bluuebunny/crossref_metadata_2025 huggingface dataset:
Novel Catalyst Support Based on Mesoporous Cobalt and Molybdenum Carbide for Enhanced ORR Kinetics in Alkaline Fuel Cells
Authors: Nikolic Vladimir M , Perovic Ivana M , Saponjic Aleksandra P , Kokunesoski Maja , Tosic Milos P , Milovanovic Dubravka , Marceta Kaninski Milica P
Published: February 2020

Abstract

The kinetics of the oxygen reduction reaction (ORR) greatly influences the performance and the costs of electrodes in fuel cells. Commercial platinum based electrocatalysts exhibit the highest performance, but also increase the cost of the fuel cells. On the other hand, carbon based platinum catalyst support is subject to oxidation, which causes agglomeration of the Pt nanoparticles and decrease in performance of the fuel cells. In order to overcome these issues, we have developed and prepared novel catalyst support based on cobalt and molybdenum carbides. Synthesis of CoMoC was done by chemical method, followed by the high temperature treatment. Since the high temperature preparation of the carbides usually produces low surface area materials, we have developed and prepared a specific carbon based matrix, where Co and Mo based precursors were added. High temperature treatment was done at several temperatures, from 750°C to 1200°C. Structural and morphological characterizations were done using XDR, SEM/EDX and BET analysis, and the obtained results show the formation of the mesoporous non-stoichiometric CoMoC, having the highest value of surface area of 97 m2g. It was shown that the increase in the preparation temperature leads to increase in the mixed carbide stoichiometry, which was ascribed to the high temperatures needed to transform molybdenum oxide into stable carbide. This catalyst support was used to deposit platinum nanoparticles via boron hydride reduction method to obtain several electrocatalyst having 1-10% Pt on the catalyst support. The electrochemical measurements with the prepared CoMoC and different Pt/CoMoC were done using cyclic voltammetry and linear sweep voltammetry on the RDE in alkaline solution. The stability of the prepared catalyst support and catalysts was evaluated using accelerated test procedure by cycling the potential between 0.65 and 1.0 V in oxygen rich alkaline solution. The ORR kinetics parameters were determined and we found comparable performance of the 10% Pt/CoMoC with the commercial Pt/C (40%), which was ascribed to the non-stoichiometric nature of the catalyst support.
Degradation of Polymer Electrolyte Membrane Fuel Cell By Siloxane in Biogas
Authors: Kim Tae-Young
Published: February 2020

We studied the degradation and durability of polymer electrolyte membrane fuel cell (PEMFC) at membrane-electrode-assembly (MEA) level by injection of octamethylcyclotetrasiloxane (D4) as a representative siloxane, which has been found in many industrial and personal products. Specifically, i) GC/MS analysis demonstrated that the ring-opening polymerization of D4 could result in the formation of various linear and cyclic siloxanes in both electrodes of MEA; ii) post-test analysis revealed that the transformed siloxanes were transported from the anode to the cathode via free-volumes in the polymer membrane; iii) RDE measurement and DFT calculation revealed that D4 was not directly responsible for the electrocatalytic activity of Pt; iv) electrochemical analysis demonstrated that the residual methyl groups of siloxane and various siloxanes did not hinder the proton transport in the polymer membrane; and v) siloxanes accumulated in the primary and secondary pores with the exception of an external surface of carbon, causing an increase in the oxygen reactant’s resistance and resulting in a decrease of the cell performance. In addition, we confirmed that injection of D4 did not affect the carbon corrosion adversely because the siloxane had little influence on water sorption in the catalyst layer.
Development on Measuring Methods of Catalyst Layer Structure Parameters Under Operating Condition of PEFCs
Authors: Yokoyama Koji , Oyama Junpei , Ohashi Masato , Sugawara Seiho
Published: February 2020

In order to familiarize fuel cell vehicles (FCV), its system cost reduction is a critical issue. High current density operation is an effective approach, but it is necessary to reduce activation overvoltage, IR loss and concentration overvoltage. In particular, the concentration overvoltage must be decreased to increase oxygen flux at high current density. In order to reduce concentration overvoltage, oxygen gas diffusion characteristics in gas diffusion layer (GDL) and microporous layer (MPL), oxygen dissolution and diffusion characteristics in ionomer, reduction of oxygen local flux to platinum and influence analyses of water retention in MEA to PEFC performance have been researched by various approaches. In recent years, not only water retention in GDL / MPL but the effect of water retention in catalyst layer (CL) on cell performance has been reported by advancement of MEA inside visualization technique. However, relationship between the catalyst layer structure parameters and liquid water retention characteristics has not been found out yet. In this study, to find out this relationship and influence of water retention in CL to oxygen transport resistance and PEFC performance, porosity/tortuosity (ε/τ) measuring method has been developed under operating condition of PEFC as first step. The ε/τ of CL did not change with temperature (25-80 deg.C) under dry environment, which is reasonable due to low thermal expansion coefficient of each CL material. Currently, we are planning this technique to humidity control and the results will be reported.

