Symposium EN16 : Combining Materials, Technologies and Societal Awareness to Harvest Natural and Human-Made Energy Sources

In order to have an impact at scale, next-generation photovoltaics have a formidable challenge: to lower the levelized cost of electricity by increasing efficiency and annual energy yield. Likewise, in order to become a scalable technology, photoelectrochemical generation of solar fuels and chemicals, such hydrogen or reduced products of carbon dioxide, must borrow from the accumulated photovoltaics knowledge base about low-cost large-area electronic device manufacturing. Tandem photovoltaic structures are compelling candidates for the next photovoltaic technology generation; of special interest are tandem structures utilizing existing Si solar cells and manufacturing methods for the bottom cell device in a two-junction tandem photovoltaic structure. Such a “X-on-Si” approach puts great pressure on the top cell device, since it must generate a significant fraction of the power at a very marginal added cost. I will discuss and compare several strategies for tandem photovoltaic module design, including i) epitaxy-free III-V/Si integration, perovskite on Si, transition metal dichalcogenides/Si, and a luminescent solar concentrator/Si. Tandem photoelectrodes are also necessary for integrated photoelectrochemical solar fuel generators to enable the photovoltage to exceed the thermodynamic reaction potential for fuel formation, as well as the kinetic overpotentials for balanced oxidation and reduction reactions. While record solar-to-fuel efficiencies can be achieved in flat-plate, single crystal, monolithic tandem multijunction III-V heterostructure photoelectrodes, such structures are likely too expensive for scalable solar fuel generation. Approaches to photoelectrochemical device architecture based on alternative tandem architectures will be discussed, including the tandem structures noted above as well as oxide-based photoelectrodes, partially-integrated photoelectrochemical structures and separately-wired photovoltaic and electrolyzer structures, with a focus on device efficiency and long-term durability.

The importance of having an optimal material for fabricating Optically Transparent Antennas (OTAs) is crucial for designing highly efficient antennas that can be integrated with photovoltaics. Transparent Conductor Oxides (TCOs) are promising for OTA
fabrication due to their capability of being simultaneously transparent at optical frequencies and conductive within the radio frequency (RF) range.

In this work, a new material was developed and optimized to be used for fabricating
an optically transparent antenna on a solar cell. Aluminum and Zinc Oxide were co-sputtered onto Si and onto a polycrystalline photovoltaic cell and then annealed between 350°C and 450°C for 24 and 48 h in N2 ambient. The annealing process ensured the
formation of the Aluminum Zinc Oxide (AZO) with a DC conductivity of 3.48×10Siemense over centimeters and a transparency of 86% for a thickness of 350 and 750 nm.

This new AZO material was shown to be capable of yielding high levels of conductivity at RF frequencies and excellent transmittance at optical frequencies. The material was tested and validated by performing RF characterization, and by fabricating
and testing different optically transparent antennas. The details of the fabrication process, its optimization process, the design of the optical antennas are presented in details and discussed. The material was tested and validated by performing RF characterization, and by fabricating and testing different OTA designs

The power factor (PF) of thermoelectric materials, S²σ, where S is Seebeck coefficient and σ is electrical conductivity, requires high charge density at an energy level ca. 0.1 eV below the transport level, and high mobility of charge carriers in that level. Semiconducting polymers are considered for this purpose because of their possible contribution to thermoelectric composites based on sustainable materials and fabrication processes. Creating stable charge carriers in a semiconducting polymer structure that maintains mobility is a materials chemistry challenge. This talk will discuss two approaches to this challenge. For hole conductivity, we modified a standard thiophene polymer structure (PQT12) with electron donating sulfur atoms between the dodecyl chains and thiophene rings, and with ethylenedioxy substitution on half the thiophene rings. Both of these modifications are intended to stabilize holes and achieve unusually high nonionic polymer conductivity. For each of the modifications, one particular dopant yielded the highest σ and PF.^[1] For electron conductivity, we employed an emerging n-type polymer with enhanced electron accepting properties and air-stable ionic dopants, one an inorganic salt and another a particle made from common elements to achieve the first step toward air stability of electron σ and PF.^[2] One notable aspect of both of these investigations is the consistent correlations of S and σ with predictions of recently published models, which indicates high mobility of doped forms of the polymers. A second aspect is the constancy of S over the minutes time scale following imposition of a temperature difference, decreasing the likelihood of a major ionic contribution to S. Spectroscopic measurements were used as alternate means of observing charge carriers, transistor data provided estimations of mobility, and x-ray scattering revealed the effects of doping on polymer chain packing.

[1] H. Li, M.E. DeCoster, R.M. Ireland, J.; Song, P.E. Hopkins, H.E. Katz,
J. Am Chem. Soc. 2017, 139, 11149-11157.
[2] X. Zhao, D. Madan, Y. Cheng, J. Zhou, H. Li, S.M. Thon, A.E. Bragg, M.E. DeCoster, P.E.; Hopkins, H.E.; Katz, Adv. Mater. 2017, 29, 1606928; D. Madan; X. Zhao; R.M. Ireland; D. Xiao; H.E. Katz, APL Materials 5, 086106 http://dx.doi.org/10.1063/1.4990139

Over the decades of PV development, a key barrier has been the cost of photovoltaics. Yet in the last few years, photovoltaics has changed from one of the more expensive photovoltaic technologies to recent power purchasing agreements as low below $0.03/kWh, photovoltaics has rapidly evolved into the lowest cost electricity generating technology. The possibility of rapid increases in PV generation has intensified debate over the impact of higher PV penetration rates on conventional grid electricity. However, California has more than 20% of its electricity from photovoltaics, demonstrating that the many solutions to dealing with variable generation on the grid are effective at enabling high penetration rates. The paper will examine approaches to mitigating the negative impacts of photovoltaics on the grid, showing that there is a range of tools to choose from, and the advances in areas intersecting with PV technology change the
A central question then arises what the critical remaining challenges are in photovoltaics. For example, one set of challenges relates to improving more outcomes from the electricity grid that generation-related reliability. For example, the power outages from weather / extreme events are increasing, and exceed those from other outage events. While improving generation reliability has little effect on such events, moving towards distributed generation can have a substantial impact. This then suggests, that barriers relate to inverters which can operate independent of the grid as well as grid-connected. It also has strong impacts on solar cell technology, such that generation of a substantial component of the load becomes desirable, placing a greater focus on higher efficiency. The inclusion of a battery also mitigates “duck curve.” Other benefits of increased and higher efficiency. Overall, examination of the advantages and drivers for photovoltaics show that innovation in photovoltaic technology is critical to achieve the benefits of photovoltaics.
Having demonstrated the importance and value of improved functionality and higher efficiency for photovoltaics to realize its benefits to the electrical grid, the final component of the paper examines approaches to high efficiency. Conventional technologies have focused on either flat plate or high concentration photovoltaics. However, in additional to these, a hybrid between them, focusing on tandems in lower cost commercial structures is emerging as an approach to overcome the efficiency and performance limitations of conventional structures. The paper gives an overview of technology paths and approaches to reach >35% for photovoltaic systems while increasing functionality and reducing costs.

Very efficient light emitting diodes (LED's), surprisingly, do actually become cold as they operate, since LED light carries away entropy. This cooling requires superb LED efficiency.
Of course, we now know that the photovoltaic cell and the LED are really the reciprocal of one another. The slogan: "A great solar cell has to be a great LED" has led to all the new solar cell efficiency records.
What if the electrical output of a photovoltaic cell drives an LED, and the LED light in turn drives the photovoltaic cell? You might fear that it would become a perpetual motion machine. Instead it becomes a heat engine in which a small amount electricity can efficiently provide refrigeration, or conversely a small temperature difference can generate electricity. Such an electro-luminescent heat engine, in which photons are the working fluid, can be more efficient than the competing science, thermo-electrics, in which electrons are the working fluid.

Photovoltaic (PV) systems are increasingly being used to harvest solar energy for increased sustainability. Because solar energy is ubiquitous, electricity can be generated closer to where it is required, thereby reducing transmission costs and losses and diminishing our reliance on traditional fossil fuels. Photovoltaic generators can come in the form of large-scale solar farms or small-scale systems integrated within the built environment. Either way it is more economical to use power closer to where it is generated. For example, many freeways have large noise minimising facades on the sides of the freeway. These, surfaces like residential roofs, can be used to harvest solar energy for the purposes of lighting, tunnel ventilation and electric vehicle charging.

However PV generators can exhibit high-frequency variations in their power output due to the intermittence of illumination. These variations can reduce the power quality and reliability in mini or micro-grids due to the reduced synchronous inertia. Energy storage can be used to address these issues by decoupling generation from demand. In addition to enabling load shifting, stored energy can be used to reduce the ramp rate of generated power and also improve power quality by reacting to detected frequency variations when the PV inverter connects to the grid. However, these latter fast frequency responses require that the energy storage system can be charged and discharged quickly and is capable of many cycles to be cost effective.

This paper will discuss the potential of integrating electrochemical capacitors capable of fast charging and discharging with the electronics of PV inverters or module-level power maximisers. With the increased availability of internet connectivity, the power buffering provided by these fast responding storage systems can be tuned according to expected shading events and/or external control systems for increased efficiency and power quality. Reliable local power can make possible a range of distributed power applications and allow higher PV penetration levels.

Photon upconversion is a process in which two or more low-energy photons are sequentially absorbed in a material and a high-energy photon is emitted. Efficient upconversion has numerous potential applications in energy harvesting. For example, a well-known challenge in photocatalytic water splitting for hydrogen fuel generation is that only a small portion of the incident solar spectrum has photon energies larger than the bandgap of the photocatalyst. Similar limitations apply to photovoltaics, where the inability to harvest low-energy photons is a significant contributor to the Shockley-Quessier limit on the net solar energy harvesting efficiency for a single junction device. Photon upconversion provides an exciting opportunity to "recycle" these wasted low-energy photons by converting them into higher-energy photons that can be harvested by the photovoltaic or photocatalytic material. Such photon upconversion technologies could also be employed to generate visible light from near-infrared solar energy or waste heat (thermal radiation) generated by electronics.

