Performance Of Polymer Electrolyte Fuel Cells Research Articles

The Effects of Contact By the Catalyst Layer and the Gas Diffusion Layer Interface on the Performance of a PEFC with Marimo-like Carbon

1. IntroductionThe Pt catalyst supported on carbon black (Pt/CB) is widely used as a catalyst for the polymer electrolyte fuel cell (PEFC). As an alternative carbon support to the CB, we have used Marimo-like carbon (MC). MC has a spherical structure comprised of carbon nanofilaments (CNFs) radially grown from an oxidized diamond core 1). There are voids of several hundred nanometers between the CNFs of the MC. These voids are expected to improve the mass diffusivity. The cell components of the PEFC mainly include the catalyst layer (CL), gas diffusion layer (GDL), flow field plates, and gaskets. When the cell is assembled, the CL and GDL are compressed to the gasket thickness. This compression improves the contact between the CL and GDL interface and between the CL and the flow field plate interface. The CB has a particulate structure, whereas the MC has a fibrous structure. This difference in the structure of the carbon support transforms the interface structure of the CL. Therefore, in this study, we investigated the optimum conditions of the interface contact for the CL of the MC. The conditions of the interface contact were adjusted by controlling the gasket thickness. The results were evaluated based on the PEFC performance.2. ExperimentalThe catalyst supports were the CB and MC. Pt/CB was prepared as a commercial Pt nanoparticle supported on the CB. MC was synthesized by thermal chemical vapor deposition (CVD). For the synthesis, methane (CH4) gas was used as the carbon source of the CNFs and Ni supported diamond oxide (Ni/O-dia.) was used as a growth catalyst. The Ni/O-dia. was contacted with the CH4 gas in a reactor rotating at 5 rpm. The CH4 gas was supplied at the flow rate of 100 CCM for 3 hours. During this time, the rotary reactor was heated to 550°C. Pt/MC was synthesized by the wet process2). To the suspension containing the MC, platinum chloride (H2PtCl6) and citric acid were added and stirred. Next, NaBH4 was added to the solution to reduce the Pt ions. The membrane electrode assembly (MEA) was fabricated by transferring the CL to the Nafion membrane by hot pressing. The area of the catalyst layer was 50 mm x 50 mm 3). The compression of the CL and GDL when the cells were assembled was evaluated by calculating the compression ratio. The compression ratio was calculated from the ratio of the total thickness of the CL and GDL before and after the cell assembly. The total thickness of the CL and GDL before the cell assembly was the sum of the measured interface contact of the CL and GDL thicknesses, respectively. The total thickness of the CL and GDL after cell assembly was regarded as the gasket thickness. The gaskets were fabricated from PTFE sheets with the gasket thicknesses ranging from 100 to 180 µm. The gaskets of various thicknesses were adopted for the sealing in order to control the compression ratios. The PEFC performance was evaluated at the cell temperature of 80°C. Humidified hydrogen gas was supplied to the anode and humidified air was supplied to the cathode.3. Results and discussionThe compression ratio was calculated from the thickness of the CL, the GDL and the gasket. The thinner the gasket thickness, the higher the compression ratio. The compression ratio varied from 50 to 2% by using gasket thicknesses of 100 to 180 µm. The change in the compression ratio as a function of the gasket thickness was almost the same for the Pt/MC and Pt/CB.The compression ratio affected the PEFC performance. Under-compression and over-compression conditions reduced the PEFC performance. The decrease in the PEFC performance under the low compression ratio condition was due to the reduced contact area of the interface between the CL and GDL and between the GDL and flow field plate. By increasing the compression ratio, the contact area was increased and the PEFC performance was improved. In the case of an excessive compression, the reduced gas diffusion paths in the CL and GDL decreased the PEFC performance. The change in the PEFC performance affected by these compression conditions was particularly high by the MC.1) N. Hirata, et.al., RSC Adv., 11, 39216-39222 (2021)2) M. Eguchi, et.al., Jpn. J. Appl. Phys., 52, 06GD06 (2013)3) C. Simon, et.al., J. Electrochem, 164, F591-F599 (2017)

The Performance of PEFC with Marimo-like Carbon Considering Relative Humidity

1.IntroductionThe Pt catalyst supported on carbon black (Pt/CB) is widely used as a catalyst for the polymer electrolyte fuel cell (PEFC). As an alternative carbon support to the CB, which is a type of particulate carbon, we have used Marimo-like carbon (MC) which is a type of fibrous carbon. MC has a spherical structure comprised of carbon nanofilaments (CNFs) radially grown from an oxidized diamond core 1). The CNFs in the MC have a cup-stack primary structure of graphene sheets; thus, the sheet edge can effectively function as a supporting site for the Pt catalyst. There are voids of several hundred nanometers between the CNFs of the MC. These voids are expected to improve the diffusion of hydrogen gas and air and the removal of produced water. The diffusivity of hydrogen gas and air and the removal of produced water are different between the MC and CB due to their structural differences. These differences in mass diffusivity changed the state of water management and the PEFC performance. In this study, the performance of a PEFC with Pt/MC while considering the relative humidity (RH) of the hydrogen gas and air were investigated.2.ExperimentalThe catalyst supports were CB and MC. MC was synthesized by thermal chemical vapor deposition (CVD). For the synthesis, methane (CH4) gas was used as the carbon source of the CNFs and Ni supported diamond oxide (Ni/O-dia.) was used as a growth catalyst. The Ni/O-dia. was contacted with the CH4 gas in a reactor rotating at 5 rpm. The CH4 gas was supplied at the flow rate of 1000 CCM for 3 hours. During this time, the rotary reactor was heated to 550°C. Pt/MC was synthesized by reducing Pt ions in a suspension with MC. Pt/CB was prepared as a commercial Pt nanoparticle supported on the CB. Nafion was used as the ionomer in this study. The catalyst ink was prepared by adding a Nafion dispersion to the Pt/MC and Pt/CB. The ionomer/carbon (I/C) ratio was 0.1 and 0.7 for the MC and CB, respectively 3). The catalyst layer (CL) was prepared by spraying the catalyst ink onto a 50 mm × 50 mm PTFE sheet. The membrane electrode assembly (MEA) was fabricated by transferring CL to the Nafion membrane by hot pressing. The PEFC performance was evaluated at the cell temperature of 80.0°C. Hydrogen gas and air were saturated at this temperature using a humidifier. Humidified hydrogen gas was supplied to the anode and humidified air was supplied to the cathode. The relative humidity of the hydrogen gas and air was controlled by the humidifier temperature.3.Results and discussionThe structure of the MC was observed by a scanning electron microscope (SEM). SEM images of the MC showed numerous CNFs. Voids of several hundred nanometers were observed between the CNFs. The Pt supporting state of the Pt/MC was observed by a transmission electron microscope (TEM). TEM images of the Pt/MC showed Pt supported on the surface of the CNFs.The PEFC performances of the Pt/MC and the Pt/CB were compared. At a high current density, the PEFC performance of the Pt/MC was higher than that of the Pt/CB. The PEFC performance in the high current density region consumed a large amount of hydrogen gas and air. This result was due to the high hydrogen gas and air diffusivity and the high removal of produced water from the MC.The relative humidity of the air was controlled from 20 to 100% RH by supplying it to a humidifier at a temperature of 42.7 to 80.0°C. The humidification conditions of the air affected the PEFC performance of both the Pt/MC and the Pt/CB. Under the low humidification condition, the PEFC performance was reduced due to the decrease in proton conductivity of the Nafion membrane. Under the high humidification conditions, the PEFC performance decreased due to flooding. The change in the PEFC performance under these humidifying conditions was particularly high by the MC.1) N. Hirata, et.al., RSC Adv., 11, 39216-39222 (2021)2) M. Eguchi, et.al., Jpn. J. Appl. Phys., 52, 06GD06 (2013)3) M. Eguchi, et.al., polymers, 4, 1645-1656 (2012)

Durability and Structural Transformation of Pt Alloy Nanowires and Nanoparticles for the Oxygen Reduction Reaction in Acidic Media

