bluuebunny/crossref_metadata_2025 huggingface dataset:
(Keynote) Devices in CMOS for Terahertz Circuits and Systems
Authors: O Kenneth K. , Ahmad Zeshan , Choi Wooyeol , Sharma Navneet , Zhang Jing , Zhong Qian , Kim Dae-Yeon , Chen Zhiyu , Zhang Yaming , Han Ruonan , Shim Dongha , Sankaran Swaminathan , Seok Eun-Yong , Kshattry Sandeep , Cao Changhua , Mao Chuying , Schueler Robert , Medvedev Ivan , Lary David , Nam Hyun-Joo , Raskin Philip , De Lucia Frank , McMillan James P. , Neese Christopher , Kim Insoo , Momson Ibukunoluwa , Yelleswarapu Pavan , Dong Shenggang , Byreddy Pranith , Chen Zhe
Published: February 2020

CMOS (Complementary Metal Oxide Silicon) integrated circuits (IC’s) technology is emerging as a means for realization of capable and affordable systems that operate at 300GHz and higher. Despite the fact that the unity maximum available gain frequency, fmax of NMOS transistors has peaked at ~320GHz somewhere between 65 and 32-nm technology nodes, signal generation up to 1.3THz and coherent detection up to 410GHz and incoherent detection up to ~10THz have been demonstrated using CMOS integrated circuits. Furthermore, a highly integrated rotational spectroscopy transceiver operating up to near 300GHz and imaging array operating at 820GHz have been demonstrated in CMOS. The nonlinear devices and circuit techniques that enable the operation at these frequencies beyond fmax will be described. Lastly, emerging applications, electronic nose/smelling using rotational spectroscopy that can detect and quantify concentrations of a wide variety of gases, imaging that can enable operation of autonomous systems in a wide range of weather conditions (rain, dust, snow, fog .. ), high-bandwidth communication over 1-2m long dielectric waveguides that can rival the bandwidth of optical communication systems, and electronic detectors that can make thermal imaging/night vision affordable will be discussed.
Development of Advanced Cobalt CMP Slurry Platform By Electrochemical Screening and Polishing Studies
Authors: Theivanayagam Murali Ganth , Wang Hongyu , VanHanehem Matthew , Auger Robert
Published: February 2020

In advanced nodes at 10 nm and below, cobalt (Co) metal is under investigation to replace copper (Cu) in metal lines and vias in back end of line (BEOL) for the first few metal layers due to its better electromigration performance, lower resistivity, conformal coverage in high aspect ratio features, and overall improved device performance. These new integration schemes require Co chemical mechanical planarization (CMP) steps to achieve planarity and desired target thickness. However, Co metal is prone to corrosion in aqueous solutions and its lower redox potential makes it an easy galvanic corrosion target under CMP conditions. Commercial Cu bulk slurries are not compatible for Co polishing as they show significant corrosion defects such as pitting, rough surface, and missing lines after CMP. Advanced Co bulk CMP slurries, therefore, need to contain effective chelator, corrosion inhibitor, and additives at suitable slurry pH to eliminate corrosion defects, while still delivering high Co removal rates (2000 to 5000 Å/min).

In this presentation, we will discuss the systematic approach taken to develop a Co bulk CMP slurry formulation platform that can provide high removal rates and low/no corrosion, through development of fundamental understanding of Co surface chemistry, chelation, and corrosion properties. Results and key learnings from the extensive screening techniques such as Tafel polarization, galvanic corrosion, static-etch corrosion, quartz crystal microbalance, and X-ray photo electron spectroscopy analyses used to identify the ideal slurry pH, effective chelators, and corrosion inhibitors will be shared. In addition, Co blanket and pattern wafer polishing performances will be presented for the leading slurry formulations.
(Invited) Flexible Electronics for Low-Cost Applications: Process and Assembly Challenges
Authors: Bringans Ross
Published: February 2020

Flexible electronics promises to enable many novel applications in which the form factor and utility of the final device or product is no longer limited by the rigid nature of most electronic components. There are a number of challenges that need to be overcome for this technology to succeed. In this presentation, we will describe two key aspects: (i) process considerations for fabrication of flexible components and circuits and (ii) approaches to assembly of different components into the sub-systems that will make up the final device.
(Invited) Transition Metal Dichalcogenide Semiconductor Growth and Large Area Devices for Optoelectronics and Sensing
Authors: Garg Sourav , Mollah Abu Shahab , Waters Joseph L , Kim Seongsin M , Kung Patrick
Published: February 2020

Atomically-thin transition metal dichalcogenides (TMDCs), such as MoS2 and WS2, are topological materials with physical characteristics of great interest for the realization of nano-electronic and photonic devices, with promising applications for chemical and biological sensing. These semiconductors possess a bandgap that changes from indirect to direct as a result of the disappearance of inversion symmetry when the material is reduced into a sub-nanometer monolayer form (2D). Although TMDC monolayers were originally and are continuing to be widely realized through mechanical exfoliation, there are accelerating material synthesis efforts to achieve more controllably sized and shaped TMDC in order to facilitate the realization of devices with greater consistency.

In this work, we present the growth and characterization of continuous monolayer TMDC followed by the realization and measurement of both photoconductive and Schottky detectors, as well as chemical sensor devices. The materials were grown by chemical vapor deposition on sapphire substrates. The monolayers were subsequently characterized using Raman spectroscopy, photoluminescence, fluorescence imaging, optical absorption, atomic force microscopy, and electron microscopy. Large area metal-2D semiconductor-metal interdigitated devices were realized by conventional photolithography, with 20 μm wide and 4000 μm long metal fingers separated by a distance ranging from 5 to 50 μm.

