Unit 4 Batteries PDF

ENGINEERING CHEMISTRY ELECTROCHEMISTRY Unit 4:Energy System and Battery Technology Battery is an electrochemical cell or often several electrochemical cells connected in series that can be used as a source of direct electric current at constant volt...

ENGINEERING CHEMISTRY ELECTROCHEMISTRY Unit 4:Energy System and Battery Technology Battery is an electrochemical cell or often several electrochemical cells connected in series that can be used as a source of direct electric current at constant voltages. A device which converts chemical energy to electrical energy is called battery cells, connected together electrically in series. Batteries are commercial electrochemical cells. ADVANTAGES OF BATTERIES: (1) Batteries act as a portable source of electrochemical energy. (2) The portability of electronic equipment in the form of handsets has been made possible by batteries. (3) A variety of electronic gadgets have been made more useful and popular with the introduction of rechargeable storage batteries having reliability, better shelf life and tolerance to service. (4) For all commercial applications, batteries are constructed for their service. For example batteries for automotives and aircrafts, stand by batteries etc. The following requirements should be possessed by the batteries. (1) High capacity, which is very small variation of voltage during discharge. (2) High energy efficiency, which is calculated as % of efficiency = energy released on discharge/energy required for charge x100 (3) High cycle life is required which is the number of charging and discharging cycles before failure. (4) Long shelf-life is required. (5) Tolerance to different service conditions such as variation in temperature, vibration shock etc. (6) Reliability is another important criteria. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 27 ENGINEERING CHEMISTRY ELECTROCHEMISTRY Differences between Primary, Secondary and Fuel cells: Primary cell Secondary cells Fuel Cell It acts as a simple It acts as a galvanic cell while It acts as a simple galvanic galvanic cell. discharging and electrolytic cell cell. while charging. Cell reaction is not Cell reaction can be reversed. Cell reaction is not reversible. reversible Cannot be recharged. Can be recharged Do not store energy Can be used as long as Can be used again and again by Energy can be withdrawn the materials are active in recharging the cell indefinitely as long as their composition. outside supply of fuel is maintained E.g: E.g: E.g: Leclanche or dry cell. 1. Lead storage cell 2. Nicol or H2-O2, CH3OH-O2 Zn/NH4Cl (20%), ZnCl2/ Nickel cadmium battery emf MnO2/C. emf =1.5V. =1.4 Applications: Applications: Applications: Electronic Space vehicles due to their Radios, torches, calculators, electronic flash light weight and the bi transistors, hearing aids. units & cordless electronic product H2O produced is a shavers etc. valuable source of fresh water for astronauts. PRIMARY CELLS: Dry Cell: (Leclanche Cell) It consists of cylindrical zinc container that acts as an anode. A graphite rod placed in the center (but not touching the base) acts as a cathode. The space between the anode cathode is packed with the paste of NH4Cl and ZnCl2, and the graphite rod is surround by powdered MnO2 and carbon. The cell is called dry cell because of the absence of any liquid phase, even the electrolyte consists of NH4Cl, ZnCl2and MnO2 to which starch is added to make a thick paste which prevents leakage. The graphite rod is fitted with a metal cap and the cylinder is sealed at the top with a pitch. Cell representation: Zn-/ NH4Cl//MnO2+ C/ C+ At Anode: (oxidation) Zn → Zn2+ + 2e- At cathode: (reduction) 2MnO2+ 2H2O +2e-→ Mn2O3 + 2OH- The net reaction is: Zn+2MnO2+ H2O → Zn2++ Mn2O3+2OH- DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 28 ENGINEERING CHEMISTRY ELECTROCHEMISTRY The resulting OH- ions react with NH4Cl to produce NH3 which is not liberated as gas but immediately combines with the Zn2+ and Cl- ions to from a complex salt [Zn(NH3)2]Cl2. 