| Recently, rapid-developing mobile broadband technologies have spurred theevolution of high performance portable electronic devices (such as smart phones andtablets), and therefore promoted the demand for high efficient and reliable micro powersources. Conventional Li-ion batteries suffered from low energy density and can’tfunction for a long time without recharging. The successful implementation of microdirect methanol fuel cells (μDMFC) was also hindered by the complicated technicalchallenges (e.g., water management and membrane degradation). Alternatively,membraneless microfluidic fuel cells (MMFCs) exploited the co-laminar flow ofmultistream in microchannel to segregate the fuel and the oxidant, and thereforeeliminated the physical membrane and membrane-related problems. Consequently,MMFCs have been regarded as one of the promising micro power sources for portableapplications.At the present stage, however, the MMFC performance was mainly limited bymass transport at both the anode and the cathode. It has been demonstrated that theair-breathing microfluidic fuel cell (AMFC) can eliminate the oxidant transportlimitation at the cathode. Nevertheless, the fuel transport to the anode was still hinderedby the fuel concentration boundary layer, which in turn limited the cell performance. Inaddition, the released CO2bubbles during the cell operation can perturb the co-laminarflow interface, resulting in convective mixing and fuel crossover. Besides, thecomputational model for MMFCs was not sufficient, which cannot provide insightfulguidance for structural design and operational optimization.In this study, in order to enhance mass transport and improve MMFC performance,novel cell architectures with various anode configurations were proposed and tested.The characteristics of mass transport and cell performance were studied bothexperimentally and numerically. The research contents include:(1) An AMFC with aflow-through anode was proposed. The effects of fuel/electrolyte concentration and flowrate on the cell performance were studied. The characteristics of fuel transport and fuelcrossover of the cells with either flow-over or flow-through anodes were alsonumerically investigated.(2) A novel electrodeposition method (RENC) for fabricatingPd catalyst layer on graphite rod electrodes for direct formic acid oxidation wasdeveloped. A series of physicochemical and electrochemical measurements were performed to characterize the electrodes. In addition, an AMFCs with a cylinder anodewas proposed and tested in acidic and alkaline electrolytes. The dynamic behavior ofCO2bubble was visualized and its influence on the discharge curve was discussed.(3)An AMFC with an array of cylinder anodes was developed. The effects of operationparameters, bubble movement as well as rod configuration on the species transport andcell performance were investigated. The cell performance was also evaluated in thealkaline electrolyte.(4) A3D computational model for alkaline AMFCs with an array ofcylinder anodes was developed. The distribution of fluid velocity, fuel concentration,anode current and ionic potential were discussed in detail. Furthermore, the effects ofgeometric parameters and scale-up strategy on the fuel transport and cell performancewere also studied. The main results were summarized as follows:1) For the AMFCs with a flow-through anode, the cell performance increasedwith the fuel/electrolyte concentration and then decreased. The reactant transport can beenhanced by improving the flow rate, but hydrodynamic instability at higher flow rate(>20mL h-1) reduced the cell performance. The flow-through anode can convectivelyenhance the fuel transport and enable uniform current distribution. The flow-throughcase suffered less parasitic (crossover) current density than the flow-over case at lowflow rates.2) The electrode fabricated by RENC method showed an “island” morphologyand a multi-layer structure. Moreover, a superior performance and durability for formicacid electro-oxidation were obtained, mainly due to the combined beneficial effects ofthe increased electrochemical surface area and catalyst utilization rate, and thepredominant Pd(111) crystallite plane presented on the electrode.3) For the AMFCs with a cylinder anode, the generation and coalescence of CO2bubbles can reduce the anode catalytic surface area and hinder fuel transport. In addition,the Electrochemical Impedance Spectroscopy (EIS) measurement also revealed that thegas column improved the inner ohmic resistance by reducing the proton conduction.Much higher and more stable cell performance were obtained in the alkaline electrolyte.4) For the AMFCs with an array of cylinder anodes, removal of the two spacersadjacent to the cathode significantly improved the cell performance. Most CO2bubbleswere constrained within the anodes array and their periodic movement can fluctuate thecell performance periodically. The presented fuel cell yielded a peak power density of21.5mW cm-3and a maximum current density of118.3mA cm-3. A maximum fuelutilization rate up to87.6%was obtained at1mL h-1. The cell with in-line anode/spacer arrangement produced lower performance.5) With identical fuel concentration and flow rate, the cell operated in the alkalineelectrolyte produced a maximum power output of36.7mW and a limiting currentoutput of229.0mA,171.0%and207.3%higher than that in the acidic electrolyte,respectively. The cell performance can be improved by increasing the fuel andelectrolyte concentration, and flow rate, but the cell performance was ohmic resistancelimited at higher current density. The cathode potential reversed at coupled low flowrate and high current density. A world-class maximum power output of50.4mW wasobtained.6) Fuel crossover effect was minimized by the fast electrolyte flow in the vicinityof the cathode. The current output of cylinder anodes was uneven and was inherentlyaffected by inner ohmic resistance correlated with the distance from the cathode. Fueltransfer limitation occurred at low flow rates but alleviated at higher flow rates. Furtherperformance increment was limited by the ohmic resistance and reaction kinetics. Thecathode potential reversed at coupled low flow rate and high current output, which canbe induced by the reversed electrolyte potential under fuel starvation conditions. Acompensation mechanism can improve current production of bottom anodes atdownstream.7) The cell with in-line rods arrangement produced lower power density. The cellperformance varied nearly linearly against the cell length, the maximum power outputvaried at the rate of0.99mW mm-1, and the maximum volumetric power density variedat the ratio of-0.46mW cm-3mm-1. The cell suffered downstream fuel transportlimitation at combined thick anodes and thin spacers. The feasibility of non-spacerarchitecture was validated without inducing severe fuel crossover. The correspondingpower density was nearly doubled. The horizontal anodes extension was moreapplicable for future scale-up. |
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