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Randall C, Zou L, Wang H, Hui J, Rodríguez-López J, Chen-Glasser M, Dura JA, DeCaluwe SC. Morphology of Thin-Film Nafion on Carbon as an Analogue of Fuel Cell Catalyst Layers. ACS APPLIED MATERIALS & INTERFACES 2024; 16:3311-3324. [PMID: 38212130 PMCID: PMC10811627 DOI: 10.1021/acsami.3c14912] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/06/2023] [Revised: 12/24/2023] [Accepted: 12/29/2023] [Indexed: 01/13/2024]
Abstract
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.
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Affiliation(s)
| | - Lianfeng Zou
- Clean
Nano Energy Center, State Key Laboratory
of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, Hebei, China
| | - Howard Wang
- University
of Maryland, College
Park, Maryland 20742, United States
| | - Jingshu Hui
- University
of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | | | | | - Joseph A. Dura
- NIST
Center for Neutron Research, Gaithersburg, Maryland 20899, United States
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2
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Olbrich W, Kadyk T, Sauter U, Eikerling M, Gostick J. Structure and conductivity of ionomer in PEM fuel cell catalyst layers: a model-based analysis. Sci Rep 2023; 13:14127. [PMID: 37644035 PMCID: PMC10465542 DOI: 10.1038/s41598-023-40637-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2023] [Accepted: 08/11/2023] [Indexed: 08/31/2023] Open
Abstract
Efforts in design and optimization of catalyst layers for polymer electrolyte fuel cells hinge on mathematical models that link electrode composition and microstructure with effective physico-chemical properties. A pivotal property of these layers and the focus of this work is the proton conductivity, which is largely determined by the morphology of the ionomer. However, available relations between catalyst layer composition and proton conductivity are often adopted from general theories for random heterogeneous media and ignore specific features of the microstructure, e.g., agglomerates, film-like structures, or the hierarchical porous network. To establish a comprehensive understanding of the peculiar structure-property relations, we generated synthetic volumetric images of the catalyst layer microstructure. In a mesoscopic volume element, we modeled the electrolyte phase and calculated the proton conductivity using numerical tools. Varying the ionomer morphology in terms of ionomer film coverage and thickness revealed two limiting cases: the ionomer can either form a thin film with high coverage on the catalyst agglomerates; or the ionomer exists as voluminous chunks that connect across the inter-agglomerate space. Both cases were modeled analytically, adapting relations from percolation theory. Based on the simulated data, a novel relation is proposed, which links the catalyst layer microstructure to the proton conductivity over a wide range of morphologies. The presented analytical approach is a versatile tool for the interpretation of experimental trends and it provides valuable guidance for catalyst layer design. The proposed model was used to analyze the formation of the catalyst layer microstructure during the ink stage. A parameter study of the initial ionomer film thickness and the ionomer dispersion parameter revealed that the ionomer morphology should be tweaked towards well-defined films with high coverage of catalyst agglomerates. These implications match current efforts in the experimental literature and they may thus provide direction in electrode materials research for polymer electrolyte fuel cells.
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Affiliation(s)
- W Olbrich
- Theory and Computation of Energy Materials (IEK-13), Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany.
- Robert Bosch GmbH, Corporate Research, 71272, Renningen, Germany.
- Chair of Theory and Computation of Energy Materials, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062, Aachen, Germany.
