1. Fuel Cells and Carnot Cycle Heat Engines--Relationship to Physics. One fascination of fuel cells (see discussion below) is that they are not heat engines like current fossil or nuclear fuel-based electric energy plants and automobile engines. Heat engines are all limited in their efficiency by the Carnot cycle; the difference between the operating temperature of the engine (typically a steam turbine for electricity generation or the inside of the cylinder for autos) and the environment (typically the outside world) divided by the temperature of the environment gives the maximum theoretical efficiency of a heat engine if everything works perfectly and there is no friction.
For example, an automobile engine running at 627 °C (9000 K) on a hot day of 27 °C (300 K) would have an ideal maximum efficiency of 30%. Because real automobiles aren't frictionless or otherwise perfectly designed, the real fuel efficiency is less than 30%. This limit holds regardless of the kind of fuel used. By contrast, the efficiency of a fuel cell (and batteries, for that matter) are governed strictly by thermodynamic properties of the reactants--the electrochemical reaction's free energy change divided by its enthalpy change provides the theoretical efficiency of a fuel cell or battery.
2. High-Tech Batteries/Fuel Cells and High-Temperature Super Conductors. A challenge for many advanced battery systems researchers is finding materials that will work at high temperature and pressure. One approach is to make conductive ceramic electrolytes that will allow ions to move through them. Many rare earth and heavy metal oxides that are used to prepare high temperature superconductors (e.g., yttrium oxide) are also used in preparing conductive ceramic electrolytes for advanced battery systems.
3. Biology and Electrochemistry. As mentioned in the introduction to the laboratory activities in this module, biochemical systems are electrochemical in nature. Two examples include photosynthesis and the respiratory chain of the mitochondrion. There are also electric eels, ion currents and action potentials in nerves, brain electricity as measured by the electroencephalogram, or EEG, and the ion currents in heart muscle measured by the electrocardiogram or EKG.
If your students are mathematically oriented, it might be interesting for them to calculate human power output based on the oxidation of glucose as a fuel as described by Chirpich (1975). An outstanding resource on bioelectricity and a number of other interesting topics is the book by Becker and Selden (1985).
4. Modern Electrochemical Instrumentation. Electrochemical instruments operate both in voltaic and electrolytic modes. Like all electrochemical cells, a minimum of two electrodes are employed. In some cases, a third half-cell is used as an internal reference cell; this half-cell determines the potential from which all voltages are measured. The most common reference cell is the calomel half-cell, sometimes referred to as the calomel electrode. The half-reaction for the calomel electrode is:
Electrochemical instrumentation is commonly used to determine the concentration of ionic species in aqueous solution. In the simplest case, an electrode is held at a constant potential vs. the reference. The potential is chosen so that an electrolysis reaction occurs that involves the species of unknown concentration. The number of electrons transferred at this electrode (the current) is related to the concentration of the species of interest. This technique is called polarography.
In addition, quantitative measurements can also be made in voltaic cells. The cell potential of a voltaic cell depends on the solution concentration of the species undergoing reaction at the electrode. (See a discussion of the Nernst equation in any standard college chemistry text for further information on this relationship.) Electrochemical measurements of pH and various other specific ion concentrations are examples of this voltaic technique.