The oxygen reduction reaction (ORR) is catalyzed at the cathode in polymer electrolyte fuel cells (PEFCs). Since the ORR is sluggish and the electrocatalytic activity of the ORR catalyst determines the overall performance of PEFCs, highly active and durable ORR electrocatalysts have been intensively studied. The combination of alloying and nanostructuring of Pt is one of the promising approaches to develop such ORR electrocatalysts. Among the Pt-based shape-controlled electrocatalysts, Pt or Pt–M alloy nanowires (M = Ni and Co) are known to exhibit high ORR activity in the half cell [1,2]. Since nanowires have larger contact areas and therefore potentially higher adhesion to the carbon support than conventional nanoparticles, Pt-based nanowires should have higher durability. However, details of the effect of durability testing (potential cycles) on the structure of nanowires remain unclear.In this work, we report synthesis of PtNi alloy nanowires at different temperatures under different atmospheres, and their structural transformation after potential cycles. PtNi nanowires were prepared at 433 K under Ar and air [3]. The difference in atmosphere affects the presence or absence of PtNi nanoparticles with PtNi nanowires: both PtNi nanowires and nanoparticles are produced under the inert condition of Ar, whereas PtNi nanowires are exclusively produced under the oxidative condition of air. PtNi nanowires with and without PtNi nanoparticles were converted into branched nanostructures after durability tests. PtNi nanowires were also prepared at different temperatures of 433, 493 and 553 K under Ar [4]. The Ni content in the PtNi nanowires increases from <5 at.% up to about 15 at.% with increasing synthesis temperature. The number of PtNi nanoparticles also decreases with increasing synthesis temperature. The PtNi nanowires in the co-presence of PtNi nanoparticles prepared at 493 K gave highly durable beads-on-nanowires with the Pt skin via Ostwald ripening of coexisting PtNi nanoparticles. This structural transformation is coupled with changes in the surface electronic structure, confirmed by in situ X-ray absorption spectroscopy. Understanding such a structural transformation will help us to design and develop highly durable Pt alloy nanostructured electrocatalysts for the ORR.References.[1] M. Li et al., Science, 354, 1414 (2016).[2] Z. Zhao et al., Adv. Mater.,31, 1808115 (2019).[3] M. Kato, Y. Iguchi, T. Li, Y. Kato, Y. Zhuang, K. Higashi, T. Uruga, T. Saida, K. Miyabayashi, I. Yagi, ACS Catal., 12, 259 (2022).[4] Y. Zhuang, Y. Iguchi, T. Li, Y. Kato, M. Kato, Y. A. Hutapea, A. Hayashi, T. Watanabe, I. Yagi, ACS Catal., 14, 1750 (2024).

Coarse-Grained Molecular Dynamics Simulation of Ionomer-Mediated Carbon Cluster Bonding in Polymer Electrolyte Fuel Cell Catalyst Layers

In the field of sustainable energy technologies, Polymer Electrolyte Fuel Cells (PEFCs) represent a crucial advancement for achieving high-efficiency power generation. This study focuses on the specific behavior of ionomers within PEFCs, particularly how they facilitate the bonding of carbon clusters during the evaporation process, critical for the structural integrity and functionality of the catalyst layer.Amid the global shift toward a hydrogen economy, the development of robust and efficient PEFC technologies is imperative. The Nafion membrane, developed by DuPont, is the standard bearer for proton exchange membranes due to its superior proton conductivity, chemical stability, and durability. Despite these advantages, the performance of PEFCs under high temperature and low humidity conditions remains a challenge due to the reduced efficiency of proton transport. This research concentrates on the micro-mechanics of how ionomers assist in the bonding between carbon clusters during the drying stages of film formation, aiming to overcome this efficiency bottleneck.Employing Coarse-Grained Molecular Dynamics (CGMD) simulations, our study delves into the detailed interactions between ionomers and Pt-carbon clusters during evaporating solvents. The simulations use a refined computational model to capture the dynamic interplay during evaporation conceptualized as micro-differentiated boxes within the system as illustrated in Figure 1. These boxes allow for the analysis of ionomer behavior on a flat, graphene-sheet-modeled cluster surface adorned with Pt particles, assessing how different configurations influence structural and transport properties.This approach not only elucidates the role of ionomers in promoting the aggregation of carbon clusters during evaporation but also provides insight into the nuanced processes that govern the formation and stability of the catalyst layer. Initial simulations pointed out the need for adjustments in the force field settings, which were subsequently optimized to enhance the accuracy of the observations regarding ionomer distribution and bonding effectiveness during evaporation.With the foundational simulations set, future work will extend to exploring how various experimental conditions—such as solvent composition, and the size and density of platinum particles—affect ionomer bonding between carbon clusters. By adjusting these parameters systematically, we aim to define the optimal conditions that enhance the structural cohesion and functional performance of the catalyst layer.This investigation is expected to yield significant insights into the fundamental interactions and mechanisms at play in the bonding process, contributing to a deeper understanding of PEFC technology and leading to advancements in the production methods of these fuel cells. Ultimately, this research will help to refine and optimize the procedures for manufacturing more efficient and durable PEFCs, marking a substantial step toward more sustainable energy systems.AcknowledgmentThe New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work under Grant number NP20003.A part of the results was obtained by supercomputer system of Institute of Fluid Science, Tohoku University. Figure 1

Synthesis and Evaluation of Polyphenylene-Based Copolymers with Superacid Groups (IV)-Effect of Superacid Structure

In order to improve the performance of polymer electrolyte fuel cells (PEFCs), polyelectrolytes with higher proton conductivity under low humidity and high temperature conditions will be required. It is well known that hydrocarbon polyelectrolytes with superacid groups show high proton conductivity at low relative humidity (RH) conditions compared to typical sulfonated polyelectrolytes due to the strong acidity providing a high efficiency of carrier generation. In this study, random copolymers consisting of the hydrophilic poly(4’-phenoxybenzoyl-1,4-phenylene) (SNF-PPBP) unit with sulfonyl(trifluoromethyl sulfonyl)imidate as a super acid group, and the hydrophobic poly(benzoyl-1,4-phenylene) (PBP) unit were synthesized. The effect of the hydrophilic structure and phase separation on electrolyte membrane properties was investigated. Two types of random copolymers, PPBP m -PBP n with different copolymerization ratios (m : n) were synthesized by Ni(0) coupling reaction. The copolymerization ratios of PPBP m -PBP n were 1.2 : 1.0 and 2.3 : 1.0, respectively. The number-average molecular weights were determined by GPC to be 2.22×104 g mol-1 (PPBP1.2-PBP1.0) and 3.03×104 g mol-1 (PPBP2.3-PBP1.0), respectively. After chlorosulfonation of PPBP m -PBP n s (SC-PPBP-PBPs), trifluoromethyl-bis(sulfonyl)imide (SNF) groups were introduced by using trifluoromethansulfonamide. FT-IR measurements suggested that the sulfonate groups were also introduced as by-products during the synthesis of SC-PPBP-PBP. The introduction rate of sulfonyl chloride groups (X SC) and sulfonate groups (X SO3H) to PPBP units of SC-PPBP-PBP and that of SNF groups (Y SNF) and sulfonate groups (Y SO3H) were estimated from elemental analysis data. The X SC and X SO3H for PPBP units of SC-PPBP-PBPs were 75% and 62% (SC-PPBP1.2-PBP1.0), and 61% and 51% (SC-PPBP2.3-PBP1.0), respectively. The Y SNF and Y SO3H for PPBP units of SNF-PPBP-PBPs were 75% and 62% (SNF-PPBP1.2-PBP1.0), and 62% and 50% (SNF-PPBP2.3-PBP1.0). The ion exchange capacity (IEC) of SNF-PPBP-PBPs determined by back titration was 2.41 meq g-1 (SNF-PPBP1.2-PBP1.0) and 2.23 meq g-1 (SNF-PPBP2.3-PBP1.0). TG-MS measurements exhibit that the 5% weight loss temperature (T d-5%) of both polymers was about 230℃, indicating that both polymers have sufficient thermal resistance for PEFC applications. Despite the high IEC value, SNF-PPBP1.2-PBP1.0 showed higher thermal resistance because of the dense introduction of ion exchange groups and these electron-withdrawing properties. The proton conductivity was evaluated in the range of 30 - 80%RH at 80oC, and SNF-PPBP-PBPs showed higher humidity dependence and proton conductivity compared with Nafion®211 membrane under high humidification conditions. SNF-PPBP-PBPs also showed higher proton conductivity than that of the homopolymer SNF-PPBP (IEC = 2.39 meq g-1). These results suggest that the superacid groups have high carrier generation ability, especially in high humidity regions, and the random copolymer structure contributes to the improvement of proton conduction. The PEFC performance was evaluated at 80oC and 90%RH using membrane assembly electrodes (MEAs) fabricated with SNF-PPBP-PBPs, Nafion®211, and SNF-PPBP (IEC = 2.51 meq g-1) membranes. The limiting current densities (I lim) of MEAs with SNF- PPBP1.2-PBP1.0, SNF-PPBP2.3-PBP1.0, Nafion®211, and SNF-PPBP membranes were 1825, 1904, 1924, and 1429 mA cm-2, respectively. The membrane resistance (R m) of MEAs with SNF- PPBP1.2-PBP1.0, SNF-PPBP2.3-PBP1.0, and Nafion®211 membranes were 29, 17, and 21 mΩ, respectively. SNF-PPBP1.2-PBP1.0 and SNF-PPBP2.3-PBP1.0 MEAs showed high power generation properties comparable to Nafion MEAs regardless of the copolymerization ratio.

