bluuebunny/crossref_metadata_2025 huggingface dataset:
Multimodal Microscopy Characterisation of 2D Materials
Authors: Flack Angela
Published: February 2024

Spatial characterisation of transition metal dichalcogenides (TMDCs) is crucial for understanding their optoelectronic properties and optimising film growth conditions. In this work, we present a multimodal microscopy platform combining conventional widefield, Raman, photoluminescence and second harmonic generation imaging for the characterisation of 2D materials.

The platform was used to characterise CVD grown WSe2 crystals. Raman imaging identified the presence of both monolayer and multilayer WSe2 through a change in the intensity and position of the E12g / A1g phonon modes. Photoluminescence imaging confirmed the presence of monolayer WSe2 with emission at 780 nm and identified two distinct multilayer regions through changes in the photoluminescence wavelength and intensity. Lastly, second harmonic generation imaging under femtosecond laser excitation was used to obtain the relative growth orientation of the three identified domains in the crystal.

Figure 1 highlights Raman imaging of WSe2. (a) Intensity of the E12g / A1g (250 cm-1) Raman band, (b) Peak position of the E12g / A1g (250 cm-1) Raman band, (c) least squares spectral matching revealing three distinct Raman spectral areas, (d) Averaged Raman spectra from areas A, B and C.

Figure 1
3-Terminal Igzo FET Based 2T0C DRAM Combined Bit-Line Structure
Authors: Kim Ji-Hun , Kim Hyeon-Jun , Kim Ki-Jun , Shim Tae-Hun , Hong Jin-Pyo , Park Jea-gun
Published: February 2024

The IGZO (InGaZnO)-based two-transistor zero-capacitor(2T0C) DRAM has attracted much attention as an alternative memory to overcome the scale-down limit of current 1T1C DRAM due to its low power consumption and monolithic 3D stacking capability. In particular, its low power consumption and the feasibility of low temperature process make it highly implementable for 3D DRAM. For operating the 2T0C DRAM, four metal lines are necessary, i.e., write word line and write bit line (WBL) for operating write transistor (WTR), and read word line and read bit line (RBL) for operating read transistor (RTR).

In this study, we propose a 3-terminal 2T0C DRAM to enhance its memory characteristics, where RBL was combined with WBL, enabling 2T0C memory operation with only three metal lines as shown in Fig. 1(b). In addition, we investigated the dependency of retenetion time of the proposed 2T0C DRAM on channel length. In particular we evaluated the dependency of the sneak current of 2T0C DRAM on off-current of transistor. For the fabrication of 3-terminal IGZO 2T0C DRAM, a top gate IGZO transistor was fabricated using RF sputtering and two transistors were connected by storage node and bit line bridge as shown in Fig. 1(d).The storage node was formed by connecting the gate of RTR to drain of WTR. And source electrodes of both transistors were coupled. To determine the voltage values for memory operation, the transfer curve of each transistor was characterized and the threshold voltages for both RTR and WTR were around 1 V. The write and read voltages were set to 3 and 2 V, respectively. The retention time was measured by performing periodic read operations after writing and measuring the RTR currents. In addition, the retention time of transistors for varying channel length and gate capacitance was characterized. The retention time was defined as the time to be taken for decreasing the currents by 90 %. Furthermore, the disturbance of 2T0C DRAM with 2x2 array structure was investigated. The RTR currents of selected cells were measured depending on the memory state of adjacent cells.

It was found that the retention times of transistors with 10- and 100-um RTR channel lengths were approximately 15 and 30 seconds, respectively. It was observed that the memory state was not changed from “0“ to “1“ by disturbant currents, but, RTR currents of “0“ state cell increased slightly when an adjacent cell was turned on. However, as shown in Fig.1(i) and (j), when off-currents of transistor of 2T0C DRAM decreased from 20 nA to 200 pA, it was confirmed the sneak current of 2T0C decreased. Finally, several technical apporaches how to suppress the disturbant currents will be presented in detail.

Acknowledgement

This research was supported by BrainKorea21 Four.

Figure 1
Mixed Oxide High-K Dielectrics - from Gate Dielectrics to Non-Volatile Memories to SSI-LEDs
Authors: Kuo Yue
Published: February 2024

Metal oxides were first used as the high-k gate dielectrics in thin film transistors (TFTs). In late 1990s, they were applied to MOSFETs to solve the high leakage current problem in the ultra-thin SiO2 gate dielectric. HfO2 was identified as a viable solution for this application due to its relatively large effective k value and low leakage current. However, HfO2 is subject to crystallization at a relative low temperature of about 600ºC. We introduced the doping method, i.e., by adding a third element to the metal oxide, to solve this problem. The crystallization temperature, the effective k value, the interface state density, and the interface layer thickness of the new film were influenced by the concentration of the added element (1). Eventually, we demonstrated that the 12% Zr doped HfO2, i.e., ZrHfO, had a crystallization temperature higher than 900ºC and many good dielectric properties. The ZrHfO thin film can be sputter deposited.

In this talk, structures, preparation methods, operation principles, and characteristics of the following devices will be discussed and new developments will be addressed.

