Polymer Electrolyte Fuel Cell Catalyst Research Articles

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

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

Structural Effects on ORR Performances of Carbon-Free Connected Platinum–Cobalt Catalysts with Chemically Ordered Superlattice Structures

Polymer electrolyte fuel cells (PEFCs) have been considered as the promising next-generation energy-conversion technology, owing to their high energy efficiency and clean emissions. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. To address these issues, our group has developed a carbon-free, connected Pt1–Fe1 catalyst with a porous hollow capsule structure. This catalyst comprises the nano-sized network formed by the connection of Pt1–Fe1 nanoparticles and exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt/C) as well as excellent durability against start-stop operations due to the carbon-free structure.[1,2] However, during the load cycle operation, iron is dissolved from the catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes caused by the Fenton reaction.In this work, carbon-free and iron-free connected Ptx-Co1 catalysts with chemically ordered superlattice structures have been developed for the first time, as shown in Fig. 1(a). In addition, the metal compositions and chemically ordered (superlattice) degrees in the catalysts were controlled to investigate the structural effects on ORR activity and durability of the connected Ptx-Co1 catalysts.Herein, we employed the silica-coating method for the catalyst synthesis, which involves the combination of silica coating and high-temperature annealing. As shown in Fig. 1(b), the surface of the Ptx-Co1 nanoparticles/SiO2 sample was coated with thin silica layers to prevent the detachment of Ptx–Co1 nanoparticles from the SiO2 template as well as the large catalyst agglomeration and coalescence during the annealing process. We controlled the chemically ordered degrees of the connected Pt1–Co1 and Pt3–Co1 catalysts by using different annealing temperatures. The XRD patterns of the catalysts showed the peaks corresponding to L10 type and L12 type chemically ordered structures for the connected Pt1–Co1 and Pt3–Co1 catalysts annealed at 600°C, respectively. These two catalysts possessed ordered degrees (S) of ca. 50–60%. The TEM images of the connected Ptx–Co1 catalysts showed the formation of connected Ptx–Co1 nanonetwork and porous hollow capsule structures.The electrochemical measurements of ORR activity and durability were conducted in a 0.1 M HClO4 electrolyte solution. The connected Pt1–Co1 and Pt3–Co1 catalysts achieved 6- and 10-times higher ORR specific activities than that of Pt/C, respectively. Subsequently, the ORR activity and load cycle durability (0.6 V for 3 s Û 1.0 V for 3 s at 60 °C) of the connected Pt3–Co1 catalyst with different ordered degrees (S = 0%, 28%, and 58%) were evaluated. As the ordered degrees in the connected Pt3–Co1 catalysts were higher, the ORR specific activity was increased. Also, the retention of the specific activity of the ordered catalyst (S = 58%) was higher than that of the disordered catalyst (S = 0%) after 10,000 load cycles. The STEM-EDX line mapping results showed that 83% of the alloyed Co retained in the catalyst with a higher ordered degree even after 10000 load cycles, whereas the disordered and lower ordered catalysts retained lower Co ratios after the test, indicating the suppression effect of an ordered structure on metal dissolution. In addition, the L12 ordered Pt3–Co1 structure remained in the connected catalyst (S = 58%) after load cycle test, confirmed by the TEM diffraction analysis.In summary, we successfully developed the connected Ptx-Co1 catalysts with chemically ordered superlattice structures. In addition, we clarified the structural effects of metal composition and ordered degree in connected Ptx-Co1 catalysts on ORR activity and load cycle durability. These results provide useful guidelines for the design of advanced ORR catalysts towards high-performance PEFCs. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

(Poster Award - 3rd Place) Synthesis of Spherical Mesoporous Carbon (SMPC) Support and Electrochemical Properties of Pt/SMPC Catalyst