Effects of Selected Fuel Cell Operating Conditions on the Distribution of Liquid Water in the Cathode Gas Diffusion Layer

Effective water management is required to enable polymer electrolyte fuel cells (PEFCs) to operate efficiently, particularly at the high current densities needed to make them relevant for transportation and stationary applications. The presence of liquid water can influence the performance of a PEFC when the rate of water production exceeds the rate of water removal [1]. More specifically, accumulation of water within the cell becomes problematic when the pores within the gas diffusion layer (GDL) become filled, resulting in restriction of the transport of reactants to the catalyst layer [2]. Identifying operational conditions that minimize mass transport losses associated with excess water accumulation is crucial for improving the performance of PEFCs, especially at high current densities. Additionally, GDL design plays an important role in reducing mass transport losses, with thermal conductivity being a key factor influencing the distribution of liquid water.In this work, miniaturized fuel cells were operated under various conditions and imaged in-operando using X-ray Computed Tomography (XCT) [3]. This imaging technique facilitates the evaluation of the distribution of liquid water at steady state over a range of operational conditions. The influence of current density on liquid water distribution was assessed at selected cell temperatures between 40°C and 70°C. The results of this study showed high saturations under both the land and the channel regions at 50 °C. However, the channels appeared to be dry at 70 °C, with liquid water present only under the lands. These data were compared to predictions generated using a one-dimensional (1-D) model of the through-plane GDL, which considers specific operating conditions and GDL characteristics, such as thermal conductivity [4]. These results demonstrate that the 1-D model can be used to predict the presence or absence of liquid water within the GDL (and, by extension, PEFC performance) under a broad range of operating conditions. Hence, the 1-D model may be useful for focusing future laboratory investigations. Keywords – operando, X-ray tomographic microscopy, GDL, water visualization Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Simon Fraser University, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Canada Research Chairs.

Evaluating Effects of Cerium Concentration and Water Content on Diffusivity of Cerium Ions in PEFC Using Molecular Dynamics Simulations

Polymer electrolyte fuel cell (PEFC) is highly regarded as a promising technology. High durability is needed to achieve the large scale spread of PEFC. Chemical degradation of polymer electrolyte membrane (PEM) is one of the most critical problems that decreases the longevity and reduce functionality of a PEFC. Hydroxyl radicals and hydroperoxyl radicals are generated by the decomposition of hydrogen peroxide generated during PEFC operation. These radicals have high reactivity and degrade the polymer membrane. A method of adding a substance that quench radicals before reacting with the polymer (radical scavenger) has been put into practical use. One of the most useful radical scavengers is a cerium ion. Cerium ions in the PEM have been reported to migrate and the distribution become non-uniform which causes local degradation. Furthermore, if we add exceeded amount of cerium ions, proton conductivity decreases. There are three factors that can make cerium ions migrate in PEM: concentration gradient, water flux, and electrical potential. These factors are considered to be influenced by cerium concentration, temperature, and water content. Separating these effects in a real system and numerically evaluating the effects of many more variables is difficult. Therefore, nanoscale numerical analysis is a realistic approach to elucidate the transport properties of cerium ions. In this study, molecular dynamics simulations were used to evaluate the transport properties of each factor of cerium ions in polymer electrolyte membranes.Simulation system is composed of Nafion chains, cerium ions, and solvent molecules (water molecules and hydronium ions). The number of water molecules was determined by the water content λ which represents the number of solvent molecules per sulfonate group. The water content was set at λ = 3, 5, 7 that correspond the real conditions. Cerium ion concentrations are set at 5.38, 7.53, and 9.69 mg/cm3, which is the typical concentration mentioned in previous studies [1]. The number of hydronium ions was adjusted according to the concentration of cerium ions so that the positive charge is balanced by the negative charge of the sulfonic acid group of Nafion in the system. Three-dimensional periodic boundary condition was applied to simulation box. The temperature in the system was 323, 363, and 403 K. After equilibration, production run was performed for 50 ns using the NVE ensemble to calculate diffusion coefficient. To analyze the effect of water flux, a force in the x direction to the water molecules was applied. The force magnitude was determined by confirming that there was no change in water molecule polarization and membrane structure. After steady state was confirmed, and a 50 ns sampling run was performed using the NVT ensemble. The time step was set at 1 fs, and the sampling interval was 10 ps.The diffusion coefficients of cerium ions were obtained using mean square displacements. Fig.1(a) shows that the higher temperature, the greater the diffusion coefficient of cerium ions. Fig.1(b) indicates that when λ=3, the diffusivity was higher at higher concentrations, but the rate of increase in diffusivity with increasing water content was higher at lower concentrations, and at λ=7, the diffusion coefficient of cerium ions was larger at lower concentrations. This result indicates diffusivity vary depending on the concentration of cerium ions, which has not been previously considered by experiment. To evaluate water flux influence, the ratio of the molar flux of cerium ions to the molar flux of water molecules (K conv) in the x direction was taken. Molar fluxes were obtained from the mean displacement of the molecules. The fluxes were compared under conditions of cerium ion concentrations at 9.69 mg/cm3, because they cannot be accurately evaluated when the concentrations are low. Fig.1 (c) shows that K conv takes lower value as temperature increases and the lowest value when λ=5 at all temperatures. As Fig.1(d) shows, the reason for the larger K conv at lower temperatures is that the mean displacement of cerium ions is not so different at 323 K~363 K, but the mean displacement of water at 323 K is significantly smaller than at other temperatures. Fig.1 (e) indicates that at water contents of 3 and 5, the mean displacement of water increases dramatically, even though the change in the mean displacement of cerium ions is small. These results can lead to optimization of water and temperature management for control of cerium ion distribution.AcknowledgmentThe New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work under Grant number JPN20003.Reference[1] A. M. Baker., et al., Cerium Migration during PEM Fuel Cell Accelerated Stress Testing, J. Electrochem. Soc. 163, F1023 Figure 1

Synthesis and Evaluation of Polyphenylene-Based Copolymers with Phosphonium Group (III) – Relationship between Copolymerization Ratio and Film Formability

