Systhesis And Property Of Pva/Pdda Alkaline Anion-Exchange Membranes And Their Application To Fuel Cell

Alkaline polymer electrolyte membrane (PEM) fuel cells, employing alkaline anion (OH)-exchange membranes (AAEMs) offer an attractive option as power sources for stationary and portable applications due to their faster electrode kinetics, lower fuel crossover, reduced CO poisoning, and use of non-precious metal catalysts. However, since the mobility of OH" is only1/4of that of the H+transportation (membranes that conduct H+cations), the membranes that can have high OH-conductivity are highly desired for obtaining a higher power density. In cases where much effort has been undertaken for the above purpose, the problem of low membrane stability, particularly alkaline stability, has also identified as one of the major barriers that affects their application in alkaline fuel cells.This work reports promising performances of a new type of alkaline anion-exchange membranes by incorporating poly(diallyldimethylammonium chloride)(PDDA) as anion charge carriers. The technique of thermal and chemical cross-linking was used to modify the blended polymer membrane. The membranes are characterized in detail at structural and hydroxyl ion (OH-) conducting property by FTIR spectroscopy, scanning electron microscopy (SEM), thermal gravity analysis (TGA), mechanical property, AC impedance technique, water uptake, swelling ratio, oxidation and alkaline stability to evaluate their applicability in alkaline fuel cells. The main points are summarized as follows:(1) The FT-IR spectra and SEM pictures showed that the novel alkaline anion-exchange membranes from PVA/PDDA-OH-composites were successfully prepared by a combined thermal and chemical cross-linking ways. All membranes possessed compact structure due to the formation of high dense cross-linkages in the membranes.(2) High OH-conductivity was achieved of PVA/PDDA-OH" alkaline anion-exchange membranes. The OH-conductivity initially increased with increasing PDDA content, then reached a plateau (0.025S cm-1) at a PVA/PDDA polymer composition of1:0.5by mass. Further addition of PDDA to the polymer led to a decrease in conductivity. The OH-conductivity also increased with increasing molecular weight of PDDA, and PVA/PDDA-HMw membrane reached a maximum σOH-value of0.027S cm-1along with the high WU around1.32g g-1.(3) The strong alkaline stability of the membranes has been achieved in8M KOH at80℃for360h. Meanwhile, the membranes exhibited excellent thermal stability with onset degradation temperature high above170℃and, also a relatively high oxidative durability at60℃for almost240h. The above results may be due to the formation of interpenetrating polymer networks of PDDA in the highly cross-linked PVA network by a combined thermal and chemical cross-linking method, which improves not only the OH-conductivity but also the stability of the membranes.(4) With the addition of multi-walled carbon nanotubes (MWCNTs), the stability of membranes improves significantly, especially high tensile strength of40.3MPa, tensile elongation of12.3%and high Young’s modulus of782.8MPa with addition of3wt.%MWCNTs. At the same time, the additional MWCNTs binds the polymer chains in the membranes more compactly, resulting in the decrease of water uptake.(5) The MEAs fabricated with three PVA/PDDA-OH" membranes depending on PVA/PDDA mass ratio showed the power densities around11.5(PVA/PDDA=1:0.25),14.8(PVA/PDDA=1:0.75) and32.7(PVA/PDDA=1:0.5) mW cm-2along with the maximum current densities at64,134.9and73.6mA cm-, respectively. Also, an increase in molecular weight of PDDA leads to a dramatic improvement in the cell performance due to increased σOH-.They showed the peak power densities around18.2(PVA/PDDA-ULMw),23.4(PVA/PDDA-LMw),28.5(PVA/PDDA-MMw) and35.1mW cm-2(PVA/PDDA-HMw), along with the maximum current densities at67.5,69.4,85.1and98.9mA cm-2, respectively. A promising H2/O2fuel cell test with PVA/PDDA/MWCNTs membranes showed the peak power density of41.3mW cm"2at room temperature and greatly increased to66.4mW cm-2at40℃on a low metal loading on both the anode and the cathode of0.5mg (Pt) cm-2at ambient temperature. The membrane also showed a promising peak power density of9.1mW cm-2at room temperature and14.0mW cm-2at40℃v with Co-phthalocyanine cathode catalyst. The long-term stability of single-cell performance showed that the PVA/PDDA membrane could approximately last80h on the fuel cell with a decrease of0.15V in cell potential.