 Acknowledgments

This research was financially supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.
Enthalpy Analysis and Heat Exchanger Sizing of an Air-Cooled Proton Exchange Membrane Fuel Cell System
Authors: Gao Xin
Published: February 2020

Xin Gao
[1]
, Torsten Berning, Søren Knudsen Kær

Department of Energy Technology

Aalborg University

Pontoppidanstraede 111, 9220 Aalborg, Denmark

Proton exchange membrane fuel cells (PEMFC’s) are becoming increasingly popular for uninterrupted power supply especially in remote areas. In the case of telecom back-up operations, PEMFC systems are often placed in areas of extreme climates, e.g. in Norway or Canada where the temperatures drop below -20 °C in the winter which make liquid-cooled fuel cells impossible. In such cases, air-cooled fuel cell systems are deployed where the air that is fed to the fuel cell serves both as reactant supplier and coolant to remove the waste heat that is generated during fuel cell operation. In some cases the warmer exhaust air is used to pre-heat and also humidify the incoming colder and dryer air stream using an enthalpy wheel.

It is important to thermodynamically understand such a fuel cell system, and in this work the enthalpy streams and the humidity stream are followed throughout the fuel cell system in order to optimize the operating conditions and the performance of such a system. The adjustable parameters include the fan speed that determines the amount of air that is brought into the system, and the size and rotating speed of the rotating enthalpy wheel. In addition, computational fluid dynamics simulations have been carried out to better understand the distribution of the reactant air over the fuel cell stack and the resulting temperature distribution across the stack. These results suggest that the humidifying function of the current enthalpy wheel is negligible and a smaller enthalpy wheel or an ordinary heat exchanger can fulfill the heat recovery demand. Despite the fact that the air enters the stack at a cold temperature, even the forefront of the stack is at a much elevated and desired stack temperature with the help of supplying an acceptable amount of power to an electric stack heater. So a preheating of the air stream by the enthalpy wheel might be unnecessary during steady-state operation.

[1] Corresponding Author: xga@et.aau.dk
Long Life Operating Algorithm for 5kW High Temperature Polymer Electrolyte Fuel Cell Stack
Authors: Kim Minjin , Sohn Young-Jun , Kim Seung-Gon , Lee Won-Yong
Published: February 2020

PBI(polybenzimidazole) based HT-PEFC(high temperature polymer electrolyte fuel cell) has been received a lot of attention as a great alternative of the LT-PEFC(low temperature polymer electrolyte fuel cell). Currently, life time of HT-PEFC was shorter than LT-PEFC, so life extension of HT-PEFC has been studied. But existing research works on the life time of HT-PEFC focus on finding out the causes of its performance degradation in the single cell or small size of sub-stack levels. Researches on the life time of HT-PEFC in a full stack level have nearly been conducted. In this paper, effective operational algorithm for HT-PEFC such as minimizing OCV conditions, selecting the proper operation temperature and purging the water inside of the HT-PEFC stack were established to avoid a performance degradation of HT-PEFC in a full stack level. 5kW HT-PEFC stack was manufactured by ourselves and conducted long-term operating test using the proposed operating algorithm. As a result, HT-PEFC stack was successfully operated with a less degradation rate during the target life time.
Analysing Gas-Liquid Flow in PEM Electrolyser Micro-Channels
Authors: Sadeghi Lafmejani Saeed , Olesen Anders Christian , Kær Søren Knudsen
Published: February 2020

PEM water electrolysis is a key component for closing the loop of the renewable energy eco-system. These high response water electrolysers are particular suitable for fluctuating power sources. Conventional PEM water electrolysers are typically operated at a current density of around 1 A/cm2 and are fairly expensive. One means of increasing the hydrogen yield to cost ratio of such systems, is to increase the operating current density. However, at high current densities, management of heat transfer and fluid flow in the anode GDL and channel becomes crucial. This entails that further understanding of the gas-liquid flow in both the porous media and the channel is necessary for insuring proper oxygen, water and heat management of the electrolysis cell.

In this work, the vertical upward gas-liquid flow pattern in a 0.5×1×94 mm micro-channel is both numerically and experimentally analysed. A sheet of titanium felt is used as a permeable wall for permeation of air to the column of water similar to the phenomenon encountered in Oxygen Evolution Reaction (OER).

The transparent setup is operated in situ and the gas-liquid flow regimes are identified using a high-speed camera. The picture shows how the transparent cell is made which consists of a channel for the inlet air and a channel for the water-bubble flow. The transparent material is Plexiglas that is sealed with a sheet of silicon.

The conventional co-current gas-liquid two-phase flow patterns, such as bubbly flow, slug flow and annular flow, are observed in the present micro-channel. The phenomenon is also analysed numerically in 3D, unsteady, euler/euler multiphase method using the commercial software ANSYS FLUENT 17 and the results show good agreement with the experimental data.

Influence of each multiphase flow regime is described in the study as well as the recommendation for improving the performance. A well management of the multiphase flow regime along the whole micro-channel length can assure a proper distribution of water inside the titanium felt.

