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Highly Hydroxide-Conductive Nanostructured Solid Electrolyte via Pre-Designed Ionic Nanoaggregates Guangwei He, Mingzhao Xu, Zongyu Li, Shaofei Wang, Shentao Jiang, Xueyi He, Jing Zhao, Zhen Li, Xingyu Wu, Tong Huang, Chaoyi Chang, Xinlin Yang, Hong Wu, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05400 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces


Highly Hydroxide-Conductive Nanostructured Solid Electrolyte via Pre-Designed Ionic Nanoaggregates Guangwei He

a,b✝

, Mingzhao Xu a,b✝, Zongyu Li

a,b

, Shaofei Wang a,b, Shentao Jiang d,

Xueyi He a,b, Jing Zhao a,b, Zhen Li a,b, Xingyu Wu a,b, Tong Huang a,b, Chaoyi Chang a, Xinlin Yang b,c, Hong Wu a,b, Zhongyi Jiang* a,b a

Key Laboratory for Green Chemical Technology of Ministry of Education, School

of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin, 300072, China c

Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of

Polymer Chemistry, Nankai University, Tianjin, 300071, China d

School of Civil & Environmental Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, USA ✝

These two authors contributed equally

Corresponding author: Zhongyi Jiang, E−mail: [email protected]

ABSTRACT: The creation of interconnected ionic nanoaggregates within solid electrolytes is a crucial yet challenging task for fabricating high-performance alkaline fuel cells. Herein, we present a facile and generic approach to embedding ionic nanoaggregates via pre-designed hybrid core-shell nanoarchitecture within nonionic polymer membranes: i) synthesizing core-shell nanoparticles composed of SiO2/densely quaternary ammonium functionalized polystyrene. Due to the spatial confinement effect of the SiO2 “core”, the abundant hydroxide-conducting groups are locally aggregated in the functionalized polystyrene “shell”, forming ionic nanoaggregates bearing intrinsic continuous ion channels; ii) Embedding these ionic nanoaggregates (20-70 wt%) into polysulfone matrix to construct interconnected 1

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ACS Applied Materials & Interfaces hydroxide-conducting channels. The chemical composition, physical morphology, amount and distribution of the ionic nanoaggregates are facilely regulated, leading to highly connected ion channels with high effective ion mobility comparable to that of the state-of-the-art Nafion. The resulting membranes display strikingly high hydroxide conductivity (188.1 mS cm-1 at 80 oC), which is one of the highest results to date. The membranes also exhibit good mechanical properties. The independent manipulation of the conduction function and the non-conduction function by the ionic nanoaggregates and the nonionic polymer matrix, opens a new avenue free of microphase separation to designing high-performance solid electrolytes for diverse application realms. KEYWORDS: solid electrolytes, alkaline fuel cells, ionic nanoaggregates, hydroxide conductivity, membrane

2


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ACS Applied Materials & Interfaces Introduction Anion exchange membranes (AEMs) have been triggering escalating interest owing to their grand application potential in energy- and environment-associated fields such as fuel cells, flow batteries, electrodialysis.1-3 AEM fuel cell is regarded as a promising alternative to proton exchange membrane fuel cell due to its faster kinetics for fuel oxidation and oxygen reduction reactions under alkaline conditions, which allows for the utilization of non-noble metal catalysts.4 However, the inadequate hydroxide conductivity sets an impediment to the commercialization of AEM fuel cell , which primarily arises from the low intrinsic mobility of hydroxide ions.5-6 To surmount this challenge, creating highly conductive ionic nanoaggregates within membrane has been recognized as the most feasible strategy.7-8 A considerable number of hydrophobic segment and ionized segment combinations have been explored to generate the ionic nanoaggregates through self-assembly of the polymer architectures via microphase separation.9-10 The ionic nanoaggregates are dispersed within membrane matrix and become 3D interconnected after the membrane crossing an ion exchange capacity (IEC) threshold, resulting in drastically improved physical microenvironment (continuous pathways with low energy barrier for ion conduction) and chemical microenvironment (high local ion concentration) for fast hydroxide conduction.11-12 Representative polymer architectures include block copolymer, graft copolymer and densely functionalized copolymer.13-17 For instance, Xu and coworkers reported that poly(quaternary 4-vinylbenzyl chloride) grafted copolymer membrane exhibited microphase separated structures with interconnected ionic nanoaggregates owing to the favorable enthalpy change associated with the demixing of incompatible segments and thus acquired a high hydroxide conductivity (86 mS cm-1 at 60 oC).18 Despite these great achievements, it remains a critical challenge to implement precise control over the microphase-separation process to construct interconnected nanoaggregates

with

long-range

continuity

because

the

complex

microphase-separation process is highly sensitive to the strong coupling effect from 3



