Manipulation Of Nanofibrous Composite Membranes Microstructure And Intensification Of Through-plane Proton Conduction

Proton exchange membrane fuel cell?PEMFC?,as a representative of new clean energy technologies,can directly and efficiently convert the chemical energy of fuel into electrical energy.Proton exchange membrane?PEM?is the core component of PEMFC,protons need to pass through the PEM from the anode to the cathode to complete the energy conversion.The nanofiber prepared by the electrospinning technology has the characteristics of large specific surface area,high porosity,and nanoscale diameter and is considered to be a good proton conductor.Meanwhile,in order to meet the requirement of low fuel permeability,dense nanofiber composite membrane?NFCM?can be prepared by filling the gaps among nanofibers with polymer matrix.However,these NFCMs confront serious proton transfer anisotropy,i.e.,the through-plane conductivity is generally much lower than that of in-plane conductivity,due to the preferential horizon-direction alignment of nanofibers.While,it is the through-plane proton conductivity that,determines the fuel cell performance in practical application.Therefore,intensifying the through-plane proton conductivity is the key to the preparation and application of high-performance NFCM.This paper focused on the construction of efficient proton transfer channels and optimization of proton transfer sites in the vertical direction of the membrane.First,through the preparation of porous nanofibers,nanopores were formed on the surface and inside of the nanofibers and the proton-conducting groups inside nanofiber were induced to enrich along pore walls.And thus formed low-energy-barrier transfer channel in the vertical direction of the membrane;In addition,conductive inorganic nanofillers were uniformly dispersed in the nanofiber to prepare hybrid nanofiber.In such way,the number of transfer sites inside the nanofiber were increased and the vertical direction“transfer dead zone”was connected.The effect of the porous structure,the arrangement of pore wall groups,the hydrophilic and hydrophobic properties of the inorganic proton conductor,and the synergistic effects of acid-base pairs at the interface on the vertical proton transfer performance and proton transfer anisotropy was investigated.The specific research contents and main conclusions are summarized as follows:?1?The preparation of porous NFCM and study on proton conduction intensification.Firstly,porous Nafion nanofiber was fabricated through ionic liquid?IL?soft template methods.The resultant porous nanofiber mat was then impregnated with basic chitosan?CS?matrix to prepare porous NFCM.The abundant pores inside porous nanofiber provided numerous vertical transfer channels at interfaces between CS and pore walls.Meanwhile,the–NH/NH₂ groups on CS formed acid-base pairs with-SO₃H group on Nafion along pore walls.These stable vertical pathways significantly facilitated the through-plane proton conduction at both hydrated and anhydrous conditions.The results indicated that the through-plane proton conductivity of porous NFCM were increased by 3.2 and 2.7 times of that of NFCM at 90 ^oC and 100%RH,and 120 ^oC and 0%RH,respectively,and the proton transfer anisotropy was significantly decreased.?2?The preparation of hybrid NFCM and study on proton conduction intensification.First,two types of quantum dots?QDs?with a size of 2-5 nm were prepared:hydrophilic polymer quantum dots?PQD?and hydrophobic graphene oxide quantum dots?GQD?.PQD contained a large number of–CO₂H and–NH–/–NH₂groups,and most of the functional groups on GQD were carbonized and had strong hydrophobicity.Then,the SPEEK/QDs hybrid nanofiber was prepared by blending QDs and sulfonated polyether ether ketone?SPEEK?.The resultant hybrid nanofiber mat was then impregnated with CS matrix to prepare hybrid NFCM.It was found that PQD and the-SO₃H group on the SPEEK had strong interaction,and thus achieved PQD uniform dispersion and high loadings in the nanofiber.And then additional transfer channel were constructed at interfaces between nanofiber and inorganic conductor.Additionally,the–NH-/-NH₂ group on PQD formed acid-base pairs with-SO₃H group on SPEEK and thus formed low-energy-barrier proton transfer site.And the vertical direction“transfer dead zone”of nanofiber was connected.The results showed that CS/SP/PQD-30%achieved a through-plane proton conductivity of 399mS cm^-1 at 100%RH and 90 ^oC,which was 224%higher than that of plain membrane under indentical conditions.Under anhydrous conditions?120 ^oC?,the through-plane and in-plane conductivity of CS/SP/PQD-30%were 451 and 525 mS cm^-1,respectively,which was 3.2 and 1.8 times higher that of plain membrane,respectively.And the anisotropy decreased remarkdly,from 2.11 for the plain membrane to 1.16for CS/SP/PQD-30%.