Figure 1
Origins of Suppressed Swelling and Increased Mechanical Properties of Ultrathin Nafion
Authors: Stafford Christopher M. , Frieberg Bradley R. , Jeong Cheol , Rowe Brandon W. , Page Kirt A. , Soles Christopher L.
Published: February 2020

We have previously shown1,2 that there is a suppression in both swelling and water diffusivity when Nafion is confined to thin and ultrathin films. We have also shown that the modulus of ultrathin Nafion films increases upon confinement3,4, as well as upon thermal annealing. Interestingly, we also observe a decrease in swelling and an increase in modulus for ultrathin films aged in high humidity environments. Similar behavior has been shown to occur in bulk Nafion films.5,6 Some researchers suggest that the root cause of such a decrease in swelling and increase in modulus is due to the formation of sulfonic anhydrides, while others suggest that it is due to internal hydrogen bonding of dehydrated sulfonic acids.

In this talk, we will present our measurements results on thin and ultrathin Nafion films. We apply polarization-modulation infrared reflection-absorption spectroscopy to study the kinetics of dehydration and formation of sulfonate species in thin Nafion films. We also employ cantilever bending during humidity-induced swelling to probe the mechanical properties of these films. Grazing-incidence small-angle x-ray scattering is used to elucidate changes in morphology upon annealing and/or aging. By combing the results from these techniques, we can paint a more holistic picture of the origins of suppressed swelling and increased mechanical properties in thin and ultrathin Nafion films.

S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager. Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films, Macromolecules, 45, 7920-7930 (2016).

E. M. Davis, C. M. Stafford, and K. A. Page. Elucidating Water Transport Mechanisms in Nafion Thin Films, ACS Macro Letters, 3, 1029-1035 (2014).

K. A. Page, J. W. Shin, S. A. Eastman, B. W. Rowe, S. Kim, A. Kusoglu, K. G. Yager, and G. R. Stafford. In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles, ACS Applied Materials and Interfaces, 7, 17874-17883 (2015).

K. A. Page, A. Kusoglu, C. M. Stafford, S. Kim, R. J. Kline, and A. Z. Weber. Confinement-Driven Increase in Ionomer Thin-Film Modulus, Nano Letters, 14, 2299-2304 (2014).

F. M. Collette, C. Lorentz, G. Gebel, F. Thominette. Hygrothermal aging of Nafion, Journal of Membrane Science, 330, 21-29 (2009).

S. Shi, T. J. Dursch, C. Blake, R. Mukundan, R. L. Borup, A. Z. Weber, and A. Kusoglu. Impact of Hygrothermal Aging on Structure/Function Relationship of Perfluorosulfonic-Acid Membranes, Journal of Polymer Science B: Polymer Physics, 54, 570-581 (2016).
Systematic in Situ Infrared Study of the Electro-Oxidation of C3 Alcohols on Carbon-Supported Pt and Pt-Bi Catalysts for Energy Applications
Authors: Gonzáles-Cobos Jesus , Baranton Stève , Coutanceau Christophe
Published: February 2020

The electrooxidation of C3 alcohols (1-propanol, 2-propanol, 1,2-propanediol, 1,3 propanediol and glycerol) has been studied in alkaline medium on Pt/C and Pt9Bi1/C catalysts by cyclic voltammetry and in situ FTIR spectroscopy. The modification of Pt by 10 at% of Bi decreases the oxidation onset potentials of C3 alcohols down to ca. 200 mV. In situ FTIR spectroscopy measurements indicated clearly that the presence of Bi also led to avoid the C-C bond cleavage during alcohol electrooxidation reactions. Systematic evaluation of the positions and intensity changes of absorption bands as a function of the electrode potential allowed determining reaction pathways of electrooxidation of C3 alcohols. It has been shown that the secondary alcohol groups are more reactive than the primary ones, but also that in the case of glycerol steric limitations due to the presence of two primary alcohol groups could be responsible of the higher oxidation onset potential. At potentials above 0.6 V, the linear1-propanol bearing a single primary alcohol group leads to the highest activity due to lower steric hindrance of the surface compared with the other alcohols studied.

Figure 1
Investigation of CeO2/Graphene and Nb2O5/Graphene Supported Pt and PtCo Catalysts Towards Methanol and Ethanol Oxidation and Oxygen Reduction Reactions
Authors: Kepeniene Virginija , Stagniunaite Raminta , Tamasauskaite-Tamasiunaite Loreta , Selskis Algirdas , Norkus Eugenijus
Published: February 2020

In this study the CeO2/graphene and Nb2O5/graphene supported Pt and PtCo catalysts denoted as PtCeO2/GR, PtNb2O5/GR, PtCoCeO2/GR and PtCoNb2O5/GR were prepared by polyol method using the simple and rapid microwave heating method. To obtain the catalysts, required amounts of CeO2/graphene or Nb2O5/graphene, H2PtCl6 and CoCl2 were heated in ethylene glycol at 170 oC for 30 min in the microwave reactor. It was found that Pt nanoparticles were successively deposited onto the surfaces of CeO2/graphene and Nb2O5/graphene. For comparison, the PtCo/graphene (PtCo/GR) and Pt/carbon (Pt/C) catalysts were prepared in the same manner. The composition and morphology of catalysts were detected by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Field Emission Scanning Electron Microscopy (FESEM), and Transmission Electron Microscopy (TEM). The synthesized PtCeO2/GR, PtNb2O5/GR, PtCoCeO2/GR, PtCoNb2O5/GR, PtCo/GR and Pt/C catalysts were examined as electrocatalysts towards the electro-oxidation of methanol and ethanol, and the reduction of oxygen.

