Recent Developments in Ion-Exchange Nanocomposite Membranes for Energy Applications

DOI: https://doi.org/10.14579/MEMBRANE_JOURNAL.2016.26.6.432

1. Introduction ¹⁾

In the last decade, highly efficient energy storage and energy conversion systems have become attractive for applications in the smart grid system and other eco-friendly energy harvesting systems such as solar power, wind power, and salinity gradient power. The U.S Department of Energy (DOE) has lead several re-

search projects to develop next generation grid scale energy storage systems, including various types of flow batteries, which were funded with $1.3 billion by more than 30 focused ARPA-E program from 2007[1], as well as energy conversion systems (e.g. hydrogen pro- duction, fuel cell)[2].

Among next-generation energy conversion and har- vesting systems, membrane-based energy harvesting

†

Corresponding author(e-mail: [email protected], http://orcid.org/0000-0002-5797-2703)

에너지용 이온 교환 복합막 최근 연구 개발 동향

황 두 성⋅티파니 청⋅통슈아이 왕⋅김 상 일^†

시카고 일리노이 주립대학교 화학공학과

(2016년 12월 21일 접수, 2016년 12월 27일 수정, 2016년 12월 27일 채택)

Recent Developments in Ion-Exchange Nanocomposite Membranes for Energy Applications

Doo Sung Hwang, Tiffany Chung, Tongshuai Wang, and Sangil Kim

Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States (Received December 21, 2016, Revised December 27, 2016, Accepted December 27, 2016)

요 약: 최근 이온 교환 고분자 전해질 막을 활용한 고효율 에너지 전환 및 저장 장치에 대한 연구가 활발히 이루어지고 있다. 고분자 전해질 연료전지, 레독스 흐름전지 및 역전기투석 등 다양한 에너지 시스템에서 에너지 효율 향상을 위해 이온 교환 전해질 막의 양/음이온의 선택적 수송 거동이 중요시되고 있다. 본 총설은 각각의 고효율 전해질 전지 시스템에 따라 요 구되는 다양한 이온 교환막의 선택적 양/음이온 투과 거동의 한계점을 고찰하고, 한계를 극복하기 위한 다양한 구조의 고분 자 이온 교환 복합막의 장점 및 단점을 정리하였다. 고분자 가교법 및 다공성 지지체 복합막 이외에 다양한 구조의 신규다공 성 무기 나노입자를 유-무기 이온교환 복합막에 도입하는 시도가 이루어지고 있는 동시에, 이온 선택도 향상을 위한 다양한 형태의 표면 개질 방법이 개발되고 있으며, 이를 통해 이온 교환 복합막의 선택적 양/음이온 거동의 한계를 극복하는 전략을 제시하고 있다.

Abstract: In the last decade, various types of energy harvesting and conversion systems based on ion exchange mem- branes (IEMs) have been developed for eco-friendly power generation and energy-grid systems. In these membrane-based energy systems, high ion selectivity and conductivity properties of IEMs are critical parameters to improve efficiency of the systems such as proton exchange membrane fuel cells, anion exchange membrane fuel cells, redox flow batteries, water elec- trodialysis for hydrogen production, and reverse electrodialysis. This article suggests variable approaches to overcome trade-off limitation of polymeric membrane ion transport properties by reviewing various types of composite ion-exchange membranes including novel inorganic-organic nanocomposite membrane, surface modified membranes, cross-linked and pore-filled membranes.

Keywords: Ion exchange membranes, Polymer composite membranes, Ion selective transport, Energy storage,

Energy conversion

systems have shown great potentials due to their low cost, high energy efficiency, and large-scale applic- ability for fuel cells[3], redox flow batteries[4,5], and reverse electrodialysis (RED)[6]. For high efficiency energy conversion systems, high ion conductivity and selectivity properties of ion exchange membranes (IEMs) has been critical requirements to enhance elec- trochemical performances of electrolyte cells. High cat- ion/anion conductivity in aqueous conditions or partial hydration can lower the ohmic resistances of the poly- mer electrolyte membrane fuel cell stack or RED stack cell system. On the other hand, ion selectivity against fuels (i.e, methanol) and co-ions/counter-ions can cause fuel crossover through the membranes, resulting in the loss of electric potential or capacity between the anode and cathode[7].

Considering these requirements for ion exchange membranes, high ion conductivity as well as ion se- lectivity properties are critical to achieving high elec- trochemical power density and energy efficiency. These properties rely on intrinsic membrane properties for ion transport through hydrophilic nanochannels due to the Donnan exclusion effect specifically from cationic or anionic functionalized polymer membrane matrix[8].

However, there is a trade-off between high ion con- ductivity (transport) and selectivity of co/counter-ions and fuels in conventional polymeric ion exchange membranes (IEMs), such as perfluorinated polymer membranes and hydrocarbon aromatic membranes [9,10]. For instance, in the direct methanol fuel cell, high water uptake widen hydrophilic ion conductive channels which enables high proton conductivity, but it also increases methanol diffusion through membranes, which leads to a drastic open circuit potential drop[11].

Similarly, high cation conductivity results in low resist- ance of RED, while its high co-ion flux crossover causes potential loss, particularly in high concentration gradients[12]. As a result, the trade-off relationship be- tween ion conductivity (membrane resistance) and se- lectivity has hindered attempts to improve the energy efficiency of energy harvesting and conversion systems.

Various types of composite membranes have been

investigated to achieve a higher ion selectivity/con- ductivity as well as enhanced mechanical/chemical sta- bility through modification of the polymer membrane morphology macroscopically and microscopically [3,13,14]. According to the type, size, and shape of nanoparticle fillers or reinforced polymer membrane structures, the electrochemical properties and physical properties of IEMs can be enhanced by optimizing se- lective ion transport properties.

This review investigates several types of ion ex- change composite membranes to improve selective ion transport properties to overcome the trade-off limi- tations for energy conversion and harvesting systems.

Several composite membranes fabricated using various nano-sized particles such as metal oxide nanoparticles [14], porous carbon particles[15], and polymers[3] will be discussed. In addition, the effect of the novel hybrid structure design on electrochemical performance of com- posite membranes will be introduced with examples of porous substrate reinforced membranes in consideration of highly selective ion transport behaviors[14,15].

2. Trade-off Property of IEM for Energy Systems

2.1. Property of IEM

The most widely used class of cation exchange

membrane (CEM) material consists of perfluorosulfonic

acid polymers (PFSAs) such as Nafion

, Hyflon

,

Selemion

, and Aquivion

. They are composed of pol-

ytetrafluoroethylene (PTFE) hydrophobic main-chains

and super acidic sulfonic acid groups containing per-

fluoroether hydrophilic side-chains[16]. This unique

structural class of PEMs forms hydrophilic ionic clus-

ters, well connected by short and narrow ionic path-

ways within the hydrophobic matrix of PTFE, which

can provide high proton conductivities below 90°C de-

spite its low ion exchange capacities (IECs). In addi-

tion, the fluorocarbon-based polymers possess good

chemical and mechanical properties[8,17]. However,

PFSAs allow fuel and co-ion crossover through

well-developed water channels and exhibit relatively poor thermal stability due to its fluorocarbon aliphatic main-chain[18].

