Use In Polymer Electrolyte Fuel Cells Research Articles

Development of PEFC Low Pt-Loading Graphene Catalyst Layer By Electrospray Method for Increasing Output Power

Polymer electrolyte fuel cells (PEFCs) have various positive aspects for achieving carbon neutrality because carbon dioxide is not emitted during operation. However, there are several issues for practical use of PEFCs. One of the problems is its high cost due to the use of Platinum (Pt) for catalyst. Our previous study tried to reduce the amount of Pt in PEFC catalyst layer (CL) and realized low Pt-loading CL by using graphene as Pt support material (1). The graphene CL did not contain ionomer which prevent oxygen transport to the Pt surface. Therefore, we considered that graphene CL realizes smooth oxygen transport. However it was found that the proton transport resistance of graphene CL was about ten times that of CL containing ionomer (2). Since the proton transport resistance depended on the CL thickness, present study employed an electrospray (ES) method to fabricate thin CL. If the proton resistance in graphene CL is reduced, Pt can be used more efficiently. In this study, graphene CLs with various Pt-loading (mg/cm2) were fabricated by decal transfer method, gas diffusion electrode (GDE) method or ES method. The graphene CL was used in the cathode side. The performance and the proton resistance of fabricated CL were respectively evaluated by I-V measurement and impedance measurement. These were also compared with fabricated CLs using commercial Pt-supported Ketjen Blacks (KB/Pt CL). At the anode side, the CL with the Pt loading 0.20mg/cm2 and I/C 0.8 was used. Operation temperature was 80℃, relative humidity (RH) of supply gases was 80%, the flow rate of cathode gas was 2000 sccm with 1% oxygen concentration, and the flow rate of anode gas is 100 sccm. Relationship between fabrication method and the I-V performance was investigated. Figure 1 shows the I-V performance of KB/Pt CL with different fabrication method at 1% oxygen concentration. The CLs with the same Pt-loading (0.047mg/cm2) but different fabrication methods (transfer, ES) show similar I-V performance. This result indicates that fabrication method does not affect the I-V performance in case of KB/Pt CL. Figure 2 shows the I-V performance of graphene CL with different fabrication method at 1% oxygen concentration. The performance of graphene CL fabricated by ES method was higher than that with GDE method despite similar Pt-loading and proton resistance (Figure 3). In case of graphene CL, the structure is considered to depend on the fabrication method. The CL structure fabricated by ES method can improves water and oxygen transport in case of graphene CL. Figure 3 shows the proton transport resistance of the fabricated catalyst layers. The proton resistance is reduced in the smaller amount of Pt-loading CL. Additionally, the Pt-loading in graphene CL was proportional to the thickness of the CL because the weight of Pt relative to the that of graphene was constant (17.2wt%). Therefore, it was suggested that the proton resistance was reduced by fabricating a thinner catalyst layer.Comparing the I-V performance of KB/Pt CL 0.047mg/cm2 (Figure 1) and graphene CL 0.01mg/cm2 (Figure 2), the graphene CL shows high performance even though the Pt weight was about 1/5. These results indicate that fabrication of thin graphene CL with ES method enables to use Pt effectively. Higher performance of graphene CL was also confirmed under higher oxygen concentration condition. Figure 4 shows the I-V performance of graphene CL with ES (Pt-loading: 0.01mg/cm2) and KB/Pt CL with transfer method (Pt-loading: 0.2mg/cm2) at 21% oxygen concentration. Both of limiting current density were over 1 A/cm2. Even though the difference in Pt-loading was 1/20, the difference in I-V performance was small.References(1) K.Yada, et.al, ECS , #101A-1389, (2019).(2) M.Okano, et al., The 25th National Symposium on Power and Energy, C223(2021)(in Japanase). Figure 1

(Digital Presentation) Investigation of the Active Site on Rhodium Oxide for the Oxygen Reduction Reaction Using In-Situ XAFS

