1 of 30 Boardworks Ltd 2010

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2 of 30 © Boardworks Ltd 2010

Electrochemical cells Redox reactions involve the transfer of electrons between two species. The flow of electrons is an electrical current. Redox reactions can therefore be used to generate electrical current. This is the basis of batteries and fuel cells. An electrochemical cell is a store of chemical energy in a closed system. All reactants and products are contained within the casing of the cell. 3 of 30 © Boardworks Ltd 2010

Rechargeable and non-rechargeable cells There are two types of cell: non-rechargeable and rechargeable. They are also known as primary and secondary cells. Primary cells can only Secondary cells can be be used once. recharged and reused. Smaller cells contain fewer reactants and produce less electrical energy. However, the reaction voltage doesn’t change with cell size. 4 of 30 © Boardworks Ltd 2010

How does a cell work? 5 of 30 © Boardworks Ltd 2010

Primary cells A primary cell can only be used once because it transfers stored chemical energy into electrical energy by a nonreversible chemical reaction. Primary cells are usually cheaper to buy. They are more reliable as they do not discharge much when they are not in use. This makes them more useful for applications such as smoke detectors. 6 of 30 Carbon–zinc dry cell Alkaline dry cell © Boardworks Ltd 2010

Secondary cells 7 of 30 © Boardworks Ltd 2010

Reactions in a zinc–carbon cell 8 of 30 © Boardworks Ltd 2010

Primary or secondary cells? 9 of 30 © Boardworks Ltd 2010

Batteries A battery is more than one cell connected together in a series arrangement. When a conductor is connected across the two terminals, a circuit is formed and electrons flow through it. By connecting more than one cell together, the electromotive force is increased. The battery voltage is the sum of the voltages of each of the cells. A battery is a closed system which contains the high energy reactants and the low energy products in a sealed unit. 10 of 30 © Boardworks Ltd 2010

Lead-acid battery 11 of 30 © Boardworks Ltd 2010

Calculate the e. m. f 12 of 30 © Boardworks Ltd 2010

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Fuel cells A fuel cell converts chemical energy into electrical energy in a similar fashion to a battery. However, a fuel cell is an open system – reactants flow in and products flow out, rather than being stored in the cell. Fuel cells are used in spacecraft and in hydrogen powered cars. 14 of 30 © Boardworks Ltd 2010

Hydrogen fuel cells have been used in spacecraft for many years. New developments are incorporating them into cars to replace the internal combustion engine. In a hydrogen fuel cell, hydrogen and oxygen gases react to form water. At the anode: 2 H 2(g) 4 H+(aq) + 4 e– At the cathode: 4 H+(aq) + O 2(g) + 4 e– 2 H 2 O(l) Overall equation: 2 H 2(g) + O 2(g) 15 of 30 2 H 2 O(l) © Boardworks Ltd 2010

How does a hydrogen fuel cell work? 16 of 30 © Boardworks Ltd 2010

Label the fuel cell 17 of 30 © Boardworks Ltd 2010

Production of hydrogen Hydrogen is a very reactive element and must be produced by a chemical reaction before it can be used in a fuel cell. There are currently two main methods of manufacturing hydrogen: l reaction of hydrocarbons with steam l electrolysis of acidified water. Although a hydrogen fuel cell is not directly polluting, processes used to produce hydrogen can be. Electricity for electrolysis is often produced by fossil fuel power stations. Carbon dioxide is produced from the reaction of hydrocarbons with steam. 18 of 30 © Boardworks Ltd 2010

Transporting and storing hydrogen 19 of 30 © Boardworks Ltd 2010

Hydrogen-rich fuels With the difficulty in transporting and storing hydrogen, some fuel cell vehicles use hydrogen-rich fuels which are converted to hydrogen by an onboard ‘reformer’. The reformer uses temperatures of 250– 300°C to favour reactions generating hydrogen gas. Hydrogen-rich fuels include methanol, natural gas and petrol. 20 of 30 © Boardworks Ltd 2010

Direct methanol fuel cells 21 of 30 © Boardworks Ltd 2010

Reactions in a direct methanol fuel cell 22 of 30 © Boardworks Ltd 2010

Ethanol fuel cell Ethanol is a less toxic and more energy-dense alcohol than methanol. Research into developing an ethanol fuel cell is ongoing. Currently, platinum-based catalysts are used to oxidize ethanol, but don’t achieve complete oxidation. Cheaper, more efficient catalysts capable of fully oxidizing ethanol are needed. Ethanol can be made from renewable sources, such as the fermentation of sugar. These energy resources are considered carbon neutral – the amount of carbon dioxide they release is equal to the amount absorbed during growth. 23 of 30 © Boardworks Ltd 2010

Fuel cell vs. conventional vehicles 24 of 30 © Boardworks Ltd 2010

Hydrogen economy 25 of 30 © Boardworks Ltd 2010

Modern breath alcohol testers Fuel cell technology has been applied to breath alcohol testers. Ethanol in a person’s breath is oxidized into carbon dioxide and water, producing a measurable electrical current ethanol from breath oxygen water platinum anode 26 of 30 acid–electrolyte membrane platinum cathode © Boardworks Ltd 2010