There are a few existing photon upconversion materials, but they do not meet the performance metrics necessary for significant impact on energy harvesting applications. Moreover, it is difficult to engineer these materials for specific applications. Semiconductors have inherently broadband absorption, capturing essentially all photons with energy above their bandgap. The absorption and emission wavelengths in semiconductor nanostructures can be controlled with both composition and size through quantum confinement effects. Moreover, semiconductor materials can be combined in complex heterostructures to guide the transfer of charges. For these reasons, semiconductors provide an appealing platform for photon upconversion.

We have developed a new semiconductor nanoparticle approach to photon upconversion. We will describe the underlying design principles and present numerical simulations that demonstrate the potential impact of such upconversion materials on solar energy harvesting. We will describe the synthesis and characterization of colloidal upconversion nanoparticles that make the creation of an “upconversion paint” a realistic possibility. We will describe how theses structures can be engineered to alter absorption and emission wavelengths and improve efficiency. We will demonstrate the viability of this new approach to photon upconversion and discuss the challenges and prospects for reaching efficiency levels that will make this technology commercially viable.

Accumulated soil on the surface of PV modules (“soiling”) is a universal phenomenon that can reduce annual energy production of PV systems by as much as 25%. The economics of this energy loss has implications across the entire PV system value chain, from module design to site selection, finance and operations and maintenance (O&M). This provides strong motivation to develop a deeper scientific understanding of the environmental processes that lead to soil accumulation, the detailed effects on PV performance and effective mitigation strategies. Due to the interplay of the various factors involved, multidisciplinary approaches are often required. In this presentation, we will present an overview of many of the challenges associated with soiling of PV panels that may be approached from the perspective of materials science.

Soiling is most commonly caused by deposition and adhesion of suspended atmospheric particulates. While influenced by the local soil, suspended particulate also contains pollutants, pollen and other organic matter, particulates generated by nearby construction, etc. Deposition strongly depends on particle size and is governed by the processes of particle diffusion toward the surfaces, of particular significance for very small particles, and of gravitational sedimentation, significant for larger particles. Adhesion of the deposited particulate to the module surface may be further influenced by factors such as tilt angle, surface texture and surface energy.

Composition of local suspended particulate affects the details of energy loss in addition to the soiling rate. Soils containing significant mineral content may act as a color filter, modifying the spectrum of the transmitted light that reaches the PV cell. Presence of minerals may also enhance cementation, hindering natural cleaning processes. In contrast, soils with a high soot content cause neutral attenuation of sunlight. However, soot can have as much as a 10x impact on transmission loss vs silica. As a consequence, PV systems deployed in areas with significant pollution from combustion may be more susceptible to soiling losses than systems located in rural or agricultural areas.

Many PV systems - particularly residential and commercial – are never cleaned, effectively leading to a permanent energy loss. The traditional mitigation strategy has been to clean systems periodically, either on a preset time interval or when monitored system data drops below a predetermined threshold. Cleaning is not always practical due to increased labor costs or local availability of water. Alternative mitigation strategies include the application of anti-soiling coatings and the use of dry, robot-based cleaning. However, these approaches may be mutually exclusive due to the potential for abrasive damage to the coatings during cleaning.

With any innovative technology there are technical and societal challenges. Specifically, once Research & Development creates a prototype that demonstrates proof-of-concept, the technical challenge is to manufacture that technology in a scalable and cost-effective manner. However, being able to produce something efficiently does not guarantee consumers will adopt it—that is the societal challenge: to educate the public about the technology so that they not only adopt it when available, but also that they invest in the industries relevant to its production and distribution. This investment will ensure, through competition, that the technology will undergo continuous improvement and that costs will be driven down for both producers and consumers. Ideally you want to educate the public about a technology in parallel with its research & development so that consumers will adopt it once the technical problems are solved and the product is ready for release.

My general research focuses on the societal challenge of how to educate the public about innovative technologies. The specific technological focus of this research is two-fold: (1) energy harvesters that convert solar, electromagnetic waves, thermal, and vibration into electricity; and (2) photovoltaic cells that incorporate nanostructures for improved energy efficiency and reduced material costs.

The societal challenges are especially difficult for energy innovations, for several reasons. First, people want proof of technology claims (like improved energy efficiency), yet much of the innovation is invisible to consumers since it occurs at the nanoscale. Second, although prototypes may exist, the product in its final form and packaging does not. The research question is: How do you convince the public to adopt a technology whose benefits they cannot see, and whose function they cannot experience?

The approach we have taken to answer this question is to develop and disseminate immersive interactive visualizations. We divide these visualizations into three categories: inner space, virtual space, and outer space.

Inner-space visualizations use 3D animations and virtual reality technology to allow users to experience representations & simulations of structures and processes at scales that are too small for the eye to see. Through these visualizations, users get an appreciation of the scale and the complexity of nanomanufacturing.

Virtual-space visualizations use videogame technology to develop interactive virtual worlds that allow users to see, albeit in a simulated environment, the benefits and costs of innovative technologies that do not yet exist. The benefits experienced are at the individual and community levels.

The final class of visualization is the outer-space visualization. As the name implies, the goal of these visualizations is to show applications of innovative technology beyond the earth.

We will present these visualizations and our use of social networking for viral distribution.

Life cycle assessment (LCA) is an internationally standardised technique for assessing the environmental and social impacts of a product, process or service from raw material supply through to point of use and disposal. The importance of life cycle thinking is recognised around the world. The European Commission, for example, has concluded that LCAs provide the best framework for assessing the potential environmental impacts of today’s products, while the United Nations Environment Programme (UNEP) has developed an initiative to turn life cycle thinking into practical action around the world.

The EU-funded Double Side Contacted Cells with innovative carrier-selective contacts (DISC) project addresses the need to reduce the consumption of fossil fuels by developing key technologies for the next generation of high-performance photovoltaic (PV) solar cells and modules, allowing ultra-low solar electricity costs with minimum environmental impact. The DISC device architecture enables reduction of non-abundant material consumption, enhancement of the energy yield, and a modern module design ensuring outstanding durability. In such a way DISC will provide the key elements for achieving very low levelized costs of electricity in Europe with potential for further reduction, making solar one of the cheapest electricity sources. DISC aims to contribute towards mitigating the impacts of climate change and towards bringing Europe back to the forefront of PV cell science, technology and manufacturing. More information is available at www.disc-project-h2020.eu

This presentation will explore the application of environmental and social LCA techniques to assess the sustainability performance of a novel technology using DISC as an example. The presentation will also discuss how LCA can be used to tackle societal challenges as well as enable the transition to and development of more sustainable energy pathways.

High-efficiency and low-cost photovoltaic technologies are still required to successfully replace fossil fuels in the US. As recently suggested by the National Renewable Energy Laboratory (NREL) Renewable Electricity Futures Study, by the year 2050, 80% of the power generated in the US could be from renewable sources. For that, photovoltaic generation must increase to >10% of the energy produced today. The main challenge to achieve this goal is the current high cost/Watt: $5.3/W for <10kW systems, and $2.5/W to $4.0/W for utility-scale systems. Thus, we are currently focusing on advancing the understanding of the physical behavior of materials with potential for high-performance photovoltaics. These materials are often composed by mesoscale constructs, which require identifying the causes for spatial variations and degradation (failure). I will present a platform for functional imaging of materials for sustainability, ranging from photovoltaics to energy storage. Further, in order to draw immediate attention of lay audiences we correlate scientific findings to animated visualizations, which will be shared during the presentation.

Renewables recently represented almost two-thirds of new net electricity capacity additions, with solar photovoltaics (PV) additions growing faster than any other fuel. Installed PV capacity recently surpassed 300 GW and the PV market will reach a volume of about 90 GW in 2017, mainly driven by China with almost half of the new installations. Today, auction prices for PV power plants reached record-low values of even less than 2c$/kWh. For the next five years, solar PV will represent the largest annual capacity additions for renewables, driven by continuous cost reductions and power conversion efficiency (PCE) improvements. Such rapid growth of PV capacity requires an even stronger focus on using abundant and sustainable materials as well as scalable production processes for solar cells and modules.
Silicon solar cell technology dominates the PV market since years, with thin films technologies (cadmium telluride (CdTe), copper indium gallium selenide (CIGS), others) reaching a total market share of less than 10%. This domination is continuously fuelled by advances in manufacturing excellence and increasing PCE. One example is the recent introduction of the Passivated Emitter and Rear Cell (PERC) technology in mass production that improves the PCE of the standard Aluminum Back-Surface Field (Al BSF) solar cells by 0.5-1% absolute, reaching about 22% PCE in production. Following the general trend to higher PV panel power conversion efficiencies to reduce overall PV systems costs, there is also a stronger emphasis on advanced solar cell architectures such as silicon heterojunction (SHJ) and Interdigitated Back-Contact (IBC) cells, accompanied by improvements in module technology.
The next technology steps are already being prepared at laboratory level: for example, tandem solar cells with a higher band gap top cell and lower band gap bottom cell better utilize the solar spectrum and have the potential to overcome the PCE limit of silicon solar cells of ca. 29%. Novel materials such as perovskites that also have the potential to be sustainable may play a crucial role in realizing such high-efficiency, low-cost tandem solar cells in production, but could also be used as a single-junction thin film technology.
This paper will review recent PV technology advances and will give an outlook on future technologies both in laboratory and production, with a discussion of the environmental impact.

History abounds with examples of the interdependence of science and engineering, especially in energy. The invention by James Watt of the steam engine led to the scientific theory of thermodynamics. The invention of the light bulb by Edison and the work of Tesla and Westinghouse on transformers for long distance electricity transmission led to the emergence of the field of electrical engineering for power generation. Einstein’s seminal scientific work on relativity eventually led to the discovery of nuclear fission and the development of nuclear power. America’s growth as an economic superpower is in many ways connected to its frontier spirit and superior ability to exploit the virtuous cycle of scientific discovery and engineering inventions to meet societal goals. This spirit is still alive and well in some places (e.g. the information technology sector), and yet the level of investment necessary to advance both science and technology in the energy space is prohibitively large for the self-funded “tinkerer.”

