Contents

Basics

Fuel Cell Technology and the MFC

As you may or may not have noticed, the Earth is, well, sort of polluted. The situation is not helped by the world's huge dependence on gasoline-powered cars. What the world needs, one would suppose, is a car that not only doesn't emit greenhouse gases, but also is efficient, and one would hope, practical. The thing about hydrogen cars is that, yes, they only emit water vapour, but they're not feasible yet because nobody knows how to extract pure hydrogen gas without it being grossly expensive or even more detrimental to our environment.

Well, until recently anyway.

A diagram of a PEM fuel cell.

The varying nature of bacteria make it impossible for a single model of fuel cell work for all species. Since there are so many different bacteria that live in so many different habitats, the variations and possibility for revolutionary innovation seem endless. Some MFC's take bacteria out and culture them within the cell, but some of them are installed right into the bacteria's natural habitat.

General Biology

First of all, many of the words presented will be chemicals such as "acetate." When something ends with "ate," it means that it is an organic acid. Organic acids are usually weak acids, unlike inorganic acids like HCl. This means that it can maintain equilibrium for a solution by absorbing and "giving off" hydrogen ion, the thing that determines how acidic a substance is. Actually, all you really need to know about organic acids at the end of the day is that when you give a proton to acetate (this is called "reduction"), it becomes acetic acid.

An electron mediator or shuttle is a molecule that is very large and complex, which can take electrons from another molecule, float over to an anode, and release the electron. These are used in fuel cells to increase its efficiency, and they are often manufactured by the bacteria themselves. Most people prefer not to use them, however, because they are often toxic to humans.

Speaking of which, this would be a good time to explain a little bit about electricity. Often on this site, the words "anode" and "cathode" come up. What are they? These two things are collectively known as electrodes. They conduct electrons from one side of the cell to the other. What happens is, a reaction takes place in the anode side of the fuel cell and some electrons fly out. These would normally get immediately placed in some other part of the raction process, but instead they are stolen by the anode, made to go through a circuit to the cathode, and then joined to where it was supposed to go. This way, we get current. My chemistry teacher from my not-so-distant past gave my class an incredibly corny, and therefore unforgettable, nmemonic for remembering which is which.

The "Cat"hode is "pussy"tive. (He was British, if that explains anything).

A redox reaction is a coupled reaction, that is, it's the coupling of oxidation and reduction. Oxidation used to be used to describe when an element turned into an oxide, such as aluminum combining with oxygen to form aluminum oxide. Reduction does not mean "to take away," but "to lead back," which is what the meaning of its Latin root is. In other words, if magnesium oxide combines with carbon, you are "lead back" to solid magnesium, and you also get carbon dioxide.

Now that we know about electrons, reduction and oxidation are talking about electron exchange. If the reaction takes electrons away from something, it's an oxidation reaction. If the reaction gives electrons to something, it's a reduction (because it "leads back" to its original state, I suppose). Another nmeonic I picked up is OIL RIG. Oxidation is Less, Reduction is More. Simple, effective.

Remember that since matter cannot just disappear or sppear, whenever there is oxidation, there is reduction somewhere else. That is why the combined reactions are called redox reactions.

A coupled reduction and oxidation reaction.

What is respiration?
Most people think of respiration as "breathing," but that is only part of the overall molecular process. In order to understand a lot of this site, you need to be familiar with the basic working of cellular respiration, but you don't need to memorize all the protein complexes and such. What you need to know is that cellular respiration begins with a fuel, like say, glucose. This glucose molecule gets warped around until it turns into carbon chains of three carbons each. In the process, a molecule called ATP, the energy storage molecule of the cell, gets created. The carbon chains, called pyruvate, also gets warped aroun, this time in a process called the Kreb's cycle. The important thing about the Kreb's cycle, besides the production of more ATP, is the reduction of several molecules of NAD+ into NADH. These molecules are electron transport molecules. In aerobic metabolism (involving oxygen), these NADHs go through somehtign called the Electron Transport Chain, where the electrons are successively passed on to a molecule with a higher potential energy. The final destination is oxygen. If the electron was to go straight to exygen, the energy released from the reaction might blow up the cell. This way, it is released gradually.

What is important to know about this information when looking at fuel cells is a) What fuels it. b) What gets reduced. c) What gets oxidized. Bacteria do not follow the exact same metabolic pathways as human cells, so some details of the above scenario, such as the three factors I listed above, change. Those changes are crucial to understanding the workings behind each different MFC.

Microbiology

What are bacteria? What do their names mean?
Bacteria are single-celled organisms that come in a variety of shapes, colors, colonization patterns, gram types, etc. Since bacteria reproduce so fast and so many get killed (but just enough survive), evolution has shaped bacteria into countless amounts of forms. Bacteria, since they're all so different, have to be classified under a very orderly taxonomy system so we can keep track of them all.

The order for taxonomy goes: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Whenever we see a bacteria's name, we see something like Escherichia coli. The first one, Escherichia, is the name of the genus, and coli is the name of the species. Sometimes microbiologists talk about an entire order of bacteria. When they do this, they usually talk about very general characteristics such as temperature tolerance.

Why are bacteria used for batteries (as opposed to chemicals and enzymes)?
For once thing, the reason why bacteria are used over inorganic substances is their ability to reproduce and adapt. This means MFCs can be left along for a very long time, provided they are given a constant source of food.

The reason why bacteria are used over enzymes is similar. Plus, bacteria can tolerate higher pH than the enzymes needed for EFCs. This gives more flexibility with the environments and conditions that MFCs can be put into.

What kind of bacteria are used for batteries?
Any bacteria that displays an ability to either produce a useful byproduct, such as hydrogen, or to use something that is either very expendable or unwanted like sugar and wastewater can be used as microbes in a fuel cell. In fact, since the criteria is so loose and the variety of bacteria so high, the listed "target" bacteria on this site are by no means the only or even necessarily the best bacteria for MFCs.