To overcome the drawbacks of PFSAs, many re- searchers have developed sulfonated aromatic polymers based on sulfonated derivatives of poly(arylene ether)s (SPAEs)[19,20], poly(arylene sulfide)s (SPASs)[21,22], polyimides (SPIs)[23,24], and non-sulfonated polyben- zimidazoles (PBIs)[25]. These sulfonated aromatic poly- mers can be synthesized via various synthetic routes using diverse monomers to obtain a variety of struc- tures[3,26]. Most sulfonated aromatic polymers are thermally and mechanically stable up to 200°C due to their more rigid polymer chain structure than PFSAs, and excellent fuel barrier properties, which make them as attractive CEM materials. However, for a given ion exchange capacity (IEC), sulfonated aromatic polymers generally have lower proton conductivities and higher dimensional swelling compared to PFSAs because of their lower acidity, rigid aromatic polymer backbone, and weaker phase separation between hydrophilic and hydrophobic moieties, resulting in a less effective mi- crophase-separated morphology for water channel for- mation[3]. The homogeneous polymer membranes have trade-off limitations between ion conductivity and ion selectivity. For enhanced ion conductivity, high water uptake in the polymer matrix is required, which lessens barrier property of the membrane. Thus, there are con- siderable efforts to solve these problems for applica- tions of energy conversion systems in aqueous con- ditions, especially with intentions to gain fundamental understanding of proton conduction and morphology effects on the IEM properties[24,27]. Consequently, electrochemical properties of IEMs should be consid- ered when an ion exchange polymer is selected as a polymer matrix for composite membrane fabrication.

Polymer structural properties, ion conductive channel morphology, and swelling behaviors can greatly affect compatibility between different materials, ion con- ductivity/selectivity, and dimensional stability of com- posite membranes for specific energy applications.

2.2. Proton conductivity and methanol sel- ectivity for proton exchange membrane fuel cell (PEMFC)

In proton exchange membranes, protons are trans- ported through proton conductive nanochannels formed by hydrophilic and hydrophobic morphology phase-sep- aration by way of the Grotthus mechanism and vehicle mechanism[28]. Typically, within the polymer matrix, the hydrophilic ion conductive channel morphology makes transitions from an isolated state to cylindrical channels or a laminar layered structure with water mol- ecule clusters of anionic functional groups in polymer chains[26,29]. The size and connectivity of hydrophilic ion channels can be controlled by optimizing the volu- metric water uptake, chain mobility, and structure of polymer matrix[29]. However, as discussed in 2.1, pol- ymeric IEMs (e.g. PFSAs and hydrocarbon aromatic polymer membranes) have shown trade-off limitations between proton conductivity and methanol cross- over[3,20]. For example, hydrocarbon aromatic polymer membranes such as poly(ether ether ketone), poly(ar- ylene ether sulfone), and polysulfone require relatively higher amounts of water to develop well-connected wide ion conductive channels of over 10 nm for proton con- ductivities above 0.1 Scm

, which is typical of per- fluorinated polymer membranes (Nafion

). However, this wide ion conductive channel morphology also allows methanol diffusion and other co-ion diffusion through ion exchange membranes in aqueous conditions.

2.3. Cation/anion conductivity (membrane resistance) and permselectivity against counter ions for reverse electrodialysis membranes

Selective counter ion transport against co-ions in

IEMs is also crucial for electrodialysis and RED sys-

tems to generate electric potential from the salinity

gradient. For high efficient RED systems, high cati-

on/anion conductivity or low resistance is one of the

key properties to reduce ohmic resistances since so-

dium ion and potassium ion diffuse more slowly than

proton[30]. High permselectivity against co-ions is also

required to prevent the loss of electric potential, espe- cially in high concentrated feed solutions for next-gen- eration brine (over 4.0 M NaCl)-seawater (0.5 M NaCl) RED systems[12,31,32]. The permselectivity properties depend on the fixed charge density of IEMs, a parameter of the ion exchange capacity (IEC) nor- malized by its water uptake ratio[10]. It indicates that the suppressed swelling behavior of IEMs with high IEC can increase their permselectivity by maximizing the charge density in membrane matrix related to the Donnan exclusion effect[9].

Fig. 1(a) shows how commercial homogeneous/heter- ogeneous reinforced electrodialysis membranes (e.g.

Fumasep

, Neosepta

, Ralex

, Selemion

) are gov- erned by the trade-off relationship between membrane resistances and permselectivity. These commercial IEMs are fabricated by pore-filling cross-linked electro- lyte polymers into woven/non-woven porous substrates (e.g. polyethylene (PE), Polyvinyldifluoride (PVDF), polytetrafluorideethylene (PTFE)). However, their mem- brane resistances and permselectivities are still coupled in binary ion separation systems because of the loss of ion conductivity due to undeveloped ion conductive channel morphology. In particular, anion exchange membranes exhibit permselectivities of under 94% at 20°C in 0.5 M-0.1 M NaCl solutions due to their high- er swelling ratios, which is lower than that of CEM.

In homogenous electrolyte IEMs, high swelling be- havior affects the ion conductivity and permselectivity more than in pore-filled composite membranes with suppressed swelling behaviors. Thus, the volumetric charge density normalized by the swelling ratio are in- dicative of its selective ion conductivity properties.

Geise et al. suggested that the volumetric swelling ra- tios of hydrocarbon aromatic anion exchange mem- brane can be controlled by polymer structures of ami- nated polylether sulfone (a-Radel), aminated poly- phenyleneoxide (a-PPO), and side branched PPO[9]. As the water volume fraction in hydrated polymer mem- branes increases, reduced volumetric fixed charge den- sity results in a decreases in the permselectivity while the anion conductivity increases because of well-devel-

oped water channels and high water uptake. Fig. 1(b) shows the trade-off relationship between the anion con- ductivity and permselectivity in anion exchange mem- branes with varying volumetric charge densities and water volume.

2.4. Membrane resistances and vanadium ion diffusivity in aqueous redox flow battery

The intrinsic trade-off relationships of IEMs can also be applied to energy storage applications. In redox flow battery (RFB) cells, PEMs require high proton

(a)

(b)

Fig. 1. Trade-off relationship between membrane resistance

and permselectivity for (a) commercial cation/anion ex-

change membranes in 0.5 M NaCl-0.1 M NaCl aqueous

solution at 20°C[10,32] and (b) homogenous anion ex-

change polymer membranes with various polymer main

chain structures in 0.5 M NaCl-0.1 M NaCl aqueous sol-

ution at 20°C[9]. Copyright at 2013 American Chemical

Society.

conductivity and low diffusivity of multivalent electro- lyte ions (i.e. vanadium ions, iron ions, etc.). However, PEMs for RBF applications suffer from the trade-off relationship between membrane resistances and electro- lyte ion selectivity[33,34]. PEMs require low resistance for high voltage efficiency (VE), however, multivalent ion crossover through PEMs decreases capacity, which decreases decreases coulombic efficiency (CE) and en- ergy efficiency (EE). For example, Nafion, a repre- sentative PEM for vanadium RFBs, shows a high pro- ton conductivity and low membrane resistance (below 1.06 Ωcm

) in hydrated conditions, but it also shows high vanadium ion diffusivity (15.5~36.5 × 10

cm

min

) through well-developed ion conductive chan- nels[35]. This high vanadium ion crossover leads to a large CE drop[33]. On the other hand, hydrocarbon ar- omatic PEMs (e.g. SPAES, SPEEK, PI, etc.) exhibit lower vanadium ion diffusivity (2~5 × 10

cm

min

) and enhanced CE value of 95~97% because of its bar- rier property due to the rigid polymer main chain.

Despite having low vanadium ion diffusivity, their area specific resistances are higher than that of Nafion[36,37].

3. Various Types of Composite Membranes for Energy Application

Chemical/physical modifications have been applied

to homogenous IEMs to improve the trade-off relation-

ships and dimensional stability. Based on the ion con-

ducting properties of several polymer structures and

designs, various types of composite membranes have

been developed for efficient ion selective transport by

using a variety of reinforcing inorganic/organic materi-

als such as non-ionic polymer blending[38-40], porous

woven/non-woven substrates[41,42], inorganic nano-

particles [15], and surface coating[43,44] as summar-

ized in Fig. 2. Depending on the fabrication method,

the structure and morphology of IEMs can be tuned to

compensate for the drawbacks of the homogeneous poly-

Fig. 2. Various strategies for composite membrane fabrication for energy applications.

mer matrix for specific energy applications. For example, pore-filling methods is an effective way to enhance the dimensional stability and chemical/mechanical stability, while surface modification and polymer blending can re- inforce higher ion selectivity in homogeneous membranes.

