Mechanical Behavior of Free-Standing Fuel Cell Electrodes on

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Mechanical Behavior of Free-Standing Fuel Cell Electrodes on Water Surface Sanwi Kim, Jae-Han Kim, Jong-Gil Oh, Kyung-Lim Jang, Byeong-Heon Jeong, Bo Ki Hong, and Taek-Soo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03854 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Mechanical Behavior of Free-Standing Fuel Cell Electrodes on Water Surface Sanwi Kim,†,§ Jae-Han Kim,†,§ Jong-Gil Oh,‡ Kyung-Lim Jang,† Byeong-Heon Jeong,‡ Bo Ki Hong,*,‡ and Taek-Soo Kim*,† †

Department of Mechanical Engineering, KAIST, Daejeon, 305-701, Republic of Korea

‡

Fuel Cell Vehicle Team 1, Eco-Technology Center, Research & Development Division,

Hyundai Motor Company, Yongin-si, Gyeonggi-do, 446-716, Republic of Korea KEYWORDS: fuel cell, electrode, mechanical properties, young’s modulus, ice-assisted separation

ABSTRACT: Fundamental understanding of the mechanical behavior of polymer electrolyte fuel cell electrodes as free-standing materials is essential to develop mechanically robust fuel cells. However, this has been a significant challenge due to critical difficulties, such as separating the pristine electrode from the substrate without damage and precisely measuring the mechanical properties of the very fragile and thin electrodes. We report the mechanical behavior of freestanding fuel cell electrodes on the water surface through adopting innovative ice-assisted separation method to separate the electrode from decal transfer film. It is found that doubling the ionomer content in electrodes increases not only the tensile stress at the break and the Young’s

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modulus (E) of the electrodes by approximately 2.1–3.5 and 1.7–2.4 times, respectively, but also the elongation at the break by approximately 1.5–1.7 times, which indicates that stronger, stiffer, and tougher electrodes are attained with increasing ionomer content, which have been of significant interest in materials research fields. The scaling law relationship between Young’s modulus and density (ρ) has been unveiled as E ~ ρ1.6 and it is compared with other materials. These findings can be used to develop mechanically robust electrodes for fuel cell applications.

INTRODUCTION Over the past few decades, the world has been shifting away from its dependence on fossil fuels toward cleaner and eco-friendlier energy sources. One promising candidate for eco-friendly fuels is hydrogen energy, which can be used in a variety of different applications. Polymer electrolyte fuel cells (PEFCs) powered by hydrogen have garnered significant attention in recent years as potential power sources for eco-friendly vehicles due to their high power density and zero emission features.1,2 For a fuel cell electric vehicle (FCEV) to become commercially viable, however, the significant challenges of insufficient performance and low durability should be solved for PEFCs, because most FCEVs are operated under harsh conditions such as cold startup/shut-down, freeze/thaw cycles, dry-wet cycles, and so on.2-6 Among the principal components of PEFCs, the electrode of the membrane-electrode assembly (MEA) is a crucial component that generates electricity through a hydrogen oxidation reaction at the anode and an oxygen reduction reaction at the cathode. For commercial MEAs for FCEV applications, the electrode is most commonly composed of platinum catalysts that are supported on carbon (Pt/C) and perfluorinated sulfonic acid (PFSA) ionomer binders such as Nafion®.2,7,8


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In general, MEAs have been fabricated using either catalyst-coated membrane (CCM) or catalyst-coated gas diffusion layer (CCG) methods.8-11 In FCEV applications, the CCM method has garnered more attention than the CCG method due to its higher catalyst utilization and better interfacial contact between the membrane and catalyst layer.8-11 Among the different fabrication methods for CCM-formed MEAs, e.g. spraying,10,11 direct-coating,12,13 and decal transfer methods,14-16 the decal transfer method is the most commonly used method for mass-produced MEAs due to its high efficiency in the manufacturing process.2,16 In order to develop an advanced MEA with excellent performance and durability, lifetime durability tests of fuel cells are normally required in order to understand the fundamental mechanisms of degradation and failure,17,18 which are costly and very time-consuming, e.g. up to six months for a full freeze/thaw durability test.5 It is noteworthy that the mechanical properties of electrodes are mainly attributed to those of ionomer binders which act as proton conductors between membrane and catalytic sites in the electrodes.11 Thus if the mechanical properties of the electrodes are not sufficient enough to resist external forces or deformations, they may cause a significant damage or even breakage of the proton conducting passages provided by the ionomer binders, finally resulting in the reduction of fuel cell performance. Therefore, a fundamental understanding of the mechanical behavior of pristine electrodes for MEA fabrication is essential to develop high performance electrode and screen inappropriate electrode structures prior to fabricating them onto expensive membranes and proceeding with long-term durability tests, which are particularly critical to FCEVs employing large quantities of MEAs. In the past few decades, extensive research into MEA electrodes has focused on investigating the electrochemical characteristics such as catalyst activity,19-21 durability,22 and microstructural imaging23-25 under a variety of operating conditions. However, research on the mechanical


