Electrolyte Membrane Fuel Cell Catalyst Research Articles

Nanoscale in-Situ Characterization of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers for Improved Water Management

Polymer electrolyte membrane fuel cells (PEMFCs) facilitate a sustainable energy infrastructure by offering emission-free electricity generation using renewably sourced hydrogen gas. The electrochemical reactions in a fuel cell occur at platinum-loaded catalyst layers (CLs), which are susceptible to degradation. Obstructing catalyst sites (triple phase boundaries1) with reaction by-products, such as water2,3, is the primary degradation mechanism in PEMFCs. Although various CL designs have been tested for improved electrochemical performance, in situ nanoscale visualization of platinum degradation and water accumulation in the CL at controlled temperature and relative humidity (RH) values are required to understand fundamental water mechanics through platinum and carbon nanostructures.In this work, we employed scanning transmission X-ray microscopy (STXM) and X-ray absorption fine structure spectroscopy (XANES) to evaluate the effect of temperature on pristine and broken-in fuel cell CLs. We developed a novel CL in-situ sample cell to enable nanostructured characterization of catalyst layers using STXM in a controlled temperature environment. Carbon 1s, Fluorine 1s, and Oxygen 1s spectral edges were probed utilizing near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to reveal chemical degradation mechanisms and differentiate material structure. Through our custom in-situ cell, we demonstrate that the chemical composition and water accumulation throughout the PEMFC CLs at low, intermediate, and high operating temperatures (25°C, 40°C, and 60°C, respectively) can be quantified, along with thermal expansion analyses of the CL and ionomer membrane. Moreover, we reveal realistic structural and chemical characteristics of nanoscale catalysts by distinguishing regions of Nafion® Ionomer, catalyst carbon support, and platinum throughout the PEMFC’s membrane electrode assembly at industrially relevant fuel cell temperatures. The insights from this work will inform strategies to mitigate water flooding from a nanoscale perspective in addition to the novel in-situ spectromicroscopy characterization of current CL features.1. Fouzaï, I., Gentil, S., Bassetto, V. C., Silva, W. O., Maher, R., & Girault, H. H. (2021). 9(18), 11096-11123.2. Li, H., Tang, Y., Wang, Z., Shi, Z., Wu, S., Song, D., ... & Mazza, A. (2008). 178(1), 103-117.3. Kumsa, D. W., Bhadra, N., Hudak, E. M., Kelley, S. C., Untereker, D. F., & Mortimer, J. T. (2016). 13(5), 052001.

Effect of Solvent on Ionomer Adsorption Behavior of Polymer Electrolyte Membrane Fuel Cell Catalyst

