Preparation And Properties Of N Heterocyclic Basic Anion Exchange Membranes For Fuel Cells

Fuel cells have become a research hotspot in recent years due to their low operating temperature,fast start-up,easy operation and maintenance.Fuel cells can be divided into two broad categories according to their working principles:proton exchange membrane fuel cells（PEMFCs）and anion exchange membrane fuel cells（AEMFCs）.The ion exchange membrane plays a role in separating the anode-cathode chambers,assists ion conduction,blocks fuel penetration,and is an indispensable part of the fuel cell.Proton exchange membrane（PEM）has the advantages of excellent electrochemical performance and strong alkali resistance stability,due to its early development and its technology is relatively mature.However,due to the shortcomings of the membrane matrix,which usually contain toxic substances such as fluorine and the use of expensive platinum metal as catalysts,the further development of PEMFCs is limited.Because AEM’s working environment is alkaline,it overcomes many shortcomings of PEMs,so its manufacturing cost is low,creating more opportunities for the spread of fuel cells worldwide.But the current research on AEM is still in the development stage.The problems of low conductivity and poor chemical stability still plague many scholars.Therefore,how to improve these problems becomes the key to improve the performance of AEM.In this paper,N heterocyclic materials with good chemical stability were selected as conductive groups.A variety of AEMs were prepared by copolymerization,doping,cross-linking,etc.,and their properties were tested,and then their practicability was evaluated.Experiment 1,due to the low conductivity and poor chemical stability of AEM,2,3-epoxypropyltrimethylammonium chloride was selected as the quaternizing agent,and polyvinyl alcohol（PVA）was modified to prepare QPVA.Methyl iodide（CH₃I）was selected as the ionization reagent,and N-vinylpyrrolidone（NVP）was ionized by a two-step method to obtain the ionic liquid（CVPnI）.The CVPnI and styrene（St）were subjected to polymerization in an aqueous solution of QPVA,and then a crosslinking agent glutaraldehyde（GA）was added to obtain an AEM having a semi-interpenetrating network structure（Semi-IPN）.The addition of St and GA alleviates the problem of faster degradation in an alkaline environment,and enhances the dimensional stability and mechanical properties of the composite membrane.The quaternization of PVA and the addition of ionic liquids increase the content of N⁺in the composite membrane,and improve the overall performance of the composite membrane.Experiment 2,Methyl iodide was used as an ionization reagent to ionically modify N-methylpyrrolidine,and a basic methylpyrrolidine ionic liquid（[DMPy]OH）was synthesized by a two-step method.Considering the problem that the ionic liquid doped into the membrane matrix PVA is easily lost in a high temperature and full humidity environment,resulting in a decrease in the overall performance of the AEM.Inorganic nano-titanium dioxide（Nano-TiO₂）is introduced into the membrane,interaction between Ti⁴⁺,O^2-on the surface of Nano-TiO₂ and[DMPy]OH is used to reduce the loss rate of ionic liquid in the membrane,increase the number of"active sites",accelerate the migration rate of OH^-,and increase the OH^-conductivity.In addition,the addition of inorganic Nano-TiO₂ effectively improves the alkali resistance and the overall performance of the composite membrane is also greatly enhanced.Experiment 3,a series of anion exchange membranes were prepared by using pyridine functionalized polyvinyl alcohol as the membrane matrix,1,4-dichlorobutane as the crosslinking agent and methyl iodide as the quaternizing reagent.When the amount of 1,4-dichlorobutane was 4.0%,the composite membrane had the maximum tensile strength（44.2 MPa）and the OH^-conductivity at 70°C was 1.05 x 10^-2 S cm^-1.The alkali resistance test showed that the OH^-conductivity of the composite membrane decreased by only 20.4%after immersion in 3M KOH solution for 120 h.In the experiment,1,4-dichlorobutane converts the pyridyl group into a quaternary ammonium group and simultaneously generate a crosslinking reaction,the content of quaternary ammonium groups in the membrane is ensured,so it has higher conductivity.The FT-IR results show that the PVA-PY-DLx composite membrane was successfully prepared.SEM showed that the cross-section and surface of the composite membrane had a smooth and dense structure.The data indicates that the introduction of the crosslinked structure improves the dimensional stability,mechanical properties and alkali stability of the composite membrane.In this paper,polyvinyl alcohol and chitosan were modified by copolymerization,blending and cross-linking to prepare various AEMs,and their physical and chemical properties were measured.The results show that a variety of modification methods have enhanced the comprehensive performance of the composite membrane,and have a positive effect on the industrial development of AEM.