bluuebunny/crossref_metadata_2025 huggingface dataset:
Low Temperature Growth of Wafer-Scale 2D MoS2 Thin Films By Pulsed Plasma-Enhanced Chemical Vapor Deposition
Authors: Choi Jeong-Hun , Lee Seung Won , Kim Hyo-Bae , Ha Min-Ji , Ahn Ji-Hoon
Published: December 2020

Layered two-dimensional molybdenum sulfide (MoS2) has attracted great interest for a promising candidate material for opto-electronic, sensing and catalysis applications due to its outstanding mechanical, electrical and optical properties. In order to apply MoS2 to the industrial field, significant efforts have been placed in obtaining a wafer-scale uniform MoS2. There have been many progressions in various methods such as thermal-CVD [1] and ALD [2], but these methods have crucial limitation that they demand either high growth temperature or post-thermal treatment. In the present study, to overcome the limitation of traditional method, pulsed plasma-enhanced chemical vapor deposition was adopted for the growth of MoS2 thin films. By using H2S plasma reactant, the growth temperature could be significantly lower than in the previous way. The pulsing of Mo precursor injection properly regulates the Mo precursor partial-pressure ratio that is crucial for the synthesis of MoS2. Through these strategies, mono to few layers of MoS2 thin films was deposited uniformly on SiO2/Si substrates. X-ray photoelectron spectroscopy and transmission electron microscopy demonstrated the composition and crystallinity of MoS2 thin films depends on the plasma conditions. Furthermore, through optimizing the plasma conditions, the potential of MoS2 for electric device component was investigated.

Fig1. (a) Raman spectra of MoS2 thin films (b) XPS spectra of Mo 3d(left), S 2p(right) obtained from MoS2 thin films deposited by pulsed PECVD.

References [1] Hyun Cheol-Min, et al. Journal of Alloys and Compounds, 2018, 765: 380-384.

[2] TAN, Lee Kheng, et al. Nanoscale, 2014, 6.18: 10584-10588.

Figure 1
Plasmonic Hot-Electron-Assisted Piezocatalyst for Highly Effective Piezo-Photo Catalytic Hydrogen Evolution Via Au Nanoparticles@MoS2 Nanoflowers
Authors: Tu Chia-Yu , Wu Jyh-Ming
Published: December 2020

This work, Au nanoparticles@MoS2 nanoflowers (Au NPs@MoS2 NFs) heterostructure sonicated with visible light has been confirmed it can enhance piezo-photo catalytic activity for hydrogen evolution reaction. MoS2 displays prominent photocatalytic properties due to its narrow bandgap and obsesses unique piezoelectricity at its edge sites. However, the high rate of electrons and holes recombination and lack of active sites limit efficiency for hydrogen evolution. Herein introduction of Au NPs into MoS2 NFs has been synthesized by a facile hydrothermal process and forms heterojunction structure. The transmission electron microscopy (TEM) images evidence the as-synthesized Au@MoS2 heterostructure exhibits a high surface to volume with abundant nanopetals. As compared to Au NPs decorated MoS2, utilizing Au NPs burying beneath MoS2 NFs eliminates the shielding effect of MoS2 active edge sites. Based on the cathodoluminescence (CL) spectrum, Au@MoS2 achieved to reduce the recombination possibility of electrons and holes. Through gas chromatography (GC) analysis, the hydrogen evolution of 40 nm Au@MoS2 shows over 150% better than pristine MoS2. The significant improvement arises from piezoelectricity inducing hot electron of surface plasmon resonance (SPR) effect as observed by the absorption spectrum of ultraviolet-visible (UV-Vis) spectrophotometer. The COMSOL software was employed to simulate the electric field with strain on MoS2. The mechanism on SPR-enhanced piezo-photo catalytic hydrogen evolution will be investigated.
Enhanced Thermoelectric Performance of Porous SrTiO3 with Exsolved Ni Nanoparticles
Authors: Ohtaki Michitaka , Hirata Shinji , Suekuni Koichiro
Published: December 2020

Nanocomposite structuring is one of the key directions to improve the thermoelectric performance of bulk materials by enabling a significant reduction of the thermal conductivity and a possible enhancement in the power factor at the same time. In this paper, we report a strongly enhanced thermoelectric performance of porous SrTiO3 (STO) doped with Nb and Ni, which subsequently contained Ni nanoparticles formed after an exsolution reaction caused by a reducing post-treatment.

Samples with a representative composition of Sr0.95(Ti0.8Nb0.2)0.95Ni0.05O3 (STNNO) were prepared by conventional solid state reaction of SrCO3, TiO2, Nb2O5, and NiO. The mixed raw powders were calcined once at 1273 K and twice at 1623 K in air to achieve the single phase of the STO structure. The resulting powder was mixed with a graphite powder and heated in air to remove graphite, forming loosely sintered bodies with a porous structure. The samples were finally heated at 1693 K for 10 h in air for sintering, and subsequently reduced at 1623 K for 20 h in 0% - 20% H2/N2.

