Mechanism of Water Content on the Electrochemical Surface Area of the Catalyst Layer in the Proton Exchange Membrane Fuel Cell

Probing Impact of Support Pore Diameter on Three-phase Interfaces of Pt/C Catalysts by Molecular Dynamic Simulation

Investigating how the size of carbon support pores influences the three-phase interface of platinum (Pt) particles in fuel cells is essential for enhancing catalyst utilization. This study employed molecular dynamics simulations and density functional theory calculation to examine the effects of mesoporous carbon support size, specifically its pore diameter, on Nafion ionomer distribution, as well as on proton and gas/liquid transport channels, and the utilization of Pt active sites. The findings show that when Pt particles are located within the pores of carbon support (Pt/PC), there is a significant enhancement in the spatial distribution of Nafion ionomer, along with a reduction in encapsulation around the Pt particles, compared to when Pt particles are positioned on the surface or in excessively large pores of the carbon support. While increasing pore diameter improves the construction of proton transport channels formed by sulfonate groups of Nafion ionomer and water molecules, it also raises the risk of obstructing oxygen diffusion. To achieve maximum Pt utilization and exchange current density, it is essential to balance the proton and gas/liquid transport channels. Among the models investigated, the Pt/PC-8 nm system demonstrates the highest Pt utilization. This work offers valuable insights for designing carbon support materials to optimize three-phase interfaces in the catalytic layer of fuel cells.

The Lifetime of Hydroxyl Radical in Realistic Fuel Cell Catalyst Layer

The lifetime of hydroxyl radicals (⋅OH) in the fuel cell catalyst layer remains uncertain, which hampers the comprehension of radical-induced degradation mechanisms and the development of longevity strategies for proton-exchange membrane fuel cells (PEMFCs). In this study, we have precisely determined that the lifetime of ⋅OH radicals can extend up to several seconds in realistic fuel cell catalyst layers. This finding reveals that ⋅OH radicals are capable of carrying out long-range attacks spanning at least a few centimeters during PEMFCs operation. Such insights hold great potential for enhancing our understanding of radical-mediated fuel cell degradation processes and promoting the development of durable fuel cell devices.

Porous Flow Field for Next-Generation Proton Exchange Membrane Fuel Cells: Materials, Characterization, Design, and Challenges

Porous flow fields distribute fuel and oxygen for the electrochemical reactions of proton exchange membrane (PEM) fuel cells through their pore network instead of conventional flow channels. This type of flow fields has showed great promises in enhancing reactant supply, heat removal, and electrical conduction, reducing the concentration performance loss and improving operational stability for fuel cells. This review presents the research and development progress of porous flow fields with insights for next-generation PEM fuel cells of high power density (e.g., ∼9.0 kW L^-1). Materials, fabrication methods, fundamentals, and fuel cell performance associated with porous flow fields are discussed in depth. Major challenges are described and explained, along with several future directions, including separated gas/liquid flow configurations, integrated porous structure, full morphology modeling, data-driven methods, and artificial intelligence-assisted design/optimization.

Pt utilization in proton exchange membrane fuel cells: structure impacting factors and mechanistic insights

It is essential to realize an expected low usage of platinum (Pt) in proton exchange membrane fuel cells (PEMFCs) for the large-scale market penetration of PEMFC-powered vehicles. As well as seeking Pt-based catalysts with a high specific activity, improving Pt utilization through structure optimization of the catalyst layer (CL) has been the main route and apparently a more practical way so far to develop high-performance low-Pt PEMFCs. Despite the significant progress achieved in the past 2-3 decades, a visible gap remains between the current Pt demand of automobile PEMFCs and the target value. To further increase Pt utilization, insights from previous studies are necessary. This review analyzes the structural factors that impact the current-generation efficiency of Pt in PEMFC electrodes in great detail, with emphasis particularly put on the mechanistic and molecule-level insights into the structural effects. The contents include the so-called local transport resistance associated with the permeation and diffusion of oxygen molecules in the ionomer film covering the Pt surface, regulation of ionomer aggregation through molecular interactions between ink components, modulation of ionomer distribution through pore size exclusion and surface electrostatic interaction of the carbon support, optimization of the coupling between the reaction and transport processes through graded composition, and the formation of highways of protons, electrons, and gas molecules through component alignment. We provide a critical analysis of the measurement methods and theoretical models assessing the local transport resistance, which is considered as a crucial issue in the current-generation efficiency of Pt in ultralow-Pt CL. Finally, new opportunities toward the further promotion of Pt utilization are proposed. These subjects and discussions should be of great significance in the rational design and precise fabrication of PEMFC electrodes, and may also inspire similar subjects in other electrochemical energy technologies such as water electrolysis, CO₂ reduction, and batteries.

How Does a Microfluidic Platform Tune the Morphological Properties of Polybenzimidazole Nanoparticles?

