sustainable energy & fuels - dicppemfc.dicp.ac.cn/201706.pdf · 2019. 12. 11. · sulfonated...

SustainableEnergy & Fuels

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A novel porous s

aFuel Cell System and Engineering Laborato

Chinese Academy of Sciences, 457 Zhongsh

[email protected]; [email protected] of Chinese Academy of Sciences,

† Electronic supplementary information (and membrane characterization includinTGA, SEM, TEM, water uptake, swelling r10.1039/c7se00240h

Cite this: Sustainable Energy Fuels,2017, 1, 1405

Received 7th May 2017Accepted 19th June 2017

DOI: 10.1039/c7se00240h

rsc.li/sustainable-energy

This journal is © The Royal Society of C

ulfonated poly(ether etherketone)-based multi-layer composite membranefor proton exchange membrane fuel cellapplication†

Yongyi Jiang,ab Jinkai Hao,ab Ming Hou, *a Shaojing Hong,ab Wei Song,a Baolian Yia

and Zhigang Shao *a

An advanced sulfonated poly(ether ether ketone) (sPEEK)-based multi-layer composite membrane with

high performance and durability is fabricated, which consists of a porous sPEEK base membrane, two

transition layers (TLs) and two PFSA outer layers (PLs). These porous sPEEK base membranes with

nanoscale pores are prepared first through a vapor induced phase inversion (VIPI) method. Owing to the

higher porosity and the denser distribution of sulfonic acid clusters, the cell performance and physical

properties of porous sPEEK membranes are superior to those of sPEEK membranes prepared by

a solvent casting method. The multi-layer structure of this composite membrane results in reduced

swelling and improved water uptake, and eventually brings about a high proton conductivity. Single cell

tests indicate that the multi-layer composite membrane has a higher cell performance and more

outstanding durability in comparison with sPEEK membranes. The growth rate of hydrogen crossover

current density of this composite membrane is much lower than that of sPEEK membranes, proving the

effectiveness of PLs in improving the chemical durability of sPEEK-base membranes. After long-term

stability tests, the sPEEK multi-layer composite membrane still shows a good cell performance,

especially at low relative humidity (RH).

Introduction

Research on proton exchange membranes (PEMs), especially onhigh performance and low-cost PEMs, is always the focus in thelow temperature PEMFC (LT-PEMFC) eld.1–3 Peruorosulfonicacid (PFSA) ionomer membranes, in particular the Naon®type, are the state-of-the-art PEMs due to their excellentperformances and stabilities in low temperature PEMFCs(around or below 80 �C).4 However, their high cost is stilla major obstacle to the widespread use of LT-PEMFCs.

Over the last few decades, many hydrocarbon polymermaterials, including polystyrene-sulfonic acid (PSSA),5 poly(arylene ether)s (PAEs),6,7 polysulfones8,9 and their derivatives,etc.,10,11 have been studied as PEMs. As a typical kind of PAE,sulfonated poly(ether ether ketones) (sPEEKs) have a rigidpolymer main chain of a polyaromatic backbone with many

ry, Dalian Institute of Chemical Physics,

an Road, Dalian 116023, China. E-mail:

Beijing 100039, China

ESI) available: Materials and chemicalsg FTIR, mercury intrusion porosimetry,atio and proton conductivity. See DOI:

hemistry 2017

short side chains of sulfonic acid groups, and are considered tobe a promising candidate for alternative PEM materials owingto their low-cost, good gas barrier properties, high thermalstability and mechanical strength.12,13 Recently, improvementon sPEEK-base membranes involves modication with otherpolymers,14,15 or formation of cross-linked networks by func-tional groups,16,17 and blending inorganic llers into PEMs.18–20

However, the common sPEEK membranes, prepared bya conventional solvent casting method, have a limited protonconductivity and cell performance unless their degree ofsulfonation (DS) is improved to some extent, which bringsabout the dramatic swelling of sPEEK.10,21,22

Another challenge that remains is that sPEEKs are prone tochemical degradation, so they cannot withstand long-term useunder PEMFC operations. Gonon et al. considered that thedegradation of sPAEK membranes initially occurred in non-sulfonated phenyl ether aromatic rings resulting from theattack of hydroxyl radicals, and further caused scissions in thesPAEK main chain.23 Zhang et al. conrmed the above degra-dation mechanism through investigating the degradationbehavior of sulfonated PEEK under strongly acidic andoxidizing conditions.24 Furthermore, Zhao et al.25 investigatedthe degradation mechanism of hydrocarbon ionomers forPEMFCs using density functional theory (DFT), and pointed out

