A Simple Self-Cross-Linking Strategy for Double-Layered Proton

Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

Letter

A simple self-cross-linking strategy for double-layered proton exchange membranes with improved methanol resistance and good electrochemical properties for passive direct methanol fuel cells Jifu Zheng, Lei Dai, Shenghai Li, Ce Shi, Yunqi Li, Suobo Zhang, Hui Yang, and Tauqir Ali Sherazi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00311 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

A Simple Self-Cross-Linking Strategy for DoubleLayered

Proton

Improved

Exchange

Methanol

Electrochemical

Membranes

Resistance

Properties

for

with

and

Good

Passive

Direct

Methanol Fuel Cells Jifu Zhenga, Lei Daia, Shenghai Lia, Ce Shib, Yunqi Lib, Suobo Zhanga,c,* , Hui Yang d,*, and Tauqir A. Sherazi e a

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

b

Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nangjing 211816, China d

Shanghai Institute of Microsystem and Information Technology, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 200050, China

e

Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan Corresponding author. *E-mail: [email protected]; *E-mail: [email protected]. KEYWORDS: proton conductivity, self-cross-linking, double-layered proton exchange membrane, methanol resistance, passive direct methanol fuel cells

ACS Paragon Plus Environment

1

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT

A series of new double-layered proton exchange membranes (PEMs) with improved methanol resistance has been successfully prepared through the self-cross-linking of cyano-groups catalyzed by super acid. The experimental results indicate that the variation of mixed solvents and cross-linking time have important effects on the morphology of the membrane. Moreover, the presence of a methanol barrier layer with appropriate thickness can significantly improve the performance of cross-linking PEMs, including proton conductivity, proton selective permeability and electrochemical property. The passive direct methanol fuel cells (DMFCs) based on these cross-linking membranes present a maximum power density of 45.0 mW cm-2 at 25°C, which is superior to the value of a pristine membrane (24.5 mW cm-2) and a commercial Nafion® 115 membrane (29.3 mW cm-2). The present study provides a new idea for the design and development of high performance PEM materials for DMFC applications.


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In recent decades, direct methanol fuel cells (DMFCs) have attracted increasing attention, specifically passive DMFCs, which can potentially be used as portable power sources due to their simple structural design, easy refueling, and low pollution output.1However, the slow reaction kinetics of methanol oxidation and unreactive crossover of methanol across proton exchange membranes (PEMs) have hindered large-scale commercial use of passive DMFCs. Particularly, methanol crossover is the most serious factor that could lead to a remarkable energy loss in passive DMFCs.2-4To reduce inefficiency caused by methanol crossover, a significant effort has been made to modify existing commercial Nafion® membranes by blending inorganic components and various polymers in the matrix with methanol barrier layers,5-9 and by using a layer-by-layer (LBL) deposition technique with thin polyelectrolyte films (or multilayer) on the surface.10-12The proper thickness of methanol barrier layers on the surface (or in the matrix) of membranes has an increasing obstruction effect and results in the tortuosity of the fuel diffusion path, which can effectively improve the proton/methanol selectivity.13-15However, blending usually causes dispersion and interface separation of the components, and the process of LBL deposition is often complex in which many factors need to be considered, including the impact of pH, ionic strength and hydrophilic/hydrophobic character of the polymer backbones. Thus, the design of new PEMs with high proton/methanol selectivity is a hot topic in the field of passive DMFCs. Currently, the following two common methods are used to reduce the methanol crossover and improve proton/methanol selectivity: developing novel sulfonated aromatic polymers with appropriate water swelling16 and selecting an appropriate cross-linking procedure for modifying the hydrophilic water channels of the PEMs17-19. In general, the use of the cross-linking agent


