Cathode Catalyst Layer Research Articles

Probing multiphase reactive transport interactions in the polymer electrolyte fuel cell catalyst layer degradation

The multiphase, multicomponent reactive transport plays a critical role in the degradation of the cathode catalyst layer in Polymer Electrolyte Fuel Cells. This warrants a fundamental understanding of the pore-scale, mechanistic interactions in the catalyst layer. Herein, the interfacial and transport interactions due to the flooding dynamics in the presence of carbon support corrosion, which is a primary degradation mode in the cathode catalyst layer, are delineated based on a comprehensive mesoscale model. The mechanistic interrogation of the degradation-transport interactions reveals the spatio-temporal heterogeneity of the pristine and aged microstructures. An electrode-averaged saturation front of 80 % unveils a critical limit of percolation which results in an onset of limiting current density. Further, it is observed that the reactant severity is amplified at progressively higher stages of corrosion owing to the appearance of dead pores and a thicker ionomer film. We also shed light on the fluid displacement patterns which reveal the existence of a capillary fingering regime. The interplay of the operating current density and carbon corrosion in governing the substrate-ionomer interaction is described through an electrochemical Damköhler number.

Monitoring changes in catalyst ink morphology during the primary drying process for polymer electrolyte fuel cells using cryogenic transmission electron microscopy

To control the microstructure formation of a catalyst layer in a polymer electrolyte fuel cell for improving its performance, we observed the real morphology changes of the catalyst ink during the primary drying process using cryogenic transmission electron microscopy. Samples of the thin film catalyst ink, applied onto holey carbon grids, were prepared with different drying times (t = 0, 10, and 30 s). At t = 0, the sample exhibited platinum/carbon agglomerates surrounded by an ionomer thin film in the solvent. At t = 10, the ionomer rods became thicker, as confirmed by high magnification images. At t = 30, light spots were observed in the ionomer/solvent areas, indicating local thinning of the ionomer thin film due to solvent evaporation during the drying process. Moreover, as the drying time increased, the ionomer morphology became more visible with significantly thicker rods. This change in morphology can be attributed to the composition change of the solvent (fraction of water/normal propyl alcohol) in the catalyst ink during drying. Based on the new findings, this study discusses process design guidelines for cathode catalyst layers.

Experimental Design of High-Performing Open-Cathode Polymer Electrolyte Membrane Fuel Cells

Open-cathode polymer electrolyte membrane fuel cells (PEMFCs) utilize a unique air-cooled system design to eliminate the humidifiers, air compressor, and liquid cooling loop of conventional, liquid-cooled PEMFC systems, thereby greatly reducing system cost. However, the open-cathode PEMFC performance is restricted by poor humidification, high membrane and charge transfer resistances, and overheating due to inefficient thermal and water management. This work aims to strategically modify the membrane electrode assembly (MEA) design to overcome these issues and achieve high open-cathode PEMFC performance that approaches that of liquid-cooled systems. The use of thinner membrane along with short side chain ionomer is found to elevate the cell performance due to increased water retention at the cathode catalyst layer (CCL) and decreased ohmic losses. Thinner gas diffusion layers with high porosity enable additional cell performance increment by improving oxygen availability at the CCL. An overall current density rise of 88% at 0.6 V and 53% at 0.4 V is achieved by the strategically designed MEA for open-cathode cells. The enhanced power density enabled by the custom MEA can both reduce the stack cost and expand the power range of open-cathode PEMFCs, thus expanding their potential use for low-cost fuel cell system applications.

Structural and Transport Properties of Hydrophilic and Hydrophobic Modified Ionomers in Proton Exchange Membrane Fuel Cells.

The sluggish commercial application of proton exchange membrane fuel cells (PEMFCs) with low Pt loading is chiefly hindered by concentration polarization loss, particularly at high current density regions. Addressing this, our study concentrates on the ionomer membranes in the cathode catalyst layer (CCL) and explores the potential of incorporating additional hydrophilic or hydrophobic components to modify these ionomers. Therefore, an all-atom model was constructed and for the ionomer and hydrophilic and hydrophobic modifications were implemented via incorporating SiO2 and PTFE, respectively. The investigation was conducted via molecular dynamics (MD) simulations to predict the morphology and structure of the ionomer and analyze the kinetic properties of oxygen molecules and protons. The simulation results elaborate that the hydrophilic and hydrophobic modifications favor the phase separation and the self-diffusion coefficients of oxygen molecules and protons are enhanced. Considering the hydration level of the ionomer films, hydrophilic modification facilitates mass transfer under low-hydration-level conditions, while hydrophobic modification is more effective in optimizing mass transfer as the hydration level increases. The optimal contents of SiO2 and PTFE for each hydration level in this work are 9.6% and 45%, respectively. This work proposes a reliable model and presents a detailed analysis of hydrophilic and hydrophobic modifications, which provides theoretical guidance for quantitative preparations of various composite membranes.

