bluuebunny/crossref_metadata_2025 huggingface dataset:
Invited: Approaches to the Formation and Integration of Large Lattice Mismatched Materials: Metamorphic and Non-Conventional 'buffer' Layers
Authors: Kuech Thomas F , Babcock Susan E , Mawst Luke J , Wood Adam , Kim Tae Won , Rajeev Ayushi , Schulte Kevin , Guan Yingxin
Published: February 2020

The growth of large lattice mismatched materials has been studied for many years in order to understand mismatch-derived defects, their mode of introduction, and their mitigation. Traditional approaches focused on the formation of ‘buffer’ layers. ‘Buffer’ layers are intermediate layers within a growing structure which allow a chemical or structural transition between two epitaxial materials. The choice and design of these layers depend critically on the nature of the material transition. Controllably varying the composition across the buffer layer has been the approach most often used in this context. As the composition is graded from the substrate or a lattice matched compound to the substrate to a material which has the desired lattice parameter, the structurally required mismatch dislocations are introduced with a minimum of residual threading dislocations.

The approach of transitional or graded layers has undergone continual development with new effects being realized which can reduce the buffer layer thickness and achieve the dislocation density desired for growth of device layers. Composition grading in compound semiconductors can be accomplished by the change in the cation composition, as in the grading of InxGa1-xAs to transition from GaAs to InAs, or on the anion sublattice using InAsyP1-y to transition from InP to InAs. Often these graded layers are too rough to use in device applications due to the presence of a surface cross-hatch derived from the introduction of mismatch dislocations. Chemical-mechanical polishing has offered a means to recover the required planar surface. There are alternative approaches to graded buffer layers which are beginning to be studied and utilized. The intent of all these approaches is to generate the required misfit dislocations, have them reside at an appropriate interface and, finally, leave no threading dislocation line segments residing within the device layer. A self-assembled block co-polymer approach to nanoscale patterning, which offers rapid and cost-effective full wafer patterning at the 20-nm length scale, was used to achieve improvements in heteroepitaxial growth of large lattice mismatched materials. The x-ray spectra taken from a 600-nm or less thickness GaSb film grown on a patterned GaAs substrate show a sharp reduction in the full width at half maximum (FWHM). The FWHM of the GaSb peak in this initial study was reduced by at least a factor of two as compared to the film grown to the same thickness on a non-patterned wafer. A reduction of this magnitude is extremely significant for films of this thickness. This improvement in materials properties through nano-patterned growth should be applicable to a wide range of materials and lattice mismatch situations. Views on the strengths, challenges, similarities and applicability of these various approaches will be presented.
Continuum Modelling of Silicon Diffusion and Activation in in0.53Ga0.47As
Authors: Aldridge Henry Lee , Lind Aaron Gregg , Law Mark E , Hatem Chris , Jones Kevin Scott
Published: February 2020

There is significant research interest in the behavior of silicon as a dopant in InGaAs during processing conditions for possible integration into future CMOS devices. A major objective is obtaining low resistances for source and drain regions, which require very high doping concentrations near the limits of solid solubility (>1×1020cm-3). Understanding dopant behavior and activation near these limits are of paramount importance in order to fully integrate InGaAs and related III-V materials into systems previously dominated by silicon.  

In this work, a Si diffusion and activation model in In0.53Ga0.47As using the Florida Object Oriented Process and Device Simulator (FLOOXS) will be presented. Similar to previous studies in GaAs, the group-III vacancy-Si pair is implemented as the principle mechanism for silicon diffusion in InGaAs. Fermi level considerations concerning concentration-dependent diffusion are also taken into account in the presented model, building upon simpler previous models.  

Experimental studies that provided the data include Si-implanted InGaAs samples and MBE-grown delta-doped layers of Si in InGaAs. Both sets of samples were capped with Al2O3 and annealed at temperatures ranging from 550 to 750°C. As indicated through SIMS data, post annealed profiles for silicon exhibit very sharp shoulders characteristic of concentration dependent diffusion.  The extracted power of concentration dependence from the SIMS results are relatively high (greater than n=4). This sharp dependence differs from prior continuum-based modelling procedures, where power dependence is believed to closely correspond to the charge state of defects responsible for diffusion. Ultimately the connection between diffusion and activation will be explored, along with developments in prediction and modelling.

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(Invited) Fabrication and Applications of High-Efficiency, Lightweight, Multi-Junction Solar Cells by Epitaxial Liftoff
Authors: Elarde Victor Christopher , Miyamoto Haruki , Chan Ray , Stender Chris , Youtsey Chris , Adams Jessica G J , Wibowo Andree , Tatavarti Rao , Osowski Mark , Pan Noren
Published: February 2020

MicroLink Devices has developed an epitaxial liftoff (ELO) technology for removing thin device layers from the surface of a semiconductor wafer using a flexible metallic sheet to provide structural support. The wafer is left intact and available for use in a subsequent growth cycle. This technology has been applied to high-efficiency, multi-junction solar cells. Removal of the substrate allows the devices to achieve very high specific power of approximately 1000W/kg. For large area devices, the cost of the substrate can be a significant component of the overall device manufacturing cost. By utilizing ELO technology to reuse the substrate for multiple growth cycles, the manufacturing cost of the device can be dramatically reduced. A flexible solar cell fabricated using ELO is shown in Figure 1.

