Proton Exchange Membrane Fuel Cell Catalysts Research Articles

Research Progress of Pt-Based Catalysts toward Cathodic Oxygen Reduction Reactions for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have been considered by many countries and enterprises because of their cleanness and efficiency. However, due to their high cost and low platinum utilization rate, the commercialization process of PEMFC is severely limited. The cathode catalyst layer (CCL) plays an important role in manipulating the performance and lifespan of PEMFCs, which makes them one of the most significant research focuses in this community. In the CCL, the intrinsic activity and stability of the catalysts determine the performance and lifetime of the catalyst layer. In this paper, the composition and working principle of the PEMFC and cathode catalyst layer are briefly introduced, focusing on Pt-based catalysts for oxygen reduction reactions (ORRs). The research progress of Pt-based catalysts in the past five years is particularly reviewed, mainly concentrating on the development status of emerging Pt-based catalysts which are popular in the current research field, including novel concepts like phase regulation (intermetallic alloys and high-entropy alloys), interface engineering (coupled low-Pt/Pt-free catalysts), and single-atom catalysts. Finally, the future research and development directions of Pt-based ORR catalysts are summarized and prospected.

Challenges and Opportunities in Preparing Fe-N-C Layers Supported on Non-Carbon Supports

Global energy demand will be continuously rising in the foreseeable future. To mitigate the results of climate change it is necessary to reduce the emittance of greenhouse gases. One promising approach is to switch to hydrogen as energy carrier and use proton exchange membrane fuel cells (PEMFCs) to efficiently convert H2 into electric power. Here, hydrogen and oxygen/air are used to directly generate electrical energy with only water as byproduct. Both reactions need to happen efficiently at the same time. The oxygen reduction reaction (ORR), however, is a very sluggish four-electron-reaction and needs efficient catalysts.The state-of-the-art ORR catalyst in PEMFC is platinum but that is a very scarce and expensive metal. Hence, it is important to switch to precious-metal-group (PGM) free catalysts. In recent years, bio inspired iron-based FeNC catalysts have been shown to approach ORR activities that are competitive with those of platinum, even though they need a higher catalyst loading than Pt to reach the desired high current densities.1–3 Those catalysts feature single Fe atoms surrounded by four N atoms in graphitic carbon. The drawback of those materials is their lower stability compared to PGM materials. In a nutshell, the carbon matrix oxidizes during operation in PEMFCs, changing the morphology of the catalytic center and rendering it less active, or inactive, depending on the carbon oxidation extent. Direct demetallation phenomena are also possible, leaving a N4 cavity without metal cation center. It was shown recently by Wu et al.4 that it is possible to raise durability by adding an additional carbon layer on the catalyst. One alternative approach to reduce those degradations is to interface Fe-N4 sites with a carbon-free support. The targeted support material must be immune to corrosion in acid and withstand the typical ORR electrochemical potentials. It also has to be electrically conductive to facilitate electron transfers, both locally and on long scale.In this work, conductive ceramic metal oxide materials (Ta doped SnO2, TTO) was investigated as a possible alternative catalyst supports. This material is resistant to acid and can withstand prolonged potentials at operating conditions in PEMFCs.5 During the synthesis of the FeNC catalysts, high temperatures are generally applied. Such treatments can be detrimental to the conductivity of ceramics due to metal aggregation (in case of doped materials) and may also lead to morphological changes as well as the reduction of the ceramics to metallic materials. A robust method to synthesize FeNC catalysts is by mixing the separate Fe, N and C precursors and using optimized thermal treatment. In this study, the effect of the pyrolysis conditions as well as the ratio of FeNC precursors to TTO was investigated with various characterization methods, e.g. X-ray absorption spectroscopy, Mossbauer spectroscopy, and high resolution TEM. The characterization techniques confirm the presence of single atom Fe as well as different Sn species that might contribute to the activity. The resulting materials were electrochemically characterized with rotating disc electrode techniques and in PEM fuel cells for their ORR-activity and durability. It was shown that the activity of materials with a high FeNC precursor content relative to TTO can outperform a reference FeNC synthesized similarly but without TTO, in both RDE (Figure) and PEMFC.The presentation will report in detail the effects of pyrolysis conditions and FeNC precursor/TTO content on the structure, activity and stability of materials.(1) Mehmood, A. et al. High Loading of Single Atomic Iron Sites in Fe–NC Oxygen Reduction Catalysts for Proton Exchange Membrane Fuel Cells. Nat. Catal. 2022, 5 (4), 311–323. https://doi.org/10.1038/s41929-022-00772-9.(2) Gasteiger, H. A.; Marković, N. M. Just a Dream—or Future Reality? Science 2009, 324 (5923), 48–49. https://doi.org/10.1126/science.1172083.(3) Zion, N. et al.. Electrocatalysis of Oxygen Reduction Reaction in a Polymer Electrolyte Fuel Cell with a Covalent Framework of Iron Phthalocyanine Aerogel. ACS Appl. Energy Mater. 2022, 5 (7), 7997–8003. https://doi.org/10.1021/acsaem.2c00375.(4) Liu, S.et al. Atomically Dispersed Iron Sites with a Nitrogen–Carbon Coating as Highly Active and Durable Oxygen Reduction Catalysts for Fuel Cells. Nat. Energy 2022, 7 (7), 652–663. https://doi.org/10.1038/s41560-022-01062-1.(5) Jiménez-Morales, et al. Strong Interaction between Platinum Nanoparticles and Tantalum-Doped Tin Oxide Nanofibers and Its Activation and Stabilization Effects for Oxygen Reduction Reaction. ACS Catal. 2020, 10 (18), 10399–10411. https://doi.org/10.1021/acscatal.0c02220. Figure 1

