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Reactive and Functional Polymers

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Preparation and properties of amorphous TiO2 modified anion exchangemembrane by impregnation-hydrolysis methodFeng Xiea,b, Xueqiang Gaoa,b, Jinkai Haoa, Hongmei Yua, Zhigang Shaoa,⁎, Baolian Yiaa Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, ChinabUniversity of Chinese Academy of Sciences, Beijing 100039, China

A R T I C L E I N F O

Keywords:TiO2 additiveAnion exchange membraneIonic conductivityMicrophase separationRelatvie humidity

A B S T R A C T

Amorphous TiO2 was introduced into the anion exchange membrane (AEM) derived from vinylbenzyl chloride-divinylbenzene copolymers. The structures and morphologies were characterized by FT-IR, XRD and SEM. Theproperties such as ion exchange capacity, hydroxide conductivity, swelling ratio and water uptake were cal-culated, and the fuel cell performance was tested and compared. The results showed that the hydrophilic/hydrophobic phase separation of the AEM was improved, the water uptake of the AEM also increased from22.5% to 40.6% and the hydroxide conductivity increased from 35 mS/cm to 43 mS/cm at 30 °C in deionizedwater, and the fuel cell performance was greatly enhanced under unsaturated humidification conditions.

1. Introduction

Fuel cell technology is considered to be one of the most promisingclean energy technologies due to its high efficiency and low emission,among which the proton exchange membrane fuel cell (PEMFC) is themost developed one [1]. However, the dependency on precious metalsuch as Pt as the catalysts makes PEMFC very costly, which limits thelarge-scale commercial applications [2]. Alkaline anion exchangemembrane fuel cell (AEMFC) has the potential to use noble metal freecatalysts, and also saves the cost of bipolar plates due to its weak cor-rosivity environment, thus it is expected to be the promising low-costfuel cells of the next generation [3].

Anion exchange membrane (AEM) is one of the key components ofAEMFC. A qualified AEM should be of high hydroxide conductivity andmechanical strength, as well as low gas permeability. In recent years,the AEMs have gained remarkable progresses [4–9]. The hydroxideconductivities of the AEMs have exceeded 100 mS/cm in many publicreports [10–15], and the performance of the AEMFCs has also beengreatly improved, which is comparable to that of the PEMFCs [8,9,16].However, the high power densities of most AEMFCs are achieved insingle fuel cell under gas flow rate far exceeding the stoichiometricratio with saturated humidification, which would be an obstacle inAEMFC stacks [17]. Adequate water supply at the cathode is a key pointfor the high performance [18,19], because water is one of the reactantsat the cathode, and OH– migrates from cathode to the anode in the formof hydrated ions. Insufficient water at the cathode will lead to mass

transfer polarization and also greatly reduce the hydroxide conductivityboth in the membrane and the catalyst layers [20–23].

Since the OH– is transported in the form of hydrated ions, the hy-droxide conductivity is closely related to the water content in AEMs[24]. The same phenomenon exists in proton exchange membranes(PEMs). The relationship between proton conductivity and water con-tent in Nafion membrane has already been well studied. It is reportedthat the proton conductivity of Nafion membrane is closely related tothe hydration number of sulfonate ions [25,26]. Also some inorganichydrophilic additives such as SiO2 and TiO2 are added into the Nafionmembrane to achieve the goal of self-humidification [27,28]. As for theAEMs, Derbali et, al. uses SiO2 and TiO2 to improve the mechanicalstrength and stability [29], Jiang et, al. also reports that adding SiO2

into the organic phase of PSF improves the ionic conductivity [30].However, little research focuses on employing inorganic oxide additivesto improve the microphase separation and water retaining property ofAEMs [31]. Improving the water concentration of the AEM is of greatsignificance. Both the oxygen reduction reaction (ORR) and the electro-osmotic effect caused by the transport of OH– from cathode to anodeconsume water at the cathode, resulting in a relative low humidity stateand doing harm to the ORR and ionic conductivity [17,23].

