bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Invited) Multi-Functional Memristive Transistors Based on Van Der Waals Heterostructures
Authors: Park Hyunik , Kim Jihyun 
Published: February 2020

In recent years, the demand for biologically inspired systems for diverse applications, such as object recognition, autonomous vehicles, and humanoid robot, has been continuously increasing. Particularly, neuromorphic computing, imitating the energy-efficient neural network in the human brain, has become powerful candidate that can overcome the limitations of the conventional von Neumann architecture, named ‘von Neumann bottleneck’. To realize neuromorphic computing, a concept of crossbar array of memristors was suggested. A memristor is a two-terminal circuit element that can store binary information within the active layer in a nonvolatile fashion by the electrical stimulation. In addition to its nonvolatility, it has received a remarkable attention for future memory and neuromorphic applications owing to its high endurance, fast switching speed, small form factor, low energy consumption, and multi-state switching capability. Nevertheless, two-terminal memristors cannot fully provide the complex functions of neurons, and thus, they are still not enough to operate complex and massive datasets. In this regard, tremendous efforts have been dedicated to the development of miscellaneous functional circuit elements for neuromorphic and in-memory computing. One way to enable this is combining the memristor with various functional circuit elements such as capacitor, inductor, or transistor.
In our study, we demonstrated a programmable and multi-functional memristive transistor by integrating a hexagonal boron nitride (h-BN) memristor and a molybdenum disulfide (MoS2) transistor. The device was fabricated by sequential stacking semiconducting MoS2, semi-metallic graphene, and insulating h-BN on a SiO2/p++-Si substrate in order, followed by deposition of contact metal. The memristive transistor consisted of a vertical graphene/h-BN/Ti/Au memristor and a lateral graphene/MoS2/Ti/Au transistor as shown in the figure. Mechanically exfoliated h-BN-based memristors showed stable resistive switching behavior as the conducting filament(s) formed and annihilated through the breakdown path within the h-BN layer. The behavior of the memristive transistor can be programmably modulated by switching the resistance state of the memristor, and it showed extremely low standby power consumption of ~16 pW. Notably, multi-state resistance and tunable current on/off ratio can be attained by controlling the synaptic weight of the device. This novel architecture can also perform logical operation by changing the electrical circuitry. The proposed device provides a higher degree of functionality in both logic and memory applications, thereby a step toward the achievement of energy-efficient in-memory computing is expected. The details of the experimental results will be presented.

Figure 1
(Invited) A Leaf-Inspired Photon Management Scheme for Graphene/Silicon Solar Cells
Authors: Roy Tania 
Published: February 2020

Graphene/silicon (Gr/Si) Schottky junction solar cells have attracted much attention due to the ease and low cost of fabrication, along with the lucrative properties of high electron mobility, transparency and mechanical flexibility of graphene as a transparent conducting electrode.1 Utilizing its inherent mechanical flexibility, graphene can be integrated with thin flexible crystalline Si substrates opening up a new regime of applications in flexible and wearable electronics. Reducing Si absorber thickness below 50 um offers advantages of reduced material cost, along with mechanical flexibility and light weight.2 But Si at such thicknesses suffers from low photon absorption in the solar spectrum. To compensate for the low light absorption in such thin substrates, light management schemes become essential. Light trapping in Gr/Si solar cells is enabled by engineering the Si surface to form nanopillars, nanowires etc., which decreases the reflection loss and allows more light to couple in to the Si substrate. The structured Si absorber increases surface area and surface recombination, which is detrimental to the solar cell efficiency. Thus, it is imperative to use a light-trapping scheme devoid of Si structuring to enhance the photo-conversion efficiency.
We present an all-dielectric light-trapping scheme on planar Gr/Si Schottky junction solar cells with the use of bottom layer of titania spheres and top layer of silica spheres.3 An optimal Si thickness coupled with an optimized light-trapping scheme leads to efficient electron-photon harvesting. The photo-conversion efficiency of a 20 um thick nanosphere-decorated Gr/Si solar cell improves to 9%, which is 1.3x higher than the pristine cell’s PCE of 7%. FDTD simulations are performed for optimizing the diameter of nanoparticles in each of the layers. The ratio of size of nanoparticles in the top to bottom layer plays a crucial role in advanced light management. An optimized structure of silica spheres, having diameter larger than that of titania spheres, suppresses reflection over wide angles of incidence and increases absorption in active Si layer over AM1.5G solar spectrum. The non-absorbing dielectric spheres can be easily realized by the well-known Stober technique. Additionally, the photovoltaic characteristic of the laminated solar cell shows negligible change after several bending cycles having bend radius ranging from 5 mm to 10 mm. After continuous bending and straightening, the ultra-thin solar cell can retain its performance, revealing the excellent stability and flexibility of the device. Such simple, low-cost light trapping schemes are universal in nature, devoid of recombination losses and are potentially viable for any solar cell technology.

