NMR and Electrochemical Investigation of the Transport Properties of Methanol and Water in Nafion and Clay-Nanocomposites Membranes for DMFCs

Abstract

Water and methanol transport behavior, solvents adsorption and electrochemical properties of filler-free Nafion and nanocomposites based on two smectite clays, were investigated using impedance spectroscopy, DMFC tests and NMR methods, including spin-lattice relaxation and pulsed-gradient spin-echo (PGSE) diffusion under variable temperature conditions. Synthetic (Laponite) and natural (Swy-2) smectite clays, with different structural and physical parameters, were incorporated into the Nafion for the creation of exfoliated nanocomposites. Transport mechanism of water and methanol appears to be influenced from the dimensions of the dispersed platelike silicate layers as well as from their cation exchange capacity (CEC). The details of the NMR results and the effect of the methanol solution concentration are discussed. Clays particles, and in particular Swy-2, demonstrate to be a potential physical barrier for methanol cross-over, reducing the methanol diffusion with an evident blocking effect yet nevertheless ensuring a high water mobility up to 130 °C and for several hours, proving the exceptional water retention property of these materials and their possible use in the DMFCs applications. Electrochemical behavior is investigated by cell resistance and polarization measurements. From these analyses it is derived that the addition of clay materials to recast Nafion decreases the ohmic losses at high temperatures extending in this way the operating range of a direct methanol fuel cell.

New Insights into Properties of Methanol Transport in Sulfonated Polysulfone Composite Membranes for Direct Methanol Fuel Cells

“…Concerning the behavior of such membranes with respect to methanol, they were swelled by immersion in aqueous methanol solutions at two different alcoholic concentration, 1 and 5 M. Table 1 shows the uptake values obtained at 20 • C. Compared with the water uptake values seen previously, there are some differences, in particular at higher methanol concentration, reaching for the composites an uptake of about 30 wt%, and 20 wt% for the filler-free sPSf. However, if we compare these values with those usually obtained in Nafion based electrolytes (34 wt% with 4 M methanol) [48], the swelling here is quite lower, confirming a lower affinity of the methanol to polysulfone polymer compared to perfluorosulfonic acid membranes. This parameter is important to overcome the major limitation of using high concentrations of methanol solutions in DMFCs.…”

Nuclear Magnetic Resonance Spectroscopic Investigation of Anode Exhaust of Direct Methanol Fuel Cells Without Isotope Enrichment

“…To evaluate the performance of our DMFCs, current–voltage curves were measured and are shown in Figure ; the maximum values of power and current density are 29 mW/cm 2 and 150 mA/cm 2 , respectively. With PtRu anode catalysts, commercially available power and current density values typically range from 20 to 40 mW/cm 2 and 100 to 200 mA/cm 2 , respectively . Therefore, the power density and current density observed in this study are reasonable.…”

Selectivity of Direct Methanol Fuel Cell Membranes

“…However, there are still specific drawbacks for DMFCs in terms of high cost and low performance, and the stability needs to be improved [ 2 , 3 ]. The main objective of the research in the field of direct methanol fuel cells [ 3 , 4 , 5 , 6 , 7 ] is to develop cost-effective catalysts and membranes with enhanced properties in order to reduce stack costs and improve performance and durability. Concerning the membrane, the main purpose of the developmental activities is to provide enhanced conductivity at a reduced methanol crossover [ 8 ].…”