Polymer electrolyte membrane fuel cell (PEMFC) is a promising power source with zero local carbon emissions that may aid in realizing the development of low-carbon societies. Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in PEMFC. To control the structure of the CL, especially for the localization of the ionomer, understand the adsorption behavior on carbon surface is important. The quantitative analysis of ionomer adsorption isotherm is a powerful tool for analyzing the adsorption behavior, however, existing experiments have been carried out in water, and the effects of alcohol are yet to be considered. Because the mixtures of water/alcohol are used in the actual fabrication of PEMFC, the adsorption isotherm using the binary mixtures of water/alcohol as the solvents could provide practical information to optimize the PEMFC performance.Principal project of this study is to investigate and evaluate the effects of the alcohol contents on the interaction between the ionomer (Nafion) and the surface of the carbon support. The control of the interaction between ionomers and catalyst composites is as well as crucial in controlling the PEMFC performance. As the catalyst composites, commercial Vulcan/Pt (CB/Pt) and Pt-loaded polybenzimidazole (PBI) polymer-wrapped Vulcan (CB/PBI/Pt) catalysts were compared. For the detailed Nafion adsorption experiment, using synthesized CB/PBI/Pt and commercial CB/Pt disperse in aqueous ethanol solutions (87 or 20 wt%) with different amounts of Nafion (i.e., carbon-to-ionomer weight ratio, I/C=0.1~1.4). Then the dispersions were filtered with a syringe filter, and the filtrates were measured with 19F-NMR to calculate the amount of adsorption. For 19F-NMR, filtrates were added to trifluoroacetic acid and heavy water used as internal standard and deuterium solvent, respectively. The amount of Nafion was calculated based on the integral ratio of the largest peaks of Nafion ranging from −115 to −130 ppm in reference to the trifluoroacetic acid peak at −75.5 ppm. To prepare the adsorption isotherm, the adsorption amount (Γ [mg·mg−1]) of Nafion was plotted as a function of the equilibrium concentration (C e). The maximum adsorption amount (Γ max [mg·mg−1])) and Langmuir constant (K L) were determined based on the Langmuir equation. This paper will also discuss the difference in CL structure and cell performance at I/C=0.72 which formulation in low (water-) and high ratio alcohol (alcohol-rich) in ink solvent.PBI-coated CB (CB/PBI) was prepared according to our previous reports1, and the adsorption isotherms were measured in water- and alcohol-rich solvents (Figure.1), in which the BET values of 88 and 119 m2·g-1 for CB/PBI and CB/PBI/Pt, respectively, were used for the calculation of Γ’ (mg·m−2) in place of Γ (mg·mg−1). For the first layer adsorption, K L was calculated as 82 and 55 in the water- and alcohol-rich solvents, respectively. Comparing these values with those of CB, clear increase of the K L in the alcohol-rich solvent from 32 (CB) to 55 (CB/PBI) was obtained, whereas the K L values in the water-rich solvent were almost comparable between CB (89) and CB/PBI (82). In addition, the Γ max of the first layer in the alcohol-rich solvents increased from 0.024 mg mg‒1 (CB) to 0.034 mg mg‒1 (CB/PBI), while that in water-rich solvents were comparable between CB (0.043 mg mg‒1) and CB/PBI (0.043 mg mg‒1). Both results clearly indicate the stronger interaction between PBI and Nafion compared to the interaction between CB and Nafion in alcohol-rich solvents, while strong interaction in water-rich solvents was still maintained. A similar trend was also observed for Pt-loaded CB/PBI (CB/PBI/Pt), namely, the K L (43) and Γ’max (0.218 mg·m−2) were larger than those of CB/Pt (K L = 25 and Γ’max = 0.179) in the alcohol-rich solvent, while in water-rich solvent, K L (44) and Γ’max (0.277 mg·m−2) of CB/PBI/Pt were very high similar to CB/Pt (K L = 29 and Γ’max = 0.188 mg·m−2). On the other hand, when we compare Γ’max values of CB/PBI and CB/PBI/Pt, the decrease of the values after Pt loading in the water-rich and alcohol-rich solvents were observed, suggesting the Nafion adsorption onto Pt surface is inferior for PBI systems regardless of alcohol content.Furthermore, the performances of different catalysts were evaluated and discussed to understand the effect of alcohol content on Nafion adsorption and CL structure. Figure. 2 represents a performance curve that maximum power density of MEA having CB/Pt was 580 and 750W cm-2, while that of CB/PBI/Pt was 720 and 810 W cm-2, in alcohol and water-rich solvents, respectively. The high current density range (1.2~2A cm-2) also showed higher potential in water-rich conditions that related to O2 diffusion in CL.Reference 1. S. M. Jayawickrama,T. Fujigaya.et al. Electrochim. Acta. 312, 20 (2019). Figure 1

PGM Recovery: Maximizing PGM Dissolution through Minimizing Ostwald Ripening

Efficient recovery of platinum group metals (PGMs) through electrochemical means using mild conditions is of significant impact from both the industrial and the environmental points of view. Owing to their large surface-to-volume ratio, fast dissolution of PGM nanoparticles is possible through potential cycling between oxidizing and reducing potentials, leading to formation and dissolution of surface-oxide layer (transient dissolution). However, at reducing potentials, reduction of the dissolved metal species on the source nanoparticles leads to enhance the Ostwald ripening process1 and hence to decrease the overall dissolution efficiency significantly. In this talk, we present strategies to accelerate dissolution of PGM nanoparticles by limiting Ostwald ripening through (1) use of surface switching species (SSS), which selectively block the PGM surface to inhibit redeposition induced particle growth at reducing condition and expose the PGM surface to facilitate dissolution at oxidation condition, respectively2. (2) use of an auxiliary deposition system to continuously remove the dissolved PGM species through reaction kinetic manipulation3. Such surface relegation property to depress unwanted process and assist desired reaction documents an alternative and efficient approach for precious metal electrochemical recovery. References Sharma, R.; Andersen, S. M., Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during an Accelerated Stress Test. ACS Catal. 2018, 8 (4), 3424-3434.Sharma, R.; Simonsen, S. B.; Morgen, P.; Andersen, S. M., Inhibition of Ostwald ripening through surface switching species during potentiodynamic dissolution of platinum nanoparticles as an efficient strategy for platinum group metal (PGM) recovery. Electrochim. Acta 2019, 321, 11.Sharma, R.; Gyergyek, S.; Andersen, S. M., Environmentally and Industrially Friendly Recycling of Platinum Nanoparticles Through Electrochemical Dissolution-Electrodeposition in Acid-Free/Dilute Acidic Electrolytes. ChemSusChem 2018, 11 (21), 3742-3750.

