Xiaoming Yan ,Wenji Zheng ,Xuehua Ruan ,Yu Pan ,Xuemei Wu ,Gaohong He ,*

1 State Key Laboratory of Fine Chemicals,School of Petroleum and Chemical Engineering,Dalian University of Technology,Panjin,LN 124221,China

2 Research and Development Center of Membrane Science and Technology,School of Chemical Engineering,Dalian University of Technology,Dalian,LN 116024,China

1.Applications of Ion Conductive Membranes in Energy Conversion and Storage Devices

Ion conductive membranes(ICMs),including proton exchange membrane(PEM)and anion exchange membrane(AEM),are good ion-selective conductors with low electronic conductivity[1].Therefore,ICMs are frequently used as separators for energy conversion and storage technologies of proton exchange membrane fuel cells,alkaline anion exchange membrane fuel cells, flow battery,and hydrogen pump.

PEM,as an excellent proton-selective conductor and fuel/oxidant separator,is a good choice as a separator for fuel cells.PEM fuel cells(PEMFCs)show high energy conversion efficiency and high power density due to those advantages of PEM[2,3].The schematic principle of PEMFCs is shown in Fig.1(a).Fuel cells with proton exchange membranes as the separators run in the acidic media.The electrode electrochemical kinetics are slow so that precious metal Pt is required as the catalyst.Moreover,the stability of nanosized Pt catalyst in the acidic operation media also needs to be improved[4].This triggered some researcher to look for alternative non-precious metal catalysts[5-7].On the other hand,by switching the operation media from acidity to basicity,alkaline AEM(AAEM)can be used as the separator in fuel cells[8].In the basic media,electrode kinetics are enhanced[9-12],and non-Pt metals(e.g.,silver,nickel,and Pd)are allowed to be used as catalyst[13-17],which are more stable[18-20].In addition,the direction of hydroxide conduction opposes that of alcohol fuel crossover,consequently alleviating or even eliminating alcohol fuel crossover,particularly at the high current density[21].The schematic principle of AAEM fuel cells is illustrated in Fig.1(b).

Fig.1.The schematic principle of PEMFC and AAEMFCs[3].

PEM is also central to electrochemical hydrogen pump,which produces pure hydrogen(99.7%and more)in an environmentally friendly way[22],thus receiving more attentions.With an external potential applied,hydrogen in the mixture is oxidized to protons at the anode,which are transported across the proton exchange membrane and reduced to hydrogen again at the cathode.Other advantages for hydrogen pump separation are related to the ability of compressing hydrogen up to 50 bar at the cathode and the capture of concentrated CO2effluent at the anode without further regeneration.The significant challenges for hydrogen pump separation are CO poisoning and the total cost consisting of the cost of the catalyst(Pt)and PEM(Nafion)[22,23].In addition,ICMs are also the key components of flow batteries which is an electricity energy storage device.Vanadium redox flow battery(VRB)is one of the most promising flow batteries.ICMs play the roles of isolating electrolytes(vanadium ions)while still transferring ions(protons or SO42-)to complete the circuit[24].The schematic principle of VRB is shown in Fig.2.

In general,ICMs are required to possess some critical properties,including good thermal stability,high ion conduction,and low fuel/vanadium permeability,excellent chemical stability,and outstanding mechanical properties[3,25].Highly selective ion conduction essentially determines the energy efficiency and power density.Mechanical properties of fuel cell membranes are important,especially for the fuel cell durability,since variations in temperature and humidity can cause cyclic stresses and strains(mechanical loading)in the membrane during the fuel cell operation.In addition,it is desirable for ICM materials to be soluble in low-boiling-point water-soluble solvents,in order to build an efficient triple phase boundary by adding them into the catalyst layers,thus drastically reducing the internal resistance for use in fuel cells[18,26].

2.Molecular Design and Synthesis of ICM Materials

2.1.Proton exchange membranes

Dupont's Nafion is the most frequently used PEM material,because of its excellent proton conductivity,oxidative resistance,chemical stability and commercial availability[27].However,it suffers from high cost and great loss of conductivity at high temperature(＞80°C)due to dehydration.These limitations have stimulated many efforts in the modification of per- fluorinated membranes and the development of alternative membrane materials.

