bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and Evaluation of Core-Shell Electrocatalysts for Oxygen Reduction Reaction
Authors: Zhu Shangqian , Yue Jeffrey , Qin Xueping , Shao Minhua
Published: February 2020

The slow kinetics of the oxygen reduction reaction (ORR) at the cathode and high cost of electrocatalysts (commonly Pt and Pt-based alloys) for this reaction are major hurdles that hinder the large scale production of proton exchange membrane fuel cells (PEMFCs).[1] A core-shell structure consisting of an atomically thin layer (up to several atoms thick) of Pt deposited on Pd has been recognized as a promising ORR electrocatalysts to replace bulk Pt-based materials.[2] Ideally, all of the Pt atoms will be fully utilized since they are all on the surface in the case of an atomically thin layer of Pt on a core material. The large scale synthesis of Pd@Pt core-shell catalysts, however, has not been fully realized. This study reported two synthesis methods to prepare gram scale batch of Pd@Pt/C electrocatalysts with the assistance of citric acid.

The first is Cu-mediated-Pt-displacement method involving the displacement of an underpotentially deposited (UPD) Cu monolayer by Pt.[3] In this method, the Cu monolayer deposited on Pd then is displaced by Pt via a surface limited redox replacement (SLRR) reaction: Pd@Cu + PtCl4
2- → Pd@Pt + Cu2+ + 4Cl-. Recent studies have shown that the SLRR reaction is rather complicated and poorly-controlled resulting in formation of 3D Pt islands instead of a uniform overlayer, especially in the large batch size synthesis. By adding citric acid in the displacement reaction, the activity of Pd@Pt/C can be enhanced by 2 times compared with that synthesized without citric acid.

The second method is direct chemcial reduction of PtCl4
2- with the assistance of citric acid without preformation of a Cu UPD layer. Comparing with other reducing agents, the reducing power of citric acid is neither too strong nor too week and allows the reduction reaction to occur at the room temperature, which significantly simplifies the reaction process. In addition, the strong interaction of citric acid with metals may reduce the Pd dissolution and introduce charge transfer from adsorbed citric acid molecules to Pd surface, which can also be used to reduce Pt cations. 

Both methods resulted good quality of core-shell catalyts in terms of activity and durability. The Pt activities of Pd@Pt/C preapred by the Cu-mediated-Pt-displacement and chemcial reduction methods were 0.95 and 0.78 A mg-1 at 0.9 V, respectively. Fuel cell performance will be also reported and the role of citric acid will be discussed in the presentation. 

References

[1] in: M. Shao (Ed.) Electrocatalysis in Fuel Cells: A Non- and Low- Platinum Approach, Springer, London, 2013.

[2] R.R. Adzic, Electrocatalysis, 3 (2012) 163-169.

[3] R.R. Adzic, J. Zhang, K. Sasaki, M.B. Vukmirovic, M. Shao, J.X. Wang, A.U. Nilekar, M. Mavrikakis, J.A. Valerio, F. Uribe, Top. Catal., 46 (2007) 249-262.
Thermal Modelling and Validation of PEM Fuel Cell System for Automotive Applications
Authors: Liso Vincenzo , Nielsen Mads Pagh , Al Shakhshir Saher
Published: February 2020

In this study a validated model for a PEM fuel cell system is developed to actively control the supplied reactants and to manage water within the cell structure and correctly provide system cooling. Due to the substantial difference in response times of each of the system volumes, a model of each components system is developed for analysis and controller design to achieve thermal tracking while adequately rejecting system disturbances.

Control-oriented transient models have been developed to account for the formation of liquid water within the gas channels or within both the channels and the GDL, however the advantages of having a validated model will help achieving adequate humidity regulation. Due to the notoriously slow response of humidity transducers, especially near saturated conditions, this is an advances the field of fuel cell reactant pre-treatment. In developing this strategy, a critical step will be accomplished by properly selecting the controller references used for temperature feedback. Although this may at first seem like a simple step, the selected temperature references have a profound impact on the resulting thermal and humidity regulation. If not properly considered, the system response could be unnecessarily slow or produce an undesirable excursion in humidity. The establishment of this relationship allows for accurate dynamic voltage estimation under a range of operating conditions, and is a necessary step towards further understanding physical phenomena.
Resonance-Stabilized Guanidinium-Tethered Poly(Phenylene)s
Authors: Lee Kwan-Soo , Fujimoto Cy , Kim Yu Seung
Published: February 2020

Anion exchange polymer electrolyte is a key element for alkaline membrane fuel cell (AMFC) applications. In fuel cells, the polymer electrolytes require alkaline stable covalently tethered cationic group to conduct hydroxide ion. Benzyltrimethyl ammonium cationic group has been mostly often employed in anion exchange polymer electrolytes because it is reasonably basic, stable and easily synthesized. However, the demands on more stable and less catalyst poisoning cationic groups are growing.