Investigating the effect of solvent composition on ink structure and crack formation in polymer electrolyte membrane fuel cell catalyst layers

To improve the performance of polymer electrolyte membrane fuel cells (PEMFCs), controlling the microstructure of the membrane electrode assembly (MEA) catalyst layer is crucial. Ink design, which includes a catalyst, an ionomer, and a solvent, serves as the starting point for controlling the microstructure of the catalyst layer. However, there is a significant lack of understanding of the ink structure required for this purpose. In this study, we investigated the effect of the solvent, a key component that determines the ink structure. The ink comprises 20 wt% Pt/C, short-side-chain (SSC) Aquivion ionomer, and a solvent mixture of 1-propanol (NPA) and water. Three types of inks with different compositions of NPA and water are manufactured, and their stability and rheological properties were measured to infer and compare the ink structures. Furthermore, the crack characteristics of the catalyst layer were compared by directly coating the ink onto the electrolyte membrane using the doctor-blade method. In the ink with a high water content, we observed a gel-like elastic behavior dominated by network structures formed by ionomers adsorbed between catalyst particles. In contrast, the ink with a high NPA content exhibited a liquid-like viscous behavior dominated by well-dispersed catalyst particles and ionomers. These properties of the inks directly influence the crack formation characteristics after coating. Specifically, the strong liquid properties of the NPA-rich ink were found to suppress crack formation in the catalyst layer. These findings provide important insights into how the solvent composition affects ink structure and how it, in turn, influences crack formation in the catalyst layer, which can help optimize the ink design to improve the performance of PEMFCs.

Effect of alcohol content on the ionomer adsorption of polymer electrolyte membrane fuel cell catalysts

Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in polymer electrolyte membrane fuel cells (PEMFCs). In particular, the preparation conditions of the CL, starting from the dispersion of the catalyst composite dispersion with an ionomer, largely affect the PEMFC performance. In this study, the effects of alcohol content in the dispersion solvent were investigated using two binary mixtures composed of water and ethanol. In addition, Pt-loaded carbon black (CB) and Pt-loaded polymer-wrapped CB were used as the catalyst composites to study the effects of the alcohol contents on the interaction between ionomer and surface of the carbon supports. The CL prepared using the water-rich (80 wt% water) solvent achieved a higher PEMFC performance compared to that using the alcohol-rich (13 wt% water) solvent, which is ascribed to the stronger interaction between the ionomer and CB surface under water-rich conditions. Using the polymer-wrapped CB, the difference of the PEMFC performance between the CLs from the water-rich and alcohol-rich dispersions was minimal because of the comparable interaction between the ionomer and wrapping polymer surface in both solvents. Therefore, the control of the interaction between the ionomer and catalyst composites is crucial to controlling the PEMFC performance.

Platinum/graphene nanoplatelets/silicone rubber composites as polymer electrolyte membrane fuel cell catalysts

