Multi-scale Simulation Approach for Polymer Electrolyte Fuel Cell Cathode Design

Polymer Electrolyte Fuel Cells (PEFCs) are key devices, producing electricity from hydrogen and oxygen, pivotal for sustainable energy systems with no CO2 emissions. They are considered for various uses, including power sources for mobility, stationary cogeneration and off-grid power generation. However, the transportation sector, especially Heavy-Duty Vehicles (HDVs) and aviation, which have high emissions, experiences difficulties with decarbonization and cannot rely on existing batteries due to high load, high output, and long usage time. This scenario places high expectations on the broader application of PEFCs. To improve the performance of PEFCs, it is necessary to improve the performance of cathode ionomers, and the development of high oxygen permeability ionomers (HOPIs), which mitigate the oxygen transport rate-limiting process of the oxygen reduction reaction, has attracted much attention.1-5 In this study,6 we developed a new synthesis method for a cyclic monomer with a fluorosulfonyl group (SMD-E4) and succeeded in synthesizing a new HOPI composed entirely of cyclic monomer by copolymerization with a hydrophobic cyclic monomer (MMD). The basic electrolyte properties of the HOPI were compared with those of Nafion, and HOPI showed comparable proton transport performance with excellent oxygen permeability, suggesting its potential use as a PEFC material. By employing the HOPI as a cathode ionomer in MEAs, improved performance was observed, especially under low humidity and high current density operating conditions. Improvements were observed in both activation overvoltage and concentration overvoltage, as well as in electrochemical surface area, mass activity, specific activity, and oxygen transport-related factors. Analysis of the catalyst layers revealed that the property of the HOPI to maintain a relatively high pore volume within the catalyst layers may mitigate the direct contact between Pt and ionomer and contribute to the lowering of activation overvoltage. Furthermore, the separation of the oxygen transport resistance component suggests that the HOPI not only lowers the oxygen transport resistance of the ionomer itself, but also contributes to lowering the oxygen transport resistance derived from Knudsen diffusion by maintaining a high pore volume in the catalyst pores.Moreover, the performance of MEAs with the HOPI applied as a cathode ionomer was evaluated in a structure similar to that of a practical PEFC with a high-performance membrane and low Pt usage, and it was found that a power density improvement of more than 140% compared to that of Nafion was achieved under low humidity conditions (20% RH). Furthermore, the unique property of the HOPI that concentration overvoltage decreases under low humidity suggests the possibility of achieving MEAs with high robustness to humidity changes.These results indicate that HOPIs can fully demonstrate their performance under the harsh operating conditions of next-generation PEFCs, such as low humidity and high current density, and will play an important role in improving the performance of PEFCs. Acknowledgement This work was partially based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References 1) K. Yamada et al., ECS Trans., 50, 1495 (2013).2) R. Shimizu et al., J. Electrochem. Soc., 165, F3063 (2018).3) R. Jinnouchi et al., Nat. Commun., 12, 4956 (2021).4) J. P. Braaten et al., J. Power Sources, 522, 230821 (2022).5) N. Macauley, et al., Adv. Energy Mater., 12, 2201063 (2022).6) T. Hirai et al., ACS App. Energy Mater. (2024), https://doi.org/10.1021/acsaem.4c00177. Figure 1