Low-Electronegativity Mn-Contraction of PtMn Nano-Dendrite Boosts Oxygen Reduction Durability

Although proton exchange membrane fuel cells (PEMFCs) are emerging as an efficient and green technology, their heavy reliance on scarce and exorbitant price platinum catalysts for the oxygen reduction reaction (ORR) hampers large-scale commercialization1. The strain effect, induced through the alloying of Pt with transition metals, can enhance the catalytic activity as well as reduce the platinum loading2. While this has been well documented, these catalysts’ durability has received limited attention3. In this work, we report a new class of PtMn alloy catalysts with low-electronegativity Mn-contraction which boosts the oxygen reduction durability of fuel cells. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) reveal that the PtMn catalyst with moderate strain shows the best activity (0.53 A mg-1 at 0.9 V RHE). The PtMn alloy catalyst achieves an outstanding mass activity retention of 96% after 10,000 degradation cycles in liquid electrolyte due to the low-electronegativity of Mn, with far superior performance than highly electronegative PtNi alloys. Finally, the PtMn catalyst in PEMFC achieves an outstanding peak power density of 1.4 W cm-2 using half the loading of commercial Pt/C, and good stability over 50 h at 0.6 V in H2-O2. Figure 1

Identification of the environmental hotspots of a recycling process - Case study of a Pt PEMFC catalyst closed-loop recycling system evaluated via life cycle assessment methodology

Proton-exchange membrane fuel cell (PEMFC) technology using green H2 as a fuel to produce decarbonized electricity is considered as a promising substitute for fossil fuels. However, its development is impeded by the use of critical raw materials, such as platinum which catalyzes the hydrogen oxidation and oxygen reduction reactions. In this paper, a novel closed-loop system for the re-manufacturing of platinum-based catalyst electrode from recycled material is developed. Three different recycling alternatives are compared using the life cycle assessment (LCA) methodology. The most promising one, which combines the extraction step with an innovative synthesis of recycled Pt/C catalyst, leads to the manufacturing of an electrochemically active Pt/C catalyst with a high recovery efficiency of 96 %. Sensitivity analyses display that the recycling processes significantly relieve the environmental burdens associated with platinum mining and that their efficiencies directly relate to a decrease in environmental impacts.

Atomically Dispersed Ce Sites Augmenting Activity and Durability of Fe-Based Oxygen Reduction Catalyst in PEMFC.

In the cathode of proton exchange membrane fuel cells (PEMFCs), Fe and N co-doped carbon (Fe-N-C) materials with atomically dispersed active sites are one of the satisfactory candidates to replace Pt-based catalysts. However, Fe-N-C catalysts are vulnerable to attack from reactive oxygen species, resulting in inferior durability, and current strategies failing to balance the activity and stability. Here, this study reports Fe and Ce single atoms coupled catalysts anchored on ZIF-8-derived nitrogen-doped carbon (Fe/Ce-N-C) as an efficient ORR electrocatalyst for PEMFCs. In PEMFC tests, the maximum power density of Fe/Ce-N-C catalyst reached up to 0.82Wcm-2, which is 41% larger than that of Fe-N-C. More importantly, the activity of Fe/Ce-N-C catalyst only decreased by 21% after 30000 cycles under H2/air condition. Density functional theory reveals that the strong coupling between the Fe and Ce sites result in the redistribution of electrons in the active sites, which optimizes the adsorption of OH* intermediates on the catalyst and increases the intrinsic activity. Additionally, the admirable radical scavenging ability of the Ce sites ensured that the catalysts gained long-term stability. Therefore, the addition of Ce single atoms provides a new strategy for improving the activity and durability of oxygen reduction catalysts.

