Synthesis And Property Of Chitosan-Based Alkaline Anion-Exchange Membranes And Their Application To Fuel Cell

Anion exchange membrane (AEM) fuel cell have attracted considerable attentions owing to the advantages of fast reaction kinetics, low permeability, low CO toxicity, and the use of non platinum catalyst. However, the diffusion rate of the hydroxyl ion (OH-) is only one fourth of the hydrogen ion (H+). It’s particularly important for the study of anion exchange membrane to achieve higher conductivity, lower activation polarization. Moreover, the stability of the membrane, especially alkaline resistance and dimensional stability in high pH 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 1-Ethenyl-3-methyl-lH-imidazoliumchloride polymer with 1-ethenyl-2-pyrrolidone (abbreviated as EMImC-Co-EP) 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, and AC impedance technique, water uptake, 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 Chitosan/EMImC-Co-EP-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 Chitosan/EMImC-Co-EP alkaline anion-exchange membranes. The OH- conductivity initially increased with increasing of EMImC-Co-EP content, then reached a plateau (0.011 S cm-1) at a Chitosan/EMImC-Co-EP polymer composition of 1:0.75 by mass. Further addition of EMImC-Co-EP to the polymer led to a decrease in conductivity.(3) With Chitosan/EMImC-Co-EP ratio exceeding 1:0.75 by mass, the tensile strength of Chitosan/EMImC-Co-EP-OH- membranes decreased to less than 2 MPa. This may be explained by the fact that the phase separation phenomena become more evident when in so high content of EMImC-Co-EP in the membranes. This has been verified in SEM pictures of the cutview of Chitosan/EMImC-Co-EP alkaline membranes.(4) The Chitosan/EMImC-Co-EP membranes showed excellent thermal stability with onset degradation temperature high above 200℃. In particular, the high alkaline stability was achieved for the Chitosan/EMImC-Co-EP membranes in hot 8.0 M KOH at 85℃ without losing their integrity and OH- conductivity during 300 hours of evaluation, and also a relatively high oxidative stability. The above results may be due to the formation of interpenetrating polymer networks of EMImC-Co-EP in the highly cross-linked Chitosan network by a combined thermal and chemical cross-linking method, which improves not only the OH- conductivity but also the stability of the membranes.(5) With the addition of graphene oxide (GO), the stability of membranes improves significantly, especially high tensile strength of 33.63 MPa, tensile elongation of 9.77% while the tensile strength and elongation of the Chitosan/EMImC-Co-EP without GO were 1.68MPa and 1.57%, respectively. After the addition of PVA, the mechanical properties of the Chitosan-PVA/ EMImC-Co-EP membrane have much improved. With Chitosan/PVA ratio at 2:3 by mass, the tensile strength was 7.39MPa, and the elongation was 50.5%, respectively.(6) The MEAs fabricated with three Chitosan/EMImC-Co-EP-OH- membranes depending on Chitosan/EMImC-Co-EP mass ratio showed the power densities around 21.7 (Chitosan/ EMImC-Co-EP=1:0.5) and 11.4 (Chitosan/EMImC-Co-EP=1:0.75) mW cm-2 along with the maximum current densities at 51.4 mA cm-2 and 50.8 mA cm-2, respectively. They showed the peak power densities around 28.3 mW cm-2 (Chitosan-PVA/EMImC-Co-EP-OH- membranes with Chitosan/PVA ratio at 2:3 by mass), along with the maximum current densities at 85.3 mA cm-2. A promising H2/O2 fuel cell test with Chitosan/EMImC-Co-EP-OH- membranes with 1 wt% GO showed the peak power density of 37.3 mW cm-2 at room temperature on a low metal loading on both the anode and the cathode of 0.5 mg (Pt) cm-2 at ambient temperature.