3


Page 4 of 25

and the participation of sulfonic groups not only can significantly alter the ion exchange capacity (IEC) values of the polymers before and after the cross-linking procedure, but also cause poor connectivity of the ion channels and decreased mobility of the polymer chains, as well as the decrease of proton conduction in many cases.20 Hydrated proton and methanol possess the similar transport mechanism and diffusion path in the polymer matrix, which makes the decrease of methanol crossover more difficult in the premise of maintaining good proton conduction. Therefore, a new design strategy to prepare cross-linking PEMs with good resistance against methanol crossover and high proton/methanol selectivity needs to be developed. The polymers containing cyano groups are of current interest in the synthesis of ionic conducting membranes since the strong dipole force of the nitrile suspended on the polymer chains can promote the adhesion of the membrane to the catalyst layer,21limit the swelling in aqueous22 and form physical cross-liking23, resulting in improved comprehensive performances of PEMs.4, 24Moreover, as potential sites for chemical cross-linking, the cyano groups can form triazine groups, which possess high thermostability and chemical stability. Although polymers containing cyano groups have been used in gas separation membranes25 and nanoporous organic polymer networks,26 so far there is no report of the chemical cross-linking of cyano groups to construct PEMs. Therefore, we tried to use cyano-containing polymers to prepare novel PEMs with methanol barrier layers and successfully synthesized a series of new cross-linking doublelayered PEMs by the self-cross-linking of the cyano groups. Note that this new cross-linking strategy only used super acid as the catalyst, and the thickness of the cross-linking layer at the micro/nanometre scale could be adjusted by controlling the cross-linking time. Herein, details of the self-cross-linking strategy, and the morphology, methanol permeability and proton


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductivity of cross-linking membranes with methanol barrier layers, as well as their potential applications in passive DMFCs are investigated.

Scheme 1. The preparation protocol for the cross-linking membranes with double-layered structure through a self cross-linking strategy. Sulfonated microblock copolymers (SPP-co-PAEKs) containing cyano-groups, as previously reported by our groups, exhibited well-developed nanophase morphologies and larger ionic clusters, as well as good interface compatibility;24 therefore, they were selected to prepare crosslinking PEMs with methanol barrier layers. Scheme 1 shows the preparation protocol of crosslinking PEMs by a simple self-cross-linking strategy. First, the sulfonated copolymers m-SPPco-PAEK 1.80 were cast onto a glass plate from their N,N-dimethylacetamide (DMAc) solution 8% (wt/v), heated in an oven at 65 °C for 12 h and at 120 °C under a vacuum for 12 h to form the homogeneous membranes. The homogeneous membrane m-SPP-co-PAEK 1.80, coated on a glass plate was immersed in a solution of CF3SO3H and organic solvents (v/v =1/4) at 110 °C for


5


Page 6 of 25

a specific amount of time to produce different cross-linking membranes. Although dense layers are readily visible in the cross-section of the scanning electron microscope (SEM) of all prepared membranes (Figure S2a-c, e), only c-m-SPP-co-PAEK 1.80-x membranes (where x refers to the treatment times) prepared in a solution of CF3SO3H/CH3COOH show a non-porous structure (Figure S2d). It is evident that the solvent has a significant effect on the polymer morphology during the immersion process. This effect may be due to the different diffusivity and plasticizing effect of the solvents in the matrix. Under the condition of optimized solvents (CF3SO3H/CH3COOH), the thickness of the dense layer in c-m-SPP-co-PAEK 1.80-x increases with the increase of the thermal treatment time (Figure S2d-f). The presence of triazine rings and the partial cross-linking of cyano groups in these dense layers can be confirmed by attenuated total reflection infrared spectra (ATR-IR) (Figure 1a, Figure S3) and X-ray photoelectron spectra (XPS) (Figure 1b, Figure S4), respectively.25,27 It is also found that c-mSPP-co-PAEK 1.80-x membranes are not completely soluble in DMAc, and that the weight of the insoluble gel content of c-m-SPP-co-PAEK 1.80-15.0 is up to 13.4 % (Figure S5, Table S1). These results demonstrate that the cyclotrimerization occurrs during the immersion process. We selected 2,6-dimethoxybenzonitrile with a weak steric hindrance effect as the model compound to confirm the cross-linking of cyano-groups in sulfonated microblock copolymers. Meanwhile, CF3SO3H was used as the catalyst to ensure that the cyclotrimerization would proceed smoothly. The chemical structure of the trimers was identified by MALDI-TOFMS spectra (Figure S6). Then, benzonitrile derivative, with a strong steric hindrance, and a structure similar to the hydrophobic segments of m-SPP-co-PAEK 1.80, was used to verify the possibility of cyclotrimerization of cyano-containing polymers. The results show the cyclotrimerization still occurred. Specifically, the formation of the triazine ring via the step-by-step dehydration was