| | - T Kadyk
- Theory and Computation of Energy Materials (IEK-13), Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany
- Jülich Aachen Research Alliance, JARA Energy, 52425, Jülich, Germany
| | - U Sauter
- Robert Bosch GmbH, Corporate Research, 71272, Renningen, Germany
| | - M Eikerling
- Theory and Computation of Energy Materials (IEK-13), Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany
- Chair of Theory and Computation of Energy Materials, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062, Aachen, Germany
- Jülich Aachen Research Alliance, JARA Energy, 52425, Jülich, Germany
| | - J Gostick
- Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada
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3
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Yoshimune W. Dependence of oxygen transport properties of catalyst layers for polymer electrolyte fuel cells on the fabrication process. RESULTS IN CHEMISTRY 2023. [DOI: 10.1016/j.rechem.2022.100738] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
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4
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Yoshimune W, Kikkawa N, Yoneyama H, Takahashi N, Minami S, Akimoto Y, Mitsuoka T, Kawaura H, Harada M, Yamada NL, Aoki H. Interfacial Distribution of Nafion Ionomer Thin Films on Nitrogen-Modified Carbon Surfaces. ACS APPLIED MATERIALS & INTERFACES 2022; 14:53744-53754. [PMID: 36416068 PMCID: PMC10806603 DOI: 10.1021/acsami.2c14574] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2022] [Accepted: 11/11/2022] [Indexed: 06/16/2023]
Abstract
Chemically modified carbon supports for the cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) show considerable promise for boosting the oxygen reduction reaction. This study evaluated the ionomer distribution of Nafion ionomer thin films on nitrogen (N)-modified carbon surfaces along their depth direction. Neutron reflectivity (NR) measurements performed using the double-contrast technique with H2O and D2O revealed that the introduction of N functional groups to carbon thin films promoted ionomer adsorption onto the surface under wet conditions (22 °C, 85% relative humidity). Molecular dynamics (MD) simulations conducted to verify the origin of the robust contact between the ionomer and N-modified carbon surface revealed an ionomer adsorption mechanism on the N-modified carbon surfaces, which involved Coulomb interactions between the positively charged carbon surface and the ionomer side chains with negatively charged sulfonic acid groups. The positive surface charge, which was determined using the contents of the N functional groups estimated by X-ray photoelectron spectroscopy, was found to be sufficient as an impetus for ionomer adsorption. This strategy involving NR measurements and MD simulations can provide insights into the solid-ionomer interfacial structures in a cathode catalyst layer and can therefore be extensively employed in studies on PEFCs.
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Affiliation(s)
- Wataru Yoshimune
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Nobuaki Kikkawa
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Hiroaki Yoneyama
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Naoko Takahashi
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Saori Minami
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Yusuke Akimoto
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Takuya Mitsuoka
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Hiroyuki Kawaura
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Masashi Harada
- Toyota
Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi480-1192, Japan
| | - Norifumi L. Yamada
- Institute
of Materials Structure Science, High Energy
Accelerator Research Organization, Naka-gun, Ibaraki319-1106, Japan
| | - Hiroyuki Aoki
- Institute
of Materials Structure Science, High Energy
Accelerator Research Organization, Naka-gun, Ibaraki319-1106, Japan
- Materials
and Life Science Division, J-PARC Center, Japan Atomic Energy Agency, Naka-gun, Ibaraki319-1195, Japan
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5
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Effect of Catalyst Ink and Formation Process on the Multiscale Structure of Catalyst Layers in PEM Fuel Cells. APPLIED SCIENCES-BASEL 2022. [DOI: 10.3390/app12083776] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The structure of a catalyst layer (CL) significantly impacts the performance, durability, and cost of proton exchange membrane (PEM) fuel cells and is influenced by the catalyst ink and the CL formation process. However, the relationship between the composition, formulation, and preparation of catalyst ink and the CL formation process and the CL structure is still not completely understood. This review, therefore, focuses on the effect of the composition, formulation, and preparation of catalyst ink and the CL formation process on the CL structure. The CL structure depends on the microstructure and macroscopic properties of catalyst ink, which are decided by catalyst, ionomer, or solvent(s) and their ratios, addition order, and dispersion. To form a well-defined CL, the catalyst ink, substrate, coating process, and drying process need to be well understood and optimized and match each other. To understand this relationship, promote the continuous and scalable production of membrane electrode assemblies, and guarantee the consistency of the CLs produced, further efforts need to be devoted to investigating the microstructure of catalyst ink (especially the catalyst ink with high solid content), the reversibility of the aged ink, and the drying process. Furthermore, except for the certain variables studied, the other manufacturing processes and conditions also require attention to avoid inconsistent conclusions.
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6
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Maruyama H, Maeda M, Fujimori A. Interfacial film conformation and its molecular arrangement of s-triazine derivatives containing three fluorocarbons without hydrophilic groups. J Fluor Chem 2021. [DOI: 10.1016/j.jfluchem.2021.109880] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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7
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Chowdhury A, Bird A, Liu J, Zenyuk IV, Kusoglu A, Radke CJ, Weber AZ. Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes. ACS APPLIED MATERIALS & INTERFACES 2021; 13:42579-42589. [PMID: 34490780 DOI: 10.1021/acsami.1c07611] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Transport phenomena are key in controlling the performance of electrochemical energy-conversion technologies and can be highly complex, involving multiple length scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions. We examine the electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low-equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic acid groups. These effects of equivalent weight are also observed via in situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex situ ellipsometry and X-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading, highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.
