Fractal Analysis of Gas Diffusion Layer in PEM Fuel Cells

The aim of this study is to show how fractal analysis can be effectively used to characterize the texture of porous solids. The materials under study were carbon papers, the backing material of the gas diffusion layer （GDL） in Proton Exchange Membrane Fuel Cell （PEMFC）. The fractal dimensions were calculated by analyzing data from mercury porosimetry. The polytotrafluoroethylene （PTFE） treated carbon paper shows a significantly high fractal dimension value than pare sample, and the high fractal dimension signifies that the physical complexity of the pore surface is enhanced. The fractal dimension can be used as a valid parameter to monitor the textural evolution of the samples as the treatment progresses, as this behaves in a similar way to other textural parameters. The use of fractal analysis in conjunction with the results of classical characterization methods leads to a better understanding of textural modificatious in the processing of materials.

Lattice Boltzmann Simulations of Water Transport from the Gas Diffusion Layer to the Gas Channel in PEFC

Water management is a key to ensuring high performance and durability of polymer electrolyte fuel cell(PEFC),and it is important to understand the behavior of liquid water in PEFC.In this study,the two-phase lattice Boltzmann method is applied to the simulations of water discharge from gas diffusion layers(GDL)to gas channels.The GDL is porous media composed of carbon fibers with hydrophobic treatment,and the gas channels are hydrophilic micro-scale ducts.In the simulations,arbitrarily generated porous materials are used as the structures of the GDL.We investigate the effects of solid surface wettabilities on water distribution in the gas channels and the GDL.Moreover,the results of X-ray computed tomography images in the operating PEFC are compared with the numerical simulations,and the mechanism of the water transport in PEFC is considered.

A Study on the Transport Process in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

Gas diffusion layer(GDL) plays a great important role in proton exchange membrane fuel cell(PEMFC).Water transport mechanism in GDL is still not clear.In the present study,an ex-situ transparent setup is built to visualize the transport phenomena and to measure the threshold pressure of water in GDL at different temperatures.It is found that the relationship between the breakthrough pressure and the temperature is nearly linear(i.e.the pressure decreases linearly with the increase of temperature).To avoid the problems faced by the continuum models,the pore network model is developed to simulate the liquid water transport through the carbon paper.A uniform pressure boundary condition is used in simulation and the results are similar to the ones obtained in the experiment.The reason is that the contact angle and surface tension coefficient of water in GDLs change accordingly with the change of temperature.

Evaluating Breakthrough Pressure in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer(GDL) of a proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.The breakthrough pressure,at which the liquid water penetrates the GDL and starts to form a droplet,was measured.The breakthrough pressure was found to be different for the GDLs with different porosities and thicknesses.The equilibrium pressure,which is defined as the minimum pressure required maintaining a constant flow through the GDL,was also recorded.The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL.A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses.The breakthrough pressure increases for the thick GDL with smaller micro-pore size.

Influence of gas-diffusion-layer current collector on electrochemical performance of Ni(OH)_(2) nanostructures

We report the electrochemical performance of Ni(OH)_(2) on a gas diffusion layer(GDL).The Ni(OH)_(2) working electrode was successfully prepared via a simple method,and its electrochemical performance in 1 M NaOH electrolyte was investigated.The electrochemical results showed that the Ni(OH)_(2)/GDL provided the maximum specific capacitance value(418.11 F·g^(−1))at 1 A·g^(−1).Furthermore,the Ni(OH)_(2) electrode delivered a high specific energy of 17.25 Wh·kg^(−1) at a specific power of 272.5 W·kg^(−1) and retained about 81%of the capacitance after 1000 cycles of galvanostatic charge–discharge(GCD)measurements.The results of scanning electron microscopy(SEM)coupled with energy-dispersive X-ray spectroscopy(EDS)revealed the occurrence of sodium deposition after long-time cycling,which caused the reduction in the specific capacitance.This study results suggest that the light-weight GDL,which can help overcome the problem of the oxide layer on metal–foam substrates,is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.

