promotion, which, cooperating with the steric interference from Ti3C2Tx, affects mechanical behaviors of the membranes as reflected by their stress−strain curves in Figure 3c,d. As expected, the incorporation of Ti3C2Tx working as a reinforcer strengthens mechanical stabilities of composite membranes. For instance, the Young’s

Nevertheless, the reduction of polymer content and steric interference generated by Ti3C2Tx weaken the intensity of aforementioned bands of composite membranes compared with plain membrane (for both Nafion- and CS-based membranes). The inner morphologies of membranes are further probed by WXRD and SAXS. The amphipathic feature of Nafion chains results in nanophase-separated structure for Nafion-based membranes, where the backbones form hydrophobic domains (tested by WXRD) and the −SO3H groups aggregate into ionic clusters (tested by SAXS). Figure S2c indicates that the absence of strong interaction (chemical bond) between Ti3C2Tx and backbones allows composite membranes to display almost the same WXRD curves as recast Nafion with the typical crystalline band at 2θ = 17° and 40° for the hydrophobic domains. For another, the presence of Ti3C2Tx gives the composite membranes characteristic bands at 2θ = 9.2°. Different from the unapparent influence on hydrophobic domain, the embedded Ti3C2Tx alters the structure of ionic clusters of composite membranes as shown by their q values of 3.05−3.44 nm−1 in SAXS curves under dry state (Figure 2e). Although all the membranes exhibit the characteristic bands, those of composite membranes shift to higher values, that is, narrower ionic clusters. This is reasonably due to the generated interfacial interaction between polymer and Ti3C2Tx (mainly hydrogenbonding interaction originating from −SO3H groups of Nafion and −OH groups on Ti3C2Tx), which restrains the assembly of sulfonic acid groups to ionic clusters by retarding chain motion. For practical application, the SAXS data under wet state are more important as PEM usually works under a humidified environment. Figure 2f exhibits that under the wet state the q value of each membrane shifts to a lower location (larger ionchannel size) compared with that under dry state as a result of the enrichment of water into ionic clusters as expected. Likewise, the composite membranes attain slightly higher q values than that of plain membrane under wet state. However, the presence of water reduces the difference between the q E


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Figure 4. Water uptakes and area swellings of (a) Nafion-based membranes and (b) CS-based membranes at 40 °C. (c) IEC values of Nafion- and CS-based membranes at 30 °C.

Figure 5. (a) Proton conductivities of Nafion-based and CS-based membranes at 40 °C and 100% RH. Temperature-dependent conductivities of (b) Nafion-based membranes and (c) CS-based membranes under 100% RH. (d) Activation energy of proton conduction through Nafion-based and CSbased membranes under 100% RH.

of −SO3H in Nafion near nanosheet interface and thus forming a hydrophilic domain. This domain provides additional space for water storage, hence yielding increased water uptakes to composite membranes. Meanwhile, increasing Ti3C2Tx content produces more hydrophilic interfacial domains and affords gradually increased water uptake. As expected, the composite membranes exhibit higher area swellings than that of recast Nafion, and further increase is observed as Ti3C2Tx content increases. Figure 4b shows the water uptakes and area swellings of CS-based membranes, which display similar trends with those of Nafion-based membranes. Additionally, the activated motion of polymer chains and solvation effect at elevated temperature give continuous increscent of water uptake and area swelling to plain and composite membranes with the increase of temperature (see Figure S3a−d). These results also indicate that Ti3C2Tx has an inapparent effect on the dimensional stability of polymer membrane according to their similar increase ratios of water uptake with temperature. For PEM, its IEC affects the proton conduction especially Grotthuss-type transfer as it indicates the loading amount of ionizable groups (hopping sites) in membrane. Figure 4c

