Comparison of Direct Methanol Fuel Cells Against Conventional Batteries

Abstract

High performance batteries currently used in laptop computers, cell phones, and other portable electronic devices have been making small improvements for decades. Nevertheless, such power supplies have inherent limitations which restrict the advancement of the technical capability of these electronic devices. For instance, the designer must strike a balance between compactness and run time. As batteries age, run time typically drop significantly, so the designer must oversize the battery system initially, thus adding weight and increasing the cost. Overall efficiency is also an issue, as losses occur during generation of electricity at the power plant, in transmission, in charging, in slow discharge during storage, and finally in conversion of stored chemical energy into electrical energy at time of use. Direct methanol fuel cells (DMFCs) are an emerging technology which has the potential to improve on several of these limitations. In the DMFC, methanol/water mixtures react and produce hydrogen ions and electrons electrochemically. The electrons flow through the load circuit, while the ions diffuse through a membrane electrolyte, completing the reduction reaction at the cathode in the presence of oxygen from air. This paper aims to compare the DMFC systems against conventional batteries in respect of energy capacity degradation, gravimetric and volumetric energy densities. It is concluded that, although the initial mass and volume of a DMFC system dominate the gravimetric and volumetric energy densities profile respectively for a short operation time, the methanol conversion efficiency becomes the dominant factor as the operation time increases. Depending on the initial system mass and volume and conversion efficiency, commercially available DMFC systems can offer weight advantages over current battery technology for operation times longer than 10 hours and volume advantages for the operation time longer than 50 hours. Near-term advances in these break-even times are expected to be an order of magnitude, given the relatively rapid development rate of the DMFC. Additionally, although the energy capacity of both the DMFC system and the battery degrades with operation time, the degradation for the battery is typically higher, especially for long time of service. For these reasons, DMFC systems appear to be highly promising for portable electronic devices.