Multi-objective optimization of ice distribution uniformity and startup failure time for cold start of PEMFC catalyst layer based on CTnet and PSO-GA

The distribution of ice in the cathode catalyst layer (CCL) of a proton exchange membrane fuel cell (PEMFC) has a significant impact on cold start. A three-dimensional multiphase numerical model for fuel cell cold start was developed to investigate the first-order finite difference sensitivities of seven selected parameters, namely gas diffusion layer (GDL) porosity, CCL porosity, ionomer volume fraction, inlet temperature, initial membrane water content, catalyst layer (CL) surface area, and initial current, with respect to the uniformity of ice distribution and cold start failure time. Six of these parameters were chosen as inputs for a CNN-Transformers based neural network (CTnet), which was developed using Convolutional Neural Networks (CNN) and the multi-head attention mechanism of Transformers as a surrogate model for cold start. The optimization algorithm PSO-GA was used to find the optimal operating and geometric parameters, aiming to achieve the most uniform ice distribution in the CCL and extend the final cold start failure time. The optimized ice distribution uniformity was found to be 0.204446, showing a 1.1163 % improvement compared to the baseline model. The final cold start failure time was extended by 15.625 % to 185 s compared to the baseline model. The optimized model exhibited a more uniform distribution of ice during cold start, resulting in an extended cold start failure time. This indicates a reduced performance impact on the CCL, thereby improving the durability of the fuel cell. The optimization provides additional time for future auxiliary cold starts, while also enhancing the starting speed of auxiliary cold start through improved ice distribution uniformity.

Effect of Engineered Cracks in Catalyst Layers on PEMFC Catalyst Layer Durability

Proton exchange membrane fuel cells (PEMFCs) are expected to play a pivotal role in decarbonizing the transportation sector, and particularly heavy-duty vehicles (HDVs). However, improvements in durability are needed for PEMFCs to compete with state-of-the-art power sources for HDVs. Here, we examine how catalyst layer (CL) cracks that are engineered affect the CL durability by using patterned silicon templates to control the CL crack density at the micrometer scale. Electrochemical analyses show that the initial PEMFC performance is relatively unaffected by crack density, but the performance after durability testing was strongly affected. Specifically, CLs with high crack density showed higher performance relative to CLs without cracks after application of a carbon corrosion accelerated stress test. Electrochemical analyses coupled with X-ray computed tomography and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy showed that the cracks provide shorter oxygen diffusion pathways to reaction sites, leading to decreased oxygen transport resistance. Additionally, we observed that the catalyst durability is unaffected by cracks. Our results provide a mechanistic explanation of the role of cracks in CL durability.

Highly dispersed ultrafine PtCo alloy nanoparticles on unique composite carbon supports for proton exchange membrane fuel cells.

The design of highly efficient and robust platinum-based electrocatalysts is pivotal for proton exchange membrane fuel cells (PEMFC). One of the long-standing issues for PEMFC is the rapid deactivation of the catalyst under working conditions. Here, we report a simple synthesis strategy for ultrafine PtCo alloy nanoparticles loaded on a unique carbon support derived from a zeolitic imidazolate framework-67 (ZIF-67) and Ketjen Black (KB) composite, exhibiting a remarkable catalytic performance toward the oxygen reduction reaction (ORR) and PEMFC. Benefitting from the N-doping and wide pore size distribution of the composite carbon supports, the growth of PtCo nanoparticles can be evenly restricted, leading to a uniform distribution. The Pt-integrated catalyst delivers an outstanding electrochemical performance with a mass activity that is 8.6 times higher than that of the commercial Pt/C catalyst. Impressively, the accelerated durability test (ADT) demonstrates that the hybrid carbon support can significantly enhance the durability. Theoretical simulations highlight the synergistic contribution between the supports and the PtCo nanoparticles. Moreover, hydrogen-oxygen fuel cells assembled with the catalyst exhibited a high power density of 1.83 W cm-2 at 4 A cm-2. These results provide a new opportunity to design advanced catalysts for PEMFC.

