Fuel Cell Operating Conditions Research Articles

In situ electrochemical grazing incidence small angle X-ray scattering: From the design of an electrochemical cell to an exemplary study of fuel cell catalyst degradation

Nowadays, electrochemistry has a considerable technological impact, involving fuel cells, super capacitors and batteries. These devices are based on complex architectures, which complicates monitoring their evolution in situ under operating conditions to reveal the reasons for reduced lifetime and performances. Here, we present a design of a multipurpose electrochemical cell for grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) where the environment for operating conditions can be recreated. We focus on proton exchange membrane fuel cells (PEMFCs) which operational conditions are simulated by means of potentiodynamic-based accelerated stress tests, applied to a thin film of Pt nanoparticles representing a model system of a benchmark catalyst. Two different upper potentials are used to mimic fuel cell operating conditions: at 1.0 V RHE the catalyst film preserves its initial morphology, while at 1.5 V RHE (simulating fuel cell start-up/shut-down cycles) significant coarsening has been observed. The initial dimension of the Pt particles of 4.0 nm increases to 8.7 nm due to the predominant process of coalescence and final Ostwald ripening. In parallel, the distance between the particles increases, the catalyst film (9 nm thick) becomes thinner at first and exhibit a higher roughness at the end.

Solid‐State Alkaline Fuel Cell Performance of Pentablock Terpolymer with Methylpyrrolidinium Cations as Anion Exchange Membrane and Ionomer

AbstractIn this study, pentablock terpolymers with methylpyrrolidinium cations were characterized and investigated as anion exchange membranes and ionomers for solid‐state alkaline fuel cells. The pentablock terpolymer (with methylpyrrolidinium cations) membranes exhibited higher fuel cell power density and durability than commercial FuMA‐Tech (with quaternary ammonium cations) membranes at 30 °C, 100% relative humidity (RH). Optimization of the catalyst ink composition (i.e., solids and solvent ratio) and fuel cell performance of membrane electrode assemblies (MEAs) with pentablock terpolymers as both the membrane and ionomer were also investigated. Optimization of the fuel cell operating conditions corroborates with the in situ electrochemical impedance spectroscopy results. The pentablock terpolymer MEA exhibited a maximum power density of 83.3 mW cm−2 and voltage decay rate of 0.7 mV h−1 after 100 h of operation under 40 °C, 100% RH. These results show promise for pentablock terptolymers with methylpyrrolidinium cations as a commercially attractive low‐cost alternative anion exchange membrane and ionomer for solid‐state alkaline fuel cells.

Titanium Dioxide Grafted on Graphene Oxide: Hybrid Nanofiller for Effective and Low-Cost Proton Exchange Membranes.

A nanostructured hybrid material consisting of TiO2 nanoparticles grown and stabilized on graphene oxide (GO) platelets, was synthesized and tested as nanofiller in a polymeric matrix of sulfonated polysulfone (sPSU) for the preparation of new and low-cost nanocomposite electrolytes for proton exchange membrane fuel cell (PEMFC) applications. GO-TiO2 hybrid material combines the nanoscale structure, large interfacial area, and mechanical features of a 2D, layered material, and the hygroscopicity properties of ceramic oxides, able to maintain a suitable hydration of the membrane under harsh fuel cell operative conditions. GO-TiO2 was synthetized through a new, simple, one-pot hydrothermal procedure, while nanocomposite membranes were prepared by casting using different filler loadings. Both material and membranes were investigated by a combination of XRD, Raman, FTIR, thermo-mechanical analysis (TGA and Dynamic Mechanical Analysis) and SEM microscopy, while extensive studies on the proton transport properties were carried out by Electrochemical Impedance Spectroscopy (EIS) measurements and pulse field gradient (PFG) NMR spectroscopy. The addition of GO-TiO2 to the sPSU produced a highly stable network, with an increasing of the storage modulus three-fold higher than the filler-free sPSU membrane. Moreover, the composite membrane with 3 wt.% of filler content demonstrated very high water-retention capacity at high temperatures as well as a remarkable proton mobility, especially in very low relative humidity conditions, marking a step ahead of the state of the art in PEMs. This suggests that an architecture between polymer and filler was created with interconnected routes for an efficient proton transport.