Introduction In Japan, extremely high cell performance is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. Therefore, it is of great importance to improve not only ORR activity, but also oxygen diffusivity of Pt and Pt-based cathode catalysts in PEFCs. In this study, a spherical mesoporous carbon (SMPC) with central mesopore’s diameter of 7 nm was synthesized and used for support of the Pt NP catalyst. Electrochemical properties of the Pt/SMPC catalyst were investigated. Experimental The SMPC support was synthesized by silica template method in which SiO2 precursor was in-situ formed [2]. 7 mL of tetrapropyl orthosilicate was added in a mixed solution (6 mL conc. NH3, 150 mL EtOH and 10 mL H2O) and stirred for 15 min. Then, 0.8 g resorcinol and 1.12 mL formaldehyde (37 wt.% aq.) were added and stirred at room temperature for 24 h. The precipitate was filtered and dried at 60°C overnight in air. The powder was heat-treated in a tube furnace at 850°C for 3 h in Ar atmosphere to obtain SiO2/SMPC. The SiO2/SMPC was dispersed in 10% HF aq. and stirred at room temperature for 2 h to remove silica template. The Pt/SMPC catalyst was synthesized by alcohol reduction method [3]. 300 mg SMPC was dispersed in a EtOH/H2O mixed solution (37.5 mL/250 mL), and Pt(NO2)2(NH3)2 (corresponding to 300 mg metallic Pt) dissolved in aqueous HNO3 was added, followed by refluxing at 90°C for 4 h in N2 atmosphere. The catalyst was filtered, washed and dried at 60°C overnight in air. Characterization of SMPC and Pt/SMPC was conducted by N2 gas adsorption, TG-DTA, XRD, TEM and TEM-EDX. The catalyst was loaded on GC-RDE. CV was recorded in Ar-saturated 0.1 M HClO4 at 25oC and at 50 mV/s. The ORR activity was evaluated by LSV performed in O2-saturated 0.1 M HClO4 at 25oC at 10 mV/s in the positive direction while rotating the GC-RDE at 1,600 rpm. Results and Discussion The weight percentage of silica template was 70% in SiO2/SMPC. Figure 1 shows SEM and TEM images of SiO2/SMPC before and after HF treatment. The SEM images clearly show that SiO2/SMPC and SMPC have spherical shape with mean diameter of 200 nm. In TEM images of SiO2/SMPC, center of the sphere is darker than edge and the edge became darker than the center after HF treatment, suggesting that the silica template NPs resided at the center of the sphere. TEM-EDX composition analysis of SiO2/SMPC is demonstrated in Fig. 2. Before HF treatment, Si was detected at the center of the sphere. After HF treatment, only C was detected with high intensities at the edge of the sphere. These results suggest that the SiO2/SMPC has a silica rich core-carbon rich shell structure. The thickness of the carbon rich shell was ca. 60 nm. The surface area of SiO2/SMPC was 510 m2/g and the area increased to 1,590 m2/g after HF treatment. Figure 3 shows pore size distribution of SiO2/SMPC. After HF treatment, SPMC support exhibited clear existence of mesopores with a central diameter of 7 nm.A cross-sectional TEM image of Pt/SMPC catalyst is depicted in Fig. 4. Pt NPs with Scherrer’s diameter of 2.3 nm were well dispersed over the carbon-rich shell, which indicates that mesopores in the carbon shell were well interconnected. The ECSA and the ORR activity of the Pt/SMPC catalyst were 92 m2/g and 450 A/g@0.9 V, respectively, the latter of which was similar to that of a commercially available Pt/C (TKK, TEC10E50E). Single cell tests using Pt/SMPC is now underway, and the results will be presented at the meeting.This study was partly supported by NEDO, Japan. Reference [1] M. Suzuki, https://www.nedo.go.jp/content/100888556.pdf (in Japanese).[2] Z. Deng et al., Nano Lett., 22, 9551 (2022).[3] M. M. Rahman et al., RSC Adv., 11, 20601 (2021). Figure 1

Design of Carbon Aggregation Structure in Polymer Electrolyte Fuel Cell Catalyst Ink by Solvent Hydrophilicity

Controlling the porous structure of the cathode catalyst layer in a polymer electrolyte fuel cell is essential to reduce the reaction and mass transport resistance of the oxygen reduction reaction. As the catalyst layer is a coating of a catalyst ink, the aggregation structure of platinum-supported carbon (Pt/C) and ionomer in ink is a key property for achieving the optimal catalyst layer.1 Previous studies reported that the weight fraction of water in solvents could control aggregation structure in ink.2-4 For example, Kumano et al.2 reported that increasing the water ratio to 1-propanol enhances the ionomer adsorption on Pt/C (Γ) and dispersion of Pt/C, causing the lowering of the viscosity of the inks. Based on the results, they speculated that water repels ionomers and promotes their adsorption on Pt/C particles. They further speculated that the ionomers adsorbed on the Pt/C particles induce repulsive interactions between Pt/C particles via electrostatic repulsive interactions among sulfonate groups of adsorber ionomers. By the enhanced repulsive interactions, Pt/C particles are dispersed, and viscosity decreases. The discussion in the previous study likely indicates that the hydrophilicity of the solvent is an essential factor controlling the aggregation structure of catalyst ink. Here, we verify this hypothesis. As a universal indicator of the hydrophilicity, we adopt the hydrogen-bond term HSP-δH of the Hansen solubility parameters. We examined how HSP-δH is correlated to the ionomer adsorption ratio to Pt/C (Γ), viscoelasticity of ink, and structural properties of ink measured by ultra-small angle X-ray scattering (USAXS).The catalyst inks were prepared by mixing Pt/C (TEC10V30E, TKK), Nafion 20 wt% solution (DE2020, Chemours), ultrapure water, and three different alcohols [ethanol (EtOH), 1-propanol (PrOH) and diacetone alcohol (DAA)]. The water weight fraction in the solvents was set to 25 - 85 wt%. Γ is plotted as a function of HSP-δH in Fig. 1. Regardless of the alcohol species, Γ increases with increasing HSP-δH. The trend is consistent with the previously reported trend observed in the mixtures of water and 1-propanol. Figures 2 (a) and (b) show the equilibrium storage modulus G’0 obtained by the rheometer and the fractal dimension D 2 of agglomerates of Pt/C particles in ink as functions of Γ. Here, D 2 is determined by fitting the unified model5 to the USAXS spectrum obtained by the synchrotron radiation (Spring-8, Japan). Both decreases with the increase of Γ. The lowering of D 2 indicates that the agglomerates of Pt/C particles are dispersed and turn to less-structured particles, as schematically shown in Fig. 2 (c). By the dispersion, G’0 decreases. Accordingly, the hydrophilicity, indeed, promotes the ionomer adsorption, and the adsorption induces the dispersion. The hydrophilicity is a key factor controlling the structure and rheology of the catalyst ink, and HSP-δH is a universal descriptor of hydrophilicity. Reference K. B. Hatzell, M. B. Dixit, S. A. Berlinger, and A. Z. Weber, J. Mater. Chem. A, 5 (39), 20527-20533 (2017). N. Kumano, K. Kudo, Y. Akimoto, M. Ishii, and H. Nakamura, Carbon, 169 429-439 (2020). S. Takahashi, T. Mashio, N. Horibe, K. Akizuki, and A. Ohma, ChemElectroChem, 2 (10), 1560-1567 (2015). T. Van Cleve, S. Khandavalli, A. Chowdhury, S. Medina, S. Pylypenko, M. Wang, K. L. More, N. Kariuki, D. J. Myers, A. Z. Weber, S. A. Mauger, M. Ulsh, and K. C. Neyerlin, ACS Appl Mater Interfaces, 11 (50), 46953-46964 (2019). G. Beaucage, H. K. Kammler, and S. E. Pratsinis, J. Appl. Crystallogr., 37 (4), 523-535 (2004). Figure captions Figure 1 Dependence of the ionomer adsorption ratio Γ on the HSP-δH of the catalyst ink solvent. Figure 2 (a) Γ dependence of the equilibrium storage modulus G’0 of the catalyst inks. G’0 is plotted on a logarithmic scale. G’0 values of inks that are not measurable due to being less than the measurable lower limit are conventionally described as 0. (b) Γ dependence of the mass fractal D 2 of Pt/C aggregates in the inks. (c) Schematic illustration of the aggregation structure of Pt/C in the solvents. Figure 1

