Preparation, Characterization Of Proton Exchange Membranes Based On Bacterial Cellulose And Their Application In Fuel Cells

Bacterial cellulose (BC) is a kind of natural macromolecular nano-material obtained by the fermentation of some microorganisms. Compared to the plant cellulose (PC), BC is chemically same but distinguished by its special3D nanostructure morphology, high tensile strength, high crystallinity, high chemical purity, high water binding capacity, low gas permeation and good biocompatibility. Hence, BC has been studying in various fields, such as food additives, high quality audio membranes, functional papers, pervaporation membranes and biomedical materials. Most recently, the applications of BC in key devices of proton exchange membrane fuel cell (PEMFC) including carbon support catalysts, gas diffusion layer (GDL) and proton exchange membranes have been investigated. However, researches on fabricating the proton exchange membrane from BC are not systematic and in-depth. Therefore, in this paper, we prepared a new highly proton-conductive, cost-effective and stable blend BC membranes utilized in PEMFC based on acid-doping methodology from references at first. Thereafter, several fabricating methods of combining BC with Nafion into blend or composite proton exchange membranes were developed to apply in direct methanol fuel cell (DMFC).By investigating effect of the states of BC, different kinds of acids and the drying method on film-forming property, the dried BC membranes were chosen to immerse into phosphoric acid (H3PO4) and phytic acid (PA) solution followed by drying under low temperature into H3PO4/BC and PA/BC membranes at first. The membrane properties are characterized based on such points of view as AC impedance technique, FTIR spectra, TG profiles, doping level, water uptake and mechanical properties. Some suitable samples were sandwiched with the catalysts and GDLs and hot-pressed into membrane electrode assemblies (MEAs) under the optimal hot-pressing conditions in order to evaluate power performances in a real H2/O2fuel cell. The results indicated that the hydrogen bonds between H3PO4, PA and cellulose did exist in the H3PO4/BC and PA/BC membranes which were both outstanding in thermostability, mechanical strength and flexibility, water uptake, acid doping level and proton conductivity. Then,8M H3PO4/BC and1.6M PA/BC membranes were selected as candidates for single cell test after they were hot-pressed with PTFE sheets as supporting material under the condition of80℃,10MPa,6min and80℃,6MPa,3min respectively, which generated the biggest power density of30.7mW cm-2(60℃) and20.6mW cm-2(80℃) separately.After establishing the methodology of using gas chromatograph to determinate the methanol permeability of proton exchange membrane, the excellent methanol-resistance property of pure BC film was verified and it became more remarkable in case of concentrated methanol. Thus, the subject was reorientate to the development of the proton exchange membranes used in DMFC leading to the preparation of the blend membranes of BC pulp in alcohol and Nafion solution (BC-pulp/Nafion). Also, the discovery of marked color changes through annealing boosted the study over the effect of annealing on the properties of BC-pulp/Nafion membranes, including ATR-FTIR spectra, thermo behaviors (TGA, DSC and DMA), mechanical property (DMA),water uptake, proton conductivity and methanol permeability. The ATR-FTIR spectra results indicated that there were hydrogen bonds existing among BC, water and Nafion which were strengthened by increasing the proportion of Nafion whilst weakened after annealing. The other test results implied that BC-pulp/Nafion membranes are heterogeneous blend membranes with two significant glass transition stages, good thermostability (below120℃), excellent properties in Young's modulus, water uptake, proton conductivity and methanol permeability. After annealing, most of aformentioned properties kept unchanged or enhanced slightly except water uptake and methanol permeability decreasing dramatically, which contributed to higher selectivity, i.e. meant more suitable for applying in DMFC.Furthermore, five different composite films of BC and Nafion were developed. With analyzing the preliminary experimental phenomena and results, it could be concluded that the BC/Nafion composite films fabricated by immersing the lyophilized BC membranes into Nafion solution, the laminated films of Nafion/BC/Nafion formed with the help of layer-by-layer self-assembly technology and the sandwiched films BC/Nafion/BC prepared by placing a Nafion membrane between two BC gels had feasibility as proton exchange membranes in DMFC, whilst the BC/Nafion hybrid membranes made by using proper solvent system were not feasible as well as the sandwiched films BC/Nafion/BC with dry BC pulp coatings on Nafion membranes.