堿性固體燃料電池堿性聚合物電解質(zhì)膜的最新研究進展

侯宏英

(昆明理工大學材料科學與工程學院,昆明 650093）

1 Introduction

The rapid development of fuel cells was triggered by the global fossil energy crisis,and many breakthroughs have been made.However,there still exist some challenges prior to wide com-mercialization.For example,successful application of proton exchange membrane such as Nafion?(Dupont）significantly promoted the development of proton exchange membrane fuel cell(PEMFC）.However,the coming commercialization of PEMFC was also postponed by some challenges such as high fuel permeability and high cost of Nafion?membrane,the heavy dependence on the expensive scarce Pt-based catalysts,and high susceptibility to CO-like species poison effect.1-4In the case of conventional alkaline fuel cell(AFC）,it has a successful and glorious history to land on the moon together with Apollo spacecraft in 1960s due to fast dynamics and low dependence on Pt-based catalyst,but alkali leakage and carbonation usually disable its long term operation.Therefore,the combination of the advantages of PEMFC and AFC naturally aroused the emergence of alkaline polymer electrolyte membrane fuel cell(APEMFC）.For example,APEMFC can possess the compact structure,avoid the leakage of liquid alkaline electrolyte,and inhibit the susceptibility to CO-like species poison effect.Meanwhile,it can also get rid of the heavy dependence on expensive scarce Pt-based catalyst,because more inexpensive non-Pt catalysts exhibited similar activity in alkaline electrode reactions.5,6It was reported that electrochemical reactions in both electrodes were more facile in the alkaline medium than in the acid medium.7-9Therefore,some cheap available non-platinum catalysts such as Pd,10-12Ru,13Au,14,15Ag,16,17Co,18,19Cu,20Ni,21MnO2,22even metal-free carbon nanotubes and graphene nanosheets23-26can exhibit satisfying catalytic activity in the alkaline electrochemical reactions.And also,the loading of catalyst can be as low as possible.27Additionally,the charge carrier in APEMFC is not the proton but the anion,which moves from the cathode to the anode,opposite to the moving direction of the proton in PEMFC.Fuel crossover due to electro-osmosis drag can be avoided accordingly,which can be confirmed by experiment results and theory calculation.28,29Low sensibility to CO-like species poisoning effect of the catalysts in alkaline medium also expanded the range of the fuels suitable for APEMFC,not only including H2and low carbon alcohols(methanol,ethanol,and glycerol）,but also other liquid fuels such as sodium borohydride,hydrazine,formate,and ammonia.30-35Therefore,the investigation about APEMFC has been a hot topic.As one of the key components,alkaline polymer electrolyte membrane played a vital role in maintaining the normal operation of APEMFC,and thus the availability of desirable alkaline polymer electrolyte membrane for APEMFC was very crucial and important.In principle,ideal alkaline polymer electrolyte membrane for APEMFC should possess high ionic conductivity,excellent thermal and mechanical stability,chemical inertness,easy availability as well as low cost.Unfortunately,there has not been available benchmark alkaline polymer electrolyte membrane for APEMFC yet,because no actual alkaline membrane material can meet all the requirements as mentioned above.Especially,high alkali-resistance of alkaline membrane is still a challenge,that is,functional quaternized group responsible for the ionic conductivity is easily subjected to the decomposition and degradation in alkaline medium.For this purpose,the intensive efforts to develop the alkaline polymer electrolyte membrane have been made.36-39Especially within recent three years,newer and better results were reported,such as metal-cation-based anion exchange membrane,40water management,41in situ durability,42-44electrode binder for triple-phase structure,45theory analysis46and so on.Herein,in this review,the state-of-the-art alkaline polymer electrolyte membranes for APEMFC mainly in recent three years were covered,and the next development prospects were also predicted.Alkaline polymer electrolyte membranes for APEMFC can be classified into three kinds:(i）quaternized anion exchange membrane;(ii）metal-cation-based anion exchange membrane;(iii）inorganic/organic hybrid alkaline membrane.These membranes will be introduced one by one in terms of synthesis strategy,accompanied with the structure-property relationship as well as the applications in APEMFC.

