Pore-scale Modeling Of The Two-phase Flow,Transport And Reaction In Porous Electrode Of Air-breathing Microfluidic Fuel Cells With Flow-through Anode

With the arrival of 5G era,the performance of micro portable electronic devices improves increasingly,which puts forward higher requirements for the performance of micro energy devices.Micro fuel cell（MFC）has the advantages of stable operation,good endurance,and high energy density,having a bright application prospect.Membraneless Microfluidic Fuel Cells（MMFCs）exploit the co-laminar flow phenomenon in microscale,separating fuel and oxidant naturally and eliminating the PEM.Thus,the cell’s structure is simplified,and manufacturing cost is reduced.Benefitted from these features,MMFC has attracted extensive attention of researchers.However,operating with high output current density,a series of complex two-phase flow transport problems arose in MMFC,such as cathode flooding and gas CO₂ evolving in anode,which would lead to insufficient reactant transport,limiting the further improvement of cell performance.Therefore,it has great academic and practical significance in studying the two-phase flow and transport phenomenon in the porous electrode of MMFC and reveals the coupling mechanism between two-phase flow transport and the electrode performance.In this paper,the air-breathing cathode and flow-through anode which are widely used for MMFC,are selected as the study object.Devoted in the two-phase flow and multicomponent mass transport in porous electrode,pore-scale numerical simulation was carried to reveal the coupling mechanism between two-phase mass transport and electrochemical reaction.The transport mechanism of O₂（reactant of cathode）,formic acid（reactant of anode）,and CO₂（product of anode）were revealed.The effects of electrode pore structure,wettability,and the rate of flow and reactions on the transport and reaction characters,such as effective diffusivity,relative permeability,and electrode output current density were analyzed.The models in this dissertation were developed and solved by self-programming with MATLAB and Python.The main research works and research results are as follows:（1）For the air-breathing cathode of MMFC,the pore network model（PNM）was reconstructed by numerical method.The PTFE coating process and its influence on structure and wettability were considered.A pore-scale numerical model coupling liquid electrolyte displacement and oxygen interphase transport was established to study the effects of PTFE mass fraction on the two-phase flow transport in the air-breathing cathode.It was found that the increase of the PTFE weight percent content would reduce the liquid saturation inside the gas diffusion layer but reduce its porosity in the meantime.Hence,the effective oxygen diffusivity increases first and then drops down.The cathode achieves the best performance with PTFE mass fraction of 20 wt.%,where the limiting current density can reach 670 m A cm^-2.In addition,the study showed that the in-plane hydrophobic PTFE distribution and the negative gradient PTFE distribution can effectively limit the liquid electrolyte transport in the through-plane direction and reduce the coalescence in the in-plane direction.Thus,the flooding in cathode is alleviated,and cathode achieves better performance.Finally,an empirical correlation formula for effective diffusion coefficient and limiting current density of air-breathing cathode was proposed based on the results.（2）For the flow-through anode of MMFC,the scanning electron microscope（SEM）and nano computed tomography（Nano-CT）were used to characterize the microstructure of the flow-through anode.Then,the 3D reconstruction was carried out based on the characterization results,and the pore network model was extracted by the SNOW algorithm.The numerical model coupling formic acid diffusion advection,the formic acid oxidation reaction,and the two-phase drainage process caused gas CO₂ evolving was established to study the two-phase flow mechanism in the flow-through anode.It was shown that the Carman-Kozeny model,which is based on the homogeneity assumption,overestimates the absolute permeability of the flow-through anode.The gas CO2 can drain reversely from the top region to the middle region of the anode,resulting in the gas phase volume fraction in the anode exceeding 0.6.Meanwhile,the liquid relative permeability decreases by 90%,and the capillary pressure exceeds 1×10⁴ Pa after the gas drainage process.With the increase of Peclet number,the gas phase volume fraction decreases by 8%,and the liquid relative permeability increases by nearly 4 times.The increase of Damkohler number increases the gas volume fraction by 11%and decreases the liquid relative permeability by 80%.Thus,by increasing the Peclete number,the gas CO₂ evolving can be effectively prevented,and the anode can operate in the single-phase flow state,reducing the pump power supplying the fuel and thus enhancing the net power output.（3）The mass transport,charge transfer,and electrochemical reaction in the interval of nanowires were coupled,and the 3D-1D mixed model was developed to further study the nonuniform electrochemical reaction characteristic under the synergistic action of formic acid concentration field and electric potential field.The results showed that the formic acid oxidation reaction rate in the nanowire interval is related to its local concentration and local overpotential.During the reaction,formic acid concentration loss along the nanowire is about 10%,and the local overpotential maintains nearly a constant.The increase of the nanowires’diameter and length can expand the reaction area,and the increase of nanowire interval can enhance the formic acid mass transport.Both ways can effectively improve the formic acid oxidation reaction rate.Moreover,the reaction kinetics in the nanowire interval is controlled by both the Tafel process and mass transport,and the reaction rate cannot be accurately described by the conventional Butler-Volmer equation based on the REV scale.Under high Peclet number,the transport performance of formic acid and CO₂ is better.The anode performance is mainly affected by proton transport under high overpotential conditions,and the anode performance can be improved by 20%improving proton conductivity.