Design And Fabrication Of Full-aliphatic-based Alkaline Anion Exchange Membrane For Energy Devices Application

The excessive use of fossil fuels has caused serious global warming and increasing energy crisis.It is extremely urgent to reduce CO₂ emissions and transform and utilize them through electrochemical processes while seeking for efficient new energy technologies.As the core component of CO₂ electrochemical reduction system,the membrane is a key to separate the anolyte and cathodlyte of the electrolyzer and mediates the flow of ions between the two electrodes while simultaneously curbing product crossover.Some studies found that different kinds of membranes had obviously different influences on product current efficiencies and their distributions in process of electroreduction of CO₂.Currently,two types of ion-conductive membranes used in the H-cell CO₂ electrolyzer,include hydroxide-conductive membranes(for example OH^-),cation-exchange membranes,such as Nafion^TM.Nafion membranes are quite commonly used in most electrochemical CO₂ reduction reaction process because of their high ionic conductivity and good mechanical strength.Unfortunately,their widespread application is currently hampered by their high production cost,because of difficulties in processing and synthesis,highly permeable to formate and preferred hydrogen evolution reaction.Therefore,the development of new membranes with high performance,simple preparation process and low cost is an important step to improve the electrochemical reduction of CO₂ to formic acid.Alkaline anion exchange membranes(AAEMs),which enable enhanced product formate selectivity and suppress hydrogen formation and formate permeability,are an ideal membrane material.However,the research of AAEMs for CO₂ electrochemical reduction to formate is just in its infancy.As a kind of new energy systems,supercapacitors(SCs),zinc-air batteries and fuel cells have been employed in electric vehicles etc.However,the majority of SCs and zinc-air batteries up to now have employed liquid electrolytes,which are accompanied by bulky and heavy configurations and inevitable safety issues such as leakage and fire etc,greatly limiting their applications in portable ones.Hence,one approach is to replacing liquid electrolytes with solid-state alternatives;the employment of solid polymer electrolyte can not only enhance security and lighten the devices,but also immensely reduce maintenance of system thanks to no need to deal with corrosive solutions.In addition,alkaline anion exchange membrane fuel cells(AAEMFCs)have attracted much attention due to their advantages such as the use of non-platinum catalysts and the effective inhibition of fuel penetration.As the core part of AAEMFCs,AAEMs is confronted with some prominent problems,such as the chemical stability of cationic groups in the structure,compared with proton exchange membrane,the low conductivity and the poor mechanical properties etc.Therefore,developing AAEMs with high performance,easy preparation and low cost is still a long way to go.In this work,the PVA/GG alkaline anion exchange membrane(PVA/GG-OH^-)was facilely fabiricated by mild solution blending,followed by heat treatment,chemical cross-linking and ion-exchange process.The effects of binary cross-linking strategy and different binary cross-linking agents on the physicochemical properties and microstructure of PVA/GG-OH^-membrane were investigated and their effects on the electrochemical reduction of CO₂ to formic acid were studied.The effects of PVA molecular weight,pyrrole-2-carboxaldehyde main cross-linking agent and GO doping on the microstructure,ionic conductivity,mechanical properties,chemical stability and dimensional stability of the membrane were investigated.As solid electrolyte and diaphragm,it is applied to all-solid supercapacitors,flexible rechargeable zinc air batteries and fuel cell to explore its practical application performance.The main contents and conclusions in this work are as follows:(1)The PVA/GG-OH^-membrane was facilely fabiricated by mild solution blending,followed by heat treatment,chemical cross-linking and ion-exchange process.For binary cross-linking process,4-Hydroxybenzaldehyde(HBA),7-Methoxy-3,7-dimethyloctanal(MDO),and 2-Thiophenecarboxaldehyde(TCA)were used as main cross-linking agents,respectively,and glutaraldehyde(GA)was used as an auxiliary cross-linker.Compared with the conventional unary cross-linking(with GA as the cross-linking agent only),the binary cross-linking membranes exhibited higher conductivity,which were