Published on September 27, 2014
Seminar on Nanogenerators Presented by : Siddhant H Pathak Roll No. - 20 M.Sc. – 3rd Sem. Department of : Materials Science
Index Introduction 3 basic types of nanogenerators Hybrid Generators Advance Material for nanogenerators Nanosized Hydrogen Generator References
Introduction Renewable energy technologies consists of two distinct processes: Energy generation (using sources such as solar, wind, tidal etc.) energy storage (batteries, fuel cells). Accomplishment of these two processes 1) Process converting the original form of energy to electricity. 2) Converting electricity to chemical energy(for storage). For the first time a device has been made that converts mechanical energy(any vibration) directly to chemical energy bypassing the intermediate step of electricity generation. The device hence acts as a hybrid(generator-battery unit) or in other words, a self-charging power cell. View slide
For this the researchers started with a coin-type Li-ion battery and replaced the polyethylene separator that normally separates the two electrodes with PVDF (Polyvinylidene difluoride) film As a piezoelectric material, PVDF film generates a charge when under an applied stress. PVDF film causes positive Li ions to migrate from the cathode to the anode in order to maintain a charge equilibrium across the battery. this ion migration process charges the battery without the need for any external voltage source. The battery is hence SELF-CHARGING View slide
Nanogenerators Nanogenerator is a device that converts ambient energy(sound, muscle movement, heat, fluid flow etc) into electricity. Nanogenerators are typically of three types -: 1) Piezoelectric nanogenerators uses mechanical energy 2) Triboelectric nanogenerators uses frictional energy 3) Pyroelectric nanogenerators uses fluctuation in temperature
Piezoelectric nanogenerator Electrode- Typically made of Si coated with Pt to increase conductivity ZnO nanowires have a diameter of 100 to 300 nm and produce 45mV on pulling back and forth
Working Schottky contact must be formed between the counter electrode and the tip of the nanowire since the ohmic contact will neutralize the electrical field generated at the tip. In order to form an effective schottky contact, the electron affinity(Ea) must be smaller than the work function(φ) of the the counter electrode. For the case of ZnO nanowire with the electron affinity of 4.5 eV, Pt(φ=6.1eV) is a suitable metal to construct the schottky contact. By constructing the schottky contact, the electrons will pass to the counter electrode from the surface of the tip when the counter electrode is in contact with the regions of the negative potential, whereas no current will be generated when it is in contact with positive potential [Rectifier]
Triboelectric Generator A device that takes advantage of static electricity to convert movement—like a phone bouncing around in your pocket— into enough power to charge a cell phone battery. When thin films of PET plastic and a metal come into contact with one another, they become charged. And a current flows between them, which can be harnessed to charge a battery. When the two surfaces are patterned with nanoscale structures, their surface area is much greater, and so is the friction between the materials—and the power they can produce. A fingernail-sized square of this triboelectric nanomaterial can produce eight milliwatts enough to run a pacemaker. A patch that’s five by five centimeters can light up 600 LEDs at once, or charge a lithium-ion battery that can then power a commercial cell phone.
Principle: works on the basis of triboelectric effect. It is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature and strain. Mechanism 1) Vertical Sliding Mode
2) Lateral Sliding Mode : By sliding friction, a periodic change in the contact area, which creates a voltage drop for driving the flow of electrons. Because of the large difference in the ability to attract electrons, the triboelectrification will leave one surface with net positive charges and the other with net negative charges with equal density. Once the top plate with the positively-charged surface starts to slide outward, the in-plane charge separation is initiated due to the decrease in contact surface area. The separated charges will generate an electric field pointing from the right to the left almost parallel to the plates, inducing a higher potential at the top electrode. This potential difference will drive a current flow from the top electrode to the bottom electrode .
Pyroelectric Effect More than 50 percent of the energy generated by us goes to waste, much of it as heat released to the environment by everything from computers to cars to long-distance electric transmission lines. This heat can be converted to electricity using something called the pyroelectric effect Harvesting thermoelectric energy relies on the Seebeck effect that utilizes a temp. difference between two ends of the device for driving the diffusion of charge carriers. Environment is spatially uniform without a temp. gradient and the Seebeck effect can not be used to harvest thermal energy. The mechanism is based on the thermally induced random wobbling of the electric dipole around its equilibrium axis, the magnitude of which increases with increasing temp.
When temp. increases the electric dipoles oscillate within a larger degree of spread around their respective aligning axes. Total average spontaneous polarization is decreased due to the spread of the oscillation angles and the quantity of induced charges is reduced, resulting in a flow of electrons. If the nanogenerator is cooled instead of heated, the spontaneous polarization will be enhanced the amount of induced charges in the electrodes will increase and the electrons will then flow in an opposite direction. Pyroelectric nanogenerator can be used as an active temperature sensor, which can work without a battery i.e utilizing the energy that it produces [SELF-CHARGING TEMP. SENSOR]
Hybrid Nanogenerator Now researchers have combined a nanogenerator with a solar cell to create an integrated mechanical- and solar-energy-harvesting device. This hybrid generator is the first of its kind and might be used, for instance, to power airplane sensors by capturing sunlight as well as engine vibrations for when solar energy isn’t available The top layer here consists of a thin-film solar cell embedded with dye-coated zinc oxide nanowires. The large surface area of the nanowires boosts the device’s light absorption The solar cell and the nanogenerator are electrically connected by the silicon substrate itself, which acts as both the anode of the solar cell and the cathode of the nanogenerator
Advanced material for nanogenerator So far, efforts to make nanogenerators have focused on ZnO nanowires. But barium titanate (BaTiO3)could lead to better generators because it shows a stronger piezoelectric effect, Lab experiments show that a barium-titanate nanowire can generate “16 times” as much electricity as a ZnO nanowire from the same amount of mechanical vibrations High Sensitive accelerometer of BaTiO3 But zinc oxide has its own advantages. It is nontoxic to biological systems making it better suited than barium titanate for implantable devices. Also, it is easier to control zinc-oxide growth in order to fabricate nanowire arrays compared to Barium Titanate
Nanosized Hydrogen Generator Hydrogen is virtually everywhere on the planet, but is typically bonded with other elements and must be separated (Like in H2O) to produce free hydrogen. The process available for this right now consumes too much electricity and releases Co2(A green house gas). N.H.G does not extract but make Hydrogen by combining an electron and a proton Principle: It has been long known that some single-celled organisms use a protein called bacteriorhodopsin (bR) to absorb sunlight and pump protons through a membrane using the green end of sunlight. At the same time electrons can be produced by combining these proteins with titanium dioxide and platinum and then exposing them to ultraviolet light. Combining this we can literally manufacture Hydrogen(for fuel)
Problem: Titanium dioxide only reacts in the presence of ultraviolet light, which makes up a mere 4% of the total solar spectrum Solution [Materials Science] For this the researchers looked for a new material. The new material would need enough surface area to move electrons across quickly and evenly and boost the overall electron transfer efficiency. The researchers also needed a platform on which biological components, like bR, could survive(biologically inert) and connect with the titanium dioxide catalyst: in short, a material like graphene. Along with this Graphene is also super strong, super light, transparent and the best conductor of electricity so far. Its very presence allows the other components to self-assemble around it, which totally changes how the electrons move throughout our system.
References: Phys.orgnews 20th september 2014 Wikipedia Google images Nanogenerators for self powered devices and systems by Zhong Lin Wang, School of Materials Science and Engineering Georgia Institute of Technology, Atlanta GA USA(first edition, June 2011) Technologyreview.com
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