Metals Tech

Scientists have dedicated intense work in recent years to convert environmental energy into electricity to meet the ongoing demands for a cleaner and more sustainable power source. Harvesting environmental mechanical energy as an eco-friendly method is a promising solution and plays a significant role in building wearable electronics and sensor networks in the Internet of Things (IoTs). A triboelectric nanogenerator (TENG) is a self-powered, feasible solution to convert mechanical energy into electricity and specifically satisfy the increasing demand of the internet of things (IoTs).

In the present work, Di Liu and co-workers at the Departments of Nanoenergy and Nanosystems, Materials Science and Engineering, and Nanoscience and Technology in China and the USA, developed a next-generation TENG to realize constant current output by coupling the triboelectrification effect and electrostatic breakdown. They obtained a triboelectric charge density (430 µC m-2), much higher than those with conventional TENG—that were limited by electrostatic breakdown. The findings of the study are now published in Science Advances, to promote the miniaturization of self-powered systems for use in IoTs and provide a paradigm shift technique to harvest mechanical energy.

Lightweight and wearable power supply modules with high energy storage performance are desirable for wearable technology in materials science. They can be conventionally achieved by directly integrating a rechargeable energy storage device, i.e. a battery or supercapacitor into fabrics. Mechanical energy harvesting has attracted much attention as explored through the techniques of electromagnetic generators (EMGs), piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs).

While EMGs are based on Faraday’s law of electromagnetic induction, suitable for large-scale power generation, PENGs can convert tiny physical deformations into electricity in self-powered, small-scale devices. Conventional TENGs have demonstrated cost-effective, clean and sustainable features, on the basis of triboelectric effects and electrostatic induction to convert energy into electricity. The TENGs also offer lightweight, small size, a wide choice of materials and high efficiency even at low frequencies.

Conventional TENGs are withheld due to the requirements of a rectifier (corrector), such as a rotary rectifier bridge to generate a DC output, which limits its portability. In addition, AC powered TENGs require electromagnetic shielding through sensor integration, which can reduce the degree of its accommodation in a miniaturized device. The pulsed output can give rise to a very high crest factor, which is a key metric to output instability influencing the performance of energy storage and electronics, where constant input is preferred. While a constant DC output was realized very recently using the sliding Schottky nanocontact technique, the output voltage was too low to directly drive electronics. In the present work, Liu et al. therefore invented DC-TENG, to address these issues and generate constant DC by directly coupling triboelectrification effect and electrostatic breakdown as a paradigm-shifting technique.

The working principle of DC-TENG relied on triboelectrification or charge transfer between two surfaces in contact in ambient environments, resembling the same natural principle behind the amber effect and lightning. For this, Liu et al. induced artificial lightning with a charge-collecting electrode (CCE), frictional electrode (FE) and triboelectric layer in the next-generation DC-TENG setup. In the experiment, the scientists used copper electrodes for both CCE and FE, and a polytetrafluoroethylene (PTFE) film attached to an acrylic sheet as the triboelectric layer.

Based on the initial alignment between the electrodes and the PTFE film, Liu et al. generated a quasi-permanent electric charge on the PTFE film. They moved a slider in the medium to build a very high electrostatic field between the CCE and negatively charged PTFE film. When the electrostatic field exceeded the dielectric strength between them at an approximate value of 3 kV/mm, the nearby air became partially ionized to begin conducting. This technique resulted in the flow of electrons from the PTFE to CCE in the experiment to rationally induce air breakdown and create artificial lightning.

Unlike with conventional TENGs that did not tap the energy of air breakdown, Liu et al. used the CCE to effectively collect these charges. In brief, in their experimental setup, the electrons on the FE transferred to PTFE via triboelectrification, then transported to the CCE via electrostatic breakdown and ultimately to the FE via an external circuit. When the slider returned to its initial state in the experiment, there was no current flow in the external circuit due to the absence of a potential difference across the CCE and PTFE film.