Sustainable Energy Fuels, 2017, 1, 1405–1413 | 1405

http://crossmark.crossref.org/dialog/?doi=10.1039/c7se00240h&domain=pdf&date_stamp=2017-07-20

http://orcid.org/0000-0002-5341-8975

http://orcid.org/0000-0003-2777-6362

http://dx.doi.org/10.1039/c7se00240h

http://pubs.rsc.org/en/journals/journal/SE

http://pubs.rsc.org/en/journals/journal/SE?issueid=SE001006

Scheme 1 Illustration of the fabrication process of the sPEEK multi-layer composite membrane.

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that the aryl sulfonated bond and the aryl ether bond are twopossible weak sites susceptible to OH radical attack.

There have been some research studies on the chemicaldurability of hydrocarbon polymer membranes in a PEMFCenvironment. Several approaches can be summarized. One is toadd a free radical scavenger,26–28 such as CeO2, MnO2 and vitaminE, or other nanocomposites,29–31 which can scavenge reactiveoxygen species (ROS) or decrease gas crossover. Another is tosynthesise a novel PAE derivative by reducing the electron densityof the polymer backbone by attaching bulky steric groups,32 orintroducing strong electron withdrawing groups.33 SulfonatedPAEKs with a cross-linked structure also had a higher chemicalstability.7,17,34 Lee et al.34 considered that this semi-inter-penetrating polymer network resulting from cross-linkingdecreased the hydrogen crossover evidently and promoted thechemical stability of fuel cells. Apart from these, bi-layer andmulti-layer composite membranes are an effective strategy toobtain high durability of PEMs, especially for sulfonated hydro-carbon materials.35–38 Lien and Lee et al.39 prepared a novelsulfonated polyimide/Naon multi-layer membrane, in whichNaon layers were adhered to either side of the SPI by thermalimidization, and durability tests revealed a relatively improvedstability compared to that of native SPI. Watanabe et al.38 re-ported a bi-layer ionomer membrane, a thin-layer Naon ona sulfonated aromatic copolymer (SPK-bl-1), which exhibiteda higher cell performance despite no signicant change of wateruptake and proton conductivity compared with the original SPK-bl-1 membrane, but long-term durability was not involved.

In this paper we designed an advanced sPEEK-based multi-layer composite membrane consisting of a porous sPEEK basemembrane, two transition layers (TLs) and two PFSA outerlayers (PLs). PLs formed by PFSA act as protective layers, whichcan protect the inner sPEEK base membrane against oxidationdegradation from reactive oxygen species such as hydrogenperoxide, hydroxyl and hydroperoxyl radicals.40,41 TLs are fabri-cated by the mixture of PFSA and sPEEK. Owing to the hydrogenbond from the sulfonic acid groups of sPEEK and PFSA, TLsimprove the interfacial compatibility between the sPEEK basemembrane and PLs effectively as well as mitigate the delami-nation problem of the composite membranes.42

Here, the porous sPEEK base membrane with nanoscalepores was prepared by using a vapor induced phase inversion(VIPI) method, which was scarcely reported. During the VIPIprocess, dibutyl phthalate (DBP), a pore-forming additive witha low molecular weight,43 was added to form the nanoscalepores and adjust the porosity of the porous sPEEK membranes.It was reported that the porous sPEEK membrane had anadvantage over the membrane prepared by a solvent castingmethod in terms of the cell performance and physical proper-ties. Based on this, a sPEEK-based multi-layer compositemembrane was prepared and investigated. Its long-termstability under accelerated PEMFC operations was also studied.