2 Anion exchange membrane

As a counterpart of cation exchange membrane,typical anion exchange membrane(AEM）usually contains cation-anion pairs:fixed positive group and movable anion,thus allowing for the passage of the anion but rejecting the cation.47-49In fact,it was developed prior to alkaline fuel cell and applied in many other fields such as salt electrodialysis,50anion selective electrode,51water treatment,52recovery of waste organic acids53and battery.54Only in recent years,increasing efforts have been made to study the application for APEMFC.Usually,a desirable membrane for APEMFC should possess a stable hydrophobic matrix for the structural mechanical integrity and some continuous hydrophilic channels for anion transport.55Among all the properties,enough ionic conductivity is the most vital to normal operation of fuel cell,because too poor ionic conductivity can result in too high inner resistance and disable the fuel cell.Generally speaking,there are three major strategies to improve the ionic conductivity of anion exchange membrane:(i）increasing the density of cationanion pairs,that is,ion-exchange capacity(IEC）of the polyelectrolyte;(ii）increasing the effective mobility of the charge carrier(OH-）;(iii）improving the alkali-resistance of functional groups for ionic conductivity.

The next is the summary of intensive efforts to pursue desirable anion exchange membranes with potential application for APEMFC.

2.1 Quaternized anion exchange membrane

Generally,quaternized anion exchange membrane(QAEM）is composed of three parts:(i）the polymer matrix;(ii）the fixed quaternized group with positive charge;(iii）the movable anion.,,andare three kinds of common positive quaternized groups.QAEM withgroups was the most common anion exchange ionomer,although it was reported that-based anion exchange membrane had higher thermal and chemical stability thanand.56Commercial QAEMs are available from several companies such as Tokuyama?in Japan,57-62Selemion?AMV in Japan,63Morgane?-ADP Solvay S.A.in Belgium,64,65Dupont?in American,66and Fumatech?in Germany,67although many of these materials are not optimized for alkaline fuel cell applications.

Most of the commercial QAEMs are based on crosslinked polystyrene or their blends with other inert polymers.For example,Tokuyama?anion exchange membrane consists of polystyrene backbone and side chains with terminated―groups,and it is the most popular commercial QAEM.Like in Nafion?membrane,the Schroeder′s paradox phenomenon was also found in Tokuyama?anion exchange membrane,and the water diffusivity of Tokuyama?anion exchange membrane showed same order of magnitude(10-10m2·s-1）as that of Nafion?membrane.68In early literature,the peak power density of alkaline direct alcohol fuel cell with Tokuyama?membrane was low.69,70Recently,new breakthrough in peak power density of 130 mW·cm-2at 80 °C was achieved with 5 mol·L-1KOH and 3 mol·L-1ethanol into the anode.71More surprisingly,in situ stability without significant decline of Pt-free alkaline ethanol fuel cell with this kind of membrane at 60°C can sustain for 520 h,as shown in Fig.1.72

Quantitative product analysis of ethanol oxidation in alkaline direct ethanol fuel cell with Tokuyama?anion exchange membrane was also performed.It was found that incomplete ethanol oxidation to acetate prevailed over complete oxidation to CO2.73But increasing the operation temperature from 60 to 100°C can improve the current efficiency of CO2from 6.0%to 30.6%.Additionally,considering that the susceptibility of alkaline medium to acid CO2,Suzuki et al.74evaluated the influence of CO2on APEMFC performances.It was confirmed that the anode was more vulnerable than the cathode when CO2was supplied to the anode and cathode,respectively,and high CO2concentration significantly reduced the single cell performance.This result indicated that it was important to remove the carbonate ion species in the triple-phase boundary in order to alleviate the electrode over-potential.Studies on water management showed that cathode flooding also occurred because the water diffusion flux from the anode to the cathode was surplus during the operation of APEMFC.75

Fig.1 In situ stability test for 520 h of alkaline direct ethanol fuel cell with Tokuyama?A201 membrane at a constant current density of 50 mA·cm-2 at 60 °C72

Morgane?-ADP membrane is a cross-linked fluorinated polymer with quaternary ammonium group,and it exhibits a higher resistance but a lower methanol diffusion coefficient compared to Nafion?membrane.At 60 °C,the peak power density of alkaline direct methanol fuel cell with this membrane was about 16 mW·cm-2,when the solution containing 2 mol·L-1methanol and 2 mol·L-1NaOH was fed into the anode.65

