bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Ge Epitaxial Growth on Si1-XGeX Buffer Layer
Authors: Jang Hyunchul , Kim Byongju , Koo Sangmo , Ko Dae Hong
Published: February 2020

Ge has higher electron and hole mobility than Si. Therefore, it has been considered as a new channel material in metal oxide semiconductor devices to replace Si. However, Ge substrates are of high cost as well as are difficult to work with. Epitaxial growth of Ge on Si is therefore preferred for fabricating Ge films. Heteroepitaxy of Ge films on bulk silicon is challenging because the lattice constant of Ge is 4.18% greater than that of Si. We investigate epitaxial growth of Ge on SiGe buffer layers. Ge epilayers and SiGe buffer layers were grown on a Si (001) substrate using ultrahigh vacuum chemical vapor deposition with Si2H6 and GeH4 as precursors at the growth temperature of 600 °C. For the SiGe buffer layer, the Si2H6 flow rate was fixed at 20 standard cubic centimeters per minute (sccm) and the GeH4 flow rate was varied from 30 to 70 sccm. The Ge epilayer was grown at the GeH4 flow rate of 200 sccm. Scanning electron microscopy (SEM) images show that the roughness of the SiGe layer increases with the GeH4 flow rate. The SiGe layer forms islands at the GeH4 flow rate of 70 sccm. The Ge epilayer was continuous when grown on the SiGe buffer layer at the GeH4 flow rate of 30 sccm. For the growth over the SiGe buffer layer at the GeH4 flow rate of over 50 sccm, the Ge epilayer formed islands.
Electrochemical Transfer Doping: A Novel Phenomenon Seen in Diamond, Gallium Nitride, and Carbon Nanotubes
Authors: Chakrapani Vidhya
Published: February 2020

Intrinsic diamond with its band gap of 5.5 eV is an extremely good insulator.  However, for undoped, hydrogen-terminated diamond exposed to air, a p-type conductive surface layer appears that is mediated by adsorbates from the ambient atmosphere.1  Since its discovery, several explanations of this effect have been proposed, none has received wide acceptance and the mechanism remained controversial. 

Recent studies have shown indisputably that the conductivity arises as result of electron exchange between atmospheric oxygen and diamond through an electrochemical redox reaction occurring in an adsorbed water layer:

4H+ + 4e- + O2 = 2H2O

This results in the formation of a hole accumulation layer on the surface.2,3  This novel phenomenon, referred to as “surface transfer doping” depends only on the band line up of the semiconductor with respect to the ambient.  For diamond, the Fermi level is higher than the electron chemical potential of oxygen redox couple, which results in electron transfer into water layer and generation of p-type surface conductivity.  See Figure 1.  The electron chemical potential (Fermi energy) of the redox couple, which can be estimated from the Nernst equation.

Since water layers form spontaneously on surfaces exposed to humid air, electrochemical transfer doping is a very general phenomenon that has been unrecognized so far.  Since its discovery in diamond, the effect has been observed in other technologically important materials such as in single-walled semiconducting carbon nanotubes, graphene, gallium nitride and zinc oxide.4  The process not only affects the electrical properties but also optical properties of the semiconductor.  For example, in both GaN and ZnO, electron exchange between oxygen redox couple and mid-gap states results in dramatic changes in the ubiquitous yellow and green luminescence respectively. This phenomenon opens the door to novel and robust class of devices and processes.

1.  M. I. Landstrass, and K. V. Ravi, Appl. Phys. Lett., 55 (1989) 975.

2.  F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett.
85, 3472 (2000)

3.  V. Chakrapani, J. C. Angus, A. B. Anderson, S. D. Wolter, B. R. Stoner and G. U. Sumanasekera, Science
318, 1424 (2007)

4.  V. Chakrapani, C. Pendyala, K. Kash, A. B. Anderson, M. K. Sunkara, J. C. Angus, J. Am. Chem. Soc.
130, 12944 (2008)

Figure 1
(Invited) Growth of Non-Polar Cubic GaN on Common Si
Authors: Durniak Mark , Bross Adam , Elsaesser David , Chaudhuri Anabil , Lee S C , Brueck Steven R.J. , Wetzel Christian
Published: February 2020

By some accounts group-III nitride may be the widest disseminated man-made compound semiconductors. Yet, what is it that makes it so successful? Maybe the wurtzite structure which allows for piezoelectric polarization? We know about the Stokes shift it induces and its associated troubles. So we rush to reduce polarization in non-polar growth. But how about the cubic form of GaN. Shouldn't that be free of polarization? Would it be the better light emitter even? To find out we prepare GaN and GaInN/GaN heterostructures in cubic lattice form. Yes, that has been done before but in MBE. No, we use MOVPE, the method responsible for all the bright emitters in the nitrides. By virtue of some patterning we achieve cubic growth on the more common form of Si, the (001) orientation, which further allows for conceptional integration with the semiconductor of industry choice. We discuss details of the epitaxial growth and progress in characterization results including first green LEDs.
(Invited) The Effect of Thermally Induced Piezoelectricity on GaN HEMT Device Characteristics
Authors: Sengupta Jonah , McCluskey Patrick , Khanna Sumeer
Published: February 2020