3.1. Physiochemically tuned ion exchange membranes

Blending non-functionalized polymer chains in the interpenetrating network structure and cross-linking can physiochemically alter the electrochemical properties of perfluorinated polymer or hydrocarbon aromatic poly- mer membranes, which can enhance their dimensional stability and ion selective properties[38-40,45]. Several nonfunctionalized polymers such as polyvinyldifluoride (PVDF), polytetrafluoroethylene (PTFE), polysulfone (PSF), and polyimide (PI), have been used to develop full/semi-interpenetration networks in the functionalized polymer matrix. This physically cross-linked structure increases their mechanical properties and suppresses their swelling behaviors in hydrated states. However, while this network structure enhances the dimensional stability of membranes, the hydrophobic domain of the non-functionalized polymer chains within the blended polymer matrix can interfere with the formation of connected ion channels and the interfacial defects be- tween the two different phases, leading to reduced pro- ton conductivity[40]. In acid-base composite mem- branes, the acid-base complex forms interactions be- tween negatively charged sulfonic acid groups of the sulfonated polymer chain and the N-H site of the basic polymer chain (i.e. polybenzimdazole (PBI))[40,46].

Despite the ionic cross-linkage between polymer chains, these acid-base composite membranes often fail to reduce high swelling behaviors because ionic cross-linking is not sufficient enough to suppress volu- metric expansion compared to covalent cross-linking.

Cross-linking via covalent bonding between polymer main chains is another approach to enhance fuel barrier properties and dimensional stability[21]. However, most chemically cross-linked membranes have shown a de- crease in ion conductivity through removal of func-

tional groups via sulfone alkylation, causing reduced ion conductivity[47,48]. To prevent low ion selectivity, end-group cross-linking via azide-alkyne click chem- istry reaction has been suggested, as the cross-linked network polymer structure enables high proton con- ductivity due to its phase-separation morphology by thermal rearrangement and maintenance of functional groups[49,50]. Despite its high proton conductivity and enhancement of mechanical strength, the end-group cross-linked ion exchange membrane is limited by its low dimensional stability and reduced ion selectivity.

To compensate for the drawbacks of ion transport properties of cross-linked membranes with enhanced mechanical properties and barrier properties, complex types of cross-linked membranes were recently sug- gested as ion exchange membranes for energy harvest- ing systems[51,52]. In dually cross-linked sulfonated poly(arylene ether sulfone) (SPAES), ionic cross- link- ing of cationic PAES blending contributes to favorable barrier properties, and due to end-group cross-linkage, proton conductivity is maintained[52]. In other related works, Guler et al.[51] and Han et al.[53] suggested that crosslinking PVDF/polyepichrolohydrin (PECH)- based anion exchange blend membrane with 1,4-dia- zabicyclo[2.2.2] octane (DABCO) could improve anion permselectivity for applications in ED and RED systems. These AEMs achieved minimal ion con- ductivity loss due to functional groups containing cross-linked bridges (DABCO) and enhanced fixed charge densities due to suppressed water swelling be- havior by their interpenetrating network structure.

3.2. Pore-filled reinforced ion exchange membranes

Ion selectivity and fuel barrier properties of IEMs

are critical factors for highly efficient energy harvest-

ing and conversion systems. Most homogeneous IEMs

must be modified to compensate for their drawbacks

such as excessive fuel crossover, low mechanical

strength, and poor chemical stability. Conventional

pore-filled membranes (e.g. CMX) based on non-func-

tionalized porous substrates generally exhibit reinforced

mechanical strength, low fuel crossover, and enhanced durability thanks to mechanically excellent porous support. However, the polymer substrates occupy about 60%~80% of the volume of IEMs and inhomogeneous macro-phase separation can cause a loss of ion con- ductivity[10]. Despite loss of ion conductivity, their fa- vorable mechanical/chemical stability and barrier prop- erties reduce membrane resistances by decreasing membrane thicknesses below 20 µm in fuel cell and RED stack cell systems.

To balance these contradictory characteristics of pore-filled membranes, several pore-filling membrane fabrication methods have been studied to design vari- ous membrane structures with high ion conductivity and optimal mechanical properties[41,42,54-56]. Pore- filled IEMs have been fabricated by several pore-filling methods including conventional impregnation [41,57], radio-chemical polymerization[42], and hot- pressing methods[55]. The Yamaguchi group fabricated high performance pore-filled membranes in which sulfonated polyimide polymer or tetra-methylammonium polymer electrolytes were impregnated into porous polyethylene or polysulfone supports[57]. UV cross- linking was al- so introduced into an anion exchange pore- filled membrane based on a polyolefin porous substrate to fabricate a highly durable membrane with high anion conductivity[22]. The cross-linking agent of N,N’-bis(acryloyl)piperazine successfully crosslinked

poly(vinylbenzylammonium) in the porous matrix with- out consuming cationic functional groups, achieving a higher anion conductivity as well as greater chemical stability in strong basic conditions. Nafion/poly(phenyl- sulfone) reinforced membrane was also fabricated with dual fiber electrospun porous support and hot-press- ing[54,55]. The method enables void-free composite membrane fabrication with thicknesses below 20 µm.

This reinforced membrane showed a much lower in-plane swelling ratio than Nafion without significant proton conductivity loss[55] as well as reduced hydro- gen crossover[54], leading to a higher power density in the PEMFC single cell test.

3.3. Mixed matrix membranes

Introducing inorganic nanoparticles into the polymer membranes can modify the nanochannel morphology and ion conductive pathway as well as its water up- take, further enhancing the dimensional stability of the membrane. Inorganic-organic mixed matrix membranes (MMMs) have been widely utilized as an approach to improve transport properties and mechanical properties of polymeric membranes by employing superior prop- erties of inorganic nanoparticles[14,15,58]. Many dif- ferent types of inorganic nanoparticles have been used as fillers for fabrication of proton and anion conduct- ing MMMs, such as metal oxides (SiO

, TiO

, ZrO

, SnO

, ZrP etc.)[59-61], mesoporous materials (silica, carbon, zeolite, metal organic frame (MOF))[10,62-65], and advanced carbon materials (graphene, graphene ox- ide (GO), single- and multi-walled carbon nanotube (CNT)) [66-72].

Fig. 3 shows the trade-off relationships for Nafion nanocomposite membrane modified with inorganic nanoparticles (e.g. mesoporous aluminosilicate, func- tionalized carbon nanotubes, zeolites)[14]. These nano- composite membranes have demonstrated great poten- tials to overcome the trade-off limitation by altering polymer membrane properties in favor of suppressed methanol cross-over with high proton conductivity. In particular, the functionalized zeolite/Nafion composite membrane exhibited superior proton transport as well Fig. 3. Trade-off relationship between proton conductivity

and methanol selectivity for proton exchange membranes

in DMFC[14]. Copyright 2016 Elsevier Ltd.

as ion selectivity due to its well-dispersed interfacial phase and high proton conductivity of inorganic par- ticles in the membrane matrix[14,58,73].

High ion conductivity of PEMs in low relative hu- midity (RH) can be beneficial to the development of fuel cell stack systems at low RH, a practical applica- tion in automobiles requiring sufficient water content without bulky and heavy balance of plants (BOP).