In this way, the scientists produced cyclic DC by periodically moving the slider, they measured the DC resulting from the unidirectional dielectric breakdown of the capacitor to produce an ongoing conduction current. Liu et al. showed that the charge quantity harvested by the DC-TENG via dielectric breakdown was greater than that harvested by conventional TENG using electrostatic induction and aimed to use this new paradigm as a prototype to harvest lightning energy. They intend to investigate the detailed mechanism of the process and form a precise theoretical model in the future.

In the present study, Liu et al. designed two modes of DC-TENG: a sliding mode and a rotary mode. To implement the sliding process the scientists used a linear motor and used a commercial motor to drive the rotary process. They used scanning electron microscopy (SEM) images to view the nanowire electrodes (CCE and FE) on the PTFE surface. When they moved the slide along the electrified layer, the scientists captured the phenomenon of corona discharge as a green glow during air breakdown between PTFE and CCE as solid evidence of air breakdown during device operation.

They measured the surface potential of PTFE to show electrostatic charge discharge by electrostatic breakdown using an Isoprobe electrostatic voltmeter, followed by the measurement of short-circuit current and transferred charges of the DC-TENG, using a programmable electrometer. To measure the open-circuit voltage of the sliding mode DC-TENG, they used a mixed domain oscilloscope—all results exhibited characteristics of good DC output.

Liu et al. showed that the initial charge density of the DC-TENG was higher (330 µC m-2) than conventional TENG (~ 70 µC m-2). To enhance the charge density, the scientists introduced nanostructures on the PTFE surfaces using inductively coupled plasma processes to modify the material and achieve a six-fold charge density enhancement at 430 µC m-2. The work showed that the output performance of the system could be enhanced by simple structural optimization of the PTFE film surface. When Liu et al. measured the long-term output current of the DC-TENG after 3000 cycles, the DC output current remained nearly stable, confirming excellent stability of the setup.

In parallel, the scientists similarly measured the output performance of rotary mode DC-TENG. The structure of the setup contained a stator and a rotator, and much like the sliding mode DC-TENG the Fes and CCEs were connected. As before, the scientists conducted measurements to show how electricity generation relied on the relative rotation between the rotator and the stator for better performance compared to conventional DC-TENG.

Due to their continuous DC output generation, Liu et al. demonstrated applications of novel DC-TENGs to drive electronic devices without using a rectifier. For device functionalization, the self-powered DC-TENGs were able to drive electronics directly by converting mechanical energy. As a proof-of-principle, the scientists formed an electronic watch driven directly by a sliding mode DC-TENG and a scientific calculator driven by a rotary DC-TENG. In addition, they formed a light-emitting diode (LED) bulb array, which could be lit up by the rotary mode of DC-TENG, and unlike LED driven via conventional TENG, these LED lights remained without flickering at a constant luminescence.

In this way, Liu et al. achieved the conversion of mechanical energy into constant output current by designing next-generation DC-TENGs based on the coupled effect of triboelectrification and electrostatic breakdown. They used a sliding mode DC-TENG and rotary mode DC-TENG to demonstrate the mechanism, resulting in a charge density value much higher (430 µC m-2) than that of the conventional device. The crest factor of rotary TENG was close to one, indicating a constant current output.

The novel DC-TENG is an effective strategy to harvest mechanical energy and power electronics or charge an energy storage unit directly without a rectifier. The paradigm-shift in converting mechanical energy to electricity can also promote the miniaturization of self-powered systems in wearable electronics and sensor networks in the IoTs. Liu et al. further envision the device as a prototype to harvest lightning energy in the future.

A new type of light-emitting diode has been developed at TU Wien. Light is produced from the radiative decay of exciton complexes in layers of just a few atoms thickness.

When particles bond in free space, they normally create atoms or molecules. However, much more exotic bonding states can be produced inside solid objects.