Polymer electrolyte fuel cells (PEFCs) are attracting attention as a next-generation power source with low environmental impact. However, the low durability and high cost of Pt-based catalysts used as electrocatalysts have hindered the widespread use of PEFCs. Currently, many studies are being conducted to improve the durability of Pt-based catalysts, such as alloying, core-shelling, and the use of oxide supports. Although these studies have achieved certain results, the high cost of raw materials cannot be avoided as long as Pt, which is a noble metal, is used to some extent. Therefore, research on non-Pt catalysts that do not use Pt is attracting attention as the next generation of electrocatalysts.Non-Pt catalysts can be broadly classified into oxide-based and carbon-based catalysts, but considering the start-up and shutdown potentials of PEFCs, oxide-based non-Pt catalysts are expected to show higher durability. It is known that the introduction of oxygen-deficient sites in the oxides of Group 4 and Group 5 elements in the oxide-based non-Pt catalysts results in a high oxygen reduction reaction rate (ORR). Therefore, oxygen-deficient sites are considered to be active sites for ORR, and the introduction of oxygen-deficient sites in the crystal lattice is a guideline for catalyst design in non-Pt catalysts based on oxides. On the other hand, it has been reported that noble metal oxides show high ORR activity even without clear oxygen-deficient sites. It is easy to imagine the hypothesis that the ORR proceeds by the adsorption of oxygen molecules on the oxygen-deficient sites, but the same hypothesis holds true for low-coordinated metal atoms on the oxide surface without the introduction of oxygen-deficient sites. Furthermore, it has been reported that in alkaline solution, oxygen atoms in the perovskite structure are the active sites. These discrepancies are due to the fact that the ORR mechanism on oxides is still unknown.In this study, in-situ XAFS measurements were performed to identify the active sites of ORR on oxides, and changes in the chemical states of metal elements during the ORR were investigated from changes in XANES spectra. A noble metal oxide (Rh2O3), whose valence changes with potential, was used as the target oxide.The target oxide, Rh2O3, was prepared by the Adams method. The prepared sample was placed on a glassy carbon plate at 100 g cm-2 and placed in a tripolar electrolytic cell for in-situ XAFS. The in-situ XAFS cell was designed to detect X-ray fluorescence by irradiating X-rays from the back of the sample. Energy of the incident X-ray was set near the Rh K edge. In-situ XAFS measurements were performed at five points: 0.4 V, where the ORR is in progress; 0.6 V and 0.8 V, near the ORR starting potential; 1.0 V and 1.2 V, where the ORR is not in progress. To investigate the effect of oxygen partial pressure, the oxygen partial pressure varied between 0%, 25%, 50%, 75%, and 100%.Comparing the height of the white line in nitrogen at 0.4 V and at 1.2 V, which is the most reducing state, the height of the white line was lower when the applied voltage was 0.4 V. This is because the average valence of Rh was higher at 0.4 V than at 1.2 V, which is the most reducing state. This indicates that the average valence of Rh changes with applied voltage. In other words, at 0.4 V, when the ORR is fully advanced, the oxidation number of Rh is considered to be in a reduced state. This behavior is consistent with the electrochemical behavior and with previous reports.In contrast, when the oxygen partial pressure was set to 25%, this difference in the applied voltage was hardly observed. The difference in the white lines between the high and low potential sides became smaller as the oxygen partial pressure increased, and under 100% oxygen, the XANES spectra obtained at the applied voltage of 0.4 V and 1.2 V almost overlapped. Therefore, unlike under a nitrogen atmosphere, the oxygen gas adsorbs on the Rh2O3 surface at 0.4 V, where the ORR proceeds under oxygen distribution. As a result, it is considered that the oxidation number of Rh increased. From the above, we predict that in Rh2O3, the ORR progresses in the state where the valence of Rh is decreased.

