Search

Over the past decade, the direct methanol fuel cell (DMFC) has been considered as a potential candidate of power sources for portable electronics due to its high energy density and environmental friendly operation. However, the use of diluted fuel loading and active fuel mixer/pump limits the DMFC's efficiency and operating time dramatically. The disadvantages become more severe in microscale DMFCs which require high conversion efficiency within a small physical space to meet the strict working specifications. In this paper, a low-power micro fuel supplier with programmable concentration regulation was designed and fabricated by using microelectromechanical system technology. The prototype adopted surface-tension pumping as the primary driving force to deliver fuel at zero power consumption. In addition, a well-arranged microfluidic capillary network was used to regulate the fuel concentration. A pulse-power actuated bubbler switch was embedded in the capillary network to achieve the liquid flow control dynamically and to adjust the fuel concentration for micro-DMFC ( \muDMFC) reaction. Both electrolysis and thermal bubble actuations were investigated in this paper for their effective operations in the prototype. The advantage of precise and low-power fuel regulation within a compact system volume makes the proposed fuel delivery subsystem a suitable candidate for future on-chip \muDMFC development. [ABSTRACT FROM AUTHOR]

Abstract: For the past decade, extensive mathematical modelling has been conducted on the design and optimization of liquid-feed direct methanol fuel cells (DMFCs). Detailed modelling of DMFC operations reveals that a two-phase flow phenomenon at the anode and under-rib convection due to the pressure difference between the adjacent channels both contribute significantly to mass-transfer in a DMFC and its output performance. In practice, comprehensive simulations based on the finite volume technique for two-phase flow require a high level of numerical complexity in computation. This study presents a complexity-reduced mathematical model that is developed to cover both phenomena for a realistic, but fast, in computation for the prediction and analysis of a DMFC prototype design. The simulation results are validated against experimental data with good agreement. Analysis of the DMFC mass-transfer is made to investigate methanol distribution at anode and its crossover through the proton-exchange membrane. From a comparison of the influence of two-phase flow and under-rib mass-transfer on DMFC performance, the significance of gas-phase methanol transport is established. Simulation results suggest that both the optimization of the flow-field structure and the fuel cell operating parameters (flow rate, methanol concentration and operating temperature) are important factors for competitive DMFC performance output. [Copyright &y& Elsevier]

Abstract: The existing direct methanol fuel cell (DMFC) systems are fed with a fixed concentration of fuel, which are either a diluted methanol solution or an active fuel delivery driven by an attached active pump. Both approaches limit the power conversion density or degrade the overall efficiency of the DMFC system significantly. Such disadvantages become more severe in small-scale DMFCs, which require a high conversion efficiency and a small physical space suitable for portable electronics. In this paper, passive fuel delivery based on a surface tension driving mechanism was designed and integrated in a laboratory-made prototype to achieve consumption depending on fuel concentration and power-free fuel delivery. Unidirectional methanol-to-water smooth flow is achieved through the capillaries of a Teflon PTFE (polytetrafluoroethylene) membrane based on the difference in liquid surface tension. The prototype was demonstrated to exhibit a better polarization performance and to last for an extended operating time compared to conventional DMFCs. Its high efficiency and load regulation performance were also demonstrated in contrast to an active DMFC supplied with a constant concentration fuel. The fuel delivery driven by the liquid surface tension effect demonstrated here is believed to be more applicable for future small-scale DMFCs for portable electronics. [Copyright &y& Elsevier]