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behavior of MEA electrodes has been scarce even though most mechanical failures are associated with surface cracks, tears, and punctures of the MEA electrodes.26 Therefore, quantifying the mechanical properties of pristine electrodes is essential for the efficient and effective design of robust electrode structures. However, there have been no attempts to investigate the fundamental nature of the mechanical behavior of a pristine electrode itself through separating the electrode from its support, e.g. a decal transfer substrate, which would lead to a free-standing form. Attempts to achieve this have been hindered due to the following two critical difficulties: first, the pristine electrode with an intrinsically porous and brittle nature is too difficult to separate from its substrate without causing significant damage to the electrode; second, the pristine electrode is too fragile to be free-standing, which creates significant difficulties in mounting and testing it using the conventional testing methods. Thus, to date, only indirect measurements based on experimental-numerical hybrid techniques have been implemented in order to extract the mechanical properties of an electrode from those of a membrane and an MEA.27 Thus, despite its great necessity and importance, the mechanical behavior of pristine electrodes in a free-standing form has not been quantitatively investigated yet. In this paper, we present a novel approach that can be used to effectively separate a pristine electrode from the decal transfer substrate and to quantify the intrinsic mechanical properties of the pristine electrode in a free-standing form. First, a pristine electrode coated on a decal transfer substrate is frozen on a water surface, and the frozen electrode layer is separated from the decal transfer substrate through the maximized interaction force between the electrode and the ice. Then, the electrode is gradually thawed on the ice surface, which creates a free-standing electrode layer floating on the water surface. Finally, using the novel mechanical testing method


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proposed in our previous study,28 we quantitatively measure the intrinsic mechanical properties of the pristine electrode.

RESULTS AND DISCUSSION The standard ice-assisted separation method was performed as follows. First, the electrode side was floated on the deionized (DI) water (resistivity > 18 MΩ cm, Wide Ocean Water Solution Co., Korea) surface for 24 h in order to sufficiently soak the electrode with DI water (Figure 1A1D). Second, the soaked electrode was frozen for 6 h in a laboratory refrigerator at a temperature of –45 °C (Figure 1E). Third, the polyimide (PI) decal transfer substrate was manually separated from the frozen electrode using tweezers (Figure 1F). Finally, the separated electrode on the ice surface was kept intact at room temperature for 8 h in order for the ice to thaw into liquid (Figure 1G). Using the soaking-freezing-thawing process, the pristine electrode was separated from the PI decal transfer substrate without significant damage due to the mechanical and chemical characteristics of the ice-holding platform. The free-standing electrodes were considered to be intact or not significantly damaged when neither broken pieces of or cracks in the electrodes nor residues or traces of electrodes on the PI decal film surface were found via visual inspection after the soaking-freezing-thawing process (see Figure S1). In general, the PFSA ionomer consists of hydrophobic backbone chains of tetrafluoroethylene and hydrophilic side chains of perfluorosulfonate with sulfonic acid functional groups (SO3- H+). In the presence of water, the hydrophilic clusters with SO3- H+ can absorb a large quantity of water to form hydrated hydrophilic regions with hydronium ions of H3O+ or H·(H2O)x+, where x represents the number


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of water molecules that are hydrating the proton.29,30 It is known that the PFSA ionomer surface is hydrophobic when it is in contact with water vapor, but it becomes hydrophilic when in contact with liquid water.29-33 When liquid water approaches the PFSA ionomer surface, some hydrophilic sulfonic acid groups that are initially inside the PFSA ionomer move toward the liquid water. This causes the PFSA ionomer surface to become more hydrophilic; thus, more water absorption occurs when the liquid water wets the ionomer surface. Goswami et al. reported the restructuring of the PFSA Nafion® surface from hydrophobic to hydrophilic through exposure to liquid water.32 Thus, the hydrophobic PFSA ionomer surface became hydrophilic after soaking in the liquid water for 24 h, and this phenomenon facilitated the filling of liquid water into the pore regions of the electrode, as depicted in Figure 1B to 1D. The freezing point of water in the small pores of the electrodes or catalyst layers might become slightly decreased to approximately –1 °C due to the enhanced surface dynamics of the water.29,34,35 Because the electrode is fully wet in this study through floating it on the water surface, a sufficient amount of water appears on the surface of the PFSA ionomer in the electrode and the water in the porous electrodes mostly freezes at 0 °C or –1 °C, as described above. Therefore, the freezing temperature of –45 °C used in this study appears to be sufficiently low to freeze the liquid water in contact with the electrodes. Recently, Plazanet et al. reported that below 0 °C, i.e. between –93 °C and –3 °C, ice forms outside the PFSA Nafion® ionomer and it crystallizes in a hexagonal form.36 Pineri et al. also investigated the structure of waterswollen Nafion® ionomer membranes upon cooling to –70 °C and concluded that the water state at sub-zero temperatures is glassy inside the ionomer and ice crystals were only observed outside the ionomer, i.e. on the ionomer surface.37 Therefore, in this study, upon freezing at –45 °C, the mobile liquid water interacting with the sulfonic acid groups of PFSA ionomers in the electrodes


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appears to be immobilized in solid ice through strong hydrogen bonding,38 which causes the interaction force between the electrode and frozen water to be significantly higher than that between the electrode and PI decal transfer substrate. This enables the PI decal transfer substrate to be easily peeled from the frozen electrode. Novel quantitative information about the mechanical properties of the pristine electrodes would facilitate the structural design of more optimized electrodes with well-balanced properties through considering the potential trade-offs of performances in advance. Most previous works have been performed in order to gain a fundamental understanding of the effects of the ionomer content11,39-42 and Pt loading7,43-45 of electrodes on fuel cell performance, because these two factors are crucial to the optimization of fuel cell performances. In particular, it has been postulated that the mechanical properties of PEFC electrodes would generally improve with increases in the amount of ionomer binders even though it has not been quantitatively verified until this work. From the perspectives of the electrochemical performances of PEFCs, however, it is well known that optimum ionomer contents exist for PEFC electrodes, i.e. 30 wt%,39 33 wt%,40 or 20-50 wt%41 depending on the Pt loadings. For example, if the amount of PFSA ionomers in the electrodes is insufficient to fully connect the Pt/C catalysts particles, some Pt/C catalysts cannot be efficiently utilized to facilitate the electrochemical reactions in PEFCs, whereas if the amount of PFSA ionomers in the electrodes is too high, the electronic conduction pathways (i.e. Pt/C catalysts) and mass transport passages (i.e. pores) can be hampered or blocked by the presence of excessive ionomers and/or flooded water in the electrodes.11,39-42 Thus, this study focuses on quantitatively measuring the mechanical properties of the pristine electrodes as a function of the ionomer content with different Pt loadings (that is, electrode thickness) in the electrodes.