Gas Transport Properties in Gas Diffusion Layers:A Lattice Boltzmann Study

The lattice Boltzmann method is applied to the investigations of the diffusivity and the permeability in the gas diffusion layer(GDL)of the polymer electrolyte fuel cell(PEFC).The effects of the configuration of water droplets,the porosity of the GDL,the viscosity ratio of water to air,and the surface wettability of the GDL are investigated.From the simulations under the PEFC operating conditions,it is found that the heterogeneous water network and the high porosity improve the diffusivity and the permeability,and the hydrophobic surface decreases the permeability.

Atomic layer deposition of ultrathin layered TiO_2 on Pt/C cathode catalyst for extended durability in polymer electrolyte fuel cells

This study shows the preparation of a TiO2 coated Pt/C（TiO2/Pt/C） by atomic layer deposition（ALD）,and the examination of the possibility for TiO2/Pt/C to be used as a durable cathode catalyst in polymer electrolyte fuel cells（PEFCs）. Cyclic voltammetry results revealed that TiO2/Pt/C catalyst which has 2 nm protective layer showed similar activity for the oxygen reduction reaction compared to Pt/C catalysts and they also had good durability. TiO2/Pt/C prepared by 10 ALD cycles degraded 70% after 2000 Accelerated degradation test, while Pt/C corroded 92% in the same conditions. TiO2 ultrathin layer by ALD is able to achieve a good balance between the durability and activity, leading to TiO2/Pt/C as a promising cathode catalyst for PEFCs. The mechanism of the TiO2 protective layer used to prevent the degradation of Pt/C is discussed.

Improved properties of carbon fiber paper as electrode for fuel cell by coating pyrocarbon via CVD method

The fabrication of a pyrocarbon coated carbon paper and its application to the gas diffusion lay(GDL) of proton exchange membrane(PEM) fuel cell were described.This carbon paper was fabricated by using conventional carbon paper as the precursor,and coating it with pyrocarbon by pyrolyzing propylene via the chemical vapor deposition(CVD) method.For comparison,conventional carbon paper composites were also prepared by using PAN-based carbon fiber felt as the precursor followed by impregnation with resin,molding and heat-treatment.SEM characterization indicates that pyrocarbon is uniformly deposited on the surface of the fiber in the pyrocarbon coated carbon paper and made the fibers of carbon felt bind more tightly.In contrast,there are cracks in matrix and debonding of fibers due to carbonization shrinkage in the conventional carbon paper.Property measurements show that the former has much better conductivity and gas permeability than the latter.In addition,current density-voltage performance tests also reveal that the pyrocarbon coating can improve the properties of carbon paper used for electrode materials of fuel cell.

电催化CO_(2)还原的稳定性问题:评论性综述

电催化二氧化碳还原反应(CO_(2)RR)能够利用可再生电能将CO_(2)转化为高附加值化学品和燃料,是一项有望缓解当下环境挑战的技术方案.本文总结了CO_(2)RR稳足性下降的根本原因,并探讨了解决这一问题的有效策略以及最新的研究进展.首先,在外加电位的作用下,催化剂会发生结构转变,且碳沉积或气体杂质引起的催化剂中毒都会降低CO_(2)RR的稳定性.其次,气体扩散层(GDL)附近发生水淹和盐沉淀会阻碍反应物/生成物的传输,三相界面会受到破坏.在长时间服役时,液压和电流的作用会导致GDL开裂.在电解质方面,碱性电解质会与CO_(2)中和,形成碳酸盐沉淀;而酸性介质腐蚀催化剂,并伴随严重的析氢副反应.另外,电解质中的金属离子杂质的优先还原进一步加剧了稳定性的衰减.深入理解CO_(2)RR中各类失活现象对设计稳定、高效的CO_(2)电催化系统具有指导意义.针对CO_(2)RR稳定性研究目前所面临的困境,本文最后展望了该领域未来的研究方向.

