bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Nanostructured Conjugated Materials in Metal-Organic Frameworks
Authors: Kitao Takashi , Uemura Takashi
Published: December 2020

As conjugated polymers play crucial roles in advanced nanoscale devices, future applications require a deeper understanding optoelectrical properties of conjugated polymers in low-dimensional assemblies. Confinement of polymer chains in nanoporous materials is a feasible method to control the polymer assemblies because this approach can prevent the entanglement and conformational disorder of polymer chains in the bulk state. The regulated and tailor-made pore characteristics of metal-organic frameworks (MOFs) allow the achievement of precise molecular assemblies in the nanopores, permitting fundamental studies on polymer properties that, until now, have remained elusive.1 Here, we report that the hybridization of MOFs and polythiophene (PTh) provides a great insight into inherent optoelectrical properties of PTh as well as functional nanomaterials for electronics and sensors.2 Controlled assembly of PTh donor and acceptor molecules is a key structure for photovoltaics and photocatalysis. We also demonstrated that unique interfaces between PTh and acceptor molecules, such as fullerene and titanium oxide, mediated by MOFs, enabled remarkable photoelectronic properties superior to conventional PTh systems.3

Other than conjugated polymers, a variety of conjugated materials, including graphene nanoribbons, could be prepared in MOFs, providing distinctly properties to those prepared in the bulk state because of the formation of specific assemblies.4

REFERENCES

1. Chem. Soc. Rev. 2017, 46, 3108.

2. (a) J. Am. Chem. Soc.,
2015, 137, 5231. (b) Angew. Chem. Int. Ed.,
2016, 55, 708. (c) Polym. Chem., 2017, 8, 5077. (d) Eur. Polym. J., 2018, 109, 162. (e) J. Am. Chem. Soc., 2019, 141, 19565.

3. (a) Nat. Commun., 2018, 9, 1660. (b) J. Phys. Chem. C, 2018, 122, 24182.

4. J. Am. Chem. Soc.,
2020
, 142, 5509.

Figure 1
Analysis of Pristine and Tcnq-Loaded MOF FILMS By Contactless Microwave Dielectric Spectroscopy Technique
Authors: Amoah Papa K. , Hassan Zeinab Mohamed , Redel Engelbert , Baumgart Helmut - , Obeng Yaw
Published: December 2020

Metal-organic frameworks (MOFs) are porous and crystalline Coordination Network Compounds (CNCs), also referred to as porous coordination polymers (PCPs), which consist of inorganic metal (or metal/oxo) clusters connected by organic linkers. Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline MOF films. For this study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution.

We report on localized charge transfer processes at microwave frequencies of porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 samples. As part of this study we will compare measurements done in the HKUST-1 pristine form of the material with open pores and HKUST-1, where the pores have been infiltrated with tetracyano-quinodimethane (TCNQ), resulting in a hybrid structure with enhanced electrical conductivity and a significant measurable effect in the microwave propagation characteristics [1]. We present data and analysis from an evaluation of pristine and loaded HKUST-1 SURMOF films using the contactless broadband microwave dielectric spectroscopy (BDS). The technique enables the accurate operando evaluation of the electrical and magnetic properties of materials without compromising the kinetic conditions of the experiment. The BDS method is uniquely suitable for investigating charge transfer reactions, regardless of the nature of the charge carriers, i.e., electrons and holes [2]. As such, these material state-sensitive parameters could be used to monitor charge transfer changes in the SURMOF and SURMOF-TCNQ hybrid analytes.

[1] G. M. Espallargas and E. Coronado, “Magnetic functionalities in MOFs: from the framework to the pore”, Chem. Soc. Rev., 2018,47, 533, DOI: 10.1039/c7cs00653e

[2] A. M. Wernbacher et al, Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts, J. Phys. Chem. C 2019, 123, 8005−8017, DOI: 10.1021/acs.jpcc.8b07417
(Invited) Defective Hierarchical Porous Copper-Based Metal-Organic Frameworks Synthesised Via Facile Acid Etching Strategy
Authors: Doan Huan
Published: December 2020

Introducing hierarchical pore structure to microporous materials such as metal-organic frameworks (MOFs) can be beneficial for reactions where the rate of reaction is limited by low rates of diffusion or high pressure drop. This advantageous pore structure can be obtained by defect formation, mostly via post-synthetic acid etching, which has been studied extensively on water-stable MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a corresponding manner by using phosphoric acid as a size-selective etching agent and a mixture of dimethyl sulfoxide and methanol as a dilute solvent. Interestingly, we demonstrate that the etching process which is time- and acidity- dependent, can result in formation of defective HKUST-1 with extra interconnected hexagonal macropores without compromising on the bulk crystallinity. These findings suggest an intelligent scalable synthetic method for formation of hierarchical porosity in MOFs that are prone to hydrolysis, for improved molecular accessibility and diffusion for catalysis.