< 1 nm EOT ZrHfO gate dielectric layers (2),

nanocrystals embedded ZrHfO NVMs in which electrons and holes could be stored separately based on the type of nanocrystal material (3), and

solid state incandescent LEDs (SSI-LEDs) which are formed from the thermal excitation of the simultaneous formation of a large number of nano-resistors during the dielectric breakdown process (4,5,6).

J.-Y. Tewg, Y. Kuo, and J. Lu, Electrochem. Solid-State Letters, 8(1), G27-G29 (2005).

Yan, Y. Kuo, and J. Lu, Electrochem. Solid-State Letts., 10(7), H199-H202 (2007).

Zhang and Y. Kuo, ECS Trans., 66(4), 195-202 (2015).

Y. Kuo and S. Zhang, ECS J. Solid State Sci. Technol., 8(12), Q223-Q225 (2019).

W.-S. Lin and Y. Kuo, ECS J. Solid State Sci. Technol., 10, 126001 (2021).

A. Samuel and Y. Kuo, Solid State Electronics, 204, 108651 (2023).
Fabrication and Electrical Characterization of GaAs/GaN Junctions
Authors: Ishimi Shota , Hirose Makoto , Shimizu Yasuo , Ohno Yutaka , Nagai Yasuyoshi , Liang Jianbo , Shigekawa Naoteru
Published: February 2024

GaAs/GaN heterojunctions are likely to be an ideal platform for high-frequency/high-output-power electron devices because of excellent electrical properties of GaAs and GaN as well as the similarity in their coefficients of thermal expansion. We have to note, however, that heteroepitaxial growth of GaAs/GaN junctions is quite difficult because of difference in crystal structures (in case of GaAs/wurtzite GaN junctions) and large mismatch in lattice constants (GaAs/zincblende GaN junctions). In this work we fabricate p+-GaAs/n-GaN heterojunctions by bonding GaAs and GaN epi wafers to each other using surface-activated-bonding technologies [1,2], selectively etching off the GaAs substrate used in the epitaxial growth, making ohmic contacts on the exposed surface of GaAs layer, and forming GaAs mesas. Ohmic contacts on the backside of GaN epi wafer are formed before bonding. The junctions are annealed at 400 ℃ in contact formation on GaAs surfaces. The temperature is much lower in comparison with the temperature in a wafer-fusion process of GaAs and GaN (750 ℃), which were reportedly applied for fabricating group-III arsenide/nitride heterojunction bipolar transistors [3]. Observation using cross sectional transmission electron microscope (TEM) of as-bonded and 400-℃ annealed GaAs/GaN interfaces shows that no voids are formed at the GaN/GaAs interfaces by the 400-℃ annealing. We also fabricate n+-GaAs/n-GaN heterojunctions for comparison.

We measure the capacitance-voltage characteristics and current-voltage characteristics for reverse-bias voltages of the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions and find that the characteristics of the two junctions are close to each other, which suggests that the band profiles of GaN layers in the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions are almost the same, i.e., the Fermi-level pinning occurs at the GaAs/GaN interfaces. More importantly, the reverse-bias characteristics are measured up to -60 V. The reverse-bias voltage corresponds to an electric field of as high as ~2 MV/cm, which is comparable to the breakdown field of GaN (3-4 MV/cm) [4]. We also excite minority electrons in the p+-GaAs layer using a 488-nm Ar laser and successfully observe the photo current due to the transport of minority electrons across the reverse-biased GaAs/GaN interfaces.

Acknowledgement: TEM samples were fabricated under the Inter-University Cooperative Research in IMR of Tohoku University. Epi wafers used in the work were provided from Sciocs (present Sumitomo Chemical) Co., Ltd.

[1] H. Takagi, et al. Appl. Phys. Lett. 68, 2222 (1996). [2] S. Yamajo, et al., Jpn. J. Appl. Phys. 57, 02BE02 (2018). [3] C. Lian, et al. Appl. Phys. Lett. 91, 063502 (2007). [4] T. Maeda, et al., IEEE Electron Device Lett., 43, 96 (2022).
Surface Activated Si-Si Wafer Bonding Using Different Ion Species
Authors: Danner Matthias , Rebhan Bernhard , Kerepesi Péter , Werner Wolfgang S. M.
Published: February 2024

Surface activated bonding (SAB) on wafer level is enabled by the EVG ComBond® system for irradiation with different ion species. In this process, the native oxide of 200 mm Si wafers is sputter-removed, and an amorphous layer is generated. After ion treatment, the wafers are bonded in ultra-high vacuum (UHV) at room temperature. The impact of Ar, Kr and Xe ion irradiation on the amorphous layer thickness (ALT) was investigated with the goal of optimizing the bonding process. This was i.a. motivated by the fact, that the presence of an amorphous layer reduces the electrical conductivity across the wafer interface [1].

The ALT of unbonded wafers was evaluated via spectroscopic ellipsometry (SE) for varying ion species, ion energies and angles of incidence. The results show that heavier ions generate a thinner amorphous layer than light ions (see Fig. 1). This is most likely due to the higher scattering cross section of heavy ions resulting in a lower penetration depth. For each ion type there is a minimum in the ALT as a function of the ion energy which can be attributed to sputter-implantation dynamics. Furthermore, the ALT is decreasing with the angle of incidence as the incoming ions are projected into the material.