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Figure 2A is a photograph of the tensile testing equipment28 and Figure 2B depicts an electrode specimen floating on liquid water. DI water was used as an underlying support for the electrode, and all specimen preparation and testing procedures were performed on the water surface. The intrinsic characteristics of water, which has a high surface tension with a low viscosity, enable almost frictionless sliding and thus easy handling of the fragile fuel cell electrode floating on the water surface without significant damage or wrinkling. For the tensile testing, the floating electrode specimen was first gripped using a polydimethylsiloxane (PDMS)-coated Al grip as depicted in Figure 2C. This gripping technique using the van der Waals adhesion between the PDMS coating and specimen surface provided easy handling and a strong gripping force without slippage or delamination during the thin film tensile test. The tensile test was performed through applying the tensile force through the linear stage with a strain rate of approximately 6.0 × 10-5 s1

until a fracture occurred in the specimen, as depicted in Figure 2D.

During the tensile test, a digital image correlation (DIC) device and a high-resolution load cell (LTS-10GA, KYOWA, Japan) ensured accurate measurements of the real-time strain and load data, respectively. The average and standard deviation values of all mechanical properties were obtained through testing five electrode specimens per electrode formulation. The electrode thickness was measured using a sub-micron micrometer for each electrode specimen before being soaked on the DI water surface. It was found that the thicknesses of the electrodes were highly dependent on the Pt loadings, but that they were significantly less sensitive to the PFSA ionomer contents. For example, the average thicknesses of the electrodes with an ionomer content of 30 wt% with 0.05, 0.1, and 0.2 mg-Pt cm-2 were 2.3 ± 0.1, 3.9 ± 0.3, and 7.7 ± 0.2 µm, respectively. In order to quantify the tensile properties of the electrodes using swollen


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thicknesses, the degree of swelling of the electrodes was measured quantitatively as described in more detail in Methods Section. Figure 2E to 2G present the representative stress-strain curves of the electrodes with a Pt loading of 0.2 mg-Pt cm-2 having PFSA ionomer content of 20, 30, and 40 wt%, respectively. It was observed that all stress-strain curves of the electrodes exhibited a substantially linear elastic behavior with little plastic deformation, and the values of the elongation at the break were significantly less than 1%, which indicates typical brittle fracture behavior. In order to examine the fracture behavior of the electrodes more closely, the fractured electrode specimens were gently placed on a small piece of silicon wafer surface, followed by observation of the fractured surface morphology using a scanning electron microscope (SEM, FE-SEM Sirion Model, FEI Company, USA). Each inset in Figure 2E to 2G demonstrates that the fractured surfaces did not exhibit necking or signs of ductile failure. With increasing the ionomer content from 20 to 40 wt%, it was observed that the Young’s modulus (E) of the electrode with 0.2 mg-Pt cm-2 Pt loading increased by 2.4 times from 75 ± 11 to 177 ± 11 MPa, the elongation at the break increased by 1.5 times from 0.37 ± 0.04 to 0.57 ± 0.05 %, and the tensile stress at the break also increased by 3.5 times from 251 ± 62 to 870 ± 79 kPa. Similar trends were also observed for the other electrodes with 0.05 and 0.1 mg-Pt cm-2 Pt loadings (Figure 3A-C). It was clearly observed that all mechanical properties of the pristine electrodes in free-standing form were significantly enhanced with the ionomer content in the electrodes. For all electrode samples with 0.05–0.2 mg-Pt cm-2 Pt loadings, doubling the ionomer content in the electrodes increased not only the tensile stress at the break and the Young’s modulus of the electrodes by approximately 2.1–3.5 and 1.7–2.4 times respectively, but also the elongation at the break by approximately 1.5–1.7 times. This quantitatively validates that stronger, stiffer, and tougher


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electrodes were obtained as the ionomer content increased, which have been of significant interest in materials research fields.46,47 This result is primarily attributed to the polymeric nature, i.e. connectivity of the polymer chains, of the PFSA ionomer binder, which normally provides electrodes with the capability to bind Pt/C catalysts and ionomers in order to withstand external tensile forces, whereas individual Pt/C catalyst particles cannot resist it. The PFSA ionomers in electrodes generally have crucial functions in determining the fuel cell performances and durability because they serve as proton conductors to allow proton transfer and binders to improve the mechanical stability and robustness through holding the Pt/C particles together.11,3942,48

As schematically illustrated in Figure 1B and 3D, it appears reasonable to assume that the

ionomer binders surrounded the carbon-supported Pt catalyst and linked these particles securely.7,15,16,24,25 Recently, an electron tomography analysis revealed that doubling the content of the Nafion® ionomer in the electrodes results in a twofold increase in its degree of coverage of the carbon, while the average thickness of the ionomer layer of approximately 7 nm is not significantly changed.25 Therefore, as depicted in Figure 3D, fuel cell electrodes with higher ionomer content (i.e. 40 wt%) are likely to have more interconnected Pt/C structures due to the larger coverage of the ionomers in the electrodes than those with lower ionomer content (i.e. 20 wt%) have. Thus, when the electrodes are strained by an external stress, the electrodes with higher ionomer content could resist more against the external stress due to the larger number of ionomer binder links than those with lower ionomer content, which leads to increase in both mechanical strength and elongation at the break. The PEFC electrodes exhibit a unique combination of several structural features7,15,16,24,39-42 that differ from other conventional materials in that the electrodes form a unique structure composed of complex multi-components such as Pt/C catalysts, PFSA ionomer