Figure 1
Subfilamentary Networks and Their Dynamics As Source of Cycle-to-Cycle Variability in Reram Devices
Authors: Dittmann Regina , Hensling Felix V. E. , Rieck Jan Lennart , Valenta Richard , Bäumer Christoph
Published: December 2020

One major obstacle for the implementation of redox-based memristive devices in non-volatile memory or neuromorphic computing is the large cycle-to-cycle and device-to-device variability [1]. Here, we combine different complementary analysis methods such as atomic force microscopy (AFM), thermography and operando photoelectron emission spectroscopy (PEEM) [2] to monitor the filament shape and position in memristive SrTiO3 cells and correlate it to the observed variability of the resistance values. Our operando PEEM analysis revealed that some devices exhibit cycle-dependent variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament [3]. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub-)filaments observed by transmission electron microscopy [4] and that the active, current-carrying filament may change from cycle to cycle. Besides this, thermal imaging enabled us to monitor the position of the filament during repeated cycling. During cycling of some devices, we observed a movement of the filament towards the edge of the device that goes along with a drift of the low resistive state. As soon as the filament reaches the boarder, the resistance value stabilizes and remains constant during the following cycles. By modifying the interface at the top electrode, we could engineer the position of the filament and its stability against cycling. Our studies thereby represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

References

[1] R. Dittmann and J. P. Strachan, Perspective article,
APL Mater. 7, 110903, 2019

[2] C. Bäumer et al., Nature Commun. 7, 12398 2016

[3] C. Bäumer et al., ACS Nano 11 (7), 6921, 2017

[4] H. Du et al., Chem. Mat. 29, 7, 3173, 2017
Electrical Characteristics of Phase Change Memory By Adding HfO2 Layer between Phase Change Materials and Heater Layer
Authors: Yun Hee Ju , Choi Seok , Choi Byung Joon
Published: December 2020

Phase change memory (PCM) is a next-generation memory storing data with a large difference in resistance between crystalline and amorphous of phase change materials. PCM has outstanding properties such as scalability, high operation speed and non-volatility. However, conventional PCM has some issues to overcome, such as high reset current and set/reset asymmetry. Among them, poor adhesion between the phase change materials and SiO2, interfacial layer dielectric, which can cause problems during memory operation. Also, reset behavior takes higher current consumption due to its melt-quenching process, so improving the thermal efficiency is necessary. The thermal conductivity and interfacial properties between the heater and the phase change materials affect nucleation, so that electrical properties may change by inserting the interfacial layer.

In this study, we examine the effects of the interfacial layer between GeTe and heater on crystallization and electrical properties. 1.5nm-thick HfO2 layer was deposited with atomic layer deposition (ALD) on the bottom electrode contact surrounded by SiO2, and then 20nm-thick GeTe was deposited thereon with ALD. The diameter of bottom electrode contact is varied from 120 to 2000 nm. TiN as top electrode was grown by sputtering. The electrical properties were characterized with varying pulse width and bias polarity. GeTe-based PCM exhibits the abrupt set and reset switching, while more gradual transition is observed in the device with HfO2 insertion layer. Additionally, GeTe film was not grown without insertion layer, whereas, ALD growth of GeTe film on HfO2 layer was facile with improved adhesion. As a result, HfO2 insertion layer affects both adhesion and the electrical properties of PCM.
(Invited) (NH4)2WS4 precursors for Electrochromic Devices
Authors: Kim Soo Young , Jang Ho Won
Published: December 2020

In this research, we investigate the effect of metal doping on the electrochromic (EC) performance of tungsten trioxide (WO3) films. These films were prepared by a novel method involving solution processing and thermal annealing. In this procedure, ammonium tetrathiotungstate ((NH4)2WS4) was dissolved in dimethylformamide at a high concentration ratio (200 mg/mL) to obtain a homogeneous solution and then spin-coated onto the indium thin oxide (ITO) substrate for use as a working electrode. Subsequently, the film was annealed at the different temperatures (200, 300, 400, and 500 oC) to form a crystal structure of WO3. X-ray diffraction, Raman, and X-ray photoelectron spectroscopic results confirm the crystal formation of WO3. Moreover, in order to improve the electrochromic performance, different concentrations (10, 20, 30, and 40 mM) of different metal chlorides such as PtCl4, PdCl2, AuCl3, AgCl, CuCl2, NiCl2, NaCl, and KCl were added into the (NH4)2WS4 precursor and the mixture was coated on the ITO substrate and annealed at 500 oC. The results indicate that the performance of the Au-doped WO3 film was better than those with other dopants, likely as a result of the plasmonic effect. Furthermore, nanocrystal quantum dots (QDs) provide tunable optoelectronic properties on the basis of their dimension. CdSe QDs, which are size-dependent colloidal nanocrystals, are used for efficient electrochromic devices owing to their unique properties in modulating quantum confinement, resulting in enhanced electron insertion during the electrochromic process. Incorporating a well-known metal oxide electrochromic material such as WO3 into CdSe QDs enhances the redox process. Herein, we propose a facile method for producing and optimizing CdSe QDs doped in WO3. The fabrication of the electrochromic film involves a solution and annealing process. Moreover, the effect of the QD size to optimize the electrochromic layer is studied. As a result, the coloration efficiency of WO3 and optimized CdSe QD-WO3 are obtained as 68.6 and 112.3 cm2/C, respectively. Thus, size-tunable nanocrystal QDs combined with a metal oxide yield high-performance electrochromic devices and are promising candidates for producing smart windows.