Lightweight, flexible, high-efficiency solar cells have a broad range of applications in both space and terrestrial markets. MicroLink has demonstrated the use of these devices to extend the flight time of electric unmanned aerial vehicles (UAVs). The lightweight and flexible solar cells can be embedded in the wings of the vehicle, adding only minimal weight and conforming to the flight surfaces to avoid introducing addition drag. Figure 2 contains an image of a Raven UAV with MicroLink’s lightweight flexible solar cells integrated into the surface of the wings. The solar array contributes an additional 40W of power to the aircraft under typical operating conditions leading to a 62% increase in flight time.

MicroLink has also demonstrated a Mobile Solar Power (MSP) panel designed for use by soldiers in the field which is capable of recharging a standard issue BB-2590 battery. This panel is shown in Figure 3 lying on top of a conventional CIGS panel. MicroLink’s MSP panel dramatically reduces the size of the panel, allowing it to be used on-the-go. This panel has demonstrated a specific power of 117W/kg, more than double the specific power of CIGS based panels (typically 43W/kg).

By reducing the weight and cost of high-efficiency photovoltaic devices, MicroLink’s novel approach to device fabrication allows for a wide array of previously impractical applications of solar power.

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(Invited) InAs/InAs1-XSbx Type-II Superlattices for High-Performance Long-Wavelength Infrared Medical Thermography
Authors: Razeghi Manijeh , Haddadi Abbas , Chen Guanxi , Chevallier Romain , Hoang Ahn Minh
Published: February 2020

After being proposed by Sai-Halasz et al. in 1977, type-II superlattices (T2SLs) have demonstrated themselves as a very important material system in various fields such as lasers, photodetectors and transistors. In the field of infrared detection and imaging, InAs/GaSb T2SLs have become a viable candidate to compete with the state-of-the-art mercury-cadmium-telluride (MCT) compounds material system. By spatially separating electrons and holes into different quantum wells, the type-II band alignment enables the full control of the electronic bandstructure and thus, allows for great flexibility in hetero-structure device designs. Beside InAs/GaSb type-II superlattices, InAs/InAs1-xSbxT2SLs have been proved to have a longer carrier lifetime than InAs/GaSb T2SLs and have been proposed as an alternative for infrared photodetectors.

We report demonstration of a high performance long-wavelength infrared nBn photodetectors based on InAs/InAs1-xSbx type-II superlattices on GaSb substrate. The photodetector’s 50% cut-off wavelength was ~10 mm at 77K. The photodetector with a 6 mm-thick absorption region exhibited a peak responsivity of 4.47 A/W at 7.9 mm, corresponding to a quantum efficiency of 54% at -90 mV bias voltage under front-side illumination and without any anti-reflection coating. With an R×A of 119 Ω·cm2 and a dark current density of 4.4×10-4 A/cm2 under -90 mV applied bias at 77 K, the photodetector exhibited a specific detectivity of 2.8×1011 cm./W.
(Invited) Synchrotron White-Beam X-Ray Topography Analysis of the Defect Structure of HVPE-GaN Substrates
Authors: Kirste Lutz , Danilewsky Andreas N. , Sochacki Tomasz , Köhler Klaus , Zajac Marcin , Kucharski Robert , Boćkowski Michal , McNally Patrick J.
Published: February 2020

For demanding GaN-based applications like laser diodes or high mobility transistors there is a need for substrate material with low defect densities, since threading dislocations act as centers of non-radiative recombination, increasing the leakage current, reducing the room temperature mobility and thereby limiting the efficiency, performance and lifetime of devices. Hydride vapor-phase epitaxy (HVPE) is the most common technique for the growth of large area freestanding GaN wafers with diameters up to 50 mm and a threading dislocation density below 107 cm−2. An alternative technique with great potential for the growth of GaN bulk crystals is the ammonothermal growth method. For this method GaN threading dislocation densities as low as 5 × 103 cm−2were reported [1]. We are interested examining significant differences in the defect structure of the GaN substrates which can be observed in samples obtained from different manufacturers or as a function of the growth technique.

In this work we present a detailed analysis of defect structures of freestanding GaN (0001) crystals grown by HVPE and the ammonothermal method from different vendors. Synchrotron white-beam X-ray topography (SWXRT), high resolution X-ray diffraction (HRXRD) and microphotoluminescence (µPL) were used for this combined study. SWXRT in two principal X-ray topography geometries was used in the course of this analysis, namely the large-area back-reflection mode and section transmission mode. HRXRD reciprocal space mappings and in-plane measurements were used for mosaicity analyses. The combination of these methods allows a detailed statement of the nature, density and distribution of typical defects in GaN bulk crystals with a large field of view. As an example, Fig. 1 shows a large-area back-reflection topograph (0006 reflection) of a HVPE GaN substrate. The topograph shows dislocation free areas, surrounded by rather sharp lines, indicating good material quality with low defect density. These features are interpreted as dislocation cell walls, suggesting the formation of a cellular defect network within this GaN sample, analogous to what was commonly observed in LEC GaAs [2].