Transmission electron microscopy (TEM) measurements were conducted on bonded wafer pairs and confirm the trend of heavy ions generating a thinner amorphous layer. Moreover, the TEM measurements show that post-bond annealing at 450°C for 5 h reduces the ALT due to recrystallization near the bonding interface.

To ensure mechanical stability for further wafer processing and for the final device, bonding energy measurements were conducted via the Maszara method [2]. Although maximum bonding energy can be achieved for specific parameters with each ion species, Xe irradiated samples tend to have a higher overall bonding energy.

[1] P.A. Stolk et al., “Contribution of defects to electronic, structural, and thermodynamic properties of amorphous silicon”, in Journal of Applied Physics, vol. 75, no. 11, pp. 7266-7286 (1994).

[2] W.P. Maszara et al., “Bonding of silicon wafers for silicon-on-insulator”, in Journal of Applied Physics, vol. 64, no. 10, pp. 4943-4950 (1988).

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(Invited) Deformation of 2D Semiconductors By Interfaces with Organic Materials
Authors: Kiriya Daisuke
Published: February 2024

2D semiconducting materials gather attention for realizing atomically thin optoelectronic devices and a platform for emerging quantum phenomena. Especially monolayer 2D materials are fascinating because of their ultrathin crystalline structure with various physicochemical characteristics such as direct-band gap and asymmetric crystalline structures. In this presentation, I will show our recent results related to the deformation of transition metal chalcogenides (TMDCs) morphology by interfaces with organic materials.

The mechanical exfoliation method is a simple method to prepare TMDCs and has been widely applied in many research studies. However, the exfoliated TMDCs on substrates show a bunch of bulk flakes and very few target monolayers. In the worst case, the exfoliated monolayer is surrounded by bulk flakes and is difficult to apply for fabricating devices. We developed a method of selectively removing bulk flakes, furthermore, isolating monolayer TMDCs on substrates. The method uses ultrasonication in organic solvents with a consideration of surface chemistry. Surprisingly, bulk flakes are selectively removed, and the adjacent monolayers can be effectively isolated on substrates. [1]

Monolayer TMDCs show emission properties because of their direct-band gap nature. The emission strength, e.g., photoluminescence (PL) intensity, is often low. The limitation factor includes various parameters, for example, the over-carrier concentration is one limiting factor in sulfur-based compounds (MoS2 and WS2). So far, p-type doping to reduce the electron concentration is demonstrated to enhance their PL intensity, however, the method is not applicable to selenium-based compounds, such as MoSe2 and WSe2. In addition, a universal method to enhance PL intensity for the selenium-based compounds is still few. We found a general PL enhancement method for both sulfur- and selenium-based compounds, MoS2, WS2, MoSe2, and WSe2, by applying a hydrocarbon, paraffin. The method simply uses paraffin on the surface of TMDCs. I would show the details of the results and methods in the presentation. [2]

REF:

[1] T. Nakamoto, et al., " Selective isolation of mono to quad layered 2D materials via sonication-based solution engineering", ChemRxiv, 2022, DOI: 10.26434/chemrxiv-2022-lgp8l

[2] T. Nakahara, et al., " Spontaneous crystal fluctuation in hydrocarbon polymer–coated monolayer MoS2, MoSe2, WS2, and WSe2 with strong photoluminescence enhancement", ChemRxiv, 2023, DOI: 10.26434/chemrxiv-2023-gxzqw
(Invited) Manipulating Light Emission from Rare Earth Doped Nanoparticles for Applications in Theranostics
Authors: Vetrone Fiorenzo
Published: February 2024

Luminescent nanomaterials that can be excited, as well as emit, in the near-infrared (NIR) have been investigated for use in a plethora of applications including nanomedicine, nanoelectronics, biosensing, bioimaging, photovoltaics, photocatalysis, etc. The use of NIR light for excitation mitigates some of the drawbacks associated with high-energy (UV or blue) excitation, for example, little to no background autofluorescence from the specimen under investigation as well as no incurred photodamage. Moreover, one of the biggest limitations is of course, that of penetration. As such, NIR light can penetrate tissues much better than high-energy light especially when these wavelengths lie within the three biological windows (BW-I: 700-950, BW-II: 1000-1350, BW-III: 1550-1870 nm) where tissues are optically transparent. At the forefront of NIR excited nanomaterials are rare earth doped nanoparticles, which due to their 4f electronic energy states can undergo conventional (Stokes) luminescence and emit in the three NIR biological windows. However, unlike other classes of nanoparticles, they can also undergo a multiphoton process (known as upconversion) where the NIR excitation light is converted to higher energies resulting in anti-Stokes luminescence spanning the UV-visible-NIR regions. Perhaps the biggest impact of such materials would be in the field of disease diagnostics and therapeutics, now commonly referred to as theranostics. Due to the versatility of their optical properties, it now becomes possible to generate high-energy light (UV or blue) in situ to trigger other light activated therapeutic modalities (i.e. drug release) while using the NIR emission for diagnostics (i.e. bioimaging, nanothermometry). Here, we present the synthesis of various NIR excited (and emitting) rare earth doped core/shell (and multishell) nanoparticles and demonstrate how their luminescence properties can be exploited for potential use in diverse biomedical applications.
(Invited) Anti-Ambipolar Phototransistors Based on Mixed-Dimensional Heterojunctions
Authors: Ho Johnny C
Published: February 2024