2NH4Cl + 2OH- → 2NH3 + 2Cl- +2H2O Zn2+ + 2NH3 + 2Cl-→ [Zn(NH3)2]Cl2 Net reaction: Zn(s) + 2NH4Cl + 2MnO2(s) → Mn2O3(s) + [Zn(NH3)2]Cl2(s) + 2H2O. Applications 1. These cells have voltage ranging from 1.25V to 1.50V. 2. Primary cells are used in torches, radios, transistors, hearing aids, pacemakers, watches. 3. Price is low. Disadvantages: 1. These cells do not have a long life, because the acidic NH4Cl corrodes the container even when the cell is not in use. 2. When current is rapidly drawn from the cell, voltage drop takes place due to the building up of production on the electrodes. Lithium cells: Lithium cells are primary cells in which lithium acts as anode and the cathode may differ. Lithium metal is used as anode because of its light weight, high standard oxidation potential (≥3V) and good conductivity. As the reactivity of lithium in aqueous solution is more, lithium cells use non-aqueous solvents as electrolyte. Lithium cells are classified into two categories: (a) Lithium cells with solid cathode: The electrolyte in this system is a solid electrolyte. The most widely used cell is lithium – manganese dioxide cell (3V). MnO2 should be heated to over 3000C to remove water before keeping it in the cathode, thereby increasing the efficiency of the cell. Anode: Lithium Metal, DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 29 ENGINEERING CHEMISTRY ELECTROCHEMISTRY Cathode: MnO2 as an active material. Electrolyte: LiBF4 salt in a solution of propylene carbonate and dimethoxy ethane. Reactions: At anode: Li⟶ Li + + e- At cathode: e- + MnO2 ⟶ MnO2- Net reaction: Li + MnO2⟶ LiMnO2 Applications: 1. The coin type cells are used in watches and calculators. 2. Cylindrical cells are used in fully automatic cameras. (b) Lithium cells with liquid cathode: Lithium–sulphur dioxide cell is an example of liquid cathode. The co-solvents used are acrylonitrile or propylene carbonate (or) mixture of the two with SO2 in 50% by volume. Cell reaction: 2Li + 2SO2⟶ Li2S2O4. Lithium thionyl chloride cell is another example of liquid cathode. It consists of high surface area carbon cathode, a non – woven glass separator. Thionyl chloride acts as an electrolyte and as a cathode. Cell reaction: At anode: Li ⟶ Li+ + e- At cathode: 4Li+ 4e-+ 2SOCl2⟶ 4LiCl + SO2 + S Net reaction: 4Li + 2SOCl2⟶ 4LiCl + SO2 + S In this cell no co-solvent is required as SOCl2 is a liquid with moderate vapor pressure. The discharging voltage is 3.3 -3.5V. USES: 1. They are used for military and space applications. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 30 ENGINEERING CHEMISTRY ELECTROCHEMISTRY 2. In medicinal devices such as neuro-stimulators, drugdelivery system, lithium batteries are widely used. 3. They are also used in electric circuit boards for supplying fixed voltage for memory protection and standby functions. Advantages: 1. The energy output of a lithium cell is 2-4 times better than that of conventional zinc anode batteries. 2. Lithium batteries can work over temperature range of 40-700C. 3. They have higher voltages of about 4V when compared to other primary cells with 1.5 V only. Secondary cell: E.g.: Lead – Acid cell: If a number of cells are connected in series, the arrangement is called a battery. The lead storage battery is one of the most common batteries that are used in the automobiles. A 12 V lead storage battery is generally used, which consists of six cells, each providing 2V. Each cell consists of a lead anode and a grid of lead packed with lead oxide as the cathode. These electrodes are arranged alternately, separated by a thin wooden piece and suspended in dil. H2SO4 (38%), which acts as an electrolyte. Hence, it is called lead acid battery. Anode: Pb Cathode: PbO2 Electrolyte: H2SO4 (20-22%) EMF = 2 V Lead storage cells: To increase the current output of each cell, the cathode and the anode plates are joined together, keeping them in alternate positions. The cells are connected parallel to each other. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 31 ENGINEERING CHEMISTRY ELECTROCHEMISTRY The cell is represented as Pb/PbSO4(s), H2SO4/PbSO4(s),Pb In the process of discharging, i.e., when the battery produces current, the reactions at the electrodes are as follows: Discharging reactions: At anode: Pb → Pb2+ + 2e- Pb2+ + SO42- → PbSO4 At cathode: PbO2(s) + 4H+ + 2e-→ Pb2++2H2O Pb2+ + SO42- → PbSO4 Therefore, the overall reaction is: Pb(s) +PbO2+4H2SO4(aq) → 2PbSO4(s) + 2H2O + Energy During discharging the battery, H2SO4 is consumed, and as a result, the density of H2SO4 falls. When it falls below 1.20 g/cm3, the battery needs recharging. In discharging, the cell acts as a voltaic cell where oxidation of lead occurs. Recharging: During recharging, the cell is operated like an electrolytic cell, i.e. electrical energy is supplied to it from an external source. The electrode reactions are the reverse of those that occur during discharge. PbSO4 + 2e- → Pb + SO42- (Reaction at cathode) PbSO4 + 2H2O → PbO2 + 2H2SO4 + 2e- (Reaction at anode) ------------------------------------------------------------------------- 2PbSO4 + 2H2O + Energy → Pb + PbO2 + 2H2SO4 During this process, lead is deposited at the cathode, PbO2 is formed at the anode and H2SO4 is regenerated in the cell. Advantages: Lead-acid batteries are used for supplying current to railways, mines, laboratories, hospitals, automobiles, power stations, telephone exchange, gas engine ignition, UPS. Other advantages are its recharge ability, portability, and relatively constant potential and low cost. Disadvantages: Use of conc. H2SO4 is dangerous. Use of lead battery is fragile. Nickel – Cadmium Cell: It is a rechargeable secondary cell. It consists of cadmium as the negative electrode (anode) and NiO2 as the positive electrode (cathode). Potassium hydroxide (KOH) is used as an electrolyte. The cell reaction during charging and discharging are as follows. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 32 ENGINEERING CHEMISTRY ELECTROCHEMISTRY Anode: Cd Cathode: NiO(OH) Electrolyte: KOH EMF = 1.4 V At anode: Cd + 2OH Cd(OH)2(s) + 2e- At cathode: NiO2 + 2H2O + 2e- 2 Ni(OH)2 + 2OH- Overall reaction: Cd + 2NiO(OH) + 2H2O Cd(OH)2 + 2Ni(OH)2 Uses: 1. The nickel-cadmium cell has small size and high rate of charge/discharge capacity, which makes it very useful. 2. It also has very low internal resistance and wide temperature range (up to 700C). 3. It produces a potential of about 1.4 V and has a longer life than lead storage cell. 4. These cells are used in electronic calculators, electronic flash units, electrical shavers, transistors, etc. 5. Ni–Cd cells are widely used in medical instrumentation and in emergency lighting, toys, etc. 6. It is also used in aircraft and space satellite power system. Advantages: 1. Ni-Cd batteries last longer, in terms of number of charge/discharge cycles, than other rechargeable batteries. 2. Ni-Cd batteries have much higher energy efficiency. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 33 ENGINEERING CHEMISTRY ELECTROCHEMISTRY FUEL CELLS Definition: A fuel cell is an electrochemical which converts chemical energy contained in readily available fuel oxidant system into electrical energy. Principle: The basic principle of fuel cell is as same as that of an electrochemical cell. The fuel cell operates like a galvanic cell. The only difference is that the fuel and the oxidant are stored outside the cell. Fuel and oxidant are supplied continuously and separately to the electrodes at which they undergo redox reaction. Fuel cells are capable of supplying current as long as reactants are replenished. Fuel + Oxidant → Oxidation products + Electric Energy Examples: 1. H2-O2 fuel cell 2. CH3OH-O2 fuel cell Hydrogen oxygen fuel cell: 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 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 two half-cell reactions are as follows; At anode: 2H2 (g) + 4OH- (aq) 4H2O (l) + 4e- At cathode: O2 (g) + 2H2O (l) + 4e- 4OH- (aq) The net reaction: 2H2 (g) + O2 (g) 2H2O (l) The EMF of this cell is measured to be 1.23V. A number of such fuel cell are stacked together in series to make a battery. DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 34 ENGINEERING CHEMISTRY ELECTROCHEMISTRY Advantages: 1. The energy conversion is very high (75-82%). 