With the combination of the results from SWXRT, HRXRD and µPL a consistent model for different defect structures of the freestanding GaN samples grown by HVPE and the ammonothermal method is discussed.

References

P.J. R. Doradzinski, R. Dwilinski, J. Garczynski, L.P. Sierzputowski, Y. Kanbara, in Technology of Gallium Nitride Crystal Growth, edited by D.Ehrentraut, E. Meissner, and M. Bockowski, (Springer-Verlag, Heidelberg, ISBN 978-3-642-04828-9), 137-158, (2010).

P.J. McNally, P.A.F. Herbert, T. Tuomi, M. Karilahti, and J.A. Higgins, J. Appl. Phys. (1996) 79, 8294.

 

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(Invited) High Efficiency Green-Yellow Emission from InGaN/GaN Quantum Well Structures Grown on Overgrown Semi-Polar (11-22) GaN on Regularly Arrayed Micro-Rod Templates
Authors: Gong Y , Xing K , Xu B , Yu X , Li Z , Bai J , Wang Tao
Published: February 2020

Last two decades have seen major developments in the field of III-nitride materials and optoelectronics, which is mainly based on polar GaN on sapphire. So far, the growth of c-plane GaN on sapphire has almost approached its theoretical limits and the major limit to the performance of GaN-based LEDs is due to the growth on c-plane. Consequently, this polar orientation leads to a number of fundamental issues, such as efficiency droop, and green and yellow gaps in wavelength coverage. One clear way forward, meeting the fundamental challenges, is to grow along semi-polar/non-polar direction. Semi-polar orientations, in particular, (11-22), offer another major advantage, namely, it can also significantly enhance indium incorporation into GaN, which cannot be achieved using c-plan GaN and thus is extremely important for growth of long wavelength emitters, such as green and even better for yellow. However, the current challenge is due to the crystal quality of semi-/non-polar GaN, which is far from satisfactory. So far, the best performing semi-polar emitters are grown on semi-polar GaN substrates which are typically ~ 10x10 mm in size. Therefore, it is necessary to develop a cost-effective approach to developing large size semi-polar GaN templates with major improvement in crystal quality but still based on sapphire substrates.

The Sheffield team has developed an overgrowth approach for 2" (11-22) semi-polar GaN with significantly enhanced crystal quality on nanorod templates fabricated by self-organised nickel nano-mask approach. Very recently, a new overgrowth approach based on optical lithography mask pattern technique has been developed, where regularly arrayed micro rod templates can be cost-effectively fabricated and the size of the micro rods can be well controlled. The new approach has led to further improvement in both crystalline quality and surface morphology. This has been confirmed by high resolution transmission microscopy (TEM) studies, demonstrating further significant reduction in both dislocation density and basal stacking fault density.

InGaN/GaN multiple quantum well structures with high In compositions have been grown on such semi-polar GaN templates, and we have achieved strong emission at long wavelengths, even down to ~600 nm. For the green-yellow emission at 555 nm, we have obtained an internal quantum efficiency (IQE) of up to 14%, and 7-10% for the pure yellow emission at ~570 nm. Con-focal photoluminescence (PL), micro-PL, time-resolved PL (TRPL), and micro-TRPL have been performed
(Invited) Power Loss Reduction in Perforated-Channel HFET Switches
Authors: Shur Michael , Gaevski Mikhail , Gaska Remis , Simin Grigory , Wong Hugh Yung , Braga Nelson , Mickevicius Rimvydas
Published: February 2020

The key factors affecting the efficiency of power electronic systems are the conduction and switching losses in semiconductor power switches. For field-effect transistor switches, the conduction power loss is proportional to the product of the squared current, ION
, flowing through the switch in the on-state and the switching loss is proportional to the squared maximum switch voltage, VMAX,
the effective device output capacitance, COUT
, and the switching frequency, fSW
. Therefore, the RONCOUT

product is an important figure of merit of a FET-type switch determining the overall power efficiency. Our proposed and implemented Perforated Channel (PC) FET design 1 allowed us to significantly reduce RON
,
but not COUT. We now propose and demonstrate the improved PC-HFET design that allows us to reduce both RON and COUT
. This design reduces the power losses and significantly improves the efficiency of power electronics systems. In the PC-HFET, the portions of the channel under the gate are removed and the remaining channel forms a dash-line of islands WG1
wide with WGG spacing between them. The channel “filling factor” in PC-HFET is kF = WG1/(WG1+WGG
). Compared to conventional HFET with the same total width W, the total channel width and, hence, the device gate capacitance, of the PC-HFET is kF
times smaller. However, our analytical estimates, numerical simulations (using the 3D simulation in Synopsys Sentaurus device simulator), and experimental data all show the on-resistance of the PC-HFET can be only 10-15% higher than that of the conventional HFET of the same width. This is achieved due to a strong current spreading effect in the source-to-gate and, especially, in a larger gate-to-drain spacing. The gate capacitance of PC-HFET reduces proportionally to the kF
factor as proven by our simulations and experimental data presented in [1]. However our 3D simulations show that the CGD
scaling in PC-HFET at high drain voltage is nearly the same as that for conventional HFET. To reduce this capacitance, we proposed the PC-HFET device design with the holes extending into the gate-drain spacing. Our simulations show that the hole extension allows for nearly the same CGD
reduction as the gate metal removal over the holes. We used the obtained RON
and CGD
data to calculate the power loss in AlGaN/GaN PC-HFET and showed that the PC-HFET design filling factor kF
= 0.25 reduces the total conversion loss in the power switch by more than a factor of 2.