Platinum and platinum-alloy oxygen reduction reaction (ORR) catalysts have been implemented in the prototype, and by now also in commercial fuel cell power systems for automotive applications, successfully meeting the demanding conditions of relatively low temperature (80°C) and high acidity operating conditions of the polymer electrolyte fuel cell (PEFC). However, due to the high price of Pt, the catalyst cost is projected to account for as much as a half of the total PEFC stack cost.1 In this context, the potential replacement of Pt-based catalysts, especially in the fuel cell cathode, with non-precious metal catalysts (NPMCs) is expected to significantly reduce PEFC stack cost. Achieving such a cost benefit is, however, contingent upon reaching sufficient activity and durability with NPMC for the ORR, a requirement that the electrocatalysis research community has been striving to meet. In this presentation, we expand upon the idea of combining nitrogen precursors during NPMC synthesis to improve the performance and performance durability of non-precious metal ORR catalysts. Previously, we introduced a new non-precious metal ORR catalyst derived from two nitrogen precursors with different decomposition temperatures: easily decomposing cyanamide (CM) and more thermally stable polyaniline (PANI).2 In this approach, CM acts as an efficient pore-forming agent that allows for the formation of a highly porous structure, which in particular benefits the high-current response of the resulting electrocatalyst. Compared to the alternative approach utilizing metal-organic frameworks (MOFs),3,4 the use of cyanamide promises a very significant reduction in the NPMC cost, without sacrificing ORR activity. In an effort to further improve the PEFC performance of the two nitrogen-precursor NPMCs, we have explored approaches involving the use of ammonia treatment, the addition of various carbon supports, and inclusion of multiple transition metals. In this presentation, we will summarize the results of these studies, which have involved the extensive use of characterization techniques, such as X-ray photoelectron spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, X-ray diffraction, surface area/pore distribution measurements, and nano-scale X-ray computed tomography. In the conclusion, we will also define critical factors for enhancing the performance of NPMCs in the PEFC cathode and outline directions for future research. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research supported through a user project at ORNL’s Center for Nanomaterials Sciences, which is an Office of Science User Facility. References Spendelow et al., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2013, (2013).Hoon T. Chung et al., “Combined nitrogen precursor approach to develop highly active non-precious metal oxygen reduction reaction catalyst.” in preparation.S. Ma et al., “Cobalt Imidazolate Framework as Precursor for Oxygen Reduction Reaction Electrocatalysts,” Chem. Eur. J. 17, 2063 (2011).Proietti et al., “Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells,” Nat. Commun. 2, 416 (2011)

Non‐precious metal catalysts of the oxygen reduction reaction are highly favored for use in polymer electrolyte fuel cells (PEFC) because of their relatively low cost. Here, a new carbon‐black‐supported pyrolyzed Co‐corrole (py‐Co‐corrole/C) catalyst of the oxygen reduction reaction (ORR) in a PEFC cathode is demonstrated to have high catalytic performance. The py‐Co‐corrole/C at 700 °C exhibits optimized ORR activity and participates in a direct four‐electron reduction pathway for the reduction of O2 to H2O. The H2‐O2 PEFC test of py‐Co‐corrole/C in the cathode reveals a maximum power density of 275 mW cm−2, which yields a higher performance and a lower Co loading than previous studies of Co‐based catalysts for PEFCs. The enhancement of the ORR activity of py‐Co‐corrole/C is attributable to the four‐coordinated Co‐corrole structure and the oxidation state of the central cobalt.

Metalloporphyrins and related metallomacrocycles as electrocatalysts for use in polymer electrolyte fuel cells and water electrolyzers

Fuel cells, as devices for direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions, are among the key enabling technologies for the transition to a hydrogen-based economy. Of several different types of fuel cells under development today, polymer electrolyte fuel cells (PEFCs) have been recognized as a potential future power source for zero-emission vehicles. However, to become commercially viable, PEFCs have to overcome the barrier of high catalyst cost caused by the exclusive use of platinum and platinum-based catalysts in the fuel-cell electrodes. Here we demonstrate a new class of low-cost (non-precious metal)/(heteroatomic polymer) nanocomposite catalysts for the PEFC cathode, capable of combining high oxygen-reduction activity with good performance durability. Without any optimization, the cobalt-polypyrrole composite catalyst enables power densities of about 0.15 W cm(-2) in H2-O2 fuel cells and displays no signs of performance degradation for more than 100 hours. The results of this study show that heteroatomic polymers can be used not only to stabilize the non-precious metal in the acidic environment of the PEFC cathode but also to generate active sites for oxygen reduction reaction.

In this article, we focus on the understanding of the electrocatalysts degradation in polymer electrolyte fuel cell (PEFC) environments. A multiscale atomistic/kinetic model is derived providing mechanistic insights on the impact of the nanostructure and operating conditions on nanoparticles durability. On the basis of ab initio (AI) data, we identify favorable pathways of the oxygen reduction reaction (ORR) on nanoparticles and of the competitive Pt–Co dissolution in acidic media. The derived AI kinetics is coupled to a description of the atomic reorganization at the nanoparticle level as a function of the cumulated Pt and Co mass losses. This nanoscale model is coupled with a transport microscale model of charges and through a PEFC cathode, and simulation sensitivity studies to operating conditions and initial compositions/morphologies are performed and complimented by microstructural and electrochemical characterizations carried out on aging direct liquid injection metallorganic chemical vapor deposition elaborated model electrodes detailed in our experimental companion paper.