In order to pursue more desirable single cell performance of APEMFC,more quaternized anion exchange membranes were designed,optimized,and synthesized in the laboratory.76-84Typically,QAEMs were synthesized via chloromethylation of the pristine polymers and subsequent exposure to trimethylamine(TMA）to form benzyltrimethyl-type quaternary ammonium(QA）head-groups.85In principle,quaternized anion exchange membranes can be synthesized by simple one-step nucleophilic substitution reaction of an aminated polymer(or monomer）with a halogenated monomer(or polymer）,and then the resultant materials could be readily quaternized to form a potentially inexpensive QAEM if necessary.86,87Therefore,for functional starting materials with N-containing,Cl-containing,or Br-containing groups,the synthesis of QAEM seems very facile and simple.In the case of nonfunctional starting materials,the functionalizations such as cholormethylation,halogenation,or ammonification are indispensible.Considering the toxicity of chloromethyl methyl ether during the cholormethylation,quaternized anion exchange membrane can be synthesized by two methods:one is prepared with chloromethylether,and the other is prepared without chloromethylether.

2.1.1 Quaternized anion exchange membrane with chloromethylether

Generally,the preparation route with chloromethylether includes three steps:(i）the non-functionlized starting polymer is chloromethylaed;(ii）the resultant polymer is quaternized with trimthylamine,and thus quaternary ammonium groups are introduced into the polymer matrix;(iii）the membrane is dipped into KOH or NaOH solution for OH-form.Obviously,this synthesis strategy belongs to post-functionalization,in which some available thermoplastic engineering plastics(such as poly(ether ether ketone）,poly(ether sulfone）,polyimide and so on）or copolymers from monomers can be the starting polymer materials.88-96During the chloromethylation of the polymer,the most common and classical chloromethylating agents are chloromethyl methyl ether(CMME）and bis-chloromethyl ether(BCME）,which can allow for high conversions and yields.For example,a quaternary ammonium polymer was synthesized by chloromethylation of commercial polysulfone,followed by amination process,as shown in Fig.2.97The ionic conductivity of the as-synthesized anion exchange membrane was 0.032 S·cm-1in deionized water at 24 °C,and the alkali-resistance of the polymer in 8.0 mol·L-1KOH solution was also satisfying.

Fig.2 Preparation route of quaternized commercial polysulfone with chloromethylether97

Novel anion conductive multi-block copolymers with sequential hydrophobic/hydrophilic structure were synthesized via polycondensation,chloromethylation,quaternization,and final alkalization.98-100Compared with the first method in Fig.2,such a method can offer more choices to design and optimize the structure-property relationship for desirable anion exchange membrane,such as excellent nano-phase separation morphology.Fig.3 showed the synthesis route of anion-conductive multiblock copoly(arylene ether sulfone）s with different chain lengths,hydrophilia and hydrophobicity.101The newest satisfying result was that H2-fueled APEMFC with crosslinked quaternized polysulfone membrane via this method can output the peak power density of 342 mW·cm-2at 70 °C.102

2.1.2 Quaternized anion exchange membrane without chloromethylether

Although the route with chloromethylether as mentioned above is efficient and typical,unfortunately,chloromethyl methyl ether and bis-chloromethyl ether used in most cases are well known to be carcinogenic,and thus have been restricted since 1970s.Therefore,more friendly approaches towards their avoidance have been developed.Firstly,some safer alternative chloromethylating agents have been investigated.It was reported that high reactive long chain chloro methyl octyl ether(CMOE）,103-106inexpensive 1,4-bis(chloromethoxy）butane,107,108paraformaldehyde and chlo-rotrimethylsilane109-111were applied to perform the chloromethylation.For example,Han et al.112recently achieved chloromethylation of poly(ether ether ketone）(PEEK）by using sulfonated PEEK and chlorotrimethylsilane as the starting polymer and chloromethylating agents,respectively,in which―SO3H groups of sulfonated PEEK can be partially substituted by quaternary group.The ionic crosslinking between remaining―SO3H groups and quaternized groups occurred and resulted in an outstanding alkali-resistance to 1 mol·L-1KOH solution at 80 °C for 30 days.