Acknowledgement

This research was supported in part by a Creative Materials Discovery Program through the NRF funded by Ministry of Science and ICT (2017M3D1A1039379) and in part by the Basic Research Laboratory of the NRF funded by the Korean government (2018R1A4A1022647).
Finite Element Model Based Determination of Local Membrane Conductivity of Polymer Electrolyte Fuel Cells
Authors: Schuller Arnaud , Eller Jens
Published: December 2020

Polymer Electrolyte Fuel Cells (PEFCs) are an essential technology for future fast refill, long range and clean mobility. However, for better market penetration, their efficiency still needs to be improved. An important aspect, leading to different kinds of losses is the water management. Indeed, there exists a humidity gradient between the gas inlet and gas outlet in PEFCs with active areas of some hundred cm². This can lead to uneven membrane conductivity over the cell area which results in current density inhomogeneities, resulting in lower cell efficiency and shorter lifetime. Therefore, the measurement of localized membrane conductivity in technical cells is of great importance. Different to the typical invasive membrane conductivity measurement techniques, such as segmented flow field plates or shunt resistor boards, we propose a non-invasive method [1] which is able to provide localized information about the ionic conductivity in the membrane using electrical impedance tomography (EIT). EIT is based on the relationship that exists between one object’s conductivity distribution and the surface potential that one could measure when an alternating current is applied to the object. The objects surface potential is measured during the injection of an alternating current between two electrodes at the remaining electrodes (see Figure 1). Once enough injection-measurement pairs have been acquired, the objects inner conductivity can be reconstructed.

This presentation will discuss the results of a numerical feasibility study of the application of EIT for the determination of the membrane conductivity in PEFCs. A finite element model (FEM) based representation of a simplified fuel cell geometry is used to solve the surface potential distribution as function of the membrane conductivity distribution and current injection. Using a minimization approach the membrane conductivity distribution is adjusted such that the mismatch between the surface potential of the FEM solution and synthetic measurement data is reduced. In order to simplify the problem and reduce the degree of freedoms in the minimization formulation, so far, the conductivity distribution in the membrane is estimated by 1D interpolations from cell inlet to cell outlet (see Figure 2). Finally, the working principle of the method is experimentally demonstrated using fuel cell relevant materials.

[1] 2019P04382EP patent application

Figure 1
Water Distribution in Fuel Cell Gas Channels Using a Mechanistic Discrete Particle Model
Authors: Niblett Daniel , Niasar Vahid J. , Holmes Stuart
Published: December 2020

Water removal and accumulation in fuel cell gas channels is a complex phenomenon which is a consequence of the strength of the capillary forces and inertial forces (1 < Re < 1000). The presence of water in the channel restricts the access of oxygen to diffuse through the gas diffusion layers to the catalyst layer. The air supplied exchanges momentum to the droplets and water clusters, causing detachment when the adhesion force is overcome.

Water emerging from discrete locations as droplets interact with both the channel walls and each discrete water cluster, depending on the configuration and operating conditions. Complexity increases as multiple detachment and coalescence events occur over large channel lengths and time scales. Modelling these systems with traditional method computational fluid dynamic (CFD) methods is computationally expensive so we have developed a new mechanistic discrete particle model to handle the liquid water two-phase flow.

Taking inspiration from interface resolving volume of fluid (VoF) simulations of water flow in gas channels, we use analytically equations determined by the equilibrium contact angle in different configurations on the channel wall to predict detachment times and coalescence events. We compare the prediction of water area coverage ratio and channel saturation to the VoF simulations and can also estimate dynamic pressure drop.

Using this model, we are able to investigate the dynamic effects of liquid water accumulation and distribution in the channel, with often periodic detachment events arising from the formation of liquid plugs. Consequently, the model is able to investigate the effect of the channel wall wettability, from hydrophilic to hydrophobic and the impact that has on the distribution and accumulation of water in the system.

We compare the speed of this model against the VoF simulations to show that this type of modelling of two-phase flow in the channel can enable higher resolution accuracy when coupled with continuum scale approaches. Furthermore, with this approach it is possible to investigate the effect of: channel dimensions, liquid water flow rate and spatial location of liquid water emergence. This should allow for designed fuel cell structures to be evaluated.

