Fuel Cell Conditions Research Articles

Nanophase-Segregated Structures and Transport in Aquivion Membrane for Polymer Electrolyte Membrane Fuel Cell

In this study, molecular dynamic simulation method was employed to study the structure and transport properties of Aquivion in comparison to Nafion. From experimental and computational work, there is a general consensus that the favored properties of Polymer electrolyte membrane (PEM) system are a result of nano-phase segregation of hydrophilic clusters in a hydrophobic matrix. Aquivion has a similar chemical structure to Nafion with hydrophobic backbone and hydrophilic sulfonate group. As such, we investigate the structure formed by Aquivion, to check for the desired nanophase segregation, when simulated in conditions similar to fuel cell conditions of temperature and humidity. We also characterize water diffusion and proton transport in this segregated structure, and compared them with hydrated Nafion membrane system.

How stable is LiNi0.8Co0.15Al0.05O2 under high-temperature hydrocarbon ceramic fuel cell conditions?

LiNi0.8Co0.15Al0.05O2 (LNCA) is an important cathode material optimized for room-temperature operation of lithium-ion batteries (LIBs). As an emerging research area, LNCA was recently applied as electrodes (both cathode and anode) for high-temperature ceramic fuel cells (CFCs) with impressive performances. This raises a critical question: How stable is LNCA as a CFC anode under methane-fueled conditions (i.e., in CH4 and CO2 at high temperatures)? Herein, we investigated the structure stability of LNCA in CH4 and CO2 at a wide temperature range of 100–800 °C. The corresponding gas-solid reaction equations under these conditions were proposed for the first time. It was found that LNCA started to react with CH4 at 500 °C, leading to its decomposition into Li2CO3, NiO/CoO solid solution, Ni/Co alloy, and LiAlO2. The NiO/CoO solid solution was further reduced into Ni/Co by CH4 at 600 °C. Furthermore, CO2 could make LNCA decomposed into Li2CO3, NiO/CoO, and Al2O3 at 600 °C or above. These findings can be used to design Ni/Co-based anodes for CFCs with LNCA.

Ultrasmall, Coating-Free, Pyramidal Platinum Nanoparticles for High Stability Fuel Cell Oxygen Reduction.

Ultrasmall (<5 nm diameter) noble metal nanoparticles with a high fraction of {111} surface domains are of fundamental and practical interest as electrocatalysts, especially in fuel cells; the nanomaterial surface structure dictates its catalytic properties, including kinetics and stability. However, the synthesis of size-controlled, pure Pt-shaped nanocatalysts has remained a formidable chemical challenge. There is an urgent need for an industrially scalable method for their production. Here, a one-step approach is presented for the preparation of single-crystal pyramidal nanocatalysts with a high fraction of {111} surface domains and a diameter below 4 nm. This is achieved by harnessing the shape-directing effect of citrate molecules, together with the strict control of oxidative etching while avoiding polymers, surfactants, and organic solvents. These catalysts exhibit significantly enhanced durability while, providing equivalent current and power densities to highly optimized commercial Pt/C catalysts at the beginning of life (BOL). This is even the case when they are tested in full polymer electrolyte membrane fuel cells (PEMFCs), as opposed to rotating disk experiments that artificially enhance electrode kinetics and minimize degradation. This demonstrates that the {111} surface domains in pyramidal Pt nanoparticles (as opposed to spherical Pt nanoparticles) can improve aggregation/corrosion resistance in realistic fuel cell conditions, leading to a significant improvement in membrane electrode assembly (MEA) stability and lifetime.

Effects of Reinforcement Type on the Structure and Properties of Perfluorosulphonic Acid Membranes for Polymer Electrolyte Membrane Fuel Cells

Fuel cell membrane durability remains one of the key challenges limiting the wide scale adoption of fuel cell technology. Membrane degradation in polymer electrolyte membrane (PEM) fuel cells limits the operational lifetime of the fuel cell and prevents the industrial targets from being met. A common approach to increasing the lifetime of membranes includes the addition of a reinforcement layer to increase the mechanical durability of the membrane while not adversely affecting the performance [1,2]. Although reinforced membranes are widely used in industry, there is a literature gap considering membrane structure and properties in relation to durability. This work contributes to characterizing the effects of reinforcement type on the membrane properties. In this study a selection of perfluorosulphonic acid (PFSA) ionomer membranes with expanded polytetrafluoroethylene (ePTFE) reinforcements were tested, these include two novel DMR100 membranes with different reinforcements and Nafion XL compared to a conventional, non-reinforced Nafion NRE-211 for reference. All of these membranes contain common PFSA ionomer and differ primarily in the type of reinforcement layer and membrane thickness. The study addresses comparison between the membrane chemical composition, water uptake, crystallinity, and mechanical strength. Methods of ex situ characterization include solid state nuclear magnetic resonance (SS_NMR), small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Membrane chemical structure and properties are measured by SS-NMR and FTIR, whereas the water uptake, domain spacing, and crystallinity are assessed by SAXS/WAXS study of hydrated and dry membranes [3-5]. Tensile mechanical properties are measured under room temperature (23°C, 50% RH) and fuel cell conditions (80°C, 90% RH) by DMA [6]. Overall, this paper contributes both qualitative and quantitative understanding of the key structural properties of membranes with different reinforcement resulting in novel knowledge about membranes that can be leveraged for improved fuel cell durability.