Fig.3 Synthetic route of quaternized multiblock poly(arylene ether sulfone）with chloromethylether101

Beside the strategy of using alternative chloromethylating agents,chloromethylation can also be achieved by directly radiation-grafting vinylbenzyl chloride(VBC）onto the polymer matrix113or by bottom-up poly-condensation of functional VBC and other monomers.114The resultant membrane can be quaternized with tertiary ammonium and ion-exchanged with KOH.In the case of the radiation-grafting method,some available fluorine-containing polymer films such as poly(vinylidenefluoride）(PVDF）115,poly(ethylene-co-tetrafluoro-ethylene）(ETFE）,116,117poly(tetrafluoro-ethene-co-hexafluoro-propylene）(FEP）,118and poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether）(PFA）119,120membranes were pre-irradiated via plasma bombardment,electron-beam,or γ-ray source in order to induce the formation of the radicals.Then vinylbenzyl chloride was grafted onto these preactivated polymer films,followed by quaternization with amine and alkalization.For example,ETFE-based membrane exhibited the ionic conductivity of 20-40 mS·cm-1at room temperature,moreover,the ionic conductivity did not significantly degrade during the storage for 16 months.121When this kind of membrane was used as anion exchange membrane for H2/O2alkaline fuel cell,the output peak power density was as high as 230 mW·cm-2.122Recently,superior peak power densities of 823 mW·cm-2at 60 °C,718 mW·cm-2at 50 °C,and 648 mW·cm-2at 20 °C forAPEMFC were also achieved as shown in Fig.4.123

Fig.4 Peak power densities of APEMFCs with the radiationgrafting membranes(5,19,and 23 μm）in the temperature range from 20 to 60°C under air and oxygen123

Compared to acid Nafion?membrane,these anion exchange membranes exhibited lower fuel permeability,solvent uptakes,and swelling degrees in methanol,ethanol,and ethylene glycol,thus allowing for the use of thinner membrane with lower ionic resistance and offsetting the intrinsically lower OH-conductivity accordingly.In addition,Ko et al.124compared the effects of the base films on the properties of QAEM.It was found that partially fluorinated films such as ETFE and PVDF exhibited better radiation-resistance than fully fluorinated films,because the radiation-inducing radicals were produced mainly via C―H fission in the partially fluorinated films,while it was formed by undesirable C―C fission in the fully fluorinated films.For example,it was reported that polytetrafluoroethylene(PTFE）exclusively underwent main chain fission because C―C bonds are weaker than C―F bonds and break prior to C―F bonds.125Additionally,high cost is a concern of this kind of materials too.It seems that fluorinated polymers may be less suitable polymer substrates for radiation-induced grafting.Therefore,other alternative substrate polymers were recently used for radiation-grafting QAEM.126-129For example,polyethylene-based radiation-grafting QAEM was synthesized,and the obtained QAEM exhibited a maximum ionic conductivity of 47.5 mS·cm-1at 90 °C,while the order of methanol permeability was as low as 10-8cm2·s-1,significantly lower than that of Nafion?membrane,implying potential feasibility for APEMFC.

Apart from radiation-grafting technology,VBC can be also introduced into the polymer matrix by direct bottom-up polymerization.VBC is an excellent monomer for synthesis of QAEMs via polymerization because of its double functional groups:vinyl group for polymerization and benzyl chloride group for quaternization.130-134Based on this conception,Xu et al.135synthesized the copolymer from the monomers of VBC,methyl methacrylate(MMA）and ethyl acrylate(EA）by free radical polymerization,as shown in Fig.5.By this method,some desirable properties of the as-synthesized QAEM,such as ionic conductivity,ion exchange capacity,mechanical properties,and water uptake,can be easily achieved by tuning the stoichiometric ratio of different monomers.Sometimes,macromolecular crosslinker or PTFE can be used to further improve the mechanical and thermal stability of PVBC.136In addition,considering that a large excess of the solvent used in polymerization and membrane formation may bring about the toxic risk to the environment,it is desirable to develop simple friendly methods for production of quaternized anion exchange membrane.Herein,Wu et al.137,138developed a friendly solvent-free synthesis strategy.In detail,the polymerization started in liquid monomers instead of additional solvents,followed by in situ polymerization and then quaternization for preparation of QAEM.