In this work, TiO2 was introduced into the pore-filled AEM by im-pregnation-hydrolysis method. The microstructure, physical and che-mical properties of the obtained membranes were tested and compared,as well as the fuel cell performance under different relative humidityconditions.

https://doi.org/10.1016/j.reactfunctpolym.2019.104348Received 16 June 2019; Received in revised form 16 August 2019; Accepted 25 August 2019

⁎ Corresponding author.E-mail address: zhgshao@dicp.ac.cn (Z. Shao).

Reactive and Functional Polymers 144 (2019) 104348

Available online 26 August 20191381-5148/ © 2019 Published by Elsevier B.V.

T

2. Experimental

2.1. Materials

The porefilled AEM was prepared according to our previous report[4]. A liquid mixture of 4-vinylbenzyl chloride (VBC, Aldrich), 2, 2, 3,4, 4, 4-hexafluorobutyl methacrylate (HFM, Aldrich), divinylbenzene(DVB, Aldrich) and benzoylperoxide (BPO) was stirred to be homo-genous. Porous polyethylene (PE) substrate was immersed in the solu-tion and then sandwiched between two pieces of glass. Polymerizationwas carried out at 100 °C for 12 h. Then the product was ammoniated in30 wt% trimethylamine solution for 48 h, and alkalized in 1 M KOHsolution for 48 h. The finally obtained porefilled membrane (marked asAEH membrane) was washed and dried in an oven at 60 °C.

The TiO2 modified AEM (marked as TiO2-AEH membrane) wasprepared by impregnation-hydrolysis method. AEH membrane wassoaked in tetrabutyl titanate/n-propanol (volume ratio is 1/100) solu-tion for 12 h. Then the membrane was taken out and hydrolyzed indeionized water (DI water) for 24 h. After that, the hydrolyzed mem-brane was alkalized with 1 M KOH for 24 h. The finally obtainedmembrane was washed with DI water and dried in an oven at 60 °C.

2.2. Physico-chemical characterization

2.2.1. FT-IR, XRD, SEM and EDS characterizationFT-IR spectroscopy of the synthesized AEH and TiO2-AEH mem-

brane was obtained on a JASCO FT-IR 4100 spectrometer with an ATRaccessory containing a Ge crystal with a wavenumber resolution of4 cm−1 and range of 400–4000 cm−1. The structures of the AEMs werecharacterized by X-ray diffraction (XRD, GeminiUltra, accelerationvoltage 45 kV, wavelength 0.154 nm) with a range of 10–90°. Themorphologies of the surface of the AEMs were observed by a scanningelectron microscope (SEM, JEOL IT-300 LA) with an acceleration vol-tage of 10 kV, and with the same device the EDS spectra were obtained.

2.2.2. Ion exchange capacity, swelling ratio and water uptakeThe methods in the literatures to evaluate the physical properties of

the membrane were adopted [32]. The AEMs with OH– as the anion wassoaked in 0.01 M HCl solution for 48 h at 30 °C. Subsequently, HCl wastitrated with 0.01 M aqueous solution of KOH with phenolphthalein asthe indicator. The IEC was calculated as follows:

=IECn n

mKOH KOH1, 2,

(1)

where n1,KOH and n2,KOH were moles of KOH consumed to titrate theequivolumetric HCl solutions before and after soaking the AEMs inthem, and m was the weight of the dry membrane.

In order to calculate the water uptake, the membranes were im-mersed in DI water at 30 °C for 24 h, then taken out and wiped with atissue paper, and quickly weighed on a microbalance. The weights ofthe dry membranes were obtained after drying the membranes at 60 °Cunder vacuum for 24 h. The water uptake was calculated as follows:

= ×WUW W

W% 100%wet dry

ryd (2)

Where Wwet and Wdry were respectively the weight of hydrated anddry membranes.