Hygroscopic metal oxide particles (SiO

, TiO

, ZrO

, SnO

, and ZrP) were incorporated into the perfluoric acid polymer and hydrocarbon polymer matrix to in- duce a water retention effect[60,74-76]. Metal oxide nanoparticles with hydrophilic surfactant (PEO) and Pt nanoparticles also can attract water molecules, forming water cluster channels in the polymer membrane matrix even in dehydrated conditions. This water retention or self-humidification can enhance the ion conductivity at low RH conditions. However, these metal oxide par- ticles are not enough to suppress fuel crossover through the membrane because of its high water uptake and relatively low dimensional stability.

For modification of selective ion transport, size, type, and pore structure of inorganic particles greatly influence ion diffusion through ion conductive channels in MMMs. Karthikeyan et al. proved the effect of the inorganic particle shape on ion diffusion in SPEEK matrix by using different types of silicate, such as col- loidal Aerosil

, disc-shaped Laponite, and MCM-41 [77]. Nanoparticles with 2D layered structures reduced both methanol diffusivity and proton conductivity due to the increased tortuosity of the diffusion path, while colloidal spherical particles increased its proton con- ductivity without a significant decrease in pro- ton/methanol selectivity. Kim et al. also fabricated 2D layered silicate AMH-3/Nafion nanocomposite membrane for RFB applications[78]. The AMH-3/Nafion mem- branes show reduced vanadium ion crossover due to tor- tuous pathway effect of AMH-3 layers in Nafion matrix.

This leads to the enhancement in the CE. However, ad- dition of the AMH-3 layers increases resistance of the membranes, which decrease both VE and EE.

Dissimilar to 2D materials, the mesoporous silica particle-based composite membrane achieved relatively high proton conductivity with suppressed methanol dif- fusivity beyond trade-off limitations[79,80]. Mesopo- rous structures of nanomaterials can provide high sur- face area and transport channels for protons but can block bigger molecules (e.g. methanol, multivalent ions)[81]. Moreover, these nanoparticles can be easily functionalized by various ion conductive functional groups such as carboxylic acid groups, sulfuric acid groups or quaternized ammonium groups[62,82-85].

The ion conductive functional groups of mesoporous particles contribute not only to favorable dispersion of the nanoparticles but also to well-developed ion con- ductive channels in polymer matrix, which leads to en- hanced ion conductivity. In DMFC application, sulfuric acid functionalized Beta zeolite nanocrystals (particle size > 50 nm)-Nafion composite membrane (NAFB) [62] enables selective proton transport over methanol as illustrated in Fig. 4. Thanks to the enhanced proton selectivity, the NAFB composite membrane showed drastically reduced methanol crossover (about 40%

compared to that of Nafion 115) in 1 M methanol sol-

ution without decrease in proton conductivity (0.088

Fig. 4. Functionalized zeolite in the Nafion mixed matrix

[62]. Copyright 2006, American Chemistry Society.

Scm

). This enables the NAFB membrane to achieve 93% higher maximum power density even in con- centrated methanol feeds (5 M), indicating a high bar- rier performance of the functionalized zeolite nano- crystals against methanol.

Novel carbon materials such as CNT and GO have also been employed to enhance ion transport properties of polymeric IEMs. Recently, sub-1-nanometer CNTs have been spotlighted for its ultrafast selective proton transport properties in mono- and multivalent ions [86,87]. Confinement effects of small diameter CNTs can promote the formation of 1D water strings which can support proton transport rates exceeding those of bulk water by an order of magnitude. The CNT with 0.8 nm diameter showed high proton conductance val- ues, 3.30 × 10

± 7.20

nS per CNT, exceeding that of Nafion (Fig. 5 (a) and (b)). In another related work, 2.6 nm wide CNT pores preferentially transport mono- valent ions over multivalent ions due to the Donnan exclusion theory as shown in Fig. 5(c)[87]. These stud-

ies strongly suggest great potential for application of CNT materials in hydrogen-based energy applications or next- generation RED system with reduced electric potential drop.

Polysiloxane-functionalized CNTs were blended with Nafion to construct a high proton conductive MMM.

Hydrophilic polymer layers of poly(oxyalkylene)-di- amines and tetraethyl orhosilicate (TEOS) reinforced pol- ysiloxane were functionalized in a layer-by-layer fashion on the surface of CNTs[82]. The interaction between amine groups and sulfuric acid groups can improve com- patibility of CNTs with the polymer matrix as well as provide well-developed hydrophilic channels surrounding the CNTs. The functionalized CNT/Nafion composite membranes showed fast proton conductivity and main- tained high values at high temperatures (ó = 2.8 × 10

Scm

at 30°C and 6.3 × 10

Scm

at 130°C). The sul- fonated CNTs also enhances the enhanced dispersion in the proton conductive polysulfone matrix[88]. Consequently, these hydrophilic functionalized CNTs prevent the loss of proton conductivity while retaining the barrier effect due to the presence of carbon fillers.

Graphene oxide (GO)/Nafion composite membranes also showed similar barrier effects of 2D layered mate- rials on proton conductivity and vanadium ion cross- over in IEMs[89]. The CE and EE of the composite membrane was higher than that of pristine Nafion 117 because of the more reduced vanadium ion perme- ability of the composite membrane compared to de- creased proton conductivity.

Various organically modified quaternized GOs have been recently introduced into anion exchange polymer membranes for direct methanol alkaline fuel cells [90-93]. Dai et al. incorporated octadecylamine func- tionalized graphene oxide (GOA) and partially quater- nized GOA (GOAN) into quaternized poly(styrene- b-isobutylene-b-styrene) (SIBS) triblock copolymer membrane[90]. The SIBS/GOA composite membrane showed a reduced methanol permeability of 1.7 × 10

cm

sec

in a 2 M methanol solution, while sustaining a similar proton conductivity to Nafion 115 of 0.0195 Scm

, which is beyond the trade-off limitation. In ad- Fig. 5. (a) Schematic of ion transport through carbon

nanotube. (b) Fast proton transport CNT aquaporin mem-

brane and proton diffusion activation energy[86]. Copy-

right 2016, Nature Publish Group. (C) monovalent-divalent

selective ion transport through CNTs[87]. Copyright 2010,

American Chemical Society.

dition, dopamine-modified GO[91], imidazolium doped GO[92], and quaternized GO[93] composite membranes exhibited enhanced anion conductivities and mechanical properties without increasing swelling ratio. The organ- ic modified 2D graphene nanoparticles can also tend to discrete hydrophobic/hydrophilic domain in polymer matrix, resulting in an enhanced anion conductivity.

The barrier properties of various functionalized nano- particles also influence the selective proton transport properties of IEMs in RFB applications[94-96].

Compared to pristine PEMs, submicron-sized silica im- mobilized phosphotungstic acid (Si-PGA)-poly(vinyl al- cohol)[94] and SPEEK-GO composite membranes[95]

presented reduced V

diffusivities as well as enhanced electrochemical stability without decay of capacity of more than 100 cycles. In particular, the organically modified silicate (ORMOSIL)/Nafion composite mem- brane prepared via in-situ sol-gel reaction of TEOS ex- hibited a drastic reduction in vanadium ion cross-over, which led to enhanced CE up to 95.8% at 80 mAcm

and increased EE of 80% compared to Nafion[96].