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binders, and pores. In general, it is reasonable to assume that larger amounts of PFSA ionomers in porous electrodes have a critical function in improving the mechanical properties of the electrodes as follows: on the macroscopic level, a larger amount of PFSA ionomers might provide electrodes with a larger number of binder links to interconnect the Pt/C catalyst particles (see Figure 3D) and also result in electrodes with less porous structures through filling the pores,11,40 which can function as defective points in the electrodes; on the microscopic or molecular level, by using various models of ionomer structures, i.e. ionic cluster model,49 elongated polymer aggregates model,50 bundle-cluster model,51 etc.,52-54 it can be reasonably proposed that a larger amount of PFSA ionomers in the electrodes results in a larger amount of ionic clusters that can function as physical cross-links between the polymer backbones,49,51-54 and thus this might contribute to the increased stiffness (i.e. Young’s modulus) of the electrodes at low strains. Furthermore, it is also conceivable that larger amounts of PFSA ionomers could enable the electrodes to have more bundles of hydrophobic polymer aggregates that can function as pseudosprings and might rotate at low strains before yielding, which also contributes to the increased stiffness.50,51 In contrast, as the strain continues to increase after the initial elastic deformation region during the tensile test, the larger amount of water, which can function as a plasticizer at room temperature and high RH,52-57 in the ionic clusters of the electrodes with a larger amount of PFSA ionomers appears to contribute to the increased elongation at the break, which leads to tougher electrodes. In addition, the electrodes with larger amounts of PFSA ionomers have larger amounts of hydrophobic polymer aggregates that can be better aligned within the bundles at larger strains,50,51 which facilitates reorientations, realignments, or disentanglements of the polymer aggregates and thus they last longer during tensile tests. These might also contribute to the increased elongation at the break of the electrodes.


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The Young’s moduli of many porous materials decrease significantly with decreasing density (ρ) (or with increasing porosity),58-62 exhibiting a power-law scaling behavior with a scaling exponent (n). For example, the Young’s moduli scale at ρ3.0 ~ 3.6 for carbon-based foams and highly porous dried gels (i.e. aerogels),59,60,62 while those scale at ρ2.0 for many natural cellular solid materials.60,61 The scaling law relationships are attributed to the fact that for highly porous materials with initially less interconnected structures, such as dried aerogels, a greater portion of the structure becomes load-bearing parts as the density increases, compared with that of cellular solid materials.60 This could be problematic for practical applications because if the mechanical properties of the porous materials of interest are too sensitive to the density variation, it would cause difficulty in maintaining uniform quality. Therefore, in order to design a highly robust structure for PEFC electrodes, it is critical to have an insight into the power-law scaling behavior between Young’s modulus and the electrode density. In Figure 4, for all electrode specimens investigated here, it was found that the Young’s modulus scales at ρ1.6 which indicates that the Young’s modulus of the PEFC electrodes, whose major component is also carbon, is less sensitive to the density variation than that of carbon-based foams or natural cellular solids. It is noteworthy that there are some examples that demonstrate the relationship of E ~ ρ1.1 due to their isotropic structures with high structural connectivity, as in the case of ultralight mechanical metamaterials,58 which indicates that the Young’s modulus of the PEFC electrodes should be more sensitive to density variations than that of these ultralight materials. Therefore, quantitative information on the relative sensitivity of the Young’s modulus of the PEFC electrodes to the density in comparison with a variety of existing materials might be useful in facilitating advanced designs of mechanically robust electrodes.


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A trend can be inferred from these examples: more connected and ordered microstructures result in decreasing the scaling exponent, which leads to less sensitive physical structures and mechanical properties with the variation of density. Furthermore, no significant macro-scale structure change was noticed in the electrodes with ionomer content of 20 and 40 wt%, as seen in the electrode’s cross-sectional images observed using the focused ion beam-scanning electron microscopy (FIB-SEM, Helios Nanolab 450 F1, FEI Company, USA) (see Figure 4, inset), which indicates that the ionomers primarily provide linkages between Pt/C catalysts without causing significant structural alteration. The present results of the scaling law behavior for the mechanical properties obtained using the well-known standard electrode materials (i.e. Pt/C catalyst and Nafion® ionomer binder) can be used as a benchmark and should be useful in optimizing the design of electrodes and MEAs for a variety of PEFC applications such as FCEVs.

CONCLUSION In conclusion, we have developed a novel approach to effectively separate pristine electrodes from decal transfer substrates using a soaking-freezing-thawing process on the water surface, and quantifying the intrinsic mechanical properties of the pristine electrode floating on the water surface in a free-standing form. Water is effective for this purpose due to its high interaction force with the PFSA ionomers in the electrodes as well as the physical interlocking via freezing. Furthermore, the water surface provides an ideal tensile testing platform for the fragile fuel cell electrodes due to its high surface tension with low viscosity, which enables almost frictionless sliding. The mechanical properties of the electrode exhibited an enhanced Young’s modulus, elongation at the break, and tensile stress at the break with increasing ionomer content in all Pt


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loading conditions. The scaling law relationship of E ~ ρ1.6 between Young’s modulus and density was also revealed, which indicates the less sensitive structures to the density variations than other materials such as carbon-based foams and aerogels. Even though the present study focuses mainly on the development of a novel methodology to quantify the mechanical properties of the pristine electrodes, our findings can be used in the study of the degradation of the mechanical properties of the electrodes upon ageing through freeze-thaw and dry-wet cycles as well as development of mechanically robust electrode design for energy conversion applications.