It has been found that the CeO2/GR or Nb2O5/GR supported Pt and PtCo catalysts show an enhanced electrocatalytic activity towards the oxidation of ethanol and methanol in an alkaline medium as compared with that of the Pt/C catalyst. Depending on catalysts composition, ethanol and methanol oxidation current densities were found to be ca. 3 – 12 times higher at the CeO2/GR and Nb2O5/GR catalysts, respectively, in comparison with those at the Pt/C catalyst.

In the case of oxygen reduction, the PtCoCeO2/GR and PtCoNb2O5/GR catalysts outperform the PtCeO2/GR, PtNb2O5/GR, PtCo/GR and Pt/C catalysts towards oxygen reduction and shows higher onset potential, as well as higher current density towards the oxygen reduction reaction as compared with those at the aforementioned catalysts.
Oxide-Coated Carbon Black for Cathode Catalyst Support
Authors: Nako Yuki , Sonoda Takashi , Itoh Tetsuya , Kinumoto Taro
Published: February 2020

Carbon black is an essential material for PEFC. Due to its specific surface area, conductivity, pore-making structure and stability for acidic environment, carbon black is so far the best choice as the catalyst support for cathode and anode. However, carbon black is being one of the critical elements which limit the lifetime of PEFC. Especially in cathode, oxidative degradation of carbon black support is resulting in the depression of catalyst activity.

In order to prevent carbon black support from its oxidative degradation, surface coating with silicon oxide (silica) on acetylene black was tried using sol-gel method. Nanometer-thick amorphous silica layer was formed on almost the whole surface on the acetylene black. Its effect on resistivity against oxidation, Pt supporting function and total behavior of the catalyst is to be discussed.
Statistical Analysis for the Electrochemical Performance of a 3D Reconstructed Catalytic Layer
Authors: Barbosa Romeli , Escobar Beatriz , Soberanis Victor
Published: February 2020

The structure of a catalyst layer (CL) in a proton exchange membrane fuel cell (PEMFC) varies according with the deposition technique and the chemical composition that build it up. Then, the electrical, ionic and catalytic properties depend on the final structure. These characteristics have also an impact on the overall response of the PEMFC. In this work, stochastic reconstruction of the microstructure of a PEMFC CL and a scaling method are used to determine the effective transport coefficients and a statistical electrochemical surface area of platinum associated with the active sites. The case study contains three stages: 1) the charge transport continuity equation is used to determine conduction efficiency. 2) The platinum surface is determined by a statistical algorithm that considers the percolation and the three phase union condition. 3) Finally, an analytical model for electrochemical behavior is applied. These numerical and analytical models are applied to hypothetical microstructures of the carbon support, ionomer electrode load and platinum to carbon weight ratio.
Resolving Active Sites and Porosity in PGM-Free Catalysts By Electron Microscopy
Authors: Cullen David A. , Sneed Brian , More Karren L. , Brewster James H , Odgaard Madeleine , Chung Hoon T , Zelenay Piotr
Published: February 2020

Electrodes based on platinum group metal-free (PGM-free) catalysts are under development as an alternative to costly Pt-based cathodes for automotive fuel cell systems.[1,2] Novel PGM-free catalysts with much improved activity and durability must be developed in order to become competitive with their PGM nanoparticle counterparts for transportation applications. Analytical characterization by transmission electron microscopy plays an important role in identifying electrochemically active sites at the atomic scale as well as to exploring electrode morphology and porosity at the microscale.

In this work, a hybrid PGM-free catalyst developed at Los Alamos National Laboratory was studied by multiscale and multidimensional electron microscopy methods. This promising catalyst, branded CM-PANI-Fe-C, has been synthesized by heat-treating two nitrogen precursors, polyaniline (PANI) and cyanamide (CM). The catalyst is comprised of fibrous carbonaceous agglomerates intermixed with few-layer graphene sheets. Low voltage aberration-corrected scanning transmission electron microscopy (STEM) and atomic resolution electron energy loss spectroscopy (EELS) were utilized to identify single Fe atoms coordinated with nitrogen within the few-layer graphene sheets (Fig. 1a,b). These atomic features have been identified by modeling as a possible active site which exhibits high four-electron selectivity.

The thick electrode structures required to accommodate high non-PGM loadings also play a critical role in fuel cell performance. Hierarchal pore structures (spanning the micro to macro) are necessary for maintaining high mass transport through the thick electrodes. Complementary imaging and compositional mapping by energy dispersive X-ray spectroscopy (EDS) has been utilized to study the bulk electrode structures, as shown in Fig. 1c. These analytical studies will be extended to understand the role of different process parameters on catalyst and electrode structures at multiple length scales.

References

G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 332 (2011) 443.

Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy Environ. Sci. 4 (2011) 3167.