A one-dimensional numerical model of carbon corrosion in catalyst layers of proton exchange membrane fuel cells

Understanding interaction between carbon corrosion and physicochemical processes inside proton exchange membrane fuel cells (PEMFCs) is important. A one-dimensional carbon corrosion model considering cathode catalyst layer (CCL) structure evolutions is established and coupled with one-dimensional PEMFC performance model. The coupled model is validated and employed to predict CCL structure evolution and PEMFC performance degradation during accelerated stress test. The results find that high relative humidity can enhance performance but accelerate carbon corrosion. Increasing carbon loading and decreasing ionomer weight can improve both performance and durability. Low Pt loading leads to reduced performance and durability. There exists an optimal carbon diameter leading to the best performance. Small carbon diameter can enhance cell durability. Proper gradient CCL design can enhance durability at cost of small initial performance reduction. Finally, an optimized structure is proposed with 21.7 % higher current density at 0.6 V and 5.2 % lower performance loss compared with base case.

Unveiling the impact of pore structure of cathode catalyst layer on proton exchange membrane cell performance

This work quantitatively explores the impact of pore structure of cathode catalyst layer (CCL) on the overall performance of low-Pt loading electrodes in Proton Exchange Membrane Fuel Cell (PEMFCs), providing guidelines for designing high-performance CCL. The model predictions align closely with experimental measurements. The results reveal that the best cell performance occurs when the ratio of secondary to primary pores is 2, which is attributed to the balance between liquid water saturation and the diffusivity of gaseous species within the catalyst layer. In addition, higher total porosity or higher Pt loading require more primary pores to achieve a remarkable improvement in performance. Furthermore, the influence of total porosity on the peak power density of fuel cell depends on pore size distribution. The optima total porosities are 0.5 and 0.7 when the ratios of secondary to primary pores are 2 and 1, respectively.

New insights into the performance and durability degradation mechanisms of PEMFCs caused by Cr3+ contaminant

It is known that contamination ions stemming from the corrosive deterioration of vital subassemblies, such as bipolar plates (BPs) and sealing gaskets, exert a profound influence on the performance and durability degradation of PEM fuel cells. The impacts of Cr3+ on the catalyst layers (CLs) and single-cell performance were investigated by immersing the cathode catalyst layer (CCL) and anode catalyst layer (ACL) in a Cr3+ solution with different concentrations for contamination treatment. During the fuel cell test, polarization curves (PCs), electrochemical impedance spectroscopy (EIS), H+ transfer impedance, and cyclic voltammetry (CV) curve results were recorded. The effects of Cr3+ contamination on the kinetic for oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) were analyzed in a three-electrode system. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to explore the degradation mechanisms of Cr3+ contamination on fuel cell activity. The results indicate that the presence of Cr3+ increased the ohmic impedance from 0.200 Ω cm2 to 0.449 Ω cm2, resulting in a voltage attenuation rate as high as 55.21 %. Moreover, the presence of Cr3+ slowed the ORR reaction rate and changed the ORR reaction pathway. Cr3+ contamination caused irreversible damage to PEMFC performance and durability.

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 exploredby introducing nano calcium carbonate of varying particle sizes for pore-forming. Physicochemical characterizationsfor are employed to examine the electrode structure, while in situ electrochemical measurements are used to scrutinize bulk oxygen transport resistance, effective oxygen diffusivity ( ) and fuel cell performance. Additionally, the CCLs are constructed with aid of Lattice Boltzmann method (LBM) simulations and for CCLs with different pore size distribution are calculated. The findings reveal that 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.

Ionomer structure and component transport in the cathode catalyst layer of PEM fuel cells: A molecular dynamics study.