References

1 B Li, et al. Adv. Mater. 22, 2743 (2010).

2 K E Petersen, Proc. IEEE 70, 420 (1982).

3 S. Das, et al. Nano Energy 58, 47 (2019).
Rutile to Cubic Phase Transformation Induced By Electron Beam
Authors: Tian Mengkun , Zhang Lizhi , Mahjouri-Samani Masoud , Sachan Ritesh , Yoon Mina , Puretzky Alexander A. , Geohegan David B. , Eres Gyula , Duscher Gerd 
Published: February 2020

Cubic TiO2 was predicted to be a great visible light absorption material partially due to the smallest bandgap among all the TiO2 polymorphs. The cubic TiO2 can be synthesized using the common TiO2 polymorph i.e. anatase as a precursor, but it mixes with other metastable phases as byproducts including baddeleyite, TiO2 (OI) and TiO2 (OII). The traditional syntheses of cubic TiO2 have to be performed at a high-temperature and high-pressure conditions. Such harsh condition limits the potential applications of the cubic TiO2.
In this work, cubic TiO­2 was grown on the surface of a rutile nano-template at room temperature in vacuum. The rutile precursors were made by pulsed laser deposition (PLD). As shown in Figure 1, the growth of cubic TiO2 was induced by the electron beam in an aberration corrected scanning transmission electron microscopy (STEM). Monochromated electron energy loss spectroscopy (EELS) was used to investigate the phase transformation under the electron illumination. The STEM images show that lattice matching relationships are (001)R // (001)C and [100]R // [100]C with only 4.6% lattice mismatch, where R and C refer to rutile and cubic, respectively. Density functional theory (DFT) reveals that rutile-to-cubic TiO2 transformation can be highly favorable if the strain is > 10% at the interface. The driving forces originate from three resources: (1) lattice mismatch at the interface; (2) defective structure of the black rutile; (3) oxygen vacancies created by the electron beam. In addition to knock out oxygen, the electron beam also provides energy for the Ti atomic displacement in the rutile during phase transformation.
Figure 1. Phase transformation from rutile to cubic TiO2 by electron beam exposure.