To improve the performance of PEMFC at elevated operation temperature,short-side-chain perfluorosulfonic acid ionomer(SSC-PFSA)has been considered as good alternative materials[28],owing to their higher crystallinity and glass transition temperature(Tg)than longside-chain(LSC)PFSA ionomer.More importantly,SSC-PFSA possesses better water retention at elevated temperature(＞100°C),which is essential to maintain higher conductivity[29].As a result,the PEMFCs with both SSC-PFSA membrane and ionomer exhibit higher performances than those with long-side-chain(LSC)PFSA at high temperatures(＞100°C).At a potential of 0.6 V,the current density of the Aquivion?E87-05S SSC-PFSA cell(0.524 A·cm-2)is 2.43 times higher than that of Nafion 212 LSC-PFSA cell at 140°C and 20%relative humidity(RH)[30].

Fig.2.The schematic principle of VRB.

The main route of developing new PEMs is the sulfonation of nonfluorinated aromatic polymers with low cost,high thermal stability,excellent chemical resistance,good mechanical properties,and low methanol/vanadium ion crossover[31].To date,a lot of sulfonated polymers have been used as the PEM materials,such as sulfonated poly(ether ether ketone)(SPEEK)[32-34],sulfonated poly(phthalazinone ether sulfone ketone)(SPPESK)[35],sulfonated poly(p-phenylene)(SPPE)[36-39],sulfonated poly(4-phenoxybenzoyl-1,4-phenylene)(SPPBP)[40],sulfonated polysulfone(SPSf),sulfonated poly(arylene ether sulfone)(SPAES)[41-43],sulfonated polybenzimidazole(SPBI)[44-46],sulfonated poly(aryl ether nitrile)(SPAEN)[47],sulfonated polyimide(SPI)[48-52],and sulfonated poly(2,6-dimethyl-1,4-phenylene oxide)(SPPO)[53,54].The conductivity of non- fluorinated PEMs is usually lower than thatofper- fluorinated ones,due to theirweak acidity and narrow proton transport channel(Fig.3)[55].The proton conductivity of non- fluorinated PEMs could be enhanced by increasing degree of sulfonation,but the swelling increases simultaneously.It means that there is a trade-off between high proton conductivity and good swelling resistance.For instance,SPPESK has excellent thermal stability(DS=28%-132%;Td:300-305°C;Tg:287-301°C).PEMs based on SPPESK exhibit methanol permeability at 15°C from 1.3×10-7to 9.7×10-8cm2·s-1depending on DS from 0 to 99%,which are reduced by a factor of 32-42 for that of Nafion115 membrane(4.2×10-6cm2·s-1)at the same conditions.The conductivity of SPPESK membranes increases with the degree of sulfonation and temperature increasing.When the DS is 91%,it can reach 3.0×10-2S·cm-1at 80°C,comparable to that of Nafion115(3.1×10-2S·cm-1)at the same conditions.However,above 50°C,the extreme swelling of SPPESK membrane would occur,resulting in the collapse of its solid morphology.In addition,the apparent activation energy of the conductivity of SPPESK 91%is 22.2 kJ·mol-1,much higher than that of Nafion115(10.8 kJ·mol-1)[35].

2.2.Anion exchange membranes

Fig.3.Schematic representation of the microstructures of Nafion and sulfonated polyetherketone[55].

