Nanotechnology in Energy Storage and Conversion PDF

Module - 4 NANOTECHNOLOGY IN ENERGY STORAGE AND CONVERSION Introduction Today, the world’s energy demands are satisfied mainly via the combustion of fossil fuels. The rest comes from coal, gas, biomass, and nuclear, a small amount from hydroelectric power, and an almost negligible amoun...

Module - 4 NANOTECHNOLOGY IN ENERGY STORAGE AND CONVERSION Introduction Today, the world’s energy demands are satisfied mainly via the combustion of fossil fuels. The rest comes from coal, gas, biomass, and nuclear, a small amount from hydroelectric power, and an almost negligible amount from renewable resources. Because of the limited availability of fossil fuels, there is an urgent need for alternative energy resources. The options are renewable energy sources such as solar, wind, geothermal, hydro, and so on. Every day, the Earth is hit by 165,000 TW of solar power. In the words of Nobel Prize winner Richard E. Smalley, every day “we are bathed in energy”. However, the problem is that renewable energy sources such as solar energy are not constant in time and evenly distributed geographically. Therefore, major challenges are solar energy collection, conversion, storage, and distribution. Another alternative energy carrier is hydrogen, but hydrogen fuel cell technology still faces several issues (e.g., hydrogen extraction, hydrogen storage, fuel cell lifetime, and cost) before a hydrogen economy can become a reality. Due to the high efficiency and cost-effectiveness of nanomaterials, nanotechnology is expected to play an important role in the field of ‘energy’. Alternative sustainable energy sources like solar energy or hydrogen- based fuel cells incorporated with nanotechnology are needed to satisfy the enormous requirements of future generations. Solar cells A solar cell is a p-n junction diode that converts light energy directly into electrical energy through the photovoltaic effect. The common single-junction silicon solar cell can produce a maximum open circuit voltage of approximately 0.5 to 0.6 volts. This isn’t much by itself, but when many solar cells are combined into a large solar panel, a considerable amount of renewable energy can be generated. Principle When light is absorbed by matter, photons are given up to excite electrons to higher energy states within the material. The extra energy of the excited electrons generates a potential difference or electromotive force (e.m.f.). This force drives the electrons through a load in the external circuit to do electrical work. Construction A solar cell consists of a p-n junction diode made of semiconductors. The p-n diode is packed in a can with a glass window on top so that light may fall upon p and n-type materials. The thickness of the p-region is kept very small so that electrons generated in this region can diffuse to the junction before recombination takes place. The thickness of the n-region is also kept small to allow holes generated near the surface to diffuse to the junction before they recombine. A heavy doping of p and n regions is recommended to obtain a large 58 Department of Physics, SJEC photovoltage. A nickel-plated ring is provided around the n- layer which acts as the positive output terminal. A metal contact at the bottom serves as the negative output terminal. An anti-reflective layer of silicon nitride is applied between grid lines to increase the amount of light transmitted through the front to the cell. Common materials used for solar cells include silicon, gallium arsenide (GaAs), indium arsenide (InAs), and cadmium arsenide (CdAs). Working When light radiation falls on a p-n junction diode, photons collide with valence electrons and impart sufficient energy enabling them to leave their parent atoms. Thus electron-hole pairs are generated in both the p and n sides of the junction. These electrons and holes reach the depletion region W by diffusion [Fig (a)] and are then separated by the strong barrier field existing there. However, the minority carrier electrons in the p-side slide down the barrier potential to reach the n-side, and the holes in the n-side move to the p-side [Fig (b)]. Their flow constitutes the minority current which is directly proportional to the illumination and also depends on the surface area being exposed to light. The accumulation of electrons and holes on the two sides of the junction [Fig (c)] gives rise to an open circuit voltage Voc which is a function of illumination. The p-n junction will behave like a small battery cell. A voltage is set up which is known as photovoltage. If we connect a small load across the junction, there will be a tiny current flowing through it. Solar panels Solar panels, also referred to as photovoltaic panels or PV panels are made by assembling multiple solar cells into a single unit. Their primary function is to capture sunlight and generate electricity from it. Solar panels can consist of various arrangements of solar cells, such as monocrystalline, polycrystalline, or thin-film cells, depending on the design and efficiency requirements. Most solar panels are made from crystalline silicon-type solar cells. These cells are composed of layers of silicon, phosphorous, and boron. These cells, once produced, are laid out into a grid pattern. The number of these cells used depends largely on the size of the panel being created. Once the cells are laid out, the panel 59 Department of Physics, SJEC itself is sealed to protect the cells within and covered with a non-reflective glass. This glass protects the solar cells from damage and is non-reflective to ensure sunlight to reach the cells. Once sealed, this panel is placed into a rigid metallic frame. This frame is designed to prevent deformation and includes a drainage hole to prevent water from building up on the panel as a buildup of water could reduce the efficiency of the panel. Additionally, the back of the panel is also sealed to prevent damage. Generations of solar cells The semiconducting substance from which a solar cell is formed is often the name given to the cell. Based on this, solar cells are divided