Using a PEM (Hydrogen) Fuel Cell
Fuel cells! Power source of the future!
Ever thought it would be cool to have one of these things powering your next big project? Well now you can, because I'm going to tell you how =)
I'll be explaining the basics of how a PEMFC (Proton Exchange Membrane Fuel Cell) works, and how you can make use of one. Also worth noting is that Wikipedia has a great article with more detail if you're interested. Also, PEM isn't the only type - these days there are many tested and emerging fuel cell technologies.
• HOW PEMFCs WORK •
This diagram (shamelessly stolen from Wikipedia) gives an idea of how the PEMFC functions. The electrolyte in a PEMFC is the Proton Exchange Membrane itself, which is a thin polymer sheet, typically made out of Dupont's 'Nafion'. No chemical elements can pass through the Nafion layer, with the sole exception of Hydrogen, and even then the Hydrogen can't pass through the Nafion without ditching its one-and-only electron. The process by which the Hydrogen (now just a solitary proton) is shuttled through the Nafion layer isn't totally understood, but it's a very useful trick.
The Oxygen waiting on the other side of the Nafion barrier is hungry for extra electrons, and given the chance it'd strip them right off the Hydrogen molecules - but the Nafion seperates the Hydrogen and Oxygen so that can't happen. Instead, metal terminals on either side of the Nafion layer provide a pathway for the electrons, so that they're able to travel from the Hydrogen-side to the Oxygen-side. This way everyone's happy - the Hydrogen atoms can ditch their electrons and pass through the layer to bond with the Oxygen (producing water), the Oxygen can collect the electrons coming into the Oxygen-side of the fuel cell, and the electrons that are passing through the external circuit can be used to drive any electrical devices that are connected.
If you have a reversible fuel cell (which almost all readily available small fuel cells seem to be) then you can also run this reaction in reverse. Pumping electrons into the Hydrogen-side of the fuel cell and pulling them out of the Oxygen-side will tear the water molecules apart. The Oxygen will reform as Oxygen gas, and likewise the Hydrogen will skip back across the Nafion barrier and reform as Hydrogen gas.
When used in this way, by 'charging' and 'discharging' the fuel cell, the whole unit acts very much like a rechargable battery. The main difference is that you can easily increase the energy storage capacity of the 'battery' just by making your Hydrogen and Oxygen storage tanks bigger.
• HOW TO SET UP THE PEMFC •
A typical, single layer PEMC will look something like the photos below:
This one is conveniently labelled with the gases consumed/produced on that side. Note that each side has two ports for connecting hoses to. Unsurprisingly, the black and red terminals on the top are the negative and positive cell terminals respectively. Make sure you know which side is Hydrogen/negative and which side is Oxygen/positive - most common fuel cells can be damaged if you connect them the wrong way around. Also, as with a regular battery, a short circuit will burn through your fuel supply quickly and generate possibly damaging amounts of heat, so be careful. Some people like to put a high-current diode between the positive and negative terminals to soak up current if the cell is accidentally charged in reverse, but this is optional. Connect the diode's cathode to the positive FC terminal, and the anode to the negative terminal if you decide you want to have the extra safety feature.
For the PEMFC to work, it needs high purity water on both sides of the cell. The membrane needs to be left for at least 10 minutes after adding the water so that it can soak in before you start using charging/discharging the cell. The water on the Oxygen-side will also be split into Hydrogen and Oxygen gases during charging, but a tiny amount of water produces a huge volume of gas.
Do not used tap water, mineral water, purified water, filtered water or any of that stuff - they're all full of impurities that'll slowly clog up the PEMFC and make it less efficient. Look for water that has been distilled instead, or if needed you can distill your own. I've got a big 10L container of steam distilled, ozonated reverse-osmosis treated water that's about 100 times purer than typical bottled water. $6 at the supermarket, well worth the trouble of lugging it back to the car. Also good for watering bonsai trees.
Hopefully you'll have been supplied with a few short pieces of rubber hose, a few hose stoppers, and a plastic syringe along with your fuel cell. If not you should probably find some gear along those lines (gardening supplies work well, i.e. automated sprinkler system hoses and fittings).