Energetics of Base-Acid Pairs for the Design of High-Temperature Fuel Cell Polymer Electrolytes.

The interaction energy of base-acid plays a key role in acid retention of phosphoric acid (PA)-doped polymer electrolytes under fuel cell operating conditions. Here, we investigate the energetics of proton-accepting and hydroxide-donating organic bases using density functional theory calculations. Because of their weak basicity, proton-accepting organic bases such as benzimidazole have relatively low interaction energy with the acid in the absence of water (15.3-28.0 kcal mol-1). Energetics of the proton-accepting base-PA complex increases by adding water, indicating that the interactions in the base-acid complex strengthen in the presence of water. On the other hand, hydroxide-donating organic bases, such as tetramethylammonium hydroxide, have high interaction energy with PA (∼110 kcal mol-1), which remains high in the presence of water. The chemical shifts of 31P NMR support the energetics of the base-acid complexes. This study further discusses the benefit of incorporating hydroxide-donating organic bases into the polymeric structure over proton-accepting bases as a way to increase acid retention.

Taguchi Approach in Combination with CFD Simulation as a Technique for the Optimization of the Operating Conditions of PEM Fuel Cells

In proton exchange membrane fuel cell (PEMFC) systems operating at low temperature, water management is important as it helps to improve cell performance. In this research paper, a three-dimensional, steady-state and non-isothermal model was established. The paper is aimed at investigating the effect of water flooding on the PEMFC performance. The Taguchi approach was proposed to obtain optimal levels and analyze the influence of different parameters, such as relative humidity (RH) on the anode and cathode sides, gas diffusion layer (GDL) porosity on the anode and cathode sides, temperature and pressure, respectively. To illustrate the effect of wet inlet gases, the relative humidity and the GDL porosity for the model were varied on both sides of the anode and cathode as follows: 10%, 30%, 50%, 70% and 90%. Therefore, our findings show that at the anode RH = 0% and 100%, the optimum levels were (L4, L5, L3 and L1) and (L1, L1, L5 and L4) for the relative humidity of the cathode, GDL porosity of the cathode, temperature and pressure, respectively, while at the cathode RH = 0% and 100%, the optimum levels were (L5, L5, L5 and L4) and (L1, L5, L1 and L4) for the relative humidity of the anode, GDL porosity of the anode, temperature and pressure, respectively. Consequently, the maximum power densities for the optimal combinations were found to be 0.727, 0.725, 0.135 and 0.111 for the anode RH = 0%, anode RH = 100%, cathode RH = 0% and cathode RH = 100%, respectively.

PEM Yakıt Hücreleri için Sülfone Polieter Eter Keton (sPEEK) Elektrolitlerin Sentezi ve Karakterizasyonu: Sülfonasyon Derecesi Etkisi