Polymer electrolyte fuel cells (PEFCs) are limited to operate at ~80oC because of the requirement of water absorption for the proton conduction and the low thermal properties of the proton exchange membrane. Operation of PEFCs in the middle and high temperature range (> 100oC) without humidification has many advantages such as simplify of water management system, improvement of catalytic activities, and reduction of catalytic poisoning. Ionic liquids with active protons (protic ionic liquids, PILs) are attracting attention as new proton conductors that can be used without humidification. Diethylmethylammonium triflate ([dema][TfO]) has high ionic conductivity (4.33 × 10- 2 S cm- 1 at 120oC) without humidification, excellent thermal stabilities, and wide potential windows among PILs. 1 ）In order to use PILs as electrolyte materials, it is necessary to fabricate solid membranes by complexing with a matrix polymer. In this study, poly(4-(trimethylphosphonium)propyl)benzyl)-1,4-phenylene-random-poly(4-phenoxybenzophenone) (PPr-r-PPBP) with quaternary phosphonium groups, which has excellent mechanical properties and thermal stabilities as a matrix polymer was synthesized, and these film formability and electrolyte properties were evaluated.Random copolymers PPr-r-PPBPx:y having different copolymerization ratios was synthesized by Ni (0) coupling reaction with (3-(4-(2,5-dichlorobenzoyl)phenyl)propyl)trimethylphosphonium (P-PrDBP) and 2,5-dichloro-4’-phenoxybenzophenone (DPBP). Since it was difficult to measure the molecular weight of these cationic copolymers by GPC, the approximate molecular weight of these copolymers was determined by comparing the viscosity of the homopolymer, PPBP with a known molecular weight. The 5% weight loss temperatures were 416oC for PPr-r-PPBP1.0:3.0 and 453oC for PPr-r-PPBP1.0:6.9, indicating that these copolymers have sufficient thermal stabilities for materials used in the medium and high temperature ranges. Composite membranes PPr-r-PPBP/PIL-z were obtained by casting the mixed DMSO solution of PPr-r-PPBP and [dema][TfO] on glass substrates, where z is the weight ratio of [dema][TfO] to the composite membrane (wt%). FT-IR measurements showed that all samples had absorption bands for the stretching vibration of S-O bonds at 1023 and 1154 cm-1 derived from [dema][TfO], the stretching vibration of C-F bond at 1225 cm-1, and the stretching vibration of N-H bond at 3054 cm-1. The S-O stretching vibration of PPr-r-PPBP/PIL-z shifted to the lower wavenumber than the S-O stretching vibration of [dema][TfO] before the composition. This result suggests that the cationic groups of PPr-r-PPBP interact with [dema][TfO] in PPr-r-PPBP/PIL-z. The 5% weight loss temperatures of the composite membrane were 362oC for PPr-r-PPBP1.0:3.0/PIL-60 and 344oC for PPr-r-PPBP1.0:3.0-PIL30, which decreased the thermal stability compared with the copolymer before being composited with [dema][TfO]. Proton conductivity was evaluated in the non-humidified condition from 50oC to 100oC. The proton conductivity increased with increasing the content of [dema][TfO]. The proton conductivity of PPr-r-PPBP1.0:3.0-PIL60 was 4.3 × 10-2 S cm- 1 at 100oC, which showed sufficient proton conductivity as electrolyte membranes for the use of high-temperature and non-humidified operation.

Relationship between synthesis conditions and ORR activity of PtCo nanowire/C catalysts for PEFC cathodes

Introduction Polymer electrolyte fuel cells (PEFCs) are used as power sources for automobiles because of their advantages such as the low environmental impact and high energy conversion. However, despite their convenience, they have not become widespread. One of the reasons is their high cost. In general, expensive Pt is used as the electrocatalyst for PEFCs, and high catalytic activity and durability are required with the reduced amount of Pt. Until now, Pt nanoparticles have been commonly used as the electrocatalysts, but there have been problems with Pt elution and agglomeration during PEFC operation. Recently, nanowire (NW) structure, which suppresses the aggregation of catalyst particles, has attracted attention. 1-3) In the formation of NWs, metal carbonyls, such as Co2(CO)8 and Mo(CO)6, act as strong adsorbents on Pt and the surface direction of the NWs. Oleylamine (OAm) also plays a specific role in nanostructure formation with capping ligands, using various organic or inorganic compounds as the precursors alone or in combination with other reactants. In this study, the cathodic PtCo NW/C catalysts were prepared with different amounts of OAm, and their effects on NW morphology, ORR activity, and durability were investigated. Experimental To prepare the PtCo NW, 100 mg of Pt(acac)2,75 mg of Co2 (CO)8, and 80 mg of Vulcan XC-72R were ultrasonically dispersed into OAm (25, 50, 100, and 150 mL), respectively. The mixture was then heated at 100 °C for 2 h while being stirred. Once the product was collected, it was washed multiple times with a mixture of EtOH and cyclohexane. The samples were naturally dried at room temperature and heat treated at 350 ℃ under H2/Ar flow. For removing the residual amount of Co, acid treatment was performed in acetic acid at 70 ℃ for 0.5 and 2 h. Finally, PtCo NW catalysts were prepared and noted as PtCoNW /C OAm25, PtCoNW/C OAm50, PtCoNW/C OAm100, and PtCoNW/C OAm150, respectively. The morphology and composition of PtCo NW/C were characterized by XRD, TEM, TGA, and XRF analysis. CV, CO stripping, and LSV were conducted for evaluation of ORR activity. MEA was fabricated using the PtCoNW/C catalyst as the cathode and evaluated for the I-V characteristic tests. ADTs were also performed to evaluate the durability of the PtCoNW/C catalysts. Results and Discussion XRD patterns showed that the peaks originating from the Pt (111) plane of the four PtCo NW/C catalysts were shifted to the higher angles, indicating that formation of the PtCo alloy. The XRD peaks shifted to the lower angle for the catalyst treated with acid for 2 h, compared to the catalyst treated with acid for 0.5 h. Figure 1 shows, the TEM image of PtCo NW was formed and uniformly dispersed on carbon support. The average diameter of the PtCo NW/C catalyst treated with acid for 2 h was smaller than that of the catalyst treated with acid for 0.5 h. The average diameters were changed with increasing amount of OAm25, OAm50, OAm100, OAm150 (3.6, 2.3, 2.0, and 2.1 nm), resperctively. Electrochemical active surface area (ECSA) was calculated from the CV and CO stripping curves. The ECSA of the PtCo NW/C OAm100 catalyst after 2 h of acid treatment was the largest among the four catalysts. The PtCo NW/C OAm100 catalyst had the highest mass activity (MA) calculated from the LSV. As shown in Figure 2, the cell voltage at high current density region (1.0 A cm－ 2) of the NW catalysts was lower degradation rate, compared to the commercial PtCo/C catalyst. Among all PtCo NW/catalysts, the PtCo NW/C OAm100 catalyst was showed excellent durability. This would be due to the NW morphology, which suppresses agglomeration.In conclusion, it was revealed that the addition of an appropriate amount of OAm would improve the ORR activity of the NW morphology of PtCo/C catalyst.This work was partly supported by NEDO, Japan. References (1) X. Wang, et.al., Nano Lett. 16,1467 (2016).(2) H. Zhang, et.al., Sci. Adv. 12, 2548 (2019).(3) T. Wang, et.al., Energy Mater. 9, 1803771 (2019).(4) K. Jiang, et al., Sci. Adv. 3,1601705 (2017). Figure 1

Understanding Degradation of PtCo/C Catalysts in Polymer Electrolyte Fuel Cell Subjected to Two Accelerated Stress Tests

Hydrogen is a promising fuel and chemical that has the potential to decarbonize many of the energy sectors, especially those with the largest carbon emission footprint. Hydrogen is especially valuable in the sectors of heavy-duty transportation, aviation and shipping because of its high gravimetric energy density. Heavy-duty vehicles (HDVs), although the minor portion of transportation sector at 7 % of the total vehicles emit 23% of the total greenhouse gas in the US1. Polymer electrolyte fuel cells (PEFCs) can potentially impact HDV sector because of their high efficiency, low weight and small footprint. PEFCs are already commercially deployed technologies but cost and durability are the areas that can be improved to enable large-scale adoption of the PEFCs. PEFC stack is the largest cost components of the fuel cell system and within the stack Pt-based cathode catalyst layer is large cost contributor. Pt and Pt-based catalysts are used on the cathode side to promote oxygen reduction reaction (ORR), which is sluggish.The state-of-the-art Pt-based catalyst for ORR is Pt-Co alloy, which has higher mass activity than Pt, enabling lower loadings of Pt catalyst used in PEFCs. Its increased activity towards ORR at the beginning of life (BOL) testing can also translate to the higher activity at the end of life (EOL), granted that Co dissolution from the catalyst can be mitigated. Co leaching results in reduced mass activity towards the ORR and also ionomer and membrane poisoning by Co2+ ions2. Pt shell that is formed on top of Pt-Co alloy particles should prevent cobalt dissolution but Pt dissolution also occurs during the PEFC operation, which exposes Co underneath the Pt shell resulting in further dissolution of both. Therefore, it is difficult to stabilize PtCo catalysts especially during harsh protocol of cycling during accelerated stress tests (ASTs).In this work, a membrane electrode assemblies (MEAs) with PtCo supported on Vulcan and high-surface area (HSA) carbon were fabricated. These MEAs were subjected to two ASTs: one that is the standard Department of Energy (DOE) catalyst degradation AST and the other that is introduced by M2FCT DOE consortia. The work shows that PtCo/HSA has higher degree of polarization loss, especially during the M2FCT protocol because of higher degree of Co leaching. The Co leaching was detected through HFR increase and also through proton transport resistance increase in the catalyst layer. It was further confirmed through transmission electron microscopy studies. We also observed that M2FCT protocol was harsher on the catalyst durability compared to the DOE catalyst degradation AST.