Comparison of Different Carbon Supports By Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Ink

Polymer electrolyte fuel cells (PEFCs) are one of the most possible power sources for heavy-duty vehicles like trucks due to their high conversion efficiency and low environmental impact. Since platinum which is often used as a catalyst is expensive, it is essential to reduce the amount of platinum by increasing the power density. Regarding materials, it was reported that the activity is increased by supporting platinum on porous carbon [1]. However, there is a voltage drop in the high current density region comparing non-porous carbon. This is caused by oxygen transport properties, but the relationship with the carbon structure is unknown. Oxygen moves through the voids in the catalyst layer, so it is necessary to control the porous structure that forms the voids. This porous structure depends on the conditions of the catalyst layer fabrication process. In particular, the dispersion process is important to determine the size of the carbon agglomerate, or the backbone of the catalyst layer. In this study, an agglomeration model in catalyst ink was developed, and the dispersion process was verified by simulation under various conditions to elucidate the factors that affect agglomeration.The discrete element method (DEM) was used for the simulation. In this study, aggregates were approximated into spheres, and the forces between the particles were the drag force from the fluid, Brownian motion, and particle-particle interactions. Based on DLVO theory, the particle-particle interactions were given as an attractive force due to the van der Waals interaction and a repulsive force due to the overlapping of the electric double layer. In addition, the effect of ionomer was considered. The ionomer effect was introduced as a repulsive force and modeled with the same equation as the repulsion due to the electric double layer. The dielectric dependence equation of the ionomer shape that Magnus et al. modeled [2] was incorporated. As the amount of ionomer added increases, a binder effect is observed between the particles. This effect was also introduced using the adsorption data of ionomers on carbon. The carbon was assumed to be non-porous (Vulcan) and porous (Ketjen). Considering the difference in the platinum ratio on the surface depending on the carbon porosity, the Hamaker constant, which affects the van der Waals force, was determined. Also, the zeta potential, which affects the electric double layer force, was used at different values for carbons. In this study, the calculations were started with the particles uniformly dispersed in the ink. Periodic boundary conditions were used as boundary conditions. Using the above numerical model, simulations were conducted with different water-alcohol ratios and ionomer carbon ratios (I/C).As a result, it was confirmed that differences in the agglomerate size for two carbon supports. This result implies that the composition of the ink and the type of carbon affect the catalyst layer structure and cell performance.AcknowledgmentThis study was carried out under the project of Development of Advanced PEFC Utilization Technologies supported by the NEDO.Reference[1] Y. C. Park et al., J. Pow. Sou r., 315, 179 (2016).[2] M. So et al., Int. J. Hydrog. Energy, 44, 28984 (2019).

