Research On High Concentration Micro Direct Methanol Fuel Cell Based On Carbon Aerogel

The rapid development of information technology has put forward higher and higher requirements for the power systems.Micro direct methanol fuel cells（μDMFCs）are considered to be one of the ideal solutions to the power supply for portable electronic devices due to the advantages of high energy density,high power conversion efficiency and free of pollution.However,the critical challenges of mass transfer management and catalyst poisoning forμDMFC disable its direct operation under high concentration fuel supply.Previously,most of the solutions proposed by researchers were based on the macro-scale,introducing new structures or devices into the cell,which not only reduced the specific energy density of the fuel cell,but also increased the complexity of the system.In this regard,based on the microstructure characteristics of the three-dimensional nanomaterial-carbon aerogel,this paper focuses on the micro-scale mass transfer management ofμDMFC under high concentration fuel supply on the premise of not increasing the complexity of the system.Based on the theoretical simulation of the multiphysics model,this paper develops optimization design of the inherent structures of the membrane electrode assembly and proposes a new method of fuel supply to achieve the high performance ofμDMFC under high concentration fuel supply.A two-dimensional two-phase steady-state model of mass transfer was established forμDMFC under high concentration methanol supply based on electrochemistry,thermodynamics,and multiphase flow theory in porous media.The model ran simulations for the mass transfer characteristics of methanol and water in the membrane electrode assembly.Results showed that the overpotential caused by methanol crossover and the influence of water on the anode reaction kinetics and the proton conductivity of the proton exchange membrane are decisive factors for output performance.In addition to the optimization of the membrane electrode assembly to enhance the water recovery from cathode and reduce the methanol crossover,a rational methanol/water molar ratio from fuel supply vapor is essential for the stable operation of high-concentrationμDMFC.Based on the super-hydrophilic nitrogen-doped carbon aerogel,a water management structure of cathode catalyst layer was designed.By forming a high water concentration and pressure on the cathode side of the proton exchange membrane,the recovery water flux from the cathode to the anode is increased.The cathode polarization has been significantly reduced due to the decrease of methanol crossover and the enhancement of oxygen mass transfer,leading to an increased operation concentration and a better performance and stability.At the same time,the anode polarization ofμDMFC under the vapor supply has been reduced,and the performance was improved by 31.3%.Graphene aerogel was applied to construct the microporous layer ofμDMFC at both anode and cathode,in order to optimize the gas-liquid two-phase management via the intrinsic microstructure and characteristics of graphene aerogel.ForμDMFC with graphene aerogel based anode microporous layer,the methanol crossover during the discharging has been reduced significantly,accompanied by a better emission of CO₂,which led to a 20%increase of maximum power density with the operation concentration increasing from 3 M to 6 M.ForμDMFC with graphene aerogel based cathode microporous layer,the water recovery from the cathode has been enhanced by increasing the transport resistance outward,leading to a tripled long-term discharging time.The decrease content of the binding agent enabled a further optimization of the cell performance.A novelμDMFC fuel supply design was proposed based on the porous media evaporation process to reduce the methanol/water molar ratio in the vapor of fuel supply.A two-phase non-isothermal transient model was established to explore the influence of porous media properties on the evaporation process of binary solutions.Carbon aerogel was chosen as the vaporization carrier for methanol solution.The polar and dispersion interactions between the porous media and methanol molecules has been selectively enhanced,resulting in the change of surface tension and thereby reducing the relative volatility of methanol/water.The regulation effect of the novel fuel supply design on the evaporation of methanol solution ensured the mass transfer balance of methanol and water in the membrane electrode assembly.As a result,μDMFC ran directly with 16 mol/L methanol solution,showing a maximum power density of 22 m W cm^-2.During the long-term discharging test,μDMFC maintained a high stable output of 20 m W cm^-2(±2 m W cm^-2)for 6 hours.