Structure, Process And Anodic Gas-liquid Characterizations Of Micro Direct Methanol Fuel Cells

Micro direct methanol fuel cell (μDMFC) is one of the most promising power sourcecandidates for applications in portable electronic devices in the future, because it offers manyadvantages such as high energy density, convenience in use, environmentally benign emission,etc. Comparing with those manufacturing technologies have been used in traditional largescale DMFC, micromaching and micro assembling should be adopted in order to make afurther volumetric reduction inμDMFC. To date, such researches are still in initial stagesthroughout the world. Aiming at the targets mentioned above, this thesis focuses researcheson the structure, process and anodic gas-liduid characterization of theμDMFC.Cathode and anode micro flow field plates of theμDMFC are successfully prepared onsilicon wafer by using double-side wet etching and RF sputtering. Single cell of the siliconμDMFC is packaged with a two-step packaging strategy. The siliconμDMFC is tested usingmethanol solution with different concentration, flow rate and temperature. Influences modesof these operating conditions on output performance of the siliconμDMFC are obtained.Resistivity of the Ti/Cu/Pt multi-layer current collector with different thickness is alsoinvestigated and compared with results from the classical Fuchs-Sondheimer theory.Relationship between thin film thickness and sheet resistance of the current collector as wellas the internal resistance of the siliconμDMFC are presented.Combined with CFD simulations and central composite design sampling (CCDS)strategy, the approximate founctions betweenμDMFC performance and structural parametersof anodic flow field are established, by using response surface methodology (RSM). FromRSM model developed in this paper, the quantitative relations betweenμDMFC performanceand anodic flow field structure are obtained. The RSM founctions are finally verified with aseries of experiments. It is found that the RSM anticipation results are in well accordance withthat of the experimental results. The RSM model provides an effective evaluation tool fordesign and optimization of theμDMFC micro flow field.Characterizations of anodic gas-liquid transportation of theμDMFC are carried out bymeans of in situ visual experiments. Distribution forms of CO₂ gas bubbles in the anodicmicro flow field at different discharging time and current density are recorded. Results showthat the gas-liquid transportation in anodic micro flow field is self-adapted and self-balanced.Based on momentum conservation of the fluidic element, a differential equation of pressuredrop gradient for gas-liduid transportation in capillary is built. The equation is then applied to calculate pressure drop of the anodic flow field. Friction, capillary and total gas-liquidtransportation pressure drop in anodic micro flow field are obtained.In order to satisfy power needs of different electronic devices, two-cell and six-cellμDMFC stack are designed and manufactured, respectively. Twin-fuel-cell structure withseries interconnection is adopted in the stack. Passive management strategies for reactants andbyproducts of the stack are realized. Testing results show that the performance of passiveμDMFC stack is highly depended on the methanol concentration when compared with that ofactive-fedμDMFC stacks. Equivalent normal residual stress, which is induced in majorpackaging process and imposed on the flow field plate, is also calculated based onviscoelastic constitutive relation of the epoxy-molding compound. The packaging parametersare then chosen according to the combined results of contact resistance experiment andstress-strain simulations of the packaging frame. Internal resistance of the packaged two-cellμDMFC stacks in this thesis is equal to that of the traditional large scale DMFC stack. Aftermatching the output performance of theμDMFC stacks to load features of different kinds ofelectronic devices, several sub-watt classμDMFC demonstrators are construced successfully.These demonstrators have provided opportunities for verifying the practical values of thepassiveμDMFC stacks.