modulus is elevated from 104.6 MPa for recast Nafion to 128.4 and 173.7 MPa for Nafion/Ti3C2Tx-2 and Nafion/Ti3C2Tx-10, respectively, together with the tensile strength increasing from 16.1 to 18.1 and 24.3 MPa. Meanwhile, the presence of rigid inorganic fillers within polymer matrix causes elongation reduction for composite membranes (both Nafion- and CSbased membranes). 3.3. Proton Conduction Properties of Nafion- and CSBased Membranes. 3.3.1. Water Uptakes, Area Swellings, and IEC of the Membranes. The microstructure variation of membrane is affirmed by water uptake and area swelling again. Figure 4a reveals that recast Nafion displays the water uptake of 21.3%, wherein −SO3H groups drive the absorbed water molecules to store in ionic clusters. In spite of the weaker hydrophilic of Ti3C2Tx nanosheets than Nafion matrix, their incorporation improves the water uptaking capabilities of composite membranes. Concretely, the water uptakes are increased to 23.3%, 25.1%, 29.5%, 35.6%, and 37.2% when incorporating 1%, 2%, 5%, 10%, and 20% Ti3C2Tx, respectively. The enhanced water absorbing capacity should be attributed to the −OH groups on Ti3C2Tx, which induce the configuration F


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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of Proton Transfer through Recast Nafion and Nafion/Ti3C2Tx-X

reveals that recast Nafion attains the IEC of 0.918 mmol g−1, close to the value in the literature.40 In contrast, due to the lower dissociation degree of −OH groups on Ti3C2Tx, its addition exerts a dilution effect on −SO3H concentration and then gives reduced IEC values to composite membranes, and a sustained decline is observed for composite membranes with the increase of filler content. Similar to the trend in Nafionbased membranes, CS/Ti3C2Tx-X exhibits lower IEC value compared with CS. For example, the IEC value decreases from 0.219 mmol g−1 for CS to 0.216 mmol g−1 for CS/Ti3C2Tx-1, and the value reduces to 0.187 mmol g−1 when increasing the filler content to 20%. For another, the tested IEC values for composite membranes are slightly lower than their theoretical values. This indicates that the inhibited motion of polymer chains by Ti3C2Tx also suppresses the H+ dissociation of functional groups in composite membranes. 3.3.2. Proton Conductivity under Hydrated Condition. Proton conductivity is critical for the design of PEM, as it governs the operational voltage and current output of a fuel cell. Proton conductivities under both hydrated (100% RH, 30−90 °C) and anhydrous (0% RH, 40−120 °C) conditions are tested to better understand the function of Ti3C2Tx on proton transfer in composite membrane. Figure 5a presents the proton conductivity under 100% RH and 40 °C, which reveals that recast Nafion and CS display the conductivities of 0.089 and 0.013 S cm−1, respectively, profiting from conductive acidic groups (−SO3H in Nafion) or basic groups (−NH2 in CS). And Nafion, as the commercially utilized PEM, parades its outstanding proton conduction resulting from the present ionic clusters (proton transfer highways). In comparison, the introduction of Ti3C2Tx facilitates the proton migration through membrane, yielding enhanced proton conductivities for both Nafion- and CS-based composite membranes. Nafion/ Ti3C2Tx-10 and CS/Ti3C2Tx-5 achieve the highest proton conductivities of 0.161 and 0.069 S cm−1 with the enhancements of 81% and 431%, respectively. The newly formed Ti3C2Tx−polymer interfacial domains should be responsible for this notable increase of conductivity. These domains adsorb vast water molecules, which on the one hand provide more proton carriers for proton diffusing in vehicle mechanism and on the other hand facilitate protonation and deprotonation of conductive groups (e.g., sulfonic, amino, or hydroxyl groups, etc.) for proton hopping in the Grotthuss mechanism. Additionally, the two-dimensional architecture of Ti3C2Tx contributes to creating long-range proton transfer pathways along interfacial domains (as illustrated in Scheme 1). Simultaneously, the increased area swelling and reduced crystallinity (i.e., flexible chain motion) cocontribute to the efficient proton conduction. Elevating Ti3C2Tx content will generate increasing promotion on proton conduction of

