Study On Anode Gas-liquid Flow Of The Micro Direct Methanol Fuel Cell

With the rapid development of micro energy technology, Micro Direct Methanol Fuel Cell (μDMFC) based on Micro Electro Mechanical System (MEMS) technology has been a promising application due to its high energy density, high efficiency and merit of environmental protection. Different from the traditional sized cell, the decrease of feature size has an evident effect on internal gas-liquid two-phase transport. In this paper, a three-dimensional μDMFC two-phase full cell transfer model was proposed to investigate methanol mass transfer and two-phase flow at anode with porous medium systematically. Combined with the micro scale effect of anode flow field, two phase flow in anode micro channel was simulated from a Mesoscopic perspective, and was experimentally analyzed using high-speed cameras, by which theoretical model can be validated. The innovated response surface analysis method was applied to study effect on performances and the two-phase dynamic characteristic at anode by operation parameter. At the same time, anode adaptive supply system of μDMFC was developed based on the response surface analysis results, providing effective basis to the portable application of micro direct methanol fuel cell system.When μDMFC is being operated, CO2from the anode catalyst layer transferred to anodic porous media area in the first place, forming gas liquid two phase flow joined with methanol solution. Therefore, this paper established three-phase flow model for the whole μDMFC, by which the mass transfer of methanol solution and gas-liquid two phase flow in anode porous media were comprehensively analyzed. The results show that operation parameters such as the methanol concentration, working temperature etc. have obvious influence on cell performances. When the characteristics (hydrophilicity effect, compression character) of anode porous medium change, methanol mass transfer and two phase flow in corresponding area change in orderliness. Moreover, a gradient gas diffusion layer structure, which can effectively improve two phase flow characteristics and battery performances, was proposed. Research results above provide a strong theoretical basis for optimization research of operation parameters in μDMFC.As the feature size decreases, the micro scale effects, which can be ignored under macro flow become increasingly obvious after CO2transfer to the anode flow field through the porous media. Therefore, combined with the theory of the Mesoscopic analysis, the main micro scale effect inside the anode flow field is analyzed using the Lattice-Boltzmann method. The cross section effect and hydrophilicity effect were simulated to investigate the influence on the movement process of two phase flow and cell performances. Simulation results show that: compared with traditional structure, hydrophilic channel with the length-width ratio of2:1, has obvious advantage on the removal of CO2. In order to verify the simulation results, different structure of aluminum based plates were fabricated using the Computer numerical control technique. The experiment results show that the performances are improved significantly after the optimization of anode micro channel, which certificates the simulation results.When fuel cells work, the corresponding operation parameters have a direct effect on performances and the internal two-phase flow. Based on the whole μDMFC model, the innovative response surface analysis method was applied to study the effect on performances by operation parameters and the CO2dynamic movement, which set the input variables as the methanol concentration, anode flow rate and the operating temperature, the output variables as the function of maximum output power density and open circuit voltage. Results show that the air-breathing μDMFC reaches the maximum power density of105.41mW/cm2when working at temperature of60℃and highest power density of50.59mW/cm2at room temperature. This method not only can represent the effect of the operation parameters on the cell performances, but also can analyze the overlapping effect between different operating variables, providing effective data support for portable application of μDMFC system.Based on the research achievements of the response surface analysis, anode adaptive supplying method was proposed and designed, and then applied to air-breathing μDMFC single cell and stack, forming the portable application system. Moreover, the adaptive supply module function, system dynamic performance and stability were comprehensively tested and analyzed. The results show that μDMFC single cell based on adaptive supply method has obvious improvements in dynamic characteristics. The fabricated portable stack was consist of6cells, with overall size of only13.25cm3and weight of24.075g. Test results show that the cell reaches its maximum output power under the optimal feeding pattern. In this supply mode, single units in the stack had uniform output, which showed favorable response characteristics in the dynamic testing, and were successfully applied in driving small household fan lasting for long hours.