 One promising stable cationic groups that potentially can be tethered in the polymer backbone is guanidinium. However, benzyl guanidinium tethered polymers prepared via chloromethylation and subsequent cationization of the halomethyl group showed a limited stability in high pH conditions due to the nucleophilic degradation of the benzylic carbon. In 2011, we had developed a new synthetic method to synthesize guanidinium tethered polymer via activated fluorophenyl-amine reaction.1 This new synthetic method allows to prepare phenyl guanidinium (instead of benzyl guanidinium via conventional method) tethered polymer. The direct connection between phenyl group and guanidinium can stabilize the cationic group by whole resonance structure. However, continuing study indicates that the poly(arylene ether) polymer that we employed undergoes ether-cleavage degradation under high pH conditions.2

In this study, we report the phenyl guanidinium tethered ether-free poly(phenylene)s. Activated fluorophenyl-amine reaction was used to synthesize the poly(phenylene)s. The alkaline stability of the polymer is exceptional: no chemical structural degradation was found during 1000 h in 0.5 M NaOH at 80°C. The tailored poly(phenylene) anion exchange ionomers is well suited for the ionomeric binders, particularly for hydrogen oxidation reaction in alkaline membrane fuel cells.

References:

1 D. S. Kim, A. Labouriau, M. D. Guiver, and Y.S. Kim, Chem. Mater. 2011, 23, 3795-3797.

2 C. Fujimoto, D. S. Kim, M. Hibbs, D. Wrobleski, Y.S. Kim, J. Memb. Sci. 2012, 423, 438-449.
Fibsem Tomography: Connecting the Roles of Corrosion and Microstructure to Polymer Electrolyte Fuel Cell Performance and Durability
Authors: Star Andrew G. , Fuller Thomas F
Published: February 2020

Low-temperature proton exchange membrane fuel cells (PEMFC) may become essential energy conversion devices in the automotive sector and contribute to a future hydrogen economy. Corrosion of the cathode catalyst support is known to degrade performance but many specific details of the changes in performance are still poorly understood. A focused ion beam with scanning electron microscope (FIBSEM) method of nanotomography was developed for postmortem direct observation of corroded cathode microstructures. Unique to corroded cathodes, additional charge-reduction measures were taken to ensure high-fidelity imaging and low-error rate image processing. From three-dimensional reconstructed volumes, computational microstructural characterization methods were developed to extract pertinent transport parameters (e.g. porosity and tortuosity distributions) and characterize changes between pristine and corroded samples. FIBSEM and computational methods were coupled with traditional electrochemical techniques and carbon mass loss detection. Taken together, this work helps clarify and connect the roles of microstructure, corrosion, performance, and durability in PEMFCs.

Figure 1
Nitrogen and Sulfur Co-Doped Three-Dimensional Reduced Graphene Oxide Networks-Supported Cobalt Nanoparticles with High Catalytic Activity in the Oxygen Reduction Reaction
Authors: Li Yi , Yang Juan , Zhou Yazhou , Chen Zulei , Huang Jipei
Published: February 2020

Although the big progress in developing heteroatom doped carbon catalysts for oxygen-reduction reaction (ORR), the current noble metal-free catalysts are still facing the poor ORR activity which restricts its application in fuel cell. Developing new fuel cell catalysts with excellent ORR performance may be a potential way to solve this drawback. Herein, we developed a facile and green method to synthesize nitrogen and sulfur co-doped three-dimensional reduced graphene oxide networks-supported cobalt nanoparticles (Co/N-S-3DrGO) using hydrothermal method. The doping of N and S not only provide the graphene networks with many defects, but also play significant roles in engineering the covalent coupling between cobalt nanoparticles and the 3D graphene networks. The Co/N-S-3DrGO displays electrocatalytic activity toward ORR, which is comparable to the commercially available Pt/C (20%) catalyst, in addition to their long-term stability and high methanol tolerance. Such superior performances may be attributed to the synergistic effect, which includes the high catalytic sites of N-S heteroatom, the distribution activity of cobalt nanoparticles and the high electron transfer rate of the 3D graphene networks. Our results indicate a simple strategy to design highly efficient graphene-based non-noble catalysts for fuel cell application.
Synthesis and Study of Structurally Ordered FePt Nanoparticles As Highly Active and Durable Catalysts for Oxygen Reduction Reaction
Authors: Li Junrui , Li Qing , Xi Zheng , Muzzio Michelle , Sun Shouheng
Published: February 2020

Finding a highly active and durable nanocatalyst under routinely used proton exchange membrane fuel cell operating conditions (60 - 80 oC) is urgent for the commercialization of fuel cells. Many nanocatalysts have been reported to have much improved oxygen reduction reaction (ORR) activity compared to the commercially used Pt/C catalyst. However, few of them can meet the durability requirement. In this report, monodisperse face-centered-tetragonal (fct) (L10) FePt nanoparticles with size of ~ 9 nm were synthesized from dumbbell-like face-centered-cubic (fcc)-FePt-Fe3O4 nanoparticles. The durability of C-fct-FePt nanoparticles as ORR catalysts at 60 oC was studied and it was found that these nanoparticles served as a highly active and durable ORR catalyst. Its mass activity was found to be about 5.4 times that of commercial Pt/C. Also, no obvious loss in ORR activity was observed after 10,000 cycles. A perchloric acid-treatment and subsequent annealing treatment were found to improve the durability of fct-FePt nanoparticles at 60 oC. The highly ordered L10 structure, with alternating layers of Fe and Pt atoms, as well as the Pt skin formed after the acid-treatment, were hypthesized to be the main reason for the durability enhancement. Our work demonstrates a reliable approach to structurally ordered FePt nanoparticles as a promising candidate for practical use in fuel cells.
PdFe Nanoparticles for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells
Authors: Lee Kug-Seung , Kang Yun Sik
Published: February 2020

