Multiphase flow management in confined geometries applied to the optimization of direct methanol fuel cells.

摘要

The production of CO2 gas at the DMFC anode leads to dramatic increases in pumping power requirements and reduced power output because of mass transfer limitations as bubble trains form in the channels of larger stacks. Experimental observations taken in a 5 cm2 DMFC test cell operated at 60°C, 1 atm, and with a methanol/water fuel flow-rates of 5-10 cc min-1 indicate that the rate of bubble formation can be reduced by increasing the fuel flow because more liquid is available for the CO2 to dissolve in. Further observations indicate that KOH and LiOH added to the fuel eliminates CO2 gas formation in situ at low concentrations because of the greatly increased solubility that results.;A mathematical model for the volumetric rate of CO2 gas production that includes effects of temperature and solubility is developed and extended to include the effects of hydroxide ions in solution. The model is used to predict the onset location of gas formation in the flow field as well as the void fraction at any point in the flow field. Predictions from the model agree very well with our experiments. Model predictions explain differences in the initial location of bubble formation for fuel solutions pre-saturated with CO2 as opposed to CO2-free solutions. Experiments with KOH and LiOH added to fuel solutions confirm the validity of the model extension that includes solubility that is enhanced by chemical reaction.;Experiments with LiOH, KOH, and ammonium hydroxide show that the long-term durability of standard Pt-Ru/Nafion/Pt membrane electrode assemblies is compromised because of the presence of lithium, potassium, and ammonium cations that interact with the Nafion membrane and result in increasing the ohmic limitations of the polymer electrolyte membrane. Experiments with Ca(OH) 2, while reducing gas formation, precipitate the product CaCO3 out of solution too rapidly for downstream filtering, blocking channels in the flow field. Attempts to manipulate multiphase flow by altering fuel fluid properties through the addition of glycerin, sucrose, and Triton X-100 are negative, resulting in no useful improvements due to oxidation of the additives on the platinum catalyst and poisoning by reaction intermediates.;Experiments with independent pressurization of the direct methanol fuel cell anode and cathode allow for the observation of DMFC operation with carbon dioxide gas formation suppressed. Results indicate that the limiting current density is strongly related to the applied pressure, and, therefore, to the presence of CO2 in the liquid phase. An additional experiment where CO2 is allowed to accumulate in recycled anode fuel solution over a period of time and is then stripped from solution using nitrogen gas indicates that the presence of CO2 in anode fuel solution at any pressure contributes to significant decreases in power and current density. Because CO2 bubbles are ubiquitous in direct methanol fuel cells, this finding is key to the optimization of these systems.;Principles of kinetic theory are used to model the coalescence of bubbles in horizontal and vertical upflows where bubble movements are restricted by channel geometry. There are four critical variables that determine the rate at which a swarm of small bubbles will coalesce: the initial bubble population, the average initial bubble diameter, the average relative velocity of the bubbles, and the efficiency of bubble collisions. Model descriptions of bubble population and size distributions agree well with experimental results for air/water and air/water/glycerin systems performed in a rectangular channel with an aspect ratio of 12.5 and a cross-sectional area of 2 cm2 under both vertical and horizontal orientations.