As energy demand increases and global warming progresses, expectations for fuel cells, which generate electricity through chemical reactions between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. Polymer electrolyte fuel cells (PEFCs) are expected to be used in stationary power sources for home use and fuel cell vehicles because of their low operating temperature, short start-up time, and ease of miniaturization. The cost has been an issue in the widespread use of fuel cells. The amount of platinum used in the catalyst layer (CL) must be reduced to reduce costs. However, it increases the current density per unit area of platinum. There are three factors that contribute to voltage drop: ohmic polarization, activation polarization, and diffusion polarization. At high current densities, the voltage drop due to diffusion polarization is dominant. The main cause of diffusion polarization is the transport resistance of oxygen molecules in the CL, and improving the oxygen transport properties in the CL will lead to lower cost PEFCs. In this study, we analyzed the transport properties of oxygen molecules in the CLs of PEFCs using the Monte Carlo (MC) method. The effect of oxygen scattering on the overall oxygen transport properties was investigated by considering the behavior on the surface of the ionomer film (surface diffusion) into the MC method, which simulates the overall transport in the PEFC CL. The objective of this study is to reproduce accurate oxygen transport by comparing the results of MC and molecular dynamics (MD) methods. First, we used the MC method to analyze oxygen scattering phenomena. In this study, we used the 3D structure data of CL in the MC simulations. The three-dimensional structure of the sample was reconstructed from sequential slice images. Molecules were assumed to reflect specularly on the boundaries of the simulation domain. Oxygen molecules were randomly placed in the simulation system. The time step was set at 5 ps and the simulation time was set at 4 μs. In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using various conditions of surface diffusion. We calculated the effective diffusion coefficient of oxygen in the CL as the transport property of oxygen molecules in the MC method. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the effective diffusion coefficient of oxygen increases due to a decrease in the time constant, which determines the probability distribution of the surface residence time. However, when the surface diffusion coefficient is large, a different trend emerges. As the time constant increases, the effective diffusion coefficient has a peak value and then decreases. Next, to verify the model of surface diffusion in the MC method, we analyzed the scattering phenomenon of oxygen molecules in a simulation system that simulates the pores in the CL using the MD method. We modeled the nano pores in the CL as a slit between the ionomer walls in the MD method. Nafion with the equivalent weight (EW) of 1146 was used as polymer model. The number of Nafion chains is set at 4, and the water content λ is set at 7. Three-dimensional periodic boundary conditions were applied to the simulation domain. After the system was equilibrated, 1 ns of NVT simulation was performed for production. The temperature was set at 297 K. The behavior of oxygen molecules impacting on the ionomer wall was analyzed. We analyzed the behavior of oxygen molecules colliding with the ionomer membrane using the MD method. The average residence time of surface diffusion on the ionomer thin film was of the order of 10-10s. When the calculation results were compared with the MC results, it was found that the average residence time obtained by the MD method was the value where the surface diffusion coefficient affects much on the effective diffusion coefficient. Comparing the results of the MD method with the surface diffusion model used in the MC method, there are some discrepancies between the results and the model. For example, the MC method used a model in which oxygen molecules reflect diffusively on the ionomer surface. However, in the MD method, it was found that oxygen molecules tend to reflect in the direction of travel (Fig. 1). Therefore, a more accurate model of the MC method needs to be developed in the future. Figure 1

Multiscale simulation of gas transport in mixed-matrix membranes with interfacial polymer rigidification

Polymer electrolyte fuel cell (PEFCs) is a potential power source for new-generation automobiles like fuel cell vehicles that run on hydrogen energy, it has been studied a lot for last decades in order to use as the power sources of automotive. The most important characteristics of PEFCs are low operation temperature and quick start and shutdown. However, the properties of PEFCs are still limited due to its high cost and insufficient durability and performance.Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction in PEFCs, oxygen proton and electron have to be transferred smoothly in porous catalyst layer. Under normal operating conditions, oxygen transfer is the dominant factor of limiting current density, therefore, oxygen transfer must be increased from the gas channel to the Pt surface through various porous media and various components.On the other hand, inkjet printing has been considered to be an economical and easy scale-up technology for the micro-scale patterning and fabrication as one of the on-demand methods in a variety of field because it allows for the patterning of various ink. In addition, it can reduce the process time, cost and the toxic waste created during the manufacturing process. Also, this method can be easily applied to perform ink composition studies. Especially for fuel cell applications, inkjet printing resolves many of the problems associated with previous methods of catalyst deposition by allowing a uniform distribution of catalyst ink onto the surface of GDL or membrane and by loading a very low amount of catalyst.So in this research, in order to increase the performance of polymer electrolyte cells, the oxygen diffusion resistance was tested based on the difference of surface of catalyst layer in cathode by using inkjet method. Firstly, we mixed carbon supported Pt catalyst, distilled water and 20wt% Nafion solution and NPA together and then use homogenizer to disperse the liquid, then we got the catalyst ink. The Second step was fabrication of cathode catalyst layer (CCL) by using inkjet method. Third step was to observe the morphology for CCL with catalyst ink, in order to know the length and the height of a drop of ink, we made a sample, and then we took this sample to observe through laser microscope. When we got the basic information about morphology of CCL, we fabricated catalyst layer by inkjet, and took it to do the MEA evaluation and characterization to investigate their electrochemical properties, such as I-V curves, limiting current densities and impedance. And in order to compare with the experimental results, a theoretical model of the 3D structure was also circulated and discussed. From this model and measured data, it was confirmed that the better electrochemical properties results from 3D structure of catalyst layer in cathode.