While the XRD patterns remained the same as the single phase of the STO structure even after the reducing treatment, SEM/EDS observation of the cross section of the samples revealed discretely dispersed Ni nanoparticles of ca. 50 nm in diameter inside of the sintered bodies. Whereas the electrical conductivity, σ, of the sample reduced in 20% H2 was significantly higher than that reduced in 5% H2, the Seebeck coefficient, S, of the former was just negligibly lower than the latter, being very insensitive to the sintering temperature and reducing atmosphere. However, the lattice thermal conductivity, κ
ph, was the lowest for the sample reduced in 20% H2, resulting in ZT = 0.6 at 1073 K as the highest value among the STNNO-based oxides so far reported. These results imply that the reducing treatment caused the electron doping in STO as well as the exsolution of Ni nanoparticles simultaneously, resulting in the increased σ and the suppressed κ
ph. The barely noticeable changes in S might be related to the large effective mass of electrons in the Ti 3d orbitals.
MFe2O4(M=Co, Mn)@MG as an Efficient Counter Electrode for Dye-sensitized Solar Cells
Authors: Zhang Meili , Chen Xiang , Zhou Cheng , Dong Hongzhou , Pang Beili , Yu Liyan , Dong Lifeng
Published: December 2020

Dye-sensitized solar cells (DSSCs) are considered as low-cost and eco-friendly alternatives for conventional silicon solar cells. Dye-sensitized metal oxide photoelectrode, electrolyte and counter electrode (CE) are the three major components of a DSSC. An ideal CE material should have superior electrocatalytic activity and distinguished conductivity, excellent chemical stability, and low cost. Noble metal Pt CE has an efficient capacity for collecting electrons from external circuits and facilitating the catalytic reduction of I3
- to I-. However, its high cost seriously restricts mass production and commercialization of DSSCs. Therefore, carbon composite materials, inorganic compounds, conducting polymers and their derivatives have been widely explored as CEs in DSSCs.

Recently, multiple transition metals and their derivative have been investigated as the CE materials for DSSCs. Theoretically, binary or ternary transition metals display intrinsic electrocatalytic activity toward the reduction of I3
- with rapid charge-transfer ability and cost-effectiveness. Thus, transition-metal ferric acid MFe2O4 (M=Mn, Co) composited with modified graphene (MG) is a novel strategy to achieve an advanced CE material with remarkable electrochemical performance. As we all know, graphene microstructure has a lamellar structure. Through a process of catalytic reduction, a curly structure modified graphene not only displays a large specific surface area to facilitate charge transfer and promote catalytic performance during photoelectric conversion, but also the conjugated structure with continuous sp2 hybridized carbon centers can induce electronic mobility.

Ferrite is a unique hierarchical nanostructure material mainly composed of transition metal (i.e., Co, Mn, Ni), iron (Fe), and oxygen (O). Transition-metal ions M2+ (M2+ =Mn2+, Co2+) have excellent electrocatalytic capability, and active Fe atoms can synergize with carbon atoms in graphene to significantly improve catalytic activity. This work offers a strategy to apply MFe2O4(M=Mn, Co)@MG composite as the CE of DSSCs to optimize catalytic sites and improve catalytic performance. The device based on the CoFe2O4@MG CE achieves a PCE of 7.78%, and the device based on MnFe2O4@MG CE has a PCE of 5.55%. Moreover, the device was prepared by a simple process with an excellent stability in the air atmosphere without encapsulation (maintaining 93.3% of an initial PCE after 30 days), which can facilitate large-scale production in the future.
(Invited) Programmed Assembly of Designer Solids from Molecular Building Blocks: A Route Towards Digitalization of Materials Research?
Authors: Woell Christof
Published: December 2020

Realizing multifunctional “Designer Solids” by programmed assembly of building units taken form libraries is a very appealing objective in connection with Materials Research entering the Information Era. Metal-organic frameworks (MOFs) have attracted a huge interest in this context, owing to their enormous wealth of different compounds – the number of characterized elements of this rapidly growing class of materials has just exceeded 100.000. Here, we will focus on a subset of MOF-based functional materials, including electrochemical, photoelectrochemical, photovoltaic and optical devices. In addition to 3D structures, also 2D interfaces in MOF heterostructures are of interest, e.g. with regard to photon-upconversion. Since the fabrication of reliable and reproducible contacts to MOF-materials represent a major challenge, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on a number of different substrates. The resulting films are referred to as SURMOFs [1,2]. The fabrication of hetero-multilayers (see Fig. 1) is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs and after loading their pores with functional guests. As a final point, we will discuss further applications realized by loading MOFs with nanoparticles or quantum dots. We will close with a more general discussion on the potential of programming functionalities into MOFs using in-silico screening methods.[3]

References:

[1] J. Liu, Ch. Wöll, Chem. Soc. Rev. 46, 5730-5770 (2017)

[2] L. Heinke, Ch. Wöll, Advanced Materials 31 (26), 1970184 (2019)

[3] R. Haldar, A. Mazel, M. Krstic, Q. Zhang, M. Jakoby, I. A. Howard, B. S. Richards, N. Jung, D. Jacquemin, S. Diring, W. Wenzel, F. Odobel, Ch. Wöll, Nature Comm., 10, 2048 (2019)
Nanoporous-Based Sensors for Electrical Detection of Gaseous Pollutants
Authors: Schindelholz Mara , Small Leo J. , Henkelis Susan , Krumhansl James , Nenoff Tina M
Published: December 2020

The advanced in situ detection of gaseous pollutants, such as NOx or SOx, is of great interest in many applications, such as the automotive and coal industries. Here, we leverage the previous successful development of impedance spectroscopy-based sensors for the detection of gaseous I2 to develop a novel sensor for the detection of these gaseous pollutants. During initial development of an I2 sensor, a metal-organic framework (MOF ZIF-8) material was utilized as an adsorbent layer deposited onto Pt interdigitated electrodes. The MOF was demonstrated to selectively adsorb I2, irreversibly, from a complex gas stream with a (>105x) change in measured impedance across the electrodes1. Further studies aimed at developing a robust sensor examined other materials, including zeolites2 and additional MOFs. It was demonstrated that utilizing MFM-300(X) (X= Al, Fe, In or Sc metal center) active materials enabled a reversible electrical response via gas adsorption/desorption3. The current work is focused on exploring these same nanoporous phases (MOFs, zeolites, etc.) for the real-time detection of NOx. The down-selection of candidate materials from bulk synthesis and gas selectivity testing will be discussed. Methods for deposition of these materials onto the Pt interdigitated electrodes and initial sensor performance during controlled gas exposures will be presented.