into first-generation, second-generation, and third-generation solar cells. i) First-generation solar cells are made using single-crystalline silicon wafers and are nothing but p-n diodes. Although their efficiency is around 15-22%, they are too expensive. ii) Second-generation solar cells are based on the thin films of crystalline silicon or amorphous silicon. They are cost-effective to some extent but not very efficient. iii) Third-generation solar cells are based on nanocrystalline materials. First-generation solar cells or Crystalline silicon cells Solar cells produced on the silicon wafers are the first-generation solar cells. Crystalline silicon refers to silicon in its crystalline form. Each wafer can supply 2–3 watt power. To increase power, solar modules which consist of many cells are used. Their efficiency is about 15% to 25%. They may experience efficiency loss at high temperatures or partial shading. They cover 80% of the solar market and currently are the oldest and most popular technology available for residential use as they have long lifespans. 60 Department of Physics, SJEC The silicon wafer-based technology can be divided into two categories: 1. Single/Monocrystalline silicon solar cell 2. Multi/Polycrystalline silicon solar cell Monocrystalline silicon solar cell: These cells are sliced from large crystals grown under controlled conditions during the manufacturing process. Since it is hard to develop large crystals of pure silicon in monocrystalline solar cells, the production cost of this type of panel has traditionally been the largest of all solar panel varieties. They lose productivity when the temperature rises past 25°C. Hence, they are mounted so that air will circulate over and under them to maximize performance. They have a uniform black or dark appearance. Monocrystalline materials are commonly used because they have a higher performance ratio than polycrystalline materials and long lifespans. Monocrystalline cells typically have efficiency rates between 15–25%. Polycrystalline silicon solar cell: A polycrystalline module is made up of multiple silicon crystals that are linked together in a single cell. The molten silicon is poured into a rectangular mold and cooled to form a solid block sliced into thin rectangular wafers. The wafers are treated with doping agents to form a p-n junction, coated with an anti- reflective layer, and fitted with metal contacts. Since tightly regulated growth conditions are not needed, producing silicon wafers in molds is less expensive than producing single-crystal silicon wafers but they have lower quality. Their efficiency decreases more at high temperatures compared to monocrystalline cells. They have a bluish hue with a distinct grainy texture, as individual crystals are visible. Polycrystalline cell efficiency usually ranges from 13–20 %. Nanotechnology in first-generation solar cells 61 Department of Physics, SJEC Nanotechnology is less prevalent in first-generation solar cells. However, some advancements in silicon processing involve nanoscale structures. Also, coatings containing nanomaterials are used to reduce reflection and improve light absorption in these cells. Advantages of first-generation solar cells: 1. High efficiency: Efficiency ranges from 15% to 22%, making them the most efficient among silicon- based technologies. They also perform well in direct sunlight and even in low-light conditions. 2. Proven technology: First-generation solar cells are based on mature and well-understood silicon technology. 3. Longevity and durability: High durability ensures performance for 25–30 years or more with minimal degradation. They are resistant to environmental factors like UV radiation, temperature fluctuations, and humidity. 4. Wide availability: Silicon, the primary material, is one of the most abundant elements on Earth, ensuring consistent supply and production scalability. 5. Recyclability: Silicon-based solar cells can be recycled at the end of their life, reducing environmental impact. 6. Versatility: Used in various applications, including residential rooftops, commercial buildings, agricultural installations, and space missions. Limitations of first-generation or crystalline silicon solar panels 1. Cost: They are typically more expensive than other types of solar panels due to the manufacturing process which involves growing a single crystal of silicon with precision in a cleanroom facility. 2. Size: They are made from a single piece of silicon, which limits their size. Larger installations may require more panels and space. 3. Weather dependence: They are heavily reliant on the weather. 4. Brittle material: The material used in crystalline silicon solar panels is brittle and fragile. They are relatively rigid and heavy, making them less versatile in terms of installation. 5. Recombinational loss: Crystalline silicon cells are often thicker than other types of solar cells, which can lead to recombinational loss. This decreases the overall efficiency of the cell. 6. Manufacturing waste: The production of crystalline silicon solar panels involves cutting silicon, which results in some waste material. 7. Batteries and inverters: Users need to purchase batteries and inverters separately to convert solar energy into electric energy. 