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ACS Applied Materials & Interfaces the configuration, polarity, and rigidity of hydrophilic/hydrophobic segments.9 To solve this dilemma, we propose an approach to manipulating the ionic nanoaggregates via embedding pre-designed core-shell nanoarchitecture within nonionic polymer matrix. The “core” and “shell” are composed of SiO2 nanoparticle and densely functionalized polymer brushes, respectively. The “shell” is synthesized through atom transfer radical polymerization (ATRP) and post-functionalization. Due to the spatial confinement effect of the “core”, the abundant hydroxide-conducting groups are locally aggregated in the “shell”, which is able to form ionic nanoaggregates containing intrinsic ion channels with continuity at submicron length scale.19-20 Incorporation of a sufficiently large amount of these ionic nanoaggregates into polymer matrix would readily construct interconnected ionic nanoaggregates throughout the membrane.21 Compared with the ionic nanoaggregates formed by the broadly harnessed microphase separation, the ionic nanoaggregates created by our approach can be pre-designed using more well-defined nanotechnologies, thus rendering fast and tunable hydroxide conduction channels. Furthermore, since the membrane is composed of core-shell nanoarchitecture and nonionic polymer matrix, the conduction function and the non-conduction function (mechanical strength, thermal stability, gas barrier property, etc.) of membrane can be individually regulated. Such weakly-coupled function-oriented design may offer more flexible controllability over membrane structure-property relationship. Specifically, we exploit the embedment of ionic nanoaggregates via incorporating pre-designed hybrid core-shell nanoarchitecture into nonionic polymer for the first time. Densely quaternary ammonium functionalized polystyrene grafted SiO2 core-shell nanoparticles were dispersed within polysulfone to fabricate mixed matrix membranes, as illustrated in the Figure 1. Due to the construction of interconnected ionic nanoaggregates by the core-shell nanoparticles, the membranes exhibit strikingly high hydroxide conductivity (188.1 mS cm-1 at 80 oC), which is one of the highest results to date. The membrane also exhibits sufficient mechanical properties. 4


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ACS Applied Materials & Interfaces The independent manipulation of conduction and non-conduction properties, which are separately imparted by core-shell nanoparticles and polymer matrix, opens a new avenue to designing high-performance solid electrolytes for a broad range of applications. Experimental Materials.

Divinylbenzene

(DVB,

80%

isomers)

and

3-Methacryloxypropyltrimethoxysilane (MPS) were purchased from Alfa Aesar and used

without

further

purification.

chloromethyl

styrene

(CMSt)

and

N,N,N',N'',N''-pentamethyldiethylenetriamine (PMEDTA) were purchased from J & K, China. 2, 2-Azoisobutyronitrile (AIBN), tetraethyl orthosilicate (TEOS) and tetrachloroethane was provided by Aladdin, China. Acetonitrile was supplied by Tianjin Kewei Ltd. Methacrylic acid (MAA) was provided by Aladdin (China) and distilled under vacuum. Aqueous solution of ammonium hydroxide (30%), Dimethylformamide (DMF), and sodium hydroxide and trimethylamine solution (30%) were supplied from Aladdin, China. CuCl was obtained from Aladdin and was purified by stirring it in glacial acetic acid, washed with ethanol, and then dried in a vacuum oven. De-ionized water was used throughout the experiment. Preparation of Membranes Synthesis of SiO2-MPS nanoparticles by Stöber method. 200 mL of ethanol, 20 mL of water and 4 mL of aqueous solution of ammonium hydroxide were mixed under vigorous stirring. Then, 6 mL of TEOS was slowly added into the above mixture, followed by vigorous stirring at 30 oC for 24 h. Subsequently, 1 mL of MPS was added into the mixture under stirring with a reaction time of 24 h to modify the silica nanoparticles with carbon-carbon double bonds. SiO2-MPS nanoparticles were obtained by centrifuging of the above mixture, and washing the precipitated product by repeatedly dispersion and centrifuge using water and ethanol as solvent. Synthesis of SiO2-CM nanoparticles. The SiO2-MPS was modified with chloromethyl groups by surface-initiated precipitation polymerization.22 0.5 g of 5