Researchers at TU Wien have now managed to utilise this: so-called “multi-particle exciton complexes” have been produced by applying electrical pulses to extremely thin layers of material made from tungsten and selenium or sulphur. These exciton clusters are bonding states made up of electrons and “holes” in the material and can be converted into light. The result is an innovative form of light-emitting diode in which the wavelength of the desired light can be controlled with high precision. These findings have now been published in the journal Nature Communications.

Electrons and holes

In a semiconductor material, electrical charge can be transported in two different ways. On the one hand, electrons can move straight through the material from atom to atom in which case they take negative charge with them. On the other hand, if an electron is missing somewhere in the semiconductor that point will be positively charged and referred to as a “hole.” If an electron moves up from a neighbouring atom and fills the hole, it in turn leaves a hole in its previous position. That way, holes can move through the material in a similar manner to electrons but in the opposite direction.

“Under certain circumstances, holes and electrons can bond to each other,” says Prof. Thomas Mueller from the Photonics Institute (Faculty of Electrical Engineering and Information Technology) at TU Wien. “Similar to how an electron orbits the positively charged atomic nucleus in a hydrogen atom, an electron can orbit the positively charged hole in a solid object.”

Even more complex bonding states are possible: so-called trions, biexcitons or quintons which involve three, four or five bonding partners. “For example, the biexciton is the exciton equivalent of the hydrogen molecule H2,” explains Thomas Mueller.

Two-dimensional layers

In most solids, such bonding states are only possible at extremely low temperatures. However the situation is different with so-called “two-dimensional materials,” which consist only of atom-thin layers. The team at TU Wien, whose members also included Matthias Paur and Aday Molina-Mendoza, has created a cleverly designed sandwich structure in which a thin layer of tungsten diselenide or tungsten disulphide is locked in between two boron nitride layers. An electrical charge can be applied to this ultra-thin layer system with the help of graphene electrodes.

“The excitons have a much higher bonding energy in two-dimensional layered systems than in conventional solids and are therefore considerably more stable. Simple bonding states consisting of electrons and holes can be demonstrated even at room temperature. Large, exciton complexes can be detected at low temperatures,” reports Thomas Mueller. Different excitons complexes can be produced depending on how the system is supplied with electrical energy using short voltage pulses. When these complexes decay, they release energy in the form of light which is how the newly developed layer system works as a light-emitting diode.

“Our luminous layer system not only represents a great opportunity to study excitons, but is also an innovative light source,” says Matthias Paur, lead author of the study. “We therefore now have a light-emitting diode whose wavelength can be specifically influenced – and very easily too, simply via changing the shape of the electrical pulse applied.”

UCLA researchers and colleagues have designed a new device that creates electricity from falling snow. The first of its kind, this device is inexpensive, small, thin and flexible like a sheet of plastic.

“The device can work in remote areas because it provides its own power and does not need batteries,” said senior author Richard Kaner, who holds UCLA’s Dr. Myung Ki Hong Endowed Chair in Materials Innovation. “It’s a very clever device—a weather station that can tell you how much snow is falling, the direction the snow is falling, and the direction and speed of the wind.”

The researchers call it a snow-based triboelectric nanogenerator, or snow TENG. A triboelectric nanogenerator, which generates charge through static electricity, produces energy from the exchange of electrons.

Findings about the device are published in the journal Nano Energy.

“Static electricity occurs from the interaction of one material that captures electrons and another that gives up electrons,” said Kaner, who is also a distinguished professor of chemistry and biochemistry, and of materials science and engineering, and a member of the California NanoSystems Institute at UCLA. “You separate the charges and create electricity out of essentially nothing.”

Snow is positively charged and gives up electrons. Silicone—a synthetic rubber-like material that is composed of silicon atoms and oxygen atoms, combined with carbon, hydrogen and other elements—is negatively charged. When falling snow contacts the surface of silicone, that produces a charge that the device captures, creating electricity.

“Snow is already charged, so we thought, why not bring another material with the opposite charge and extract the charge to create electricity?” said co-author Maher El-Kady, a UCLA postdoctoral researcher of chemistry and biochemistry.