3.4. Surface modified composite membranes Surface modification of IEMs is another effective approach to improve ion selectivity of co-ions or mul- tivalent counter ions, minimizing the loss of ion con- ductivity[43,97,98]. Surface morphology modification and alteration of surface charge distribution influence ion selectivity and membrane resistance without com- plex polymer structure design. Among several surface modification methods, the layer-by-layer (LBL) assem- bly method has been widely applied to IEMs in aque-

ous electrolyte solution-based energy storage/conversion systems (e.g. flow battery, ED, RED). LBL-IEMs [43,44] possess cationic/anionic charged surface layers as ion selective layers which allow transport of specific ions based on Donnan exclusion effects. Xi et al. de- veloped the LBL-Nafion membrane with alternating ad- sorption of polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate), Nafion-[PDDA-PSS]

, to improve proton se- lectivity against vanadium ions in the RFB electrolyte cell (Fig. 6)[43]. The LBL-Nafion membrane with 5~7 alternative coating layer achieved more than 5 times decreased vanadium diffusivity with slightly decreased proton conductivity of 0.048 Scm

, which shows sig- nificant improvement from the trade-off relationship between ion conductivity and selectivity. The poly- ethyleneimine (PEI)/Nafion LBL membrane[44] also showed reduced vanadium crossover (~70%) which re- sulted in enhanced columbic efficiency from 81% to 93% thanks to the impermeability of the PEI layer to multivalent cation and gas molecules.

Recently nano-crack regulated fluorocarbon plasma

coated polybiphenylsulfone (BPS) membranes[99] showed

great potential for applications in salinity gradient RED

systems by showing low membrane resistances due to

enhanced ion selectivities against co-ions without sacri-

ficing ion conductivity. The nano-crack patterned hy-

drophobic coating layer could provide sodium ion and

chloride ion transport channels in thin surface fluo-

rocarbon coating layers (< 100 nm) on the surface of

CEMs and AEMs (Fig. 7(a)). This thin surface se-

lective layer shows considerably enhanced permse-

Fig. 6. Layer-by-Layer Nafion [PDDA-PSS] membrane for enhanced vanadium ion selectivity[43]. Copyright 2008, American

Chemical Society.

lectivity of BPSH with degree of functionalization from 65% to 85% as seen in Fig. 7 (b). This enhanced permselectivity against co-ion diffusion enables the ap- plication of high energy efficient RED with low mem- brane resistances below 0.5 Ωcm

.

4. Conclusions

Highly ion selective and conductive transport proper- ties of IEMs are essential for the development of high-efficiency energy conversion and storage systems.

High cation/anion conductivity in aqueous conditions or partial hydration can lower the ohmic resistances of the polymer electrolyte membrane fuel cell stack or RED stack cell system. Low ion selectivity can cause fuel crossover through the membranes, resulting in the loss of electric potential or capacity between the anode and cathode. However, most homogenous IEMs have been known to have the trade-off limitations between ion conductivity and selectivity.

Among several approaches to overcome the trade-off limitations, many researchers have developed the novel composite IEMs via polymer blending, cross-linking, pore-filling technique, inorganic/organic MMM, and surface modification for specific energy applications.

While this review article doesn’t introduce detailed re-

sults from every paper published in the area, this over- view represents research trends in modifying IEMs to improve their ion transport properties. The primary purpose of the IEM modifications is to increase ion se- lectivity while maintaining the good ion conductivity and chemical stability of IEM matrix. However, these two transport properties are often reverse, and in most cases, increasing ion selectivity could only be achieved by sacrificing ion conductivity. Therefore, the balance between the conductivity and selectivity is the most important design criteria for the development of mem- brane-based high efficient energy conversion and stor- age systems. The future of the nanocomposite mem- branes is also dependent on the development of high ion selective inorganic nanoparticles. Among some highly attractive inorganic nanoparticles, one-atom- thick (monolayer) graphene and hexagonal boron ni- tride (hBN) have been recently proposed as the ulti- mate candidates for use as a thin selective layer in hy- brid IEMs because of their high proton conductivity, excellent chemical resistance against oxidation, thermal stability, and impermeability to other molecules (e.g.

ions, water, fuels)[100]. Only subatomic particles such as protons can transport through the tiny ‘pores’ of atomically thin electron clouds. This unique infinite proton-selective property makes these 2D membranes attractive candidates for use in various electrochemical energy conversion and production technologies.

Most nanocomposite membrane researches for energy applications are still in early stages and lots of efforts are required to make the nanocomposite membranes as next-generation IEMs for several energy applications. In this respect, particular issues to be investigated with high priority are fundamental understanding on subtle ion transport mechanisms in inorganic nanoparticles and de- velopment of commercially attractive fabrication process.

Acknowledgments

This work was supported by the University of Illinois-Chicago Research Fund. T. Chung acknowl- edges support by UIC-COE Summer Internship Fund.

Fig. 7. (a) Scheme of nano-crack surfacemodified ion ex- change membranes and (b) selective ion transport behav- iors of plasma treated cation exchange membranes BPS40 and BPS60. CMX (from Neosepta/ASTOM; 1, black square), FKD (from Fumacep/Fumatech; 2, green (square), CMV (from Selemion/AGC Asahi Glass; 3, purple square).

Copyright 2016, Nature Publishing Group.

Reference

1. E. D. Williams, “ARPA-E: First seven years, A sampling of project outcomes”, the Advanced Research Project Agency for Energy (ARPA-E), the United States (2016).

2. D. Papageorgopoulos, “2016 Annual Merit Review Proceedings Hydrogen and Fuel Cells Program Plenary”, The Department of Energy (DOE), The United State (2016).

3. C. H. Park, C. H. Lee, M. D. Guiver, and Y. M.

Lee, “Sulfonated hydrocarbon membranes for me- dium-temperature and low-humidity proton ex- change membrane fuel cells (PEMFCs)”, Prog.

Polym. Sci., 36, 1443 (2011).

4. R. M. Darling, K. G. Gallagher, J. A. Kowalski, S. Ha, and F. R. Brushett, “Pathways to low- cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries”, Energy Environ. Sci., 7, 3459 (2014).

5. M. Skyllas-Kazacos, M. H. Chakrabarti, S. A.

Hajimolana, F. S. Mjalli, and M. Saleem,

“Progress in flow battery research and devel- opment. journal of the electrochemical society”, J.

Electochem. Soc., 158, R55 (2011).

6. B. E. Logan and M. Elimelech, “Membrane-based processes for sustainable power generation using water”, Nature, 488, 313 (2012).

7. R. Darling, K. Gallagher, W. Xie, L. Su, and F.

Brushett, “Transport property requirements for flow battery separators”, J. Electrochem. Soc.,

8. K. A. Mauritz and R. B. Moore, “State of under- standing of nafion”, Chem. Rev., 104, 4535 (2004).

9. G. M. Geise, M. A. Hickner, and B. E. Logan,

“Ionic resistance and permselectivity tradeoffs in anion exchange membranes”, ACS Appl. Mater.

Interfaces, 5, 10294 (2013).

10. P. Dlługoł˛ecki, K. Nymeijer, S. Metz, and M.

Wessling, “Current status of ion exchange mem-

branes for power generation from salinity gra- dients”, J. Membr. Sci., 319, 214 (2008).

11. Z. Qi, and A. Kaufman, “Open circuit voltage and methanol crossover in DMFCs”, J. Power Sources, 110, 177 (2002).

12. A. Daniilidis, D. A. Vermaas, R. Herber, and K.

Nijmeijer, “Experimentally obtainable energy from mixing river water, seawater or brines with re- verse electrodialysis”, Renew. Energy, 64, 123 (2014).

13. S. J. Peighambardoust, S. Rowshanzamir, and M.

Amjadi, “Review of the proton exchange mem- branes for fuel cell applications”, Int. J.

Hydrogen Energy, 35, 9349 (2010).

14. E. Bakangura, L. Wu, L. Ge, Z. Yang, and T.

Xu, “Mixed matrix proton exchange membranes for fuel cells: State of the art and perspectives”, Prog. Polym. Sci., 57, 103 (2016).