The electrical and photoresponse characteristics of MoS2 based photoconductors were measured at room temperature in nitrogen ambient. The spectral response was measured in the 400-800 nm range and exhibited a rapid drop for wavelengths longer than 675 nm, corresponding to the bandgap of MoS2. The spectral response revealed clear A and B band-edge exciton related peaks, as well as a peak arising from the van Hove singularities. At a bias of 1 V, monolayer MoS2 devices exhibited a responsivity of 11 A/W corresponding to a specific detectivity of 8x1011 Jones. The high responsivity was associated with a large photoconductive gain arising from trap states/surface states in the large area MoS2 material and device. The resulting persistent photoconductivity and photoconductive time decay were subsequently studied. The use of MoS2 devices for chemical sensing was investigated. The changes in electrical resistance in the dark was monitored as a function of the concentration of the gas sensed in an inert nitrogen ambient. When tested for CO2, concentrations down to 200 ppm were able to be detected.
(Invited) Growth, Optical, and Mechanical Properties Investigations of AlN Nanowires and Nanohelices
Authors: Song Bo , Han Jiecai
Published: February 2020

The anisotropic nanostructures have demonstrated novel physical features and promising utility in nanoelectronics, nanophotonics, and electrochemical energy storage. Among them, aluminum nitride (AlN), is an attractive III-V group semiconductor because of its potential applications in UV devices. More structurally complex one-dimensional (1D) AlN nanostructures are thus great demand because the shapes of nanomaterials evidently affect their chemical and physical properties. By controlling the reaction parameters of physical vapor transport (PVT) process, we have performed a detailed investigation on the synthesis of AlN nanostructures. Moreover, we found that the growth of diverse AlN nanowires (NWs) and nanohelixes with different morphological features can be well explained by the helical growth model. Two ideal platforms, including individual AlN microhelix and AlN helices with complex-defect of (VNAli) syntheiszed by adjusting the reaction parameters, were utilized to investigate the mechanical and luminescence properties of AlN helices, respectively. We have utilized in situ tensile-bending test to investigate the mechanical properties of individual AlN helix. We show that the morphologies serves the key role in determining the mechanical featutres of the AlN nanostrictures. The detailed optical measurement exhibited a typical afterglow feature after UV irradiation in AlN helices with complex-defect of (VNAli).
(Invited) Cost-Effective Fabrication of Nanostructured Electrodes for High-Performance Flexible Supercapacitors
Authors: Fan Zhiyong , Lin Yuanjing
Published: February 2020

The blooming market of portable and wearable electronic devices has triggered research interest in energy storage devices that combine both high performance and attractive form factors. Planar supercapacitors are considered as favorable alternatives or complements to batteries, especially to serve as highly flexible, environmentally-friendly and high performance energy storage devices for portable and wearable electronics. In this work, a scalable and printable method was developed to fabricate flexible planar supercapacitors. By using a cost-effective inkjet printing technique, novel and unique hierarchical Nickel (Ni)@Manganese dioxide (MnO2) nanocoral structures were constructed as the interdigitated electrodes on flexible substrates. The as-fabricated flexible all-solid-state planar supercapcacitor achieved an areal capacitance of 52.9 mF cm-2 with enhanced specific power and energy, and such a performance parameter is among the highest in literature for the same category of devices. More interestingly, the inkjet printing technique realized pattern designs with excellent versatility and working supercapacitors with various artistically designed patterns are demonstrated here. Meanwhile, the high scalability of such printable method was also demonstrated by fabrication of large size “power papers”, which successfully served as energy storage devices in wearable self-powered system to power up light emitting and gas detection devices.
(Invited) All-Solid-State Batteries: Status and Perspectives
Authors: Kühnel Ruben-Simon , Remhof Arndt , Battaglia Corsin
Published: February 2020

All-solid-state lithium-ion batteries employing solid-state electrolytes promise higher operational safety, enhanced temperature stability, potentially higher cell voltages, and consequently higher energy density compared to traditional batteries with liquid electrolytes based on flammable organic solvents. We discuss recent advances and challenges on the path towards a competitive high-power all-solid-state battery technology.
(Invited) 2D Materials Synthesis: It’s All about the Substrate
Authors: Robinson Joshua A
Published: February 2020

To date, the majority of published works on electronic and optoelectronic properties of 2D materials has been from mechanically exfoliated flakes on silicon oxide. However, if we are to make 2D materials a viable material technology, we must be able to grow them over very large scale, with exceptional control over structure, chemistry, and doping. We have demonstrated that direct synthesis of a variety of 2D layers and heterostructures, and have found that the substrate upon which the materials are grown can ultimately dictate the properties and performance of the synthetic layers. In this talk, I will discuss recent breakthroughs in our understanding and engineering of the 2D/3D interface to achieve high quality 2D layers on sapphire and other crystalline substrates. This includes the impact of substrate chemistry, structure, and defects on achieving high moblity and high quantum-yield 2D layers.
Synthesis and Optoelectronic Performance of III-V Nanowires
Authors: Zhang Xuehong , Pan Anlian
Published: February 2020