Development, Evaluation and Characterization of Hydrophilic Oxide/Polymer Composite Membranes for Polymer Electrolyte Fuel Cells Operating at Temperatures over 80oc

The present transportation system based on the internal combustion engine is a major contributor to the issue of CO2 emission, and a paradigm shift is required toward to electrical motorization based on batteries and polymer electrolyte fuel cells (PEFCs). The motorization of heavy-duty vehicles (HDVs) by PEFCs is one of the essential targets. HDVs will require higher operating temperatures, at least 120oC, versus normal temperatures (60-80oC), for optimal heat management. A commercial perfluorosulfonic acid-based electrolyte (PFSA, e.g., NafionⓇ) shows better proton conductivity with chemical stability over a wide temperature range, although the conductivity decreases under low back pressure at 100~120 oC. In order to improve the proton conductivity in the higher temperature range, a composite membrane of NafionⓇ with a hydrophilic filler of Ta-doped TiO2 (Ta-TiO2) was synthesized. Ta-TiO2 nanoparticles with fused-aggregate network microstructure were synthesized by the flame oxide-synthesis method and were crushed by wet milling (Star Burst, Sugino Machine, Limited Co.). The fillers were mixed with NafionⓇsolution (20 wt.%, D2020 Chemours Co.) by use of a rotary ultrasonic dispersing processor (500 rpm, 60 min., PR-1, THINKY Co.), a planetary ball mill (500 rpm, 30 min., PULVERISETTE 6, FRITSCH Co.), and a defoaming processor (500 rpm、5 min., HM-400W, Kyoritsu Seiki Co.). The dispersion solution was coated in hot air by use of a die coating system (die coater, Taku-Dai Mini-50, Daimon Co., Ltd.), and were hot-pressed (140 ℃, 3 min., TCMD-2.5, TOHO KOGYO Co.) to form composite membranes (9 cm × 9 cm × 25 µmt). The cross-sectional backscattered electron image BSE) of the composite membrane was observed by scanning electron microscopy (SEM, acceleration voltage 2 kV, SU9000, Hitachi High-Tech Co.) and transmission electron microscopy (TEM, H-9500, Hitachi High-Tech Co.). The proton conductivity of the composite membrane was measured at 80 °C and 120 °C by the AC four-probe method. Water uptake of the composite membrane was measured by water vapor adsorption. The current-voltage (I-V) polarization curves of single cells using the prepared composite membrane (membrane thickness, 25 µm) were measured at 80 ℃, 100% RH and 120 ℃, 20% RH, back pressure 50 kPaG.The obtained composite membranes were soft and flexible. The BSE images indicated, based on their high contrast, that the Ta-TiO2 particles were uniformly dispersed in the composite membrane(Fig. 1(a)). The higher magnification of the cross-sectional TEM images also showed that the hydrophilic surface of Ta-TiO2 adhered to the Nafion® without any voids. The proton conductivity of the composite membrane at 80 °C and 80% RH increased with increasing Ta-TiO2 content and showed a maximum value of 0.13 S cm-1at 3 wt% and then decreased. The maximum proton conductivity was 1.3 times higher than that of Nafion®(Fig. 1(b)). The proton conductivity of the composite membrane (3 wt% of Ta-TiO2) at 120 °C, 20% RH reached 0.018 S cm-1, which was 1.8 times higher than that of a commercial Nafion® membrane, 0.01 S cm-1. The water uptake in each composite membrane was nearly independent of the concentration of the hydrophilic oxide at each humidity(Fig. 1(c)). Comparing with literature results, improvements of the proton conductivity by use of various types of additives have been reported, but the improvement of the proton conductivity by such a small percentage of hydrophilic oxide additive without affecting the water uptake in the membrane has not been reported, to our knowledge. We consider that the protonic hopping (proton mobility) might change as a result of adding the hydrophilic Ta-TiO2 in the membrane. The I-V performance of a single cell using a composite membrane preserved the same performance as that of a cell using an unmodified Nafion® membrane at 80 °C and 100% RH, and then improved at 120 °C and 20% RH(Fig. 1(d)).

Effect of Contamination in the Carbon Catalyst Support on Chemical Degradation of Polymer Electrolyte Membranes in Fuel Cells

Introduction One of the key components of polymer electrolyte fuel cells (PEFCs) is the polymer electrolyte membrane (PEM). The PEM facilitates the transport of protons from the anode to the cathode, acts as an electronic insulator to prevent short circuits, and plays a role in preventing the direct reaction of hydrogen gas and oxygen in air. One of the major technical issues with PEMs is their chemical degradation, causing decomposition of their molecular structure. This leads to reduced protonic conductivity, increased hydrogen and oxygen crossover, and reduced mechanical strength, all of which significantly affect the power generation performance and durability of PEFCs (1). In addition, in our laboratory, the use of SnO2 and mesoporous carbon (MC) as cathode catalyst supports has been suggested to suppress membrane degradation (2). However, the relationship between the cathode catalyst materials and the chemical degradation of PEM has not yet been fully understood. Here in this study, we conduct durability tests such as accelerated chemical degradation of PEM in cells using cathode electrocatalysts with different carbon support materials, aiming to clarify the effects of carbon support materials properties (structure, contaminants, and graphitization) on the chemical degradation of electrolyte membranes. Experimental A commercial Pt/KB catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) was used as the anode catalyst. Several types of cathode catalysts were applied and compared: the commercial Pt/KB; Pt/MC where Pt was directly deposited on the MC; and various electrocatalysts where Pt was directly deposited on carbon supports such as graphitized AB (acetylene black). The durability test at open circuit voltage (OCV) was performed for 100 h. Before and after the durability test, cell performance, hydrogen crossover current density, and electrochemical surface area (ECSA) were characterized. The degree of membrane degradation was evaluated by measuring fluoride-ion emission rate (FER) by ion chromatography, and membrane thickness by cross-sectional observation with a focused-ion-beam scanning electron microscope (FIB-SEM), as illustrated in Fig.1. Moreover, contaminants and graphitization of the carbon support materials were also analyzed, and their relationship to membrane degradation was considered. Results and discussion The results of OCV holding tests up to 100 h for four different MEAs are shown in Fig. 2. The voltage drop in using Pt/MC as cathode catalysts was smaller than that in using Pt/KB. The FER of Pt/MC as cathode catalysts was lower than that of Pt/KB as shown in Fig. 3, suggesting that the use of MC as carbon supports may suppress PEM degradation. In this presentation, we will show the effects of carbon support materials properties (structure, contaminants, and graphitization) in cathode catalysts on PEM degradation, and discuss possible mechanisms based on the characteristics of various carbon supports. Acknowledgements This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). An educational part for young scientists was supported by the Japan Science and Technology Agency (JST) through the program “Adopting Sustainable Partnerships for Innovative Research Ecosystem” (ASPIRE), Grant Number JPMJAP2307. The presenting author (S. Nakamura) is supported by the Miyamoto Jun-ichi Hydrogen Research Award, and the Kyushu University Q-PIT Support Program for Young Researchers and Doctoral Students. References M. Zatoń, J. Rozière, and D. J. Jones, Sustain. Energy Fuels, 1, 409 (2017).S. Nakamura, T. Ogawa, Z. Gautama, Z. Noda, M. Yasutake, S. M. Lyth, J. Matsuda, A. Hayashi, M. Nishihara, and K. Sasaki, ECS Trans., 112 (4), 315 (2023). Figure 1

