Transport Characteristics And Performance Enhancement Of Air-breathing Microfluidic Fuel Cells Based On The Regulation Of Anode Catalyst Distribution

The explosive growth of portable and wireless electronic products has stimulated the development of power supply technologies,and high power and high energy density are the development trends of today’s new power supply products.However,the lithium-ion batteries commonly used in the market have limited capacity and suffer from security issues in the charging process.In recent years,micro fuel cells（μ-FC）with high energy density have received considerable attention as a power source for portable electronic devices instead of traditional rechargeable batteries.Among them,Air-breathing membraneless microfluidic fuel cells（MMFCs）remove the proton exchange membrane and use the oxygen in the air as an oxidant,benefiting the further miniaturization and integration of the cell.Hence,it is one of the most ideal micro fuel cel s.The performance of MMFCs is mainly affected by mass flow and transport associated with electrochemical reactions.The catalytic activity,catalyst load,catalyst distribution,and its matching with the electrode structure play a vital role in the mass flow,transport and cell performance.In view of the match issue between anode catalyst distribution and electrode structure in MMFCs,this paper studied the influence mechanism and law of catalyst distribution,catalyst load in the anode,and cell operating conditions on gas-liquid flow,mass transport,and cell performance in MMFCs to improve electrode catalytic performance and enhance mass transport by experimental studies and numerical simulations.The method of electrodepositing a high-dispersion catalyst in the porous anode was presented and obtained high-performance MMFCs.The research results are as follows:（1）The planar anode with superhydrophilic wedge-shaped catalyst distribution was constructed,and the bubble dynamic behaviors and power generation characteristics in the cell were studied.The results showed that wedge-shaped catalyst distribution on the anode can introduce Laplace pressure to drive the bubbles transport from the reaction area to the superhydrophobic area,which accelerated the detachment of bubbles from the reaction area and reduced the impact of bubbles on the reaction area.Compared with the MMFC with a uniform catalyst distribution anode,the cell performance and stability were improved.The maximum power density and limiting current density were increased by 35%and 61%,respectively.（2）Three kinds of air-breathing MMFCs with flow-through anode equipped with different catalyst distributions were constructed,and the effect of catalyst distribution on the two-phase flow behavior in the microchannel,and the cell stability and the performance in acidic electrolytes were studied.The influence mechanism and law were also analyzed.It was shown that the distribution of the catalyst on the porous anode had a great impact on the two-phase flow behavior and cell performance.When the catalyst was distributed on the fuel flow channel side,the CO₂ was accumulated in the porous anode,and large CO₂ bubbles or bombs were produced in the electrolyte flow channel.When the catalyst was only distributed in the electrolyte flow channel side,a few small CO₂ bubbles existed in the electrolyte flow channel.When the catalyst was distributed on both sides of the anode,the performance was the best but the cell stability was poor.（3）The influence of catalyst distribution on carbonate accumulation on electrodes,cell stability,and performance in alkaline electrolytes were investigated.The results indicated that the catalyst distributed on the side of the electrolyte flow channel was most favorable for charge transport,and the cell performance was best.The catalyst layer on the side of the fuel flow channel easily triggered the accumulation of carbonate after the cell long time operation.In addition,the compactness of the catalyst distribution also affected the accumulation of carbonate,and the denser region of the catalyst was easy to accumulate carbonate.The accumulation amount of carbonate first increased and then decreased with the increase of the flow rate when the cell operated at a constant voltage for a long time.The cumulative amount of carbonate on the electrode was less when operated at a low and high flow rates(50μL min^-1,400μL min^-1).While,carbonate was more likely to accumulate on the electrode at a medium flow rate(100μL min^-1,200μL min^-1).（4）A three-dimensional mathematical model of an air-breathing microfluidic fuel cell with flow-through anode was established to study the influence of anode catalyst distribution thickness on cell performance and mass transport.The simulation results showed that the fuel was uniformly distributed in the anode catalytic layer at high fuel concentration,and the cell performance was mainly restricted by the ohmic loss.The increase of the catalyst distribution thickness in the porous anode can reduce the average current density in the catalytic layer,thus effectively reducing the CO₃^2-concentration in the anode catalytic layer,which was not easy to cause the accumulation of carbonate.Besides,increasing the reactant flow rate also reduced the CO₃^2-concentration in the catalytic layer,but led to lower fuel utilization.（5）The method of preparing highly dispersed catalysts in porous electrodes was developed.The electrode catalytic performance toward formate was optimized by regulating preparation parameters.A series of physicochemical and electrochemical measurements were carried out to characterize the electrode.An MMFC based on the anode with uniform catalyst distribution was constructed,and the effect of operating parameters on the cell performance was studied.The concentration of Pd²⁺in the electroplating solution,the scanning speed,and the scan cyclic number mainly affected the number of catalyst particles per unit area,the morphology of the catalyst,and the size of the catalyst particles,respectively.It was shown that the electrode had superior catalytic activity and high electrochemical active area when the electrode was prepared in 2 m M Pd²⁺electroplating solution with 5 electrodeposition cycle number at a scan rate of 5 m V s^-1.The catalyst particles on the electrode presented an island-like structure and were uniformly distributed inside and outside the carbon paper electrode.The cell performance based on the present anode had a maximum power density of 46.6 m W cm^-2,and a limiting current density of 288.4 m A cm^-2.The catalyst particles on the electrode were not covered by carbonate after a long-time operation.（6）To reduce noble metal catalyst loading on the anode,an alloy catalyst electrode was developed.The physical and chemical properties of alloy catalyst electrode were measured.Finally,the economy of cell power generation was also analyzed.The results showed that the deposition potential range affected the deposition rates of Pd and N i during the electrodeposition process.The deposition rate became faster when the deposition termination potential became more negative,causing a significant increase in the loading of Pd and N i on the electrode.It ultimately affected the morphology and catalytic activity of the catalyst.The electrode exhibited the best catalytic performance toward formate oxidation when the deposition potential ranged from-0.5 V to-0.8 V.The electrode catalytic active was improved due to the enhancement of formate adsorption on the Pd Ni alloy catalyst.The maximum power density of MMFC with Pd Ni/CP anode was 36.4 m W cm^-2 normalized to electrode area,and 18.7 W$_A^-1normalized to the cost of the anode.