Figure 1
Molecular Dynamics Study of the Nanoscale Proton Density Distribution at the Ionomer-Catalyst Interface
Authors: Spooner Jacob A , Eslamibidgoli Mohammad J , Malek Kourosh , Eikerling Michael H
Published: December 2020

The proton density in the cathode catalyst layer is a crucial variable to define the local reaction conditions in and thereby determine the performance and lifetime of polymer electrolyte fuel cells. We present a molecular modeling study to examine the proton density distribution in a water-filled, slab-like pore. The slab is confined by two distinct boundaries, the first consisting of a platinum single-crystalline surface and the second being a thin and dense skin layer of proton-conducting ionomer. Classical molecular dynamics simulations serve as a valuable tool to rationalize the impact of the molecular structure and properties of the ionomer film as well as the adsorption and charging state of the metal surface on structure and properties of interfacial water and the spatial distribution of protons. Boundary conditions at the metal surface are obtained from explicit quantum mechanical simulations at the DFT level. The oxide coverage at the metal surface and the water layer thickness are considered as crucial parameters. Results of detailed structural analyses elucidate the impact of a structured layer of near-surface water molecules at the platinum surface on accumulating protons at the interface.
Monitoring Carbon Corrosion in Anode of a PEM Fuel Cells Under Fuel Starvation Using a Hydrogen Reference Electrode and on-Line Mass Spectrometry
Authors: Park Sunjin , Lim Katie Heeyum , Lee Woong hee , Jeong Yoonjae , Kim Hansung
Published: December 2020

Polymer electrolyte membrane fuel cells (PEMFCs) has attracted great attention in transportation and stationary applications with zero emissions and high efficiency compared to conventional combustion engines. However, various factors limit the lifespan of PEMFCs, which remain a major obstacle to large-scale commercialization. In particular, corrosion of the carbon support is one of the main causes of fatal and irreversible deterioration of PEMFCs performance.

If the hydrogen fuel supply is insufficient to maintain current demand due to poor water management or fuel depletion at the anode, carbon corrosion may occur on the anode side. In this fuel starvation condition, reactions that take place in place of hydrogen oxidation, such as water electrolysis and oxidation of the carbon support, compensate for the current shortage.

The exact deterioration mechanism of PEMFCs in fuel starvation is investigated by simultaneously monitoring the potential behavior from the anode and the exhaust gas using hydrogen reference electrodes and in situ online mass spectrometry. Interestingly, water electrolysis starts at a lower potential range and carbon oxidation occurs at a higher range as an alternative reaction to hydrogen oxidation. Thus, carbon corrosion, which is detrimental to cell performance, can be reduced by extending the duration of the water electrolysis reaction. In this regard, the introductions of the graphitized carbon support and water electrolysis catalyst to the anode are effective in alleviating damage due to fuel starvation by making a dominant alternative reaction to continue electrolysis of water. And this alternative reaction is Monitored with a hydrogen reference electrode and using on-site online mass spectrometry.
Operando Analysis of Cation Migration in PEMFC Electrodes Using Nanoscale X-Ray Computed Tomography
Authors: Braaten Jonathan , Kongkanand Anusorn , Litster Shawn
Published: December 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are beginning to be accepted as the electrification technology of choice for long-range, high-power vehicles due to their clean operation, high efficiency, and the ability to independently scale their range and power. However, PEMFC vehicles are still not cost-competitive with internal combustion engines, largely because of the high cost of the platinum (Pt) used as the electrocatalyst in the cathodes of PEMFCs (1). Recent efforts have succeeded in reducing the Pt content of PEMFC cathodes by using high-performance Pt alloys like PtCo or PtNi, which have improved catalytic activities over pure Pt (2, 3). While these alloys achieve higher power densities with smaller amounts of Pt in beginning-of-life performance, they experience significant degradation over time, including loss of Pt to particle coarsening and a Pt band and ionization and dissolution of the less-stable alloying element (i.e. Co or Ni). These contaminant cations are absorbed by the ionomer, replacing protons on the sulfonate groups and inhibiting proton conduction and affecting oxygen transport, leading to reduced performance at high current densities (4). While the overall loadings of these cations with respect to the total number of sulfonate groups in the membrane is low, these cations are suspected to migrate during operation, causing high local loadings of contaminant cations in the cathode catalyst layers.

The focus of this work was the operando analysis of contaminant cation migration and accumulation using the nanoscale X-ray computed tomography (nano-CT) imaging technique. We designed a miniature nano-CT conducive fuel cell hardware with a Kapton-sealed X-ray viewing port, allowing us to monitor the temporal concentrations of contaminant cations (i.e. Co2+ and Cs+) at the catalyst layer/membrane interface using 2D absorption contrast imaging during cell polarization. The preliminary results from our method have demonstrated back-and-forth cation migration and accumulation under hydrogen pumping conditions, confirming the mobility of these cations under constant current application and demonstrating promise in the operando evaluation of their impact on proton and oxygen transport.

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

References:

S. T. Thompson, B. D. James, J. M. Huya-Kouadio, C. Houchins, D. A. DeSantis, R. Ahluwalia, A. R. Wilson, G. Kleen and D. Papageorgopoulos, Journal of Power Sources, 399, 304 (2018).

H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Applied Catalysis B: Environmental, 56, 9 (2005).

H. R. Colón-Mercado and B. N. Popov, Journal of Power Sources, 155, 253 (2006).

T. Greszler, T. Moylan and H. Gasteiger, Modeling the impact of cation contamination in a polymer electrolyte membrane fuel cell (2009).

