Structural Modification And Transport Optimization Of MEA For Direct Methanol Fuel Cells

Optimizing the energy structure,developing efficient and clean secondary energy and reducing the pollution of energy consumption to the environment are extremely important parts in China's sustainable development strategy.Direct methanol fuel cell(direct methanol fuel cell,DMFC),as a device that can continuously convert chemical energy in fuel into electrical energy,is regarded as one of the most promising clean and efficient energy production technologies.Because of its simple structure,high energy density and easy transportation,it is widely used in portable equipments.As the core component of DMFC,the membrane electrode(membrane electrode,MEA)directly affects the performance of the cell.Currently,some problems exist in Direct methanol fuel cells as follows:anode methanol oxidation reaction kinetics is slow;a large amount of precious metal catalysts need to be used per unit power,and the cost is high;methanol crossover induces mixed potential to cause cell performance to decrease;the problem of cathode flooding seriously reduces the electrochemical active area and the transmission resistance increases;poor transport property of the microporous layer causes problems such as cell performance degradation.Therefore,expanding the anode electrochemical active area,reducing methanol crossover and ensuring the normal transmission of reactants and products,building a high-performance MEA have become the key to improving the performance of direct methanol fuel cells.In this paper,the influence of the hydrophilicity/hydrophobicity of the anode diffusion layer on the cell performance and the mass transfer process under the condition of high/low concentration of methanol and the influence of temperature on the cell performance increase were compared.The results show that at low concentration of methanol,the hydrophilic anode diffusion layer has better discharge performance,while at high concentration,the hydrophobic anode diffusion layer has more advantages.In low concentration DMFC,the lower the temperature,the more significant the effect of increasing the hydrophilicity of the anode diffusion layer on the cell performance.EIS indicates that the increase of hydrophilicity of the anode diffusion layer will reduce the mass transfer resistance of the anode and improve the methanol transfer efficiency.In low concentration DMFC,the reduction of mass transfer resistance can alleviate the shortage of anode fuel and improve the cell performance.But at high concentrations,it leads to more methanol crossover,reducing cell voltage and discharge performance.A novel anode double microporous layer structure was then prepared,in which an inner microporous layer containing Nafion ionomer was added between the anode catalyst layer and the outer microporous layer containing PTFE.Experimental results show that the double-layer structure can effectively reduce the charge transfer resistance and mass transfer resistance of the cell,expand the area of electrochemical activity,and reduce the attenuation of the cell performance.After the conventional electrode runs for a long time,the catalyst sinks into the microporous layer and cannot be utilized.Nafion ionomer in the inner microporous layer has the ability to conduct protons,enabling the sinking catalyst to play a catalyst role and increasing the electrochemical reaction site.The hydrophilicity of Nafion ionomer reduces the transport resistance of methanol solution.Finally,a membrane electrode structure which can reduce the methanol crossover was proposed.The graphene layer was constructed between the anode catalyst layer and the proton exchange membrane and its material composition is optimized.The experimental results show that the graphene layer can effectively reduce methanol crossover,but it will increase the charge transfer resistance of the cell,reduce the electrochemical active area,resulting in lower discharge performance.Graphene layers adding Nafion ionomer can improve the transmission efficiency of H~+,reduce the cell charge transfer resistance and extend the three-phase interface reaction.The modified graphene layers still has a good barrier for methanol,and can effectively reduce methanol crossover caused by the cell voltage drops,improve the discharge performance of high concentration direct methanol fuel cells.