Mass Transport Characteritics And Performance Enhancement Of Thread-based Passive Microfluidic Fuel Cells

Membraneless microfluidic fuel cells（MMFCs）utilize the feature of co-laminar flow to separate fuel and oxidant in the microchannel instead of the proton exchange membrane（PEM）.Thus,no membrane is required,and membrane-related problems such as liquid water management and membrane degradation are also eliminated.However,for conventional MMFCs,external devices like pumps are needed to keep the co-laminar flow of fuel and oxidant streams in the microchannel,which not only reduces the net power output but also limits their miniaturization and portability.Therefore,it is of great significance to develop a passive（pump-free）MMFC to meet the needs of practical applications.Recently,a thread-based passive MMFC was proposed,which utilized the flexible porous media such as cotton threads as microchannel.The fuel and oxidant were driven by capillary combined with gravity forces,and the external pump was no longer needed.Moreover,thread-based passive MMFCs also have the advantages of simple structure,low cost and easy portability,and are one of the most promising portable micro power sources.At present,however,the thread-based passive MMFCs suffered from mass transport limitations and fuel/oxidant crossover issues,which resulted in a poor cell performance.Besides,the theoretical studies for the thread-based passive MMFCs were not sufficient,which cannot provide effective theoretical guidance for the structural design and operational optimization.In order to enhance mass transfer,eliminate fuel crossover and boost cell performance,novel thread-based passive MMFCs with various electrode architectures were proposed in this study.The characteristics of mass transport and electricity-generation for the proposed thread-based MMFCs were investigated.First of all,a woven thread-based microfluidic fuel cell based on graphite rod electrodes was proposed.In this fuel cell,a three-dimensional graphite rod deposited Pd catalysts was used as electrode.A woven cotton threads prepared by“three-ply braid”method was used as high-flux flow channel.The effects of electrode distance and fuel concentration on the cell performance were investigated.Next,a dual-functional flow-through electrode with woven carbon fibers as substrate was prepared.The prepared dual-functional electrode could serve as the flow channel and the electrode simultaneously.A thread-based passive MMFC with dual-functional flow-through electrode was constructed and tested.Moreover,a three-dimensional computational model for the proposed passive MMFC was developed.The distribution of fluid velocity and fuel/oxidant concentration were obtained.The characteristics of fuel/oxidant crossover of the fuel cell were also discussed.In addition,the effects of geometric parameters and operational parameters on the cell performance were also investigated.Then,a non-precious metal oxygen reduction reaction（ORR）electrocatalyst（Co@NPC/C-MWCNTs）with high catalytic selectivity was successfully prepared.A flexible single-stream thread-based MMFC using Co@NPC/C-MWCNTs as the air-cathode catalyst was constructed and tested.The issue of fuel crossover was eliminated,and the fuel cell structure was greatly simplified.Finally,a single-stream thread-based H₂O₂ MMFC was proposed for the practical application.The proposed fuel cell utilized H₂O₂ as both fuel and oxidant.Moreover,a novel self-powered H₂O₂thread-based electrochemical sensor based on the proposed fuel cell structure was further developed and characterized.The major achievements are as follows:（1）To enlarge the electrode area and improve the transport capacity of reactant,a three-dimensional graphite rod electrode and woven thread-based flow channel were prepared,respectively.A woven thread-based microfluidic fuel cell based on graphite rod electrodes was proposed.Co-laminar flow visualization verified the feasibility of the proposed thread-based MMFC.The open circuit potential of the MMFC was 1.44 V.The maximum power and current densities of the proposed MMFC were 20.7 m W cm^-2and 56.6 m A cm^-2,respectively.The optimal fuel utilization rate was 10.9%.（2）In order to enhance mass transfer and realize the integration of flow channel and electrode,a dual-functional flow-through electrode was prepared for a passive MMFC.The highest power density of the proposed MMFC was 29.9 m W cm^-2,which was higher than that of other passive MMFCs reported in previous literatures.The numerical simulation showed that the middle cotton thread-based flow channel can prevent crossover between the fuel and oxidant.The produced parasitic current density under different cell voltage can be ignored.The cell performance can be improved by reducing the distance between the anode and cathode.Future scale-up of the fuel cell can be easily realized by amplifying the electrode dimensions.（3）Traditional air-cathode catalysts（e.g.Pt/C）suffered from with high cost,low stability and poor tolerance to fuel crossover.A highly efficient and low-cost non-precious metal catalysts for ORR with high catalytic selectivity was successfully synthesized in this study.The obtained Co@NPC/C-MWCNTs displayed an excellent ORR performance with a half-wave potential of 0.79 V,effective four-electron transfer and superior formate-tolerance ability.The fuel permeation problem was also removed.A flexible single-stream thread-based MMFC using Co@NPC/C-MWCNTs as the air-cathode catalyst was proposed.The fuel permeation issue was removed.The proposed flexible MMFC yielded a peak power density of 4.32 m W cm^-2 stably.Notably,the performance of the proposed flexible MMFC can still maintain 90.9%of its initial value after repeated bending for 1000 times.Moreover,two flexible DFFCs connected in series mode can easily power some low-power electronics.（4）A single-stream thread-based H₂O₂ MMFC was fabricated for the practical application,which utilized H₂O₂ as both fuel and oxidant.The fuel/oxidant crossover issue was also eliminated in this fuel cell.The open circuit potential of the fuel cell can reach 0.66 V and the highest power density of 5.5 m W cm^-2 was produced.To widen the application of the fuel cell,a self-powered H₂O₂ electrochemical sensor was further developed based on the proposed fuel cell structure.The presented sensor showed a wide linear current response with the H₂O₂ concentration range from 5 to 50 m M,a high sensitivity of 0.0375 m A m M^-1,a low LOD of 1.44 m M（S/N=3）,a rapid respond time of<1 min as well as a good reproducibility.