EXPERIMENTAL SECTION Preparation of electrode samples. Electrode samples coated on the decal transfer substrate were prepared according to the methods commonly described in the literature.14,15 Catalyst inks were prepared through mixing a carbon-supported Pt catalyst (Pt/C: HISPEC4000®, 40 wt% Pt/Vulcan XC-72R powder, Johnson Matthey, UK), PFSA Nafion® ionomer dispersion (Nafion® D2021, 20 wt% dispersion, equivalent weight = 1100, DuPont, USA), DI water (resistivity = 18.2 MΩ cm, Millipore Cor., USA), and isopropyl alcohol (Duksan Pure Chemicals Co., Korea). The solid content in the catalyst inks was maintained at approximately 10 wt%. The ionomer contents in the electrodes were 20, 30, 35, and 40 wt% with respect to the total solid content of the dried electrode (i.e. Pt/C + ionomer), which were the most commonly used contents in the literature.11,39-42 The catalyst inks were stirred at 250 rpm with a stirrer (WiseStir®, Daihan Scientific Co., Korea) for 24 h at room temperature. Ultrasonication was performed for 1 h three times a day.


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PI (Kapton® HN, 25.4 µm thickness, DuPont, USA) film was used as the decal transfer substrate. The mixed catalyst inks were coated at 3 cm s-1 on the PI decal transfer substrate using a spreading apparatus composed of a micrometric film applicator (Elcometer® 3570, Elcometer Co., UK). The Pt loadings in the electrodes were adjusted to be 0.05, 0.1, and 0.2 mg-Pt cm-2 through changing the coating thickness of the catalyst inks. The electrodes coated on the PI decal transfer substrates were dried at room temperature for 4 h followed by oven drying at 80 °C for 2 h. Then, the electrodes on the PI decal transfer substrates were stored at room temperature until the separation test on the water surface. Separation of electrodes using the soaking-freezing-thawing process. Prior to the separation process, the electrodes coated on the PI decal transfer substrate were cut with a very sharp razor blade into smaller rectangular specimens with a width of 5 mm and length of 25 mm. The separation of the electrodes coated on the PI decal transfer substrates was performed using consecutive soaking, freezing, and thawing processes on the water surface. During the soaking process, the electrode on the PI decal transfer substrate was carefully placed on the water surface in order to minimize the amount of air trapped between the electrode and water surface. If a large amount of air was trapped, it could cause irregular crater formations on the ice surface after freezing, which would consequently lead to failed separation on that part of the electrode. However, it should be noted that a small amount of air does not result in failed separation because the amount of air trapped decreases gradually during the soaking process. The freezing process was performed at -45 °C in the freezer section of a commercial refrigerator (FD-170-SF, UNIQUE, Korea). The thawing process was undertaken through leaving the frozen electrode intact at room temperature for 8 h to ensure complete thawing. All experimental results in this


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study were obtained using the thawing process for 8 h, although the electrodes did not deteriorate while floating on water for several days. Measurement of electrode thickness. Before soaking the electrode on the water surface, the total ‘dry’ thickness of the electrode coated on the PI decal transfer substrate was measured at two points using a High-Accuracy Sub-Micron Digimatic Micrometer (MDH-25M, Mitutoyo, Japan), which can measure at the submicron level. After separation of the electrode from the PI decal transfer substrate via freezing, the thickness of the PI decal transfer substrate was measured. Thus, the ‘dry’ thickness of the pristine electrode was calculated through subtracting the thickness of the PI decal transfer substrate from the total thickness of the electrode coated on the PI decal transfer substrate. In order to calculate the tensile properties of the freestanding electrodes using the ‘wet’ thicknesses of the electrodes, independent tests were performed to measure the degree of swelling of the electrodes on water surface after the soaking time of 24336 h. For the tests, four electrodes with ionomer contents of 20, 30, 35, and 40 wt% with a constant Pt loading of 0.2 mg-Pt cm-2 were used as representative examples. The total thicknesses of the electrodes coated on the PI decal transfer substrate soaked on the liquid DI water surface were measured at six measurement points per electrode. When the thickness measurements were completed after the soaking time of 336 h, the electrode was separated from the PI decal transfer substrate via freezing according to the abovementioned method; then, the thicknesses of the PI substrate were measured at the same six measurement points. Thus, the thickness of the free-standing electrode after the soaking processes was calculated through subtracting the thickness of the PI decal transfer substrate from the total thickness of the electrode coated on the PI decal transfer substrate, and the percentages of thickness variation were reported. It was observed that after soaking for 24 h, the thicknesses of the electrodes with


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20, 30, 35, and 40 wt% ionomers increased to approximately 2%, 3%, 5%, and 6%, respectively. Then, the thicknesses of all electrodes appeared to plateau until 336 h. Therefore, the ‘wet’ thickness of an electrode after soaking was estimated by applying the corresponding swelling percentage to the ‘dry’ thickness of the electrode and then used to calculate the tensile properties of the electrodes throughout this study.


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Figure 1. Schematic of the ice-assisted separation method. (A) Soaking an electrode floating on DI water surface. (B-D) A detailed illustration of the soaking process. The electrode upon contact with DI water before soaking (B), after soaking for 24 h (C), and an ionomer surface contacting with water (D). (E) Freezing the soaked electrode at a temperature of –45 °C. (F) Separating the PI decal transfer substrate from the frozen electrode. (G) The pristine fuel cell electrode on a water surface after thawing.


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Figure 2. Schematic of the tensile testing process and tensile behavior of 0.2 mg-Pt cm-2 electrodes. (A) A photograph of the tensile testing equipment. (B) A separated electrode specimen floating on water. (C) A separated electrode attached to a PDMS-coated Al grip prior to the tensile test. (D) A fractured electrode after the tensile test. (E-G) Stress-strain curves of electrodes with the ionomer content of 20, 30, and 40 wt%, respectively. The insets are images of the fractured surface of the electrodes by SEM (scale bar: 5 µm).