Influence of Ethanol Decomposition on Dispersion of PEFC Catalyst Ink

To achieve high power density operation of polymer electrolyte fuel cells (PEFCs), it is required to realize high-performance catalyst layer with low oxygen transport resistance, high proton and electron conductivities, and high electrochemical surface area (ECSA) with low Platinum loading. Because dispersion structure of catalyst ink strongly affects porous structure of the catalyst layer, it is crucial for realization of high-performance catalyst layer to understand the dispersion mechanism of the catalyst ink. Our group has reported that decomposition of ethanol, as a solvent, by platinum catalyst significantly affects aggregation of the catalyst ink. [1, 2] But, the mechanism of the catalyst ink aggregation was not fully understood. In this study, effect of ethanol decomposition on the aggregation of the catalyst ink was investigated.The catalyst ink was fabricated by mixing platinum-supported carbon (TEC10V30E, Tanaka Kikinzoku), ionomer (DE1021, Sigma-Aldrich), and water/ethanol solvent (water/ethanol: 60/40 wt%). Decomposition product of ethanol within the catalyst ink solvent was analyzed by GC/MS (GCMS-QP2020NX, SHIMADZU), and presence of acetaldehyde (C2H4O) and acetic acid (CH3COOH) was detected. In order to investigate the influence of these products on aggregation of the catalyst ink, acetaldehyde or acetic acid was added to the catalyst ink. The particle size distribution was evaluated by using laser diffraction type particle size distribution meter (LA-960V2, HORIBA) without dilution, and it was confirmed that the acetaldehyde-added catalyst ink showed larger particle size and the acetaldehyde caused the aggregation of the catalyst ink (Figure 1). In addition, dispersion of the catalyst ink was observed by using an optical microscopy, and aggregation of the catalyst ink by adding the acetaldehyde was clearly observed (Figure 2). To understand the aggregation mechanism by adding acetaldehyde, ionomer adsorption fraction on the platinum-supported carbon was measured by using centrifugation method, and the decrease in ionomer adsorption fraction by adding acetaldehyde was confirmed. From these results, it was confirmed that acetaldehyde generated by the decomposition of ethanol in the catalyst ink leads to a decrease in the ionomer adsorption fraction. It is reported that the ionomer promotes the dispersion of platinum-supported carbon due to electrostatic repulsion forces, and it is suggested that the decrease in ionomer adsorption fraction results in the aggregation of the catalyst ink. Therefore, to realize high-performance catalyst layer, influence of acetaldehyde should be minimized and further research is requested to understand it. Acknowledgement This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Exploring a Facile Synthetic Procedure of Iron-Based Platinum Group Metal-Free Electrocatalyst for Polymer Electrolyte Fuel Cells

The transportation sector, mainly powered by fossil fuels, is responsible for approximately 14% of all greenhouse gas emissions1. Polymer electrolyte fuel cells (PEFCs) are promising candidates for sustainable transport as they only emit water, can be refilled fast, and enable long ranges with a single refill. Their current costs and dependence on scarce materials challenge their broad implementation in heavy-duty vehicles and maritime transportation 2. Another challenge is the catalyst degradation during operation, therefore the goal of at least 35,000 lifetime hours for heavy-duty transportation is ambitious as existing fuel cell vehicles have been engineered for ca. 5,000 hours3. The state-of-the-art catalyst layers are based on platinum nanoparticles supported on carbon and global efforts have been made to develop high-activity catalysts with reduced platinum contents, including platinum alloys such as Pt-Ni4 and Pt-Co5, alternative supports such as nanotubes6, or a combination aforementioned methods. While platinum-based materials remain the most active for the oxygen reduction reaction, their elevated cost and instability during operation motivate the exploration of alternative materials.Platinum group metal (PGM)-free catalysts have gained prominence in the PEFCs field, where Fe-N-C based catalysts have shown high oxygen reduction reaction activity in acidic media7, approaching that of platinum-based catalyst layers, yet their practical application is hindered by insufficient stability under operating conditions8. Inspired by recent efforts7,8, our goal is to develop stable, active, and low-cost catalyst layers using earth-abundant materials. In this poster presentation, I will discuss my efforts in the synthesis of Fe-N-C catalysts by the incorporation of iron in a calcinated nitrogen-doped zeolitic imidazolate framework (ZIF). ZIFs are excellent supports due to their high porosity of approximately 60% and a surface area of 1600 m2/g, leading to a high density of active sites9. After the incorporation of iron, post-treatments are applied including leaching and the utilization of secondary nitrogen precursors, where efforts are made to develop non-hazardous activation approaches. The incorporation of secondary nitrogen precursors such as cyanamide have been shown to promote the formation of metal-N-C bonds and increase the performance towards the oxygen reduction reaction10,11. We employ microscopic, spectroscopic and electrochemical techniques, such as the rotating disc electrode, to correlate the catalyst composition with electrochemical performance and durability. With this work, we hope to gain an understanding of composition-structure relationships to enhance the performance of iron-based platinum-free catalysts for PEFCs. Acknowledgements SH2IPDRIVE has received funding from the Ministry of Economic Affairs and Climate Policy, RDM regulation, carried out by the Netherlands Enterprise Agency. References W.F. Lamb et al., 2021, Environmental Research Letters vol. 16.S.T. Thompson et al., 2018, Solid State Ion 319, 68–76.D.A. Cullen et al., 2021, Nature Energy vol. 6 462–474.S.I. Choi et al., 2013, Nano letters 13(7), 3420-3425.P.R. Kumar et al., 2018, Surfaces and Interfaces 12,116-123.D. Schonvogel et al., 2017, Journal of the Electrochemical Society 164(9), F995.A. Mehmood et al., 2022, Nature Catalysis 5(4), 311-323.X. Wan, and J. Shui, 2022, ACS Energy Letters 7(5), 1696-1705.J.C.Tan et al, 2010, Proceedings of the National Academy of Sciences 107(22), 9938-9943.X. Wu et al., 2022, Journal of The Electrochemical Society 169.1, 016507.N.D. Leonard et al., 2018, ACS Catalysis 8(3), 1640-1647.