The unity of “basic” and “applied” research activities was a major factor in the highly productive corporate R&D activities in the 20^thcentury. Examples abound from AT&T Bell Labs, IBM, Xerox, DuPont and General Electric. My own experience at Bell Labs, alongside the history of the invention of the transistor and the discovery of the transistor effect, illustrates the importance of breaking down barriers between science and engineering. In the 1970’s and 1980’s, Bell Labs made enormous strides in artificially-tailored thin-film materials, which led to new scientific discoveries in semiconductor quantum physics. These advances led simultaneously to the creation of devices like high-mobility transistors, which are in every cell phone, and tiny communications lasers, which allow high speed fiber optic communications across the globe.

To enhance the public benefits of DOE’s energy R&D, a closer integration is needed between activities typically managed by the Office of Science and the technology offices. The appointment in 2014 of a single Undersecretary for Science and Energy is a step in the right direction for DOE R&D management; this organizational structure should be maintained. This move enabled the creation of crosscutting initiatives, wherein multi-office teams coordinate funding for a set of specific technical challenges, such as grid modernization. Further steps to improve the structure at the Assistant Secretary level may be needed. One possible improvement is to create a new “Office of Energy Research” that would combine activities across the full spectrum of energy-related research. This office could coordinate initiatives that fill existing departmental functions, including core research programs in science and engineering that span the range from distant commercial relevance (e.g. condensed matter and atomic physics) to a strong technology focus (e.g. energy storage).

The Durable Module Materials (DuraMAT) Consortium brings together the national lab and university research infrastructure with the photovoltaic (PV) and supply-chain industries. DuraMAT is led by NREL and Sandia, with key collaborators at SLAC and Berkeley Lab. These four labs form the core capability network, with contributions expanding from additional labs in the near future. University research consortia and industry-led collaborations are engaged with the capabilty network on 11 DuraMAT funded projects with more opportunities for engagement in FY18 and FY19.
Our goal is to discover, develop, de-risk, and enable the commercialization of new materials, designs, and accelerated tests for PV modules—with the potential for a levelized cost of electricity of less than 3 cents per kilowatt-hour.
DuraMAT is part of the Department of Energy’s Energy Materials Network, which leverages federal funding to facilitate industry's access to the unique scientific and technical resources at DOE's national labs in high-performance computing, synthesis and characterization of new materials, and high-impact experimentation.
We envision doubling the rate at which companies can implement new materials in PV modules by coupling an Energy Materials Network architecture, with PV durability science and state-of-the-art analysis.

In today's world, we are all responsible for being aware of and reducing our environmental footprint. The larger the entity, the greater the responsibility. This is doubly true for schools, who not only create their own footprint, but have the power and responsibility of shaping the next generation and their awareness. At Explore Academy, a small team of students and teachers worked to determine 1. to do what degree can we reduce our impact? 2. to what degree can we create more sustainably-minded citizens? Over the course of the school year, the team implemented a number of measures to adjust our community's practices around waste, energy use, and energy production; as well as began to integrate curriculum to educate our students about sustainable energy in a practical, real-world context. From all of this, we present to you our proposal for how any school is not only able to, but responsible for being a model of sustainable practices and education that looks toward the future.

A steady increase in new photovoltaic installations and thus a rising contribution of solar energy to the world-wide energy production can only be ensured by a continuous reduction of the $/Watt costs for photovoltaic systems. Silicon single-junction solar cells are close to reaching their practical efficiency limit which will boost the need for higher-efficient photovoltaic technologies. Efficiencies over 30% were demonstrated by combining Si solar cells with wide-bandgap III-V top cells. Despite these impressive efficiencies, the application of solar cells including III-V semiconductors has been limited by the high fabrication costs mainly caused by the MOCVD growth and required substrates. Other, potentially lower-cost top cell materials like perovskite semiconductors have so far not been able to reach tandem cell efficiencies of 30%. The costs of III-V top solar cells can be drastically reduced when using GaAs nanowire arrays fabricated by Aerotaxy™. Due to the superior geometry, high theoretical single-junction efficiencies of >25% can be achieved for an area coverage of <20%, enabling tandem cell efficiencies over 30% when applied in a tandem cell with Si bottom cell. The fabrication by Aerotaxy™ [1] does not require a growth substrate, allows an efficient use of the precursors and is conducted at higher throughputs. In this presentation, more insights into the concept of low-cost GaAs nanowire/Si tandem solar cells will be given and the great potential of this innovative technology will be discussed.

[1] M. Heurlin, M. H. Magnusson, D. Lindgren, M. Ek, L. R. Wallenberg, K. Deppert, L. Samuelson,
”Continuous gas-phase synthesis of nanowires with tunable properties”, Nature 492, p. 90–94, doi:10.1038/nature11652

Heterogeneous integration of dissimilar semiconductor materials via epitaxial lift-off and membrane transfer proved to be an effective technique to integrate structurally mismatched materials with minimal alteration of their structural and electrical properties. Based on this recent progress, we integrate photovoltaic devices fabricated on single-crystalline GaSb membranes with Si substrates. The potential applications of this material system are multi-fold ranging from IR detectors to solar cells. In the field of photovoltaics, integrating a narrow band-gap semiconductor cell with a Si sub-cell will broaden the absorption from the visible to the infrared regions of the solar spectrum. The epi lift-off of GaSb membranes from GaSb substrate is achieved by a combination of selective etching of sacrificial layer and directional etch of the substrate followed by a stamp assisted transfer onto the Si substrate
A grand challenge in incorporating epitaxial lift-off technique to III-Sb systems relates to the poor selectivity of available etching solutions between the sacrificial layer and the GaSb membrane itself. Hence this process is challenging in terms of successfully achieving epi lift-off without affecting the active layer. Al_0.4Ga_0.6Sb (~40nm) is chosen as the optimum sacrificial layer preventing the occurrence of threading dislocations into the membrane as well as exhibiting some degree of selectivity with respect to the active layer. An etch stop layer is also incorporated on top of the sacrificial layer (N periods of InAs/GaSb) to prevent the etching of GaSb membrane. After growth, the GaSb membrane was patterned in a 5x5 mm² array of pixels with each pixel area of ~30 µm². Upon capping the membrane to protect it from the etching solution, a combination of the sacrificial layer etch and a directional etch of the substrate in the wet etchant (diluted HF) facilitate the pixels to be weakly bonded to the growth substrate. A stamp (e.g., water-soluble tape) is then used to transfer the pixelated GaSb membranes onto Si.
A comparative analysis of the structural and functional characteristics of III-V devices bonded to the native growth substrates and to Si establishes the effect of the process and the interface with the new host on device performance.
ACKNOWLEDGEMENT. This work was primarily supported by the National Science Foundation (NSF) and the Department of Energy (DOE) under NSF CA No. EEC-1041895.

Narrow-bandgap semiconductors allow for the conversion of near-infrared (NIR) photons into electricity. These are typically used for thermophotovoltaic (TPV) cells that convert blackbody radiation from thermal sources to electric power as well as for photovoltaic subcells in mechanically stacked systems or in spectral splitting systems that are designed to absorb the NIR tail of the solar spectrum. Examples of such cells include GaSb, InGaAsSb/GaSb, Ge and InGaAs/InP based cells. There is additional benefit from being able to fabricate these cells as thin film cells with complete substrate removal because of the reduced weight of the cells and the possibility of photon recycling. There is the added benefit of better thermal management of such thin-film cells by bonding them to CVD diamond. The elimination of the substrate’s thermal resistance helps to remove the heat from the cells more efficiently.

In this presentation, we demonstrate the development of thin-film GaSb and InGaAs PV cells that are freestanding after completely removing the substrate. The only method for realizing large-area thin-film GaSb PVs is to grow the structure on GaAs substrates metamorphically. However, this results in performance degradation due to the presence of threading dislocations in the material. InGaAs on InP is however shown to be very favorable for fabrication of such thin-film structures with the final devices showing slight improvement in performance compared to devices that are processed on the substrate. We will present solar cell characterization data for the devices described including J-V and quantum efficiency plots. We will also include extensive material characterization data including transmission electron microscopy and high resolution X-Ray diffraction data.

The cost of solar cells is still too high for them to be widely used in consumer applications, and hence succeed as a commercial technology. Many research efforts focus on reducing processing and material costs or increasing the efficiency of the photovoltaic conversion (i.e., the conversion of the optical power radiated from the sun into electrical power). Thus, although photovoltaic devices are still mostly based on silicon (Si), a multitude of other materials and device architectures have been investigated. Engineering the band gap of the absorber to convert a large portion (i.e., a broad range of wavelengths) of solar radiation in electrical power is a major area of research in the field of renewable energy.
I will illustrate how ultra-thin semiconductors offer a new avenue to band gap engineering. Direct band gap semiconductors can be isolated as large-area sheets or membranes with thickness varying between ~0.7 nanometers and few microns. These new structural elements range from monolayer transition metal dichalcogenides to sheets of single-crystalline III-V and III-N semiconductors. Freestanding membranes can be transferred and bonded to other hosts. In addition, a large variety of local and global strain fields can be established in sheets with nanoscale thickness due to their high fracture limit. I will present a few examples of broad band absorbers of solar radiation relying on these unique properties of semiconducting sheets. For instance, I will describe integration of III-Sb and Si photovoltaic devices by direct bonding to obtain a solar cell operating in the visible and in the infrared range. In another example, I will show how few-layered MoS₂ can be strain-engineered to establish a spatially varying profile of the band gap and exciton confinement in a single material.