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Figure 3. Mechanical properties and behavior mechanism of the pristine electrodes as a function of the ionomer content with different Pt loadings. (A) Young’s modulus, (B) elongation at break, (C) tensile stress at break of the electrodes with the ionomer contents of 20, 30, 35 and 40 wt% and the Pt loadings of 0.05, 0.1, and 0.2 mg-Pt cm-2. (D) The role of ionomer binders in enhancing the mechanical properties the electrodes. The ionomer binder provides more mechanical linkages between Pt/C particles in higher ionomer content.


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Figure 4. Modulus-density scaling relationships of fuel cell electrode and other materials. The scaling relationship of fuel cell electrode is described by E ~ ρ1.6. The insets are cross-sectional images of fuel cell electrodes with ionomer content of 20 and 40 wt% by FIB-SEM (scale bar: 1 µm).

.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Photographs of the free-standing electrode on water surface, delaminated PI decal film, and bare decal film (PDF)

AUTHOR INFORMATION Corresponding Author *(B.K.H.) E-mail: [email protected]. *(T.-S.K.) E-mail: [email protected]. Author Contributions § S. K. and J.-H.K. contributed equally.

ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031569), the Basic Science Research Program (2015R1A1A1A05001115) of the National Research Foundation under the Ministry of Science, ICT & Future Planning of Korea, the High-Risk and High-Return (HRHR) project from KAIST, and the R&D Collaboration Program (R-164691.0001) of Hyundai Motor Company.

REFERENCES


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345-352. 2. Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. 3. Bashyam, R.; Zelenay, P. A Class of Non-Precious Metal Composite Catalysts for Fuel Cells. Nature 2006, 443, 63-66. 4. Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107, 3904-3951. 5. Han, K.; Hong, B. K.; Kim, S. H.; Ahn, B. K.; Lim, T. W. Influence of Anisotropic Bending Stiffness of Gas Diffusion Layers on the Degradation Behavior of Polymer Electrolyte Membrane Fuel Cells Under Freezing Conditions. Int. J. Hydrogen Energy 2011, 36, 12452-12464. 6. Kim, S.; Mench, M. Physical Degradation of Membrane Electrode Assemblies Undergoing Freeze/Thaw Cycling: Micro-Structure Effects. J. Power Sources 2007, 174, 206-220. 7. Litster, S.; McLean, G. PEM Fuel Cell Electrodes. J. Power Sources 2004, 130, 61-76. 8. Mehta, V.; Cooper, J. S. Review and Analysis of PEM Fuel Cell Design and Manufacturing. J. Power Sources 2003, 114, 32-53. 9. Hu, M.; Sui, S.; Zhu, X.; Yu, Q.; Cao, G.; Hong, X.; Tu, H. A 10kW Class PEM Fuel Cell Stack Based on the Catalyst-Coated Membrane (CCM) Method. Int. J. Hydrogen Energy 2006, 31, 1010-1018.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

10. Sun, L.; Ran, R.; Shao, Z. Fabrication and Evolution of Catalyst-Coated Membranes by Direct Spray Deposition of Catalyst Ink Onto Nafion Membrane at High Temperature. Int. J. Hydrogen Energy 2010, 35, 2921-2925. 11. Kim, K.-H.; Lee, K.-Y.; Kim, H.-J.; Cho, E.; Lee, S.-Y.; Lim, T.-H.; Yoon, S. P.; Hwang, I. C.; Jang, J. H. The Effects of Nafion® Ionomer Content in PEMFC MEAs Prepared by a Catalyst-Coated Membrane (CCM) Spraying Method. Int. J. Hydrogen Energy 2010, 35, 2119-2126. 12. Wilson, M. S.; Valerio, J. A.; Gottesfeld, S. Low Platinum Loading Electrodes for Polymer Electrolyte Fuel Cells Fabricated Using Thermoplastic Ionomers. Electrochim. Acta 1995, 40, 355-363. 13. Chun, Y.-G.; Kim, C.-S.; Peck, D.-H.; Shin, D.-R. Performance of a Polymer Electrolyte Membrane Fuel Cell with Thin Film Catalyst Electrodes. J. Power Sources 1998, 71, 174-178. 14. Cho, H. J.; Jang, H.; Lim, S.; Cho, E.; Lim, T.-H.; Oh, I.-H.; Kim, H.-J.; Jang, J. H. Development of a Novel Decal Transfer Process for Fabrication of High-Performance and Reliable Membrane Electrode Assemblies for PEMFCs. Int. J. Hydrogen Energy 2011, 36, 12465-12473. 15. Suzuki, T.; Tsushima, S.; Hirai, S. Effects of Nafion® Ionomer and Carbon Particles on Structure Formation in a Proton-Exchange Membrane Fuel Cell Catalyst Layer Fabricated by the Decal-Transfer Method. Int. J. Hydrogen Energy 2011, 36, 1236112369. 16. Jeon, S.; Lee, J.; Rios, G. M.; Kim, H.-J.; Lee, S.-Y.; Cho, E.; Lim, T.-H.; Hyun Jang, J. Effect of Ionomer Content and Relative Humidity on Polymer Electrolyte Membrane