The transport of water and protons in the cathode catalyst layer (CCL) of proton exchange membrane (PEM) fuel cells is critical for cell performance, but the underlying mechanism is still unclear. Herein, the ionomer structure and the distribution/transport characteristics of water and protons in CCLs are investigated via all-atom molecular dynamics simulations. The results show that at low water contents, isolated water clusters form in ionomer pores, while proton transport is mainly via the charged sites of the ionomer side chains and the Grotthuss mechanism. Moreover, with increasing water content, water clusters are interconnected to form continuous water channels, which provide effective paths for proton transfer via the vehicular and Grotthuss mechanisms. Increasing the ionomer mass content can enhance the dense arrangement of the ionomer, which, in turn, increases the density of charge sites and improves the proton transport efficiency. When the ionomer mass content is high, the clustering effect reduces the space for water diffusion, increases the proton transport path, and finally decreases the proton transport efficiency. By providing physics insights into the proton transport mechanism, this study is helpful for the structural design and performance improvement of CCLs of PEM fuel cells.

Investigating the transient electrical behaviors in PEM fuel cells under various platinum distributions within cathode catalyst layers

The spatial distribution of platinum (Pt) within the cathode catalyst layer (CCL) is vital for the electrochemical reactions and mass transport in fuel cells. Though important, the transient effects of these distributions are seldom explored. This study examines the impact of Pt distribution on transient electrical behaviors, including voltage and local current distribution (LCD) uniformity, using a transient, two-dimensional, two-phase, non-isothermal fuel cell model that incorporates catalyst agglomerate. Three Pt distribution types are investigated: uniform, MPL-side biased, and PEM-side biased. Results indicate a voltage undershoot occurs during current loading. Compared to the homogeneous CCL, PEM-side bias reduces this undershoot by 12.5% due to shortened proton transfer paths and decreased ohmic loss, while MPL-side bias increases it by 18.8% due to the inverse effect. Additionally, Pt distribution affects both oxygen transport and reaction resistance within the agglomerate, influencing LCD uniformity. Under loading conditions, gradient CCLs show inferior LCD uniformity than homogeneous ones. Peak non-uniformity values of 0.22, 0.59, and 0.71 are observed for homogeneous, MPL-biased, and PEM-biased CCLs, respectively. From the perspective of voltage and LCD uniformity, the MPL-side biased CCL is not found to enhance dynamic characteristics, whereas the PEM-side bias improves voltage undershoot but at the cost of LCD uniformity. This study provides a novel perspective on fuel cell dynamics, emphasizing the transient effects of Pt distribution and their potential for optimizing dynamic performance by adjusting the Pt gradient.

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.

Morphology of Thin-Film Nafion on Carbon as an Analogue of Fuel Cell Catalyst Layers.

Species transport in thin-film Nafion heavily influences proton-exchange membrane (PEMFC) performance, particularly in low-platinum-loaded cells. Literature suggests that phase-segregated nanostructures in hydrated Nafion thin films can reduce species mobility and increase transport losses in cathode catalyst layers. However, these structures have primarily been observed at silicon-Nafion interfaces rather than at more relevant material (e.g., Pt and carbon black) interfaces. In this work, we use neutron reflectometry and X-ray photoelectron spectroscopy to investigate carbon-supported Nafion thin films. Measurements were taken in humidified environments for Nafion thin films (≈30-80 nm) on four different carbon substrates. Results show a variety of interfacial morphologies in carbon-supported Nafion. Differences in carbon samples' roughness, surface chemistry, and hydrophilicity suggest that thin-film Nafion phase segregation is impacted by multiple substrate characteristics. For instance, hydrophilic substrates with smooth surfaces correlate with a high likelihood of lamellar phase segregation parallel to the substrate. When present, the lamellar structures are less pronounced than those observed at silicon oxide interfaces. Local oscillations in water volume fraction for the lamellae were less severe, and the lamellae were thinner and were not observed when the water was removed, all in contrast to Nafion-silicon interfaces. For hydrophobic and rough samples, phase segregation was more isotropic rather than lamellar. Results suggest that Nafion in PEMFC catalyst layers is less influenced by the interface compared with thin films on silicon. Despite this, our results demonstrate that neutron reflectometry measurements of silicon-Nafion interfaces are valuable for PEMFC performance predictions, as water uptake in the majority Nafion layers (i.e., the uniformly hydrated region beyond the lamellar region) trends similarly with thickness, regardless of support material.