8. Reflection: Some sunlight is reflected away from the surface of the solar cell. 9. Non-uniform absorption: Silicon cells may not absorb all wavelengths of light effectively, leading to energy loss. Second-generation solar cells or Thin-film solar cells 62 Department of Physics, SJEC  Obtaining pure silicon is a complex and costly process. The cost of solar cells can be reduced if thin films of silicon (1 μm thickness) can be deposited. A second-generation solar cell known as a ‘thin-film solar cell’ is created by depositing one or more thin layers of photovoltaic material over a substrate made of glass, plastic, or metal. Amorphous thin-film silicon, copper indium gallium diselenide, and cadmium telluride are among the commercially available thin- film solar cell materials. Homo-junction, heterojunction, and multi-junction solar cells are manufactured using thin film technology. Compared to crystalline Si PV technology with about 94% production, thin film PV technology production is only 6%. Thin film solar cells have simpler manufacturing processes and lower production costs. They also have greater flexibility, allowing for lightweight and potentially rollable or flexible solar panels. Thin-film cells perform better in diffuse light, partial shading, or cloudy conditions compared to crystalline silicon cells. They have an efficiency of about 11% to 20% which is a little less than first-generation solar cells. Nanotechnology in second-generation solar cells Second-generation solar cells incorporate nanoscale materials and structures to improve performance. Nanotechnology is used to enhance light absorption and reduce reflection, increasing overall efficiency. For example, nanostructured layers and anti-reflective coatings are employed. Thin film cells have a top layer called the window layer made of a large band gap material that helps with high light transmission and low light absorption and a bottom layer called the absorber layer made of a smaller band gap material that absorbs the photons passed by the window layer. This design allows for an inherently better efficiency. Advantages of thin film solar cell technology Low material consumption Large area modules Tunable material properties Low-temperature processes Transparent modules can be made Thin films can be packaged into flexible and lightweight structures, which can be easily integrated into building components. Limitations Materials that these cells are made of are 63 Department of Physics, SJEC  1. increasingly rare 2. more expensive (Indium) 3. highly toxic (Cadmium) Applications This solar cell technology is being used in pocket calculators to power homes and some remote facilities. in panels that can power traffic, street lights, solar fields, and forest regions. in buildings as a semi-transparent photovoltaic glazing material that can be laminated onto windows. Third-Generation Solar Cells This is the most recent solar cell technology that combines the best aspects of thin-film and crystalline silicon solar cells to deliver high efficiency and enhanced usability. They have several junctions of layers of various semiconducting materials made of amorphous silicon, gallium arsenide, organic polymers, quantum dots, or perovskite crystals. These semiconductors are doped with nanomaterials like silicon wires and solar inks using conventional printing press technologies, organic dyes, and conductive plastics for increasing efficiency. To increase the efficiency of solar cells, it is needed to absorb all photons in incident sunlight. Single- junction solar cells will not solve this purpose. Thus, multi-junction solar cell is considered as a solution to this problem. More theoretical advancements include frequency conversion (i.e., converting light frequencies that the cell cannot use to those the cell can use – thus producing more power), hot-carrier effects, and other multiple-carrier ejection techniques. Some third-generation technologies achieve efficiencies of up to 33% or more in experimental conditions. This technology focuses on simpler, scalable, and low-energy manufacturing techniques, like printing or chemical deposition. Some technologies (e.g., organic PVs and perovskites) are lightweight and flexible, enabling applications in portable devices, wearable electronics, and curved surfaces. Nanotechnology in third-generation solar cells Third-generation solar cells use nanoscale semiconductor particles (silicon wires, solar inks, quantum dots) to absorb a broader spectrum of sunlight, maximizing light capture and conversion. With nanotechnology, materials can be made with tailorable bandgap for specific applications. Also, the third generation of solar cells is a multi-exciton generation where materials absorbing one photon can produce multiple electrons that can increase the performance of the solar cell. Materials like TiO₂ nanowires and CNTs provide direct pathways for electron transport, reducing recombination losses and improving efficiency. 64 Department of Physics, SJEC Advantages of third-generation solar cells Raw materials are easy to find. Easier fabrication process rather than the other two technologies. Technologies like perovskites and OPVs have lower production costs due to simpler manufacturing methods. Flexible and lightweight cells enable integration into vehicles, wearable devices, and building materials. Technologies like DSSCs and OPVs can be transparent or semi-transparent, allowing integration into windows or facades. Some materials like perovskites and quantum dots, perform well under low-light or indoor conditions. Limitations of third-generation solar cells Low efficiency compared to other technologies. Many third-generation technologies, particularly perovskites and OPVs degrade quickly under environmental conditions. Transitioning from laboratory-scale production to large-scale manufacturing can be challenging. Some materials, like lead in perovskites and cadmium in quantum dots pose environmental and health risks as they are toxic. Multi-junction cells and quantum dots can involve expensive or rare materials. Applications of third-generation solar cells 1. Transparent or semi-transparent third-generation cells can be used in windows, walls, or roofs. 2. Lightweight and flexible cells are ideal for powering portable and wearable electronics. 3. High-efficiency multi-junction cells are preferred for satellites and space exploration. 4. Integrated solar panels for electric vehicles or drones. 