Production of high temperature core-sheath nanofiber proton exchange membranes via electrospinning method

Fuel cells are electronic transducers which continuously generate electricity by the means of the reaction of two hydrogen atoms with an oxygen atom; where their by- products are water and emitted heat. Nowadays, the emerging need of clean energy resources has gained particular attention to tackle environmental issues such as excessive greenhouse gasses and global warming. Fuel cells offer electricity generation with no vibration, noise, particulate, or, greenhouse gasses emissions depending on the technology. Fuel cells are classified depending their operation temperature, as of low temperature (50–100 °C) and high temperature (100–200 °C) fuel cells, that each has their advantages and disadvantages associated to their proton conducting mechanism. For instance, to overcome the drawbacks of the low- temperature proton exchange membrane fuel cell (PEMFC) such as CO catalyst poisoning, necessity of humidification, heat management, and low diffusion rates of protons, polybenzimidazole (PBI) membranes have received research attention due to their superior proton conductivity at high temperatures (up to 200°C), lack of necessity of water presence in proton transport, and good mechanical properties. Sulfonated polyether ether ketone (SPEEK) is another promising proton exchange membrane (PEM) material for high-temperature fuel cells because of its good thermal stability, moderate mechanical strength, and good proton conductivity. Hydrophilic SPEEKs have been often used as membrane materials for the separation of liquids. However, several studies have explored their use in high-temperature PEM. Earlier studies showed that the introduction of functional groups, such as sulfonate groups, into a polymer entity promoted miscibility with PBI due to intermolecular interactions between -NH group on the PBI and the sulfonate groups on the SPEEK. On the other hand, enhanced direct methanol fuel cell (DMFC) performance has recently been measured using an acid-base SPEEK/PBI composite membrane in comparison to commercial Nafion®. Recent efforts have focused on the preparation of membranes using polyelectrolyte nanofibers to construct efficient ion-conductive networks. These nano-scaled fibers have very high specific surface to volume ratio, better mechanical performance than films, and good pore interconnectivity, which makes them potential candidates in PEMFCs. Different nanofiber structures such as core/sheath, bicomponent, hollow and porous structures, can be produced by using specially designed spinnerets. Core/sheath nanofiber morphology facilitates dual function, where sheath region behaves differently than the core. In this thesis, bicomponent nanofibrous core/sheath structured proton conductive membranes were prepared and used as PEM in hydrogen fuel cells. Their atomistic models studied by molecular dynamic (MD) modeling were constructed for a better understanding of their interactions, their solubility, and proton conductivity behaviors. Neat and bi-component polymer membranes' proton conductivity and actual fuel cell tests were performed. This thesis began with molecular modeling investigations performed by Materials Studio® were used to shed light on the solubility of PBI and SPEEK polymer and various solvents such as N-Methyl-2-Pyrrolidone (NMP), Dimethylformamide (DMF), Dimethylacetamide (DMAc) and Dimethyl sulfoxide (DMSO). The solubility parameters of each polymer and different solvents were calculated using cohesive energy density (CED) calculations by MD simulation method in order to better select the proper solvent for each polymer for nanofiber production. MD studies showed that DMF and DMAc were determined to be the more proper solvents for SPEEK and PBI respectively. After the selection of proper solvent for each polymer, first neat PBI nanofibrous membranes were fabricated using the electrospinning method; their electrospinning parameters were optimized systematically by altering the polymer concentration, applied voltage and feeding rate at constant nozzle-collector distance of 15cm and by performing morphological studies using scanning electron microscope (SEM) micrographs. Out of the 15 systematically investigated neat PBI electrospun systems, the system with 15kV applied voltage, 15 wt% polymer concentration, 100 μL/hr flow rate, and 15 cm nozzle-collector distance was selected as optimum. Neat PBI nanofibrous membranes were doped in phosphoric acid (PA) for 24, 48, 72 and 96 hour (hr) to gain proton conductive properties; an optimized doping time of 72 hr was determined using Fourier transform infrared spectroscopy (FT-IR), thermogravimetric (TGA) and proton conductivity analysis, and the membranes' water-uptake and dimensional changes were recorded. In morphological studies, the neat PBI and acid-doped PBI membrane's process parameters were optimized using morphology analysis based on the density of bead structure, and the mean and deviation in fiber diameter. The morphological changes associated with PA doping indicated that acid doping caused significant swelling and a 2-fold increase in mean fiber diameter. The tensile strength of the membranes was found to be increased by the doping level, whereas the strain at break (15%) decreased because of the brittle nature of the H-bond network. PBI membranes doped for 72 hr demonstrated the highest proton conductivity (123 mS/cm; the proton conductivity of