Module 4.pptx

Engineering Chemistry (BCHY101L) Module 4 Energy Devices  Contents…. (6 ▪ Electrochemical andhours ) electrolytic cells ▪ Electrode materials with examples Semi-conductors ▪ Chemistry of Li ion secondary batteries ▪ Supercapacitors ▪ Fuel ce...

Engineering Chemistry (BCHY101L) Module 4 Energy Devices  Contents…. (6 ▪ Electrochemical andhours ) electrolytic cells ▪ Electrode materials with examples Semi-conductors ▪ Chemistry of Li ion secondary batteries ▪ Supercapacitors ▪ Fuel cells: H2-O2 and solid oxide fuel cell (SOFC) ▪ Solar cells: Photovoltaic cells (silicon based), Photoelectrochemical cells, Dye- sensitized cells.  Electrochemic ▪ A device that is used al toCell generate electricity from a spontaneous redox reaction or, conversely, that uses electricity to drive a non-spontaneous redox reaction. ▪ An electrochemical cell typically consists of Two electronic conductors (also called electrodes >> anode and cathode) An ionic conductor (called an electrolyte) the electron conductor used to link the electrodes is often a metal wire, such as copper wiring ▪ The electrochemical cells are broadly classified into two ΔG types: 0reaction to electrical  Galvanic or voltaic ▪ A galvanic cell uses cell the energy released during a spontaneous redox reaction (ΔG < 0) to generate electricity. ▪ This type of electrochemical cell is also called a voltaic cell after its inventor, the Italian physicist Alessandro Volta. ▪ Anode is written on the left-hand side >> oxidation Electrode on Electrode on occurs Metal the (or left solid phase)| Electrolyte the right ▪ Cathode is written on the right-hand side(whole Electrolyte formula or >> reduction (whole formula or ion) ion) | Metal (or solid phase) occurs Zn|Zn(NO3)2 (1M) Cu(NO3)2 (1M)|Cu Oxidation: M1 → M1n+ + n e- Reduction: M2n+ + n e- → M2 Overall representation of Galvanic cell  Cell Representation  Cell Representation Convention Regarding the Sing of EMF: Cell is not feasible if EMF = -Ve Cell is feasible if EMF = # Electrons flow from anode to cathode +Ve current flows from cathode to while anode. The potential of cathode is more than the anode. Cell will be feasible To Calculate Standard EMF of Cell (Eo Cell): Calculate Standard EMF of Cell (Eo Cell) of following cell if standard reduction potential of Zn and Ag are -0.76 V and + 0.80 V: Solution : Example 1: Example 2: Nernst Equation: Numerical Problems: Numerical Problems: To Calculate Eq. Const. K: Problem Construction of No.1 cell Write the half cell reaction, Construct the cell, and explain the the net reaction and cell EMF of the following cell: Cd|Cd2+ (0.01 M)║Cu2+ (0.5 M)|Cu Cd The standard reduction potentials are ̶ 0.40 V and 0.34 V respectively. ▪ The half reactions: At anode: Cd → Cd2+ + 2 e- Standard CdSO4 reduction potential = ̶ 0.40 V At cathode: Cu2+ + 2 e- → Cu Standard reduction potential = 0.34 V ▪ Net reaction: Cd + Cu2+ → Cu + Cd2+ ▪ Cell EMF: The Electrolysis of Molten NaCl ▪ Here Na+ ions gain electrons and are reduced to Na at the cathode. ▪ As Na+ ions near the cathode are depleted, additional Na+ ions migrate in from molten salt. ▪ Similarly, there is net movement of Cl- ions to the anode where they are oxidized to Cl2. ▪ Positive terminal of the battery is connected to the anode. ▪ The negative terminal is connected to the cathode which forces electrons to move from the anode to the cathode. Steel ▪ Electrolysis is performed at ~ 801 °C. Graphit rod ▪ Here inert electrodes are used. e Anode: 2 Cl ̶ (l) → Cl2 (g) + 2 e- Overall: 2 NaCl (l) → 2 Na + (l) + Cl2 (g) Electrolytic Decomposition of Water ▪ A pair of inert electrodes are dipped H2 into the solution and applying a Power O2 voltage between them results in the supply rapid evolution of bubbles of H 2 and O2. ▪ Due to very poor electrical conductivity of pure water, a small amount of an ionic solute (such as H2SO4 or Na2SO4) is added to increase its electrical conductivity. ▪ The electrolytes are chosen such a way that the ions that are harder to oxidize or reduce than water: Anode: 2 H 2O → O2(g) + 4 H+ + 4 e- Platinum Cathode: 4 H (aq) + + 4 e- → 2 electrodes Electroplati ng ▪ Uses electrolysis to deposit a thin layer of one metal on another metal to improve beauty or resistance to corrosion. ▪ Electroplating was first discovered by Luigi Brugnatelli in 1805 through using the electrode position process for the electroplating of gold. ▪ Both ferrous and non-ferrous metals are plated with Ni, Cr, Cu, Zn, Pb, Al, Ag, Au, Sn etc. ▪ The base metal to be plated is made cathode of an electrolytic cell. ▪ The anode is either made of the coating metal itself or an inert material of good electrical conductivity. ▪ A water-soluble salt of the plating metal is used as electrolyte. ▪ Often non-participating electrolytes, such as, Na2SO4, are added to the bath solution to increase the conductivity of the medium. ▪Copper plating: Anode reaction: Cu (s) → Cu2+ (aq.) + 2 e- Cathode reaction: Cu2+ (aq.) + 2 e- → Cu (s) ▪Nickel plating: Electrolyte: Aqueous CuSO2+4 Anode reaction: Ni (s) → Ni solution (aq.) + 2 e- Cathode reaction: Ni2+ (aq.) + 2 e- → Ni (s) ▪Gold plating: Aqueous Electrolyte: NiSO4 Anode reaction: Au (s) → Au + solution (aq.) + e- Cathode reaction: Au+ (aq.) + e- → Au (s) ▪ Silver plating 2: ▪Silver plating 1: Electrolyte: Aqueous K[Au(CN)+2] Anode reaction: Ag (s) → Ag+ (aq.) Anode reaction: Ag (s) → Ag + e solution (aq.) + e- Cathode reaction: Ag+ (aq.) + e → Cathode reaction: Ag+ (aq.) + Ag (s) Electrolyte: Aqueous K[Ag(CN)2] E.g. ELECTROPLATING of copper AIM (i) To increase the resistance to corrosion and chemical attack of the plated metal. (ii) To obtain a polished surface (iii) To improve hardness and wear resistance Uses : (i) It is often used in electronic industries for making printed circuit boards, edge connectors, semiconductor lead-out connection (ii) It is also used in the manufacture of jewelry, refrigerator, electric iron etc. 19 Electroplating of Cu PRINCIPLE Electroplating is the process in which the metal to be coated is deposited on the base metal (substrate) by passing a direct current in the presence of electrolytic solution containing the soluble salt of the metal to be coated. Required materials, Electrolyte : (3-5%) H2SO4 / (15-30%) CuSO4 Anode : Pure Cu metal or Graphite (inert) 20 Cathode : Metal part (Object to be Electroplating of Cu DC battery Cu (Anode) Steel object + ve Cu2+ (cathode) (- Cu 2+ ve) Cu2+ SO42- Cu deposited surface Proces s Ionization reaction of electrolyte: H2SO4 2H+ + SO42- CuSO4 Cu 2+ + SO 4 2- ; On passing current: Cu2+ + 2e- Cu (at cathode) Cu + SO42- CuSO4 + 2e- (at anode) 21 Factors affecting electroplating  Surface cleaning – for strong adherent  Concentration of electrolyte.  