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


confirmed by MALDI-TOFMS spectra (Figure S7). The possible mechanism is shown in Figure S8. We speculate that a hot solution of CF3SO3H and CH3COOH has two important effects during the cross-linking process as follows: to plasticize the hydrophobic aromatic domain and then to swell the ion conducting sulfonated domain. Cyano-groups on the surface of pristine membranes are catalyzed by CF3SO3H to form triazine rings, which, in turn, determine the initial packing of the hydrophobic aromatic domain. At the same time, the ion-conducting domain quickly takes up mass, generating osmotic pressure, which is a common phenomenon for PEMs with nanometresized hydrophilic channels.28,29 This accumulated osmotic pressure allows CF3SO3H to continuously permeate the membranes. This continuous permeation means that the cross-linking is not confined to the direct contact region between the membranes and the solution, and it also may be the reason for the rapid formation of the cross-linking layer. As shown in Figure S2f, the thickness of the cross-linking layer can reach 5 µm in 15 minutes. This demonstrates that alteration of the solution type and cross-linking time can effectively control the variety of membrane morphology and the thickness of the dense layer. To our knowledge, this is the first report of chemical cross-linking of cyano-groups being used to construct PEMs containing dense cross-linking layers. Moreover, merely by simple immersion, the homogeneous membrane can be directly converted to the cross-linking membrane with methanol barrier layers during the preparation procedure. In particular, the cross-linking process only occurs on a single side of the homogeneous membrane that is in direct contact with the solution of CF3SO3H and CH3COOH; thus, the prepared cross-linking membranes possess a peculiar double-layer. Therefore, this strategy is notably different from traditional methods for methanol resistance, such as the LBL deposition technique, composite membrane or blending process.


7


Page 8 of 25

Figure 1. a) ATR-IR spectra of m-SPP-co-PAEK 1.80-10.0 membrane (black) and c-m-SPP-coPAEK 1.80-10.0 membrane (blue) in the range of 750 to 1700 cm-1; b) The N1s core-level XPS spectra of m-SPP-co-PAEK 1.80-10.0 membrane (bottom) and c-m-SPP-co-PAEK 1.80-10.0 membrane (top); c) SAXS patterns of m-SPP-co-PAEK 1.80 membrane and c-m-SPP-co-PAEK 1.80-x membranes; d) DSC thermograms of m-SPP-co-PAEK 1.80-10.0 membrane and c-mSPP-co-PAEK 1.80-x membranes. The microstructures of prepared membranes were further characterized by small-angle X-ray scattering (SAXS). The occurrence of intense ionomer peaks of all cross-linking membranes at higher q values indicates the connectivity of the ion cluster domains is maintained, even if the ionomer peaks shift to a higher q range with increasing the cross-linking time. Notably, the ionic


8

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


domain for all cross-linking membranes is slightly smaller than that of the pristine membrane, which may help to improve selective permeability of the mass transport. In contrast, there is no significant change in the intensity and the position of the matrix knee peak in all cross-linking membranes. This shows that the sequence-distribution structures of the cyano groups cannot be altered by the self-cross-linking of partial cyano groups; thus, the crystalline domains remain intact. The significant interdomain interaction peak is found in scattering curves, thus, change of the hydrophobic matrix can be further observed at the low-q regime (Figure 1c). This variation is evident by the value of fractal dimension (Df), which is evaluated for the double-layer structure by the equation: l(q) = k·x(-Df) . The Df values of the cross-linking membranes are higher than that of the pristine membrane (Table S2) with an increase of the cross-linking time. This suggests that the formation of bulky triazine rings can constrain the polymer chains and prevent them from moving around during the cross-linking process, which accounts for the chain tightening effects. There is a close relationship between the morphology and the proton conductivity or methanol permeation of cross-linking membranes. As summarized in Table 1, the proton conductivities of cross-linking membranes containing different thicknesses of cross-linking layers exhibit minor changes upon cross-linking. This finding may be due to the maintained connectivity of the ionic cluster domains and the local sulfonic acid density in the per unit volume of the ionic cluster domains being improved, even though the chain tightening effects constrain the size of the ionic cluster domains. Moreover, since sulfonic acid groups do not participate in the cross-linking reaction, there is no obvious change for the value of IEC before and after the cross-linking (Table S1). The cross-linking membranes also show low methanol permeability at room temperature, which is lower than the value of the pristine membrane (4.18 ×10-7 cm2 S-1).