obtained by forming the main chains using TCA,MDO and HBA as cross-linkers where only one end was cross-linked with PVA and the other end was free,which was easy to motion as the main chain in the polymer matrix,facilitating hydroxide ion transfer.Compared with MDO and HBA,the heterocycle in the structure of the main cross-linking agent TCA had the highest activity in the presence of S atoms,so the PGG-GT membrane(with GA and TCA as binary cross-linking agents)displayed the highest conductivity(0.105 S cm^-1,at room temperature),and also showed the optimal physical and chemical properties.Compared with heterocyclic ring TCA,benzene ring from aromatic compound HBA had larger steric hindrance,greatly limiting movement as the main chain in the polymer matrix,which was not conducive to the improvement of conductivity.The activity of aliphatic chain of MDO was between heterocyclic ring and benzene ring.As a result,the conductivity of PGG-GM(binary crosslinkers GA and MDO)and PGG-GH(binary crosslinkers GA and HBA)membranes were 0.033 and0.024 S cm^-1,respectively.In addition,due to the formation of dense and porous binary cross-linking network structure,the mechanical properties and dimensional stability of the membranes were enhanced,and the PVA/GG-OH^-membrane also showed excellent alkaline stability.The practical performance of the PGG-G,PGG-GH,PGG-GM,and PGG-GT membranes are demonstrated in the electrochemical reduction of CO₂to formate under ambient conditions.The PGG-GT membrane exhibits the highest Faradaic efficiency of formate(FE_HCOO-)at 71.5%over that of PGG-GM,PGG-GH,and PGG-G membranes,owing to its excellent physicochemical properties,and all of them are superior to the commercial Nafion 212membranes(57.6%).This is because the alkaline membrane system can effectively inhibit formate permeability and suppress by-product hydrogen formation.The PGG-GT membrane demonstrates superior stability with 61.7%of FE_HCOO-after 22h continuous electrolysis(Nafion 212 with 39.5%of FE_HCOO-).(2)PVA/GG-OH^-membranes with different molecular weights of PVA were prepared by using high(205000 g mol^-1),middle(146000-186000 g mol^-1)and low(89000-98000 g mol^-1)molecular weights of PVA as the substrate and TCA-GA as binary cross-linking agent.The microstructure,mechanical strength,thermal stability,crystallinity and conductivity etc of PVA/GG-OH^-membranes can be effectively controlled by changing the molecular weight of PVA.In LM_W-PVA/GG membrane(low-molecular weight PVA),a dense and porous three-dimensional semi-interpenetrating network structure was formed.With the increase of PVA molecular weight,the number of micropores gradually decreased,and the internal structure of the membrane became more compact,so the mechanical strength showed an increasing trend.PVA was a semi-crystalline polymer,the membrane based on PVA of high molecular weight had more intramolecular hydrogen bonds compared with the PVA of middle and low molecular weight,resulting in the crystallinity of the membrane increased with the molecular weight of PVA.Thanks to the structure of porous three-dimensional semi-permeable network and the expansion of amorphous region,LM_W-PVA/GG membrane exhibited the highest conductivity of 0.105 S cm^-1 at room temperature over that of MM_W-PVA/GG(0.101 S cm^-1)and HM_W-PVA/GG(0.056 S cm^-1).LM_W-PVA/GG,MM_W-PVA/GG and HM_W-PVA/GG membranes showed excellent thermal stability and dimensional stability with less than 2 of the anisotropic swelling degree.PVA/GG-OH^--based SCs with electrochemical stability window of 0-1.4 V has good rate capability and the specific capacitance was still nearly 70%of its initial value increasing of the current density from 0.5 to 10A g^-1.Compared with MM_W-PVA/GG and HM_W-PVA/GG membranes,LM_W-PVA/GG-based SCs exhibited the highest specific capacitance and energy density.Specific capacitance was retained about 85.1%of the initial value after 8000 cycles.The higher conductivity promoted the transfer of hydroxide ions and speeded up the reaction kinetics,thus improving the performance of the SCs.