Wide bandgap semiconductor-based High Electron Mobility Transistor (HEMT) power amplifier devices are coming into widespread use in a variety of applications, including hybrid electric vehicles.  These devices dissipate considerable power, which, when combined with shrinking package size, leads to high densities of heat flux.  To remove this heat, novel cooling approaches are used which can create significant thermal gradients.   This study will discuss these packaging and thermal management approaches focusing on the piezoelectric defect generation in AlGaN/GaN high electron mobility transistors’ (HEMTs) resulting from thermomechanical stresses produced by these thermal gradients and the effect on electrical reliability. Two different multiphysics modeling software packages were used to determine the thermal gradients in a model of a HEMT device, convert them to thermomechanical stresses and then voltage gradients, and then assess defect creation resulting from the voltage gradient.  First a thermal gradient of between 30°C to 50°C was imposed across the device.  For this condition, a principal stress of 0.73GN/m2 was seen in the device along with a 9V drop across the AlGaN/GaN barrier. Next, devices were probe tested at a power density of 6.9W/mm in the GaN and a baseline temperature of 120°C in the substrate. Evident drain and gate current and on-resistance degradation were observed after 30 minutes.
(Invited) Plasmonic Terahertz Detectors
Authors: Shur Michael
Published: February 2020

We review the achieved and potential performance of plasmonic terahertz detectors implemented in III-N, III-V, Si, and graphene materials systems and operating in the overdamped and resonant regimes. Such detectors have been using single transistors and transistor arrays. They operated in a wide temperature range – from cryogenic to elevated temperatures – and in the frequency range from 0.2 THz to 5 THz with Noise Equivalent Powers as low as 1 pW/Hz1/2. The modulation frequency over 10 GHz has been achieved. The predicted modulation frequencies reach into the THz range.  Each materials system (III-N, III-V, and Si) has its optimum frequency range of operation. High mobility InGaAs plasmonic HEMT detectors are expected to operate efficiently up to 10 THz and higher. For the III-N plasmonic transistors, the operation range could reach or exceed 1 or 2 THz. Si CMOS could work as efficient plasmonic THz detectors up to 1 THz or higher. The optimum channel length for each frequency range depends on the materials system. The device responsivity peaks near or slightly below the threshold voltage and is greatly enhanced by driving drain or gate currents due to the increased non-linearity (in the case of the drain current) and due to the induced phase shift (in the case of the gate current). The detection of single THz pules with plasmonic detectors has also been achieved. These THz detectors are expected to find numerous applications in sensing, imaging, wireless communications, and security systems.
Early Detection of Lung Cancer Using High Electron Mobility Transistors
Authors: Sarangadharan Indu , Chu Chia Ho , Hsu Chen-Pin , Wang Yu-Lin
Published: February 2020

Lung cancer is the leading cause of cancer deaths in the world, among both men and women. It claims more lives each year than do colon, prostate, ovarian and breast cancers combined. Currently, diagnosis of lung cancer in stages I and II is very difficult and hence prognosis is delayed, leading to increased mortality rate. Carcinoembryonic antigen (CEA), is a common cancer biomarker and the expression of CEA above 5 ng/mL is suggestive of the presence of cancer. In this study, we utilize immunology, which is antibody/antigen reaction, with the combination of High Electron Mobility Transistor (HEMT) as the transducer, to develop a biosensor for detecting lung cancer biomarkers, electronically, as opposed to traditional ELISA techniques. HEMTs have already been demonstrated in gas, ion and biomolecule sensing purpose. When analytes accumulate on the gate area, the net charge on the HEMT surface changes. This electrical detection technique is simple, fast, and convenient. The detecting signal from the gate is amplified through the drain-source current, making the sensor very sensitive.

Figure 1 (a) and (b) show the schematic of AlGaN/GaN HEMT and the top-view of the device. To create a high quality HEMT device, a 3 μm-thick undoped GaN buffer layer and a 150 Å-thick undoped Al0.25Ga0.75N layer were deposited by metal-organic chemical vapor deposition (MOCVD) on the substrate. Mesa isolation was formed by Inductively Coupled Plasma (ICP) etching system for 28 seconds. Ti, Al, Ni and Au were deposited by Electron Beam Evaporation and were annealed at 850 °C for 45 seconds under N2 flowing to form ohmic contacts. Passivation was provided by encapsulating the device with photoresist, with only the gate region open for antibody immobilization using photolithography.

After sensor fabrication, surface functionalization is carried out by thiol binding on gold. We mixed bi-functional thio-carboxylate with the CEA antibody and let the antibody bind with the carboxyl functional group by EDC-sulfo NHS coupling reaction. Then the device is incubated in this solution for 12 hours at 4 ºC. BSA is used for surface blocking and as a protein dilution buffer. For the sensor measurement, source-drain bias is fixed at 0.5 V and the current change induced by binding of CEA was measured by applying a 0.5 V gate voltage.

Figure 1 (c) shows the total charge accumulated for various concentrations of CEA. This can be used as an index to quantify the amount of CEA present in sample as it can distinguish various protein concentrations even in the physiological blood/serum concentration (1x PBS). The change in total charge with varying concentration of CEA can thus represent sensor calibration curve. Comparing to the conventional methods, the electronic detection of CEA using HEMT has the advantages of high sensitivity and low response time. We believe this method has the potential to further the research in the field of electronic biosensors.