Acknowledgements

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

References

1) Small, L.J and Nenoff, T.M., ACS Appl. Mater. Interfaces, 2017, 9 (51), 44649-44655.

2) Small, L.J.; Krumhansl, J.L.; Rademacher, D.X.; Nenoff, T.M. Micro. Meso. Mater., 2019, 280, 82-87.

3) Small, L.J.; Hill, R. C.; Krumhansl, J. L.; Schindelholz, M. E.; Chen, Z.; Chapman, K.W.; Zhang, X.; Yang, S.; Schroder, M.; Nenoff, T.M., ACS Applied Materials and Interfaces, 2019, 11(31), 27982-27988.
Coordinated Molecule-Modulated Magnetic Phase with Metamagnetism in Metal–Organic Frameworks
Authors: Oh Hyunchul
Published: December 2020

Metal Organic Frameworks (MOFs) are a very prominent research topic because of their potential uses in applications such as molecular devices, gas storage systems, and especially single chain magnet (SCM) materials. Among these MOFs, those that feature open metal sites (Ni2+) inherently possess spin states that are tunable via organic molecular coordination. They have demonstrated the potential of SCM materials, exhibiting the possibilities for the spin-canting, magnetic phase transition, and field-induced spin slow dynamics behavior.

In this paper, we experimentally and theoretically investigated the magnetic properties of a MOF possessing an open metal site, and its derivatives. It exhibits a magnetic ordered state at low temperatures, which can be altered by altering its imidazole (IM) coordination. Our results show that coordinated IM clearly influences the spin state of Ni(II) ions with orbital geometries, resulting in shifts in critical temperature and field. These obtained results imply that the magnetic behavior in MOFs with IM reveals the coexistence of spin-canting, metamagnetism, magnetic phase transitions, and field-induced spin slow dynamics.

To the best of our knowledge, this is the first example of a MOF exhibiting the influence of the spin state of Ni(II) ions with orbital geometries via organic molecular coordination, which shows the coexistence of spin-canting, metamagnetism, magnetic phase transition, and field-induced spin slow dynamics.
(Invited) Investigation on Coloring/Self-Bleaching Mechanism in Photochromic Device Using Raman Spectroscopy
Authors: Chun So Yeon , Park SungHyeok , Lee Seok In , Hong Sungjun , Han Chi-Hwan , Cho Minhaeng , Kwak Kyungwon , Choi Han-Kyu
Published: December 2020

Smart windows have been of great interest for a wide variety of energy saving application to control the penetration of solar energy into buildings by switching between a coloring and bleaching states. For dynamic transmittance control, various smart devices are developed, such as electrochromic (EC), photochromic (PC), and photoelectrochromic (PEC) devices. Although, the coloring/bleaching mechanisms of EC and PEC devices are well-known to achieve better smart window performance, it is still lacking information about coloration/decoloration mechanism in PC device. In this regards, unrevelling photochromic mechanism during coloring/self-bleaching could help to improve their performance to advance smart window application.

In this talk, we investigate the coloring/self-bleaching mechanism using Raman spectroscopy. The Raman spectroscopy help to understand the photochromic mechanism upon light irradiation and self-bleaching stage. Our approach is based on the PC device, which is composed of the tungsten oxide (WO3) and titanium oxide (TiO2), and 5-methyl salicylic acid (5-MSA) as a cost effective ligand. The Raman spectroscopy allows us to understand the correlation between coloring/bleaching process and intercalation/deintercalation of Li ion.

Figure 1
Tunable Emission in Lanthanide-SURMOFs Heteroepitaxial Thin Films
Authors: Chen Dong-Hui
Published: December 2020

By using a layer-by-layer (LbL) approach, we have fabricated lanthanide-based, monolithic metal-organic framework (MOF) thin films for optical applications. The thickness of the Ln-SURMOFs can be easily controlled, thereby maintaining its transparent properties, which makes them attractive for the field of optical coatings. Moreover, employing a hetero-epitaxy process, we demonstrated that Ln-SURMOF can be deposited on the top of other Ln-SURMOFs with different lanthanide ions, maintaining the crystallinity and orientation. The hetero-epitaxial multilayer architectures of Ln-SURMOF reduced the efficient energy transfer between different lanthanides ions and optimized the fabrication of Ln-MOF thin film with a straightforward modulation of the emission color. Meanwhile, the distance between lanthanide ions in Ln-SURMOFs was extended by replacing the linkers. These novel hetero-epitaxial Ln-SURMOF architectures, possess a bright future in the field of optics, photonics and optoelectronics.
Development of Targets for Heavy Duty Fuel Cell Vehicles with Application-Driven System Modelling
Authors: Padgett Elliot , Kleen Gregory J , Mukundan Rangachary , Wang Xiaohua , Ahluwalia Rajesh , Papageorgopoulos Dimitrios C
Published: December 2020