Carbon-supported PdFe nanoparticles (NPs) were prepared with composition-controlling via a modified chemical synthesis and a heat-treatment at high temperature under a reductive atmosphere. The synthesis combined the polyol reduction method and hydride reduction method, which was used to obtain monodispersed PdFe NPs. In addition, to induce structural modifications, the as-prepared PdFe NPs received heat-treatment under a reductive atmosphere. Results of structural characterizations including highresolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analysis, indicated that heat-treated PdFe NPs exhibited a higher degree of alloying and surface Pd atomic composition compared with as-prepared ones. Furthermore, new crystalline phases were detected after heat-treatment. Thanks to the structural alterations, heat-treated PdFe NPs showed ~3 and ~18 times higher mass- and area-normalized oxygen reduction reaction (ORR) activities, respectively than commercial Pt/C. Single cell testing with heat-treated PdFe catalysts exhibited a ~2.5 times higher mass-normalized maximum power density than the reference cell. Surface structure analyses, including cyclic voltammetry (CV), COad oxidation, and XPS, revealed that, after heat-treatment, a downshift of the Pd d-band center occurred, which led to a decrease in the affinity of Pd for oxygen species, resulting in more favorable ORR kinetics.
Evaluating in Situ Platinum Deposition As a Preparative Step for Fibsem Tomography of PEMFC Cathodes
Authors: Star Andrew G. , Fuller Thomas F
Published: February 2020

In polymer electrolyte fuel cells (PEFC), cathode catalyst layers may experience high overpotentials during cell start up/shut down and even during normal operation. High overpotentials undesirably facilitate corrosion kinetics of the carbon catalyst support which results in a severe increase in mass transport resistance in the cathode. Focused ion beam and scanning electron microscope tomography (FIBSEM) is an emerging technique for nanometer-resolution direct observation of three-dimensional microstructure and shows promise for improving understanding of microstructural changes to PEFC cathodes in response to corrosion. However, the high porosity of PEFC cathodes leads to challenges in downstream image processing. Filling the pore space prior to tomography may allow circumvention of image processing challenges but has not yet been definitively shown to provide good results. The present paper assesses the viability of an organoplatinum electron beam induced deposition (PtEBID) step in order to fill the pore space prior to FIBSEM. PtEBID methods were investigated and validated with complementary AFM and STEM to quantify precursor diffusion into porous cathodes, kinetics of adsorption and cracking of organoplatinum on the carbon support surface, and damage induced by the focused ion beam used during FIBSEM.
A Hybrid of Palladium-Graphitic Carbon Nitride-Reduced Graphene Oxide for Methanol and Formic Acid Electrooxidation
Authors: Zhang Wenyao , Wang Xin , Zhang Lili , Zhang Weiguang
Published: February 2020

A novel nitrogen-rich support material (CNNF-G) consisting of graphitic carbon nitride (g-C3N4) nanoflakelets (CNNF) and reduced graphene oxide (rGO) is designed and fabricated for loading Pd nanoparticles. Structural characterizations indicates that the CNNF is formed via splitting decomposition of the g-C3N4 polymer on rGO at higher temperatures and the resulting CNNF is intimately coupled to the rGO sheets. The CNNF can provide more exposed edge sites and active nitrogen species for the high dispersion of Pd NPs. It is found that the Pd NPs with an average diameter of 3.92 nm are uniformly dispersed on CNNF-G sheets. DFT computations reveal that CNNF can trap Pd adatom and thus act as a Pd nucleation site at which Pd atoms tend to accumulate to form Pd clusters. The Pd-CNNF-G nanocatalyst exhibits excellent electrocatalytic activity for both formic acid and methanol oxidation reactions, including large electrochemically active surface area (ECSA) values, significantly high forward peak current densities, and reliable stability and durability, far outperforming the Pd-graphene, commercial activated carbon-supported Pd catalyst or Pd-carbon nanotubes. Such a stable Pd/CNNF-G nanocatalyst may bring new design opportunities for high-performance direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) in the future.

Figure 1
Dissolution Behavior Analysis of Model for PEFC Electrodes Using Eqcm
Authors: Jozuka Tetsuta , Ando Taishi , Asakura Noriaki , Kaito Takamasa , Matsuda Toshihiko
Published: February 2020

In electrode catalysts used for solid polymer fuel batteries, reducing the usage quantity of Pt has become a critical issue. For that reason, catalysts with greater oxygen reduction activity are developed daily. Among those catalysts, Pt alloy catalysts using transition metals are known for their property of high oxygen reduction activity. On the other hand, such Pt alloy catalysts, which use transition metals, have a problem in that their performances degrade depending on the usage conditions or environment.Conventionally, the degradation behavior of catalysts has been analyzed on the basis of the changes in oxygen reduction performance or the structure observation and composition reduction after their measurements. Though those methods can verify the performances before and after the experiment, it is difficult for them to analyze what behaviors the gradations occur with. In the study here, dissolution behaviors of Pt model electrodes were analyzed by the EQCM method, which can perform in situ measurement on the mass change caused by electrochemical reactions.
Peald-TiN Protective Coatings for Metallic Bipolar Plates of PEMFC
Authors: Yun Eun-Young , Kim Da-Young , Lee Han-Bo-Ram , Kwon Se-Hun
Published: February 2020

Proton Exchange Membrane Fuel Cell (PEMFC) is an electrochemical device that convert the chemical energy of reactants directly to electrical energy. The water-based, acidic polymer membrane usually used as its electrolyte. Therefore, the bipolar plate, as one of the most important components in PEMFC system, is required to be high corrosion resistance in the acidic condition. Currently, researchers have tried to develop metallic bipolar plate using a stainless steel. In case of using stainless steel, a passivation or protective coatings layers should be applied to improve its stability in the harsh environment. However, the general intrinsic defects (columnar structures, pinholes, pores, discontinuities) in the protective films, which can greatly affect its high performance. Therefore, a passive thin film with low defects and stable property was needed to be developed.