Figure 1
Elucidating the Mass Transport Properties of Additively Manufactured Electrodes Using Spatially Resolved Simulation
Authors: Beck Victor A , Ivanovskaya Anna N , Chandrasekaran Swetha , Forien Jean-Baptiste , Baker Sarah E , Duoss Eric B , Worsley Marcus A.
Published: December 2020

Flow through electrodes are the core, active component in a number of electrochemical systems including fuel cells and flow batteries. Controlling fluid flow and mass transport is a core engineering challenge in these systems and has a dramatic impact on cell power, utilization, and efficiency. Most advanced electrode materials are composed of microscale, disordered porous media. The length scales of these materials allow for very large surface area but at the cost of low flow speeds, low surface mass transfer rates, high pressure drops, and added complexity and cost from the need to use external flow distribution systems like flowfields. The multiscale control over electrode architecture enabled by using additively manufactured electrodes could provide a potential resolution to these trade-offs. To elucidate the mass transport of these electrodes, we use resolved, continuum computation to analyze the fluid flow and species transport in direct ink-write printed, carbon aerogel electrodes. We determine the mass transfer correlations (Sh ~ Pea) across two different lattice aerogel geometries and validate these results against the experimentally manufactured and tested flow-through electrodes operated at limiting current. In contrast to conventional electrodes, the mesoscopic length scales in our electrodes lead to finite Reynolds numbers and we find an increase in the mass correlation exponent (a) as inertial effects become important. We analyze the local mass transfer coefficients in the electrode and correlate regions of enhanced mass transfer with secondary flows in the electrode. Our work provides a promising new pathway to increased mass transfer rates in flow-through electrodes.

LLNL-ABS- 810719

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Performance and Durability of Mn-Based Platinum Group-Metal Free Catalyst in Membrane Electrode Assemblies
Authors: Yang Fan , Stracensky Thomas , Zhong Sichen , Chen Mengjie , Guo Lin , Wang Yun , Wu Gang , Wang Guofeng , Xu Hui
Published: December 2020

Proton exchange membrane fuel cells (PEMFCs) have drawn increasing attention in automotive and residential applications. To significantly reduce the cost of the PEMFC stacks, current platinum-group metal (PGM) catalysts must be replaced by PGM-free catalysts. Although Fe-based PGM-free catalysts (Fe-N-C) have exhibited promisingly high activity for the oxygen reduction reaction (ORR)1, they suffer from insufficient stability. Additionally, a major issue associated with Fe-containing catalysts is that dissolved iron ions can facilitate Fenton’s reaction thus forming a strong radical known to degrade the ionomer and membrane2.

Mn-based catalysts have recently been developed, aiming to improve the catalyst durability and mitigate the Fenton’s reaction3. However, the catalyst stability and integration into membrane electrode assemblies (MEAs) have yet to be studied. In this work, density function theory (DFT) has been applied to study the active sites and their degradation mechanism. Computational fluid dynamic (CFD) modeling was also applied to study water, oxygen and proton transport in the electrode. Electrode fabrication optimizations were conducted based on the guidance from the CFD results. The optimizations include ionomer content, electrode porosity and MEA fabrication approaches It has been found that Mn-based catalysts showed improved MEA durability compared to Fe-based catalysts due to the higher carbon graphitization degree, reduced de-metallization and mitigation of peroxide radical formation. However, there is still a significant performance degradation compared to PGM-based MEAs. X-ray absorption has been applied to examine the catalyst and corresponding MEAs before and after durability. The degradation is primarily attributed to the oxidation and agglomeration of active sites.

Acknowledgement: The project is financially supported by the Department of Energy’s HFTO under the Grant DE-EE0008075.

Reference:

1. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science, 332, 443, 2011.

2. F Coms, H Xu, T. McCallum, C. Mittelsteadt, ECS Trans. 64, 389, 2014.

3. Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nature Catalysis
2018,
1 (12), 935-945.
(Invited) Development of Durable and Low Cost Membrane Humidifiers for Fuel Cell Systems
Authors: Huizing Ryan Nicholas
Published: December 2020

In order to operate effectively, currently commercialized fuel cell proton exchange membranes require sufficient hydration to ensure good proton transport. Hydration also ensures that membrane in the fuel cell stack will have a longer operational lifetime. In order to ensure adequate hydration in the stack, most liquid-cooled PEM fuel cell systems use external humidification. Without external humidification, the fuel cell must be operated at a lower operating temperature to ensure that the stack will not dry out. Operating the stack at lower temperature requires that the coolant loop in the system must reject a greater amount of heat, necessitating a large radiator, which increase system volume. Furthermore, operating at lower fuel cell stack temperatures leads to a significant trade-off in terms of stack power output and acceptable environmental operating ranges.

Membrane humidifiers are an efficient form of external humidification that transport humidity through a membrane from the cathode exhaust to the incoming cathode supply air stream. In these humidifiers, membranes with a dense selective layer are used to transport water vapour without allowing oxygen and nitrogen to diffuse through the membrane. Since the membrane humidifier is coupled to the stack cathode exhaust and supply, in order to maintain its function, the membrane humidifier must not leak air or else reactant oxygen will bypass the stack through the humidifier. The humidifier must maintain a high level of water vapour transport and low air leakage over the lifetime of the device.