Niobium Oxide Coated Mesoporous Platinum Nanoparticles for the Oxygen Reduction Reaction

Effective and economic methods of converting chemical energy into electricity or vice versa (i.e., through the use of fuel cells) must be developed to aid in the transition to renewable energy sources. One of the major barriers in the commercialization of fuel cells is the disparity in speed and efficiency between the two fuel cell reactions: the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) of which the latter is orders of magnitude slower.1 Mesoporous Pt prepared by electrodeposition has been shown to have increased activity towards the ORR than the industry standard Pt nanoparticles (NPs) supported on carbon.2 Platinum based catalysts, however, lack long term stability under acidic fuel cell conditions. One method that has been found to mitigate the long-term loss of Pt catalyst stability and activity is coating platinum with nanoscale thin films of niobium oxide.3 In this work, niobium oxide was investigated as a stabilizing coating for mesoporous Pt NP catalysts.Mesoporous Pt NPs were synthesized via electrodeposition to form ~50 nm diameter NPs. The Pt NPs were then coated with niobium oxide films with a varying nanoscale thickness that ranged from 3 to 50 nm. The niobium oxide films were fabricated using atomic layer deposition (ALD) or electrodeposition.3,4 Scanning electron microscopy and transmission electron tomography were used to evaluate the porosity and surface morphology of the Pt NPs before and after coating with niobium oxide. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were used to confirm the presence and composition of the niobium oxide films. Electrochemical characterization by cyclic voltammetry and electrochemical impedance spectroscopy under acidic conditions was used to determine an optimal thickness of the niobium oxide film that stabilizes the size and morphology of the Pt NPs while maintaining catalyst activity. The Pt NPs coated with <10-nm thick niobium oxide films were observed to have an increased long-term performance in comparison to their uncoated counterparts. Stabilizing NP catalysts for improving their long-term activity is a vital step towards creating affordable alternatives to fossil fuels by maximizing renewable energy resources. Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43–51.Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci. Rep. 2019, 9, 4161.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.Zhitomirsky, I. Electrolytic Deposition of Niobium Oxide Films. Mater. Lett. 1998, 35, 188–193.

Modeling and Simulation of the Mechanical Properties of Reinforced Fuel Cell Membranes

Polymer electrolyte fuel cells (PEFCs) have gained popularity over internal combustion engines due to their zero CO2 emission, low working temperature, and high efficiency. However, transportation markets generally require high durability and reliability, which remains a significant challenge for PEFC developers. For instance, the thin, ion-conducting membranes used in PEFCs must be able to withstand both chemical and mechanical stresses during dynamic operation. Reinforced membranes even still there are durability challenges that remain even though it is more robust. These membranes have limited mechanical strength, and micro-cracks may allow hydrogen permeation and cause ultimate fuel cell failure. Dynamic stresses caused by temperature and humidity cycles are causing micro-cracks to form and propagate. Because the membrane is constrained by the other parts of the membrane-electrode assembly (MEA), any change in temperature and humidity can create swelling strain and thermal strain in the membrane, resulting in residual stress [1]. The first step in understanding this damaging phenomenon is to simulate the membrane's visco-elastic and visco-plastic behavior while taking temperature, humidity, and strain rate into account. The objective of the present work is to develop such a constitutive mechanical model for mechanically reinforced membranes, which are commonly used in modern PEFCs.Khorasany et al. [2] developed a fatigue lifetime model based on the elastic-plastic constitutive method and Smith-Watson-Topper (SWT) fatigue equilibrium for conventional, non-reinforced fuel cell membranes. In the present work, for strains below the yield point, the linear elasticity by Hooke's law has been considered and the visco-elastic and visco-plastic behaviors of the membrane have been neglected, but for every temperature and humidity, the related Young’s modulus and Poisson’s ratio have been obtained from prior experiments, and for plastic yield response, the Von Mises yield criterion has been selected. Khattra et al. [3] used the G’Sell-Jonas theory for the constitutive model of a non-reinforced membrane, which can only be employed for tensile stresses, while in this study, a generalized G’Sell-Jonas approach; equation 1, has been developed for the reinforced membrane that can also be used for compressive stresses that are common during in-situ fuel cell conditions. This phenomenological theory accounts for the effects of temperature, humidity, and strain rate on membrane mechanical properties and, when combined with thermal and swelling strains, provides a comprehensive model of membrane behavior for both ex-situ and in-situ conditions. For the reinforced membrane, tensile stress-strain tests in two principal in-plane directions have been done that showed its isotropic behavior and, therefore, provide input data for the parameters of the proposed generalized G’Sell-Jonas model. σ(ε,T,H)generalized G'Sell-Jonas=step(ε)K(T,H)(1-e-w(H)abs(ε))eh(H)ε^2 (1) step(ε)=+1 ε≥0, -1 ε<0 In the above equations, K, w, and h are empirical parameters dependent on temperature (T) and humidity (H), and ε is strain. Elastic modeling based on Von-Mises has been studied for constitutive models with stresses below 1 MPa, while isotropic work hardening based on the generalized G'Sell-Jonas theory has been considered for higher stresses in order to follow plastic behavior. In elastic mode, the membrane's Young’s modulus is calculated using a function of humidity and temperature derived from tensile tests. Tensile tests under four environmental conditions and two strain rates in two principal in-plane directions, as well as fatigue tests for extracting S-N curves for this membrane, have been obtained using dynamic mechanical analysis (DMA).Based on Figure 1, the mechanical strength of this isotropic reinforced membrane decreases with increasing temperature and humidity, but the effect of temperature is greater. Because of the viscoelastic nature of the membrane, by increasing the strain rate, the membrane stiffness has been increased too. In the fatigue tests done in DMA, the force track is 150% (= ×100%), R-value = , and frequency is 10 Hz.FEM modeling based on G’Sell-Jonas’s theory shows a strong agreement with the experiments that has been illustrated in Figure 1. The fatigue lifetime distribution based on generalized G’Sell-Jonas’s theory and SWT parameters extracted from Khorasany’s paper [2] have been simulated that reveals when the maximum stress in fatigue cycles is more than 17-18 MPa, a huge drop in fatigue lifetime is observed. Acknowledgments Funding for this research has been provided by AVL Fuel Cell Canada and Mitacs. References Alavijeh, A.S., et al., Effect of hygral swelling and shrinkage on mechanical durability of fuel cell membranes. Journal of Power Sources, 2019. 427: p. 207-214.Khorasany, R.M., et al., Mechanical degradation of fuel cell membranes under fatigue fracture tests. Journal of Power Sources, 2015. 274: p. 1208-1216.Khattra, N.S., et al., Residual fatigue life modeling of fuel cell membranes. Journal of Power Sources, 2020. 477: p. 228714. Figure 1