In this work, TiN thin films were deposited on 316L stainless steel as the protective layer by using plasma enhanced atomic layer deposition (PEALD). For comparison, tetrakis-dimethylamido-titanium (TDMAT) and titanium tetrachloride (TiCl4) were used as different Ti sources, respectively. The microstructure, chemical composition and the corrosion behavior of TiN protective layer deposited by different sources were investigated by 4-point probe, XRD, FESEM, AES and potentiodynamic polarization equipment. The SEM results showed that the uniform TiN protective layers were successfully deposited on the 316L stainless steel substrate by both two sources. and the FCC crystal structure was confirmed by the XRD analysis. In addition, the TDMAT-deposited TiN has lower the corrosion current density (Icorr) and higher corrosion potential (Ecorr) than those of TiCl4-deposited TiN, which could be inferred that the TDMAT-deposited TiN will have a higher corrosion resistance and protective efficiency during the fuel cell application.
A Study on Degradation of Membrane Electrode Assembly By Operating Conditions of Fuel Cell Vehicle
Authors: Yoo Seung Eul , Kim Myong-Hwan , Goo Youngmo
Published: February 2020

In generally, degradation of membrane & electrode assembly(MEA) according to driving conditions of fuel cell electric vehicle focused on analyzing the state of catalyst, membrane, gas diffusion layer before and after durability evaluation . At present, the exact mechanism for degradation of membrane electrode assembly(MEA) according driving conditions of fuel cell vehicle does not hold and the mechanism were approximately the level that analogize the whole out of a part.

In case of the degradation of MEA by operating modes, it was shown on three sources; (1) the loss of apparent catalytic activity according to the time(ηa), (2) conductivity loss by decreasing ion conductivity per the time, (3) the loss of rate of mass transfer according to the time. In other words, the equation about degradation of MEA can be seen that the basic form can be created by analyzing the polarization according to time.In this study, the 10 driving modes were devised by cell voltage behavior according to driving conditions of fuel cell vehicle. The operating modes were basically designed by high current, middle current and low current. Also, these modes were considered about the most commonly used driving power and time variation in accordance with the current change. The result of analyzing the MEA degradation by current/voltage variation was as below.

The activation overpotential shows a tendency to be rapidly decreased in early state and decrease or increase gradually without reference to operating conditions. The ohmic overpotential tends to decrease in proportion to the time. OCV(Open Circuit Voltage) shows the result of decreasing rapidly in high current mode than low current mode. In this paper, we conducted an analysis of the equation for degradation of MEA according to the time, current/voltage variation.

In conclusions, the system of fuel cell vehicle must be operated in high current range in order to prevent degradation of the MEA in similar load change condition. It is considered to be occurred in oxidation of electrode catalyst under high voltage condition.
Nickel/Carbide Based Catalysts for Oxygen Evolution Reaction in Alkaline Solution
Authors: Song DongHoon , Kim MinJoong , Cho EunAe
Published: February 2020

Hydrogen, produced by electrolysis, is one of the most promising means to store electricity coming from renewables and to address its intermittency. Due to the increase of global electricity share from renewable energy sources, large-scale storage systems are required. Alkaline electrolysis can be a suitable solution for the large-scale energy storage system. However, high overpotential and slow kinetics, particularly of the oxygen evolution reaction (OER), still limit the efficiency of the system. To enhance the efficiency of alkaline electrolysis cells, catalysts with high electrocatalytic activity for OER should be developed.

 Platinum-like behavior of tungsten carbide in surface catalysis was first discovered by Levy and Boudart in 1973[1]. In addition, tungsten carbide was reported to have a Pt-like electronic structure by Colton in 1975[2]. Since then, tungsten carbide has been studied widely and expected to replace noble metal catalysts which are used in many electrochemical reactions. In this study, we researched tungsten carbide as a support for nickel, and formation of bi-metallic surfaces for developing a high activity OER catalyst.

  WC was synthesized by two-steps. First, WC-Co was synthesized by heat treatment. Carbon nitride (g-C3N4) and tungsten chloride were used as a carbon and tungsten precursor. Cobalt precursor was also used to help carburization of tungsten. After synthesizing WC-Co, the acidic leaching process was followed by using sulfuric acid (H2SO4) to eliminate cobalt. Through additional mixing with 3d metal precursors and heat treatment, Ni/WC and NiFe/WC were synthesized. Furthermore, Ni/C was also prepared using same precursors as a counter group. Figure 1 shows the SEM images of the synthesized WC and Ni/WC. Cyclic voltammetry was performed to evaluate OER activity in 1 M KOH in potential range from 1 to 1.8 V.

  Figure 2 shows the OER catalytic activities of synthesized catalysts and Ir/C. Pure WC and Ni/C had 430 mV and 394 mV overpotential at 10 mA∙cm-2 respectively. In 3d Metal/WC cases, Ni/WC and NiFe/WC showed only 335 mV and 314 mV overpotential at 10 mA∙cm-2. Both catalysts had higher electrocatalytic activity for OER than Ir/C in alkaline media.  