[1] G. Simin, M. Islam, M. Gaevski, J. Deng, R. Gaska, and M. Shur, “Low RC-constant Perforated-Channel HFET”, IEEE El. Dev. Letters, V. 35, pp.449-451, 2014

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In-Situ Characterization of Defect Dynamics in 4H-SiC Power Diodes under High-Voltage Stressing
Authors: Shenai Krishna , Raghothamachar Balaji , Dudley Michael , Christou Aris
Published: February 2020

Wide bandgap (WBG) semiconductor power switching devices, especially those made on silicon carbide (SiC) and gallium nitride (GaN), promise transformative advances in electrical power switching systems because of superior electrical and thermal properties of these materials compared to the semiconductor silicon [1]. However, the progress has been slow despite intense scientific and industrial development. Both SiC and GaN semiconductors contain a high density of crystal defects and the role of defects on the performance and reliability of electrical power switching devices under extreme operating environment is not clear. Using synchrotron white beam X-ray topography (SWBXT), it is shown that the breakdown mechanism in 4H-SiC is initiated at the threading screw dislocations present in the high field regions of a power diode. To avoid this phenomenon from occurring, commercial 4H-SiC high-voltage diodes are rated for punch-through leakage currents rather than for avalanche breakdown condition. Thus, crystal defects in 4H-SiC present a major roadblock for performance and reliability optimization of power switching devices.

[1] K. Shenai et al, “Current status and emerging trends in wide bandgap (WBG) semiconductor power devices,” ECS J. Solid State Sci. and Tech.
2(8), N3055-N3063, Jul. 2013.
Optically Transparent Flexible IGZO TFTs Fabricated with a Selective Wet-Etch Process
Authors: Tari Alireza , Lee Czang-Ho , Wong William S.
Published: February 2020

The emerging transparent thin-film transistor (TFTs) technology has recently received great attention in the active-matrix organic light-emitting diode (AMOLED) flexible displays. This TFT technology also has the potential to enable enhanced functionality in conventional large-area electronics applications ranging from smart windows and heads-up displays to flexible sensor arrays. During the past few years, amorphous transition-metal oxides have been suggested to be one of the best candidates in this application. Amorphous InGaZnO (IGZO) TFTs are the most promising transparent-metal oxide semiconductor due to its high optical transparency and field-effect mobility (μFE) in TFT devices compared to conventional amorphous silicon (a-Si:H). The amorphous structure is more suitable for large-area processes than other polycrystalline metal-oxide TFTs. These unique advantages have resulted in the rapid development of IGZO TFT technology. Given the relatively poor selectivity to common semiconductor etchants, IGZO TFTs are typically fabricated using lift-off processes to define the electrical contacts. However, the success of the emerging transition-metal oxide materials system for flexible electronics requires compatibility with conventional TFT microfabrication manufacturing processes.

Flexible IGZO TFTs having 85% optical transparency in the visible regime were fabricated on polyethylene napthalate substrates using a selective wet etching process to eliminate the need for lift-off processing. The devices were processed directly onto the plastic platform at a maximum temperature of 150°C. The TFT structure consisted of a sputtered 100 nm aluminum doped zinc oxide (AZO) gate electrode, a 300nm SiNx gate dielectric deposited using plasma enhanced chemical vapor deposition (PECVD) method at 150ºC, and a 50nm IGZO layer deposited using RF sputtering at room temperature. A 150 nm AZO source/drain (S/D) electrodes were deposited by using RF sputtering at 150ºC and patterned wet etching process in diluted HCl solution having a selectivity of 11 between AZO and IGZO. The channel width and length of the studied TFTs were 200 and 100 µm, respectively.

The fabricated TFTs exhibited a field-effect mobility of ~10 cm2/V.sec, threshold voltage of ~5.0 V, a sub-threshold swing of 0.8 V/decade, and an Ion/Ioff ratio of 107. No current crowding behavior was observed for the TFTs at the low drain-source voltage (VD) regime. The I-V characteristics showed excellent ohmic contacts can be fabricated using AZO S/D electrodes through a wet etching process and promises a more conventional approach to large-area flexible electronics fabrication. The highly selective etching process provides a means to fabricate IGZO based circuits with low processing complexity that will enable system-on-“plastic” integration of flexible transparent flexible displays. In addition, the effect of long-term electrical bias on the electrical stability of the devices will also be presented.
Electrodeposited Sb- Doped ZnO Nanorod Arrays and Electrical Characterization Based on Single Nanorod Field Effect Transistors
Authors: Liang Jin-Kun , Su Hai-Lin , Kuo Chun-Liang , Wu Yu-Cheng , Huang Jung-Chun Andrew
Published: February 2020

Sb-doped ZnO nanorods were fabricated via electrochemical methods. SbCl3,

 Zn(NO3)2 and hexamine mixed solution were used as a precursor. The morphology, composition

 and crystallinity of nanorods were characterized using a field emission scanning electron microscope, 

energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The transmitted spectrum

 was used to investigate optical properties of Sb-doped ZnO arrays. Single Sb-doped ZnO nanorod field effect transistors were constructed via standard photolithography process. 

