Going round the exhibition of the Association for Science Education always

leaves me with a few small challenges, such as “how much can I really take in of

what an exhibitor is telling me?” and “how much school science do I really

understand?”  For example, on the same table as the Toilet

Roll Fungus, part of the NBCE exhibit, I came upon

two fuel cells.  I’ve had to think quite hard before writing this one

up

The first of these was the yeast cell.  It’s sitting there in the

middle of the picture, a small tank full of dark blue liquid with two wires

coming out of it to power an odd motor-driven device at the bottom.  Behind it

is a tin of Allinson dried yeast, a popular brand for baking in the UK.  Top and

left are bottles of very dark methylene blue

solution,

(indent)

Methylene Blue is a

dye of very low toxicity, deep blue in its oxidized form and colourless when

reduced.  It has so many applications, it could be called ‘the chemical of a

thousand uses’.  Further to the left there is a yellow solution of what used to

be called potassium ferricyanide, but is now more properly called potassium

hexacyanoferrate(III).

From an article The Microbial Fuel

Cell comes this diagram.  The yeast is respiring, with glucose as fuel. 

Given sufficient heat, the glucose would burn in atmospheric oxygen anyway, but

the respiratory system does two things: (1) it is catalytic, and so allows the

overall reaction to proceed at lower temperatures, (2) it taps off the energy

released to be used for the yeast cell’s own purposes, and in animals for

movement and digestion.  In this cell, electrons from the methylene blue at a

higher potential are passed through an electrical circuit in order to do some

work — not much in this case, the motor is there for show rather than doing very

much.

The

overall reaction in this case is glucose being oxidized by the

hexacyanoferrate(III), which becomes hexacyanoferrate(II).  I suppose that in

the process, the yeast is losing much of the value of the glucose.

This

cell serves only educational purposes, albeit very well — it’s given me insight

into things I studied as a student but didn’t grasp well because of a forest of

details.  The next one perhaps has perhaps less immediate educational value in

terms of science studied, but it’s more fun and has a practical application. 

One goes to one’s nearest pond or lake, retrieves some smelly mud from the

bottom (wet garden soil won’t do) and makes these mud cells.  Here, in the main

part of the diagram, are two connected in series.  They are filled with mud from

the bottom of the university lake.

To see what they

are made of, first look at the inset bottom left of picture.  E is a

carbon fibre electrode, long and thin but with a square paddle hidden at the

bottom of the pot.  Behind it is a single made up cell, with a red wire coming

out of the cathode (positive) and a black wire from the anode

(negative)*.

However,

the working component of these cells is again a microorganism, this time Geobacter.  The genus was

first discovered in 1987 in the Potomac River, which sticks in my mind as

marking the frontier between the Union and Confederate forces in the American

Civil War.  The species pictured right is the first discovered,

G.metallireducens.  This is surrounded by a mass of flagella,

which enable it

to swim and find food.  But it also has a few long pili, but originally

it was not known what these are for.  These bacteria are anaerobic, and

different species derive their energy by transferring electrons along their respiratory chain to a variety of oxidizers, such as ferric oxide (rust) and uranium.  At present the star of the

show is Geobacter

sulfurreducens, found in 2003 in a soil sample contaminated by

hydrocarbons in Oklahoma.  It uses organic matter for food, but instead of

oxygen can burn its food using among other things sulfur or sulfates, giving

rise to smelly hydrogen sulfide.

The first major use of Geobacter was

seen to be bioremediation, since they can live in all sorts of muck.  But

G.sulfurreducens can be used in microbial fuel cells that are much

more efficient than the yeast cells above.  A new strain KN400

might even power batteries attached to turtles to track their migration.  This

arises because the pili, although made of protein, are conductive like metallic

nanowires.  Indeed, G.sulfurreducens might even bridge

the gap between solid state electronics and biological

systems.

But back to the cells, particularly the one at top

middle of picture.   The paddle of the cathode is just under the mud at the top,

attached to the electrode shaft by a clip C, while the paddle of the

anode is at the bottom, away from oxygen so the bugs can do their

work.

Such wonderful things in science education these days, perhaps with

things like this I might have had a better grasp of many subjects.  Kids today —

they don’t know when they’re well off!

* I used to find the

definitions of Cathode and Anode quite

confusing.  For example, the anode of a battery is the negative terminal,

whereas in a vacuum tube the anode or ‘plate’ is positive.  Even worse, in a

recharging battery, the two terminals swap roles as anode and cathode.  The two

Wikipedia links just given are helpful for sorting this

out.

Further Reading

Bacteria

power, from the American Chemical Society

Potential use in

supercapacitors, from the Geobacter Project.