Optimization of Realistic Accelerated Stress Tests for PEM Fuel Cells Using Standardized Automotive Driving Cycles

Proton exchange membrane (PEM) fuel cells have so far not achieved the targeted lifetime necessary to be an economical alternative for automotive applications. This is mainly caused by their high degradation rates. To better understand the degradation mechanisms, the use of accelerated stress tests (ASTs) at the component level is widespread. However, it is still a challenge to transfer their results to real-world applications.To address this problem, the approach of this work is to derive load profiles from standardized driving cycles for passenger cars. In combination with a fuel cell system model that is used to define the operating conditions of the media, such as temperature or pressure, a test procedure is obtained that results in a realistic degradation of the fuel cell. The proposed test procedure exposes the PEM fuel cell (PEMFC) to the most important degradation conditions in mobile applications. Dynamic operation is responsible for deterioration processes within the PEMFC and is the underlying cause of typical hygrothermal and voltage cycling events. Such cycling also occurs during start-stop operations, causing severe damage to the PEMFC catalysts. In addition, idling conditions may prompt open-circuit voltage (OCV) conditions in the PEMFC, resulting in dissolution of the Pt catalyst and subsequent chemical degradation of the PEM. Additionally, high load operation may also lead to catalyst degradation. Finally, the inertia of the auxiliary components, such as the humidifier, during load changes causes not ideal conditions, such as relative humidity changes, resulting in further mechanical stress on the membrane.Due to the targeted PEMFC lifetime of up to 8,000 hours for automotive applications [1], realistic degradation test procedures have a very long test duration, making them particularly challenging and costly to conduct. To overcome this hurdle, the load profile of the proposed test procedure is shortened by using an in-house developed optimization algorithm. For this purpose, four operation modes are defined as degradation-accelerating: voltage cycling, relative humidity (RH) cycling, OCV/idling, and high load operation. Based on state-of-the-art AST procedures, a sub-algorithm is developed for each operating mode. Such sub-algorithms are capable of identifying all occurrences within a given test procedure that may lead to accelerated fuel cell degradation. For each sub-algorithm, the degradation parameters (namely upper and lower potential limits for voltage cycling, membrane tensile stress for RH cycling, fluoride emission rate for OCV/idling, and voltage degradation rate for high load) are specified by the user. This allows the comparison of different fuel cells in terms of their resistance to conventional degradation mechanisms.The developed algorithm can be used to optimize various driving cycles such as the worldwide harmonized light vehicles test cycle (WLTC) for various mobile applications. All cycle events identified as accelerating degradation are connected with a point connector function based on the fuel cell polarization curve. This ensures moderate voltage steps between the identified phases and thus prevents the algorithm from inducing further degradation of the fuel cell.The optimization algorithm obtained can reduce the test duration by up to 40%. The method can be repeated frequently during AST operation by repeatedly measuring the polarization curve to consider the continuous degradation in the test procedure. Decreasing fuel cell performance leads to more severe voltage drops during load changes and thus an increase of degradation-accelerating phases over the test period. Furthermore, start-stop ASTs developed by Bisello et al. [2] are also included due to their strong contribution to fuel cell degradation in automotive applications.Finally, the optimized cycles are analyzed using two simulation tools to evaluate the degradation. The first tool by Ao et al. [3] uses the electrochemical surface area (ECSA) to quantify the proceeding degradation during cycle operation. A characteristic voltage and the voltage change rate are extracted from the driving cycle. For both values, the resulting decrease in ECSA is modeled and aging is defined as the ECSA loss rate. The second tool by Pei et al. [4] examines the driving cycle according to the four degradation-inducing conditions and combines them with experimentally determined degradation rates. Both tools allow an analysis of the degradation derivative between the initial and optimized test procedures. By adjusting the degradation rate for the different degradation mechanisms, the AST procedure is improved in terms of its representativeness for real-world applications.