All current commercial anion exchange membranes(AEMs)cannot match the requirement of AAEMFCs and VRB,due to their low conductivity and poor stability.Therefore the research and development of high-performance AEMs have become one of the most challenging works.In general,AEM materials can be synthesized by introducing ammonium[56],imidazolium[57-67]or phosphonium[18,68-71]functional groups onto high-performance engineering polymers,predominantly through nucleophilic-substitution reaction between halogenomethyl(typically,chloromethyl and bromomethyl)groups and tertiary-amine alkyl imidazole or tertiary-phosphine molecules[72].Since these nucleophilic-substitution reactions are very reactive,the introduction of halogenomethyl group onto polymer matrix has been considered as the most critical step[73].The halogenomethyl group could be introduced onto polymer matrix through three routes:(1)radiation-grafting chloromethyl-containing vinyl benzyl chloride(VBC)monomer onto fluorinated polymers[74-76],(2)the bromization of methyl groups in benzene rings of polymers,and(3)the direct chloromethylation of aromatic polymer matrixes.The bromization of methyl groups in benzene rings of polymers is a simple method to introduce bromomethyl groups,but it requires the polymers to possess methyl groups in their benzene rings.Poly(2,6-dimethyl-1,4-phenylene oxide)has been widely used as the matrix to prepare AEMs through this bromization route[64,77-81].The direct chloromethylation route is easier to conduct and more flexible to choose polymer matrixes than the other two routes.The chloromethylation reaction usually can be carried out by the reaction of polymers and chlromethyl alkyl ethers with chloride hydrocarbons as solvents and Lewis acids as catalysts,or concentrated sulfuric acid/methylsulfonic acid as solvents and concentrated sulfuric acid as catalyst[56,82,83].Based on the direct chloromethylation route and following quaternization,many high-performance polymers have been modified to prepare AEMs[77,84-87],including polysulfone(PSf)[20,62,68,88-90],poly(ether ether ketone)(PEEK)[56],polyethersulfone cardo(PES-C)[91],poly(phthalazinone ether sulfone ketone)(PPESK)[92],poly(phthalazinone ether ketone)(PPEK)[93],polyphenylsulfone(PPES)[94,95],polyetherketone cardo(PEK-C)[96],polyetherimide(PEI)[97,98],poly(phenylene)(PPE)[99],poly(arylene ether sulfone)(PAES)[100,101],poly(styrene ethylene butylene styrene)(PSEBS)[87,102],poly fluorenyl sulfone(PFES)[103],and so on.The polymer matrix provide many other important HEM properties,including but not limited to dimensional stability,thermal stability,and mechanical property.As shown in Fig.4,the imidazolium-functionalized PEEK(PEEK-ImOH)membrane shows lower swelling ratio than the imidazolium-functionalized PSf(PSf-ImOH)membrane with the same ion exchange capacity(IEC)[67].The difference in swelling behavior may arise from the inter-chain interactions in PEEK and PSf.Thus enhancing the van der Waals interactions among polymer chains could improve dimensional stability[70].

Anion conducting functional groups are responsible for the solubility,conductivity,and stability of AEMs[18,104].Most AEMs functionalized by the traditional ammonium groups have poor solubility,low conductivity,and terrible stability.Some novel functional groups,e.g.,quaternary phosphonium[18,68-71],guanidinium[95,105-107],and imidazolium[57-67,108-113],have been introduced to HEM materials to improve the solubility,conductivity,and/or stability.All the three functional groups can form the polymer which are soluble in low boiling-point solvents,facilitating processing to make membranes or incorporation into the catalyst layers in fuel cells.The effect of functional groups(thrimethylamine-constructed ammonium(QAOH)[56],tris(2,4,6-trimethoxyphenyl)-constructed phosphonium(QPOH)[71],and 1-imidazole-constructed imidazolium(ImOH)[67])onthe properties of PEEK-based AEMs were systematically investigated,and their chemical structures are shown in Fig.5.The order of water absorption capacity is QPOH＞QAOH＞ImOH,which supports that the order of functional group basicity is QPOH＞QAOH＞ImOH.As seen in Fig.6,the ion conductivity order of the membranes with the same IEC is PEEK-QPOH＞PEEK-QAOH＞PEEK-ImOH,which is consistent with that for water uptake.The sharp comparison in ion conductivity again demonstrates that the basicity of QPOH group is much higher than those of QAOH and ImOH groups.For instance,PEEK-QPOH membrane showed a high conductivity of 89 mS·cm-1at 60 °C,but it also underwent a large swelling ratio of 61%.All these results indicate that there is also a trade-off between high ion conductivity and good swelling resistance.

Fig.4.Swelling ratio(60°C)of PEEK-ImOH and PSf-ImOH membranes[67].