5. Quantum dots and perovskites can efficiently convert indoor lighting into electricity. Types of third-generation solar cells 1. Nanocrystal-based solar cells 2. Polymer-based solar cells 3. Dye-sensitized solar cells 4. Quantum dot solar cells 5. Perovskite solar cells 6. Organic solar cells 7. Hot carrier solar cells 8. Photochemical solar cells Dye-Sensitized Solar Cells (DSSC) A dye-sensitized solar cell is a thin-film solar cell that mimics natural photosynthesis. It is a photoelectrochemical cell that converts sunlight into electricity using a dye that absorbs light and initiates 65 Department of Physics, SJEC electron transfer. It is semi-flexible and semi-transparent. The modern version of DSSC was invented by Michael Gräetzel and coworkers in 1991 and hence called Gräetzel cells. Construction: DSSC can be made using simple roll-printing techniques or laser ablation technology. There are five different components of a DSSC: 1. Transparent conducting substrate: A glass substrate with a transparent conducting coating like FTO (fluorinated tin oxide) or ITO (indium tin oxide) which also works as fixed mechanical support. 2. Photoanode: TiO2 particles are deposited on a glass substrate that provides a large surface area for dye adsorption and facilitates electron transport. 3. Dye sensitizer: Dye sensitizers are responsible for sunlight harvesting and electron injection. E.g.: Ruthenium complexes, organic dyes, or natural dyes. The dye’s function is similar to that of chlorophyll in plants. 4. Electrolyte: Electrolyte transfers electrons between the photoanode and the counter electrode to regenerate the dye. Usually, an iodide layer acts as an electrolyte. 5. Counter electrode: A glass substrate coated with platinum serves as the counter electrode. The two electrodes are connected externally through a load to obtain the current. Working: 1. Light passes through the transparent electrode and excites electrons in the dye molecules. 2. The excited electrons in the dye molecules are injected into the conduction band of the TiO2. This creates a flow of electrons within the TiO₂ network, leading to the anode. Using nanocrystalline semiconductor material coating increases the effective surface area for the dye particles to adsorb and allows a wider range of the visible spectrum to be absorbed. 3. The oxidized dye molecules are regenerated by the redox electrolyte, which donates electrons. This allows it to continue absorbing light. 4. The electrons conducted away by nanocrystalline TiO2 flow through the external circuit to the platinum counter electrode. The movement of electrons creates electricity that can be used to perform work. 66 Department of Physics, SJEC 5. The electrons from the counter electrode enter the iodide electrolyte where they reduce triiodide (I₃⁻) to iodide (I⁻), completing the cycle. Advantages of DSSCs: DSSCs have advantages such as transparency (allowing for use in semi-transparent solar windows), flexibility, and ease of manufacturing. They perform well in low-light conditions and indirect sunlight, making them suitable for indoor applications or cloudy climates. DSSCs can be manufactured using low-cost and environmentally friendly materials and processes. They are ideal for portable devices and applications with weight constraints. Can be made in various colours or transparent for integration into windows or facades. Since there is no permanent chemical change for dye-sensitized solar cells, the estimated lifetime of these devices is 20 years. Limitations of DSSCs: DSSCs typically have lower efficiency compared to traditional silicon-based solar cells, though research continues to improve their performance. Various hybrid solar cells using nanoparticles, CNTs, graphene, polymers, and other materials are under intense research. Liquid electrolytes are prone to leakage and evaporation, reducing long-term reliability. High temperatures can degrade the dye and electrolyte, impacting performance. The TiO2 nanoparticles in a dye-sensitized solar cell are randomly deposited. This is one of the obstacles to the generated charge carriers reaching the appropriate electrodes, reducing the efficiency of the cell. Quantum Dot-Sensitized Solar Cells Quantum dot-sensitized solar cells (QDSSCs) are a type of third-generation solar cell that utilize quantum dots (QDs) as the light-absorbing and light-converting material. These cells are inspired by dye-sensitized solar cells (DSSCs) but replace organic dyes with quantum dots. 67 Department of Physics, SJEC Construction: 1. Quantum dots (QDs): Quantum dots are nanoscale semiconductor particles (2 to 10 nm in diameter) with size-tunable electronic and optical properties due to quantum confinement effects. They can be made from various semiconductor materials, such as cadmium selenide (CdSe), lead sulfide (PbS), or indium phosphide (InP). The size of quantum dots determines their energy levels and the wavelengths of light they can absorb or emit. 2. Transparent conducting substrate: A glass substrate with a transparent conducting coating like FTO (fluorinated tin oxide) or ITO (indium tin oxide) which also works as fixed mechanical support. 3. Photoanode: They are made of wide-bandgap nanoporous semiconductors like titanium dioxide (TiO₂) or zinc oxide (ZnO) which provide a surface for QD attachment. Nanostructures (e.g., nanorods, nanotubes) are often used to enhance the surface area for QD sensitization. 4. Electrolyte: These serve as the medium for charge transport between the photoanode and counter electrode. Redox couples like polysulfide, iodide, or organic electrolytes are often used to regenerate the QDs. 5. Electron transport layer: This assists in moving electrons generated by light absorption in quantum dots. 6. Hole transport layer: This facilitates the movement of holes (positively charged vacancies left when electrons are excited) to the electrode. 7. Counter electrode: A glass substrate coated with platinum, carbon, or conductive polymers serves as the counter electrode. This facilitates the reduction of the oxidized electrolyte. The two electrodes are connected externally through a load to obtain the current. Working: 1. Upon light irradiation, QD absorbs solar energy, and electrons in the valence band of QDs are excited to the conduction band, generating electron-hole pairs. 2. The excited electrons move to the electron transport layer, while the holes move to the hole transport layer. This charge separation creates a flow of electrons and holes. 