Preparation Of Nanofibrous Composite Proton Exchange Membranes And Adjustment Of The Microstructures

Proton exchange membrane(PEM) is the heart of Proton exchange membrane fuel cell(PEMFC), and its proton conductivity could decide the performance of PEMFC. As the main mechanisms for proton transfer are Grotthuss mechanism and Vehicle mechanism, the construction of continuous proton transfer pathways is critical to the enhancement of proton conductivity. Electrospun nanofibers are forced-assembly three-dimentional network with nanoscale diameter and high specific surface area, which enables them to act as natural long-range continuous proton transfer pathways. Meanwhile, polymer matrix could be filled into the holes between nanofibers to obtain dense nanofibrous composite membrane(NFCM),which possess the bi-continuous structure composed of nanofiber phase and matrix phase. Acid-base pairs formed by acid and base groups could work as low-energy-barrier proton hopping sites for efficient proton transfer. The functional groups from the nanofibers and matrix could be independently adjusted, which makes the proton hopping environment easy to control. These features endow NFCMs distinct advantages over the membranes obtained by directly blending polymers.Besides, nanofibers could work as the backbone of NFCMs, and the interactions between nanofibers and matrix would restrict the chain motion of polymer, thus reinforcing the structural stability of NFCMs.To explore the effects of physical structure and chemical environment of membrane on proton transfer property, a series of nanofibers were fabricated and then different kinds of NFCMs were prepared, and the interactions within them were adjusted controllably. The detailed research contents and main results are listed below:(1) Influences of different coupling of conductive groups from nanofibers and matrix Firstly, a series of nanofiber mats bearing different functional groups(neutral,acidic, or basic groups) were fabricated by electrospinning the mixture of poly(vinyl alcohol)(PVA) and tetraethyl orthosilicate(TEOS) and then modifying them with different kinds of silane coupling agents. Afterwards, acidic sulfonated poly(ether ether ketone)(SPEEK) or basic chitosan(CS) matrix was incorporated into the nanofiber mat to obtain NFCMs bearing three types of composite proton carriers(I-type: acid-neutral or base-neutral, II-type: acid-acid or base-base,III-type: acid-base or base-acid) at the interfacial domains of the nanofiber/matrix.The strong acid-base-pair interactions endowed NFCMs containing III-type proton carries with excellent thermal and structural stabilities. Through the investigation of proton conductivities under different conditions, it was found that I-type proton carriers lacked efficient proton hopping sites. By comparison,II-type carriers displayed an increase of carrier loading amount but still lacked synergistic effect. III-type proton carriers(acid-base pairs) exhibited a distinct induction effect and resulted in superior low-energy-barrier proton hopping pathways. The highest conductivities of SP/NF-NH2 containing III-type proton carriers under hydrated and anhydrous conditions were 0.094 S cm-1(65 o C) and0.027 S cm-1(120oC)。(2) Construction of bi-continuous proton transfer pathways in matrix-nanofiber and matrix-nanofiller interfaces Firstly, the above-mentioned nanofibers fabricated with PVA and TEOS were modified by dopamine to form a basic PDA layer and extra PDA nanoparticles,followed by incorporating acidic SPEEK matrix to obtain NFCMs. The characterization results revealed that the polydopamine nanoparticles disperse homogeneously in SPEEK matrix and adjusted the chain mobility and proton transfer barrier of polymer. Meanwhile, acid-base pairs were constructed by polydopamine and SPEEK, acting as efficient proton pathways. By measuring the proton conductivities and activation energy, it was reasonable to state that the construction of bi-continuous interfacial proton pathways enhanced the proton transfer ability obviously. The obtained highest proton conductivities of NFCMs(SP-82/NF-PDA-8.5) under hydrated and anhydrous conditions were 0.127 S cm-1(60 o C) and 0.012 S cm-1(120 o C).(3) Preparation of CS/SPEEK nanofibrous composite membranes and the enhancement of proton transfer properties by acid-base pairs Firstly, a series of SPEEK nanofibers with controllable sufonation degrees and diameters were electrospun. Afterwards, CS matrix was filled in the pores between nanofibers to get dense NFCMs. Cross-sectional SEM images revealed that SPEEK nanofibers disperse homogeneously within the CS matrix, and FTIR results suggested that acid-base pairs were formed within the NFCMs. The enhancement of acid-base pairs for the proton transfer were explored by systematically measuring the proton conductivities under both hydrated and anhydrous conditions. The increase of SPEEK sulfonation degree or the decrease of nanofiber diameter could obviously improve the amount of acid-base pairs.NFCMs displayed obviously enhanced proton conductivities due to the continuous proton transfer pathways in the nanofiber/matrix interfacial regions constructed by the sufficient acid-base pairs. For example, the highest proton conductivity of CS/SP-80-1 under hydrated condition was 0.153 S cm-1(80 o C)which was comparable with that of commercial Nafion membrane; the highest proton conductivity of CS/SP-80-1 under anhydrous condition was 0.060 S cm-1(120 o C), which was much higher than that of Nafion.