Hydrogen fuel cells are an attractive technology to power zero-emissions heavy-duty vehicles, including road vehicles, such as trucks and buses, as well as marine, rail, and mining applications. Hydrogen fuel cell powertrains can offer several advantages over incumbent technologies, such as diesel engines, including higher efficiency, reduced emissions, higher torque, and no noise pollution. Additionally, they offer fast fueling and adequate fuel storage for applications demanding longer range. In comparison to light duty fuel cell vehicles, which have begun early commercialization, fuel cell systems for heavy duty applications have important differences in their requirements and typical operation. Heavy duty fuel cell systems must offer high durability and efficiency to provide a competitive total cost of ownership considering capital costs, fuel costs, and lifetime. Furthermore, load-hauling applications, such as trucks, rail locomotives, and mining vehicles, must provide higher average and peak power while meeting heat rejection and onboard packaging constraints.

Efforts to develop effective fuel cell materials, components, stacks, and systems for heavy duty applications must be guided by new targets and testing protocols. Fuel cell system modeling is essential to determine how application requirements translate to specific demands for the fuel cell system and components.

The Department of Energy has set system-level targets for long-haul class 8 fuel cell trucks at 25,000 hour lifetime, 68% peak efficiency, and $80/kWnet fuel cell system cost by 2030 and ultimate targets of 30,000 hour lifetime, 72% peak efficiency, and $60/kWnet fuel cell system cost.[1] This presentation will discuss the system-level targets as well as the development of new targets and testing protocols for heavy duty fuel cell components and materials. This includes specific performance tests for different operational requirements, accelerated stress tests, and targets combining durability, performance, efficiency, and cost. We will also discuss differing needs between heavy duty applications, each of which presents unique requirements for fuel cell systems. It is desirable to address these unique requirements with standardized, cross-platform fuel cell stacks and components to enable high volume, low cost manufacturing, which requires the development of suitable cross-platform targets.

[1] U.S. Department of Energy. “Hydrogen Class 8 Long Haul Truck Targets”. DOE Hydrogen and Fuel Cell Technologies Program Record. December 12, 2019: https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf
(Invited) Liquid Chromatography/Mass Spectrometry Analysis of Effluent Water from Perfluoroalkylsulfonic Acid (PFSA) Membrane Degradation Studies at Accelerated Durability Test Conditions
Authors: Yandrasits Michael , Komlev Andrei , Kalstabakken Kyle , Lindell Matthew , Kurkowski Michael
Published: December 2020

Perfluorosulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Analyzing effluent water for fluoride ion release rate is a primary method for evaluating the membrane degradation rate. However, many proposed degradation reaction pathways should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer or Nafion™ membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The results of this study imply significant degradation occurs at the tertiary fluoride on the polymer backbone or the ether oxygen connecting the side chain to the backbone. In Nafion™, these structural features also exist in the ionomer sidechain.
Cumulative Damage Modeling of Fuel Cell Membranes
Authors: Khattra Narinder , Bhattacharya Sandeep , Kjeang Erik , Lauritzen Michael Victor
Published: December 2020

The strongly varying humid environment under which fuel cell membranes operate introduces detrimental mechanical stresses that inflict damage to the membrane1. The damage incurred by the membrane during each humidity cycle when accumulated over time may lead the membrane to fatigue failure. The residual fatigue life of the membrane is determined here by using a cumulative-damage model2. A time-temperature-humidity dependent constitutive model and a multi-physics fuel cell finite element model are first developed to characterize the material properties and estimate the coupled mechano-hygral-thermal stress field in the membrane. Using this multi-level modeling approach, and with the aid of experimentally obtained fatigue data, fail-safe regions (see Figure) are identified for the case of accelerated stress test loading. For general loads simulating fuel cell operation, the distribution of stress-reversals is calculated for the load history using rain-flow counting algorithm. The regions of membrane that are more susceptible to fatigue damage are identified. It is also found that high amplitude stress cycles with low occurrence percentage are more detrimental than high occurrence low-amplitude cycles.

This work was supported by Natural Sciences and Engineering Research Council of Canada, Simon Fraser University Community Trust Endowment Fund, Canada Research Chairs, Mitacs through the Mitacs Accelerate program, and Ballard Power Systems.

1 Borup, R., et al., Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical Reviews, 2007. 107(10): p. 3904-3951.

2 Fatemi, A. and L. Yang, Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials. International Journal of Fatigue, 1998. 20(1): p. 9-34.

Figure 1
Measurement of Proton Conductivity and Water Diffusivity of Nafion Membrane at Subzero Temperatures
Authors: Tajiri Kazuya , Shrivastava Udit , Verma Saumitra
Published: December 2020

Water management is critical for the high performance of proton exchange membrane fuel cells (PEMFCs). The knowledge of PEM properties such as water content, proton conductivity, and water diffusivity is necessary for water management. In literature, PEM properties are very well documented at temperatures above room temperatures but at subzero temperatures PEM properties are scarcely reported. The acquisition of PEM properties at sub-zero temperatures is very important to the quick start-up of fuel cell electric vehicles in below freezing conditions.

The objective of this study is to obtain the water transport characteristics of a membrane at subzero temperatures for automotive applications. In this study, three different properties of a Nafion membrane are quantified: water content, proton conductivity, and water diffusivity.