The dependence of nanorod resistance on the temperature from 5K to 300K and the current

 versus voltage curve of the field effect transistor at different temperature from 100K to 300K were measured. 

The Sb-doped nanorod showed different electrical properties following the change of temperature.

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(Invited) The Joint Center for Energy Storage Research (JCESR): A New Paradigm for Energy Storage Research
Authors: Crabtree George
Published: February 2020

The Joint Center for Energy Storage Research (JCESR) pursues high performance, low cost beyond lithium ion electricity storage that will transform transportation and the electricity grid. JCESR will leave three legacies:

a library of fundamental knowledge of the materials and phenomena of energy storage at the atomic and molecular level

two prototypes, one for the grid and one for transportation, that, when scaled to manufacturing will be able to deliver five times the energy density at one-fifth the cost

a new paradigm for battery R&D that combines discovery science, battery design, research prototyping and manufacturing collaboration in a single highly interactive organization, accelerates the pace of discovery and innovation and significantly shortens the time from discovery to commercialization.

An introduction to JCESR’s vision, mission and legacies will be followed by research highlights illustrating its advances in fundamental science and the promising pathways to transformational battery designs and prototypes.

This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences
(Invited) Techno-Economic Analysis of Batteries and Key Considerations for Performance Comparisons
Authors: Gallagher Kevin G.
Published: February 2020

Energy storage and conversion technologies range from those currently in widespread application to those that exist mainly in the research laboratories around the world. Evaluating technologies against each other requires unbiased and defensible tools and metrics even within the same class of storage technology (e.g. transportation batteries). Techno-economic modeling is a powerful tool that can be used to compare both across platforms as well as within [1 ,2]. This talk will focus on electrochemical energy storage in the form of enclosed, hybrid, and flow batteries for both transportation and stationary applications. Key performance indicators will be discussed and values compared for existing technologies against promising new chemistries under development. The role of the analysis framework will be highlighted particularly in light of considering market size and maturity. Finally, comparison to other non-battery technologies will be presented.

[1] K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu, V. Srinivasan “Quantifying the promise of lithium-air batteries for electric vehicles” Energy & Environmental Science, 7 (5), 1555 - 1563 (2014)

[2] R. M. Darling, K. G. Gallagher, J. Kowalski, S. Ha, F. Brushett “Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries” Energy & Environmental Science, 7 (11), 3459 – 3477 (2014)
(Invited) Standardizing Experimental Methods and Metrics in the Field of Photoelectrochemical (PEC) Water Splitting
Authors: Chen Zhebo , Dinh Huyen N , Miller Eric Lars
Published: February 2020

Photoelectrochemistry is a particularly rich field of study that combines principles and concepts from both semiconductor physics as well as electrochemistry.  Although its intellectual depth provides incredible opportunities for technical development, it also presents a steep learning curve for scientists new to this field who wish to leverage PEC in their efforts to advance low-carbon energy conversion research.  As a result, PEC studies are not always performed consistently from one laboratory to the next, limiting the ability to accurately compare results in the literature and amongst researchers.  In order to address this challenge, the U.S. Department of Energy formed the PEC Standards Working Group to identify the most readily accessible experimental methodologies as well as the most meaningful metrics required to advance materials development.  Herein, we discuss the path taken towards establishing benchmarks in PEC water splitting as well as the key concepts and methodologies identified as being the most pertinent within the field.  These include various efficiency definitions in addition to techniques such as illuminated 3- and 2-electrode electrochemical measurements.  Such measurements can elucidate important material properties including band structure and charge transport.  Understanding the advantages and the limitations of these methods is a critical step towards enabling researchers to more accurately evaluate the performance capability of PEC devices and to advance progress in the field.