3.Macro/Micro-structure Control

Thougha lot of new non- fluorinated ICMs have been developed,it is difficult for most of them to be applied in practice because it is hard to maintain high conductivity and good dimension stability simultaneously.Increasing IEC could enhance the ion conductivity of the membranes.Unfortunately,much higher IEC results in extreme swelling of these membranes,even loss of dimension stability.It is because higher IEC would make hydrophilic channels easier to be continuous,which inevitably expands the overall hydrophilic domains,reduces the hydrophobic domains,weakens the interaction between polymer chains,increases the mobility of polymer chains,and eventually induces larger swelling.Limiting the mobility of polymer chains could control the micro-scale expansion and macro-scale swelling of the membranes with high IEC.Based on this strategy,some efficient techniques have been developed,including covalent crosslinking,semi-interpenatrating polymer network,and blending.

3.1.Covalent crosslinking

Covalent crosslinkinghas been indicated toresist the swelling of the membranes with high IEC,since covalent bond is much stronger than hydrogen bond,static interaction as well as van-der-waals interaction in general[114-117].For PEMs,the polyatomic alcohols are good crosslinkers,and the crosslinking bonds could be formed by the condensation between the sulfonic acid(-SO3H)and hydroxyl groups(-OH).Polymer alcohols have better crosslinking efficiency than small molecule alcohols.The crosslinked SPPESK membranes were prepared through covalent crosslinking by condensation of-SO3H in SPPESK with-OH in glycol,glycerol and poly(vinyl alcohol)(PVA)[118].The crosslinking reaction of-OH in PVA and-SO3H in SPPESK is illustrated in Fig.7.Compared to glycol-and glycerol-crosslinked membranes,PVA-crosslinked membrane has much stronger stability in water,and it could resist hot water(even 95°C)and retain fairly good morphology.A reasonable explanation is that long molecular chain not only can facilitate crosslinking(as mentioned above),but also can make much more SPPESK crosslinked to form a larger network so as to dramatically increase molecular bulk, finally considerably enhance the stability in water[118].All the swelling ratio of the membranes(SPEESK/PVA:85/15-65/35)are at a lower level(≤36%)even at elevated temperature(80°C).The conductivity of the membrane with SPEESK/PVA of 85/15 reaches 2.00 × 10-2S·cm-1,which is higher than that(1.72× 10-2S·cm-1)of unstable SPPESK DS 91%membrane(can not sustain too long in60°C water due to extremely swelling),even comparable to that(2.24 × 10-2S·cm-1)of Nafion115[118].

Fig.5.Chemical structures of QPOH-,QAOH-,and ImOH-functionalized PEEK[56,67,71].

Fig.6.Hydroxide conductivity(20°C)as a function of IEC for QPOH-,QAOH-,and ImOH-functionalized PEEKs[71].

For AEMs,there are four covalent bonds around centered N or P atom in the anion conducting groups of ammonium,imidazolium or phosphonium.Based on this structure,the di-amine,diimidazole or di-phosphine could be appropriate crosslinkers.Each crosslinking could create an anion conducting group.There is no sacrifice of the amount of anion conductive sites through crosslinking.The simultaneous quaternization and cross-linking of chloromethylated ploymers could be accomplished using 1,4-diazabicyclo[2,2,2]octane(DABCO)as the crosslinker[119].This crosslinking structure dramatically limited the excessive swelling of the membranes with high IEC.The crosslinked membranes have much higher[N+OH-]than non-crosslinked membranes[119].In addition,the halogenomethylated PPO could be highly efficient macromolecular cross-linkers due to its high Friedel-Crafts alkylation activity.The crosslinked PPO membrane(Fig.8)achieved high hydroxide conductivity of 120 mS·cm-1and outstanding dimensional stability at 90°C[120].The Friedel-Crafts alkylation reaction also could be used to accomplish the selfcrosslinking between the benzyl chloride and benzene ring with high electron donating groups,even at the moderate temperature[69].The thermal crosslinking of the unsaturated side chains during the membrane formation process is another efficient way to enhance the conductivity and swelling resistance[121].

3.2.Semi-interpenatrating polymer network(sIPN)

Fig.7.Crosslinking of SPPESK with PVA[118].

Fig.8.The structure of the crosslinked PPO[120].