Influence of Ethanol Decomposition on Dispersion of PEFC Catalyst Ink

To achieve high power density operation of polymer electrolyte fuel cells (PEFCs), it is required to realize higher performance catalyst layer. Because dispersion structure of catalyst ink strongly affects the catalyst layer structure, it is crucial to understand the dispersion mechanism of PEFC catalyst ink. Though water/ethanol solution is used as solvent of the catalyst ink, decomposition of ethanol by Platinum catalyst strongly affect dispersion of the catalyst ink. In this study, influence of ethanol decomposition on dispersion of catalyst inks were investigated. Among the decomposition byproducts of ethanol, results of rheology characteristics and direct observation by scanning electron assisted dielectric microscopy clearly showed that acetaldehyde has a significant impact on aggregation of catalyst ink. To reveal the mechanism of aggregation, particle size measurement and ionomer adsorption fraction measurement of the catalyst ink were carried out. The results suggested that the acetaldehyde impede adsorption of the ionomer on Platinum catalyst.

Experimental and in silico simulation of slot-die coating with a polymer electrolyte fuel cell catalyst slurry

The causes of coating irregularities in high-speed slot-die coating of catalyst slurries for polymer electrolyte fuel cells (PEFCs) have not been determined. Accordingly, in this study, slot-die-coating experiments and computer simulations of two-phase gas–liquid flow were conducted to explore coating instability in the high-speed coating of PEFC catalyst slurries applied through slot-die coating. The results obtained from experiments and simulations were consistent. A thinner target layer and a faster coating, causing non-uniform coating. This is caused by movement of the gas–liquid interface on the upstream side and side of the die, which entraps the gas phase. Viscosity and surface tension are important, and a lower ethanol content in the solvent increases surface tension and lowers viscosity, improving coating stability.

The Ionomer Molecular Structure Effect in the PEFC Ink & Applications

The interaction between the perfluorosulfonic acid (PFSA) ionomer and platinum group metal (PGM) catalyst in polymer electrolyte fuel cell (PEFC) inks is a critical issue for designing high-performance PEFC electrodes. During the ink fabrication, the complex interparticle interaction of the ink components determines the agglomerate morphology and size distribution. Among the ink components, the ionomer, which has amphiphilic in nature due to its hydrophobic backbone and the hydrophilic ionic group, mostly effective parameter for interparticle interaction. Therefore, understanding the interaction and molecular structure of ionomer is required for a high-performance catalyst layer. The backbone length of the ionomer which controls the spacing between ionic pendant groups is the main parameter of the ionomer character. In this study, the three different ionic spacing ionomers, made in the Solvay industry: Aquivion D72-25BS, Aquivion D83-24BS, and D98-25BS, were used for investigating the adsorption behavior of the catalyst ink and PEFC catalyst layer morphology. During the fabrication of PEFC ink and catalyst layer, all samples have same sulfonic acid molality to avoid the ionic clustering effect in the side chain. Ex-situ measurements were carried out to quantify the interparticle interaction and agglomerate size distribution in the PEFC ink. The dynamic lights scattering (DLS) carry out to determine the aggregation size distribution. The rheological behavior was conducted to understand the interparticle interactions and the agglomeration behavior of the inks by steady-shear and dynamic-oscillatory-shear measurements. It is noted that increasing the ionomer backbone length, which favors interaction via hydrophobic interaction on a carbon support, suppresses the agglomeration of ink components and decreases ink viscosity. The in situ electrochemical tests were carried out to figure out the relationship between ink formulation and catalyst layer morphology. It was found that the long ionomer backbone length helps to suppress the O2 transport resistance and ORR kinetic loss by uniform distribution from ionomer backbone-carbon supports interaction.

(Digital Presentation) Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Inks

Polymer electrolyte fuel cells (PEFCs) are expected to use as a power source for new generation automobiles, especially heavy-duty vehicles because of their low environmental affection. To be widely spread, it is essential to increase their power output and reduce the cost of catalyst platinum. Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction, there is a need to improve the oxygen transport and proton conductivity. In recent years, porous carbon has been studied and it has been found its high activity and durability. However, the catalyst layer made of porous carbon gets a lower voltage than that made of non-porous carbon in the high current density area. This output reduction results from high oxygen transport resistance, but it is unclear how the porous structure influences the performance. So in this study, we assumed the following relation. Firstly, the difference in voltage at a high current density between carbons results in oxygen transport properties. Secondly, their properties depend on an effective oxygen diffusion coefficient, which is determined by the porous structure in the catalyst layer. And finally, the factor that attributes the catalyst layer structure is the size of the agglomerate in the catalyst ink because the catalyst layer is fabricated by the wet process. Therefore, we focused on the particle agglomeration process in catalyst ink and calculated the agglomerate behavior under various ink conditions using a numerical model. The differences in the agglomeration and its factors were also examined for carbon with different internal pores.A three-dimensional discrete element method (DEM) was used to calculate agglomeration. Initially, aggregates, the smallest unit of carbon black, were placed randomly in the cubic region. In this simulation, aggregates were regarded as spheres. The distance of movement was determined by solving the equation of motion. For each particle, the fluid drag, Brownian motion, and particle interactions were considered. The particle-particle interaction included the forces based on DLVO theory [1] and the ionomer effect. Several studies show that ionomer adsorbed on the carbon aggregates improves their stability. So it was assumed that the ionomer effect was the repulsive force and modeled using the same equation as for the EDL force. The dielectric constant dependence of this effect was used in the function previously reported by Magnus et al. [2]. The carbon particles assumed here were non-porous (Vulcan) and porous (Ketjen), and were coated with Pt particles. The initial average diameter of the aggregates was 300 nm. Hamaker coefficient was expressed as an average of the physical properties of platinum and carbon black, which was weighted by the percentage of platinum loaded on the surface of the carbon particles. The solvent was a mixture of 1-propanol (NPA) and water, and the dielectric constant of the ink was expressed as a weighted average of the composition ratio of water and NPA. The simulation was conducted under various water/alcohol ratios and ionomer/carbon ratios (I/C). From this model, it was confirmed that the carbon internal pores affect the size of the agglomerate in the catalyst ink.