Conductivity and stability of electrolyte  Thickness of the deposit – for decorative purpose thin coating and for corrosion protection multiple coating  Current density (current per unit of the base metal) should be low for uniform controlled deposition  Additives: Ensure strong adherence and mirror smooth coating  pH of the electrolytic bath 22 Semiconduct ors ▪ A semiconductor is a substance, usually a solid chemical element or compound that can conduct electricity under some conditions, making it a good medium for the control of electrical current. ▪ It has almost filled valence band, empty conduction band and very narrow energy gap i.e., of the order of 1 eV. Energy gap of Silicon (Si) and Germanium (Ge) are 1.0 and 0.7 eV respectively. Consequently Si and Ge are semiconductors. E E E Conductio n band Electron Conduction distributi band Conduction on band Valance Valance Valance band band band Insulato Semiconduc Conduct r tor or Semiconduct Conduction ors band Conduction band: Empty First empty band above band Band gap the highest filled band Electron Outermost band energy Partially containing electrons Valence Valence band: filled band Outermost band containing electrons Nucleus Classification of solids: 1. Conductors C 2. Insulators u 3. 1. Semiconductors Conductors  Outermost (orbital) band is not completely filled  Essentially no band gap easy overlap lots of available energy states if field is applied e.g Metals and Alkali metals 24  2. Insulators 3. Semiconductors:  Valence band full or nearly full  Similar to insulators but narrow band gap  At electrical temperatures some electrons can be  Wide band gap with empty conduction promoted to the conduction band band  Essentially no available energy states to which electron energies can be increased Conduction band Empty Conduction band Almost Empty Eg Band gap Eg Wide band Valence gap band Almost Full T > 0K Valence band Full Some common band gaps: Element gap (ev) Ge 0.6 Si 1.1 GaAs 1.4 25 SiO2 9.0  Effect of temperature on conductivity of semiconductors: ▪ At 0 K electrons freeze at valence band and hence all semiconductors are insulators. ▪ Electrical conductivity of a semiconductor material increases with increasing temperature as resistivity decreases. ▪ At higher temperature transition from the valence band to the conduction band gets facilitated ⇒ higher conductivity or lower resistivity. Temperatu re  Types of semiconductors: Intrinsic Semiconductors: Intrinsic Semiconductors: It is a Pure state (conductive on its own) E.g. Covalently bonded, tetravalent Si lattice  Promotion of an electron to the conduction band leaves “hole” in the valence band (electron-hole pair)  When an electric field is applied the electron will migrate to positive (+) terminal  The hole will migrate to negative (– )terminal  i.e the electron next to the hole will be attracted to the (+) terminal , leaving a hole toward (-) terminal - + Ec Eg Ev Net propagation of hole 28  Extrinsic Semiconductor. ▪ Extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process called doping, which alters the electrical properties of the semiconductor and improves its conductivity. ▪ Semiconductors are available as either elements or compounds. ▪ Silicon and Germanium are the most common elemental semiconductors. ▪ Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, and ▪ Doping GaN. produces two process ▪ Introducing impurities (Doping) into the semiconductor groups of semiconductors: materials can control The negative theirconductor charge conductivity. ▪ (n-type) the positive charge conductor  n-type ▪ Semiconductor An n-type semiconductor is an intrinsic semiconductor doped with pentavalent impurity, such as, P, As, Sb, etc. ▪ If a small amount of phosphorus is added to a pure silicon crystal, one of the valence electrons of phosphorus becomes free to move around (free electron) as a surplus electron.  p-type ▪Semiconductor An p-type semiconductor is an intrinsic semiconductor doped with trivalent impurity, such as, B, Al, In, etc. ▪ If a small amount of boron is doped to a single crystal of silicon, valence electrons will be insufficient at one position to bond silicon and boron, resulting in holes that lack electrons. Si lattice with n-type dopant Ec Ed Ev  Localized energy levels just below conduction band which facilitates Si easy conduction Ev = Energy level of valence P (Donor bond 32 atom) Ec = Energy level of p-type Doping of Si with trivalent indium (3 valence electrons) Incomplete bonding with Si Nearby electron from Si can fill hole Majority carriers are holes in the valence band Minority carriers are electrons in the conduction band Lattice doped with acceptor atoms localized energy levels just above valence band 33 Si lattice with p-type dopant Ec Ea Ev localized energy levels just above valence band which makes the conduction easy Si Ev = Energy level of valence B (Acceptor bond 34 atom) Ec = Energy level of  Czochralski crystal pulling ▪ Ge technique or Si obtained for by zone Si refining method is polycrystalline, i.e., there is no regularity in their crystal structure and contain crystals of different sizes. ▪ This method involves growing the crystal on a single crystal seed; thereby the atoms reproduce identical atomic arrangement as that of the seed crystal. Silicon crystal with a diameter of 300 mm and a weight exceeding 250 kg  Doping ▪techniques Epitaxy: Involves in unified crystal growth or deposition of a thin crystal on another substrate. Si or Ge wafer (kept in graphite boat) is placed in a long cylindrical quartz tube reactor, which is then heated (by RF induction coil). Then gases containing compounds of Ge or Si mixed with calculated/appropriate quantities of dopant over the wafer results. For getting Si epitaxial film, SiCl4, H2 and N2 mixture is used. For n-type doping ⇒ above mixture is used with phosphine (PH 3) For p-type doping ⇒ Diborane (B2H6) is employed. ▪Diffusion technique: Conversion of a region of semiconductor material by solid or gaseous diffusion of impurity atom into the crystal lattice of the Batte ry ▪ Battery is a device that consists of one or more electrochemical cells connected in series or parallel or both and converts the chemical energy into electrical energy. ▪ The cell consists of three major components: The anode: Reducing electrode which gives up electrons to the external circuit The cathode: Oxidizing electrode which accepts electrons from the external circuit The electrolyte: It is the ionic conductor ▪ Types of Cells/Batteries: Primary battery (Primary cells): The cell reaction is not reversible. When all the reactants have been converted to product, no more electricity is produced and the battery is dead. Example: Lechlanche Cell (Dry Cell), Alkaline Cell and Lithium batteries. Secondary battery (secondary cells): The cell reactions can be Lithium-Ion (Li ion) Batteries ▪ Lithium-ion battery is a secondary battery. ▪ It does not contain metallic lithium as anode. ▪ As the name suggests, the movement of lithium ions are responsible for charging & discharging. ▪ Lithium ion battery technology was first proposed in the 1970s by M Whittingham who used titanium sulphide for the cathode and lithium metal for the anode. ▪ The Nobel Prize in Chemistry 2019 is awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino.  