Lattice Boltzmann Pore-Scale Investigation of Coupled Physical-electrochemical Processes in C/Pt and Non-Precious Metal Cathode Catalyst Layers in Proton Exchange Membrane Fuel Cells

Impact of Gas Diffusion Layer Compression on Electrochemical Performance in Proton Exchange Membrane Fuel Cells: A Three-Dimensional Lattice Boltzmann Pore-Scale Analysis

Proton exchange membrane fuel cells (PEMFCs) are being pursued for applications in the maritime industry to meet stringent ship emissions regulations. Further basic research is needed to improve the performance of PEMFCs in marine environments. Assembly stress compresses the gas diffusion layer (GDL) beneath the ribs, significantly altering its pore structure and internal transport properties. Accurate evaluation of the PEMFC cathode's electrochemical performance at the pore scale is critical. This study employs a three-dimensional multicomponent gas transport and electrochemical reaction lattice Boltzmann model to explore the complex interplay between GDL compression and factors such as overpotential, pressure differential, porosity, and porosity gradient on PEMFC performance. The findings indicate that compression accentuates the reduction in oxygen concentration along the flow path and diminishes the minimum current density. Furthermore, compression exacerbates the reduction in current density under varying pressure conditions. Increased local porosity near the catalyst layer (CL) enhances oxygen accessibility and water vapor exclusion, thereby elevating the mean current density. Sensitivity analysis reveals a hierarchy of impact on mean current density, ranked from most to least significant: overpotential, porosity, compression, porosity gradient, and pressure difference. These insights into the multicomponent gas transfer dynamics within compressed GDLs inform strategic structural design enhancements for optimized performance.

Boosting Bulk Oxygen Transport with Accessible Electrode Nanostructure in Low Pt Loading PEMFCs

Despite the high potential for reducing carbon emissions and contributing to the future of energy utilization, polymer electrolyte membrane fuel cells (PEMFCs) face challenges such as high costs and sluggish oxygen transport in cathode catalyst layers (CCLs). In this study, the impact of pore size distribution on bulk oxygen transport behavior is explored by introducing nano calcium carbonate of varying particle sizes for pore-forming. Physicochemical characterizations for are employed to examine the electrode structure, while in situ electrochemical measurements are used to scrutinize bulk oxygen transport resistance, effective oxygen diffusivity (D O 2 eff $D_{{{\mathrm{O}}}_2}^{{\mathrm{eff}}}$ ) and fuel cell performance. Additionally, the CCLs are constructed with aid of Lattice Boltzmann method (LBM) simulations andD O 2 eff $D_{{{\mathrm{O}}}_2}^{{\mathrm{eff}}}$ for CCLs with different pore size distribution are calculated. The findings reveal thatD O 2 eff $D_{{{\mathrm{O}}}_2}^{{\mathrm{eff}}}$ initially increases and then decreases as the most probable pore size increases. A "sphere-pipe" model is proposed to describe practical bulk oxygen transport in CCLs, highlighting the significant role of not only the pore size of secondary pores but also the number of primary pores in bulk oxygen transport.

Boosting the Performance of Low-Platinum Fuel Cells via a Hierarchical and Interconnected Porous Carbon Support

The design of a low-platinum (Pt) proton-exchange-membrane fuel cell (PEMFC) can reduce its high cost. However, the development of a low-Pt PEMFC is severely hindered by the high oxygen transfer resistance in the catalyst layer. Herein, a carbon with interconnected and hierarchical pores is synthesized as a support for the low-Pt catalyst to lower the oxygen transfer resistance. A H2-air fuel cell assembled by Pt/hierarchical porous carbon shows 1610 mW/cm2 peak power density, 2230 mA/cm2 current density at 0.60 V, and only 18.4 S/m local oxygen transfer resistance with 0.10 mgPt/cm2 Pt loading at the cathode, which far exceeds those of various carbon black supports and commercially used Pt/C catalysts. Both the experimental and simulation results have shown the advancement of hierarchical pores toward the high efficiency of oxygen transportation.

A circle/sphere populating method to generate 2D/3D stochastic microstructures

A circle/sphere populating method is proposed to generate 2D/3D stochastic microstructures. The proposed method uses circles/spheres as the basic elements and generates microstructure features through the populating process of the circles/spheres. In the populating process, the cores are first generated randomly and circles/spheres start to populate around the cores or the previous generation's circles/spheres. The populating process is controlled by the input parameters including the volume fraction, core number, circle/sphere size distribution, circle/sphere populating distance distribution, circle/sphere populating number, and populating direction constraint angle. The proposed method was compared with the QSGS method and random circle/sphere method in 2-dimensional (2D) and 3-dimensional (3D) cases. The proposed method shows advantages in generating microstructures with clear feature geometries and boundaries. Furthermore, parametric studies are conducted in 2D and 3D to investigate the effect of input parameters on the generated microstructures. With the consideration of circle/sphere spatial distributions, the proposed method can achieve different degrees of feature clustering and agglomerating. A wide range of microstructure morphologies can be achieved by adjusting the input parameters. A more accurate description of the features in the microstructures can be achieved without the involvement of the annealing-based optimization process. As a case study, the proposed method was used to generate sandstone microstructures with different grain size distributions and spatial distributions, and the permeability of generated sandstone was analyzed. Furthermore, the proposed method was applied to generate the microstructure model with a target radial distribution function to demonstrate its computational efficiency by comparing it with the random sphere method and simulated annealing based method.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Interlink among catalyst loading, transport and performance of proton exchange membrane fuel cells: a pore-scale study