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fuel Cell (PEMFC) Performance of Membrane-Electrode Assemblies (MEAs) Prepared by Decal Transfer Method. Int. J. Hydrogen Energy 2010, 35, 9678-9686. 17. Wu, B.; Zhao, M.; Shi, W.; Liu, W.; Liu, J.; Xing, D.; Yao, Y.; Hou, Z.; Ming, P.; Gu, J.; Zou, Z. The Degradation Study of Nafion/PTFE Composite Membrane in PEM Fuel Cell Under Accelerated Stress Tests. Int. J. Hydrogen Energy 2014, 39, 14381-14390. 18. Li, Y.; Dillard, D.; Case, S.; Ellis, M.; Lai, Y.; Gittleman, C.; Miller, D. Fatigue and Creep to Leak Tests of Proton Exchange Membranes Using Pressure-Loaded Blisters. J. Power Sources 2009, 194, 873-879. 19. Genorio, B.; Strmcnik, D.; Subbaraman, R.; Tripkovic, D.; Karapetrov, G.; Stamenkovic, V. R.; Pejovnik, S.; Marković, N. M. Selective Catalysts for the Hydrogen Oxidation and Oxygen Reduction Reactions by Patterning of Platinnum with Calix[4]Arene Molecules. Nat. Mater. 2010, 9, 998-1003. 20. Greeley, J.; Stephens, I.; Bondarenko, A.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552-556. 21. Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum-Cobalt Core-Shell Nanoparticles

with

Enhanced

Activity

and

Stability

as

Oxygen

Reduction

Electrocatalysts. Nat. Mater. 2013, 12, 81-87. 22. Liu, Z.; Brady, B.; Carter, R.; Litteer, B.; Budinski, M.; Hyun, J.; Muller, D. Characterization of Carbon Corrosion-Induced Structural Damage of PEM Fuel Cell Cathode Electrodes Caused by Local Fuel Starvation. J. Electrochem. Soc. 2008, 155, B979-B984.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

23. Xin, H. L.; Mundy, J. A.; Liu, Z.; Cabezas, R.; Hovden, R.; Kourkoutis, L. F.; Zhang, J.; Subramanian, N. P.; Makharia, R.; Wagner, F. T. Atomic-Resolution Spectroscopic Imaging of Ensembles of Nanocatalyst Particles Across the Life of a Fuel Cell. Nano Lett. 2012, 12, 490-497. 24. Epting, W. K.; Gelb, J.; Litster, S. Resolving the Three-Dimensional Microstructure of Polymer Electrolyte Fuel Cell Electrodes using Nanometer-Scale X-ray Computed Tomography. Adv. Funct. Mater. 2012, 22, 555-560. 25. Lopez-Haro, M.; Guétaz, L.; Printemps, T.; Morin, A.; Escribano, S.; Jouneau, P.-H.; Bayle-Guillemaud, P.; Chandezon, F.; Gebel, G. Three-Dimensional Analysis of Nafion Layers in Fuel Cell Electrodes. Nat. Commun. 2014, 5, 5229. 26. Luo, Z.; Li, D.; Tang, H.; Pan, M.; Ruan, R. Degradation Behavior of MembraneElectrode-Assembly Materials in 10-Cell PEMFC Stack. Int. J. Hydrogen Energy 2006, 31, 1831-1837. 27. Lu, Z.; Santare, M.; Karlsson, A.; Busby, F.; Walsh, P. Time-Dependent Mechanical Behavior of Proton Exchange Membrane Fuel Cell Electrodes. J. Power Sources 2014, 245, 543-552. 28. Kim, J.-H.; Nizami, A.; Hwangbo, Y.; Jang, B.; Lee, H.-J.; Woo, C.-S.; Hyun, S.; Kim, T.-S. Tensile Testing of Ultra-Thin Films on Water Surface. Nat. Commun. 2013, 4, 2520. 29. Jiao, K.; Li, X. Water Transport in Polymer Electrolyte Membrane Fuel Cells. Prog. Energy Combust. Sci. 2011, 37, 221-291. 30. Li, X. Principles of Fuel Cells; Taylor & Francis, New York, 2006.


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31. Zawodzinski, T. A.; Derouin, C.; Radzinski, S.; Sherman, R. J.; Smith, V. T.; Springer, T. E.; Gottesfeld, S. Water Uptake by and Transport Through Nafion® 117 Membranes. J. Electrochem. Soc. 1993, 140, 1041-1047. 32. Goswami, S.; Klaus, S.; Benziger, J. Wetting and Absorption of Water Drops on Nafion Films. Langmuir 2008, 24, 8627-8633. 33. Majsztrik, P. W.; Satterfield, M. B.; Bocarsly, A. B.; Benziger, J. B. Water Sorption, Desorption and Transport in Nafion Membranes. J. Membr. Sci. 2007, 301, 93-106. 34. Jiao, K.; Li, X. Effects of Various Operating and Initial Conditions on Cold Start Performance of Polymer Electrolyte Membrane Fuel Cells. Int. J. Hydrogen Energy 2009, 34, 8171-8184. 35. Ge, S.; Wang, C.-Y. Characteristics of Subzero Startup and Water/Ice Formation on the Catalyst Layer in a Polymer Electrolyte Fuel Cell. Electrochim. Acta 2007, 52, 48254835. 36. Plazanet, M.; Sacchetti, F.; Petrillo, C.; Demé, B.; Bartolini, P.; Torre, R. Water in a Polymeric Electrolyte Membrane: Sorption/Desorption and Freezing Phenomena. J. Membr. Sci. 2014, 453, 419-424. 37. Pineri, M.; Gebel, G.; Davies, R. J.; Diat, O. Water Sorption-Desorption in Nafion® Membranes at Low Temperature, Probed by Micro X-Ray Diffraction. J. Power Sources 2007, 172, 587-596. 38. Stillinger, F. H. Water Revisited. Science 1980, 209, 451-457. 39. Qi, Z.; Kaufman, A. Low Pt Loading High Performance Cathodes for PEM Fuel Cells. J. Power Sources 2003, 113, 37-43.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