Experimental

The source of materials and chemicals used in this study andmembrane characterization are included in the ESI.†

1406 | Sustainable Energy Fuels, 2017, 1, 1405–1413

PEEK sulfonation

sPEEK was prepared by the post-sulfonation method.44–46 8 g ofPEEK powder was dissolved in 100 mL sulfuric acid (95–98 wt%)in a three-neck ask with a vigorousmechanical agitation undera nitrogen atmosphere. The sulfonation reaction was carriedout at 65 �C for two hours. Aer that, this sulfonation reactionwas terminated through precipitating the polymer solution intodeionized (DI) water, and then washed with DI water severaltimes until the pH of the rinse water was in the range of 6–7.Subsequently, the sPEEK polymer was dried at 50 �C for severalhours, and further dried at 100 �C for 12 h. The DS of the sPEEKpolymer was determined to be 0.65 by 1H NMR spectroscopy(Fig. S1, ESI†).44,45

Membrane preparation

The fabrication process of the sPEEK multi-layer compositemembrane is illustrated in Scheme 1.

Firstly, porous sPEEK membranes were prepared througha vapor induced phase inversion (VIPI) method.47 Specially, theprepared sPEEK was dissolved in DMAc to obtain 16 wt%sPEEK solution. Then the sPEEK solution was cast onto a glassplate and placed it in a constant temperature and humiditychamber at 50 �C and 60% relative humidity (RH) for 0.5 h.Aer that, the membrane was peeled off from the above glassplate, and washed with the mixture of water–ethanol and DIwater several times. During the VIPI process, differentamounts of DBP, such as 0 wt%, 0.5 wt% and 1.0 wt%, wereadded into the above sPEEK solution to improve the porosityof the porous sPEEK membranes. The corresponding sPEEKmembranes were named SP2, SP3 and 3P4 membrane,respectively.

Secondly, indirect hot-spraying was used to fabricate PLs andTLs on the surface of silicon substrates by spraying Naondispersion and the solution mixture. Finally, the sPEEK multi-layer composite membrane was prepared by a thermal transfermethod via sandwiching the prepared sPEEK membranebetween two silicon substrates at 130 �C and 0.4 MPa. Thesolution mixture was made up of 5 wt% Naon dispersion and 5wt% sPEEK solution with the ratio of 1 : 1. The amounts ofsolution mixture for TLs and Naon dispersion for PLs were 8.5mL cm�2 and 20 mL cm�2, respectively.

As a comparison, the sPEEK membrane prepared by usingthe solvent casting method was named the SP1 membrane. Thethickness of all pure sPEEK membranes was 16–17 mm and thatof the sPEEK multi-layer composite membrane was controlledto be 24 � 2 mm and was marked as SP3–NF.

This journal is © The Royal Society of Chemistry 2017


Fig. 1 I–V performance and ohmic resistance of MEAs fabricated with different sPEEK membranes at different relative humidities (30% RH, 50%RH, 80% RH and 100% RH): (a) SP1 membrane, (b) SP2 membrane, (c) SP3 membrane, and (d) SP4 membrane. Open symbol represents thecorresponding ohmic resistance.

Table 1 DBP amount, porosity, OCV and hydrogen crossover currentdensity of the sPEEK membranes

Membrane SP1a SP2 SP3 SP4

DBP amount (wt%) — 0.0 0.5 1.0Porosity (%) — 63.5 71.7 73.9OCVb (V) 0.986 0.968 0.930 0.886Hydrogen crossover (mA cm�2)c 1.92 3.96 4.4 26

a SP1 membrane prepared by the solvent casting method without DBPaddition. b The OCV data testing at 65 �C and 100% RH. c Hydrogencrossover current density testing at 65 �C and 100% RH.

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Membrane characterization

The physical properties of the porous sPEEK membranes werecharacterized, such as the porosity, water uptake (WU) andswelling ratio. Fourier Transform Infrared Spectroscopy (FTIR),thermogravimetric analysis (TGA) and proton conductivitymeasurements of the membranes and scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM)were performed. All of the detailed experimental procedures arealso included in the ESI.†

Assembly of fuel cell and performance tests

Membrane electrode assemblies (MEAs) were fabricatedthrough sandwiching the membrane between gas diffusionelectrodes (0.5 mg Pt per cm2 for the cathode and 0.3 mg Pt percm2 for the anode). The effective area of the MEAs was 5 cm2.The fabricated MEAs were rst activated by feeding H2 and O2 at65 �C for several hours. I–V curves were recorded using a fuelcell impedance meter (KFM2030, Kikusui, Japan) at 65 �C anddifferent RHs with 0.1/0.2 L min�1 of H2/O2.

