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Fuel cells

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  1. Fuel cells • Learning objectives: • Revise the work covered so far on module 2 of EEE. • Explain that a fuel cell uses the energy from the reaction of a fuel with oxygen to create voltage. • Explain the changes that take place at each electrode in a hydrogen-oxygen fuel cell. Starter Spend a few minutes looking through the work covered previously on module 2 of EEE (pages 182 – 190).

  2. Combine the following two half-equations to form a redox equation: • Cu  Cu2+ + 2e- • Ag+ + e-  Ag • Use oxidation states to construct a redox equation for the following reaction: • HBr is oxidised to Br2 by concentrated H2SO4, which is reduced to SO2 • Draw a labelled diagram of a simple electrochemical cell made from a copper and a zinc half-cell combined together.

  3. Draw a labelled diagram of a simple electrochemical cell made from a copper and a zinc half-cell combined together.

  4. 4. Which is the negative electrode? 5. Does oxidation or reduction happen at the negative electrode? 6. What is used as the standard electrode? 7. Draw a labelled diagram of a standard hydrogen electrode.

  5. 4. Which is the negative electrode? • 5. Does oxidation or reduction happen at the negative electrode? • 6. What is used as the standard electrode? • Draw a labelled diagram of a standard hydrogen electrode. • Why is platinum used as the electrode in this example?

  6. Calculate the standard cell potential for the Fe2+/Fe and Cr3+/Cr cell if: • Fe2+ (aq) + 2e Ý Fe (s) E = -0.44 V • Cr3+ (aq) + 3e-Ý Cr (s) E = -0.74 V • Determine the overall cell reaction. • What are the limitations of predicting whether a reaction will occur based on electrode potentials? • State an everyday use of electrochemical cells.

  7. Fuel cells • A fuel cell uses energy from the reaction of a fuel with oxygen to create a voltage. • They operate continuously (assuming fuel is continuously added) and do not need to be recharged. • They consist of the reactants flowing in, the products flowing out and an electrolyte which stays in the cell. Next lesson we’ll talk about how they’re used and the advantages and disadvantages of fuel cells.

  8. Hydrogen – oxygen fuel cell (alkaline) Write an equation showing the overall reaction. H2 + ½O2  H2O These fuel cells can contain either acidic or alkaline electrolytes. Your exam is more likely to focus on the alkaline one (OH- ions). Which reactant is reacting with the OH- ions? It will be the one undergoing oxidation as this is the only way to construct a balanced half-equation (think about the charges involved)? H2 + 2OH-  2H2O + 2e -Negative terminal The other reactant must be forming the OH- ions (as the electrolyte is effectively acting as a catalyst – it is not used up). ½O2 + H2O + 2e-  2OH-Positive terminal

  9. Hydrogen – oxygen fuel cell (alkaline) H2 + ½O2  H2O H2 + 2OH-  2H2O + 2e-Negative terminal ½O2 + H2O + 2e-  2OH-Positive terminal Check that the two half-equations add together to give the overall equation. If the standard electrode potentials are as follows, calculate Ecell 2H2O + 2e-  H2 + 2OH--0.83 V ½O2 + H2O + 2e-  2OH- +0.40 V Ecell = 0.4 + 0.83 = +1.23 V

  10. Hydrogen – oxygen fuel cell (acidic) H2 + ½O2  H2O Now lets try the same for an acidic electrolyte. Acid = H+ ions. Which of the reactants will react with H+ ions? ½O2 + 2H+ + 2e-  H2O Positive terminal Which of the reactants will form H+ ions? H2  2H+ + 2e-Negative terminal

  11. Hydrogen – oxygen fuel cell (acidic) H2 + ½O2  H2O ½O2 + 2H+ + 2e-  H2O Positive terminal H2  2H+ + 2e-Negative terminal Check that the two half-equations add together to give the overall equation. If the standard electrode potentials are as follows, calculate Ecell 2H+ + 2e-  H20.00 V ½O2 + 2H+ + 2e-  H2O +1.23 V Ecell = 1.23 – 0.00 = +1.23 V

  12. Hydrogen – oxygen fuel cell (acidic) porous titanium/ platinum anode to catalyse the reaction of O2 porous titanium/ platinum cathode to catalyse the ionisation of H2 e– e– e– e– e– e– e– e– Acidic membrane allows H+ ions to flow through it H H H+ O H H+ H O H+ H H H+ O O H H

  13. Methanol fuel cell (acidic) Write the equation for the combustion of methanol. CH3OH + 1½O2  CO2 + 2H2O Assuming we have an acidic electrolyte. Acid = H+ ions. Which of the reactants can form H+ ions? CH3OH + H2O  CO2 + 6H+ + 6e- Negative Which of the reactants will react with H+ ions? 1½O2 + 6H+ + 6e-  3H2O Positive Check that the two half-equations combine to give the overall equation.

  14. Ethanol fuel cell (acidic) Write the equation for the combustion of ethanol. C2H5OH + 3O2  2CO2 + 3H2O Assuming we have an acidic electrolyte. Acid = H+ ions. Which of the reactants can form H+ ions? C2H5OH + 3H2O  2CO2 + 12H+ + 12e- Negative Which of the reactants will react with H+ ions? 3O2 + 12H+ + 12e-  6H2O Positive Check that the two half-equations combine to give the overall equation.