III-V semiconductor nanowires (NWs), such as InAs, GaAs, InP, are promising materials for applications in infrared optoelectronic devices because of their typically high carrier mobilities and narrow band gap. In view of expanding the application fields, a major opportunity for III-V NWs and their applications lies in the tunability of bandgaps. Here, the complete composition range InAsxP1-x alloy NWs with potential bandgap tunability from the near-IR region to the mid-IR region were synthesized by a new route combining the vapor-liquid-solid (VLS) growth mechanism with an additional ion-exchange (IE) process. Based on these as-grown InAsxP1-x NWs, broadly band tunable nanometer-scale infrared photodetectors (PDs) have been achieved. In addition, high quality GaSb/GaInSb p–n heterojunction semiconductor nanowires were synthesized successfully via a simple chemical vapor deposition (CVD) method. Based on these nanowires, high-performance photodetectors were constructed with an external quantum efficiency of 104, a responsivity of 103 A/W, and a short response time of 2 ms in the infrared optical communication region (1.55 μm). These III-V NWs shows promising potential applications in integrated photonics and optoelectronics devices or systems.

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Enhanced Piezoelectric Property in Strained Anisotropic Layered Tungsten Ditelluride Nanoflowers
Authors: Masimukku Srinivaas , Wu Jyh Ming
Published: February 2020

Layered transition metal dichalcogenides (TMDS) possess enormous attention in the field of catalysis, optoelectronic devices, sensors, and piezotronics owing to its wide range of chemical, electrical and mechanical properties. Numerous efforts have been made to develop nanostructured TMDS in solution-based methods. Here, we successfully synthesised layered tungsten ditelluride ( WTe2 ) nanoflowers (NFs) in solution phase approach. We further characterised WTe2 NFs and evaluated the piezoelectric properties using atomic force microscopy. The layered WTe2 NFs exhibited significant piezoresponse which may result due to its non-centrosymmetric structure. The unique piezoelectric property of the layered WTe2 is opening up the new field applications such as low-power consumption devices, ultrasensitive sensors, and piezoelectric nanogenerators.

Reference

Wenzhuo Wu, Lei Wang, Nature, 514, 470-474.
Recent Progress in SiC and GaN Power Devices
Authors: Sheng Kuang , Yang Shu , Guo Qing , Xu Hongyi
Published: February 2020

Owing to low conduction/switching loss, high switching frequency and high operating temperature, wide bandgap SiC and GaN power devices are attractive for next-generation power electronic systems with increased efficiency and power density. Because of the superior performance, SiC and GaN power devices could be in position to compete with the mainstream Si power devices in a variety of applications, including household appliances, photovoltaic inverters, electric vehicles, data centers, etc. Despite numerous research efforts devoted to material growth, device fabrication and circuit demonstration in the past decades, wide bandgap power semiconductor devices are still confronted with challenges that need to be addressed through new structure design and advanced process development. In this talk, we will review recent progress in SiC and GaN power devices, including the following. (1) High-density of interface traps usually exist at the gate-oxide/SiC interface in SiC MOSFETs, presenting a critical challenge to the channel mobility and device stability. Interface engineering techniques for SiC MOSFETs (e.g. NO/N2O annealing, P incorporation, metal interfacial-layer, high-k dielectric deposition, etc.) and their impact on interface trap distribution as well as channel mobility will be discussed. (2) Long-term reliability of SiC MOSFETs with special focus on time-dependent dielectric breakdown (TDDB) will be presented. (3) We will introduce our latest progress in design and fabrication of super junction SiC devices, which can break the fundamental limit of the conventional unipolar devices. (4) In GaN-based lateral power devices, the 2DEG channel is an inherent normally-on channel, due to the spontaneous and piezoelectric polarization effects. To achieve fail-safe operation and simpler gate drive circuit, normally-off device technology including p-(Al)GaN cap, barrier recess and plasma treatment in the gate region will be introduced and compared. (5) Gate stack engineering techniques toward high-reliability GaN MIS-HEMTs/FETs will also be presented.
Effect of Wafer and Damaged Layer Thickness on Residual Stress and Bow of Free-Standing Gallium Nitride Wafers during Wafering Process
Authors: Park Jin-Seong , Shim Jae-Hyoung , Chun Kyung-Bin , Lee Jae-Eon , Shim Tae-Hun , Park Jea-Gun
Published: February 2020

In recent years, free-standing gallium nitride (FS-GaN) wafers were researched due to high demand for power device, high brightness LED (HB-LED) and laser diode (LD). The GaN wafers grown by hydride vapor phase epitaxy (HVPE) have been used for the substrate of power device, HB-LED and LD because they demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD). However, they still have residual stress and wafer bow, which lower the device productivity in mass production. To overcome these issues, the control of crystallographic quality and wafer bow are the key technologies. The crystallographic quality of the GaN wafer is mostly determined at the growth process. The bow of the wafer is determined by the difference in thermal expansion coefficient (TEC) between sapphire substrate and GaN, and it can be controlled at the wafering process. In this study, therefore, we investigated how the bow of the FS-GaN wafer is changed during wafering process. In particular, the effect of the abrasive size on the damaged layer thickness during lapping process was investigated and residual stress was calculated theoretically.

Two inch GaN wafers were fabricated on (0001) sapphire substrates by HVPE and separated by the laser lift-off (LLO) process. The bow of as-grown GaN wafer after LLO process was ranged from -300 to -100 um, which was determined by the residual thermal stress. The GaN wafer bow is changed during the wafering process, especially the lapping process. From the Stoney’s formula, it is notable that the thickness of substrate (tS) and sub-surface damaged layer (tSSD) can change the wafer bow.

