| Micro direct methanol fuel cell（μDMFC）is one of proton exchange membrane fuel cell which use methanol/water solution as the fuel directly.It is a promising power source candidate for miniaturized devices and military equipment due to its unique advantages such as high energy density,small size,abundant sources,high security,convenience in fuel replacement.However,the block of mass transportation is a key problem to restrict theμDMFC performance.To relief the problem of mass transfer including capillary blocking caused by CO2 bubbles in anode flow field and water flooding in cathode flow field,a novel flow field containing auxiliary superhydrophilic/superhydrophobic channels and fuel channels is designed on Ti plates.And the influence of auxiliary channels on μDMFC is studied by experiment.Based on the traditional serpentine and spiral anode flow field（AFFs）,a novel AFF structure with nested layout of fuel channels and superhydrophobic degassing channels which can remove most of CO2 from AFF before it is released to the fuel channels was presented.A water management system consist of cathode flow field with superhydrophilic capillary network structure and carbon papers are designed for air-breathing μDMFCs to eliminate excessive water in the cathode and avoid water flooding.The structure size of channels are determined according to the affect of channels geometric parameters on μDMFCs performance.Ti surface will form a large number of micro-nano hybrid roughness structures after it is anodized,WCA of Ti surface can be facilely changed from hydrophilic（53°）to superhydrophilic（0°）.Then,the WCA of Ti surface changed to superhydrophobic（160°）by using the FAS modification techniques.The novel μDMFC flow field plates are fabricated on Ti substrates by using micro photochemical etching combined with anodization and fluorination treatments.Effect of superhydrophobic degassing channels on removal process of CO2 bubbles in anode and μDMFC performance are studied.μDMFCs with superhydrophobic degassing channels AFFs and referenced AFFs are assembled.Performance of the μDMFCs with different AFFs is is comparatively studied.Results show that output power density ofμDMFC with superhydrophobic degassing channels in AFF is ultimately increased by 91.84%compared to those μDMFCs with reference flow fields.Also,the results of pressure drop measurement and motion properties of anode bubbles show that most of CO2 bubbles can be excluded from anode flow fields by superhydrophobic degassing channels without thoroughthe fuel channel,which reduced the mass-transportation resistance of the methanol and thereby lead to a solid enhancement of the μDMFCs performance.Effect of superhydrophilic capillary network structures on eliminating μDMFC water flood in cathode is verified by numerical simulation and prototyping experiment.The distribution condition of liquid drops in cathode flow field（CFFs）with superhydrophilic capillary network structure are simulated by COMSOL,results show that superhydrophilic capillary channels can assimilate and eliminate water from cathode.The CFFs with superhydrophilic channels are designed and fabricated and they are assembled in singleμDMFCs.The degradation of μDMFC power density caused by water flood in cathode is measured when the μDMFCs discharged at a constant voltage of 0.15 V for 4h.Results indicate that the output power descend range of μDMFCs applying novel CFFs after discharged for 4h is only about one third of μDMFC with conventional CFF compared to initial discharge condition.In situ observation finds that water drops in the cathode were dragged into carbon papers by gradient capillary force and removed from cathode after water flowed into superhydrophilic channels,indicating that superhydrophilic channels and carbon papers can improve the water management ability of μDMFCs. |