Nafion 115® in same conditions was 78 mS/cm), while a decrease in conductivity for PBI membranes doped for 96 hr was observed, which could be attributed to the morphological changes. Then, neat SPEEK nanofibrous membranes were produced by electrospinning; their process and material parameters were systematically optimized by varying the polymer concentration, feeding rate, and applied voltage. The optimum system of 30 wt% polymer concentration, 15 μL/hr flow rate and 15 kV applied voltage was selected, considering its lowest fiber diameter distribution and average fiber diameter (239 ± 16.8 nm). The neat SPEEK membrane's thermal behaviors were investigated using FT-IR, SEM and differential scanning calorimetry (DSC) analyses; thermal cross- linking of polymer chains at a temperature below their glass transition temperature (Tg) were observed. The thermal cross-linking of SPEEK membranes was increased with temperature and time. Since the proton conductivity of SPEEK was inversely proportional to its degree of cross-linking because of the reduction of active sulfonate groups, the minimum cross-linking degree, obtained by conditioning the membranes at 160 °C for 24 hr, was chosen. More importantly, 100% sulfonated SPEEK membranes were soluble in hot water; thus, their thermal behavior was noted systematically using FT-IR, DSC, and a SEM analysis to optimize the hot press conditions. In the next stage, bi-component electrospun nanofiber production was systematically studied, where SPEEK formed the core while sheath PBI polymer co-axially surrounded SPEEK. Biaxial electrospinning parameters such as flow rate, applied voltage and polymer concentrations were optimized utilizing SEM and transmission electron microscopy (TEM) micrographs, keeping the nozzle-collector distance constant at 20 cm. Solution shear viscosity measurements of different polymer solution concentrations were performed with a rotational viscometer, using different polymer and solvent pairs to gain a better understanding of the rheological behavior of solutions and to find best matched viscosity profile in two different polymer solution systems. The viscosity measurement results demonstrated that in co-axial electrospinning created by the existence of two different solutions and a single electric field, the viscosities of core and sheath solutions should be the same or close to each other for successful core-sheath fiber formation. The core/sheath membranes exhibited higher tensile strength than neat SPEEK membranes and lower elongation at break than both PBI and SPEEK membranes. Fabricated core/sheath membranes displayed almost the same water uptake as neat PBI membranes (213 ± 25 wt%) and almost half of the mass change after doping of neat PBI membranes (13.60 ± 4.9 wt%). Morphology studies of the core/sheath structured membranes showed a flatter, more ribbon-like structure in thermally treated bicomponent nanofibers compared to the untreated membranes. Due to the lower applied treating temperature than the polymer's Tg, this deformation was not a consequence of polymer melting or glass transition, but the thermal cross- linking of the core component. Proton conductivity measurements showed that the proton conductivity of the core/sheath membranes increased with temperature and was almost ten times greater than the values measured for commercially available PBI films. Actual fuel cell performance studies showed that when increasing the cell humidity from 0% to 100% relative humidity (RH), fuel cell performance surprisingly decreased. Since this decrease in ionic conductivity of the PEM at high humidity was not expected, diffusion limitations within the catalyst layers was a likely reason for the increased low-frequency resistances at higher RH. Hydration of the gas streams resulted in excess water within the membrane, flooding of the electrodes, and limited O2 diffusion in the cathode catalyst layer. Comparing the fuel cell results measured using core/sheath SPEEK/PBI nanofibrous membranes with those of the literature, it was clear that SPEEK/PBI nanofibrous membranes showed better fuel cell performance than pristine SPEEK membranes by almost three orders of magnitude at 60 °C in anhydrous environment, and slightly higher performance (10%) than pristine doped PBI membranes at 120 °C and again in anhydrous environment. Comparing the fuel cell performance and proton conductivity results of the fabricated core/sheath membranes with commercially available PBI, and Nafion based films, it was evident that these membranes had great potential to compensate these commercial membranes' disadvantages particularly as high-temperature proton exchange membranes (HTPEM) in fuel cells. Last, this thesis also underlined the governing ion diffusion and proton transport mechanism in atomic scale using MD simulations. In SPEEK, PBI and SPEEK/PBI systems, the interactions of H+, H3O+ and H2PO4- ions and H2O molecules with each other and with PBI, SPEEK polymers and SPEEK/PBI bi-polymer. Radial distribution function (RDF) and radius of gyration (RG) analysis results showed that there was an attractive interaction between PBI and SPEEK polymer chains which prevented the interface separation of core/sheath nanofibers and increased the interfacial strength of produced membranes; and calculated proton conductivity values in molecular scale, at 1 atm and 298 K, also verified the order of with experimental values of PBI, SPEEK and SPEEK/PBI nanofibrous membranes' proton conductivity.