The importance of carbonaceous catalyst support materials in polymer electrolyte membrane (PEM) fuel cell is well known. In this study, we used two different types of commercial graphene nanoplatelets (GNPs) that have respectively 181 m2/g (G1) and 745 m2/g (G2) surface areas and their pyrolyzed forms with and without melamine as platinum (Pt) catalyst support. The GNPs pyrolyzed with melamine were called as M-G1, M-G2; while pyrolyzed without melamine were called as P-G1, P-G2 through the study. Solely, M-G2 sample was nitrogen (N)-doped (2.3%) via pyrolysis by means of melamine. Pt nanoparticles (NPs) were loaded on these support materials by using microwave irradiation technique. PEM fuel cell performance tests were conducted in order to determine the electrochemical activity of the Pt/GNPs catalysts along with polydimethylsiloxane (PDMS) polymer as catalyst binder. Pt/G2 and Pt/M-G2 provided the better PEM fuel cell performance results according to the Pt/G1 and Pt/M-G1 with PDMS polymer. Both of Pt/M-G1/PDMS and Pt/M-G2/PDMS composites gave the lower performance values compared to their pristine cases. Beside, conventional Nafion solution was also used as catalyst binder in the content of catalyst inks of the best two catalysts (Pt/G2, Pt/M-G2). PDMS and Nafion were compared with each other in terms of PEM fuel cell performance. Based on the results (@0.6 V), Pt/G2/PDMS (531.8 mA/cm2) prevailed the Pt/G2/Nafion (473.7 mA/cm2). However, performance sequence changed inversely for the Pt/M-G2/PDMS (62.6 mA/cm2) and Pt/M-G2/Nafion (329.9 mA/cm2). The highest current density value belongs to the Pt/G2/PDMS composite. The higher surface area (for the well distribution of Pt NPs) and the larger pore volume (for the better transfer of reactant gases) provided by the G2 are effective in its superiority. Under catalyst binder mission, the high oxygen permeability and hydrophobicity of the PDMS polymer is also thought to be beneficial for the improved PEM fuel cell performance. Consequently, Pt/G2/PDMS composite is the best candidate for being high-performance catalyst/binder couple in order to be used in the cathode catalyst layer (CL) of PEM fuel cell.

Water management implications for ALD-modified polymer electrolyte membrane fuel cell catalysts

Device testing of polymer electrolyte membrane fuel cell catalysts is essential for electrocatalyst development, yet most reported catalysts fabricated with nanoceramic coatings via atomic layer deposition (ALD) have not been thoroughly examined in this fashion. Platinum nanoparticle catalysts supported on functionalized XC72 carbon black that had been modified with sub-monolayer WN films and subjected to various thermal treatments were incorporated into membrane electrode assemblies and tested for oxygen reduction reaction activity in a fuel cell. The mass activities of the catalysts calculated at low current densities had comparable trends to those previously recorded in aqueous half-cell testing; however, significant deviations in catalyst performance became apparent at high current densities in the mass transport region of the polarization curves. The poor performance of the ALD-modified catalysts was attributed to water flooding induced by the low hydrophobicity of these catalyst layers, as determined by contact angle measurements on the cathode surfaces. It was seen that hydrophobicity was increased by thermal treatment and removal of surface groups known to be prone to hydrogen bonding, thereby improving the catalyst performance in the mass transport region. A heat-treated WN-coated ALD catalyst was found to have superior performance to commercial Pt/C. The field of research on fuel cell catalyst design via ALD is rapidly expanding, and this work demonstrates the importance of understanding the impact of ALD-deposited nanostructures on materials properties beyond the pure electrocatalytic activity.

The ex-situ and in-situ gas diffusivities of polymer electrolyte membrane fuel cell catalyst layer and contribution of primary pores, secondary pores, ionomer and water to the total oxygen diffusion resistance

Oxygen transfer resistance in catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFCs) is the limiting factor under high current density operation. The gas diffusivity of the CL has been studied ex-situ and in-situ in the literature, however, an order of magnitude difference between values of ex-situ and in-situ diffusivities and contribution of pores, ionomer and water to the total mass transfer are yet remain to be investigated. In this study ex-situ and in-situ gas diffusion of ten different CL designs are measured. The ex-situ measured resistance is a part of the total resistance measured in-situ, and having that, the constituting parts of total diffusion may be obtained. The diffusion resistance of ionomer-water accounts for about 80% of the total resistance, and the rest is almost all due to secondary pores. Although, primary pores’ direct contribution to the diffusion resistance is negligible, their size has a notable impact on the ionomer-water relative diffusivity and increasing their diameter can linearly increase the ionomer-water diffusivity. Moreover, porosity of CL affects ionomer-water diffusivity by power of three. This study shows the efforts to reduce the total oxygen diffusion resistance should focus on increasing the CL porosity and size of primary pores.