Figure 1
The Photoluminescence Properties of CuInS2 and AgInS2 Nanocrystals Synthesized in Aqueous Solutions
Authors: Borkovska L. , Romanyuk A , Strelchuk V. , Polishchuk Yu. , Kladko V. , Stroyuk O. , Raevskaya A. , Kryshtab T.
Published: February 2020

The CuInS2 and AgInS2 are the low-toxic semiconductor materials with direct band gap and high absorption coefficient in the visible and near-infrared regions. The nanocrystals (NCs) based on these materials demonstrate enhanced photoluminescence (PL) tunable over the whole visible spectral range resulting in growing attention for their application in light-emitting devices, solar cells and bio-markers.

Despite the fact that different strategies have been developed for the synthesis of I-III-VI NCs, it still remains a challenge to develop a facile, general, and inexpensive method for the preparation of I-III-VI NCs with controllable luminescence spectrum.

Herein, we report the results of investigations of the photoluminescent, optical and structural properties of CuInS2 and AgInS2 NCs synthesized in mild conditions in aqueous solutions in the presence of aliphatic mercapoacids. The pristine colloidal solution of the NCs, the powdered NCs as well as the NC embedded in the polymer films of gelatin or polyvinyl alcohol were studied. The X-ray diffraction patterns and the Raman scattering spectra of powdered NCs indicated their tetragonal phase. The average size of the NCs was estimated to be about 4 nm for the AgInS2 and about 2 nm for the CuInS2 ones. The optical absorption spectra of the NCs solution as well as of the nanocomposite films showed the band onset larger than the band gap of the corresponding bulk chalcopyrite materials indicating a quantum confinement effect.

The PL spectra showed a wide band with maximum at about 2.0 eV for the AgInS2 NCs and 1.8 eV for the CuInS2 NCs and a Stokes shift in the range of 0.3-0.7 eV. The PL band is caused by carrier recombination via the levels of defects in the NCs. No exciton emission was observed. As the temperature decreased, the PL intensity increased, the PL band broadened and the PL maximum shifted to the blue on about 0.02 eV for the CuInS2 NCs and on about 0.27 eV for the AgInS2 NCs. It is shown that the PL spectra of the CuInS2 NCs can be well fitted with single Gaussian function, while those for the AgInS2 NCs by two. The PL band spectral position did not change essentially with the excitation power, but varied with the excitation energy (wavelength). The latter is explained by strong dependence of the PL excitation spectra on the emission position in the PL spectrum. For the AgInS2 NCs this dependence was found to be different for two PL components which allowed supposing that these PL components were connected with two different ensembles of the NCs.

It is shown that the PL quantum yield in the NCs studied can reach around 15% that makes them suitable for future optoelectronic and bio-medical applications.
Investigation of Trapezoidal Well for Improving the Light Efficiency in Algainp-Based LEDs
Authors: Oh Hwa Sub
Published: February 2020

(AlxGa1-x)0.5In0.5P-based LEDs have a small conduction band offset that limits their electron confining potential.1 This weaker electron confinement subsequently leads to electron heterobarrier leakage in AlGaInP heterostructure LEDs, especially in short-wavelength devices, where a fraction of the electrons injected into the active region have a sufficient thermal energy to escape into the cladding layer. To overcome this problem, trapezoidal well structures are introduced in the active region in order to prevent carrier overflow and to gain a higher light output power (Pout). In the field of AlGaInP-based LEDs that emit a short peak wavelength at around 590 nm, however, the effects of the trapezoidal well profile in the active region on LED performance has yet to be systematically studied.

In this study, we investigate the behaviors of optical and electrical characteristics according to their trapezoidal well profile in the active region through the analysis of optical behaviors and device performances.

To investigate the optical behaviors according to the trapezoidal well shape, the sloped barrier (SB) is inserted between well and flat barrier (FB) and the thickness of SB (dSB) is changed from 2.5 nm to 10 nm. And, the FB thickness is adjusted from 15 to 0 nm to maintain the total barrier thickness 20 nm. The schematic diagram of MQW structures of a trapezoidal-shaped well is depicted in Fig. 1(b).

Fig. 2 shows the device performances having different sloped barrier profiles in the MQW LED structures. As the current is increased to 600 mA, the current-voltage (I-V) curves show different behavior according to the SB thickness. In Fig. 1(a) inset, the forward bias voltage at 350 mA decreases from 2.86 V to 2.62 V with an increase in the thickness of SB from 0 to 10 nm. Fig. 1(b) then shows the light output power (Pout)-current characteristics of LEDs on the dSB and the normalized efficacy of the same LEDs at 350 mA operating current is shown in the figure inset. As the dSB between the well and the barrier is increased from 0 to 5.0 nm, the normalized efficacy improves from 1.0 to 1.16 and the maximum Pout increases from 400 to 500 mA, though at a further increase of the dSB to 10 nm, the normalized efficacy degrades to 0.98.
How Predictive Are Reactor-Scale Simulations of Wbg Semiconductor Epitaxy? a Sensibility Analysis of SiC and Nitride Epitaxy Models
Authors: Yanguas-Gil Angel , Shenai Krishna
Published: February 2020

Process modeling can be the perfect complement to process development: in addition to offering a cost-effective way of exploring the parameter space of a given process or new reactor configurations,  models are able to provide information regarding details of surface and gas phase chemistry that is not readily available experimentally. This information can be extremely useful in helping us rationalize trends in yield, epitaxial quality and defect concentration, or why a particular set of experimental conditions leads to the highest quality materials.