9


Page 10 of 25

Moreover, with the increase in thickness of the cross-linking layer, methanol permeability gradually decreases, from 3.98 ×10-7 to 0.91×10-7 cm2 S-1. Therefore, compared with pristine membrane, the relative selectivity parameters (SP) of cross-linking membranes are clearly improved. The state of water molecule in the membranes was also studied by differential scanning calorimetry (DSC) because it is directly related to the chemical structure of the membrane.13 The results of DSC show that the content of freezable water in c-m-SPP-co-PAEK 1.80-x membranes is lower than that of m-SPP-co-PAEK1.80, while the amount of non-freezable water in all cross-linking membranes is comparable to that of pristine membrane (Figure 1d, Table 1, Figure S9). It is well known that, the content of freezable water is strongly influenced by the size of the ionic cluster and the free volume for the occupation of a water molecule. Therefore, low content of freezable water in c-m-SPP-co-PAEK 1.80-x membranes can be attributed to the formation of the cross-linking network. Moreover, the cross-linking membranes hold more strongly bound water molecules per acid group, although the size of ionic clusters was reduced. The confinement of non-freezable water within nanoscale ion channels could facilitate ionic transport through a Grotthus-type mechanism. To our knowledge, methanol crossover is strongly influenced by the portion of freezable and non-freezable water. Therefore, the reduced methanol permeability of cross-linking membranes can be primarily attributed to the larger portion of non-freezable water than freezable water. These results indicate that the formation of a cross-linking network not only can efficiently inhibit methanol crossover and improve proton selective permeability. Additionally, this can cause chain-tightening effects, efficiently reducing the effects of oxidizing radical species on the polymer chains, improving the oxidation stability of cross-linking membranes (Table S1). Concerning the mechanical property of cross-linking


10

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


membranes, the mechanical strength shows that the tensile modulus in the range of 37.2-47.9 MPa, suggesting a good mechanical property (Table S3). Table 1. Methanol permeability, and proton conductivity of the pristine membrane and crosslinking membranes. Membrane

m-SPP-coPAEK 1.80 a

c-m-SPP-coPAEK 1.80-5.0



N115

PC b±0.2

41.5

43.0

39.5

27.2

77.0

4.18

3.98

2.97

0.91

18.30

9.93

10.80

13.30

29.89

4.21

22.1

20.3

18.3

15.6

18.8

10.2

2.9

2.2

0.2

-f

11.9

17.4

16.1

15.4

-f

(σ, mS cm-1) MP c±0.05 (P, cm2 s-110)

7

SP d (S s cm-3104) WU e±0.5 (wt %) Freezable water (wt %) Bound water (wt %) a b c

Previously synthesized and added for comparison.24 Proton conductivity (PC), measured at 30°C.

Methanol permeability (MP), measured at 25 °C.

d

Selectivity parameter (SP), the ratio of the proton conductivity to methanol permeability (σ/P). e

Water uptake (WU), measured at 25°C.

f

Not determined


11


Page 12 of 25

To evaluate the performance of cross-linking membranes, a membrane electrode assembly (MEA) was prepared according to published methods.30 Figure 2a shows the comparison of passive DMFC single cell performance of these cross-linking membranes and the Nafion® 115 (N115) membrane at a methanol concentration of 4M. The power density of the cross-linking membrane is clearly higher than that of pristine membrane. Highlighting the excellent cell performance derived from the c-m-SPP-co-PAEK 1.80-10.0 membrane, the maximum current density and power density reach 210 mA cm-2 and 45.0mWcm-2, respectively. Notably, the power density of the c-m-SPP-co-PAEK 1.80-10.0 membrane is approximately 80 % higher than that of pristine membrane (24.4 mWcm-2). This increase in cell performance can be attributed to the decrease in methanol crossover, which may decrease the occurrence of a mixed potential at the cathode.7,

12

Further, as the thickness of the cross-linking layer increases, the maximum

power density of the cell gradually decreases. This change may be due to the decrease of proton conductivity caused by the increased thickness of the cross-linking layer, and to poor adhesion of the membrane to the catalyst layer due to the increased Tg (Table S1). The performance of the cm-SPP-co-PAEK 1.80-10.0 membrane is superior to that of commercial N115 (29.3 mWcm-2) and other types of reported PEMs (Table S4).