Why ▪ Currently, most lithium? portable electronic devices, including cell phones and laptop computers, are powered by rechargeable lithium-ion (Li-ion) batteries, because: ▪ Lithium is a very light element. ▪ Li-ion batteries achieve a high specific energy density which is the amount of energy stored per unit mass. ▪ Because Li+ has a very large negative standard reduction potential, Li-ion batteries produce a higher voltage per cell than other batteries. ▪ A Li-ion battery produces a maximum voltage of 3.7 V per cell, nearly three times higher than the 1.3 V per cell that nickel–cadmium and nickel–metal hydride batteries generate. ▪ As a result, a Li-ion battery can deliver more power than other batteries of comparable size, which leads to a higher Evolution of secondary batteries History of battery specific energy Future 300 (Wh/kg) Energy 250 Li-polymer 200 Specific Energy 150 Li-ion 100 Ni-Cd Lead 50 Acid Ni-MH 0 1850 1900 1950 2000 Year 41  Construction of Lithium-Ion (Li ion) ▪ Cathode: This Batteries is the positive electrode and it is typically layers of lithium-metal oxide Ch (LiCoO2, LiNiO2, LiMn2O4, ng argi LiNiMnCoO2) and lithium metal Li+ polyanionic materials (LiFePO4, LiMnPO4, LiFeSO4F, etc.). ▪ Anode: The negative electrode is made from graphite, usually with composition Li0.5C6. i rg c ha ▪ Electrolyte: Mixture of organic Di s ng carbonates such as ethylene Li+ carbonate, diethyl carbonate. ▪ Separator: Prevents touching two electrodes. This absorbs the electrolyte, and enables the passage of ions, but LiCoO2 Graphit  Charging and Discharging Reactions e- e- e- e- Chargi - Dis- + - + Anod ng Cathod Charging e e Co Co O O Li+ Li+ LiCoO LiCoO Graphi Graphi Li ion te 2 permeable te 2 Li ion permeable membrane membrane Electroly Electroly te te  Charging Reaction and Charging Reaction: Discharging Discharging Reaction: ▪ When the cell is being ▪Li+ ions move out of the charged, cobalt ions are anode and migrate through oxidized and release the electrolyte where they electrons. enter the spaces between ▪ Simultaneously Li+ ions the cobalt oxide layers. migrate out of LiCoO2 and ▪ Simultaneously electrons into the graphite. flow through the external ▪ Electrons flow from the circuit. positive electrode to the ▪ Electrons reduce cobalt at negative Anode: electrode. LiCoO 2 → Li1-nCoO2 + Anode: LinC ions at the 6 → C6 + positive n e- + n electrode ▪ The electrons n Li+ + and n e- Li+ ions to regenerateLi LiCoO + 2. combine C at+ the Cathode: negative n e- + n Li+ → Cathode: Li1-nCoO2 + n Li+ + n 6 electrode. Li C e- → LiCoO2 n 6  Lithium ion battery NAME variants CONSTITUEN ABBREVIATI MAJOR APPLICATIONS TS ON CHARACTERIST ICS Lithium Cobalt LiCoO2 LCO High capacity Cell phones, laptops, cameras Lithium LiMn2O4 LMO Lower capacity Power tools, Manganese medical, hobbyist Oxide Lithium Iron LiFePO4 LFP Lower capacity Power tools, Phosphate medical, hobbyist Lithium Nickel LiNiMnCoO2 NMC Lower capacity Power tools, Manganese medical, hobbyist Cobalt Oxide Lithium Nickel LiNiCoAlO2 NCA Electric vehicles Cobalt Aluminium and grid storage Oxide Lithium polymer (Poly-Carbon monofluoride) batteries have an output of 2.8 V and moderately high energy density.  Lithium-ion battery applications ▪ Portable power packs: Li-ion batteries are lightweight and more compact than other battery types, which makes them convenient to carry around within cell phones, laptops and other portable personal electronic devices. ▪ Uninterruptible Power Supplies (UPSs): Li-ion batteries provide emergency back-up power during power loss or fluctuation events to guarantee consistent power supply. ▪ Electric vehicles: As Li-ion batteries can store large amounts of energy and can be recharged many times, they offer good charging capacity and long life spans which creates high demand for Li-ion battery packs for electric, hybrid or plug-in hybrid electric vehicles. ▪ Marine vehicles: Li-ion batteries are emerging as an alternative to gasoline and lead-acid batteries in powering work or tug boats and leisure craft like speed boats and yachts. ▪ Personal mobility: Lithium-ion batteries are used in wheelchairs, bikes, scooters and other mobility aids for individuals with disability or mobility restrictions. ▪ Renewable energy storage: Li-ion batteries are also used for storing energy  Advantages & Disadvantages of Lithium ▪ Advantages: Ion Battery High energy density: High energy density is one of the biggest advantages of lithium ion battery technology. This higher power density offered by lithium ion batteries is a great advantage for their use in electronic gadgets and electric vehicles. Low self-discharge: Lithium ion cells is that their rate of self-discharge is much lower than that of other rechargeable cells such as Ni-Cad and NiMH forms. Low maintenance: Lithium ion batteries do not require active maintenance. High cell voltage: The voltage produced by each lithium ion cell is about 3.6 volts. This ensure less number of cells in many battery applications. Variety of types available: There are several types of lithium ion cell available. This ensures the right technology can be used for the particular application needed. No requirement for priming: Lithium ion batteries are supplied operational and ready to go. Load characteristics: These provide a reasonably constant 3.6 volts per cell ▪ Disadvantages: Protection required: Lithium ion cells and batteries are not as robust as some other rechargeable technologies. They require protection from being over charged and discharged too far. Ageing: Lithium ion batteries suffer from ageing. Often batteries will only be able to withstand 500-1000 charge discharge cycles before their capacity falls. High Cost: A major lithium ion battery disadvantage is their cost. Typically they are around 40% more costly to manufacture than Nickel cadmium cells. Chances of explosion: Bad design or manufacturing defects: In that case, there wasn’t enough space for the electrodes and separator in the battery. When the battery expanded a little as it charged, the electrodes bent and caused a short circuit. Overcharging: When overcharged, lithium cobalt oxide releases oxygen which can react with flammable electrolyte leading to overheating. Electrolyte breakdown: On overheating, Dimethyl carbonate decompose  Supercapacitor or ▪ A supercapacitor is aUltracapacitors type of capacitor that can store a large amount of energy, typically 10 to 100 times more energy per unit mass or volume compared to electrolytic capacitors. ▪ These can deliver and accept charge more quickly than batteries. Ultrathin supercapacitors for wearable devices Batterie Compariso s n ▪ Batteries and capacitors do a similar job—storing electricity—but Capacit in completely different ways. ▪ Batteries ▪ Aors capacitor is a device used to store have two electrical terminals (electrodes) separated electrical charge and electrical by a chemical substance called an energy. electrolyte. ▪ Capacitors use static electricity ▪ When power is on, chemical (electrostatics) rather than chemical reactions happen involving both substances to store energy. the electrodes and the electrolyte. These reactions convert the ▪ Inside a capacitor, there are two chemicals inside the battery into conducting metal plates with an other substances, releasing insulating material called a dielectric electrical energy as they go. in between them - it's a dielectric ▪ Once the chemicals have all been sandwich. depleted, the reactions stop and ▪ Positive and negative electrical the battery is flat. charges build up on the plates and ▪ In a rechargeable battery, such as the separation between them, which a lithium-ion power pack used in a laptop computer or MP3 player, the prevents them coming into contact, is Applications of Supercapacitors and Market  Supercapac ▪itors A supercapacitor differs from an ordinary capacitor in two important ways: Its plates effectively have a much bigger area and the distance between them is much smaller, because the separator between them works in a different way to a conventional dielectric. Although the words "supercapacitor” and "ultra capacitor” are often used interchangeably, there is a difference: they are usually built from different materials and structured in slightly different ways, so they store different amounts of energy. ▪ Like an ordinary capacitor, a supercapacitor has two plates that are separated. ▪ The plates are made from metal coated with a porous substance such as powdery, activated charcoal, which effectively gives them a bigger area for storing much more charge Construction of (1) Electrodes: The electrodes of the Supercapacitors supercapacitor is made of Activated Carbon material. The electrodes have Porous nature which helps to store more charge carriers. if the electrodes are able to store more charge carrier then the capacitance will be increased. There are two electrodes one is the positive electrode and another is the negative electrode. - + (2) Current Collectors: The current collectors are used to connect the electrodes and the terminals of the capacitor. The current collectors are generally made up with foil metals. Mostly aluminum is used. There are two current ng wi collectors in the supercapacitor one for the positive llo electrode and another is the negative electrode. fo e s (3) Separator: The separator is used to provide th or ct as insulation or separate the electrodes to prevent the lle n rh ol to t co s tio de short-circuit. The separator mainly made up to El ep r ro lu ci n cu ect with Kapton material. The separator is very thin So yt r pa ec a re l e 1. rt s rc a tr a Tw o E like paper. The separator provides insulation r pa pe Tw o , between the electrodes but it allows to the flowing Su S of charge carrier through it. A 2..  Working of Supercapacitors Chargi ▪ When the plates are charged up, an ng opposite charge forms on either side of the separator, creating what's called an electric double-layer. ▪ This is why supercapacitors are often referred to as double-layer capacitors, also called electric double-layer +ve capacitors or EDLCs). -ve electrode electrode ▪ The capacitance of a capacitor increases as the area of the plates increases and as the distance Dis- between the plates decreases. (C α charging Area/distance) ▪ When we apply a voltage across the electrodes of the supercapacitor then it starts charging. The electrodes start to attract the ions of opposite polarity. That means the positive electrode attracts the negative Ions or charges and the negative electrode attracts the  Characteristics of Supercapacitors ▪ Fast charging speed and it can reach more than 95% of its rated capacity within minutes ▪ Cycle life is long and the number of charge and discharge cycles can reach 10,000 to 500,000 times, without "memory effect“. ▪ The high current discharge capacity is super strong, the energy conversion efficiency is high, the process loss is small, and the high current energy cycle efficiency is ≥90%; ▪ High power density, up to 300W/KG ~ 5000W/KG, equivalent to 5~10 times of battery; ▪ The raw material composition, production, use, storage and dismantling process of the product are not polluted, and it is an ideal green environmental protection power source; ▪ The charging and discharging circuit is simple, no charging circuit like rechargeable battery is needed, and the safety factor is high, and the maintenance is long-term maintenance-free; ▪ Good ultra-low temperature characteristics, temperature range -40°C - +70°C; ▪ Easy to detect, the remaining power can be read directly; ▪ The capacity range is usually 0.1F – 1000F.  Comparison of supercapacitors with batteries and ordinary capacitors ▪ Unlike battery, supercapacitors can store and release energy almost instantly. ▪ That's because a supercapacitor works by building up static electric charges on solids, while a battery relies on charges being produced slowly through chemical reactions. ▪ Batteries have a higher energy density (they store more energy per unit mass) but supercapacitors have a higher power density (they can release energy more quickly). ▪ Supercapacitors are suitable for quick storing and releasing large amounts of energy. ▪ Although supercapacitors work at relatively low voltages (maybe 2–3 volts), they can be connected in series (like batteries) to produce bigger voltages for use in more powerful equipment. ▪ Since supercapacitors work electrostatically, rather than through reversible chemical reactions, they can theoretically be charged and discharged any number of times. ▪ They have little or no internal resistance, which means they store and release energy without using much energy—and work at very close to 100 percent efficiency.  