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ACS Applied Materials & Interfaces SiO2-MPS nanoparticles were dispersed in 200 mL of acetonitrile after ultrasonic treatment for 1 h. Then, 0.8 mL of MAA, 0.5 mL of CMSt, 0.3 mL of DVB, and 0.032 g of AIBN was dissolved into the mixture. Subsequently, the above solution was heated to boiling state in 10 min, followed by refluxing for 70 min. The resulting SiO2 modified with chloromethylated groups (SiO2-CM) were purified by several centrifugation/washing cycles in water or ethanol. Synthesis of SiO2-PSt core-shell nanoparticles. Polystyrene brushes were in situ grown on SiO2-CM nanoparticles via ATRP.23 1.0 g of SiO2-CM nanoparticles, 0.2 mg of PMEDTA, 5.85 mL of styrene, and 5.85 mL of toluene were added into a Schlenk flask, followed by ultrasonic treatment of the mixture for 30 min. The flask was degassed for 30 min with N2, and then 58.5 mg of CuCl was added as catalyst. Subsequently, the flask was degassed for 30 min, and then transferred to an oil bath at 110 oC to perform the polymerization for 1 h. The resulting SiO2-g-polystyrene (SiO2-PSt) core-shell nanoparticles were repeatedly washed using THF and ethanol as solvent, and then dried at 60 oC for 48 h. Chloromethylation of SiO2-PSt core-shell nanoparticles. 1 g of SiO2-PSt nanoparticles were dispersed in 30 mL of tetrachloroethane under ultrasonic treatment. Then, 0.66 g of ZnCl2 and 30 mL of chloromethylmethylether was added dropwise into the reaction solution. (Caution: chloromethylmethylether is a carcinogenic compound, and the reaction must be performed in a fume hood.) The reaction solution was stirred at 60 oC for 48 h. The resulting SiO2-g-chloromethylated polystyrene (SiO2-CMPSt) nanoparticles were obtained by centrifuging, and washed several times with water and ethanol, followed by drying at a vacuum oven for 48 h. To determine the potential IEC of the SiO2-CMPSt nanoparticles after the reaction with trimethylamine, the SiO2-CMPSt nanoparticles were treated with the following procedures: (i) SiO2-CMPSt nanoparticles were immersed into trimethylamine solution (30%) at 30 oC for 48 h to transform the chloromethyl groups into quaternary ammonium groups; (ii) The quaternary ammonium functionalized SiO2-CMPSt 6


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ACS Applied Materials & Interfaces (SiO2-QPSt) nanoparticles were dried at 60 oC for 24 h in a vacuum oven; and (iii) the N content in the SiO2-QPSt nanoparticles was found to be 5.89 wt% using a combustion method (PerkinElmer 2400-II). The Elemental analysis: Found: C: 67.1; H: 8.09; N: 5.89. Membrane preparation. The mixed matrix membranes were fabricated by a solution casting method.24 A certain amount of SiO2-CMPSt nanoparticles were dispersed into 6 mL of DMF under ultrasonic treatment for 2 h, and then a certain amount of PSU was dissolved into the solution under stirring. The total amount of nanoparticles and PSU was 0.3 g. The mixed solution was treated using ultrasonic sound for 5 min, and then cast onto a glass plate and dried at 60 oC for 12 h. The obtained membrane was peeled off, and immersed into trimethylamine solution (30%) at 30 oC for 48 h to transform the chloromethyl groups into quaternary ammonium groups. The quaternary ammonium functionalized mixed matrix membrane was immersed in 1 M NaOH solution at room temperature for 24 h, and then washed with water until no Cl- can be detected by AgNO3. The obtained mixed matrix membranes were designated as PSU/SiO2-QPSt-X, where X (=20, 30, 40, 50, 60, 70) represents the weight percentage of SiO2-CMPSt nanoparticles with the total amount of SiO2-CMPSt and PSU. Characterizations High resolution TEM (Tecnai G2 20 S-TWIN) was utilized to obtain the morphology of SiO2-MPS, SiO2-CM and SiO2-CMPSt nanoparticles. Field emission SEM (Nanosem 430) was utilized to observe the cross-section morphology of the PSU/SiO2-QPSt membranes. FTIR spectra of the PSU/SiO2-QPSt membranes were obtained using a Bruker Vertex 70 FTIR spectrometer with a horizontal attenuated transmission accessory. TGA (NETZSCH-TG209 F3) was utilized to measure the thermal properties of the SiO2-MPS, SiO2-CM and SiO2-CMPSt nanoparticles, and PSU/SiO2-QPSt membranes at a heating rate of 10 oC min−1 under a N2 flow. For isothermal TGA, the sample was heated to 200 oC at a rate of 10 oC min−1 and 7