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Affiliation(s)
- Anamika Chowdhury
- Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
- Department of Chemical and Biomolecular Engineering, University of California Berkeley, Berkeley, California 94720, United States
| | - Ashley Bird
- Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
- Department of Chemical and Biomolecular Engineering, University of California Berkeley, Berkeley, California 94720, United States
| | - Jiangjin Liu
- Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | - Iryna V Zenyuk
- Department of Chemical and Biomolecular Engineering, University of California Irvine, Irvine, California 92697, United States
| | - Ahmet Kusoglu
- Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | - Clayton J Radke
- Department of Chemical and Biomolecular Engineering, University of California Berkeley, Berkeley, California 94720, United States
| | - Adam Z Weber
- Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
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8
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Mabuchi T, Huang SF, Tokumasu T. Influence of Ionomer Loading and Substrate Wettability on the Morphology of Ionomer Thin Films Using Coarse-Grained Solvent Evaporation Simulations. Macromolecules 2020. [DOI: 10.1021/acs.macromol.0c01303] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Affiliation(s)
- Takuya Mabuchi
- Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira,
Aoba-ku, Sendai, Miyagi 980-8577, Japan
| | - Sheng-Feng Huang
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira,
Aoba-ku, Sendai, Miyagi 980-8577, Japan
| | - Takashi Tokumasu
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira,
Aoba-ku, Sendai, Miyagi 980-8577, Japan
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9
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Ito K, Harada M, Yamada NL, Kudo K, Aoki H, Kanaya T. Water Distribution in Nafion Thin Films on Hydrophilic and Hydrophobic Carbon Substrates. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2020; 36:12830-12837. [PMID: 33085483 DOI: 10.1021/acs.langmuir.0c01917] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We performed H2O and D2O double-contrast neutron reflectivity measurements on ∼25 nm thick Nafion thin films on hydrophilic and hydrophobic carbon in water and 80% relative humidity vapor to investigate the depth profile of the water and Nafion distribution. We found a dense Nafion layer at the air or water interface regardless of the carbon hydrophilicity. On the other hand, a water-rich Nafion dense layer was observed at the carbon interface only for hydrophilic carbon. The double-contrast measurements provided quantitative information about the depth profile but simultaneously indicated that the sum of the volume occupancies of water and Nafion in the film was less than unity. We assessed the problem based on two possibilities: voids in the film or "residual water", which cannot be exchanged or is difficult to exchange with water outside.
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Affiliation(s)
- Kanae Ito
- Materials and Life Science Division, J-PARC Center, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
| | - Masashi Harada
- Toyota Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan
| | - Norifumi L Yamada
- Materials and Life Science Division, J-PARC Center, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
- Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
| | - Kenji Kudo
- Toyota Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan
| | - Hiroyuki Aoki
- Materials and Life Science Division, J-PARC Center, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
- Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
| | - Toshiji Kanaya
- Materials and Life Science Division, J-PARC Center, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
- Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan
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10
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Farzin S, Johnson TJ, Chatterjee S, Zamani E, Dishari SK. Ionomers From Kraft Lignin for Renewable Energy Applications. Front Chem 2020; 8:690. [PMID: 33005600 PMCID: PMC7480228 DOI: 10.3389/fchem.2020.00690] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2020] [Accepted: 07/03/2020] [Indexed: 01/03/2023] Open
Abstract
Converting industrial/agricultural lignin-rich wastes to efficient, cost-effective materials for electrochemical devices (e.g., fuel cells) can aid in both bio- and energy economy. A major limitation of fuel cells is the weak ion conductivity within the ~2-30-nm thick, ion-conducting polymer (ionomer)-based catalyst-binder layer over electrodes. Here, we strategically sulfonated kraft lignin (a by-product of pulp and paper industries) to design ionomers with varied ion exchange capacities (IECs) (LS x; x = IEC) that can potentially overcome this interfacial ion conduction limitation. We measured the ion conductivity, water uptake, ionic domain characteristics, density, and predicted the water mobility/stiffness of Nafion, LS 1.6, and LS 3.1 in submicron-thick hydrated films. LS 1.6 showed ion conductivity an order of magnitude higher than Nafion and LS 3.1 in films with similar thickness. The ion conductivity of these films was not correlated to their water uptake and IECs. Within the three-dimensional, less dense, branched architecture of LS 1.6 macromolecules, the -SO3H and -OH groups are in close proximity, which likely facilitated the formation of larger ionic domains having highly mobile water molecules. As compared to LS 1.6, LS 3.1 showed a higher glass transition temperature and film stiffness at dry state, which sustained during humidification. On the contrary, Nafion stiffened significantly upon humidification. The smaller ionic cluster within stiff LS 3.1 and Nafion films thus led to ion conductivity lower than LS 1.6. Since LS x ionomers (unlike commercial lignosulfonate) are not water soluble, they are suitable for low-temperature, water-mediated ion conduction in submicron-thick films.