A perspective on influences of cathode material degradation on oxygen transport resistance in low Pt PEMFC

A large-scale industrial application of proton exchange membrane fuel cells(PEMFCs)greatly depends on both substantial cost reduction and continuous durability enhancement.However,compared to effects of material degradation on apparent activity loss,little attention has been paid to influences on the phenomena of mass transport.In this review,influences of the degradation of key materials in membrane electrode assemblies(MEAs)on oxygen transport resistance in both cathode catalyst layers(CCLs)and gas diffusion layers(GDLs)are comprehensively explored,including carbon support,electrocatalyst,ionomer in CCLs as well as carbon material and hydrophobic polytetrafluoroethylene(PTFE)in GDLs.It is analyzed that carbon corrosion in CCLs will result in pore structure destruction and impact ionomer distribution,thus affecting both the bulk and local oxygen transport behavior.Considering the catalyst degradation,an eventual decrease in electrochemical active surface area(ECSA)definitely increases the local oxygen transport resistance since a decrease in active sites will lead to a longer oxygen transport path.It is also noted that problems concerning oxygen transport caused by the degradation of ionomer chemical structure in CCLs should not be ignored.Both cation contamination and chemical decomposition will change the structure of ionomer,thus worsening the local oxygen transport.Finally,it is found that the loss of carbon and PTFE in GDLs lead to a higher hydrophilicity,which is related to an occurrence of water flooding and increase in the oxygen transport resistance.

Integration of multi-physics and machine learning-based surrogate modelling approaches for multi-objective optimization of deformed GDL of PEM fuel cells

The development of artificial intelligence(AI)greatly boosts scientific and engineering innovation.As one of the promising candidates for transiting the carbon intensive economy to zero emission future,proton exchange membrane(PEM)fuel cells has aroused extensive attentions.The gas diffusion layer(GDL)strongly affects the water and heat management during PEM fuel cells operation,therefore multi-variable optimization,including thickness,porosity,conductivity,channel/rib widths and compression ratio,is essential for the improved cell performance.However,traditional experiment-based optimization is time consuming and economically unaffordable.To break down the obstacles to rapidly optimize GDLs,physics-based simulation and machine-learning-based surrogate modelling are integrated to build a sophisticated M 5 model,in which multi-physics and multi-phase flow simulation,machine-learning-based surrogate modelling,multi-variable and multi-objects optimization are included.Two machine learning methodologies,namely response surface methodol-ogy(RSM)and artificial neural network(ANN)are compared.The M 5 model is proved to be effective and efficient for GDL optimization.After optimization,the current density and standard deviation of oxygen dis-tribution at 0.4 V are improved by 20.8%and 74.6%,respectively.Pareto front is obtained to trade off the cell performance and homogeneity of oxygen distribution,e.g.,20.5%higher current density is achieved when sacrificing the standard deviation of oxygen distribution by 26.0%.

Numerical Investigation on the Effects of Design Parameters and Operating Conditions on the Electrochemical Performance of Proton Exchange Membrane Water Electrolysis

Proton exchange membrane electrolysis cell(PEMEC)is one of the most promising methods to produce hydrogen at high purity and low power consumption.In this study,a three-dimensional non-isothermal model is used to simulate the cell performance of a typical PEMEC based on computational fluid dynamics(CFD)with the finite element method.Then,the model is used to investigate the distributions of current density,species concentration,and temperature at the membrane/catalyst(MEM/CL)interface.Also,the effects of operating conditions and design parameters on the polarization curve,specific electrical energy demand,and electrical cell efficiency are studied.The results show that the maximum distribution of current density,hydrogen concentration,oxygen concentration,and temperature occur beneath the core ribs and increase towards the channel outlet,while the maximum water concentration distribution happens under the channel and decreases towards the channel exit direction.The increase in gas diffusion layer(GDL)thickness reduces the uneven distribution of the contour at the MEM/CL interface.It is also found that increasing the operating temperature from 323 K to 363 K reduces the cell voltage and specific energy demand.The hydrogen ion diffusion degrades with increasing the cathode pressure,which increases the specific energy demand and reduces the electrical cell efficiency.Furthermore,increasing the thickness of the GDL and membrane rises the specific energy demand and lowers the electrical efficiency,but increasing GDL porosity reduces the specific electrical energy demand and improves the electrical cell efficiency;thus using a thin membrane and GDL is recommended.