In the case of physical vapor and chemical vapor epitaxy, the fundamental aspects of reactor-scale simulations are well established. However, it is our knowledge of homogeneous and heterogeneous processes what limits the predictive nature of our models. Every reactor rate and transport coefficient comes with an associated error that propagates into the model solution.  In addition to this, the complexity of the overall kinetics of the growth process complicates our ability to identify rate limiting processes exclusively based on experimental trends of growth rates as a function of experimental parameters. Consequently, knowing how sensitive our model is to the different assumptions will help us both better understand the margin of error of model predictions and identify the key steps in the growth process.

In this talk I will summarize the sensitivity analysis studies that we have carried out on the heterogeneous and homogeneous processes during SiC, GaN, and AlN epitaxy. Using datasets compiled from different models available in the literature, we will compare the predicted results in both 0.5D and simplified 1D and 2D models that will allow both local and non-local sensitivity studies. The advantage of 0.5D models is that they can be used as a first order approximation to simulate epitaxy on planetary systems where wafer to wafer variability is lower than in-wafer variability. We will use these models to identify both  the key steps and  the main sources of error as a function of the experimental conditions. Finally, we will study the impact that reduction on complexity has on model predictions.
(Invited) Developing Periodically Oriented Gallium Nitride for Frequency Conversion
Authors: Hite Jennifer K , Freitas Jaime A. , Mastro Michael A , Vurgaftman Igor , Meyer Jerry R. , Brown Christopher G. , Kub Francis J , Bowman Steven R. , Eddy Charles R.
Published: February 2020

Gallium nitride is a semiconductor widely used in both optical and electronic devices. The polarity of GaN (+/- c-direction) influences many properties of the resultant material, including chemical reactivity and electric field in these ‘spontaneously polarized’ materials. By engineering inversion layers, we have demonstrated control of GaN polarity on both polar faces of GaN. By employing a selective growth method to deposit the IL, the lateral polarity of the GaN can be alternated, thus enabling structures referred to as periodically oriented (PO) GaN.

On N-polar substrates, we demonstrated that optimization of the MOCVD growth rates resulted in sharp, vertical interfaces and smooth surfaces. This work has moved the technology substantially closer to practical non-linear optic emitters by using HVPE to extend the PO GaN templates on N-polar substrates to total thicknesses of up to 500 mm, while faithfully maintaining the pattern of alternating polarity. Additionally, cross-sectional cathode-luminescence (CL) imaging of such an extension shows that the large initial dislocation densities occurring in the original inversion layers greatly decreased after about 25 mm of regrowth.

For growth on Ga-polar substrates, we have demonstrated that inversion layers can be created using atomic layer deposition (ALD) of Al2O3. This new capability is especially relevant because Ga-polar films are more prominent in devices, as they result in lower impurities, higher quality and smoother films. In this case, GaN grown over the inversion layer is N-polar. This inversion layer was used to form laterally-patterned stripes of alternating Ga- and N-polar films. We find that annealing the ALD films crystallizes the Al2O3, thereby allowing N-polar GaN to be grown over the new sapphire-like surface. Transmission electron microscopy shows that the inversion layer in a PO GaN structure is crystalline, a-plane oriented, and a-phase. TEM characterization further indicates that the GaN layers, both above and below the Al2O3 inversion layer, are c-oriented without any rotation between them. The optimization of this process has enhanced the surface smoothness.

These methods of GaN polarity inversion offer the promise of engineered materials with custom lateral and vertical polarity variations for applications in novel electronic and optoelectronic devices, a subset of which are expected to be suitable for non-linear optics.
Weak Quantum Confinement and Polaritons in ZnO and ZnO Cu Nanocrystals Prepared by Electrochemical Method
Authors: Torchynska Tetyana V. , El Filali Brahim , Díaz Cano Aaron Israel , Shcherbyna Lyudmula V.
Published: February 2020

Photoluminescence (PL), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman scattering have been used for the optical and structural characterization of ZnO nanocrystals (NCs) of various sizes. The samples prepared by an electrochemical method have a size of NCs from the range 60 to 600 nm after a heat treatment for 2 hours at 400 oC in ambient air. The Raman scattering technique presents the several active modes including the surface phonon mode. The Raman intensity increases with decreasing NC size and the Raman spectrum shifts to low energy. Simultaneously the X-ray diffraction diagrams present a small shift of XRD peaks that testifies on a change in the lattice parameters in ZnO NCs with decreasing NC size.

Photoluminescence spectra show a free exciton and defect-related emission. The intensity stimulation of exciton-related PL bands with NC size decreasing up to 60 nm is attributed to the realization of the week confinement and the exciton-light coupling with the formation of polariton in small size of ZnO NCs. The numerical calculation of recombination times in the frame of this model has confirmed the presented explanation.
Development of Wide Bandgap Multifunctional NiO Nanostructures and Thin Films for Sensing Applications
Authors: Tomar Monika , Tyagi Manisha , Sharma Anjali , Gupta Vinay
Published: February 2020

Realizations of transparent pn homo-junction have become more important for the development of transparent electronics for the next-generation optoelectronic devices. The tunability of the electrical conductivity of NiO thin ﬁlms with both P-type and N-type conduction by varying growth kinetics using RF sputtering technique with good optical and properties in the visible region have been demonstrated. The pn homo-junction of NiO thin ﬁlms has been fabricated. The room-temperature conductivities for n-type and p-type NiO thin ﬁlms have been studied using Hall measurement. Glancing angle deposition (GLAD) configuration of rf sputtering technique is utilized to fabricate the array of vertically align nanorods of NiO and are exploited for the development of homojunction devices. The fabricated homo-junction exhibits good rectifying behaviour with efficient ultra-violet (UV) photodiode characteristics, and providing suitable solution for the development of low-cost UV photo-detectors with enhanced response characteristics. The obtained results on the developed UV photo-detector will be elaborated.