Platinum Nanoparticles on Plasma-Pretreated Carbon Nanotubes Prepared By Atomic Layer Deposition As Polymer Electrolyte Membrane Fuel Cell Catalysts

Polymer electrolyte fuel cells (PEMFC) are attracting attention as the next generation power supply for electric vehicles, unmanned aerial vehicles and submarines. However, the high price of fuel cell catalysts i.e. platinum (Pt) or platinum alloys is considered a major hurdle to the development of economical fuel cell electric vehicles. Another problem with Pt catalysts is the low stability of long term operation due to separation and aggregation of the catalyst from the carbon support resulting from repeated on-off switching. Recently, atomic layer deposition (ALD) has been proposed as a technique for synthesizing metal nanoparticles. ALD is considered to be the most advanced deposition technology that can control material size and composition in the atomic scale. In addition, ALD has demonstrated large scale conformal deposition of uniform films and particles over high aspect ratio trenches and complex three dimensional contours. In this study, Pt nanoparticles were successfully deposited on carbon nanotubes (CNTs) by ALD. The key to the success of high quality ALD Pt production has been found to be a plasma pretreatment that introduces functional groups that make the CNT surface ready to initiate the ALD nucleation. ALD Pt-CNT catalysts were evaluated as cathode catalysts for PEMFC membrane-electrode-assemblies (MEAs). As a result, even though the Pt load is significantly reduced, the performance is significantly improved compared to commercial PEMFC MEAs in terms of power and stability.

Electrospun carbon nanofiber catalyst layers for polymer electrolyte membrane fuel cells: fabrication and optimization

Current polymer electrolyte membrane fuel cell catalyst support in a form of carbon black is linked with poor Pt utilization, non-optimized triple-phase boundary, mass transport losses, non-uniform microstructure, and durability issues. Little improvement has been made in creating a novel catalyst support microstructure and design to address the challenges associated with the use of carbon blacks. Of particular importance is the ability to control the microstructure of the catalyst layer. There is a lack of fundamental understanding of the relationship between the well-controlled catalyst layer microstructure and fuel cell performance. As catalyst layers require a structure that offers large porosity and surface area, it is envisioned that electrospun nanofibers are an excellent choice, providing a number of structural parameters that could be controlled, i.e., porosity, fiber diameter, fiber alignment, layer thickness. In this work, the fabrication parameters of electrospun carbon nanofibers (CNF) are optimized by factorial design to target key membrane electrode assembly design criterion. Validation of the structural and material properties concludes that optimized CNF surpassed design targets, achieving ~ 80% porosity, ~ 30 S cm−1 in-plane electrical conductivity, and ~ 330 nm fiber diameter. The optimization reveals that CNF properties were successfully tailored and appear feasible as novel catalyst support. The results from the detailed and systematic design of experiments provide a basis for correlating the material and structural properties to key fuel cell performance factors.

Correction to Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during Accelerated Stress Test

RETURN TO ISSUEPREVCorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during Accelerated Stress TestRaghunandan SharmaRaghunandan SharmaMore by Raghunandan Sharma and Shuang Ma Andersen*Shuang Ma AndersenMore by Shuang Ma Andersenhttp://orcid.org/0000-0003-1474-0395Cite this: ACS Catal. 2018, 8, 6, 4979Publication Date (Web):May 1, 2018Publication History Published online1 May 2018Published inissue 1 June 2018https://doi.org/10.1021/acscatal.8b01558Copyright © 2018 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views652Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (198 KB) Get e-AlertsSUBJECTS:Membranes,Transmission electron microscopy,Fuel cells,Materials Get e-Alerts

Quantification on Degradation Mechanisms of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers during an Accelerated Stress Test

The long-term durability of the catalyst layers of a low-working temperature fuel cell such as a polymer electrolyte membrane fuel cell (PEMFC) is of significant scientific interest because of their operation criteria and high initial cost. Identification of degradation mechanisms quantitatively during an accelerated stress test (AST) is essential for assessing and improving the durability of such catalyst layers. In this study, we present a quantitative analysis of the degradation mechanisms such as (i) electronic connectivity loss due to carbon support corrosion, (ii) proton connectivity loss due to ionomer/catalyst interface loss, (iii) catalyst loss due to dissolution or detachment, and (iv) physical surface area loss due to particle growth that is responsible for the electrochemical surface area (ECSA) loss in Pt-based catalyst layers for PEMFCs during an AST performed through potential cycling (linear sweep cyclic voltammetry) between 0.4 and 1.6 V for 7000 cycles in Ar-saturated 1 M H2SO4. Using a ...