Figure 1
(Invited) Fermentative Hydrogen Production from Biomass in the Cellulose-Degrading Bacterium Clostridium Thermocellum
Authors: Magnusson Lauren , Chou Katherine , Maness Pin-Ching
Published: February 2020

Hydrogen can be produced from diverse energy sources, using a variety of process technologies. One promising route for biological hydrogen production is via the fermentation of renewable lignocellulosic biomass. However, due to its crystallinity, biomass has to undergo a thermochemical pretreatment step in order for microbes to ferment its cellulose and hemicellulose constituents. Clostridium thermocellum is a thermophilic (55 – 60oC) and anaerobic bacterium which displays one of the highest growth rates on cellulose.  C. thermocellum is capable of simultaneous biomass hydrolysis and fermentation in one integrated step, which could be a potential cost saver. During cellulose fermentation, the bacterium produces hydrogen, ethanol, formate, lactate, acetate, and CO2.  To reduce the cost of hydrogen production, we aim to overcome several technical barriers identified by the DOE Fuel Cell Technologies Office. To reduce the feedstock cost barrier, we optimized cellulose fermentation in bioreactors, operating in either batch or fed-batch mode. In the latter mode we tested parameters including hydraulic retention time, and amount and frequency of liquid replacement. To circumvent the low hydrogen molar yield barrier (mol H2/mol hexose), we conducted genetic engineering by redirecting metabolic pathways toward maximal hydrogen production lieu of reduced carbon byproducts. Lastly, a techno-economic analysis is underway to set the targets and to guide future research direction to deliver low cost hydrogen. This effort would rely on standardization of metrics in the fermentation technology to assess more accurately as to its economic viability.
(Invited) Re-Energizing Waste CO2 to Fuels with the Sun: Efficiency, Scale, Resource Utilization, and Economics
Authors: Stechel Ellen B , Miller James E
Published: February 2020

The interconnected challenges posed by the strategic and economic importance of petroleum and the increasing concentration of atmospheric carbon dioxide are now widely recognized. A transition away from the fossil fuel dominated global economy toward a system with ever-lower carbon intensity but commensurate with energy demand is necessary. Addressing these challenges requires thinking differently, beginning with recognizing that from a technological point of view, liquid hydrocarbon fuels remain an ideal transportable energy storage medium, and that CO2 and H2O are simply the energy depleted, oxidized form of hydrocarbons. Further, it is instructive to acknowledge that conventional fossil fuels are in fact ‘‘stored sunlight’’. This perspective suggests searching for large-scale options that convert and store modern sunlight via reactions of carbon dioxide and water to more usable and transportable forms as nature did, but do so as efficiently, effectively, affordably, and sustainably as possible – including being able to produce at the same rate as the product is being consumed. Conversion of solar energy to chemically or biologically reduce waste CO2 into hydrocarbon fuels is an attractive option, but it is not without significant resource, economic, and technical challenges. Herein we present a general examination of resource and economic considerations and conclude that solar-to-fuel efficiency is a key metric that drives not only the potential impact (scalability) of a technology for storing (contemporary) sunlight and sequestering carbon above ground as energy dense fuels, but also the economics. In this presentation, we will talk about a number of comparative metrics of significance and concern to accelerating a transition to a low carbon sustainable transportation future.
Design and Cost Considerations of Solar-Fuel Devices
Authors: Modestino Miguel A. , Rodriguez Claudia A. , Psaltis Demetri , Moser Christophe
Published: February 2020

Recent environmental concerns have led to an increased interest in the deployment of renewable energy technologies. Electrochemical energy conversion devices that can directly capture and store solar energy in the form of fuels are a promising alternative to increase the share of renewables in our current energy landscape. In order to deploy solar-fuel technologies in large scale, practical devices need to operate continuously and stably over long periods of times, need produce more energy over their lifetime than they require for their fabrication and need to generate fuels in an economically viable way. To this end, significant research efforts have been devoted towards the development of photoelectrochemical components that are able to spontaneously split water in the presence of solar irradiation. These efforts have resulted in major advances in the solar-fuels field over the past decade, while only limited attention has been given to understanding the factors that drive economically viable solar-fuel generators. In this presentation we will describe a system agnostic approach that allows for the understanding of economic factors behind the design of solar-hydrogen generators.(1) Using this framework, we evaluate the effects of the material selection for the light absorption and water splitting components on the cost of the generated fuel ($/Kg of H2). Furthermore, guidelines for optimal designs for solar water-splitting devices are formulated in order to minimize the cost of hydrogen production. The results presented here provide insights into engineering aspects related to device design and the use of light concentration components that, when optimized, can lead to costs below $2.90 per kilogram of hydrogen after compression and distribution (reaching comparable cost levels to hydrogen produced from fossil fuel). In order to achieve these low hydrogen production costs, optimized devices will need to be fabricated with water splitting components that are significantly smaller in area than light absorption units (by a factor 10-100’s). Moreover, the analysis demonstrates that in optimized devices the cost of hydrogen is primarily driven by the light-absorbing component while the material selection for the electrolysis components has minor effects. This suggests that the development of highly efficient and inexpensive photovoltaic components for solar water-splitting can greatly facilitate the implementation of the technology. The findings discussed here can help direct research and development efforts towards the fabrication of scalable solar-hydrogen generators that are cost competitive with commercial energy sources.       