Bu araştırma, proton değişim membran (PEM) yakıt hücrelerinde proton taşıyıcı olarak kullanılan ticari membranlara alternatif olabilecek sülfone polieter eter keton (sPEEK) membranların sentezi ve karakterizasyonu ile ilgilidir. Çalışmada saf PEEK membran matrisinin farklı sürelerde (1, 2 ve 3 saat) sülfonasyonuyla, sülfonasyon derecesi farklı membranlar üretilmiş, hazırlanan membranların yapısal ve termomekanik karakterizasyonu gerçekleştirilerek standart yakıt hücresi çalışma koşullarında proton iletkenlikleri belirlenmiştir. Hazırlanan sülfone polimerik membranların termomekanik davranışları ve proton iletkenlikleri sülfonasyon derecelerine bağlı olarak değerlendirilmiştir. Membranların termal dayanımları termogravimetrik analizlerle (TGA) incelenmiş, tüm ürünler standart yakıt hücresi çalışma koşulları için yüksek termal kararlılık sergilemişlerdir. Proton iletkenlik değerleri ve iyon değişim kapasiteleri sülfonasyon süresi ve sülfonasyon derecesiyle artmış, en yüksek iletkenlik değerleri % 81.44 sülfonasyon derecesi ile hazırlanan sPEEK-3 membranlarında gözlenmiştir. sPEEK-3 membranları için 300-330 K sıcaklık aralığında en yüksek proton iletkenlik değerine (3.38x10 -2 S/m) 330 K sıcaklıkta ulaşılmıştır. sPEEK-3 membranların mekanik mukavemeti dinamik mekanik analizle (DMA) incelenmiş, depolama modülü (E’: 0.23 GPa) ve camsı geçiş sıcaklığı (T g : 143 o C) değerlerinin ticari Nafion 212 proton taşıyıcı membranlarla mukayese edilebilir seviyelerde olduğu gözlenmiştir. Bu çalışma sonucunda, hazırlanan sPEEK-3 membranlarının termomekanik kararlılıklarının ve iletkenlik değerlerinin standart PEM yakıt hücresi işletme koşullarında alternatif olabilecek düzeyde olduğu görülmüştür.

Operating Conditions Effect on PEM FC Durability Using Design of Experiment Approach

The current state of art PEM fuel cell cathode catalyst is represented by Pt-alloy nanoparticles dispersed on high surface area carbon (HSC) supports. [1] Mass activity > 0.6 A/mgPt has been reported consistently for such catalysts. Unfortunately, the high BOL activity of the Pt-alloy nanoparticle catalysts can suffer fairly rapid degradation in mass activity (MA) over time on exposure to fuel cell operating conditions, especially under voltage cycling (V-cycling). The prevalent mechanism for loss of specific activity is due to dissolution of the non-noble alloying element and thickening of Pt shell during voltage cycling. [2]. While numerous efforts have focused on stabilizing the PtCo nanoparticle, the improvements so far have been marginal. Under severe environment of the fuel cell operation, the less noble alloying element tend to dissolve rapidly and contaminate the MEA. The actual operating conditions of the PEMFC can have a significant impact on the PtCo degradation. In our initial study, we had shown that the relative humidity of fuel cell operation can have a significant impact on Pt electrochemical surface area (ECSA) loss and Co dissolution [3]. 35% ECSA loss was observed for 30,000 voltage cycles in H2-N2 at 100% RH, vs. zero ECSA loss for similar voltage cycling test at 25% RH. Given the criticality of this issue, a careful systematic study of this phenomenon with best-in-class materials in a state of art (SOA) membrane electrode assembly (MEA) is desired.To study the impact of operating conditions in depth, a design of experiment approach was utilized. The design of experiments approach allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. A 20-run design was created to study the various factors, such as relative humidity, temperature, upper potential limit, upper potential hold time etc at three different levels. The factors and its corresponding levels studied are listed in Table 1. Voltage cycling tests was conducted in H2/N2 environment at different operating conditions up to 60,000 voltage cycles. In-situ electrochemical diagnostics such as electrochemical surface area (ECSA), mass activity (MA), specific activity (SA) was measured to understand the kinetics and degradation mechanism. Transport properties such as proton transport resistance and oxygen transport resistance were measured using H2-N2 impedance [4] and oxygen limiting current measurements [5]. The end of test (EOT) MEAs were characterized by ex-situ characterization measurements such as TEM for particle size distribution, electron micro probe analysis (EPMA) and EDS measurements to quantify the cobalt composition and loss up on voltage cycling at different operating conditions. A fundamental model was developed to predict the ECSA and activity loss as a function of operation conditions.