2. Fuel cell minimizes expensive transmission lines and transmission losses. 3. It has high reliability in electricity generation. 4. The byproducts are environmentally acceptable. 5. Maintenance cost is low for these fuels. 6. They save fossil fuels. 7. Noise and thermal pollution are very low. 8. They have low maintenance cost. 9. They have quick start system. Disadvantages: 1. The major disadvantage of the fuel cell is the high cost and the problems of durability and storage of large amount of hydrogen. 2. The accurate life time is also not known. APPLICATIONS: 1. The most important application of a fuel cell is its use in space vehicles, submarine or military vehicles. 2. The product H2O is valuable source of fresh water by the astronauts. 3. It is hoped that fuel cell technology will bring a revolution in the area of energy production. 4. Fuel cell batteries for automotive will be a great boom for the future. Limitations: 1. The life time of fuel cells is not accurately known 2. It cannot store electricity 3. Electrodes are expensive ad short lived. 4. Storage and handling of H2 gas is dangerous because it is inflammable. Methyl Alcohol- Oxygen (Alkaline Fuel Cell): In this fuel cell, CH3OH is used as a fuel and O2 as oxidant to generate electrical energy. The methyl alcohol–oxygen fuel cell has two electrodes. The anode consists of porous nickel electrode impregnated with Pt/Pd catalyst. Porous nickel electrode coated with silver catalyst constitutes a cathode of the cell. The electrolyte, KOH, is taken in between the two DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 35 ENGINEERING CHEMISTRY ELECTROCHEMISTRY electrodes. CH3OH and O2 are sent continuously into their respective electrodes as shown in Fig. and the electrical energy is produced with the continuous replenishment of the fuel, CH3OH at the anode. At anode: CH3OH + 6OH-→ CO2 + 5H2O + 6e- At cathode: 3/2 O2 + 3H2O + 6e- → 6OH- Overall reaction: CH3OH + 3/2 O2 → CO2 + 2H2O Advantages of methyl alcohol-oxygen fuel cell: 1. Methanol fuel cells are reasonably stable at all environmental conditions. 2. Easy to transport 3. Do not require complex steam reforming operation. 4. These fuel cells are targeted to portable applications. 5. Because of high hydrogen concentration in methanol. it is an excellent fuel. 6. Methanol poses less risk to aquatic plants, animals and human beings than gasoline 7. Because methanol possess lower inflammability limit than gasoline it poses less fire risk than gasoline. 8. There is zero emission by the cells hence the fuel cells are eco friendly. Application of alcohol-oxygen fuel cell: 1. The major application of methyl alcohol oxygen fuel cells is a fuel for fuel cell motor vehicles like NECAR-5 in Japan, USA etc. ---------------------------------------------------------------------------------------------------------------- Numerical Problems: 1. A solution of salt (1.0 N) surrounding platinum electrodes 2.1cm apart and 4.2 cm2 in area was found to offer a resistance of 50 Ω. Calculate the equivalent conductivity of the solution. Solution: l = 2.1 cm, C = 1.0N a = 4.2cm2 R= 50 Specific conductance (K) = -1 -1 Equivalent conductance ( λ eq) = = =10Ω-1cm2equiv-1. 2. Specific conductance of a decinormal solution of KCl is 0.0112 ohm-1 cm-1. The resistance of a cell containing the solution was found to be 56 Ohms. What is the cell constant. Solution: K = 0.0112Ω-1cm-1 R = 56Ω DEPARTMENT OF HUMANITIES & SCIENCES ©MRCET (EAMCET CODE: MLRD) 36