Numerical Study Of Microfluidic Membraneless Fuel Cell

In this work, the performances of a Y-shaped microfluidic membraneless fuel cell are investigated by computational study.Microfluidic membraneless fuel cells define a group of fuel cells with fuel delivery and removal, reaction sites and electrodes structures all confined to a microfluidic channel. They operate without the need of a physical membrane to separate the anode and cathode flows. The fuel cell consists of a Y-shaped microfluidic channel with liquid fuel and liquid oxidant entering at separate inlets and flowing in parallel without turbulent or convective mixing.Experimental studies have established the proof-of-concept of this kind of fuel cells and theoretical models have been implemented in previous studies. This work presents a mathematical model providing insight into the running parameters of the cell. This model contains the flow kinetics, species transport and electrochemical reactions as well as the influence of the chemical reaction within the channel and the influence of the ionic concentration related electrical conductivity, which have not been taken into account in previous studies at the author knowledge. The model is implemented in a commercial CFD package FLUENT 6.3 with custom developed user defined functions (UDF).Through numerical simulations, we first put into evidence the effect of Peclet number on the mixing zone which is at the interface of the two streams, where species mix by diffusion. The width of this zone extends gradually as Peclet number decreases. To avoid the fuel crossover we have to prevent this zone to reach the electrodes. Thus it is better to use high Peclet number, that is to say either high inlet velocity or low channel height. The performances of the fuel cell are shown to be completely cathode limited by the low flux of oxygen transport to the electrode. An increase in the fuel concentration has almost no effect on the polarization curve. The conductivity is shown to be linked to a great extent to the electrolyte concentration (sulfuric acid), but even if the concentration, then the conductivity, is multiplied by 10 the performances of the cell do not improve significantly. Crucial factors are given by the volumetric flow rate and the oxygen concentration. Increasing the volumetric flow rate up to 8ml/min enhances the maximum current density as well as the maximum power density generated but lowers the consumption ratio of oxygen. Above this critical value the consumption ratio is constant and around 0.1%. An augmentation of the oxygen concentration improves linearly the maximum power density of the cell. Thus this model enables us to determine which configuration is the most wanted based on appropriate priorities for the use of the cell: high power density or saving of oxygen. The effect of the chemical reaction inside the channel is also considered. This reaction leads to a deviation of the diffusive zone towards the corresponding electrode (cathode for oxygen and anode for fuel) and can prevent or delay fuel and oxygen crossover.Further studies should focus on how to find a more efficient way to improve the transport of the oxidant to the cell: by using another electrolyte for the oxygen (with higher diffusivity), by using air breathing membraneless fuel cells or other geometries, such as cylindrical channels. Phenomena as electrophoresis effect or electrical force were not taken into account in our model by lack of time and it could be also of great interest to define to which extent they have an influence on the fuel cell behavior.