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ACS Applied Materials & Interfaces maintained at 200 oC for 10 h. Measurements Hydroxide conductivities of the PSU/SiO2-QPSt membranes were measured by alternating current (AC) impedance spectroscopy using an electrode system connected with a frequency response analyzer (FRA, Compactstat, IVIUM Tech.).25 Hydroxide conductivities under 100% RH was measured under a N2 flow in a temperature-controlled water-bath chamber. Hydroxide conductivity (σ, S cm−1) was calculated using the equation: σ = l / AR, where R is the resistance, A the cross-sectional area of the membrane sample, and l the length between the electrodes. The weight-based ion exchange capacity (IEC) of the membranes was tested by titration method.18 Ion concentration and effective ion mobility we’re calculated according to our previous literature.25 The alkaline stability of the PSU/SiO2-QPSt membranes was evaluated in 1 M N2-saturated NaOH solution at 60 oC. During the period of 0-480 h, the hydroxide conductivities were measured after complete removal of residual alkaline solution at set intervals. The oxidative stability of the membranes was measured by calculating the residual weight after soaking in Fenton’s reagent (3 wt% H2O2 + 2 ppm FeSO4) at 80 oC for 1 h. Rectangular membrane samples were dried at 60 oC under vacuum for 24 h. The weight (Wdry) and length (Ldry) of the samples were tested. Then, the samples were fully hydrated in water, followed by measuring the weight (Wwet) and length (Lwet). The water uptake and in-plane swelling were calculated using the equations: water uptake (%) = (Wwet–Wdry)/Wdry × 100, and ∆L (%) = (Lwet − Ldry)/Ldry × 100, respectively. The mechanical properties of the membrane samples in dry state were tested using an electronic tensile machine (WDW-2, Yangzhou Zhongke Measuring Apparatus Co., China) at a stretching rate of 5 mm min−1 at room temperature. 8


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Result and Discussion Characterization of the core-shell nanoparticles. The synthesis of the SiO2-CMPSt core-shell nanoparticles is illustrated in Figure 1. The morphology of the SiO2 modified with 3-Methacryloxypropyltrimethoxysilane (SiO2-MPS), SiO2 modified with chloromethylated groups (SiO2-CM) and SiO2-CMPSt nanoparticles was observed by transmission electron microscopy (TEM), as shown in Figure 2. The SiO2-MPS nanoparticles are monodisperse with diameter about 90 nm. The methacrylate groups on SiO2-MPS initiated the polymerization of methacrylic acid (MAA), chloromethyl styrene (CMSt), and divinylbenzene (DVB) on the surface.26 The resulting SiO2-CM nanoparticles contain abundant chloromethyl groups, which are able to initiate ATRP of styrene on the surface of SiO2-CM.27 Poly(CMSt-co-MAA-co-DVB) is selected as the donor of initiator for ATRP because the surface density of ATRP initiator is much higher, and the initiator is anchored in an polymer network with higher alkaline stability when compared with the commonly used bromine-based initiator with amide bonds.28 The surface of SiO2-CM becomes much rougher than that of SiO2-MPS nanoparticles (Figure 2b), arising from the coating of a thin polymer layer.29 After the graft of CMPSt on the nanoparticles through surface-initiated ATRP and chloromethylation, the SiO2-CMPSt nanoparticles (Figure 2c and Figure S1) show a core-shell structure with a diameter of around 180 nm and a “shell” thickness of around 45 nm, verifying the growth of long polymer brushes on SiO2 surface.

Figure 1. Schematic illustration of the fabrication of SiO2-CMPSt nanoparticles and PSU/SiO2-QPSt membranes. 9



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Figure 2. TEM images of (a) SiO2-MPS, (b) SiO2-CM and (c) SiO2-CMPSt nanoparticles. The chemical structures of the nanoparticles were characterized by Fourier transform infrared (FTIR) (see Figure S2). In the SiO2-MPS sample, the characteristic absorption bands were observed at 1110 cm-1 and 800 cm-1 (Si-O-Si stretching vibrations), 1630 cm-1 (C=C stretching vibrations. After surface-initiated precipitation polymerization of poly(CMSt-co-MAA-co-DVB), the resulting SiO2-CM shows stronger absorption bands at 1714 cm-1 (C=O stretching vibrations in polyMAA segments). New absorption bands at 1449 and 1508 cm-1 are ascribed to the stretching vibrations of phenyl groups in polyDVB segments. After grafting CMSt, the SiO2-CMPSt nanoparticles show stronger absorption bands at 1449 and 1508 cm-1 due to more phenyl groups in SiO2-CMPSt nanoparticles. The absorption bands of C=C stretching vibrations at 1630 cm-1 disappear because the thick CMPSt layer shield the signal of C=C from the inner SiO2-CM. The absorption band at 2925 cm-1 (C-H stretching vibrations in CH2-Cl) and 675 cm-1 (C-Cl stretching vibrations in CH2-Cl) become stronger in comparison with that in SiO2-CM because SiO2-CMPSt nanoparticles have much more chloromethyl groups.30 According to the elemental analysis of N (N:5.89 wt%; C: 67.1 wt%; H: 8.09 wt%), the IEC of the SiO2-CMPSt nanoparticles after quaternization reaction is 4.2 mmol g-1. The high IEC manifests that the quaternized SiO2-CMPSt nanoparticles are able to form ionic nanoaggregates, rendering densely distributed ionic functionalities for fast conduction of hydroxide ions. 10