Comparison of Supercapacitor, Ordinary Capacitor Battery Parameter Super Capacitor Ordinary and Battery Capacitor Energy storage Watt-second Watt-second energy Watt-hour energy energy Power supply Fast discharge, Fast discharge, Maintain a linear or linear or constant voltage exponential voltage exponential voltage for a long time decay decay Charging/ milliseconds to Picosec. to milli sec. 1 to 10 hours discharging time seconds Dimensions Small Small to Large Large Weight 1g to 2g 1g to 10kg 1g to ＞ 10kg Energy density 1 to 5Wh/kg 0.01 to 0.05Wh/kg 8 to 600Wh/kg Power density High, ＞ 4000W/kg High, ＞ 5000W/kg Low, 100- 3000W/kg Operating 2.3V to 2.75V 6V to 800V 1.2V to 4.2V  Application of Supercapacitors ▪ Supercapacitors have been widely used as the electrical equivalents of flywheels in machines—"energy reservoirs" that smooth out power supplies to electrical and electronic equipment. ▪ Supercapacitors can also be connected to batteries to regulate the power they supply. ▪ In wind turbines, where very large supercapacitors help to smooth out the intermittent power supplied by the wind. In electric and hybrid vehicles, supercapacitors are increasingly being used as temporary energy stores for regenerative braking (where the energy a vehicle would normally waste when it comes to a stop is briefly stored and then reused when it starts moving again). Fuel Cells ▪ A fuel cell is a device that converts chemical potential energy (energy stored in molecular bonds) into electrical energy ▪ Electricity is generated without combustion by combining hydrogen and oxygen to produce water and heat ▪ They offer higher electrical efficiency (≥ 40%) compared to conventional power generation systems. Types of Fuel Cells ▪ There are eight main types of fuel cells, based mainly on the type of electrolyte: PEMFCs, proton exchange membrane or polymer electrolyte membrane fuel cells AFCs, alkaline fuel cells PAFCs, phosphoric acid fuel cells MCFCs, molten carbonate fuel cells SOFCs, solid oxide fuel cells DMFCs, direct methanol fuel cells DAFCs, direct ammonia fuel cells DCFCs, direct carbon fuel cells ▪ Apart from DAFCs, DMFCs, and DCFCs, other types of fuel  Hydrogen – oxygen fuel cells (HOFC) ▪ This cell is a common type of fuel cell. Similar to a galvanic cell, fuel cell also have two half cells. ▪ Both half cells have porous graphite electrode with a catalyst (platinum, silver or a metal oxide). ▪ The electrodes are placed in the aqueous solution of NaOH or KOH (alkaline fuel cells-AFC) or H2SO4 (acidic fuel cell) which acts as an electrolyte. ▪ Hydrogen and oxygen are supplied at anode and cathode respectively at about 50 atmospheric pressure, the gases diffuse at respective electrodes. ▪ The overall chemical reaction in a hydrogen fuel electrochemical cell involves the oxidation of hydrogen by  Principle of Operation ▪ A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) and oxygen to create electricity by an electrochemical process. ▪ Hydrogen and oxygen (air) are supplied to anode and cathode, respectively. ▪ When hydrogen is led to the anode, the hydrogen molecules are split into proton and an electron. ▪ The protons migrate through the electrolyte to the cathode, where they react with oxygen to form water. ▪ At the same time, the electrons are forced to travel around the electrolyte to the cathode side, because they cannot pass through the electrolyte. This movement of electrons thus creates an electrical current. Alkaline Fuel Cells (or) H2- O2 Fuel Cells H2 O2 An electrochemical cell in which the energy of a Unreacted Unreacted reaction between a fuel H2 + H2O - + Oxygen (such as liquid hydrogen) Vapour e- e- and an oxidant (such as liquid oxygen) is converted OH- directly and continuously into electrical energy. H 2O K+OH- Cathode: Porous Anode : Porous carbon with Pt carbon with Pt catalyst catalyst Proc ▪ess Hydrogen fuel is processed at the anode where electrons are separated from protons on the surface of a platinum- based catalyst. ▪ The protons pass through the membrane to the cathode side of the cell while the electrons travel in an external circuit, generating the electrical output of the cell. ▪ On the cathode side, another Pt electrode combines the protons and electrons with oxygen to produce water. At anode: H2 (g) → 2 H+ + 2 e- At cathode: O2 (g) + 4 H+ + 4 e- → 2 H 2O Net reaction: O2 (g) + 2 H2 (g) → 2 H2O (I) E ˚= 1.23 V ▪ Advantages The energy conversion is very high (75-82%). Fuel cell minimizes expensive transmission lines and transmission losses. It has high reliability in electricity generation and the by-products are environmentally acceptable. Maintenance cost is low for these fuels and they save fossil fuels. Noise and thermal pollution are very low. ▪ Disadvantages: The major disadvantage of the fuel cell is the high cost and the problems of durability and storage of large amount of hydrogen. The accurate life time is also not known and It cannot store electricity. Electrodes are expensive ad short lived. Storage and handling of H2 gas is dangerous because it is ▪ Applications: inflammable. The most important application of a fuel cell is its use in space vehicles, submarine or military vehicles. The product H2O is valuable source of fresh water for the astronauts. Fuel cell batteries for automotive will be a great boon for the future. Polymer Electrolyte Membrane Fuel Cell (PEMFC): General features: Fuel : Hydrogen Electrolyte: Ion exchange polymer membrane (Nafion) Nafion is a perfluorinated cation exchange polymer membrane. It is a solid state electrolyte. Electrodes: Metal based or made of porous carbon Catalyst : Platinum Charge carrier: H+ ions Operating temperature: 80 °C Fuel cell efficiency: 60 % Size & Cost: Small & Low Better performance: If gases are humidified Application: Mainly for transportation like Buses & Cars Polymer Electrolyte Membrane Fuel Cell (PEMFC):  Solid Oxide Fuel Cell (SOFC) ▪ SOFC is a high-temperature FC that utilizes solid ceramic inorganic oxide as an electrolyte; e.g., zirconium oxide stabilized with yttrium oxide, instead of a liquid or membrane, also known as Yttria-stabilized Zirconia (YSZ). ▪ SOFC is also referred to as ceramic FC. ▪ Both hydrogen and carbon monoxide are used as fuels. ▪ Solid oxide fuel cells work at very high temperatures, the highest of all the fuel cell types at around 800 °C to 1,000 °C. ▪ Efficiency: over 60% when converting fuel to electricity ▪ This cell relatively resistant to small quantities of sulphur Structure of SOFC ▪ Anode or fuel electrode: Nickel mixed with YSZ (Yttria stabilized Zirconia) or called Nickel-YSZ cermet (a cermet is a mixture of ceramic and metal). It is a porous ceramic layer to allow the fuel to flow towards electrolyte. ▪ Cathode or air electrode: The cathode is usually a mixed ion-conducting and electronically conducting ceramic material. It is a thin porous ceramic layer coated over the solid electrolyte where oxygen reduction takes place. One example being, strontium doped lanthanum manganite (LSM). ▪ Electrolyte: Oxide ion (O2-) conducting ceramic. The most popular electrolyte material is a bilayer composite Solid oxide fuel cells (SOFC) Anode, cathode and electrolyte all made up of ceramic substances and these cells are generally operated at high temperature. Ceramics are employed as functional elements. Both electrodes are separated by Solid Impermeable Electrolyte which conduct oxygen ions from cathode to anode that reacts chemically with fuel. Anode: Porous material, made up of oxides of Ni-Zr or Ni-Y stebilized by zirconium oxide. Cathode: Thin porous layer made up of lanthanum manganate (LaMnO3) at which reduction of oxygen takes place. Electrolyte: YO2 and ZrO2 Construction of SOFC ▪ At anode (oxidation): H2 (g) + CO (g) + 2 O2- → H2O (g) + CO2 (g) + 4e- ▪ At cathode (reduction): O2 (g) + 4e- → 2 O2- ▪ Net reaction: H2 (g) + CO (g) + O2 (g) → H2O (g) + CO2 (g) ▪ Advantages of SOFC: SOFCs have a number of advantages due to their solid materials and high operating temperature. Since all the components are solid, as a result, there is no need for electrolyte loss maintenance and electrode corrosion is eliminated. Also because of high-temperature operation, the SOFC has a better ability to tolerate the presence of impurities as a result of life increasing. High efficiencies: Due to high-quality waste heat for cogeneration applications and low activation losses, the efficiency for electricity production is great. Low emissions. Releasing negligible pollution. It is the ▪ Disadvantages: High operating temperature (500 to 1,000 °C) which results in longer start up times and mechanical/chemical compatibility issues. The cost and complex fabrication are also significant problems that need to be solved. Applications: SOFCs are being considered for a wide range of applications, such as working as power systems for trains, ships and vehicles; supplying electrical power for residential or industrial utility. Stationary power generation By product gases are channeled to turbines to generate more electricity: cogeneration of heat and power and Differences between Primary, Secondary and Fuel cells  Solar Energy Potential ▪ Theoretical: 1.2x10 TW solar energy potential 5 (1.76 x105 TW striking Earth; 0.30 Global mean albedo (The amount of energy reflected back to space) Energy in 1 hr of sunlight ↔ 14 TW for a year ▪ Practical: ≈ 600 TW solar energy potential (50 TW - 1500 TW depending on land fraction etc.; WEA 2000) Onshore electricity generation potential of ≈ 60 TW (10% conversion efficiency) Photosynthesis: 90 TW Types of solar energy conversion cells  Photovoltaic Cells  Photoelectrochemical cells  Dye-sensitized solar cells  Photovoltaic Why Cells ▪ A solar cell is a device that converts Silicon? Silicon is considered as the most suitable the energy of sunlight directly into material for solar energy conversion electricity by the photovoltaic because: effect. ▪ Second most abundant element (~ 28% ▪ The photovoltaic effect involves by mass) after oxygen creation of a voltage (or a corresponding electric current) in a ▪ Highly pure silicon can be readily material upon exposure to electro- synthesized from sand or quartz by magnetic radiation. heating them at high temperature in furnace ▪ Though the photovoltaic effect is directly related to the photoelectric SiO2 + C → Si + CO2 effect, the two processes are different. ▪ There are several different types of ▪ Silicon is an excellent semiconductor PV cells which all use with optimum band gap of 1.23 eV at semiconductors to interact with 300 K incoming photons from the Sun in ▪ Cost effectiveness order to generate an electric current. ▪ Silicon can be easily doped with Introduction to Photovoltaic Consistent annual growth in solar power markets coupled with a global shortage in crystalline silicon needed for traditional solar power has propelled the market for thin-film photovoltaics (PV), which is expected to grow from $220 million in 2006 to over $3 billion in 2013 (Ref:NanoMarkets, 2006). However, for cadmium-telluride (CdTe) thin-film PV, the market projections are tempered by concern over global environmental policies restricting electronic products containing cadmium, a known human carcinogen (IARC, 1993). The global solar energy market was valued at $52.5 billion in 2018 and is projected to reach $223.3 billion by 2026, growing at a CAGR of 20.5% from 2019 to 2026. Therefore, alternate materials are being searched……. The photovoltaic effect was first recognized in 1839 by French physicist Alexandre-Edmond Becquerel. However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 77 1946 (U.S. Patent 2,402,662) Solar Spectrum Photons do not have sufficient energy to Photons have free e- from Si sufficient energy to free e- from Si  200 to 2000 nm  About 46% of the spectral energy is distributed in the visible region  About 49% in near IR,  About 3% in UV region and  Rest in far IR regions 78 Applications of Solar Cells It is a Renewable energy It Can be powered for remote locations No pollution and environmentally friendly etc., R15-1200- Rollable Solar Charger $274.00 79 Photovoltaic effect (cell) It is a physical phenomenon in which light energy is converted to electrical energy Photo = light Materials for photovoltaic cells- Voltaic = Produce Voltage 1. Encapsulate of c s 2. Contact Grid e i 3. Antireflective i pl lta Coating in c ov o 4. N-type Silicon Pr hot 5. P-type Silicon p 6. Back Contact Top electrode e n-type semi conducting - layer - p-type semi conducting + layer Bottom electrode 80 Working 3. The photons (yellow dot) carry their energy down through the cell. 4. The photons give up their energy to electrons (green dot) in the lower, p-type layer. 5. The electrons use this energy to jump across the barrier into the 1. A solar cell is a upper, n-type layer and sandwich of n-type escape out into the silicon (blue) and p- circuit. type silicon (red). 6. Flowing around the 2. When sunlight shines on circuit, the electrons the cell, photons (light make the lamp light up. particles) bombard the 81 upper surface. Efficiency of a solar cell A solar cell's energy conversion efficiency (η, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of Pm, divided by the input light irradiance under "standard" test conditions (E, in W/m²) and the surface area of the solar cell (Ac in m²). Efficiency (%) : Pm Pm= Power (W)   E= irradiance or electromagnetic E x Ac radiation (W/m2) Ac= The surface area of the solar cell (m²) 82 Materials for photovoltaic cells Oxygen 46.1% Table 1. Natural abundance of elements in Silicon 28.2% earth crust Monocrystalline Si, 38% Aluminiu 8.2% m Iron 5.6% Calcium 4.2% Sodium 2.4% Cd Magnesiu 2.3% S m CuGaS Potassiu 2.1% m e2 ZnO CuInSe2 Titanium 0.57% Hydrogen 0.14% Materials for solar cell - 2006 Copper 0.005% Silicon is extracted from naturally occurring SiO2 or from its halides. It exists in two allotropic forms Total 99.8% 1. Amorphous – A brown powder 2. Crystalline – grey crystals 3. Single crystal silicon 83 Photovoltaic Cells using silicon: Materials such as silicon, germanium exhibit a property of semi-conduction and are called semiconductors. These semiconductors have electrical conductivity lower than metals and higher than insulators. Silicon is an ideal material for PV as it is a cheap material for fabrication of photovoltaic cells. (p-type) When pure silicon is doped with an acceptor element such as boron, it becomes a positively charged (p-type) semiconductor. (n-type) On the other hand, when it is doped with a donor element such as phosphorus, it becomes a negatively charged (n-type) semiconductor. Silicon has a valency of four and each silicon atom is bonded to four other silicon atoms as shown in Figure. When it is doped with boron that has a valency of three, silicon is replaced 84  Band gap vs The efficiency