In this presentation we present our work developing low-cost selective membranes for fuel cell humidification. These membranes are functional over a wide temperature range (-30 to +110°C) and have high water vapour permeance. They are resistant to contamination, relative humidity cycling, freeze-thaw cycling, oxidation, hydrolysis, and acidic conditions that are present in the cathode exhaust.

This presentation will also discuss the incorporation of these flat-sheet membranes in to heat and mass exchangers (membrane humidifier cartridges). These cartridges are compact with a high membrane-to-volume ratio, high water vapour transport, and have low pressure loss. The membrane humidifiers are robust to a wide range of operating conditions. Furthermore, the design of these cartridges allows the humidifier to be manufactured at a low cost in high manufacturing volumes.

Finally, this presentation will discuss membrane-based heat and mass exchanger performance at different operating conditions. A performance model will be presented which summarizes humidifier performance as a function of membrane properties, exchanger design and architecture, and fuel cell operating conditions.
Voltage Control for a Polymer Electrolyte Fuel Cell System By Model Predictive Control Controller
Authors: Qi Yuanxin , Espinoza-Andaluz Mayken , Andersson Martin
Published: December 2020

Due to its high power density, fast start up ability, low operating temperature, noiseless and environmental friendliness, polymer electrolyte fuel cells (PEFCs) have been regarded as the most promising alternative power sources for a variety of applications, such as transportation system, domestic system, stationary system and so on. Stable and effective control strategy is essential for the efficient and safe operation of PEFC systems. Voltage control is always a research hotspot in PEFCs’ control study. When using a PEFC system to supply power in a real application, it is of great significance to maintain a stable output voltage to avoid an adverse voltage drop under sudden load change, which can reduce the maintenance costs of the PEFC generation system. The output voltage of the PEFC system can be easily affected by the inlet flow rate of the reactant gas, i.e., the reactions inside the fuel cell system will be more sufficient with higher input flow rate of the reactant gas to produce higher electrical voltage, otherwise, less electrical voltage is generated. Therefore, the focus of this study is to control the output voltage of a PEFC system by regulating its hydrogen and air flow rates, which stands for a multi input and single output control problem.

Model predictive control (MPC) is an advanced method of process control with stronger robustness and faster response compared with conventional controllers. Its main advantage is that it allows the current timeslot to be optimized while keeping future timeslots in account. Moreover, the MPC is rather suitable for solving systems with constraints, which makes a good choice for the voltage control of the PEFC system since the fuel cell system is a complicated nonlinear and strongly coupled system, with many constraints that need to be considered to ensure the coordination and balance between every subsystem.

The first part of this study is to develop a PEFC system dynamic model, which is based on the NedStack PS6 of 6kW rated power. Its polarization behavior based on different reactant flowrates are evaluated. Then based on the developed model, a novel MPC controller is designed to control the PEFC system’s output voltage at 48V, assuming that the PEFC system can be an alternative power source for an electric bicycle with 48 V by regulating the input hydrogen and air flow rates. The following figure gives an overview of the voltage control of the PEFC system. It can be seen the input of the MPC controller contains the reference stack voltage, the state vector, the actual stack voltage and the current. It should be noted that the current is the our test signal.

Figure 1
Online Measurements of Hydrogen Impurities in PEMFC Single Cells and Short Stacks
Authors: Becker Hans , Smith Graham , Bacquart Thomas , Hinds Gareth
Published: December 2020

Impurities can easily contaminate pure hydrogen at various points in the supply chain. The maximum allowed impurity concentrations in hydrogen, sold for automotive PEMFC use, are regulated by the ISO 14687 standard. Imposing too strict limits on hydrogen quality increases the cost of hydrogen unnecessarily but too lax limits risk the performance and durability of PEFMCs. It is therefore imperative that the specified limits are based on rigorous and realistic studies of impurity behaviour in fuel cells. As part of the European Union funded projects HYDRAITE and MetroHyVe2, NPL is investigating the impact of fuel impurities on the performance and durability of fuel cells. The overall aim is to develop new testing standards and provide more reliable information to ISO committees for the continuous revision of fuel quality standards.

Impurity studies are being carried out primarily on commercial short stacks tested with a hydrogen recirculation loop and a dead-end anode. This operation mode is typical in real systems as it gives a very high fuel utilisation, however it also causes fuel impurities to concentrate well above their inlet level. Standard fuel cell testing is coupled with online measurements of the concentration of a range of contaminants inside the anode recirculation loop. For example, a Gas Chromatograph with a methanizer and flame ionising detector (GC-methanizer-FID) is used to provide information on CO2, CO, and CH4; while Selected Ion Flow Tube - Mass Spectrometry (SIFT-MS) is used to analyse isotopically labelled impurities and probe the concentrations of other species as a function of time.