15. C. Laberty-Robert, K. Valle, F. Pereira, and C.

Sanchez, “Design and properties of functional hy- brid organic-inorganic membranes for fuel cells”, Chem. Soc. Rev., 40, 961 (2011).

16. K.-D. Kreuer, S. J. Paddison, E. Spohr, and M.

Schuster, “Transport in proton conductors for fuel-cell applications: Simulations, elementary re- actions, and phenomenology”, Chem. Rev., 104, 4637 (2004).

17. R. Souzy and B. Ameduri, “Functional fluoropol- ymers for fuel cell membranes”, Prog. Polym.

Sci., 30, 644 (2005).

18. K. D. Kreuer, “On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells”, J. Membr. Sci., 185, 29 (2001).

19. B. Bae, K. Miyatake, and M. Watanabe, “Effect of the hydrophobic component on the properties of sulfonated poly(arylene ether sulfone)s”, Macromolecules, 42, 1873 (2009).

20. Y. S. Kim, M. A. Hickner, L. Dong, B. S.

Pivovar, and J. E. McGrath, “Sulfonated poly(ar-

ylene ether sulfone) copolymer proton exchange

membranes: composition and morphology effects

on the methanol permeability”, J. Membr. Sci.,

21. D. S. Phu, C. H. Lee, C. H. Park, S. Y. Lee, and Y. M. Lee, “Synthesis of crosslinked sulfonated poly(phenylene sulfide sulfone nitrile) for direct methanol fuel cell applications”, Macromol. Rapid Commun., 30, 64 (2009).

22. Z. Bai, J. A. Shumaker, M. D. Houtz, P. A.

Mirau, and T. D. Dang, “Fluorinated poly(ar- ylenethioethersulfone) copolymers containing pendant sulfonic acid groups for proton exchange membrane materials”, Polymer, 50, 1463 (2009).

23. K. H. Lee, S. Y. Lee, D. W. Shin, C. Wang, S.-H. Ahn, K.-J. Lee, M. D. Guiver, and Y. M.

Lee, “Structural influence of hydrophobic diamine in sulfonated poly(sulfide sulfone imide) copoly- mers on medium temperature PEM fuel cell”, Polymer, 55, 1317 (2014).

24. Y. Yin, O. Yamada, Y. Suto, T. Mishima, K.

Tanaka, H. Kita, and K.-i. Okamoto, “Synthesis and characterization of proton-conducting copolyi- mides bearing pendant sulfonic acid groups”, J.

Polym. Sci. Part A Polym. Chem., 43, 1545 (2005).

25. Q. Li, J. O. Jensen, R. F. Savinell, and N. J.

Bjerrum, “High temperature proton exchange membranes based on polybenzimidazoles for fuel cells”, Prog. Polym. Sci., 34, 449 (2009).

26. N. Li and M. D. Guiver, “Ion transport by nano- channels in ion-containing aromatic copolymers”, Macromolecules, 47, 2175 (2014).

27. T. J. Peckham and S. Holdcroft, “Structure-mor- phology-property relationships of non- per- fluorinated proton-conducting membranes”, Adv.

Mater., 22, 4667 (2010).

28. K.-D. Kreuer, A. Rabenau, and W. Weppner,

“Vehicle mechanism, a new model for the inter- pretation of the conductivity of fast proton con- ductors”, Angew. Chemie Int. Ed., 21, 208 (1982).

29. K.-D. Kreuer and G. Portale, “A critical revision of the nano-morphology of proton conducting ion- omers and polyelectrolytes for fuel cell applica-

tions”, Adv. Funct. Mater., 23, 5390 (2013).

30. J. Veerman, R. M. de Jong, M. Saakes, S. J.

Metz, and G. J. Harmsen, “Reverse electro- dialysis: Comparison of six commercial mem- brane pairs on the thermodynamic efficiency and power density”, J. Membr. Sci., 343, 7 (2009).

31. M. Tedesco, A. Cipollina, A. Tamburini, and G.

Micale, “Towards 1 kW power production in a reverse electrodialysis pilot plant with saline wa- ters and concentrated brines”, J. Membr. Sci.,

31. E. Fontananova, D. Messana, R. A. Tufa, I.

Nicotera, V. Kosma, E. Curcio, W. van Baak, E.

Drioli, and G. Di Profio, “Effect of solution con- centration and composition on the electrochemical properties of ion exchange membranes for energy conversion”, J. Power Sources, 340, 282 (2017).

32. E. Fontananova, W. Zhang, I. Nicotera, C.

Simari, W. van Baak, G. Di Profio, E. Curcio, and E. Drioli, “Probing membrane and interface properties in concentrated electrolyte solutions”, J.

Membr. Sci., 459, 177 (2014).

33. B. Schwenzer, J. Zhang, S. Kim, L. Li, J. Liu, and Z. Yang, “Membrane development for vana- dium redox flow batteries”, ChemSusChem, 4, 1388 (2011).

34. D. J. Kim and S. Y. Nam, “Research trend of polymeric ion-exchange membrane for vanadium redox flow battery”, Membr. J., 22, 385 (2012).

35. Q. Luo, H. Zhang, J. Chen, D. You, C. Sun, and Y. Zhang, “Preparation and characterization of Nafion/SPEEK layered composite membrane and its application in vanadium redox flow battery”, J. Membr. Sci., 325, 553 (2008).

36. Z. Mai, H. Zhang, X. Li, C. Bi, and H. Dai,

“Sulfonated poly(tetramethydiphenyl ether ether ketone) membranes for vanadium redox flow bat- tery application”, J. Power Sources, 196, 482 (2011).

37. S. Kim, J. Yan, B. Schwenzer, J. Zhang, L. Li, J.

Liu, Z. Yang, and M. A. Hickner, “Cycling per-

formance and efficiency of sulfonated poly(sul-

fone) membranes in vanadium redox flow bat- teries”, Electrochem. Commun., 12, 1650 (2010).

38. B. Kosmala and J. Schauer, “Ion-exchange mem- branes prepared by blending sulfonated poly(2,6- dimethyl-1,4-phenylene oxide) with polybenzimi- dazole”, J. Appl. Polym. Sci., 85, 1118 (2002).

39. C. Hasiotis, V. Deimede, and C. Kontoyannis,

“New polymer electrolytes based on blends of sulfonated polysulfones with polybenzimidazole”, Electrochim. Acta., 46, 2401 (2001).

40. Y. T. Hong, C. H. Lee, H. S. Park, K. A. Min, H. J. Kim, S. Y. Nam, and Y. M. Lee,

“Improvement of electrochemical performances of sulfonated poly(arylene ether sulfone) via in- corporation of sulfonated poly(arylene ether ben- zimidazole)”, J. Power Sources, 175, 724 (2008).

41. T. Yamaguchi, F. Miyata, and S. Nakao,

“Polymer electrolyte membranes with a pore- fill- ing structure for a direct methanol fuel cell”, Adv.

Mater., 15, 1198 (2003).

42. M.-S. Lee, T. Kim, S.-H. Park, C.-S. Kim, and Y.-W. Choi, “A highly durable cross-linked hy- droxide ion conducting pore-filling membrane”, J.

Mater. Chem., 22, 13928 (2012).

43. J. Xi, Z. Wu, X. Teng, Y. Zhao, L. Chen, and X.

Qiu, “Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vana- dium ion crossover for vanadium redox flow bat- teries”, J. Mater. Chem., 18, 1232 (2008).

44. J. G. Austing, C. N. Kirchner, L. Komsiyska, and G. Wittstock, “Layer-by-layer modification of Nafion membranes for increased life-time and ef- ficiency of vanadium/air redox flow batteries”, J.

Membr. Sci., 510, 259 (2016).