“While snow likes to give up electrons, the performance of the device depends on the efficiency of the other material at extracting these electrons,” he added. “After testing a large number of materials including aluminum foils and Teflon, we found that silicone produces more charge than any other material.”

About 30 percent of the Earth’s surface is covered by snow each winter, during which time solar panels often fail to operate, El-Kady noted. The accumulation of snow reduces the amount of sunlight that reaches the solar array, limiting the panels’ power output and rendering them less effective. The new device could be integrated into solar panels to provide a continuous power supply when it snows, he said.

The device can be used for monitoring winter sports, such as skiing, to more precisely assess and improve an athlete’s performance when running, walking or jumping, Kaner said. It also has the potential for identifying the main movement patterns used in cross-country skiing, which cannot be detected with a smart watch.

It could usher in a new generation of self-powered wearable devices for tracking athletes and their performances.

It can also send signals, indicating whether a person is moving. It can tell when a person is walking, running, jumping or marching.

The research team used 3-D printing to design the device, which has a layer of silicone and an electrode to capture the charge. The team believes the device could be produced at low cost given “the ease of fabrication and the availability of silicone,” Kaner said. Silicone is widely used in industry, in products such as lubricants, electrical wire insulation and biomedical implants, and it now has the potential for energy harvesting.

Inside modern cell phones are billions of nanoscale switches that flip on and off, allowing the phone to function. These switches, called transistors, are controlled by an electrical signal that is delivered via a single battery. This configuration of one battery to power multiple components works well for today’s technologies, but there is room for improvement. Each time a signal is piped from the battery to a component, some power is lost on the journey. Coupling each component with its own battery would be a much better setup, minimizing energy loss and maximizing battery life. However, in the current tech world, batteries are not small enough to permit this arrangement—at least not yet.

Now, MIT Lincoln Laboratory and the MIT Department of Materials Science and Engineering have made headway in developing nanoscale hydrogen batteries that use water-splitting technology. With these batteries, the researchers aim to deliver a faster charge, longer life, and less wasted energy. In addition, the batteries are relatively easy to fabricate at room temperature and adapt physically to unique structural needs.

“Batteries are one of the biggest problems we’re running into at the Laboratory,” says Raoul Ouedraogo, who is from Lincoln Laboratory’s Advanced Sensors and Techniques Group and is the project’s principal investigator. “There is significant interest in highly miniaturized sensors going all the way down to the size of a human hair. We could make those types of sensors, but good luck finding a battery that small. Current batteries can be round like coin cells, shaped like a tube, or thin but on a centimeter scale. If we have the capability to lay our own batteries to any shape or geometry and in a cheap way, it opens doors to a whole lot of applications.”

The battery gains its charge by interacting with water molecules present in the surrounding air. When a water molecule comes in contact with the reactive, outer metal section of the battery, it is split into its constituent parts—one molecule of oxygen and two of hydrogen. The hydrogen molecules become trapped inside the battery and can be stored until they are ready to be used. In this state, the battery is “charged.” To release the charge, the reaction reverses. The hydrogen molecules move back through the reactive metal section of the battery and combine with oxygen in the surrounding air.

So far, the researchers have built batteries that are 50 nanometers thick—thinner than a strand of human hair. They have also demonstrated that the area of the batteries can be scaled from as large as centimeters to as small as nanometers. This scaling ability allows the batteries to be easily integrated near transistors at a nano- and micro-level, or near components and sensors at the millimeter- and centimeter-level.

“A useful feature of this technology is that the oxide and metal layers can be patterned very easily into nanometer-scale custom geometries, making it straightforward to build intricate battery patterns for a particular application or to deposit them on flexible substrates,” says Annie Weathers, a staff member of the laboratory’s Chemical, Microsystem, and Nanoscale Technologies Group, who is also involved in the project.

The batteries have also demonstrated a power density that is two orders of magnitude greater than most currently used batteries. A higher power density means more power output per the volume of the battery.