Materials science has made progress in maximizing or minimizing the thermal conductivity of materials, however, the thermal effusivity – related to the product of conductivity and capacity – has received limited attention, despite its importance in the coupling of thermal energy to the environment. We design materials that maximize the thermal effusivity by impregnating high surface area copper (Cu) and nickel (Ni) foams with conformal chemical vapor deposited graphene (G) and octadecane (OD) as a phase change material, achieving values exceeding J m^-2 (s K)^-1/2 – the highest value in the literature, to date, for isotropic, ambient phase change materials. The graphene is shown to increase thermal conductivity up to 20% by enhancing interfacial thermal conductance and providing transport bridges over grain boundaries of the metal foams, whereas the foam porosity houses the OD for enhanced capacity in the form of latent heat. These composite materials are ideal for ambient energy harvesting in the form of what we call broadband thermal resonators to generate persistent electrical power from ambient thermal fluctuations over large ranges of frequencies. Theory and experiment demonstrate that the harvestable power for these devices is directly proportional to the thermal effusivity of the dominant thermal mass. To illustrate, we experimentally measure persistent energy harvesting from diurnal frequencies at an outdoor location in Cambridge, Massachusetts, extracting as high as 350 mV and 1.3 mW from approximately 10 ^oC diurnal temperature differences continually over 16 days using high thermal effusivity materials. Thermal resonance devices of this type may provide renewable and persistent energy sources over extended periods.

Widespread energy harvesting, generating self-sufficient power from the surrounding environment, such as wind, solar and geothermal, have attracted increasing attention in the past decade due to the energy crisis and global warming. Most recently, a new type of power generating device, named as triboelectric nanogenerator (TENG) based on triboelectric effects coupled with electrostatic effects have been demonstrated as powerful means of harvesting mechanical energy from living environment. Here, the polyimide (PI) polymer was synthesized by changing different dianhydride and diamine monomer to control dielectric properties and work functions. We report a new PI based TENG for effective dielectrics, resulting in 4.5 times higher output voltage and current of 30.2 V and 8.4 μA/cm^{<span style="font-size:10.8333px">2</span>} at PI from Bis(3-aminophenyl) Sulfon (APS) based TENG compared to general Kapton based TENG due to the increase of dielectric constant of negatively charged layer and surface potentials on the triboelectric surfaces. Therefore, aromatic polyimides show a great promise in controlling dielectric properties of negatively charged layer in TENG and tuning the performance of polymer-based triboelectric devices for effective mechanical energy harvesting.

Triboelectric nanogenerator (TENG), which converts the ambient mechanical energy to electricity by friction, is one of the most promising methods for powering up portable devices. Despite of the successful development so far, there still remain some issues to be improved. Here, we focus on the generation and maintenance of high output-power under harsh environments, and the noise cancellation of the device to keep a stable operation for a long period of time. The TENG, fabricated using a 3D printer, is a fully-packaged, cylinder-shaped device with polydimethylsiloxane (PDMS) bumpy balls inside and linearly patterned Al film on the inner surface. The new design and optimization remarkably increases the output power up to 45 mW, operable under harsh environments such as water, providing enough DC power for charging a battery of smart watch. The noise occurring during the operation is canceled to the noise level (~ 50 dB) of a normal conversation, by approximately 20 dB, with no degradation of the output power by using highly compressible, conductive Ag nanowires-embedded polyurethane sponge instead of Al. This helps people around the device feel comfortable during the operation. Finally, as a large-scale power supply, windmill composed of three TENGs, are also developed.

For harvesting eco-environment energy, triboelectric nanogenerator (TENG) was fabricated which can generate electric energy from ambient mechanical energy. Although the TENG consists of cheap and affordable materials, the ingredients of TENG would be still difficult to obtain for the Third World people. Therefore, silica-based powder TENG (S-TENG) was demonstrated, which utilizes sand as a freestanding dielectric layer for triboelectrification. Sand is mainly consisted of silicon and oxygen which are constituting the earth’s crust. Sand has advantage of easy availability on earth. To demonstrate general electrical output, silicon and silicon dioxide are utilized with the sand for fabricating TENG. PTFE film which tends to be negatively charged was utilized as a counter triboelectric contact material. The highest electrical output was generated when using silicon dioxide as a dielectric layer. Because silicon dioxide tends to be positively charged than sand and silicon do. The result showed an open-circuit voltage as high as 15 V and a short-circuit current of 350 nA when using silicon dioxide and shaken by electrodynamic shaker. In case of hand shaking, the power density shows the peak value of 0.3 mW/m². Besides, five commercial LEDs are simultaneously turned on when using half fist sized S-TENG device. The electrical energy which is sufficient for the people in the Third World can be generated by several number of this S-TENG device.

The most important aspect of photovoltaics sustainable development is the use of environmentally friendly, low-cost materials and technologies for manufacturing PV station components. The main material for solar cell production is silicon.
The paper presents the research results of a new technology for solar-grade silicon production by the method of plasma-chemical purification of metallurgical-grade silicon with the use of magnetohydrodynamic mixing, based on plasma interaction with the silicon melt surface in the crucible. The method ensures the ecological safety and scalability of the technology, the combination of the purification process and the subsequent direct crystallization of silicon melt into one process, the silicon cost reduction.
The paper studies the purification process regularities by the method of mathematical modeling:
the effect of the plasma flow rate on the surface mass transfer rate of silicon melt;
the dependence of the impurity diffusion in silicon on the surface mass transfer rate;
the effect of magnetohydrodynamic mixing of silicon melt on silicon purification process.
The analysis of the cost of silicon produced by the investigated method was carried out.
Comsol calculation programs are used for mathematical modeling.
The research results are as follows:
the interconnection between the plasma flow rate and the surface mass transfer rate is established;
the dependence of the diffusion rate of impurities (metals, phosphorus, boron) on the surface mass transfer rate is established;
the processes of silicon melt movement under the conditions of its interconnection with plasma and the effect of magnetohydrodynamic mixing on the processes of silicon purification are investigated.
It is established that the cost of silicon obtained as a result of the new technical process amounts to no more than USD 9.0 per kg.

Triboelectrification is a commonly experienced phenomenon, and it is usually considered a negative effect and avoided in development of many technologies. The triboelectric nanogenerator (TENG) is a novel invention that converts mechanical energy into electricity through the effects of contact triboelectrification and electrostatic induction. In the inner circuit of the TENG, a potential is created by the triboelectric effect due to charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons flow between two electrodes attached on the back sides of the films in order to balance the potential. Within a year from the birth of the TENG in 2012, the output power density of TENG was enhanced by at least five orders of magnitude. The area power density can reach 500 W/m², and an energy conversion efficiency of ~85% has been demonstrated. TENGs can harvest all varieties of mechanical energy that are otherwise wasted in our daily lives, such as human motion, ambient and machine-related vibration, rotation of vehicle tires, wind, water currents, and more. Alternatively, TENGs have great potential as self-powered active sensors for detecting static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, with potential applications for smart user authentication, security systems, and smart skin technologies. The TENG is a new energy technology with boundless applications and potential to contribute to the world energy supply in the near future.


References:

Z.L. Wang, Materials Today, 20 (2017) 74-82.
“Nanogenerators for Self-Powered Devices and Systems”, by Z.L. Wang, published by Georgia Institute of Technology (first book for free online down load): http://smartech.gatech.edu/handle/1853/39262
Z.L. Wang, L. Lin, J. Chen. S.M. Niu, Y.L. Zi “Triboelectric Nanogenerators”, Springer, 2016. http://www.springer.com/us/book/9783319400389
Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.
Z.L. Wang, J. Chen, L. Lin “Progress in triboelectric nanogenerators as new energy technology and self-powered sensors”, Energy & Environmental Sci, 8 (2015) 2250-2282.

Ni is widely used as active phase in hydrogenation catalysts for its high activity and relatively low cost compared to noble metals. The the presence of unsaturated hydrocarbons (especially in biomass derived feed gases) are known to promote coking of the catalyst [1]. Ni-containing catalysts may be oxidatively regenerated at elevated temperatures by burning off carbon and cleaning the catalyst surface from coke and/or poison species. However, during regeneration current Ni catalysts with high metal loadings undergo microstructural changes like sintering. The aim of this work is to develop a redox stable and catalytically active material allowing catalysts that suffered from the above mentioned deactivation mechanisms to be oxidatively regenerated without unwanted activity loss due to Ni particle sintering. Incorporation of Ni in perovskite-type mixed oxide (PMO) LaFe_1-xNi_xO_3±δ (LFNO) allows to exploit the self-regenerating property of PMOs [2-4]. In the current study, we explore the structural reversibility of Ni in LFNO for CO₂ hydrogenation. It is shown that Ni can be selectively reduced and segregates from the perovskite lattice at elevated temperatures forming dispersed and active metallic Ni particles on the surface. Reoxidation causes Ni to reenter the support lattice. This principle allows C-deposits and poison species to be removed simultaneously, whereas inhibition of Ni sintering by re-dispersion of the active phase on the surface upon reduction is a beneficial consequence.
It was demonstrated that LFNO exhibits the exceptional property of reversible nickel segregation. After reduction in 10 vol% H₂/Ar at 600 °C Ni for 1 h, Ni was preferentially reduced and segregated to the PMO surface where it formed catalytically active, metallic Ni particles of few nm in size. These Ni particles were again fully reincorporated into the perovskite lattice after reoxidation in air at 650 °C. Amongst other methods like X-ray diffraction and temperature programmed reduction, the state of nickel was assessed over a redox cycle using X-ray absorption spectroscopy around the Ni K-edge. After reduction 50 % of all Ni was reduced and all Ni was reincorporated into the host lattice after reoxidation at 650°C for 2 h from which it could be segregated again at no loss of catalytic activity. The stability could be attributed to the constant particle size due to the Ni incorporation and segregation mechanism. Other Ni catalysts which do not show this mechanism suffer from particle sintering over the number of redox cycles (e.g. Ni/Al₂O₃). It is shown in this work that the preservation of catalytic activity over redox cycles at high temperatures allows the material to be regenerated after it has suffered from severe coking during reaction.