Semi-interpenetrating polymer networks(sIPNs)are polymeric composites obtained by interpenetrating of a linear or branched polymer within a network of another cross-linked polymer.These mixtures of cross-linked polymers are held together by permanent entanglement and chain interlocking,which endow the sIPNs with forced compatibility and synergistic effect.It is indicated that the sIPNs have the ability to improve the dimensional stability,thermal and mechanical properties as well as reduce the phase separation of the membranes[122,123].The sIPNs also have the capacity of improving proton conductivity of PEMs at low relative humidity(RH).e.g.,crosslinked poly(styrene sulfonic acid)(CrPSSA)is incorporated into sulfonated poly(ether ether ketone)(SPEEK)to prepare SPEEK/CrPSSA sIPN membranes,as shown in Fig.9[124].The sIPN membrane with 40 wt%CrPSSA has a proton conductivity of 10-3S·cm-1at 25%RH,which is comparable to that of Nafion 115 and is 2 orders of magnitude higher than that of the pristine SPEEK membranes(10-5S·cm-1at RH 50%).The percolation threshold for proton conduction occurs at lower hydrophilic volume fractions with the increasing content of CrPSSA,suggesting different packing behaviors of-SO3H groups in the SPEEK/CrPSSA sIPN membranes as compared with SPEEK and Nafion membranes[124].

3.3.Blending

The polymer blending is another potential approach to restrict the swelling of the membranes[125].Polymer blend consists of both a hydrophilic sulfonated polymer to provide proton conductivity and a hydrophobic polymer to provide flexibility and dimension stability.Poly(vinylidene fluoride)(PVdF)has strong hydrophobicity,good mechanical strength,sufficient chemical stability,and high dimension stability.More important,it has the excellent compatibility to form blend pairs with various hydrophilic polymers containing oxygen atom.Based on all these good properties,PVdF was used to prepare the blend membranes with polymers containing functional group.These PVdF/functionalized polymer blend membranes exhibited enhanced dimension stability e.g.,the PVdF/SPPESK blend membranes have good dimension stability because the swelling ratios are at a fairly low level(8%-22%at 80°C).The conductivity of blend membrane with PVdF content of 10%reaches 3.6×10-2S·cm-1,which is even higher than that(3.4×10-2S·cm-1)of Nafion115 under the same test condition[126].Furthermore,the swelling of sulfonated polymer membranes could be limited by blending with the aminated polymers,because of the acid-base interactions between the sulfonic acid and amine groups[127].

4.Macro/Micro-structure Optimization

The optimization of macro/micro-structure could improve the performance of ICMs.An optimal design of macro-structure could enhance the mechanical properties of the membranes,while the optimization of micro-structure could construct more efficient ion transport channels,improving the ion conductivity of the membranes.Therefore some efficient methods have been developed to optimize the macro/microstructure of the membranes,including multiple functionalization,small molecule inducing micro-phase separation,electrospun nanofiber,organic-inorganic hybrid,and multilayer composite.

4.1.Long side chain

Fig.9.Synthetic schematics of SPEEK/CrPSSA sIPN[124].

Introducing a long side chain with head functional groups could increase the mobility of the functional groups,promoting their aggregation during the membrane formation.As a result,the hydrophilic/hydrophobic micro-phase separation structure in the membrane could be improved.A long flexible spacer was introduced onto PPO via the Suzuki-Miyaura coupling reaction as shown in Fig.10.The obtained ammonium functionalized PPO membrane showed enhanced hydroxide conductivity and suppressed water swelling as compared to the one prepared by the traditional bromination-functionalization method[128].In addition,grafting a hydrophobic long side chain also could enhance the hydrophilic/hydrophobic micro-phase separation structure in the membrane.As shown in Fig.11,elongating the hydrophobic side chain led to the gradual enhancement in hydrophilic/hydrophobic microphase separation.Whereas no microphase separation were observed in the pristine QAPS(Fig.11(c)),aQAPS-S8 showed the very clear aggregation of hydrophilic species(Fig.11(e)).In a QAPS-S14(Fig.11(f)),the assembly of hydrophilic species was extended,forming partitioned hydrophilic micro-domains[129].A side-chain-type sulfonated poly(ether ketone/ether benzimidazole)containing sidechain sulfonate groups and main-chain benzimidazole rings(SPEKEBI-2)were successfully synthesized via benzimidazolization and acylation in a one-pot method.The methanol fuel cell with SPEKEBI-2 membrane showed higher performance than Nafion117 membrane as shown in Fig.12[46].