Influences of Ni content and porosity on mechanical properties of Ni–YSZ composites under solid oxide fuel cell operating conditions

Mechanical properties of nickel and yttria-stabilized zirconia (Ni–YSZ) anodes of solid oxide fuel cells undergo changes not only because of extrinsic conditions but also because of Ni, YSZ and pore proportions. In this study, elastic modulus and fracture behavior of Ni(NiO)–YSZ composites are investigated with changing Ni and pore volume fractions as the main parameters. NiO–YSZ composite stiffness at 800 °C monotonically increases with increasing NiO. However, the reduced form of Ni–YSZ shows drastic changes in elastic modulus and strength around 30 vol% Ni where the deformation behavior and fracture made transition from brittle-like to ductile. Both stiffness and strength show linear decreases with porosity, of which the degree of deterioration at 800 °C was duller than at room temperature. The influences of Ni content and porosity on elasticity are quantified using multiple-linear regression analysis. Pore volume fraction shows large negative effects on elasticity at room temperature in air. However, at 800 °C, Ni volume shows larger negative effects than pore volume under reducing atmospheres.

Effects of Fuel Cell Operating Conditions on Proton Exchange Membrane Durability at Open‐Circuit Voltage

AbstractProton exchange membrane fuel cells (PEMFCs) continue to face cost and durability challenges which need to be addressed before their large scale commercialization. The PEM is an essential component of the fuel cell stack and its durability is thus a critical factor for the overall fuel cell reliability. Significant membrane degradation leads to the development of internal transfer leaks and cell short circuiting irreversibly affecting the fuel cell's functionality. In this study, perfluorosulfonic acid (PFSA) membranes were investigated for the effects of operating temperature and relative humidity on membrane durability using an open circuit voltage (OCV) accelerated stress test. The response surface methodology (RSM) was used to evaluate and optimize the effects of the operating temperature and humidity. As a result, the optimum fuel cell operational region was mapped and suggested as an alternative approach to maintain membrane durability without modifying membrane materials. The mapping could provide valuable guidelines for PEMFC designers and system engineers to optimize the operating conditions during idling to achieve a targeted membrane lifetime.

Assessment of Glucose Oxidase Based Enzymatic Fuel Cells Integrated With Newly Developed Chitosan Membranes by Electrochemical Impedance Spectroscopy

AbstractConsiderably stable enzymatic fuel‐cells (single cell and 5‐cells stack) were prepared by using chitosan based membranes along with glucose oxidase attached bioanode. Continuous operation of fuel‐cells were monitored under short circuit conditions reaching half‐life over a week. Detailed analysis for the effects of pH, temperature, buffer types and concentration on different type of in‐house produced chitosan membranes were performed by electrochemical impedance spectroscopy (EIS). EIS was utilized to observe, total electrolyte resistance, charge transfer properties, mass transfer and double layer effects on integrated fuel cells (single cell and 5‐cells stack). Performance of the fuel cells was also analyzed by the polarization experiments. Current density of the fuel cell increased at higher operation temperatures not only due to better enzyme kinetics, but also due to increase in electrolyte (membrane+buffer solution) conductivity. Buffer concentration in the fuel (glucose) solution was found as an important parameter. Under optimum fuel cell operation conditions (i. e. 30–40 °C, pH 5 0.3 M buffer solution), maximum current densities of 3.0–3.2 mA cm−2 were reached. Low‐power devices (i. e. a calculator, step motor) were powered with 5‐cell stack producing 3 mW at 1.3 V.

Origin of High Activity and Durability of Twisty Nanowire Alloy Catalysts under Oxygen Reduction and Fuel Cell Operating Conditions.