1. C.H. Bartholomew, Catal. Rev. 24 (1982)
2. Steiger, P., et al., ChemSusChem 10 (2017)
3. Nishihata, Y., et al. Nature 418 (2002)
4. Burnat, D., et al. A. J. Mater. Chem. A 4 (2016)

Tilt sensor is a device that detects angular variation and make electrical signal. It gives us information about the horizontal and vertical state. It is related with stability so tilt sensor is very important part of structure. There are some special applications using tilt sensors such as airplanes, smart phones, game machines, automobile security systems, bridges and drones.
To sense angular changes, sensor needs critical characteristics. First, sensor have to work not only constantly for a long time but also stably. Most of tilt sensor include liquid because it flows downward due to gravity, this is the simplest mechanism. The solution used sensor should not be interacting with physically and chemically to maintain performance. Second, tilt sensor has return capability. When the angle is no longer changed and reached flat condition, sensor have to point horizontal value exactly.
Because of some essential feature, there is some limitation to make existing sensors. We have to consider about the internal material and also supply external power continuously. In this paper, we deal with a new triboelectric sensor (TS) as a self-powered sensor. The shape of TS is hexahedron with PTFE balls inside. TS consist of PTFT balls, acrylic box, and eight aluminum electrodes. Because of the use of solids as internal material, there is no need to consider environmental and chemical degradation. When PTFE balls rolls inside the acrylic box, electric potential difference will be made. Because triboelectric follows triboelectric series and caused by electrostatic induction and contact-electrification. When one PTFE ball is moved by gravity, the output voltage and current are 2 V and 20 nA, respectively. So TS does not require external power. It can act self-powered active system. The inclination and angle can be estimated by integrating the change of the voltage of 8 electrodes. A multi-channel system was designed and measured to observe the voltage change of 8 electrodes at one time. TS has simple structure, low-cost, economical, eco-friendly and self-powered system. This study will expand the application of triboelectric sensor.

This study presents an evaluation of quartzitic bodies as potential deposits of high-purity quartz (HPQ) for use as raw materials for special applications in high-technology industries. HPQ is increasingly considered a strategic mineral in the world market due to its applications in solar, chemical, breast-tissue implants, fiber optic and other industries. In this study, we characterize quartz-vein samples in terms of their microchemical signatures and textures. Electron microprobe analysis (EMPA), particle-induced X-ray emission (PIXE) and laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) analyses were performed on the quartz. These methods revealed strong Si and O signatures. Cathodoluminescence spectroscopy (CLS) integrated with back-scattered electron (BSE), secondary electron images (SEI) and energy-dispersive spectroscopy (EDS) revealed low trace-element concentrations in some regions of the samples, and higher concentrations in some mineral inclusions. EMPA, PIXE, LA-ICPMS and cathodoluminescence studies revealed processes that led to the presence of low concentrations of trace-elements in the quartz.

Quartzitic bodies have been mapped in southwestern Cameroon but the textures and the purities of the quartz varieties in these veins are not known. Typically, the quartz is homogeneous and anhedral with mineral inclusions disseminated within the quartz matrix. The quartz contains medium to coarse sized grains, and varies in type from transparent to milky quartz. Euhedral mineral inclusions of mica flakes, dark radiating tourmaline and rutile needles are present in clusters within the vein. Electron microprobe analysis (EMPA), particle-induced X-ray emission (PIXE) and Laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) analysis were performed on the quartz. These methods revealed strong Si and O signatures typically associated with the materials being >99.89 % SiO₂. In this study we use cathodoluminescence spectroscopy (CLS) integrated with backscattered electron (BSE) imaging, secondary electron imaging (SEI), and energy dispersive spectroscopy (EDS) to reveal complex growth textures and fracture histories of the quartz. CLS textures indicates very low trace-element concentrations that are also mapped by electron microprobe analysis (EMPA) of the quartz and higher concentrations in some mineral inclusions. These mineral inclusions can be easily sorted out after mining, and before the beneficiation process. Consequently, the quality and potential quantity of HPQ veins in this area indicate a potential for HPQ exploration in the region. This discovery, together with a better knowledge of the processes leading to the formation of HPQ in quartzitic bodies, could enhance development of the mineral sector in Cameroon. This will also be useful for the exploration of HPQ in Sub-Saharan Africa, to help provide raw materials for high-tech industries such as the manufacturing of solar panels (the use of which are currently on the rise).

Thermochromic materials are those which change color as a result of a temperature change. A thermochromic coating can passively regulate the temperature of a surface, for example to reduce costs and energy consumption for heating and cooling a building or to hold a component within a desired temperature range. This type of coating enables cross-disciplinary, passive energy savings. The thermochromic pigments used in this research are melamine-microencapsulated leuco dyes based on 7-anilino-3-diethylamino-6-methylfluoran. Three thermochromic samples with transition temperatures at 20, 25, and 30° C were prepared by the suspension of dye powder in acrylic resin. Below the characteristic temperature each thermochromic coating has strong absorption across the visible spectrum, but above this point the pigment becomes transparent. The spectral reflectance above and below the transition temperature was assessed for coated and uncoated substrate materials including aluminum and polyvinylchloride. The temperatures of the coating surface and substrate were measured as a function of time before, during, and after illumination with an incandescent lamp at up to 1000 watts per square meter. The data demonstrates the temperature rise is held lower with a thermochromic coating compared to the uncoated substrate material. An analytical model for temperature rise of the coating and substrate was derived and compared against the experimental data.

In most situations, neutralization of an acid and an alkali is carried out in the bulk and the energy released in the form of heat. Here, we demonstrate that the neutralization energy of the two electrolytes can be directly utilized to produce electricity or split water with high efficiency. We reported a dual-electrolyte microfluidic fuel cell system (DEFC), which could stably deliver an open circuit voltage up to 1.76 V at room temperature under atmospheric pressure, and a peak power density of 145 mW/cm², improved by 3 times compared with that in single electrolyte mode. When operated in the reverse mode, the microfluidic cell can be used for water electrolysis, demonstrating a water split voltage of ~0.74 V and a round trip voltage efficiency of 83.5% at a current density of 100 mA/cm². The neutralization energy was both experimentally and theoretically proved to be well utilized as electrochemical energy in dual electrolyte systems at the electrodes both in fuel cell mode and water electrolyte mode. These results revealed that electrochemical neutralization can proceed if protons are consumed at the cathode and hydroxide ions reacted separately at the anode, accompanied by electron transfer via an external circuit. Given the DEFC's features of high open-circuit voltage, low water splitting voltage, and room-temperature operation condition, the reported DEFCs technology presents a superior route for high-efficiency energy conversion and storage system that could revolutionize the fields of large-scale energy storage and portable power systems.

Triboelectric nanogenerator (TENG) which converts ambient mechanical energy into electrical energy is a spotlighted energy harvesting technology for a portable power generator. In previous researches, the electrical power output of TENG has been improved via applying new materials or combining with other energy harvesting technologies. However, the electrical power generated by the sliding-type TENG is proportional to the size of the internal electrode, and the vertical contact-separation TENG requires a hollow space for moving the dielectric to create the electrical potential difference. Therefore, it is necessary to overcome the spatial limitation of TENGs for developing portable generators. Recently, many studies have shown miniaturization of TENG for portable applications, and there has been continuing efforts to invent portable generating devices which are capable of harvesting low input energy.
In this work, we demonstrate Roll-type sheet TENG (RTS-TENG) that can harvest mechanical energy through an extraction and self-retraction process without additional components for retraction. RTS-TENG utilizes the elastic force of the sheet itself and is thereby operate as a spiral spring. Besides, in the process of the extraction and retraction, it generates additional multiple power peaks in a single input, from a stacking/fluttering motion and changes of the sheet shape. We designed RTS-TENG considering two major factors for maximizing the output: the dimension of the casing cylinder and the length of the sheet. Furthermore, we established that the output energy is changed by patterning the electrodes of each part. And an electrical circuit is designed to efficiently combine the output harvested from each electrode. Finally, we demonstrate portable RTS-TENG which is applicable to any flat board devices without spiral springs for self-retraction process. This study shows possibility of practical applications for RTS-TENG which is usable when attached to portable electronic devices in the future. Additionally, it provides a potential solution as portable energy generators.

As our society continues to develop, various electronic devices have appeared in our lives. However, with these developments, environmental pollution and energy shortage also have been highlighted. So, the technology that converts green energy into available energy has become the most important research field. In this regard, many wireless electronics, such as drone, UAV, and electronic car, require lightweight, low cost, and high efficiency self-powered sensors and generators. To charge and generate electricity wireless electronics itself, various wind energy harvesting methods had been studied through the variety of researches such as electromagnetic generator, piezoelectric generator, and triboelectric generator. Among the wind energy harvesting methods, a triboelectric nanogenerator (TENG) is very important generating mechanism thanks to its simple design, input sensitive output, and high power density. A lot of studies indicated that TENGs would be able to use as wind power generators and self-powered sensors by inducing rotation or vertical contact-separation motions using windmills and the fluttering behaviour flexible substrates¹. However, the robustness and durability of the wind-driven TENGs continue to mount challenges due to the wear and fatigue failure caused by friction between two dielectric materials and a repeatedly applied load during flutter motion. Recently, the wind-rolling triboelectric nanogenerator (WR-TENG) was proposed that could generate the electric energy in a wide wind speed range without fracture or critical damage by using the vortex whistle design and lightweight dielectric materials². However, its qualified design that the presence of inlet and outlet shows the limitation of application like the uni-directionality and non-compact design.
To overcome these drawbacks, this study demonstrated the advanced design of wind-driven triboelectric nanogenerator, Omnidirectional wind-rolling triboelectric nanogerator (OWR-TENG). As shown in the Figure, the whole inlet of OWR-TENG can function as the inlet itself and outlet at once. Due to its functional design, the OWR-TENG has the omni-directionality and compact design that can be applied in variety application. The computational fluid dynamics (CFD) simulations are used to improve the efficiency of mechanical energy conversion and the sensitivity of self-powered anemometer. The analysis of CFD can visualize the complex flow and provide the design parameters with consideration for the applicability such as the velocity of lightweight dielectric materials, robustness, and drag force. The Omnidirectional wind-rolling TENG is a novel approach for a sustainable wind-driven TENG design, and for various practical applications that can be used in wind tunnel, automobiles, airplanes, and drones.

