Enhancement Of Mass Transfer And Performance In Microfluidic Fuel Cells With Persulfate As Oxidant

Microfluidic fuel cell（MFC）is a micro energy conversion device,converting chemical energy directly into electrical energy.More than two steams of fluid flowing in the microchannel will form parallel laminar flow,which naturally separates anodic and cathodic solutions.It can eliminate proton exchange membrane（PEM）and some technical issues related with the membrane（water management,membrane degradation,and cost,etc.）in the PEM fuel cell.In recent years,MFCs have become a potential power source for non-implantable medical devices and microfluidic chips,due to their improved performance and reduced size.Therefore,MFC is one of the research hotspots in micro energy technology.The application of microfluidic fuel cells still faces the problems of low performance and high cost.And the cell performance is determined by anode and cathode.At present,the anode performance in MFC is limited by mass transfer.Moreover,CO₂ bubbles produced in anode under the acidic condition will disturb the parallel laminar interface in the microchannel,resulting in convective mixing of fuel and oxidant or even reactant crossover,which also deteriorates cell performance.Although high cathode performance is achieved by using the noble metal catalyst,this increases the cost of fuel cell significantly.In addition,the cathode performance is also limited by oxidant mass transfer.To solve above problems,this study utilizes mass transfer and fluid flow theory in the engineering thermophysics to improve cell performance and reduce cell cost.MFCs with persulfate as the oxidant were presented in this dissertation,which directly used carbon paper without noble metal catalyst as cathode.And the characteristics of two phase flow,mass transport and cell performance in the MFCs were studied.The following main research contents were included.（1）A flow-over MFC with filter paper structure was constructed.The dynamic behavior of bubbles in the MFC with or without filter paper structure and its influence on cell discharge and performance were studied.（2）A flow-through MFC with a new flow configuration was constructed.The dynamic behavior of bubbles in the MFC with the new or conventional flow configuration and its effect on cell discharge and performance were studied.（3）The oxidation properties of persulfate in acidic,neutral and alkaline media were studied.A flow-through MFC under dual-electrolyte system was constructed.The effect of the operating parameters on cell performance was investigated.A three-dimensional mathematical model was established,and the flow and transport characteristics of the reactant in the MFC with flow-through electrodes were studied.（4）The low-cost carbon paper（CP）cathode owning carbon nanosheet for MFC was prepared by in-situ electrochemical exfoliation.The characteristics of microstructure,material and electrochemistry for the prepared CP cathode were characterized.A flow-through MFC with the CP cathode owning carbon nanosheet was constructed.The cell performance of MFC with different CP cathodes was also studied.（5）The nitrogen catalyst was prepared on the electrochemically exfoliated CP cathode by in-situ doping.The characteristics of physical and electrochemical for the N-doped cathode were characterized.The effect of experimental conditions on the electrochemical performance for the N-doped CP cathode was studied.A flow-through MFC with the N-doped CP cathode was also constructed,and its cell performance was compared with that without the N-doped CP cathode.The main research results are as follows:1)In order to solve the problem that bubbles disturb the laminar interface in acidic condition,a layer of filter paper was embedded in the middle of the microchannel for the MFC with flow-over electrodes,which can effectively limit the size of bubbles,weaken the disturbance of bubbles on the parallel laminar interface,and accelerate gas removal.The bubble removal frequency of the MFC embedded with paper separator was twice higher than that of the MFC without paper separator.Compared with the MFC without paper separator,the maximum power density and limiting current density of the MFC embedded with paper separator were increased by 25.2%and 130%,respectively,because of the enhanced fuel transfer and the smaller ohmic resistance.With the increase in the flow rate,the gas removal was accelerated and the fluctuation of current density decreased,resulting in a superior performance.2)The new flow configuration was used to optimize the distribution of bubbles in the flow-through MFC,greatly reduce the ionic transport resistance between anode and anode,and improve the fuel transport to the anode catalyst surface.The MFC with new flow configuration still maintained stable operation at medium and high current densities.The maximum power density of the MFC with new flow configuration was52.9%higher than that with conventional flow configuration at 200μL min^-1.Compared with conventional flow configuration,the reactant flow rate had less influence on the bubble dynamic behavior,discharge curve and cell performance of the flow-through MFC with new flow configuration.3)A flow-through MFC under dual-electrolyte system with alkaline anode and acidic cathode was constructed,and a high open circuit voltage（OCV）of 2 V was yielded.After the optimization of operation parameters,the maximum power density of the MFC was 148 m W cm^-2,and the limiting current density was 450 m A cm^-2 under the fuel concentration of 4 M,the oxidant concentration of 1.5 M,and the reactant flow rate of 200μL min^-1.And a three-dimensional theoretical model of the flow and mass transfer in the MFC was established.The calculation results showed that,under the optimum operation parameters,the oxidant and fuel could maintain high concentration in most of the cathode and anode regions,respectively.And the reaction rate of reactant on the electrode was mainly affected by reaction kinetic parameters,not limited by the mass transfer.4)To increase the reaction active sites of cathode,porous carbon nanosheets were successfully prepared on the surface of CP electrode by in-situ electrochemical exfoliation.Compared with the pristine CP electrode,the exfoliated CP electrode enlarged specific surface area,introduced abundant defects and oxygen functional groups,and improved the hydrophilicity.The charge and mass transfer resistances of the exfoliated CP electrode were much lower than those of the pristine CP and hydrophilic CP electrodes.And the exfoliated CP electrode owned good electrochemical performance and long-term discharge stability.The highest power density of the flow-through MFC with the CP cathode owning carbon nanosheet was 188.6 m W cm^-2,which was 93.4%and 27.3%higher than that with the pristine CP cathode and the hydrophilic CP cathode,respectively.5)To further improve the reaction kinetics of the cathode,the nitrogen functional groups were prepared on the electrochemically exfoliated CP cathode by in-situ doping.The electrocatalytic activity of CP cathode for persulfate reduction reaction was obviously improved after N doping,and the good stability of N-doped cathode was obtained.Under the hydrothermal time of 24 h and the precursor concentration of 40%,the N-doped cathode had good electrical conductivity and electrocatalytic activity due to the optimal content of pyridine nitrogen and graphite nitrogen,resulting in excellent electrode performance.The highest power density of the flow-through MFC with the N-doped CP cathode reached 241.0 m W cm^-2,which was 27.8%higher than that before N doping.