The porefilled membranes had very low swelling ratios at lengthand width dimensions. The swelling ratio in thickness direction wascalculated as follows:

= ×L L

LSR% 100%wet dry

dry (3)

Where Lwet and Ldry were the thicknesses of membranes under hy-dration and dry conditions, respectively. The hydrating and dryingprocedures were the same as above.

2.2.3. Ionic conductivity measurementThe ionic conductivity was determined in a cell with a pair of Ti

electrodes coated with Pt. The resistance of the membrane was mea-sured through electrochemical impedance spectroscopy (EIS, solartron1260A). Signal amplitude of 100 mV in the frequency range of 1 M Hzto 1 Hz was applied. The test cell was placed in DI water to maintain therelative humidity. Besides, to eliminate the effect of the carbonation asfar as possible [33–35], the test system was placed in a glove box full ofnitrogen. The ionic conductivity (σ, mS/cm) was calculated as follows:

= LRS (4)

Where L (cm) was the distance between the working electrode andreference electrode, S (cm2) was the cross sectional area of the mem-brane and R (kΩ) was the membrane resistance from the EIS data.

2.2.4. MEA preparation and fuel cell testsMEAs are prepared according to the literature [16]. 0.5 g 70 wt%

Pt/C (JM) was mixed with 0.1 mL water and 4 mL ethanol and home-made ionomer of functionalized SEBS copolymer [32]. The weight ratioof the catalyst and the ionomer was 4:1. Then the mixture was soni-cated to obtain a homogenous ink. The catalyst ink was brushed onto agas diffusion layer (GDL, Toray-60) to form a gas diffusion electrode(GDE). The Pt loading was 0.4 mg/cm2. MEA was comprised of amembrane sandwiched between two GDEs. The active area of the MEAwas 5 cm2. Fuel cell test was carried out employing H2 and O2 at0.2 MPa and 60 °C with 100% and 80% relative humidity (RH), re-spectively. The flow rates of H2/O2 were 200/300 SCCM (standardcubic centimeters per minute).

3. Results and discussion

3.1. FT-IR spectra of AEH and TiO2-AEH membranes

The FT-IR spectra of AEH and TiO2-AEH were shown in Fig. 1. Thepeaks at 2916 cm−1 and 2848 cm−1 was ascribed to the stretching vi-bration of eCH3 and eCH2. The peak at 1623 cm−1 was ascribed toC]C stretching vibration in the aromatic rings, and the peak at1380 cm−1 is a typical CeN vibration peak. The broad band around3400 cm−1 was related to the quaternary ammonium stretching vi-bration. The FT-IR spectra of AEH and TiO2-AEH membranes nearlycoincided, indicating the same functional groups in the two mem-branes, and TiO2 particles did not change the chemical properties of thefunctional groups.

Fig. 1. FT-IR spectra of AEH and TiO2-AEH.

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3.2. XRD test and analysis

The XRD spectra of TiO2, AEH and TiO2-AEH were shown in Fig. 2.The TiO2 obtained by hydrolysis of tetrabutyl titanate was amorphous,and there were no characteristic peaks. The AEH and TiO2-AEH mem-branes both had two peaks at 22.4° and 24.7°, and the peak areas of thelatter were larger than those of the former. The larger peak area of theTiO2-AEH membrane indicated more crystalline phases in the mem-brane. The hydrophilic phases of the AEMs, which contain quaternaryamine functional groups, were usually amorphous. The hydrophobicphases, containing the CeC main chains and fluorine side chains, whichwere functioned to afford the strength of the membranes, were tend tobe crystallize. In the AEH membrane, some hydrophilic quaternaryamine groups might be encapsulated by hydrophobic chains, whichdestroyed the orientation of hydrophobic phase and lowered the crys-talline phase content. When AEH was impregnated in tetrabutyl tita-nate/n-propanol solution, the butyl group entered the hydrophobicregion of the membrane, which swelled and reoriented. The hydrophilictitanate group aggregated the isolated hydrophilic quaternary aminegroups to form a larger hydrophilic phase. The rearrangement of hy-drophilic phase and hydrophobic phase in the membrane promoted themicrophase separation. After hydrolysis of tetrabutyl titanate, TiO2

remained in the hydrophilic phase, while hydrophobic phase reorientedto be crystalline, thus increasing the content of the crystalline phase.Therefore, the corresponding peak areas of crystalline phase in the