1.         C. A. Rodriguez, M. A. Modestino, D. Psaltis, C. Moser, Design and cost considerations for practical solar-hydrogen generators. Energy & Environmental Science,  (2014). DOI: 10.1039/c4ee01453g
(Invited) Stabilized Bismuth Oxide As a Component in the Air Electrodes of Reversible Solid Oxide Electrochemical Cells
Authors: Haile Sossina M
Published: February 2020

The high ionic conductivity of the high-temperature, cubic phase of bismuth oxide has been recognized for several decades. Significant effort has been directed over the years to stabilize this phase at temperatures of interest for operation of solid oxide electrochemical cells, with Y and Er having emerged as promising substituents to achieve this goal. While stabilized bismuth oxide is not suitable as the membrane material for an electrochemical cell due to its instability under reducing atmospheres, its high conductivity, one to two orders of magnitude greater than that of yttria-stabilized zirconia, renders it attractive as a component in a composite air electrode. Here we describe oxygen electrocatalysis in both random composite structures of yttria-stabilized bismuth oxide (YSB) and lanthanum strontium manganite (LSM) and defined structures with patterned metal electrodes on YSB. The results show that, beyond the high ionic conductivity of YSB, the material provides inherent electrochemical activity for oxygen reduction/evolution reaction.
(Invited) Enhanced Steam Electrolysis at Exsolution Titanates
Authors: Neagu Dragos , Tsekouras George , Shin Tae-Ho , Irvine John
Published: February 2020

B-site doped, A-site deficient perovskite titanate oxides with formula  (M = Ni2+ or Fe3+; x = 0.06; g = (4-n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis (HTSE) at 900 °C. A proportion of B-site dopants were exsolved at the surface in the form of metallic nanoparticles under SOEC operating (reducing) conditions, due in part to the inability of the host lattice to accommodate vacancies (introduced (d) oxygen vacancies () and fixed A-site () and inherent (g) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Ni0 or Fe0 nanoparticles and higher  concentrations dramatically lowered the activation barrier to steam electrolysis and led to sharp rises in oxide ion mobility compared to the parent material (x = 0). La0.4Sr0.4Ni0.06Ti0.94O2.94 demonstrated discontinuous temperature dependence possibly due to oxygen vacancy trapping at lower temperatures.
The Effect of Mixed-Conductivity on Electrolysis Cells Involving BaCexZr0.9-XY0.1O3-d (x = 0, 0.1 and 0.2) Protonic Ceramic Membrane
Authors: Babiniec Sean M , Dippon Michael , Ricote Sandrine , Sullivan Neal P
Published: February 2020

Planar, fuel electrode-supported cells were fabricated utilizing a thin (~25 μm) BaCexZr0.9-xY0.1O3-δ electrolyte on a 1-mm-thick BCZY27/Ni fuel electrode [1]. Three electrolyte compositions were studied: x = 0, 0.1 and 0.2 referred to as BZY10, BCZY18 and BCZY27 respectively. Two different air electrodes were prepared: painted Au or a 40-μm-thick porous BCZY27 backbone infiltrated with La2NiO4+δ nano-particles as an electro-catalyst. As shown in Figure 1, which is an example of the cell microstructure for the infiltrated backbone, the adhesion between the electrolyte and the electrodes as well as the density of the electrolyte are satisfactory.

These devices were tested over a wide range of conditions. Both fuel-cell and electrolyser modes of operation were explored over a range of temperature spanning 550-750 °C. Flux of hydrogen was continuously monitored with a gas chromatograph and a mass spectrometer in parallel. Figure 2 displays the hydrogen fluxes in electrolysis operation measured with a BCZY27 electrolyte at 700 °C with 5% H2, 3% H2O (balance Ar) at the fuel electrode and 21%, 7% H2O (balance Ar) at the air electrode. The measured hydrogen fluxes are far below the Faradaic fluxes. The difference between the measured fluxes and the Faradaic fluxes increases with increasing current. Similar results were reported by Sakai et al. [2] and Matsumoto et al.[3]. 

Oxide ion conduction is negligible as no increase of steam content was measured at the fuel electrode. Therefore, the low Faradaic efficiency can only be compensated by an electronic leakage. This work draws a comparison of the hydrogen fluxes with the percentage of cerium in the BaCexZr0.9-xY0.1O3-δ membrane.

 

References:

1. S.M. Babiniec, S. Ricote, N.P. Sullivan, submitted to Journal of Power Sources

2. T. Sakai, S. Matsushita, H. Matsumoto, S. Okada, S. Hashimoto, T. Ishihara, Int. J. Hydrogen Energy 34 (2009) 59

3. H. Matsumoto, T. Sakai, Y. Okuyama, Pure Appl. Chem. 85 (2013) 427

Figure 1
Steam Electrolysis By Proton-Conducting Solid Oxide Electrolysis Cells (SOECs)
Authors: Bi Lei , Traversa Enrico
Published: February 2020