An improved agglomerate model with adsorption isotherm for oxygen transport in CCLs with low Pt loading

Understanding of oxygen transport in cathode catalyst layers with low Pt loading is of great importance to the proton exchange membrane fuel cells. The resistance to oxygen transport within the catalyst layers (CCLs) increases significantly as the Pt loading decreases. Thus, it is necessary to develop an accurate continuum-scale model for the oxygen transport and reaction in catalyst layers with low Pt loading so as to guide the optimization of catalyst layers. To address this issue, a modified agglomerate model is developed in the present study. In the model, the CCLs are conceptualized as packings of agglomerate particles composed of the carbon, ionomer, and catalyst. Each of agglomerate particle is covered by a thin layer of ionomer. The main features of our model are two folds: (1) adsorption isotherm is employed instead of Henry's law, commonly used the previous agglomerate models, to depict the oxygen adsorption at the surface of ionomer; (2) non-uniform oxygen transfer flux across the ionomer layer surrounded each agglomerate is considered. The results predicted by our developed model are in good agreement with the experimental data for the polarization curve and the variations of the oxygen transport resistance with the Pt loadings. Especially, our model is superior to the previous agglomerate models in regarding to the variation of oxygen transport resistance with mass fraction of Pt in the catalyst. The modified agglomerate model developed in this work provides an effective tool for investigation of oxygen transport and reaction in CCLs with low Pt loading.

Effects of gradient structures of cathode catalyst layers on performance and durability of proton exchange membrane fuel cells

Improving performance and durability of proton exchange membrane fuel cells (PEMFC) is crucial for promoting commercial applications. Deep understanding of the underlying degradation mechanisms is still lacking. In this study, a two-way coupling performance-degradation model is established by incorporating one-dimensional (1D) Platinum (Pt) degradation model into 1D PEMFC performance model, which can predict Pt degradation phenomenon considering Pt dissolution, Pt2+ diffusion and reprecipitation in cathode catalyst layers (CCLs) and resulting performance loss. Effects of gradient CCL structures in terms of ionomer weight, Pt loading and Pt particle size on degradation and performance are investigated. The results show that multi-layer CCLs with more ionomer, more Pt loading, and/or smaller particles near membrane have higher performance, while the opposite gradients present enhanced anti-degradation capacity. Wave-like spatial distributions of Pt mass retention are observed in gradient CCLs, with significant difference across the sub-layer interface. Combined effects of two gradient distributions are also investigated. It is found that for performance, simply add-up roles are followed, namely combining two gradient distributions with higher performance can obtain higher performance, and vice versa. For durability, synergy effects are identified, in which two gradient structures with opposite final Pt mass distribution have competitive influence on Pt2+ local transport, with local Pt2+ diffusion suppressed, the catalyst distribution uniformity improved and performance loss alleviated. The gradient CCL structure considering the balance of performance and durability is identified based on 28 structures studied. Such add-up roles for performance and synergy effects for durability provide new insights for improving the design of PEMFCs.

Optimal MEA structure and operating conditions for fuel cell reactors with hydrogen peroxide and power cogeneration

The cogeneration of hydrogen peroxide (H2O2) and power in proton exchange membrane fuel cell (PEMFC) reactors via two-electron oxygen reduction reaction on the cathode is an economical, low-carbon, and green route for the on-site production of H2O2. However, in practice, the H2O2 that cannot be collected timely will accumulate and self-decompose in the catalyst layer (CL), reducing the H2O2 generation efficiency. Thus, accelerating the mass transport of H2O2 within the cathode CL is critical to efficient H2O2 generation in PEMFC. Herein, we investigated the effects of the membrane electrode assembly (MEA) fabrication process, cathode CL thickness, and cathode carrier water flow rate on H2O2 generation and cell performance in a PEMFC reactor. The results show that the catalyst-coated membrane-type MEA exhibits high power output due to its lower proton transport resistance. However, the formed CL with a dense structure significantly limits H2O2 collection efficiency. The catalyst-coated gas diffusion electrode (GDE)-type MEA formed macroporous structures in the cathode CL, facilitating carrier water entry and H2O2 drainage. In particular, carbon cloth GDE with thin CL could construct rich macroscopic liquid channels, thus maximizing the generation of H2O2, but will impede fuel cell performance. These results suggest that the construction of a well-connected interface between CL and proton exchange membrane (PEM) in MEA and the establishment of a macroscopic pore structure of the CL are the keys to improve the cell performance and H2O2 collection efficiency.