Influence of different carbon materials on electrical properties of epoxy-based composite for bipolar plate applications

During the last years, polymer composite bipolar plates have been developed from appropriate carbon materials and polymers. Considering these materials, several factors influence the effective electrical properties of bipolar plates for polymer electrolyte fuel cell (PEFC) applications. This research focuses on the development of a composite of graphite/epoxy resin/aliphatic amine, which be technically feasible as a material for use in PEFC and meet international commercial standards. A synergic mixture of Expanded Graphite (EG) as a primary filler and different secondary fillers was analyzed. The effect of Carbon Black (CB) and Graphite Nanoplatelets (GNP) as secondary fillers in proportions of 0, 0.5, 1, 2, and 3 wt% on EG with particle size greater than 300 µm and content of 40 wt% are analyzed. The methodology in this composite processing did not include any solvent and relatively low temperature processing is performed. The processing conditions were studied to understand the impact on the electrical properties of the composite. The best results were obtained with carbon black at 1%, obtaining an in-plane electrical conductivity at about 50 S/cm. The appropriate dispersion of the CB through the Epoxy/EG matrix creates bridges connecting plates that improve the flow of electrons compared to GNP composites.

Study of Active Center Structures for Fe/N/C Type Oxygen Reduction Catalyst Using Semi-Empirical Molecular Orbital Calculations

Polymer electrolyte fuel cells (PEFCs) are one of the most important devices in realizing hydrogen society. Platinum, which is used as the catalyst of PEFCs, is expensive and less abundant. Thus, the development of a non-platinum oxygen reduction catalyst is required for the wide-spread use of PEFCs. Fe/N/C non-platinum oxygen reduction catalysts are promising platinum alternative catalyst candidates. We have reported new metal/nitrogen/carbon-electrocatalysts synthesized by the pyrolysis of precursors consisting of Ketjenblack®, a Zn complex with 4,4’-bipyridine, 1,10-phenanthroline (phen), and iron(II) chloride1)．The density of active sites on the catalysts need to be increased for the improvement of for oxygen reduction reaction (ORR). Although we obtained that pyridine type nitrogen is related to the ORR activity, the chemical structure of the active center is still unknown. The conclusion does not match whether Fe atoms function as activity centers2-3．In this study, the structures of active center for Fe/N/C ORR catalysts were studied using semi-empirical molecular orbital calculations． Assuming that four electrons were injected into the compound, the distance between an O2 molecule and atoms was stepwise shortened with the parameters4 using MOPAC 20165. The result of the calculation was visualized by Jmol6. We used model compounds modeling a part of the edge of a graphite layer (Fig. 1(a)). Fig. 1(b) shows the dependence of ΔH on the distance between O and target atoms during shortening distance. The activation energies of route (i) was the smallest of the four routes. The cleavage of O-O bond occurred at routes (i) and (iv). The supply of further proton via H3O+ is necessary in the route (ii) and (iii). References and Notes Shimbori, H. Shiroishi, R. Ono, S. Kosaka, M. Saito, T. Yoshida, H. Katagiri, K. Miyazawa, Y. Tanaka, Mat. Sci. Eng. B, 228, 190 (2018). Lefèvre, J.P. Dodelet, P. Bertrand, J. Phys. Chem. B , 106, 8705 (2002). Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science, 351, 361 (2016).Typical calculation parameters of MOPAC used in the simulations: PM6-DH2 EF PRECISE PULAY CAMP ITRY=9999 SHIFT=50 SYMMETRY STEP=-0.025 POINT=74 NOINTER GRAPHF EPS=78.5 NSPA=200 (UHF) CHARGE=(2 to -2).MOPAC2016, James J. P. Stewart, Stewart Computational Chemistry, Colorado Springs, CO, USA, HTTP://OpenMOPAC.net (2016).Jmol: an open-source Java viewer for chemical structures in 3D. http://www.jmol.org/ Figure 1