References

[1] R. B. Levy, M. Boudart, Science, New series, Vol. 181, No. 4099 (1973) 547-579

[2] Richard J. Colton, Jan-Tsyu j. Huang, J.Wayne Rabalais, Chemical Physics Letters, Vol. 34, No. 2 (1975) 337-339

Figure 1
Structural Effects of Non-Uniform Deformation of Porous-Streaming Paths on Transport Phenomena and Electrochemical Reactions in Polymer Electrolyte Fuel Cells
Authors: Kim Ah-Reum , Park Joo-Hee , Liu Jiawen , Um Sukkee
Published: February 2020

Unified fuel cell reaction modeling was proposed to predict the effects of local deformation of porous-streaming paths (i.e. gas diffusion media (GDM)) on transport phenomena of polymer electrolyte fuel cells (PEFCs) by structural and electrochemical approaches. First, structural analysis is conducted to figure out non-uniform behavior of GDM with bipolar plate channels under the stack clamping pressure. The deformation results enables the exact determination of flow properties of porous media (i.e. permeability and porosity). After recalculation process of flow paths, computational fuel cell analysis in the deformed regions is performed to visualize flow patterns and estimate the electrochemical performance of PEFCs. The results show that water transport in deformed GDM restricts the smooth transport of reactant gas leading to significant mass transport losses. Therefore, combined structural-fluidic analyses with electrochemical reaction provide insight into high-current density PEFC system.
Influence of GDL Properties on Liquid Water Distribution and Temperature at CL Surface
Authors: Araki Takuto , Kobayashi Koki , Minami Ryotaro , Hashimura Sota
Published: February 2020

Proper water management in proton exchange membrane fuel cell (PEMFC) is critical to achieve a high power density and durability of construction materials. The gas transport is blocked by liquid water remained in GDL, and it causes performance decrement with increase of concentration overpotential. The location of water accumulation is influenced by the temperature distribution inside GDL. At the same time, the temperature distribution is affected by the liquid water distribution, because the liquid water inside GDL decrease the effective thermal resistance. Then, the objective of this study was set to clear the mutual interaction between the liquid water distributions and temperature fields. For this purpose, temperature at the CL surface was measured using thin film thermocouple fabricated by MEMS based technology, and liquid water inside the GDL was visualized by X-ray CT.

The experimental conditions were shown in the table. The GDL thermal property, especially the thickness, was changed intentionally to get symmetrical temperature distributions, and the influence of the GDL thermal property on the liquid water distributions and temperature have been examined.

The thickness of the sensor was only about 9µm and the width is about 100µm. The electromotive force of the MEMS based sensor was confirmed to be proportional to the temperature difference. The MEMS sensor was inserted at the boundary between MPL and CL. The temperature rise from OCV was plotted in the bottom left figure. The temperature increased quadratically except for the high current density region of condition 4.

The visualized liquid water distributions were shown in right hand side figures. The accumulated water was observed in cathode side GDLs in all conditions, however, the highest saturation of water was observed near the rib surface of condition 4. The accumulated water at the rib surface was supposed to reduce the contact thermal resistance and decrease the temperature at the CL surface.

Figure 1
Wettability of Gas Diffusion Layers for Proton Exchange Membrane (PEM) Fuel Cells
Authors: Sow Pradeep Kumar , Prass Sebastian , Zahiri Beniamin , Mérida Walter
Published: February 2020

Water transport within the gas diffusion layer (GDL) depends on the material’s wettability. In a PEM fuel cell, hydrophobic substrates facilitate water transport at the cathodes under high current density operation. We report a new approach to quantify the relative, ex situ GDL wettability by measuring the solid- liquid (S-L) interfacial area. We applied electrochemical techniques to measure the double layer capacitance, and exploited the proportionality between this quantity and the S-L interfacial area. We combined this electrochemical approach with traditional techniques (e.g., contact angle, and capillary pressure) to characterize commercial GDL materials. The measurements demonstrate that the fibre structure and morphology affect the wetting behaviour of the GDL materials. We describe the experimental approach and the underlying physical mechanisms, as well as the method’s applicability to characterize water intrusion processes for GDL materials.
(Invited) Structure-Property Relathionships of Proton Conducting Membranes Studied By Synchrotron Radiation
Authors: Portale Giuseppe
Published: February 2020

The properties of ion conducting polymers (ionomers) used as proton conducting membranes are strongly dictated by their nanostructure. Due to the strong difference in polarity between the acidic sulfonic groups and the apolar main polymer chain, these materials exhibit strong phase separation. X-ray and neutron scattering are among the best experimental techniques to investigate the ionomer nanostructure. In particular, synchrotron based small and wide angle X-ray scattering (SAXS and WAXS) are particularly suited to perform in-situ investigation on ionomers.

In this contribution, I will show our most recent results obtained by using synchrotron SAXS and WAXS on the investigation of the structure-property relationships in different ionomers. The structural knowledge acquired on the benchmark Nafion membranes is compared to that of its short-side chain counterpart (Aquivion) and to other non-perfluorosulfonated ionomers like sulfonated poly(ether ether ketone)s and sulfonated polysulfones. The nano morphology of Nafion and Aquivion can be qualitatively and (almost) quantitatively explained by a locally flat morphology, describing the morphology as an alternated assembly of water/ions and polymeric 2D domains. The same morphological model can be applied successfully to other fluorine-free ionomers, suggesting that a local flat morphology could be common to many proton conducting ionomers.