4D imaging of polymer electrolyte membrane fuel cell catalyst layers by soft X-ray spectro-tomography

4D imaging - the three-dimensional distributions of chemical species determined using multi-energy X-ray tomography - of cathode catalyst layers of polymer electrolyte membrane fuel cells (PEM-FC) has been measured by scanning transmission x-ray microscopy (STXM) spectro-tomography at the C 1s and F 1s edges. In order to monitor the effects of radiation damage on the composition and 3D structure of the perfluorosulfonic acid (PFSA) ionomer, the same volume was measured 3 times sequentially, with spectral characterization of that same volume at several time points during the measurements. The changes in the average F 1s spectrum of the ionomer in the cathode as the measurements progressed gave insights into the degree of chemical modification, fluorine mass loss, and changes in the 3D distributions of ionomer that accompanied the spectro-tomographic measurement. The PFSA ionomer-in-cathode is modified both chemically and physically by radiation damage. The 3D volume decreases anisotropically. By reducing the incident flux, partial defocusing (50 nm spot size), limiting the number of tilt angles to 14, and using compressed sensing reconstruction, we show it is possible to reproducibly measure the 3D structure of ionomer in PEM-FC cathodes at ambient temperature while causing minimal radiation damage.

Biomimetic design of monolithic fuel cell electrodes with hierarchical structures

Despite the significant improvement of polymer electrolyte membrane fuel cell catalyst activities, a cost-effective and stable membrane electrode assembly is still lacking, which greatly inhibits the commercialization of this efficient and environmental friendly technology in stationary and transportation applications. The main reason is that the engineering of different components of an electrode, such as catalytically active metals, electron transport and reactant diffusion paths in a compatible way is very challenging. Here we show the design and preparation of a monolithic fuel cell electrode with a compatible wire on wire structure that mimics the configuration of a pine tree. We developed a procedure to make a flexible carbon thin film composed of porous nanofibers with a thickness of ~100nm and centimeter scale lengths. Platinum nanowires (ca. 3nm diameter) were deposited on these microscale carbon nanofiber films, resulting in a hierarchical structure. The platinum nanowires were then decorated with a porous bismuth coating to modulate the atomic structure and induce catalytic activity toward formic acid electrooxidation. The end result is a monolithic structure used as a fuel cell electrode that combines microscale diffusive pathways and nanoscale catalyst structures. Prepared by a process that is readily scalable, this design strategy offers a new way to tailor catalytic functions at a system level.

Optimization of polymer electrolyte membrane fuel cell catalyst layer with bidirectionally-graded composition

The catalyst layer (CL) of the polymer electrolyte membrane (PEM) fuel cell is comprised of multiple constituent materials including platinum catalyst, carbon support and ionomer. The loading and spatial distribution of these materials must be optimized to improve fuel cell performance and reduce cost. In this study, the CL composition is optimized simultaneously in both the in-plane and through-thickness directions for the first time. An improved agglomerate approach is employed within a 2D cathode model and optimal CL constituent (Pt, C, and ionomer) distributions are obtained to maximize the current density under different operating regimes. The optimum distributions are found to depend significantly on the operating regime, with regions of high constituent loading located in the high reaction-rate zones. In comparison to unidirectional grading (either in the in-plane or through-thickness direction), higher performance is obtained with a bidirectionally-graded CL composition. We also conducted two-component optimization by considering the joint distributions of Pt with Nafion®, and C with Nafion®. Higher performance improvements are obtained for two-component optimization.

Quantification of artifacts in scanning electron microscopy tomography: Improving the reliability of calculated transport parameters in energy applications such as fuel cell and battery electrodes

Focused ion beam and scanning electron microscopy tomography (FIB-SEMt) is commonly used to extract reactant transport relevant parameters from nano-porous materials in energy applications, such as fuel cells or batteries. Here we present an approach to virtually model the errors in FIB-SEMt which are caused by the FIB cutting distance. The errors are evaluated in terms of connectivity, solid volume fraction (SVF), conductivity, diffusivity, as well as mean grain and pore sizes. For state-of-the-art FIB-SEMt experiments, where a hydrogen fuel cell catalyst layer with 60 nm mean grain size and 40% SVF is sectioned with a cutting distance of 15 nm, the error in our simulation ranges up to 51% (conductivity), whereas other parameters remain largely unaffected (Laplace diffusivity, 4%). We further present a method, employing virtual coarsening and back interpolation, to reduce FIB cutting distance errors in all investigated parameters. Both error evaluation and correction are applicable to sphere based porous materials with relevance for the energy conversion and storage sector such as polymer electrolyte membrane fuel cell catalyst layer (PEMFC CL), battery carbon binder domain (CBD) or supercapacitor electrodes.