In sub-zero condition at different RHs, the membrane water content and proton conductivity were quantified from gravimetric and electrochemical measurements. A novel measurement technique using segmented cell and methodology were established to estimate water diffusion coefficient as a function of water content and temperature (see figure below). Further, particular to the Nafion membrane, an empirical relationship between water diffusivity and water content in temperature range of 243K to 303K was developed.

Figure 1
Electrocatalytic Activity and Durability of Pt–Ni Nanowires for Oxygen Reduction
Authors: Kato Masaru , Iguchi Yoshimi , Li Tianchi , Zhuang Yu , Nakahoshiba Ryota , Yagi Ichizo
Published: December 2020

Oxygen reduction reaction (ORR) is a key reaction in future energy generation devices such as polymer electrolyte fuel cells (PEFCs) and metal–air batteries. To drive the ORR in PEFCs, Pt–M alloy nanoparticles (M = Ni) on carbon supports have been used as catalysts. Under electrocatalytic conditions, these electrocatalysts are known to degrade involving particle detachment, agglomeration and metal dissolution.[1] These phenomena can be suppressed by increase metal/support interactions: using heteroatom-doped carbon supports[2] and/or nanostructured Pt–M alloy catalysts.[3]

Herein, we report synthesis, activity and durability of Pt–Ni nanowires (NWs) for the ORR in acidic media. The Pt–Ni NWs showed higher ORR electrocatalytic activity than Pt/C. The Pt–Ni NWs showed almost no activity loss even after 50,000 potential cycles in acidic media. Physicochemical measurements including in situ Pt L3-edge X-ray absorption spectroscopy [4] revealed that the Pt surface oxide formation under potential control was suppressed after potential cycles, relative to the initial state. This finding suggests that potential cycles could induce strong metal/support interactions, resulting in the formation of highly durable NW catalysts.

References.

[1] N. Hodnik, G. Dehm, K.J.J. Mayrhofer, Acc. Chem. Res., 49, 2015–2022 (2016).

[2] M. Kato, K. Ogura, S. Nakagawa, S. Tokuda, K. Takahashi, T. Nakamura, I. Yagi, ACS Omega, 3, 9052–9059 (2018).

[3] J. Li et al., Science, 354, 1414–1417 (2016).

[4] K. Nagasawa et al., J. Am. Chem. Soc., 137, 12856 (2015).

Acknowledgements.

This work was supported by NEDO.
Effect of Global Fuel Starvation on Reversal Tolerant Anode Materials – Pulsed Versus Continuous Cell Reversal Events
Authors: Maric Robert , Gebauer Christian , Hasché Frédéric , Strasser Peter
Published: December 2020

A broad scientific consensus exists that anthropogenic emissions are major factors fueling global warming and its potential impacts on the earth’s ecosystem. On the way to a sustainable and decarbonized energy supply, "green" hydrogen is an emission-free alternative to conventional energy carriers. In this context, proton exchange membrane fuel cells (PEMFCs) offer promising properties for the electromobility by a simple scalability of the performance through the modular design of fuel cell stacks, a refueling time and range comparable with conventional combustion engines and zero emissions if "green" hydrogen is used. Despite all scientific work on this field – performance, cost and durability issues still hamper the wide commercialization of PEMFCs.

Concerning the durability, global fuel starvation causes destructive and irreversible degradation at the anode catalyst. A lack of fuel supply leads to the anode potential being raised to levels where the oxygen evolution reaction (OER) and carbon oxidation reaction (COR) take place instead of the hydrogen oxidation reaction (HOR). Thus, the overall cell voltage reverses compared to normal operation. In consequence, the widely employed carbon-supported anode catalyst corrodes and the overall cell performance is reduced if appropriate mitigation strategies are absent [1]. Among other remedies, incorporating a second catalyst component with an enhanced OER activity, such as IrO2 [2], is a promising material-specific strategy. Within this study we present a technical approach on single cell level (50 cm2
geo) to investigate and evaluate the impact of short-pulsed versus long-lasting fuel starvation events on the degradation of a reversal-tolerant anode catalyst layer comprising Pt/C and IrO2 [3]. By utilizing an on-line mass spectrometer, the anode exhaust gas stream is examined in terms of O2- and CO2-amounts to reveal the origin of the degrading effect.

References:

[1] T. Ioroi and K. Yasuda, J. Power Sources, 450, 227656 (2020).

[2] K. H. Lim, W. H. Lee, Y. Jeong, and H. Kim, J. Electrochem. Soc., 164, F1580 (2017).

[3] R. Marić, C. Gebauer, M. Nesselberger, F. Hasché, and P. Strasser, to be submitted.
Fuel Cell Automotive Drive-Cycle Cathode Catalyst (Pt/C) Layer Durability: Dependence on Low Vs. High Carbon Supports and Ionomer-to-Carbon-Ratio
Authors: Rice Cynthia A.
Published: December 2020

The typical urban automotive drive-cycle for proton exchange membrane fuel cells (PEMFC) consists of frequent idle-to-peak power cathode voltage transients, with extremes ranging from 1.2 V to 0.6 V, respectively. Understanding the platinum electrochemical surface area (ECSA) loss mechanisms dependence on material and operational conditions have been extensively investigated with the objective of minimizing performance losses over the lifetime of PEMFC powered automobiles. Commonly, automotive accelerated stress tests (AST) are performed ex-situ on thin catalyst films. The standard catalyst consists of carbon supported platinum nanoparticles (Pt/C). This study compares triangle waveform (TW) ASTs monitoring ECSA loss for ex-situ (thin film in 0.1 HClO4) and in-situ (catalyst layers bond with ionomeric electrolyte in a complete membrane electrode assembly (MEA)). The ionomer-to-carbon ratios (I/C) of the MEAs is varied from 0.6 to 1.5 on either low surface area carbon (LSAC) or high surface area carbon (HSAC) supports. The AST profiles included 25k cycles at 10 sec intervals, between the upper potential limit of either 0.9 V or 1.2 V and the lower potential limit of 0.6V, with intermediate ECSA evaluations from the H-desorption current (0.02 V → 0.4 V).

