The Influence of Chitosan Membrane Properties for Direct Methanol Fuel Cell Applications

Abstract

The chitosan membranes with different degrees of deacetylation (dda), prepared from Cape rock lobster collected from the surroundings of Cape Town, South Africa were characterized for suitability in methanol fuel cell applications. A comparison of chitosan membranes characteristics and that of conventional Nafion 117 membranes were made. Following this, the chitosan membranes were chemically modified with sulfuric acid to improve its proton conductivity and mechanical properties. A mass balance on proton transfer across the membrane resulted in a second order differential equation. Experimental data fitted into the equation gives a linear curve that was used to determine the membrane resistance. It was found that the dda of the chitosan membranes affected the water uptake, thereby affecting the proton flow. At a temperature of 20°C, chitosan membranes with a difference of 10% dda have a difference of about 5% water content. Chitosan membranes with a lower dda were found to have higher water content resulting in lower membrane resistances to proton flow. The water content of chitosan membranes was higher than Nafion membranes. The average resistance to proton flow for chitosan membrane was 53 min/cm and a Nafion membrane was 78 min/cm. Thermogravimetry analysis shows that chitosan membrane with higher dda is more thermally stable than chitosan with lower dda, Nafion membranes were more stable at higher temperature than chitosan membranes, Nafion membranes could decompose at temperature of 320 °C while chitosan membranes at 230 °C. Methanol permeability through chitosan membrane of higher dda was more than with one lower dda, however, the permeability through chitosan was three times lower when compares to Nafion membranes under the same temperature and pressure conditions. The performance of chitosan membranes and Nafion 117 membranes measured from a single cell DMFC with Pt-Ru/C anode catalysts and Pt/C cathode catalysts showed that Nafion membranes have a better performance. This was because the current and peak power densities determined for Nafion membranes were 0.56 A/cm2 and 0.075 W/cm2, respectively, and for Chit-I, were 0.22 A/cm2 and 0.0274 W/cm2, respectively, and for Chit-II membrane, were 0.26 A/cm2 and 0.0424 W/cm2, respectively.