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Affiliation(s)
| | | | | | | | - Shudipto K. Dishari
- Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States
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11
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Stoev K, Sakurai K. Recent Progresses in Nanometer Scale Analysis of Buried Layers and Interfaces in Thin Films by X-rays and Neutrons. ANAL SCI 2020; 36:901-922. [PMID: 32147630 DOI: 10.2116/analsci.19r010] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
In the early 1960s, scientists achieved the breakthroughs in the fields of solid surfaces and artificial layered structures. The advancement of surface science has been supported by the advent of ultra-high vacuum technologies, newly discovered and established scanning probe microscopy with atomic resolution, as well as some other advanced surface-sensitive spectroscopy and microscopy. On the other hand, it has been well recognized that a number of functions are related to the structures of the interfaces, which are the thin planes connecting different materials, most likely by layering thin films. Despite the scientific significance, so far, research on such buried layers and interfaces has been limited, because the probing depth of almost all existing sophisticated analytical methods is limited to the top surface. The present article describes the recent progress in the nanometer scale analysis of buried layers and interfaces, particularly by using X-rays and neutrons. The methods are essentially promising to non-destructively probe such buried structures in thin films. The latest scientific research has been reviewed, and includes applications to bio-chemical, organic, electronic, magnetic, spintronic, self-organizing and complicated systems as well as buried liquid-liquid and solid-liquid interfaces. Some emerging analytical techniques and instruments, which provide new attractive features such as imaging and real time analysis, are also discussed.
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12
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Kosakian A, Urbina LP, Heaman A, Secanell M. Understanding single-phase water-management signatures in fuel-cell impedance spectra: A numerical study. Electrochim Acta 2020. [DOI: 10.1016/j.electacta.2020.136204] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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13
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Transport and Electrochemical Interface Properties of Ionomers in Low-Pt Loading Catalyst Layers: Effect of Ionomer Equivalent Weight and Relative Humidity. Molecules 2020; 25:molecules25153387. [PMID: 32722653 PMCID: PMC7435395 DOI: 10.3390/molecules25153387] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2020] [Revised: 07/02/2020] [Accepted: 07/18/2020] [Indexed: 11/17/2022] Open
Abstract
Catalyst layer (CL) ionomers control several transport and interfacial phenomena including long-range transport of protons, local transport of oxygen to Pt catalyst, effective utilization of Pt catalyst, electrochemical reaction kinetics and double-layer capacitance. In this work, the variation of these properties, as a function of humidity, for CLs made with two ionomers differing in side-chain length and equivalent weight, Nafion-1100 and Aquivion-825, was investigated. This is the first study to examine humidity-dependent oxygen reduction reaction (ORR) kinetics in-situ for CLs with different ionomers. A significant finding is the observation of higher ORR kinetic activity (A/cm2Pt) for the Aquivion-825 CL than for the Nafion-1100 CL. This is attributed to differences in the interfacial protonic concentrations at Pt/ionomer interface in the two CLs. The differences in Pt/ionomer interface is also noted in a higher local oxygen transport resistance for Aquivion-825 CLs compared to Nafion-1100 CLs, consistent with stronger interaction between ionomer and Pt for ionomer with more acid groups. Similar dependency on Pt utilization (ratio of electrochemically active area at any relative humidity (RH) to that at 100% RH) as a function of RH is observed for the two CLs. As expected, strong influence of humidity on proton conduction is observed. Amongst the two, the CL with high equivalent weight ionomer (Nafion-1100) exhibits higher conduction.