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ACS Applied Materials & Interfaces Figure S2 shows the thermogravimetric analysis (TGA) curves of SiO2-MPS, SiO2-CM and SiO2-CMPSt nanoparticles. All samples have three stages of weight loss: (i) the evaporation of free water and bound water in the first region (40-150 oC), (ii) the degradation of functional groups in the second region (150-380 oC), and (iii) the decomposition of main chains of organic polymer in the third region (380-800 oC). With the increase of organic moieties in the nanoparticles, the residual mass decreases. In contrast, the SiO2-CMPSt sample displays a more remarkable weight loss in the region from 150 to 800 oC with a much lower residual mass of 41.2%, indicating the high content of organic polymer in the SiO2-CMPSt core-shell nanoparticles (in agreement with the TEM image and elemental analysis). Characterization of the membranes The PSU/SiO2-QPSt membranes show similar absorption bands, as shown in Figure S3. The absorption bands at 1589, 1505 cm-1 are arisen from the stretching vibrations of phenyl groups, while the bands at 1239 and 1172 cm-1 are arisen from the stretching vibrations of C−O and S=O in PSU, respectively. The absorption band at 1072 cm-1 is assigned to the stretching vibration of C−N, indicating the formation of quaternary ammonium groups after the reaction between chloromethyl groups with trimethylamine through an efficient Menshutkin reaction. The cross-sectional morphologies of the PSU/SiO2-QPSt-X membranes are observed by scanning electron microscope (SEM), as shown in in Figure 3, Figure S4, Figure S5. The PSU/SiO2-QPSt-X (20, 30, 40) membranes show two kinds of domains, that is, continuous, smooth domain and granular domain. According to the decreasing volume of the smooth domain with decreasing PSU content as well as the smooth morphology observed in the PSU membrane (Figure S4), it can be deduced that the smooth domain is the PSU polymer matrix. The diameter of the round particles observed in the granular domain is around 180 nm, in good agreement with the size of SiO2-QPSt shown in the TEM image (Figure 2). Thus, it can be concluded that the granular morphology in the PSU/SiO2-QPSt membranes is SiO2-QPSt 11



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ACS Applied Materials & Interfaces nanoparticle aggregation. Obvious aggregation of SiO2-QPSt nanoparticle fillers can be observed in all the mixed matrix membranes due to the high loading of fillers. With the increase of filler content within the membranes, the aggregation of fillers becomes more pronounced. After the filler loading reaching up to 50 wt%, large-area polymer phase disappears in the corresponding membranes, indicating that 50 wt% is a threshold of SiO2-QPSt nanoparticle interconnection; above this threshold, the nanoparticles become highly interconnected, which is expected to buildup highly continuous ion channels to afford high conductivity.

Figure 3. Cross-sectional SEM images of the PSU/SiO2-QPSt membranes: (a) PSU/SiO2-QPSt-20,

(b)

PSU/SiO2-QPSt-30,

(c)

PSU/SiO2-QPSt-40,

(d)

PSU/SiO2-QPSt-50, (e) PSU/SiO2-QPSt-60, and (f) PSU/SiO2-QPSt-70. Properties of the membranes Thermal properties. The thermal properties of the PSU/SiO2-QPSt membranes are determined by TGA, as shown in Figure S6a. All the membranes exhibit four stages of weight loss: (1) water evaporation in the range of 40-150 oC; (2) degradation of functional groups (quaternary ammonium) in the range of 150-300 oC; (3) degradation of aliphatic chains in the range of 300-470 oC; and (4) degradation of aromatic chains in the range of 470-800 oC. The PSU/SiO2-QPSt membranes with higher filler 12


