Fabrication Of PVA-based Alkaline-exchange Polymer Membrane Electrolyte For Energy Devices Application

The excessive use of fossil fuels not only brings about irreversible energy crisis,but also leads to the increase of global averageCO₂ content and the increasingly severe greenhouse effect.In order to alleviate the energy crisis and improve the environment,electrochemical conversion and energy storage strategies have attracted extensive attention.For example,the electrochemical reduction technology of CO₂ can not only realize the resource utilization of CO₂ and effectively transform it into high value-added chemicals or fuels,but also make a great contribution to reducing the greenhouse effect.In addition,zinc-air battery,the energy conversion device,is considered to be the most valuable and potential high-performance,safe and environmental protection power generation technology in the 21st century.It not only has high power density and specific capacity,but also has certain stability and safety guarantee.In these energy storage devices or electrochemical devices,ion-exchange membrane electrolytes as the core components play an important role.In CO₂electrochemical reduction,alkaline-exchange polymer membrane electrolyte(AEMs)has higher formic acid selectivity and inhibits the occurrence of side reaction(hydrogen evolution reaction),so it has become a hotspot of current research.Among zn-air batteries,AEM stands out among many alkaline electrolytes because of its advantages such as small size,portable and foldable,high specific capacity,high energy density,safety and pollution-free.Therefore,the preparation and development of green,high performance,low cost AEM is particularly important.In this paper,a series of AEMs were prepared by using polyvinyl alcohol(PVA)as skeleton and different quaternary ammonium salts as charge carriers through mild and simple solution blending,physical cross-linking,chemical cross-linking and ion exchange processes.The effects of mass ratio of raw materials,physical crosslinking conditions,chemical crosslinking agents,crosslinking time,KOH concentration and doping effect of nano GO on alkaline membranes were explored and discussed.The physical and chemical characterization of different AEMs were compared and analyzed,and they were applied to various electrochemical devices to test and analyze the electrochemical performance and the application prospect and value.The main conclusions are as follows:(1)Infrared spectroscopy(FTIR)and scanning electron microscopy(SEM)showed that PQ44was successfully"captured"into the polymer framework of PVA,forming a three-dimensional porous semi-interpenetrating network structure.The conductivity of PVA/PQ44-OH^-membrane was 21.47 m S/cm at room temperature and it had excellent alkali resistance.We further prepared Cu/Sn bimetal cathode catalyst by electrodeposition with carbon cloth as substrate and,studied theCO₂ electrochemical reduction effect of PVA/PQ44-OH^-in different electrolyte systems.Both in0.5 M KHCO₃ and KOH electrolyte,the PVA/PQ44-3 membrane with the highest conductivity and unique microstructure shows the best electrochemical performance.Compared with the two commercial membranes,PVA/PQ44-3 has good practical value and application prospect.The comprehensive electrochemical performance is ranked as follows:PVA/PQ44-3>commercial Nafion117>commercial A201 membrane.When the electrolyte is KOH,a careful comparison shows that its current density is relatively higher,but its continuous electrolysis stability is worse and the efficiency of the product formic acid degrades faster,which seriously limits the long-term and stable application of this polymer membrane electrolyte in practical process.Comprehensive comparison shows that KHCO₃ solution is superior to KOH solution and has more extensive practical application value and potential.(2)PVA and PDDA polymers were selected as raw materials to prepare composite membranes with different nano GO doping amounts.The PVA/PDDA@GO-OH^-using binary crosslinking strategy has a high conductivity of 43.47 m S/cm and excellent flexibility,mainly because the crosslinking agent pyrro-2-formaldehyde(2-PCA)reacts with aldehyde group and hydroxyl group at one end,and the other end is still a free-moving pyrro-group,which not only enhances the flexibility of the membrane.Concurrently,the conductivity of the membrane is improved.Physical characterization SEM and FTIR verified that PDDA was successfully"coupled"to the polymer PVA/PDDA@GO-OH^-framework,forming a uniformly distributed three-dimensional semi-interporous network structure.The rich and active oxygen-containing functional groups of nano GO and the hydroxyl group of PVA produce strong hydrogen bond and van der Waals force,which promotes the formation of a large number of interconnected layered transport channels,which not only improves the conductivity of the composition,but also is beneficial to maintain the stability of the polymer membrane.(3)PVA/PDDA@GO-OH^-composite membrane was applied to study the electrochemical energy storage characteristics of CO₂ reduction and flexible zinc-air battery.Compared with the two commercial membranes,self-made alkaline membrane showed obvious advantages in Faraday efficiency and stability of the product formic acid.In addition,the self-made alkaline membrane and commercial A201 were applied to the flexible zinc-air battery for testing and comparison.2%GO membrane showed higher open circuit voltage(1.4V),power generation(140 m W cm^-2),constant current charge and discharge(8.4 h)and constant discharge(6.17 h)than A201.