DP Chemistry: Electrochemistry, rechargeable batteries & fuel cells
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Electrochemistry, rechargeable batteries & fuel cells

C.6 Electrochemistry, rechargeable batteries & fuel cells
(4 hours)

Pause for thought

The first crude fuel cells were actually invented as long ago as 1838 by William Grove, a Welshman and independently by Christian Schönbein from Germany.

It was the NASA space programme that really developed them to become a viable technology. The space shuttle contains three fuel cells, which operate as independent power sources. These use oxygen and hydrogen gas and an electrolyte of potassium hydroxide. Each one is capable of supplying 12 kW peak and 7 kW maximum continuous power supply which is ample for the average power consumption of the shuttle. As well as providing electrical power and heat, the product, water, is used by the astronauts for drinking.

Cut-away of the Toyota Mirai showing the fuel cell stacks beneath the driver and passenger seats and the yellow hydrogen fuel tanks in the rear.

Only now are fuel cells being seriously used to power cars. This year Toyota plans to build 700 cars running on hydrogen fuel cells. The car, the Toyota Mirai, has a range of 312 miles (502 km) from a full tank. The first cars went on sale in the US in August 2015 at a basic cost (before any government subsidies) of $57 500. The relatively high cost and the current lack of hydrogen fuel stations are two initial hurdles that will need to be overcome. Ultimately vehicles powered by fuel cells (FCVs) may well prove to be the best answer to providing transport for the masses that does not rely on fossil fuels and so significantly reduce the emission of polluting greenhouse gases.

Nature of science

Redox reactions can be used as a good source of electricity but the disposal of batteries can cause environmental problems.

Learning outcomes

After studying this topic students should be able to:

Understand:

  • Electrochemical cells have internal resistance because of the finite time it takes for ions to diffuse.
  • The internal resistance of a cell limits the current it can produce.
  • The voltage of a battery primarily depends on the type of materials used, while the total work output depends on their quantity.
  • In primary cells the electrochemical reaction is non-reversible. Rechargeable cells involve redox reactions that can be reversed using electricity.
  • Fuel cells convert the chemical energy that is contained in the consumed fuels directly into electrical energy.
  • Microbial fuel cells, MFCs, which use different carbohydrates or substrates present in waste waters as the fuel, are a possible sustainable energy source.
  • The potential of a half-cell under non-standard conditions can be calculated from the Nernst equation:
  • Concentration cells contain the same electrodes but the concentration of the electrolyte solutions at the cathode and anode are different.

Apply their knowledge to:

  • Distinguish between fuel cells and primary cells.
  • Deduce the half equations for the electrode reactions in a fuel cell.
  • Compare fuel cells with rechargeable batteries.
  • Discuss the advantages of different types of cells in terms of size, mass and voltage.
  • Solve problems using the Nernst equation.
  • Calculate the thermodynamic efficiency (ΔG/ΔH) of a fuel cell.
  • Explain the workings of rechargeable batteries and fuel cells (including diagrams and relevant half-equations).

Clarification notes

Consider a battery to be a portable electrochemical source comprising of one or more voltaic (galvanic) cells connected in series.

The Nernst equation is given in Section 1 of the data booklet.

Consider hydrogen and methanol as fuels for fuel cells and consider the operation of fuel cells under both acid and alkaline conditions. Familiarity with proton-exchange membrane, PEM, fuel cells is expected.

Use the Geobacter species of bacteria as an example that can be used in some cells to oxidize ethanoate ions, CH3COO−, under anaerobic conditions.

Consider the lead–acid storage battery, the nickel–cadmium, NiCd or NiCad, battery and the lithium–ion battery.

Familiarity with the half-equations at the anode and cathode and the uses of the different cells is expected.

International-mindedness

The ways in which spent batteries are disposed of and/or recycled vary from country to country.

Teaching tips

This sub-topic follows on neatly from Topics 9 & 19 from the core/AHL. Essentially it is the practical application of voltaic cells so does require some memory work as well as a deeper understanding. The key to the sub-topic is either knowing or being able to deduce the equations for the half-reactions taking place in the different types of rechargeable batteries and fuel cells.

I start by revising how voltaic cells work and how to calculate the electromotive force for the whole cell (EMF or E⦵total) from the standard electrode potentials of the two half-cells. Then go into the specific details for the lead-acid, Ni/Cd and Li-ion rechargeable batteries. Students should realise that when batteries recharge the reaction occurring is the reverse of when it discharges.

Fuel cells are expensive but have considerable potential. Start with the hydrogen fuel cell and stress that the same overall reaction occurs irrespective of whether the electrolyte is acidic or alkaline but that the half-reactions are different. When methanol is used as the fuel carbon dioxide and water are the products of oxidation. However if the oxidation is carried out anaerobically, as in a microbial fuel cell, then carbon dioxide, hydrogen ions and electrons are formed instead. By using a membrane to allow the passage of the hydrogen ions and the external circuit to allow the flow of electrons a microbial fuel cell can be used to provide a sustainable energy source using waste water and bacteria. Explain the thermodynamic efficiency of a fuel cell including why, in the hydrogen fuel cell, ΔG⦵ is smaller in quantitative value than ΔH⦵ as work needs to be done to overcome the negative entropy change when gaseous hydrogen and oxygen turn into liquid water. One way to ensure they fully understand this is to get them to work out the value for ΔS⦵ (see 2(d) in the questions).

The syllabus states that the Nernst equation can be used to calculate the potential of a half-cell in an electrochemical cell, under non-standard conditions. This is not strictly true. It can be used to calculate the potential for non-standard concentrations but not when just the temperature is changed as then the expression – (RT / nF) ln Q = 0. Give your students plenty of practice at working out the EMF for cells with different concentrations including concentration cells (where only the concentration of the ions in the solutions of otherwise identical half-cells changes).

Study Guide

Page 163

Questions

For ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Electrochemistry, rechargeable batteries & fuel cells.

For short-answer questions see Electrochemistry, rechargeable batteries & fuel cells questions  together with the worked answers on a separate page   Electrochemistry, rechargeable batteries & fuel cells answer .

Vocabulary list

Nernst equation
concentration cell
microbial fuel cell, (MFC)
proton exchange membrane (PEM) fuel cell
Nickel-cadmium (NiCd or NiCad) battery
Lead-acid battery
Lithium-ion battery

Practical work

Making a microbial fuel cell - this could make a nice project for your own students.

Teaching slides

Teachers may wish to share these slides with students for learning or for reviewing key concepts.

  

Other resources

1. A good description by BASF on how lithium-ion batteries work.

  How lithium-ion batteries work

2. Another good description - this time on how a lead-acid battery works by Bill Hammack from the University of Illinois.

  How lead-acid batteries work

3. An animated overview of fuel cells by the Naked Science Scrapbook which provides a good introduction to the different types of fuel cells and why they are useful (but does not include the half-equations).

  Fuel cells

4. This American Chemistry Society video shows how waste water can be used to produce electricity using microbial fuels cells.

  Using microbial fuel cells to generate electricity

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