The long-term stability of the sPEEK multi-layer compositemembrane was evaluated by constant-current tests using anelectric load PLZ152WA (Kikusui, Japan). During these stabilitytests, the ow rates of H2 and O2 with 30% RH were 0.06/0.1 Lmin�1, respectively. At the same time, the linear sweep vol-tammetry (LSV) method was performed to record the H2

crossover current density using a Solartron SI 1287 equippedwith a potentiostat 1260 frequency response analyzer.

Results and discussion

Before investigating this sPEEK-based multi-layer compositemembrane, we explored rst the cell performance of the sPEEK


base membrane. Fig. 1a–d show the I–V performance of theMEAs fabricated with the SP1 membrane (denoted as SP1 MEA)and porous sPEEK membranes. It was observed that theseporous sPEEK MEAs exhibited a better I–V performance thanthe SP1 MEA regardless of the open circuit voltage (OCV) andhydrogen crossover, especially at high relative humidity (RH).The cell performance of the porous sPEEK membranesincreased with the increasing amount of DBP (from SP2 to SP4).In addition, the SP1 MEA showed a slightly higher ohmicresistance compared to the porous sPEEK MEA at inadequatehumidication, whereas all of the membranes showed a similarohmic resistance at 100% RH. The high cell performance of SP3and SP4 membranes might be attributed to their porousstructure which helped to keep more water and improve theproton conductivity.

A comparison of OCV and hydrogen crossover currentdensity for these MEAs is shown in Fig. S2 (ESI),† and Table 1also lists the addition amount of DBP, porosity, OCV andhydrogen crossover current density of the sPEEK membranes. Itwas seen that SP1 prepared by the solvent casting method had




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the highest OCV value (0.986 V) and the lowest hydrogencrossover current density (1.92 mA cm�2), agreeing with itsdense structure. The porosity of SP2, SP3 and SP4 was 63.5%,71.7% and 73.9%, respectively. It showed an increasing trendwith the increasing amount of DBP, coupling with the decreaseof OCV and the corresponding increase of H2 crossover currentdensity. The OCV of SP4 was just 0.886 V and its hydrogencrossover current density reached 26 mA cm�2. High gascrossover resulted from high porosity, so SP2 and SP3 hadhigher OCV and lower H2 crossover current density than SP4due to their lower porosity. Therefore, we concluded that (i) thehigher porosity caused by the addition of DBP could bring abouthigher gas crossover, leading to decreased OCV and increasedH2 crossover current density. (ii) The porous sPEEK membranewith high porosity had an outstanding cell performanceregardless of the OCV and H2 crossover.

The water uptake and swelling ratio are of importance forPEMs, which have a signicant inuence on the protonconductivity and dimensional stability.48 Fig. 2 shows the wateruptake and swelling ratio of these sPEEK membranes. InFig. 2a, the porous sPEEK membranes showed a higher watercontent than SP1. This was explained by the porous structurewhich facilitated the collection of more water in the poroussPEEK membrane and promoted the proton transport. Addi-tionally, the water uptake of porous sPEEK membranes, fromSP2 to SP4, tends to increase with the increasing amount ofDBP. It was noteworthy that the water uptake exhibited a slightdifference when the testing temperature ranged from 25 �C to55 �C. Once the temperature reached or exceeded 65 �C,a dramatic increase of water uptake had taken place, especiallyfor SP4, so the water uptake and swelling ratio were notmeasured properly. The swelling ratio of the above membranesexhibited a similar result to the water uptake in Fig. 2b. Whentaking into account the initial cell performance and physical

Fig. 2 (a) Water uptake and (b) swelling ratio dependence ontemperature for the sPEEK membranes.


properties of the above membranes, we found that (i) althoughSP4 had a higher cell performance, the higher water uptake andswelling ratio at elevated temperatures hindered the use of SP4.(ii) SP2 had a better swelling ratio compared to sPEEKmembranes, but its cell performance was low. So, SP3 was usedas the base membrane for the following research.

