The Anode in the Direct Methanol Fuel Cell

University dissertation from Stockholm : Kemiteknik

Abstract: The direct methanol fuel cell (DMFC) is a very promisingpower source for low power applications. High power and energydensity, low emissions, operation at or near ambientconditions, fast and convenient refuelling and a potentiallyrenewable fuel source are some of the features that makes thefuel cell very promising. However, there are a few problemsthat have to be overcome if we are to see DMFCs in our everydaylife. One of the drawbacks is the low performance of the DMFCanode. In order to make a better anode, knowledge about whatlimits the performance is of vital importance. With theknowledge about the limitations of the anode, the flow field,gas diffusion layer and the morphology of the electrode can bemodified for optimum performance.The aim of this thesis is to elucidate the limiting factorsof the DMFC anode. A secondary goal is to create a model of theperformance, which also has a low computational cost so that itcan be used as a sub model in more complex system models. Toreach the primary goal, to elucidate the limiting factors, amodel has to be set up that describes the most importantphysical principles occurring in the anode.In addition, experiments have to be performed to validatethe model. To reach the secondary goal, the model has to bereduced to a minimum. A visual DMFC has been developed alongwith a methodology to extract two-phase data. This has provento be a very important part of the understanding of thelimiting factors. Models have been developed from a detailedmodel of the active layer to a two-phase model including theentire three-dimensional anode.The results in the thesis show that the microstructure inthe active layer does not limit the performance. Thelimitations are rather caused by the slow oxidation kineticsand, at concentrations lower than 2 M of methanol, the masstransport resistance to and inside the active layer. Theresults also show that the mass transfer of methanol to theactive layer is improved if gas phase is present, especiallyfor higher temperatures since the gas phase then contains moremethanol.It is concluded that the mass transport resistance lower theperformance of a porous DMFC anode at the methanolconcentrations used today. It is also concluded that masstransfer may be improved by making sure that there is gas phasepresent, which can be done by choosing flow distributor and gasdiffusion layer well.Keywords: direct methanol fuel cell, fuel cell, DMFC, anode,model