The ability to control the surface composition and morphology of alloy catalysts is critical for achieving high activity and durability of catalysts for oxygen reduction reaction (ORR) and fuel cells. This report describes an efficient surfactant-free synthesis route for producing a twisty nanowire (TNW) shaped platinum-iron (PtFe) alloy catalyst (denoted as PtFe TNWs) with controllable bimetallic compositions. PtFe TNWs with an optimal initial composition of ∼24% Pt are shown to exhibit the highest mass activity (3.4 A/mgPt, ∼20 times higher than that of commercial Pt catalyst) and the highest durability (<2% loss of activity after 40 000 cycles and <30% loss after 120 000 cycles) among all PtFe-based nanocatalysts under ORR or fuel cell operating conditions reported so far. Using ex situ and in situ synchrotron X-ray diffraction coupled with atomic pair distribution function (PDF) analysis and 3D modeling, the PtFe TNWs are shown to exhibit mixed face-centered cubic (fcc)-body-centered cubic (bcc) alloy structure and a significant lattice strain. A striking finding is that the activity strongly depends on the composition of the as-synthesized catalysts and this dependence remains unchanged despite the evolution of the composition of the different catalysts and their lattice constants under ORR or fuel cell operating conditions. Notably, dealloying under fuel cell operating condition starts at phase-segregated domain sites leading to a final fcc alloy structure with subtle differences in surface morphology. Due to a subsequent realloying and the morphology of TNWs, the surface lattice strain observed with the as-synthesized catalysts is largely preserved. This strain and the particular facets exhibited by the TNWs are believed to be responsible for the observed activity and durability enhancements. These findings provide new insights into the correlation between the structure, activity, and durability of nanoalloy catalysts and are expected to energize the ongoing effort to develop highly active and durable low-Pt-content nanowire catalysts by controlling their alloy structure and morphology.

Simulations of the Oxidation and Degradation of Platinum Electrocatalysts.

Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand-canonical Monte Carlo scheme. 2-4 nm cuboctahedral particles serve as model systems, for which electrochemical potential-dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80-0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt6 O8 ]4- clusters is observed. Density functional theory calculations confirm that [Pt6 O8 ]4- is indeed stable and hydrophilic. These results suggest that the formation of [Pt6 O8 ]4- may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt2+/4+ ions in fuel cells.

Enhancement of Electrochemical Properties, Impedance and Resistances of Micro‐tubular IT‐SOFCs with Novel Asymmetric Structure Based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ Proton Conducting Electrolyte

AbstractAnode‐supported micro‐tubular SOFCs based on a proton and oxide ion mixed conductor electrolyte, BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb), have been successfully fabricated via phase‐inversion process. Typical micro‐tubular cell with the configuration of Ni‐BZCYYb|BZCYYb|La0.6Sr0.4Co0.2Fe0.8O3–δ, consists of anode support, electrolyte and cathode with thicknesses of 200, 12, and 10 μm, respectively. The cells show excellent electrochemical performance with the peak power densities of 1.040, 0.902, 0.721, and 0.590 W cm−2 at 750, 700, 650, and 600 °C, respectively, with humidified (3 vol.% H2O) hydrogen as fuel and ambient air as oxidant. The AC impedance spectroscopy result shows low concentration polarization values of 0.016–0.021 Ω cm2 at 750–600 °C. Further, significant performance enhancement is observed during long‐term stability test at 600 and 750 °C under constant voltage of 0.5 and 0.7 V over 120 h, respectively, indicating excellent stability of BZCYYb under fuel cell operating condition. The high performance is attributed to high‐quality cell, validated by the dense, crack‐free BZCYYb electrolyte film supported by the anode with micro sponge‐like pores.

Characterization of water management in metal foam flow-field based polymer electrolyte fuel cells using in-operando neutron radiography

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the ‘land/channel’ geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water ‘maps’ with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm−2, exhibiting a ~50% increase in voltage, ~127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC.