45. J.-J. Woo, S.-J. Seo, S.-H. Yun, R.-Q. Fu, T.-H.

Yang, and S.-H. Moon, “Enhanced stability and proton conductivity of sulfonated polystyrene/PVC composite membranes through proper copoly- merization of styrene with á-methylstyrene and acrylonitrile”, J. Membr. Sci., 363, 80 (2010).

46. J. Kerres, A. Ullrich, F. Meier, and T. Häring,

“Synthesis and characterization of novel acid-base

polymer blends for application in membrane fuel cells”, Solid State Ion., 125, 243 (1999).

47. J. Kerres, W. Cui, and M. Junginger,

“Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU crosslinked PSU blend mem- branes by alkylation of sulfinate groups with di- halogenoalkanes”, J. Membr. Sci., 139, 227 (1998).

48. S. D. Mikhailenko, K. Wang, S. Kaliaguine, P.

Xing, G. P. Robertson, and M. D. Guiver,

“Proton conducting membranes based on cross- linked sulfonated poly(ether ether ketone) (SPEEK)”, J. Membr. Sci., 233, 93 (2004).

49. N. R. Kang, S. Y. Lee, D. W. Shin, D. S.

Hwang, K. H. Lee, D. H. Cho, J. H. Kim, and Y. M. Lee, “Effect of end-group cross-linking on transport properties of sulfonated poly(phenylene sulfide nitrile)s for proton exchange membranes”, J. Power Sources, 307, 834 (2016).

50. S. Y. Lee, N. R. Kang, D. W. Shin, C. H. Lee, K.-S. Lee, M. D. Guiver, N. Li, and Y. M. Lee,

“Morphological transformation during cross-link- ing of a highly sulfonated poly(phenylene sulfide nitrile) random copolymer”, Energy Environ. Sci.,

51. E. Guler, Y. Zhang, M. Saakes, and K. Nijmeijer,

“Tailor-made anion-exchange membranes for sal- inity gradient power generation using reverse electrodialysis”, ChemSusChem., 5, 2262 (2012).

52. W. H. Lee, K. H. Lee, D. W. Shin, D. S.

Hwang, N. R. Kang, D. H. Cho, J. H. Kim, and Y. M. Lee, “Dually cross-linked polymer electro- lyte membranes for direct methanol fuel cells”, Power Sources, 282, 211 (2015).

53. B. Han, J. Pan, S. Yang, M. Zhou, J. Li, A.

Sotto Díaz, B. Van der Bruggen, C. Gao, and J.

Shen, “Novel composite anion exchange mem- branes based on quaternized polyepichlorohydrin for electromembrane application”, Ind. Eng.

Chem. Res., 55, 7171 (2016).

54. J. B. Ballengee and P. N. Pintauro, “Preparation

of nanofiber composite proton-exchange mem-

branes from dual fiber electrospun mats”, J.

Membr. Sci., 442, 187 (2013).

55. J. B. Ballengee and P. N. Pintauro, “Composite fuel cell membranes from dual-nanofiber electro- spun mats”, Macromolecules, 44, 7307 (2011).

56. D.-H. Kim and M.-S. Kang, “Preparation and characterizations of ionomer-coated pore-filled ion-exchange membranes for reverse electro- dialysis”, Membr. J., 26, 43 (2016).

57. H. Jung, K. Fujii, T. Tamaki, H. Ohashi, T. Ito, and T. Yamaguchi, “Low fuel crossover anion ex- change pore-filling membrane for solid-state alka- line fuel cells”, J. Membr. Sci., 373, 107 (2011).

58. B. P. Tripathi and V. K. Shahi, “Organic-in- organic nanocomposite polymer electrolyte mem- branes for fuel cell applications”, Prog. Polym.

Sci., 36, 945 (2011).

59. M. Watanabe, H. Uchida, and M. Emori,

“Polymer electrolyte membranes incorporated with nanometer-size particles of Pt and/or metal-oxides:

Experimental analysis of the self-humidification and suppression of gas-crossover in fuel cells”, J.

Phys. Chem. B, 102, 3129 (1998).

60. X. Zhu, H. Zhang, Y. Zhang, Y. Liang, X.

Wang, and B. Yi, “An ultrathin self-humidifying membrane for PEM fuel cell application:

Fabrication, characterization, and experimental analysis”, J. Phys. Chem. B, 110, 14240 (2006).

61. C. Bi, H. Zhang, Y. Zhang, X. Zhu, Y. Ma, H.

Dai, and S. Xiao, “Fabrication and investigation of SiO

supported sulfated zirconia/Nafion®

self-humidifying membrane for proton exchange membrane fuel cell applications”, J. Power Sources, 184, 197 (2008).

62. Z. Chen, B. Holmberg, W. Li, X. Wang, W.

Deng, R. Munoz, and Y. Yan, “Nafion/zeolite nanocomposite membrane by in situ crystal- lization for a direct methanol fuel cell”, Chem.

Mater., 18, 5669 (2006).

63. S. Meenakshi, A. K. Sahu, S. D. Bhat, P.

Sridhar, S. Pitchumani, and A. K. Shukla,

“Mesostructured-aluminosilicate-Nafion hybrid mem-

branes for direct methanol fuel cells”, Electrochim. Acta., 89, 35 (2013).

64. U. Mueller, M. Schubert, F. Teich, H. Puetter, K.

Schierle-Arndt, and J. Pastre, “Metal- organic frameworks-prospective industrial applications”, J.

Mater. Chem., 16, 626 (2006).

65. S. Mikhailenko, D. Desplantier-Giscard, C.

Danumah, and S. Kaliaguine, “Solid electrolyte properties of sulfonic acid functionalized meso- structured porous silica”, Microporous Mesoporous Mater., 52, 29 (2002).

66. Y.-H. Liu, B. Yi, Z.-G. Shao, L. Wang, D. Xing, and H. Zhang, “Pt/CNTs-Nafion reinforced and self-humidifying composite membrane for PEMFC applications”, J. Power Sources, 163, 807 (2007).

67. J.-M. Thomassin, J. Kollar, G. Caldarella, A.

Germain, R. Jérôme, and C. Detrembleur,

“Beneficial effect of carbon nanotubes on the per- formances of Nafion membranes in fuel cell ap- plications”, J. Membr. Sci., 303, 252 (2007).

68. L. Wang, D. M. Xing, H. M. Zhang, H. M. Yu, Y. H. Liu, and B. L. Yi, “MWCNTs reinforced Nafion

membrane prepared by a novel sol- ution-cast method for PEMFC”, J. Power Sources, 176, 270 (2008).

69. Y.-C. Cao, C. Xu, X. Wu, X. Wang, L. Xing, and K. Scott, “A poly (ethylene oxide)/graphene oxide electrolyte membrane for low temperature polymer fuel cells”, J. Power Sources, 196, 8377 (2011).

70. H.-C. Chien, L.-D. Tsai, C.-P. Huang, C.-y.

Kang, J.-N. Lin, and F.-C. Chang, “Sulfonated graphene oxide/Nafion composite membranes for high-performance direct methanol fuel cells”, Int.

J. Hydrogen Energy, 38, 13792 (2013).

71. Y. Heo, H. Im, and J. Kim, “The effect of sulfo- nated graphene oxide on Sulfonated Poly (Ether Ether Ketone) membrane for direct methanol fuel cells”, J. Membr. Sci., 425-426, 11 (2013).

72. Z. Jiang, Y. Shi, Z.-J. Jiang, X. Tian, L. Luo,

and W. Chen, “High performance of a free-

standing sulfonic acid functionalized holey gra-

phene oxide paper as a proton conducting poly- mer electrolyte for air-breathing direct methanol fuel cells”, J. Mater. Chem. A, 2, 6494 (2014).

