Mass Transport And Performance Enhancement Of Eletrode/Electrolyte Interface In Membrane-Less Microfluidic Fuel Cell

Owing to the rapid development of information technology,the increasing portable micro-electronic devices have urgent demand for micro power source with superior performances,including high power density,long-period operation and low-cost.Membraneless microfluidic fuel cells（MMFCs）are considered as promising micro power sources due to their advantages of competitive cell performance,easy fabrication and integration,flexible cell design and broad range of applications.However,the gas-liquid two-phase flow and mass transport mechanism on the electrode/electrolyte interface in MMFCs are still unclear.The development of related research work possesses important scientific significance and engineering application value,and can provide theoretical guidance for the construction of MMFCs.Based on the perspective of engineering thermophysics,this paper aims to understand the gas-liquid two-phase flow behavior and mass transfer mechanism at the electrode/electrolyte interface,improve the gas-liquid two-phase flow and mass transport in the flow channel,and then enhance the cell performance.First,a novel air-breathing MMFC is proposed,which can realize high-resolution in-situ observation of the dynamic behavior of gas-liquid two-phase flow in the microchannel.The important factors affecting the dynamic behavior of bubbles are comprehensively analyzed via mechanical models.The cell performance and two-phase flow are investigated under various parameters.The influence mechanism of the anode surface microstructure on the bubble escape behavior is also analyzed.Secondly,a microfluidic fuel cell with asymmetrically wetted anode flow channel is proposed.The dynamic behaviors of gas bubbles are visualized and the bubble growth and gas emission mechanisms are analyzed.The effects of fuel flow rate and flow channel width on the two-phase flow and cell performance are investigated.A superhydrophilic/superaerophobic electrode with fern-shaped Pd-nanoarray is proposed.The gas bubble release behavior is visualized and its release mechanisms are discussed.And the cell performance is also obtained.Finally,an immersed micro-jet microfluidic fuel cell based on enhanced fuel transport is proposed.The effect of structure and operating parameters on the fuel transport,cell performance and fuel utilization are discussed in detail.Furthermore,a microfluidic fuel cell with high specific surface area discrete-holes film fueling anode is proposed.The effect of operating parameters on the cell performance are studied.In addition,a three-dimensional mathematical model of the MMFC is established to analyze the fuel flow and transport characteristics on the anode surface.The main outcomes of this thesis are summarized as follows.（1）The performance of the fuel cell based on the hydrophilic flow channel is 18.1%higher than that of the hydrophobic flow channel.The stability of the fuel cell based on hydrophilic flow channel is better than that of the hydrophobic flow channel under different discharge currents.The voltage fluctuation period of the fuel cell with hydrophobic flow channel is shortened by nearly 50%at a discharge current of 30 m A in comparison with 20 m A.For the hydrophobic flow channel,the air bubbles undergo the process of nucleate,grow,and coalesce to gas slug.For the hydrophilic channel,the gas bubbles move to the cathode under the effect of buoyancy.The gas bubble discharge rate increases with the increased flow rates.A high electrolyte concentration will inhibit the coalescence of gas bubbles.The cell performance increases with the increased slant angles and then decreases.For porous electrode,increasing the catalyst loading can reduce the pore size of electrode surface,which is beneficial to promoting the detachment of small bubbles,reducing the adhesion,enhancing fuel transport,and improving the power generation and stability of fuel cell.（2）The gas bubbles in the asymmetrically wetted anode flow channel are discharged in situ through the bubble-trap layer.The MMFC with asymmetrically wetted anode flow channel shows better voltage stability and an improvement of 10%in power density,compared to the conventional fuel cell.The dynamic behaviors of gas bubbles are also specifically discussed.The gas phase pressure and surface tension are the key factors for the in-situ gas emission.It is demonstrated that the two bubbles coalescence mode could benefit the rapid gas bubble removal.The results show that high flow rate can promote the discharge of gas bubbles from the channel outlet,but inhibits the gas emission from the bubble-trap layer.In addition,increasing the flow rate can improve the power generation and stability of the fuel cell,but it will lead to a decrease in fuel utilization.The gas-slug emission mode is observed in narrow microchannel and the cell voltage fluctuation is more obvious.（3）The prepared Pd-nanoarray@CP electrode exhibits superhydrophilicity in air and superaerophobicity under water.The electrode with a catalyst loading is 0.42 mg exhibits optimal electrochemical performance.Its unique superaerophobic feature successfully facilitates the CO₂ bubble releasing from the catalyst surface in a significantly small size.The mass activity and specific activity of superhydrophilic/aerophobic electrode are 1.6-and 4.9-times higher than that of the basic commercial electrode,respectively.In addition,we also demonstrate the as-prepared electrode possesses relatively low charge resistance and robust stability.The peak power density of 35.8 m W cm^–2 is obtained,which is 1.5 times that of fuel cell with Pd-black@CP anode.The Pd-black@CP electrode shows large gas bubble departure mode but Pd-nanoarray@CP electrode shows small gas bubble departure mode.（4）Part of the fresh fuel is jetted perpendicular to the anode,enabling targeted convective fuel transport.Compared with the conventional MMFC structure,the maximum power density of the cell is increased by 25%,and the fuel mass transfer resistance is reduced by 17.4%at the same conditions.The micro-jet located at the middle of flow channel can balance the trade-off between replenishment and benefitted anode area.The effect of total fuel flow rate,micro-jet/lateral flow rate ratio and fuel concentration on the fuel transport and cell performance are also investigated.At optimal conditions,the cell performance improvement of 40.9%is obtained by the immersed fuel micro-jet as compared to the flow-over mode,and the highest power density reaches 119.3 m W cm^-3.（5）The prepared high specific surface area discrete-holes film fueling anode has large electrochemical active area and high electrocatalytic activity.The visualization experiments show that the annular film flow is the ideal flow pattern for improving fuel transport and power density.Under the same total fuel supply,the best cell performances are obtained in the case of 50μL min^–1&2.0 M.To achieve higher power density,the cell performance is evaluated under various operating conditions including fuel concentration and flow rate.As a result,the maximum power density of 361.9 m W cm^–3 is obtained under the condition of 4.0 M fuel concentration and 100 min^–1 fuel flow rate.The simulation results show that the uneven velocity distribution and fuel concentration distribution in the main channel.The lower electrolyte flow rate and the larger discrete pore size are beneficial to improving the fuel concentration distribution on the anode surface and cell performance.