the materials Efficiency increases as the band gap 40 increases but beyond c-Si CdT certain value of the band e gap the efficiency decreasesThe– Shockley–Queisser why? CZT limit gives the maximum Efficiency S possible efficiency of a single- junction solar cell under un- 30 Cd concentrated sunlight, as a function of the semiconductor (%) CuInS CuGaS S band gap. If the band gap is too e2 e2 high, most daylight photons ZnO cannot be absorbed; if it is too low, then most photons have much more energy than necessary to excite electrons 20 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 across the band gap, and the rest is wasted. The Egap semiconductors commonly used (eV) in commercial solar cells have band gaps near the peak of this curve, for example silicon Efficiency (%) : Pm (1.1eV) or CdTe (1.5eV). Pm= Power10(W) E= irradiance or electromagnetic radiation  (W/m2) 0 A = The surface area of the solar cell (m²) E x Ac 85 Unique Features of solar cell 7. Require little 1. Have no moving parts (in the maintenance if properly classical mechanical manufactured and installed sense) to wear out 8. Can be made from silicon, 2. Contain no fluids or gases the second most abundant (except in hybrid systems) that element in the earth's crust can leak out, as do some solar- 9. Have a relatively high thermal systems conversion efficiency giving 3. Consume no fuel to operate the highest overall 4. Have a rapid response, conversion efficiency from achieving full output instantly sunlight 5. Can operate at moderate to electricity temperatures 10. Have a high power-to- 6. Permitting a wide range of weight ratio making them solar-electric applications such suitable for roof application as 11. Are amenable to on-site - Small scale for remote installation applications and residential use 12. Produce no pollution while - Intermediate scale for producing electricity (but waste business and neighborhood products from the 86  Disadvantages of Solar Photovoltaic Cell ▪ Some toxic chemicals, like cadmium and arsenic, are used in the PV production process. These environmental impacts are minor and can be easily controlled through recycling and proper disposal. ▪ The conversion of light energy into heat energy is one of the limitations. ▪ Solar energy is somewhat more expensive to produce than conventional sources of energy due in part to the cost of manufacturing PV devices ▪ Solar power is a variable energy source, with energy production dependent on the sun. ▪ Solar panels efficiency levels are relatively low (between Photo Electro Chemical cell (PECs) It is a cell containing a semiconductor and a metal electrode immersed in an electrolyte solution and when light falls on the surface of the semiconductor, the light energy is converted in to electrical energy The main difference between a PEC cell and the photovoltaic cell Light In PEC photo- generated whereas in the later, carriers transfer the energy there is no such from light to chemical process. species in the solution Electricity Fuels O H 2 2 CO 2 e e Sugar sc M sc M H2O H2O O 2 Semiconductor/Liquid Photosynthesis Junctions Solar cell Photovoltaic 88  Photo electrochemical Cells Types 1. Liquid Junction Solar Cell (LJSC) The cell, which is used for the conversion of solar energy into electrical energy, with the help of photoactive semiconductore-and the electrolyte. 2. Photoelectrosynthesis (PES) cells Solar energy is converted into chemical energy in the form of fuels 89 Photoelectrochemical Cell-LJSC Material requirements Band gap must be at least 1.8-2.0 eV but small enough to absorb most 2hν + H2O ---> H2(g) + ½ O2 (g) sunlight Band edges must straddle 2H2O + 2e- = 2OH- +H2 Redox potentials e- EC H2O/H2 Fast charge transfer Ehν > Stable in aqueous Eg solution 1.8-2.0 eV The simplest LJSC 1.23 eV consists of two electrodes e- Eg Counter (one of them a SC and the electrode other a metal) dipped in an electrolyte containing a redox couple. EF Both the electrodes must H2O/O2 be inert, i.e., the EV h electrode material itself + H2O + 2h+ = 2H+ + should not take part in p-type the electrochemical h = + 1/2O2 semiconductor reactions. hole Pt One of the important wire requirements for the Light energy is Converted to operation of a LJSC is the presence of depletion Briefly working of Photo Electro Chemical cell (PECs) 1. A photoelectrochemical cell consists of a photoactive semiconductor (SC) working electrode (either n- or p-type) and counter electrode made of metal (M) (e.g. Pt). Both electrodes are immersed in the electrolyte containing suitable redox couples. 2. If the junction of the semiconductor-electrolyte is illuminated with a light having energy greater than the bandgap of the semiconductor, photogenerated electrons/holes are separated in the space charge region. 3. With the help of a connecting wire, photogenerated majority carriers are transported via a load to the counter electrode where these carriers electrochemically react with the redox electrolyte. 91 Working Contd..(For Reading, Explanation) 1. A typical type of the photocurrent-generated device has a semiconductor in contact with an electrolyte, and this is often referred as photoelectrochemical cells. 2. A photoelectrochemical cell consists of a photoactive semiconductor (SC) working electrode(either n- or p-type) and counter electrode made of metal (M) (e.g. Pt). Both electrodes are immersed in the electrolyte containing suitable redox couples. 3. In a metal-electrolyte junction, the potential drop occurs entirely on the solution side, whereas in a semiconductor- electrolyte junction, the potential drop occurs on the semiconductor site as well as the solution side. 4. The charge on the semiconductor side is distributed deep in the interior of the semiconductor, creating a space charge region. If the junction of the semiconductor-electrolyte is illuminated with a light having energy greater than the bandgap of the semiconductor, photogenerated electrons/holes are separated in the space charge region. 5. The photogenerated minority carriers (holes) arrive at the interface of the semiconductor-electrolyte. Photogenerated majority carriers (electrons) accumulate at the backside of 92 the semiconductor. hemical reactions - Working -LJSC 1. Absorption of light near the surface of the semiconductor creates electron-hole pairs. The non-equilibrium electrons in the conduction band are produced by illumination of light will have energy Ehv ≥Eg. 2. Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen (Oxidation) 2h+ + H2O ----> ½ O2(g) + 2H+ 3. Electrons (majority carriers) are conducted to a metal electrode (typically Pt) where they combine with H+ ions in the electrolyte solution to make H2 (Reduction) 2e- + 2H+ ----> H2(g) 4. Transport of H+ from the anode to the cathode through the electrolyte completes the electrochemical circuit. 