 Further, Nickel oxide (NiO) nanoparticles (NiO-NPs) have been synthesized by a simple chemical route and exploited successfully for the fabrication of urea biosensors. A thin layer of NiO nanoparticles deposited on indium tin oxide (ITO) coated glass serves as efficient platform for the immobilisation of Urease (Ur) biomolecules for the detection of Urea. The array of NiO nanorods prepared by GLAD was also used as a matrix for making bio-electrode for the efficient detection of Urea using electrochemical techniques. The prepared amperometric bio-electrodes based on NiO nanostructures (Ur/NiO/ITO/glass) exhibits a high sensitivity of about 21.0 mA/(mM-cm2) and good linearity over a wide range (up to 16.65 Mm) of urea concentration.
(Invited) Analytic Methods for Benchmarking Hydrogen and Fuel Cell Technologies
Authors: Melaina Marc , Saur Genevieve , Ramsden Todd , Eichman Joshua
Published: February 2020

The National Renewable Energy Laboratory (NREL) is involved in a number of low carbon energy conversion projects for transportation and stationary applications. The overall focus is on sustainable, economic solutions that also provide lower carbon emissions for the environment. The range of projects provide cross application insight into transforming how we think about and use energy.

NREL has worked on low carbon energy projects from renewable hydrogen production to integration of hydrogen technologies with electricity markets to end-use applications in the transportation and stationary fuel cell markets. Several different analysis tools and studies have focused on energy resource potential, costs, emissions, and logistical requirements for deployment. These projects bridge traditional market boundaries by integrating electricity services, energy storage, and transportation applications that also can lower the carbon footprint of these energy sectors. In this presentation we will review four types of crosscutting metrics to used to benchmark low-carbon technologies: (1) resource potential, (2) greenhouse gas emissions, (3) cost of delivered energy, including the impacts of different control strategies, and (4) influence of revenue streams from electrolysis units providing grid services. The results of these various analytic approaches highlight the complexity and interdisciplinary nature of efforts to benchmark low-carbon hydrogen and fuel cell technologies.
(Invited) Benchmarking a Metal Oxide-Based Thermochemical Cycle for Solar Hydrogen Production
Authors: McDaniel Anthony H. , Ermanoski Ivan
Published: February 2020

It is estimated that growth in global population and continued industrialization of developing countries will double world energy consumption by 20351. This rising demand for energy will be largely met by burning fossil fuels, thereby increasing anthropogenic carbon in the atmosphere and fuelling the socioeconomic strife associated with climate change. Developing transformational energy technologies that efficiently and cost-effectively convert solar energy into simple chemical fuels is therefore necessary in order to realize a carbon-neutral, sustainable energy future.

In this talk we will critically assess a two-step, high temperature thermochemical water splitting (WS) cycle powered by concentrated solar energy. This conceptually simple technology, outlined in Figure 1, can theoretically achieve greater solar-to-hydrogen (STH) conversion efficiency than alternative WS routes such as photosynthesis (natural or artificial) or photovoltaic-driven electrolysis2. However, many engineering and material science challenges must be overcome to demonstrate economic viability at large scales.

Cost and scalability issues for solar thermal hydrogen production will be addressed within the context of a particle-based solar receiver reactor concept3–5developed at Sandia through a multi-year research initiative sponsored by the US Department of Energy. Key design principles will be reviewed and create a framework for discussing the synergistic interplay of materials chemistry, reactor design, and system operation necessary for this technology to achieve optimal STH conversion efficiency. The successful implementation of this solar energy conversion technology is predicated on meeting stringent requirements for cost and scalability that are deeply rooted in STH efficiency.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

(1)   International Energy Outlook 2011; DOE/EIA-0484(2011); U.S. Energy Information Administration, DOE/EIA-0484(2011) | September, 2011.

(2)   Siegel, N. P.; Miller, J. E.; Ermanoski, I.; Diver, R. B.; Stechel, E. B. Ind. Eng. Chem. Res.
2013, 52, 3276–3286.

(3)   Ermanoski, I.; Siegel, N. P.; Stechel, E. B. J. Sol. Energy Eng.
2013, 135, 031002.

(4)   Ermanoski, I.; Miller, J. E.; Allendorf, M. D. Phys. Chem. Chem. Phys.
2014, 16, 8418.

(5)   Ermanoski, I. Int. J. Hydrog. Energy
2014, 39, 13114–13117.