The electrolyte environment alters, liquid vs. ionomer, the dominant AST decay mechanism, Figure 1A. The decay in the liquid electrolyte is most sever upon initiation of the AST and the rate of decay decrease after the first 2000 cycles. While the MEA decay is less initial, but maintains the same rate of initial decay over 5000 cycles resulting in a higher overall loss of ECSA. ECSA decay is exacerbated at temperatures of 70°C. Increasing the I/C increases the interfacial acidity at the surface of the Pt/C and results in exacerbated ECSA loss, Figure 1B. In accordance with the Pourbaix diagram for Pt, it is less stable in a more acidic environment. The relative rate of ECSA loss is reduced on HSAC supports.

Figure 1
Co3O4/CeO2/C As Electrocatalyst for Ethylene Glycol Oxidation Reaction
Authors: Kepeniene Virginija , Ruškytė Linora , Stagniūnaitė Raminta , Drabavičius Audrius , Tamašauskaitė-Tamašiūnaitė Loreta , Norkus Eugenijus
Published: December 2020

In recent decades, alkaline direct alcohol fuel cells have been interested due to their advantages over hydrogen fuel cell. Alcohols such as methanol and ethanol are the most common types of fuel used in direct liquid-feed fuel cells however alternative alcohols are increasingly being investigated. One of the choices for alkaline alcohol fuel cell is ethylene glycol. Ethylene glycol in an alkaline medium generates, as intermediates, oxalates, glyoxylate, glycolate, glycolaldehyde, glyoxal and carbon dioxide and it is considered an energy carrier for fuel cells. The importance of this fuel is attributed to several factors: low molecular weight, biomass origin, high energy density, high boiling point and lower toxicity than methanol.

One more challenge is to find the most efficient electrocatalysts for various fuel oxidation reaction which could replace platinum in fuel cell anode materials in relation to its limited resources and high price.

Lately, the most research is based on the development of different, new, rare earth metals and metal oxides compositions, the improvement of known materials such as carbon with various additives with the aim to reduce or completely refuse of noble metal and to find low cost, higher efficient and great performance catalysts.

In this study, the Co3O4 was deposited onto the CeO2/C substrates prepared by two different ways. The substrates were prepared as follow: a) dry powder of CeO2 was mixed with carbon powder (mass ratio being 1:1) in a 2-propanol solution by ultrasonication for 30 min with further drying of the mixture (CeO2/C-1); b) CeCO3 were precipitated onto the carbon surface using the appropriate concentrations of CeCl3 and Na2CO3 solutions with stirring on a magnetic stirrer, followed by drying the mixture (CeO2/C-2). Then Co3O4 was deposited onto the CeO2/C-1 and CeO2/C-2 substrates using microwave irradiation and β–cyclodextrin as reducing agent. The synthesized Co3O4/CeO2/C-1 and Co3O4/CeO2/C-2 catalysts were characterized by Inductively Coupled Plasma Optical Emission Spectroscopy, Field Emission Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray Diffraction. Electrocatalytic activity of the prepared catalysts was evaluated towards ethylene glycol oxidation reaction in an alkaline medium using cyclic voltammetry and chrono-techniques.

It has been found, that the Co3O4 supported catalysts showed higher electrocatalytic activity for the ethylene glycol oxidation reaction than the pure CeO2/C substrates. Moreover, the preparation way of the substrate has had the influence on the electrocatalytic activity of prepared catalysts. It has been found that Co3O4/CeO2/C-2 catalyst where CeO2/C substrate was obtained from CeCl3 salt solution showed higher electrocatalytic activity towards ethylene glycol oxidation reaction than Co3O4/CeO2/C-1 catalyst where CeO2 was directly mixed with carbon powder.
(PEFC&E 2nd Place Best Poster Winner) Investigation on Methanol Tolerance of Pt/C Cathode Catalyst with Heteroatom-Doped Carbon Layer for Direct Methanol Fuel Cell
Authors: Choi Jung-goo , Ham Kahyun , Lee Jaeyoung
Published: December 2020

Direct methanol fuel cell (DMFC) is one of the most commercialized technology in direct liquid fuel cells due to various advantages of rapid refueling and high energy density, which are crucial characteristics for portable applications. DMFC consists of methanol oxidation reaction at anode and oxygen reduction reaction (ORR) at cathode on PtRu-based and Pt-based catalyst, respectively. Most challenging problem toward DMFC is methanol crossover. Methanol at the anode permeates the cathode and reacts with cathode catalyst, resulting in voltage drop. Pt/C is an excellent catalyst, but it actively reacts with methanol at the cathode. Thus, it is crucial to design a methanol-tolerant Pt-based catalyst to increase the ORR selectivity at the cathode.