Acknowledgements

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

Figure 1
An Examination of Voltage Losses for the Hydrogen Oxidation and Reduction Reactions in an Alkaline Membrane Fuel Cell
Authors: Park Andrew Michael , Yang-Neyerlin Ami C. , Shiau Huai-Suen , Alia Shaun M , Weber Adam Z , Neyerlin K.C. , Pivovar Bryan S
Published: February 2020

Recent advances in highly conductive and base stable anion exchange membrane chemistries have enabled the widespread fabrication of alkaline membrane fuel cells (AMFC).(1) A significant limitation to AMFC commercialization, however, is the sluggish hydrogen oxidation reaction (HOR) kinetics of current anode catalysts. Much work has been performed to understand the fundamental limitations of HOR in an alkaline environment(2, 3) and to study the HOR in an alkaline membrane electrode assembly (MEA)(4). It is of significant interest to the AMFC community to continue studying the HOR reaction to understand the in-situsources of voltage losses, and to develop a diagnostic tool to test the kinetics of new/alternative catalysts in an alkaline MEA.

In this study, a series of MEAs were fabricated where the membrane (Tokuyama A201) and binder polymer (Tokuyama AS-4) are held constant, but where the catalyst content and composition are varied. A hydrogen pump technique was utilized where an external current source drives HOR on one electrode of the MEA and hydrogen evolution (HER) on the other. It should be noted that there is no traditional reference electrode in this system, as each electrode reaction contributes a significant overpotential. The polarization curves acquired from the hydrogen pump are analyzed for sources of voltage losses including mass transport, ohmic, and kinetic overpotentials. The causes of such losses will be discussed and presented with insight gained from modeling and experimental work. Exchange current densities of HOR and HER for both platinum on carbon and platinum ruthenium on carbon will also be presented, where all data was taken in an in-situalkaline MEA. The potential for hydrogen pump as a diagnostic method for future anode catalyst testing will also be included.

Figure 1. HOR and HER overpotentials for Pt/C electrodes between 0.119 and 0.8 mg/cm2 loading as a function of current density per Pt site, with the corresponding Butler-Volmer model fit

1. J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy & Environmental Science(2014).

2. J. Durst, A. Siebel, C. Simon, F. Hasche, J. Herranz and H. A. Gasteiger, Energy & Environmental Science, 7, 2255 (2014).

3. Y. Wang, G. Wang, G. Li, B. Huang, J. Pan, Q. Liu, J. Han, L. Xiao, J. Lu and L. Zhuang, Energy & Environmental Science, 8, 177 (2015).

4. M. D. Woodroof, J. A. Wittkopf, S. Gu and Y. S. Yan, Electrochemistry Communications, 61, 57 (2015).

Figure 1
Electrochemical Property of Ni-Ba(Zr,Y)O3 Anode for Direct Ammonia-Fueled SOFC
Authors: Miyazaki Kazunari , Okanishi Takeou , Muroyama Hiroki , Matsui Toshiaki , Eguchi Koichi
Published: February 2020

Hydrogen carriers have attracted much attention as alternative fuels to hydrogen. This is because the hydrogen fuel has technological difficulties in transportation and storage, which impedes the wide application. Among various hydrogen carriers, ammonia is one of the promising candidates because of various advantages such as ease in liquefaction, high energy density, large hydrogen content, and zero emission of CO2during reforming or decomposition. In addition, the infrastructure for ammonia utilization has been established because ammonia has been used as a fertilizer and commodity chemicals worldwide.

In the case of solid oxide fuel cells (SOFCs) directly supplied with ammonia fuel, ammonia decomposes into hydrogen and nitrogen on the anode and subsequently hydrogen is electrochemically oxidized. Therefore, the anode materials for ammonia-fueled SOFCs need to be active for the electrochemical hydrogen oxidation as well as the catalytic ammonia decomposition. The mixture of nickel and Ba(Ce,Y)O3−δ
(BCY), which is one of the promising proton conductors, has been already reported as a highly active catalyst for ammonia decomposition[1].

In this study, then, the activity tests for ammonia decomposition were performed for various nickel catalysts supported on proton conductors. Ammonia decomposition proceeded at lower temperature over Ni−proton conductor cermets compared with the conventional anode of Ni−yttria-stabilized zirconia (YSZ) cermet. Ammonia decomposed completely over Ni−proton conductor cermets at ca. 600ºC, while a complete ammonia decomposition was accomplished over Ni−YSZ at higher temperature. In particular, the Ni−Ba(Zr,Y)O3−δ
(BZY) catalyst exhibited superior activity. In order to evaluate the anode performance of ammonia-fueled SOFC, a single anode-supported cell composed of Ni−BZY|BZY|Pt was fabricated. Electrochemical measurements were conducted at 600−700ºC by using ammonia or hydrogen as a fuel and oxygen as an oxidant. The partial pressure and flow rate of hydrogen in the hydrogen-containing fuel gas were controlled to be the same as those after the complete decomposition of ammonia in the ammonia-containing one. At each temperature, the open circuit voltages (OCVs) under both fuel gas flows were almost the same. From this result, it can be tentatively concluded that ammonia was oxidized via the two-step reaction on the anode as mentioned above. At 700ºC, the current density at the terminal voltage of 0.5 V under hydrogen and ammonia fuel conditions were 280 mA cm−2 and 250 mA cm−2, respectively. The difference in performance between ammonia and hydrogen-containing fuels was small. Consequently, the Ni−BZY catalyst is a promising anode for ammonia-fueled SOFCs.