New Advances in Graded PEMFC Catalyst Layers

Ohmic and mass transport overpotentials remain investigation and optimization areas for Proton Exchange Membrane Fuel Cell (PEMFC) applications. One potential optimization approach involves the preferential distribution of the chemical substituents spatially in the catalyst layer. This would ultimately result in graded catalyst layers (GCLs). The spatial grading directions can thereby be in the in plane and/or through plane direction(s). Chemical constituents often graded in both directions include the binder and the catalyst[1, 2]. The ionomer, an essential constituent for PEMFC applications has also been a key focus area for many research institutes and groups. Novel optimization approaches include finding new ionomers with low equivalent weights (EWs).Typically, in a PEMFC, the highest proton conduction rate is at the membrane interface and decreases towards the GDL[3, 4]. Hence, previous studies with gradients containing high ionomer contents close to the membrane and lower ones at the GDL interface resulted in improved performances. The higher ionomer content at the membrane interface resulted in better protonic conductions, and the bigger void spaces in the GDL interface resulted in improved mass transport properties[1, 5]. Though precise electrochemical analysis is lacking, it is commonly agreed that the graded layers have lower mass transport and ohmic resistances. However, the studies encountered in the literature are almost exclusively based on high EW ionomers, such as Nafion. In contrast, low EW ionomers, which have higher protonic conductivity and water uptake capabilities have not yet been a focus of researchers. Additionally, the few reports encountered in the literature focus solely on GCLs while often disregarding other contributing effects to the overall cell voltage such as the equivalent weight.Low EW ionomers offer a wider operation range for PEMFCs in terms of relative humidities. By combining the low EW ionomers with graded catalyst layers, the operation range becomes even more flexible, and the limits are further stretched. In this study, we push the operating range limits of low EW graded catalyst layers and compare them to non-graded ones. We investigate different manufacturing techniques and use suitable scalable coating methods for the manufacturing of the electrodes. Afterwards, suitable in situ and ex situ analysis techniques are used in detail to trace down the improvements for the gradients. Among the key findings, we notice that potential improvements for low EW ionomer graded catalyst layers become apparent at considerable low relative humidities. The improvements are visible in the kinetic, ohmic, and mass transport regions.

The Porous Structure Design of Catalyst Layer by Controlling Particle Size Distribution of PEMFC Catalyst Ink

A general catalyst ink of proton exchange membrane fuel cells(PEMFC) consists of a catalyst, its support, an ionomer, and a solvent. These elements are made in the form of membrane electrode assembly(MEA) through grinding, dispersing, and coating processes. Therefore, the preparation of catalyst ink is an important step that directly affects the structure formation of the catalyst layer and the cell performance. In this study, by controlling the particle size distribution of the catalyst ink, we aim to achieve optimized pore structure in the catalyst layer that leads to improved performance of the fuel cell. Particle size and its distribution of the catalyst ink were confirmed by using particle size analyzer(PSA). Mercury intrusion porosimetry(MIP) was used to determine porous structure, including the pore diameter and total pore volume of MEA. Segmented tomographic evaluation is utilized to evaluate the 3D porous carbon structure in terms of local surface area, pore size distribution, and their 3D networking. MEA were tested to ascertain the influence of the catalyst ink properties on electrochemical character such as H2/air polarization curves, electrochemical impedance spectroscopy(EIS). Also, to analyze the resistance contributed by the molecular and Knudsen diffusion and permeation through the ionomer film, we tested MEA using limiting-current measurements. As a result, We are able to confirm that the particle size has a high correlation with the pore characteristics of the electrode layer that affect the cell performance. The enhanced electrochemical performance is attributed by decreasing fine particles about 0.1um in size. We figured out that the gas diffusion resistance in the catalyst layer increases as the fine particles increase. The results of this study will provide important insights into the design and fabrication of the catalyst layers in PEMFCs and could potentially lead to improvements in the efficiency of fuel cells.

Mechanism study of the improved catalytic activity of PEMFC catalyst layer by short-side-chain ionomer: Focusing on the ionomer/Pt interface

In the field of proton exchange membrane fuel cells (PEMFCs), short-side-chain (SSC) ionomers are widely considered to enhance the catalytic activity of the catalyst layer, but the mechanism of enhancement is controversial. In this work, the density functional theory (DFT) calculations and practical membrane electrode assemblies (MEAs) experiments were performed to explore the mechanism of the improved catalytic activity by SSC ionomers. The interactions between Pt representative crystal planes and low-molecular-weight model anions/PFSA fragments were investigated by DFT calculations, which revealed and divided the effects of sulfonic groups, ether groups, and side-chain structures on the adsorption energy and adsorption state. The calculations showed that the ether group of the short side chain has difficulty in interacting with the Pt surface, while the ether group around the midpoint of the long side chain could still interact with the platinum. Therefore, the ionomers having longer side chains more strongly block ORR. In practical catalyst layers, the SSC ionomer significantly improved the MEA performance especially at high voltages, and increased the rated power density by up to 56.3% compared with conventional Nafion®. This work provides a reference for developing desirable ionomers to enhance the catalytic activity of PEMFC catalyst layers.