4.2.Multiple functionalization

The multiple functionalization of polymers endows each pendent side chain with more functional groups clustered.Such multifunctionalized moieties could provide higher hydrophilicity and ionic interaction than the mono-functionalized ones.With the same amount of functional groups,the multi-functionalized polymer has much longer hydrophobic polymer segments between the hydrophilic pendent sites,which are also helpful in the aggregation of hydrophobic domains.Such multi-functionalized polymer facilitates the formation of good hydrophilic-hydrophobic micro-phase separation,resulting in an improvement in the efficiency of hydroxide ion conductive channels in the membranes[130-134].Poly(ether ether ketone)(PEEK)with two quaternary ammonium groups on pendent side chains was synthesized through the chloromethylationdiquaternization route,using bi-functional 1,4-diazabicyclo[2,2,2]octane(DABCO)as quaternization reagent.The di-quaternized membrane had bigger ionic clusters than the mono-quaternized membrane.The hydroxide conductivity is about 2 to 3 fold higher for the diquaternized membranes than for the mono-quaternized membranes with similar IEC[132].A tri-ammonium functionalized PPO was synthesized as shown in Fig.13.The highly flexible and hydrophilic multi-functionalized side chains make the functional group easy to aggregate together to facilitate the formation of well-defined hydrophilic-hydrophobic microphase separation in the membrane,leading to the superior hydroxide conductivity of 69 mS·cm-1at room temperature[135].A multi-functionalized copolymer,possessing rigid hydrophobic main chains and soft hydrophilic graft chains was synthesized by the ATRP graft polymerization of QVBC monomers onto the brominated PPO.The relatively long graft chains induced the hydrophobic/hydrophilic nanophase separation,which creates a convenient pathway for high hydroxide ion mobility[136].A comb-shaped copoly(arylene ether sulfone)s with multisulfonated PPO side chains(Fig.14)achieved high conductivity at low relative humidity[137].

4.3.Small molecule inducing micro-phase separation

Ion conductivity strongly depends on the morphology of ion conductive channels within the membranes.Connected,wide,and less branched hydrophilic channels greatly favor the ion conduction.It is found that some small molecules,including 3,4-dimethylbenzaldehyde(DMBA)[138,139],n-butyl alcohol[140],carbon tetrachloride[141],and tetrachloroethylene[142],can assist the self-organization of the functional groups during the membrane preparation,improve the micro-phase separation,and enhance the proton conductivity.These self-organization inducers are liquid molecules with small molecular weight and relatively low boiling point.They can be evacuated completely during high temperature annealing,resulting in pure polymer membranes.e.g.,modification of proton conductive channels(PCCs)in Nafion has been achieved with the assistance of 3,4-dimethylbenzaldehyde(DMBA)[138].TEM images of pristine Nafion and Nafion/DMBAmembranes with different annealing times are shown in Fig.15.During annealing,ionic clusters develop from small isolated spheres(1.72 nm)to wide continuous channels(5.15 nm),and the crystallinity of Nafion/DMBA membranes is also improved from 17%to 32%as shown by X-ray diffraction.Molecular dynamic simulation reveals that hydrogen bonding and hydrophobic interaction between DMBA and Nafion work synergistically to achieve better phase separation.

Fig.10.Synthesis of ammonium functionalized PPO with a long flexible spacer[128].

Fig.11.TEM images of ammonium functionalized PSf with a hydrophobic long side chain[129].

Fig.12.Polarization curves of methanol fuel cells with SPEKEBI-2 and Nafion 117 membranes[46].

The morphology-property relationship shows that,versus various PCCs width,the corresponding proton conductivities vary greatly from 0.079 to 0.139 S·cm-1at 80°C.By carefully tuning the width of PCCs,the proton conductivity displays an improvement of 22%-34%as compared with pristine Nafion.As shown in Fig.16,a significant enhancement on the maximum power density is achieved for the membrane electrode assembly on Nafion/DMBA-8 h(as high as 1018 mW·cm-2),yielding an enhancement of 39%on pristine Nafion-8 h(730 mW·cm-2)[138].