Investigation of Liquid Water Behavior and Performance of PEFC Catalyst Layers Using a Microdevice and in-Operando Infrared Microscopy

Studies on water behavior in the vicinity of a catalyst layer (CL) and its effect on the cell performance are important challenges because water management is a key factor to achieve higher performance of polymer electrolyte fuel cells (PEFCs). In this study, a microdevice that contains a catalyst coated membrane (CCM) was developed (1) and in-operando infrared (IR) microscopy was conducted to visualize water behavior on the CL. Two types of CLs contain either carbon (2) or tin oxide (3) as supporting material to investigate the effects of materials on the water behavior and cell performance.The microdevice, which was made of a silicon wafer, contains two microchannels (100 μm-width, 200 μm-pitch) and a current-collecting layer (30 μm-width) on a rib of the cathode plate. The CCM was sandwiched by two plates. The anode channel faced the rib of the cathode. The surface of a cathode catalyst layer was coated with thin Au by sputtering to enhance IR reflectance, which did not change the porous structure of the CL significantly. The surface of the cathode catalyst layer was observed by IR microscopy through the cathode plate.Figure 1 shows liquid water generated during the constant current operation of the device. The catalyst layer contains carbon as the supporting material in this case. The effects of the materials and flow conditions on the water behavior and cell performance were investigated.AcknowledgmentsThis work was supported by NEDO ECCEED’30.References(1) T. Suzuki et al., J. Therm. Sci. Technol., 11 (2016), 16-00370.(2) T. Suzuki et al., Int. J. Hydrogen Energy, 41 (2016), 20326.(3) Y. Chino et al., J. Electrochem. Soc., 162 (2015), F736. Figure 1

(Digital Presentation) Investigation on Effects of I/C Ratio on Dispersion Structure of PEFC Catalyst Ink By Scanning Electron Assisted Dielectric Microscopy

To achieve high power density operation of polymer electrolyte fuel cells (PEFCs), it is required to realize higher performance catalyst layer with low oxygen transport resistance, high proton conductivity, and low Platinum loading. Because dispersion structure of catalyst ink strongly affects the catalyst layer structure, it is crucial to understand the dispersion mechanism of PEFC catalyst ink. We have reported that that solvent composition (ethanol concentration) of the catalyst ink strongly affect dispersion of the catalyst ink [1, 2], but effects of other components on the dispersion of the catalyst ink were not fully understood yet. In this study, effect of I/C ratio (amount of ionomer) on the dispersion of the catalyst ink was investigated.The catalyst inks were fabricated by mixing platinum-supported carbon (TEC10V30E, Tanaka Kikinzoku), ionomer (DE1021, Sigma-Aldrich), and water/ethanol solvent. To investigate the effect of ionomer on dispersion of the catalyst ink, solvent compositon (ethanol/water = 20/80 wt%), amount of the platinum-supported carbon and solid content of the catalyst ink were kept constant., and the I/C ratio was changed from 0.25 to 1.25. Viscosity characteristics were measured at each shear rate in the range of 0.01 to 1000 1/s by a rotary rheometer (MCR302, Anton-Paar), and particle size distribution of the catalyst ink was measured by the laser diffraction type particle size distribution meter (LA-960V2, HORIBA) without any dilution. In addition, we have succeeded to directly observe the dispersion structure of the catalyst ink by using a scanning electron assisted dielectric microscopy (SE-ADM) for the first time. SE-ADM enabled observation of the catalyst ink with very little radiation damage and high-contrast imaging without staining or fixation at an 8-nm spatial resolution, and distribution of the platinum-supported carbon, ionomer, and solvent were clearly observed.Figure (a) showed the effects of I/C ratio on aggregation size. The result clearly showed that the catalyst inks with too less (I/C=0.25) or too much (I/C=1.25) amount of ionomer were aggregated. Figure (b) showed the results of SE-ADM imaging. Because the dielectric constant of materials within the catalyst ink were different, the image clearly visualized distribution of the platinum-supported carbon, ionomer, and solvent, and the visualization results of the catalyst ink with different I/C ratio were consistent with the results of the particle size distribution measurement. Though the ionomer works as a surfactant within the catalyst ink, too much ionomer makes the dispersion of the catalyst ink worth, and tuning the amount of ionomer is important to realize the high performance catalyst layer.AcknowledgementThis presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).[1] Kaname Iida et al. 2020 ECS Trans. 98, 497.[2] Takashi Sasabe et al. 2021 ECS Trans. 104, 191. Figure 1