Fig.5 Direct bottom-up copolymerization of VBC and other monomers for quaternized anion exchange membrane135 AIBN:azobisisobutronitrile

Fig.6 Synthesis of poly(amimCl-MMA）membrane in OH-form:co-polymerization and alkalization154

The fourth preparing route without chloromethylether can be actualized via some available nitrogen-containing,halogen-containing,or oxygen-containing starting materials such as 4-vinylpyridine,halide,chitosan,poly(vinyl alcohol）and so on.139In the case of nitrogen-containing starting materials,the quaternization can be easily achieved by direct SN2 nucleophilic substitution reaction with halohydrocarbon.140,141For example,Wan et al.142synthesized a series of quaternized chitosan derivatives(QCDs）with glycidyltrimethylammonium chloride as a main quaternized agent.After tuning the degree of crosslinking and quaternization,the as-synthesized quaternary chitosan membranes showed controllable crystallinity,swelling behavior,ionic conductivity,and thermal stability,indicating potential applications for APEMFC.Likewise,the bottom-up copolymerization of N-containing monomer and other monomers provided an alternative synthesis route.143,144The resonance effect of the guanidine moiety or imidazole ring can reduce the charge density of the cation,weaken the hydroxide attack to the cation,and improve the alkali-resistance of QAEM.145-148For example,Zhang et al.149designed and prepared a pore-filling membrane via polymerization of 3-methacryloylamino propyl trimethylammonium chloride in a porous substrate.The water movement of APEMFC with this membrane was accurately observed by a water collection method.It was experimentally confirmed for the first time that the water moved from the anode to the cathode,opposite to the moving direction of the proton in acid polymer electrolyte membrane fuel cell.The model calculation also showed that the Ohm resistance of the membrane can reduce as much as possible when the relative humidity was high enough by desirable water movement.149

Ionic liquids,a new class of benign organic electrolytes,have remarkable physicochemical properties,such as ideal conductivity,wide electrochemical window,low volatility,and excellent thermal and chemical stability.Imidazolium-type ionic liquids have been used as the monomers for QAEM preparation due to their high ionic conductivity,stability and solubility.150-153As shown in Fig.6,novel anion exchange membranes were prepared by copolymerization of imidazolium-type ionic liquids and alkyl acrylates.154The hydroxyl ionic conductivity of the as-synthesized membrane can reach as high as 3.33×10-2S·cm-1in deionized water at 30°C.The ex situ stability tests showed that the onset of mass loss was above 200°C,and the membranes still maintained good ionic conductivity in 6 mol·L-1NaOH solution at 80 °C for 120 h.Such membranes exhibited superior thermal stability and alkali resistance than alkyl quaternary ammonium polymers.

Halogen-containing starting polymer can be directly quaternized in an aqueous trimethylamine solution for some time,and then final quaternary ammonium anion exchange membrane was obtained via ion-exchanging with KOH solution.155Additionally,nonfunctional polymer plastics such as poly(ether ether ketone）(PEEK）,polyether sulfone(PES）,and polyphene oxide(PPO）or other copolymers from the monomers can also be used as the starting materials,but it is necessary to perform the halogenations in advance,that is,bromination or chlorination in most cases.156-164By this method,comb-shaped PPO-based QAEM was designed and prepared,and the resultant membrane can maintain the initial ionic conductivity in 1 mol·L-1NaOH solution for as long as 2000 h at 80°C.165A novel PES-based QAEM was also successfully prepared,in which bromination reaction was carried out for preparing the target membranes.The corresponding ionic conductivity at 30 and 80 °C in deionized water was 6.00×10-2and 13.00×10-2S·cm-1,respectively,while its methanol permeability was as low as 1.02×10-9cm2·s-1at 30 °C.166Considering that the flexibility of pendant side chain can promote better micro-phase separation,higher conductivity,and dimensional stability of the conductive polymer,Shen et al.167designed and prepared poly(aryl ether ketone）with methyl groups on pendant phenyls,in which N-bromosuccinimide(NBS）was used for the bromination of the methyl groups on pendant phenyls.The ionic conductivity of the final polymer was 0.024 S·cm-1at 20 °C and 0.0307 S·cm-1at 80°C in deionized water,respectively.Likewise,Lin et al.168,169synthesized PPO-based novel anion exchange membranes containing pendant guanidinium or benzimidazolium groups.The resultant QAEMs exhibited high ionic conductivity,excellent thermal stability and alkali-resistance attributed to the p electron conjugated system of the pendant guanidinium or benzimidazolium head-groups.