Deeper Understanding of Ternary Eutectic Carbonates/Ceria-Based Oxide Composite Electrolyte through Thermal Cycling

Due to a high conductivity of about 0.1 S·cm−1, Li-Na-K carbonate eutectic and Sm-doped ceria composite material is a good electrolyte candidate for hybrid fuel cells operating between 500 °C and 600 °C. The present paper aims at a deeper understanding of the species and mechanisms involved in the ionic transport through impedance spectroscopy and thermal analyses, in oxidizing and reducing atmospheres, wet and dry, and during two heating/cooling cycles. Complementary structural analyses of post-mortem phases allowed us to evidence the irreversible partial transformation of molten carbonates into hydrogenated species, when water and/or hydrogen are added in the surrounding atmospheres. Furthermore, this modification was avoided by adding CO2 in anodic and/or cathodic compartments. Finally, a mechanistic model of such composite electrical behavior is suggested, according to the surrounding atmospheres used. It leads to the conclusions that cells based on this kind of electrolyte would preferably operate in molten carbonate fuel cell conditions, than in solid oxide fuel cell conditions, and confirms the name of “Hybrid Fuel Cells” instead of Intermediate Temperature (or even Low Temperature) Solid Oxide Fuel Cells.

Effect of Two Different ZnO Addition Strategies on the Sinterability and Conductivity of the BaZr0.4Ce0.4Y0.2O3−δ Proton-Conducting Ceramic Electrolyte

A comparative study has been made to relate the effect of different addition strategies for the sintering aid of ZnO (∼1 wt %) on the performance of the BaZr0.4Ce0.4Y0.2O3−δ (BZCY442) electrolyte in order to improve the sinterability and conductivity of this material more efficiently. Two samples, Zn-doped BaCe0.4Zr0.4Y0.15Zn0.05O3−δ (BZCYZn, internal doping) and BZCY–ZnO (BZCY442 mixed with ZnO, external addition), were synthesized. The BZCYZn crystallized at a single phase after sintering at 1350 °C, and the segregation of Y2O3 was observed in BZCY–ZnO in the same conditions. The relative densities of the BZCYZn and BZCY–ZnO samples sintered at 1350 °C for 5 h were 98 and 94%, respectively. The high total conductivity in BZCYZn benefited from the low barrier height at the grain boundary core determined by the Mott–Schottky space charge layer model, which was 0.15 V for BZCYZn and 0.26 V for BZCY–ZnO. A cell with BZCYZn as the electrolyte generated a power density of 414.5 mW cm–2 at 700 °C. The total ionic transport number of BZCYZn was determined to be ∼0.90 at 600 °C under fuel cell conditions.