Fig. 1 shows the wafer bow (black triangle), residual stress (blue square) of experimented GaN wafer and wafer bow calculated by the Stoney’s formula (red triangle). It was confirmed that the wafer bow was reduced as the thickness of GaN wafer reduced. In particular, the bow was inverted from concave to convex at the thickness of around 180 um. As the lapping process progressed, tS is reduced and tSSD is generated, which led to the change of bow to convex.

Fig. 2 shows the sub-surface damaged layer of the GaN wafer after lapping with 2-um diamond abrasives. It exhibits that the crystalline GaN layer was damaged by the diamond abrasives and changed to polycrystalline phase at the top surface. In addition, it indicates that the microcracks propagated into the wafer under the polycrystalline layer. Total thickness of polycrystalline layer and microcracks, tSSD, was around 1.09 um. Thus, the tSSD should be removed for achieving the damage-free GaN wafer.

In our presentation, we report how we achieved zero-bow FS-GaN wafer shown in Fig. 3, and how to control residual stress and optimize wafering process.

* This work was financially supported by the Brain Korea 21 Plus Program in 2017.

Figure 1
Evaluation of Hvpe GaN Layers Grown on Ammonothermal GaN Substrates By Synchrotron X-Ray Topography
Authors: Wu Shuang , Raghothamachar Balaji , Dudley Michael , Freitas Jaime A. , Sochacki Tomasz , Bockowski Michal
Published: February 2020

For obtaining low defect density GaN substrates with controllable electronic properties for high performance electronic and optoelectronic devices, quasi-bulk growth by hydride vapour phase epitaxy (HVPE) on ammonothermal grown GaN substrates appears to be a feasible approach [1]. Synchrotron X-ray topography (white beam and monochromatic beam) has been employed to characterize dislocation configurations in such GaN substrates that were grown by HVPE on ammonothermal GaN substrate, and then slicing the substrate. Threading dislocations and basal plane dislocations (BPDs) are observed in these GaN substrates. Compared with GaN grown on sapphire and silicon carbide substrates [2], HVPE GaN on ammonothermal GaN [3] is obviously of higher crystalline quality with lower defect densities. Threading screw dislocation (TSD) density was measured to be about 880 cm-2 while threading edge dislocation (TED) density was about an order higher (~ 5824 cm-2). The distribution of BPDs which are induced by deformation, was found to be highly non-uniform with most regions of the wafer nearly BPD-free as well as high densities concentrated near one edge. Also, besides the surface features, like the surface scratches, arrays of threading dislocations can be seen from the transmission and grazing incidence topographs, which likely originate from scratches on the surface of the ammonothermal substrate. Correlation of the defect distributions with the HVPE growth process and their implication for devices will be discussed.

References

[1] J. A. Freitas, Jr., J. C. Culbertson, N. A. Mahadik, T. Sochacki, M. Bockowski, and M. Iwinska, Cryst. Growth Des., 15, 4837 (2015).

[2] Tian. Y, et al. Direct growth of freestanding GaN on C-face SiC by HVPE. Sci. Rep. 5, 10748; doi: 10.1038/srep10748 (2015).

[3] S. Sintonen. Synchrotron radiation x-ray topography of crystallographic defects in GaN [D]. Finland: School of Electrical Engineering, 2014. 15-24
Sensitivity of Breakdown Voltage of Power Transistors to Dopant Impurities
Authors: Zhu Chen , Andrei Petru
Published: February 2020

To increase the breakdown voltage (BV) and, in the same time, decrease the on-state voltage (OSV) of wide-band gap transistors it is essential to carefully design the doping profile of these transistors in order to decrease the resistivity of the material during the on-state current flow and decrease the impact ionization generation rate in regions with high electric field during off-state. In general, the two problems depend on each other and there is trade-off between the increasing the BV and decreasing the OSV [1]. In this article we will analyze this tradeoff by using the formalism of doping sensitivity functions of the BV and OSV. These functions show how sensitive the BV and OSV are to infinitesimal variations of the acceptor or donor impurities inside the semiconductor. For instance, the doping sensitivity function of the BV, gBV(r), shows how much the BV increases by adding one donor impurity at location r inside the device. Similarly, the doping sensitivity function of the OSV, gOSV(r), shows how much the OSV increases by adding one donor impurity at location r inside the device. As we have shown in [2, 3], the doping sensitivity functions are instrumental in the optimization of WBG power devices.

The doping sensitivity functions of the BV and OSV can be computed efficiently by using the adjoint method. Once these functions are computed they can be coupled with gradient-based optimization methods to estimate the optimum doping profiles for acceptors, Na(r), and donors, Nd(r), in order to increase the BV and decreases the OSV. At the conference we will describe the numerical implementation of the adjoint method for the optimization of WBG power devices and present simulation results for a vertical and horizontal SiC MOSFETs. More importantly, we will show that in general, in wide-band-gap MOSFETs and IGBTs, the doping sensitivity functions of the BV and OSV, gBV(r) and gOSV(r), respectively, and are not linearly dependent. Therefore, from a mathematical point of view, the BV and OSV are quantities that can be controlled independently by changing the doing profiles inside the transistor. Different equivalent ways to define and compute the doping sensitivity functions of BV and OSV will also be presented. The different definitions of the doping sensitivity functions will be compared to each other and the computational complexity will be discussed in each case.