12

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. a) Polarization curves and power density curves of the DMFCs using a N115 membrane, pristine membrane and prepared membrane measured at 25°C. Operating conditions: feeding with 4 M CH3OH under passive operation; b) Polarization curves and power density curves of the DMFCs using a N115 membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane measured at 25°C. Operating conditions: feeding with 6 M CH3OH under passive operation; c) Transient discharging curves of three passive DMFCs with N115 membrane, pristine membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane at a constant voltage of 0.35 V fueled with 4.0 mL of 4 M methanol solution at 25°C. d) Discharging curve of a DMFC with c-m-SPP-co-PAEK 1.80-10.0 membrane based MEA at a constant current of 40 mA cm-2 at 25°C.


13


Page 14 of 25

It is well known that high methanol concentration can increase specific energy of the cell, reduce cell weight, and have an important effect on the stability of the PEM.31 Therefore, the effect of fuel concentration on cell performance was evaluated at a methanol concentration of 6 M. As shown in Figure 2b, for the N115 membrane, a remarkable decrease in the cell performance is observed. This decrease may be the result of a combination of swelling under a high methanol concentration, and a high charge concentration within nanoscale domains leading to the ion channels losing their selectivity.4, 12 Moreover, methanol molecules permeate through the membrane resulting in a mixed potential and catalyst poisoning at the cathode. By contrast, the polarization curves of c-m-SPP-co-PAEK 1.80-10.0 membrane-based MEA exhibit almost no drop in the power density with the increase of methanol concentration. With pristine membrane, c-m-SPP-co-PAEK 1.80-10.0 membrane, and N115 membrane as PEMs and 4.0 mL of 4 M methanol as fuel, three passive DMFCs were discharged at a constant voltage of 0.35 V to study the effect of cross-linking membranes on the cell’s Faraday efficiency and energy efficiency. As shown in Figure 2c, the Faraday efficiency and energy efficiency for the passive DMFC with the c-m-SPP-co-PAEK 1.80-10.0 membrane are 66.9 % and 19.8%, respectively, where as those of passive DMFC with pristine membrane are ca. 60.6 % and 18.0 %, respectively. Meanwhile, the energy efficiency of the c-m-SPP-co-PAEK 1.80-10.0 membrane is also higher than that of the N115 membrane (18.8 %). These results demonstrate that the cross-linking membranes containing double-layers exhibit better fuel cell performances than the pristine membrane, due to their effective methanol-blocking and improved proton/methanol selectivity. In addition, the experimental results for the long-term stability of the cell show no dramatic attenuation in performance within 270 hours (Figure 2d), demonstrating that the stability of the MEA with cross-linking PEMs is suitable for practical applications.


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In summary, we demonstrated an efficient preparation method for cross-linking PEMs with methanol barrier layers through the self-cross-linking of homogeneous membrane containing cyano-groups catalyzed by super acid. This strategy not only avoids the use of a cross-linking agent or other nanofillers, but also allows for the thickness of the methanol barrier layer at the micro/nanometre scale to be adjusted. In addition, the preparation process of the membrane is simple due to none of any volatile byproducts being liberated. More importantly, the prepared membranes using this strategy can inhibit methanol permeability at the anode side and maintain interfacial compatibility of the anode. A passive DMFC with c-m-SPP-co-PAEK 1.80-10.0 membrane has a maximum power density of 45.0 mW cm-2 at 25 °C, which is superior to the value of the N115 membrane. The present study demonstrates that appropriate physical and chemical adjustment of the pathway can effectively control ion and mass transport. This efficient strategy provides a new method for the design and development of high performance PEM materials for DMFC applications.