Figure 1
Understanding the Degradation of Pt Electrocatalysts in Proton Exchange Membrane Fuel Cells By 3D Identical Location STEM
Authors: Yu Kang , Li Chenzhao , Xie Jian , Ferreira Paulo Jorge 
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are promising energy conversation devices due to their high power/energy density, high efficiency and the absence of carbon dioxide emission. Yet, one of the largest obstacles for the commercial application of PEMFC is the degradation of platinum group metal (PGM)-based electrocatalysts during long term operation. Four degradation mechanisms have been widely reported, namely, 1) Ostwald ripening; 2) particle migration and coalescence; 3) carbon corrosion and 4) Pt dissolution and reprecipitation in the membrane. However, the role played by each degradation mechanism is not fully understood.
In this work, scanning transmission electron microscopy (STEM) tomography coupled with identical location TEM (IL-TEM) are utilized to delineate the roles of each degradation mechanism for Pt catalysts on Vulcan XC72 carbon support. Compared with conventional TEM, 3D STEM tomography is capable of identifying the following: 1) interparticle distances; 2) 3D structure of carbon black, 3) specific positions of Pt catalysts on carbon support; and 4) shapes and morphologies of Pt catalysts[1]. In addition, IL-TEM allows a comparison of Pt catalysts and carbon support of the same region of interest (R.O.I), at the beginning of life and after potential cycling, which provides direct insight into the effects of potential cycling on the degradation of Pt catalysts.
Figure 1(a-b) shows the 3D illustration of Pt catalysts on the Vulcan XC72 carbon support before and after potential cycling, respectively. We observe that volume shrinkage is insignificant in the bulk of the carbon black particles and that there are no Pt catalysts falling off after potential cycling (0.6V-1.0V vs RHE, 6000 cycles). In addition, we note the appearance of newly-formed Pt nanoparticles or ionic clusters (<1nm in diameter). A careful inspection regarding the formation mechanisms of these particles shows that particle migration and coalescence is not involved. Instead, these newly-formed particles are likely due to Pt dissolution and re-precipitation. The 3D IL-TEM experiments also show the coalescence of Pt NPs. However, the coalescence is unlikely due to long migration distances (>0.5nm). Instead, the growth of Pt nanoparticles result from Ostwald ripening and subsequent sintering.
In summary, 3D STEM tomography coupled with IL-TEM (~1nm spatial resolution) provides unique insight into the roles of each degradation mechanisms for Pt electrocatalysts on Vulcan XC72 during potential cycling.
Reference:
[1]Horn, S. Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.;Morgan, D. Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells. Top. Catal. 2007, 46, 285-305.

Figure 1
Impact of Porosity Gradients within Catalyst Layer and MPL of a PEM Fuel Cell on the Water Management and Performance: A Neutron Radiography Investigation
Authors: Kartouzian Dena , Mohseninia Arezou , Markötter Henning , Langner Philipp , Scholta Joachim , Manke Ingo 
Published: February 2020

The influence of porosity modifications of either microporous layer (MPL) or the catalyst layer of a low temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) on the performance of the cell has been investigated in many studies. However, we have been focusing in our study on the interactive impact of this morphological modification of both cathode MPL and catalyst layer in the water distribution and transport inside the cell and consequently also on the performance of the cell.
In this study, three types of cathode MPLs with different porosity distributions and three type of membrane electrode assemblies (MEA) with different porosity of the catalyst layer are manufactured. Five one-cell fuel cells are investigated using combinations of those in-house made layers. To modify the porosity of the cathode MPL polymer particle in two sizes of 30 µm and 1.5 µm from company Chemisnow® are introduced into the carbon slurry, which are then evaporated out of the MPL during the sintering process. For the catalyst layers, 0.5 µm Polystyrene particles are mixed in the electrode slurry with variable weight fractions and then washed out of the dried MEA. A more detailed description of the production procedure of these layers will be demonstrated in the presentation.
Performance of fuel cells using these five combination of cathode MPL and electrode are measured using a Fuel Cell with 25 cm² active area and 3 serpentine channel flow fields. Neutron Radiography is used to investigate the water distribution dynamics and water content of the fuel cell. For this purpose, a specially designed single cell with 8 cm² active area and 3 serpentine channel flow fields is used. The measurements are obtained at two different humidity values on both cathode and anode inlet gases of RH=70% and RH=120% and different current densities. With an exposure time of 10 s, images with 12.4 µm pixel size are achieved.
The quantitative analysis of the water thickness in the cathode GDL and cathode electrode of 5 cells at RH=120% for an increasing current density is shown at an exemplary radiogram for water distribution inside the cell with our reference cathode MPL and electrode (without porosity modification). Also the Current (ICell) and Voltage (VCell) evolution of the same cell is shown in figure 1.
All cells have reached a water content plateau even before reaching the highest current density of 1A/cm². The cell with a porous catalyst layer and a double layer cathode MPL shows the highest water content on the cathode side of the cell, whereas the cell with a highly porous catalyst layer and a porous MPL shows the lowest water content of all cells, comparable to the water thickness observed for the cell without modified layers. Further results of both water content and performance dependent on porosity changes will be provided in the presentation.