In the case of oxygen-containing starting materials,poly(vinyl alcohol）(PVA）is a kind of common starting material due to its outstanding features:low cost,good film-forming ability,chemical stability,and availability of crosslinking sites.In this case,QAEM can be synthesized by open-ring polycondensation of PVAand bifunctional compounds with epoxy and quaternized groups,in which epoxy group was responsible for combination with PVA main chain,while quaternized group was responsible for the ionic conductivity.As displayed in Fig.7,Ye et al.170quaternized PVA(QPVA）by directly grafting 4-methyl-4-glycidylmorpholinium chloride onto PVA.After crosslinked with glutaraldehyde,the resultant quaternized PVA membrane exhibited high ionic conductivity(7.34×10-3S·cm-1）and low methanol permeability.In addition,in order to further improve the transport property and durable performance of PVA-based QAME,quaternized SiO2was added into the QPVAmatrix.171,172

Fig.7 Synthetic reaction route for the preparation of QPVA170

As mentioned above,intensive efforts were made to investigate quaternary anion exchange membranes for APEMFC application,and many significant progresses were made.For example,the highest power density of H2-fueled APEMFC with irradiationgrafting QAEM was as high as 823 mW·cm-2at 60 °C;and the in situ durability for 520 h of commercial Tokuyama?membrane was also achieved.However,up to now,there have not been wide and deep in situ stability tests for APEMFC yet,although a large quantity of new QAEMs was designed and synthesized.Awidely quoted concern with quaternary anion exchange membranes is the stability in the alkaline environment,that is,alkali-resistance,especially at elevated temperatures.173The recognized degradation mechanisms of quaternized groups include:(i）Hofmann elimination;(ii）nucleophilic substitution;(iii）Ylide formation.Hoffman elimination reaction usually results in the loss of quaternized group,wherein hydroxyl ion attacks β-hydrogen to the cation forming a double bond between α-carbon and β-carbon.Density function theory(DFT）calculation also suggested that Hofmann elimination was the most vulnerable pathway to degradation.174Direct nucleophilic displacement occurs via the attack to α-carbon atom of the cation,which can either detach the cation groups from QAEM or convert quaternary ammonium groups into tertiary amines.Degradation through the Ylide pathway begins with hydroxyl ion′s attack to a methyl group of the cations and produces a water molecule along with an Ylide intermediate.

In order to obtain QAEM with high stability,some strategies were applied:(i）some conjugated head-groups including guanidinium and imidazolium are introduced into QAEM in order to weaken the hydroxide attack to the cation and improve the alkaliresistance,as mentioned above;175(ii）crosslinking among polymer chains for stronger mechanical property.176,177In the case of the second strategy,some reinforced inorganic or organic fillers such as TiO2,178SiO2,179,180ZrO2,181crosslinked polyvinyl alcohol(PVA）182-184or PTFE185-189were also added into the matrix of QAEM for high mechanical stability.For example,Zhao et al.189synthesized polyvinyl benzyl chloride membrane,which was crosslinked with diethylamine and reinforced by PTFE matrix.The obtained target membrane exhibited satisfying ex situ ionic conductivity stability and in situ durability for about 230 h with mild degradation,as shown in Fig.8.

Additionally,the possible reason resulting in low in situ durability may be also due to the absence of anion exchange ionomer binder for triple-phase boundary in the electrode,which is similar to Nafion?solution in proton exchange membrane fuel cell.As shown in Fig.9,alkaline polymer electrode binder can not only bond the catalyst particles together but also facilitate the ionic transport by building triple-phase boundary.190Therefore,the development of alkaline polymer bond should be vital to assemble metal-cation-free alkaline membrane electrode assembly with long-term stability.191However,for the moment,although there have existed different types of QAEMs in literature as discussed above,the corresponding suitable catalyst binder for APEMFC is not easily available yet.In early literature,192PTFE or Nafion?ionomers were usually used as a temporary strategy to deal with this challenge.For example,Qiao et al.192assembled metal-cationfree APEMFC still with Nafion?solution as electrode binder for constructing triple phase interface of electrochemical reaction,and the peak power density was only 32.7 mW·cm-2at 25 °C.No doubt,this value would be better if suitable alkaline binder counterpart was used.As expected,the latest results showed that all-solid-state H2/O2APEMFC can output the peak power density of 823 mW·cm-2at 60 °C with poly(vinylbenzyl chloride）as electrode binder,which can rival with that of proton exchange membrane fuel cell.123Additionally,layered double hydroxides,aminosilane-based and chloride conductive layer can also be in-corporated into the electrode structure to improve the triple-phase boundary.193-195