(Digital Presentation) Rotating Ring-Disk Electrodes Studies on Oxygen Reduction Reaction at Hydrophobic Ionic Liquids Layer Modified Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are expected as one of the clean, highly effective energy conversion systems, and have been actively studied. Pt nanoparticles on carbon supports (Pt/C) are commonly used as a cathode catalyst for PEFCs, because of the high activity on oxygen reduction reaction (ORR). However, its durability in long term driving and high mass activity for ORR have not sufficiently fulfilled at the present. More improvements of Pt/C catalyst for ORR activity and the durability are required for the spread of PEFCs To improve the activity and durability of Pt/C, ionic liquids (ILs) modified catalysts, called Pt/C-Solid catalysis with an IL layer (SCILL) catalysts, have recently attracted much attention [1]. ILs are liquid salts around room temperature, which have high ionic conductivity and electrochemical stability. We focused on hydrophobic ionic liquids with quaternary phosphonium cations [2]. Quaternary phosphonium cations ionic liquids have higher ionic conductivity and hydrophobicity than quaternary ammonium ionic liquids with the same structure and the same counter ions. Because of interference in the oxygen reduction reaction on platinum by water, increasing the hydrophobicity of the surface is expected to improve the activity on Pt/C catalysis. Hence, by using the quaternary phosphonium cations ILs for Pt/C-SCILL catalysts, it promises to improve the ORR activities for the catalyst. On the other hand, proton conduction and oxygen permeation rate are also important factors for ORR activity enhancement. In this study, we report the ORR activity for Pt/C-SCILL catalysts with ILs composed of highly hydrophobic cations and hydrophobic anions. Quaternary phosphonium based ILs (PXXXY+TFSA-[alkyl chain X= 4 Y= 1, 12, 16]) were used for Pt/C-SCILL modified electrodes. Commercially available 20 wt% or 30 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The ultrasonicated Pt/C catalyst suspension was dropped on the mirror polished glassy carbon disk electrodes. Pt/C-SCILL modified electrodes were prepared by recasting the ILs-2-propanol solution on the bare Pt/C modified electrodes or adding ILs to the catalyst suspension. Moreover, the mixture of the ILs and bis(trifluoromethanesulfonyl)imide (HTFSA) were also examined. The thickness of the ILs layer was calculated relative to the BET surface area of the carbon support. A platinum wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. CV and LSV were carried out in 0.1 M HClO4 solution under Ar (CV) or O2 (LSV) atmosphere, respectively. The Pt/C-SCILL catalyst was prepared by recasting (thin layers, ~0.4 nm) or mixing (thick layers, 1nm~) methods. In the case of the thin layer (~0.4 nm) of ILs, the ORR activity increased as the size of the cations increased. However, for the thicker (1 nm ~) ILs layers, the ORR activity was lower than for the bare Pt/C catalysts. It seems that the aprotic ILs interfered with the proton supply, making the four-electron reduction reaction of oxygen less likely to occur. Therefore, we attempted to supply protons to the thick ILs layer by mixing an ionic liquid with a proton source. HTFSA was dissolved in P444(12)TFSA, which showed relatively good ORR activity in thin film studies. The Pt/C-SCILL catalyst was prepared by adding the mixture with a catalyst dispersion. This proton mixing Pt/C-SCILL catalyst showed a significant improvement in ORR activity compared to bare Pt/C catalyst.

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

Investigation of a Method of Evaluating Proton Transport Resistance in PEFC Catalyst Layers

Introduction In polymer electrolyte fuel cells (PEFCs), the catalyst layers, consisting of electrocatalysts and ionomers, greatly affect current-voltage (IV) performance. In other words, not only catalytic activity and electronic conductivity, but also proton conductivity become important to achieve higher performance in PEFCs. Even though the proton conductivity within the catalyst layer is one of the most important factors, there is no method to directly measure the proton conductivity in catalyst layers. Therefore, the objective of this study is to establish a method to directly measure the proton resistance and to quantitatively evaluate the proton conductivity within the catalyst layer. Experimental A schematic diagram for the proton conduction measurement used in this study is shown in Fig. 1. A commercial cell holder commonly used for PEFCs was utilized in our measurement system. As show in Fig. 1, Pt/C gas diffusion electrodes and water-repellent GDLs were used for both the anode and cathode sides. The sample to be analyzed was sandwiched between two Nafion® membranes in order to block electrons. In detail, the 1 cm2 gas diffusion electrode was prepared by spray printing 46.5% Pt/KB (TEC10E50E) on each Nafion®212 film. Then, for one of the two Nafion® membranes, the sample to be analyzed was also spray-printed on the reverse side. Finally, two Nafion® membranes were hot pressed first and assembled with two GDLs as described in Fig. 1. With this setup, ohmic resistance was measured using a current interrupt method, in which direct current (DC) was applied to the sample but electrons were blocked. Since it is important to select a reaction with a small overvoltage for the DC reaction, we have used hydrogen oxidation/reduction reaction in this study and flowed hydrogen on the anode side and nitrogen on the cathode side. Results and Discussion Before the actual sample measurement, a Nafion®212 was first sandwiched instead of the sample to be analyzed and tested. When the H2 generated at the cathode side was quickly removed along with the N2 flow, a gradient of hydrogen concentration was produced. The resulting concentration cell generated the additional voltage. Also, unlike a fuel cell reaction, since water was not produced during the reaction, the Nafion® membrane dried up as the reaction proceeded, leading to increase in resistance. Therefore, in order to avoid formation of a concentration cell, the flow rate of gases was minimized and also N2 gas was stopped at the cathode right before the measurement. Additionally, the water-repellent GDLs normally used in PEFCs were replaced with GDLs without a water-repellent treatment. Using the optimized measurement system described in the above, two samples, a catalyst layer containing Pt/Vulcan, and a Vulcan carbon support layer without Pt, were tested. As a result, ohmic resistance of the Pt/Vulcan layer was very small like 0.011 Ω. On the other hand, the Vulcan catalyst layer showed 0.095 Ω. The reason for this is most likely because the presence of Pt in the layer brings up some other chemical reactions, causing the electron transfer within the sample layers, and the electron movement cannot be completely blocked.Consequently, by directly evaluating the Pt-free carbon support layer, deriving the proton conductivity in the catalyst layer becomes the best way. Figure 1