Air oxidation of carbon supports boosts the low-humidity fuel cell performance.

We introduce a straightforward, yet effective strategy to combat the performance decline of proton-exchange membrane fuel cells in low-humidity environments. Our method centers on air-oxidizing carbon supports, significantly improving proton and oxygen transport within the cathode catalyst layer.

Preparation and characterization of palladium nanoparticle-embedded carbon nanofiber membranes via electrospinning and carbonization strategy.

Carbon nanofiber membranes (CNMs) are expected to be used in many energy devices to improve the reaction rate. In this paper, CNMs embedded with palladium nanoparticles (Pd-CNMs) were prepared by electrospinning and carbonization using polyimide as the raw material. The effects of carbonization temperature, carbonization atmosphere, and heating rate on the physicochemical properties of the as-obtained Pd-CNMs were studied in detail. On this basis, the electrocatalytic performance of Pd-CNMs prepared under optimal conditions was characterized. The results showed that highly active zero-valent palladium nanoparticles with uniform particle size could be distributed on the surface of carbon nanofibers. Under vacuum conditions, at a carbonization temperature of 800 °C and a heating rate of 2 °C min-1, Pd-CNMs have lower H2O2 yield, lower Tafel slope (73.3 mV dec-1), higher electron transfer number (∼4), and superior durability, suggesting that Pd-CNMs exhibit excellent electrocatalytic activity for ORR in alkaline electrolyte. Therefore, polyimide-derived CNMs embedded with Pd nanoparticles are expected to become an excellent cathode catalyst layer for fuel cells.

Optimized mass transfer in a Pt-based cathode catalyst layer for PEM fuel cells

Different strategies of mass transfer enhancement in a Pt-based cathode catalyst layer for PEM fuel cells.

Unraveling the Effects of Carbon Corrosion on Oxygen Transport Resistance in Low Pt Loading Proton Exchange Membrane Fuel Cells.

Cost and durability have become crucial hurdles for the commercialization of proton exchange membrane fuel cells (PEMFCs). Although a continuous reduction of Pt loading within the cathode catalyst layers (CCLs) can lead to cost savings, it also increases the oxygen transport resistance, which is further compounded by key material degradation. Hence, a further understanding of the mechanism of significant performance loss due to oxygen transport limitations at the triple phase boundaries (TPBs) during the degradation process is critical to the development of low Pt loading PEMFCs. The present study systematically investigates the impact of carbon corrosion in CCLs on the performance and oxygen transport process of low Pt loading PEMFCs through accelerated stress tests (ASTs) that simulate start-up/shutdown cycling. A decline in peak power density from 484.3 to 251.6 mW cm-2 after 1500 AST cycles demonstrates an apparent performance loss, especially at high current densities. The bulk and local oxygen transport resistances (rbulk and Rlocal) of the pristine cell and after 200, 600, 1000, and 1500 AST cycles are quantified by combining the limiting current method with a dual-layer CCL design. The results show that rbulk increased from 1527 to 1679 s cm-2, Rlocal increased from 0.38 to 0.99 s cm-1, and the local oxygen transport resistance with the normalized Pt surface area (rlocal) exhibited an increase from 18.5 to 32.0 s cm-1, indicating a crucial impact on the structure collapse and changes in the chemical properties of the carbon supports in the CCLs. Further, the interaction between the ionomer and carbon supports during the carbon corrosion process is deeply studied via electrochemical quartz crystal microbalance and molecular dynamics simulations. It is concluded that the oxygen-containing functional groups on the carbon surface could impede the adsorption of ionomers on carbon supports by creating an excessively water-rich layer, which in turn aggravates the formation of ionomer agglomerations within the CCLs. This process ultimately leads to the destruction of the TPBs and hinders the transport of oxygen through the ionomer.

Performance of a PEM fuel cell cathode catalyst layer under oscillating potential and oxygen supply

A model for impedance of a PEM fuel cell cathode catalyst layer under simultaneous application of potential and oxygen concentration harmonic perturbations is solved. The solution demonstrates strong lowering of the layer impedance under increase in the amplitude of oxygen concentration perturbation. Small in–phase oscillations of the overpotential and oxygen concentration lead to formation of a sub–layer with negligible oxygen transport loss. Working as an ideal non–polarizable electrode, this sub–layer dramatically reduces the system impedance.