In the second part of my talk, I will show some recent in-situ SAXS experiments on microstructural changes occurring in different ionomer membranes under controlled temperature and humidity conditions. The data suggest local re-distribution of water inside the membranes that can lead either to membrane swelling (low temperature and high humidity) or to local ionic structure collapse (high temperature and low humidity). These experiments mimic very closely what may happen in a real fuel cell and the understanding of the results is crucial to improve the membrane performances.
Impact of Equivalent Weight and Side-Chain on Structure/Functionality of PFSA Ionomers and Thin Films
Authors: Kusoglu Ahmet , Shi Shouwen , Dudenas Peter , Weber Adam Z
Published: February 2020

Perfluorosulfonic-acid (PFSA) ionomers play key roles in polymer-electrolyte fuel-cell (PEFCs), primarily as a proton-exchange membrane (PEM), and also as an electrolyte film in porous catalyst structures binding the catalytic agglomerates.1 Despite their wide adoption in PEFCs, most studies on PFSAs have thus far focused on Nafion, and studies on other PFSAs, in particular with varying side-chain chemistries and equivalent weights (EWs), have been relatively scarce. Therefore, it is of interest to elucidate how such changes in chemistry and EW influence the PFSA behavior, not only as a bulk membrane (i.e., PEM), but also as a catalyst-ionomer film (i.e., thin-film). In particular, the latter phenomenon could have important implications on explaining the mass-transport limitations in catalyst layers, which have been recently shown to be strongly related to the transport resistances at the ionomer thin-film.1,2

In this talk, the impact of EW and side-chain chemistry on structure/property relationship of various PFSAs will be explored as bulk membranes (> 20 μm) and dispersion-cast thin films (< 100 nm). PFSAs owe their combination of good transport properties and mechanical stability to their phase-separated morphology of conductive hydrophilic ionic domains and the hydrophobic backbone, which are connected via side-chains. It will be shown how this PFSA morphology is controlled by the fraction of backbone (via EW) and the side-chain length (via chemistry), which together govern the interrelation between transport and stability. Moreover, our investigations demonstrate the existence of universal correlations between the water-uptake and conductivity (chemical energies), and backbone crystallinity (mechanical energies), with a key role of the side-chain length in altering this balance between the chemical-mechanical energies. We will report the results of a systematic investigation on hydration, conductivity, mechanical properties and crystallinity of various types and EWs of PFSA ionomers to develop universal structure/function relationships. Lastly, it will be explored how these EW and side-chain effects influence the structure and properties of PFSA thin films when confined to nanometer thicknesses, and control their deviation from bulk membrane behavior. Our results provide new insights into the understanding and optimizing the PFSA ionomers' functionalities in fuel-cells, both as bulk membranes and as thin-films.

References

1. A. Z. Weber and A. Kusoglu, Journal of Materials Chemistry A, 2, 17207 (2014).

2. A. Kongkanand and M. F. Mathias, The Journal of Physical Chemistry Letters, 1127 (2016).

Acknowledgement

This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy under contract number DE-AC02-05CH11231 and Program Development Manager Dimitrios Papageorgopoulos.
Gram-Scale Synthesis of Nitrogen-Doped Carbon Foams As Template-Free, Metal-Free, Non-Precious Electrocatalysts for Oxygen Reduction and CO2 Conversion
Authors: Lyth Stephen Matthew , Liu Jianfeng , Mufundirwa Albert , Smid Bretislav , Ma Sichao , Kenis Paul J.A. , Sasaki Kazunari
Published: February 2020

Carbon foams are ideal materials for electrochemical applications due to their large surface area, high electrical conductivity, and stability over a wide electrochemical potential window. However, many of these materials are expensive; use complex multi-step synthesis protocols; and require the use of nanostructured template materials which must be removed with strong acid or alkaline. It is often difficult to control the porosity (and thereby optimize mass diffusion); and it is difficult to dope effectively with heteroatoms. We synthesize carbon foams at low cost and gram-scale, with large micron-scale pores, very thin walls, and very large surface area (e.g. 2500 m2/g), by thermal decomposition of sodium ethoxide.[1,2]

Crucially, by synthesizing and subsequently decomposing nitrogen-containing metal alkoxides, nitrogen-doped carbon foams can be made.[3] The nitrogen content can be varied over a wide range (e.g. <0.5 to 15 at%) by simply changing e.g. the precursor ratios. In addition, the ratio of e.g. pyridinic to tertiary nitrogen bonding can be tailored by changing the pyrolysis temperature. These materials do not contain transition metal contamination (confirmed by ICP-AES), unlike carbon nanotubes or carbon black. Therefore they can be used to probe the fundamental oxygen reduction reaction (ORR) activity of nitrogen-doped carbon in acid – a relatively under-represented topic, plagued by contamination issues.[4,5] We observe surprisingly high mass activity and onset potential for the ORR, with high electron transfer number. This work shows that 4-electron ORR is possible even in the absence of transition metals, most likely associated with tertiary nitrogen sites.[6] In alkaline medium, these catalysts are comparable to Pt/C, and undergo negligible degradation even over 60,000 load potential cycles.[7]

Finally, these materials are investigated for their electrochemical CO2 conversion activity to carbon monoxide, formic acid, methane, and ethane.

References

[1] S. M. Lyth, H. Shao, J. Liu, K. Sasaki, Int. J. Hydrogen Energy, 2014, 39, 376

[2] J. Liu, D. Takeshi, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F838

[3] S. M. Lyth, Y. Nabae, N. M. Islam, et al., eJournal of Surface Science and Technology, 2012, 10, 29-32.