Fig.8 (a）Ionic conductivity stability test,(b）voltage-current density polarization curves,and(c）in situ stability test of alkaline H2/O2fuel cell with crosslinked PTFE-reinforced polyvinyl benzyl chloride membrane189

2.2 Metal-cation-based anion exchange membrane

Beside traditional quaternized anion exchange membrane,anion exchange membrane can also be synthesized via the chelation of metal cations with N-containing or O-containing materials,in which metal cations such as Ru2+and K+act as fixed positive groups,and anions acts as movable charge carriers.Based on this idea,novel metal-cation-based anion exchange membrane containing ruthenium complex was designed and synthesized recently.As shown in Fig.10,40each Ru2+can combine with six nitrogen atoms containing lone electron pairs to form ruthenium complex,and such a complex can have two associated counter anions,different from most ammonium-based and phosphoniumbased anion exchange membranes only with single cation-anion pairs.Such a new metal-cation-based anion exchange membrane exhibited higher anion conductivity,mechanical properties,alkaliresistance as well as methanol tolerance than traditional quaternary anion exchange membrane.40

Fig.9 Schematic for the architecture of ionomer-bindercontaining membrane electrode assembly190 MEA:membrane&electrode assembly

Crown ethers are ring-shaped heterocyclic compounds containing several ether groups,and their repeating unit is ethyleneoxy group(―CH2CH2O―）.Important members of this series are the tetramer(n=4）,the pentamer(n=5）,and the hexamer(n=6）.Crown ethers can strongly combine with certain cations to form complexes.When metal atoms such as sodium or potassium pass through the center of the ring,they can attach themselves to the exposed oxygen atoms like a key in a lock.The denticity of the crown ether influences its affinity for various metal cations.For example,18-crown-6 ether has high affinity for K+,15-crown-5 ether for Na+,and 12-crown-4 ether for Li+,respectively.196The microstructure and ball-and-stick model of 18-crown-6 ether coordinating a potassium ion are shown in Fig.11.Based on this property of crown ether,metal-cation-based anion exchange membrane can be synthesized by chemically grafting pendant 18-crown-6 ether onto the polymer backbone,followed by combination of KOH.197Similar to ruthenium complex,metal cation K+can be fixed into the polymer matrix by chelating with 6 oxygen atoms,while OH-is unsolvated or naked and thus acts as the movable charge carrier.The obtained membrane displayed higher thermal stability,alkali-resistance,and ionic conductivity than quaternized anion exchange membrane,implying a potential application in APEMFC.

Fig.10 Synthesis of metal-cation-based anion exchange membrane40

polybenzimidazole(PBI）membrane is well known to possess excellent endurance both in alkaline medium and at high temperature.198,199It is a weak basic polymer due to two imidazole rings in one repeating unit of PBI matrix.Inherently,PBI is an electronic and ionic insulator,but it can become an ionic conductor by being functionalized with the acids200and inorganic hydroxides,201,202apart from quaternization with halogenated hydrocarbons as mentioned above.Compared with acid-doped PBI membrane,there are fewer literature about alkali-doped PBI membrane for alkaline fuel cell.The first investigation about alkali doped PBI membrane for alkaline H2/O2fuel cell was reported in 2000.203Recently,the application of PBI/KOH membrane for alkaline direct ethanol fuel cell was carried out and the satisfying results were achieved in our laboratory.204,205The obtained membrane exhibited excellent thermal stability as well as in situ durability for 336 h in air-breathing Pt-free alkaline direct ethanol fuel cell at 60°C(Fig.12）.206When this kind of membrane was used in alkaline direct ethylene glycol fuel cell,the corresponding peak power density can reach as high as 80 mW·cm-2at 60 °C and 112 mW·cm-2at 90 °C,respectively.The power output at 60°C was 67%higher than that of acid direct ethylene glycol fuel cell,mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline medium.207The interaction between KOH and PBI matrix was also explored,and scanning electron microscope(SEM）and energy dispersive X-ray spectrometer(EDX）results showed that the concentration of K element within PBI/KOH membrane was much higher than that of O element.Possible reason may be due to double resources of K element:(i）K+combined with―NH―in PBI matrix,meanwhile,H2O was produced and left as a result of a neutralization between H in―NH―and OH-in the medium(Fig.13）,206(ii）K+chelated with pyridine―N in PBI membrane by complexation with OH-as movable charger carrier,thus endowing the membrane with the ionic conductivity.

