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A multiscale approach involving both density functional theory (DFT) and molecular dynamics (MD) simulations was used to deduce an appropriate binder for Pt/C in the catalyst layers of high-temperature polymer electrolyte membrane fuel cells. The DFT calculations showed that the sulfonic acid (SO3−) group has higher adsorption energy than the other functional groups of the binders, as indicated by its normalized adsorption area on Pt (− 0.1078 eV/Å2) and carbon (− 0.0608 eV/Å2) surfaces. Consequently, MD simulations were performed with Nafion binders as well as polytetrafluoroethylene (PTFE) binders at binder contents ranging from 14.2 to 25.0 wt% on a Pt/C model with H3PO4 at room temperature (298.15 K) and operating temperature (433.15 K). The pair correlation function analysis showed that the intensity of phosphorus atoms in phosphoric acid around Pt ( ρ P g Pt - P r ) increased with increasing temperature because of the greater mobility and miscibility of H3PO4 at 433.15 K than at 298.15 K. The coordination numbers (CNs) of Pt–P(H3PO4) gradually decreased with increasing ratio of the Nafion binders until the Nafion binder ratio reached 50%, indicating that the adsorption of H3PO4 onto the Pt surface decreased because of the high adsorption energy of SO3− groups with Pt. However, the CNs of Pt–P(H3PO4) gradually increased when the Nafion binder ratio was greater than 50% because excess Nafion binder agglomerated with itself via its SO3− groups. Surface coverage analysis showed that the carbon surface coverage by H3PO4 decreased as the overall binder content was increased to 20.0 wt% at both 298.15 and 433.15 K. The Pt surface coverage by H3PO4 at 433.15 K reached its lowest value when the PTFE and Nafion binders were present in equal ratios and at an overall binder content of 25.0 wt%. At the Pt (lower part) surface covered by H3PO4 at 433.15 K, an overall binder content of at least 20.0 wt% and equal proportions of PTFE and Nafion binder are needed to minimize H3PO4 contact with the Pt. [ABSTRACT FROM AUTHOR]

Oxygen diffusion in the cathode catalyst layer (CCL) is crucial to the high performance of polymer electrolyte membrane fuel cells (PEMFCs), especially in high current density or concentration loss regions. Recently, PEMFC performance has been reported to be enhanced by increasing CCL pore size and pore volume due to the reduction of diffusion resistance by capillary water equilibrium [Yim et al., Electrochimica Acta 56 (2011) 9064–9073]. Herein, we simulate these experimental results utilizing a new one-dimensional PEMFC model considering the effects of accumulated water film in CCL on oxygen diffusion. Two CCL microstructures were numerically generated based on agglomerate models to examine the experimental results obtained for two membrane electrode assembly (MEA) samples with different CCL porosity. The effective diffusivity of oxygen in the CCL was estimated by performing auxiliary simulations of oxygen concentration in CCL microstructures covered by a film of liquid water, with exponential correlation obtained between effective diffusivity and the thickness of the above film. Polarization curves predicted by the present model were in good agreement with experimental results. In agreement with the results of Yim et al., the present model predicts that the MEA featuring a CCL with smaller pores (which are more easily filled by liquid water) should exhibit a larger concentration loss at high current densities. [ABSTRACT FROM AUTHOR]