Energy harvesting technologies are defined as a converting ambient energy into electric energy. Among these technologies, triboelectric nanogenerator (TENG) is a promising method which converts mechanical energy into electric energy. However, some existing TENGs are hard to be used in the practical application field, because they have complicated structure and instantaneous output. In this paper, we introduce an innovative and simple freestanding type roly-poly TENG (RP-TENG) that can sustainably harvest mechanical energy from oscillating motions. RP-TENG is mainly composed of the hemisphere shell with attached electrode and base plate with another attached electrode. The principal mechanism of RP-TENG is that it oscillates like a roly-poly motion when the hemisphere shell is tilted to any direction. At that time, electric potential is continuously generated owing to coupling effect of triboelectrification and electrostatic induction between the hemisphere shell and base plate. Simple structure makes manufacturing process simple and maintenance easy while working principle that gains sustainable output is quite simple. Because prototype of RP-TENG gains multiple oscillations from one input, another distinguished advantage of RP-TENG is multiple electricity output, experimentally measured 17, gained from just a single input. It suggests that users can use RP-TENG properly in desired condition by adjusting oscillation duration time. Furthermore, it can be widely used as self-powered acceleration sensor, which indicates orientation and quantity of acceleration, owing to sustainable output and energy harvesting possibility in all directions. For this purpose, we construct meshed cells on the base plate in rows numbering cells. When hemisphere shell is oscillated by external input, we sense electric measurements between contact cells and non-contact cells during duration time. With these advantages, we expect RP-TENG to be used as an energy harvesting device or self-powered sensors that can be easily used in various situations.

Owing to surface modifications, triboelectric nanogenerators (TENGs) have seen increases in their electrical power and have successfully powered portable devices on their own. However, modifying the material and its surface may place limitations on the duration of device operation, and most of the portable applications demonstrated in previous studies have excessive input conditions. In this study, we developed a capacitor-integrated TENG (CI-TENG) that utilizes the fundamental mechanisms of the Leyden jar. The Leyden jar was the first form of capacitor that can store electrostatic charge and then discharge when two electrodes are sufficiently close for electrons to flow. In this device, a long sheet metal (capacitor electrode)–polymer–metal composite (TENG electrode) is rolled inside the casing cylinder, and a capacitor unit is fabricated at the end of the sheet composite. This is the first to integrate capacitor unit inside the TENG to generate higher ouput. This new operating mechanism of the CI-TENG is analyzed through the dielectric constant of the capacitor unit and the metal-to-metal contact between electrodes. Through the instantaneous charging and discharging of the capacitor unit inside the CI-TENG, it can generate a peak open-circuit voltage (V_OC) of 156 V and a peak closed-circuit current (I_CC) of 4.3 mA with a hand input. Compared with the conventional TENG, it charges a capacitor more than three times faster. Furthermore, the internal impedance of CI-TENG has shown to be decreased to 200 kΩ without any external circuit. The novel CI-TENG developed in this study provides a potential solution for CI-TENGs with a high-power output.

Negatively charged nanogenerator (TENG) has some critical limitation for accomplishing electric generator role such as low current density, power continuity, etc. To supplement weak point of TENG, here, we introduce parallel-contacted negatively charged nanogenerator (PCTENG) with additional metal middle layer acting as ground layer. Aluminum is used as electrode and middle layer, Polytetrafluoroethylene (PTFE) is used as active layer and etched by reactive ion etching (RIE). The working mechanism of PCTENG is based on triboelectrification of PTFE and Al, aluminum middle layer enhances performance of PCTENG. Using this PCTENG, we also demonstrate portable self - generator for application. Performances of basic structure PCTENG are 150 µA/cm². (Active layer area is 4 4 cm²)

The importance of employing more efficient energy storage systems for high energy density applications such as hybrid cars and portable electronic devices is remarkably growing at an increasing speed. Among proposed structures for Li-ion batteries, lithium/sulfur (Li/S) batteries are a promising candidate due to their very high specific energy density (~2600 W.h.kg^-1), abundance in nature, low cost, and environmental friendliness in comparison to the conventional Li-ion batteries. However, fast capacity fading due to shuttling phenomenon, as a drawback in these cells, has hindered their wide application as an efficient energy storage today. Here a rigorous semi-empirical model is developed to predict the capacity fading of Li/S batteries for different nanostructures embedded in the cathode by taking into account the polysulfide (PS) shuttling effect and discharge rates. In our numerical model, capacity fading of the cell is considered to be affected by the concentration of sulfur dissolved into the electrolyte and deposited on the anode as a Solid Electrolyte Interphase (SEI) layer. Our approach considers SEI layer formation as the main factor that dominates capacity fading over initial cycles (50 cycles). The mean value of percentage error between simulation results and experimental capacity in all analyzed structures is less than 5%. According to our model, adding a graphene layer to the separator with using a hollow carbon nanotube structure in the cathode retain >99% of initial capacity over 50 cycles which is promising for more efficient Li-S batteries.

Gallium nitride (GaN) is an attractive piezoelectric material as same as aluminum nitride (AlN). These wurtzite nitrides have a good output voltage coefficient rather than oxide piezoelectric materials such as lead zirconium titanate Pb(Zr, Ti)O₃. They have been regarded as attractive candidates for featuring a sensor, an energy harvester and a bulk acoustic wave resonator. However, it has not been progressed the investigation of GaN as piezoelectric materials, compared with AlN. Indeed, high quality GaN films can be prepared by MOCVD, but they have not been prepared by RF-sputtering, a common method of MEMS. The lattice mismatch is large between GaN and silicon (Si) which is standard substrate material. Then, it is not easy to grow GaN on Si directly.
In this study, we investigated some metal layers as a buffer layer. (002) oriented hafnium (Hf) was most effective. The FWHM of rocking curve for GaN 002 diffraction was about 1°, which is sufficient narrow compared with those of general (002) oriented AlN MEMS films. This would be caused by that the a-axis of hexagonal Hf (3.196 Å) is close to that of GaN (3.19 Å). This film showed a good piezoelectric coefficient, which was comparative to that of single crystal GaN.
Some calculation papers predict that the scandium (Sc) doping increase the piezoelectric coefficient of GaN. However, it has not been demonstrated experimentally. We prepared the Sc-doped GaN films using Hf buffer layer. While the crystallinity of films directly-grown on Si was very low, those of doped films on Hf layer was good as same as above non-doped films. The piezoelectric coefficient significantly increased by Sc-doping. The maximum value was 4-times of non-doped GaN. This significantly increase indicates experimentally the possibility of GaN as a piezoelectric material as same as AlN.

Triboelectric nanogenerators (TENGs) are one of the energy harvesters that generated by common mechanical energy. These devices show outstanding performances by light weight, eco-friendliness, low cost, and portability. Nonetheless, the commercialization of TENGs meets its fabrication method’s requirements such as cost-effective, simple processing, scalable. Even these are fabricated in textile-type compatible with a various textile-type devices. In this study, we report for the first time, the layer-by-layer (LbL) assembly of graphene-polymer multilayers for cost-effective, scalable, simple-processing, and wearable TENGs. We could fabric TENGs with graphene-polymer multilayers on flexible polymer substrates with flat, undulated, and textile surfaces by LbL technique. Graphene-polymer multilayers roles as a positive triboelectric material and as an electrode, where the polymer substrate role as a negative triboelectric material. We assemble targeted number of graphene-polymer bilayers and its properties are analyzed by using the morphological and electrical properties. Due to advantages of LbL technique such as simple-processing, scalable, graphene-polymer multilayers could be uniformly deposited on undulated 3D surfaces, even on large-scale fabric textiles. These LbL assembled graphene-polymer multilayers called graphene based-TENGs (G-TENGs) with high mechanical durability and outstanding triboelectric performances. As a result, textile sample with graphene-polymer multilayers shows wearable and scalable textile-based G-TENG (TG-TENGs). Due to dual roles of graphene-polymer multilayers, TG-TENGs operated in a single electrode mode with cost-effective and compatibility with textile products such as cloths, bags, etc. Advantages of LbL technique can enable the fabrication of TENGs on various types of substrates. Thus, LbL assembled G-TENGs have variety of applications such as portable personal microelectronic device (e.g., self-powered wireless sensors). The LbL assembled graphene-polymer multilayers show outstanding triboelectric performances and high durability (over 20,000 times for bending and rolling) by cost-effective, scalable, and wearable G-TENG’s properties. A 3 BL graphene-polymer thin film shows the maximal output performance (~100 V and ~5 μA at 9 N) due to its suitable morphological and electrical properties. We expect LbL technique for G-TENG fabrication to be a powerful technique for the real-world commercial application of TENGs, particularly in the electronic textiles industry.

Triboelectric nanogenerators(TENGs) have been intensively studied due to their simple device structures, high output voltages, and high energy conversion efficiency. The use of environmental triggers, such as friction, vibration, and deflection, serve as mechanical sources to power TENGs. In addition to these advantages, materials chosen for TENGs are not limited to wurtzite structures. General materials, including dielectric and polymer materials, are also commonly seen in TENGs, which largely broadens the application prospects, compared to the conventional semiconductor devices.
Generally, two materials with different tribopolarity result in a potential difference upon contact, leading to a contacted-induced charge transfer process, which is known as the basic mechanism of TENGs. The material structures fabricated can vary from nanowires to thin films. Recently, much attention paid to the research of TENGs focused on the device structure modification and the use of different materials. Here, we develop a simple structure based on nonpolar ZnO thin film. It has been shown that nonpolar facet of semiconductor has a huge impact on piezotronic effect of nanowire-based devices. Our work indicated that the nonpolar facet, or textured surface of ZnO thin film, can enhance the induced charges, which further increases the output power of TENG, compared to the same material under polar facet. The obliquely aligned ZnO thin film is grown by home-made magnetron sputtering. And the texture characteristics are carefully examined through XRD,SEM, and TEM. The physical mechanism is systematically investigated. To the best of our knowledge, this work provides the first evidence of how the nonpolar ZnO thin film improves the output power of TENGs.