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14
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Kimura T, Kawamoto T, Aoki M, Mizusawa T, Yamada NL, Miyatake K, Inukai J. Sublayered Thin Films of Hydrated Anion Exchange Ionomer for Fuel Cells Formed on SiO 2 and Pt Substrates Analyzed by Neutron Reflectometry under Controlled Temperature and Humidity Conditions. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2020; 36:4955-4963. [PMID: 32310665 DOI: 10.1021/acs.langmuir.0c00440] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Anion-conductive ionomers are used for electrolyte membranes in membrane-electrode assemblies and for binders in catalyst layers in anion exchange membrane fuel cells (AEMFCs). The conformations of these ionomers as well as their water distribution are important for designing new efficient/durable anion-conductive ionomers for AEMFCs. For a deeper understanding of the distribution of deuterium oxide (D2O) as a function of depth, neutron reflectometry (NR) was carried out on thin films of an anion exchange ionomer, BAF-QAF, with a thickness of approximately 60 nm formed on a thermally formed SiO2 film on Si(100) and on a 20 nm Pt layer deposited on the SiO2 film at a temperature of 60 °C and relative humidities of 0, 50, 70, and 90%. Clear NR modulation was obtained under each condition. The NR data were fit very well with a three-sublayered model parallel to the substrate with different densities of BAF-QAF and D2O. The influence of the SiO2 and Pt substrates was observed not only at the BAF-QAF/substrate interface but also on the entire thin film. The D2O absorption/desorption behavior in each sublayer differed in the BAF-QAF films cast on SiO2 and Pt. The BAF-QAF/SiO2 interface was rather hydrophilic, while the BAF-QAF/Pt interface was very hydrophobic.
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Affiliation(s)
- Taro Kimura
- Integrated Graduate School of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan
| | - Teppei Kawamoto
- Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu 400-0021, Japan
| | - Makoto Aoki
- Division of Life, Medical, Natural Sciences and Technology, Organization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 658-8501, Japan
| | - Takako Mizusawa
- Comprehensive Research Organization for Science and Society Neutron Science and Technology Center, 162-1 Shirakata, Tokai, Ibaraki 319-1106, Japan
| | - Norifumi L Yamada
- Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan
| | - Kenji Miyatake
- Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu 400-0021, Japan
- Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
| | - Junji Inukai
- Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu 400-0021, Japan
- Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
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15
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Mabuchi T, Huang SF, Tokumasu T. Dispersion of Nafion Ionomer Aggregates in 1-Propanol/Water Solutions: Effects of Ionomer Concentration, Alcohol Content, and Salt Addition. Macromolecules 2020. [DOI: 10.1021/acs.macromol.9b02725] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Takuya Mabuchi
- Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan
| | - Sheng-Feng Huang
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan
| | - Takashi Tokumasu
- Institute of Fluid Science, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan
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16
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Shrivastava UN, Suetsugu K, Nagano S, Fritzsche H, Nagao Y, Karan K. Cross-correlated humidity-dependent structural evolution of Nafion thin films confined on a platinum substrate. SOFT MATTER 2020; 16:1190-1200. [PMID: 31898714 DOI: 10.1039/c9sm01731c] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Nanometer thin films of Nafion ionomer interfaced with platinum form the functional electrodes in many electrochemical devices including fuel cells and electrolyzers. To impart facile proton conduction in a Nafion ionomer, sufficient hydration of the Nafion ionomer is necessary to create a percolating network of water-filled nanometer-sized hydrophilic domains that manifest as macroscopic swelling. This hydration behavior of the ionomer thin films is poorly understood especially for films confined on electrochemically relevant Pt substrates. In this work, we present the evolution of hydration-dependent microscopic hydrophilic domains and macroscopic expansion of a 55 nm thin Nafion film on a Pt substrate. The cross-correlation among the film macro-expansion from ellipsometry, the micro-expansion from GISAXS, and the water distribution from neutron reflectometry (NR) explains the observed non-affine behavior of the film which can be attributed to the randomly and spatially non-uniform distribution of water domains. A correlation between the macroscopic factor (ε/τ) for protonic conductivity, and the domain size and swelling is presented for the first time. In addition, interfacial water between Pt and the ionomer interface is estimated at 75% and 84% RH, and an increase in domain size with RH is discussed to explain the increased activity and oxygen diffusivity with RH.
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Affiliation(s)
- Udit N Shrivastava
- Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada.