Abstract: Electrochemical corrosion of carbon in a membrane electrode assembly (MEA) is a critical issue to be resolved to satisfy the durability targets for commercialization of proton exchange membrane (PEM) fuel cells. In order to understand the dominant mechanisms of MEA degradation by carbon corrosion, accelerated stress tests (ASTs), which consisted of holding at 1.3V for 50h, were conducted in a single cell, and the microstructural changes of the cathode catalyst layers (CCLs) and polarization losses were analyzed. The results of carbon corrosion were an increase in CCL porosity and 50% loss in the surface area of the Pt, leading to a 70% loss of the initial MEA performance at 0.6V. Quantitative analyses of polarization losses for the MEA showed that oxygen diffusion was reduced significantly, while there were somewhat smaller increase of the kinetic and high frequency resistance (HFR) losses after carbon corrosion. Carbon oxide species that form on carbon particles during the carbon corrosion process make the surfaces of the CCLs more hydrophilic, and this is believed to diminish oxygen diffusion through the water-filled pores of the CCLs, leading to dramatic degradation of the fuel cell performance. However, another proposed mechanism of diffusion loss, which involves structural collapse and the loss of porosity in the CCLs due to carbon corrosion did not occur in the present study. These findings are anticipated to contribute to the development of more durable MEAs for PEM fuel cells. [Copyright &y& Elsevier]

Abstract: In this study, we significantly enhanced the oxygen diffusion in catalyst layer (CL) based on hydrocarbon ionomer containing dimethyl silicone oil (DSO) for polymer electrolyte membrane fuel cells. The enhancement of cell performance is attributed to increase of oxygen solubility in DSO as well as increase of hydrophobicity induced by DSO in CL. The oxygen diffusion flux in DSO is higher than that in water due to higher oxygen solubility. Furthermore, the addition of DSO in CL enhances the water transport from cathode to anode through polymer membrane due to hydrophobicity of DSO. This phenomenon helps to increase the hydration of polymer membrane and decrease the water flooding in cathode CL. [Copyright &y& Elsevier]

Abstract: Although cathode catalyst layers (CCLs) are at the center of water management in polymer electrolyte fuel cells (PEFCs), the understanding of water movement in CCLs and their roll on fuel cell performance is still limited. In this present study, several CCLs with controlled microstructure, including main pore size, pore volume and porosity ranging from 30 to 70nm, 0.443 to 0.962cm3/gPt/C, and 45.4 to 64.4%, respectively, were prepared by changing the hot-pressing pressure in a decal process, and their water management ability and cell performance were evaluated. The electrochemical analyses reveal that, as the pore size and pore volume of CCLs increase, the diffusion resistance mainly arising from water accumulation in the pores is evidently reduced by capillary water equilibrium, which leads to better cell performance. Water balancing between accumulation and discharging in the pores also depends on the CCL pore structure, and the CCLs with greater pore sizes and larger pore volumes reveal more stable cell performance by better water management in steady state operation, even under extremely humid conditions. Based on these MEA technologies such as fabrication of CCLs, further study will be performed to understand microscopic phenomena in nano pores of CCLs by combining the experimental approach with CCL numerical modeling. [Copyright &y& Elsevier]

Abstract: The combination of Nafion-based electrode and hydrocarbon-based membrane is an ideal choice for researcher in making membrane electrode assemblies (MEAs) containing alternative membranes replacing Nafion for polymer electrolyte fuel cells (PEFCs) due to their intrinsic properties. This advantage, however, is limited by the incompatibility between the membrane and the electrode, which results in MEA performance decay and low durability. In this study, we propose fabrication of MEA made of sulfonated poly(aryl ether sulfone) (SPES) membrane and Nafion-based electrode using the decal process. The decal process was found to be very effective in forming good interface between SPES and the electrode, although hot pressing temperature was relatively low (140°C). The SPES-MEA revealed comparable performance to conventional Nafion-MEA at high humidity, indicating negligible contact resistance in the SPES–electrode interface. Open circuit voltage (OCV) drop of SPES-MEA during OCV holding at 40% RH for 200h was from 0.975V to 0.8V, implying slight chemical degradation of SPES leading to increased hydrogen crossover in the membrane. However, it seems that the interfacial damage between the SPES and Nafion electrode in the SPES-MEA is negligible during the OCV test. Nonetheless, further investigation is necessary to confirm the long-term stability of the SPES-MEA fabricated by the decal process under harsher conditions such as dry/wet and freeze/thaw cycling. [Copyright &y& Elsevier]