Improvement in the durability of carbon black-supported Pt cathode catalysts by silica-coating for use in PEFCs

Commercially available Pt metal catalysts supported on carbon black (Pt/CB) for polymer electrolyte fuel cell (PEFC) cathodes were covered with silica layers to improve their durability under the severe cathode operating conditions. The Pt metal particles in the Pt/CB catalyst grew in size during the accelerated durability tests (potential cycling between 0.6 and 1.0 V vs. RHE in an aqueous HClO4 electrolyte). Thus, the Pt/CB catalyst was seriously deactivated for the oxygen reduction reaction over the course of the durability tests. In contrast, the silica layers, which wrapped around the Pt metal particles in the silica-coated Pt/CB catalyst, prevented the migration of the Pt metal particles on the carbon supports and the diffusion of Pt cations out of the silica layers. Thus, the silica-coated Pt/CB catalysts maintained a high activity for the oxygen reduction reaction over the course of the durability tests. In addition, the silica-coated Pt/CB prepared from methyltriethoxysilane showed a higher activity than that prepared from tetraethoxysilane. The porous structures and hydrophobicity of silica prepared from methyltriethoxysilane promoted the diffusion of oxygen and water molecules in the silica layers of the silica-coated Pt catalysts.

Water Transport in Ionomer and Ice Formation during Cold Startup with Supercooled State in PEFC

In cold regions, ensuring startup ability and durability of polymer electrolyte fuel cells (PEFCs) at subfreezing conditions is one of critical issues for the practical use of PEFC. At cold startup of PEFCs near 0 °C like –10 °C, the produced water remains as supercooled state before it freezes, and then the shutdown occurs with ice layer formation between the cathode catalyst layer (CL) and micro-porous layer (MPL) (1). It was estimated from our previous study (2) that water transport in the CL occurs by concentration gradient in ionomer. This study investigated the water transport in ionomer and the ice formation behavior by measurement of cold start characteristic and observation of membrane electrode assembly (MEA) by a cryo-SEM. To evaluate the water transport, we measured change in polymer membrane resistance corresponding to water content during the supercooling release. Further, to improve the cold start characteristic, we introduced a hydrophilic MPL and confirmed that the hydrophilic MPL makes the startup period longer and improves ice distribution. A single cell with an active area of 25 cm2(5 cm × 5 cm) was used in this study. After a preconditioning process to enhance the performance of a MEA, the initial conditions of the water in the cell were carefully controlled by the procedure described in Ref. 1. Then, the cell and the chamber were cooled to –10 °C, and cell operation was started and maintained at a constant current density. The anode and the cathode supplied gases were dry hydrogen and dry air. For the cryo-SEM observation, the sample of the MEA as removed from the cell after the stop of operation was immediately immersed in liquefied nitrogen, and the sample was set to a sample holder. The sample was cut by a cold knife in a vacuumed chamber, and the cut section area was coated with gold-palladium alloy (Au-Pd), and observed at –150 °C. To estimate the changes in the water content of a polymer electrolyte membrane from supercooled water to ice, we measured the change in the in-plane high frequency resistance of the polymer membrane. It is expected that when supercooled water changes to ice, the chemical potential of water decreases and the water content of the polymer membrane contacting with ice also decreases. Fig.1 shows change in resistance and temperature of the polymer electrolyte membrane. Supercooling release occurs after around 70 min at –7 °C, and the temperature slightly rises due to heat of solidification. Here, resistance increases even after the temperature becomes again –7 °C. This resistance increase suggests the decrease in the water content of the polymer electrolyte membrane due to ice formation. ΔR is the difference between the resistance contacting with ice and with supercooled water. Fig.2 shows relationship of ΔR and supercooling degree (ΔT). ΔR increases as ΔT increases, and this supports the above estimation that ice freezing from the supercooled state takes up water from the ionomer. Secondly, we introduced a hydrophilic MPL made by the gas diffusion electrode (GDE) method (3) to prevent water from freezing at the interface between the CL and MPL and to improve the cold start characteristics. In the previous study (1)(2), we observed the ice layer at the interface with a hydrophobic MPL. Fig.3 shows change in cell voltage and resistance at 0.02 A/cm2, –10 °C. This result suggests that hydrophilic MPL increases the startup period than conventional hydrophobic MPL. Fig.4 is observation results of cross-section of the cathode MEA with hydrophilic MPL after the 0.02 A/cm2shutdown. Figs.4(a) and (b) are the cryo-SEM images with low magnification and with high magnification. Fig.4(a) shows that there is no ice layer at the interface and water freezes in the MPL. Fig.4(b) shows that there is little ice in CL, and this suggests that the hydrophilic MPL enhances the migration of water through ionomer in the CL. These results also suggest that the hydrophilic MPL is effective to improve the characteristics at cold startup. Reference (1) Y. Tabe, et al. , Journal of Power Sources208 (2012), 366. (2) Y. Ishima, et al. , National Symposium on Power and Energy Systems(2015), 101. (3) Y. Aoyama, et al. , ECS Trans. 69 (17) (2015), 743. Figure 1