Our results will first show the power and limitations of using online gas analysis with isotopically labelled impurities to monitor processes related to CO poisoning in single cells. We will then demonstrate that certain hydrocarbon impurities have little impact on stack performance despite being stringently regulated. Finally, we will present information on rates of O2 and CO2 crossover, drawing conclusions relevant for setting the next iteration of hydrogen fuel standards. This work provides vital underpinning metrological support for the development of more robust business cases for large scale deployment of hydrogen in automotive applications.
Investigation of Multilayered Catalyst Layers for Polymer Electrolyte Fuel Cells by Electrospray Deposition
Authors: Arai Hikaru , Irita Masaru , Katayama Noboru
Published: December 2020

Due to problems such as environmental pollution and global warming, fuel cells are attracting attention as an energy source with low environmental load. As a type of fuel cells, there is a growing demand for polymer electrolyte fuel cells that can operate at low temperatures and are suitable for miniaturization. Improved power generation performance is required to increase the penetration of this technology.

The performance of a fuel cell depends on the structure of the catalyst layer (CL), which is directly involved in the reaction. The structure of the CL mainly depends on the composition and the coating method. Recently, a method has been proposed in which ion and gas transportation is made more efficient and the performance is improved by changing the ionomer distribution in the thickness direction. However, in order to change the ionomer distribution, precise control of the coating amount is required, and there are few actual formation examples and the coating method is limited. Therefore, coating methods that have not yet been investigated should also be considered. In this study, the electrospray deposition (ESD) method was used to prepare the CLs, and the comparison of the power generation performance and the effect on the CL structure were investigated.

In order to change the ionomer distribution in the thickness direction, multi-layered CLs with different ionomer to carbon ratio (I/C ratio) were formed, and the power generation performance was compared with that of the homogeneous CLs. Several homogeneous CLs with different I/C ratios were formed and the CLs were observed with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The maximum output power density was 274 mW cm-2 and 312 mW cm-2 for the homogeneous and three-layer CLs, respectively. Therefore, it was found that the structure in which the distribution of ionomers is changed in the thickness direction is effective in improving the performance of the CL formed by ESD. The SEM-EDS observation revealed that the agglomeration of particles became more pronounced and the CL became thinner as the I/C ratio increased. The result suggest that it is necessary to take into account the change in thickness with the amount of ionomer in order to form an ideal CL structure using ESD.
Welcome Remarks - I01B: Polymer Electrolyte Fuel Cells & Electrolyzers 20 (PEFC&E 20) - Cells, Stacks and Systems
Authors: Rice Cynthia A.
Published: December 2020

Welcome to I01B: Polymer Electrolyte Fuel Cells & Electrolyzers 20 (PEFC&E 20) - Cells, Stacks and Systems! Take a moment to listen to welcome remarks from the lead organizer, Cynthia Rice.
Investigating the Influences of Polystyrene-Based Hydroxide Ion Conducting Ionomer on Electrode in Anion Exchange Membrane Fuel Cells
Authors: Chae Ji Eon , Lee So Young , Song Kwang Ho , Kim Hyoung-Juhn
Published: December 2020

Alkaline anion exchange membrane fuel cells (AEMFCs) are an attractive alternative to proton exchange membrane fuel cells because they have the advantage of cost savings due to the use of non-noble metal catalysts. Since AEMFCs has a charge carrier of hydroxide ions, anion exchange membranes having various cationic groups substituted on the hydrocarbon polymer backbone have been actively studied for high hydroxide conductivity, mechanical properties and durability. In addition, the electrode ionomer of the core material of the membrane electrode assembly acts as a binder for uniformly dispersing the catalyst and fixing the catalyst layer and anion exchange membrane. It also serves as a transport pathway for hydroxide ions produced from the cathode to the anode. In particular, it is important that the electrode ionomer greatly affects the catalyst layer type and fuel cell performance.

Herein, polystyrene-based copolymers with variable molecular weight are prepared by radical polymerization of styrene, grafted with bromo-alkyl chains of different length (C3 or C6) through Friedel-Crafts acylation, and quaternized to afford a series of hydroxide ion–conducting ionomers. The performance of anion-exchange membrane fuel cells (AEMFCs) constructed using the prepared electrode ionomers is affected by several ionomer properties, and a maximal power density of 407 mW cm−2 and a durability exceeding that of a reference cell with a commercially available ionomer are achieved under optimal conditions. Thus, the developed approach is concluded to be well suited for the fabrication of next-generation electrode ionomers for high-performance AEMFCs.
Novel Composite Membranes Reinforced By Vertically Aligned Electrospun PTFE Nanofibers for Enhancing Proton Conduction and Dimensional Stability in PEM Fuel Cells
Authors: Yoon Ki Ro
Published: December 2020