Abstract: The small PEMFC stack to be integrated with miniaturized fuel reformer is usually operated under very poor operating conditions such as low relative humidity and reformed gas conditions including trace amounts of CO. Hence, for the stable operation of the stack under such real conditions, the effect of reactants feeding method and stack operating conditions such as stack temperature and gas humidity were experimentally investigated and the optimal operating strategy was suggested. The external-manifolds configuration was not efficient compared to the internal-manifolds due to their poor water management. The fuel introduction to the stack by opposite directions between air and hydrogen enhanced the stack stability, particularly under low humidified conditions, by enhancing water exchange between the two fuel streams. Even 10ppm of CO in the reformed gas deteriorated the stack performance seriously under low stack temperature or low relative humidity conditions. Hence, for the stable operation of the small PEMFC stack, particularly under practical operating conditions, air-bleeding method seems to be a promising strategy. [Copyright &y& Elsevier]

In this study, we address the catalytic performance of variously sized Pt nanoparticles (NPs) (from 1.7 to 2.9 nm) supported on magnéli phase titanium oxide (MPTO, Ti4O7) along with commercial solid type carbon (VXC-72R) for oxygen reduction reaction (ORR). Key idea is to utilize a robust and electrically conductive MPTO as a support material so that we employed it to improve the catalytic activity and durability through the strong metal-support interaction (SMSI). Furthermore, we increase the specific surface area of MPTO up to 61.6 m2 g−1 to enhance the SMSI effect between Pt NP and MPTO. After the deposition of a range of Pt NPs on the support materials, we investigate the ORR activity and durability using a rotating disk electrode (RDE) technique in acid media. As a result of accelerated stress test (AST) for 30k cycles, regardless of the Pt particle size, we confirmed that Pt/MPTO samples show a lower electrochemical surface area (ECSA) loss (<20%) than that of Pt/C (~40%). That is explained by the increased dissolution potential and binding energy of Pt on MPTO against to carbon, which is supported by the density functional theory (DFT) calculations. Based on these results, we found that conductive metal oxides could be an alternative as a support material for the long-term fuel cell operation. [ABSTRACT FROM AUTHOR]

The objective of this study is to determine the durability of polymer electrolyte membrane fuel cells (PEMFCs) in constant current operation incorporated with regular recovery protocol for eliminating reversible performance loss of membrane electrode assemblies. Effects of operation 'shunt current protocol' on PEMFC durability are studied through analyses of the main degradation mechanism based on results of electrochemical characterizations and post-mortem investigations. The voltage of the protected cell using the shunt current protocol is stably preserved under the applied current density for 700 h with less degradation (4.2% of decay ratio), while the performance of the unprotected cell steadily decreased with time (15.1% over 700 h). The substantial performance deterioration of the unprotected cell is mainly attributed to morphology deterioration of the cathode catalyst layer with oxidation of Pt catalysts and chemical degradation of ionomers caused by generation of excessive water from electrochemical oxygen reduction reactions under high-humidity operating conditions. In contrast, the shunt current protocol plays an important role in sustaining high oxygen activity at the cathode catalyst surface, detaching partially covered OH− species on Pt active sites from water oxidation during cell operation caused by periodically applied shunt current for a very short period of 1 s every 5 h. We hope to provide insight into the operation protocol to extend the lifetime of PEMFCs, minimizing conversion from recoverable performance loss to irreversible (permanent) degradation during operation. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The effect of gas diffusion layer (GDL) compression on fuel cell performance was experimentally studied in a 5-cell proton exchange membrane fuel cell (PEMFC) stack. Two stacks with different GDL compression, 15% and 30%, were prepared using SGL 10BC carbon-fiber felt GDL and Gore 57 series MEA. The PEMFC stack with high GDL compression (30%) showed better performance than low GDL compression (15%) at all current ranges. This implies that the decrease of contact resistance by high GDL compression affects more dominantly on the stack performance than the increase of mass transport resistance in the present stack configurations. The degree of GDL compression also affected the stack stability, particularly, under the reformed gas conditions as anode fuel. The stack with low GDL compression could not be operated at low reformed gas flow rates showing sudden decrease of stack voltage. Hence, in case of reformed gas condition, GDL compression should be higher than the case of pure hydrogen for the stable stack operation and to enhance the gas utilization at anode. [Copyright &y& Elsevier]