Enhanced Durability and Catalytic Performance of Pt–SnO2/Multi‐Walled Carbon Nanotube with Shifted d‐Band Center for Proton‐Exchange Membrane Fuel Cells

Worldwide, significant efforts are made to identify energy sources that can help achieve carbon neutrality and promote sustainable development. The development of a catalyst that combines durability and high performance is essential for the commercialization of proton‐exchange membrane fuel cells (PEMFCs). In a fuel cell, carbon corrosion occurs during startup and shutdown due to improper local flooding caused by inadequate water management. In this study, a Pt‐based catalyst is designed with excellent durability and high activity. Introducing a metal oxide layer modified with Pt/multi‐walled carbon nanotubes reduces the direct contact between carbon and the fuel cell environment. This helps prevent carbon corrosion and inhibits the separation, aggregation, and growth of Pt nanoparticles. Moreover, the catalyst exhibits enhanced oxygen reduction activity due to the electronic effect of the metal oxide layer that is coated on it. In this study, by implementing a carbon erosion acceleration protocol, excellent catalytic properties during a load‐cycling experiment consisting of 5,000 cycles are reported. The practical application of the developed catalyst in PEMFCs offers an effective approach to developing Pt‐group metal catalysts with exceptional activity.

Proton Transport Functionality-Enabled Carbon Support for Improved Fuel Cell Performance and Durability.

A novel Monarch carbon material with proton conduction capability due to the presence of sulfone/sulfoxide/sulfonic groups on the surface was evaluated as a potential cathode catalyst support to enable an electrode design with low ionomer content in proton exchange membrane (PEM) fuel cells. X-ray photoelectron spectroscopy of the carbon support confirmed the sulfonic acid functionality, while dynamic vapor sorption measurements proved higher water uptake. Electrochemical impedance spectroscopy of the PtCo/Monarch electrodes showed higher proton conductivity than state-of-the-art PtCo/C with decreasing ionomer to carbon (I/C) content due to the presence of sulfonic acid functional groups on the carbon support surface. Fuel cell performance and durability measurements showed better high-current density performance for PtCo/Monarch with a 75% lower ionomer content in the electrode compared to that of PtCo/C. Our studies indicate that Monarch carbon could be a viable alternative support for PEM fuel cell catalyst applications.

Enhance the durability of the oxygen reduction reaction catalyst through the synergy of improved graphitization and nitrogen doping of carbon carrier

The energy crisis has highlighted the need for low-cost, highly active, and durable catalysts for Proton Exchange Membrane Fuel Cells (PEMFCs). Accordingly, we have developed a highly durable composite carbon carrier material, ECP600@NC, through pyrolyzing polyaniline coated on commercial carbon black ECP600, and supported the Pt nanoparticles (NPs) on ECP600@NC to prepare catalyst Pt/ECP600@NC. Compared with the catalyst Pt/ECP600, the electrochemical active surface area (ECSA) of Pt/ECP600@NC (94.1 m2·gPt−1) increased by 15% and the mass activity (MA) of Pt/ECP600@NC (343.9 mA·mgPt−1) increased by 142%. After 30,000 cycles carbon and Pt attenuation tests, the ECSA retention rate of Pt/ECP600@NC increased by 40% compared with Pt/ECP600@NC, and the MA retention rate of Pt/ECP600@NC increased by 15%. It is the synergistic effect of higher graphitization degree and nitrogen doping of the carbon carrier that improves the performance and durability of Pt/ECP600@NC. This work provides a promising method to optimize the catalysts for PEMFCs.

Durability of Pt nanowire supported on carbon nanotubes under simulated start-up/shut-down conditions for polymer electrolyte membrane fuel cells

The primary issue for the commercialization of proton exchange membrane fuel cells (PEMFCs) is the carbon corrosion of support and dissolution agglomeration of platinum nanoparticles under start-up/shut-down (SU/SD) conditions. Carbon nanotubes (CNTs)-supported Pt nanowire (PtNW) catalyst toward the ORR was synthesized via the soft template method. Electrochemical characterization demonstrated that the PtNW/CNTs showed higher activity than Pt/C because the anisotropy of the 1D structure strengthened its ability to transport electrons. An accelerated stressed test between 1.0 and 1.5 V for carbon corrosion was performed to examine durability at a single cell under practical SU/SD conditions. After 5000 cycles, the voltage drop was 0.142 V, and the voltage decay rate was 0.0284 mV cycle−1 for the PtNW/CNTs catalyst at 1600 mA cm−2; the voltage drop was 0.417 V, and the voltage decay rate was 0.0834 mV cycle−1 for the Pt/C catalyst at 1600 mA cm−2. The Pt/C catalyst demonstrated severe Pt nanoparticle aggregation and growth with a wide size distribution, whereas the PtNW/CNTs catalyst showed a slight variation. The PtNW/CNTs catalyst showed remarkable durability after 5000 SU/SD cycles. Consequently, PtNW/CNTs is a promising electrocatalyst, which can replace traditional Pt/C catalyst in PEMFCs.