A new Sc-doped La0.5Sr0.5MnO3-δ cathode allows high performance for proton-conducting solid oxide fuel cells

Sc ions are doped at the Mn site in the ABO3-type perovskite La0.5Sr0.5MnO3-δ (LSM), while the investigation from the density functional theory calculations makes the guidance that the doping of Sc into LSM could be beneficial for its application in proton-conducting solid oxide fuel cells (H-SOFCs) with decreased oxygen formation energy and hydration energy as well as improved oxygen reduction reaction ability. The subsequent experimental studies indicate that the Sc-doped LSM (LSMSc) could greatly boost its performance in H-SOFCs, leading to a high fuel cell output of 1221 mW cm−2 at 700 °C, which is about 75% higher than that of the cell using Sc-free LSM cathode. This fuel cell performance is also a record-high performance for LSM-based H-SOFCs compared with the literature reports. Furthermore, the good fuel cell performance is compatible with the excellent chemical stability of the material. The LSMSc material shows excellent stability against CO2 and H2O, allowing its stable operation in the fuel cell condition.

FEATURES OF ZnO APPLICATION IN THE STRUCTURE OF HYDROGEN FUEL CELL

The work is devoted to zinc oxide and the prospects of its use for the needs of hydrogen energy. The influence of zinc oxide on electrolyte materials for ceramic fuel cells is determined. The properties of ceramics based on 8YSZ were investigated by adding 0.5 wt.% nanopowder of zinc oxide according to the me­thod of the research of zinc oxide ceramics. The electrical conductivity of 8YSZ in an oxy­gen atmosphere shows a better conductivity characteristic, but 8YSZ-ZnO ceramics have better mechanical properties and higher reactivity in real fuel cell conditions.&#x0D; It was found that zinc oxide has a positive effect on the open voltage of SOFC in the temperature range from 320 °C to 600 °C. The maximum value of the open voltage for the electrolyte 8YSZ-ZnO was obtained at a temperature of 520 °C with a value of 1.02 V, in turn for the electrolyte of pure 8YSZ the maximum value was 0.92 V at a temperature of 600 °C. The small amount of data on the use of pure zinc oxide as an electrolyte opens up opportunities for thorough analysis and determination of optimal technological parameters that will accelerate the introduction of hydrogen energy technologies with operating tempe­ratures below 600 ºC. From a brief overview of existing zinc oxide-based materials in fuel cells, materials with mixed conductivity and low sintering temperatures are the most promising and effective for implementation in real systems.&#x0D; Data on the effect of sintering temperature on the porosity of ceramics with 8YSZ-ZnO and 8YSZ determined that the porosity of samples of both types decreases significantly with increasing sintering temperature of powders, but the addition of zinc oxide to 8YSZ-ZnO composite allows to obtain dense ceramics at temperatures below 100 °C than for pure 8YSZ. Regarding the strength of 8YSZ-ZnO ceramics, it increases with increasing sintering temperature and has higher values ​​relative to pure 8YSZ. Both porosity and strength change rapidly in the range of 1200–1300 °C and slowly in the range of 1300–1400 °C, due to the sintering temperature of ceramics made of pure zinc oxide 1100–1200 °C. Improving the sintering conditions of ceramics and mechanical properties reveal its advantage in the addition of zinc oxide.

Multi-Objective Optimization of Operating Condition and Geometry of a Modified Tapered Parallel Flow Field Proton Exchange Membrane Fuel Cell

Multi-Objective Optimization of Operating Condition and Geometry of a Modified Tapered Parallel Flow Field Proton Exchange Membrane Fuel Cell

Materials Engineering toward Durable Electrocatalysts for Proton Exchange Membrane Fuel Cells

AbstractProton exchange membrane fuel cells (PEMFCs) have penetrated many commercial markets, especially in the automotive market as Toyota has launched the first commercially mass‐produced fuel cell vehicle, the Mirai in 2014. Electrocatalysts play an irreplaceable role in determining the PEMFCs, performance and account for half of the total cost. Despite substantial progress in exploiting highly active platinum group metal (PGM) and PGM‐free electrocatalysts, current electrocatalysts are faced with significant durability challenges, i.e., high‐performance electrocatalysts usually suffer from rapid degradation during PEMFC operation. The lifetime of the reported electrocatalysts is far from the requirement of performing steadily over the 8000 h of operation in commercialized PEMFCs. To this end, addressing the durability issues of oxygen reduction reaction (ORR) electrocatalysts is imperative for their practical employment in PEMFCs. Herein, the state‐of‐the‐art advances in understanding the durability issues of PGM and PGM‐free catalysts for ORR under fuel cell conditions and the materials engineering strategies to tackle these issues are summarized. The insights into the durability issues, involving the degradation mechanisms and the impact of operation conditions are reviewed. Establishing strategies to mitigate catalyst degradation through rational design of stable PGM and PGM‐free catalysts is highlighted.

