Research on Mass Transfer Barrier Layer of Concentrated Direct Methanol Fuel Cell

Author:Xue Rui

Supervisor:zhang yu feng


Degree Year:2019





With the rapid development of micro power supplies and increasing market demand,direct methanol fuel cell(DMFC)based on microelectro mechanical system(MEMS)technology has become a research hotspot due to its great application prospect in the future.Compared with other types of fuel cells,DMFC has the characteristics of high energy density,rich fuel source,low cost,easy storage and carrying,and high safety.Micro-direct methanol fuel cells,especially passive micro-direct methanol fuel cells,with the increase in the concentration of methanol solution,the energy efficiency of the battery increases.However,the high concentration of the anode is usually accompanied by the problem of methanol permeation.In order to achieve performance values that can be used,a dilute methanol solution(<10 wt%)is typically used as a fuel to minimize the permeate flux of methanol through the proton exchange membrane.However,when a diluted methanol solution is used,the specific energy density of the fuel cell is significantly reduced.Due to the phenomenon of methanol permeation,fuel waste caused by infiltration and cathode overpotion become obstacles to the increase of methanol solution concentration.In addition to the existence of cathode flooding,it is necessary to find an improved solution for high-concentration passive micro-direct methanol fuel cells.In this paper,the theoretical analysis and experimental verification of the anode methanol mass transfer barrier of micro-direct methanol fuel cell under high concentration conditions are completed.The DMFC full-cell model with anode mass transfer barrier is established and compared with the traditional model at high concentration.This model comprehensively considered the material transfer,electron and proton transfer,heat transfer and electrochemical reaction dynamics inside DMFC,and can accurately analyze the changes of performance after the addition of mass transfer barrier layer as well as methanol crossover,CO2 gas distribution and other key factors affecting the performance.The simulation results show that the anode high concentrations of 10.0 M methanol solution after mass transfer barrier layer buffer decreased significantly,arriving at catalytic layer with 2.0 M,which is a low concentration.The simulation results show that the cathodic overpotential caused by methanol permeation at high concentration has much less impact on the performance of the battery than the conventional battery model.Through the modeling and simulation of the mass transfer barrier layer,various performance indexes of different porosity barrier layers for DMFC are obtained.The impact of the study provides a theoretical basis for subsequent experiments.The composite structure of stainless steel sintered felt as anode collector-diffusion layer was prepared.The polarization data show that the new anode structure improves the performance at high methanol concentration.Stainless steel fiber felt(SSFF)was used as the anode diffusion layer aa well as flow field plate to replace the gas diffusion layer and anode current collector of conventional structure,eliminating the carbon paper based anode diffusion layer.In addition,the fine pores of stainless steel felt can facilitate the exhaust of anode carbon dioxide and prevent the lack of reactant caused by gas clogging pores from causing performance loss.This composite structure not only optimizes the composition of the battery,but also lays a foundation for the preparation of the subsequent mass transfer barrier layer.Different barrier preparation schemes have been developed for different applications.The stainless steel sintered felt was prepared as an anode current collecting plate-diffusion layer integrated single cell to achieve the optimized battery structure and at the same time reduce the methanol permeation effect.The battery with the new mass transfer barrier reached a maximum power density of 20.8 mWcm-2 at a concentration of 7.0 M,while the conventional battery reached a maximum of 20.4 mW cm-2 at 3.0 M.On the context of keeping a stable output power density,the novel DMFC improves the methanol concentration at anode.The graphene-stainless steel felt composite structure was then introduced to further buffer the methanol mass transfer,and different composite methods were used for different applications.Finally,a high performance cathode water-backing effect of self-breathing DMFC was designed and fabricated.The battery has a novel water"mass transfer barrier layer",which can effectively drain the cathode and alleviate the phenomenon of"water flooding".The influences of cathode backwater structure,material thickness,porosity and hydrophobicity on the performance of concentrated passiveμDMFC were analyzed.The water transmission coefficient is 1/3 lower than that of normal batteries,indicating that rgo-ssff can reduce the water flux from anode to cathode and avoid water flooding.It can improve the mass transfer efficiency of cathode oxygen,prevent the blocking of oxygen mass transfer channel,as well as make the cathode reduction reaction faster and output voltage higher.The new type of reduced graphene oxide-stainless steel fiber felt(rGO-SSFF)composite material is used as the cathode diffusion layer and the DMFC of the collector plate,and its peak power density is 20%higher than the discharge time of the conventional structured single cell.In this paper,the mass transfer of cathode water is also blocked.On the premise of not affecting the oxygen transfer of cathode,the water pressure on one side of cathode is increased to finally achieve the effect of"water backing"and further increase the concentration of DMFC in high concentration.