In exploring the solid oxide fuel cell (SOFC) sealing applicability of boroaluminosilicate glass, the crystallization of 4K₂O-1Li₂O-12MgO-10B₂O₃-40SiO₂-16Al₂O₃-12MgF₂ composition with and without containing 5PbO/BaO/ZnO (wt.%) content were studied by means of dilatometry, DSC, XRD, FTIR, SEM and microhardness analysis. Density of base K-Li-Mg-B-Si-Al-F glass (2.59 g.cm^–3) is found to be increased on addition of the network modifier oxides PbO, BaO and ZnO content. Addition of Pb²⁺, Ba²⁺ and Zn²⁺ furthermore increased the glass transition temperature (T_g) and decreased coefficient of thermal-expansion value (<7.55×10^-6/K, 50-500°C). A characteristic exothermic peak is found to be appeared in DSC thermograph at the temperature range 800-900°C; and that is ascribed to the formation of crystalline phase fluorophlogopite mica, KMg₃(AlSi₃O₁₀)F₂. Opaque glass-ceramics were prepared from K-Li-Mg-B-Si-Al-F glasses (with and without containing PbO, BaO and ZnO content) by controlled heat-treatment at 1000°C. Interlocked type microstructure combined of flake like fluorophlogopite mica crystals is obtained in ZnO-containing K-Li-Mg-B-Si-Al-F glass-ceramic; and such microstructural pattern is ascribed to cause large thermal-expansion (10.91-13×10^-6/K, 50-800°C). ZnO-containing boroaluminosilicate glass-ceramic is, hence, considered with potential interest as they can exhibit the microcrack resistivity in high temperature recycling operation (like SOFC).

Keywords: Boroaluminosilicate glass; crystallization; fluorophlogopite mica; microstructure; SOFC

Reference:
1. M. Garai, N. Sasmal, A. R. Molla, A. Tarafder, B. Karmakar, J. Mater. Sci. Tech., 31 110-119 (2015).
2. M. Garai, N. Sasmal, A. R. Molla, B. Karmakar, Solid State Sci., 44 10-21 (2015).

We have developed a new soft wave energy converter (SWEC) platform by combining soft material and piezoelectric fibers, and demonstrated 30 µW power generation from the waves. With the abundance of ocean coverage on the earth, wave energy conversion platforms can provide a high percentage of green energy demands. However, it would only be viable if the low efficiency of energy conversion can be overcome. Currently, there are multiple types of wave energy converters (WEC), such as point absorber buoys, surface attenuators, oscillating water columns, and overtopping devices. Nearly all of these converters are very large, require a high cost of deployment and maintenance and hence degrade their real-world applicability. To decrease the maintenance cost and increase the power generation efficiency, we propose to develop a SWEC by integrating soft material with piezoelectric fibers as a low cost, no maintenance device. A Piezoelectric Fiber Composite energy converter and bubble wraps, for floatation, are integrated into a soft elastomer ecoflex sheet. The power generation efficiency of the device has been investigated by testing the SWEC in a wave tank and demonstrating its energy harvesting ability from water waves. The promising wave energy harvesting capability combined with the low cost, no maintenance advantage could potentially generate an impact on the SWEC platform development and its real-world wave energy harvesting applications.

Thermoelectric Properties of Carbon Nanotube Composites with Nonionic Insulating and Conducting Polymers

Mechanical energy harvesting has received substantial attention as a sustainable power generation technology for self-powered wireless sensor network systems in biomedical, wearable as well as industrial health monitoring applications. There are abundant mechanical sources available in nature including sound, vibration, ultrasonic waves and human-based kinetic energies. These sources can be converted into useful electrical energy via piezoelectric materials and devices that are advantageous due to its high efficiency and direct conversion mechanism. Although piezoelectric energy harvesting is a very attractive technology, insufficient power generation still remains as an issue to overcome in order to realize a self-powered system for practical use. So far, development of high efficient piezoelectric materials, devices, and electrical circuits have been the key research approaches in order to enhance harvesting performance. In this work, we will present a new paradigm work, that is, enhancement of metamaterial-based energy harvesting. Metamaterials, artificially engineered materials, exhibit exotic properties including negative refractive index and bandgap, which thus enable us to manipulate mechanical wave propagations. In order to amplify input mechanical wave energy into energy harvesting systems, metamaterials can be utilized to guide and focus acoustic, elastic, vibration energies towards the desired position for harvesting, Recently, several research efforts on metamaterial-based enhancement of energy harvesting have been reported, but mostly based on intuitive design or with little experimental support. We propose an optimized design of phononic crystals with defect as metamaterial for piezoelectric energy harvesting system. Systematic design through geometric and bandgap optimization process is performed and followed by theoretical analysis and experimental verification. Drastic enhancement of energy harvesting performance via metamaterials, more than 20 times of power enhancement, is demonstrated and thoroughly investigated both analytically and experimentally.

The vast majority of power generation in our country today is produced through the same process as it was in the late-1800s: heat is applied to water to generate steam, which turns a turbine, which turns a generator, generating electrical power. In out lab, we are developing a solid-state power generation process that is more befitting the 21^st-century. Thermophotovoltaic (TPV) cells directly convert radiated thermal energy into electrical power, through a process similar to how the more familiar photovoltaics work. TPV generators, however, include more “host-machinery” that solar cells do not incorporate. These components, selective-emitters and filters, shape the way the radiated heat is transferred into the TPV cell for conversion and are critical for its efficiency. Here, we will discuss the work we are performing to improve each of the components in these systems, which will enable TPV generators to be used with nearly any thermal source for both primary power generation and waste heat harvesting.

Thermoelectric (TE) power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions as well as for some unique terrestrial applications where unattended operation in remote locations is required. They are static devices with a high degree of redundancy, no electromagnetic interferences, with well-documented “graceful degradation” characteristics and a high level of modularity and scalability. They are also tolerant of extreme environments (temperature, pressure, shock and radiation). The development of new, more efficient materials and devices is the key to improving existing space power technology and expanding into efficient, cost-effective systems using high-grade heat sources, generated through fossil fuel combustion or from waste exhaust streams in transportation, industrial and military applications.
The Thermoelectric Technology Development Project, one of two technology development projects of NASA’s Radioisotope Power Systems Program, has established a roadmap for the advancement and maturation of higher performance TE materials and devices. This roadmap starts with collaborative research efforts to identify advanced bulk thermoelectric materials, capable of quadrupling current state-of-practice average ZT values over the available operating temperature range of 1275 K to 475 K, through the exploration of structurally complex compounds. The roadmap continues with device-level experimental performance validation followed by advancing high temperature couple and multi-couple module technologies in order to establish their long term performance characteristics. We describe how some of these technologies might be infused into next generation space power systems with significantly higher conversion efficiencies and specific power, and could facilitate the development of modular system architectures thanks to highly versatile common device building blocks. We also discuss how these technology investments have helped renew interest in potential terrestrial waste heat recovery and energy harvesting applications using high grade heat sources (above 800 K), and we highlight some recent efforts, near term opportunities, and the unique challenges in terrestrial applications whose solutions can further benefit and promote NASA’s technology goals.

Solar energy is perhaps the most abundant renewable energy source, but its conversion into electric power is still challenging for photovoltaic (PV) cells because they have not achieved efficiencies and cost that can that put solar cells in a competitive advantage. Achieving a larger efficiency by collecting from the UV to IR spectrum is the motivation for a proposed ultra-wide solar spectrum photovoltaic-thermoelectric (PV-TE) solar power cell. The understanding of the Photo-thermoelectric effects and charge transfer characteristics for the proposed PV-TE solar cell is fundamental and covered in this work. The PV-TE solar cell is composed of a p-type BiSbTe layer deposited over n-type GaAs. The TE component (p-type BiSbTe) will capture the thermal energy of the infrared (IR) region producing carrier diffusion and accumulation on the cold side of the device cell. Charge separation will be observed between the n-GaAs and the p-type BiSbTe thermoelectric leg establishing the contribution to power generation by both: photovoltaic and thermoelectric elements. The proposed technology provides an alternative to single device with a high efficiency due to a wider use of the solar spectrum.

Annually billions of small batteries are replaced and thrown away causing potential environmental threat due to hazardous materials in the batteries. At the same time, their replacement can be expensive as one replacement round may easily exceed the cost of e.g. wireless sensors due to labour cost. Consequently, energy harvesters have been under active research and technology development where the aim has been to replace batteries as a power source or extend their lifetime. In addition, another driving force is to enable truly wireless measurements in applications where conventional solutions are not feasible e.g. rotating parts where cables cannot be used.

In this scheme, piezoelectric energy harvesters have been extensively researched as they provide compact designs and high power density to be utilised in small size electronic devices to power wireless sensors. However, state of the art piezoelectrics contain lead which malign health issues have been previously addressed in Europe by legislations thus possibly compromising the development of future emerging energy harvesting applications. Therefore, new high performance lead-free materials for energy harvesting should be discovered.

In this research, energy harvesting performance of lanthanum and titanium doped flexoelectric BaSrTiO₃ ceramics were investigated. Cantilever type harvester components were tested under different vibration and energy levels and stress conditions. The results and power densities were analysed and compared to corresponding PZT ceramic based cantilever harvester at the same conditions. Furthermore, the results were compared against state-of-the-art piezoelectric energy harvesters under their operating schemes.