Proton conductivity and fuel cell performance of organic–inorganic hybrid membrane based on poly(methyl methacrylate)/silica

Organic–inorganic hybrid membranes based on poly(methyl methacrylate) (PMMA)/silica have been synthesized using a sol-gel technique for use in polymer electrolyte fuel cells (PEFCs). The properties of these membranes were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. The results indicate that these membranes are formed through hydrogen bonds between the carbonyl group of PMMA and the uncondensed alcohol functional groups of the inorganic clusters. The proton conductivity of these membranes is on the order of 10−1 S cm−1, and the 60PMMA–30SiO2–10P2O5 membrane displays the highest proton conductivity of 3.85 × 10−1 S cm−1 at 90 °C and 50% RH. The performance of a fuel cell using these membranes was tested. A maximum power density of 370 mW cm−2 is obtained at 80 °C, and the current density at 0.4 V remains almost unchanged during the 100-h test time under the test conditions. This class of hybrid membranes is an extremely promising material for use in PEFCs.

Ice Formation and Current Distribution in the Catalyst Layer of PEM Fuel Cell at Cold Start

In polymer electrolyte fuel cells (PEFCs), the freezing of produced water induces the extreme deterioration of the cell performance below zero. This phenomenon is serious problem in cold regions and is needed to be solved to achieve the practical use of PEFCs. In this study, we investigated the ice formation and the reaction rate in the cathode catalyst layer (CL) using a CRYO-SEM and a three-phase boundary model to clarify the freezing mechanism in the cold start at -20oC. The observation results showed that the ice distribution formed in the CL is changed by the operation time and the current density. In particular, in a case of large current density, it was shown that ice grows from the membrane side to the gas diffusion layer (GDL) side. This process can also be analyzed in detail by the model that calculates the reaction rate in the CL.

Anhydrous Proton-Conducting Properties of Nafion–1,2,4-Triazole and Nafion–Benzimidazole Membranes for Polymer Electrolyte Fuel Cells

Recently, fuel cells have gathered a lot of interest due to their potential as future sources of clean energy. Polymer electrolyte fuel cells (PEFCs) require high operating temperatures for high conversion efficiency. Anhydrous proton-conducting polymers employing Nafion–1,2,4-triazole- and Nafion–benzimidazole-blend membranes were studied for use in PEFCs above . The solvents 1,2,4-triazole and benzimidazole are thermally stable, organic reagents with low volatility and are used to replace water as the proton acceptor in the perfluorinated ionomer membrane. The Nafion–benzimidazole blend had high thermal stability above and proton conductivity of at under nonhumidified conditions. Fuel cell tests using a single membrane electrode assembly cell with the Nafion-blend membranes were successfully performed.