The incapability of modulating the photoresponse of the assembled heterostructure devices has remained a challenge for developing optoelectronics with multi-functionality. In this presentation, we report a gate-tunable and anti-ambipolar phototransistor based on 1D GaAsSb nanowire/2D MoS2 nanoflake mixed-dimensional van der Waals (vdW) heterojunctions. The resulting heterojunction shows asymmetric control over the anti-ambipolar transfer characteristics, possessing the potential to implement electronic functions in logic circuits. Meanwhile, the anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a remarkable on/off current ratio of 4×104, fast response of 50 μs and high detectivity of 1.64×1011 Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is also explored in the heterojunction. Finally, a visible image sensor based on this hybrid device is demonstrated with excellent imaging capability, suggesting promising applications in future optoelectronic systems.
(Poster Award - 1st Place) Emerging Effect of Vertical-Oriented WSe2 Interfacial Layers in the CIGSe Thin Film Solar Cells
Authors: Chung Chia-Chen , Liu Jun-Nan , Yang Tzu-Yi , Lai Chih-Huang , Chueh Yu-Lun
Published: February 2024

Copper indium gallium selenide (CIGSe) thin film solar cells become popular due to their potential as a solution for green energy and space technology recently. These solar cells have a similar electrical performance to traditional silicon solar cells, making them ideal for light, flexible, and high-performance applications. Molybdenum is a popular material for the rear electrode due to its conductivity and low cost. During the selenizing process of the absorber layer, MoSe2 will be formed between Mo and CIGSe layer, which improves adhesion between Mo and CIGSe and facilitates carrier transport from the absorber layer to the rear electrode. Therefore, previous studies show that oxides or nitride-based materials, such as Titanium Nitride and Alumina, are used as the passivation layer at the back contact to address the trade-off of the critical thickness of the MoSe2 layer. That is, designing a nanostructure and creating a contact hole can attract carriers and avoid current blocking. Similarly, Oxide-based material interfacial layers, such as Molybdenum oxide and tungsten oxide, can also play a role in the obstacle of selenium diffusion and modify the band structure. In this work, a few layers of WSe2 interfacial layer were introduced between the CIGSe absorber layer and Mo electrode. First, 20 nm WOx was deposited on Mo by E-beam evaporation. Second, WOx was selenized by a vertical furnace with ICP coils plasma system to form WSe2. By tuning the substrate temperature of plasma-enhanced selenization process, the full vertically lamellar-oriented WSe2 was formed on Mo. The results show that inserting a few layers of WSe2 improved the efficiency of the CIGS solar cell from 12.5 to 14.3%, with a significant increase in short circuit current density up to 38mA/cm2. This improvement was attributed to the improved carrier transportation through the backside interface, facilitated by the specific vertically lamellar-orientated WSe2 and the induced Gallium gradient measured by TEM, SIMS, and XPS. The remarkable enhancement by WSe2 provides the potential for further applications.

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Quantifying the Thermochemistry of Kinetic Transitions in a Solid-State Charged Li/Llzo/LCO Cell Using Differential Scanning Calorimetry
Authors: Bhargava Bhuvsmita , Johnson Nathan B , Zaman Samaa , Albertus Paul
Published: February 2024

It is a general understanding that removal of combustible organic solvent with the introduction of solid-state electrolytes would make a battery safer. However, safety in solid state batteries is multifaceted and involves analysis of self-heating rates, total generated energy, thermal runaway onset temperature, maximum temperature, and thermal runaway prevention mechanisms1,2. The use of reactive lithium metal as the anode, and cathode material such as LiCoO2 that releases O2 when heated in its delithiated state, results in conditions in which the exothermic reaction 4Li+O2→2Li2O may occur if these reactants come into contact. We measure heat flow data using a differential scanning calorimeter (DSC) and hermetically sealed DSC pans to establish a reaction thermochemistry responsible for self-heating of LixCoO2|LLZO|Li metal cell, with x=0.43 (which corresponds to 4.3V vs. Li). While the main exothermic reaction is 4Li+O2→2Li2O when only Li0.43CoO2 is present in the cathode3, a practical solid-state cathode will almost certainly contain conductive carbon and a fluorinated binder. We carry out careful DSC analysis with a range of components and ratios of components to quantify the expected reaction pathway, and amount of heat released, for a cathode that contains Li0.43CoO2, carbon additive, and PVDF binder. We find that the carbon and PVDF strongly influence the reaction pathway upon heating to 500°C in our DSC experiments. Hence, the presence of even small weight fraction components in a solid-state cathode should be considered when evaluating their thermochemistry upon heating.

