Lattice Boltzmann Simulation Of Anode Mass Transfer Characteristics In Cotton Thread-based Microfluidic Fuel Cells

Among new portable micro fuel cells,membraneless microfluidic fuel cells,which employ the characteristics of parallel laminar flow of fuel and oxidant in the microchannel to separate the anode and cathode,has many advantages such as eliminating the proton exchange membrane and avoiding a series of problems related to it,lowering cost,easy to be miniaturization,and high energy density.Therefore,it has become research hotspot in the field of energy.However,microfluidic fuel cells based on cotton threads utilize the capillary force of porous materials and gravity to drive fluids flow.It has the advantages of simple structure,low cost and small occupation space because of no external pumps and could maintain the continuous flow driven by the gravity,showing a promising power source for portable micro devices.However,the cell performance is limited by the fuel transfer at the anode,and it is very important to understand the transport mechanism to improve the cell performance.Moreover,there has been no reported study on the theoretical research of microfluidic fuel cells based on porous fibers.At present,the numerical study of microfluidic fuel cells is mostly based on computational fluid dynamics,and it is unable to elaborate the effects of the porous structure on mass transport and electrochemical reactions.In recent years,the Lattice Boltzmann method(LBM)has developed into an effective tool to deal with fluid transfer process in complex porous media.The LBM based on the representative elementary volume(REV)scale is suitable for a larger computing domain,which is efficient to explore the impact of various operating and structural parameters on cell performance.Therefore,the LBM based on REV scale is used to study the flow and the mass transfer of coupled electrochemical reaction in the anode flow channel of microfluidic fuel cells based on cotton threads.The main research results of this study are as follows:(1)A three-dimensional theoretical model of the reaction interface at the bottom of the flow channel and on the surface of the cotton thread was constructed for the flow channel unit with a single cotton thread.The effects of porosity,permeability and diameter of the cotton thread on the fluid flow and mass transfer in the flow channel unit were studied.The numerical results indicated that the fluid velocity in the cotton thread was evenly distributed and there was an order of magnitude difference between the internal and the external velocity of the cotton thread,and the reactant consumption when the reaction interface was on the surface of the cotton thread was much larger than that when the reaction interface was at the bottom of the flow channel.The average concentration along the flow direction was lower when the porosity,permeability or diameter of the cotton thread was lower.That was because the flow velocity was smaller resulting in the consumed reactants could not be replenished in time.Moreover,the mass transfer of reactants was mainly affected by the convection mass transfer when the reaction interface was at the bottom of the flow channel,while it was mainly affected by the diffusion when the reaction interface was on the surface of the cotton thread.(2)A three-dimensional theoretical model of flow channel unit with multiple cotton threads and reaction at the bottom was built for a cotton thread-based microfluidic fuel cell with carbon paper anode.The effects of different inlet fuel concentrations,flow rates,cotton thread arrangements and channel lengths on fuel transfer and the anode performance were studied.The fuel flow and the mass transfer coupled electrochemical reaction at anode flow channel unit were obtained.The results indicated that the fuel concentration decreases along the flow direction under different anodic overpotentials and larger decrement was found at higher anodic overpotentials.An increasing average current density was obtained with an increase in the inlet fuel concentration,leading to a better anode performance.Moreover,with an increase in the inlet fuel flow rate,a greater difference in the fuel concentration was observed between the location where the cotton thread contacts with reaction interface and other regions.However,when the inlet fuel flow rate decreased,both the fuel concentration difference and the fuel concentration at the downstream of the flow channel reduced.The trend of the average current density of the anode for the inverted triangle and the equilateral triangle arrangements was consistent and the difference was small.With an increase of the flow channel length,the local current density along with the flow direction on the bottom reaction interface decreased,and further led to a lower anode performance.(3)A three-dimensional theoretical model of flow channel unit with multiple cotton threads and reaction inside cotton threads was built for a cotton thread-based microfluidic fuel cell with fiber-integrated anode.The effects of different inlet fuel concentrations,flow rates,porosities and catalyst layer thicknesses on fuel transfer and anode performance were studied.The fuel flow and the mass transfer coupled electrochemical reaction in the fiber-integrated anode were obtained.The results showed that the fuel concentration outside cotton threads was higher than that inside cotton threads.An increasing average current density was obtained with an increase in the inlet fuel concentration due to a high electrochemical reaction rate,leading to a better anode performance.The fuel concentration was higher when the inlet flow rate was greater,resulting in a greater local current density and a better anode performance.A high porosity was beneficial to fuel transport and improvement of the anode performance,while the variation of porosity smaller than 0.2 had a little impact on fuel transport and anode performance.The change of the thickness of catalyst layer had a little impact on the velocity distribution of the fuel.However,the thicker catalyst layer could provide a larger electrochemically active area,and more fuel participated in the reaction to produce larger local current density,leading to a better anode performance.