Improvement of Mesoporous Carbon Supports for PEFC

Introduction Polymer electrolyte fuel cells (PEFCs) are one of the energy technologies to realize a carbon neutral society, and further improvement of IV performance is essential in order to expand the application into heavy-duty vehicles. Catalyst layers with three dimensional porous structures have an important role since they mostly determine the IV performance. To make such three dimensional structures, carbon supports are important. An attention to mesoporous carbon (MC) supports has been increased since MC is used in 2nd generation MIRAI. Our group have also been working on MC materials, MC bulk1 and MC fibers (MCF) 2. Based on our previous studies, we have found that the size of carbon particles affects the IV performance of PEFC as well as the structure of mesopores. In this study, we have successfully synthesized new MC that is composed of bigger mesopores and smaller particles than conventional MC bulk we developed. Effect of this difference in MC materials was evaluated in terms of IV performance by developing MEAs made by two MC materials. Experimental New MC was similarly synthesized to our previous study2 with modification. Pluronic F127 was used as the template to build up mesopores in addition to carbon precursors being composed of phloroglucinol dihydrate, formaldehyde, and ethyl orthoacetate. After well mixing of the template and precursors in acidic water/ethanol solvent, PVA solution was added and stirred. The heat treatments for polymerization of precursors in the air and then for decomposing the template and carbonizing carbon precursors in N2 were performed. Resulting new MC was ball-milled into small particles and used for catalyst supports. Pt deposition was done in two ways. In the conventional way1, Pt(acac)2 was used as a Pt precursor and thermally reduced. On the other hand, in the new method3, Pt(NO2)2(NH3)2 with alcohol reduction was used. Then, membrane electrode assemblies (MEAs) were prepared by spray printing 0.3 mg Pt/cm² using TEC10E50E and Pt/MCs for the anode and cathode, respectively, within 1 cm² area of Nafion 212 membrane. Results and Discussion N2 sorption measurements were done for new MC. Resulting N2 sorption isotherms and corresponding pore size distributions were shown in Fig. 1 (a) and (b), respectively, with those of MCs previously developed in our group. As shown in Fig. 1, new MC have bigger mesopores, and the pore size distribution is wider than other MC supports. The specific surface area of mesopores and external results in 350 cm2/g, which is almost equivalent to MC bulk. This result suggests that new MC also has a capability of Pt deposition. Regarding to Pt deposition, two methods were used as described in Experimental. Slightly smaller Pt particles were found in the case of the alcohol reduction method. MEAs using Pt/MC cathode synthesized in different conditions were prepared and evaluated. The best IV performance was obtained with 45% Pt/new MC cathode made by the alcohol reduction method. In comparison to 30.6% Pt/MC bulk, ohmic overvoltage was reduced, and the increase of IV performance was found especially at high current density. At the high current density region, IV performance was even better than that of MEA with TEC10E50E cathode. In order to investigate which factors have contributed to improvement of IV performance, electrocatalysts were prepared by varying %Pt, Pt deposition methods, and the pore structures of MCs, and then evaluated in details. References A. Hayashi et al., Electrochim. Acta, 53(21), 6117 (2008).W. Huang et al., Molecules, 26(3), 30 (2021).M. Rahman et al., RSC Adv., 11, 20601 (2021). Figure 1

High Oxygen Permeability Ionomer Composed Entirely of Cyclic Monomers: Synthesis, Properties, and Application to Polymer Electrolyte Fuel Cell Cathodes

Polymer Electrolyte Fuel Cells (PEFCs) are key devices, producing electricity from hydrogen and oxygen, pivotal for sustainable energy systems with no CO2 emissions. They are considered for various uses, including power sources for mobility, stationary cogeneration and off-grid power generation. However, the transportation sector, especially Heavy-Duty Vehicles (HDVs) and aviation, which have high emissions, experiences difficulties with decarbonization and cannot rely on existing batteries due to high load, high output, and long usage time. This scenario places high expectations on the broader application of PEFCs. To improve the performance of PEFCs, it is necessary to improve the performance of cathode ionomers, and the development of high oxygen permeability ionomers (HOPIs), which mitigate the oxygen transport rate-limiting process of the oxygen reduction reaction, has attracted much attention.1-5 In this study,6 we developed a new synthesis method for a cyclic monomer with a fluorosulfonyl group (SMD-E4) and succeeded in synthesizing a new HOPI composed entirely of cyclic monomer by copolymerization with a hydrophobic cyclic monomer (MMD). The basic electrolyte properties of the HOPI were compared with those of Nafion, and HOPI showed comparable proton transport performance with excellent oxygen permeability, suggesting its potential use as a PEFC material. By employing the HOPI as a cathode ionomer in MEAs, improved performance was observed, especially under low humidity and high current density operating conditions. Improvements were observed in both activation overvoltage and concentration overvoltage, as well as in electrochemical surface area, mass activity, specific activity, and oxygen transport-related factors. Analysis of the catalyst layers revealed that the property of the HOPI to maintain a relatively high pore volume within the catalyst layers may mitigate the direct contact between Pt and ionomer and contribute to the lowering of activation overvoltage. Furthermore, the separation of the oxygen transport resistance component suggests that the HOPI not only lowers the oxygen transport resistance of the ionomer itself, but also contributes to lowering the oxygen transport resistance derived from Knudsen diffusion by maintaining a high pore volume in the catalyst pores.Moreover, the performance of MEAs with the HOPI applied as a cathode ionomer was evaluated in a structure similar to that of a practical PEFC with a high-performance membrane and low Pt usage, and it was found that a power density improvement of more than 140% compared to that of Nafion was achieved under low humidity conditions (20% RH). Furthermore, the unique property of the HOPI that concentration overvoltage decreases under low humidity suggests the possibility of achieving MEAs with high robustness to humidity changes.These results indicate that HOPIs can fully demonstrate their performance under the harsh operating conditions of next-generation PEFCs, such as low humidity and high current density, and will play an important role in improving the performance of PEFCs. Acknowledgement This work was partially based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References 1) K. Yamada et al., ECS Trans., 50, 1495 (2013).2) R. Shimizu et al., J. Electrochem. Soc., 165, F3063 (2018).3) R. Jinnouchi et al., Nat. Commun., 12, 4956 (2021).4) J. P. Braaten et al., J. Power Sources, 522, 230821 (2022).5) N. Macauley, et al., Adv. Energy Mater., 12, 2201063 (2022).6) T. Hirai et al., ACS App. Energy Mater. (2024), https://doi.org/10.1021/acsaem.4c00177. Figure 1

One Million Atoms Large-Scale Reactive Molecular Dynamics Simulations for Design of Cathode Catalyst Layer in Polymer Electrolyte Fuel Cell Toward Boosting Its Performance