Figure 1
(Invited) Renewable Hydrogen Evolution on Nickel Phosphide Electrocatalysts: A Comparative Study of Efficiency and Tolerance to Corrosion
Authors: Dismukes G. Charles , Laursen Anders , Liu Bin , Patraju Kelly , Greenblatt Martha
Published: February 2020

The intermittent nature of renewable energy sources like photovoltaics and wind mills requires storage of large quantities of energy. Renewable hydrogen produced by electrochemically water splitting into its elements pose an unlimited resource for energy storage. Gaseous hydrogen is not only a carbon-free energy carrier which could replace fossil fuels, but also an important bulk chemical of which ~108 ton/year is produced from natural gas. Platinum and nickel (or alloys thereof) are the electrocatalysts used today for hydrogen evolution in acidic and alkaline solution, respectively.  However, the scarcity of platinum and the low efficiency of nickel, have greatly restricted the development of renewable hydrogen from water splitting. Since 2013 the number of reports on the use of transition metal phosphides as a replacement for Pt for hydrogen evolution have virtually exploded. Ni3P and Ni2P were the first phases shown to be highly electrochemically active for hydrogen evolution. Ni2P was unfortunately was shown to deactivate in acid and alkali. We recently reported, that another crystalline phase of the nickel phosphide family, Ni5P4, outlperforms Ni2P and Ni3P and is comparable to platinum on a geometric basis in its electrical efficiency at hydrogen evolution (kinetics and intrinsic overpotential). Moreover, unlike Ni2P this catalyst is corrosion resistant, being stable in both strong acid and alkali during electrocatalysis. Here, we will present our most recent work on multiple nickel phosphides as well a structural basis for the unusually high catalytic activity and corrosion resistance. The work was supported by AFOSR, the NSF and NATCO Pharma.

Figure 1
Fabrication of a Novel BaCe0.8Y0.2O3-δ - Cu Ceramic-Metallic Composite Membrane for Hydrogen Separation
Authors: Rosensteel Wade A , Sullivan Neal P
Published: February 2020

This presentation details the first report of a dense BaCe0.8Y0.2O3-δ–Cu (BCY-Cu) ceramic-metallic (cermet) membrane for high-temperature hydrogen separation. Such a composite may have applications in catalytic partial oxidation of methane (CPOX), steam reforming of methane, the gasification of carbonaceous materials, and methane dehydroaromatization (MDA). BCY serves as the proton conductor, while Cu provides electronic conductivity for electronic-charge compensation. A novel molten-copper infiltration technique is used to form a dense cermet membrane by infiltrating a porous BCY skeleton.

The BCY skeleton fabrication begins with mixing appropriate amounts of BaCO3, CeO2, and Y2O3 powders with a mortar and pestle, followed by calcination in air. The phase-purity of the resulting powder is verified by X-ray diffraction spectroscopy. Binder is added to the powder via wet-milling with water, followed by pan drying. The powder is crushed with a mortar and pestle, and sieved to achieve a uniform particle size distribution. Pellets are formed by uniaxial dry-pressing, and subsequently sintered at 1600 °C in air, creating a skeleton with approximately 50 % open porosity (Figure 1a). 

This porous BCY skeleton is then infiltrated with copper. Cu and CuO powders are mixed with a mortar and pestle to form a powder containing 8 at.% O. This powder is uniaxially pressed into pellets, which are placed on top of the BCY skeleton. The skeleton and Cu-CuO pellet are placed into a controlled-atmosphere furnace, and heated to 1200 °C in a 330-ppm-oxygen environment (balance argon). Under these conditions, the liquid Cu spontaneously infiltrates the BCY skeleton. While dwelling at 1200 °C, the gas environment is changed to a 10 mol.% hydrogen environment (balance argon), and the sample is cooled.

Cu will not wet, nor infiltrate, a BCY skeleton in a reducing environment, which necessitates the initial heating in the oxidizing environment. Once the Cu-CuO alloy infiltrates the BCY skeleton at high temperature, the environment can be switched to a reducing environment without the Cu-CuO alloy defiltrating the BCY skeleton, due to capillary pressure. The reducing environment removes the O from the alloy, leaving pure Cu metal. An electron micrograph of a polished-cross section of the resulting membrane (Figure 1b) reveals that the cermet is nearly fully dense. Our current efforts focus on hydrogen permeation measurements through these novel cermets.

Figure 1: (a.) Scanning electron micrograph of BCY skeleton after sintering, and (b.) scanning electron micrograph of BCY-Cu membrane, in which the dark phase is Cu and the light phase is BCY.

Figure 1
(Invited) Electrochemical CO2 Reduction to Formic Acid on Crystalline SnO2 Nanosphere Catalyst
Authors: Fu Yishu , Liu Yuyu , Li Yanan , Qiao Jinli , Zhou Xiao-Dong
Published: February 2020

The electrochemical reduction of CO2 is a reaction of much current interest as a possible reaction for energy storage.1 To date, formic acid, carbon monoxide, methanol, and oxalic acid have been prepared by this way. However, the key technological challenge for electrochemical reduction of CO2 is the preparation of the electrode with high catalytic activity, high selectivity and long term stability. Considering these difficulties, developing new material synthesis technology to give innovative new catalysts with optimal performance is the priority.