Figure 1
Understanding Carbon Corrosion Damage to Fuel Cell Electrodes: Towards Reversal Invincible
Authors: Hu Leiming , Hong Bo Ki , Oh Jong-Gil , Litster Shawn 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) have the advantage of low carbon dioxide emission and high efficiency over internal combustion engines in automobile applications1. However, durability of PEMFCs is one of the major obstacles for a wide range of applications. One such durability issue is the cell reversal caused by hydrogen starvation. It results in severe carbon support corrosion in the anode catalyst layer and dramatically decreases the fuel cell performances2. Material-based solutions to increasing the cell’s tolerance to reversal are preferable since it will not increase the system control complexity. Effective material-based solutions include using more corrosion-resistant carbon supports3 and adding water electrolysis catalysts to promote the oxygen evolution reaction (OER) over carbon corrosion4,5. However, these current solutions are still not resilient enough to guarantee long-term durability of PEMFCs under extreme hydrogen starvation conditions.
This present work has examined different anode configurations and their effectiveness in preventing cell performance degradation during hydrogen starvation events. Three different anode structures are studied and compared, including i) a conventional Pt/C gas diffusion electrode (GDE), ii) an anode made of platinum (Pt) black and gas diffusion layer (GDL), and iii) an anode made of Pt black, a thin titanium (Ti) protection layer (10 μm in thickness), and a GDL. Figure 1a shows the change in cell voltage of PEMFCs when hydrogen was cut off while keeping a constant current density of 0.2 A/cm2. During reversals, the Pt/C GDE anode and the anode made of Pt black and a GDL experienced a sudden voltage drop within 5 min which was attributed to severe carbon corrosion damage. In contrast, the cell with Ti protection layer between the Pt black and the GDL was stable for 60 min of reversal without a large voltage drop. As shown in Figire 1b, the cell voltage of the fuel cell with an anode made of Pt black, a Ti protection layer, and a GDL decreased less than 10 % even after the accumulated voltage reversal time of 240 min. Thus the anode with Ti protection layer is much more effective compared with the common reversal tolerant anode approach of adding OER catalyst (i.e., 50 wt% IrO2) to the anode5. Both hydrogen starvation tests and post-reversal electrochemical characterizations demonstrate that using a carbon-free anode with a corrosion-resistant barrier layer is highly effective in preventing cell reversal damage over long periods of time.

Figure 1
Co and Ce Cation Impact on PEM Fuel Cell Performance
Authors: Cai Yun , Gu Wenbin , Kongkanand Anusorn 
Published: February 2020

PtCo/C is often used in the cathode in proton exchange membrane fuel cells to enhance performance. However, Co2+ cations can be released from the catalyst into the ionomer during operation. Additionally, Ce3+ cations are often introduced into the membrane as a chemical stabilizer. These cations replace protons of the ionomer acid cites and can redistribute during operation, resulting in local proton deficiencies, especially, in the cathode at high current densities [1-2]. While studies have shown severe performance loss due to Co2+ migration [3-4], the simultaneous impact of both Co2+ and Ce3+ has not been reported. Moreover, state-of-the-art cathodes often use high-surface-area carbon (HSC) with numerous PtCo particles within carbon internal pores. In absence of ionomer within these internal pores, it is unclear how the cations affect the proton activity and hence fuel cell performance. In this study, we performed controlled experiments where known amounts of Co and Ce cations were exchanged into the membranes and investigated the effect of cation concentration at different operating conditions. We also include the comparison of cation impact on electrodes with Vulcan and HSC supported cathode catalysts. Impedance spectroscopy was used to study the effect of cations on membrane and electrode proton resistances. The experimental results provide direct assessment of the cation impact on the fuel cell performance, and will also be used to improve our fuel cell performance model and provide direction for future fuel cell development.
This work was partially supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.
References

T. A. Greszler, T. E. Moylan and H. A. Gasteiger, Chapter 49, Handbook of Fuel Cells – Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Harumi Yokokawa, Hubert A. Gasteriger. Volume 6: Advances in Electrocatalysis, Materials, diagnostics and Durability. 2009 John Wiley & Sons, Ltd.