The distribution of sulfonic acid groups of the sPEEKmembranes stained with silver ions was characterized by TEM.In comparison with Naon, the sPEEK membrane has a similarmicro-phase separation structure, which is closely associatedwith water uptake and proton conductivity and further inu-ences the PEMFC performance.49,50 In the TEM images (Fig. 3),the dark spots were hydrophilic domains formed by sulfonicacid clusters, while the bright regions represented the hydro-phobic domains composed of polymer backbones.51,52 As shownin Fig. 3a and b, both SP1 and SP3 displayed a homogeneousdispersion of sulfonic acid clusters, but SP3 had denser ionclusters. Fig. 3c and d are the high magnication images of SP1and SP3, respectively. The size of sulfonic acid clusters in SP3was 8–11 nm, which was larger than that of SP1 (7 � 2 nm). Thelarger sulfonic acid clusters as well as the denser distribution inSP3 contributed to higher water uptake and proton conduc-tivity, and eventually improved the PEMFC performance, whichwere in accordance with the previous results.

The FTIR spectra of the sPEEK base membranes and SP3–NFcomposite membrane are shown in Fig. 4a. In the spectrum ofsPEEK, the typical absorption peak at 1648 cm�1 correspondedto the carbonyl group (C]O), and the peaks at 1023 and 1074–1078 cm�1 were assigned to asymmetric and symmetricstretching vibrations of O]S]O.44,53,54 Additionally, the peaksat 1074 cm�1 and 1078 cm�1 were associated with SP1 andporous sPEEK membranes, respectively, which might haveresulted from the variation of the pore structure. SP3–NF hada similar infrared absorption to sPEEK membranes except forthe peaks at 982 cm�1 and 1054 cm�1 assigned to the stretchingvibration of C–O–C and the symmetric stretching vibration ofthe –SO3– group in the side chain of PFSA, respectively.55,56 Theabsorption peak of ether groups (Ar–O–Ar), marked with theblack-dotted line in Fig. 4a, shied from 1150 cm�1 to 1130cm�1. The reason was attributed to the inuence of the PFSAionomer on sPEEK. Therefore, the chemical interaction among

Fig. 3 TEM images: (a and c) SP1 and (b and d) SP3.




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TLs, PLs and sPEEK base membranes was conrmed due to thehydrogen bond from sulfonic acid groups of PFSA and sPEEK,which also indicated the effectiveness of TLs in improving theinterfacial compatibility between sPEEK base membranesand PLs.

The proton conductivity of the PEMs has a vital inuence onthe PEMFC performance. Fig. 4b shows the proton conductivityof the membranes, in which all of the membranes exhibitincreasing proton conductivity with the increase of tempera-ture. At the same time, the proton conductivity of the poroussPEEK membranes tended to increase with increasing porosity,and their value was superior to that of SP1, especially for SP3and SP4. Additionally, SP3–NF had a higher proton conductivitycompared to SP3. At 25 �C, the proton conductivity of SP3 was0.058 S cm�1, but that of SP3–NF reached 0.068 S cm�1. Higherproton conductivity was observed in Naon 211. While thetesting temperature increased to 65 �C, the proton conductivityof SP3–NF could even surpass that of Naon 211, which wasattributed to the multi-layer composite structure of SP3–NF.

Fig. 4d shows the corresponding Arrhenius plots for theproton conductivity of the membranes, and their activationenergy (Ea) for proton transport in the membranes is calculatedand represented as well. It was seen that the Ea of Naon 211was 14.4 kJ mol�1, and all of the sPEEKmembranes and SP3–NFhad a higher Ea than 14 kJ mol�1, which agreed with that of theGrotthuss mechanism which was reported to be around 14–40kJ mol�1,57 that is to say, the proton transport mainly took place

Fig. 4 (a) FTIR absorbance spectra of the membranes. (b) Variation of presponding Arrhenius plots for the proton conductivity of the membrana heating rate of 10 �C min�1 in nitrogen.


via the Grotthuss mechanism where the protons transfer fromone solvent molecule to another through the hydrogen bonds.Shukla et al. considered that the proton transport via the formerwas faster than the diffusion of water (vehicular mechanism)due to the presence of high water uptake.58 The above alsoexplained the higher proton conductivity of SP3–NF. Asmentioned above, high porosity resulted in high water uptakewhich promoted proton transport, but the correspondingswelling also increased. Here, as shown in Fig. S3 (ESI),† SP3–NF achieved an improved water uptake and a reduced swellingratio due to the presence of a multi-layer structure, whichcontributed to an elevated proton conductivity.

