Effect Of Ultrasonic Spray On Direct Methanol Fuel Cell Performance

With the advantages of high theoretical density,simple construction,a lower fuel costs,etc.,direct methanol fuel cell(DMFC)has a promising application prospect.However,there are still many problems impeding its promotion and application,such as the methanol crossover,catalyst poisoning,and carbon dioxide emission.Based on our preliminary research result,a new kind of high-performance DMFC structure combined with ultrasonic spray technology is proposed to relieve the methanol crossover phenomenon in fuel cell.In this paper,a lower-power,high spray volume ultrasonic atomization nozzle is developed and manufactured firstly.Then,we improve the structure of a conventional DMFC,and accomplish the design and manufacture of all the critical components.After assembled with the nozzle,the prototype DMFC is compared with the conventional liquid fed one under a performance testing to verify the effectiveness of this new cell design solution.Finally,we design a series of experiment to further study the rules about how the methanol concentration,ultrasonic power,atomizing rate and orientation parameters influence the dynamic open-circuit voltage and discharge polarization properties in the ultrasonic spray DMFC,and analysis the functional mechanism from the mass transfer perspective.The main research and experimental conclusion are as followings:(1)A whole improvement about the conventional ultrasonic spray is proposed,and each size of the spray is calculated by acoustics equation.After conducted the modal and piezoelectric steady dynamic simulation in Abaqus software,the spray is manufactured and assembled.We establish a testing platform to complete the resonant frequency and atomization testing experiments on the spray.The actual resonant frequency tested in experiment is 53.42 KHz,which contains about 2.87% deviation between the target frequency 55 KHz and could be accepted.The spray works well and the maximum atomization amount reaches 15mL/min under 2W of effective power.(2)The ultrasonic spray fed DMFC can effectively improve the open circuit voltage(OCV)and the discharge performance of the fuel cell relieving the methanol crossover.At 2M and 8M methanol,the OCV fed by ultrasonic spray is 0.007 V and 0.029 V respectively higher than liquid feed.And the maximum power fed by ultrasonic spray is higher than liquid by 20.55% at 2M and 31.27% at 8M respectively.(3)As the methanol concentration improving,the OCV drops significantly meaning that the excessive concentration is the main factor of causing methanol crossover.But the discharge polarization performance rises firstly and then gets worse with the improvement of fuel concentration,and the best concentration is 4M in this DMFC.At low concentration,the DMFC performance is mainly limited by the concentration polarization;at high concentration,the severely methanol crossover effects the DMFC performance badly.(4)The OCV and discharge polarization performance get better together with the improvement of ultrasonic power.As the fuel steam distribution experiments show,the atomization result gets better and the more droplets distribute in the atomization cavity well with the improvement of ultrasonic power,which can reduce the stacking of the liquid fuel on the MEA releasing the methanol crossover.At the same time,the dispersion angle of ultrasonic spray also grows and the distribution of droplet amount on the cross section gets more homogeneous,which is beneficial for the DMFC discharge performance.(5)There is an optimal atomizing rate value that can keep the DMFC in the best discharge performance,which is 2mL/min in our experiment.When the atomizing rate is too small,the supply of fuel will be insufficient causing serious concentration polarization.Oppositely,it will increase the amount of liquefied fuel promoting the methanol crossover and cost more wasted fuel.(6)At low concentration,the OCV when DMFC get positive position just get a slight improvement than inversion,but the improvement increase significantly fed by high concentration fuel.In the polarization discharge experiment fed by ultrasonic spray,the performance of positive orientation is much better than inversion,but the performance of inversion position keep improving even better than positive finally as the concentration of fuel getting higher.Thus,the variation of fuel concentration would change the DMFC internal mass transfer mechanism's demands for the orientation of cell.