Interplay between temperature and biaxial loading on creep behavior of perfluorosulfonic-acid membranes

The durability and performance of polymer-electrolyte fuel cells largely depend on the mechanical durability of perfluorosulfonic-acid membranes. With the increasing demand for operating at higher temperature for fuel cells, creep is more detrimental to the mechanical durability of the membrane. Nevertheless, even though the membrane is in a biaxial stress state during fuel cell operation conditions, numerous studies on creep responses of the membrane are conducted under uniaxial loading, leaving the biaxial creep behavior unexplored. In this paper, the uniaxial and biaxial creep behavior at different temperatures are investigated. The biaxial stress states is found to suppress the delayed elastic strain at temperatures below 65 °C and retard the viscous flow strain at 80 °C. Master curves are constructed based on both uniaxial and biaxial creep tests, and it is found that even though the equilibrium time is much longer for biaxial creep tests, the activation energy is the same, indicating the same creep deformation mechanism from molecular level. In addition, increasing biaxial stress ratio from 0 to 0.1 causes substantial decrease in creep strain. The findings here are expected to provide new insights for understanding membrane failure and be helpful for fuel cell optimization.

Intermetallic PtBi Nanoplates Boost Oxygen Reduction Catalysis with Superior Tolerance over Chemical Fuels

The crossover issue of small‐molecule organic fuels usually makes the Pt‐based nanocatalysts exhibit low efficiency, and poor tolerance and durability under the practical fuel cell operating conditions. Here, a new approach to prepare a class of intermetallic hexagonal PtBi nanoplates with superior tolerance over chemical fuels for boosting oxygen reduction catalysis is reported. The time‐dependent morphology studies reveal that the well‐defined hexagonal PtBi nanoplates are transformed from a new reaction immediate (triangular/truncated triangular bismuth oxides nanoplates). Different from conventional Pt‐based catalysts, the hexagonal PtBi nanoplates exhibit much improved tolerance over CH3OH, HCOOH, and CO. X‐ray photoelectron spectroscopy results reveal that the charge displacement resulting from Pt–Bi bonding is the main factor in enhancing the tolerance. The PtBi nanoplates also exhibit much higher ORR activity than all the reported PtBi nanoparticles. Considering their exceptional antipoisoning ability and high activity, these hexagonal nanoplates show great potential in the development of commercial fuel cells.

(Invited) Modeling Mechanical Behaviors and Lifetime of a Polymer Electrolyte Membrane in Fuel Cell Dynamic Operations

Mechanical failure of a polymer electrolyte membrane (PEM) such as pin-hole and delamination has been identified as a major degradation source in a PEM fuel cell. The hydration/dehydration cycles of a membrane during the fuel cell operation, and the resulted mechanical stress, are in general responsible for the mechanical failures. For example, the cyclic mechanical stressor has shown a significant role in propagating a crack through the thickness of a membrane [1], although there is also the synergistic acceleration from the chemical stressor [2]. It is of interest to thoroughly investigate the mechanical behaviors of a membrane under in-situ cyclic conditions, so that alleviating strategies focusing on the fuel cell operating conditions could be developed. We have interfaced a two-dimensional transient fuel cell transport/performance model in Comsol [3] with a viscoelastic-plastic membrane mechanical model in the fuel cell geometry [4]. Using the combined model we have studied the impacts of dynamically cycling hydration, current and voltage on a membrane [5]. Furthermore, we have developed a model-based method to predict the crack propagation rate. It is expected that the crack propagation rate could be used as a metric to differentiate various dynamic driving cycles from the mechanical durability aspect, thereby creating a pathway to a transfer function for mechanical durability. The modeling results will be validated with RH cycling data. The role of the chemical stressor will also be discussed in using this mechanical durability transfer function.