93 The overall reaction : 2hν + H2O ----> H2(g) + ½ O2 (g) Dye sensitized Solar Cells- Gratzel cells Professor Michael Gratzel Swiss Federal Institute of Technology Lausanne Switzerland Michael Grätzel and Brian O’Regan invented “Dye-sensitized solar cells”, also called “Grätzel cells”, in 1991. Dr. Brian O’Regan Research Lecturer Imperial College London 94  Difference between PEC and Gratzel Cell Photo electrochemical cell 1. Semiconductor electrode 2. Counter electrode 3. Electrolyte solution Gratzel Cell 1. Dye adsorbed Semiconductor electrode 2. Counter electrode 3. Electrolyte solution 95 (Iodide) Why?/How? - Dye sensitization of a semiconductor for solar energy conversion  Chlorophyll is a best example for a organometallic (Mg containing organic compound) dye that uses solar energy for photosynthesis – reported by Moser 1887  Cue was taken from this and proved that dye adsorbed on semiconductor surface can enhance the photo-electrochemical cell’s performance for solar energy conversion  In a dye molecule, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied level (LUMO) is analogous to the conduction band - valence band gap in a semiconductor  Dye-sensitized solar cells (DSCs) are potential low-cost solar energy conversion devices  Efficiency of DSCs is 11.1% at present  The life time of DSCs exceeds 10 years based on the durability and stability towards light and chemicals organometallic porphyrin system 96 ▪ Transparent and Conductive Substrate Substrate for the deposition of the semiconductor and catalyst, acting also as current collectors Characteristics of a substrate: More than 80% of transparency Should have a high electrical conductivity. The fluorine-doped tin oxide (FTO, SnO 2: F) and indium-doped tin oxide (ITO, In2O3: Sn) are usually applied as a conductive substrate in DSSCs. These substrates consist of soda lime glass coated with the layers of ITO and FTO. The ITO films have a transmittance > 80% and 18 Ω /cm 2 of sheet resistance, FTO films show a lower transmittance of ~ 75% in the visible region and sheet resistance of 8.5 Ω /cm2 ▪ Working Electrode (WE) Working electrodes (WE) are prepared by depositing a thin layer of oxide semiconducting materials such as TiO 2, Nb2O5, ZnO, SnO2 (n- type), and NiO (p-type) on a transparent conducting glass plate made of FTO or ITO These oxides have a wide energy band gap of 3 – 3.2 eV Due to its non-toxicity, and easy availability, TiO 2 is mostly used as a semiconducting layer To enhance its activity the TiO2 semiconducting layers are immersed in a mixture of a photosensitive molecular sensitizer and a solvent Due to highly porous structure and the large surface area of the electrode, a high number of dye molecules get attached on the nanocrystalline TiO2 surface, and thus, light absorption at the semiconductor surface increases. ▪ Photosensitizer or Dye Dye are responsible for the maximum absorption of light. These should have the following photophysical and electrochemical properties: Dyes should be luminescent. Their absorption spectra should cover UV-vis and NIR regions. The HOMO should be located far from the surface of the conduction band of TiO2. LUMO should be placed as close to the surface of the TiO 2, and should be placed higher than the TiO2 conduction band potential. The periphery of the dye should be hydrophobic to enhance the long-term stability of cells. Co-absorbents like chenodeoxycholic acid (CDCA) or anchoring ▪ Electrolyte An electrolyte, such as I−/I3−, Br−/Br2−, SCN−/SCN2, and Co(II)/Co(III) has five main components, i.e., redox couple, solvent, additives, ionic liquids, and cations. The following properties should be present in an electrolyte: Redox couple should be able to regenerate the oxidized dye efficiently. Should have chemical, thermal, and electrochemical stability. Should be non-corrosive with DSSC components. ▪ Counter Should Electrode be able (CE) to permit fast diffusion of charge CEcarriers, enhance conductivity, and create effective in DSSCs are mostly prepared by using Pt, C, CoS, contact between the working and counter electrodes. Au/GNP, alloy CEs like FeSe, and CoNi0.25. Construction and Working of Gratzel cell At Anode Dye+ (adsorbed on TiO2) + 3/2 I-  Dye (adsorbed on TiO2) + 1/2 I3- SnO2 Coated Single Dye Counter Glass ( -ve TiO2 molecule - (+ve) electrode) Electrolyt Nanoparti Adsorbed e electrode, cle on TiO2 Pt e- C e - B e- e- Sun e - light Ef e- e- 3I qV (obtainable e- e- V - I) e - ERedo B I3- e- x e- e- e-  3 e-  e- e- 2 e- e- 1 0 At Cathode I3- + 2e-  101 -  Working Principle of Gratzel cell Step 1 Sunlight (photon of light) passes through the titanium dioxide layer and strikes electrons in the adsorbed dye molecules. Electrons gain this energy and excited to conduction band. Step 2 The excited free electrons move through the titanium dioxide nanoparticle from dye molecule and accumulate at the conducting glass electrode (-ve plate TiO2-dye plate). Step3 The free electrons from the negative electrode flows through the external circuit to the counter electrode which produces electric current which can be measured. Step4 On the surface of the counter electrode (+ve) iodide ion (I - reduced form) receive the electron and gets oxidized to I 3- ion after transferring the electrons to dye molecule which regains its electron from the iodide electrolyte. The dye is then regenerated for second cycle. Step5 The free electrons at the metal/graphite (+ve) plate then reduce the tri- iodide (I3-) molecules to I-. The dye molecules are 102 then ready for the next excitation. In the next cycle step1 to step 5 Dye Sensitized solar cell - working 1. Sunlight energy (photon of light) passes through the titanium dioxide layer and strikes electrons within the adsorbed dye molecules. Electrons gain this energy and become excited because they have the extra energy. 2. The excited electrons escape the dye molecules and become free electrons. These free electrons move through the titanium dioxide and accumulate at the -ve plate (dyed TiO 2 plate). 3. The free electrons then start to flow through the external circuit to produce an electric current. This electric current powers the light bulb. 4. To complete the circuit, the dye is regenerated. The dye regains its lost electrons from the iodide electrolyte. Iodide (I-) ions are oxidised (loss of 2 electrons) to tri-iodide (I3-). The free electrons on the graphite plate then reduce the tri-iodide molecules back to their iodide state. The dye molecules are then ready for the next excitation/oxid/red cycle. 103  Molecular engineering of dyes. cis - isomer 0.8 COOH HOO C COOH HOOC N N Trans - 0.6 N isomer Absorbance N NCS SCN Ru NCS Ru N NCS N N HOOC N 0.4 HOO C O OCH HOO C 0.2 Very small changes to a dye structure can have significant influence on the optical 0.0 properties. In the present case compare the absorption 400 500 600 700 800 300 spectra of two isomers of Energy [nm] dithiocyanato bis(4,4’- 104 dicarboxylic acid-2,2’-  Naturally Occurring Dyes Advantages and disadvantages of DSSC