[1] J. Baliga, Fundamentals of Power Semiconductor Devices: Springer, 2008.

[2] P. Andrei, "Using Doping Sensitivity Functions to Optimize Power Transistors," IEEE Workshop on Microelectronics and Electron Devices (WMED), Boise, ID, 2015.

[3] C. Zhu and P. Andrei, “Adjoint Method for Increasing the Breakdown Voltage and Reducing the On-State Resistance in Wide Band”, 231 ECS Meeting, New Orleans, 2017.

[4] P. Andrei and L. Oniciuc, "Suppressing Random Dopant-Induced Fluctuations of Threshold Voltages in Semiconductor Devices," Journal of Applied Physics, vol. 104, Art. No. 104508, 2008.
(Invited) Microstructural Modeling of PEFC Catalyst Layer Performance and Durability
Authors: Ogawa Shohei , Komini Babu Siddharth , Chung Hoon T , Zelenay Piotr , Padgett Elliot , Muller David A. , Kongkanand Anusorn , Litster Shawn
Published: February 2020

Improving our understanding of the role that microstructure has on the performance and durability of polymer electrolyte fuel cell (PEFC) cathodes is critical to advancing this technology. Modeling is an important tool in interpreting imaging and experimental characterization of microstructure effects because it allows us to quantify the impact that specific microstructure characteristics have and also allows us to visualize processes and quantities that are otherwise inaccessible in the physical system. Various continuum-level and volume-averaging approaches are available to incorporate idealized microstructural effects, but they unfortunately are generally limited to highly idealized geometries and decoupled and simplified transport and reaction models. Herein, we will present recent advances in directly modeling transport, reaction, and degradation mechanisms in catalyst layers where a spatially-resolved multi-dimensional representation of the microstructure is used to capture the impact of anisotropic and heterogeneous features. In these works, the microstructure geometry is often generated through 3D computed tomography (CT), stochastic reconstruction, or a combination thereof. For example, high resolution scanning transmission electron microscope CT (STEM-CT) images can be combined with X-ray nano-CT images to stochastically reconstruct the catalyst layer microstructure with both electrode and catalyst length scales resolved. In addition, correlative methods of incorporating different imaging modalities can be used to resolve the properties and distribution of multiple material phases, including spatial ionomer loading heterogeneity in the cathode. In addition to advances in the field, this work will highlight progress in our group in applying microstructural modeling to both Pt/Pt-alloy and Pt group metal-free (PGM-free) catalyst cathodes. The modeling domains we consider in our work span from Pt particles embedded in high surface area carbon supports to transport and reaction across the electrode thickness. The modeling results range from characterizing microstructural effects on the effective transport properties for the bulk electrode to particle-scale modeling of catalyst metal dissolution and redistribution.

Figure 1
Novel Fuel Cell MEA Based on Pt-C Deposited by Magnetron Sputtering
Authors: Ostroverkh Anna , Johanek Viktor , Dubau Martin , Kus Peter , Veltruska Katerina , Vaclavu Michal , Fiala Roman , Smid Bretislav , Ostroverkh Yevhenii , Matolin Vladimir
Published: February 2020

The presented research is focused on Pt-C composite catalysts for proton exchange membrane fuel cell (PEMFC) applications. Nanostructured Pt-C thin films (thickness 25-150 nm, Fig 1d) were deposited on carbon paper gas diffusion layer (Sigracet 29 BC) substrates by magnetron sputtering method. Total Pt loading in the thin films varied in the range 5.5-30 µg/cm2as determined by XPS. These catalysts were examined for their activity in hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR).

A fuel cell with a membrane electrode assembly (MEA) based on Nafion 212 membrane with our Pt-C anode (Pt loading 5.5-30 µg/cm2) and a commercial state-of-the-art Pt/C cathode (Pt loading 300µg/cm2) generated output power up to 1.0 W/cm2 (Fig 1a), value comparable to MEA with commercial powder anode using much higher Pt loading 200-300 µg/cm2. This result demonstrates significant improvement in platinum utilization for HOR due to high dispersion of Pt provided by the magnetron sputtering deposition method. Similarly, a MEA with the commercial Pt/C cathode and our Pt/C thin film (Pt loading 5.5-30 µg/cm2) as anode yielded stable power up to 0.73W/cm2(Fig 1 b), proving its high efficiency for ORR.

Based on the above findings, a MEA with both magnetron sputtered Pt/C electrodes using total Pt loading 20 µg/cm2 was tested with a 4 cm2 cell operating at 70˚C in H2/O2-feed regime for 24 hours. The platinum content on cathode and anode side was chosen 14.4 µg/cm2 and 5.5 µg/cm2, providing highest efficiency for HOR and ORR, respectively. I-V polarization and power density curves were measured with fresh catalyst as well after a 24-hour duration test without any significant lost of efficiency during this period of time (Fig 1c). The magnetron sputtered Pt-C with fine metal dispersion proved to be a promising catalytic material with high Pt utilization for hydrogen-feed PEMFC applications. An additional significant benefit of the Pt-C composite is its lower requirement with respect to feed humidity.