Effect of Water Sorption on the Electronic Conductivity of Porous Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

A method is described for measuring the effective electronic conductivity of porous fuel cell catalyst layers (CLs) as a function of relative humidity (RH). Four formulations of CLs with different carbon black (CB) contents and ionomer equivalent weights (EWs) were tested. The van der Pauw method was used to measure the sheet resistance (RS), which increased with RH for all samples. The increase was attributed to ionomer swelling upon water uptake, which affects the connectivity of CB aggregates. Greater increases in RS were observed for samples with lower EW, which uptake more water on a mass basis per mass ionomer. Transient RS measurements were taken during absorption and desorption, and the resistance kinetics were fit using a double exponential decay model. No hysteresis was observed, and the absorption and desorption kinetics were virtually symmetric. Thickness measurements were attempted at different RHs, but no discernible changes were observed. This finding led to the conclusion that the conducting Pt/C volume fraction does not change with RH, which suggests that effective medium theory models that depend on volume fraction alone cannot explain the reduction in conductivity with RH. The merits of percolation-based models were discussed. Optical micrographs revealed an extensive network of "mud cracks" in some samples. The influence of water sorption on CL conductivity is primarily explained by ionomer swelling, and its effects on the quantity and quality of interaggregate contacts were discussed.

Carbon-embedded carbon nanotubes as supports of polymer electrolyte membrane fuel cell catalysts.

Carbon shells embedded carbon nanotubes can facilitate Pt nanoparticles loading and dispersing on the core-shell nanostructural support. Carbon shells are prepared by coating polyaniline layers on the core (carbon nanotubes) with in-situ polymerization and subsequent carbonization. The carbon shell embedded carbon nanotube supported Pt catalyst reveals high electrochemical active surface area and mass activity, which are the factors of 1.4 times and 2.2 times higher than that of the pristine carbon nanotube supported Pt, respectively. In addition, the carbon shell embedded carbon nanotube supported Pt catalyst has a higher stability than the carbon nanotube supported Pt catalyst. The improved catalytic activity and stability of our new catalyst can be ascribed to the improved dispersion of Pt nanoparticles on surfaces of carbon nanotubes and the interaction between Pt nanoparticles and carbon shells.

Investigation of a polymer electrolyte membrane fuel cell catalyst layer with bidirectionally-graded composition

The catalyst layer (CL) of the polymer electrolyte membrane (PEM) fuel cell must be modeled accurately in order to resolve the effects of complex interactions between charge and mass transport on the fuel cell's electrochemical reactions. In previous work, we developed an agglomerate model [1] which correctly accounts for variations in the agglomerate surface area as the CL constituents are varied to provide a better estimate of diffusion losses. Here, this improved agglomerate model is employed to investigate a PEM fuel cell catalyst layer with a functionally-graded composition. We present results for varying catalyst and ionomer loadings in both the through-thickness and in-plane directions. In agreement with experimental observations, we find that a higher catalyst and/or ionomer loading at the membrane/CL interface improves performance especially in the ohmic loss regime. Similarly, improved performance is observed for higher catalyst and/or ionomer loadings under the channel in the mass transport loss regime. In addition, we investigated bidirectionally graded CLs for the first time. It is observed that higher performance can be obtained with bidirectionally graded CLs in both ohmic and mass transport loss regimes.

Study of thermal conductivity of PEM fuel cell catalyst layers

Thermal conductivities and compression of differently composed Polymer Electrolyte Membrane Fuel Cell (PEMFC) Catalyst Layers (CLs) were measured, both when dry and when containing liquid water. The results were compared using a 1-D thermal model.Dry CLs, and CLs with low water content, were found to have thermal conductivity values of 0.07–0.11 W K−1 m−1 when compressed in the range of 5–15 bar compaction pressure. When adding water up to 70 mol of water per mole of sulphonic group, it was observed that the water only had an effect on the thermal conductivity with a water content significantly beyond the capacity of the ionomer. This means that water held by and between the catalyst particles contribute to the increase in thermal conductivity rather than water in the ionomer. The CLs were all found to compress irreversibly and to become incompressible above 10 bar compaction pressure.When considering wet porous transport layers (PTL) and moderately humidified CL, the PEMFC maximum internal temperature difference increased by 33% when compared to the commonly assumed CL thermal conductivities.