Ion conductivities of non- fluorinated ion exchange membranes are usually limited by weak phase separation and narrow hydrophilic channels.Smallmolecule inducing also can improve the micro-phase separation in the non- fluorinated membranes.A highly-conductive SPEEK membrane was prepared by n-BuOH inducing self-organization[140].Multiple hydrogen bonds between SPEEK and n-BuOH facilitate the clustering of hydrophilic domains,and enlarge the ionic clusters from 1 nm to 3 nm.SPEEK/n-BuOH membrane(IEC=1.5 mmol·g-1)exhibits a proton conductivity as high as0.314 S·cm-1at80°C,yielding increases of 161%and 88%on pristine SPEEK and Nafion 115 membranes,respectively[140].

4.4.Electrospun nanofiber

Fig.13.Synthetic routes of the tri-ammonium functionalized PPO[135].

Fig.14.Structure and simplified illustration of fully aromatic comb-shaped copolymers[137].

Electrospinning is proposed as an effective method to prepare threedimensional interconnected nanofiber networks,which favors the formation of long-range ionic conductive channels[143,144].It is reported that the proton conductivity can be enhanced by more than 10 folds by means of the single electrospun polyelectrolytic nanofiber,as compared with its bulk membrane[145,146].However,these electrospun membranes often show trade-off between proton conductivity and mechanical stability[147-150].The reasons are the reduced fraction of the polyelectrolytic components,and the limited interfacial compatibility between the dissimilar electrospun fibers and inter fiber voids filler matrix,like in the case of polymer blend systems.Electrospun SPPESK nanofiber enhanced PEMs were prepared by using SPPESK electrospun fiber mats,in which the inter fiber voids were also filled by SPPESK,in order to improve interfacial compatibility[151].Images of SPPESK electrospun fiber mat,electrospun PEM and cast PEM are shown in Fig.17.Fiberization promotes proton conductivity,swelling resistance as well as mechanical and thermal stabilities of SPPESK materials.Larger and ordered ionic clusters aggregated along the interfaces between SPPESK nanofibers and matrix were observed,resulting in as high as 1.3 times of proton conductivity as compared to the cast membranes.The strategy of electrospun fiber mats with higher IEC filled by those with lower IEC fibers contributes to the performances of electrospun SPPESK PEMs greatly.The IEC 2.01 fiber/IEC 1.72 filler electrospun membrane exhibits high proton conductivity(186.4 mS·cm-1)and controlled swelling ratio(30.0%)at 80°C[151].In addition,the electrospun nanofiber having a symmetrically pore- filled structure could be used as the porous substrate to reinforce the non- fluorinated composite membrane[144].Sulfonated PPO(SPPO)was mechanically reinforced by electrospun and crosslinked bromomethylated PPO(cBPPO).The SPPO membrane with a high water swelling of 90%was significantly suppressed to 20%due to the mechanical support of cBPPO porous substrate.As shown in Fig.18,the H2/O2fuel cells employing the composite membrane exhibited the greatly enhanced performances[152].

Fig.16.Polarization curves of MEAs based on pristine Nafion-8 h(27 mm)and Nafion/DMBA-8 h membrane(27 mm)with dry H2 and O2 at 80°C[138].

Fig.15.TEM images of pristine Nafion and Nafion/DMBA membranes with different annealing time.(a)Pristine Nafion-0 h.(b)Pristine Nafion-8 h.(c)Nafion/DMBA-0 h.(d)Nafion/DMBA-1 h.(e)Nafion/DMBA-2 h.(f)Nafion/DMBA-4 h.(g)Nafion/DMBA-8 h[138].

Fig.17.Images of SPPESK electrospun fiber mat,electrospun PEM and cast PEM(Optical photographs of electrospun PEM(b),cast PEM(c),electrospun fiber mat(d);SEM of electrospun SPPESK fiber mat with inserted histogram of the fiber diameter distribution(a),PEM surface with pore filler of about 25 wt%(e),PEM with pore filler of about 45 wt%surface(f),crosssection(g).[151].

Fig.18.Single cell performances of the Nafion?112 and SPPO/nanofiber composite membranes[152].