In injection moulding processes, the melt undergoes a complex deformation and cooling history, which results in an inhomogeneous distribution of crystalline superstructures in semi‐crystalline thermoplastics, which significantly influences their final mechanical and thermal properties. In this paper we describe the determination of local effective thermal properties of a moulded part via a multiscale simulation approach. First a macroscopic filling and heat transfer simulation is performed followed by a microstructure evolution calculation on the micro‐scale. Then, the effective thermal properties are derived via a two‐level homogenization scheme. The results show that the effective thermal conductivity is anisotropic and that it varies asymmetrically over the analysed plate sections.

The lack of sensory feedback represents one of the main drawbacks of commercial upper limb prosthesis. Transcranial Focused Ultrasound Stimulation (tFUS) seems to be a valid non-invasive technique for restoring sensory feedback allowing to deliver acoustic energy to cortical sensory areas with high spatial resolution and depth penetration. This paper aims at studying in simulation the use of tFUS on cortical sensory areas to evaluate its effects in terms of latency ad firing rate of the cells response, for understanding if these parameters influence the safety and the efficacy of the stimulation. In this paper, in order to study the propagation of the ultrasound wave from the transducer to the cortical cells, a multiscale approach was implemented by building a macroscopic model, which estimates the pressure profile in a simplified 2D human head geometry, and coupling it with the SONIC microscale model, that describes the electrical behaviour of a cortical neuron. The influence of the stimulation parameters and of the skull thickness on the latency and the firing rate are evaluated and the obtained behaviour is linked to the sensory response obtained on human subjects. Results have shown that slight changes in the transducer position should not affect the efficacy of the stimulation; however, high skull thickness leads to lower cells activation. These results will be useful for evaluating safety and effectiveness of tFUS for sensory feedback in closed-loop prosthetic systems.

The long-term behavior and safety performance of hydraulic concrete structures are influenced by chemo-mechanical deterioration due to leaching and frost. The single-scale experimental investigations on the certain deterioration phenomenon of concrete structure submitted to leaching or frost have been performed. In this work, the multi-scale numerical simulation approach is introduced to analyze the deterioration phenomena due to leaching or/and frost in hydraulic concrete and the structural responses to the above deterioration phenomena. The three-dimensional (3D) multi-scale modeling and homogenization methods of hydraulic concrete submitted to leaching or/and frost are developed. The effect analysis of leaching or/and frost on the mechanical properties of hydraulic concrete is implemented. First, according to the multi-scale and multi-phase characteristics of hydraulic concrete as composite material, the scale separation standard of hydraulic concrete is given. The hydration model and random aggregate model are introduced to reconstruct the representative volume element model of hydraulic concrete with different scales, namely micro-scale (cement paste), meso-scale (mortar), macro-scale (concrete). The numerical homogenization method is presented to determine the mechanical properties of the hydraulic concrete. Second, the methods are developed to implement the 3D multi-scale simulation for random leaching process and thermo-mechanical coupling frost damage process of hydraulic concrete. Regarding seepage leaching as the leading role, a numerical method investigating the combined effect of leaching and frost on the mechanical properties of hydraulic concrete is proposed. Finally, the water-saturated hydraulic concrete is taken as an example. The deterioration phenomena due to leaching or/and frost in selected concrete are analyzed. The structural responses to the above deterioration phenomena are identified. It is indicated that the proposed multi-scale progressive analysis approach can capture the potential deterioration phenomena of hydraulic concrete. The variation mechanism of concrete structural behavior can be given a deeper insight. It can be seen that the nonlinear superimposed effect of leaching and frost can easily cause serious damage of water-saturated hydraulic concrete.