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ACS Applied Materials & Interfaces contents show higher ratio of water loss due to the fact that the membrane with more hydrophilic groups absorbs more water. With the increase of filler contents in the membranes, the membrane tends to exhibit higher weight loss at the second stage and third stage, indicating more quaternary ammonium groups and aliphatic chains are incorporated, which is in accordance with filler contents. Figure S6b shows that the PSU/SiO2-QPSt-60 membrane can be stable at 200 oC. The result indicates that the membranes have good thermal properties for fuel cell applications. Oxidation stability. The oxidation stability of the membranes was measured by the accelerated oxidative stability test in Fenton’s reagent31, and the result is shown in Table 1. The residual weights of the membranes are all above 90 wt% after 1 h exposure to Fenton’s reagent, which can be comparable to many aromatic ion exchange membranes.32-33 The degradation weight may primarily come from the unstable polystyrene chains in the membranes. Table 1. The IEC, water uptake, swelling ratio, hydroxide conductivity and oxidative stability of PSU/SiO2-QPSt membranes IEC

Water uptake

Swelling ratio

Hydroxide conductivitya

Oxidative stabilityb

Membrane mmol g-1

30 oC

80 oC

30 oC

80 oC

mS cm-1

wt%

PSU/SiO2-QPSt-20

1.15

6.98

8.32

0.59

1.29

7.5

97.0

PSU/SiO2-QPSt-30

1.39

7.27

12.45

1.32

2.36

15.3

97.4

PSU/SiO2-QPSt-40

1.62

19.86

23.18

4.88

5.87

24.6

97.3

PSU/SiO2-QPSt-50

1.83

32.5

42.73

6.06

8.62

36.9

96.6

PSU/SiO2-QPSt-60

1.93

48.84

64.19

7.41

11.98

49.3

92.5

PSU/SiO2-QPSt-70

2.31

94.12

114.81

11.26

19.82

59.7

90.0

a

measured at 30 oC;

b

Residual weight percent of the membrane samples after

Fenton’s test.

Mechanical properties. The mechanical properties of the mixed matrix membranes (Table 2) were assessed by the stress-strain curves, as shown in Figure S6. 13



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ACS Applied Materials & Interfaces With the increase of filler content, the Young’s modulus of mixed matrix membranes decreases

from

1175.6

MPa

(PSU/SiO2-QPSt-20)

to

174.34

MPa

(PSU/SiO2-QPSt-70). The tensile strength of the mixed matrix membranes decreases from 42.28 to 14.18 MPa with the increase of filler content. Nevertheless, all the membranes except PSU/SiO2-QPSt-70 maintain good mechanical properties, which arise from two reasons: (i) the SiO2-QPSt nanoparticles have a small size (big surface area), which is advantageous to the formation of extensive interfacial interactions. Moreover, small-size particles have less chance to damage the continuous PSU cohesive phases9; (ii) The SiO2-QPSt has a polymer shell, which has a good compatibility with the PSU matrix. With further increase of filler content, the aggregation of SiO2-QPSt fillers becomes more pronounced (as shown in the SEM images in Figure 3), leading to the deterioration of interfacial morphologies and interactions, thus resulting in reduced Young’s modulus and Tensile strength. Moreover, since the mechanical properties are independently imparted by the PSU matrix, the mechanical properties decrease with the reduction of PSU volume. Table 2. The mechanical properties of PSU/SiO2-QPSt membranes. Membrane

Young’s modulus

Tensile strength

Elongation at break

（MPa）

（MPa）

（%）

PSU/SiO2-QPSt-20

1175.6

42.28

6.14

PSU/SiO2-QPSt-30

999.60

37.59

7.32

PSU/SiO2-QPSt-40

918.64

34.63

6.91

PSU/SiO2-QPSt-50

747.10

31.98

6.30

PSU/SiO2-QPSt-60

574.07

29.31

6.07

PSU/SiO2-QPSt-70

174.34

14.18

6.64

Water uptake and dimensional stability. The water uptake and dimensional swelling of PSU/SiO2-QPSt membranes are shown in Table 1. With the filler content increasing from 20 to 70 wt%, the water uptake of PSU/SiO2-QPSt membranes 14


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ACS Applied Materials & Interfaces increases from 6.98 to 94.12% at 30 oC. The PSU/SiO2-QPSt-70% membrane shows remarkable high water uptake because its low molecular interactions are unable to withstand the osmotic-pressure-driven swelling.9 The swelling ratio of the PSU/SiO2-QPSt membranes increases from 0.59 to 11.26% with increasing filler content, which resembles the variation trend of the water uptake.