Mechanical stress and strain investigation of sulfonated Poly(ether ether ketone) proton exchange membrane in fuel cells: A numerical study

Mechanical durability of hydrocarbon-based proton exchange membranes in long-term fuel cell operation is vital for their successful commercialization. Herein, a complex finite element model is developed that couples electrochemical performance of sulfonated poly(ether ether ketone) (SPEEK) membrane with its mechanical response at different operating conditions to predict the susceptible sites to mechanical failure. To this end, the impacts of cell voltage (0.1–0.9 V), operating temperature (30–80 °C) and backpressure (0.3–1.5 bar), inlet reactants relative humidity (30–100%), and clamping pressure (1–5 MPa) on hydration and consequently, stress and strain formation in the SPEEK membrane are investigated. Results suggest that SPEEK membrane, particularly at the channel outlet, undergoes the greatest swelling-induced degradation when the cell is running at low voltages, high temperature and backpressure values. Clamping pressure, however, imposes compressive stress and thinning upon the land region and is the dominant factor compared to hygrothermal swelling, albeit high hydration level shifts the maximum stress to the channel region. Despite superior resistance to different hygrothermal conditions and lower chance of thinning under the clamping pressure, Nafion membrane exhibits a relatively higher risk of wrinkle deformation under compression. This study provides insights into the mechanical response of hydrocarbon-based membranes under various fuel cell conditions.

Self-healing properties of Ce/Co-coated stainless steel under simulated intermediate temperature solid oxide fuel cell conditions

The interconnects used in solid oxide fuel cells (SOFC) are usually shaped into a corrugated form that creates gas channels. Coatings are applied onto an interconnect to increase its longevity by reducing Cr(VI) evaporation and oxide scale growth. To date many manufacturers first deform the interconnect and then apply the coating. However, the reverse (hereinafter termed pre-coating) would be more cost-effective, because large-scale roll-to-roll coating processes could then be used instead of batch coating processes. The drawback of this method is that cracks are introduced into the coating during deformation. The present work shows that the cracks heal after relatively short exposure times for the often-used Ce/Co coating (10 nm Ce and 640 nm Co) even at low operating temperatures (650 °C and 750 °C). The Cr evaporation rate of pre-coated deformed Ce/Co-coated AISI 441, even though slightly elevated in the beginning of the exposure, decreases and stabilizes to rates that are comparable to that of undeformed Ce/Co-coated AISI 441. SEM micrographs show that the cracks introduced during the shaping of the interconnect heal after roughly 70 h of exposure at 750 °C and 360 h of exposure at 650 °C.

Effect of Microstructure and the Interface on Localized Corrosion in Metallic Nitride Coated Metallic Flow Field Plate in Simulated Low-Temperature Fuel Cell Conditions

Proton Exchange Membrane (PEM) fuel cell offers clean energy on par with renewable energy generations. Graphite bipolar plate, engraved with flow field which is a key component in the PEMFC assembly suffers from volumetric and machining issues. Metallic plate is as an alternative, among which stainless steel and titanium are looked upon as promising candidates, but it suffers from the aspect of corrosion. Inherent corrosion current density (Icorr) of SS316L is 0.5 µA/cm2 which is well within the international standards (Department of Energy (DoE), US) but that of Ti-6Al-4V is 47 µA/cm2 which is not meeting the DoE targets under hydrogen purge conditions. Under fuel cell operating conditions, both the material tends to fail due to corrosion aspects. Various surface treatments are imparted to enhance corrosion in the PEMFC environment, but most of the corrosion protection coatings fail over a period of time due to various aspects.An in depth understanding of the microstructure of the coating prior and post, subjecting it to simulated fuel cell conditions and study of the interface will elude failure at earlier stages of real time operation. In this study, local investigations were carried out on metallic nitride coated SS316L flow field plate to juxtapose the changes induced in the microstructure due to change in pH of the simulated electrolyte, the effect of the interface on the adhesion of the coating. Titanium nitride particles (as seen in Fig.1) adhered to the base material and qualified with 5B rating in ASTM D3359 adhesion studies. Pre-processing and post-processing treatments resulted in the roughness of the coating (Ra) as low as 11nm. Corrosion current density of the coated specimen, Icorr is 0.455 µA/cm2, is within DoE target values and the Ecorr was positive, thus enhancing the corrosion resistance fuel cell operating conditions. Localized corrosion studies reveal the role of pitting agents, present in the simulated electrolyte, which enhance the mechanical cracking of coating layer thus undermining the durability of the coating when tested over a period of 6 months. Figure 1

Is the Degradation of Fe-N-C Catalysts in Proton Exchange Fuel Cells Caused By Hydrogen Peroxide Formation?

