Development of polystyrene sulfonic acid-polyvinylidene fluoride (PSSA-PVDF) blends for direct methanol fuel cells (DMFCS)

2014-11-11As direct methanol fuel cells (DMFCs) hold great promise for wide applications in transportation, stationary and portable power sources, the study of fuel cell materials and their contribution to the overall cost and performance are of considerable practical importance. One of the key components that receives much research attention is the proton electrolyte membrane (PEM). Perfluorosulfonic acid (PFSA) membranes such as Nafion® are considered to be the current benchmark membranes for DMFCs. Despite of the merit of high proton conductivity and stability, PFSA membranes suffer from high cost and excessive methanol crossover. Therefore, alternative cost-effective and high performance materials are strongly desired. In this work, high performance and cost-effective polymer electrolyte membranes (PEMs) based on tetra-butylammonium poly(styrene sulfonate) (PTBASS)/PVDF polymer blends are developed. The PTBASS/PVDF holds promise as a substitute membrane in DMFCs, and the methodology to fabricate membranes in the present study also provides a tool to elucidate the complexities of the structure-performance relationship in PEMs. ❧ In Chapter I, the background of fuel cells, the working principle of DMFCs and the polymer electrolyte membranes for DMFCs are reviewed and discussed. Various strategies including sulfonation of aromatic polymer, impregnation of polyelectrolytes in inert polymers, organic/inorganic composite, and polymer blending have been employed to develop alternative membranes for DMFCs. In spite of some progress have been made in terms of reduction in methanol crossover, development of a sturdy inexpensive substitute to perfluorosulfonic acid membranes is yet to materialize. ❧ In chapter II, a seemingly immiscible polystyrene sulfonic acid/polyvinylidene fluoride (PVDF-PSSA) nano-composite homogeneous composite blend was synthesized by a “polymer camouflage” approach. Terpolymerization of an ionic liquid monomer, tetra-butylammonium styrene sulfonate (BASS), with styrene and 4-chloromethyl styrene (CMS) is carried out with free radical polymerization to give a terpolymer poly(BASS-S-CMS). Polymer blend membranes consisting of poly(BASS-S-CMS) and PVDF was fabricated through solution-cast technique followed by annealing, crosslinking and ion exchange, giving highly flexible, transparent and tough PSSA/PVDF Nafion®-like membranes. Fuel cell relevant properties of PSSA/PVDF membranes, such as the methanol permeability, water uptake, ion exchange capacity (IEC), proton conductivity, membrane morphology and microstructure, and ultrafiltration property were investigated. The relationship between membrane composition and property is systematically studied. The experimental techniques including Gas Chromatography (GC), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDS), Thermogravimetric Analysis (TGA), UV-Visible Spectroscopy, and Electrochemical Impedance Spectroscopy (EIS) are included. ❧ In Chapter III, the electrochemical performance, methanol crossover, fuel utilization, and fuel cell efficiency of PSSA/PVDF modular membranes in DMFCs were extensively measured and analyzed. The effect of PSSA content on the fuel cell performance, methanol crossover and fuel cell efficiency are addressed. The impact of various operation parameters including temperature, methanol concentration, methanol and oxygen flow rate on the performance of PSSA/PVDF in DMFCs were tested in both ambient oxygen and air environment. An understanding of membrane property-performance relationship is examined and discussed. The present PSSA/PVDF membranes exhibit comparable or superior electrochemical performance compared to Nafion®-117 due to lower methanol crossover and higher proton conductivity. As a result, high fuel utilizations and overall fuel cell efficiency were achieved using PSSA/PVDF membranes in DMFCs. ❧ The polymer electrolyte membranes based on the polymer blend of PSSA/PVDF shows a promising potential in DMFC applications with impressive economic advantages. The methodology employed in membrane fabrication provides a general method for exploring many other monomer, including ammonium salts of 4-vinyl sulfonic acid (VSA), 2-acrylamido-2-methylpropane sulfonic acid) (AMPS), and 3-sulfopropyl methacrylate (SPM) as alternative polymers electrolyte materials for DMFCs applications. The conclusions and outlook are discussed in the last chapter