The activity of oxygen reduction reaction (ORR) catalysts often determine performance of polymer electrolyte fuel cells (PEFCs) as measured by their power output, open circuit voltage, and fuel conversion efficiency. Currently, Pt-nanoparticle catalysts, either supported on high surface-area carbons or prepared in a form of contiguous thin layer on conductive or non-conductive supports, represent the state of the art in ORR electrocatalysis at the PEFC cathode. However, the high and variable price and scarceness of Pt have limited its widespread implementation in the -temperature fuel cells to date, especially in automotive transportation. Under these circumstances, platinum group metal-free (PGM-free) ORR catalysts have received growing attention in recent years as a possible replacement for Pt-based formulations. The progress achieved since the development of the first nature-inspired electrocatalysts of oxygen reduction in the seminal work by Jasinski in the 1960s (Nature 201, 1212, 1964), which mostly happened through the broad implementation of the high-temperature synthesis approach, makes replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this this presentation, we will summarize recent progress in research targeting development of high-performance PGM-free catalysts for oxygen reduction reaction (ORR) at Los Alamos National Laboratory. Two approaches will be discussed in a greater detail: (i) the approach involving fine-tuning of the catalyst porosity and surface area using pore-forming compounds and (ii) the method specifically focusing on the development of atomically dispersed transition metal moieties and avoiding the formation of transition metal-rich nanoparticles during the heat treatment of catalyst precursors. We will demonstrate the impact that porosity/surface area optimization, through the use of either pore formers (cyanamide, ZnCl2) precursor templating (metal organic frameworks) or both, can have on the fuel cell performance of PGM-free catalysts. We will also show how modifications to the electrode structure through the change in the ionomer content and ionomer equivalent weight can lead to substantial improvements in the fuel cell performance at both low- and high current densities (aerial power density of more than 0.50 W/cm2 in H2-air testing at 80°C). Finally, we will recapitulate the challenges still facing PGM-free research that, in spite of all the progress achieved in recent years, is yet to produce materials capable of competing with the incumbent Pt-based catalysts in terms of oxygen reduction activity, performance durability, and cost (specifically, when extended to the overall cost of a fuel cell stack). Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

An increase in platinum catalyst utilization in polymer electrolyte fuel cells (PEFCs) is necessary to improve performance and reduce costs. We have previously proposed a novel electrode preparation method, based on the process of grafting a proton-conducting agent onto platinum-supported carbon before the conventional electrode fabrication process. In the present study, platinum-supported carbon grafted with methylsulfonic acid groups (-CH2SO3H) and platinum-supported carbon grafted with polyacrylamide tert-butylsulfonic acid groups (Poly-ATBS) were mixed in fabricating the membrane electrode assembly (MEA). The PEFCs' performance of the MEA made from our novel ionomer-grafted platinum-supported carbon material was superior to that of an MEA made using the conventional method using Nafion as an ionomer. The current density at 0.7 V of the MEA using grafted proton-conducting groups on catalyst-supported carbon was 600 mA/cm2, which is twice as large as the MEA using nongrafted catalyst-supported carbon.

The high cost of Pt-based catalysts for the anode and cathode has greatly limited the distribution and the commercialization of polymer electrolyte fuel cells (PEFCs). Non-precious metal catalysts (NPMCs) have shown high activity and low hydrogen peroxide yields [1, 2], and are a possible alternative to Pt-based catalysts for oxygen reduction reaction (ORR) in the PEFC cathode. In this work, we explore the development of graphite-based catalysts for fuel cell applications. The desired mechanical and chemical stability of graphite could improve durability of NPMC systems. However, implementation of graphite as a catalyst precursor presents new challenges as it is chemically inert. Given the fact that NPMCs are composed of carbon, nitrogen, and transition metals, a systematic approach will be presented to incorporate transition metal and nitrogen precursors into graphite structures. Graphite as a starting material will be treated with high temperature and pressure to intercalate transition metals and/or nitrogen compounds. The formation of active sites will be induced by the increase of defects in the material by harsh acid treatments and by microwaved synthesis. Structural, chemical, and morphological information will be presented via X-ray (XRD and XPS) techniques along with electron microscopy (SEM, TEM). The electrochemical activity for the ORR using RRDE for these graphite-based catalysts will be presented in both alkaline and acidic media. Our preliminary results indicate an improvement of the catalytic activity upon addition of defects, metal and nitrogen compounds.