The rapid advancement of intelligent wearable electronics imposes the insurgent need for power sources that are deformable, compliant and stretchable. Power sources with these characteristics are difficult and challenging to achieve. The use of liquid metals as an electrode may provide a viable strategy to produce such power sources. Here, we would like to introduce a liquid metal-based triboelectric nanogenerator (LM-TENG) by employing Galinstan as the electrode and silicone rubber as the triboelectric and encapsulation layer. The small Young’s modulus of the liquid metal ensures the electrode to remain continuously conductive under deformations, stretchable to a strain as large as ~300%. The surface oxide layer of Galinstan effectively prevents the liquid Galinstan electrode from further oxidization and permeation into silicone rubber, yielding outstanding device stability. Operating in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cm² produces an open-circuit voltage of 354.5 V, short-circuit current of 15.6 μA, and average power density of 8.43 mW/m², which represent the best performance values for any TENGs. Further, the LM-TENG maintains stable performance under various deformations, such as stretching, folding and twisting. LM-TENGs in different forms, such as bulk-shaped, bracelet-like and textile-like, are all able to harvest mechanical energy from human walking, arm shaking or hand patting to sustainably drive wearable electronic devices. Our work demonstrates a novel application of liquid metal as a stretchable electrode in fabricating stretchable power sources.

With the rapid development of wearable electronics and sensor networks, batteries cannot meet the sustainable energy requirement due to their limited lifetime, size and degradation. Ambient energies such as wind have been considered as an attractive energy source due to its copious, ubiquity, and feasibility in nature. With miniaturization leading to high-power and robustness, triboelectric nanogenerators (TENGs) have been conceived as a promising technology by harvesting mechanical energy for powering small electronics. TENG integration in large-scale applications is still unexplored considering its attractive properties.
In this work, a state of the art design TENG based on wind venturi system is demonstrated for use as a self-powered sensor system in any complex environment. When wind introduces into the air gap of the homemade TENG system, a thin flexible polymer repeatedly contacts with and separates from electrodes. This device structure makes the TENG suitable for large scale harvesting without massive volume. Multiple stacking not only amplifies the output power but also enables multi-directional wind utilization.
The sensor system converts ambient mechanical energy to electricity with 400V peak voltage by charging of a 1000mF super capacitor rapidly with 50mW power. Its future implementation in an array of applications aids in environment friendly clean energy production in large scale medium and the proposed design performs with an exhaustive material testing. The relation between the wind and the electrical performance enhancement is comparatively studied.
By considering these merits of simple fabrication, outstanding performance, robust characteristic and low-cost technology, we believe that TENG can open up great opportunities not only for powering small electronics, but can contribute to large-scale energy harvesting through engineering design being complementary to solar energy in remote areas.

This study presents the energy harvesting characterization of piezoelectric ceramics Lithium Niobate (LNB) and Lead Zirconate Titanate (PZT). PZT and LNB have been used for sensing and energy harvesting applications in the past due to its high piezoelectric and pyroelectric coupling coefficient in the case of PZT and high Curie temperature in the case of LNB. However, characterization of power output of these materials under different thermal and mechanical loads has not been performed in depth. In this study, power output of LNB at different electrical loads due to several thermal stresses was measured. These thermal stresses were introduced using a custom testing setup. In addition, a custom testing setup was fabricated to characterize the response of PZT under thermal and mechanical loads combined. LNB was tested at three different temperature ranges. Pyroelectric coefficient of LNB, as well as peak power at these ranges is reported. Furthermore, PZT was tested under pure mechanical, pure thermal, and combined loads. Power response from the PZT sample was measured using a varying electrical resistance to find the optimum load and its correlation to the testing conditions. Pyroelectric coefficient of LNB was found to increase when temperature was incremented, with a maximum value of -196 μC.m^-2°C^-1 at the 200°C to 225°C. However, it was observed that power decreases as higher temperatures are introduced. A peak power density of 111.84 nW/cm³ was found at the 200 °C to 225 °C temperature range. Finally, the introduction of combined thermal and mechanical loads to the PZT sample yielded overall higher power outputs compared to when only one of these loads was introduced. These results provide a better insight of electrical power generation by piezoelectric ceramics under different thermal and mechanical loads for energy harvesting applications.

Through billion years of evolution, Nature provided us with a myriad of materials with incredibly different colors, structures, and response to external stimuli such as thermal, mechanical or electrical. Nature could be the source of abundant and environmentally benign materials to be used in next generation electronics, paving a way towards a sustainable use of resources. At present, electrical and electronic devices use massive amounts of energy for their manufacturing. Human and environmentally benign natural materials and low-temperature fabrication processes are the first choice for a number of applications, including ubiquitous device networks in the Internet of Things.
Natural molecular materials are usually immersed in aqueous saline media such that their electrical response includes a significant ionic contribution. Depending on their molecular structures, natural materials can also feature electronic transport, on distance ranges that depend on their supramolecular aggregation and consequent p-p stacking.
Systematic investigation of structure-property relationships are needed to generate technologies based on natural materials in sustainable electronics. Because natural organic materials are complex in their chemical composition and molecular structure as well as charge transport mechanisms, comparative studies with well-defined chemically controlled analogues are imperative.
Our work focuses on eumelanin, a dark-brown subclass of redox-active melanin pigments, ubiquitous in animals and plants. Eumelanin, exhibits intriguing hydration-dependent electrical properties. It also possesses photoprotective, thermoregulative, metal ion chelating (ion binding affinity) and free radical quenching properties.
We report about how the eumelanin pigment (polymer) forms from building blocks (monomers) and how the structure of the pigment affects its electrical response, in view of the fabrication of eumelanin-based devices for energy storage (1-3). We also explore, in our studies, the biodegradability of materials and devices, in compost conditions.

(1) E. Di Mauro et al, MRS Communications 7, 141, 2017.
(2) P Kumar et al, JMCC, 4, 9516, 2016.
(3) A. Pezzella et al. 2, 212, 2014

ReaxFF based molecular dynamics (MD) simulation provides opportunities for fundamentally understanding pyrolysis of lignocellulose biomass through precisely controlled reaction conditions and monitoring of reaction evolution processes. Despite demonstration of simulating the pyrolysis process of dry lignocellulose, MD investigation of this process assisted by water is yet to be performed. This is important considering that most pyrolysis experiments are performed under wet conditions. In this paper, roles of water on the pyrolysis process of the lignocellulose were investigated by ReaxFF MD simulation. In the simulation, both dry cellulose and lignin systems as well as their systems containing 33% and 66% water by weight were studied at a temperature range of 1250 K-2000 K at a time scale of 5 ns. Products were characterized by studying their phases, H/C, O/C ratios, and their higher heating values (HHV) that are used to evaluate their value as fuels. Time evolutions of water and other chemical products were investigated to determine the role of water in the reactions and to reveal the reaction mechanism. Compared with dry systems, pyrolysis of the cellulose in presence of water shows several interesting trends, including enhanced breakdown of the cellulose polymer, increased oxygenation of the products, and shift of the final products from char to oil. In contrast, lignin remains largely unaffected by water, and simulation has reproduced experimental results of lignin char formation at elevated temperature in liquefaction. Moreover, it is found that the temperature plays an important role in the reactions. As temperature increases the water’s oxygenating effects in the cellulose is decreased. These theoretical results provide solid evidence for unveiling the reaction mechanism of biomass pyrolysis, offering useful guidance for processing wet biomass to liquid fuels.

Sulfur poisoning is a common problem for Ni catalysts and various industrial processes, including solid oxide fuel cells (SOFC). Especially in the case of biomass derived syngas feeds sulfur species like H₂S are present in large concentrations and can cause irreversible damage to a SOFC anode. It is widely accepted that this is largely due to the poisoning of Ni sites active for the water gas shift reaction (WGS, CO + H₂O → CO₂ + H₂), which provides additional H₂ as fuel to the cell[1]. Conventional Ni/Yttria-stabilized zirconia (Ni/YSZ) anode cermets cannot be regenerated from poisoning by reoxidation due to fatal anode fracturing. This work addresses anode regenerability by substituting the conventional cermet with a self-regenerating perovskite type metal oxide (PMO). The development of such a material can significantly increase overall device lifetime and reduce costs of feed gas desulfurization.
Some PMO compositions exhibit the remarkable reversible segregation of catalytically active metals from and back into the host lattices. This was found to be a highly efficient way to inhibit catalyst deactivation by metal particle sintering in alternating red/ox conditions [2-4]. A material was developed in this work which is completely regenerable from S-poisoning. Simultaneously, vital SOFC application targeted material requirements such as high ionic and electronic conductivity are fulfilled.
Phase pure materials were prepared by a citrate-gel method and characterized using X-ray diffraction (XRD), N2-physisorption and electron microscopy (SEM and STEM). La_0.3Sr_0.55Ti_0.95Ni_0.05O_3-δ (LSTN) and impregnated 1.6 wt% Ni/La_0.3Sr_0.55TiO_3-δ (Ni/LST) are compared to a conventional 50 wt% Ni/YSZ SOFC anode material. Ni reduction, segregation and reincorporation were followed by means of H₂-temperature programmed reduction as well as XRD and X-ray absorption spectroscopy (XAS). Catalytic activity towards WGS was determined on powder samples. It is shown that at 800 °C Ni can be selectively reduced and segregated from the bulk LSTN to the material surface where it forms catalytically active, metallic Ni particles of few tens nm in size [4]. After reoxidation Ni is fully reincorporated into the host lattice as shown by Ni K-edge XAS. Catalytic data obtained over a number of redox cycles and Ni particle size analysis show the excellent redox stability of this material and demonstrates inhibition of Ni particle sintering due to the reversible Ni reincorporation during oxidation. This is displayed by neither reference material (Ni/LST and Ni/YSZ). Catalytic activity after sulfur poisoning could be completely restored by two redox cycles thus demonstrating catalyst regenerability. Furthermore, the material was successfully implemented and tested in a redox stable SOFC anode.

1. Kuhn, J.N., et al., J. Mol. Catal. A 282 (2008)
2. Nishihata, Y., et al., Nature 418 (2002)
3. Steiger, P., et al., ChemSusChem 10 (2017)
4. Burnat, D., et al., A. J. Mater. Chem. A 4 (2016)