 Tin (Sn) and copper (Cu) are considered as promising electrocatalysts to convert carbon dioxide (CO2) to fuels (e.g. formate, methanol or hydrocarbons) because of their low cost, easy availability and reasonable overall Faradaic efficiency towards fuel production.2 However, the deactivation of Sn metal electrodes during CO2 reduction is very fast, and requires at least 0.86 V of overpotential to attain a CO2 reduction partial current density of 4−5 mA cm-2 in an aqueous solution saturated with 1 atm CO2.3 Combined with the advantages of metal tin, we here report a simple one step synthesis of crystalline SnO2 nanosphere with good electrochemical performance of high catalytic activity and high selectivity.
Effect of Acid Leaching on the Catalytic Activity of Co-Salen/C Non-Precious Metal Catalysts for Oxygen Reduction Reaction
Authors: Qiao Jinli , Xu Pan
Published: February 2020

To overcome the barrier of  high cost caused by the exclusive use of Pt-based catalysts, the development of non-precious  metal catalysts (NPMCs) to replace Pt in polymer electrolyte membrane fuel cells has become the goal of intensive research in recent years.1,2 Some of these NPMCs have shown remarkable catalytic activity towards ORR. Among these NPMCs, carbon-supported transition metal/nitrogen ( M/N/C, M = Fe, Co, Mn, etc.) materials have gained increased attention due to their promising catalytic activity and high durability.3-5 However, the role of transition metal playing in the catalysts’ active sites is still a subject of controversy. In order to further clarify the nature of the active sites of NPMCs, in this, with Co(SO4)7H2O as the metal precursor and N,N-bis (salicylidene) ethylenediamine (Salen) as the nitrogen precursor, carbon-supported non-precious metal catalysts, Co-Salen/C, were synthesized using a facile thermal annealing approach. The catalysts were heat-treated at different temperatures (from 600- 1000oC) to optimize oxygen reduction reaction (ORR) activity. To clarify the significance of metal (Co) content for the ORR enhancement, the catalyst was further processed by acid leaching.  

The electrocatalytic activity and electron transfer mechanism were demonstrated in oxygen-saturated alkaline electrolyte by cyclic voltammetry (CV), linear sweep voltammetry (LSV) as well as rotating disk electrode (RDE) techniques. Scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements were used to identify the structure and composition of the catalysts.
Synthesis and Conductive Property of Alkaline Anion-Exchange Membranes Based on Poly(vinyl alcohol)/Bis(2-chloroethyl) Ether-1,3-bis[3-(dimethylamino)propyl] Urea Copolymer Composite
Authors: Chen Shuli , Song Yanxi , Song Feifei , Zhao Xiaoxiang , Qiao Jinli , Zhou Xiao-Dong
Published: February 2020

Anion-exchange membrane fuel cells (AEMFCs) researches have recently made a noticeable comeback from proton-exchange membrane fuel cells (PEMFCs) dominant era, since the AEMFCs can get faster electrokinetics, lower fuel crossover, reduced CO poisoning, and use of non-precious metal catalysts1,2. In the development of AEMFCs, the conductivity of  alkaline anion-exchange membranes (AEMs) is considered to be the most chanellagen, since the mobility of OH- is only 1/4 of that of the H+ transportation (membranes that conduct H+ cations), the membranes that can have high OH- conductivity are highly desired for making a breakthrough in AEMFCs performances3. In cases where much effort has been undertaken for the above purpose and some promising candidates have been proposed such as quaternized polysulphane (PS)4, poly(ether imide) (PEI)5 and radiationgrafted PVDF and FEP6. However, these polymers were generally of high price and their quaternization is complex process. 

Chitosan (CS) is a low-cost biopolymer, which has very good mechanical and chemical stability along with good film formation property7. However, since CS is just a weak alkaline conductivity, it cannot be directly used to fuel cell membrane. In this work, we report a novel series of alkaline anion-exchange membranes: Chitosan /Bis(2-chloroethyl)ether-1,3-bis[3-(dimethylamino)propyl]urea copolymer (PUB) membranes. By incorporation of PUB, the OH- conductivity of the CS/PUB membrane was found to be greatly improved and reached high up to 8.610×10-
3S cm-1 at 70oC.
Noble Metal Aerogel Design for Bio-/Fuel Cell Applications
Authors: Wen Dan , Zhu Chengzhou , Eychmüller Alexander
Published: February 2020

The considerable interest in energy technology has sparked a sustained research effort on fuel cells because of their high energy efficiency and outstanding environmental compatibility.[1] Aerogels derived from noble metal nanoparticles (NPs) are a new kind of materials, which not only retain the inherent characteristics of the aerogels, including high surface area, interconnected porosity in the meso and macropore size range, and nanosized primary units, but also possess high electrical conductivity and intrinsic catalytic activity.[2] These unique properties endow them with an enormous potential in applications, such as electrocatalysis.

  Our recent efforts have been focused on the design of noble metal aerogels with controllable morphologies or composition and their exploration in bio-/fuel cell applications. Via a cation-induced or a spontaneously running route, we realized the controlled growth of Pd aerogels with different porosities and surface areas and obtained aerogels composed of Pd-Ferrocene. They provided a three-dimensional matrix with high electrical conductivity and high surface area for enzyme loading, showing a promoted bioelectrocatalysis and biofuel cell application.[3,4] On the other hand, a series of highly efficient AuxPty bimetallic aerogel electrocatalysts derived via a host-guest interaction-induced method exhibited high activity, low overpotential, and high CO tolerance toward the direct oxidation of methanol, ethanol and glucose.[5] Additionally, PdxNiy aerogels from kinetically controlled synthesis displayed excellent performance toward the methanol oxidation.[6] Here the non-precious metal component was introduced to lower the cost of the fuel cell electrocatalyst and to increase its activity due to the bimetallic synergistic effect.

References

[1] M. Debe, Nature
2012, 486, 43–51.