An optimum balance between performance and Pt loading is critically important for the commercialization of proton exchange membrane (PEM) fuel cells. This research aims to investigate the interlink among Pt loading, reactive transport, and performance. An advanced pore-scale model is developed to describe the coupled reactive transport in the catalyst layer (CL) with the reactant gas, protons, and electrons all considered. The CL microstructure is stochastically reconstructed as a computational domain, and the physicochemical phenomena inside CLs are resolved by a multi-component lattice Boltzmann (LB) model. The results show that the electronic potential drop is not sensitive to Pt loading, while the ionic potential drop is much higher. The distributions of local overpotential and the reaction rate are similar with peak values near the membrane, indicating the importance of proton conduction. A high Pt loading could decrease the local transport loss for a shorter path to catalyst sites, but increases the overall transport resistance for a thicker structure. Although a larger electrochemical surface area (ECSA) is provided under a high Pt loading, a low Pt loading (0.1 mg cm^-2) is suggested for high current conditions (2 A cm^-2) where the transport loss is the main factor restricting the performance.

Impact of the Cathode Layer Printing Process on the Performance of MEA Integrating PGM Free Catalyst

In this work, platinum group metal (PGM) free-based cathode active layers were prepared using different printing techniques. The membrane electrode assemblies (MEAs) integrate a PGM free catalyst based on Fe, N and C atoms at the cathode side. Scanning electron microscopy (SEM) images of MEA cross sections showed the strong impact of the fabrication process on the cathode structure, the porosity and the ionomer repartition. The MEAs were characterized in a 25 cm2 single cell using cyclic voltammetry under H2/N2. The performance of the MEAs and the double layer capacity of the cathodes were also shown to be linked to the process used. The comparison of the electrochemical accessible surface of the catalyst and of its surface area (SBET) led to the determination of a utilization factor. The coated membrane (CCM) made using the decal transfer process gives the best performances.

Numerical study of mass transfer and desorption behaviors in deformable porous media using a coupling lattice Boltzmann model

The aim of this paper is to investigate the pore-scale mass transfer and desorption behaviors in deformable porous media using a coupling immersed boundary method (IBM)-lattice Boltzmann (LB) scheme. In this numerical model, a three-dimensional multiple-relaxation-time LB model is used to simulate fluid flow in porous media consisting of movable rigid adsorbent particles. To consider the effect of dynamic deformation of a porous structure, an improved immersed boundary method scheme is introduced to describe the fluid-structure interaction at the interface between the carrier gas and moving absorbent particles. Moreover, a LB model for the convection diffusion equation is adopted to consider the mass transfer of adsorbate into the macropores and micropores of the porous adsorbent. This coupled IBM-LB model is used to illustrate the mass transfer and desorption processes in shrinkage deformation of the porous structure caused by the movement of rigid adsorbent particles along different directions. At the initial time, these adsorbent particles have a saturation adsorption amount, and the adsorbate in the macropores has the uniform concentration distribution. The numerical results show that the time history curve of the adsorbate concentration in the macropores can be divided into an upturn period and a downturn period during the dynamic desorption process. In the concentration upturn period governed by Langmuir adsorption kinetics, the shrinkage deformation of the porous structure along different directions has no remarkable effect on the mass transfer and desorption behaviors. However, during the concentration downturn period governed by the mass transfer rate of the adsorbate, the shrinkage deformation of the porous structure obviously decreases the efficiency of the desorption process. In addition, the roles of the deformation direction and morphology of the porous media in the desorption process are illustrated in this work.

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

The review provides a comprehensive understanding of the atomically dispersed metal–nitrogen–carbon cathode catalysts for proton-exchange membrane fuel cell applications.

Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel-cell technology is the high content of platinum-group-metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low-cost PGM-free electrocatalysts synthesized from inexpensive, earth-abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe-N-C). PGM-free Fe-N-C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel-cell technology costs. Herein, recent developments in PGM-free electrocatalysis, demonstrating increased fuel-cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM-free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active-site densities, while at the same time establishing methods to mitigate catalyst degradation.

Lattice Boltzmann simulation of multicomponent noncontinuum diffusion in fractal porous structures

A lattice Boltzmann method (LBM) of multicomponent diffusion is developed to examine multicomponent, noncontinuum mass diffusion in porous media. An additional collision interaction is proposed to mimic the Knudsen diffusion caused by the collision interaction between gas molecules and solid pore walls. Using the improved LBM model, the ternary mixtures diffusion is simulated in fractal porous structures which are reconstructed by the random midpoint displacement algorithm. The effects of fractal characteristics and Knudsen diffusion resistance on the multicomponent diffusion in porous structures are investigated and discussed. The results indicate that the smaller fractal dimension enhances the diffusion rate of gas mixtures in fractal porous structures. When the dimensionless Knudsen diffusion coefficient is less than 20, the presence of Knudsen diffusion resistance reduces the rate of mass diffusion in porous structures obviously, especially for the species with larger molecular weight.