“What I think made this project work is the fact that none of us are battery people,” says Ouedraogo. “Sometimes it takes somebody from the outside to see new things.”

Currently, water-splitting techniques are used to generate hydrogen for large-scale industrial needs. This project will be the first to apply the technique for creating batteries, and at much smaller scales.

The research firm expects the market to generate $1.75 billion by 2025 from $194.5 million in 2017.

The rapid growth is a result of an increase in investments by governments towards the development and adoption of fuel cells and the growing adoption of clean energy. The main aim is to reduce carbon emissions

Owing to the massive investment made by the US Department of Energy towards R&D of fuel cells and infrastructure, North America had the largest market share in 2017 and is expected to record an annual growth of 36.0% during the forecast period.

Europe, which accounted for 3% of the overall market in 2017, is expected to be the fastest growing region with CAGR of more than 38.0% over the forecast period.

ABB and the SINTEF Ocean laboratory in Trondheim will assess how fuel cells and batteries can best function together for short-distance ferry operations, and how Fiskerstrand can integrate them with other engine room systems. The tests will also provide insight into the introduction of hydrogen fuel cells for future reviews of the rules covering shipboard use of hydrogen.

The tests will simulate the conditions the ferry is expected to encounter on a high frequency 10km route to ensure that the propulsion systems including fuel cells are robust enough for repetitive, short-burst service duties.

“We expect to get a realistic view of what we need to do to achieve our objectives in delivering a ferry equipped with hydrogen fuel cell propulsion as part of ourHYBRIDship project,” says Kåre Nerem, Project Manager, Fiskerstrand. “ABB’s system integration know-how, combined with SINTEF Ocean’s long-standing experience in the field of marine propulsion systems, as well as SINTEF Industry’s expertise in fuel cells technology will be key in solving the challenges ahead. This is a pioneering project, and together we will ensure the solution is optimized for the specific ferry route and vessel.”

The HYBRIDshipproject, started in 2017 and driven by Fiskerstrand Holding, is supported by Norway’s “Pilot-E” technology accelerator program funded by the Research Council of Norway, Innovation Norway and Enova Norwegian government enterprise. The project is envisaging a zero-emissions passenger ship retrofitted with fuel cells operating on a domestic route by the end of 2020.

“The project is a major step towards the practical use of the hydrogen fuel cell as a maritime propulsion technology,” says Jostein Bogen, product manager for energy storage and fuel cells at ABB Marine & Ports. “Fuel cells combined with batteries are an important part of ABB’s ‘Electric. Digital. Connected.’ vision for a sustainable maritime future. The true significance of these tests will be in defining the optimum engine room configuration for hydrogen fuel cells to be installed and work day-in, day-out with other systems onboard.”

ABB first invested in SINTEF Ocean’s hybrid marine laboratory in 2014, recently strengthening its collaborative commitment by injecting a second round of funding to expand the laboratory’s facilities for future development work. “Together with the Norwegian Fuel Cell and Hydrogen Test Centrehosted by SINTEF Industry, the extended hybrid lab will help us to further develop, validate and optimize control strategies for advanced maritime energy system” says Anders Valland, Research manager for maritime energy systems at SINTEF Ocean.

“These facilities have become an important catalyst for the ongoing evolution of hydrogen-based power generation,” comments Bogen. “The ability to carry out real-life testing of different configurations accelerates the development process and ensures we arrive at an optimized solution that satisfies vessel design criteria and operational profile.”

Hydrogen fuel cells are regarded as a promising option for radically reducing vessel greenhouse gas emissions. Combined with more established shipboard battery technology, they have the capability to enhance energy density in zero-emission marine operations while also improving vessel endurance.

The joint ABB/SINTEF development program will also focus on finding solutions to support the hydrogen supply and bunkering infrastructure. In addition, outputs from the new tests are expected to accelerate Norwegian Maritime Authority (NMA) work in modifying regulations to better accommodate and approve hydrogen as a fuel.