Fig. 4c shows TGA curves of four membrane samples. It wasseen that Naon showed a two-step weight loss pattern.59 Therst weight loss started around 280 �C, which was attributed tothe splitting-off of sulfonic acid groups on Naon side-chains.The second weight loss occurred above 355 �C, which wasattributed to the decomposition of the main chains. SP1 andSP3 exhibited a three-step weight loss.60 The rst weight losswas before 250 �C attributed to the dehydration of absorbedwater or the evaporation of residual DMAc. The second and thethird weight loss were the loss of sulfonic acid and the mainchain decomposition of sPEEK, respectively, which started ataround 320 �C and 480 �C. It was noteworthy that the SP3–NFcomposite membrane showed a similar degradation processcompared to SP1 and SP3 in spite of a small change in thetemperature of weight loss. At 550 �C, the residual weight of

roton conductivity with temperature for the membranes and (d) cor-es. (c) TGA curves of the membrane samples from 50 �C to 900 �C at



Fig. 5 SEM images of the SP3–NF multi-layer composite membrane:(a and b) the cross section; (d) the surface; (c) EDS linear scanninganalysis of fluorine element in the cross section.


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SP3–NF (48 wt%) was much higher than that of Naon,demonstrating the outstanding thermal stability of the SP3–NFcomposite membrane.

The cross-sectional structure and surface morphology of theSP3–NFmulti-layer composite membrane were characterized bySEM. As shown in Fig. 5a, the multi-layer structure of the sPEEKcomposite membrane was observed and it agreed with the EDSlinear scanning of uorine (F) element in the cross section(Fig. 5c). The thickness of the sPEEK base membrane is 16–17mm, and the thickness of TLs and PLs is about 1 mm and 4 mm,

Fig. 6 (a) I–V curves, ohmic resistance and (b) power density curves of tRH and 100% RH); open symbol represents the corresponding ohmic resiand SP3–NF MEA at 65 �C, 30% RH supplying with H2/N2. (d) Stability test�C and 30% RH with 0.06/0.1 L min�1 of H2/O2.


respectively. So, the lower cost of SP3–NF can be achieved due tothe lower content of Naon. At the same time, a good contactamong layers of SP3–NF indicates the effectiveness of TLs inimproving the interfacial compatibility between sPEEK basemembranes and PLs. Fig. 5b shows the high magnicationimage of the sPEEK base membrane in SP3–NF. It was observedthat the sPEEK base membrane prepared by the VIPI methodhad nanoscale pores, not microscale pores. Additionally, thedense surface morphology of the SP3–NF multi-layer compositemembrane is observed in Fig. 5d, which contributed to miti-gating the gas crossover caused by the porosity of the sPEEKbase membrane.

Fig. 6a shows the I–V curves and ohmic resistance of the MEAfabricated with the SP3–NF multi-layer composite membrane(SP3–NF MEA) at different humidity levels, and the corre-sponding power density curves are given in Fig. 6b. It was foundthat the SP3–NF MEA exhibited a higher cell performance thanthe SP3 MEA (Fig. 1c), especially at 80% RH and 100% RH. TheOCV of the SP3 MEA at 100% RH was 0.92 V, but the value of theSP3–NFMEA increased to 0.96 V. In comparison with that of theSP3 MEA, the maximum power density of the SP3–NF MEAincreased by 23% (867 mW cm�2) and 17% (1176 mW cm�2) at80% RH and 100% RH, respectively. It is indicated that theintroduction of the multi-layer composite structure effectivelyimproves the cell performance of SP3 and brings about anelevated OCV, mitigating the gas leakage from the poroussPEEK base membrane.

In addition to high cell performance, long-term stability isalso a vital factor for the practical use of PEMs.61,62 Generally,chemical degradation of PEMs will happen during long-term

he SP3–NF MEA at different relative humidities (30% RH, 50% RH, 80%stance. (c) Comparison of H2 crossover current density for the SP3 MEAs of the SP3–NF MEA at 400 mA cm�2 in constant-current mode at 65




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cell operations, coupled with mechanical degradation.59,63,64

Humidity changes usually lead to the aggravated mechanicaldegradation of PEMs, while low RH operation (inadequatehumidication) always brings about the accelerated failure ofPEMFCs due to the severe chemical degradation of PEMs.65

Considering this, during the stability tests the cell temperatureof 65 �C and the gases supplied with 30% RH were providedwith an aim to accelerate the chemical degradation of PEMs,lowering the inuence of mechanical degradation.

