Chapter 27 – Cells and Batteries

Primary Cells Batteries such as dry cells, alkaline cells and button cells have one common feature; they are non-rechargeable. Cells that cannot be recharged are called primary cells. In primary cells, the products slowly migrate away from the electrodes and are consumed by side reactions occurring in the cells.

Zinc-Carbon Dry Cell This was the first small scale source of electrical energy. An electrolyte composed of a moist paste of zinc chloride and ammonium chloride and it plays the same role as the salt bridge. At the anode (-), oxidation of the zinc case produces the electrons. At the cathode (+), reduction of manganese dioxide.

Zinc-Carbon Dry Cell cont… A new cell produces about 1.5 volts, but this diminishes during use. To maintain a net forward reaction, the soluble reaction products must migrate away from the electrodes. During use, the build up of products at the electrodes slows and can stop the forward reaction. This is known as polarisation. If a cell is allowed to rest, some of the products migrate away from the electrodes. However, once the cell reaches equilibrium, the cell will be flat.

Alkaline Cells Have largely replaced Zinc-Carbon Cells. The chemical reaction within an alkaline cell is similar to the Zinc-Carbon cell but the construction is totally different. The alkaline cell is optimised for performance and longevity. At the anode, zinc powder around the central metal rod is oxidised and once the ion is formed it reacts immediately with the OH- ions in the electrolyte to form zinc hydroxide. At the cathode, manganese dioxide is reduced. The improvements in this cell give it about five times the life of the Zinc-Carbon cell.

Button Cells Are used in very small devices. There are two main types: silver-zinc cells and lithium cells. Lithium button cells produce about 3 volts during discharge and silver-zinc give an almost constant 1.6 volts.

Rechargeable Cells and Batteries Rechargeable cells are known as secondary cells or accumulators. To recharge a cell, the products of the reaction must be converted back into the original reactants. This is done by connecting the cell to a charger, a source of electrical energy, which has a potential difference greater than the potential difference of the cell. Electrical energy supplied is converted into chemical energy in the cell. In order for it to be possible to regenerate the reactants, the products formed in the cell during discharge must remain in contact with the electrodes in a convertible form.

Car Batteries Lead-acid batteries are the most widely used secondary cells. This is a car battery. Although they appear to be a single unit, they are actually six separate cell connected together in a series. Each cell contains three positive electrodes sandwiched between four negative electrodes.

Car Batteries cont… The electrodes are separated by a porous separator. The positive electrodes consist of a lead grid packed with lead oxide. Negative electrodes consist of a lead grid packed with powdered lead. A solution of sulfuric acid acts as the electrolyte. Each cell has a potential difference of just over 2 volts, a car battery has six of these so the total potential difference is about 12volts.

Car Batteries cont… At the anode: At the cathode: Pb(s) + SO42-(aq)  PbSO4(s) + 2e- At the cathode: PbO2(s) + SO42-(aq) + 4H+(aq) + 2e-  PbSO4(s)+ 2H2O(l) The overall equation: Pb(s) + PbO2(s) + 2SO42-(aq) + 4H+(aq)  2PbSO4(s) + 2H2O(l) The product of both electrode reactions forms a solid on the surface of the electrodes, enabling the battery to be recharged.

Car Batteries cont… To recharge the battery, the electrode reactions are reversed. The alternator is used to force electrons into the battery’s negative terminal and draw them out at the positive terminal.

Nickel-based Cells and Batteries The nickel based cells consist of a coiled anode, porous separator and cathode immersed in a concentrated KOH electrolyte. At the anode: For nickel-cadmium cells: Cd(s) + 2OH-(aq)  Cd(OH)2(s) + 2e- For nickel-metal hydride cells: MH(s) + OH-(aq)  M(s) + H2O(l) + e- At the cathode: NiOOH(s) + H2O(l) + e-  Ni(OH)2(s) + OH-(aq)

Nickel-based Cells and Batteries The electrode reactions are fully reversible. This enables the reactants to be regenerated when the cell is recharged. Most brands of nickel-cadmium cells are capable of 1000 discharge-recharge cycles. Nickel-metal hydride cells will do around 500 cycles.

Fuel Cells Cells can be constructed in which the reactants are supplied continuously allowing constant production of electrical energy. These devices are called fuel cells. Fuel cells transform chemical energy directly into electrical energy. At the anode: H2(g) + 2OH-(aq)  2H2O(l) + 2e- At the cathode: O2(g) + 2H2O(l) + 4e-  4OH-(aq) The overall reaction: 2H2(g) + O2(g)  2H2O(l)

Fuel Cells cont… Each cell produces about one volt. Higher voltages are obtained by connecting a number of fuel cells in series. The only by-products are water and heat. The has been development of an acid fuel cell – this cell uses concentrated phosphoric acid as the electrolyte and air as the source of oxygen. It uses hydrogen but does not require it to be pure.

Fuel Cells cont… The use of fuel cells in transportation improves fuel efficiency and reduces greenhouse gas and other emissions. Stationary fuel cell systems are used to generate electricity for domestic, commercial and industrial purposes. A methanol powered fuel cell has been developed for use with portable electronic equipment such as mobile phones.

Advantages of Fuel Cells Convert chemical energy directly to electrical energy, better efficiency. Water is a by-product, no greenhouse gases released. Will generate electricity for as long as fuel is supplied, in conventional batteries electricity production stops once the reaction reaches equilibrium. Use a variety of different fuels. Electricity can be generated on site.

Disadvantages of Fuel Cells Require a constant fuel supply. They are expensive. Some types of fuel cell use expensive electrolytes and catalysts. Fuel cells generate a direct current (DC), electrical appliances used in homes and industry rely on alternating current (AC). The effectiveness of some fuels cells are affected by impurities in the hydrogen fuel. Use of fuel cells in transport is limited by lack of facilities for hydrogen storage and distribution.