Water Transport in the Gas Diffusion Layer of PEM Fuel Cells: An Energy Approach

The design of a fibrous gas diffusion layer (GDL) that manages water effectively over the full range of fuel cell operating conditions has to account for the morphology of the gas-liquid interfaces under complex geometrical and wetting configurations, encountered within the fiber matrix. An energy minimization approach is taken to capture the interface evolution of water breaking through the pores. Water in contact with fibers can take the shape of an axisymmetric or non-axisymmetric droplet depending on the fiber diameter, the droplet volume and the contact angle. The energy barrier between the two morphologies as well as the percolation pressure of water through the pores, are determining factors in the process of liquid fingers formation. We consider a droplet breakthrough model which sets the contact line delimiting the protruding gas-liquid interface along both wetting and non-wetting fibers, yielding mixed wetting conditions. The breakthrough pressure of water through a mixed wetting pore is determined to be 40% larger than through a wetting pore, a ratio which is consistent with experimental results. ESEM images indicate that the model captures accurately the interface evolution while accounting for the specific geometrical features and wetting properties of the GDL. It is advantageous to corroborate a realistic pore model such as the one proposed in this study, with existing models including the pore network model to characterize and design optimal transport features of the porous medium.

Analysis and Optimization of Transport Losses in Hydroxide Exchange Membrane Fuel Cells

As an emerging alternative to proton exchange membrane fuel cells (PEMFCs), hydroxide exchange membrane fuel cells (HEMFCs) are more cost-effective due to PGM-free catalysts options, more affordable bipolar plate production, and other potential savings.1 However, unlike PEMFCs, the HEMFC technology is in its early phases and there is a continuous effort to improve the performance and durability through the advancement of materials and optimization of operating conditions. The polarization curve is one of the most common methods of testing a fuel cell. A typical polarization curve for a fuel cell consists of open circuit potential (crossover losses), low-current behavior (kinetics losses), moderate-current behavior (ohmic losses), and high-current behavior (transport losses). Crossover and ohmic losses are determined by electrolytes, kinetics losses depend on catalysts and transport losses involve gas diffusion and water management. In addition, a triple phase boundary comprised of an electrolyte, an electrode and a gaseous reactant is crucial for the electrochemical reactions. Thus, the optimization of all these aspects is critical for HEMFC performance improvement. Here, we focus our work on transport phenomena in a fuel cell. The materials we used are hydroxide exchange membranes (HEMs) and ionomers (HEIs) based on poly(aryl piperidinium) (PAP).2 The specific chemical structure (shown in Figure 1) endows PAP HEMs with excellent chemical stability, high conductivity and mechanical robustness, making PAP HEMs/HEIs one of the best choices for HEMFC electrolyte. Furthermore, the use of PAP HEMs/HEIs enables operating HEMFCs at 95 °C, which accelerates reaction rates and decreases heat rejection. Optimization of fuel cell operating conditions is vital for water management. Water production by hydrogen oxidation reaction (HOR) tends to cause flooding in the anode, while water consumption from oxygen reduction reaction causes cathode to dry-out. In addition, the electro-osmotic drag pulls water from the cathode to the anode (in the reverse direction of PEMFCs), thus worsening the water management problem further. In this work, observable characteristics in the polarization curve for a flooded HEMFC were found, and a model was developed to describe this issue. Then we demonstrate that balanced water management can be obtained by adjusting the anode relative humidity and back pressure. Limiting current technology is widely employed for PEMFCs to characterize the transport resistance in order to reduce mass-transport losses; however, no one has done this systematically to HEMFCs, although there is a difference between limiting current behavior of PEMFCs and HEMFCs. We have found this technique useful to screen catalysts, optimize catalyst loading and adjust ionomer loading to improve the efficiency of gas diffusion, which is very helpful when switching from oxygen to air on cathode. Our optimized HEMFC can achieve a peak power density of 2.02 W cm-2 in H2/O2 and 1.33 W cm-2 in H2/air with platinum group metal (PGM) based catalysts (shown in Figure 2). The current density at 0.663 V for cell voltage is about 1.70 A cm-2, which is suitable for hydrogen fuel cell vehicles. References B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat Nanotechnol, 11 (12), 1020-1025 (2016).J. Wang, Y. Zhao, B. P. Setzler, S. Rojas-Carbonell, C. Ben Yehuda, A. Amel, M. Page, L. Wang, K. Hu, L. Shi, S. Gottesfeld, B. Xu, and Y. Yan, Nature Energy, (2019) doi:10.1038/s41560-019-0372-8. Figure 1

