Research On The Design Of Flow-through Electrode Architecture And Performance Of Membraneless Microfluidic Fuel Cell

Technical development and miniaturization efforts in the area of portable electronicshave generated enormous demand for small and efficient power supplies. Membranelessmicrofluidic fuel cell (MMFC), also called laminar flow-based fuel cell, provide analternative way towards miniaturized power supplies. MMFC is defined here as a devicethat confines all fundamental components of cell to a single microstructured manifold. Inthe case of active sites, such fuel cell exploit the relatively low rates of diffusion mixinginherent to co-laminar microfluidic flow to maintain the separation of fuel and oxidantstreams flowing side-by-side, thereby eliminating the demand for a physical barrier such asa proton exchange membrane (PEM). Ionic conduction is facilitated by a supportingelectrolyte contained within the reactant streams. Mixing of the two streams occurs bytransverse diffusion only, and is restricted to an interfacial width at the center of thechannel. The MMFC avoids many of the issues encountered in polymer electrolytemembrane-based fuel cell (PEMFC), including membrane humidification, membranedegradation and liquid water management. Although some progress has been made in thedevelopment of MMFC, the commercialization of MMFC is, however, still impeded by anumber of basic problems, including the poor kinetics of both the anode and cathodereaction, the poor durability, and the cross-over of methanol from the anode to the cathode.To avoid these problems, some strategy are the development of novel fuel cell architectures,methanol oxidation catalysts and oxygen reduction catalysts. The work in this dissertationis devoted to these issues.To improve the catalytic performance of Pt/CNTs catalysts, three catalysts wereprepared based on Pt precursors reduced by (i) hydrogen plasma (Pt/CNTs-HP),(ii)hydrogen (Pt/CNTs-H) and (iii) NaBH4(Pt/CNTs-N), respectively. We found that thehighly dispersed Pt/CNTs catalysts with smaller particle size (2.1nm) and the higher percentage of Pt (0) could be obtained with the reduction of hydrogen plasma. Threecatalysts were compared for performance in the oxidation of methanol. The Pt/CNTs-HPpossessed of the higher electrocatalytic activity as compared with the other two catalysts.The nitrogen-doped carbon naotubes (NCNTs) were synthesized via microwaveplasma chemical vapor deposition (MPCVD) using ammonia as nitrogen precursors.During the synthesis of NCNTs, the total reaction pressure in the chamber was kept at4kPa and the flow rates of CH4, H2and NH3were3.3sccm,55sccm and2sccm,respectively. The input power of microwave plasma was400w with a growth time of60min. The resulting NCNTs have shown superior oxygen reduction reaction (ORR)performance.Some routes for the improvement of the catalytic performance of Pt/CNTs catalystswere investigated. Pt precursors reduced by hydrogen plasma technology, and the effects ofmicrowave power and reduction time on Pt/CNTs catalysts were analyzed. Results showedthat the highly dispersed Pt/CNTs catalysts with smallest particle size could be obtainedwith the microwave power of200w under the60min reduction time. Such Pt/CNTscatalysts exhibit optimally catalytic performance toward the oxidation of methanol andalso show a long-term stability.The flow-through porous electrodes were fabricated on the basis of carbon paper withthe CNTs using MPCVD technology, and the PDMS pool was made by cast molding. Thecell was sealed with a flat layer of PDMS pool, with porous electrodes, with punched holesfor the inlets, outlets, and electrical contacts, which renders hydrophilic channel walls andfacilitates covalent binding upon assembly. The effect of flow rates on fuel cellperformance was investigated. Experimental results showed that high overall powerdensities was obtained through a combination of relatively high levels of fuel utilizationand cell voltage. The overall power density increases considerably with flow rate. Thehighest power density obtained here was7mW·cm-2at100ul·min-1. The power density of100ul·min-1was4times that of1ul·min-1. Operation at higher flow rates would bepossible, but with limited gain due to the parasitic ohmic voltage loss caused by series resistance in the cell.