Numerical Simulation Study On Vanadium Microfluidic Fuel Cell

In addition to the supplying of uninterruptible power to industrial production,people also need some portable power sources for portable devices, such as electricalvehicles, laptop and so on. With the continuous development of new energytechnologies, various forms of energy storage were invented. Among them, thechemical energy storage applications using vanadium as the couple of oxidation andreduction are widely used. This dissertation developed battery mathematical modelwhich based on physical and chemical processes including electrochemical reactionkinetics, fluid flow and mass transport. Using this mathematical model, some typicalcharacteristics about vanadium redox flow battery and microfluidics fuel cell werestudied.The temperature in the vanadium redox flow battery has great impacts on theelectrolyte solubility and stability. When temperature stays below5°C, the redoxcouple V²⁺/V³in catholyte will condense; On the contrary, thermal subsidence ofVO⁺₂happens at higher temperature (above40°C) in the positive electrolyte.Without critical temperature control, chemical thermal subsidence ofVO⁺₂willblock electrolyte flow channel and the pores of carbon electrode, which can causebattery performance degradation. Hence, the temperature is a very important controlparameter in the redox flow battery system. In this article, the combined effects of thetemperature at the inlets and air, volume flow rate and applied current were studiedthrough establishment of the non-isothermal model.(1) High flow rate could be able to improve the uniform of the concentrations inelectrode surface effectively. The distribution of concentration is affected obviouslyby temperature at a low flow rate; degree of influence by temperature will decreaserelatively with increasing flow rate. As the electrolyte flow rateu0increasescontinuously, the difference contours of reactants between adjacent lines is graduallyreduced, which indicates the effect on the gradient distribution ofVO²⁺becomessmaller by temperature with the increase of flow rate.(2) Over-potential and temperature have their higher value with applied current increases. In the case of higher current, the range of over-potential value is also largei.e. the distribution of over-potential on the electrode surface is non-uniform.(3) The contours ofVO²⁺concentration shows the distribution trends which ishigher at right than left (near the current collector plate), and this tendency becomesmore serious gradually with the increase of temperature T_inat the inlets. When T_air(airtemperature) and T_inat the value of293K and323K respectively, the concentrationgradient of tetravalent vanadium ions reaches maximum value and has lowerconcentration at the side of collector plate, which results in side reactions easily.Microfluidic fuel cell has been developed based on the concept of chemical labon a chip and micromachining technologies, and unlike the traditional fuel cell it doesno longer need a polyelectrolyte membrane to separate the fuel and oxidant. Whentwo laminar flows encounter in microchannel simultaneously, a liquid-liquid interfacebetween them as a 'virtual membrane' will be developed. Due to the membraneneglected, issues related to membrane hydration and water management areeliminated; structure is more simple and compactable. However, the trade-off betweenfuel utilization and power density of microfluidic fuel cell is a limited factor. In thispaper, a three-dimensional numerical model was developed to determine the effects ofsome important factors on cell performance and optimize the fuel utilization. In thiswork, two methods have been employed including recycling fuel repeatedly andintroducing the porous electrode. The main contents of this article are listed asfollows.(1) A double-Ψ structure of microfluidic fuel cell was first proposed in thisarticle, which can be used to separate fuel from outlet in the terminal. And itsperformance was studied in details.(2) The introduction of ion conductive fluid in the middle of microfluidic caneffectively prevent the cross-over between fuel and oxidant. In addition, there is alsoan important phenomenon that the fuel concentration near the electrode surface hasincreased significantly. This is of great significance to the vanadium redox micfluidiccell which suffered great limitation from the mass transport.(3) Fuel utilization of double-Ψ structure microfluidic fuel cell is higher thanthat of Y-shaped, i.e. cell performance has been improved significantly. Through8cycling separately, the maximum fuel utilization was achieved in45%at the peakoutput power density. (4) Fuel utilization of microfluidic fuel cell with through-flow porous electrodeis related to the porosity. When the flow rate is below1μL·min^-1or above10μL·min-1,the maximum fuel utilization was achieved under flow rate at the porosity value of0.85or0.65. When the flow rate is below1μL·min^-1, the fuel utilization is nearlyclose to85%at the state of peak output power density.