Study on Key Technology of the Passive Micro Direct Methanol Fuel Cell Cathode

Author:Cao Zuo Mu

Supervisor:liu xiao wei


Degree Year:2018





Direct methanol fuel cell(DMFC)has become a research hotspot in the field of fossil fuels alternatives due to the many advantages including high energy density,high efficiency,environmentally friendly,simple structure,high portability,and low temperature start-up.In recent years,with the popularization of the micro electro mechanical system(MEMS),the micro methanol fuel cell(μDMFC)development rapidly,become one of the international research hotspot in the field of micro energy technology.According to the reactant supply mode theμDMFC can be divided into active and passive type.Since the activeμDMFC need to use the pump,valve,and fan to control fuel transport,the passive type(fuel transportation by diffusion and natural convection)is more advantageous to reducing the volume,weight,and energy consumption and improve energy density.However,compared with the activeμDMFC on a micro scale,the problem of gas-liquid two-phase mass transport,such as the passiveμDMFC cathode flooding and methanol infiltration,becomes more and more obvious.On the other hand,the performance ofμDMFC depends on the kinetics rate of the cathode reaction.Therefore,raising cathodic catalysis efficiency and reducing catalyst cost are also obstacles to the commercialization ofμDMFC.In order to solve the above problems,this paper will carry out theoretical and experimental research from the aspects of membrane electrode and collector plate in the passiveμDMFC cathode structure,so as to improve the output efficiency of the fuel cell.First,it is proposed to create an ultrathin hydrophilic structure between the cathode side of theμDMFC proton exchange membrane and the catalytic layer,which can improve the water flooding and methanol permeation of the fuel cell cathode.The results show that the hydrophilic structure can reduce the methanol permeation and flooding of theμDMFC by the simulation analysis of the gas-liquid two-phase mass transfer model.By using the technology of micro-processing such as spraying,the carboxyl carbon nanotubes were prepared into ultrathin hydrophilic layer and used for membrane electrode preparation,then theμDMFC assembly,test and performance analysis were carried out.The experimental results show that introduce a hydrophilic layer at the cathode side allows the water diffused from the anode or produced at the cathode to gather there and the super hydrophobic interface of the cathode catalytic layer is used to increase the concentration gradient of water forcing it back to the anode.Secondly,a MoS2-Pt/C binary catalyst was proposed in order to improve the slow ORR kinetics of cathode and the high amount of precious metal in the cathode catalyst.By establishing a fuel cell cathode Pt and MoS2-Pt model,using materials Studio software to simulate and calculate the catalyst,the author analyzes the ORR catalytic activity of different catalyst models and expounds the mechanism of the reaction,revealing that the introduction of MoS2 can promote the reaction of pure Pt catalyst to ORR.Provide a theoretical basis for the experiment.The Pt and MoS2-Pt models of theμDMFC cathode were established.Through the establishment of fuel cell cathode and MoS2-Pt model,Pt catalyst to make use of the Materials Studio software simulation calculation and analysis of different catalyst for ORR catalytic activity and reaction mechanism of the model,reveal the introduction of the MoS2pure Pt catalyst for ORR reaction with the mechanism of the promoting effect,as to provide theoretical basis for the experiment.In the experiment,the MoS2-Pt/C binary catalyst was prepared with carbon black as the carrier and characterized by microwave reduction.The test results show that the prepared MoS2-pt/C binary catalyst has higher ORR catalytic activity and stability than Pt/C,which is consistent with the theoretical calculation results.The structure of MoS2-Pt/C binary catalyst provides a new way for the development of a low cost and high activity DMFC catalyst.Finally,a passive DMFC cathode collector structure with the capacity of water collection and directional transport was proposed in this paper to solve the problem of passive DMFC cathode flooding under longtime discharge.Part of the cathode reaction produces water at the cathode catalyst interface will be in the form of water vapor through the porous diffusion electrode outward transport,spread to the cathode current collector condense into liquid water attached to the diffusion electrode surface,the longtime accumulation formation water droplets in diffusion electrode inlet channel congestion,reduced the amount of oxygen into the,causes the cathode electrochemical reaction efficiency drop will affect the cell performance.Using surface erosion,surface modification and laser etching process was prepared with the combination of super hydrophilic and hydrophobic water collecting plate interface,and the planning of super hydrophilic collection interface and superhydrophobic,super hydrophilic drainage channel interface.On the basis of this,a fuel cell collector plate with a storage tank is realized and a cathodic water collection structure which can collect and transmit water directionally is introduced in the cathode.The fuel cell was assembled and tested,and the results showed that theμDMFC performance with a new cathode collector plate structure was significantly improved,and its performance did not decrease after a long period of discharge.At the same time,the structure can also realize the collection,directional transportation,and storage of cathode water,which lays a foundation for the reuse of the passiveμDMFC cathode water.