Figure 1
Development of a Common Differential Fuel Cell Test Fixture and Protocols to Expedite Material Development
Authors: Arisetty Srikanth , Rock Jeffrey , Dabel Jeremy , Lakshmanan Balsu , Kjell Eggen , Kennette Claudette , DeBolt Michael , Kinthali Suresh , Cherat Naajein , Naidu Dodla , Roth Mark , Harper Michael
Published: February 2020

Many test fixtures used for Membrane Electrode Assembly (MEA) development by the fuel cell community have major design drawbacks that both reduce the confidence in data sets and make the sharing of results difficult. A new common hardware design that incorporates the learnings and best practices from various technical leaders is needed to improve build robustness and reduce cell-to-cell variation. Such common hardware can yield improved data quality and accurate comparisons, thus accelerating materials development. Further, creating a common set of differential cell protocols along with corresponding data processing tools can facilitate sharing of results and researcher confidence in data quality. The design objective of the single cell hardware is to ensure MEAs are exposed to uniform conditions during testing of both performance and durability with minimal influence due to the flow field.

In this talk, we will present the design and results from a simplified test hardware that can improve the overall quality of testing and facilitate comparisons from lab-to-lab. The design ensures as much as possible a uniform distribution of all physical quantities such as velocity, pressure, temperature, and oxygen concentration in the flow field. Minimal oxygen concentration gradient across land-channel is ensured in the design with parallel flow fields, having fine land of 0.24 cm, and operating at high (>10) stoichiometry. Pressure drop across the inlet and outlet ports in the hardware is also reduced by designing the ports for minimal restriction. Temperature and compression uniformity is also ensured in the design. The hardware is set to operate with stress control through a pneumatic system integrated into the anode end-plate with the additional capability to regulate compression during operation.

To investigate the uniformity of key paramaters, FEA, CFD, thermal, and ohmic analyses were performed on the designed cell. Hardware replicates will be tested at multiple labs, and an operating procedure will be developed and shared with the community. Assuming successful development, we propose broad adoption of this hardware across the community as a common MEA testing platform.

Figure 1
Using Multi-Scale Modeling to Understand Transports inside PEMFC Under Different Configurations
Authors: Shimpalee Sirivatch , Satjaritanun Pongsarun , Weidner John W. , Hirano Shinichi , Lu Zijie , Shum Andrew , Zenyuk Iryna V. , Ogawa Shohei , Litster Shawn
Published: February 2020

The objective of this study is to use multi-scale/multi-dimensional model to enhance the development of a fuel cell stack and electrochemical cells with increasing in performance at high current densities. Experimental investigation of oxygen transport has been limited by an inability to resolve water saturation-dependent properties. This work shows the successful in development of a multi-scale calculation technique that incorporates detailed structure of each scale dimension in every component of a fuel cell and simultaneously performed a prediction. The effect of operating conditions under different types of flow-fields, gas diffusion layers, and compression pressures on the transports inside PEMFC will be the primary focus of this work.

Figure 1 gives the idea of multi-scale modeling approach at each component of the fuel cell. The flow-field bipoplar plates and MEA models are calculated using traditional CFD method with exisitng PEMFC model [1-3]. The micro-structure of gas diffusion layer (GDL) and/or microporous layer (MPL) are numerically predicted by Lattice Bolzmann method (LBM) [4-6]. Therfore, during the calcuation, the solutions of each iteration from CFD and LBM are required to simulteneously exchange for the next iteration until all solutions are converged, especially at the interfaces [7]. Figure 2 shows liquid water inside GDL and GDL w/ MPL from the simulation when the macroscopic model is combined with the microscopic model under baseline operation.

References:

S. Shimpalee, D. Spuckler, and J. W. Van Zee J. Power Sources, 167 (2007) 130-138.

S. Shimpalee, M. Ohashi, C. Ziegler, C. Stoeckmann, C. Sadeler, C. Hebling, J. W. Van Zee, Electrochemica Acta, 54 (2009) 2899-2911.

S. Shimpalee, J. of Electrochem. Soc., 161 (2014) E3138-E3148.

U. Frisch, B. Hasslacher, Y. Pomeau, Physical review letters 56 (14) (1986) 1505-1508.

G. R. McNamara, G. Zanetti, Physical Review Letters 61 (1988) 2332-2335.

P. Satjaritanun et al., submitted J. of Electrochem. Soc., (2017).

S. Shimpalee et al., abstract # 2387, Pacific Rim Meeting on Electrochemical and Solid-State Science, October 2, 2016, Honolulu, Hawaii.

Figure 1
Development of 200W Class Direct Formic Acid Fuel Cell Stack Using Bi-Modified Pt/C Catalyst By Irreversible Adsorption
Authors: Choi Mihwa , Yang Seugran , Kim Jungsuk , Jin Hyun-Jin , Rhee Choong Kyun
Published: February 2020

Formic acid has been attracting attention as an energy storage medium because of its excellent ability to store hydrogen. In addition, technologies to convert CO2, a greenhouse gas, into formic acid are being developed. It is expected that a large amount of environment-friendly formic acid from recycled CO2 will be produced in the near future. DFAFC has been actively developed in recent years because of high electro-motive force (EMF), low fuel crossover through membrane, non-flammable, and low toxic. However, the electrooxidation performance of formic acid is degraded by CO poison on platinum catalyst in DFAFC. In this study, the characteristics of DFAFC using Bi-modified Pt/C catalyst by irreversible adsorption was investigated in order to improve the performance of platinum catalysts. Bi-modified Pt/C (Bi/Pt/C) catalyst was used for the anode catalyst layer at a loading of 1.2 mg·cm-2. The cathode consisted of unsupported Pt black at a loading of 3.0 mg·cm-2. The DFAFC stack was comprised of 35 cells with 50 cm2 electrode. For single-cell with 5.06 cm2, the performance of DFAFC based on Bi/Pt/C catalyst was more than 2 times higher than that of Pt/C catalyst. The maximum and the average power output of 35-cell stack was 301 W and 200 W at 60 oC, 6 M formic acid and air, respectively. The maximum power density of the cell with the best performance among the cells in the stack was 202.4 mW·cm-2. The high performance of the DFAFC stack demonstrates that it has a great potential for the power supply of portable electronics or small transport vehicles such as electric bicycles.
The Influence of CO on Hydrogen Evolution
Authors: Banas Charles Joseph , Bonville Leonard J. , Pasaogullari Ugur , Molter Trent M.
Published: February 2020

