A Study On The Electrolyte Membranes In Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFC) are characterized by simple structure, convenient fuel supply and high energy density. DMFC will be one of the ideal power sources in the future for mobile, portable and stationary applications. Present electrolyte membrane, electrode materials and catalysts in DMFC are not satisfactory. Commercial Nafion? electrolyte membrane, Pt or Pt alloy catalysts are expensive, and methanol crossover in Nafion? electrolyte membrane leads to not only a waste of fuel but also poor fuel cell performance. The aim of this study is to explore alternative membranes based on the fact that catalyst of non-noble metals shows high activity in alkaline media. Metal carbonate is an alkali salt with moderate basicity. When a metal carbonate, e. g. K2CO3 is used as electrolyte in DMFC, the potential advantage is both widened selection of catalysts and resistance of CO2 uptake in the electrolyte. We initiated the idea of using membrane composed of polymer matrix and embedded metal carbonate as the electrolyte in DMFC and K2CO3 electrolyte membranes have been prepared through three methods. Firstly, the gel electrolyte membrane of K2CO3 doped polyacrylamide is prepared by dispersing ionic species, K2CO3, to a solution of acrylamide and bisacrylamide followed by polymerization and gelation. Their ambient temperature conductivity is in the range 10-2 to 10-1S/cm. Secondly, to enhance the strength of the above-mentioned gel electrolyte, microporous membrane of polyethersulfone (PES) is used as supporting material and gelation takes place in the micropore of PES membrane. The reinforced gel electrolyte membrane of K2CO3 doped polyacrylamide is thus obtained. The third method is to K2CO3 powder directly dispersed in polymer. In this method, we use two ways. In one way, casting membranes are prepared by using K2CO3 in aqueous PAA solution or PTFE emulsion. In the other, membranes are prepared by K2CO3 directly being doped with thermoplastic polyurethane elastomer (TPU) and nylon 6, respectively, and that a composite membrane is prepared by polycaprolactam loading K2CO3 prior to K2CO3 being doped with polysulfone (PSF). The results show that K2CO3 doped solid electrolyte membranes have rather high electricity at ambient temperature and electricity increase with the increasing of temperature. The stable performance of the membranes used in DMFC can be maintained. The component concentration in an equilibrium system of aqueous carbonate solution and CO2 under varying temperature is analyzed. The equilibrium relation in such a system determines the exclusion of CO2 by an aqueous metal carbonate electrolyte. On the basis of the above analysis, the mechanism of a DMFC with carbonate electrolyte is discussed.