ASSOCIATED CONTENT Supporting Information. Experimental section, SEM image, ATR-IR spectra, XPS spectra, Photographs, MALDITOFMS spectra, DSC thermograms, IEC values, SAXS profiles, DMFCs data AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected]. Notes


15


Page 16 of 25

The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (No. 2015CB655302), the National Natural Science Foundation of China (No. 21774123, 21374116, 51473163, 51661145024), and the Development of Scientific and Technological Project of the Jilin Province (No. 20160101316JC, 20160519006JH, 20170204025GX). REFERENCES (1) Kakati, N.; Maiti, J.; Lee, S. H.; Jee, S. H.; Viswanathan, B.; Yoon, Y. S. Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We Have Any Alternative for Pt or Pt-Ru? Chem. Rev., 2014, 114, 12397-12429. (2) Xu, K.; Li, K.; Ewing, C. S. Hickner, M. A.; Wang, Q. Synthesis of Proton Conductive Polymers with High Electrochemical Selectivity. Macromolecules 2010, 43, 1692-1694. (3) Masud, J.; Nath, M. Co7Se8 Nanostructures as Catalysts for Oxygen Reduction Reaction with High Methanol Tolerance. ACS Energy Lett. 2016, 1, 27-31. (4) Li, Q.; Chen, Y.; Rowlett, J. R.; McGrath, J. E.; Mack, N. H.; Kim, Y. S. Controlled Disulfonated Poly (Arylene Ether Sulfone) Multiblock Copolymers for Direct Methanol Fuel Cells. ACS Appl. Mater. Interfaces 2014, 6, 5779-5788. (5) Chen, Z. W.; Holmberg, B.; Li, W. Z.; Wang, X.; Deng, W. Q.; Munoz, R.; Yan, Y. S. Nafion/Zeolite Nanocomposite Membrane by in Situ Crystallization for a Direct Methanol Fuel Cell. Chem. Mater., 2006, 18, 5669-5675.


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Pandey, R. P.; Thakur, A. K.; Shahi, V. K. Sulfonated Polyimide/Acid-Functionalized Graphene Oxide Composite Polymer Electrolyte Membranes with Improved Proton Conductivity and Water-Retention Properties. ACS Appl. Mater. Interfaces 2014, 6, 16993-17002. (7) Mochizuki, T.; Uchida, M.; Uchida, H.; Watanabe, M.;Miyatake, K. Double-Layer Ionomer Membrane for Improving Fuel Cell Performance. ACS Appl. Mater. Interfaces 2014, 6, 13894-13899. (8) Auimviriyavat, J.; Changkhamchom, S.; Sirivat, A. Development of Poly(Ether Ether Ketone) (Peek) with Inorganic Filler for Direct Methanol Fuel Cells (DMFCS). Ind. Eng. Chem. Res., 2011, 50, 12527-12533. (9) Zheng, J. F.; He, Q. Y.; Liu, C. L.; Yuan, T.; Zhang, S. B.; Yang, H. Nafion-microporous organic polymer networks composite membranes. J. Membr. Sci., 2015, 476, 571-579. (10) Argun, A. A.; Ashcraft, J. N.; Hammond, P. T. Highly Conductive, Methanol Resistant Polyelectrolyte Multilayers. Adv. Mater., 2008, 20, 1539-1543. (11) Xiang, Y.; Lu, S. F.; Jiang ,S. P. Layer-by-Layer Self-assembly in the Development of Electrochemical Energy Conversion and Storage Devices from Fuel Cells to Super capacitors. Chem. Soc. Rev., 2012, 41, 7291-7321. (12) Yuan, T.; Pu, L. J.; Huang, Q. H.; Zhang, H. F.; Li, X. M.; Yang, H. An Effective Methanol-Blocking Membrane Modified with Graphene Oxide Nanosheets for Passive Direct Methanol Fuel Cells. Electrochimica Acta 2014, 117, 393-397.