 

[1] J. Yang, A.F.S. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi. ACS Appl. Mater. Inter., 7, 7406 (2015).
Electro-Chemo-Mechanical Studies of Perovskite-Structured Mixed Ionic-Electronic Conducting SrSn1-XFexO3-x/2+d
Authors: Kim Chang Sub , Bishop Sean R. , Perry Nicola H. , Tuller Harry L.
Published: February 2020

High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive for conversion of fuels to electricity. Reduced operating temperatures, desirable for reduced costs and extended operation, however result in significant losses in efficiency. This has been traced primarily to slow cathode surface reaction kinetics. In this work, we extended previous studies on the promising mixed ionic and electronic conducting perovskite-structured SrTi1-xFexO3-x/2+δ (STF) materials system whose exchange kinetics were correlated with the minority electron charge density by replacing Ti with Sn, due to its distinct band structure and higher electron mobility.

Oxygen nonstoichiometry and defect chemistry of the SrSn1-xFexO3-x/2+δ (SSF) system were examined by thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure while surface kinetics were examined by means of AC impedance spectroscopy. The electro-chemo-mechanical properties of SSF were found not to differ significantly from the corresponding composition in STF. Key thermodynamic and kinetic parameters for SrSn0.65Fe0.35O2.825+δ (SSF35) were derived including the reduction enthalpy, high electronic band gap, anion Frenkel enthalpy, oxygen vacancy migration energy, electron and hole mobilities. Though slightly shifted by Sn’s larger size, the defect equilibria and the cathode area specific resistance differed only in a limited way from those in STF. This was attributed to properties being largely dominated by Fe and not by the substitution of Ti with Sn.
Chromium Impurity Effects on SOFC Cathodes Using Half-Cell Measurements
Authors: Gong Yiwen , Zhu Yuexing , Wang Ruofan , Sun Zhihao , Pal Uday Bhanu , Basu Soumendra Nath , Gopalan Srikanth
Published: February 2020

Solid oxide fuel cells (SOFCs) offer a clean, efficient and environmental-friendly technique to convert chemical energy to electricity . In order to achieve high performance at lower operating temperature, anode-supported planar solid oxide fuel cells are usually preferred. The lower operating temperature enables the deployment of metallic interconnects based on ferritic stainless steel. The leading candidate alloy is Crofer 22 APU which is a state-of-the-art material with good high-temperature stability, high electrical conductivity and low coefficient of thermal expansion. However, the chromia oxide scale that forms on the surface of the Crofer 22 APU alloy reacts with the oxygen and water vapor to form volatile chromium species, chromium trioxide, CrO3, and chromium oxyhydroxide, CrO2(OH)2which deposit on the electrochemically active regions of the cathode. In order to understand the mechanism of chromium poisoning in SOFCs, electrolyte-supported half-cells have been fabricated. 8mol%-yttria-stabilized-zirconia pellets are used as electrolyte by uniaxial pressing. Lanthanum strontium manganite is screen printed on both side of the YSZ pellet, and a silver wire is used as the reference electrode.

These half-cells have been tested at different temperatures with different humidity levels in the air, with and without contacting the Crofer 22 APU alloy. Galvanostatic current interruption (GCI) has been employed during the electrochemical test, and after the test, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) have been used to detect the composition near the TBPs.

Figure 1
Hodrogen Oxidation Reaction Mechanisms in Mixed Ionic and Electronic Conducting Oxide Anode for Solid Oxide Fuel Cells
Authors: Li Wenyuan , Liu Xingbo
Published: February 2020

Ceramics anode offers certain advantages in terms of impurity tolerance and other properties, as compared to conventional Ni/YSZ anode for solid oxide fuel cells (SOFC). However, ceramic anodes typically have much lower electrochemical performance, and there is very few studies on the hydrogen oxidation reaction (HOR) mechanisms and kinetics in ceramic anodes. In this research, using Doped YCrO3 as baseline material, we systematically investigated the HOR mechanisms in mixed ionic and electronic conducting (MIEC) ceramic anodes, with the focus on the effect of electronic conductivity, electro-catalytic activity, triple-phase-boundary (3PB), and electrolyte, on the HOR kinetics.