Solid oxide electrolysis cells (SOECs) are considered as an effective way of converting renewable energies to chemical energy in the form of hydrogen. Using this conversion as energy storage, we can solve the site-specific and intermittent problems for renewable energies, such as solar and wind energy. During the SOEC working condition, H2O is split into H2 and O2 by applying voltage. Compared with low temperature electrolysis cells, SOECs that work at high temperatures can save electricity with the compensation from heat sources [1]. However, conventional SOECs using oxygen-ion conducting electrolytes have several problems. First, the working temperature is quite high due to the use of yttria-stabilized zirconia (YSZ) as electrolyte, which possesses adequate conductivity only at high temperatures. Second, the produced H2 is mixed with H2O, needing further separation. Third, the Ni-based fuel electrode materials trend to be oxidized by H2O during operation. To solve these problems, proton-conducting oxides are proposed as alternative electrolytes that show several advantages and can avoid the mentioned problems occurring for conventional oxygen-ion SOECs [2]. However, current proton-conducting SOECs focus on the use of BaCeO3-based electrolytes, which have been demonstrated to be unstable in the presence of water. In this talk, chemically stable BaZrO3-based electrolyte material used for proton-conducting SOECs is presented. Proton-conducting SOECs with BaZrO3-based electrolyte show a good chemical stability, together with reasonable cell performance and a superior long-term stability. The possibility of applying proton-conducting SOECs for synthesizing CH4 by co-electrolyzing CO2 and H2O will be also discussed.

Reference

1. A. Hauch, S. D. Ebbesen, S. H. Jensen and M. Mogensen, J. Mater. Chem., 2008, 18, 2331-2340.

2. Lei Bi, Samir Boulfrad and Enrico Traversa, Chem. Soc. Rev., 2014, 43, 8255-8270.
Phase Dependent Selectivity of Electrochemical CO2 Conversion to Fuels on TiO2 nanoparticles
Authors: Wright Christopher J. , Sharma Pranav P. , Ke Fu-Sheng , Peterson Andrew A. , Zhou Xiao-Dong
Published: February 2020

We report the activity and selectivity of electrochemical CO2 reduction to carbon monoxide and formate on rutile (110) and anatase (101) TiO2. Our studies show that the rutile TiO2 phase was found to be more selective, with Faradaic efficiencies of ~ 15% and ~ 18% for CO and formate, while the anatase efficiencies were less than ~2%.  The phase dependent efficiency exhibited by the electrochemical reaction is opposite to previously reported photochemical CO2 reduction efficiency, which showed a greater activity from anatase TiO2. In order to evaluate the role of oxygen vacancy and Ti3+ sites in the catalytic activity XPS was performed on the catalysts after CO2 reduction. The XPS showed oxygen vacancies and Ti3+ defects in both the anatase and rutile phases. This implies that while the presence of surface defects may be necessary for the electro reduction of CO2 on TiO2 the efficiency determining property is the catalyst’s underlying atomic structure. Our recent results using DFT to elucidate a potential mechanism for this phenomenon, comparing the binding energies of CO2 reduction adsorbates on rutile and anatase TiO2, will also be discussed.
(Invited) Optimizing Electrocatalyst Selectivity for CO2 Reduction over H2 Evolution
Authors: Peterson Andrew A.
Published: February 2020

The electrocatalytic reduction of CO2 to fuels or chemicals is an attractive alternative to fossil- or biomass-based feedstocks. However, since the reduction typically requires protons in the reaction pathway, the evolution of H2 is a common side reaction; in practice H2 often dominates the observed products. In this talk, I will discuss ways in which our group is understanding and optimizing electrocatalysts for selectivity between these reactions. We will discuss ways in which we identified and optimized the expression of active edge sites in nanostructured Au catalysts for CO evolution, providing extremely high selectivities at low overpotentials. Also discussed will be the fundamental adsorbate-adsorbate interactions that drive reactivity and selectivity across various catalyst surfaces, including on Cu, on Mo, and on carbides. Strategies to overcome these limitations and provide low-overpotential CO2 reduction will be discussed.
(Invited) Two-Dimensional Materials for Electrochemical Synthesis of Fuels
Authors: Wu Jingjie , Liu Mingjie , Hackenberg Ken , Liu Yuanyue , Yadav Ram Manohar , Sharma Pranav P. , Zhou Xiao-Dong , Wood Brandon , Yakobson Boris I. , Lou Jun , Ajayan Pulickel M
Published: February 2020

The sustainable civilization and long-term economic development rely on the security of energy. The conversion of carbon dioxide into fuels or commodity chemicals, incorporated with intermittent renewable energy sources like solar and wind, is an attractive venture that could offer an alternative solution to both the contemporary energy crisis and environmental issues. The recycling of carbon dioxide together with water by utilizing renewable electricity in the low temperature electrolysis process provides a diversity of carbon-neutral fuel products depending on the catalyst system.The development of active and stable catalyst system with affordable cost for the low temperature electrolysis remains a major challenge. We developed carbon-based two dimensional materials as metal-free catalysts for efficient, selective and sustainable electroreduction of carbon dioxide into carbon monoxide.

Hydrogen is not only an ideal energy carrier, but also an important agent for many industrial chemical processes. The economical production of hydrogen from electrochemical water splitting is strongly dependent on the affordable catalysts with promising activity to replace platinum-group metals. Recently, layered molybdenum and tungsten have attracted substantial interest to catalyze hydrogen evolution reaction (HER) with their suitable differential free energies for intermediate adsorbed *H at the edge sites. We aim to develop a novel series of transition-metal dichalcogenides with surface activity for HER with a combination of theoretical prediction and experimental verification.