In fuel cells chemical energy is directly converted into electricity. Based on this, fuel cells are attractive for automotive propulsion. However, a major drawback that hinders the widespread use of fuel cells are the costs. A major cost contributor in today’s state of the art fuel cells is the catalyst; namely platinum and its alloys that are used to catalyse the hydrogen oxidation reaction and the oxygen reduction reaction (ORR) [1]. Most of the expensive metal is required for the cathodic ORR. However, Fe-N-C catalysts are a potential alternative to Pt/C as they reach promising activity while the stability still needs to be improved. There seems to be a trade-off between activity and stability for this kind of catalysts. Beside other causes of degradation, it is discussed that hydrogen peroxide formation is one of the driving forces [2]. While molecular FeN4 centres are assumed to reduce oxygen to water, other phases are suggested to contribute to hydrogen peroxide formation. Such side phases are e.g. iron or iron carbide which are found in the catalysts after high temperature pyrolysis or if insufficient acid leaching was applied [3].However, there is no systematic study if under fuel cell conditions indeed hydrogen peroxide formation from these side phases is at the origin of instability or maybe the leaching of iron species (from these side phases) with subsequently induced Fenton’s reaction might cause the degradation. The knowledge on this is required for future catalyst design and therefore given attention in this work.In order to address this point, small quantities of an FeNC catalyst were post-modified with different metals as e.g. Pt, Pd, Ag in order to tune the selectivity for hydrogen peroxide formation of the resulting catalyst while still keeping the iron-related composition the same.The catalysts were characterized by cyclic voltammetry and rotating ring disc electrode experiments in order to see to what extent specific adsorption occurs and to determine the ORR activity and selectivity in aqueous electrolyte (0.1M H2SO4).Besides, polarization curves were measured in H2/O2 fuel cells and potentiostatic stability tests (12h) were performed at U = 0.6 V to identify activity and stability under operating conditions.We will discuss to what extent hydrogen peroxide formation correlates with the degradation of FeNC catalysts in proton exchange fuel cells.

(PEFCE&E 21 Best Student Poster - Third Place) Molecular Dynamics Study of Proton Conductivity at an Interface between Nafion and Graphene Sheet

In order to improve the power generation efficiency of polymer electrolyte fuel cells (PEFC), it is necessary to increase the flux of proton transport inside a proton exchange membrane (PEM) under high temperature and low humidity conditions. Nowadays, the Nafion membrane is used as a reference for all proton exchange membranes. This membrane developed by DuPont company demonstrates a high proton conductivity value, chemical stability and prolonged using in fuel cell conditions in comparison with other membranes. However proton transport properties are greatly reduced when the membranes are degraded. Researches have been actively conducted to solve the problem of proton transport reduction in PEM under high temperature and low humidity conditions by two approaches. One of the research focuses is to design alternative materials based on those engineering polymers, for example, polybenzimidazole, poly(ether sulfone), and poly(ether ketone). Another research focus is the introduction of additives. Among the many additives, carbon nanotubes (CNTs) which are next-generation materials with a high aspect ratio and excellent thermal durability and mechanical properties, have been attracting attention. Previous studies have reported that the PEEK (poly ether ether ketone ) / CNT composite membranes show higher durability than Nafion membranes. And the proton transport properties in Nafion / sulfonated CNT composite membranes are improved over the that of Nafion membranes. It is said that the proton transport property is improved by forming a proton path which is made by the influence of the structural characteristics of CNT. However, the proton transport mechanism at the interface between the carbon surface of CNT and the polymer has not been fully understood. The purpose of this study is to clarify the proton transport mechanism in the interface between the carbon surface region and the polymer region and to evaluate the effects of functional groups with carbon surface by using molecular dynamics (MD) simulations. The surface of a CNT was modeled with a graphene sheet (GS). The diameter of the CNTs used in the experiment is about 10-20 nm, which is beyond capability of our MD simulations, and thus the CNT surface was assumed to be a flat surface. The simulation system is shown in Fig. 1. GSs were placed at the top and bottom of the simulation box, and polymer, water molecules, and hydronium ions were placed between them. In order to evaluate the influence of distances between GSs on structural characteristics and transport property, we prepared calculation systems at the wall distances of 4, 6, 8 and 10 nm. We calculated the density distribution in the direction vertical to the GS and the self-diffusion coefficient of the protons at the GS surface region and the center region. Furthermore, a hydrophilic sulfonated GS was prepared by putting sulfo groups to the hydrophobic GS to investigate the effects of the hydrophilic and hydrophobic GS interface on proton transport. The results of the density distribution showed that the surface of pristine GS and sulfonated GS had water clusters, and that proton self-diffusion coefficient of pristine GS and sulfonated GS were larger than that of bulk Nafion. From these facts, it is considered that the diffusion by the grotthuss mechanism increased by improving the connectivity of the proton path on the GS surface and reducing the tortuosity. As a result, the proton transportability is considered to be improved. Figure 1

(PEFCE&E 21 Best Student Poster - First Place) Oxygen Reduction Reaction Properties and Microstructures of Pt/Nb:SnO2 Model Catalysts Fabricated on Pt Single Crystal Surfaces