17


Page 18 of 25

(13) Choi, B. G.; Hong, J.; Park, Y. C.; Jung, D. H. Hong, W. H.; Hammond, P. T.; Park, H. S. Innovative Polymer Nanocomposite Electrolytes: Nanoscale Manipulation of Ion Channels by Functionalized Graphenes. ACS Nano, 2011, 5, 5167-5174. (14) Tseng, C.-Y.; Ye, Y.-S.; Cheng, M.-Y.; Kao, K.-Y.; Shen, W.-C.; Rick, J.; Chen, J.-C.; Hwang, B.-J. Sulfonated Polyimide Proton Exchange Membranes with Graphene Oxide show Improved Proton Conductivity, Methanol Crossover Impedance, and Mechanical Properties. Adv. Energy Mater. 2011, 1, 1220-1224. (15) Nicotera, I.; Simari, C.; Coppola, L.; Zygouri, P.; Gournis, D.; Brutti, S.; Minuto, F. D.; Aricò, A. S.; Sebastian, D.; Baglio, V. Sulfonated Graphene Oxide Platelets in Nafion Nanocomposite Membrane: Advantages for Application in Direct Methanol Fuel Cells. J. Phys. Chem. C 2014, 118, 24357-24368. (16) Shin, D. W.; Guiver, M. D.; Lee, Y. M. Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability. Chem. Rev., 2017, 117, 4759-4805. (17) Lee, K.-S.; Jeong, M.-H.; Lee, J.-P.; Lee, J.-S. End-Group Cross-Linked Poly(arylene ether) for Proton Exchange Membranes. Macromolecules 2009, 42, 584-590. (18) Li, N. W.; Zhang, S. B.; Liu, J.; Zhang, F.; Synthesis and Properties of Sulfonated Poly[bis(benzimidazobenzisoquinolinones)] as Hydrolytically and Thermooxidatively Stable Proton Conducting Ionomers. Macromolecules 2008, 41, 4165-4172.


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Zhang, G.; Li, H. T.; Ma, W.J.; Zhang, L.Y.; Lew, C. M.; Xu, D.; Han, M. M.; Zhang, Y.; Wu, J.; Na, H. Cross-Linked Membranes with a Macromolecular Cross-Linker for Direct Methanol Fuel Cells. J. Mater. Chem., 2011, 21, 5511-5518. (20) Chikh, L.; Delhorbe, V.; Fichet, O. (Semi-) Interpenetrating polymer networks as fuel cell membranes. Journal of Membrane Science 2011, 368, 1-17. (21) Gao, Y.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li, X.; Kaliaguine, S. Synthesis of Copoly(aryl ether ether nitrile)s Containing Sulfonic Acid Groups for PEM Application. Macromolecules 2005, 38, 3237-3245. (22) Kim, D. S.; Kim, Y. S.; Guiver, M. D.; Pivovar, B. S. High Performance Nitrile Copolymers for Polymer Electrolyte Membrane Fuel Cells. J. Membr. Sci., 2008, 321, 199-208. (23) Shin, D. W.; Lee, S. Y.; Kang, N. R.; Lee, K. H.; Guiver, M. D.; Lee, Y. M. Durable Sulfonated Poly(arylene sulfide sulfone nitrile)s Containing Naphthalene Units for Direct Methanol Fuel Cells (DMFCs). Macromolecules 2013, 46, 3452-3460. (24) He, Q. Y.; Xu, T.; Qian, H. D.; Zheng, J. F.; Shi, C.; Li, Y. Q.; Zhang, S. B. Enhanced Proton Conductivity of Sulfonated Poly(p-phenylene-co-arylether ketone) Proton Exchange Membranes with Controlled Microblock Structure. J. Power Sources 2015, 278, 590-598. (25) Li, F. Y.; Xiao, Y. C.; Chung, T.-S.; Kawi, S. High-Performance Thermally Self-CrossLinked Polymer of Intrinsic Microporosity (PIM-1) Membranes for Energy Development. Macromolecules 2012, 45, 1427-1437.


19


Page 20 of 25

(26) Ren, S. J.; Bojdys, M. J.; Dawson, R.; Laybourn, A.; Khimyak, Y. Z.; Adams, D. J.; Cooper, A. I. Porous, Fluorescent, Covalent Triazine-Based Frameworks Via Room-Temperature and Microwave-Assisted Synthesis. Adv. Mater., 2012, 24, 2357-2361. (27) Zhan, Y. Q.; Yang, X. L.; Guo, H.; Yang, J.; Meng, F. B.; Liu, X. B. Cross-Linkable Nitrile Functionalized Graphene Oxide/Poly(arylene ether nitrile) Nanocomposite Films with High Mechanical Strength and Thermal stability. J. Mater. Chem., 2012, 22, 5602-5608. (28) Van Honschoten, J. W.; Brunets, N.; Tas, N. R. Capillarity at the nanoscale. Chem. Soc. Rev., 2010, 39, 1096-1114. (29) Park, M. J.; Downing, K. H.; Jackson, A.; Gomez, E. D.; Minor, A. M.; Cookson, D.; Weber, A. Z.; Balsara, N. P. Increased Water Retention in Polymer Electrolyte Membranes at Elevated Temperatures Assisted by Capillary Condensation. Nano Lett., 2007, 7, 3547-3552. (30) Chen, M.; Chen, J.; Li, Y.; Huang, Q. H.; Zhang, H. F.; Xue, X. Z.; Zou, Z. Q.; Yang, H. Cathode Catalyst Layer with Stepwise Hydrophobicity Distribution for a Passive Direct Methanol Fuel Cell. Energy Fuels 2012, 26, 1178-1184. (31) Li, X. L.; Faghri, A. Review and Advances of Direct Methanol Fuel Cells (DMFCs) Part I: Design, Fabrication, and Testing with High Concentration Methanol Solutions. J. Power Sources 2013, 226, 223-240.