composite membrane. For instance, the conductivities of Nafion/Ti3C2Tx-X and CS/Ti3C2Tx-X continuously increase from 0.109 to 0.156 S cm−1 and from 0.026 to 0.069 S cm−1 as Ti3C2Tx content increases from 1% to 5%. However, when further increasing Ti3C2Tx content, the conductivity promotion effect becomes obviously less or even reversed, especially for the content over 10%. Like many other composite PEMs, this is most likely due to the aggregation of Ti3C2Tx, which decreases the amount of interfacial domains and reduces their contribution to proton transfer.31,39 To further elucidate the transport properties, temperaturedependent conductivities of membranes are collected and plotted in Figure 5b,c. All the membranes display gradual improvement in proton conduction with the increase of temperature, which should be originated from the promoted motion of polymer chains and activated water molecules at elevated temperature (resulting in smaller enthalpy change for proton hopping). Additionally, the incorporation of Ti3C2Tx endows composite membranes with enhanced proton conductivities than those of recast Nafion and CS over the entire temperature range, and Nafion/Ti3C2Tx-10 and CS/Ti3C2Tx-5 acquire the highest conductivities, respectively. The Arrhenius activation energy (Ea) is calculated by linear regression analysis of temperature-dependent conductivity curves (see Figure S4a,b) and depicted in Figure 5d. As is well-known, the Grotthuss and vehicle mechanisms jointly govern the proton conduction in Nafion membrane under hydrated condition, and the Ea values for these membranes (ranging 6.1−9.6 kJ mol−1) are in compliance with this conclusion. Meanwhile, composite membranes display lower Ea values than recast Nafion, meaning the easier proton migration. Together with the enhanced conductivity value, they corroborate the effective transfer ability of Ti3C2Tx−polymer interfacial pathways. Although all the composite membranes display lower Ea values than plain membrane, Ea exhibits the trend of first decrease and then increase with the increase of filler content, in accordance with the alteration of conductivity. Meanwhile, the Ea values for CSbased membranes are calculated and depicted in Figure 5d, which suggests that the proton conduction through CS-based membranes is a little more difficult when compared with that through Nafion-based membranes as testified by their higher Ea values (ranging 13.7−15.2 kJ mol−1). Apart from this, a similar case of Ea tendency is observed for CS-based membranes and CS/Ti3C2Tx-5 attains the lowest Ea of 13.7 kJ mol−1. 3.3.3. Proton Conductivity under Anhydrous Conditions. For PEM fuel cell, although high-temperature system under anhydrous condition possesses unique advantages, the insufficient proton conduction of most PEMs impedes its practical application, especially for commercial Nafion. The anhydrous conductivities at various temperatures are invesG


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Figure 6. (a) Proton conductivities of Nafion- and CS-based membranes at 120 °C and 0% RH. Temperature-dependent conductivities of (b) Nafion-based membranes and (c) CS-based membranes. (d) Activation energy of proton conduction through the Nafion- and CS-based membranes under 0% RH.

probed in Figure 6b,c. Akin to the behavior of hydrated conductivity, these two types of composite membranes display the gradual increase of anhydrous conductivity with the testing temperature increasing. The Ea values of membranes are determined (see Figure S4c,d) and depicted in Figure 6d. It should be noted that as the proton could just transfer via the Grotthuss mechanism with the absence of water, the membranes exhibit higher Ea values than those under hydrated condition. Again, the composite membranes exhibit lower Ea values than that of recast Nafion or CS, which implies the accelerated proton transfer by the additional pathways on the Ti3C2Tx surface. The serious influence of water disappearance makes Nafion-based membranes require high energy to complete proton hopping, leading to higher Ea values than those of CS-based membranes. Collectively, these results demonstrate that the incorporation of Ti3C2Tx provides additional conduction pathways and enhances the proton conductivity especially under anhydrous condition. 3.3.4. Single Cell Performances of Nafion-Based Membranes. Considering the superior proton conductivity and structural stability, the single cell performances of Nafion-based membranes are examined to further investigate the function of Ti3C2Tx on membrane performance. Figure 7 shows the polarization and performance curves for recast Nafion and Nafion/Ti3C2Tx-10 at 90 °C under a humidified environment. During the measurement, the thicknesses of both membranes are kept almost the same, and the MEAs are fabricated in the same manner to ensure a good comparison of the intrinsic properties of membranes. The open circuit voltages of the two cells are both above 0.95 V, indicating that there are no problems related to gas (H2 and O2) crossover.43 Recast Nafion attains the maximum current density and power density of about 491.4 mA cm−2 and 152.6 mW cm−2, respectively. By comparison, cell performance of the composite membrane is remarkably improved. Nafion/Ti3C2Tx-10 possesses the