Fig.11 Microstructure and ball-and-stick model of 18-crown-6 ether coordinating a potassium ion

2.3 Organic/inorganic hybrid alkaline membrane

Fig.12 Lifetime of air breathing Pt-free alkaline direct ethanol fuel cell with PBI/KOH at 60°C 206

Fig.13 Bond lengths and bond energies of N―K(a）and N―H(b）calculated by density functional theory206

Recently,organic/inorganic hybrid anion exchange membrane have gained much interest because suitable combination of two components can endow the composite with some remarkable changes in mechanical,thermal,and electrical properties compared to single material especially for those via molecular-level integration.Generally,organic polymers offer structural flexibility,convenient processing,and good film-forming ability,while inorganic phase is responsible for crystalline,high carrier mobility,porous structure for mass transport,thermal and mechanical stabilities.In the case of application for APEMFC,polyvinyl alcohol and chitosan usually act as the organic polymer phase,while KOH is the second inorganic phase.208-211Sometimes,in order to further optimize mechanical property and the porous structure for better transport properties,the third inorganic phase such as SiO2,212carbon nanotubes,213layered double hydroxide,214graphene nanosheets,or organic polymer215-217etc.was also introduced into the composite membrane.For example,hybrid membrane containing graphene nanosheets and poly(vinyl alcohol）(PVA）was designed and prepared for alkaline direct methanol fuel cell,and the transport property of hybrid membrane was explored.The results suggested that graphene nanosheets were uniformly dispersed,resulting in continuous,well-connected and tortuous ion transport channels.The addition of only 0.7%graphene nanosheets into PVAmatrix brought about the increase by 126%in ionic conductivity and 55%reduction in methanol permeability,while the loading of 1.4%graphene increased by 73%the tensile strength.For special consideration into high ionic conductivity,the further quaternization of the third phase was also performed.218Chitosan is a main derivative of chitin and it can be mainly obtained by deacetylating chitin with an alkaline treatment.Although it is a weak-alkaline polymer,its ionic conductivity can be close to 0.001 S·cm-1after full hydration,which is still not high enough for fuel cell.Therefore,chitosan/KOH hybrid membrane for alkaline fuel cell was preliminarily prepared and evaluated.219,220The obtained membrane had a three-layer structure,consisting of a porous intermediate layer and two crosslinked solid surface layers.After being hydrated for 1 h at room temperature,this membrane showed ionic conductivity near 0.01 S·cm-2.

3 Conclusions and prospect

Significant progresses about alkaline polymer electrolyte membrane for APEMFC have been made:up to now,the highest peak power density of H2/O2APEMFC with irradiation-grafting QAEM was as high as 823 mW·cm-2at 60 °C,and in situ durability tests for 520 h of commercial Tokuyama?membrane were achieved too.Among various alkaline polymer electrolytes,quaternized anion exchange membrane was still the most promising membrane for APEMFC,although the thermal stability and alkali-resistance need be further enhanced;metal-cation-based anion exchange membrane and organic/inorganic hybrid membrane may be fit for high temperature APEMFC due to their high stability.221

On the other hand,there still existed some challenges to deal with:(i）soluble anion exchange ionomer electrode binder similar to commercial Nafion?soultion for building triple-phase interface may be one of the effective solutions to achieve better performances of metal-cation-free APEMFC,however,the corresponding investigation need be intensively carried out because there still is not authoritative alkaline counterpart to Nafion?solution for the moment;(ii）although many synthesis strategies of various alkaline polymer membranes were deep and widely investigated,however,there is still no alkaline benchmark counterpart to Nafion?membrane for PEMFC.Additionally,the corresponding in situ durability tests for APEMFC are still scarce and not very satisfying,therefore,the corresponding investigation will be a continuous hot topic in the future.After all,in situ durability is final standard to judge whether an alkaline polymer membrane is suitable for fuel cell application.

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