[4] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Nanosci. Nanotechnol. 2012, 12, 4887

[5] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, S. Miyata, J. Electrochem. Soc. 2011, 158, B194

[6] J. Liu, D. Takeshi, D. Orejon, K. Sasaki, S. M. Lyth, J. Electrochem. Soc. 2014, 161, F544

[7] J. Liu, K. Sasaki, S. M. Lyth, ECS Trans. 2014, 64, 1161
Mechanistic Analysis of Electrocatalytic CO Oxidation By a Rh Porphyrin on a Carbon Support
Authors: Yamazaki Shin-ichi , Maeda Yasushi , Asahi Masafumi , Siroma Zyun , Ioroi Tsutomu
Published: February 2020

Electrochemical CO oxidation (CO + H2O → CO2 + 2H+ + 2e–) is important to counteract CO poisoning of Pt-based anode catalysts, since the reaction can reduce CO concentration on an anode of PEFC. However, the CO oxidation at anode potentials hardly occurs on conventional Pt-Ru electrocatalysts.

From these backgrounds, we have developed Rh-porphyrin-based electrocatalysts for CO oxidation [1, 2]. In the catalysts, molecules of Rh porphyrin are dispersed on a carbon black support. These catalysts can oxidize CO at much lower overpotentials than the Pt-Ru electrocatalysts [1-3]. A membrane-electrode assembly (MEA) that uses a Rh porphyrin as an anode catalyst gave significant power (>40 mW/cm2) when CO was supplied as a fuel [4]. A combined catalyst of a Rh porphyrin and Pt-Ru catalyst functions as a CO-tolerant anode catalyst [5].

However, the reason why Rh porphyrins can catalyze CO electro-oxidation at low overpotentials remains unclear. Mechanistic analyses on carbon-supported Rh porphyrin are needed to clarify the reason. The analysis would also contribute to further improvement of this catalyst. In this work, we report a possible reaction mechanism of CO oxidation by the catalyst. First, we examined the reactivity of Rh porphyrins dissolved in solution. This analysis enables us to discuss the CO activation mechanism by Rh porphyrins without the effect of carbon black support. A Rh porphyrin was treated with CO gas in dichloromethane to generate CO adduct of the Rh porphyrin. We characterize this CO-adduct, a possible intermediate of CO oxidation, by IR, NMR, and X-ray crystallography. The results indicate that CO coordinated on Rh atom is feasible to nucleophilic attack by water. Actually, the CO-adduct reduces an soluble electron acceptor in the presence of water. A possible reaction mechanism is shown in Fig. 1. An electrode works as an electron acceptor in the electrocatalytic CO oxidation.

In actual electroctalysts using Rh porphyrins, molecules are adsorbed on a carbon black support. Configuration of molecules on a carbon support would be also important for the catalytic activity. Then, we examined possible effects of the configuration by observing molecules on a highly oriented pyrolytic graphite with an atomic force microscope. The results show that certain ligand structure facilitates the configuration desirable for the high catalytic activity.

These analyses would be an answer for the question; why does Rh porphyrin can oxidize CO at lower potentials than conventional Pt-based catalysts.

This work was supported by Grant-in-Aids for Scientific Research (B) (No. 15H03853).

References

[1] S. Yamazaki et al., Phys. Chem. Chem. Phys. (2010) 12,8968.

[2] S. Yamazaki et al., Electrochem. Solid-State Lett. (2011) 14,B23.

[3] J. F. van. Baar et al., Electrochim. Acta (1982) 27,57.

[4] S. Yamazaki et al., Angew. Chem. Int. Ed. (2006) 45,3120.

[5] S. Yamazaki et al., J. Phys. Chem. C (2010) 114, 21856.

Figure 1
(Invited) Multi-Element-Doped Ceria-Based Metal Oxides for Advanced Proton Conducting SOFCs
Authors: Thangadurai Venkataraman , Singh Kalpana
Published: February 2020

The ability of solid oxide fuel cells (SOFC) to convert the chemical energy of various kinds of fuels into electricity at high efficiency, and with reduced environmental impacts makes them an attractive technology for current and future plans for clean power generation. State-of-the-art yttria-stabilised ZrO2 (YSZ) electrolyte shows high conductivity (10-2 S/cm) at about 1000 °C [1]. However, the high operating temperature leads to durability and cost issues and hinders its full market implementation. Operating temperature of SOFCs can be lowered by employing ceramic proton conducting electrolytes based on doped- BaCeO3 which exhibits high proton conductivity (10-2 S/cm) in the intermediate temperature (IT, 400-700 °C) range [2]. However, the poor chemical stability of doped-BaCeO3 under SOFC by-products CO2, and H2O limits its use as stable electrolyte [2].