Introduction Considering strong acidic and severe potential fluctuation environments for operating conditions of Polymer Electrolyte Fuel Cells (PEFC), electrochemical degradation of electrode materials, such as elution and/or oxidation, is crucial for development of high-performance electrode materials, e.g., Pt-based nanoparticles (NPs) and C-based supports. Recently, replacing the C-based supports with metal-oxides aimed for improving electrochemical stabilization has attracted much attention. Among the candidates of metal-oxides used as support materials, X (=Nb, Sb, Ta etc.) doped tin dioxide (X:SnO2) shows relatively high electrical conductivity. Actually, Kakinuma et al. [1,2] demonstrated that superior oxygen reduction reaction (ORR) activity and durability of the Pt NPs supported on Nb-doped SnO2 (Pt NPs/Nb:SnO2). The results clearly show that such the metal-oxide supports are one of the powerful candidates for the next-generation cathode materials. Also, it is considered possible to use Nb:SnO2 as a core material (Pt@Nb:SnO2) for highly durable Pt-shell / metal-oxide-core type catalyst, although influences of microstructures of the Pt-based materials and metal-oxide interface on the ORR properties remains unclear. Therefore, in this study, we prepared Pt NPs/Nb:SnO2 and Pt@Nb:SnO2 model catalysts with atomically-controlled Pt/SnO2 interface under ultra-high vacuum (UHV; ~10-8 Pa) and investigated relationship between the surface and interface microstructures of the model catalysts and their ORR properties. Experimental Nb-doped Sn layer with 5 nm thickness was deposited on the surface-cleaned Pt(111) substrate (Ar+-sputtering and annealing) by arc-plasma deposition (APD) of Sn-Nb alloy target (composition ratio 96:4 (at.%)) at the substrate temperature of 300 K. Subsequently, the prepared sample was transferred to a furnace and thermally-oxidized in dry air (Nb:SnO2 / Pt(111)). Then, the Nb:SnO2 / Pt(111) sample was re-introduced into the UHV-chamber, Pt was vacuum-deposited by the APD for Pt NPs / Nb:SnO2 / Pt(111) and by e-beam evaporation for Pt-shell on the Nb:SnO2/Pt(111) (Pt shell / Nb:SnO2 / Pt(111)) (Fig1-(a) and Fig1-(b), respectively). The microstructures of the catalyst model samples were analyzed by STM, cross sectional STEM-EDS, XPS etc. The ORR properties were evaluated by CV and LSV with the RDE method conducted in N2-purged and O2-saturated 0.1 M HClO4, respectively. The ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky-Levich equation. The electrochemical stability was discussed by the ORR activity changes during applying the potential cycles (PCs) of 0.6-1.0 V vs. RHE (each 3 s) in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion The STM image (Fig. 2-(a)) of the Pt NPs / Nb:SnO2 / Pt(111) for the Pt/SnO2 model catalyst showed Pt NPs with an average size of 3.6 nm were distributed over the Nb:SnO2 substrate surface. The ORR activity and durability for the Pt NPs / Nb:SnO2 / Pt(111) clearly outperformed relative to the Pt NPs formed on a highly oriented pyrolytic graphite (HOPG)[3] formed under the same Pt deposition condition: increase in Pt particle sizes and size distribution of Pt NPs by the 10000 PCs loading was suppressed compared to that of the Pt NPs / HOPG.A cross-sectional HAADF-STEM image of the Pt-shell / Nb:SnO2 / Pt(111) model core-shell sample (Fig. 2-(b)) reveals that the surface Pt layers with (111) orientation (ca. 2.5 nm in thickness) and SnO2(101) lattice of ca. 4 nm-thick Nb:SnO2 layer are grown on the Pt(111) substrate surface. This is probably due to lattice matching between the Pt(111) (0.48 × 0.56 nm) and SnO2(101) (0.48 × 0.58 nm)(Fig. 2-(c)). Such the lattice matching might correlate with the forementioned suppression of nanoparticle migration [4] in the Pt NPs / Nb:SnO2 / Pt(111) model catalyst. Furthermore, as shown in Fig. 2-(d), the Pt shell / Nb:SnO2 / Pt(111) shows that the ORR activity enhancement factor is at most × 4.1 vs. clean Pt(111) at 500 PCs and × 3.2 even after 5000 PCs loading. The ORR activity enhancement might stem from so-called the ligand effect of Sn and/or Nb on the surface Pt(111) layer through thermal diffusion such the elements into the surface Pt(111) layer during the model catalyst UHV preparation. At the meeting, I will show ORR properties of the Pt / Nb:SnO2 / Pt(110) and discuss influences of the lattice orientations of the surface Pt and underlaid SnO2 layer. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan.Reference[1] K. Kakinuma et al., Electrochim. Acta., 110, 316-324 (2013).[2] K. Kakinuma et al., J. Electrochem. Soc., 165, 15, J3083-J3089 (2018).[3] S. Takahashi et al., Phys. Chem. Chem. Phys., 17, 18638-18644 (2015).[4] M. Wakisaka, et al., Phys. Chem. Chem. Phys., 12, 4184-4190 (2010). Figure 1