It is found that both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5 eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3 ~ 1/2 for Ni doped yttrium chromites. A model of H2

oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

The influence of electrolyte to electrode performance was investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain.
High-Performance Low-Temperature Solid Oxide Fuel Cells By Atomic Layer Deposition of Yttria-Stabilized Zirconia on Silver Cathode
Authors: Koo Junmo , Li You Kai , Choi Hyung Jong , Neoh Ke Chean , Jang Dong Young , Shim Joon Hyung
Published: February 2020

There have been lots of effort to lower the operating temperature of solid oxide fuel cell (SOFC) below intermediate temperature (IT) regime (< 500 °C) because the high operating temperature (> 800 °C) of SOFCs causes various problems such as thermal stress, mechanical degradation and long startup time. At sub-IT, sluggish oxygen reduction reaction (ORR) at the cathode surface is considered as the rate determining step and it limits the practical use of SOFC at sub-IT region. Although Pt is known as the best catalyst for ORR, it is uneconomical due to its high price. Ag is considered as a great alternative for Pt cathode because of its high ORR activity and its relatively lower price compared to Pt. However, Ag agglomerates easily even at the low temperature region due to its low thermal stability. This agglomeration causes radical decrease of triple phase boundary (TPB) region where ORR occurs. In this study, we introduce atomic layer deposited yttria-stabilized zirconia (ALD YSZ) nano-particulate on Ag cathode in order to expand the TPB region of the cathode. ALD YSZ nano-particulate was deposited on the surface of porous Ag cathode made by DC sputtering. Fuel cell performance of Ag-ALD YSZ cathode was evaluate at the region of 250–450 °C. We will discuss the enhanced performance of ALD YSZ-treated Ag cathode in terms of morphology, electrochemical impedance and fuel cell performance.
Evidence of the Space Charge Layer Evolution at the YSZ Grain Boundaries upon Long Term Electrochemical Operation of Solid Oxide Fuel Cells
Authors: Song Xueyan , Chen Yun , Liang Liang , Abernathy Harry , Hackett Gregory , Gerdes Kirk
Published: February 2020

This work presents the first experimental evidence of nanostructure of space charge layer from the grain boundaries of YSZ ionic conductors and it evolution upon Solid Oxide Fuel Cell (SOFC) long term operation. For SOFC with Ni/YSZ composite anode, in stark contrast to the virgin cell and thermally aged cell, the cell operated for 2640 hours exhibit distinct ~20-50 nm sized low-mass core-shell structures on the YSZ/YSZ grain boundary plane only. TEM imaging supported by analytical composition analysis reveals that the low mass core-shell structure is with the core having the oxygen vacancies accumulation and the very thin shell having Y enrichment. The formation of the core-shell structure at the YSZ/YSZ grain boundaries was explained in term of the space charge effect at the grain boundaries. The impact of core-shell structure at the YSZ/YSZ grain boundaries on the SOFC performance was discussed.
Oxygen Activation and Dissociation on Transition Metal-Free Perovskite Surfaces. Isights on the Surface Reactivity of SrTiO3
Authors: Staykov Aleksandar , Akbay Taner , Druce John , Téllez Helena , Ishihara Tatsumi , Kilner John A.
Published: February 2020

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO3) and iron-doped strontium titanate (SrTi1–x
FexO3−δ). Our study reveals that while O2 dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O2 molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O2 molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O2 molecules. Our focus on the SrO-terminated surface, rather than the TiO2 layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO3 after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO2-perovskite layer is not exposed at the gas–solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction. Further calculations were performed using the non-equilibrium Green's function method combined with density functional theory (NEGF-DFT) to study the ballistic electron transport through a few layers of iron-doped strontium titanate. This computational method can provide critical information for the surface reactivity and surface active sites which is comparable with the STM experiments. Our study shows that the electron transport is enhanced if oxygen vacancies are present in the surface layed and the electron transport is significantly suppressed by overlayers of SrO. We believe that this study provides useful information for the control over the surface activity of SrTiO3 and explains the mechanisms of the material degradation with SrO surface segregation. Finally, we investigate the properties of the SrTiO3 / SrO interface to elucidate the mechanism of oxygen ion transport between the perovskite and the oxide phase.

Figure 1
Chemical and Electrochemical Stability on Perovskite Oxides As a Function of Surface Reducibility
Authors: Lu Qiyang , Tsvetkov Nikolai , Sun Lixin , Crumlin Ethan J , Yildiz Bilge
Published: February 2020

Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO3) surfaces is detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H2O and CO2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants by the positively charged oxygen vacancies enriched at the surface. Here we show that decreasing the surface reducibility (oxygen vacancy concentration) improves the oxygen surface exchange kinetics and stability significantly. This appears contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O2 molecules. We take La0.8Sr0.2CoO3 (LSC) as a model perovskite oxide, and modify its surface with additive cations that are more and less reducible than Co on the B-site of LSC. By using ambient pressure X-ray absorption and photoelectron spectroscopy, we proved that the dominant role of the less reducible cations is to suppress the enrichment and phase separation of Sr while making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30x acceleration of the oxygen exchange kinetics, after 54 hours in air at 550 oC achieved by Hf addition onto LSC. Finally, the results revealed a “volcano” relation between the oxygen exchange kinetics and the oxygen vacancy formation enthalpy of the binary oxides of the additive cations. This volcano relation highlights the existence of an optimum surface oxygen vacancy concentration that balances the gain in oxygen exchange kinetics and the chemical stability loss.
(La,Sr)(Cr,Ni)O3/CeO2-Based Composite Anode with Various Ni Content and Mixing Ratio for SOFC
Authors: Fukunaga Hiroshi , Saito Makoto , Takei Yuta , Shimada Iori , Osada Mitsumasa , Takahashi Nobuhide
Published: February 2020

