Questions on Electrolysis and Fuel Cells

[adoado]

Hey all,

This is just a post about some questions I had regarding electrolysis and fuel cells. I know that when electricity is forced through water the water molecules split, and produce hydrogen and oxygen gas. I was wondering how things like the electrode material and how far apart they are affect the rate of reaction?

If I used zinc metal, would it be different from copper? If they are one centimeter apart, will it be faster than 5 cm apart?

I am also curious about why platinum seems to be a good choice in hydrogen fuel cells as the catalyst. I have always wondered, what makes a specific element a good catalyst -i.e. can I substitute platinum with graphite?

Cheers,
Adrian

 

[Borek]

If I used zinc metal, would it be different from copper?

Yes, you want electrode from the inert material - neither is inert enough here.

If they are one centimeter apart, will it be faster than 5 cm apart?

Speed of the reaction depends in this case mostly by the current that you are able to force. Large distance between electrode means large resistance, which in turn means you will need higher voltage for the same speed. That's not all, but that's the first effect to take care of.

 

[Ygggdrasil]

Speed of the reaction depends in this case mostly by the current that you are able to force. Large distance between electrode means large resistance, which in turn means you will need higher voltage for the same speed. That's not all, but that's the first effect to take care of.

Will the speed depend more directly on the current or the power? (not that it matters for the answer since both models predict that the speed of the reaction will decrease with increasing resistance).

 

[Borek]

I would say on the current - after all, it is all about charge & Faraday's law of electrolysis. Voltage is the driving force, but it is current that does the job.

 

[Topher925]

The rate at which the reaction occurs is directly proportional to current, as borek stated. The major parameters that effect that rate are overpotential or voltage (obviously), temperature, and of course catalysts.

A catalyst is a material that lowers the activation energy of a chemical reaction yielding a greater average probability that a reaction will actually occur. In laymens terms, it reduces the size of the "energy hill" that the molecules will have to clime before they can do anything. A good catalyst is a material that makes this hill as small as possible.

What makes Pt a good catalyst for fuel cells is that it has a good intermediate bond-strength for the hydrogen charge transfer reaction. If the bond strength between the metal and the hydrogen is to weak, then the hydrogen won't bond to the surface of the catalyst. If the bond strength is to strong, then it won't leave the surface of the catalyst. So a good catalyst essentially has that "goldylocks" bond strength between itself and the reactants. For hydrogen, this peak catalytic activity is in the platinum group metals. So metals like Pt, Pd, Ir, and Rh will all make good catalysts for fuel cells or electrolysis cells.

A sort of similar thing happens on the other side of cell where oxygen is reduced but its much more complicated and not well understood so I won't go into it.