Here we report, perovskite–type Ba0.5Sr0.5Ce1-x-y-z
Zr
x
Gd
y
Y
z
O3-δ as proton conductors for IT-SOFCs [3]. A-and B-site of BaCeO3 were doped by more electronegative elements to improve its chemical stability under H2O and CO2 at elevated temperature. In terms of chemical stability and conductivity, Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ seems to be the optimal composition with conductivity of 10-3 S/cm at 700 °C in 3% H2O/H2. Open circuit voltage of 1.15 V at 700 °C for H2-air cell suggests pure ionic (proton) conduction in Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ [3]. The effect of sintering temperature on bulk and grain boundary conductivity of these oxides was investigated using dielectric loss spectroscopy [4]. Moreover, the difference in the relaxation times in the current study suggests that short-range and long-range proton dynamics seems to be differing to previous studies on Y-doped BaZrO3 systems [5]. Additionally, layered perovskite-type Gd0.5Pr0.5BaCo2O5+δ were characterised as cathode for H-SOFCs [6]. Symmetrical cell measurements under air and wet air gave an area specific resistance of 2.4 Ω cm2 and 1.9 Ω cm2 for oxygen reduction reaction at 700 °C [6]. The effect of phase purity and synthesis methods on the electrochemical performance of Ni+Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3
-δ anode composites was investigated through symmetrical cell measurements in 3% H2O/H2.

References

1) J. W. Fergus, R. Hui, X. Li, D. P. Wilkinson, J. Zhang, Solid Oxide Fuel Cells: Materials Properties and Performance, CRC Press, New York (2009).

2) K. D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333.

3) R. Kannan, K. Singh, S. Gill, T. Fürstenhaupt, V. Thangadurai, Sci. Reports. 3 (2013) 2138.

4) K. Singh, A. Baral, V. Thangadurai, J. Am. Ceram. Soc. 99 (2016) 866.

5) Y. Yamazaki, F. Blanc, Y. Okuyama, L. Buannic L, J. C. Lucio-Vega, C. P. Grey, S. M. Haile, Nat. Mater. 12 (2013) 647.

6) K. Singh, A. K. Baral, V. Thangadurai, Solid State Ionics (2016) DOI:10.1016/j.ssi.2015.12.010.
Effects of Electrolytes on Electrochemical Properties of Cathode Material LaNi0.6Fe0.4O3-d based Fuel Cells
Authors: Yang Peng , Sui Jing , Dong Lifeng
Published: February 2020

Unique electrochemical properties of LaNi0.6Fe0.4O3-δ (LNF) have made it a better choice of cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with high power density. To improve electrochemical properties of LNF and analyze the effects of different electrolytes on the performance of LNF-based IT-SOFC, half-cells are fabricated with Ce0.9Gd0.1O2 (GDC) electrolyte or porous GDC backbone/GDC electrolyte. Ag paste is used as a current collector deposited on the cathode material surface. Additionally, LNF and GDC powders are synthesized by sol-gel method leading to the polarization resistance (Rp) and current density of half-cells with two kinds of electrolytes to be measured by a three-electrode method. Experimental results at 550-750 oC show that the polarization resistance (Rp) of LNF/GDC backbone/GDC is 0.376 Ω·cm2 at 750 oC in air, a reduction by 67.02 % compared with that of pure GDC electrolyte at the same temperature. And LNF/GDC backbone/GDC has a higher current density, 0.377 A·cm-2 at 750 oC in air. Systematic analysis suggests that porous GDC backbone improves electrochemical properties of LNF cathode materials.
Developing Cost-Effective Dense Continuous SDC Barrier Layers for SOFCs
Authors: Hardy John S , Coyle Christopher A , Nguyen Hoang V. P. , Lu Zigui , Stevenson Jeffry W
Published: February 2020

Solid oxide fuel cells (SOFCs) with lanthanum strontium cobalt ferrite (LSCF) cathodes often employ a samaria-doped or gadolinia-doped ceria (SDC or GDC, respectively) barrier layer between the cathode and the yttria-stabilized zirconia (YSZ) electrolyte to inhibit Sr from migrating to the YSZ surface where it reacts to form electrically-resistive strontium zirconate during cathode sintering.  Because the SDC must be sintered at limited temperatures (generally ≤1200°C) to avoid forming a CeO2-ZrO2 solid solution that has significantly lower ionic conductivity, screen printed barrier layers are often porous after firing.  Electrochemical tests of cells with dense, continuous pulsed laser deposited (PLD) SDC found that these cells exhibited significantly higher performance than those with the conventional screen-printed porous SDC layers.  Thus, PNNL began investigating the fabrication of dense, continuous SDC barrier layers using cost-effective deposition by screen printing which is amenable to industrial production of SOFCs.  Many approaches to improve the SDC density have been explored including the use of powder with reduced particle sizes, inks with increased solids loading, and doping with sintering aids.  While some of these tests were successful in enhancing the density of the SDC, the stresses induced by constrained sintering on the surface of previously-fired YSZ caused cracks and discontinuities in the SDC barrier layer.  These results together with steps that have been taken to address the constrained sintering issue will be presented.
Fundamental Understanding of Electro-Chemo-Mechanical Properties of Layered Rare Earth Cuprate-Based Systems
Authors: Kim Chang Sub , Tuller Harry L.
Published: February 2020

This study is part of an investigation of the factors contributing to the performance of rare earth cuprates RE2CuO4 (RE = rare earth) as promising SOFC cathode materials. The layered cuprate system changes its crystal structure from T (octahedral) to T*(pyramidal) to T’(square-planar) with decreasing size of the A-site cation, and incorporates oxygen interstitials as well as vacancies depending on the A site chemistry. The three crystal structures show surprisingly different oxygen diffusivities and electronic conductivities. Additionally, layered perovskite systems show anisotropic properties, such as oxygen diffusivity, which differs by orders of magnitude along different axes. In this study the defect chemistry, transport properties, chemical expansion and surface kinetics of these anisotropic materials are investigated to assess the role of crystal structure, orientation and types of defect carriers on the material’s bulk and surface exchange properties.