Microfluidic synthesis methods are among the most promising approaches for controlling the size and morphology of polymeric nanoparticles (NPs). In this work, for the first time, atomistic mechanisms involved in morphological changes of polybenzimidazole (PBI) NPs in microfluidic media are investigated. The multiscale molecular dynamic (MD) simulations are validated with the literature modeling and experimental data. A good agreement is obtained between the molecular modeling results and experimental data. The effects of mixing time, solvent type, dopant, and simulation box size at the molecular level are investigated. Mixing time has a positive impact on the morphology of the PBI NPs. Microfluidic technology can control the mixing time well and engineer the morphology of the NPs. In the process of morphological changes, at the optimum time (about 11.5 ms), the attraction energy between the polymer molecules is at the highest level (-37.65 kJ/mol). The size of the polymer NPs is minimal (2.3 nm), and the aspect ratio and entropy are at the lowest level, equal to 1.07 and 11.024 kJ/mol·K, respectively. It was found that the presence of water leads to the precipitation of polymeric NPs owing to the dominance of hydrophobic forces. Both dimethylacetamide (DMA) and phosphoric acid (PA) improve the control of the size and morphology of NPs. However, the addition of PA has a greater impact; PA acts as a cross-linker, making PBI NPs finer and more spherical. In addition, MD simulation reveals that PA increases the proton diffusion coefficient in PBI and enhances its efficiency in fuel cells. This study paves a new efficient way for morphological engineering of polymeric NPs using microfluidic technology.

Effect of Stratification of Cathode Catalyst Layers on Durability of Proton Exchange Membrane Fuel Cells

To decrease the cost of fuel cell manufacturing, the amount of platinum (Pt) in the catalyst layer needs to be reduced. In this study, ionomer gradient membrane electrode assemblies (MEAs) were designed to reduce Pt loading without sacrificing performance and lifetime. A two-layer stratification of the cathode was achieved with varying ratios of 28 wt. % ionomer in the inner layer, on the membrane, and 24 wt. % on the outer layer, coated onto the inner layer. To study the MEA performance, the electrochemical surface area (ECSA), polarization curves, and electrochemical impedance spectroscopy (EIS) responses were evaluated under 20, 60, and 100% relative humidity (RH). The stratified MEA Pt loading was reduced by 12% while maintaining commercial equivalent performance. The optimal two-layer design was achieved when the Pt loading ratio between the layers was 1:6 (inner:outer layer). This MEA showed the highest ECSA and performance at 0.65 V with reduced mass transport losses. The integrity of stratified MEAs with lower Pt loading was evaluated with potential cycling and proved more durable than the monolayer MEA equivalent. The higher ionomer loading adjacent to the membrane and the bi-layer interface of the stratified catalyst layer (CL) increased moisture in the cathode CL, decreasing the degradation rate. Using ionomer stratification to decrease the Pt loading in an MEA yielded a better performance compared to the monolayer MEA design. This study, therefore, contributes to the development of more durable, cost-effective MEAs for low-temperature proton exchange membrane fuel cells.

A Molecular Model of PEMFC Catalyst Layer: Simulation on Reactant Transport and Thermal Conduction

Minimizing platinum (Pt) loading while reserving high reaction efficiency in the catalyst layer (CL) has been confirmed as one of the key issues in improving the performance and application of proton exchange membrane fuel cells (PEMFCs). To enhance the reaction efficiency of Pt catalyst in CL, the interfacial interactions in the three-phase interface, i.e., carbon, Pt, and ionomer should be first clarified. In this study, a molecular model containing carbon, Pt, and ionomer compositions is built and the radial distribution functions (RDFs), diffusion coefficient, water cluster morphology, and thermal conductivity are investigated after the equilibrium molecular dynamics (MD) and nonequilibrium MD simulations. The results indicate that increasing water content improves water aggregation and cluster interconnection, both of which benefit the transport of oxygen and proton in the CL. The growing amount of ionomer promotes proton transport but generates additional resistance to oxygen. Both the increase of water and ionomer improve the thermal conductivity of the C. The above-mentioned findings are expected to help design catalyst layers with optimized Pt content and enhanced reaction efficiency, and further improve the performance of PEMFCs.

Protonated state and synergistic role of Nd3+ doped barium cerate perovskite for the enhancement of ionic pathways in novel sulfonated polyethersulfone for H2/O2 fuel cells

In recent decades, there has been a need for novel advancement of sustainable non-fluorinated polymer electrolyte membranes for proton exchange membrane fuel cell (PEMFC) applications. The set forth strategy aims to ameliorate proton conduction of sulfonated polyethersulfone (SPES) polymer membranes with a distinct mixture of barium cerate (BCO) and neodymium-doped barium cerate (BCNO) perovskites developing cationic composite membranes (CCMs) prepared through a technique of solvent casting. The CCMs were subjected to analysis of their mechanical, structural, chemical compositional, thermal, morphological, oxidative, physicochemical, electrochemical and fuel cell polarization performance respectively. Acceptor doping of the trivalent neodymium group at the B site of BCO increases the number of oxygen vacancies and improves ionic conduction. The CCM of neodymium-doped barium cerate demonstrates a higher proton conductivity of 42.2 mS cm-1 with a lower activation energy of 6.80 kJ mol-1 at 80 °C. The maximum current density and power density with the OCV of 0.93 V for the neodymium-doped barium cerate membrane are 397 mA cm-2 and 117 mW cm-2, which is 1.8 times greater than that of the pure SPES membrane. On the basis of polarization performance, the SPES membrane with neodymium-doped barium cerate has great potential in highly-efficient PEMFC applications.