4.5.Organic-inorganic hybrid

The ion conductivity dramatically depends on the water uptake within the membranes.In current fuel cell systems,external humidifiers are required to endow the fuel and oxygen with 100%RH to keep the membranes hydrated.However,those humidifiers make the fuel cell equipment heavier,more complicated and expensive.Moreover,water management is hard for fuel cells.The fuel cells often suffer from nonuniform distribution of water in the membranes and even flooding of cathodes,which would reduce the performance of fuel cells.If the fuel cells could run in the low RH or non-humidified condition,the water management would be significantly simplified and the durability would be improved.Recently,organic-inorganic hybrid has been indicated to be a good way to enhance the ion conductivity of the membrane in the condition of low RH or non-humidification.Self-humidifying membranes have emerged as an intriguing design,where nanosized Pt catalysts embedded in the membrane catalyze the reaction of crossovered H2and O2to produce water for hydration of PEMs[153,154].In addition,the ion conductivity of the membrane could be enhanced by adding some water retentive inorganic oxides,such as SiO2[155,156]and graphene oxide[157,158].Introducing the sulfonic acid,phosphonic acid orcarboxylic acid onto these inorganic oxides could further improve the ion conductivity of the hybrid membranes[159-162].

4.6.Multilayer composite

Multilayer composite is an efficient way to improve the mechanical strength and the dimensional stability of the membranes[163-165].It could also decrease the content of perfluorosulfonic acid resin in the membrane,reducing the cost of the per- fluorinated ICMs[166].Polytetra fluoroethylene(PTFE)porous film has been the mostfrequently used composite material because of its very high mechanical strength[167-170].We prepared a Nafion/PTFE composite membrane by a solution-spray process.The composite membrane has higher stiffness and strength as well as lower swelling than that of Nafion211.The conductivity of 0.375 S·cm-1at 85°C is relatively high in comparison to that of 0.300 S·cm-1for Nafion211.The 20 kW stack with the composite membranes is evaluated.The performance of fuel cell stack with Nafion/PTFE composite membrane is shown in Fig.19.The mean single cell voltage is 0.67 V at 1000 mA·cm-2.The stack has behaved performance uniformity and steadily operated under low humidifying condition[166].

Fig.19.The performance of fuel cell stack with Nafion/PTFE composite membrane[166].

In addition,a design of an asymmetric structure comprised of a defect-free thin skin layer supported on a highly porous substructure could reduce the area resistance of the membrane and improve the voltage efficiency of VRB[171].An integrally thin skinned asymmetric anion exchange membrane with low IEC in the range of 0.72 to 1.01 mmol·g-1was designed as the separators for VFBs.The schematic cross-section of an integrally thin skinned asymmetric anion exchange membrane is shown in Fig.20.This design demonstrated higher stability,lower AR,and better overall electrochemical performance as shown in Fig.21[171].

5.Conclusions

Ion conductive membranes(ICMs)are key components for energy conversion and storage technologies of fuel cells, flow battery,and hydrogen pump.Firstly,this article reviews the recent studies on the development of new non- fluorinated ICMs and their macro/microstructure control.Non- fluorinated ICMs usually have lower conductivity than commercial per- fluorinated ones,due to their poor ion transport channels.Increasing IEC would create more continuous hydrophilic channels,thereby enhancing the conductivity.However,it inevitably expands the overall hydrophilic domains,increases the mobility of polymer chains,and eventually induces larger swelling.The micro-scale expansion and macro-scale swelling of the ICMs with high IEC could be controlled efficiently by limiting the mobility of polymer chains via covalent crosslinking,semi-interpenatrating polymer network,and blending.Secondly,this review introduces the optimization of macro/microstructure of both per- fluorinated and non- fluorinated ICMs for improving the performance.Macro-scale multilayer composite could efficiently promote the mechanical strength and the dimensional stability of the ICMs.Long side chain,multiple functionalization,small molecule inducing micro-phase separation,electrospun nanofiber,and organicinorganic hybrid could construct more efficient ion transport channels,thus enhancing the ion conductivity of ICMs.In the future studies,multi-scale design of ICMs in molecular,micro,and macro scales should play important roles in developing new high-performance ICMs.

Fig.20.Schematic cross-section of an integrally thin skinned asymmetric anion exchange membrane[171].

Fig.21.Electrochemical performance of VFBs employing different membranes as a function of charge/discharge current density[171].

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