Introduction

Fuel cells are being developed as one of the power sources in the near future. Among them, solid oxide fuel cell (SOFC) is known for its high efficiency due to its high operating temperature. On the other hand, because of the high temperature, problems such as sintering, material compatibility, and mismatch of thermal expansion occurs. Generally, a so-called cerment is used for the anode material, which is a composite of metals and ceramics. Ni-YSZ (Yttria Stabilized Zirconia) is the most common cermet, but metal particles such as Ni would show degradation due to the sintering of the particles and/or due to expansion and contraction by thermal or redox cycling. As an alternative to Ni-YSZ anode, perovskite-type oxide has been attracting attention. Some perovskite type oxide show deposition of metal particles on the surface of the oxides in a reducing atmosphere, which is expected to increase catalytic activity. The deposited metal particles would dissolved in the structure in an oxidizing atmosphere. We have reported this repeating deposition and dissolution would recover the degradation using Pd doped lanthanum-chromite based perovskite type oxide. In this study, LSCN with different Ni composition ratio (La,Sr)(Cr,Ni)O3 were prepared, and their performance was compared. The performance of anode with different ratio of LSCN and doped ceria was also investigated.

Experimental

LSCN was prepared by citrate process. Crystal structure of the sample was determined using X-ray diffraction. Mixture of LSCN and GDC was used as the anode. Sm0.5Sr0.5CoO3 was used for the cathode. Electrochemical performance was investigated by measuring current-voltage characteristic at 900 ˚C. Terminal voltage change over time was measured during power generation at constant current for 16 h, followed by oxidation treatment, and another 16 h of power generation.

Results

Fig. 1 shows current density vs. time during power generation at constant current using anode with different mixing ratio of La0.85Sr0.15Cr1-x
Ni
x
O3 (LSCNX , X = 100x). LSCN with higher Ni composition showed higher power generation performance, whereas LSCN with lower Ni composition showed higher resistance to degradation.

The performance of anode with different ratio of LSCN and doped ceria was measured. Anode with mixing ratio of 60 % LSCN and 40 % GDC showed highest performance and resistance to degradation.

References

1) Y. Nishihata et al, Nature, 418, 164 (2002)

2) H. Fukunaga et al, J. Chem. Eng. Jpn, 40, 1183 (2007)

Figure 1
Enhanced Charge Transfer at Electrolyte/Electrode Interfaces in SOFCs
Authors: Choi Mingi , Hwang Sangyeon , Byun Doyoung , Lee Wonyoung
Published: February 2020

We systematically investigated the effect of Ag grids at the electrolyte/electrode interfaces on the electrochemical performance of solid oxide fuel cells (SOFCs). Electrohydrodynamic jet printing was employed to fabricate Ag grids with a precise control of the grid geometry. A substantial reduction in the polarization resistance was observed when the pitch of the Ag grids decreased from 400 to 50 mm, indicating enhanced charge transfer through efficient supply and distribution of electrons along the Ag grids at the electrolyte/electrode interfaces. Our results demonstrate the possibility of engineering interfaces with metallic grids to achieve enhanced electrochemical performances for SOFCs operating at intermediate temperatures.
Processing of Solid Oxide Fuel Cell Anodes for Enhanced Intermediate Temperature Catalytic Activity at High Fuel Utilization
Authors: Gasper Paul Joseph , Lu Yanchen , Basu Soumendra Nath , Pal Uday Bhanu
Published: February 2020

Despite many years of research on solid oxide fuel cell (SOFC) electrolyte and electrode materials, the conventional anode formulation of nickel and yttria-stabilized zirconia (Ni-YSZ) cermet has not changed much, which highlights the effectiveness of nickel as a catalyst for the fuel oxidation reaction occurring at the triple phase boundaries (TPBs). However, most studies on single cells report results only under fuel rich atmospheric conditions in the anode, which is not representative of how fuel cell stacks are operated. SOFC stacks can easily reach fuel utilizations of 85%. High fuel utilization conditions result in lower hydrogen and higher water vapor partial pressures and an increase in anodic polarization, reducing the electrochemical performance of SOFC anodes. It is postulated that fuel oxidation reaction kinetics can be improved by increasing the TPB length via the deposition of sub-micron sized nickel particles in the cermet anode. This research analyzes the effects of various such nickel infiltration and deposition processes on the performance of anode-supported SOFC over a wide range of hydrogen and water vapor partial pressures at intermediate temperatures (600-800 Celsius). Infiltration and stability or coarsening of sub-micron nickel particles in Ni-YSZ cermets is a well-documented problem. So the impacts of various processing techniques derived from literature and developed internally were examined through long-term or accelerated testing. The cell performance was characterized using I-V and EIS techniques. The cell microstructure was characterized using SEM and image analysis techniques. Out-of-cell measurements on electrodes were also conducted to model its performance. The results were then all combined to identify the role of the sub-micron size nickel particles and the deposition process in enhancing the anode performance at high fuel utilization.