3. Electrons are then injected into the mesoporous film of TiO2 from where they are transferred to the transparent conductive oxide substrate and then get to the counter electrode through the external circuit. 4. Meanwhile, the oxidized QDs are regenerated by reduced species of the redox couple in the electrolyte, and the oxidized species of the redox couple are then reduced by the electrons from the external circuit. 5. Apart from these desired charge transport processes, some other unwanted processes, also known as charge recombination occur simultaneously and significantly deteriorate the solar cell performance. Advantages of QDSSCs: 1. High efficiency: Quantum dots can be engineered to absorb a wide range of wavelengths, allowing for the capture of a broader spectrum of sunlight. This can lead to higher energy conversion efficiency. 2. Tunable bandgap: The bandgap of quantum dots can be tuned by controlling their size and optimizing the cell for specific wavelengths enabling the customization of solar cells for specific applications. 68 Department of Physics, SJEC 3. Multiple exciton generation: This means that a single photon can generate multiple electron-hole pairs (excitons), leading to increased photocurrent potentially improving efficiency. 4. Versatility: Quantum dots can be incorporated into various types of solar cell architectures, including tandem solar cells to enhance performance. 5. Cost-effectiveness: QDSSCs can be fabricated using low-cost, scalable techniques like spin-coating or chemical bath deposition. Limitations of QDSSCs: 1. Stability: Quantum dots can degrade under prolonged exposure to light, oxygen, and moisture, limiting the lifespan of QDSSCs. 2. Toxicity: Many high-performance QDs (e.g., cadmium or lead-based) pose environmental and health hazards. 3. Charge recombination: High recombination rates of electrons and holes can reduce efficiency. 4. Efficiency limits: Practical efficiencies are still lower than silicon or perovskite solar cells, despite the theoretical potential of multiple exciton generation. 5. Scalability: Challenges exist in scaling up the production of high-quality QDs and integrating them into large-area devices. 69 Department of Physics, SJEC Batteries Batteries are devices that convert chemical energy into electrical energy. A battery has an electrochemical cell that generates DC through a coupled set of reduction-oxidation (redox) reactions. The battery consists of a positive electrode facing a negative electrode divided by a porous separator, which prevents the electrodes from touching, and an ionic electrolyte, a conducting medium that ensures the movement of the ions from one electrode to the other. The positive electrode (cathode) is reduced (captures electrons) and the negative electrode (anode) is oxidized (donates electrons). Classification of batteries 1. Primary batteries: A primary cell is one in which electrical energy can be obtained at the expense of chemical energy only as long as the active materials are still present. Once these have been consumed, the cell cannot be readily recharged and must be discarded. Eg.: Dry cell 2. Secondary batteries: A secondary cell, once used can be recharged by connecting it to a DC power supply and passing an electric current in the opposite direction. It can be used over and over again. The reactions taking place during the discharge of a secondary battery are reversible. E.g.: Lead-acid battery, Ni-Cd battery, Li-ion battery 3. Reserve batteries: The batteries that may be stored in an inactive state and made ready for use by activating them before the application are referred to as reserve batteries. In this type, a key component is separated from the rest of the battery before activation. In most of the batteries, the component separated from the battery is the electrolyte, thus preventing the chemical reaction in the battery. For example: 1. Lead-acid battery is activated only after the electrolyte sulphuric acid solution is added. 2. Zinc-air battery is activated only after air is allowed to enter the battery by opening the air inlet. Lithium-ion batteries Any battery that uses lithium metal as the anode material is a lithium battery. Lithium has been used in high- performance primary and secondary batteries. Primary batteries involve Li metal whereas secondary batteries involve Li ion (as in LiCoO2/graphite). Lithium metal is used as anode material in the battery because of its lightweight, high energy density (the amount of energy a battery can store in a given mass or volume), lowest electrode potential (-3.7 V), high electrochemical equivalence (the amount of a substance that is deposited or liberated at an electrode when a specific amount of electricity passes through it), and good conductivity. 70 Department of Physics, SJEC Components: Anode material: Lithium intercalated (reversibly included between the layers) in graphite. Cathode material: A lithium metal oxide such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), or other variants. Electrolyte: The electrolyte is a conductive solution or solid material that allows lithium ions to flow between the anode and cathode while preventing direct electrical contact between them. Usually, Lithium salt such as LiPF6 (lithium hexafluorophosphate) dissolved in a non-aqueous binary organic solvent mixture such as ethylene carbonate-dimethyl carbonate is used. Separator: A porous separator physically separates the anode and cathode to prevent direct contact while allowing the passage of lithium ions. It must be chemically stable and non-conductive to electrons. E.g. polyethylene or polypropylene membrane. Working: The cyclic process of charging and discharging occurs as below. Charging: When the battery is connected to an external power source (e.g., a charger), a voltage is applied, and lithium ions are forced to migrate from the cathode to the anode through the electrolyte. Electrons released from the cathode flow through an external circuit to the anode, creating an electric current. Lithium ions are stored in the anode's structure. Discharging: When the battery is used, the stored lithium ions dissociate from the anode and move back to the cathode through the electrolyte. Electrons are released from the anode and travel through an external circuit to the cathode, generating electrical power. The flow of lithium ions completes the circuit, and the battery powers the connected device. Nanotechnology in lithium-ion battery 1. Nanomaterials in electrodes: a) Electrodes are constructed using nanoscale materials to increase lithium-ion storage capacity and improve electrochemical performance. Carbon nanofiber electrodes show four times the storage capacity of current lithium-ion batteries. Silicon-coated carbon nanotubes for anodes show an increase in the capacity of Li-ion batteries by up to 10 times. 