73. S. Kango, S. Kalia, A. Celli, J. Njuguna, Y.

Habibi, and R. Kumar, “Surface modification of inorganic nanoparticles for development of organ- ic-inorganic nanocomposites-A review”, Prog.

Polym. Sci., 38, 1232 (2013).

74. Y.-H. Su, Y.-L. Liu, Y.-M. Sun, J.-Y. Lai, D.-M.

Wang, Y. Gao, B. Liu, and M. D. Guiver,

“Proton exchange membranes modified with sul- fonated silica nanoparticles for direct methanol fuel cells”, J. Membr. Sci., 296, 21 (2007).

75. H. Hagihara, H. Uchida, and M. Watanabe,

“Preparation of highly dispersed SiO

and Pt par- ticles in Nafion

112 for self-humidifying electro- lyte membranes in fuel cells”, Electrochim. Acta,

76. M.-N. Kim, Y.-W. Choi, T.-Y. Kim, M.-S. Lee, C.-S. Kim, T.-H. Yang, and K.-S. Nam,

“Characterization of sulfonated ploy(aryl ether sulfone) membranes impregnated with sulfated ZrO

”, Membr. J., 21, 30 (2011).

77. C. S. Karthikeyan, S. P. Nunes, L. A. S. A.

Prado, M. L. Ponce, H. Silva, B. Ruffmann, and K. Schulte, “Polymer nanocomposite membranes for DMFC application”, J. Membr. Sci., 254, 139 (2005).

78. J. Kim, J.-D. Jeon, and S.-Y. Kwak,

“Nafion-based composite membrane with a perm- selective layered silicate layer for vanadium redox flow battery”, Electrochem. Comm., 38, 68 (2014).

79. E. Vijayakumar and D. Sangeetha, “A quaternized mesoporous silica/polysulfone composite mem- brane for an efficient alkaline fuel cell applica- tion”, RSC Adv., 5, 42828 (2015).

80. V. Elumalai and S. Dharmalingam, “Synthesis characterization and performance evaluation of ionic liquid immobilized SBA-15/quaternised pol- ysulfone composite membrane for alkaline fuel cell”, Microporous Mesoporous Mater., 236, 260 (2016).

81. N. Qian, Z. Duan, Y. Zhu, Q. Xiang, and J. Xu,

“4,4’-Diaminodiphenyl sulfone functionalized SBA-15: Toluene sensing properties and improved proton conductivity”, J. Physic. Chem. C, 118, 1879 (2014).

82. W.-F. Chen, J.-S. Wu, and P.-L. Kuo, “Poly(oxy- alkylene)diamine-Functionalized Carbon Nanotube/

Perfluorosulfonated Polymer Composites: Synthesis, Water State, and Conductivity”, Chem. Mater.,

83. Y. Wang, Z. Jiang, H. Li, and D. Yang,

“Chitosan membranes filled by GPTMS-modified zeolite beta particles with low methanol perme- ability for DMFC”, Chem. Eng. Process.: Process Intensification., 49, 278 (2010).

84. L. Zhiting, D. Xuezhi, Q. Gang, Z. Xinggui, and Y. Weikang, “Eco-friendly one-pot synthesis of highly dispersible functionalized graphene nano- sheets with free amino groups”, Nanotechnol., 24, 045609 (2013).

85. C. W. Jones, K. Tsuji, and M. E. Davis,

“Organic-functionalized molecular sieves as shape- selective catalysts”, Nature, 393, 52 (1998).

86. R. H. Tunuguntla, F. I. Allen, K. Kim, A.

Belliveau, and A. Noy, “Ultrafast proton transport in sub-1-nm diameter carbon nanotube porins”, Nat. Nanotechnol., 11, 639 (2016).

87. F. Fornasiero, J. B. In, S. Kim, H. G. Park, Y.

Wang, C. P. Grigoropoulos, A. Noy, and O.

Bakajin, “pH-Tunable ion selectivity in carbon nanotube pores”, Langmuir, 26, 14848 (2010).

88. S. H. Joo, C. Pak, E. A. Kim, Y. H. Lee, H.

Chang, D. Seung, Y. S. Choi, J.-B. Park, and T.

Kim, “Functionalized carbon nanotube-poly (arylene sulfone) composite membranes for direct methanol fuel cells with enhanced performance”, J. Power Sources, 180, 63 (2008).

89. K. J. Lee and Y. H. Chu, “Preparation of the graphene oxide (GO)/Nafion composite membrane for the vanadium redox flow battery (VRB) sys- tem”, Vacuum, 107, 269 (2014).

90. P. Dai, Z.-H. Mo, R.-W. Xu, S. Zhang, X. Lin,

W.-F. Lin, and Y.-X. Wu, “Development of a cross-linked quaternized poly(styrene-b-isobutylene- b-styrene)/graphene oxide composite anion ex- change membrane for direct alkaline methanol fuel cell application”, RSC Adv., 6, 52122 (2016).

91. Z. Luo, Y. Gong, X. Liao, Y. Pan, and H.

Zhang, “Nanocomposite membranes modified by graphene-based materials for anion exchange membrane fuel cells”, RSC Adv., 6, 13618 (2016).

92. J. Li, X. Yan, Y. Zhang, B. Zhao, and G. He,

“Enhanced hydroxide conductivity of imidazolium functionalized polysulfone anion exchange mem- brane by doping imidazolium surface-func- tionalized nanocomposites”, RSC Adv., 6, 58380 (2016).

93. L. Liu, C. Tong, Y. He, Y. Zhao, and C. Lü,

“Enhanced properties of quaternized graphenes re- inforced polysulfone based composite anion ex- change membranes for alkaline fuel cell”, J.

Membr. Sci., 487, 99 (2015).

94. J. Pandey and B. R. Tankal, “Performance of the vanadium redox-flow battery (VRB) for Si-PWA/

PVA nanocomposite membrane”, J. Solid State Electrochem., 20, 2259 (2016).

95. W. Dai, Y. Shen, Z. Li, L. Yu, J. Xi, and X.

Qiu, “SPEEK/Graphene oxide nanocomposite membranes with superior cyclability for highly ef- ficient vanadium redox flow battery”, J. Mater.

Chem. A, 2, 12423 (2014).

96. X. Teng, Y. Zhao, J. Xi, Z. Wu, X. Qiu, and L.

Chen, “Nafion/organically modified silicate hy- brids membrane for vanadium redox flow bat- tery”, J. Power Sources, 189, 1240 (2009).

97. S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, and H. Matsuyama, “Simultaneous improvement of the monovalent anion selectivity and antifouling properties of an anion exchange membrane in an electrodialysis process, using polyelectrolyte mul- tilayer deposition”, J. Membr. Sci., 431, 113 (2013).

98. M.-K. Park, S. Deng, and R. C. Advincula,

“pH-Sensitive bipolar ion-permselective ultrathin films”, J. Am. Chem. Soc., 126, 13723 (2004).

99. C. H. Park, S. Y. Lee, D. S. Hwang, D. W. Shin, D. H. Cho, K. H. Lee, T.-W. Kim, T.-W. Kim, M. Lee, D.-S. Kim, C. M. Doherty, A. W.

Thornton, A. J. Hill, M. D. Guiver, and Y. M.

Lee, “Nanocrack-regulated self-humidifying mem- branes”, Nature, 532, 480 (2016).

100. S. Hu, M. Lozada-Hidalgo, F. C. Wang, A.

Mishchenko, F. Schedin, R. R. Nair, E. W. Hill, D. W. Boukhvalov, M. I. Katsnelson, R. A. W.

Dryfe, I. V. Grigorieva, H. A. Wu, and A. K.

Geim, “Proton transport through one-atom-thick

crystals”, Nature, 516, 227 (2014).