Poisoning of platinum catalyst due to adsorption of carbon monoxide (CO) severely hinders the hydrogen oxidation-evolution reactions due to loss of electrochemically active surface area and changes to the reaction kinetics on the electrodes.[1,2] This paper presents electrochemical impedance spectroscopy (EIS) data for hydrogen oxidation and evolution in presence of . The system consists of two isolated 10mil diameter platinum electrodes coated with a thin layer of Nafion electrolyte for ionic conduction. Both electrodes are exposed to the same environment. EIS measurements are taken both before and after 6 hours of CO exposure. At the beginning of testing, two distinct semicircles are visible in the Nyquist plots at both low and high potentials ( and ). Following CO poisoning, EIS measurements show a significant increase in the diameters of both semicircles. While the data obtained at high potential () shows an amplification of the original Nyquist plot, the measurements taken at the lower current () have an intermediary curve between the original two semicircles.

In this work, we will look to understand the mechanisms that are at work when CO is present during the hydrogen oxidation/evolution reaction. The impedance spectra will be reproduced using an equivalent electrical circuit (EEC) model to examine what circuit elements replicate the measured EIS data and what can be the cause the introduction of the third curve in the spectrum.

References

N.R. Elezovic, Lj. Gajic-Krstajic, V. Radmilovic, Lj. Vracar and N.V. Krstajic, Electrochim. Acta, 54, 1375 (2009).

G.A. Camara, E.A. Ticianelli, S. Mukerjee, S.J. Lee and J. McBreen, 149(6), A748 (2002).

Figure 1
(Invited) Quantifying Water Distribution and Connectivity in Ionomers: Comparison of Hydrated Nafion and Nafion/Polybenzimidazole Systems
Authors: Liu Hongjun , Cavaliere Sara , Jones Deborah J. , Rozière Jacques , Paddison Stephen J.
Published: February 2020

The design of ionomers for electrochemical and fuel cell applications would greatly benefit from a fundamental understanding of how chemical structure determines hydrated morphology. Ion transport through ionomer membrane is essentially governed by the connectivity of ‘tortuous’ water network. Quantification of the connectivity of the water domains poses a challenge in the design of highly conductive ionomers. Blending of different ionomers into a single membrane through casting or electrospinning should permit the bias of a thin film with a specific morphology exhibiting high conductivity. We are currently seeking to understanding how the electrospinning of either fibers or particles of polybenzimidazole PBI) with Nafion determines the hydrated morphology of the composite membrane. In the present work we implement dissipative particle dynamics (DPD) simulations, a coarse-grained scheme, to investigate hydrated Nafion and hydrated Nafion/PBI. Quantitative water cluster analysis confirms that increasing hydration level lead to ever-growing water domains, finally a percolating water channel for both systems. A percolating conductive network of the Nafion/PBI composite system emerges at a hydration level of 4 H2O/SO3H , which is less than that observed in a neat Nafion system. Cluster size distribution and accompanying informative configuration snapshots present a clear advantage over the widely-used radial distribution function method to characterize the ion conductive water channel. This proposed analysis can be readily applied to atomistic and various coarse grained systems. We hope an extensive usage of this quantitative tool will greatly deepen our understanding on the structure-and-function relationship and facilitate the rational design of optimal ionomers.
States of Water in Thermally Annealed Recast Nafion® Films and Impact on Fuel Cell Performance
Authors: Ren Xiaoming , Gobrogge Eric A. , Beyer Frederick
Published: February 2020

The presence of water in polymer electrolyte materials has substantial impact on the materials’ morphology, mechanical and transport properties, and thus dedicate the materials' performance in many important applications in energy production and conversion devices such as fuel cells and electrolyzers. In this study, the states and distributions of water in recast H+-Nafion® ionomer films thermally annealed at a temperature up to 120oC were determined from attenuated total internal reflection FTIR (ATR-FTIR) spectra in the O-H stretching region which were analyzed using principle component and multivariant curve resolution methods. The morphologic changes of the films indicated by the ATR-FTIR experiments are further confirmed by the small-angle X-ray scattering (SAXS) experiments. It is concluded that a Nafion film annealed at a higher temperature results in a more robust polymer matrix surrounding the ionic clusters, and thus exert a stronger restriction on the amount of water uptake in each one of the ionic clusters. The restricted amount of water in the ionic clusters limits the extent of the hydrogen bonding of the water clusters residing in the ionic clusters upon the ionomer film water uptake. These results could lend useful insights on the nature of water in the polymer electrolyte materials, and on the effects of thermal annealing treatment used in fabricating electrodes from the catalyst inks containing the ionomer solution. The impacts of the cathode catalyst layer annealing treatment on the hydrogen/air fuel cell performance are also presented.
Rotating Ring-Disk Electrode Voltammetry Considering the Quasi-Four Electron Reduction of Oxygen for Fe/N/C Catalsyts
Authors: Nabae Yuta , Muthukrishnan Azhagumuthu , Wu Yun
Published: February 2020

