| A micro direct methanol fuel cell (μDMFC) is an electrochemical energy-conversion device that converts chemical energy of liquid methanol into electrical energy directly. Because of its unique advantages, such as high energy densities, facile liquid fuel storage, and simple system structures, the μDMFC has been identified as one of the most promising power sources for portable and mobile applications. However, there are still some issues for the μDMFCs, such as methanol crossover, low-efficient methanol transport, low integration and lack of stability of the μDMFC stack, etc., leading to the poor performance and making it hard for μDMFCs to be utilized in portable application. In this paper, a two-phase mass transport model of an active μDMFC was proposed, and based on the grasp of the inner mass transport of the μDMFC, two novel anode structures were designed and optimized. Further, to study on the anode mass transport of passive μDMFC, a two-dimensional two-phase non-isothermal model considering the effect of natural convection was also established. At last, a “4-cell” μDMFC module stack was proposed, which contributed to further development of portable μDMFC systems.To solve poor mass transport efficiency in the anode flow channel, this paper presents a novel N inputs-N outputs (NINO) parallel flow pattern with convexes to reinforce methanol mass transfort and reduce concentration polarization. The simulation results show NINO parallel flow channels with the rectangle convexes are distinctly contributed to improve the performance. The μDMFCs with four anode flow patterns respectively are fabricated by MEMS technology. The testing results show that the performance of μDMFC with the rectangle convexes is twice higher than the traditional flow field structure. Moreover, to increase the supplying methanol concentration of the μDMFC, this paper also presents a novel compound anode flow field structure. Similarly, mass transport of the novel flow field structure was analyzed by numerical and experimental methods. Both the modeled and the experimental results show that compared with the conventional parallel flow field, the compound one can enhance the masstransfer resistance of methanol from the flow field to the anode diffusion layer.To study the mass transport phenomenon of the passive μDMFC, a two-dimensional, non-isothermal model is presented for a passive μDMFC. The effect of natural convection at the anode in the fuel reservoir is considered. The coupled heat and mass transport of the whole cell, along with the electrochemical reactions occurring in the passive μDMFC are modeled. The comprehensive model is solved numerically by the finite element method and validated against the experiment results reported in this paper. The numerical results show that when in vertical operation, the cell temperature increases gradually from the bottom of the cell to the top of the cell, resulting from natural convection at the anode. A higher cell temperature will lead to stronger natural convection in the fuel reservoir, which will in ture lead to a larger temperature difference across the cell. The results also indicate that the rate of methanol crossover increases with increasing methanol concentration. When incrementally increasing the current densities, the rate of methanol crossover decreased at low methanol concentration but increased at high methanol concentration. These results provide more accurately mass transport process and specifics of the passive μDMFC.To validat the anode nature convection of the passive μDMFC, a single passive μDMFC was designed and fabricated. Based on the structure of the single passive μDMFC, a “4-cell” passive μDMFC unit stack was designed and fabricated for portable application. Two electrical connection accessories were designed to connect the μDMFC unit stack in series. This novel structure could improve the stability of the μDMFC stack. The testing results show that the passive μDMFC unit stack obtained the maximum power of545mW under the5mol/L methanol concentration. The maximum power density of the passive μDMFC unit stack is29.62mW/cm2. This passive μDMFC stack showed good response with the high current and impulse discharging, which could be well applied to the portable application. |
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