In this study, we propose PtFe IM N@Pt/C, featuring a Pt shell on a nitrogen-infused PtFe intermetallic core structure, and attempt large-scale synthesis of catalysts with high metal content (>50 wt%). Using the ultrasound-assisted polyol synthesis method, we synthesize core@shell structured PtFe@Pt/C catalysts with approximately 60 wt% metal content at a 10 g scale in a single step. Post-treatments, including annealing, acid treatment, and nitriding in high-pressure NH 3 gas, follow to synthesize the target catalysts. Optimizing the annealing temperature reveals a trade-off relationship affecting catalyst activity and durability. The PtFe IM N@Pt/C_600 °C, synthesized under optimized conditions, exhibits superior electrochemical performance and durability compared to commercial Pt/C catalysts. The accelerated stability test (AST) further shows significantly enhanced durability when nitrogen is included in the core. In the single cell tests, PtFe IM N@Pt/C_600 °C demonstrates approximately four times higher mass activity than commercial Pt/C catalysts and a 29.9 mV voltage drop at 0.8 A cm−2 after AST, meeting the U.S. Department of Energy's targets. [Display omitted] • 10 g-scale synthesis of N-infused core@shell structured Pt-based catalysts for ORR. • Annealing conditions optimized to ensure optimal activity and durability. • A trade-off relationship with annealing temperature has been identified. • Optimal PtFe IM N@Pt/C_600 °C catalyst outperforms commercial catalysts. • PtFe IM N@Pt/C_600 °C meets the 2020 US DOE target for mass activity and durability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Transport properties of ultrathin ionomer films are investigated by full atomistic simulation. • Development of water channel is accelerated with increasing thickness of ionomer films and hydration level. • Solubility of oxygen decreased with the increasing thickness of the ionomer films. • Thick ionomer films hinder development of the free volume and restrict permeability of oxygen. • Hopping transport of protons is hindered by increasing thickness of the ionomer films. Understanding the transport of oxygen, proton, and water in the catalyst layer is important for improving the performance of proton exchange membrane fuel cells. In this investigation, the nanostructure of ultrathin (3–7 nm) ionomer films and the influence of the film thickness on the transport properties are investigated using molecular dynamics simulations. The development of the water channel, corresponding to the transport of water and vehicular transport of proton, is accelerated with the increasing thickness of the ionomer films and the hydration level. However, the oxygen permeability, which is associated with the solubility and diffusivity of oxygen, is affected by the free volume and has a trade-off relationship with the properties of water. Notably, the hopping transport of protons is hindered by the increasing thickness of the ionomer films because of the nanostructure of the water phase, derived from the strong interaction with the Pt particles. [ABSTRACT FROM AUTHOR]

Accelerated degradation tests (ADTs) are commonly used to assess the durability of membrane electrode assembly (MEA) components consisting of polymer electrolyte membrane fuel cells (PEMFC) under harsh stress conditions, estimating their lifetime in actual use condition and uncovering their vital degradation mechanisms. ADTs apply mechanically, chemically, or thermally combined stressors to efficiently investigate the durability of MEAs. However, combined stressors for ADTs might cause biased lifetime prediction because major deterioration mechanisms of MEA components are mixed with each other. This work proposes a method to accurately predict the lifetime of MEA through empirical modeling of its performance degradation through ADTs under potential cycle (carbon corrosion) and load cycle tests (electrocatalysts). To simulate operation modes of fuel cell electric vehicles, MEAs are tested under continuous on-off cycle testing (24 h operating and 1 h break) for 5000 h. Degradation patterns of MEAs are first modeled by the empirical model. The relationship between ADTs (potential and load cycle) and continuous on-off condition is then closely examined to accurately predict MEA lifetime under actual operation environments. The proposed idea has a potential to resolve critical durability issues of MEAs by identifying intermingling effects from other constituents. • Lifetime prediction technology based on decay mechanism of cells is developed. • Degradation patterns of cells are analyzed under load and potential cycle tests. • Degradation mechanism of cells are examined by various analysis tools. • Cell lifetime is predicted by empirical modeling of its performance degradation. • The relationship between accelerated degradation and on-off testing is examined. [ABSTRACT FROM AUTHOR]