References:

(1) Bates, A. M.; Preger, Y.; Torres-Castro, L.; Harrison, K. L.; Harris, S. J.; Hewson, J. Are Solid-State Batteries Safer than Lithium-Ion Batteries? Joule
2022, 6 (4), 742–755. https://doi.org/10.1016/j.joule.2022.02.007.

(2) Johnson, N.; Albertus, P. Modeling Thermal Behavior and Safety of Large Format All-Solid-State Lithium Metal Batteries under Thermal Ramp and Short Circuit Conditions. J. Electrochem. Soc.
2022, 169 (6), 060546. https://doi.org/10.1149/1945-7111/ac79cf.

(3) Inoue, T.; Mukai, K. Are All-Solid-State Lithium-Ion Batteries Really Safe?–Verification by Differential Scanning Calorimetry with an All-Inclusive Microcell. ACS Appl. Mater. Interfaces
2017, 9 (2), 1507–1515. https://doi.org/10.1021/acsami.6b13224.
(Invited) Culprits or Collateral Damage? The Science behind Lithium-Ion Battery Failure Investigations
Authors: Spray Ryan , Barry Michael , Hayes Troy
Published: February 2024

Thermal runaway failures of lithium-ion batteries have garnered a great deal of attention in recent years, including notable failures in the spaces of consumer products, electric vehicles, and energy storage systems. The growing public awareness of hazards associated with thermal runaway in lithium-ion batteries has coincided with increased scrutiny of lithium-ion batteries when they are found at fire scenes. Lithium-ion batteries can go into thermal runaway due to heating from an external fire, even if they are not the cause of the fire. As always, the fire investigator and other experts involved in the failure analysis need to remain guided by the scientific method when performing an origin and cause analysis, including when that analysis involves lithium-ion batteries.

This presentation will encompass many of the tools and reasoning that may be relied upon when analyzing the role lithium-ion batteries at fire scenes, as well as in the post-scene analysis of batteries and battery-containing devices. Techniques for understanding heat release and heat release rate during thermal runaway failures will be discussed, as well as the associated literature that may be relied upon to estimate heat release based on chemistry and capacity of the cells in question. Also to be discussed are metallurgical approaches and techniques that may inform the conditions that likely existed during the failure of the battery.
Macroscale Inhomogeneity of Electrochemical Lithium-Metal Plating Triggered by Gas Phase Evolution
Authors: Kim Kyoungoh , Ko Youngmin , Kang Kisuk
Published: February 2024

Promoting homogenous electrode reactions is essential to suppressing electrode degradation and thus realizing stable energy storage systems. In particular, lithium-metal anodes that undergo drastic morphological change during charging/discharging require strict homogeneity control, more so than conventional intercalation electrodes. Previous studies have mainly focused on the micrometer-scale homogeneity of lithium plating and its control by tailoring the electrolyte chemistry and following the solid-electrolyte interphase formation process. However, the larger-scale, electrode-level homogeneity issue recently emerging for conventional electrodes has received little attention. Here, for the first time, we observe clear electrode-level inhomogeneity during lithium plating that is independent from the micro-scale lithium dendritic growth. Ex-situ and in-situ experiments clearly reveal that this electrode-level inhomogeneity is triggered by gas-generating reactions and the resulting gas-phase (i.e., bubble) evolution that causes localized lithium growth. Experiments using Li|Cu half cells demonstrate that this macroscale inhomogeneity is accompanied by an overpotential increase mainly attributed to limited lithium-ion mass transport through bubbles, as revealed by continuum modeling and simulation. Full-cell experiments using lithium iron phosphate demonstrate that this bubble-inducing macroscale inhomogeneity significantly contributes to decay of the cell capacity and energy density. Our observation of this larger-scale lithium plating inhomogeneity suggests that homogeneity control and its implications for lithium-metal cell performance should be approached from a multi-scale perspective.
Deconvoluting the Impact of Early-Life Abuse Conditions on the Cyclic Degradation of Lithium-Ion Cells
Authors: Leonhardt Robert , Tichter Tim , Scharpmann Luca , Schmidt Anita , Krug von Nidda Jonas
Published: February 2024

The successful repurposing of degraded lithium-ion (Li-ion) batteries in second-life applications is a vital step towards achieving a circular economy. While reusing aged Li-cells is a promising way of mitigating their overall environmental footprint, it is crucial to anticipate their future safety and performance characteristics. Unfortunately, predicting these properties is a cumbersome task, essentially caused by limited knowledge of the interference of different degradation modes in the cells’ first life.

To still enable estimating of these parameters from a current state, the present study systematically investigates the impact of abusive conditions in an early phase of the cells’ life on their subsequent degradation behavior. For this purpose, individual Li-cells are initially stressed by different measures such as overcharging, deep-discharging, plating, and deliberate combinations of the aforementioned methods. Electrochemical performance indicators are monitored during subsequent cycling of the cells which provides insights into the interdependencies of different degradation modes induced by specific stress conditions. In this manner, it is clarified whether or not the total degradation can be determined by a convolution or a superposition of individual deterioration effects and, thus, be described as a multidimensional state function.

