Electrochemistry, rechargeable batteries and fuel cells

Nature of science:

Environmental problems—redox reactions can be used as a source of electricity but disposal of batteries has environmental consequences. (4.8)

• An electrochemical cell has internal resistance due to the finite time it takes for ions to diffuse. The maximum current of a cell is limited by its internal resistance.
• The voltage of a battery depends primarily on the nature of the materials used while the total work that can be obtained from it depends on their quantity.
• In a primary cell the electrochemical reaction is not reversible. Rechargeable cells involve redox reactions that can be reversed using electricity.
• A fuel cell can be used to convert chemical energy, contained in a fuel that is consumed, directly to electrical energy.
• Microbial fuel cells (MFCs) are a possible sustainable energy source using different carbohydrates or substrates present in waste waters as the fuel.
• The Nernst equation, E = E^° - (^RT/_nF ) In Q, can be used to calculate the potential of a half-cell in an electrochemical cell, under non-standard conditions.
• The electrodes in a concentration cell are the same but the concentration of the electrolyte solutions at the cathode and anode are different.

Applications and skills:

• Distinction between fuel cells and primary cells.
• Deduction of half equations for the electrode reactions in a fuel cell.
• Comparison between fuel cells and rechargeable batteries.
• Discussion of the advantages of different types of cells in terms of size, mass and voltage.
• Solution of problems using the Nernst equation.
• Calculation of the thermodynamic efficiency (ΔG/ΔH) of a fuel cell.
• Explanation of the workings of rechargeable and fuel cells including diagrams and relevant half-equations.

• Are battery recycling programmes equivalent in different areas of the globe?

Theory of knowledge:

• Does scientific language and vocabulary have primarily a descriptive or an interpretative function? Are the terms “electric current” and “internal resistance” accurate descriptions of reality or metaphors?

Utilization:

• Syllabus and cross-curricular links:
• Topic 9.1—redox reactions
• Topic 19.1—electrochemical cells
• Biology topic 6.5—muscle and nerve cells discussed in biology are concentration cells
• Physics topic 5.3—the relationship between electrical power, voltage, resistance and current

Aims:

• Aim 2: The conversion of chemical energy to electricity is important in a number of different technologies.
• Aim 6: The factors that affect the voltage of a cell and the lead–acid battery could be investigated experimentally.
• Aim 8: Consideration of the advantages and disadvantages of the different energy sources shows the economic and environmental implications of using science and technology. The environmental aspects of fuel cells, especially with regard to methanol, could be discussed.
• Aim 8: Disposal of primary batteries and the chemicals they use can introduce land and water pollution problems. Appreciation of the environmental impact of cadmium and lead pollution.
• Aim 8: Bacterial fuel cells use substrates found in waste water as the fuel and so can be used to clean up the environment.

• A battery should be considered as a portable electrochemical source made up of one or more voltaic (galvanic) cells connected in series.
• The Nernst equation is given in the data booklet in section 1.
• Hydrogen and methanol should be considered as fuels for fuel cells. The operation of the cells under acid and alkaline conditions should be considered. Students should be familiar with proton-exchange membrane (PEM) fuel cells.
• The Geobacter species of bacteria, for example, can be used in some cells to oxidize the ethanoate ions (CH₃COO-) under anaerobic conditions.3
• The lead–acid storage battery, the nickel–cadmium (NiCad) battery and the lithium–ion battery should be considered.
• Students should be familiar with the anode and cathode half-equations and uses of the different cells.