Advanced ORR Catalysts Based on Intermetallic Nanoparticles and Metal Organic Frameworks

Over the past decade, substantial resources have been devoted to developing proton exchange membrane fuel cells (PEMFCs). The high cost and insufficient durability of oxygen reduction reaction (ORR) catalysts are the main factors delaying the commercialization of high-efficiency fuel cells. Development of new catalysts with enhanced activity and durability would have a transformative effect on fuel cells and related energy devices. Catalyst design for ORR requires creation of catalytic sites with high ORR activity, but these sites must also have good accessibility for transport of reactants (oxygen, protons, electrons) and products (water). They must also have high durability, with lifetimes of up to 30,000 hours required for heavy duty vehicle applications. The most successful ORR catalysts for PEMFCs are PtM/C, i.e., nanoparticles of Pt alloys with a base metal (e.g., Co, Ni) dispersed on a nanostructured carbon catalyst support. Tuning properties of both the PtM nanoparticles and the C support is necessary to achieve high performance and durability. In recent years, intermetallic PtM nanoparticles in which Pt and M occupy distinct sites in a highly ordered structure have emerged as a leading candidate for PEMFC applications. However, development of nanostructured carbon supports with improved control of morphology and pore properties is needed to fully reap the benefits of advanced intermetallic nanoparticle catalysts. Herein, we report recent advances in development of both intermetallic PtM nanoparticles and nanostructured carbon supports with controlled morphology and pore seize distribution. By combining novel nanostructured materials synthesis with advanced characterization and fuel cell testing, we demonstrate a path to produce highly active catalysts with sufficient durability to meet the challenging requirements of heavy duty trucking applications.

Atomically Dispersed Single Mn Site-Rich Carbon for PtCo Intermetallic Catalysts with Promoted Performance and Durability for Heavy-Duty Fuel Cells

Promoting the activity and durability of the Pt-based catalysts with a reduced platinum group metal (PGM) loading holds significance to the deployment of proton-exchange membrane fuel cells (PEMFCs) in the transportation market. In this work, synergistic catalysts, which integrate ultrafine Pt-based nanoparticles with ORR-active single atom catalysts (SACs), were studied to meet the activity and stability milestones for Pt-based catalysts. The synergistic effect between different SACs and ultrafine Pt nanoparticles were firstly elucidated via DFT calculations. It indicates that MnSA-NC catalyst with high surface area and abundant single atoms sites can improve the activity and stability of Pt catalyst via a strong electronic interaction. Ultrafine Pt and L12 Pt3Co intermetallic nanoparticles were uniformly anchored in the MnSA-NC via a facile strategy. The structures of the Pt@MnSA-NC and Pt3Co@MnSA-NC were comprehensively studied by multiple transmission microscopy and X-ray absorption spectroscopy. It reveals that ultrafine Pt and L12 Pt3Co intermetallic nanoparticles are anchored amongst the atomically dispersed MnNx sites. The Pt@MnSA-NC catalyst achieved a mass activity (MA) of 0.698 A mgPt −1 (@0.9ViR-free) and maintained 86 % of its initial performance after 30,000 cycles of accelerated stress test (AST, 0.6 and 0.95 V), exceeding the DOE target for 2025. By intercalating Co into the Pt nanoparticles, the L12 Pt3Co@MnSA-NC catalyst achieved significantly improved activity, achieving a MA of 0.905 A mgPt −1 and 1.631 A cm−2 (@0.7 V, H2-air fuel cell) at 150 kPaabs with 0.10 mgPt cm−2 in the cathode. The loss in the performance at 0.7 V is only 27.2 %, manifesting its high intrinsic stability. Furthermore, the performance of L12 Pt3Co@MnSA-NC catalyst (20 wt.% Pt) was studied under the heavy-duty vehicle (HDV) conditions with a higher Pt loading (0.20 mgPt cm−2) and higher backpressure (250 kPaabs). Encouragingly, the current densities at 0.67 and 0.7 V are 2.12 and 1.80 A cm−2, respectively. After 30,000 cycles of AST, the loss in MA is only 17.4 %, 87.0 % of performance were retained at 0.67 and 0.7 V. The compelling performance holds a great potential to meet the DOE target set for HDVs (>1.07 A cm−2 at 0.7 V after 250,000 hours equivalent operations). The synergistic effect revealed in this work shed light on the design of highly efficient catalysts for PEMFCs.