Understanding the kinetics of the oxygen reduction reaction (ORR) for fuel cell applications is quite important but difficult because the four-electron pathway is often overestimated by including a quasi-four electron pathway that consists of the formation and reduction of H2O2. To solve this problem, here we demonstrate a novel analysis method with experimental data over a Pt-free Fe/N/C cathode catalyst. In this study, H2O2 voltammetry was conducted separately to evaluate the rate constant of the H2O2 reduction more accurately, and the obtained data were combinatorally analyzed with those from the ORR experiments. First, experimental and mathematical modification of the conventional Damjanovic approach was performed, and then the effect of the catalyst loading density was carefully studied by utilizing a novel reaction model with consideration of the quasi-four-electron pathway to avoid overestimation of the four-electron pathway kinetic parameters. Figure 1 shows the reaction current, I
1, I
2 and I
3, evaluated with different analysis methods. With the conventional Damjanovic-Hsueh approach (left), the evaluated reaction currents are very noisy and do not correspond to the measured current. After the experimental and mathematical modification (middle), the noise in the analysis results are much reduced. Furthermore, by considering the quasi-four electron reduction, the overestimation of the four-electron pathway can be avoided (right). In the most overestimated case, the percentages contribution of four-electron pathway over the Fe/N/C catalyst was estimated as 85% with the modified Damjanovic approach, while that from the newly proposed model is 51%. This method will be applicable for many other cathode catalysts and will facilitate understanding the nature of the ORR.

Figure 1. The comparison of conventional Damjanovic-Hsueh approach, modified Damjanovic approach and newly proposed model for ORR over Fe/N/C catalyst.

Figure 1
Engineering Metal-Organic Framework-Derived PGM-Free Catalysts for Oxygen Reduction in Acidic Media
Authors: Wu Gang
Published: February 2020

To significantly reduce the cost of proton exchange membrane (PEM) fuel cells, current Pt must be replaced by platinum-metal-group (PGM)-free catalysts for the oxygen reduction reaction (ORR) in acid [1-5]. We presented a new class of high-performance atomic iron carbon catalysts through controlled chemical doping of iron ions into zinc-zeolitic imidazolate framework (ZIF), a type of metal-organic framework (MOF). The novel synthetic chemistry enables accurate size control of Fe-doped ZIF catalyst particles with a wide range from 20 to 1000 nm without changing chemical properties, which provides a great opportunity to increase the density of active sites that is determined by the particle size. We elucidated the active site formation mechanism by correlating the chemical and structural changes with thermal activation process for the conversion from Fe-N4 complex containing hydrocarbon networks in ZIF to highly active FeNx sites embedded into carbon. A temperature of 800oC was identified as the critical point to start forming pyridinic nitrogen doping at the edge of the graphitized carbon planes. Further increasing heating temperature to 1100oC leads to increase of graphitic nitrogen, generating possible synergistic effect with FeNx sites to promote ORR activity. The best performing catalyst, which has well-defined particle size around 50 nm and abundance of atomic FeNx sites embedded into carbon structures, achieve a new performance milestone for the ORR in challenging acidic media including a half-wave potential of 0.85 V vs RHE. This has been approaching to Pt and holds great promise for future PEM fuel cells.

References:

(1) H. Zhang, H. Osgood, X. Xie, Y. Shao and G. Wu, Nano Energy, 31, 331-350, 2017.

(2) G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogoke, H. Zhang, H.-L. Wang and L. Dai, Nano Energy, 29, 83–110, 2016.

(3) S. Gupta, S. Zhao, O. Ogoke, Y. Lin, H. Xu and G. Wu, ChemSusChem, 10, 774–785, 2017.

(4) X.-J. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng and G. Wu, Nano Energy, 25, 110–119, 2016.

(5) X. Wang, Q. Li, H. Pan, Y. Lin, Y. Ke, H. Sheng, M. T. Swihart and G. Wu, Nanoscale, 7, 20290-20298, 2016.
Structural Tuning of PGM-Free Electrocatalyst for the High Performance of Membrane Electrode Assembly in PEFCs
Authors: Hwang Sun-Mi , Sohn Young-Jun , Yim Sung-Dae , Park Seok-Hee , Park Gu-Gon
Published: February 2020

A non-platinum group metal (PGM) catalyst is one of the promising candidates to reduce the cost of polymer electrolyte fuel cell (PEFC) systems. In our previous work, we reported an excellent ORR activity of porphyrinic carbon based PGM-free catalysts. However, we found a discrepancy of performance between the half-cell and the membrane electrode assembly (MEA) evaluation. Our previous PGM-free catalyst showed unsatisfactory cell performance in the MEA because of several technical barriers such as a low electrical conductivity, insufficient effective active site, and poor oxygen mass transport through the small pore structure. In this study, we redesigned the morphology of PGM-free catalysts which can enhance the effective active sites and electrical conductivity by modifying the design of synthetic process. As a result, we could get a dramatically enhanced MEA performance showing over 600 mW/cm2 of peak power density which can be considered as one of the highest PGM-free MEA performance in this field till now.