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17
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Nagao Y. Progress on highly proton-conductive polymer thin films with organized structure and molecularly oriented structure. SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 2020; 21:79-91. [PMID: 32158509 PMCID: PMC7033726 DOI: 10.1080/14686996.2020.1722740] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/28/2019] [Revised: 01/23/2020] [Accepted: 01/24/2020] [Indexed: 05/08/2023]
Abstract
Several current topics are introduced in this review, with particular attention to highly proton-conductive polymer thin films with organized structure and molecularly oriented structure. Organized structure and molecularly oriented structure are anticipated as more promising approaches than conventional less-molecular-ordered structure to elucidate mechanisms of high proton conduction and control proton conduction. This review introduces related polymer materials and molecular design using lyotropic liquid crystals and hydrogen bond networks for high proton conduction. It also outlines the use of substrate surfaces and external fields, such as pressure and centrifugal force, for organizing structures and molecularly oriented structures.
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Affiliation(s)
- Yuki Nagao
- School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Japan
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18
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Karan K. Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:13489-13520. [PMID: 30753782 DOI: 10.1021/acs.langmuir.8b03721] [Citation(s) in RCA: 33] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Ion-containing perfluorinated polymers possess unique viscoelastic properties, excellent proton conductivity, and nanophase-segregated structure all arising from the clustering of hydrophilic sulfonic acid groups within a matrix of hydrophobic fluorocarbons. When these ionomers are confined to nanothin films, a broad swathe of structural organization imparting a rich variety of surface, interfacial, and bulk characteristics can be expected. However, our understanding of perfluorinated ionomer thin film behavior is still in a rudimentary stage, and much of the research focus to date has been on its hydration-related structure and properties pertinent to electrochemical applications. Thus, many hidden gems-their interesting surface and interfacial properties-have been overlooked. In this Invited Feature Article, which is a summary of the key contributions by the author's group, including several collaborative publications on ionomer thin films, we unravel many of these facets. In addition, the article attempts to integrate knowledge acquired from a variety of investigations of different aspects of the ionomer thin films to refine and develop a consistent picture of their structure and behavior. First, we focus on the self-assembly of ionomers and show that dispersion media and hydrophobicity/hydrophilicity of the substrate can result in partial or even no coverage of substrates, shedding light on the complexity of polymer-substrate, polymer-solvent, and polymer-polymer interactions, an insight completely obscured when the spin-coating method is adopted for film creation. We demonstrate that the same ionomer can be used to create a variety of surfaces ranging from superhydrophilic to highly hydrophobic by controlling the film thickness or through the choice of substrate material. The ultrathin, hydrophilic surfaces of self-assembled Nafion ionomer films exhibit wettability switching behavior which opens the door to creating stimuli-responsive smart surfaces. The thermoresponsive behavior of the films is discussed in the context of surface (wettability) and bulk (thermal expansion) characteristics as well as a newly discovered vibrational mode. The substrate- and film thickness-dependent thermal expansion coefficients reinforce the importance of interfacial interactions and confinement on the structure/properties of these films. They also open up the potential of tuning ionomer bulk properties via substrate chemistry. The discovery of a vibrational mode that becomes thermally activated at high temperature has provided new insights into the origins of the molecular motions responsible for the α-relaxation of the Nafion ionomer as well as the underlying reason for wettability switching. Our recent neutron reflectometry study of different ionomers varying in side-chain composition/length on a platinum substrate shows that the interfacial hydration level is correlated to the side-chain length, which opens up the possibility of the controlling the interfacial electrochemistry. Finally, a systematic analysis of factors affecting proton conduction is presented to elucidate the yet-unresolved origins of the suppressed conduction of nanothin ionomer films compared to that of the bulk membrane. By revealing these interesting yet poorly understood facets of ionomer thin films, the article aims to stimulate further scientific pursuit on this topic.
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Affiliation(s)
- Kunal Karan
- Department of Chemical & Petroleum Engineering , The University of Calgary , Calgary , Alberta T2N1N4 , Canada
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19
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Zhang C, Davies M, Karan K. Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity. ACTA ACUST UNITED AC 2019. [DOI: 10.1002/polb.24792] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Chi Zhang
- Department of Chemical and Petroleum Engineering; The University of Calgary; 2500 University Dr. NW, Calgary Alberta T2N 1N4 Canada
| | - Michael Davies
- Department of Mechanical and Manufacturing Engineering; The University of Calgary; 2500 University Dr. NW, Calgary Alberta T2N 1N4 Canada
| | - Kunal Karan
- Department of Chemical and Petroleum Engineering; The University of Calgary; 2500 University Dr. NW, Calgary Alberta T2N 1N4 Canada
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