Numerical Study Of Flow And Transport Characteristics On Air-breathing Microfluidic Fuel Cells With A Flow-through Anode

Recently,with the development of internet communication technology,portable mobile devices are used widely.There are more requests in well performance,high stability and environmentally friendly for micro power sources.With lithography and soft-lithography technologies becoming mature gradually,membraneless microfluidic fuel cells(MMFC)considered as a kind of micro power sources.MMFCs separate the fuel and oxidant using the controllable co-laminar flow in the microchannel.Using this method,the proton exchange membrane in the conventional fuel cell is removed,reducing the cost of cell and eliminating problems related to the membrane.Therefore,the microfluidic fuel cell has received much attention.Researchers evaporated sequentially chromium and Au onto the sidewalls of Y-shaped microchannel using an electron bean evaporator,fabricating the original MMFC.There is a big problem in this kind of MMFC,that is,mass transport limitation.It causes concentration boundary layers on the surfaces of electrodes,resulting in a low fuel utilization and poor cell performance.The concept of air-breathing cathode was proposed to solve the issues of low concentration of oxidant and concentration boundary layer.And the anode performance is improved to a certain extent by using a flow-through anode.Generally,carbon paper or carbon cloth are used as the material of flow-through electrodes.However,it doesn't take full advantage of the flow-through electrode due to the two-dimensional surface catalytic layer loaded on it.The three-dimensional graphite felt with large porosity has a large specific surface area.Using the graphite felt as a flow-through electrode,catalyst could be loaded in it by some methods,such as electrochemical deposition.The forming of the structure of three-dimensional catalyst layer provides more possibility to improve the performance of MMFC.Therefore,the present study aims at an air-breathing microfluidic fuel fells with a flow-through graphite felt anode,studied systematically flow and transfer characteristics in the conditions of the different electrolytes by numerical simulations.Single-phase flow and two-phase flow models under acidic electrolyte condition,as well as a single-phase flow model under alkaline electrolyte condition are established for the microfluidic fuel cells.The simulation results including velocity field,concentration field and phase distribution in the MMFC are obtained.The influence of fuel concentration and flow rate of reactants on the cell performance are discussed.By changing the size and structure of the cell,performance are studied at different conditions.The main conclusions of this study are as follows:(1)The air-breathing microfluidic fuel cell with a flow-through graphite felt anode in the acidic electrolyte,comparing with carbon paper and carbon cloth used as electrode material,can improve fuel utilization and mitigate fuel crossover and thus improve cell performance.Increasing flow rate of reactants,inlet concentration of fuel and thickness of graphite felt,the cell performance increases firstly,then trends to be steady.Remaining the same catalyst layer volumes,electrode structure is changed to the cylinderic electrode,the power density of the cylinderic MFFC is twice as the planar MFFC,with a fuel utilization of up to 80%.But at the same time,it increase in the fuel crossover especially at applied voltage is high.(2)The graphite felt anode MMFC in the acidic electrolyte produces carbon oxidant(CO2)in the anode catalyst layer.According to the result of the two-phase flow model,the gas fraction was up to 0.4.CO2 in the pore of the catalyst layer and it caused the decrease of the cell performance.The maximum current density and power density are 36.4 mA/cm2 and 3.0 mW/cm2,respectively.Because CO2 gases are removed with the liquid,they have no effect on the cathode performance.Changing reactant flow rate,cell performance do not change prominently;changing fuel concentration,cell performance increases with the increasing of the fuel concentration.(3)The air-breathing microfluidic fuel cell with a flow-through graphite felt anode in the alkaline electrolyte could achieve a high current density and power density.The MMFC with a 2mm-thickness graphite felt anode yielded at a maximum current density of 136.9 mA/cm2 and a maximum power density of 23.5 mW/cm2.Comparing with the acidic MMFC in the same condition,the current density and the power density of the alkaline MMFC increased 6.6 times and 7.6 times,respectively.Increasing fuel concentration and thickness of graphite felt,cell performance increases,but the latter's increment decreasing gradually.With increasing of flow rate of reactants,the performance increased,and then trends to be steady.