Fig. 6c compares the hydrogen crossover current densitybetween the SP3MEA and SP3–NFMEA. The initial H2 crossovercurrent densities of the SP3 MEA and SP3–NFMEA were 0.5 and0.46 mA cm�2, respectively. Aerwards, the H2 crossover of theSP3 MEA exhibited a sharp growth. Its value increased 3-foldaer 24 h, and reached 65.6 mA cm�2 aer 100 h. However, theH2 crossover of the SP3–NF MEA showed a slow growth, and itsvalue was just 3.96 mA cm�2 aer 113 h. During the whole tests,the growth rate of H2 crossover current density for the SP3–NFMEA was 12.5 mA cm�2 h�1 approximately, which was muchlower than that of the SP3 MEA. Fig. 6d exhibits the long-termvoltage data of the SP3–NF MEA at 400 mA cm�2 and 30% RH,indicating an outstanding stability of voltage. The initialupward tail of voltage data was attributed to the full humidi-cation of the SP3–NFMEA. Aerwards, the humidity was kept ata low level, and the oxygen partial pressure was also at a lowerlevel aer hydrogen crossover tests, which accounted for thedownward tails in the subsequent stability tests. The resultsproved that the SP3–NF composite membrane not only hasa higher cell performance, but also achieves an improveddurability compared to SP3.

The I–V performance and power density curves of the SP3–NFMEA at different RHs before and aer the stability tests were

Fig. 7 (a) I–V performance and ohmic resistance (open symbol) and(b) corresponding power density of the MEA fabricated with the SP3–NF composite membrane during stability tests at 65 �C and 80% RH.


characterized. In Fig. 7a, the I–V performance of the SP3–NFMEA at 80% RH increased aer the stability test of 40 h, anda slight decrease was observed aer 845 h. There was littlechange in the ohmic resistance of the SP3–NF MEA. However,the maximum power density (Fig. 7b) of the SP3–NF MEAincreased by 5.4% (914 mW cm�2) aer 40 h, but decreased by15% (738 mW cm�2) aer 845 h. At the same time, the OCV ofthe SP3–NF MEA also decreased. In addition, the I–V perfor-mance of the SP3–NF MEA at 50% RH and 100% RH is given inFig. S4 and S5 (ESI),† respectively. Even aer 845 h stability testsat low RH, the SP3–NF MEA displayed a better I–V performanceat 50% RH, and its maximum power density reached 651 mWcm�2. When the RHwas promoted to 100%RH, the polarizationperformance of the SP3–NF MEA showed a decrease to someextent. Nevertheless, the SP3–NF membrane still exhibitsa satisfactory result.

Conclusion

We fabricate a novel porous sPEEK-based multi-layer compositemembrane with high performance and long-term durability,which consists of a porous sPEEK base membrane, two TLs andtwo PLs. The porous sPEEK membrane with nanoscale pores isrealized rst by using a VIPI method. Owing to the higherporosity and the denser distribution of sulfonic acid clusters,porous sPEEK membranes have an advantage over sPEEKmembranes prepared by solvent casting in terms of cellperformance and physical properties. SEM tests indicate thatthis multi-layer composite membrane has an obvious multi-layer structure which can enhance water uptake, restrictswelling and promote the proton conductivity eventually. Thethin PLs and TLs indicate a lower cost for this multi-layercomposite membrane due to the lower content of Naon.Additionally, single cell tests for the SP3–NF MEA show highercell performance than the SP3 MEA. The outstanding durabilityof the SP3–NFmulti-layer composite membrane is conrmed bylong-term stability tests under accelerated PEMFC operations.The growth rate of H2 crossover current density for the SP3–NFMEA (12.5 mA cm�2 h�1) was much lower than that of the SP3MEA. This demonstrates that this multi-layer compositestrategy has a remarkable effectiveness in improving thechemical stability of sPEEK-base membranes. Consequently, weconclude that high performance and high durability PEMs canbe achieved by the multi-layer composite strategy. Thisadvanced sPEEK-based multi-layer composite membraneexhibits huge potential as an alternative PEM for PEMFCapplication.

Acknowledgements

This work was nancially supported by the National Key Tech-nology Support Program (No. 2015BAG06B00) and the MajorProgram of the National Natural Science Foundation of China(No. 61433013).




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