Optimization of Ammonium Head Groups in Partially Fluorinated Anion Exchange Membranes for Alkaline Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) have gained attention due to their possible use of non-precious metal electrocatalysts compared to proton exchange membrane fuel cells.1 However, the chemical/physical instability of anion exchange membranes under operating fuel cell conditions remains an issue.1, 2 We developed previously anion exchange membranes containing perfluoroalkyl and phenylene rings in the main chain (e.g., QPAF-C3-TMA (trimethylammonium)). The polymer main chains were alkaline stable because of the absence of oxygen ether bonds in the main chain, however, the ammonium groups suffered from alkaline degradation.3 In this work, the impact of varying head group structure was investigated through utilizing cyclic piperidine (-Pip), dimethylbutylamine (-DMB), dimethylhexylamine (-DMH), and dibutylmethyl amine (-DB) (Figure 1). All of the membranes showed phase-separated morphology as confirmed from TEM images. The DFT calculations revealed that hydrophilic domain sizes were dependent on polar surface area rather than the molecular size of the head groups. The compact piperidinium-based membrane (QPAF-C3-Pip) exhibited the most balanced properties of low water absorbability, high ionic conductivity, alkaline and mechanical stability. An H2/O2 fuel cell using QPAF-C3-Pip membrane achieved maximum power density of 232 mW/cm2, slightly higher than that of TMA-based membrane (224 mW/cm2). Furthermore, QPAF-C3-Pip membrane showed reasonable durability for 240 h in operating fuel cell at a constant current density of 100 mA/cm2 in which the cell voltage decreased from 0.75V to 0.43V with average voltage decay of 1.24 mV/h, that was more durable than the TMA-based membrane (average decay of 4.8 mV/h at 50 mA/cm2). Other membranes properties will be also discussed.Figure 1. Structure and fuel cell performance of QPAF-C3-Pip membranes

Precise Electrochemical Testing in Half-Cells: Controlling Catalyst Layer Formation and the Three-Phase Boundary in Gas Diffusion Electrodes

Novel and tailored electrocatalysts need solid benchmarking for evaluation of e.g. the oxygen reduction reaction activity (ORR), and the performance in a model approach should strictly reflect their eventual performance in the fuel cell devices. It is strongly believed that the routinely used testing methods, like rotating disk electrode (RDE) method, provide erroneous results, as they do not mimic the original fuel cell conditions in terms of water management, presence of ionomer, relevant current densities, etc. Half-cell tests of gas diffusion electrodes (GDEs) have been shown to be a good compromise as these tests exhibit comparable oxygen mass transport to the electrode as a fuel cell device, in contrast to the limited mass transport in RDEs. Realistic operational conditions, such as potential range and higher current densities in the GDE set-ups, are more akin to membrane electrolyte assemblies (MEA) testing than RDEs without the hassle and cost of full-cell tests [1-4]. In this presentation, half-cell test results will be reported for gas diffusion electrodes prepared using various methods, namely: 1) Drop-casting; 2) Spray coating with sonication; 3) Air brushing with spray gun; and 4) Hand painting. Commercially available Pt/C HiSpec 4000 was used as electrocatalyst and results of parallel RDE and MEA tests with similar Pt loadings will be discussed. Detailed electrochemically active surface area (ECSA) analysis using Hupd and CO stripping studies of various loadings from 5μg cm-2 Pt loading to 50 μg cm-2 will be reported. Drop-casting method on GDE was found to be the most efficient process to study the ECSA using Hupd (38 m2 gPt -1) and CO-stripping (61 m2 gPt -1) which were achieved at a loading of 20 μgPt cm- 2. Spray-coating with ultrasonication with a diluted catalyst ink was the ideal procedure to fabricate a uniform thin layer of catalyst with multiple deposition cycles. The current densities achieved during ORR evaluation in half-cell set-up were almost 2 orders of magnitude higher than those achieved using the RDE method. Different strategies of implementing the Nafion ionomer were utilized and compared, like: 1) Nafion 117 membrane hot-pressed on the GDE; 2) Nafion solution sprayed on the surface of the GDE reaching a loading of 2 mg cm-2 and; 3) GDE without a membrane. This report will also aim to assess how the presence of phosphoric acid in the perchloric acid electrolyte and its purity affects the catalyst’s performance for the ORR reaction. Scanning electron microscopy (SEM) was used to study the catalyst layer formation on the gas diffusion layers (GDLs). The GDEs were characterized using X-Ray Absorption Spectroscopy (XAS) [5, 6], ex-situ and in-situ, allowing us to extract structural information through the extended X-ray absorption fine structure analysis (EXAFS) (catalyst particle size, composition, morphology, and oxidation state); furthermore, by using a subtractive-normalization technique in the near-edge region (Δμ XANES) it is possible to obtain adsorbate coverage (H, O, PO4 species adsorbed on Pt surface) and binding-site information for a detailed comparison between the above samples.