71 Department of Physics, SJEC  b) The available power from a battery can be increased and the time required to recharge a battery can be decreased by coating the surface of an electrode with nanoparticles. c) Nanostructured lithium titanate electrodes have significantly improved the charge/discharge capability at subfreezing temperatures, increasing the upper-temperature limit at which the Lithium- ion battery remains safe from thermal runaway. 2. Separators: a) The shelf-life of a battery can be increased by using nanomaterials to separate liquids in the battery from the solid electrodes when there is no draw on the battery. b) Nanoporous separators improve transport, reduce internal resistance, and enhance overall battery efficiency. 3. Improved electrolytes: Solid-state electrolytes with nanoscale structures offer enhanced ionic conductivity, safety, and stability of lithium-ion batteries compared to liquid electrolytes. 4. Battery size: Nanotechnology enables miniaturization of Li-ion batteries for use in smaller devices like pacemakers, to be implanted in the eye to power the artificial retina, etc. 5. Battery safety: The possibility of batteries catching fire can be reduced by providing less flammable nanomaterial-coated electrode material. 6. Energy density: Nanocomposite-based cathodes have increased the energy density of Li-ion batteries. Requirements of anodic and cathodic materials In lithium-ion batteries, anodic and cathodic materials play vital roles in determining the performance, safety, and efficiency of the battery. Requirements of anodic materials: High lithium-ion storage capacity: Nanostructured anode materials like silicon nanowires or tin oxide nanoparticles can store more lithium ions, increasing capacity. High electrical conductivity: Anode materials often suffer from reduced conductivity which can be improved by the use of nanoscale conductive additives or coatings. Stability: Nanoscale anode materials may experience significant volume changes during charge-discharge cycles due to insertion and extraction of lithium ions, so they must be structurally stable. Low voltage: Low operating voltage to maximize energy density. Safety: Resistance to dendrite formation to avoid short circuits and thermal runaway. Requirements of cathodic materials: High energy density: Nanostructured cathode materials like lithium iron phosphate nanoparticles can enhance energy density. High capability: Nanoscale cathodes enable faster lithium-ion diffusion, making them suitable for high- power applications. 72 Department of Physics, SJEC  Chemical stability: Cathode materials must remain stable during repeated charge-discharge cycles to ensure the battery's longevity. High voltage: High operating voltage to improve energy density. Thermal stability: Stability at high temperatures to ensure safety. Cycle life: Minimal capacity fades over repeated charge/discharge cycles. Environmental safety: Use of non-toxic, sustainable materials. Lithium-ion battery classification based on ion storage mechanisms 1. Intercalation batteries: Intercalation batteries are a class of rechargeable batteries known for their ability to store and release energy through the process of intercalation. Intercalation involves the reversible insertion and extraction of ions into and from the crystal structure of specific materials used in the battery's anode and cathode without significant structural changes. This mechanism allows for efficient energy storage and retrieval, making intercalation batteries widely used in various applications. These batteries have high stability, long cycle life, and moderate energy density. They are used in smartphones, laptops, power tools, medical devices, and electric vehicles. Anode: These batteries use anodes where lithium ions can be reversibly inserted and extracted without significant structural changes. Common anode materials include graphite and various nanostructured carbon materials. Cathode: The cathode also contains intercalation compounds, such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), where lithium ions can intercalate during charging and de- intercalate during discharging. 2. Conversion batteries: In conversion batteries, Lithium ions react with the host material, causing a conversion reaction to form new phases during lithiation/delithiation. These batteries have high specific capacity but lower efficiency due to larger volume changes during cycling. They are used in flexible and wearable electronics. Anode: Conversion-type anodes involve materials like silicon or tin, which undergo significant structural changes during lithium-ion insertion and extraction. Nanotechnology helps to mitigate the structural degradation associated with these materials. Cathode: Conversion-type cathodes often involve metal oxides or sulfides that can undergo reversible conversion reactions to store lithium ions. 3. Alloy batteries: 73 Department of Physics, SJEC  In alloy batteries, Lithium ions form alloys with metals, leading to volumetric expansion during lithiation. They have high capacities and energy storage potential, but mechanical degradation occurs due to volume changes. Anodes: o Silicon (Si): Extremely high capacity but prone to cracking during cycling. It is used with nanostructuring to mitigate stress. o Tin (Sn): Moderate capacity and better mechanical stability than Si. o Germanium (Ge): High electronic conductivity but expensive. 