There is still limited understanding of ink structure and the transition process from ink to the catalyst layer (CL) in proton exchange membrane fuel cells despite recent interest in the field. This study proposes a methodology for analyzing the CLs formed after drying single- and multi-ink droplets produced using an ultrasonic spray. Two model inks, Pt/Vulcan Carbon and Pt/Ketjen Black, are prepared using these catalysts, and their characteristics are analyzed through sedimentation behavior, ionomer adsorption quantification, and rheology. The structural properties of the spray-dried CLs are examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy-Auger electron spectroscopy. These analyses reveal distinct differences in the CLs' structures, influenced by physical phenomena such as coffee ring formation and droplet coalescence, which are dependent on the ink properties. Furthermore, the structural characteristics of the CLs produced by the two model inks affect the electrochemical behavior of the 25 cm2 fuel cell. This study establishes a correlation between the ink structure, CL structure, and fuel cell performance, emphasizing the usefulness of the ultrasonic spray coating method in advancing such investigations. [Display omitted] • Investigated ink structure through the application of ultrasonic spray coating. • Noted the occurrence of the coffee ring effect in fuel cell ink droplets. • Utilized Auger electron spectroscopy for assessing ionomer distribution in CLs. [ABSTRACT FROM AUTHOR]

An effective method to enhance the durability of polymer electrolyte membrane fuel cells (PEMFCs) is reported. PEMFC performance loss is mitigated by exposing the electrodes of fuel cells to dry nitrogen gas periodically at high temperature. This method extends the lifetime of fuel cells significantly compared to their non-treated counterparts. The impact of treatment temperature and exposure time on PEMFC durability is reported, using potential cycling accelerated stress tests. The enhanced durability is attributed to the suppression of “ionomer relaxation” that occurs under the nearly water saturated operating conditions of a PEMFC cathode. [ABSTRACT FROM AUTHOR]

We synthesized novel sulfonated poly(arylene ether sulfone)s (SPAESs) multi-block copolymer membranes containing highly sulfonated hydrophilic blocks. Two different kinds of multi-block SPAESs sharing the same structure of hydrophobic blocks were proposed to investigate the membrane properties based on the effects of the hydrophilic blocks bearing sulfonic acids. The higher local concentration and acidity of the sulfonic acid groups in the hydrophilic block induced higher proton conductivity. The block SPAESs showed comparable proton conductivities to those of the state-of-the-art perfluorinated sulfonic acid (PFSA) membranes, and their transmission electron microscopy (TEM) images revealed a well-developed phase separation. In particular, the SPAES 4 X10Y10 membrane showed excellent proton conductivity, which was higher than that of the PFSA over the entire relative humidity (RH) range at 80 °C. Moreover, the multi-block SPAES membranes showed a comparable fuel cell performance to the Nafion membranes, even at 50% and 30% RH. [ABSTRACT FROM AUTHOR]

Abstract: The carbon material with tailored mesopore size was explored as an electro-catalyst in polymer electrolyte fuel cells (PEFCs). The mesoporous carbon material possesses unique structural characteristics including non-micropores, specific surface area, pore volume and well-developed interconnected mesoporosity. The good characteristics of the mesoporous carbon make it a large efficient catalyst support for PEFCs. The great mesoporous carbon supported PtRu electro-catalyst has demonstrate an improvement in electro-catalytic activity toward cyclic voltammetry in comparison with commercial Vulcan XC72 supported. [Copyright &y& Elsevier]

