促进气体扩散的双孔分布多孔流场型燃料电池
Improving gas diffusivity with bi－porousflow－field in polymer electrolyte membrane fuel cells
Masaya KozakaiKenji DateYutaka TabeTakemi Chikahisa
Abstract
The performance of polymer electrolytemembrane （PEM） fuel cells highly depends on their mass transport property athigh current density operation． To improve the mass transport property， varioustypes of flow－fields have been developed， such as serpentine， straight， grid，and porous flow－fields． Because of the limits of thin land and channel width，the authors have developed a porous type flow－field with unique pore diameterdistribution． Unlike general types of porous flow－fields， it has a double peak in the pore diameter distribution； in this paper，it is called a “bi－porous flow－field”． The structure was intended to haveorganized flow paths for liquid water and gas． The paper investigates itsperformance and impedance characteristics compared with those of theconventional flow fields． The results of the analysis on the polarization andelectrochemical impedance spectrometry revealed that the bi－porous flow－fieldexhibits the smallest gas diffusionresistance at a high current density operation regardless of humidityconditions． These results indicate that the bi－porous flow－field conductswater management well at high current density． In low humidity conditions， however， dry－out tends to occur and must beprevented．
Table 1 e Property of flow－fields
Fig． 2 e Morphologies and pore sizedistributions of porous media： （a） mono－porous and （b） bi－porous
Fig． 4 e Polarization characteristics at100％ RH and stoichiometric ratio of 1．25 for hydrogen， 2．5 for oxygen： （a） current－voltageand （b） current－power characteristics．
Fig． 5 e Cell potential characteristicswith different oxygen stoichiometric ratio for current density of （a） 0．5， （b）1．0， （c） 1．3， and （d） 1．5 A cm¡2．
Fig． 6 e Pressure drop at cathode at 100％RH and stoichiometric ratio of 1．25 for hydrogen， 2．5 for oxygen．
Fig． 7 e Nyquist plots with differentcurrent densities at 100％ RH and stoichiometric ratio of 1．25 for hydrogen， 2．5for oxygen． The current density is （a） 0．5， （b） 1．0， and （c） 1．5 A cm－2．
Fig． 8 e Resistances at 100％ RH andstoichiometric ratio of 1．25 for hydrogen， 2．5 for oxygen： （a） ohmicresistance， （b） charge transfer resistance at anode， （c） charge transferresistance at cathode， and （d） gas diffusion resistance at cathode．
Fig． 9 e Diffusion resistance from EIS atthe current density of 1．5 A cm－2 （100％ RH）．
Fig． 10 e Polarization characteristics at60％ RH and stoichiometric ratio of 1．25 for hydrogen， 2．5 for oxygen：（a）current－voltage and （b） current－power characteristics．
Fig． 11 e Resistances at 60％ RH andstoichiometric ratio of 1．25 for hydrogen， 2．5 for oxygen： （a） ohmicresistance， （b） charge transfer resistance at anode， （c） charge transferresistance at cathode， and （d） gas diffusion resistance at cathode．
Conclusions
Bi－porous flow－field was developed toincrease the current density limit of PEM fuel cells， and the cell performancewas compared with those of several types of flow－fields． The biporous media hasa double peak in the pore size distribution． The results showed that the bi－porous flow－field exhibited thehighest power density operation at high humidity condition among theflow－fields compared． It was operated with smaller oxygen stoichiometric ratios， which can be a greatadvantage for automotive and portable applications． EIS analysis confirmed thatthe improved performance was due to theimproved gas diffusivity in a large water production condition compared withthe other types of flow－fields． This suggests that the bi－porous flow－fieldseparates the flows of liquid water and gas in an organized manner by thedifferent pore size structure． These results indicate that the bi－porousflow－field appears to have a good potential to keep gas diffusivity high andincrease the power density of the cells．
However， some issues were clarified． The pressure drop was high in the sameorder of the serpentine flow－field． Additionally the cell performance was lower than those of the other types of flow－fieldsin a relatively low humidity condition， 60％ of RH in this experiment． TheEIS analysis showed that thedeteriorated performance was caused by the increased ohmic resistance andcharge transfer resistance in the cathode． Both resistances depend on thewater content in the membrane and ionomer in the catalyst． This suggests that the bi－porous flow－field has more dryingcharacteristics than the other types of flow－fields due to the higher thermalcontact resistance at the layers and the lower thermal conductivity of theporous media． These issues of dry－out and large pressure drop appear to besolved by controlling the pore diametersand the porosity of the media to have better thermal conductivity and gaspermeability while maintaining their good water management characteristics．
干态和湿态都做得好，好难两全
不知道MPL、GDL双孔分布是否也有相同的问题。