This knowledge will finally contribute to a better understanding of the performance and safety behavior of degraded Li-ion batteries which can help to successfully implement them into second-life applications.
Synchrotron X-Ray Topography Studies for Defect Formation at the Early Stage of PVT-Grown 4H-SiC Crystals
Authors: Hu Shanshan , Liu Yafei , Cheng Qianyu , Chen Zeyu , Raghothamachar Balaji , Dudley Michael
Published: February 2024

Silicon Carbide (SiC), typically 4H-SiC, is replacing conventional silicon materials for high power and high frequency applications due to its excellent electrical and thermal properties [1]. However, high quality SiC wafers with low defect density are still in urgent need for automotive and energy saving applications. SiC wafers are obtained from bulk growth by physical vapor transport (PVT) method and considerable efforts have been expended to optimize growth process to lower defect densities. One major approach is the study of initial stages of crystal growth as most defects are nucleated at this stage and propagate into the boule. Ailihumaer et al [2] demonstrated the behaviors of threading edge dislocations (TEDs), threading screw dislocations (TSDs)/threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) across the seed/newly grown layer interface in PVT-grown 4H-SiC. Sanchez et al [3] revealed the nucleation of TEDs and TSDs at the initial growth stage in PVT-grown SiC.

However, more detailed investigations are still in demand for PVT grown 4H-SiC crystals during initial stage of growth. This study investigates 4° off-axis 4H-SiC wafers with several hundred microns of initial-stage growth PVT method. Synchrotron X-ray topography (SXRT) technique is employed and shows dramatically modified defect distribution across the seed/newly grown layer interface (Figure 1). The formation of Shockley stacking fault starting at initial stage of crystal growth is suggested as partial dislocations running along <11-20> directions on basal plane are observed to form rhombus shapes (Figure 2). Prismatic slip is observed to take place near the wafer edges and prismatic dislocations undergo cross slip to form unique dislocation configurations. A statistical analysis on defect changes across the seed/newly grown layer is being carried out and the results will be reported. These results will be complemented by observations from Nomarski optical microscopy (NOM) and Raman Spectroscopy studies to propose the formation mechanism of defects across the seed/newly grown layers.

References:

G. Dhanaraj, K. Byrappa, V. Prasad and M. Dudley, Springer handbook of crystal growth, edn. (Springer, 2010)

T. Ailihumaer, H. Peng, Y. Liu, B. Raghothamachar, M. Dudley, G. Chung, I. Manning and E. Sanchez, J. Electron. Mater., 50, (2021).

E. Sanchez, J. Liu, M. De Graef, M. Skowronski, W. Vetter and M. Dudley, J. Appl. Phys., 91, (2002).

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(Invited) Challenges and Opportunities for the Computational Analysis of Electrochemical Energy Systems
Authors: Secanell Marc
Published: February 2024

Electrochemical systems are challenging to analyze and design due to their multi-dimensional, time-dependent, and multi-physics nature. For this reason, many computational electrochemical system models have been developed in the past three decades, some of which can now be found in commercial software packages, e.g., COMSOL or ANSYS Fluent. These models however: i) are based on simplifying assumptions, e.g., reduced dimensionality, infinite dilution, Tafel kinetics; ii) contain uncertain input parameters, e.g., effective transport properties and kinetic parameters; and, iii) are only validated with a limited number of experiments, e.g., polarization curves. These limitations, understandable at the time due to limited computational resources, characterization techniques, and material understanding and stability, are difficult to justify today that parallel programming and computer clusters enable scientists to perform large multi-dimensional simulations; electrochemical testing has expanded to include segmented cell testing, impedance spectroscopy, water flux estimation, and in-operando visualization; and, characterization tools and techniques have been developed to, for example, easily measure adsorption isotherms, effective proton conductivity and, within a given resolution, visualize the three-dimensional microstructure of the electrodes.

Scientists working on computational analysis of electrochemical systems must acknowledge that further model development is still needed and must take advantage of new resources to improve the robustness and accuracy of cell-level models. The path is long and crooked, but it is very likely that the opportunity for a truly useful computational model, one that can be used for design, is beyond simplistic implementations and polarization curves. The effort is great, but it can be minimized by concurrent software development, as well as, by careful experimental work dedicated to parameter estimation and model validation.

This presentation aims at outlining the limitations of state-of-the-art models, the challenges and opportunities of concurrent development and maintenance of a multi-scale, transient electrochemical energy system software, such as the open-source fuel cell simulation toolbox (OpenFCST) [1], and the new opportunities provided by combining advanced characterization tools with computational analysis. As an example, the development and validation of the transient, two-phase fuel cell and electrolyzer cell-level models in OpenFCST will be discussed [2]. The use of a hydrogen-pump model to study the effect of an active catalyst in CL effective proton conductivity measurements will also be discussed as an example of concurrent experimental/model design [3].