Ion conductivity. The ionic conductivity is determined by the ion concentration and ion mobility within the membrane (ionic conductivity ∝ ion mobility × ion concentration).34 The ion concentration and effective ion mobility of the PSU/SiO2-QPSt membranes are calculated according to our previous study,25 and shown in Figure 4a. With the increase of IEC, the ion concentration of PSU/SiO2-QPSt membranes initially increase, and then decrease when IEC reaches 1.83 mmol g-1. The decrease of ion concentration is arisen from the fact that the excessive water uptake dilutes the ion density within the PSU/SiO2-QPSt-X (X=60 and 70) membranes. The effective ion mobility of PSU/SiO2-QPSt membrane continues to rise with the increase of IEC, indicating the optimization of channel morphology within the mixed matrix membranes. This is arisen from the fact that the ionic nanoaggregates tend to interconnect with each other with increasing quantity of the core-shell nanoparticles, leading to highly continuous ion channels. Effective mobility is determined by the intrinsic mobility of charge carriers and channels morphology (e.g. tortuosity of ion channels, connectivity of channels, spatial proximity of ionic groups, etc.). Proton and hydroxide ions have exceptionally high intrinsic mobility due to the unique conduction mechanism (structure diffusion) and relatively lower mass. For example, the mobility of proton and hydroxide ions are 34.98 × 10-4 and 19.83 × 10-4 cm2 s V-1, respectively, in infinitely diluted solution 25 o

C.25 The effective ion mobility in membranes is usually much lower than the intrinsic

mobility of hydroxide due to the inferior continuity and tortuosity of ion channels. The effective hydroxide mobility in PSU/SiO2-QPSt-70 is 5.35 × 10-4 cm2 s V-1, 15



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ACS Applied Materials & Interfaces which is even comparable with that of Nafion membrane (5.51 × 10-4 cm2 s V-1; the state-of-the-art ion exchange membrane).25 Considering that the intrinsic mobility of hydroxide is only 56.7% of that of proton and the ion concentration of PSU/SiO2-QPSt-70 is close to that of Nafion25, the PSU/SiO2-QPSt-70 membrane should contain better channel morphology when compared with Nafion. The hydroxide conductivity of the PSU/SiO2-QPSt membranes are shown in Table 1 and Figure 4b. With the filler content increasing from 20 to 70%, the hydroxide conductivity of the PSU/SiO2-QPSt membranes increases from 7.5 to 59.7 mS cm-1 at 30 oC, and increases from 23.5 to 188.1 mS cm-1 at 80 oC. Figure 4c shows that the increment of conductivity of PSU/SiO2-QPSt with filler content from 50 wt% to 70 wt% is much higher than that of PSU/SiO2-QPSt with filler content from 20 wt% to 40 wt%. The drastic increase of conductivity after the filler loading reaching up to 50 wt% indicates that there is a percolation threshold at around 50 wt% filler loading. Above this percolation threshold, the ionic nanoaggregates become highly interconnected, thus enabling much higher ion mobility, which is in agreement with the observed morphology in Figure 3, and the trend of hydroxide mobility in Figure 4a. The high hydroxide conductivity of the PSU/SiO2-QPSt membranes with high filler content (50-70 wt%) is attributed to the following reasons: (i) The quaternary ammonium groups are densely distributed in the “shell” of SiO2-QPSt nanoparticles (4.2 mmol g-1), which could enable the lower energy barrier for hydroxide transport due to the smaller spatial proximity between the conducting groups.35 Moreover, the ion concentration of the mixed matrix membranes is sufficiently high to maintain high hydroxide conductivity; and (ii) the mixed matrix membranes possess percolated ionic nanoaggregates, leading to highly continuous ion channels.36-37 When the filler loading is high up to 50 wt%, the ionic nanoaggregates (composed of the “shells” of SiO2-QPSt core-shell nanoparticles), which contain intrinsic ion channels with continuity at submicron length scale (~180 nm), readily become interconnected, thus forming ion channels with much higher continuity. The hydroxide conductivity of the 16


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ACS Applied Materials & Interfaces PSU/SiO2-QPSt membranes continue to increase with temperature because ion conduction is a thermal-driven process. Remarkably, the hydroxide conductivity of the PSU/SiO2-QPSt-60 and PSU/SiO2-QPSt-70 reaches up to 136.2 and 188.1 mS cm-1, respectively. This hydroxide conductivity is one of the highest values reported to date for hydroxide exchange membrane (Figure 4c).38-40