• Clean your hose, stoppers and syringe by flushing them with distilled water.
• Connect a piece of hose (at least 30mm, or 1inch) to each of the 4 ports on the FC.
• Fill your syringe with the distilled water.
• Insert the syringe into the lower hose attachment on the Hydrogen-side, and push the water into the cel until all the air is expelled and the water starts coming out of the top hose.
• Insert a stopper into the top hose, trying not to let any air back in.
• Pinch the lower hose with your fingers and pull the syringe out, replacing it with another stopper.
• Repeat the filling part for the Oxygen-side of the FC.
Now your PEMFC should be left to soak for at least 10 minutes. After that you can actually start using it like a battery, but since there's nowhere for the accumulated gases to go the capacity will be pretty low, so the cell will charge and discharge quickly.
This next part may require some creative thinking on your part - where are you going to store the gases for the fuel cell? A few things to remember about your storage system ideas are that they must:
• Be sealed well enough to keep the gases trapped under slightly elevated pressures.
• Not be so restrictive that the fuel cell has trouble pushing the gas into the container.
• Have no problems with exposure to water, Oxygen or Hydrogen.
A common setup is to run the fuel cell gas hoses into two inverted jars/glasses/vials that are submersed inside a larger container and totally filled with distilled water. Oxygen and Hydrogen don't dissolve in water very much, so when the gas flows out of the hose it's trapped in the top of the inverted container. As more gas flows in, the water is pushed down into the larger container, so gas can be accumulated without a large increase in back pressure. Producing too much gas just causes the inverted container to overflow and bubble off into the air, so no worries there either. The main disadvantage of this setup is that you have to keep the storage arrangement upright, otherwise everything just spills out.
Alternatively you can use the balloon system I developed for Mr. Lobster, which I haven't had any trouble with.
Regardless of how you store your gases, make sure to once again wash everything with distilled water just to be safe. Also remember to keep your Oxygen and Hydrogen seperated - it's a pretty explosive mix =)
Once your storage system is prepped you can hook up the lower hoses from the fuel cell to your storage tanks, and now you're ready to go!
• HOW TO OPERATE THE PEMFC •
To 'charge' the PEMFC and produce your fuel gases, connect a DC power source (it doesn't have to be very clean, so long as there are no big voltage spikes) in the normal positive-to-positive and negative-to-negative arrangement. You should have some reccomended charging specs from your supplier, but in case you don't a common PEMFC requires 1.7V to 2.0V, at 700mA for maximum charging rate. To compensate for various internal resistances and innefficiencies, it's common to charge the FC with closer to 3V to ensure you get a good current flowing into the cell. The easiest source to charge your FC is a nice new pair of 1.5V AA alkaline cells.
Virtually the instant you connect power, you should see bubbles of gas forming on the inside chambers of the FC. The chambers will fill up, pushing the water out of the cell and into the storage tanks, and then eventually filling the tanks with accumulated Hydrogen and Oxygen.
Once you've got some fuel saved up, you can use your PEMFC to power a few devices. The output for a small typical single cell is nominally 0.6V @ 500mA, but often you'll see closer to 1V @ 800mA when the cell is freshly charged. You can connect multiple FCs in parallel or series to increase the current or voltage output respectively. Some PEMFCs, like the one used in Mr. Lobster, are actually two FCs back to back, with a shared Hydrogen or Oxygen chamber.
Like most rechargeable power storage systems, the power output from the FC will be fairly consistent at first, but gradually weakening as the fuel supply runs out. Conveniently you can visually see how much 'juice' an FC has left just by looking at the tanks and checking how full they are =D
One last note - you'll probably find that most circuits need a fair bit more than ~1V to work, and connecting multiple FCs in series can be expensive, bulky, and annoying. Since the current output of even a small FC is typically quite high, you can use a voltage booster circuit to 'trade in' some of that extra current for more voltage. Take a look at this example by OddBot to get started in the right direction.