tigated and shown in Figure 6a−d. Figure 6a reveals that both Nafion- and CS-based membranes exhibit much lower proton conductivities than those under hydrated condition due to the absence of water molecules (the main vehicle-type carriers). Conductivities of Nafion- and CS-based membranes are in the ranges of 0.075−0.152 and 1.16−2.22 mS cm−1, respectively. Accordingly, it is interesting to note that the anhydrous conduction ability of Nafion-based membranes is lower than that of CS-based membranes, contrary to the behavior under hydrated condition. The more evident reduction of conductivity of Nafion-based membrane implies that the disappearance of water generates more serious aftereffect: (i) the lack of water induces the shrinkage and size reduction of ionic clusters; (ii) the loss of water molecules will suppress the protonation and deprotonation of −SO3H groups and inactivate their proton conduction function. Different from the situation of plain membrane, the incorporation of Ti3C2Tx affords composite membranes with enhanced proton conductivities despite the IEC values reduction, similar to those found in other literatures.41,42 Such enhancement should be owing to that the large specific surface area and high aspect ratio of Ti3C2Tx allow it to interconnect the ionic channels (especially the “dead ends”) and thus maintain the transfer capacity to some extent. Meanwhile, the abundant hydroxyl and fluorine on Ti3C2Tx could enrich the sulfonic groups or amino groups in polymer phase onto sheet surface through interaction, ensuring effective proton hopping. Compared with acidic Nafion with phase-separated structure, the inexistence of phase separation and the basic conductive groups confer less influence on CS-based membranes for water disappearance. Likewise, the conductivity of CS/Ti3C2Tx-X increases first and then reduces with the increase of Ti3C2Tx content, and CS/Ti3C2Tx-5 attains the highest conductivities of 2.22 mS cm−1, 91.4% higher than that of CS. The dependency of conductivity on temperature under anhydrous condition is H


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universality of the promotion effect of Ti3C2Tx on proton conduction, another common polymer, SPEEK, is utilized as an analogue to Nafion to prepare another series of composite membranes (SPEEK/Ti3C2Tx-X). The SEM image in Figure 8a reveals that the cross section of SPEEK is dense and smooth, while by contrast, SPEEK/ Ti3C2Tx-10 becomes rough and displays detectable wrinkles for the presence of Ti3C2Tx nanosheets (Figure 8b). These nanosheets disperse homogeneously throughout the membrane due to the interfacial hydrogen-bonding interaction, similar to the situation in Nafion/Ti3C2Tx-X. Meanwhile, such interaction, combining with the steric interference from Ti3C2Tx, results in slightly decreased bond intensity in FTIR spectra (1000−1230 cm−1, Figure S5a) and crystallization peak in WXRD patterns (2θ = 19°−24°, Figure S5b) for composite membranes. As regard to the ionic cluster probed by SAXS in Figure S5c, the incorporated Ti3C2Tx makes the q values of composite membranes shift to higher values (i.e., smaller ionic clusters) under dry state. Like that in Nafion-based membranes, the interfacial interaction should be responsible for this alteration by inhibiting the assembly of sulfonic acid groups. Under wet state (see Figure S5d), the enrichment of water within ionic cluster enlarges the size of ionic clusters as proved by the smaller q value for each membrane when compared with those under dry state. Meanwhile, the existence of water reduces the discrepancy between the q values of composite membranes and plain membrane. For another, the weaker amphiphilic feature offers SPEEK-based membranes higher q values, i.e., narrower ionic clusters, than those of Nafion-based membranes. Benefiting from the interfacial interaction and excellent structural stability of Ti3C2Tx, composite membranes achieve improved thermal and mechanical stabilities as shown in Figure 8c,d. Water uptake at 45 °C (see Figure S5e) shows that SPEEK has the water uptake and area swelling of 18.6% and 17.5%, respectively. By comparison, composite membranes

Figure 7. Single H2/O2 cell performances of Nafion-based membranes at 90 °C under humidified environment (80% RH).

