Although not directly involving biofuels, the development of alternative drive systems that do not use fossil fuels for transport is important in the reduction in fossil fuel use and greenhouse gas emissions. There are a number of systems being tested including fuel cells, electric cars and hybrid systems.

Fuel cells

Fuel cells have had a long development, including use in the NASA Apollo pro­gramme in 1960, and since 1990 an experimental transportation system has been introduced. A fuel cell consists of two electrodes — the anode and cathode — divided by an electrolyte (Fig. 3.7). Hydrogen is run into the anode, where a platinum-coated

Fig. 3.7. Outline of a fuel cell.

proton-exchange membrane splits the hydrogen into hydrogen ions (protons) and electrons. The protons pass through the electrolyte to the cathode where they combine with oxygen, forming water. The electrons produce an external current which can be used to run an electric motor.

Fuel cells are classified by their operating temperature which is also determined by the electrolyte (Stambouli and Traversa, 2002). Table 3.5 gives some of the char­acteristics of fuel cells. Fuel cells can be combined in stacks, connected in series to produce the desired voltage. The number of fuel cells in a stack determines the voltage and the surface of each cell determines the current. Proton exchange and solid oxide fuel cells are the most advanced and have been fitted into experimental cars.

Two recent developments in fuel cell technology are the direct carbon fuel cell and the microbial fuel cell. In the direct carbon fuel cell, fine particles of carbon (10-1000 nm) are mixed with molten lithium, sodium, or potassium carbonate at 700-800°C (Cooper, 2006). The molten salt is introduced into the anode compart­ment and air to the cathode (Fig. 3.8). Electrons are carried from the carbonate to the cathode. Oxygen passes through a membrane which reacts with carbon, releasing electrons, forming carbon dioxide.

The microbial fuel cell derives energy from organic compounds metabolized by microorganisms. Figure 3.9 shows the layout of a microbial fuel cell. Microbes in the anode chamber oxidize substrates added to the chamber, generating electrons and protons as found in the chemical fuel cell. Carbon dioxide is formed but as organic substrates are used, the carbon dioxide released is only that fixed during photosynthesis.

Cathode

Proton

exchange

membrane

Fig. 3.8. Direct carbon fuel cell. (Redrawn from Cooper, 2006.)

The reactions are as follows when using acetate as a substrate:

CH3COOH + 2H2O ® 2CO2 + 7H+ + 8e — (3.1)

At the cathode the protons react with oxygen:

O2 + 4e — + 4H+ ® 2H2O (3.2)

To extract electrons to the anode, mediators have to be added to the anode chamber. These mediators move across the microbial cell membrane where they are reduced

and pass out of the cell, releasing the electrons to the anode. Mediators are dyes and metallorganics such as neutral red, methylene blue, thionine, Meldola’s blue and 2-hydroxy-1,4-naphthoquinone. However, the instability of the mediators limits their use, but recently a group of bacteria, the anodophiles, have been isolated. These bac­teria (including Shewanella putrefaciens, Geobacteraceae sulfurreducens, Geobacter metallireducens and Rhodoferax ferrireducens) attach themselves and transfer elec­trons directly to the anode (Du et al., 2007). Some microbial fuel cells have been inoculated with bacteria mixtures such as sewage sludge and sediments which have the advantage of a wider substrate range. The amount of electricity provided by the microbial fuel cells is still very low, but they can be stacked and used to produce hydrogen, for wastewater treatment and as biosensors.

Alternative biological fuel cells have the microbial cells replaced with enzymes. This has the advantage of having a higher volumetric catalytic capacity, and it avoids toxic oxidation products. One of the fuels tested in an enzyme-based fuel cell is gly­cerol, one of the by-products of biodiesel production. Glycerol is a non-toxic, non­volatile, high-energy density substrate (6.3 kWh/l) for a cell containing the enzymes alcohol dehydrogenase and aldehyde dehydrogenase (Arechederra et al., 2007).