(3)In view of the high activity of heterocyclic compounds,pyrrole-2-carboxaldehyde(PCL)with higher activity was selected as the main cross-linking agent to further explore its influence on PVA/GG-OH^-membrane properties.PGG-GP membrane showed a high conductivity of 0.123 S cm^-1 at room temperature.This can be attributed to the synergistic effects of the following two aspects:PCL was a new and effective cross-linking agent and highly electronegative nitrogen atom increased the density of the electron cloud on their neighboring carbon atoms leading to the heterocyclic ring more active,which facilitated the chain segment motion,contributing to the ionic transport;through the effective binary cross-linking strategy,porous three-dimensional semi-permeable network structure and the pronounced microphase-separated structure as well as larger ionic clusters which was uniformly and continuously distributed in the hydrophobic region was constructed forming ion transport pathways,which was beneficial to the transport of hydroxide ions in the polymer electrolyte membranes.In addition,the PGG-GP membrane also exhibited excellent mechanical properties(tensile strength up to 90 MPa),thermal stability,dimensional stability,and high water uptake.Compared with commercial alkaline anion exchange membrane A201,PGG-GP membrane displayed superior hydroxide ion conductivity,high water content and low anisotropic swelling,which improves its electrochemical performance as solid electrolyte in all-solid supercapacitor and flexible rechargeable zinc air battery.The SCs employing the PGG-GP membrane exhibited outstanding specific capacitance values,good rate capability and cycling stability.PGG-GP-based SCs obtained a specific capacitances of 45.2 F g^-1,maximum energy density of 11.87 Wh kg^-1 at a current density of 0.5 A g^-1.Even in the face of a high current density of 10 A g^-1,it still has the ability to maintain 75%of its initial specific capacitances value and an energy density of 4.07 Wh kg^-1 with a high power density of 2.03 k W kg^-1.Simultaneously,the specific capacitance of the PGG-GP-based SCs was retained about 87.5%of the initial value after 8000 cycles.Zinc air battery employing the PGG-GP membrane exhibited higher specific capacitance,outstanding output power and cycling stability,and also show stable battery performance in flexible devices.Better output power density can be obtained under different bending angles,and discharge capacitance has almost no attenuation.(4)In order to meet the needs of AAEMs in fuel cells,which must be required to have ideal conductivity,mechanical properties,chemical stability and thermal stability.GO/PVA/GG composite membrane was prepared by solution blending with GO as filler and PCL-GA as binary cross-linking agents.The microstructure of the composite membrane can be effectively controlled by using GO as filler.GO was evenly dispersed in the PVA substrate,and a large number of interconnected lamellar transmission channels were created in the composite membrane through hydrogen bonding,which could promote the conduction of hydroxide ions in the parallel direction,providing more paths for the transmission of hydroxide ions,which will be beneficial to the improvement of the conductivity of the composite membrane.2%GO composite membrane displayed the highest conductivity(0.138 S cm^-1)at room temperature.However,with the increase of GO loading content,the strong van der Waals force generated between the nanolayers increased the overlap of GO nanoparticles in the limited space,leading to the dominant blocking effect,which was not conducive to the further improvement of the hydroxide ion conductivity.The filler GO had a mechanical strengthening effect on PVA/GG-OH^-membrane,which could generate a strong interaction with PVA through hydrogen bond,thus making the membrane structure more strong.When the GO doping amount was 3%,the composite membrane exhibited a maximum tensile strength of 130.6 MPa.The alkaline stability,oxidation stability and dimensional stability of the composite membrane were also improved significantly.The 2%GO composite membrane was immersed in 80 ^oC,8M KOH solution for 350h,and 0.128 S cm^-1of the conductivity was maintain at room temperature;when immersed in 30 wt%H₂O₂ solution for 350h,the weight retention rate of 2%GO composite membrane was still as high as 85.7%.The zinc-air battery utilizing the 2%GO composite membrane displays a long(14h)battery life and a low(about 0.9V)charge-discharge voltage difference.Moreover,a notably stable output power density could still be obtained at a bending angle of 120^o,displaying better stability performance.At the same time,the membrane electrode based on 2%GO composite membrane showed the peak power density 50 m W cm^-2 at room temperature.Such excellent battery performance was attributed to the 2%GO composite membrane having enhanced mechanical strength(114.3 MPa),high hydroxide ion conductivity(0.138 S cm^-1,at room temperature),outstanding chemical stability and dimensional stability.