maximum current density of 644.0 mA cm−2 (increased by 31.1%) coupled with the maximum power density of 193.3 mW cm−2 (increased by 26.7%). The enhanced proton conduction should be partly accounted for such improvement, which accelerates the reduction reaction in cell cathode and thus improves the conversion of chemical energy to electrical energy. Meanwhile, it can be found that Nafion/Ti3C2Tx-10 obtains the maximum power density of 193.3 mW cm−2 at 0.51 V, whereas recast Nafion has 152.6 mW cm−2 at 0.48 V, implying that incorporating Ti3C2Tx reduces activation loss and ohmic loss of the fuel cell. 3.4. Preparation and Characterization of SPEEK-Based Membranes. In the above section, Ti3C2Tx nanosheets are employed as inorganic fillers into two kinds of composite PEMs. It is found that Ti3C2Tx could effectively enhance the proton conduction by several times for both acidic and basic polymer membranes. Consequently, the composite membrane achieves improved fuel cell performance. To further verify the

Figure 8. SEM images of cross sections of (a) SPEEK and (b) SPEEK/Ti3C2Tx-10. (c) TGA curves and (d) stress−strain curves of SPEEK-based membranes. I


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Figure 9. (a) Proton conductivities (at 40 °C) and activation energy of SPEEK-based membranes under 100% RH. (b) Proton conductivities (at 120 °C) and activation energy of SPEEK-based membranes under 0% RH.

S6b,d). Among the various filler contents, SPEEK/Ti3C2Tx-10 acquires the highest proton conductivity of 1.69 mS cm−1 coupled with the lowest Ea value of 16.4 kJ mol−1. Above all, incorporating a suitable amount of Ti3C2Tx confers significant promotion on the proton conductivity of composites membranes, in accordance with those found in Nafion- and CS-based membranes. The main performances of the three types of membranes are summarized in Table S1. All composite membranes exhibit superior thermal/mechanical stabilities to those of plain membranes, which should be attributed to the hydrogenbonding interaction between Ti3C2Tx and polymer matrix. Meanwhile, the incorporation of hydrophilic Ti3C2Tx endows composite membrane with elevated water uptake and hence area swelling. More importantly, the crucial parameter of proton conductivity exhibits remarkable enhancement upon Ti3C2Tx incorporation under both hydrated and anhydrous conditions, which is mainly owing to the additional proton carriers and transfer pathways. Furthermore, these improvements are heightened by increasing Ti3C2Tx content. Additionally, it should be noted that the achieved conductivities of polymer−Ti3C2Tx composite PEMs are comparable to or even higher than the results in the literature (see Table S2 in Supporting Information).

exhibit increased water uptakes and area swellings supported by the formed hydrophilic interfacial domains and distributed chain packing. Simultaneously, both water uptakes and area swellings of SPEEK-based membranes display gradual increase tendencies with the testing temperature (see Figure S5f,g). As found in Nafion-based membranes, elevating Ti3C2Tx content will bring magnified influence on these physicochemical properties of SPEEK/Ti3C2Tx-X. 3.5. Proton Conduction Properties of SPEEK-Based Membranes. As illustrated in Figure S5h, the IEC values of composite membranes are lower than that of SPEEK (1.150 mmol g−1) due to the lower dissociation degree of −OH groups on Ti3C2Tx. And the increase of Ti3C2Tx content leads to further reduction of IEC value to composite membrane. The proton conductivities of SPEEK-based membranes are also performed under both hydrated and anhydrous conditions. Under 100% RH and 40 °C (Figure 9a), SPEEK obtains the conductivity of 0.056 S cm−1. In comparison, composite membranes exhibit superior proton conduction ability to SPEEK. For instance, 1% Ti3C2Tx offers a 17.8% increase of proton conductivity to 0.066 S cm−1 for SPEEK/Ti3C2Tx-1. The conductivity enhancement, once more, corroborates the obvious promotion effect of Ti3C2Tx on proton conduction, helped by the formed interfacial pathways, elevated water uptake, and reduced crystallinity. Proton conductivity takes on the tendency of first increase and then decline with the filler content, and SPEEK/Ti 3C2Tx-10 possesses the highest conductivity of 0.147 S cm−1. In spite of the close behaviors, the proton conductivities of SPEEK-based membranes are lower than those of Nafion-based ones due to the relatively weak intrinsic conduction ability of SPEEK matrix. The temperature-dependent conductivity is tested to analyze the Ea value (see Figure S6a,c), and the Ea value under hydrated condition is depicted in Figure 9a. A reduction of Ea value is clearly observed after incorporating Ti3C2Tx, in other words, accelerated proton migration with low energy barrier. At the same time, higher Ti3C2Tx content will generate more promotion effect and then yield lower Ea value. Under anhydrous conditions, the reduction of vehicle-type carriers and shrinkage of ionic clusters make proton transport laborious, leading to declined proton conductivities and raised Ea values as shown in Figure 9b. For instance, the proton conductivity and Ea value of SPEEK are 1.26 mS cm−1 and 19.7 kJ mol−1, respectively. Even so, it is notable that composite membranes possess enhanced proton conduction abilities aided by the additional pathway along Ti3C2Tx surface, which as well reduce the Ea values of composite membranes. Increase of proton conductivity emerges under anhydrous condition (see Figure