In this study, we synthesized Pt/C with heteroatom-doped carbon layer. Heteroatom-doped Pt/C showed both lower activity in the methanol oxidation reaction and higher activity in ORR than pristine Pt/C. The origin of selectivity will be investigated with electrochemical tests such as CO stripping voltammetry and XPS in aspects of oxophilicity and Pt electronic structure. Finally, we will apply heteroatom-doped Pt/C on the cathode and confirm selectivity compared with pristine Pt/C. This approach suggests the way to design a Pt catalyst for ORR to improve performance in DMFC.

References

[1] Strmcnik, D., et al. Nat. chem., 2013, 5, 300-306.

[2] Wang, Y., et al. Energy Environ. Sci., 2015, 8, 177-181.

[3] Uhm, S., et al. ChemPhysChem, 2008, 9(10), 1425-1429.
(Invited) Electrochemical Energy Storage and Conversion Devices Incorporating C2-Protected Polybenzimidazoliums and Polyimidazoliums
Authors: Wei Qiliang , Chen Binyu , Cassegrain Simon , Holdcroft Steven
Published: December 2020

Alkaline anion exchange membranes which are capable of conducting hydroxide ions have broadly been applied in alkaline-based electrochemical energy storage and conversion technologies. Electrochemical systems relying on alkaline-based active components possess unique advantages over their acidic counterparts but at the same time are associated with significant challenges. There is a growing interest in C2-protected polybenzimidazoliums and polyimidazoliums as hydroxide ion conducting polymers for membranes and catalyst layers because of their high stability under caustic conditions, scalability in synthesis, and mechanical strength. This presentation will focus on recent progress in their preparation and on research trends incorporating these polymers into membranes, catalyst layers, and electrochemical energy storage and conversion devices. The challenges of integration that need to be overcome to meet critical requirements will be discussed, together with a summary and perspective for future directions.
Understanding the Mechanisms for Recoverable Vs. Unrecoverable Aemfc Degradation
Authors: Ul Hassan Noor , Zheng Yiwei , Adabi Firouzjaie Horie , Mustain William
Published: December 2020

In recent years, there has been significant improvement in the achievable current and peak power densities as well as the longevity of anion exchange membrane fuel cells (AEMFCs). However, cell performance degradation remains an area of concern for the technology and a key to reducing performance decline is understanding mechanisms at work – which includes both recoverable and irrecoverable (irreversible) losses. To date, very few studies have been done that aim to understand and counter performance degradation mechanisms, while those that have generally focused only on irrecoverable performance degradation (such as AEM and ionomer instability). The present study highlights the mechanisms for recoverable voltage loss in operating AEMFCs – mainly associated with water management and exposure to CO2. The purpose of this talk is to highlight both the fundamental principles underlying recoverable voltage loss as well as modern approaches to mitigate and control these mechanisms. Reducing the influence of water and CO2 on AEMFC behavior is very important for their performance, lifetime and operational stability, which will be shown by 2000+ hour durability experiments showing extremely low voltage decay rates.
(Invited) Uncovering the Role of Alkaline Pretreatment for Hydroxide Exchange Membrane Fuel Cells
Authors: Shi Lin , Setzler Brian , Weiss Catherine M. , Matz Stephanie , Xue Yanrong , Xu Zhiyuan , Zhuang Zhongbin , Gottesfeld Shimshon , Yan Yushan
Published: December 2020

As an emerging alternative to proton exchange membrane fuel cells, hydroxide exchange membrane fuel cells (HEMFCs) can use more cost-effective platinum-group-metal-free (PGM-free) catalysts, bipolar plates, and other components.1 Taking advantage of the recent significant progress in hydroxide exchange membranes (HEMs) and optimized electrodes based on hydroxide exchange membrane ionomers (HEIs), HEMFCs achieved unprecedented performance and the record of peak power density was updated frequently in the past few years. Interestingly, nearly all membrane electrode assemblies (MEAs) with satisfactory performances reported in the literature were pretreated with alkaline solutions such as KOH before cell assembly.

Here we show that the most crucial role of the alkaline soak is to remove carboxylates that are generated from the oxidation of primary alcohols, used as solvents for ionomers and/or catalyst inks, by platinum/palladium catalysts during membrane electrode assembly fabrication. These carboxylates are difficult or impossible to eliminate by self-purging processes. We also show that bicarbonates are an adequate replacement for the most frequently used strong bases for the pretreatment. As shown in Figure 1, the pretreatment effect can be seen in the shape of polarization curves. And Figure 2 depicts the electrochemical purging process in HEMFCs for acetate and carbonate/bicarbonate, respectively. As a result, alkaline pretreatment is unnecessary for catalysts that do not oxidize primary alcohols. In fact, membrane electrode assemblies with Pt/Pd-free catalysts deliver a better performance without the alkaline pretreatment: a voltage of 0.64 V at 1.0 A/cm2 and a peak power density of 0.69 W/cm2 in H2/O2.

References

B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat Nanotechnol, 11 (12), 1020-1025 (2016).