References

[1] M. Secanell et al., ECS Transactions 64.3 (2014) (10.1149/06403.0655ecst)

[2] M. Moore et al, Journal of the Electrochemical Society (2023) (10.1149/1945-7111/acc898)

[3] M. Mandal et al, ACS Applied Materials & Interfaces, 12 (44), 49549-49562 (2020) (10.1021/acsami.0c12111)
Numerical Reconstruction of Proton Exchange Membrane Fuel Cell Gas Diffusion Layers
Authors: Yang Danan , Garg Himani , Beale Steven B. , Andersson Martin
Published: February 2024

Flooding and dehydration reduce stability and power performance in Proton Exchange Membrane Fuel Cells (PEMFCs). The Gas Diffusion Layer (GDL) plays a crucial role in facilitating reactant gas transport and removing product water from the electrode. To suit various PEMFCs, GDLs with different shapes have been commercialized. The impact of the GDL structure on the surface-tension-driven water transport behavior remains poorly understood. However, this is one important aspect that can be controlled by proper design. In this study, the GDL performance is investigated by comparing curved and straight carbon fibers within the region. Specifically, an image-processing method extracts porosity, domain size, and fiber diameter from an experimental image-based GDL reconstruction. These parameters are utilized by in-house developed computer codes to stochastically reconstruct curved and straight carbon fiber GDLs, respectively. The real and reconstructed GDLs are compared in terms of pore size distribution, tortuosity, and permeability. Liquid transport in these GDLs and corresponding gas channels is simulated using a volume of fluid method in OpenFOAM 7.0.

Figure 1(a) presents the T-shaped simulation domain and top view of three GDLs. Figure 1(b) displays the Cumulative Density Function (CDF) of the pore size distribution for the three GDLs, revealing that the main difference between the three GDLs lies in the pore diameter range of 10-30 µm. Upon completion of the research program, we aim to identify the influence of fiber shape on the GDL transport properties as well as the water behavior inside them.

Figure 1
Numerical Simulation of the Two-Phase Flow Dynamics in an Alkaline Water Electrolyzer Considering Difference in Bubble Diameter
Authors: Kanemoto Ryo , Araki Takuto , Mitsushima Shigenori
Published: February 2024

For more efficient hydrogen production, predicting the characteristics of bubble flow inside a water electrolyzer is critical since bubbles affect ion transport, and electrode coverage by bubbles influence overpotentials and the entire electrolysis performance. Transport of bubble significantly depends on its size. In this study, a CFD model of an alkaline water electrolyzer which considers difference in sizes of generated bubbles was developed. The model treats bubbles of different sizes as different gas phases, which are independent of each other. The total bubble mass source for a given current density is divided into phases of each bubble size according to experimental visualizations. Drag, virtual mass, lift, wall lubrication, and turbulent dispersion forces are considered as interaction between gas and liquid phase.

The following three sizes of bubbles were considered: diameters d = 36 μm (small), 75 μm (medium), and 208 μm (large). The entire calculation domain represents the anode side of a zero-gap monopolar alkaline water electrolyzer. Figure 1 shows the volume fraction fields of the three gas phases (i.e., bubble sizes) at a constant current density of 0.6 A/cm2. The bubbles are generated on the electrode surface at the z=0 plane. As the liquid phase forms a circulating flow inside the electrolyzer, small bubbles are transported along the circulating downstream flow due to smaller buoyancy force compared to their viscous drag. In contrast, large bubbles flow out directly to the upper outlet because buoyancy force dominates. This result points out the need to consider the difference in bubble diameter for designing flow field inside alkaline water electrolyzer.

[Acknowledgements]

This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Fig. 1. Total gas phase volume fraction field obtained from the sum of the volume fraction field for each size bubble, and its experimental visualization.

Figure 1
Gas Diffusion Electrode for Oxygen Evolution Reaction Catalyst Testing
Authors: Puthiyapura Vinod Kumar , Zalitis Christopher Mark , Stevens James
Published: February 2024

The oxygen evolution reaction (OER) is one of the major contributors of efficiency loss in water electrolysis and consequently, development of OER catalysts to improve the electrolyser efficiency is a major reasearch theme in the field. Though the current commercial PEMWE may operate at anode potential <1.60 V, futue PEMWE may operate at potential higher than this as the drive to operate PEMWE at high current density is inceasing. Inorder to achieve this, an active and stable catalyst is required that can operate at this regime. Also, the potential experienced by the anode during the startup/shutdown of an electrolyser are different to the steady state value.

Conventional OER testing involves catalysts coated on a conductive substrate submerged in an electrolyte solution, in a three electrode cell. However, such techniques are generally limited to low current densities due to oxygen bubble formation and site blocking at high current density limiting the system to study the OER kinetics below realistic operating current density. Although Rotating Disk Electrode(RDE) is widely employed to mitigate this, the RDE system is still not effective enough to remove the bubbles1.

A floating electrode system developed by Kucernak et al2 shows that combining direct access with a lower catalyst loading improves the O2 gas mass transport and a higher current density could be achieved for the ORR. This technique also used for OER by Arenz et al.3, for easier bubble removal. Combining these two above mentioned systems, we have developed a new GDE cell system which allows screening of OER catalyst at industrially relevant current densities. This cell allows to study the OER kinetics at very realistic voltage/current regime and the information obtained helps to develop more active/stable catalyst.