4. CONCLUSIONS In summary, Ti3C2Tx nanosheets (as the typical representative of MXenes) are synthesized and employed as universal inorganic fillers to prepare composite membrane for proton conduction enhancement. Herein, three typical polymers, acidic Nafion and SPEEK (phase-separated structure) and basic CS (non-phase-separated structure), are utilized as membrane matrixes. Obvious enhancement of proton conductivity under hydrated or anhydrous condition emerges on all kinds of composite membranes by the uniformly dispersed Ti3C2Tx. Such outperformances should be owing to the following features: (i) the large specific surface area and high aspect ratio allow Ti3C2Tx to interconnect the ionic channels (especially the “dead ends”) and construct long-range interfacial pathways within membrane; (ii) the −OH groups on Ti3C2Tx induce the acidic/basic groups of polymer matrix to enrich on interface by hydrogen-bonding interaction, providing efficient hopping sites. These traits produce rapid proton migration and low-energy barrier for composite membranes, thus yielding higher hydrogen fuel cell performance. The composite membranes also attain enhanced thermal and mechanical stabilities due to the interfacial interaction between polymer matrix and the present stable Ti3C2Tx. Such promotion should be universal as J


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(8) Sorkin, V.; Pan, H.; Shi, H.; Quek, S. Y.; Zhang, Y. W. Nanoscale Transition Metal Dichalcogenides: Structures, Properties, and Applications. Crit. Rev. Solid State Mater. Sci. 2014, 39, 319−367. (9) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (10) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253. (11) Barsoum, M. W. MAX Phases: Properties of Machinable Ternary Carbides and Nitrides, 1st ed.; John Wiley & Sons: Weinheim, Germany, 2013. (12) Shein, I. R.; Ivanovskii, A. L. Planar Nano-block Structures Tin+1Al0.5Cn and Tin+1Cn (n = 1, and 2) from MAX Phases: Structural, Electronic Properties and Relative Stability from First Principles Calculations. Superlattices Microstruct. 2012, 52, 147−157. (13) Shein, I. R.; Ivanovskii, A. L. Graphene-like Titanium Carbides and Nitrides Tin+1Cn, Tin+1Nn (n = 1, 2, and 3) from De-Intercalated MAX Phases: First-Principles Probing of Their Structural, Electronic Properties and Relative Stability. Comput. Mater. Sci. 2012, 65, 104− 114. (14) Khazaei, M.; Arai, M.; Sasaki, T.; Chung, C.-Y.; Venkataramanan, N. S.; Estili, M.; Sakka, Y.; Kawazoe, Y. Novel Electronic and Magnetic Properties of Two-Dimensional Transition Metal Carbides and Nitrides. Adv. Funct. Mater. 2013, 23, 2185−2192. (15) Enyashin, A. N.; Ivanovskii, A. L. Atomic Structure, Comparative Stability and Electronic Properties of Hydroxylated Ti2C and Ti3C2 Nanotubes. Comput. Theor. Chem. 2012, 989, 27−32. (16) Wang, X. H.; Zhou, Y. C. Layered Machinable and Electrically Conductive Ti2AlC and Ti3AlC2 Ceramics: a Review. J. Mater. Sci. Technol. 2010, 26, 385−416. (17) Tzenov, N. V.; Barsoum, M. W. Synthesis and Characterization of Ti3AlC2. J. Am. Ceram. Soc. 2000, 83, 825−832. (18) Wang, X.; Shen, X.; Gao, Y.; Wang, Z.; Yu, R.; Chen, L. AtomicScale Recognition of Surface Structure and Intercalation Mechanism of Ti3C2X. J. Am. Chem. Soc. 2015, 137, 2715−2721. (19) Tang, Q.; Zhou, Z.; Shen, P. Are MXenes Promising Anode Materials for Li Ion Batteries? Computational Studies on Electronic Properties and Li Storage Capability of Ti3C2 and Ti3C2X2 (X = F, OH) Monolayer. J. Am. Chem. Soc. 2012, 134, 16909−16916. (20) Ling, Z.; Ren, C. E.; Zhao, M.-Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y. Flexible and Conductive MXene Films and Nanocomposites with High Capacitance. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 16676−16681. (21) Neef, H.-J. International Overview of Hydrogen and Fuel Cell Research. Energy 2009, 34, 327−333. (22) Ogo, S. Electrons from Hydrogen. Chem. Commun. 2009, 23, 3317−3325. (23) Zhang, H.; Shen, P. K. Recent Development of Polymer Electrolyte Membranes for Fuel Cells. Chem. Rev. 2012, 112, 2780− 2832. (24) Thanganathan, U. Structural Study on Inorganic/Organic Hybrid Composite Membranes. J. Mater. Chem. 2011, 21, 456−465. (25) Wu, W.; Li, Y.; Chen, P.; Liu, J.; Wang, J.; Zhang, H. Constructing Ionic Liquid-Filled Proton Transfer Channels within Nanocomposite Membrane by Using Functionalized Graphene Oxide. ACS Appl. Mater. Interfaces 2015, 8, 588−599. (26) He, Y.; Wang, J.; Zhang, H.; Zhang, T.; Zhang, B.; Cao, S.; Liu, J. Polydopamine-modified Graphene Oxide Nanocomposite Membrane for Proton Exchange Membrane Fuel Cell under Anhydrous Conditions. J. Mater. Chem. A 2014, 2, 9548−9558. (27) Zhao, L.; Li, Y.; Zhang, H.; Wu, W.; Liu, J.; Wang, J. Constructing Proton-Conductive Highways within an Ionomer Membrane by Embedding Sulfonated Polymer Brush Modified Graphene Oxide. J. Power Sources 2015, 286, 445−457. (28) Binsu, V. V.; Nagarale, R. K.; Shahi, V. K. Phosphonic Acid Functionalized Aminopropyl Triethoxysilane−PVA Composite Ma-

confirmed by the similar tendencies of these three kinds of polymer-based membranes. Therefore, this preliminary exploration of MXene as inorganic filler provides new insights into the potential application of MXene family for membrane separation and proton conduction.

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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.6b04800. Synthesis of Ti3C2Tx and preparation of CS-based membranes and SPEEK-based membranes; SEM image of Ti3AlC2 and FTIR spectrum of Ti3C2Tx; FTIR spectra, WXRD patterns, TGA curves, and water uptake and area swelling as a function of temperature, and Arrhenius plots of proton conductivities (under 100% RH and 0% RH) of Nafion- and CS-based membranes; FTIR spectra, WXRD patterns, SAXS curves, water uptakes, area swellings, IEC, temperature-dependent conductivities, and Arrhenius plots of proton conductivities (under 100% RH and 0% RH) of SPEEK-based membranes; summary of the main properties and performances of the prepared membranes; comparison of proton conductivity in this study with some reported in other literatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 86-371-63887135; Fax 86-371-63887135 (J.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from National Natural Science Foundation of China (21576244 and 21506232).

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REFERENCES

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Ti3C2Tx Filler Effect on the Proton Conduction Property of Polymer

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