Abstract: This study aims to investigate the effect of different gas diffusion layers (GDLs) on freeze/thaw condition durability in polymer electrolyte fuel cells (PEFCs). Three kinds of GDLs–cloth, felt and paper type—with similar basic properties except thickness and bending stiffness were used. The changes in the properties and cell performance were investigated from the −30 to 70 °C range of freeze/thaw cycles. The I–V performance degradation was observed to be negligible for the felt GDL whereas the cloth and paper GDLs showed a marked I–V performance loss. No distinctive correlation between the changes in electrochemical properties, such as active metal surface area, hydrogen crossover rates and decreased I–V performance, was observed except an increase in ohmic resistance revealed by ac-impedance spectroscopy. The physical destruction of electrodes was also shown by scanning electron microscope (SEM) analysis. The present study found that sufficient mechanical supporting force between the interfaces of materials enhances PEFC durability in sub-zero temperature conditions. [ABSTRACT FROM AUTHOR]

• We use ultrasound to synthesize of PdCo@Pt nanoparticles up to 10 g per batch. • Effect of post heat-treatment of PdCo@Pt was investigated under varied gas conditions. • Heat-treated PdCo@Pt samples shows improved ORR activity and durability. • Heat-treated PdCo@Pt samples under H 2 atmosphere shows highest durability. For the commercialization of polymer electrolyte fuel cells (PEFCs), the development of highly active and durable electrocatalysts is essential to improve the sluggish oxygen reduction reaction (ORR) and reduce the usage of precious group metals (Pt and Pd) in the catalyst layer of the membrane electrode assembly (MEA). Core–shell structured nanoparticles (NPs) have garnered significant attention because their unique structure enables the maximization of Pt utilization and catalytic performance. Moreover, ternary Pt-based core–shell electrocatalysts are being actively investigated for securing additional activity and durability in ORR. Herein, we report the sonochemical preparation of ternary PdCo@Pt NPs up to 10.0 g per batch and post-heat treatment effects based on the gas atmosphere. Based on the difference in reactivity of metal β-ketonate precursors under ultrasound irradiation, a core–shell structure with a Pt skin layer is formed and the as-prepared PdCo@Pt sample is post heat-treated under various gas atmospheres (N 2 , H 2 /N 2 , and NH 3 /N 2) to compare the catalytic activity and durability of ORR. The structural characterization of the heat-treated PdCo@Pt samples reveals that the gas atmosphere during heat treatment controls the particle size, the degree of alloying and the Pt shell thickness. In addition, electrochemical studies show that heat-treated PdCo@Pt samples under a reducing atmosphere (H 2 /N 2 , NH 3 /N 2) exhibit higher ORR activity and better durability than those heat-treated under an inert atmosphere (N 2). We demonstrated that the use of ultrasound enables mass production of core-shell NPs, and gas induced segregation under post heat-treatment help to control the structure of appropriate electrocatalyst for PEFCs. [ABSTRACT FROM AUTHOR]

Abstract: Enhancement of system durability at the sub-freezing temperature is one of critical issues for the commercialization of polymer electrolyte fuel cell (PEFC) applications. In this work, effects of residual water at the sub-freeze condition on the gas diffusion layer (GDL)/membrane-electrode assembly (MEA) were investigated for the successful cold start-up of PEFC. The performance changes of MEAs were observed by 300 times of freeze/thaw (F/T) cycles with well designed single cell. The post analysis for the (−30 to 70)°C F/T cycled MEA were conducted. The gradual decreases of I–V performance were observed after the F/T cycle number of 70. About 0.4mV/cycle of degradation at 1A/cm2 could be obtained at the controlled operating conditions. The main cause of performance decrease was in the weakened adhesion between the electrodes and membrane interface. The SEM images as well as cell resistance changes directly supported the degradation reasons of cell performance in the freeze condition. [Copyright &y& Elsevier]