20

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic Scheme 1. The preparation protocol for the cross-linking membranes with double-layered structure through a self cross-linking strategy. Figure 1. a) ATR-IR spectra of m-SPP-co-PAEK 1.80-10.0 membrane (black) and c-m-SPP-coPAEK 1.80-10.0 membrane (blue) in the range of 750 to 1700 cm-1; b) The N1s core-level XPS spectra of m-SPP-co-PAEK 1.80-10.0 membrane (bottom) and c-m-SPP-co-PAEK 1.80-10.0 membrane (top); c) SAXS patterns of m-SPP-co-PAEK 1.80 membrane and c-m-SPP-co-PAEK 1.80-x membranes; d) DSC thermograms of m-SPP-co-PAEK 1.80-10.0 membrane and c-mSPP-co-PAEK 1.80-x membranes. Figure 2. a) Polarization curves and power density curves of the DMFCs using a N115 membrane, pristine membrane and prepared membrane measured at 25°C. Operating conditions: feeding with 4 M CH3OH under passive operation; b) Polarization curves and power density curves of the DMFCs using a N115 membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane measured at 25°C. Operating conditions: feeding with 6 M CH3OH under passive operation; c) Transient discharging curves of three passive DMFCs with N115 membrane, pristine membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane at a constant voltage of 0.35 V fueled with 4.0 mL of 4 M methanol solution at 25°C. d) Discharging curve of a DMFC with c-m-SPP-co-PAEK 1.80-10.0 membrane based MEA at a constant current of 40 mA cm-2 at 25°C.


21


TOC Graphics


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. The preparation protocol for the cross-linking membranes with double-layered structure through a self cross-linking strategy. 160x88mm (300 x 300 DPI)



Figure 1. a) ATR-IR spectra of m-SPP-co-PAEK 1.80-10.0 membrane (black) and c-m-SPP-co-PAEK 1.8010.0 membrane (blue) in the range of 750 to 1700 cm-1; b) The N1s core-level XPS spectra of m-SPP-coPAEK 1.80-10.0 membrane (bottom) and c-m-SPP-co-PAEK 1.80-10.0 membrane (top); c) SAXS patterns of m-SPP-co-PAEK 1.80 membrane and c-m-SPP-co-PAEK 1.80-x membranes; d) DSC thermograms of m-SPPco-PAEK 1.80-10.0 membrane and c-m-SPP-co-PAEK 1.80-x membranes. 160x120mm (300 x 300 DPI)


Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. a) Polarization curves and power density curves of the DMFCs using a N115 membrane, pristine membrane and prepared membrane measured at 25°C. Operating conditions: feeding with 4 M CH3OH under passive operation; b) Polarization curves and power density curves of the DMFCs using a N115 membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane measured at 25°C. Operating conditions: feeding with 6 M CH3OH under passive operation; c) Transient discharging curves of three passive DMFCs with N115 membrane, pristine membrane and c-m-SPP-co-PAEK 1.80-10.0 membrane at a constant voltage of 0.35 V fueled with 4.0 mL of 4 M methanol solution at 25°C. d) Discharging curve of a DMFC with c-m-SPP-co-PAEK 1.80-10.0 membrane based MEA at a constant current of 40 mA cm-2 at 25°C. 160x115mm (300 x 300 DPI)


A Simple Self-Cross-Linking Strategy for Double-Layered Proton

Recommend Documents