40. Passalacqua, E.; Lufrano, F.; Squadrito, G.; Patti, A.; Giorgi, L. Nafion Content in the Catalyst Layer of Polymer Electrolyte Fuel Cells: Effects on Structure and Performance. Electrochim. Acta 2001, 46, 799-805. 41. Sasikumar, G.; Ihm, J.; Ryu, H. Dependence of Optimum Nafion Content in Catalyst Layer on Platinum Loading. J. Power Sources 2004, 132, 11-17. 42. Xie, J.; Xu, F.; Wood III, D. L.; More, K. L.; Zawodzinski, T. A.; Smith, W. H. Influence of Ionomer Content on the Structure and Performance of PEFC Membrane Electrode Assemblies. Electrochim. Acta 2010, 55, 7404-7412. 43. Eom, K.; Kim, G.; Cho, E.; Jang, J. H.; Kim, H.-J.; Yoo, S. J.; Kim, S.-K.; Hong, B. K. Effects of Pt Loading in the Anode on the Durability of a Membrane-Electrode Assembly for Polymer Electrolyte Membrane Fuel Cells During Startup/Shutdown Cycling. Int. J. Hydrogen Energy 2012, 37, 18455-18462. 44. Gasteiger, H.; Panels, J.; Yan, S. Dependence of PEM Fuel Cell Performance on Catalyst Loading. J. Power Sources 2004, 127, 162-171. 45. Ahn, S. H.; Jeon, S.; Park, H.-Y.; Kim, S.-K.; Kim, H.-J.; Cho, E.; Henkensmeier, D.; Yoo, S. J.; Nam, S. W.; Lim, T.-H. Effects of Platinum Loading on the Performance of Proton Exchange Membrane Fuel Cells with High Ionomer Content in Catalyst Layers. Int. J. Hydrogen Energy 2013, 38, 9826-9834. 46. Ritchie, R. O. The Conflicts between Strength and Toughness. Nat. Mater. 2011, 10, 817-822. 47. Bouville, F.; Maire, E.; Meille, S.; Van de Moortèle, B.; Stevenson, A. J.; Deville, S. Strong, Tough and Stiff Bioinspired Ceramics from Brittle Constituents. Nat. Mater. 2014, 13, 508-514.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


48. Xie, J.; Garzon, F.; Zawodzinski, T.; Smith, W. Ionomer Segregation in Composite MEAs and its Effect on Polymer Electrolyte Fuel Cell Performance. J. Electrochem. Soc. 2004, 151, A1084-A1093. 49. Hsu, W.; Gierke, T. Ion Transport and Clustering in Nafion Perfluorinated Membranes. J. Membr. Sci. 1983, 13, 307-326. 50. van der Heijden, P.; Rubatat, L.; Diat, O. Orientation of Drawn Nafion at Molecular and Mesoscopic Scales. Macromolecules 2004, 37, 5327-5336. 51. Liu, D.; Kyriakides, S.; Case, S.; Lesko, J.; Li, Y.; McGrath, J. Tensile Behavior of Nafion and Sulfonated Poly(arylene ether sulfone) Copolymer Membranes and its Morphological Correlations. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1453-1465. 52. Kundu, S.; Simon, L.; Fowler, M.; Grot, S. Mechanical Properties of NafionTM Electrolyte Membranes under Hydrated Conditions. Polymer 2005, 46, 11707-11715. 53. Shi, S.; Liu, D.; Liu, D.; Tae, P.; Gao, C.; Yan, L.; An, K.; Chen, X. Mechanical Properties and Microstructure Changes of Proton Exchange Membrane under Immersed Conditions. Polym. Eng. Sci. 2014, 54, 2215-2221. 54. Kawano, Y.; Wang, Y.; Palmer, R.; Aubuchon, S. Stress-Strain Curves of Nafion Membranes in Acid and Salt Forms. Polím.: Cienc. Technol. 2002, 12, 96-101. 55. Satterfield, M.; Majsztrik, P.; Ota, H.; Benziger, J.; Bocarsly, A. Mechanical Properties of Nafion and Titania/Nafion Composite Membranes for Polymer Electrolyte Membrane Fuel Cells. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2327-2345. 56. Majsztrik, P.; Bocarsly, A.; Benziger, J. Viscoelastic Response of Nafion. Effects of Temperature and Hydration on Tensile creep. Macromolecules 2008, 41, 9849-9862.


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Page 30 of 31

57. Satterfield, M.; Benziger, J. Viscoelastic Properties of Nafion at Elevated Temperature and Humidity. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 11-24. 58. Zheng, X.; Lee, H.; Weisgraber, T.; Shusteff, M.; DeOtte, J.; Duoss, E.; Kuntz, J.; Biener, M.; Ge, Q.; Jackson, J.; Kucheyev, S.; Fang, N.; Spadaccini, C. Ultralight, Ultrastiff Mechanical Metamaterials. Science 2014, 344, 1373-1377. 59. Pekala, R.; Alviso, C.; LeMay, J. Organic Aerogels: Microstructural Dependence of Mechanical Properties in Compression. J. Non-Cryst. Solids 1990, 125, 67-75. 60. Fan, H.; Hartshorn, C.; Buchheit, T.; Tallant, D.; Assink, R.; Simpson, R.; Kissel, D.; Lacks, D.; Torquato, S.; Brinker, C. Modulus-Density Scaling Behaviour and Framework Architecture of Nanoporous Self-Assembled Silicas. Nat. Mater. 2007, 6, 418-423. 61. Gibson, L.; Ashby, M. Cellular Solids: Structure and Properties; Cambridge University Press, 1997. 62. Qiu, L.; Liu, J.; Chang, S.; Wu, Y.; Li, D. Biomimetic Superelastic Graphene-Based Cellular Monoliths. Nat. Commun. 2012, 3, 1241.


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Table of Contents Graphic Peeling

Ice

Ice Thawing

Water

Water Tensile Test

Water


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Mechanical Behavior of Free-Standing Fuel Cell Electrodes on

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