The OER catalyst diagnostics test from our GDE cell was comparable to the standard three electrode cell with an additional advantage of extended potential window upto 1.80 V vs.RHE. Preliminary results obtained from our study shows a promising opportunity to study the OER at high current densities.

Reference

FathiTovini, A.Hartig-Weiß, H.A.Gasteigerand, H.A.El Sayed, Journal of The ElectrochemicalSociety, 2021, 168, 014512.

M.Zalitis,D.KramerandA.R.Kucernak,Phys.Chem.Chem.Phys.,2013,15,4329-4340

Schröder,V.A.Mints,A.Bornet,E.Berner,M.FathiTovini,J.Quinson,etal., JACS Au 2021,1(3),247-25
Using Magnetic Field Analysis for Localizing Defects of Fuel Cell & Electrolyzer Components
Authors: Rasuli Sistan , Witte-Bodnar Kerstin , Grunwald Erik , Naumann Volker , Porstmann Sebastian , Polster Stefan , Ilse Klemens
Published: February 2024

Production capacities of electrolysis and fuel cell technologies are expected to increase significantly in the upcoming years, reducing costs of stack components through numbering-up. To ensure reliability and durability and reduce costs along the manufacturing process, developing suitable in-line non-destructive diagnostic methods for quality control becomes essential for their industrialization.

In both fuel cell and electrolyzer stacks, the main components' electrical resistance and current density distribution greatly influence stack efficiency and longevity. Here, the flowing electric current generates a magnetic field depending on its strength and direction. Potential defects caused by the manufacturing process, including cracks, defective welding spots, coating adhesion issues, delamination, and contamination, influence the components' electric conductivity and current density distribution and hence the magnetic field distribution.

This work investigates non-destructive diagnostic methods using magnetic field analysis (MFA) to identify changes in the magnetic field distribution and, thus, local electrical resistance and current density distributions of fuel cell and electrolyzer components. The methods used are magnetic field imaging (MFI) and Eddy Current Testing (ECT). MFI analyzes the magnetic field distribution in all three spatial directions to trace electrical currents and identify electrical defects, creating a magnetic field signature that can potentially distinguish between functional and non-functional components along the manufacturing process. ECT measures variations in local conductivity by inducing eddy currents in the sample through an alternating magnetic field generated by a sender coil. These eddy currents create a detectable magnetic field in a receiver and are affected by defects and changes in electrical conductivity. Therefore, the method can characterize sheet resistance and material homogeneity through electrical anisotropy in electrically conductive components.

In initial experiments, ECT measurements on coated and uncoated hollow embossed stainless steel bipolar half plates showed systematic differences depending on quality characteristics resulting in unique patterns and inhomogeneities which require further investigation. Furthermore, experiments with both methods on other stack components will be presented and discussed. In addition, their in-line capability will be assessed and evaluated. However, these first promising results imply that MFA is potentially useful as an in-line, non-destructive diagnostic method for quality control.
Evaluating Ink Structure Using Ultrasonic Spray Coating for PEMFC MEA
Authors: Lee Seon-Ho , Woo Seunghee , Kang Yun Sik , Park Seokhee , Yim Sung-Dae
Published: February 2024

From the standpoint of improving manufacturing productivity and performance/durability of PEMFC MEAs, there is an increasing interest in ink. Ink research is centered on comprehending the interplay between the components of the ink, including catalysts, ionomers, and solvents, to control the ink structure and evaluate its influence on ink properties, catalyst layer microstructure, and fuel cell performance. As a facet of this ink research, the current study proposes ultrasonic spray coating as a methodology to indirectly evaluate the ink structure.

50 wt% Pt/C catalysts loaded on Ketjenblack (KB) and Vulcan carbon (VC) were separately incorporated to produce sprayable inks with a solid content of 4 wt% using an Aquivion ionomer (EW 720) and a mixture solvent composed of a 1:1 weight ratio of 1-propanol and water. The structural differences between the two catalyst inks were compared by quantifying the amount of adsorbed and free ionomers, as well as their rheological properties. Additionally, the advantages of ultrasonic spray coating, such as the formation of small droplets on the order of tens of micrometers and rapid drying, were utilized to coat each ink onto a silicon substrate. The structural characteristics of the resulting catalyst layers were compared through SEM images, as well as the distribution of constituent elements using EDS and Auger spectroscopy. The Pt/KB catalyst ink forms a gel-like structure due to its relatively high amount of adsorbed ionomer, resulting in a relatively uniform distribution of catalyst particles and ionomers in the catalyst layer formed from ultrasonic spray droplets. In contrast, the Pt/VC catalyst ink behaves as a liquid-like, mainly existing in the form of free ionomer, regardless of ionomer content. During the drying process of each droplet, they merge to form larger unit catalyst layers, which exhibit an uneven distribution where free ionomers tend to concentrate at the edges of the catalyst layer due to the coffee-ring effect. As a result, the catalyst and ionomers exist in a non-uniform distribution, which is observed to affect the performance and electrochemical characteristics of fuel cells. In this presentation, we will discuss in detail the series of characteristics of ink-catalyst layer-fuel cell performance that vary according to catalyst properties.