(Invited) Model-Predictive Design of Low Pt Loaded PEMFC Catalyst Layers

Proton exchange membrane fuel cells (PEMFCs) are a promising means to decarbonize the global vehicle fleet, but commercialization is limited by costly Pt catalysts, among other challenges. PEMFC designs with low Pt loading typically suffer anomalous performance losses. Previous studies have agreed that poor device performance is linked to complex physiochemical processes in the cathode catalyst layer (CL) [1]. More specifically, poor performance with low Pt-loading is attributed to sluggish proton and/or oxygen transport through the nano-thin Nafion ionomer in the CL. Previous studies have also identified that the properties of the Nafion thin films in the CL vary from those previously measured for thicker membranes. However, research has not yet definitively connected CL polymer properties with PEMFC limitations in low Pt-loaded cells, nor found a means to address the limitations for low cost, higher-performance PEMFCs.In this presentation, we will describe efforts to quantify structure-property relationships for thin-film Nafion and incorporate these relationships into PEMFC numerical simulations to identify advantageous PEMFC catalyst layer designs. We model nm-scale transport in thin-film Nafion to develop structure-property relationships for thin-film Nafion, based on in situ neutron reflectometry [2], and conductivity measurements [3] on thin film Nafion. These result in a model that calculates the conductivity as a function of temperature, local water uptake, film thickness, and underlying substrate (Pt or carbon). Incorporating these structure-property models into a continuum-scale PEMFC cathode half-cell model to locally resolve proton conductivity and O2 diffusion improves model fits to PEMFC data with varying Pt loading [4]. The fits provide new insights into the coupling between kinetic, mass transport, and Ohmic losses in low-Pt-loaded PEMFC cathodes and show that Ohmic losses in thin film Nafion play a key limiting role in low-Pt loaded cells. We conclude with a parametric study of PEMFC CL design parameters with low Pt loading. Incorporating multi-scale modelling of Nafion transport processes enhances the predictive model capabilities and unlocks new insights into CL design. We will present a detailed analysis of how best to manage coupled limiting phenomena in PEM fuel cells, and identify functionally graded catalyst layer designs that could significantly improve PEMFC power density with low Pt loading [4]. Weber, A. and Kusoglu, A. Mater. Chem. A. 2014. DOI: 10.1039/c4ta02952fDeCaluwe, S.C. et al. Nano Energy 2018. DOI: 10.1016/j.nanoen.2018.01.008Paul, D.K. et al. Electrochem. Soc. 2014. DOI: 10.1149/2.0571414jesRandall, C.R. and DeCaluwe, S.C., Electrochem. Soc. 2022. DOI: 10.1149/1945-7111/ac8cb4

The degradation of cathodic Fe/N/C catalyst in PEMFCs: The evolution of remanent active sites after demetallation

The state-of-the-art Fe/N/C catalyst has presented comparable initial cathode performance to the benchmark Pt/C catalyst in proton exchange membrane fuel cells (PEMFCs). However, the major bottleneck is its significant activity decay in real-world PEMFC cells. The superposed “fast decay” and “slow decay” have been well documented to describe the degradation process of Fe/N/C catalysts during PEMFC operation. The fast decay has been well understood in close relation to the demetallation at the initial 15-h stability test. Nevertheless, it is still unclear how the remanent active sites evolve after demetallation. To this end, the catalyst performance and evolution of a typical Fe/N/C active site were herein investigated through postmortem characterizations of the membrane electrode assemblies (MEAs) after different operations. It is presented that 1 bar pressure and 80 °C temperature are the optimized conditions for Fe/N/C MEA. Particularly, the “fast decay” in the initial 15 h is immune to the various operating parameters, while the “slow decay” highly depends on the applied temperature and pressure. According to the X-ray absorption spectra (XAS) analysis and stability test of MEA, the gradual evolution of Fe-N coordination to Fe-O is found correlated with the “slow decay” and accounts for the catalyst decay after the demetallation process.