fuel cells

[victoria]

I'm doing a project on fuel cell efficiency. I have tried to follow the directions for it found on this site - Fueling the Future. But I don't understand where to connect my wires and multimeter leads during the input and output stages. My fuel cell does'nt look like the one in the picture. It doesn't have posts to put the alligator clips on. My cell just has holes for the oxygen and hydrogen sides. I'm also having trouble figuring out how to use the multimeter and switching from volts to amps so quickly during the experiment. Thanks for any help you can give.

[paulsdecarli]

I'm afraid that isn't enough information to figure out your situation. It might help if you could give the full name (URL)that appears at the top of your screen for the web site on fuel cells. Details on your fuel cell are also needed.

[amyC]

Hi Victoria - I did a search on the Science Buddies site and found the project you're working on. I'm posting the info here in hopes that an expert will be able to take a look and help answer your questions.

Fuel Cells—Fueling the Future!
https://www.sciencebuddies.org/science- ... p002.shtml

Amy
Science Buddies

[deleted-71588]

Have you read "https://www.sciencebuddies.org/science- ... sure.shtml"? If not, then I suggest you do so as a start.

My fuel cell does'nt look like the one in the picture. It doesn't have posts to put the alligator clips on. My cell just has holes for the oxygen and hydrogen sides.

There has to be two electrical contacts of some form somewhere on your fuel cell. Look at any metal parts for something that will accept a small wire. It maybe a screw that the wire goes under, a spring clip that the wire slips into, a post, but there has to be some way to make up two electrical connections to the unit.

I'm also having trouble figuring out how to use the multimeter and switching from volts to amps so quickly during the experiment.

That is a fundamental problem with this experiment. One way to solve this problem is to use two meters, one hooked up in series to measure DC current and one hooked up in parallel with the fuel cell to measure DC voltage. Another way to solve the dual measurement problem is to utilize a 0.1 ohm resistor. Instead of placing the meter in series in its DC MA mode, you place the 0.1 ohm resistor in series with the fuel cell and either the battery or the load (DC motor per the kit described in the project). With the use of this 0.1 ohm "shunt" resistor, you can now leave the meter setup to measure DC volts. If you have a digital multi-meter that will read +/- DC volts, connect one lead to the junction (connection) between the fuel cell and one end of the "shunt" resistor. Then you simply move the other end between the other end of the fuel cell to measure the input voltage and the other end of the "shunt" resistor to measure the voltage across the know resistance. You can then convert this "shunt" voltage into current by utilizing the V = I * R ohms law equation. Divide the "shunt" voltage by 0.1 ohms and you get the current going through the "shunt" resistor. From Kirkoff's law, the current at any point in a series circuit is the same so this is the current being consumed (from the battery in "hydrogen generation mode") or produced by the fuel cell (in "fuel cell" electrical generation mode).

If you have an analog multi-meter instead, then you will have to move both leads from measuring across the fuel cell to measuring across the shunt resistor so that you keep the negative (ususally black by convention) toward the negative terminal of the battery and the positive (ususally red by convention) toward the positive terminal of the battery so that the meter always reads positive and doesn't peg the needle on the "zero" post.

[victoria]

Mr. Craig - Thanks so much for the information about the fuel cell project. Before I had a chance to see your reply, an electrician suggested I use the two multimeter idea - one to measure current and one to measure volts. I hope the resistance of two meters, rather than one, didn't affect my results too much. As for the holes on the sides of the fuel cell, I just stuck the leads of my meter I used for volts into those holes. I then clipped my battery pack to the leads. During the moto?r phase, I clipped one of the wires from the motor to a lead stuck into the fuel cell, and clipped the other wire to the lead of the meter measuring amps. Does all of this sound OK to you? But I don't understand why I had to swap my leads on the multimeter measuring current in order to get a positive reading. The leads had to be one way during the electrolysis phase and had to be swapped around during the motor/electricity phase. Thank you so much for all your help! Victoria

[deleted-71588]

A typical multi-meter on a DC voltage scale will have a very high resistance which is great because it will be in parallel with whatever you are measuring the voltage across. This means that most of the current will flow through what you are across and not through the meter.

On a DC current scale, a typical multi-meter has a very low resistance which is great because it will be in series with whatever you are trying to measure the current through so most of the voltage drop will be across the original part of the circuit and not be across the meter.

Now as to the switch in current direction seen between electrolysis (electricity being used to separate water 2 H2O into 2 H2 and 02) and the fuel cell producing power (2 H2 + O2 => 2 H2O). For Hydrogen to be released, it has to be ionized. Hydrogen in ion form is H+ so it will be attracted to and form at the negative electrylysis electrode which will give it an electron. For this to happen, electrons have to flow into this electrode. Since electrons carry a negative charge and are flowing into this electrode and current by convention is positive, current must flow in the opposite direction of the electrons, so the current is flowing out of this electrode. When you switch to fuel cell mode, the H2 molecule needs two give up two electrons to the fuel cell electrode to ionize, so the electrons have to flow out of the hydrogen side electrode. Again, current by convention is positive, so it flows into this electrode.

By this reasoning, current flows out of the hydrogen side electrode during electrolysis and into the same electrode in fuel cell mode, the current flow "reverses". This isn't surprising for a "reversing" fuel cell when you break it down.