TiO2-AEH membrane increased substantially.

3.3. Microstructure and element distribution of membranes

The SEM images of AEH and TiO2-AEH were shown in Fig. 3. Therewere many micropores on the surface of AEH membrane, which mightcome from the skeleton of the substrate PE and also the pinholes of theAEH membrane itself. After introducing the TiO2 additive into the AEHmembrane, the micropores on the surface disappeared, and the skeletonwas fully covered by TiO2, indicating that the TiO2 was successfullycomposited with the AEH membrane. The EDS spectra of TiO2-AEHmembrane were given in Fig. 4. The content of TiO2 in the TiO2-AEHmembrane was about 2 wt%, and the N and Ti elements were evenlydistributed in the membrane, which was also evidence that TiO2 wassuccessfully added to the membrane.

The TiO2 additive in the micropores of the membrane would lowerthe gas crossover, thus improving the performance of the membrane. Inaddition, some hydrophobic micropores on the surface were modifiedto be hydrophilic, which would promote the water concentration in themembrane.

3.4. Ion exchange capacity, water uptake and swelling behavior

The IEC, water uptake and swelling ratio of AEH and TiO2-AEHmembrane were listed in Table 1. The IEC of AEH and TiO2-AEH were1.36 mmol/g and 1.33 mmol/g, respectively. The lower IEC of TiO2-AEH might be resulted from the additional TiO2 in the AEH membrane.The thicknesses of AEH and TiO2-AEH were both 25 μm, and theswelling ratio both were 36% in the thickness direction. However, thewater uptake of TiO2-AEH was almost twice as much as that of the AEHmembrane. In the TiO2-AEH membrane, except for the water adsorbedby TiO2 itself, some hydrophobic micropores were changed to be hy-drophilic, and the reorientation of the microstructure might be bene-ficial for the water uptake in the micropores. The hydroxide con-ductivity of the TiO2-AEH membrane at 30 °C also increased by 23%from 35 mS/cm to 43 mS/cm.

3.5. Ionic conductivity at different temperatures and relative humidity

The hydroxide conductivities of AEH and TiO2-AEH at differenttemperatures were tested in DI water and the results were shown inFig. 5. The hydroxide conductivities of TiO2-AEH membrane at dif-ferent temperatures were higher than those of AEH membrane, and theslope of conductivity to temperature was also bigger, indicating that theactivation energy of ion transportation in the membrane was lower. The

Fig. 2. XRD spectra of TiO2, AEH and TiO2-AEH.

Fig. 3. SEM of (a) surface of AEH and (b) surface of TiO2-AEH.

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reduction of activation energy might be resulted from the change of themicrostructures of the AEM. First of all, the results of XRD and SEMcharacterization showed that the introduction of TiO2 promoted thehydrophilic/hydrophobic microphase separation. Secondly, the hydro-philic modification of the micropores in the membrane would improvethe water concentration. At last, TiO2 might improve the concentrationof quaternary amine ions by occupying a part of the space in the hy-drophilic region, which was beneficial for the transport of the hydro-xide ions [36].