Impact of Operating Conditions on PEMFC Durability

PtCo-alloy catalysts have achieved mainstream application approximately 20 years after they were first reported. The current state of art PEM fuel cell cathode catalyst is represented by Pt-alloy nanoparticles dispersed on high surface area carbon (HSC) supports. [1] Mass activity > 0.6 A/mgPt has been reported consistently for such catalysts. Unfortunately, the high BOL activity of the Pt-alloy nanoparticle catalysts can suffer fairly rapid degradation in mass activity (MA) over time on exposure to fuel cell operating conditions, especially under voltage cycling (V-cycling). The prevalent mechanism for loss of specific activity is due to dissolution of the non-noble alloying element and thickening of Pt shell during voltage cycling. [2] Not withstanding the advances and numerous impactful studies, the degradation of Pt-alloy nanoparticles and its impact on electrode performance is poorly understood. For example, a recent article reviews the conflicting results on the impact of alloying on Pt nanoparticle stability, from positive, to neutral, to negative. [3] Additionally, the existing literature on durability of PtCo alloys are focused on the activity and nanoparticle characterization rather than on associated V-loss in the composite electrode. Given the criticality of this issue, a careful systematic study of this phenomenon with best-in-class materials in a state of art (SOA) membrane electrode assembly (MEA) is desired. In this present work, the use of operating conditions as a key factor to mitigate electrode degradation is explored. This was accomplished by conducting voltage cycling tests in H2/N2 environment at different operating conditions up to 60,000 voltage cycles. To identify the factors affecting the degradation, fractional factorial design of experiments was implemented. The fractional design allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. Factors in the test matrix include temperature, relative humidity, upper potential limit and upper potential hold time etc. In-situ electrochemical diagnostics such as electrochemical surface area (ECSA), mass activity (MA), specific activity (SA) was measured to understand the kinetics and degradation mechanism. Transport properties such as proton transport resistance and oxygen transport resistance was measured using H2-N2 impedance [4] and oxygen limiting current measurements [5]. The end of test (EOT) MEAs were characterized by ex-situ characterization measurements such as TEM for particle size distribution, electron micro probe analysis (EPMA) and EDS measurements to quantify the cobalt composition and loss up on voltage cycling at different operating conditions. In addition to multi-factor design of experiments, few key factors like RH and temperature were studied independently. Figure 1 shows the impact of relative humidity (RH) on H2-air performance and measured electrochemical surface area. Up to 100 mV loss at high current density (2.0 A/cm2) and ~30% ECSA loss observed for MEAs cycled up to 30,000 voltage cycles. Both performance and ECSA loss decreases with decreasing relative humidity. Almost zero ECSA loss after 30,000 voltage cycles at 25% RH observed. This example indicates that PtCo alloy catalyst degradation can be mitigated by carefully identifying the operating condition. The effect of various such operating condition and degradation mechanisms will be discussed. References S. Kumaraguru, 2018 Annual Progress Report, https://www.hydrogen.energy.gov/pdfs/progress18/fc_kumaraguru_2018.pdf C.E. Carlton, S. Chen, P.J. Ferreira, L.F. Allard, Y. Shao-Horn, J. Phys. Chem.Lett. 3, 161 (2012)J.A. Gilbert, A.J. Kropf, N.N. Kariuki, S. DeCrane, X. Wang, S. Rasouli, K. Yu, P.J. Ferreira, D. Morgan, D.J. Meyers, J. Electrochem. Soc., 162 (14), F1487 (2015)R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5), A970 (2005)T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012) Figure 1