Boosting polymer electrolyte fuel cell (PEFC) performance is required in transportation. PEFC performance depends on the electrode reaction activity related to proton conductivity and oxygen diffusivity in catalyst layer (CL), consisting of Pt nanoparticles (Pt NPs), ionomer, water, and carbon supports. To achieve high proton conductivity and oxygen diffusivity, the optimization of CL structures such as Pt composition, ionomer/water distribution, and carbon support structures is essential by computational approaches, such as first-principles molecular dynamics (MD) and classical MD methods. The first-principles MD can describe chemical reactions, however can only simulate a small part of CL structures, consisting of a few hundred atoms. In contrast, classical MD can perform over 1 million atoms simulation to calculate whole CL structures, while the simple inter-atomic potential which enables 1 million atoms calculation, can not describe chemical reactions. Then, developing our original MD simulator "Laich", implementing MPI and OpenMP with ReaxFF inter-atomic potential, enabled us to calculate million atoms system reproducing whole CL structures and to simulate chemical reactions in whole CL structures. In this work, to design the higher-performance CL, we performed reactive MD simulations using a 1 million atoms CL model. A CL structure model was constructed with ionomers, water, Pt NPs, and carbon support. The carbon support composed of six meso pores with a size of 6 nm was constructed. To improve the hydrophilicity, hydroxyl groups and hydrogen terminated 32% and 31% of its surface carbon, respectively. Pt NPs were put on the carbon support at the exterior and in the interior of the meso pores. The Pt-supported carbon was coated with ionomers and water. Protons and oxygen were introduced into the system. Hereafter, we refer to this structure as a catalyst particle (CP) model (Fig. 1). To assess the influence of the carbon support structures on the electrode reactions, the oxygen diffusivity was evaluated from the trajectories of oxygen at the exterior and in the interior of the meso pores. Ideally, oxygen should be supplied to the Pt NPs without hindering its diffusion by obstacles. At the exterior of the meso pore, oxygen could not approach the Pt NPs, because ionomers and water were obstacles to the oxygen diffusion (Figs. 2(a)). Conversely, in the interior of the meso pores, oxygen could approach the Pt NPs via the gas phase since ionomers and water did not inhibit oxygen diffusion (Figs. 2(b)). From this analysis, the oxygen diffusivity showed higher value in the interior of the meso pores than at the exterior of the meso pores. Furtherly, it is reported that oxygen diffusivity in ionomers varies with temperature [1]. However, the influence of temperatures on oxygen diffusivity at the exterior and in the interior of the meso pores has not been analyzed. In this study, we calculated the mean squared displacement (MSD) and diffusion coefficients at 300 K and 365 K. At 300 K, oxygen diffusion coefficients (the linear regressions of MSD) at the exterior (Dexterior ) and in the interior of the meso pores (Dinterior ) were 1.67×10-10 m2/s and 3.02×10-6 m2/s, respectively (Fig. 3(a)). At 365 K, Dexterior and Dinterior were 12.3×10-10 m2/s and 2.59×10-6 m2/s, respectively (Fig. 3(b)). Therefore, Dexterior and Dinterior respectively increased 7.37 and 0.86 times with increasing temperature. Next, to investigate whether the change of Dexterior was caused only by temperatures, we calculated the oxygen diffusion coefficient at 1 atm (D1atm ) in an environment consisting of oxygen atoms only. As a result, D1atm increased 1.05 times from 300 K to 365 K. By rising temperature, the increase of the oxygen diffusion coefficient showed 1.05 time at D1atm , while it showed 7.37 time at D exterior. Therefore, other factors besides temperature affect the oxygen diffusivity. To identify the factors contributing to D exterior , we focused on the size of the water cluster, one of the obstacles to oxygen diffusion. Water clusters are defined as groups of water with an inter-molecular distance of less than 3.5 Å. The cluster size is defined as the number of water molecules forming the water cluster. Fig. 4 depicts the sizes of the 10 largest water clusters. As the temperature rose, the size of the largest water cluster decreased, while the size of other water clusters increased. These analyses indicate that the morphology of water clusters changed from continuous to discontinuous clusters with increasing temperature. Therefore, as water cluster sizes decrease by increasing temperature, oxygen diffuses through the gas phase. These results indicate that the control of the water cluster size improves the oxygen diffusivity.[1] K. Kudo et. al., Electrochim. Acta, 209, 682-690 (2016). Figure 1

Impact of Metal Oxide Nanoparticles in Anode Catalyst Layer on Membrane Degradation and Output Performance of PEFC

The chemical degradation of polymer electrolyte membranes (PEMs) is one of the most serious issues that need to be addressed to achieve a long lifetime for the polymer electrolyte fuel cells (PEFCs). The PEMs such as Nafion (perfulorinated sulfonic acid polymers) are attacked by ·OH radicals generated from the decomposition of H2O2molecules, which are predominantly produced at the Pt/C anode catalyst layer (ACL).1 The addition of metal oxide nanoparticles (NPs) such as CeO2 and MnO2 to PEM or anode side has been introduced as one of the effective solutions to demolish the chemical degradation of PEM by scavenging the ·OH radical through the redox properties of these metal ions.1‒3 Although the lifetime of the PEMs have effectively prolonged, the proton conductivity of both the PEM and the ionomer binder in the cathode catalyst layer (CCL) has dramatically decreased, resulting in an appreciable loss of the output performance,4, 5 i.e., trade-off relationship between the durability and the output performance. To suppress the membrane degradation accompanied with increased output performance, we have proposed a unique concept of addition of metal oxide NPs without releasing the metal ions into ACL. The silica NPs were the first that exhibited such excellent properties.6 In the present research, we offer a new candidate of metal(IV) oxide (MO2) NPs which played a great role in increasing the lifetime of Nafion-PEM much longer than in the case of using silica NPs. The catalyst-coated membranes (CCMs) of the current study have been prepared as reported in our previous work.6 The fabricated CCMs comprised Nafion 211 (NRE-211; 25 µm in thickness) as the PEM, heat-treated Pt/graphitized carbon black (Pt/GCB-HT, TEC10EA50E-HT, 50.2 wt%-Pt) as the cathode catalyst, and Pt supported on carbon black (Pt/C, TEC10E50E, 46.4 wt%-Pt) as the anode catalyst, in which MO2-NPs were incorporated with a changeable volume ratio to the carbon content (VMO 2/VC, ranging from 0 to 0.4). Each CCM was mounted in a single cell (geometric electrode area 29.2 cm2). The chemical stability of the PEM was examined via an accelerated stress test (AST) in which the single cell was maintained at open circuit voltage (OCV) in a 40% humidified H2/air at Tcell = 90 °C and a backpressure of 160 kPa-G.6 As a measure of the durability of the PEM, the lifetime is defined as the time at which the OCV reached 0.85 V (ca. 10% loss).Figure 1 shows the changes of the OCV and the total amount of fluoride (F−) ions emitted during the AST. The OCV of the cell without any metal oxide in the ACL (denoted as Pt/C only in Fig. 1) has rapidly dropped, reaching values below 0.85 V with a very high F− emission rate (FER, slope of the line in Fig. 1b) after ca. 150 h of durability testing. With the addition of silica at VSiO2/VC = 0.2 in the ACL, the lifetime has appreciably increased (ca. 570 h) with a low FER. It is striking for the MO2-based ACL with VMO2/VC =0.2 that the lifetime has been remarkably prolonged, reaching ca. 1650 h (ca. 11 times increase of the Nafion lifetime) together with the lowest FER.Also, it has been found that the use of silica NPs has provided a reduced ohmic resistance (increase in water content of PEM and ionomers) and an effective utilization of Pt cathode catalyst in a single cell, which contributed to increase the output I-V performance.6 A similar effect was observed for the incorporation of MO2 NPs into the ACL. Such an improvement of the output performance accompanied with the remarkable increase in the durability of PEM is quite distinct from the trade-off effect of conventional radical scavengers. The mechanism for suppression of membrane degradation, as well as the changes in the microstructure of CCM, are under progress in our laboratory.This work was supported by funds for the “R&D of novel anode catalyst project” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Endoh, ECS Trans., 16(2), 1229 (2008).Zhao, B.L. Yi, H.M. Zhang, and H.M. Yu, J. Membr. Sci., 346, 143 (2010).Wang, C. Cai, J. Tan, and M. Pan, Int. J. Hydrogen Energy, 46, 34867 (2021).Lin, C. Cao, H. Zhang, H. Huang, and J. Ma, Int. J. Hydrogen Energy, 37, 4648 (2012).H. Wong and E. Kjeang, J. Electrochem. Soc., 166, F128 (2019).Mohamed R. Berber, M. Imran, H. Nishino, and H. Uchida, ACS Appl. Mater. Interfaces, 15, 13219 (2023). Figure 1

Ionomer Adsorption Study during Electrochemical Reaction of Pt Electrode Surfaces Using Electrochemical Quartz Crystal Microbalance