Figure 4. (a) The ion concentration and effective ion mobility of PSU/SiO2-QPSt membranes; (b) Hydroxide conductivity of the membranes as a function of temperature; (c) Comparison of hydroxide conductivities at 80 oC of PSU/SiO2-QPSt membranes with other typical membranes: block copolymers14,

41-42

, graft

copolymers6, 18, 43-44, random copolymers45-47, and alternating polymers18, 48-49; and (d) Hydroxide conductivity of the membrane after immersion in 1 M NaOH at 60 oC. Alkaline Stability. The long-term stability of AEMs in alkaline conditions is of great concern because the cation groups in AEMs readily undergo Hoffmann degradation or nucleophilic substitution.50-51 To evaluate the alkaline stability, the conductivities and IEC of PSU/SiO2-QPSt membranes over time were monitored in 1 M NaOH solution at 60 oC for 480 h, as shown in Figure 4d and Figure S7. The 17



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ACS Applied Materials & Interfaces decreasing trend of hydroxide conductivity is in agreement with that of the IEC because the conductivity decrease is caused by the degradation of quaternary ammonium groups.46 The hydroxide conductivity and IEC of PSU/SiO2-QPSt respectively keep about 75% and 77% of the initial value after 480 h measurement, demonstrating its excellent long-term alkaline stability that is comparable to the state-of-the-art AEMs.18,

34, 36

The hydroxide conductivities of all PSU/SiO2-QPSt

membranes show similar changing trend. In the initial stage (0-24 h), the hydroxide conductivities slightly increase; then, the conductivities remarkably decrease from 24 to 100 h, followed by slowly decreasing over time. This trend is also observed in many other AEMs.37 The mechanical properties (Figure S8 and Table S1) of the membrane samples were measured after the alkaline stability test. Compared with the mechanical properties shown in Table 2, all the membranes showed decreased tensile strength and Young’s modulus, but increased Elongation at break after soaking in 1 M NaOH. This behavior is due to that the molecular chains were partially broken in the alkaline condition, which weakens the coherent force of the PSU matrix as well as the PSU/SiO2-QPSt interfacial interactions.52 Comparing the membrane morphologies before and after alkaline stability test (Figure 3 and Figure S9), it is found that the membrane morphologies are not obviously damaged by the alkaline treatment. Since the PSU/SiO2-QPSt membranes are functionalized by quaternary ammonium groups, the mechanism of the conductivity degradation should be the nucleophilic attack towards ammonium group by OH−1.53 Compared with the typical AEMs with quaternary ammonium groups attached on the polyarylether-based backbone, the PSU/SiO2-QPSt membranes display much better alkaline stability.54 The excellent stability is attributed to the fact that the quaternary ammonium groups are grafted on the polystyrene chains of the core-shell SiO2-QPSt nanoparticles, which is more stable than the polyarylether-based backbone.18 This finding presents a novel approach to enhancing the alkaline stability of AEMs through rational architecture design. Conclusion 18


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ACS Applied Materials & Interfaces In this study, we reported a facile approach to creating interconnected ionic nanoaggregates within polyelectrolyte membrane through embedding pre-designed SiO2-QPSt core-shell nanoarchitecture within polysulfone matrix. Due to the spatial confinement effect of the SiO2 “core”, the dense hydroxide-conducting groups are locally aggregated in the QPSt “shell”, forming ionic nanoaggregates bearing high ion concentration and intrinsic continuous ion channels. Embedment of these ionic nanoaggregates into membrane readily constructs continuous ion channels with high effective ion mobility comparable to that of the state-of-the-art Nafion membrane. As a result, the PSU/SiO2-QPSt membranes with high filler loading (50-70 wt%) exhibit strikingly high hydroxide conductivities (88.4-188.1 mS cm-1) at 80 oC, which are among the highest results to date. Moreover, PSU/SiO2-QPSt membranes are imparted sufficiently high mechanical properties. The independent manipulation of the conduction function and the non-conduction function of membranes by the core-shell nanoarchitecture and the polymer matrix, opens a new avenue instead of microphase separation to designing high-performance solid electrolytes for diverse energy-oriented emerging applications.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. The FTIR spectrum and TGA curves of the nanoparticles, FTIR spectrum, TGA curves and stress-strain curves of the membranes can be found in the SI.

Acknowledgements The authors thank the financial support from the National Natural Science Foundation of China (No. 21490583, 21621004), the National Science Fund for 19



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ACS Applied Materials & Interfaces Distinguished Young Scholars (21125627), and the Program of Introducing Talents of Discipline to Universities (B06006).

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