4. Double-layer capacitors (Supercapacitors): These are distinct from traditional lithium-ion batteries and store energy through electrostatic adsorption of ions at the electrode-electrolyte interface. Nanomaterials with high surface area, such as carbon nanotubes or graphene, are commonly used to enhance capacitance and offer fast charge/ discharge rates. These have high power density and faster kinetics due to surface storage. Electrodes: o Metal oxides (e.g., RuO₂): Fast redox reactions and high conductivity, suitable for hybrid supercapacitors. o Conductive polymers (e.g., Polyaniline): Flexible and lightweight materials for wearable technologies. Limitations of graphite anodes in Lithium-ion batteries Limited capacity: Graphite has a limited lithium-ion storage capacity, which can hinder the overall energy density of the battery. Slow charging: The intercalation process of lithium ions into graphite can be relatively slow, leading to longer charging times. Expansion and contraction: Graphite anodes expand and contract during charge and discharge cycles, which can lead to mechanical stresses and reduced battery lifespan. Potential for dendrite growth: Graphite anodes can promote the growth of lithium metal dendrites during charging, leading to short circuits. Lower operating voltage: Graphite anodes typically have a lower operating voltage compared to some newer materials, affecting the overall performance of the battery. Advances in anodic materials in Lithium-ion batteries Recent advancements in anodic materials have improved the performance of Li-ion batteries 74 Department of Physics, SJEC 1. Nanocomposite anodes: Embedding nano-sized silicon particles in a conductive matrix can mitigate volume expansion and improve cycling stability in anodes. 2. Nanostructured graphite: Engineering graphite with nanoscale structures like graphene sheets or carbon nanotubes can increase the surface area and conductivity of anodes enabling fast charge/discharge rates. 3. Hybrid anodes: Hybrid structures combining silicon, carbon, and metal oxides are used in anodes for improved performance. Silicon's high capacity along with graphene's stability, reduces volume expansion and enhances overall performance. 4. Nano-coating on graphite: Applying protective nanoscale coatings on graphite can improve stability, prevent electrolyte decomposition, and extend lifespan. 5. Metal oxide anodes: Using metal oxides in anodes can increase the capacities and better thermal stability in anodes compared to graphite. (E.g., SnO2, TiO2) Advances in cathodic materials in Lithium-ion batteries Recent advancements in cathodic materials have improved the performance of Li-ion batteries 1. High-energy cathodes: Materials like lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LiFePO4) offer higher energy density and improved cycle life. 2. Enhanced safety: New cathode materials are designed with improved thermal stability, reducing the risk of thermal runaway. 3. Fast charging: Some cathode materials enable faster charging and discharging rates, addressing the need for rapid-charging batteries. 4. Increased capacity: Advancements have led to cathodes with higher lithium-ion storage capacity, increasing the overall energy storage of Li-ion batteries. 5. Cost-effective and environmentally friendly materials. Advances in Separators Advancements in separator technology have improved battery safety and performance: 1. Nanoporous separators: Nanostructured separators with small pores enhance ion transport, reduce internal resistance, and improve overall battery efficiency by higher charge-discharge rates. 2. Improved thermal tolerance: Ceramic-coated separators are designed with nanoscale materials to enhance thermal tolerance, reducing the risk of thermal runaway and improving mechanical strength. 3. Enhanced safety: Nanotechnology is used to develop separators with improved safety features, preventing short circuits and thermal issues. Polymer Electrolyte Membranes are used to enhance safety by reducing flammability and leakage 4. Ionic conductivity: Nanoporous membranes are used which offer high ionic conductivity while preventing dendrite penetration. 5. Self-healing separators: Incorporate nanomaterials that can repair damage, prolonging battery life. 75 Department of Physics, SJEC Advantages of lithium-ion battery They have a higher energy density than other rechargeable batteries. They produce a high voltage of around 4V per cell. Low self-discharge rate. Long cycle life. Batter safety and low maintenance to ensure their performance. Disadvantages lithium-ion battery They are expensive They are not available in standard cell types. Applications of lithium-ion battery Used in portable devices like mobile phones, laptops, tablets, and digital cameras. Used in tools such as cordless drills, sanders, saws, and various garden equipment. Used in cardiac pacemakers and implantable devices. Used in telecommunication equipment, instruments, electric vehicles, backup power systems, portable radios, and TVs. 76 Department of Physics, SJEC Fuel Cells Fuel cells are electrochemical devices that convert the chemical energy of a fuel and an oxidant into electrical energy through a controlled chemical reaction. In a fuel cell, the energy of combustion of a fuel such as hydrogen, methane, carbon monoxide, methanol, etc. is directly converted to electrical energy. They offer an efficient and clean alternative to traditional combustion-based power generation methods. The fuel cell like any other electrochemical cell consists of two electrodes and an electrolyte. The reactants are not stored within the cell; they are supplied from outside to the anode and cathode separately. The fuel supplied to the anode undergoes oxidation and oxygen supplied to the cathode undergoes reduction continuously. Electrons produced at the anode during oxidation move through the external circuit to the cathode to reduce the oxygen. A notable feature of fuel cells is that they produce electrical energy as long as they are supplied with the reactants. Construction of fuel cells Components: 1. Electrodes: 1. Anode: It is the negative electrode, typically made of low-cost porous carbon electrodes incorporating platinum as a catalyst. 