Introduction: In polymer electrolyte fuel cells (PEFCs), the platinum (Pt) catalyst is covered by the solid polymer electrolyte ionomer. The ionomer used as an electrolyte at both the anode and cathode serves the function of transporting protons and dissolved oxygen to the surface of the Pt catalyst. Therefore, it is considered that the adsorption/desorption behavior of the ionomer during PEFC operation affects the catalytic activity of Pt. While perfluorosulfonic acid (PFSA) based ionomers are now widely used, previous studies have shown that the sulfonic acid groups on the side chains strongly adsorb to the Pt surface at thr cathode and suppress the oxygen reduction reaction (ORR) [1]. In this study, the relationship between the adsorption/desorption of several PFSA-based ionomers, such as NafionⓇ and AquivionⓇ, and the redox behavior of Pt electrodes was investigated by in situ monitoring with an electrochemical quartz crystal microbalance (EQCM), which is a method that can measure the potential-dependent, minute mass of 10-9 g (ng) changes on the electrode surface. Experiment: Electrochemical measurements were performed with a three-electrode cell, which was composed of Pt-QCM electrode as a working electrode, Pt wire as a control electrode, and Ag/AgCl as a reference electrode. Prior to the EQCM experiment, electrochemical cleaning of the electrode surface was carried out by using 50 mM H2SO4 solution. We used the aqueous dispersion solutions containing NafionⓇ and AquivionⓇ, and then the cyclic voltammograms (CVs) at scan rate of 50 mV s-1 were acquired at room temperature. Results and Discussion: In both aqueous dispersion solutions, we observed the mass changes during Pt oxide formation/removal and hydrogen adsorption/desorption reactions. NafionⓇ exhibits specific adsorption/desorption behavior during the Pt oxide formation/removal reaction process. At the potential approximately 0.5 V in the negative-going scan, cathodic current due to the removal of the surface Pt oxide flowed accompanied by mass increased and then decreased on Pt surface. This result suggested that OH adsorbed on the surface of the platinum oxide is reductively removed from the surface accompanied with NafionⓇ adsorption, and after that the surface Pt oxide is reductively removed accompanied with NafionⓇ desorption. On the other hand, AquivionⓇ showed adsorption/desorption behavior dependent on the reaction process of hydrogen adsorption/desorption on the Pt surface. At the potential range of 0 V or more negative, when hydrogen absorbed Pt surface in negative-going scan, the mass was greatly decreased and when hydrogen removed from surface in the positive-going scan, the mass increased again. Comparing the side chain structures of NafionⓇ and AquivionⓇ, the former has long side chain and one more ether group. It is considered that the difference in the length of these side chains corresponds to the difference in the adsorption/desorption activity on the Pt surface.

Combining Microscopy and Synchrotron X-Ray Diagnostics to Study Catalyst Degradation in PEFCs

Polymer electrolyte fuel cells (PEFCs) suffer from performance degradation during operation due to transformations in the catalyst (Pt or Pt/Co particles) and their carbon support. However, nano-structural diagnostic methods are not readily applicable in the humid, high-temperature, and gas environments of PEFC operation. This hinders elucidation of the mechanisms behind catalyst property evolution (size, shape, lattice parameters), which directly affects the electrocatalytic surface area (ECSA), a critical factor in catalyst performance.1 Here, we employ a combined methodology to study the changes in catalyst particle properties using identical location scanning transmission electron microscopy (IL-STEM)2 and Operando synchrotron X-ray measurements, particularly small angle X-ray scattering technique (SAXS)3. While IL-STEM offers high-resolution imaging for estimating local particle size, shape, and lattice parameters, SAXS provides statistically reliable data on the overall particle size distribution.4 These complementary techniques, with their inherent strengths, allow for a comprehensive understanding of the mechanisms involved during particle transformation.Electrochemical cycling of the catalysts was performed using a square wave potential pattern cycling between 0.6 and 1.0 VRHE at 6 seconds/cycle. A gold STEM grid was mounted on a specially designed working electrode for electrochemical cycling, which is performed ex-situ. STEM images, of the same areas, before and after electrochemical cycling were recorded using JEOL NEOARM operated at an accelerating voltage of 200 kV. SAXS experiments were conducted at BL40B2 beamline of SPring-8. An electrochemical cell (PECC-2, Zahner, Germany) was modified to increase X-ray transmittance. The thickness of its electrolyte layer was minimized to 1.5 mm, the upstream window was a 7.5 µm thick Kapton film. Its downstream window, a 50 µm thick graphite window onto which a catalyst film was deposited, acted as the working electrode. A PILATUS 2M detector was used to record scattering data with exposure time of 100 seconds.Scanning transmission electron microscopy revealed a comparable mean particle radius before electrochemical cycling (1.60 ± 0.44 nm) to that obtained by small-angle X-ray scattering (1.62 ± 0.32 nm), Table 1. Furthermore, STEM analysis of the same locations before (607 particles) and after (494 particles) cycling showed a similar trend in particle size growth as observed in operando SAXS measurements. However, the estimated sizes after cycling differed slightly, with SAXS data indicating a larger mean radius (2.08 ± 0.53 nm) compared to STEM (1.88 ± 0.49 nm). This discrepancy is attributed to the smaller probed volume by STEM, which was also limited to the same locations, potentially excluding other growing particles from analysis compared to the bulk characterization offered by SAXS.These results highlight the potential of combining microscopy with synchrotron X-ray SAXS for studying catalyst growth mechanisms with greater accuracy. In addition, by incorporating complementary synchrotron X-ray techniques like wide-angle X-ray scattering (WAXS) and X-ray fluorescence (XRF) during analysis, a more comprehensive understanding of catalyst degradation can be achieved. This combined approach can pave the way for the development of more informed catalyst engineering strategies.Table 1 summarising the mean particle sizes obtained from both methods. BEFORE (nm)AFTER (nm)Peak radiusMean radiusFWHMPeak radiusMean radiusFWHMSTEM1.38 ± 0.011.60 ± 0.440.86 ± 0.031.66± 0.021.88 ± 0.490.95 ± 0.04SAXS1.47 ± 0.011.62 ± 0.320.60 ± 0.021.84 ± 0.022.08 ± 0.531.04 ± 0.04 References Lyth, S. M. & Mufundirwa, A. Electrocatalysts in polymer electrolyte membrane fuel cells. Heterog. Catal. Adv. Des. Charact. Appl. 2, 571–592 (2021).Yu, H. et al. Tracking Degradation in Individual Catalyst Nanoparticles Under Fuel Cell-Relevant Cycling Conditions by Identical-Location STEM. Microsc. Microanal. 28, 2614–2617 (2022).C. Smith, M., A. Gilbert, J., R. Mawdsley, J., Seifert, S. & J. Myers, D. In Situ Small-Angle X-ray Scattering Observation of Pt Catalyst Particle Growth During Potential Cycling. J. Am. Chem. Soc. 130, 8112–8113 (2008).Mufundirwa, A. et al. Contrast variation method applied to structural evaluation of catalysts by X - ray small - angle scattering. Sci. Rep. 1–9 (2024) doi:10.1038/s41598-024-52671-7.

Numerical Model for Investigating Effects of Cracks and Perforation on Polymer Electrolyte Fuel Cell Performance

The presence of cracks in the microporous layer (MPL) of polymer electrolyte fuel cells (PEFCs), often occurring during the membrane electrode assembly manufacturing process, has a significant impact on cell performance. However, the exact influence of crack presence, density, and patterns within MPLs on cell performance and transport behaviors remains unclear. This study introduces a three-dimensional macroscale model of PEFCs aimed at investigating the effects of MPL cracks and gas diffusion layer (GDL) perforations on cell performance and transport behavior. This model offers several advantages, including the ability to potentially integrate the effects of flow channel design in future. Additionally, the model can seamlessly incorporate electrochemical reactions and explore phenomena within the catalyst layers (CLs), expanding simulation capabilities beyond water transport alone. The findings suggest that MPL cracks contribute positively to performance by facilitating water drainage. Furthermore, when combined with perforations in GDLs, MPL cracks can significantly enhance performance by providing pathways for water transport. Overall, this study provides valuable insights into developing models for optimizing PEFC performance and underscores the need for further research and development in this area.

Microstructural Design of PEFC Electrocatalyst Layer Using Mesoporous Carbon

Heavy-duty applications of polymer electrolyte fuel cells (PEFCs) require improved activity and durability of the cathode electrocatalysts. The nanostructure and microstructure of the catalyst layer both significantly affect PEFC performance. Mesoporous carbons (MC) have mesopores which have been shown to suppress ionomer poisoning, when used as a catalyst support, leading to higher catalytic activity. Here, we compare the initial performance of single cells using Ketjen black (KB) and MC catalyst supports. Furthermore, the fuel cell performance is evaluated using Pt-Ta-Co alloy-based catalyst decorated on MC. Pt-Ta-Co/MC electrocatalyst exhibits high cell performance in a low current density range. However, cell performance at high current densities has yet to be improved due to non-uniform microstructure of the Pt-Ta-Co/MC electrocatalyst layer.