[2] W. Liu, A.-K. Herrmann, N. Bigall, P. Rodriguez, D. Wen, M. Özaslan, T. Schmidt, N. Gaponik, A. Eychmüller, Acc.
Chem. Res.
2014, revised.

[3] D. Wen, A.-K. Herrmann, L. Borchardt, F. Simon, W. Liu, S. Kaskel, A. Eychmüller, J. Am. Chem. Soc.
2014, 136, 2727–2730.

[4] D. Wen, W. Liu, A.-K. Herrmann, A. Eychmüller, Chem. Eur. J.
2014, 20, 4380–4385.

[5] D. Wen, W. Liu, D. Haubold, A. Eychmüller, 2014, submitted.

[6] C. Zhu , D. Wen , M. Oschatz , M. Holzschuh , W. Liu , A.-K. Herrmann , F. Simon , S. Kaskel , A. Eychmüller, small
2014, accepted, DOI: 10.1002/smll.201401432.

Figure 1
Oxygen Reduction on Hafnium/Hafnium Carbide Nanoparticles
Authors: Baturina Olga A , Epshteyn Albert , Purdy Andrew
Published: February 2020

Non-precious metal nanoparticles are attractive candidates for oxygen electrocatalytic reduction in electrochemical energy conversion devices, such as fuel cells and batteries. Here we report for the first time electrocatalytic activity of hafnium/hafnium carbide nanoparticles towards the oxygen reduction reaction (ORR). While activity of hafnium oxynitride nanoparticles towards the ORR in acid has previously been reported   [1] a little was known about the ability of Hf/HfC nanoparticles to reduce molecular oxygen. Here we demonstrate that molecular oxygen can be reduced on Hf/HfC nanoparticles both in acidic and alkaline environment. However, the reaction mechanism changes from 4 - to 2 - electron pathway, when switching from acidic to alkaline environment. Although the ORR in base occurs through the two-electron parthway, this reaction has a potential for a practical application because Hf/HfC catalyst demonstrates an activity toward the disproportionation of hydrogen peroxide to O2 and H2O. Onset potential for the ORR in 0.1 M KOH for non-optimized catalyst (-0.10 V vs AgAgCl ) is superior to that of the best reported to date MnO2 catalyst (-0.13 V vs Ag/AgCl). In acid, moderate catalytic activity towards the ORR is observed. The results suggests that Hf nanomaterials can be considered as a promising candidate for replacing noble metal catalysts in alkaline air electrodes.

[1] CHISAKA, M. & MURAMOTO, H. 2014. Suppression and Evaluation of Hydrogen Peroxide Formation on Carbon-Supported Hafnium Oxynitride. ECS Electrochemistry Letters, 3, F1-F4.
Impregnation Based Electrodes for Solid Oxide Fuel and Electrolysis Cells, the State-of-the-Art and Perspectives
Authors: Boulfrad Samir , Da'as Eman Husni , Bi Lei , Traversa Enrico
Published: February 2020

Increasing the density of the triple phase boundary (TPB) sites has been the main focus of the microstructural optimization of the electrodes in solid oxide fuel and Electrolysis Cells (SOCs). TPB is the interface between the catalyst/electronic conductor, the ionic conductor, and the gas phase where the electrode reactions occur. Better electrode performance is achieved with increasing the TPB surface.  Classically, electrodes are prepared by mixing powders of the solid phases and then depositing them to form porous films after appropriate heat treatments. Optimizing the composition and particle size distribution of starting powders, as well as the deposition and sintering processes have led to a substantial increase in the TPB density. However, this has reached an optimum maturity. In the recent years impregnation based processes were developed to achieve larger TPB and better performance. In this scenario, one solid phase, usually the electrolyte, is sintered to form a porous backbone structure to be impregnated with a liquid precursor of the second phase. After drying and heating, the second solid phase is usually formed as a very fine structure on the top of larger particles of the porous backbone. This ensures a significant increase in the TPB, and good percolation of both solid phases. Very interesting results have been reported with a large number of cathode and anode materials in SOCs.

In this paper we will review the progress performed in our laboratories in impregnation based SOCs electrodes, including methods, materials, and results. In addition, the optimization of inkjet printing impregnation process will be presented and discussed in terms of controllability, reproducibility, and scalability.
(Invited) Measuring and Modeling Transport Processes in Porous Electrodes
Authors: Gostick Jeff T.
Published: February 2020

The porous materials found inside fuel cells and other electrochemical devices differ substantially from the traditional porous media studied by geologists and reservoir engineers (i.e. rock, sand and soil).  The list of challenging features of porous electrode materials is long: they can be extremely thin, nanoporous, electrically conductive, anisotropic, highly porous, fibrous, multilayered, fractured, compressible, and not self-supporting.  As such, the standard set of experimental tools and modeling techniques are not always suitable or applicable.  In this tutorial, progress made in porous materials characterization in the fuel cell field over the past decade will be summarized, but this tutorial will be of interest to workers in any application involving engineered porous materials.  Specific topics will include pore phase measurements of effective diffusivity and permeability, solid phase measurements such as electronic and thermal conduction, multiphase flow related processes such as capillary pressure curves and breakthrough conditions, as well as pore space characterization techniques such as porosity, pore size distributions, and X-ray tomography.  The role and value of the various transport parameters will also be discussed in relation to modeling and simulation.