2. Cathode: It is the positive electrode, typically made of low-cost porous carbon electrodes incorporating platinum or silver as a catalyst. 2. Electrolyte: 1. The electrolyte is a critical component that separates the anode and cathode. It allows the transport of ions between the two electrodes while preventing the mixing of fuel and oxidant gases. 2. Types of electrolytes include proton-exchange membranes (PEM), solid oxide, alkaline, and molten carbonate, each with unique advantages and applications. 3. Fuel: Hydrogen gas 4. Oxidant: Oxygen 5. Bipolar Plates: Bipolar plates are typically made of conductive materials like graphite, carbon composites, or metals. They serve to distribute reactants (fuel and oxidant) and collect the generated electricity. 77 Department of Physics, SJEC  They provide electrical conduction between cells and also give mechanical strength to the stack. Working of fuel cells Fuel cells operate based on a simple electrochemical process. 1. Fuel supply: Hydrogen, natural gas, methanol, or other hydrocarbons are supplied continuously to the anode compartment of the fuel cell. 2. Oxidant supply: Oxygen from the air is supplied to the cathode compartment. 3. Electrochemical reaction: At the anode, a platinum catalyst causes the fuel (e.g., hydrogen) to oxidize into positively charged hydrogen ions (H+ ions) and electrons (e-). The electrons produced at the anode cannot pass through the electrolyte, so they flow through an external electrical circuit, creating an electric current that can power devices. 4. Ion transport: The positively charged hydrogen ions (H+) migrate through the electrolyte to the cathode. 5. Oxygen reduction: At the cathode, oxygen molecules from the air combine with the electrons and hydrogen ions to form water (H2O) which flows out of the cell. 6. Generation of electricity: The electrochemical reactions at the anode and cathode produce electricity, which can be used to power electrical devices or stored in batteries. The cell gives a voltage of 1.15V as long as the supply of hydrogen and oxygen is maintained to the electrodes. The water formed in the reaction should be removed at the same rate at which it is formed. Otherwise, the electrolyte gets diluted, and the cell stops operating. Therefore, the electrolyte is kept warm so that water evaporates as fast as it is formed. 7. Emission-free energy: One of the significant advantages of fuel cells is that they produce electricity without the emission of greenhouse gases or other harmful pollutants (depending on the type of fuel used). Nanotechnology in hydrogen storage 1. Nanomaterials for hydrogen storage: Metal hydrides: Nanoscale metal hydrides offer increased hydrogen storage capacity. By reducing particle sizes, the surface area exposed for hydrogen absorption and desorption is maximized. Carbon nanotubes (CNTs): CNTs can serve as effective hydrogen storage materials due to their high surface area and tunable pore structures. Functionalized CNTs can improve hydrogen adsorption. Activated carbon and graphene: Porous nanostructures are designed for hydrogen trapping through weak van der Waals forces. 78 Department of Physics, SJEC  Nanoporous materials: Various nanoporous materials, such as metal-organic frameworks (MOFs) and zeolites are being engineered to enhance hydrogen adsorption by optimizing pore size and structure at low temperatures and moderate pressures. 2. Enhanced kinetics: Nanoscale materials enable faster hydrogen absorption and desorption kinetics, reducing the time required for refueling hydrogen-powered vehicles. 3. Improved safety: Nanomaterials can enhance safety by moderating the release of hydrogen, reducing the risk of explosions or leaks. 4. Lightweight: Nanomaterials lead to the design of low-cost, lightweight hydrogen storage systems. 5. Nanotechnology challenges: Challenges include scalability, cost, and long-term stability of nanomaterials for hydrogen storage. Research is ongoing to address these issues. Nanotechnology in Proton Exchange Membranes (PEMs) 1. Nanomaterials in PEMs: Nanocomposite membranes: Incorporating nanomaterials like graphene oxide or carbon nanotubes into PEMs improves proton conductivity, mechanical strength, and water management. Nanofibers and nanowires: Electrospun nanofiber membranes offer high surface area and improved proton transport properties. They also improve the mechanical strength and flexibility of the cell. 2. Durability and longevity: Nanotechnology can enhance the durability of PEMs, making them more resistant to chemical degradation and mechanical stress. 3. Water management: Nanomaterials can help control water content within PEMs, which is crucial for maintaining proton conductivity and preventing flooding or drying out. 4. Catalytic performance enhancement: Incorporation of platinum-based nanoparticles in membrane layers improves catalyst utilization. It reduces catalyst loading in membranes, lowering costs while maintaining efficiency. 5. Efficiency improvement: Nanostructured catalyst layers in PEMFCs can enhance electrochemical reactions, leading to higher efficiency and power density. 6. Challenges: Challenges include the integration of nanomaterials into PEM manufacturing processes, stability over time, and cost-effectiveness. Comparison of fuel cell and battery Fuel cell Battery 79 Department of Physics, SJEC  Reactants are fed from outside the cell (and are Reactants are an integral part of the battery. not an integral part of the cell). Chemical energy is not stored in the fuel cell. Chemical energy is stored in the battery. Fuel cells operate as long as the reactants are Battery operates until the reactants stored in it supplied to the electrodes from outside. are completely used up. There is no need to charge a fuel cell. A secondary battery has to be recharged once it is almost used up. Fuel cells have a very high efficiency. Efficiency of a battery is low Absence of harmful waste products, thus eco- Presence of harmful waste products friendly ************** 80 Department of Physics, SJEC