Polymer electrolyte membrane fuel cells (PEMFCs) have widely been considered environmental friendly alternative energy technologies due to their high energy conversion efficiency and zero emission. In the recent, the importance of performance and durability of electrolyte membrane, interfacial layer separating two electrodes, prohibiting fuel cross-over, while transporting only protons, has become much crucial for utilizing PEMFCs to the automobile applications such as fuel cell electric vehicles (FCEVs) and drones. Unfortunately, the current state membranes, mainly composed of perfluorosulfonic acid (PFSA) ionomers, have suffered from the high cost and low proton conductivity, and low dimensional stability during dynamic humidity changes during fuel cell operation. In order to deal with these issues, recently, a reinforced composite membranes (RCMs) have been developed in which a PFSA ionomer is impregnated into a porous support matrix consisted with nanofibrous polymer materials such as polytetrafluoroethylene (PTFE) due to their thermal/mechanical robustness. The PTFE based RCMs could provide enhanced mechanical stability and electrochemical performance with a reduced use of PFSA ionomer, but low impregnation of ionomers into nanosized hydrophobic PTFE networks, resulting in phase separation and irreversible creep deformation. Herein, we report a novel preparation of vertically alined PTFE aggregated fibers (PTFE AFs) as a reinforcement by using facile electrospinning hetero-polymer solutions containing PTFE nanoparticles and polyethylene oxide (PEO) as a carrier polymer and subsequent thermal removal of PEO. Developed PTFE AF were effectively soaked by PFSA ionomers, and PTFE AF based RCMs delivered outstanding dimensional stability and proton conductivity compared to commercial RCMs. Furthermore, PTFE AF based RCMs used MEA exhibited exceptional single-cell performance and long-term stability. The enhanced performance was attributed to the facile proton conduction through the one dimensional channels between aligned fibers without fiber scattering, and effective suppression of volume expansion of hydrated PFSA ionomer by rectangular shaped PTFE fiber network. We believe the widespread use of novel PTFE fiber based membranes with an unique pattern in a variety of membrane applications where porous supports are used.

Figure 1
Co-Tri MOF -Impregnated Aquivion® Composite Membrane for Fuel Cell Application
Authors: Paul Subir , Choi Sang-June , Kim Hee Jin
Published: December 2020

A membrane electrode assembly (MEA) is regarded as a center of the polymer electrolyte membrane fuel cell (PEMFC), and a solid electrolyte membrane is an indispensable component of the MEA. Reducing the ohmic resistance while improving mechanical stability is a challenge to membrane research and development. In polymeric proton exchange membranes (PEMs), Metal−organic frameworks (MOFs) have gained a great deal of attention as ﬁller materials because of their tenability and designability. Aquivion® is a state-of-the-art membrane used in fuel-cell and electrolysis applications because of its mechanical stability and elevated proton conductivity. Considering the theory of coordination networks, the conductivity of short-side-chain (SSC) Aquivion® has been improved by incorporating the highly proton-conductive Metal-Organic Framework (MOF), namely, Co-tri MOF {[(Co(bpy)(H2O)4](Hbtc).(H2O)1.5}n. The proton conductivity of the 1 wt.% Co-tri MOF/Aquivion® composite membrane is significantly increased by 66% than that of the pristine Aquivion® membrane, 3.5 times that of cast Nafion® membrane. The effect of Co-tri MOF on the Aquivion® membrane structural morphology and thermal properties were investigated by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermal gravimetric analysis (TGA) and Scanning Electron Microscopy (SEM). The composite membrane showed an improved in ion conductivity and cell performance compared to the pristine Aquivion® membrane.
Modeling Synergistic Fuel Cell Membrane Degradation with Mitigating Effects of Cerium
Authors: Ehlinger Victoria Marie , Kusoglu Ahmet , Weber Adam Z.
Published: December 2020

During operation, polymer-electrolyte-membrane (PEM) fuel cells undergo mechanical and chemical degradation mechanisms, which behave synergistically and lead to accelerated membrane degradation over time. This study builds upon previous modeling work on mechanical degradation as described by a pinhole in the membrane and the effects of cerium on chemical degradation.1 By combining these two models, analysis can be carried out on the coupled degradation methods and how the mitigation effects of cerium disrupt the degradation cycle. The mechanical model represents a pinhole in the membrane using an effective void fraction, which allows for increased gas crossover through the membrane and impacts the membrane transport and mechanical properties. A microkinetic model for the chemical degradation is included in the model, including attack of the membrane polymer by hydroxyl radicals as well as the mitigation reaction for quenching of hydroxyl radicals with cerium. A concentrated solution theory approach is used to model the transport of cerium ions throughout the cell.2 The model results show how the location of cerium in the cell can be used to prevent chemical degradation. In addition, the model shows how cerium slows down the rate of pinhole growth by reducing the gas crossover and membrane thinning rate. Finally, the model can be used to optimize the distribution of cerium in the membrane and catalyst layers by balancing trade-offs between lowering degradation rates and decreasing fuel cell performance.

Acknowledgements: Funding support was supplied by the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231. The authors would like to acknowledge Grace Anderson and Claire Arthurs for their work on membrane characterization experiments.

V. M. Ehlinger, A. Kusoglu and A. Z. Weber, Journal of The Electrochemical Society, 166, F3255 (2019).

A. R. Crothers, R. M. Darling, A. Kusoglu, C. J. Radke and A. Z. Weber, Journal of The Electrochemical Society, 167, 013548 (2020).