Predicting Fuel Cell Ink Aggregation

Polymer-electrolyte fuel cells (PEFCs) provide multisector decarbonization solutions including in transportation, manufacturing, and long-term energy storage. They have become increasingly popular in these areas due to their high efficiency, power density, and low (or zero) emissions compared to traditional fossil-fuel dependent processes. The PEFC catalyst layer is the most complex and key part of the cell, and is critical for optimizing PEFC performance. Several studies have explored the structure/function relationships of PEFC catalyst layers, yet the physics and interactions controlling its in-situ formation remain a mystery. PEFC catalyst layers are traditionally fabricated out of a catalyst supported on a carbon nanoparticle with an ionomer, traditionally perfluorosulfonic acids (PFSAs) such as Nafion, as a binder, which stabilizes the carbon suspensions in the ink dispersion. Recent studies demonstrated the importance of pH as an experimental parameter for both comparison and characterization of such systems.1 In this talk, we explore the interactions in the colloidal inks through detailed mathematical modeling. We propose a kinetics-based model representing species aggregation with pointwise interacting spheres that vary in charge through buried side chains for predicting the aggregation behavior of Nafion and carbon in solutions under varying conditions, such as solvent, Nafion wt% and carbon wt%. To demonstrate the accuracy and robustness of the model, we compare the results against a range of pH conditions and size distributions. The insights from the model help establish design criteria and guide future ink and process conditions.AcknowledgementsThis study was conducted under the Million Miles Fuel Cell Truck Consortium (M2FCT) funded by the Hydrogen and Fuel Cell Technologies Office in the Energy Efficiency and Renewable Energy Office of the U.S. Department of Energy under contract DE-AC02-05CH11231.References S. A. Berlinger, B. D. McCloskey, and A. Z. Weber, J. Phys. Chem. B, 122, 7790–7796 (2018).

Parametric Study of the Influence of Support Type, Presence of Platinum on Support, and Ionomer Content on the Microstructure of Polymer Electrolyte Fuel Cell Catalyst Layers

Ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, the presence of platinum on carbon, and ionomer loading on the microstructure of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs). Particle size distributions (PSDs), obtained from fitting the measured scattering data were used to interpret the size of carbon aggregates (40–300 nm) and agglomerates (>400 nm) from two-component carbon/ionomer and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were investigated: high surface area carbon (HSC) and Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer to carbon (I/C) ratios (0.2–1.2) and also with perfluoroimide acid (PFIA) ionomer were studied to evaluate the effect of ionomer on CL microstructure. The carbon type, the presence of platinum, and ionomer loading were all found to significantly impact carbon agglomeration. The extent of Pt/C agglomeration in the CL was found to increase with increasing ionomer and platinum concentration and to decrease with increasing carbon surface area. Platinum electrochemically-active surface area (ECSA) and local oxygen transport resistance (RnF) were correlated to the CL microstructure to yield relationships affecting electrode performance.

N-, F-, and Fe-Doped Mesoporous Carbon Derived from Corncob Waste and Creating Oxygen Reduction Reaction Active Centers with a Maximum Charge Density of ≥0.25 for a Polymer Electrolyte Fuel Cell Catalyst

N-, F-, and Fe-Doped Mesoporous Carbon Derived from Corncob Waste and Creating Oxygen Reduction Reaction Active Centers with a Maximum Charge Density of ≥0.25 for a Polymer Electrolyte Fuel Cell Catalyst

Insight towards kinetics and stability of carbon supported Pd–Y2O3 as cathode catalyst for polymer electrolyte fuel cells

Pd–Y 2 O 3 on carbon (Pd–Y 2 O 3 /C) in different mass ratios of Pd to Y 2 O 3 (1:1, 2:1, 3:1) were prepared and subjected as cathode electrocatalyst for polymer electrolyte fuel cells (PEFC). X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to determine the structure, crystalline size and dispersion of Pd–Y 2 O 3 /C respectively. The electrochemical characterizations of the electrocatalysts were evaluated from Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV). These electrocatalysts were evaluated for their catalytic activity towards oxygen reduction reaction (ORR) in fuel cell. Pd 2 –Y 2 O 3 /C catalyst shows higher power density of 325 mW cm −2 than Pd 1 –Y 2 O 3 /C, Pd 3 –Y 2 O 3 /C and Pd/C. • Various atomic ratios of Pd–Y 2 O 3 /C prepared and evaluated as ORR catalyst for PEFC. • Pd 2 –Y 2 O 3 /C as a cathode catalyst shows high peak power density of 325 mW cm −2 . • Pd 2 –Y 2 O 3 /C exhibits higher ORR activity due to SMSI effect.