Figure 1
(Invited) Carbon Coatings for Components in Anion Exchange Membrane Electrochemical Energy Conversion
Authors: Proch Sebastian Karl , Bexell Ulf , Moffatt Claire , Jansson Anna , Stenström Mikael , Bernuy-Lopez Carlos , Westlinder Jörgen
Published: December 2020

We at SMT are committed to using engineering and innovation to advance the world towards more sustainable business. We also understand the need to make the shift to new business models and new thinking in line with the UN’s Sustainable Development Goals and the Paris Climate Agreement. We are, moreover, fully aware that the intermittency of renewables poses significant challenges, but we believe that long-term energy storage by power-to-gas (e.g., the “Hydrogen Economy”) is an important pillar in overcoming those challenges. We imagine carbon as (almost) ideal (coating) material for low-temperature electrochemical energy conversion due to its sustainability, conductivity, low cost, and the fact that, unlike most metals and alloys, it does not form thick oxide layers increasing its contact resistance (CR). Therefore, we have evaluated a selection of materials (Ti, [carbon-coated] stainless steels, stainless nickel) towards its compatibility with alkaline environments in terms of contact resistance and dissolution properties. The outcome clearly demonstrates the superiority of the carbon-coated material at intermediate pH values (< 11) [1]. Finally, at SMT we have long outgrown the research stage and offer, today, graphite-like roll-to-roll coatings on metal substrates, like stainless steel, as convenient low-cost material for bipolar plates in all types of low-temperature electrochemical energy conversion devices.

[1] Proch S, Stenström M, Eriksson L, Andersson J, Sjöblom G, Jansson A, et al. Coated stainless steel as bipolar plate material for anion exchange membrane fuel cells (AEMFCs). Int J Hydrogen Energy. 2020;45:1313-24.
Determination of Oxygen Transport Resistance through Limiting Current Analysis in Hydroxide Exchange Membrane Fuel Cells
Authors: Weiss Catherine M. , Setzler Brian , Shi Lin , Matz Stephanie , Gottesfeld Shimshon , Yan Yushan
Published: December 2020

Development of Hydroxide Exchange Membrane Fuel Cells (HEMFC) is motivated by the promise of significantly reduced overall module costs compared to Proton Exchange Membrane Fuel Cells (PEMFC). By transitioning from a low-pH to a high-pH environment, less expensive materials become stable1. Specifically, platinum-group metal loading can be greatly reduced or potentially eliminated from the catalyst formulations2, and the bipolar plates can be manufactured more cost effectively. However, HEMFCs are currently in the early stages of development, and improvements in performance and durability are needed.

A key challenge in the design of PEM and HEM fuel cells is maximizing O2 transport from the inlet air stream to the triple phase boundary in the cathode catalyst layer. O2 transport resistance causes voltage losses at high current densities, limiting maximum power density. Limiting current analysis has been used extensively to experimentally determine O2 transport resistance in PEMFCs, and we demonstrate that this technique is valid for HEMFCs and produces similar results. Our analysis quantifies the individual O2 transport resistance contributions from several factors: molecular diffusion through the gas diffusion layer (GDL), Knudsen diffusion through the microporous layer (MPL) and catalyst layer, and diffusion through ionomer in the catalyst layer. Importantly, PEMFC and HEMFC have a different water balance. In PEMFC, an MPL is added to relieve the cathode GDL of flooding but also contributes to O2 transport losses. Conversely, in HEMFCs, the cathode dries-out and the anode floods; therefore, the MPL is an unnecessary component on the cathode GDL3. We report that the elimination of the MPL significantly decreases the O2 transport resistance and improves the performance under air, especially at high current density.

References

Setzler, B. P., Zhuang, Z., Wittkopf, J. A. & Yan, Y. Activity targets for nanostructured platinum-group-metal-free catalysts in hydroxide exchange membrane fuel cells. Nat. Nanotechnol.
11, 1020–1025 (2016).

Wang, J. et al. Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells. Nat. Energy
4, 392–398 (2019).

Kaspar, R. B., Wittkopf, J. A., Woodroof, M. D., Armstrong, M. J. & Yan, Y. Reverse-Current Decay in Hydroxide Exchange Membrane Fuel Cells. J. Electrochem. Soc.
163, F377–F383 (2016).

Figure 1
Engineering Heterometallic Bonding in Bimetallic Electrocatalysts Towards Optimized Hydrogen Oxidation and Evolution Reactions
Authors: Tsai Meng-Che , Huang Chiu-Ping , Su Wei-Nien , Hwang Bing-Joe
Published: December 2020

Tuning the type and degree of heterometallic bonding in bimetallic catalysts is crucial to achieving optimal catalytic performance. In this study density functional theory (DFT) and X-ray absorption spectroscopy (XAS) were used to investigate the use of Ru-based bimetallic electrocatalysts for hydrogen oxidation/evolution reactions (HOR/HER) under alkaline conditions. The catalyst's hydrogen absorption and OH adsorption energies can both be tuned by altering the heterometallic bonding in bimetallic systems. DFT calculations were used in conjunction with a RuNi bimetallic system to optimize the hydrogen adsorption energy and weaken the OH adsorption energy. The degree of heterometallic bonding (i.e. the alloying extent) in the Ru2.3Ni1/C catalyst was experimentally confirmed by XAS analysis. Our findings indicate that tuning the hydrogen and OH adsorption energies on the RuNi bimetallic catalyst's surface leads to superior activity and stability in the alkaline HOR/HER compared to as-prepared Ru/C, commercial Pt/C and the reported catalysts. We showed that the oxophilic effect can be manipulated by altering heterometallic bonding and that both the hydrogen and OH adsorption energies are important determinants of the efficiency of low-cost HER/HOR catalysts in alkaline media.