Y. Cai, J. M. Ziegelbauer, A. M. Baker, W. Gu, R. S. Kukreja, A. Kongkanand, M. F. Mathias, R. Mukundan, and R. L. Borup, J. Electrochem. Soc., 165, F3132–F3138 (2018)

S. Chen, H. A. Gasteiger, K. Hayakawa, T. Tada and Y. Shao-Horn, J. Electrochem. Soc., 157, A82 (2010).T. Okada, H. Satou, M. Okuno and M. Yuasa, J. Phys. Chem. B, 106 1267 (2002)

Y. Cai, A. Kongkanand, W. Gu and T. E. Moylan, ECS Trans., 69, 1047 (2015).
(Invited) Porous Materials for PEM Fuel Cells and Electrolyzers: Heterogeneous and Interfacial Considerations
Authors: Bazylak Aimy 
Published: February 2020

While in practical application, polymer electrolyte membrane (PEM) fuel cells and electrolyzers exhibit macroscale length scales spanning centimeters to meters, the inner workings of these energy conversion devices fundamentally rely on microscale and nanoscale transport phenomena and porous materials to facilitate their passive operation. In order to maximize the power density and minimize the cost of the PEM fuel cell or electrolyzer, we must optimize our surface area to volume ratios at the reaction sites; thus, state-of-the-art catalysts exhibit diameters on the order of nanometers. At the same time, we must consider the effective delivery of reactants to these small- catalyst sites, and porous materials provide a means to impart homogeneous distributions of reactants, in theory. In addition to laterally distributing reactants across the planar catalyst layer, porous materials also facilitate multiple pathways for the competing reactant and product pathways to co-exist.
The PEM fuel cell and electrolyzer are composed of multiple porous materials, including the catalyst layer, microporous layer, and substrate. Commercial materials, whether by design or not, typically exhibit highly heterogeneous material and chemical properties. For example, in our past work, we extensively examined the heterogeneous microstructure and hydrophobicity of the gas diffusion layer (GDL). In order to reach cost targets for widespread commercial adoption, we must realize materials that enable more effective multiphase flow phenomena than what currently exists. Mass transport losses in PEM fuel cells and electrolyzers are both prohibitively significant. However, designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. These are factors that are currently not fully understood in the literature. In this work, I will discuss these critical design factors (heterogeneous porous materials and nature of interfacial contacts) and how they influence the flow and mass transport behaviour in PEM fuel cells and electrolyzers. I will also discuss the new materials we have designed and fabricated, informed by in-house numerical modelling and tested through a combination of in operando and ex situ X-ray and neutron beam characterization approaches.
Optimization of Open Pore Cellular Foams for PEM Fuel Cells
Authors: Nandjou Fredy , Haussener Sophia 
Published: February 2020

Open pore cellular foams (OPCF) have attracted substantial attention for use as flow-field plates in proton exchange membrane fuel cells due to their potential to enhance the performance [1]. Moreover, they could help to mitigate membrane degradation due to the hygrothermal stresses, which are generally observed in fuel cells that use bipolar plates with the channel-rib structure [2].
In order to optimize the structure and operating conditions of OPCF for use in proton exchange membrane fuel cells, we performed coupled numerical and experimental studies. The numerical studies used a 2-D model that solved mass and momentum conservation and species transport, and coupled them to an electrochemical model in the catalyst layers for the source terms. Parametric studies indicated that some OPCF’s properties like thickness, porosity and pore diameter highly impact the global performance of the cell, and that the optimal properties could considerably be influenced by current density, humidity, and cell dimensions.
The results allowed for the design of optimized OPCF in different operating conditions, which were tested in fuel cells. It was observed that the optimized OPCF were very efficient in enhancing the performance of the cells at high currents, where mass transport limitations are generally observed. Promising solutions for heat and water management were explored.
[1] Ahrae Jo and Hyunchul Ju, Numerical study on applicability of metal foam as flow distributor in polymer electrolyte fuel cells. Int. J. Hydrogen Energy 43, 2018.
[2] Fredy Nandjou et al., Impact of heat and water management on proton exchange membrane fuel cells degradation in automotive application, J. Power Sources 326, 2016.