The increasing of hydroxide conductivity of the TiO2-AEH mem-brane at saturated humidity was not the same as that of inorganicoxides modified Nafion membrane. Introducing hydrophilic inorganicoxides, such as SiO2 and TiO2 into Nafion membrane often reduced theionic conductivity in saturated humidity conditions, since they were notionic conductive [37]. As for the TiO2-AEH membrane, the TiO2 ad-ditive promoted the microphase separation to improve hydroxidetransport channels and promoted the water concentration of themembrane, thereby promoting the hydroxide conductivity.

The hydroxide conductivities of AEH and TiO2-AEH membraneswith different relative humidity at 60 °C were also tested. The relativehumidity was controlled by setting the dew point temperature at 50 °C,40 °C and 30 °C, and the relative humidity is 61.9%, 37% and 21.3%,respectively. The results were shown in Fig. 6. With the decrease ofrelative humidity, the hydroxide conductivities of both membranesdecreased. The ionic conductivity of AEH membrane was lower thanthat of TiO2-AEH at the same relative humidity, and the hydroxideconductivity of AEH membrane decreased faster when the relativehumidity decreased.

The TiO2 itself was hydrophilic to absorb some water at low hu-midity. In addition, TiO2 in the hydrophobic micropores made them tobe hydrophilic. Owing to the capillary force, the gaseous water waspreferred to be liquid water at given relative humidity, which wasbeneficial for retaining water in the membrane. Therefore, the hydro-xide conductivity of TiO2-AEH was higher than that of AEH at low re-lative humidity, and decreased less with decreasing relative humidity.

3.6. Fuel cell performance

The fuel cell performance with AEH and TiO2-AEH as the AEMs atdifferent relative humidity was shown in Fig. 7. The open circuit vol-tages (OCV) of all cells were above 1.0 V, indicating that the gascrossover of the AEMs was limited. The OCV of TiO2-AEH cell wasslightly higher than that of AEH cell, which was consistent with thefindings of SEM characterization that the micropores were fulfilled bythe TiO2. The peak power density of TiO2-AEH single fuel cell at 100%

Fig. 4. EDS of surface of TiO2-AEH (a) element N and (b) element Ti.

Table 1Properties of AEH and TiO2-AEH membrane.

Samples IEC(mmol/g)

WU (%) SR (%) σ(30 °C, mS/cm)

Thickness(μm)

AEH 1.36 22.5 36 35 25TiO2-AEH 1.33 40.6 36 43 25

Fig. 5. Temperature dependence of hydroxide conductivities of AEH and TiO2-AEH.

Fig. 6. Relative humidity dependence of hydroxide conductivities of AEH andTiO2-AEH at 60 °C.

F. Xie, et al. Reactive and Functional Polymers 144 (2019) 104348

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relative humidity was slightly better than that of AEH single cell, whichcould be attributed to the higher ionic conductivity of TiO2-AEHmembrane. When the relative humidity decreased to 80%, the perfor-mances of both fuel cells decreased significantly. However, the peakpower density of TiO2-AEH fuel cell was much higher than that of theAEH fuel cell, indicating that the TiO2 in the membrane played a greatrole in maintaining the performance at low relative humidity.

4. Conclusions

In summary, amorphous TiO2 was successfully introduced into thepore-filled anion exchange membrane. The hydrophilic/hydrophobicphase separation were promoted and the water concentration was en-hanced, as well as the hydroxide conductivity of the AEM at differentrelative humidity. As a result, the fuel cell performance in unsaturatedhumidification was significantly improved. This work enriched themethod to improve the performance of AEMs and also the performanceof AEMFCs especially under unsaturated humidity conditions.

Data availability

The raw data required to reproduce these findings are available todownload from [INSERT PERMANENT WEB LINK(s)]. The processeddata required to reproduce these findings are available to downloadfrom [INSERT PERMANENT WEB LINK(s)].

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

This work was financially supported by the National key R&DProgram of China under Grant No.2016YFB0101205, the CAS-DOEcooperation Project (Program No. 121421KYSB20160009) and High-Level Talents Innovation Support Program of Dalian (Program No.2017RQ071).

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